Strain Gage Instnllnentation

Jeraen Lammertink

Report on practical training periodcarried out from December 2pt 1992 until March 18th 1993

under supervision of ir. R.G. van Vlietdate: March nnd 199:3

The Department of Electrical Engineering of the Eindhoven University of technologyaccepts no responsibility for the contents of this report.

Abstract

The boat lift of the yachting club of Eindhoven needed a weight installation for the determinationof the weight of the ships and the division of weight over the four winches. In the mean timethe Eindhoven University of Technology was interested in techniques for weight measurements.Questions arose about the necessity of synchronous detection as read-out method.

In this report the use and principles of strain gages were studied. Several configurationsare compared with one another in the area of measurement accuracy. Also the contributions ofseveral measurement errors are compared and it turned out that in a full-bridge configurationwith a DC-amplifier the amplifier drift is far more important than the thermo-electrical effects.

These results are used in the development of a appopriate DC-amplifier. A design of aninstrument that feeds a loadcell and converts the measurement signal into a standard currentloop signal is admitted in this report. The concerning loadcell was chosen by the yachtingcommittee out of a list which is included in chapter four.

Also an old 10 tons load cell including a measuring amplifier was made operational againand calibrated. This to check the weighing equipment after installation.

Contents

1 Objective of this report

2 Measuring with strain gages2.1 Introduction.........2.2 Principle of the strain gage2.3 Measurement errors ...2.4 Selection of a strain gage2.5 Selection of the glue . . .2.6 Instructions for cementing2.7 Wiring between bridge and instrument2.8 Quarter, half and full bridge.2.9 AC versus DC-excitation . . . .

3 The DC-Amplifier construction3.1 Introduction .3.2 The first try-out .3.3 The implemented version3.4 The final design . . . . . .

4 Loadcells4.1 The inside of the HBM Kraftmefidose .4.2 The calibration of the loadcell . . . . . . . . . . . . . . . .4.3 Overload check with the HBM loadcell and the KWSjII-54.4 Force sensor selection .

5 Conclusions

1

223466789

10

1212121416

1919212323

25

List of Figures

2.1 Resistance strain gage grills .2.2 Wheatstone bridge circuit .2.3 Worst case of vibration frequency2.4 The gage attached to an object2.5 Attachment of strain gages2.6 Three wire technique . . . . . .2.7 Supply voltage feedback . . . .2.8 Supply voltage feedback for half bridge.2.9 Principle scheme of synchronous detector

3.1 The instrumentation amplifier INAI0IM .3.2 The implemented circuit3.3 The final design . . . . . . . . . . . . . . .

4.1 Electronic circuit of the load cell from HBM .4.2 Cross-section of the Kraftmessdose from HBM4.3 Calibration graph of the loadcell with the claibration potentiometer

2457789

1011

131517

202022

List of Tables

2.1 The gage factor of some common materials

3.1 Input offset voltages INAI0l3.2 Input offset voltages JLA7413.3 Drift of the INAI0l .3.4 Drift of the JLA741 .3.5 Voltages and currents in the circuit .

4.1 Temperature effects of the load cell .4.2 Calibration of the loadcell with the KWSjII-5 .4.3 Force sensors . . . . . . . . . . . . . . . . . . .

4

1313141418

212224

1 Objective of this report

The yachting club of Eindhoven owns a self built boat crane to lift yachts out of the water toplace them on a trailer, and the other way around. This crane has four winches, each with amaximal tractive power of 5,000 kg at the tackles. This boatlift was meant to hoist up to 20,000kg, which is only possible if the weight is equally divided over the four winches. Without anyinformation about the individual loads on the tackles it nearly is impossible for the liftoperatorto lift ships which have a weight near to 20 tons.

A second problem with the crane was the overload protection, of which is thought that it actsvery inaccurately. A weight installation at every winch can provide further information aboutthe accuracy of the overload protection. The last problem is that some yacht owners are notcompletely honest about the weight of their ship, or they just don't know its weight. Weighingthe ship during hoisting will improve the safety and can prevent terrible damage.

In order to help the yachting committee with making technical choices, I tried to answer thefollowing questions:

• Which loadcells are eligible for the boatlift.

• Is it possible to apply plain strain gages; which techniques are used to attach them.

• Design the electronics for reading out the force sensors.

• What is the effect of long wires on the measurement signal.

• Calibrate a reference loadcell, so that later on, when tIle weight equipment is installed,the lift can be calibrated.

My coach, 11'. R. van Vliet, had also some questions about the modern strain gage techniques.In modern equipment strain gage signals are amplified with DC-amplifiers, while literature stillputs forward that synchronous detection is necessary to eliminate thermo-voltages. Why thisshift in insight? He asked me to repair an old tube synchronous detector and to built a DC­amplifier to compare both. To work towards an answer of all these questions I started withstudying the strain gage techniques, of which the results are given in the next chapter.

2 Measuring with strain gages

2.1 Introduction

A resistance strain gage (sometimes called strain gauge) is a folded piece of resistance wire,bonded with adhesive material to a mechanical structure. When the structure is strained, theresistance of the wire changes owing to changes in the length and the diameter of the wire.

Figure 2.1: Resistance strain gage grills

The physical phenomenon upon which the strain gage is based, is the fact that the resistanceof an electric conductor is dependent on its mechanical load. This was discovered as long ago as1856 by Lord Kelvin. Only in 1939, however, was this phenomenon turned to practical accountin the form of strain gages by the Americans Dr. A.C. Ruge and E.E. Simons.[PHI 58]

Strain gages are used for weighing (from laboratory scales to silo contents measuring), aswrist force sensors in robots [HUB 87], for engine power meters in aircrafts and for practicaltesting of objects under operating conditions, e.g. measurements on wings and propellers ofan aircraft during its flight; on the chassis of motor vehicles; on high pressure units, boilers,pipelines, nuclear reactors and conveyors; on machine parts, rotating shafts, steel frames orbuildings; in short on all objects subject to statically or dynamically often unknown tensile orcompressive forces, torsion or flexion.

One can distinguish between bonded and unbonded strain gages. Bonded strain gages arebonded with glue onto the measuring object. Unbonded gages are loops of resistance wirewinded so that the wires are in tension, even in the position of maximum deflection of themoving member.[BAR 63]

Besides resistance strain gages also piezo electric elements can be used to measure strain.

CHAPTER 2. MEASURING WITH STRAIN GAGES 3

These elements supply a voltage (e.g. between 0 and 1 Volt) depending on the strain [ELD 91].In this report I restrict myself to resistance strain gages.

Originally, only wire gages were manufactured in flat grid construction: one wire in a singleflat plane. These gages, however, have large dimensions for high resistance values and thus thereason for making so called wrap around gages: one wire in two planes. These have decreaseddimensions with the same resistance. Etching processes opened the way to manufacture foilgages: these are made from thin metal foils using a photo-etching method. One of the advantagesof this method is that there is no limit to the forms of the gages.[PHI 58]

2.2 Principle of the strain gage

The relation between strain and a measurable output voltage in the strain gage technique startswith the definition of strain. The strain

b.Le=-

L

is the relative change of length of the wire. The quantity strain e has the dimensionless unit f

also called 'strain'. The change in resistance produced exclusively by this change in length b.Lis

b.RL = R· e

where R the unstrained resistance of the gage. Now we suppose a wire having a circular crosssection. The change in resistance related to the change in area b.A is

"A _ 11"(d - J.Led)2 1I"d2 _ 1I"d2(2 2 2)L.1 - - - - - - J.Le - J.L e

444

b.A 2b.RA = -RA = Re(2Jl - Jl e)

where J.L = Poisson's ratio and d = diameter of the wire. The total change in resistance causedby the strain is

b.R = b.RL +b.RA = Re(l + 2J.L - J?e)

The ratio between the relative change of resistance and the relative change of length is definedas the gage factor [STO 61]

b.R/ R :2G = b.L/L = 1+ 2J.L - J.L e ~ 1 + 2J.L (2.1)

[BAR 63] gives a table of the gage factors of different materials, also listed in table 2.1When a strain gage is bonded to a piece of metal with a Young's modulus E and a cross­

sectional area Am, the strain e will depend on the applied force F [H&C 90].

Fe =-- (2.2)

AmE

E.g. Young's modulus of steel is EFe = 220· 109 Pa. When Am = 1 cm2, a maximum forceof 10,000 N will strain the metal 455J.Lf (micro strain).

Strain gages are usually connected in a Wheatstone bridge circuit (figure 2.2). Supposingthe meter V does not load the bridge circuit, a change in resistance of one of the arms willchange the voltage measured in the following way.

CHAPTER 2. MEASURING WITH STRAIN GAGES 4

t . 1ffT bl 21Tha e .. e ga1?:e actor 0 some common rna ena sMaterial G Material GNickel -12.1 Chromel +2.5Manganin +0.47 Iso-elastic +3.5Nichrome +2.0 Soft iron +4.2Advance +2.1 Platinum +4.8Constantan +2.1 Carbon +20Capel +2.4 Doped crystals 100 to 5,000

+~---i

Figure 2.2: Wheatstone bridge circuit

If all resistances are approximately the same, this formula can be reduced to

sv4 .L (-1 )'SRi;=1

4R(2.3)

Supposing that a number of k resistances are active strain gages and these gages are connectedin such a way that the even numbered gages give a positive contribution to the resistance whilethe uneven gages give a negative contribution. Then the formulas 2.1, 2.2, 2.3 can be combinedinto [G&W 62]

6V = kFGVsup (2.4)4Am E

Taking a maximum strain of 1000 JLf and Vsup = 5 V, G = 2, k = 2 the maximum voltagewill be SV = 5 mY.

2.3 Measurement errors

Different factors may effect the performance of the resistance strain gage.

Lateral contraction: strain perpendicular to the direction we wanted to measure strain, canaffect the measurements. This will be in the order of magnitude of 0.5%.

CHAPTER 2. MEASURJNG WITH STRAIN GAGES 5

Strain hysteresis: after stretching, the strain gage doesn't completely return to its startingposition. The amount of strain hysteresis depends on the material the strain gages aremade of [HSZ 81] [KLA 89].

Creep effect: during straining the strain gage moves a bit along the metal object [KLA 89].

Adhesive effect: because of a certain thickness of the adhesive layer, which has a certainelasticity that depends on temperature, age and moisture, the strain gage will be lessstrained as the metal object [Gre 73].

:t IT'dead'

~~~~~~~~I - resistance

::::;:'''~ ;-1= ==_

strain --I'---~.__---7l_--

1---- wavelength ------l..-j

Figure 2.3: Worst case of vibration frequency, occurring when vibration wavelength equals theactive length of the strain gage

Vibratory effect: if the metal object is vibrating and the wavelength of the strain variationsis of the same order of magnitude as the gage length, the gage will partly be stressed andpartly be compressed, see figure 2.3 [Gre 73].

Temperature resistance effect: the resistance of the gage depends on its temperature [Gre 73].

Temperature expansion effect: dissimilar temperature coefficients of expansion of the metalobject and the gage [Gre 73].

Thermo voltage effect: the use of different metals in the construction of the strain gage causesa temperature dependent voltage[Gre 73].

The small amount of change in resistance gives also problems with measuring.

Wire resistance: the wire resistance between the strain gage bridge and the voltage supply istemperature dependent.

Amplifier drift: DC-amplifiers always have some temperature dependent offset voltage at theinputs; a very low drift amplifier has about 0.1 JL VrC.

Noise: the noise from the bridge and the amplifier limits the sampling rate in the case of AD­converting or limits the bandwidth of the read-out of measuring instruments; this can bea problem with dynamic measurements.

Cable capacitance: in the case of AC excitation of the bridge an capacitive coupling betweensupply and measuring cables can cause a capacitive unbalance.

CHAPTER 2. MEASURING WITH STRAIN GAGES 6

Switch contact: Switching a common dummy strain gage in different bridges or switching themeter between different circuits introduces a variable contact resistance of the selectorswitch. When taking G = 2 and R = 120 n, 1 J.U. corresponds with 0.24 mn.

2.4 Selection of a strain gage

The selection of a strain gage depends on several factors:

Static/dynamic measurements Creep is an important factor when static measurements aremade. The larger the grid the better the creep properties. The gages with the slightestcreep are the flat gages, followed by the foil gages and closing with wrapped aroundgages, which can have a creep of less than 10 mE in the first 24 hours at 25 °C. Hightemperatures make creep even worse. With dynamic measurements creep is negligible.But in high frequency measurements the size of the gage will be limited by the vibratoryeffect, so small gages must be used.

Room for placement The smallest gage I've seen was circular with a diameter of 4 mm. Itcontained two grids: one horizontal and one vertical. The biggest was 12 cm long. Allsizes in between are available.

Resistance The standard resistances of strain gages are 120 nand 350 n. Also available is 60n, 300 n, 600 nand 1000 n. The higher the resistance, the higher the maximum supplyvoltage, which improves the sensitivity. (See formula 2.4)

Surrounding temperature For low temperature applications paper gages are cheapest. Fortemperatures up to 200 °C bakelite is suitable.

Strain limit The strain limit varies from 100 to 10000 JL€.

Fatigue life The number of strain variations a gage can perform is in the order of magnitudeof 1 . 106 cycles.

Accuracy To minimize the temperature expansion effect, temperature compensated gages areavailable for steel, concrete, stainless steel and aluminum. For rough measurements papergages can be used; paper gages are cheap and easy to handle.

Current The maximal allowable current through the gage is about 10 mA for static measure­ments and about 25 mA for dynamic measurements. The higher the current, the higherthe sensitivity (formula 2.4), so always take the maximum.

2.5 Selection of the glue

Most important factors in the selection of the glue are operating temperature, creep and easyhandling. Paper gages are cemented with cellulose glue. This glue is most easy to handle. Thedrying time of the cellulose glue, however, is 24 hours. For quick measurements bakelite gagescan be used with quick hardening cement. After using this cement dynamic measurementscan be made within half an hour and static measurements within one hour. But when timeis no object, standard glue can be used, as long as temperature is beneath 50 °C. Below thistemperature creep is negligible for the glue itself. When minimal creep is required above 50 °C,thermal hardening cement has to be chosen.

CHAPTER 2. MEASURING WITH STRAIN GAGES

2.6 Instructions for cementing

/I----------------__---t/

/IIIIIII~~I

/".. / / /

7

Figure 2.4: The gage attached to an object; a) water repeller; b) cement; c) strain gage; d) cable

Before the strain gage can be cemented onto the object, its surface has to be made smooth andafterwards a mechanical cleaning has to be done by filing, grinding, brushing and/or sanding.After these preparations the surface must show scratches which run criss-cross (do not polishthe surface!). Next the spot has to be cleaned by brushing and rinsing with ethyl acetate, oracetone. Finally it has to be swabbed with a clean wad of cotton wool, until it remains entirelyfree from dirt. Now the surface is ready for cementing. After cleaning the spot must not betouched by fingers or any greasy object. [PHI 58]

Cementing the gage should be done immediately after cleaning. A liberal amount of glue hasto be used to avoid the formation of air pockets. A well glued strain gage always shows excesscement around its periphery. The thickness of the adhesive layer should be made small to avoiderrors in the transmission of the size changes in the test object to the strain gage. This is bestperformed by covering the gage with a non-adhesive film and pressing it with a curved block,covered with rubber, see figure 2.5 [Gre 73]. Rolling with the finger is rather risky; shifting thegage during cementing must be avoided.

~rubber pad

metal plate

::-==~~~~~R=_--=~::nonOdheSIVefJlmstrain gage --,:: .:: .:.......... ",",-__ adhesive

Figure 2.5: Attachment of strain gages

Having attached the gage to the test-object it may be desirable that the gage is covered witha waterproofing agent to prevent it from the effects of a moisty environment. Waterproofingcompounds are available for this purpose but many do not adhere to the p.v.c. insulation ofleads

CHAPTER 2. MEASURING WITH STRAIN GAGES 8

commonly used to connect the gage to the measuring instrument. To overcome this difficultyclamps or crimped-on sleeves may be used over the connection leads.

2.7 Wiring between bridge and instrument

Although the resistances of the cables are relatively small, they can disturb the measurements.A change in temperature of the leads 6.Te causes a change in measured strain which is calledthe zero shift Se (in ~l The zero shift caused by a cable with a resistance Re can be calculatedby means of:

S _ Re6.Tee - GR

v

Figure 2.6: Three wire technique

where G is the gage factor and R the resistance of one gage. A quarter bridge, wired with10 m of cable with a copper core diameter of 0.7 mm has a resistance of Re = 0.45 n. The zeroshift for the two connecting leads will be 2· o.~\02go4 = 1.5 J.LEre. At a maximum strain of 1000J.LE, a temperature change of 10 0 e affects the measurements by 15%.

The three wire circuit (figure 2.6) eliminates the temperature influence caused by the cable.It's still important to use leads of the same length, diameter and material. Also the wiring toterminal strips should be in three wire technique.

The best way to connect a halve bridge is to use a three core cable and make the junctionbetween the two strain gages at the gages themselves. Also full bridges must be linked at thespot of the gages. [PKL 71]

Feedback of the supply for the half or full bridge eliminates cha.nges in resistance of thesupply wires and overcomes changes in load caused by changes in the resistance of the gages.The feedback and measuring cables do not pass a current of significance, so that changes inresistance of those cables do not matter. [PKL 79]

CHAPTER 2. MEASURING WITH STRAIN GAGES

V+

9

u

kno\oJ

n

Istanc

s

v-

v+

v-

Figure 2.7: Supply voltage feedback

2.8 Quarter, half and full bridge

A quarter bridge is a bridge as pictured in figure 2.2 in which only one resistance is a straingage. A half and a full bridge contain two and four gages respectively. Not all gages need to beactive. An active gage is cemented to the structure in a way that it is supposed to follow thestretching and contracting of the object it is bonded to. A dummy gage is cemented to the samematerial as the object of the active gage. Two good reasons to use dummy gages are lack of areaand usage of one common dummy for several strain measurements. To improve the sensitivityone better uses all gages in an active environment (formula 2.4), e.g. one gage in the directionof the strain and the other perpendi.cular to the strain.

In an environment with variable temperatures the usage of the quarter bridge is not advisable.It is more likely then, that we measure temperature instead of strain. The half bridge cancelsthe temperature expansion and temperature resistance effect, at least when the voltage source isideal and the resistances of the connecting wires are negligible. When both gages change equallyin resistance (caused by temperature expansion or temperature resistance effect), the load ofthe voltage source with connecting wires will change. This can change the supply voltage of thebridge depending on the series resistance of the voltage source and the resistance of the supplyleads.

A good method to cancel this load effect is feedback of the voltage at the bridge to thesupply circuit. Then it's important that the other half of the bridge is excited with the samevoltage. This can be done by proper electronic circuitry (see figure 2.'S) or by using a full bridge(figure 2.7). This second halve bridge can also contain two normal resistances of e.g. 10 kn, tolimit the supply current, when, at least these resistances are placed nearby the gages.

A full bridge or a dummy halve bridge close to the active half bridge is also the best wayto compensate the thermo voltage effect that can appear at the connection of the leads to thegage. The zero shift Se (in f) caused by this effect can be calculated.

CHAPTER 2. MEASURlNG WITH STRAIN GAGES

V+

u

kn

5 01: w,.. naI ,..

0ns

9 Ia s9 1:e as n

c

s

Figure 2.8: Supply voltage feedback for half bridge

10

Be = 4Fe6.TekGVsup

For a half bridge with G = 2 and Vsup = 5 V, with two active constantan gages connectedwith copper wire (Fe = 40 JLV;cC), a temperature difference of lOOC implicates a shift of 80 JL€.

For a full bridge, with the connections of the gages close to each other, a temperaturedifference between these two connections of O.25°C leads to an error of 1 JL€.

2.9 AC versus DC-excitation

The supply of the bridge can be AC as well DC. DC-excitation is rather straightforward: DC­supply and a DC-amplifier. AC-excitation implicates an oscillator for supply, an AC-amplifierwith adjustable phase, synchronous detection, demodulation and filtering. A block diagram isdepicted in figure 2.9.

In early literature AC-excitation was recommended. The advantages of AC- excitation men­tioned in literature were (1) the good filtering properties of disturbances by synchronous detec­tion, which is only possible with AC- excitation and (2) the elimination of the thermo-voltageeffect, which causes a DC component in the voltage measured. My coach Jr. R. van Vlietdou bted whether the proportions of the thermo-voltage effect are still significant, after noticingthat DC-excitation was nowadays more common. In the section 'Qttarter, half and full bridge'I gave two figure examples of the significance of this effect and I think Jr. R. van Vliet's doubtwas with reason. Although a synchronous detector still is the best filter for disturbances thiscould be rather overdone in a 'friendly' environment.

I think the real reasons for AC-excitation were (1) the considerable drift of DC tube amplifiersin those days, (2) the many extra cascades needed in DC-amplifiers for the same amplificationand (3) the amount of noise caused by the long wires connecting the bridge to the big andvulnerable measurement apparatus. Nowadays the drift of a good DC-amplifier can be within

CHAPTER 2. MEASURING WITH STRAIN GAGES

'----------+ ---+ -Phase shift Multiplier Amplifier Demodulator

---+ ---+

Bridge supply Oscillator Meter

Figure 2.9: Principle scheme of synchronous detector

11

0.1 fLV rC, high amplifier gains are no problem. Small I.C.'s can immediately generate thesupply voltages needed for the bridge circuit, process the measurement signal and be placedclose to the bridge itself.

3 The DC-Amplifier construction

3.1 Introduction

To allow a comparison between DC and AC-excitation we chose to make a DC-amplifier and torepair an aged strain gage tube amplifier which did not work properly: KWS/II-5 of 'HottingerBaldwin Messtechnik'. The malfunctioning of a dual three position switch (sch 003 see blatt 7of [KWS/II-5]) turned out to be the cause. After replacing this switch the instrument appearedto function correctly.

The wide bandwidth strain gage signal conditioner IB31 or the bridge transducer signalconditioner of Analog Devices are ready to use single chip measuring IC's. The first is meantfor dynamic measurements below 20 kHz. It is encapulated in a 28 pins package and caneasily be connected to an AD-convertor. The second one is especially designed for weighinginstruments and has a chopper-based amplifier: the excitation is DC but the signal is choppedbefore amplification. Both devices were too expensive to buy for only a comparison between DCand AC-excitated bridges. So I designed several amplifiers that could be made of componentsavailable from stock.

3.2 The first try-out

The amplifier drift seemed to be the most important property to look at when choosinga device. In the first two designs I compared two components with different drift and noiseproperties. The first was the low drift instrumentation amplifier INAI01 (figure 3.1) which issupposed to have a drift ofless than 0.25 JlV JOC and an input noise of 10-16 V 2/Hz. The secondone is the general purpose operational amplifier JlA741. The data sheets specified a drift of 15Jl VJOC and an input noise of 10-15 V 2 /Hz.

For the study of the effects of drift and input noise I made an instrumentation amplifier withthree JlA741 'so This amplifier had the same layout and resistors as the INAI01. The gain isset by an external resistor Rg • For the gain it holds that:

In figure 3.1 Rg is connected between the terminals gain set 1 and gain set 2. The reference andTCR of Rg contribute directly to the gain accuracy and gain drift.

I noticed that the elimination of the offset was a lot easier with the INAI01 than with theamplifier using three JlA741. I had to wait for about 15 minutes before the self built circuitstabilized. Hereafter I fed small DC- voltages to both amplifiers and commutaded the input toderive the offset voltages. (See table 3.1 and 3.2)

CHAPTER 3. THE DC-AMPLIFIER CONSTRUCTION 13

- n ut1k

+Ve:e:

gain 1kset 1 20k 10k

Outputgain 20k 10kset 2 1k

-Ve:e:

+1n t 1k

Off et

Figure 3.1: The instrumentation amplifier INA101M

Table 3.1: Inpu t offset voltages INAl01 Table 3.2: Input offset voltages JLA741

Input +Out -Out Gain OffsetJlV mV rnV x1000 JlV10 42.8 -44.2 4.35 -0.1620 86.1 -86.8 4.32 -0.0830 132.2 -133.9 4.44 -0.1940 167.2 -168.2 4.19 -0.1250 220.1 -221.2 4.41 -0.1260 257.0 -258.2 4.29 -0.1370 311.1 -312.5 4.45 -0.1680 363 -365 4.55 -0.2290 401 -403 4.47 -0.22100 490 -492 4.91 -0.20

Input +Out -Out Gain Offset

JLV mV mV x1000 JLV10 43.0 -46.3 4.47 -0.3720 87.6 -90.3 4.45 -0.3030 134.5 -137.9 4.54 -0.3740 170.2 -173.6 4.30 -0.4050 224.6 -228.0 4.53 -0.3860 262.7 -266.0 4.41 -0.3770 317.8 -321.4 4.57 -0.3980 371 -374 4.66 -0.3290 410 -414 4.58 -0.44

100 502 -505 5.04 -0.30

CHAPTER 3. THE DC-AMPLIFIER CONSTRUCTION 14

Table 3.3: Drift of the INA101 Table 3.4: Drift of the pA741

Time +Out -Out Offset Offset:.!h V V pV P V2

11.50 2.46 -2.35 2.75 7.612.10 2.36 -2.55 -4.75 22.612.50 2.36 -2.55 -4.75 22.613.30 2.48 -2.34 3.50 12.314.30 2.47 -2.34 3.25 10.615.00 2.51 -2.54 -0.75 0.6~ -0.75 76.1

Vn r:.. 1 (Offset2- Offset

2) 3.9 pV

Time +Out -Out Offset Offset:.!h V V pV P V2

11.50 3.55 -2.80 18.75 35212.10 3.30 -3.22 2.00 412.50 2.90 -3.55 -32.50 105613.30 2.89 -3.44 -27.50 75614.30 2.70 -3.64 -47.00 22091.5.00 2.50 -4.08 -79.00 6241~ -156.25 10618

Vnr:.. 1 (Offset2- Offsee) 36 pV

The unstable gain was caused by the resistor box I used for Rg • This shouldn't be a problemfor measurement of input offset voltages. One can see that the pA741 circuit has about 0.18pV more offset. This was the result of the difficult adjustment of the opamp. The change inoffset during the measurements is more interesting then the average offset. For both circuits thetables 3.1 and 3.2 show a difference of 0.14 pV between the maximum and the minimum offset.However, these figures do not inform us about drift, but about the measurement noise.

In cases the read-out of the strain is done immediately after nulling the amplifier there isno difference between the cheapest and most expensive instrumental amplifier! If digital dataprocessing is used, changing poles with a relay and substraction of both measurement figureswill eliminate all input offsets.

Later measurements, with a loadcell connected to the input, gave some insight in the driftof both instrumentation amplifiers. Both amplifiers had a gain of 20,000 and had a constantinput (checked by connecting the loadcell consecutive to both amplifiers and the KWSjII-5).Tables 3.3 and 3.4 show the drift of both amplifiers during three hours of measuring, the bottomline of each table showing the standard deviation.

These standard deviations give a rough comparison in drift within three hours. The standarddeviations of the strain measurements caused by this drift effect would be 3.6 W for the pA741and 0.39 W for the INA101 in the case of a full bridge with G = 2 and Vaup = 5 V. When thetime between measuring and nulling the amplifier is more than an hour, the drift propertiesbecome important. In that case it is better to use a one chip instrumental amplifier: both firststage operational amplifiers are then situated on the same chip and will have about the sametemperature, so that the drift of the individual opamps partly compensate.

3.3 The implemented version

For the boat lift I designed a low-cost quadruple strain gage amplifier. At that time I thoughtit would be more simple to make one small box that contained the amplifiers, the supply andthe meters. We did not know what force sensor we would use, so I supposed a half bridge,which should be cheapest in strain gages and electronics. I wanted to use as less as possiblecomponents to make a robust circuit.

The first stage contains a very low drift amplifier with a gain of 45 (see figure 3.2). Thesecond stage has a coarse adjustable gain of 0.1 to 1100 and an adjustable offset of ±33% of itsmaximum input voltage. This results in an adjustable offset of ±0.75% of the input voltage ofthe first stage because of the constant gain of 45 of the first stage. The data sheets of strain

CHAPTER 3. THE DC-AMPLIFIER CONSTRUCTION 15

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II'::£ N0 ::£0 N.-l -t

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+ +'f/t

NAI

0

o 0N N.-l .-l

Figure 3.2: The implemented circuit

CHAPTER 3. THE DC-AMPLIFIER CONSTRUCTION 16

gages show that such a correction is reasonable if the gages are mounted when the constructionis in starting strain, e.g. when the construction is loaded by its own weight. The third stage hasa fine gain adjustment of 0.5 to 2. The switch is for checking the connection of the half bridge.If the switch is pressed the needle must be halfway, otherwise there is a loose connection or ashort-circuit.

While testing this design appeared to be no success. The print was closely situated to thetrafo and the amplifier input picked up the 50 Hz supply signal. With low gains this was noproblem, but before the highest gain was reached the disturbance dominated the output.

3.4 The final design

The yacht-club committee had decided what sensor to chose. They choose the Scaime SD25Xwhich had 1 H1 strain gages. The full bridge can be supplied with 20 V, and with this supplythe full load (2000 kg) causes an unbalance of 24 mV. Because the load cells would be mountedbetween the winches and the frame, the SD2,5X would get an offset of - ± 150 kg == - ± 1.8 mV.

The former amplifier design had not enough range for offset adjustment, to zero an offset of7.5%. Because of this unsuitability I made the design of figure 3.3 with the following features:

• Offset adjustment range of -40% +40% of full load

• Gain adjustment range of -85% +65% of normal gain (= 24 mV == full scale)

• Use of full strain gage bridge

• Single chip, low drift e<0.25 Il'VJOC) instrumentation amplifier with high gain e200x) infirst stage

• First order 7 Hz low-pass filter in first stage

• One supply for loadcell and electronics

• Standard 4 ... 20 mA current loop output

• Small number of components: 16 pins IC, 14 pins IC, voltage regulator, transistor, 4capacitors, 21 resistors, 6 diodes

• Inputs and outputs protected against high voltages

• Low-cost three core cable connection

The four stage amplifier amplifies from stage one to three respectively 200, -1 and i. Thefourth stage is a voltage to current convertor with lout = 0.02-~. Stage three amplifies aroundthe offset of 8 V, V out = 8 + ieVin - 8), and has an adjustable gain. In stage two a voltageof 8 - y V is added to the input; y is adjustable with a potentiometer in the range -2 ... 2 V.A certain offset in the input x = ~ can be compensated this way. Table 3.5 gives five figureexamples of the voltages and currents throughout the circuit.

Two parts of this circuit are thoroughly tested: the voltage to current convertor estage 4)and the voltage regulator. Both functioned as desired. The first two stages, the instrumentationamplifier AD625, is comparable with the INAlO1. Both have the same drift qualities and similarlayout. Stage three is a standard adjustable non-inverting amplifier, so I expect no problemsconcerning this part. Because all electronics will be positioned in the open air, nearby the water,special care should be taken to protect the circuit against moisture. Special spray to protectthe print and components is available and placing the box at a place that cannot be reached byrain is vital.

(J

~."

~~

~

~::r:t'::ltl<)>~."t:-<=;;M~

(Joa~c::(J

~o~

O.lSuF

o V

Output

1Sk

4 .. 20 mA

1033

O.Sk

18 V ~

LM3900-1

18 .. 131.2 VAD62S-3

10k

10k

+

stage 2

Ok

+'10k

O •• 4.8 V1k

SO

----±IO •• 24 mV

SO

so SO,...-A../\/' Ii' i

LM 317

o V

1k

20 v120 V I I I I I I I I I I I I VO G VI

No

st

~r---'f>lk

......-.J

CHAPTER 3. THE DC-AMPLIFIER CONSTRUCTION

Table 3.5: Voltages and currents in the circuitInput stage 1 stage 2 stage 3 OutputmV V V V mA

O+x 0.0+ y 18.0 18 46+x 1.2 + y 17.8 16 812 + x 2.4+ y 1.5.6 14 1218+ x 3.6 + y 14.4 12 1624+ x 4.8 + y 13.2 10 20

18

4 Loadcells

4.1 The inside of the HBM Kraftme6dose

After the measuring amplifier KWSjII-5 was repaired, I thought I could immediately cali­brate the 'Kraftme6' dose from Hottinger Baldwin MeBtechnik. The origin of this loadcell wasrather obscure and no documentation was available. To me it seemed to be as old as the tubeamplifier, but a robust sensor as this loadcell should last forever.

The laboratory for structural fatigue of the T.U.E. possesses a machine for tensile strengthmeasurements which would fit for the calibration. Mr Overkamp operated the machine, whileII'. R. v. Vliet and I studied the meter of the synchronous detector. Even in its most sensitivestate we did not notice any unbalance of the gage bridge under forces of several ten thousands ofNewtons. I assured mI'. R. van Vliet that the electronic part was functioning, and mr Overkampwas very sure his machine was really pulling. Could the loadcell be broken?

This was the opportunity to see the inner of the loadcell. First we opened the connectionbox, that had an inlet for the cable. This box was completely filled with wax, which we had toremove with a blow-drier. Inside we found several wire wound resistors. These probably hadthe function of a calibration bridge, that should have exactly the same unbalance as the gagebridge in the case the loadcell is measuring maximum force. I think also two additional smallresistors were inserted in one half of the bridge to bring the bridge in perfect balance at zeroforce.

We sawed through the thick steel casing and removed two tinplate membranes (figure 4.2).Everything was circular, with the exception of middle piece, which had four equal flat sides.Here we found four strain gages of which two were placed with their sensitive axis horizontaland two with their sensitive axis vertical. The cross-sectional area of the frame at the place ofthe gages was only 1 em2 .

In the circuit between the supply and the bridge were two dummy gages, which were mountedon the thick part of the body. These dummy gages have probably negative temperature coeffi­cients, eliminating the positive temperature coefficients of the bridge, to load the supply equallyunder all temperature circumstances (see figure 4.1).

The bridge and the dummy gages were still functioning, so I checked the theoretical findings.I put the loadcell in the coolerjoven and regulated the temperature from -lOoe to +400 e inthree hours. I measured the zero shift with three different amplifiers: The KWSjII-5 (The mostaccurate instrument), the INA101 (a DC-instrumentation amplifier) and an instrumentationamplifier made out of JLA741's. Table 4.1 shows the results (these are the same measurementsas the measurements of table 3.3 and 3.4). The KWSjII-5 showed no zero shift and I supposethere was not any zero shift. The gain of the DC-amplifiers was 20,000, so with the supplyvoltage of 5V and a gage factor of a.pproximately 2, one volt is equivalent to 5JLt.

Apparently all temperature effects, resistance effect, expansion effect and thermo voltage

CHAPTER 4. LOADCELLS 20

tion

V-

V+

NTC gage

NTCA

gage Is >< t <

< r < <a zero Ac:~rrec:tionIn vv l

outlDut Calibra9 A A 1a zero c:orrec:tion9 >e

> s >vv I

Figure 4.1: Electronic circuit of the load cell from HBM

casing

tinplate membrane

strain gage

heavy frame ...,

screw holes

connection box

Figure 4.2: Cross-section of the Kraftmessdose from HBM

CHAPTER 4. LOADCELLS

Table 4.1: Temperature effects of the load cellTime J.LA741 INA101 KWSjII-5 temperature

h V V J.LE °C11.50 3.18 2.41 0.0 -1012.10 3.26 2.46 0.0 012.50 3.23 2.46 0.0 1013.30 3.17 2.41 0.0 2014.30 3.17 2.41 0.0 3015.00 3.29 2.68 0.0 40

21

effect, had no noticeable zero shift as a result between -lOoe and +40oe. I already showed inthe former chapter that amplifier drift was a far bigger problem than temperature effects.

4.2 The calibration of the loadcell

Now we were sure that the synchronous detector and the loadcell were both functioning we didanother try to calibrate the loadcell. We used the following procedure for this calibration.

• Set the excitation voltage switch (Sch 009) to 5 Volt.

• Put the measurement range switch (Sch 013) and the calibration potentiometer (P 002)in position 0 and the meter magnification switch (Sch 014) in position 1.

• Put bridge switch (Sch 001) in position IV and switched the power on (Sch 016).

• Bring the instrument switch (Sch 003) in position fnstr.. Now the meter must point atzero, otherwise the mechanical zero adjustment of the meter needs to be corrected. It waszero.

• Bring the measurement range switch (Sch 013) in position 2. I did not use range 1, becauseI didn't completely trust that position. Then open the tube eye as much as possible byadjusting the phase (Sch 006 and POOl) and adjust the meter to zero with the offsetcorrection (Sch 007 and P 003).

• Switch on the resistance bridge switch (Sch 008) and calibration factor switch 5 J.L (Sch010).

• Work with two separate people for operating the tensile strength measuring machine andreading out the instrument. Mr Overkamp slowly increased the load to the loadcell and Ikept the meter pointing at zero by adjusting the calibration potentiometer (P 002). Everymoment a multiple of 500 Kg passed by, mr Overkamp gave a signal, so that I could readout the calibration potentiometer.

• Go up and down in load at least two times in order to discover possible mechanical orstrain hysteresis.

The results are tabled in table 4.2. The calibration graph is depicted in figure 4.3. For normalreading out of the KWSjII-5 one better uses the meter with a proper measurement range insteadof the above procedure. Nulling and phase corrections have to be done for reliable measurements.

CHAPTER 4. LOADCELLS

Table 4.2: Calibration of the loadcell with the KWSjII-5

22

Load Up Down Up Down Average Load Up Down Up Down Average1000Kg 1000Kg

0.5 4.5 4.0 4.0 4.0 4.1 5.5 50.0 50.0 50.0 50.0 50.01 8.5 8.5 9.0 8.5 8.6 6 54.5 54.5 54.5 54.0 54.4

1.5 13.5 13.0 13.5 13.0 13.3 6.5 59.0 59.0 59.0 58.5 58.92 18.0 18.0 18.0 18.0 18.0 7 63.5 63.5 63.5 63.5 63.5

2.5 22.5 22.5 22.5 22.5 22.5 7.5 68.5 68.5 68.0 68.0 68.33 27.5 27.0 27.,5 27.5 27.4 8 73.5 73.5 73.0 72.5 73.1

3.5 31.5 31.5 31.5 31.5 31.5 8.5 78.0 77.5 77.0 77.0 77.44 36.0 36.5 36.5 36.0 36.3 9 81.5 81.5 81.5 81.5 81.5

4.5 41.5 40.5 40.5 40.5 40.8 9.5 86.5 86.5 86.5 86.5 86.55 45.0 44.5 45.0 45.0 45.1

10

90•

80 •••70 ••60 ••50 •

•40 ••30 ••20 •••10 ••0

0 1 2 3 4 5 6 7 8 9 10

x1000Kg

Figure 4.3: Calibration graph of the loadcell with the claibration potentiometer

CHAPTER 4. LOADCELLS 23

4.3 Overload check with the HBM loadcell and the KWS /11-5

In the following procedure the KWS/II-5 is adjusted for a precise verification of the overloadprotection.

• Connect the unloaded loadcell with the KWS 111-5.

• Set the excitation voltage switch of the KWS/II-5 (Sch 009) to 5 Volt.

• Put the measurement range switch (Sch 013) and the calibration potentiometer (P 002)in position 0 and the meter magnification switch (Sch 014) in position 1.

• Put bridge switch (Sch 001) in position IV and switch the power on (Sch 016).

• Bring the instrument switch (Sch 003) in position Instr.. Now the meter must point atzero.

• Bring the measurement range switch (Sch 013) in position 2. Then open the indicatortube eye as much as possible by adjusting the phase (Sch 006 and POOl) and adjust themeter to zero with the offset correction (Sch 007 and P 003).

• Switch on the resistance bridge switch (Sch 008) and calibration factor switch 5 J.L (Sch010).

• When a maximum load of 5,000 kg is expected, the calibration potentiometer must beturned anti-clockwise to position 45 (see calibration graph). Now increase the measurementrange by turning up switch 003, until the needle excursion is within the range of the meter.This will be position 20 lower scale for 5,000 kg.

• Place the loadcell between the cable and the tackle.

• As long as the weight on the loadcell is below .5,000 kg the meter will point left of thezero. When the meter points at zero the load will be 5,000 kg with an inaccuracy of 1%.Pointing right of the zero will indicate an overload.

4.4 Force sensor selection

An important part of my training period was the collection of documentation of force sensors forthe yacht-club committee. I received papers of six manufacturers and I selected nine differentforce sensors that were possible candidates for application in the boatlift weighing system (seetable 4.3).

The Microcell from Dosco and the 922 from Depex are strain links. A strain link is a thinstrip of metal with strain gages. The 922 from Depex was the cheapest solution, but nobodycould guarantee approval by any inspection authority. Because the crane is situated nearby thewater, and all equipment is always outside, a stainless steel sensor was preferable. The onlytwo good options left were SD25X from Scaime and the Krueger, the KOSD-40 from WeTe wasrather expensive and not so easy to attach. In this situation I would choose the Krueger, becauseit had already all electronics included, but after Feteris came with a large price reduction theSD25X was chosen.

The SD25X can bear a maximum voltage of 24 V. Scaime recommends to use 10 V, but Ithink that it is exaggerated caution to load a strain gage bridge with an input resistance of 1100n with only 10 V. 20 V will be a nice value; then every gage of 1000 n dissipates about 83 mW.

CHAPTER 4. LOADCELLS 24

Table 4 3' Force sensors..Manufacturer Model Capacity material resistance price (Dfl)NMB, WeTe bv T3B1 1000/2000 kg NiCrMo-Steel 35011 f 915,-

U3B1-B 1000 kg NiCrMo-Steel 35011 f 1165,-WeTe KOSD-40 2000 kg stainless steel 35011 f 1850,-Scaime Feteris SD25X 1000/2000 kg stainless steel 110011 f 1203,-

ZF 1000/2000 kg nickeled 38511 f 1020,-Depex 546 QD 1000 kg ? 40011 f 440,-

922 Al or steel ? f 210,- / f 250,-Krueger 1500/5000 kg stainless steel f 1200,- / f 2100,-Dosco Microcell nickeled ? f 1240,-

Using this high impedance will give a better measurement signal, in this case a signal betweenoand 24 mV. The amplifier of figure 3.3 discussed in chapter :3 is suitable for this purpose.

5 Conclusions

During the study of strain gage attachment and measurement errors I realized that gluing gagesis a elaborate job that demands the necessary experience. Because a save solution was demanded,the use of plain strain gages was rejected as an option.

The study of strain gages gave still an insight in the measuring errors. The effect of thethermo-voltage effect turned out to be rather small in the full bridge configuration as the tem­perature difference between the connections can be kept small. With the analysis of chapter 2one can estimate the inaccuracies of the utilization of strain gages in quarter to full bridge, withdynamic and static measurements, with AC or DC-excitation.

AC-excitation with synchronous detection is still the most accurate way to read out straingages, but with a low drift DC-amplifier in a environment with not to much noise, good resultscan be achieved. For the boat lift I would choose a DC-amplifier. To restrict the influence ofnoise, the electronics can be best placed nearby the sensor. The transport of the signal fromthese electronics to the control panel is done with a current loop. This is an industry standard.

The last day of my training period I calibrated a loadcell. The calibration graph shows a verystraight line through the origin. The quality of these results is good enough for calibration ofthe weight equipment of the boatlift. The only things that still has to be done is the installationof the loadcells under the winches and the realization of the instrumental amplifier. With theinitial impetus of the final design of chapter 3 this shouldn't be much of a problem.

Bibliography

[KWS/II-5] Hottinger Baldwin Messtechnik GMBHBedienungsanweisung KWS/II-5

[PHI 58] PhilipsGuide on strain gauges1958

[STO 61] Melville B. StoutBasic Electrical Measurements (second edition)1961, page 143-149

[G&W 62] Golding and WiddisElectrical measurements and measuring instruments1962, page 306-309

[BAR 63] Davis BartholomewElectrical measurements and instrumentation1963, page 230-237

[PKL 71] Peekel instruments B.V.Do we measure strain when we meaSU1'e strain?1971

[Gre 73J B.A. GregoryAn introduction to electrical instrumentation1973, page 236-247

[PHI 75J Philips GMBHDC Measuring Amplifier PR 93351975

[PKL 79] Peekel instruments RV.Principe van de rekstrook meettechniekMeettechnische aspecten1979

[HSZ 81] Ir. E.L.M. Haffrnans, Ir. J.J. Schrage, Ir. H. ZoeteElectrotechnische meettechniek 21981, page 139-142

[HUB 87] Jr. C. HuberSensoren in de roboticaIndustriele automatisering 1987, nr 10, page 51-54

BIBLIOGRAPHY

[KLA 89] K.B. KlaassenElectrotechnisch meten1989, page 127-130

[TML 89] TML Tokyo Sokki Kenkyujo Co., Ltd.TML Stmin Gauges1989

[H&C 90] A.D. Helfrick, W.D. CooperModern Electronic Instrumentation and Measurement Techniques1990, page 341-345

[ELD 91] Elektro-DataQuasi rebtrookje in weegapparatuurDecember 1991, page 40-41

27

Index

j.LA741 , 12

AC-excitation, 10accuracy, 6active gage, 8adhesive effect, 5amplifier drift, 5

bonded strain gages, 2

cable capacitance, 5calibration, 21cementing, 6chopper-based amplifier, 12creep effect, 5current, 6

DC-excitation, 10digital data processing, 14drift, 12dummy gage, 8dynamic,6

feedback,8foil gages, 3full bridge, 9

gage factor, 3gain drift, 12glue, 6grills, 2

half bridge, 9hysteresis, 4

INA101, 12

Krueger, 23KWS/II-5, 12

lateral contraction, 4loadcell, 19

moist, 7

noise, .5

piezo electric elements, 2Poisson's ratio, 3

quarter hridge, 8quick hardening cement, 6

resistance, 6

scaime SD25X, 16sensitivity, 6static, 6strain, 3strain gage, 2strain gauge, 2strain hysteresis, 4strain limit, 6switch contact, 5synchronous detector, 10

temperature, 6temperature expansion effect, 5temperature resistance effect, 5thermal hardening cement, 6thermo-voltage effect, 5, 10

unbonded strain gages, 2

vibratory effect, 5

waterproofing agent, 7Wheatstone bridge, 4wire gages, 3wire resistance, 5wiring, 8

Young's modulus, 3

zero shift Se. 8zero shift Se, 9