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8/7/2019 Selecting_Prox

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Inductive Proximity Sensors: Design, Selection,Specification and Implementation

Authored by Guerrino SuffiProximity Sensor Product Marketing, Omron Electronics LLC

When your application calls for metallic target sensing that falls within an inch of the sensingsurface, the inductive proximity sensor fits nicely into your design criteria. These durable sensorsare suitable for harsh environments. They have dust and dirt materials build up immunity.

Industrial inductive proximity sensors first came out in the early 1960s and today have a proventrack record in the sensing arena. They also generally have standardized behaviors.

This article discusses the rudimentary design of the inductive proximity sensor, and goes on toshow a selection method that accounts for conditional application and device requirements. Thearticle then teaches key inductive proximity sensor specifications followed by a discussion ofmounting restrictions for the sensors implementation. Together, this information will supply adesigner with the knowledge required for a successful inductive proximity sensor to objectdetection design.

Inductive Sensor DesignInductive proximity sensors operate under the electrical principle of inductance. Inductance is thephenomenon where a fluctuating current, which by definition has a magnetic component, induces

an electromotive force (emf) in a target object. In circuit design, one measures this inductance inH (henrys). To amplify a devices inductance effect, a sensor manufacturer twists wire into a tightcoil and runs a current through it.

An inductive proximity sensor has four components; The coil, oscillator, detection circuit andoutput circuit. The oscillator generates a fluctuating magnetic field the shape of a doughnutaround the winding of the coil that locates in the devices sensing face.

When a metal object moves into the inductive proximity sensors field of detection, Eddy circuitsbuild up in the metallic object, magnetically push back, and finally dampen the Inductive sensorsown oscillation field. The sensors detection circuit monitors the oscillators strength and triggersan output from the output circuitry when the oscillator becomes dampened to a sufficient level.

Designers should consider two types of inductive proximity sensors when selecting an inductive

sensor; shielded and unshielded. When current generates in the sensors coil the doughnut effectit causes the proximity sensor to trigger when any object comes behind, along side or in front ofthe device. Shielding uses a ferrite core to direct the coils magnetic field to radiate only from thesensors detection face. Unshielded inductive proximity sensors are not completely unshielded. Apeeled back ferrite core shielding in the unshielded case allows for a longer sensing distance,while still preventing sensing due to objects behind the detection face.

Understanding the operation, the magnetic nature, and the shielding of the inductive proximitysensor is helpful when considering the influences of target material, environment, and mountingrestrictions on the sensor itself and in your design.

Inductive Sensor SelectionInductive proximity sensors categorize in five specific types; cylindrical, rectangular, miniature,

harsh environment, and special purpose. 70% of all inductive proximity sensor purchases are ofthe standardized cylindrical threaded barrel type. When one considers this statistic, it is easy tounderstand why a designer would specify into their application a general-purpose (orstandardized) inductive proximity sensor. 70% of the time, he would be correct. Experience hasshown, however, that applications in need of inductive sensing usually warrant the examination ofa few additional design criteria.

These conditional criteria eliminate (or specify) the more special inductive proximity sensorsavailable first before falling upon the general purpose inductive proximity sensor. The threeguiding beliefs of inductive proximity sensor selection are target material, environment, andmounting restrictions.


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Target MaterialsIn the world of inductive proximity sensors, not all metals are created equally. A designer can findhimself looking for a quick fix to an inductive sensing problem that would vanish if a specialinductive sensor was selected. The inductive proximity sensor specification that we have allbecome familiar with in technical data sheets worldwide references a standard detectable objectmade of an iron (ferrous) material. Other metallic materials, such as stainless steel, brass,aluminum, and copper have different influence over the inductive effect and are usually lessdetectable than iron.

Is the target material an iron object? Will the target material change in future runs of theapplication?Examine the sensing distance reductions for typical inductive proximity sensors below.

Stainless Steel = Standard Sensing Distance X .8Brass = Standard Sensing Distance X .5Aluminum = Standard Sensing Distance X .4Copper = Standard Sensing Distance X .3

A designers full line sensor supplier will have a solution for his inductive proximity sensorsdetection of troublesome metallic materials. Manufacturers terms for these special inductiveproximity sensors are non-ferrous sensing or all metal sensing. Non-ferrous sensingInductive proximity sensors will detect non-ferrous metals such as aluminum better than theysense iron. All metal sensing inductive proximity sensors will detect all metallic materials at the

same sensing distance.

What separates the non-ferrous sensing and all metal sensing inductive proximity sensor fromthe standard (or general-purpose) inductive proximity sensor is the number of separate inductivecoils included in the proximity sensor head.

The non-ferrous sensing or all metal sensing inductive proximity sensor will include two orthree separate coils in the proximity sensor head while the general-purpose inductive proximitysensor will include only one coil. The main trades between a non-ferrous sensing or an allmetal sensing type proximity sensor and a general-purpose proximity sensor are the cost andbody size. Non-ferrous sensing and all metal sensing proximity sensors tend to be moreexpensive due to the increased number of coils required and have larger enclosures than theirtraditional inductive proximity sensor counterparts.

EnvironmentEnvironmental conditions can have far and sweeping effects upon the inductive proximity sensor.These effects specifically refer to sensor life, but can only be related to premature failure (falsetrigger or otherwise) of the inductive proximity sensor once installed into its component mountingposition. Nevertheless, your full line sensor supplier has many solutions to specific environmentalconditions.

Is the application one in which metallic chips or filings are prone to build up on the side orface of the inductive proximity sensor?Intelligent semi-conductor microprocessors found in some modern inductive proximity sensorshave the ability to detect the slow build up of metal filings or chips over time and teach theinductive proximity sensor to ignore their effects. Sensor suppliers call this specialized inductiveproximity sensor a chip immune type.

Another type of inductive proximity sensor that is resilient against chip build up is the flat-packproximity sensor. The slim profile of the flat pack proximity sensor when mounted with its sensingface exposed vertically is virtually unaffected by chip build up on its slim horizontal component.

Is the inductive proximity sensor exposed to cutting fluids or chemicals for prolongedperiods of time?In the face of cutting fluids or corrosive chemicals, a traditional inductive proximity sensor maybecome brittle and crack, shortening its life.


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In such cases, a designer must again turn to a specialized inductive proximity sensor. Proximitysensors dipped, coated or shot from fluoroplastic suffer no ill effects from the materialin terms of performance or reliability. Fluoroplastic's stability against cutting oils and corrosivechemicals outweigh the additional costs that come with a product manufactured with fluoroplastic.An additional benefit to this type of inductive proximity sensor is its ability to preventany build up of weld slag.

Is the sensing application in a high temperature environment?Inductive proximity sensors typically include their silicon amplifiers and detection circuitry inside

the sensor head housing. These proximity sensors are called self-contained devices. Self-contained proximity sensors are practical for most applications until environmental conditionsbegin to exceed the normal operating parameters for a silicon-based circuit. Normal operatingtemperatures for silicon based circuitry is within the realm of -25 to 70C (-13 to 158F). Under anytemperature conditions beyond these ranges, the circuitry becomes more prone to operatingfailure.

For temperature applications that exceed these requirements, look for inductive proximity sensorsthat use separate amplifiers. With separate amplifier inductive proximity sensors, the sensorhead contains the inductive coil and little more. The intelligent amplifier and detection circuitry canbe located safely away in a remote environmentally controlled area. Such sensors can resisttemperatures as high as 200C (392F).

Mounting RestrictionsWhen it comes to miniaturization, few components so strongly represent the micro-electronicrevolution that has occurred within the last 10 years as inductive proximity sensors. Today, theworlds leading manufacturer of inductive proximity sensors can manufacture a 5.5mm X 5.5mmX 19mm rectangular proximity sensor with an extended sensing range of 1.6mm.

Does your application space constraints prohibit the use of an inductive proximity sensorwith a traditional cylindrical based body?Inductive proximity sensors come in a wide variety of body types. Rectangular style inductiveproximity sensors range from the sub-miniature (5.5mm X 5.5mm X 19mm) to the flat pack style(25mm X 10mm X 50mm) all the way up to the limit switch housing size (40mm X 40mm X115mm). In the last case, the life of an inductive proximity sensor in limit switch housing will faroutweigh the life of a typical limit switch. Limit switch life is on the order of 300K cycles while thesimilarly shaped inductive proximity sensor in limit switch housing can last up to 100K hours.

Other advances in inductive proximity sensor miniaturization include separate in-line amplifiertype. Inductive proximity sensors of this type come with sensing heads as small as 3mm indiameter and robotics cabling that allow the sensor head to move if needed.

So have you considered the questions above and eliminated the need for the most populartypes of specialized inductive proximity sensors?As a designer, you can then reliably fall back on the proven success of the traditional inductiveproximity sensor. Before you specify a particular inductive proximity sensor for your application,be sure to investigate the following qualities for a long lasting and well-manufactured device.

Does the inductive proximity sensor in question have a strong enclosure?Cylindrical proximity sensors barrel housing thickness varies from manufacturer to manufacturer.

The thicker the barrel housing of the Inductive Proximity Sensor, the less likely it will be to breakdue to over zealous installation techniques or through incidental object collision.

Has the inductive proximity sensor been vacuum potted?Most proximity sensors are potted, but poor potting is almost worse than no potting at all. Thepoorly potted proximity sensors will have air bubbles trapped inside the devices that cause unduestressing which may lead to PCB cracking and failure.

Does the cable have a proper strain relief?An inductive proximity sensor with a cable that protrudes directly out of the potting material issusceptible to breakage at the potting material-cable conjunction. A proximity sensor cable with


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this design also has a much weaker pull force. If you are interested in a long life sensor, look fora strong, flexible strain relief on your Inductive proximity sensor of choice.

Inductive Sensor SpecificationIn automation design, it is necessary for one to understand the precise technical definition of acomponents behavior. The following definitions, if not the terminology itself, are unique toinductive proximity sensors. Therefore, it is important to describe and comprehend definitionsbefore implementation of the inductive component into the application at present.

Upon review an inductive proximity sensor data sheet displays many specifications that tell adesigner how to implement the inductive device for the purposes of detecting a specific object.

Standard Detectable ObjectWhen an inductive proximity sensors data sheet refers to a standard detectable object, it tells thespecified shape, size, and material which is used as the standard to examine the performance ofthe proximity sensor. This understanding is important because the detection distance of theinductive proximity sensor differs according to the shape and material of an object. Typically, thestandard detectable object will be an iron plate with a thickness of 1mm and height and width ofequal length to the diameter of the inductive sensor.

Detection DistanceDetection distance is the position at which the inductive proximity sensor operates when a

standard detectable object is moved in front of the sensor in a defined manner. For an Inductiveproximity sensor with an end (or front) detection surface, the detection distance is determined byaligning the center line of the Inductive sensor with the center line of the standard detectableobject. The standard detectable object is moved towards the face of the inductive proximitysensor until the proximity sensor changes states and the detection distance is determined.

One of the issues that is examined when considering the detection distance of an inductiveproximity sensor is the target materials capacity for conducting electricity. Materials that arehighly conductive make poor targets for traditional inductive proximity sensors. Also, the targetsthickness will have an influence of its detection. Thin materials are easier for an inductiveproximity sensor to detect then thick materials.

If one reviews the principles of operation for an inductive proximity sensor shown earlier in thisarticle, the material conductance and thickness factor to detection distance behavior falls in line

with the technology of the inductive proximity sensor. A conductive material will disperse Eddycircuits and not allow them to build up thus making it harder to detect. A thin material due to itslack of ability to move current when compared to a thicker material causes a build up of Eddycurrents, which allow for higher detection distances.

Reset DistanceThe reset distance refers to the distance at which the inductive proximity sensor releases itsoutput when the standard detectable object is removed from its field of detection. The differencein distance between the detection distance and the reset distance is called the Distance

differential. Typically, the distance differential is from 3% to 10% of the overall detectiondistance. The distance differential is incorporated into the design of the inductive proximity sensorto prevent the proximity sensor from having its output chatter due to noisy environments ordetectable object vibrations.

Todays quality inductive proximity sensors can have trigger points that are repeatable to1/10,000ths of an inch. Designers be must aware, however, that desired detectable object mustapproach the face of the inductive sensor to trip the output and then be removed from theinductive sensors field of detection by the distance differential before another precise objecttrigger can occur.


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Setting DistanceThe setting distance describes the distance at which the inductive proximity sensor will trigger anoutput with the standard detection object even if the detection distance has been decreased dueto temperature or voltage fluctuations. When implementing an inductive proximity sensor, thedetectable object to sensor face calculations should begin with the setting distance specifications.

Not every design, however, will have the luxury of detecting the standard detection objectdescribed in the inductive proximity sensor data sheets engineering section. In the cases ofirregular object detection, the detecting distance cannot be estimated from the engineering data.

In these cases, an operational check with the sample object is required. Take the detection objectin question and approach the inductive proximity sensor until the output changes state. Thedistance determined is the detection distance of the target object and inductive proximity sensorcombination.

The Setting Distance for the target object can then be calculated by the following formula: NewSetting Distance = (Detection Distance obtained by test with target object) X (Setting Distance ofthe standard detectable object)/(Standard Detection distance of the standard detectable object).In other words, the objects setting distance is proportional to the standard detecting targetsdetection distance to setting distance ratio.

Inductive Sensor ImplementationMounting requirements must be considered when implementing the inductive proximity sensor

into your design otherwise you may encounter reduced sensing distance, false triggering, or non-detection of the target.

Influence of surrounding metalsWhen an inductive proximity sensor is mounted into its sensing position, it is important to considerthe effects of the mounting hardware itself and other metallic objects that may be present in thearea of the sensor.

For the shielded type of inductive proximity sensor, the device can be embedded into a metallicmounting fixture up to the point when the sensors face is at an equivalent height as the mountingsurface. This embedded mount protects the inductive sensor from mechanical damage due toincidental contact with the target object. It is not recommended that a shielded inductive proximitysensor be recessed into a metal mounting surface. Objects, materials, or opposing surfaces thatare not to be detection objects should remain clear of the Inductive sensors face by a factor of 3

times the sensors standard detection distance.

For the unshielded type of inductive proximity sensor, the device cannot be completely embeddedinto a metallic-mounting fixture. Due to its extended sensing distance, the unshielded inductiveproximity sensor is susceptible to the influences of surrounding metals. One does not only needto consider that objects, materials, or opposing surfaces must remain clear of the sensors face bya factor of 3 times the sensors standard detection distance. In addition, one must consider thatthe inductive proximity sensor must be clear of surrounding metals by its size (diameter in thecase of a cylindrical proximity sensor) in every direction with a depth clearance of 2 times itsstandard detection distance.

Failure to meet the inductive sensors clearance requirements can lead to false detection orreduced sensing distances.

Mutual interferenceWhen multiple inductive proximity sensors are mounted in close proximity to one another eitheralong side or in an opposing directions to another inductive proximity sensor, either inductivesensor can be subject to an effect called mutual interference.

Mutual interference is created when the field of a proximity sensor couples with the detection coilfield of another closely mounted proximity sensor. The result can create an inductance that canresult in the generation of a beat frequency in one or both of the sensors. This, in turn, causes theoutput of the proximity sensor to chatter (switch on and off erratically).


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Mutual interference problems can be insidious due to their erratic nature. A sensing applicationwhere Inductive sensors are mounted side by side and closer then a manufacturers mutualinterference distance specifications can actually perform seamlessly at one time and thensuddenly display signs of chattering and false detection at another time.

Specifications for separation distance of proximity sensors that are mounted side by side can varyfrom sensor body type and by manufacturer. Always examine and adhere to the manufacturersspecification distances for mounting inductive proximity sensors to avoid potential mutualinterference problems.

If your application and sensing requirements demand your inductive proximity sensors to bemounted closer together, consider the following tips. The selection of a shielded type of inductivesensor allows for closer mounting. Of course, one could also specify a miniature inductivesensor. The smaller size means smaller sensing distances and less probably for mutualinterference.

In addition, some manufacturers of inductive proximity sensors offer alternate frequency types.Alternate frequency inductive sensors oscillate their magnetic coils at different cycle rates thantheir standard inductive proximity sensor counterparts. This prevents the inductive coupling thatleads output chattering.

Lastly, if close sensor mounting cannot be avoided, the inductive proximity sensors can bemultiplexed. Turning off and on alternate inductive proximity sensors and taking alternate readscan be a quick solution to a mutual interference problem provided that your application accountsfor the response time hit.

Inductive Proximity Sensors: ConclusionMore inductive proximity sensors are sold worldwide then any other sensing technology. Theinductive proximity sensors durability, life, and resistance to dust and harsh environments havemade it the designers prime choice in sensing technology. Newly armed with the knowledge ofselection techniques, specification nuances and implementation considerations will help youovercome the most common inductive proximity sensor application pitfalls.updated 12.05.05

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