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#Operation Manual

CanogaVehicle Detection SystemModel 701 Microloop

Model 701 Microloop Operation Manual i

Table of Contents

1 Introduction..................................................................................................................................................1

1.1 Basic Application Guide.....................................................................................................................1

1.2 Description .........................................................................................................................................2

1.3 Basic Installation Guide .....................................................................................................................2

2. Theory of Operation ....................................................................................................................................3

2.1 Basic Theory.......................................................................................................................................3

2.2 Magnetic Principles ............................................................................................................................32.2.1 Geomagnetism .....................................................................................................................52.2.2 Effects of Ferromagnetic Material on Earth’s Magnetic Field ............................................5

3. Operational Characteristics and Usage Considerations...........................................................................8

3.1 Vehicle Detection Characteristics ......................................................................................................83.1.1 Sensitivity to Vehicle Presence – Vehicle Size and Type ...................................................83.1.2 Loop Detector Sensitivity Setting........................................................................................103.1.3 Inductance Increases............................................................................................................103.1.4 PRESENCE Versus PASSAGE Detection ..........................................................................11

3.2 Microloop Probe Placement Configurations ......................................................................................123.2.1 Vehicles Centered in a Traffic Lane ....................................................................................123.2.2 Vehicles Changing Lanes and Motorcycles/Bicycles..........................................................123.2.3 Ensuring a Single Detection per Vehicle .............................................................................12

3.3 Detection Area....................................................................................................................................13

3.4 Vehicle Speed, Vehicle Spacing Considerations................................................................................13

3.5 Microloop Operating Environment Requirements .............................................................................143.5.1 Magnetic Field Intensity ......................................................................................................143.5.2 Magnetic Field Noise...........................................................................................................153.5.3 High Iron Environments ......................................................................................................16

3.6 Matching the Microloop to Digital Loop Detectors ...........................................................................16

4. Guide to Specific Applications....................................................................................................................19

4.1 Speed Measurement ...........................................................................................................................19

4.2 Counting Vehicles ..............................................................................................................................21

4.3 Advance Detection .............................................................................................................................22

4.4 Ramp Metering...................................................................................................................................23

ii Model 701 Microloop Operation Manual

Table of Contents (continued)

5. Microloop Installation ................................................................................................................................25

5.1 General ...............................................................................................................................................25

5.2 Microloop Descriptive Data ...............................................................................................................255.2.1 Physical................................................................................................................................255.2.2 Environmental......................................................................................................................255.2.3 Electrical ..............................................................................................................................25

5.3 Installation Steps ................................................................................................................................265.3.1 Installation Planning ............................................................................................................265.3.2 Magnetic Field Measurements at Installation Site...............................................................285.3.3 Hole Boring and Saw Cutting..............................................................................................295.3.4 Microloop Probe and Cable Placement................................................................................295.3.5 Resistance Checks ...............................................................................................................305.3.6 Backfilling and Sawcut Sealing...........................................................................................305.3.7 Splice to Lead-in (Home-run) Cable ...................................................................................305.3.8 Connection to Inductive Loop Detector and Final Checks ..................................................31

6. Proposed Specification for Canoga Microloop..........................................................................................33

6.1 General ...............................................................................................................................................33

6.2 Physical ..............................................................................................................................................33

6.3 Environmental ....................................................................................................................................33

6.4 Electrical.............................................................................................................................................33

Model 701 Microloop Operation Manual 1

1. Introduction

The 3M™ Canoga™ Vehicle Detection SystemModel 701 Microloop is a unique, reliable devicefor sensing vehicles passing over it. This passivedevice is contained in a small, cylindrical probe,which transforms changes in magnetic fieldintensity into changes in inductance. Vehicleschange the intensity of the earth’s magnetic field.When the Microloop is attached to a device thatsenses changes in inductance, such as an inductiveloop detector, vehicle passage over the Microloop isdetected.

Figure 1-1. Model 701 Microloop –Dual Probe Set

The Microloop is intended to be buried beneath theroadway surface and connected, via lead-in cable, toa loop detector. Installation costs may besignificantly less than those for conventional loops,because only single, straight ¼-in. sawcuts need tobe made and a 1-in. diameter hole drilled about 20inches deep. Also resulting is the opportunity forgreatly increased service life due to the reducedexposure to hazards such as road traffic, pavementmovement, pavement deterioration, and roadwork.

The fact that the Microloop is not installed in thepavement and that the lead-in is very durable allowsthe Microloop to be used in situations such ascobblestone pavements, poor pavements, dirt/gravelroads, and bridge decks. Minimal intrusion intogood pavements is another benefit.

The area of detection provided by a probe is smallerthan that provided by a 6-ft. x 6-ft. loop and may beconsidered a point detector. This characteristicallows separation of closely spaced vehicles, verygood resistance to detecting vehicles in adjacentlanes, and near immunity to “crosstalk” problems.

The Model 701 Microloop is ideally suited fordetecting the passage of moving vehicles (vehicles

moving at a 10 mph or faster) when connected to aCanoga™ Digital Loop Detector. The Model 701Microloop is not recommended for detecting slowlymoving or stationary vehicles. For details, seeSection 3, Operational Characteristics/UsageConsiderations.

1.1 Basic Application Guide

The recommended applications for the Model 701Microloop include advance detection, speedmeasurement, counting, and ramp metering passagedetection. These passage type applications arenormally served well by inductive loop detectorsoperating in the PULSE mode. It is recommendedthat all inductive loop detectors connected toMicroloops be operated in the PULSE mode.

Microloops may be used in several differentconfigurations to customize vehicle detectionresults. They may be placed in series to ensuredetection of small vehicles, such as bicycles, in alane, or for multilane detection. They may also beplaced in series with standard loops for specializeddetection configurations. (See Section 3 for detailedrecommendations).

Microloop operation with Canoga™ Digital LoopDetectors has been fully characterized. The CanogaDigital Loop Detector should be set to use PULSEmode, NORMAL recovery mode, and NORMAL orLOW frequency. The total inductance connected toa channel should be 400 microhenries or less toensure proper signal amplitude to the Microloop.

Usage of the Model 701 Microloop with otherbrands of inductive loop detectors may providesatisfactory performance. However, the user mustdetermine if performance is acceptable.

To determine if another brand of detector willperform satisfactorily several factors must beconsidered. The Model 701 Microloop inductanceis about 25 microhenries per probe, Q is less than 5,the peak-to-peak voltage (per probe) must be within.25 to 1 volt p-p, sensitivity decreases as the loopfrequency increases, and the inductive loop detectormust adequately handle inductance increases. Manymodels of inductive loop detectors do not providethe set of operating conditions required by the

2 Model 701 Microloop Operation Manual

Microloop. For guidance in using other loopdetectors with the Model 701 Microloop, seeSection 3.6, Matching the Microloop to DigitalLoop Detectors.

Adequate handling of inductance increases is thetroublesome parameter for most digital inductiveloop detectors. Often this problem will be missedduring a casual performance observation.

Model 701 Microloops decrease in inductance as themagnetic field intensity through them increasesabove the earth’s ambient magnetic field. Similarly,their inductance increases as the magnetic fieldintensity through them decreases below the earth’sambient magnetic field. Typically, as a vehiclepasses over the Microloop, the magnetic fieldintensity through the Microloop may beinstantaneously either less than or greater than theearth’s ambient magnetic field intensity. Thisresults in both inductance increases and decreasesbeing seen by the inductive loop detector.

Many inductive loop detectors almost instantlyadapt (use as the new reference frequency) to afrequency decrease caused by an inductanceincrease. If vehicles are moving very slowly, orstop at the proper position, the inductive loopdetector may quickly adjust to use the decreasedfrequency as its new reference frequency. When thevehicle then leaves, the vehicle detector willfunction as though a vehicle is actually there.PULSE mode will eliminate such a false “call” inabout 2 seconds. NEMA TS 1-1983 Section15.2.17.2 requires that a loop detector in PULSEmode become responsive to additional vehiclesentering a loop within 3 seconds after a vehicle hasstopped over a portion of the loop. In PRESENCEmode a call could remain for several minutes.

Canoga Digital Loop Detectors, in NORMALrecovery mode, adapt to frequency decreases(inductance increases) relatively slowly. If trafficspeeds always exceed 10 mph, PRESENCE modedetection can be used. If stop-and-go trafficregularly occurs, only PULSE mode should be used.This will ensure that adaptation to an inductanceincrease creates only a momentary operationalchange.

1.2 Description

The Model 701 Microloop is comprised of aninjection molded urethane cylinder (7/8-in.diameter) with cable emerging from one end as

shown in Figure 1–1. The cable is jacketed withdurable urethane and will fit into a ¼-in. sawcut.

Microloop probes are available in standard sets assingles and doubles. They are also available incustomized configurations by special order.Customized configurations may have any probeseparation, any lead-in cable length, and amaximum of four probes per set.

1.3 Basic Installation Guide

Installtion is straightforward. A 1-in. diameter hole(18-in. to 24-in. deep) is drilled for the probe(s). Ashallow, ¼-in. sawcut from the hole(s) to the edgeof the road provides a path for the lead-in wire. Theprobe(s), with lead-in, is inserted into the hole(s),and the hole(s) filled with dry sand. 3M BrandDetector Loop Sealant material is then used to fillthe slot and top potion of the hole(s).

It is important that the probe be installed in avertical position and that the vertical position of theprobe be maintained. Under some soil conditions,installation can be further simplified by installingthe Microloop inside a length of PVC pipe (not steelpipe) with an interior diameter of about 1 inch. Inthis case, the hole must be slightly larger than theoutside diameter of the PVC pipe, typically 1-5/8-in.or less. After insertion of the PVC pipe andMicroloop, all cavities are back-filled with fine, drysand and the installation completed in the normalmanner.

See Section 5 for detailed installation guidelines.


2. Theory of Operation

2.1 Basic Theory

The heart of the Microloop probe is a network ofpassive inductive components. One of thesecomponents contains a special magnetic materialthat causes the inductance of the Microloop probe tochange as the magnetic field intensity through theprobe changes. The response of a typical singleprobe is shown in Figure 2-1.

The Microloop is designed to be installed in avertical position, and it uses the vertical portion ofthe earth’s field (geomagnetic field) as its ambientbias magnetic field intensity. Most cars will causethe magnetic field intensity through the probe toincrease 20%. A typical location in the UnitedStates could have a 500 milloersted ambient field.Thus, the field intensity in the probe would move to600 milloersted. This would cause the inductanceof the probe in Figure 2-1 to change from about 36.7microhenries to 36 microhenries (a change of–700 nanohenries or a 1.9% decrease). Sensitivityis defined as a change in inductance for a change inmagnetic field intensity. A typical Microloopsensitivity is 7 nanohenries/milloersted or .000007henries/oersted.

The change in the magnetic field intensity caused bythe ferromagnetic materials in the vehicle isprimarily a function of vehicle height. Therefore,the Microloop’s ability to detect small vehicles,such as bicycles, is significantly better than that of astandard loop. On the other hand, if the vehicle ismade completely of non-ferromagnetic materials,such as aluminum, the Microloop will not sense itbecause such a vehicle does not disturb the earth’smagnetic field.

Note that the probe inductance increases as well asdecreases. Vehicles approaching a probe cause themagnetic field intensity through the probe todecrease, and the inductance of the probe toincrease. Any inductive loop detector used with theMicroloop must be capable of ignoring thisinductive increase or adjusting to it slowly.

Two other significant points are shown in Figure2-1. First, the ambient magnetic field intensity mustbe 200 milloersted or greater in magnitude. Second,the probe is not sensitive to the polarity of theambient magnetic field. It is sensitive only to themagnitude of the magnetic field intensity. TheModel 701 Microloop is equally effective north orsouth of the equator, and the Model 701 responds tochanges in the magnetic field both above and belowit.

2.2 Magnetic Principles

To fully understand Microloop performance andconsiderations in its use, it is helpful to understandthe basic magnetic principles upon which it relies.Geomagnetism, the natural magnetism of the earth,supplies the bias magnetic field upon which theMicroloop relies. This is the same magnetic fieldthat causes a compass to point north. This biasmagnetic field must be disturbed (increased), by avehicle before an inductive loop detector attached tothe Microloop can detect the passage of a vehicle.Materials that have what is called high magneticpermeability will cause a “focusing” of the earth’smagnetic field intensity. The most common suchmaterials are iron and most of its alloys. Vehicles inuse today contain significant quantities of steel andiron.


Figure 2-1. Typical Microloop Probe Response (Single Probe)


2.2.1 Geomagnetism

The earth’s magnetic field is represented in Figure2-2. Its total intensity is about .6 oersted. The forcelines of this field intersect the earth’s surface at anangle, which is a function of the earth’s latitude.The angle varies from 90 degrees at the magneticpoles to 0 degrees at the magnetic equator. A mapof the world showing the angle from horizontal ofthe earth’s magnetic field is detailed in Figure 2-3.As one example, the intersection angle isapproximately 58 degrees at Los Angeles,California, U.S.A. Thus, the vertical component ofthe magnetic field has an intensity of:

Hvert = .6*sine(58 deg.) = .51 oersted.

Note that the vertical component of the earth’smagnetic field intensity exceeds 200 milloersted(.2 oersted) for most of the world. See Figure 2-3.Also, recall that the Microloop functions the sameregardless of the direction of the magnetic fieldthrough it. Thus, the Microloop functions equallywell in either the northern or southern hemisphere.

2.2.2 Effects of Ferromagnetic Material onEarth’s Magnetic Field

Ferromagnetic material will cause the magnetic fieldintensity in the Microloop to change. It does this by“focusing” the earth’s field and by adding magneticfield from magnetized material in the vehicle to the

earth’s field. In a typical vehicle, both factors areimportant. A third source of magnetic field, currentflowing in wires, may be significant on vehicleswith electric brakes, or other electrical systems oraccessories.

Ferromagnetic materials, such as the iron and steelused to construct vehicles, have a high magneticpermeability. What this means is that it is easier forthe magnetic lines of force to go through theferromagnetic material than through the air or otherless permeable materials.

This results in a focusing (concentrating) of themagnetic lines of force beneath a vehicle. This isshown in Figure 2-4. It is important to also observethat this causes a reduction in the magnetic fieldintensity in front of the vehicle, to the rear of thevehicle, and to the sides of the vehicle. The tallerthe vehicle is, the stronger the effect is of focusing(and reducing) the earth’s magnetic field intensity.

The focusing effect caused by a vehicle (verticalpiece of ferromagnetic or high permeabilitymaterial) is most closely characterized as a percentchange in the vertical component of the earth’smagnetic field. This means that the Microloop hasgreater apparent sensitivity in areas of the world thathave a higher relative strength of the verticalcomponent of the earth’s magnetic field.

Figure 2-2. Model of the Earth and its Magnetic Field


For instance, let’s assume a vehicle causes a 10%field change. Ten percent of 500 milloersteds is 50milloersteds and 10% of 200 milloersteds is 20milloersteds. For the probe shown in Figure 2-1, itsinductance would decrease 350 nanohenries in thefirst case and 140 nanohenries in the second.Obviously the inductive loop detector sensitivitymust be set more sensitive to detect the vehicle inthe second case.

Portions of the iron and steel in a vehicle aresomewhat magnetized. This is either intentional (anormal magnet) or the natural effect of the processesused to shape steel pieces such as the body panels.The magnetic field intensity from a magnetic dropsrapidly as one moves away from the magnet.However, the sum of the magnetic field caused byfocusing the earth’s field and the fields from thesemagnetized pieces of metal can create great changesin the total magnetic field intensity near the roadwaysurface. This is also shown in Figure 2-4. For thisreason, the Microloop is installed well below the

roadway surface (about 20 inches in mostapplications). This causes the Microloop to detectprimarily the focusing of the earth’s magnetic field,rather than the widely varying magnetic field thatexists closer to the roadway surface. As long as theMicroloop is installed at a depth below the roadwaysurface, which separates it from the vehicle to bedetected by less than ½ the height of that vehicle, itwill experience the earth’s magnetic field focusingeffects caused by that vehicle.

In general cars are about twice as high as bicycles.Since the Microloop installation depth is the same,one would expect a motorcycle or bicycle to causeonly about ¼ of the focusing effect of a car, sincethe Microloop is effectively twice as far away(relative to vehicle height). Measurements confirmthis is approximately the case. Thus, increasinginductive loop detector sensitivity by a factor of 4from that required to sense a car will permit sensingall vehicle types.

Figure 2-3. Map of the Angle of the Earth’s Magnetic Field Showing Regions(Areas Not Crosshatched) Which Are Suitable for Microloop Operation


Figure 2-4. Effects of a Vehicle on the Magnetic Field Through the Microloop


3. Operational Characteristics and Usage Considerations

The Microloop has unique characteristics. Knowingand understanding these characteristics will helpensure that the device is properly used and thedesired results are achieved. This section providesinformation that must be considered in planningapplications of the Microloop.

3.1 Vehicle Detection Characteristics

The function of the Microloop, when attached to aninductive loop detector, is to detect vehicles. Allfactors that pertain to applications using wire loopsmust be considered such as: vehicle shape; vehiclespeed; vehicle type; vehicle separation; lane widths;detector capabilities; distance of “loop” from thedetector; pavement conditions; and end applicationof the vehicle detection. The Microloop lessens thedegree of many problems. It does add some uniqueconsiderations such as; ambient magnetic fieldlimits; magnetic field noise limits; and inductanceincreases (as well a normally expected inductancedecreases). To assist in clarifying explanations, theMicroloop’s characteristics will be compared to thecharacteristics of 6-ft. x 6-ft. loops where similarityexists.

3.1.1 Sensitivity to Vehicle Presence –Vehicle Size and Type

Vertical pieces of ferromagnetic material,principally iron and steel, in a vehicle cause themagnetic field intensity in a Microloop to change.This causes the inductance of the Microloop tochange. A field increase causes an inductancedecrease and, conversely, a field decrease causes aninductance increase.

The taller the vertical steel is, the more it changesthe magnetic field. Similarly, the longer the pieceof vertical steel is, the more it changes the magneticfield. In addition to its size, the location (distanceabove the road surface) of the vertical steel isimportant. The higher it is above the road, the lessit will change the magnetic field in the Microloopprobe.

Table 3-1 gives numerical illustration of the effectsof vehicle size where the vehicles pass directly overthe probe(s). For the data shown, the Microloopwas buried 20 inches deep. An automobile is abouttwice as tall and twice as long as a bicycle. Onewould expect a signal about four times larger, andthis is what was measured. The tractor for a tractor-trailer unit is about twice as tall as a car and aboutthe same length. It caused a signal about twice thatof a car. Again, this was the expected result.

Table 3-1. Detection Sensitivity Comparison

Change inL

(nanohenries)

L[loop + 200 ft. lead-in]

(microhenries)

% LChangeVehicle

TypeH vert.

(Oersted)% H

changeChange in

H vert(Milloersteds)

1Probe

2Probe 6X6

1Probe

2Probe 6X6

1Probe

2Probe 6X6

Truck .560 32.0 180 900 1800 4000 65 90 116 1.4 2.0 3.4

Car .560 16.0 90 450 900 3000 65 90 116 0.7 1.4 2.6

Motor-cycle

.560 3.6 20 100 130 100 65 90 116 0.15 0.14 0.09

Bicycle .560 4.5 25 150 200 15 65 90 116 0.23 0.22 0.01

Vehicle Signal Ratio (largest to smallest): 9.3X 14X 340X

Test Conditions: Single and Dual Across-the-Lane Microloop Probes, sensitivity of 5 nanohenries/milloersted


Because of the magnetic field intensity reductionaround a vehicle and the small size of theMicroloop, for most purposes one may assume theinductance will decrease only when the vehicle isover the Microloop. This applies regardless ofvehicle size, direction of travel, or speed. Furtherdetails are given in Section 3.2, Microloop ProbePlacement Configurations.

Table 3-1 illustrates another significantcharacteristic. Notice that the Microloop is muchmore sensitive to small vehicles, such as bicycles,than is a 6-ft. x 6-ft. three-turn loop. The ratio ofthe percent change in L caused by the largestvehicle to that caused by the smallest vehicle is onlyabout 10 for Microloops. That same ratio is about340 for 6-ft. x 6-ft. loops. This greatly simplifiesdesign of vehicle detection installations designed todetect broad types of vehicles.

Table 3-1 does not mention the trailer portion oftractor-trailer units. One should expect that atractor-trailer will be detected as two vehicles byacross the lane Microloop installations. Today asignificant portion of most trailers is aluminum orstainless steel (many grades of stainless steel are notferromagentic). The normal steel portions are thebed I-beams, the parking stand, and the rearaxle/suspension system. The bed I-beams are ofinsufficient height for their distance from theMicroloop to be detected. The rear axle/suspensionhas been detected for all trailers tested to date. Ifthe trailer contains a cargo with large pieces ofvertical steel, the tractor-trailer unit may be detectedas one vehicle.

A similar situation applies to cars pulling trailers.The lack of vertical steel between the hitch and thetrailer will cause the car and trailer to be detected astwo separate vehicles by across-the-lane Microloopinstallations.

Another situation where a single vehicle may bedetected as two vehicles is aluminum body vanswithout cargo containing vertical steel. Thissituation is somewhat similar to the typical tractor-trailer unit. Depending on the construction of thespecific van, there may be only horizontal steelbetween the cab and the rear axle/suspension. Sucha vehicle will frequently be detected as two vehiclesby across-the-lane Microloop installations.

Standard 6-ft. x 6-ft. loops also tend to detect thesetypes of vehicles as two vehicles. Special diamondshaped loops with extra turns have beensuccessfully used to detect these vehicles as a singlevehicle where such classification was a directrequirement of the traffic control software, forinstance where one of the main variables is vehiclelength.

These examples vividly illustrate the ability of theMicroloop to separate closely spaced vehicles.

Horizontal sections of steel will not likely bedetected unless significant portions are magnetized.


3.1.2 Loop Detector Sensitivity Setting

Different inductive loop detectors used differentrelationships between change in inductance and totalinductance as thresholds for vehicle detection.Canoga Digital Loop Detectors use the formula:

Detect if:

(change in L)/(square root of(L + 150 microhenries))

is greater than threshold (sensitivity)

Note that for loops with L less than 150microhenries, the sensitivity and change in L arenearly directly related. Table 3-2 gives a roughapproximation of the change in magnetic fieldintensity in a Microloop that will cause a CanogaDigital Loop Detector to register vehicle detection.

Values shown in this table assume each probedetects the same increase in magnetic field intensity.If only one probe detects the field increase, use thecolumn for a one Probe Set regardless of whetherthe actual case is a one, two, or three Probe Set.This table assumes nominal sensitivity Microloops,not minimum sensitivity Microloops. Multiplyingtable numbers (those in milloersteds) by 1.5 willapproximate the relationship for all probes atminimal sensitivity. This table is only a roughguide. The nominal magnitude of the earth’smagnetic field intensity, length of home-run cable,and other factors that affect the total inductance willcause variations.

As is customary practice with standard loops, thesensitivity on the inductive loop detector should beset so that at least 2X margin-for-error is allowed.For instance, the numbers in Table 3-1 show that fora single probe a Canoga Detector would detect thecar at SENSITIVITY 1. SENSITIVITY 2 (twice assensitive) should be used. Similarly, whileSENSITIVITY 3 will detect motorcycles,SENSITIVITY 4 should be used to achieveconsistent detection.

The sensitivity range for Model 701 Microloopprobes is about 3.5 to 8 nanohenries/milloersted.The practice of setting the loop detector to asensitivity twice as high as possibly required allowsfor probe sensitivity variations and variations inmagnetic field intensity increases caused by similarvehicles.

3.1.3 Inductance Increases

Inductance increases are a normal part of Microloopoperation. While the inductance of a 6-ft. x 6-ft.loop may also increase slightly, particularly when avehicle is about to enter a loop, the increase isnormally small relative to the inductance decreasewhen the vehicle is over the loop.

To more specifically define this inductance increasecharacteristic, the following example is provided.With a depth of 20 inches from the road surface tothe bottom of the probe, one should anticipate that avehicle could cause an inductance increase of about½ the inductance decrease caused by that vehicle.For example, if a vehicle causes an inductance

Table 3-2. Approximate Thresholds of Magnetic Field Intensity Changes for Detection

Sensitivity (CanogaDigital Loop Detector)

1 Probe Set(milloersteds)



1 60 30 20

2 30 15 10

3 15 7.5 5

4 7 3.5 2.3

*5 3.5 1.8 1.2

*5, 6, 7, 8 are too sensitive for use with MicroloopsThe change in magnetic field strength is assumed to occur at each probe in a multiple probe set, not at just one probe in the set.


decrease of 450 nanohenries, there is likely alocation, relative to the probe, where the samevehicle will cause the probe inductance to increaseby 225 nanohenries. Typically, this will be whenthe vehicle is approaching the probe but will pass tothe left or right of the probe rather than over it. Thereason for the inductance increase is that a vehiclereduces the magnetic field intensity through theMicroloop, when the Microloop is located in thereduced field area some distance to the rear, front orsides of the vehicle. See Figure 2-4, Effects of aVehicle on the Magnetic Field through a Microloop,and Figure 2-1, Typical Microloop Probe Response.

For moving vehicles, the duration of an inductanceincrease is very short relative to the duration of aninductance decrease, and the increase will beignored by the loop detector. However, when stopand go traffic is involved, this may not always bethe case. If the loop detector adjusts to thefrequency decrease (inductance increase) cased by avehicle stopped at a location near (not over) theMicroloop, the loop detector may respond as thougha vehicle is over the Microloop, when the vehicleleaves the area. This is because the inductancereturned to the “no vehicle” state after the vehicleleft, and this is an inductance decrease from the“vehicle nearby” state.

Most models of inductance loop detectors aredesigned to almost instantly “adapt” to inductanceincreases. “Adapt” here is defined as the inductiveloop detector calling any frequency decrease(assumed to be caused by an inductance increase)the new reference frequency for detecting vehicles.This feature is useful during periods of constantoccupancy of a loop during peak traffic periods. Ifthe loop is vacated, even momentarily, the ability ofthe inductive loop detector to continue sensingconstant loop occupancy is refreshed. This preventsloss of vehicle detection, due to environmental adaptfeatures in inductive loop detectors, on loops thatare constantly occupied during traffic peaks.

If the traffic is not constantly moving at anestimated 45 mph or faster, this fast adapt feature ofinductive loop detectors can cause erratic operationwhen such inductive loop detectors are connected toa Microloop. Canoga Digital Loop Detectors have afront panel switch that controls the rate at which thedetector adjusts to frequency decreases (inductance

increases). The NORMAL position isrecommended. The adjust rate in this position is ½detection threshold per second. This means that upto a 2-second inductance increase will not cause a“call” to be locked in. This allows vehicle speeds atleast as slow as 10 mph. If the “FASTRECOVERY” position on a Canoga Digital LoopDetector were used, an inductance increase as shortas 2 milliseconds could cause a “call” to be lockedin. Most new inductive loop detectors adapt toinductance increases in less than .1 seconds.

To prevent any possibility of erroneous locked in“calls” due to stop-and-go traffic, we recommendusing PULSE mode and NORMAL mode onCanoga Digital Loop Detectors. PULSE mode willcause any “locked in call” to be canceled in under 2seconds, should it ever occur. If PULSE modecancels a “locked in call”, vehicles may not bedetected for 2 to 10 seconds. However, this isusually preferable to the situation of having acontinuous detect condition for 15 to 60 minutes.

3.1.4 PRESENCE Versus PASSAGEDetection

PRESENCE applications require the detector todetect the vehicle for the entire time it is over theloop. Typical applications include occupancydetection such as stop bar detection and turn lanedetection, gap measurement, long vehicle detection,queue detection, etc.

PASSAGE applications require the detector to sensethe movement of a vehicle past a point. Typicalapplications include counting, speed measurement,advance detection, etc. In most passageapplications, the inductive loop detector may beoperated in the PULSE mode or PRESENCE modeinterchangeably.

Considered only as a self-contained sensing device,the Model 701 Microloop is capable of PRESENCEdetection or PASSAGE detection. However, whenthe characteristics of inductive loop detectors arealso considered, its use for PRESENCE detection isbest limited to locations where stop-and-go trafficwill not normally occur. The reasons for this areexplained in Section 3.1.3, Inductance Increases.


Use of inductive loop detectors configured in otherthan PULSE mode should be done only afterconsidering the possible side effects of inductanceincreases:

a. Possible long duration call (with no vehiclepresent) after inductive loop detectoradaptation to an inductance increase. The moresensitive the inductive loop detector is set, themore likely it is this will happen.

b. Depending on the amount of adaptation by theinductive loop detector to an inductanceincrease and its sensitivity setting, it is possibleto have double counting of each vehicle: onecount as it approaches the Microloop probe andone as it moves away from the probe. This isdue to inductance increases caused by themagnetic field reduction to the front and rear ofthe vehicle. These inductance increases maycause the inductive loop detector to changefrom a “detect” status to a “non-detect” status.This phenomenon will normally occur only inslow moving traffic. Note that this possibilitywill partially cancel, or at least postpone theobservation of the long duration call discussedin paragraph a. above.

3.2 Microloop Probe PlacementConfigurations

The configuration of Microloop probes to beinstalled under the road is determined by the resultsdesired. Two primary considerations are assumed:1) detect all vehicles in the lane, and 2) do not detectvehicles in adjacent lanes.

When inductive loop detector sensitivity is set asrecommended in Section 3.1.2, Loop DetectorSensitivity Setting, vehicles will usually be detectedat a distance of up to ½ the vehicle height to the leftor right of a single probe installation. In multipleprobe, across the lane installations, this detectiondistance is reduced for vehicles passing to the left orright of the installation, because the next probe inline sees a reduced magnetic field intensity and hasan inductance increase that partially offsets theinductance decrease of the nearer probe. Thisphenomenon contributes to the Microloop’sexcellent ability to avoid detection of vehicles inadjacent lanes.

3.2.1 Vehicles Centered in a Traffic Lane

A single Microloop probe centered in the traffic lanewill allow detection of all vehicles centered on thatlane, including bicycles.

Vehicles that could be missed are those centered onthe lane separation line, because they are switchinglanes, and vehicles that tend not to travel in thecenter of the lane such as motorcycles.

3.2.2 Vehicles Changing Lanes andMotorcycles/Bicycles

Two Microloop probes placed across the lane andspaced 4 ft. apart will allow detection of cars andtrucks changing lanes and most motorcycles andbicycles.

If it is necessary to allow detection of nearly allmotorcycles and bicycles, three Microloop probesspaced 3 ft. apart and going across the lane willprovide the needed sensitivity.

Use of four probes across the lane in an attempt todetect motorcycles traveling down the lane divisionstripe is not advisable, because adjacent lanerejection would be noticeably reduced.

3.2.3 Ensuring a Single Detection perVehicle

There are applications where it is desirable to haveone detection output per vehicle regardless ofvehicle length or type. An obvious extension of theuse of multiple across-the-lane probes to ensuredetection is to add multiple down-the-lane probes toguarantee car-trailer, tractor-trailer, and aluminumbody vans are classified as one vehicle. Aconfiguration using three Microloops (arranged in apyramid pattern with Microloops placed at the threepoints, with two as the base spaced 4 ft. apart acrossand centered in the lane and one as the peak 14 ft.upstream) would likely guarantee the desired result.

When Microloops are connected in series formultiple probe applications, the inductance changein each probe is additive as shown in Table 3-1. Inthe three probe Microloop configuration justproposed, the sensitivity of the inductive loopdetector would have to be set at that necessary todetect motorcycles to guarantee detecting allvehicles and to ensure one detection output from theloop detector for all vehicle types.


This system would require the ambient magneticfield noise at this location to be less than 4.0milloersted. While it is not unusual to have ambientmagnetic field noise intensities this low, it ispossible that magnetic fields generated by current innearby power lines may approach this level.

When sets of Microloop probes are used on a singleloop detector channel, it is conceivable that asituation could occur where a vehicle was stoppedclose to the Microloops, but not over any of them.If this reduced the magnetic field through several ofthe probes, a relatively large inductance increasewould occur. The inductive loop detector wouldadapt to the resulting large frequency decrease.When the vehicle left, a large frequency increasewould occur, leaving a large signal “detect”condition. This would disappear in less than 2seconds for an inductive loop detector in PULSEmode, but the “detect” condition might last forhours in PRESENCE mode.

There is an alternative to special probeconfigurations that can give a single detection pervehicle. It does, however, also increase thepossibility of not detecting vehicles “tail gating” thepreceding vehicle. This alternative is the use of aninductive loop detector with timing to extend thedetection pulse through the section of the vehiclethat is not detectable to the next portion of thevehicle that is detectable by the Microloops. SeeTable 3-3.

Using extension timing of .25 seconds gives a totaldetection pulse of .25 + .118 = .368 seconds, where.118 sec. is the normal pulse duration in PULSEmode. A truck with an undetectable section 20-ft.long would now be classified as a single vehicle atspeeds of 37 mph and faster. This operationalcondition may give acceptable results on manylimited access roadways. It is effective over areasonable range of speeds, and vehicle spacingrequirements are reasonable.

3.3 Detection Area

With most vehicles, detection will start atapproximately the front bumper and end atapproximately the rear bumper. Oddly shapedvehicles such as bicycles and trailers may appearshorter to the Microloop than their actual length.For example, some bicycles detect only for a shortperiod of time near the center of the bicycle.Aluminum or fiberglass trailers will likely detectonly for a short period of time around the axle andsuspension.

These characteristics are very similar to those of a6-ft. x 6-ft. loop. The primary difference is that theMicroloop in itself is only a point (or a line formultiple probe across-the-lane installations) ratherthan a 36 square foot area. This allows detection ofclosely spaced vehicles. It also places a new limiton maximum vehicle speed to obtain vehicledetection.

3.4 Vehicle Speed, Vehicle SpacingConsiderations

The Microloop has no built-in signal delays.Therefore, vehicle speed considerations are similarto those for a 6-ft. x 6-ft. loop. Again, the primarydifference is that the vehicle is detected for a shorterperiod of time due to the fact that the Microloop is apoint or line detection device.

Assume, for example, a vehicle 18 ft. long. A 6-ft.x 6-ft. loop has a detection length of about 12 ft.Thus for the 6-ft. x 6-ft. loop, the detection outputwill classify the vehicle as 30 ft. long. TheMicroloop will classify this vehicle as 20 ft. long.At 65 mph (95.3 ft./sec.), a 6-ft. x 6-ft. loop willthen detect this vehicle for about .32 seconds. TheMicroloop would detect that same vehicle for about.21 seconds.

Table 3-3. Effect of Pulse Width on Vehicle Speed and Spacing Limits

Pulse Width (seconds) Slowest Speed WithoutDouble Detection (mph)

Minimum Vehicle Spacing at 65 mph (ft.)

.118 116 11 (default condition)

.368 37 35 (.250 sec. extension)

.618 22 59 (500 sec. extension)

Slowest speed calculations assume the vehicle section length that is not detectable is 20 ft. long.


The maximum speed at which a vehicle will bedetected is a function of the inductive loop detectorscan rate and the effective vehicle length. Theequation below may be used to calculate theeffective vehicle length:

Maximum Vehicle Speed =Effective Vehicle Length / Scan Time

Let’s examine a practical worst case condition usinga Canoga Digital Loop Detector. Consider amotorcycle with an effective length of 4 ft. Tripleprobe sets will be assumed on each of the fourchannels with a loop detector SENSITIVITY 4 foreach channel. Scan time is then about 40milliseconds. Therefore, the motorcycle must beover the Microloop for 40 milliseconds to ensure itis being detected. This means the motorcycle mustbe traveling no faster than 4 ft./40 milliseconds =100.0 ft./sec. = 68.2 mph. This may be acceptablefor freeway detection requirements of motorcycles.

If all four detector channels were operated atSENSITIVITY 3, the maximum speed for detectingmotorcycles would double to 136 mph. Use of atwo channel Canoga Loop Detector would allow amaximum motorcycle speed of about 136 mph atSENSITIVITY 4 and 227 mph at SENSITIVITY 3.There is no effective speed constraint when a twochannel Canoga Digital Loop Detector is used.

A similar approach is taken to determine theminimum vehicle spacing that will cause theindividual vehicles to be detected. In PRESENCEmode, the equation is as follows:

Minimum Spacing = (speed)x(scan time)

If the digital loop detector has a scan time of 40milliseconds, vehicles as close together as 3.8 ft. canbe detected at 65 mph. Vehicles spaced as closelyas 1.8 ft. can be individually detected at 30 mph.

In PULSE mode the duration of the detect output isalso important. A second vehicle cannot be detectedduring the detect indication. The default setting ofpulse width in PULSE mode for Canoga DigitalLoop Detectors is 118 milliseconds. Other availableoptions are 15, 59, and 236 milliseconds. If thepulse width is longer than the scan time (the actualcase most of the time), the equation for calculatingminimum vehicle spacing while operating in thePULSE mode becomes:

Miminum Spacing = (speed)x(output pulse width)

At 65 mph, this is 11.3 ft. At 30 mph, this is 5.2 ft.Minimum vehicle spacing can be increased ordecreased by use of the optional pulse widths.Table 3-3 shows some effects of increasing pulsewidth using EXTENSION timing.

NOTE: In PULSE mode the minimum vehiclespacing length that allows detection of individualvehicles is also the maximum length of a vehiclesection, such as a tractor-trailer, that will not cause asecond detect condition. PULSE mode noticeablyreduces the number of tractor-trailers, car-trailers,and aluminum box vans that are counted as twovehicles. This occurs without any real reduction inthe ability to individually detect closely spacedvehicles.

3.5 Microloop Operating EnvironmentRequirements

3.5.1 Magnetic Field Intensity

To obtain optimum performance of the Model 701Microloop, it is essential that magnetic fieldmeasurements be done before making the actualinstallation. Unlike 6-ft. x 6-ft. loops the Microloopwill function correctly in areas containing largeamounts of iron, such as reinforcement rod, etc.(See Section 3.5.3, High Iron Environments). TheModel 701 Microloop must, to have adequatesensitivity, be located in a position that has at least.2 oersted of vertical magnetic field intensity.

Most regions of the earth have a vertical componentof magnetic field intensity that is between .2 oerstedand .6 oersted (See Figure 2-3). The excludedportion is a band across central Africa, central SouthAmerica, Malaysia, southern India, and a smallportion of southern China.

The 3M Model MM-9 Magnetic Field Analyzer wasdesigned specifically for analyzing magnetic fieldsfor magnetometer and Microloop applications. It issimple to use and accurate. Other commerciallyavailable instruments may be used, but the usermust be familiar with that instrument to ensure thecorrect measurements are made.

Measurement of Vertical Component of MagneticField Intensity:

Limits: .2 oersted minimum; .6 oerstedmaximum


Set the MM-9 to H. Place the probe vertically at thelocation where the Microloop is planned to beinstalled. The probe standing on the road surface isusually a sufficient measure, even though theMicroloop probe will generally be installed with itsbottom 20 inches below the road surface.

Nearby iron may cause the earth’s apparentmagnetic field intensity to be higher or lower thannormal ambient. Move the MM-9 probe somewhatto the left, right, forward, or back until a location isfound that is near the area’s normal magnetic fieldintensity. There is usually a location within one footof the originally intended location that will havenear normal values of the vertical component of theearth’s magnetic field.

3.5.2 Magnetic Field Noise

Set the MM-9 to Hac. Read the magnetic fieldnoise. If the noise is greater than that shown inTable 3-4 Approximate Magnetic Field NoiseLimits, false calls will likely result. This situation israre. However, if it does occur, use a vehicle-sensing device other than the Microloop.

All significant magnetic noise is man-made. Itsprimary source is currents flowing in nearbyconductors. Very few sources carry currents largeenough to disturb the Microloop, if those conductorsare 30 ft. or more away.

Sites where noise may be expected are neartrolley/bus lines, subways, elevators, main powerdistribution lines, and sometimes on overpasses(closer to a power line). Since main power linesmay be buried underground, it is always best tocheck for magnetic noise, even though the siteappears suitable.

Another type of man-made magnetic noise cancome from vehicles traveling above or below theMicroloop, but not on the road surface beingmonitored. Some examples are subway trains in atunnel below the road, vehicles traveling below anoverpass (Microloop intended to detect vehiclestraveling on the overpass), and vehicles on otherlevels of a multi-level bridge. The MM-9 scale fordelta H is used to check for this type of magneticfield noise. The same limits apply as for Hac. Tocheck, set the MM-9 to the delta H scale, place theMM-9 probe where a Microloop probe will belocated, and observe the readings. The potentiallyoffending vehicular traffic must be occurring whilemeasurements are being taken.

Table 3-4. Approximate Magnetic Field Noise Limits

Canoga Digital Loop Detector Sensitivity Approximate Magnetic Noise Limits (Oersted p-p)

1 .050/# of probes

2 .025/# of probes

3 .012/# of probes

4 .006/# of probes

5,6,7,8 Not recommended for use


3.5.3 High Iron Environments

Microloops, unlike 6-ft. x 6-ft. loops, perform in anormal manner in most high iron environments.Figure 3-1 shows the use of a Microloop on a bridgedeck. Use the MM-9 to ensure that the Microloop islocated in a position having normal magnetic fieldintensity (200 to 600 milloersteds) and lowmagnetic field noise (see Section 3.5.2, MagneticField Noise).

If the nearby iron fully shields the Microloop fromthe earth’s magnetic field, the Microloop cannot beused. One possible problem location is inside asteel lined tunnel. Always use the MM-9 tomeasure the field before assuming the Microloopwill not work. Most steel structures will not fullyshield the Microloop from the earth’s field.

3.6 Matching the Microloop to DigitalLoop Detectors

The Model 701 Microloop performs well withCanoga Digital Loop Detectors, and its performancehas been fully characterized when used with them.It is possible that other brands and models of digitaldetectors may be used. However, the details ofoperation of digital loop detectors are changing sorapidly that generalizations cannot be made. Wehave found that two inductive loop detectors fromthe same manufacturer that appear identical mayoperate differently due to component value changesand different software revision levels. Forconsistent results, use of Canoga Digital LoopDetectors with the Microloop is stronglyrecommended.

Figure 3-1. Microloop Usage on a Bridge Deck


For those who wish to test for compatibility withprecisely defined versions of other digital loopdetectors, several guidelines are offered. Thesefactors have been found significant:

• Ability to operate with low inductance loops

• Ability to operate with low Q loops• Amplitude of signal driving the Microloop

• Frequency of signal driving the Microloop

• Method used by the inductive loop detector forhandling inductance increases (frequencydecreases).

Unless your applications will never use single probeMicroloop installations, most tests should be runusing a single Microloop probe. All checks can bemade with one Microloop and a sample digital loopdetector. A fast storage oscilloscope with flexibleexternal triggering, an MM-9 Magnetic FieldAnalyzer, a probe holder (for the Microloop and theMM-9 probe spaced 3 inches apart and heldvertically), inductors (33 microhenries to 1000microhenries with Q greater than 10), and a piece ofsheet metal (about 2 ft. x 3 ft.) vertically mounted (2ft. dimension horizontal) on a non-ferromagneticmaterial cart with the height of the sheet metaladjustable above the floor. It is useful to have aCanoga Digital Loop Detector available to use as areference for performance checks.

Operational familiarity with test instrument usage isassumed and will not be described here.

Connect the Microloop to the digital loop detector,place the Microloop in its holder (vertical position)and turn the detector ON. If it completesinitialization, carry the sheet steel over theMicroloop and see if it is detected. If everythingappears normal to this point, tests are morestraightforward. Otherwise, “troubleshooting” musttake place.

If initialization does not occur, the detector probablyrequires more inductance. To check if the primaryrequirement is higher Q, place a 50 microhenryinductor parallel with the Microloop. This willincrease loop Q even though an inductance decreaseoccurs. If the detector will now initialize, itsprimary requirement is higher Q. Then find thelargest inductor that can be placed in parallel withthe Microloop and still have the detector operate. Inactual application, it is desirable to use as large avalue of inductance in parallel as practical so thatthe least amount of sensitivity reduction occurs.

If the Q requirement check is unsuccessful, placethe 50-microhenry inductor in series with theMicroloop rather than parallel. At this pointinitialization should occur. If it doesn’t, the detectormay be functioning improperly.

After getting a functional system, check foroperation by bringing the sheet steel (held in avertical position) over the Microloop (about 16inches from the probe bottom to the bottom of thesteel). This will produce a large increase in themagnetic field intensity through the Microloopprobe. The MM-9, set to the H scale, can be used toconfirm this. A 50 milloersted to 100 milloerstedchange is a large signal. The steel should bedetected. If detection doesn’t occur, recheck thedetector sensitivity setting. Reinitialize the detector(steel sheet at least 2 ft. from probe). Recheck.Movement of magnets, steel carts, etc., in the areaduring the testing can confuse results. Control yourmagnetic environment. The magnetic field changethrough the Microloop (and resulting inductancedecrease) can be increased by adjusting the height ofthe sheet steel (reducing the vertical distancebetween the sheet steel and the Microloop probe).

Next, check the amplitude of the signal beingapplied to the Microloop. It must be at least .25Volts peak-to-peak (.25 Vp-p) for full proberesponse. If it is less than .1 Vp-p, the probe willprobably have no response. The signal should beless than 1 Vp-p. If it is greater than 2 Vp-p, theprobe will probably give no response.

Assuming full functionality at this point, check forproper signal amplitude delivery to the Microloopover the range of probable total inductances. Thesignal amplitudes are required across each probe.For example, with four probes in series, theminimum amplitude level is 1.0 volt total across theMicroloop probe set. Assume, for purposes of thischeck, that each Microloop probe is 33microhenries. Add series inductors, 33microhenries at a time, until the signal amplitudeacross the test Microloop reaches .25 Vp-p. Thiswill give a reasonable estimate of the maximumtotal inductance that can be attached to this detector,when the system contains at least one Microloopprobe.


Measure the frequency of the signal applied to theMicroloop by the detector. The sensitivity of theMicroloop (amount of inductance decrease for amagnetic field intensity increase) decreases as thefrequency of the applied signal increases. If thesignal is over 100 kHz, the Microloop’s sensitivitywill probably be insufficient for many applications.This can, however, be measured in detail with thetest items you are using.

Assuming everything is fine to this point, verifydigital detector’s handling of inductance increases.Ignoring the Microloop and the detector for themoment, use the MM-9 (set to the H scale) and thecart-mounted sheet steel to determine how to locatethe sheet steel relative to the MM-9 probe (andconsequently Microloop probe) to decrease the fieldthrough the probe about 50 milloersteds. Thisposition will be with the sheet steel some distance tothe side of the probes. Make certain the plane of theMicroloop and MM-9 probes is parallel to the sheetsteel so that both probes see the same magnetic fieldintensities.

Arrange your test configuration so that you canquickly push the cart through this position (one thatprovides a significant inductance increase) relativeto the Microloop. Detector response (detector inPRESENCE mode) is to be observed as this is done.The detector should not detect the sheet steel orhave a detection indication “locked in” after thesheet steel leaves the vicinity of the Microloop (atleast 2 ft. away). Cart velocity initially should bethe equivalent of 10 mph. If the initial test issuccessful, continue repeating the test using aslower cart velocity each time. At some rate ofmovement, detection indication “lock-up” willoccur. The Microloop will give consistentperformance with this detector when used inPASSAGE detection applications where vehiclespeeds are higher than the cart speed that created aproblem.

The final step is to determine how to set sensitivitiesfor this detector to ensure vehicles are properlydetected. This can be done by connecting thedetector to an actual Microloop installation anddriving vehicles of each type to be detected over theMicroloop. An alternative is to calibrate your testconfiguration. Table 3-1 gives typical amounts ofmagnetic field intensity increases caused bydifferent vehicles. First, use the MM-9 to verifythat the vertical component of the earth’s magneticfield intensity at the calibration location if typical ofthat in your local area. Steel used in constructingbuildings can create significant variations from thefield seen at a roadway installation. The MM-9, setto the H scale, can be used to find those heights ofthe sheet steel above the bottom of the Microloopprobe(s) that will cause the same increases inmagnetic field intensity as vehicles would. The cartcan then be moved over the Microloop probe(s) athigher than minimum velocity (determined bydetector handling of inductance increases) tosimulate actual vehicles.

Since many digital detectors claim to detect the ratioof inductance change divided by total inductance,the Microloop configuration being tested forcalibration of detector sensitivity must approximatethat which would occur at an installation. Actualhome-run cable length must be used or simulated byusing an inductor. The correct number of probesmust be used. If the Microloop probe(s) is in seriesor parallel with a loop, the loop must be attached orsimulated using an inductor. When checking forlarge vehicle response (cars, trucks, etc.), align theprobes parallel (separated by at least 3 inches) withthe sheet steel on the cart so that all Microloopprobes see the same field. When checking forresponse to motorcycles and bicycles, separate theMicroloop probes as they would be in the street, andconsider the sheet steel to be the bicycle.

The detector sensitivity calibration procedure justdescribed gives an excellent approximation of actualinstallation requirements, assuming that themagnetic environment in the calibration test areawas controlled during testing.


4. Guide to Specific Applications

Use of the Microloop for PASSAGE applications isverify similar to using 6-ft. x 6-ft. loops. Theprimary differences revolve around the fact that theMicroloop is a point detector rather than an areadetector, and the fact that the microloop has greaterinductance increases than does a 6-ft. x 6-ft. loop.Microloop inductance decreases are similar incharacter to the inductance decreases of a6-ft. x 6-ft. loop.

Several specific PASSAGE applications are coveredin some detail to illustrate these differences andsimilarities.

4.1 Speed Measurement

To measure speed, two Microloop probe sets areused in the same manner as two loops would beused. A typical placement in shown in Figure 4-1.

Figure 4-1 shows use of a single Microloop probe inthe lane center for one lane of the speedmeasurement configuration. This will allowdetection of all size vehicles where some portion ofthe vehicle travels over the Microloop or within 1 ft.to 3 ft. (depending on vehicle height and length) tothe left or right of it. If the only purpose of these“loops” is speed measurement, use of a single probeper configuration portion should be adequate. In

this case it is unimportant that a motorcycle is notdetected at one or both of the Microloops because ofwhere it is traveling in the lane. Similarly, missingan occasional detection because of vehicle ischanging lanes should not matter. If one or more ofthe Microloops is also doing a counting function,the techniques shown in Section 4.2, CountingVehicles, should be used to achieve good countaccuracies.

For measuring vehicle speed, it is recommended thatthe Microloop be connected to a two-channelCanoga Digital Loop Detector for reliable andpredictable performance. The detector should beoperated in NORMAL recovery mode, MEDIUM(NORMAL) or LOW frequency, and PULSE mode.The lowest practical sensitivity should be used.This is SENSITIVITY 3 or 4 if detection ofmotorcycles is required, and SENSITIVITY 1 (twoor three probes per “loop”), or 2 (one probe per“loop”) for detecting cars, trucks, and buses.

Speed is measured by having a computerized deviceperform the following calculation:

Speed = D/(T2 – T1)

D = distance between Microloops

T1 = time vehicle detection started at Microloop 1

T2 = time vehicle detection started at Microloop 2

(T1 and T2 are times-of-day, i.e., 09:30:50)

Figure 4-1. Typical Speed Measurement PASSAGE Application


Measurement accuracy is a function of the certaintyin times T1 and T2. That certainty is dependent onconsistent detection of a vehicle at a specific pointon the vehicle, the scan time of the digital detector,the accuracy of the computerized calculating devicein determining the time of the starting edge of thedetection indication, and the vehicle speeddetermines the percent the uncertainty is of thedifference between T1 and T2).

T2 – T1 = D/Speed = .168 seconds (assumingperfect accuracy) at 65 mph and .242 seconds at 45mph. Now let’s take a look at timing uncertainties.

With the Microloop, detection begins within about 1ft. of the actual front of a vehicle. Inaccuracy inposition detection by loops is frequently assumed tonot create measurement error. Those making thisassumption say that since the position error shouldbe the same at each loop for a specific vehicle, nonet error from detection position errors results. Infact, however, it is a certainty some error occurs. Ifa positional error of 1 ft. occurred, a 6.3% error inspeed measurement would result. A positional errorof 3 inches causes a 1.6% error. It is almost certainerrors in the 2% range actually occur. If attemptsare made to measure it, error in position detectionby the Microloop will be less than that of a regularloop. The Microloop response is usually notaffected by vehicle alignment with lane center,vehicle bounce, and other factors affecting loopresponse.

With a Canoga Digital Loop Detector, a vehiclemust be over the Microloop for the duration of onescan cycle to guarantee detection. This time isshorter than for most scanning detectors. Here aresome actual measurements:

Test Conditions:

• Model P422T-905-OD Canoga Digital LoopDetector

• Channel 1 connected to dual probe Microloop(50-ft. lead-in)

• Channel 2 connected to dual probe Microloop(175-ft. lead-in)

• Digital loop detector settings – NORMALfrequency, NORMAL recovery mode, PULSEmode

Sensitivity Setting Scan Time (milliseconds)

1 5.5

2 7.4

3 10.6

4 17.1

Notice that the scan time is very short. At 65 mph(a worse case than 45 mph) the scan time creates anuncertainty range of from 3.3% at SENSITIVITY 1to 10.2% at SENSITIVITY 4. The shortest scantime on other digital loop detectors is 20milliseconds and is typically closer to 40milliseconds.

Next, let’s examine error introduced by the abilityof the speed-calculating device to ascertain exactlywhen each detection started. This is entirelydependent on the design of the computerizedcalculating device. It can range from nearly zeroerror (a dedicated hardware counter used to measureT1-T2 for each speed trap), to interrupt drivensoftware (probably .1 millisecond resolution), toscanned sampling of multiple traps (probably 10millisecond resolution). Therefore, this portion ofthe system provides from 0% to 6% error potential.

Summary of error sources (referenced to65 mph):

Positional detection due to “loop”: 1% to 6%

Canoga Digital Loop Detector scan timedelays: 3% to 10%

(Scan time delay error from other digitaldetectors: 12% to 25%)

Computerized speed calculation: 0% to 6%

Total for recommended configuration:4% to 22%

(Total for other brands of digital detectors:13% to 37%)

This is a range of +/-2% to +/-11%. It is likelyspeed measurement traps operate at an actualaccuracy of no better than +/-5%. The Microloop,as compared with a 6-ft. x 6-ft. loop, will helpenhance accuracy.


Using digital loop detectors in PULSE mode forspeed measurement places the detector in theposition of defining the minimum vehicleseparation. This is determined by the PULSE width.Vehicles must be separated, in time, by at least thePULSE width plus one scan cycle. For CanogaDigital Loop Detectors this time is .120 seconds.Therefore, at 100 mph (146.7 ft/sec), vehicles mustbe separated by 17.6 ft. At 65 mph (95.3 ft/sec), therequired separation is 11.4 ft. This should notinterfere with speed measurement performance.

4.2 Counting Vehicles

There are many reasons for counting vehicles. Allrevolve around getting data necessary to make adecision by the local intersection controller, by atraffic control system, or by a traffic engineer.Ideally, this information would include allconsiderations that relate to the final decision. Forinstance, a tractor-trailer is different from a car, afully loaded tractor-trailer is different from anempty tractor-trailer, a motorcycle is different froma car.

Vehicle counting installations cover a wide range oftechniques for attempting to deliver completeinformation. The most sophisticated installationsprobably combine axle weighing, axle counting,vehicle counting, vehicle length measurement,vehicle speed measurement, and date/time of day.This data is taken as a set for each vehicle. Thesimplest installation probably only counts axles.

Where it is not economical to have a fullinformation set to process for each vehicle, manydifferent approaches have been taken to making“good” decisions on limited data. If a decision isbeing made solely on the number of vehicles,perhaps it is desirable to count tractor-trailers andcar-trailers as two vehicles. For instance, with axlecounting installations, a common approach is to saytwo axles represent one vehicle. This will call atractor-trailer 2.5 vehicles, a car-trailer 1.5 or 2.0vehicles, etc.

Straightforward use of the Microloop in vehiclecounting applications will cause continuous vehicleswith sheet steel exteriors or constructed of verticalsteel members, such as cars, motorcycles, andbicycles to be counted as one vehicle. Non-continuous vehicles, such as car-trailers, andvehicles having a large section without verticalsteel, such as the trailer of many tractor-trailers andtrucks (vans) with aluminum boxes, will be countedas two vehicles (depending on the length of thesection not containing vertical steel and vehiclespeed).

Typical Microloop installations for counting areshown in Figure 4-2. A two-probe set is used ineach lane to catch vehicles changing lanes and mostmotorcycles. If it is necessary to detect allmotorcycles except those traveling down the lane, athree-probe set should be used in each lane.

Figure 4-2. Typical Vehicle Counting PASSAGE Application


For counting vehicles, it is recommended that theMicroloop probe set be connected to a CanogaDigital Loop Detector for reliable and predictableperformance. The detector should be operated inthe NORMAL recovery mode, MEDIUM (orNORMAL) or LOW frequency, and PULSE mode.The lowest practical sensitivity should be used.This is SENSITIVITY 3 or 4 when motorcycledetection is required, and SENSITIVITY 1 (two orthree probes per “loop”) or SENSITIVITY 2 (oneprobe per “loop”) for detecting cars, trucks, andbuses.

The down-the-lane probe configurations have beenshown to be quite reliable in providing a singledetection per vehicle or measurement of vehiclelength. These topics are discussed in Section 3.1.4,PRESENCE Versus PASSAGE Detection; Section3.2.3, Ensuring a Single Detection per Vehicle, andSection 3.3, Detection Area.

4.3 Advance Detection

Loops installed for the PASSAGE application ofadvance detection at signalized intersectionstypically have at least two uses. They allowreactivation of the extension timer for “dilemmazone” safety and allow counting of vehicles arrivingduring the RED signal indicator to provide foradded or variable initialization.

The first use simply requires detection of vehiclesmoving past the location of the Microloop. Therequirements of the counting portion of advancedetection are covered in Section 4.2, CountingVehicles.

A typical installation is shown in Figure 4-3. In thisapplication, a three Probe Set (with probes spaced 3ft. apart) would be used in each lane. Motorcyclesare as important as cars in advance detection.

For advance detection, it is recommended that theMicroloop probe sets be connected to CanogaDigital Loop Detectors for reliable and predictableperformance. The detector should be operated inthe NORMAL recovery mode, MEDIUM(NORMAL) or LOW frequency, and PULSE mode.The lowest practical sensitivity should be used.This is SENSITIVITY 3 or SENSITIVITY 4 formotorcycle detection.

Figure 4-3. Typical Advance Detection PASSAGE Application


4.4 Ramp Metering

Control of the entrance ramps uses multiplePASSAGE type vehicle detectors as well as somePRESENCE type vehicle detectors. The Microloopis well suited for the PASSAGE applications.

Relative to loops, Microloops can be installedquickly. This is of major significance for highvolume, limited access roadways. Frequently thereis no suitable alternative route for vehicles usingthat roadway. The shorter the length of timerequired to install or repair necessary detectionsensors, the less time traffic is disrupted.

Entrance ramp metering typically controls thenumber and spacing of vehicles allowed to enter theroadway. The goal is to optimize the roadway’svehicle handling capacity. Multiple measurementsand control methods are used to achieve trafficresponsive performance.

One possible ramp metering application is shown inFigure 4-4. Microloops V1 through V6 are typicallyused to measure speed, count vehicles, measureoccupancy, and project gaps in the traffic.

Typically, PRESENCE detectors are used tomeasure occupancy. Occupancy is defined as thepercent of time that a vehicle is over a loop during atotal period of time. The equation is:

Occupancy = [(sum of detection durations)/(timeperiod over which detection durations aresummed)] x 100

If sufficient accuracy is achieved by assuming anaverage vehicle length, PASSAGE detectors inPULSE mode can also measure occupancy. Theequation becomes:

Occupancy = [((Vehicle count) x (Average vehiclelength/Average vehicle speed))/(time period overwhich measurement is made)] x 100

Across-the-lane Microloop configurations will countmany long vehicles as two vehicles (due to a sectionof the vehicle that has very little vertical steel). Thedown-the-lane probe configuration can provide asingle detection per vehicle.

Figure 4-4. Typical Ramp Control Application


It is likely traffic will be moving faster than 10 mphon the limited access roadway. It is also likely thatdetection of motorcycles will not be of significance.In this instance, a Dual Probe across-the-lane, or aDual Probe down-the-lane, connected to a CanogaDigital Loop Detector set to PRESENCE mode,SENSITIVITY 2, NORMAL recovery mode, andNORMAL or LOW frequency will give goodPRESENCE detection results. Across-the-laneconfigurations will not give precise estimates ofaverage vehicle length, if a significant percentage ofthe traffic is tractor-trailers and aluminum bodyvans. However, it will give a good estimate ofoccupancy, speed, and gaps. The down-the-laneconfiguration can provide good data for allmeasurements, provided that the vehicles areseparated by 14 ft. or more.

Therefore, Microloops may be used for eitherPRESENCE or PASSAGE (detector in PULSEmode) applications for vehicle detection in the laneson a limited access roadway. The design engineermust make the choice based on design requirements.

Some other definitions are:

Gap Time = time between successivedetections at a single Microloop

Volume = (number of vehicles counted) / (timeperiod over which vehicles are counted)

Vehicle counting is covered in Section 4.2,Counting Vehicles. Vehicle speed measurement iscovered in Section 4.1, Speed Measurement. Referto those sections for more detail on those Microloopapplications.

Precisely where the Microloop should be locatedupstream from the ramp is a function of the specificramp control strategy being used and will not becovered here.

The Microloop installation at location V8 in Figure4-4 is the “check-out” detector (a PASSAGEapplication). It can be used to verify that a vehicleleft the ramp signal after a green indication.Frequently it would be located 6 ft. to 8 ft.downstream of the ramp signal.

The “check-in” detection loop at location V7 andthe “merge lane” detection loop at location V9 arearea PRESENCE detection applications. Suchapplications are well served by standard loops.

For ramp metering control applications, it isrecommended that the Microloop probe sets beconnected to Canoga Digital Loop Detectors forreliable and predictable performance. ForPASSAGE detectors in this application, the detectorshould be operated in the NORMAL recovery mode,MEDIUM (or NORMAL) or LOW frequency, andPULSE mode. The lowest practical sensitivityshould be used. This is SENSITIVITY 1 (twoProbe or three Probe installation) or SENSITIVITY2 (one Probe installation) for detecting cars, trucks,and buses. SENSITIVITY 3 or SENSITIVITY 4 isused for motorcycle detection.

Settings for limited usage of Microloops inPRESENCE mode were covered earlier in thissection.


5. Microloop Installation

5.1 General

The 3M™ Canoga™ Vehicle Detection SystemModel 701 Microloop is a small, passive, cylindricaldevice that transforms magnetic field intensitychanges into changes in inductance. It is intendedfor use with inductive loop detector units operatingin PULSE mode for detecting vehicle passage.Microloops provide point detection as compared toarea detection. When used with Canoga DigitalLoop Detectors, Microloop operation is fullycharacterized.

A single Microloop probe may be used on aninductive loop detector channel, or up to ten probesmay be connected in series on an inductive loopdetector channel for unusual detection patterns.Probes may also be connected in series withthemselves and normal wire loops on an inductiveloop detector channel for other detectionconfigurations.

5.2 Microloop Descriptive Data

5.2.1 Physical

Probe: Cylindrical – 0.88-in. diameter, 3.63-in.long. Installs vertically in a 1-in. diameter hole,nominally 20-in. deep, bored through the pavement.

Cable: AWG #22, four conductor, polyurethanejacketed, 0.20-in. outside diameter. Color codedRED, GREEN, BLACK and WHITE (BLACK &WHITE are not used. Cable fits in a ¼-in. sawcutfrom Microloop probe location(s) to roadside pullbox. Probes can be ordered with up to 200 ft. oflead-in cable.

5.2.2 Environmental

Temperature Range: -35°F to +165°F(-37°C to +74°C)

Humidity: 0% to 100% relative humidity.Withstands immersion in solutions typical ofroadway runoff.

Magnetic Field Intensity: .20 oersted to.60 oersted, vertical component of ambient magneticfield intensity.

Magnetic Field Noise: Must be less than.05 oersted. This upper limit is reduced as multipleprobes are placed in series and as inductive loopdetector sensitivity is increased.

5.2.3 Electrical

Sensitivity: Approximately 3.5 to 8microhenries/oersted at 40 kHz and .2 to .6 oerstedambient magnetic field. Sensitivity at 100 kHz isabout 60% of the sensitivity at 40 kHz.

Inductance: 25 microhenries nominal plus 21microhenries nominal per 100 ft. of cable.

Q: Approximately 3 at .4 Oe, 40 kHz,approximately 5 at .4 Oe, 100 kHz.

Resistance: 0.5 ohms nominal per probe plus 3.2ohms nominal per 100 ft. of cable.

Operating Voltage: .25 volts peak-to-peak to1.0 volts peak-to-peak. This requirement is met onCanoga Digital Loop Detectors, if the totalinductance on a channel does not exceed 400microhenries.

Operating Frequency: 10 kHz to 100 kHzSensitivity decreases with increasing frequency.


5.3 Installation Steps

Installation follows these basic steps:

1. Installation planning2. Magnetic field measurements at installation site3. Hole boring and saw cutting4. Microloop probe and cable placement5. Resistance checks6. Backfilling and sawcut sealing7. Splice to lead-in (home-run) cable8. Connection to inductive loop detector and final

checks

5.3.1 Installation Planning

Details are covered elsewhere in this Manual. Mostconsiderations are summarized here.

1. List requirements of the PASSAGE applicationto be implemented using the Microloop, vehicletypes expected, minimum and maximum speed,lane widths, specific results desired and type ofinductive loop detection to be used.

! CAUTION

If inductive loop detectors other than CanogaDigital Loop Detectors are to be used,carefully verify that they will provide theintended results by testing in a representativeinstallation under simulated or real trafficconditions.

2. Plan the physical requirements of the installationbased on the application needs. Some generalguidelines are:

Point Detection: Vehicles are detected whileover the Microloop and the Microloop is a point(or a line, in across-the-lane and down-the-laneconfigurations).

Auto, Trucks, Buses – one probe per laneORtwo probes per lane at 4-ft. intervals (to detectvehicles changing lanes)

Motorcycles, Bicycles – two probes per lane at4-ft. intervals (will detect most such vehicles)ORthree probes per lane at 3-ft. intervals (willdetect all steel-framed bikes)OR

two probes per lane at 14-ft. separation down-the-lane, on lane center (to obtain one detectionper vehicle)ORThree probes per lane, two probes across-the-lane, 14 ft. apart, and one probe 14 ft. back (toobtain one detection per vehicle and detect mostmotorcycles and bicycles)

Probe Depth – 20-in. from road surface tobottom of probe (16-in. minimum to 24-in.maximum)

Installing the Microloop closer to the roadsurface increases sensitivity, but will tend tomultiple count some vehicles. Installing itfurther from the road surface may causemotorcycles and bicycles to go undetected.

Special Configurations: Specialconfigurations can achieve area detectionand/or a single detection for all vehicle types,but should be tested to determine if desiredresults are achieved.

Other configurations depend on the specificapplication being implemented. Always keepin mind that “normal” Microloop installationconfigurations achieve point detection, i.e.,vehicles are detected for a shorter period oftime than when loops are used.

Order Microloop Probe Sets to MatchInstallation Needs: Installation is mostconvenient when the amount of cable betweenprobes, in multiple probe sets, exactly matchesthe depth and probe spacing requirements. Ifthe interconnecting cable is too short for thedesired depth and spacing, we recommendcompromising on the spacing. If the cable istoo long, some method of handling the excesscable must be devised. While some excesscable can be coiled and placed at the top of thehole, it is somewhat difficult to hold in placefor final sealing. Custom ordered sets areavailable with virtually the same lead time asstandard units. If the standard sizes don’tmatch your needs, custom ordered sets wouldbe your best choice.

A representative physical installation is shownin Figure 5-1. A second representativeinstallation on a bridge deck is shown inFigure 5-2.


Figure 5-1. Typical Microloop Intersection Installation

Figure 5-2. Microloop Usage on a Bridge Deck


5.3.2 Magnetic Field Measurements atInstallation Site

Microloops should never be installed withoutconducting a magnetic field analysis prior tocommencing boring and cutting of pavement. The3M Model MM-9 Magnetic Field Analyzer isrecommended for this purpose.

Measurement of Vertical Component of MagneticField Intensity:

Limits: .2 oersted minimum; .6 oersted maximum.

Set the MM-9 to H. Place the MM-9 probevertically at the location where the Microloop is tobe installed. The MM-9 probe standing on the roadsurface is usually a sufficient measure, even thoughthe Microloop probe will generally be installed withits bottom 20 inches below the road surface.

Nearby iron may cause the earth’s apparentmagnetic field intensity to be higher or lower thannormal. Move the MM-9 probe somewhat to theleft, right, forward, or back until a location is foundthat is near your area’s normal magnetic fieldintensity. There is usually a location within 1 ft. ofthe originally intended location that will have nearnormal values of the vertical component of earth’smagnetic field.

NOTE: On bridge checks it is important to observethe polarity of the ambient magnetic field as it isbeing measured with the MM-9. Magnetization ofthe structural I-beams can cause ambient magneticfield reading polarity to be opposite that of theearth’s magnetic field. If this is observed, theMicroloop may not be used at this location. Becauseof the “reversed” polarity, vehicles would cause

decreases in the magnetic field around the probe andincreases in the inductance. Loop detectors sensedecreases in inductance and therefore detectionwould not occur. With the MM-9, measure alternatelocations to determine if the polarity is the same asthe earth’s at other sites.

Set the MM-9 to Hac. Measure the magnetic fieldnoise at the locations just selected for installing aMicroloop probe, following the approach formeasuring H. Note that the recommended maximumpeak-to-peak noise varies with sensitivity and thenumber of probes connected in series to an inductiveloop detector channel. The numbers in Table 5-1 areappropriate for Canoga Digital Loop Detectors.

For example, at SENSITIVITY 4 and with threeprobes in series to a channel, the maximum Hac is.002 oersted peak-to-peak. The MM-9 reads .100oersted peak-to-peak full scale. There are 50divisions from 0 to full scale, so each division is.002 oersted peak-to-peak. Therefore, the magneticfield noise must be less than one division. Carefullymechanically zero the meter in the physical positionin which you will operate it, before turning theMM-9 ON.

Also, check to be certain that unanticipated vehiculartraffic in tunnels below the roadway, on roadwaysbelow the deck, or on decks above this roadway doesnot create the potential for a problem. Set the MM-9to delta H (reads +/- 10 milloersteds full-scale).Place the MM-9 probe at the locations whereMicroloops will be installed. Observe readingsduring the times potentially offending traffic ispresent. The maximum permissible readings for +delta H are the same as for Hac.

Table 5-1. Measurement of Magnetic Field Noise

Canoga Digital LoopDetector Sensitivity

Approximate Magnetic Noise Limits(Oersted p-p)

1 .050/# of probes

2 .025/# of probes

3 .012/# of probes

4 .006/# of probes

5,6,7,8 Not recommended for use


5.3.3 Hole Boring and Saw Cutting

Typically a 1-in. diameter hole is bored to a depthseveral inches deeper than final placement to allowfor debris in the bottom of the hole. If the soil issuch that the hole walls may collapse, it is suggestedthat a length of PVC pipe (plumbing or electrical) beused to line the hole. This will ensure the Microloopprobe is mounted in a vertical position. It isimperative that the probe be mounted in a verticalposition and that its position is stable. When a lineris used, the bored hole must then be somewhat largerthan the pipe outside diameter. Measure the pipe.Plumbing pipe is different than electrical pipe.Plumbing PVC pipe will probably be 1-1/16-in.diameter and electrical PVC pipe will probably be1-5/32-in. diameter.

After boring the hole (and inserting PVC tube, ifused) measure the hold depth to ensure sufficientdepth for the probe to be installed.

A ¼-in. wide sawcut is then made from the roadedge to each hole in the normal manner as for loops.The cut can be shallower than for loop leads,because the cable is only .19-in. diameter. If moreconvenient, sawcuts may be made before drillingholes for the Microloop probes.

5.3.4 Microloop Probe and CablePlacement

Before inserting the Microloop probes into theirholes, it is suggested that a ring of colored electricaltape be placed on the cable, so that the top of thetape will line up with the bottom of the sawcut whenthe bottom of the probe is at the correct depth. Insertall probes and lead-in cable. Note that when probesets are used, the probe with only a single lead goinginto it is normally the probe of that set this isinserted furthest from the road edge.

A “typical” probe installation is shown inFigure 5-3.

Figure 5-3. Typical Microloop BuriedInstallation


5.3.5 Resistance Checks

As a last check before beginning permanentinstallation, resistance checks should be made. Allprobe sets are fully checked at the factory.However, it is possible a set could have beeninadvertently damaged during installation. Suchdamage will usually be uncovered by a resistancecheck using an ohmmeter. Anticipated readings areshown in Table 5-2.

5.3.6 Backfilling and Sawcut Sealing

Fine, dry sand, such as sandblasting sand, workswell for filling the holes in which the Microloopprobes are installed. If PVC pipe sleeves were used,fill any excess area around the sleeve also. It willprobably work best to fill the tube first. Pour somesand over the probe. Then slowly lift the probe untilthe depth marking tape aligns with the bottom of thesawcut. Release the probe. If it goes back down,add more sand and repeat. After stabilizing theprobe depth, fully fill the hole to the bottom of thesawcut, also filling around the PVC tube sleeve ifused.

Complete the installation by filling the sawcut andhole tops with 3M Brand Detector Loop Sealant.

5.3.7 Splice to Lead-in (Home-run) Cable

Connections of the Microloop probes, seriesconnections and connections to the home-run cable,are made as shown in Figure 5-4. Items of particularimportance are:

• Correct series connection of probe sets (ifneeded)

• Use of twisted conductor cable for the home-runcable

• Making of electrically sound, waterproofconnections

Use of 3M Type 30003 twisted, shielded cable isstrongly recommended for the home-run cable. Itwas designed for this application and will provideexcellent reliability and system performance.

Type 30003 cable is a four-conductor #18 AWGshielded cable with a tough, high-densitypolyethylene jacket. It is resistant to abrasiondamage during pulling through conduit. The cable isfilled with an amorphous material, which preventsthe penetration of water into the cable. This protectsthe cable and system from environmental factors thatmight otherwise degrade the cable and impedesystem performance.

This cable uses twisted wires (not twisted pairs) toreject magnetic field induced noise. To achieve thisnoise rejection, opposite pairs (RED, GREEN) or(BLACK, WHITE) must be used in the samemanner as a twisted pair would be used. The cableis also shielded. Canoga Loop Detectors performbest when the shield is not terminated (left open) atboth ends of the cable and is insulated to prevent itfrom contacting other conductors.

All further connection descriptions refer to thiscable.

If desired, the Microloop probe sets may beconnected in series to a single channel of aninductive loop detector. The connections wouldlikely be made at a roadside pull box. See Figure 5-4 for an example. Note that the RED from the firstprobe set goes to RED of the home-run cable,GREEN from the first probe set goes to RED of thesecond probe set, etc., and GREEN from the probeset goes to GREEN of the home-run cable.

Table 5-2. Resistance Checks – Expected Readings

Resistance Check, Using an Ohmmeter, for Model 701 Microloops(Note that the BLACK and WHITE wires are not connected within the Microloop)

Measuring Points (Probe Cable Wire Colors) Correct Resistance*(Calculate prior to measuring)

RED – to – GREEN .5 ohms nominal per probe in series chain PLUS3.2 ohms nominal per 100 ft. of interconnecting and

lead-in cable

RED to Earth Ground or GREEN to Earth Ground Greater than 1 megohm

*Total Resistance = [(# of probes in series) * (ohms per probe)] + [# feet of cable) * (ohms per 100 feet)/(100)]


Figure 5-4. Connecting Microloop Probe Sets in Series

Also note that the BLACK and WHITE wires in theMicroloop cable are not used. These wires are cutoff at the factory and covered with a seal. They willnot be seen unless the lead-in cable has beenshortened in the field.

The BLACK, WHITE wire pair in the Type 30003home-run cable may be used to connect a secondchannel of the inductive loop detector to anotherMicroloop probe set or loop. Both channels usingthe four wires within the Type 30003 cable must goto a single inductive loop detector that sequentiallyscans its channels.

It is strongly recommended that all connections besoldered, insulated, and then sealed in a waterproofmanner. This will ensure a long, reliable life. TheCanoga 30672 Splice Kit simultaneouslyaccomplishes splice insulation and waterproofing.For more complex splicing applications, such asconnecting probe sets in series at the pull box, 3M3800 “SUPER CAN” Buried Service Wire (BSW)Splice Encapsulation Kits may also be used. Followdirections included with those kits.

5.3.8 Connection to Inductive Loop Detectorand Final Checks

Before connecting the home-run cable to aninductive loop detector, use an ohmmeter to repeatthe resistance checks described in Section 5.3.5 andTable 5-2. This should detect any wiring errors.

Connect the RED and GREEN wires to the inductiveloop detector’s loop input. Set the inductive loopdetector controls to the proper settings. For CanogaDigital Loop Detectors the usual settings are:

• NORMAL recovery mode

• PULSE mode on channels connected toMicroloops

• LOW or NORMAL frequency

• SENSITIVITY: Typically 2 for autos, trucks,buses; typically 3 or 4 for motorcycles andbicycles


Important: FAST recovery mode (typical on mostdigital detectors) will give erratic results unlesstraffic is relatively fast and continuous.

PRESENCE mode will result in long duration“locked calls” in stop-and-go traffic conditions formost useful combinations of probe configurationsand sensitivities. On Canoga Digital LoopDetectors, PRESENCE mode operation ispredictable if these sensitivity/probe configurationsare used:

• One Probe per Channel – SENSITIVITY 3(useful)

• Two Probes per Channel – SENSITIVITY 2(useful)

• Three Probes per Channel –SENSITIVITY 1(may not be useful)

The Model 701 Microloop is intended forPASSAGE applications that can be implementedusing PULSE mode. Other applications shouldreceive careful evaluation prior to implementation.

Turn on the inductive loop detector and check forproper operation. Check each probe for properoperation. This can be done best by standing a 3-ft.by 4-ft. piece of sheet steel on end over each probe,one at a time. Another alternative is to carry a barmagnet over each probe. However, the bar magnetmethod is somewhat uncertain, as care must beexercised not to inadvertently activate the adjacentmicroloop in multi-probe sets. Stronger bar magnetscan place calls 15 ft. from a Microloop probe. Useof sheet steel on end is a better approach.


6. Proposed Specification forCanoga Microloop

6.1 General

This device shall transform magnetic field intensitychanges into inductance changes. A magnetic fieldintensity increase shall cause an inductancedecrease. The device shall be a small, cylindricalunit designed to be buried beneath the road surface.When the device is connected to an inductive loopdetector with compatible operating specifications, allvehicles containing significant vertical sections offerromagnetic material shall be detectable.

A full description of operating characteristics shallbe furnished for use with at least one type ofinductive loop detector, such as Canoga DigitalLoop Detectors. This device is intended for use inPASSAGE applications of vehicle detection with theinductive loop detector in PULSE mode. This doesnot preclude furnishing of a device that will alsooperate in PRESENCE mode of vehicle detection.

6.2 Physical

Assembly shall be sealed against moisture entry.

Probe: Cylinder: Gray color, 0.88-in. outsidediameter and 3.63-in. long.

Probe Interconnecting and Lead-in Cable: 0.20-in. outside diameter, polyurethane jacketed, 4conductor, #22 AWG, RED, GREEN, BLACK,WHITE conductor color coding, bundle twisted at 4to 6 twists per ft.

6.3 Environmental

Temperature Range: -35°F to +165°F(-37°C to +74°C)

Humidity: 0% to 100% relative humidity,including submersion in solutions of chemicalstypical of roadway runoff.

Magnetic Field Noise: AC magnetic field intensitynoise must be less than 10 milloersteds peak-to-peakdivided by the number of probes connected in seriesfor most common installation configurations.

Ambient Magnetic Field Intensity: 200 to 600milloersteds operating magnetic field intensity (thepolarity must be the same as that of the earth’smagnetic field in the installation area).

6.4 Electrical

Inductance: 25 microhenries nominal per probeplus 21 microhenries nominal per 100 ft. ofinterconnecting and lead-in cable.

Resistance: 0.5 ohms nominal per probe plus 3.2ohms nominal per 100 ft. of interconnecting andlead-in cable.

Q: Nominally 3 at 40 kHz, 400 milloerstedsambient magnetic field intensity, nominally 5 at 100kHz, 400 milloersteds magnetic field intensity.

Sensitivity: 3.5 to 8.0 nanohenries per milloerstedat 40 kHz, 400 milloersteds ambient magnetic fieldintensity. Sensitivity at 100 kHz, 400 milloerstedsambient magnetic field intensity is about 60% of thesensitivity at 40 kHz.


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3Intelligent Transportation Systems3M Safety and Security Systems Division

3M Center, Bldg. 225-4N-14St. Paul, Minnesota 55144-1000Customer Service 1-800-328-7098Technical Service 1-800-258-4610Worldwide Technical Service 1-651-575-5072

3M Canada Company

P.O. Box 5757London, Ontario, CanadaN6A 4TI519-451-25001-800-3MHELPS

Printed in U.S.A.Copyright 2000, 3M IPC.All rights reserved.75-0299-7230-6 (Rev. B)

Important Notice to Purchaser:

All statements, technical information and recommendations contained herein are based on tests 3M believes to bereliable but the accuracy or completeness thereof is not guaranteed. THE FOLLOWING IS MADE IN LIEU OFALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING THE IMPLED WARRANTY OFMERCHANITABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

3M warrants that the Canoga Vehicle Detection System will be delivered in merchantable quality. 3M furtherwarrants that, provided the system has been properly installed, operated and maintained by the purchaser, 3Mwill, at its option, either repair or replace any system component or components found to be defective in materialsand/or workmanship during the first year from the date of installation or delivery, but in no case more than 24months from the date of shipment. The foregoing shall not apply to any Canoga product which has been: (1)repaired or modified by persons not authorized by 3M so that in 3M’s sole judgement, the stability or reliability ofsuch component is adversely affected; (2) subjected to misuse, neglect or accident; or (3) has been damaged byextreme atmospheric or weather-related conditions, including chemical corrosion, hail, windstorm, lightning orflooding. This warranty shall apply only to those products comprising the Canoga vehicle detection system assold by 3M.

In no event shall 3M be liable for any injury, loss, or damage, whether direct, indirect, incidental or consequential,arising out of the use or inability to use the Canoga system or any component thereof. THE REMEDIES SETFORTH HEREIN ARE EXCLUSIVE.

3M Canoga vehicle detection systems are manufactured under one or more of the following U.S. patent numbers:3,943,339; 3,989,932; 4,449,115; and other U.S. and foreign patents.

Statements or recommendations not contained herein shall have no force or effect unless in an agreement signedby officers of seller and manufacturer.

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