VOL. 15 No. 7, pp. 189-220 JANUARY 1954

Philips Technical Review1

DEALING ~TH TEC~CAL PROBLEM5RELATING TO THE PRODUCTS, PROCESSES AND INVESTIGATIONS OF

THE PHILIPS 'INDUSTRIESEDITED BY THE RESEARCH LABORATORY OF N._V. PHILIPS' GLOEILAMPENFABRIEKEN, EINDHOVEN, NETHERLANDS.' ,

AN AUTOMATIC RECORDING POTENTIOMETER FOR INDUSTRIAL USE

by H. J. ROOSDORP. 621.317.733.083.4.078: 621.317.39

In electrical equipment for the measurement and controlof industrial processes, high.çensitivity and accuracy must be combined with robust construction and reliability. Thefollowing article describes an instrument operating on the null principle and automaticallybalanced, which goes a long way towards meeting these requirements.

The great progress in electronics, which has madesuch a valuable contribution to the development oftelecommunications, has also resulted in greatimprovements in the field of electrical measurement.Much thought has also been devoted to the problemof applying these perfected measuring techniques tothe measurement of non-electrical quantities. Inorder that such measurements may be effected withthe aid of electrical equipment, it is necessary toconvert the quantity to be measured to a corres-ponding electrical quantity. Devices that will dothis are known in general as transducers, and ex-amples of these are given in the table.From this table it will he seen that most of the

electrical quantities produced by such transducersare D.C. voltage or resistance values. This is notaltogether fortuitous, for these quantities can beeasily and accurately measured: the transduceremployed for any particular purpose is chosen togive the most suitable electrical quantity formeasurement.

In this article an instrument will be 'descrihedwhich is capable of measuring very accurately directvoltages, resistances or even impedances, and which,because of its robust construction, is suitable (inconjunction with some form of transducer) for themeasurement, recording or automatic control ~fmany non-electrical quantities' in industry 1). '

1) A recording instrument somewhat similar to that describedhere was designed in the Philips Laboratories during thewar,

Quantity to be Transducer Electrical quantitymeasured- .-, ... . - - ---

Temperature Thermocouple D.C. voltageResistance thermo- ResistancemeterTotal radiation D.C. voltage orpyrometer resistance

Pressure

Force (weight) Strain gauge Resistance ..Displacement Differential Self inductance

transformer-

Rate of flow of Diaphragm with Resistance or selfgas or liquid pressure difference inductance

pick up

.Humidity Dew-point pickup Resistance

(resistance thermo-meter)Wet and dry bulb, Resistancewith resistancethermometerConductivity Resistancepickup

PH. Glass-calomel I D.C'. voltageelectrode

Luminous inten- Photo-electric cell D.C. voltagesity Barrier-layer cell Resistance .: ,,: ',;..~l

':' ~ .~!#1-.Ionizing radia- Ionization chamber Direct eurrent ' ..··tions ' - .. , . (~

Geiger counter Direct current'I ' ,

190 PHILlPS TECHNICAL REVIEW VOL. 15, No. 7

~easurüug nnethodsVoltages, currents and impedances can be meas-

ured in accordance with two different principles,viz. the deflection method and the null or compari-son method. Some instruments using the first.method are, e.g., electrostatic voltmeters for volt-ages, moving-coil and moving-iron instruments forcurrents, and the cross-field or cross-coil type ofinstrument for impedances.

The great advantage of the deflection system isthat it gives ~ direct reading, which bears a knownrelationship to the quantity to be measured. Itsuffers from the drawback, however, that themeasuring instrument itself determines the attain-able accuracy, which may not always be adequate.Another disadvantage is that a certain amount ofpower is required to produce. the needle deflection,so that, if the transducer is capable of deliveringonly a very small amount of power, an extremelysensitive instrument, or alternatively an amplifier,has to be used. In the latter instance, the measuringaccuracy is also dependent on the stability of theamplification.With the null method, the unknown quantity is

compared with another (known) quantity of thesame kind. This can be done by means of a' bridgecircuit or a potentiometer. Only a detecting instru-ment is then required and the accuracy of the measu-rement does not then depend on that of the instru-ment but only on .the comparison elements (e.g.standard resistances). Also, since the instrument isemployed only as a zero indicator, it can be madevery much more sensitive than those of the direct-reading type. (Moreover, the sensitivity can in thiscase be increased by using an amplifier, the amplifi-cation of which need not be known, or constant).The null method thus provides at once greater

sensitivity and accuracy than the direct-readingsystem. The only disadvantage of the null methodis that an adjustment has always to be made beforethe value of the unknown quantity can be obtainedfrom the known comparison quantity.

Automatic' balance

The comparison or null method is normallyemployed when precise measurements are required.The disadvantage of having to adjust the balancemanually for .each and every measurement can beelimin'ated by using a servo motor to effect theadjustment automatically 2). With such an "auto-matic bridge" the quantity to be measured is thus

9) An early example of a measuring circuit embodying thisprinciple was described in this Review by R. Vermeulen,Philips tech. Rev. 4, 354-363, 1939.

read direct, so that here we have in effect a direct-readirig instrument. Another great advantage of theservo motor is that it makes a much larger forceavailable than the conventional direct-reading sys-tem. The null method with automatic balancetherefore provides better facilities for connectingtlie measuring system to a recording mechanism toprovide a continuous record.

The following numerical example will serve toillustrate this.

In a moving-coil meter the driving torque is:

M = Bil dw, (1)

where:M = torque in Nun *),B .- magnetic induction in the air-gap, in Whjm2

(I Wbjm2 = 104 gauss),i current in Amps flowing in the coil,

_ length in m, of that part of the coil withinthe magnetic field,

d diameter of coil in m,10 number of turns.If rhe resistance of the coil is, say, 400 n, the

current i, for the measurement of a voltage of 10 mV(e.g. from a thermocouple}, is 25 [LA.Now, if wefurther assume that B = 0.3 Wbjm2, l = 3 cm,d = 2 cm and 10 = 1000, the torque will be 4.5 X10-4N.cm = 46 mg.cm. This torque must counter-balance that of the spiral springs. When the currentto be measured is varied, the difference between thetorque applied to the moving coil by the magnëticfield and that of the spiral springs is available forthe variation in the deflection of the needle.If the current varies, say 1%, a torque of 0.46 mg.

cm is available for the deflection. The force appliedto the end of a needle 10 cm in length is then 46 [Lg,which is just enough to initiate an indication, butquite inadequate for direct recording purposes.The amount of force delivered can be increased'

by means of· an amplifier, but the amplificationthat can be employed is limited by the maximumdissipation of the moving coil. Assuming this tobe 1 W, and again taking the resistance of the coilto be 400 n, the maximum current that the. coilwill carry is 50 mA. The maximum possible torqueis then 2000 times greater than in the examplegiven above; a force equal to 92 mg will now beavailable at the extremity of a needle 10 cm long.This is just sufficient to drive an inking stylus, butthe greatest care would have to be taken in thedesign and operatien of such an instrument.

*) N = Newton, the unit of force in the M.K.S. system(= 105 dynes).

JANUARY 1954 AUTOMATIC POTENTIOMETER 191

Using the null method with a servo-drivenbalancing mechanism as described below, a forceof some hundreds of grammes is available. Thisforce is supplied by an induction motor, to thecontrol winding of which a power of about 4, W isapplied by an amplifier. (To obtain so much forceat the pointer of a moving-coil direct-readinginstrument, 1010 W would have to be applied togive full-scale deflection!)

Principle of the automatic bridgeFigures 2, 3, 4 and 5 depict a number of circuits

suitable for the measurement of direct voltages, andresistances, by the null method with automaticbalance.

In fig. 2 the voltage Ex from a thermocouple iscompared with a voltage Eo from a potentiometerconnected to a source En whose voltage is preciselyknown. The difference Ei (the so-called error voltage)

Fig. 1. The automatic measuring and recording bridge (type PR 2000) user! for measuringand recording the temperature for the pasteurisation of milk.

A measuring circuit operated on the null principleand incorporating automatic balancing can be quiteeasily designed for a setting time very much shorterthan that of a sensitive direct-reading instrumentwithout an amplifier. Equipment can be made forindustrial purposes in which the time necessary fortraversing the whole scale is less than 1 sec.

For industrial applications, the null method withautomatic balancing accordingly has many advan-tages, viz. high accuracy, considerable power andhigh speed. The equipment about to be describedbased on this system, will measure voltages andimpedances and, if necessary, record them 3).

Fig. 1 shows the unit employed for measuringand recording the temperature for the pasteurizationof milk.

3) Series PR 1000 for indicating, and series PR 2000 forrecording instruments. The use of the automatic potentio-meter/bridge for the control of industrial processes willbe dealt with in a subsequent article.

between Ex and Eo is applied to an amplifier A,and the amplified voltage is used to drive a motor Mwhich is coupled mechanically to the contact ofthe slidewire Rn. This contact is always moved bythe motor in the direction that will reduce thedifference between Ex and Eo. Provided that theamplification by A IS sufficient, the motor operates

A

M 77302

Fig. 2. Basic potentiometer circuit for measuring the voltageEx of a thermocouple by the null method with automaticbalance. Ell source of reference voltage. Rn potentiometerslidewire. A amplifier. M motor. The motor sets the contactof the slidewire to the point where the difference Ei betweenEx and Eo is practically zero.

192 PHILlPS TECHNICAL REVIEW . VOL. 15, No. 7

at very low values of Ei, so that, practically speak-ing, when the motor stops, Eo is equal to Ex.Accordingly, Ex can be read from a scale mountedon Rn.

In many cases it may be desirable to have theadjustment corresponding to zero voltage at a pointother than the beginning of the scale; e.g. in measure-ments over a range of say from 50 to 150V insteadof 0-100 V, or where the range of measurementcovers both positive and negative voltages (e.g.from -10 to'+ 100 mV). To achieve this, thepotentiometer is made into a bridge circuit (fig. 3),which has the additional advantage that variableresistors (e.g, temperature-dependent resistors) canbe included in one or more of the bridge arms, inorder to render the circuit independent of certaindisturbing effects. In this way, for example, com-pensation can be provided for variation. in thee.m.f. of a thermocouple due to changes in tem-perature at the cold junction.

Ex

En 77303

Fig. 3. Potentiometer circuit for measuring the voltage Exof a thermocouple. With this arrangement, zero voltage mayhe made to correspond to any point along the length of theslidewire. The balancing voltage is obtained from a bridgecircuit.

Fig. 4 shows a circuit suitable for the measurementof the resistance Rx of a resistance-thermometer. Inthe simplest form of circuit, this resistance is in-cluded in one of the arms of the bridge. Here againthe sliding contact of the potentiometer Rn is movedby the motor M in such a way as to balance thebridge so that the error voltage Ei delivered to theamplifier A will be practically zero.In jig. 5 we illustrate a circuit that is widely

employed in conjunction with strain gauges. Thesegauges, represented in the figure by the resistancesRs, are wired up in the form of a bridge, so as tobalance out the effect of any resistance variationsin the connections. Measurement is effected withthe aid of a second bridge circuit in which themeasuring slidewire, controlled by the motor Mis included. Both bridges are fed from a single.transformer; the motor sets the slider of Rn to the.point where the voltage between points a and bis the same as that between c and d: at this pointthe adjustment is complete and the motor stops since

the error voltage Ei' applied to the amplifier isthen effectively' zero.

With sufficient amplification, the accuracy ofmeasurement is determined by the tolerance withinwhich the resistance values or the potential En

M

7ï304E

Fig. 4. Simple bridge circuit for measuring the resistance Rxof a resistance thermometer.

can be set. For industrial use this type of instrument.is usually calibrated to an accuracy within i%.

The sensitivity, that is the lowest value of Eiat which the motor will revolve, is determined bythe amplification from A. Very high amplificationcan be combined with robust construction in elec-'tronie equipment, to give an effective sensitivityof 1 (-LV.

The speed at which the adjustment is effected isgoverned by the design of the servo system. Witha high-speed motor the time can be made very short,viz. less than 1 sec, but there is then some risk ofthe balancing mechanism maintairiing an oscillationabout the point of balance. To prevent this, themechanism must be damped. The amount ofdamping required can be computed in the followingmanner.

Fig. 5. Circuit for strain gauge measurements. Thc gauges,represented liy the resistances Ro, are connected as a bridgecircuit. The slidewire Rn is included in a secondbridge circuit,and the two bridges are fed from a single transformer T.The motor sets the slider onRnto the point where the differenceEi between the voltage across a·b and the voltage across c-dis practically zero. .

JANUARY 1954, AUTO:WATIC POTENTIOMETER 193

We shall now suppose that a damping force ispresent which is proportional to the velocity of the Critical damping occurs when

lnfig. 6 the point on the slidewire Rn correspond-ing to zero voltage applied to the amplifier A,is denoted by O. Let us employ the following nota-tion:x = displacement of the slider from the point 0;vx = voltage applied to the amplifier A, corres-ponding to displacement x;a = amplification provided by A;kavx= force applied by the motor M to the movingparts when a voltage avxis applied to it;m = mass of the moving parts.

With no damping applied to the slider, theequation of motion is

d2xm--+ kavx = O.

dt2

The values of x satisfying this equation may berepresented as a function of time by an undampedoscillation of angular frequency given by

'17306

Fig. 6. When the contact of the slidewire Rn is located at adistance x from the point of balance 0, a voltage vx is appliedto the amplifier A. The amplifier then delivers a voltage av~which causes thc motor to exert a force kavx on the slider.

moving parts. For a velocity dx/dt, let this dampingforce he pdx/dt; the equation of motion IS then:

d2x dxm- + p-+kavx= O.

dt2 . dt

From this it follows that x as a function of timecan be represented by a damped oscillation whoseamplitude is proportional to e-pt/2m. As with mostmeasuring instruments, what we require is aperiodicmotion, so that the value of p will have to he highenough to introduce critical damping. This occurswhen: .

p2= 4 mkav. . . . .. (5)

Mech~nically, it is extremely difficult to obtaina reproducible value of p. Electrically, however,this is quite a simple matter; in effect, the mot.or Mis coupled to a tachometer generator G, which deli-

vel's a voltage that is proportional to the speed. Fora speed of dx/dt, let this voltage be represented bygdx/dt. This voltage from the generator G, togetherwith that from the potentiometer Rn, is supplied toth~ input of the amplifier (fig. 7). Without themechanical damping, the equation of motion is now

d2x ( dX)m-2 +ka vx+g- =0.dt dt

(6)

The solution of this equation, giving x as afunction of time, may he represented by a dampedoscillation of amplitude proportional to' e-kagtf2m.

(2)

(3)Fig. 7:A tachometer-generator G, coupled directly to the motorM, delivers a voltage that is proportional to the speed. Thisvoltage is added to the input of the amplifier, producing anelectrical damping of the balancing mechanism.

4mvu2Ö = -ka' (7)

The damping' can accordingly he adjusted to therequired value by varying 'the output from the'generator (i.e, by varying g).

To ensure high sensitivity, it is essential that the.motor, even for the smallest value xmin,'of the dis-placement x, delivers a certain minimum power [(minto drive the recording mechanism, viz.

k a v Xmin= [(mim (8)

(4) and the required amplification is, therefore,

[(mina=---.

kvXmin(9)

From (7) and (9) it follows that the value of gat which critical damping occurs is given by

., 4mV2Xming~=---.[(min

(10)

We see from (9) and (10) that the desired valuesof a and g are dependent on v, that is, on the measur-ing range. If measurements are to be made in anyother range, both g and a have to be re-adjusted, butthis can he obviated by splitting the amplifier into

.----------------------~------~------.--~-- -

194 PHILIPS TECHNICAL REVIEW VOL. 15, No. 7

two parts, Al and A2, in the manner shown in fig. 8.The equation ofmotion is now obtained by replacingv in (6) by val; and (l, by a2• According to (8) thefollowing equation must he satisfied if the recordingmechanism is to operate at a small value Xmin of thedisplacement x:

orKmin

al = ---- (11)ka2vxmin

Critical damping occurs when

4mvalg2 =, (12)ka2

from which, together with (11),

Fig. 8. With the amplifier divided in two sections, Al and A2'it is not necessary, when the measuring range is changed, tomodify both the voltage which tbe generator delivers at agiven speed (factorg) and, at the same time, the amplification.It is sufficient to vary only the amplification ai of the amplifiersection AI'

This shows that g is now independent of v. Whenthe measuring range is varied, g need not be re-adjusted; in accordance with (11) it is suffibientto .adjust al'

Construction

(1~)

The general arrangement of the automatic bridgeisshown in the diagram in fig. 9. The resistors, whichtogether with the slidewire Rn, form the bridgecircuit proper, are grouped in the form of an inter-changeable unit B corresponding to the requiredmeasuring range. The latter can then be changedsimply by replacing the unit B. The slider of Rnis mounted on a cursor W which is moved alongthe rail C by the motor M through gearing and afriction drive D. A pointer indicates the positionof the cursor on a scale S. The rotor of the tacho-meter generator G is coupled to the shaft of themotor M. The amplifiers and supply units, in theform of interchangeable units, are mounted in arack E. Unit 1 supplies a voltage EB to feed themeasuring element. Unit 2 is the amplifier denotedby Al in fig. 8. The amplifier shown in that figureas A2 is in two sections, 3 and 4, of which 3 receivesboth the output voltage from 2 and the voltage EGfrom the generator. Unit 4 is the output stage, ofwhich the output voltage Eu is applied to the motorM. Lastly, the supply voltages for the two amplifiersare furnished by unit 5.Between 4 and 5, other units can be placed in

circuit when the equipment is to be used f~r auto-matic control, but these will not be discussed here 3).

B

~I 1/ ",Rn

I ,,/ /

~/" ./ G

S MW C

-H--J(.) u-I--S L I_.I_II I I I II I ITn 1/[/

/)" , #'////L-;/ /E~ /E.-;/ ~/1"'7U G El cB

5 4 3 2 1

-c ,~ I I -I ~

77309 E

Fig. 9. Block diagram of automatic potentiometer bridge, types PR 1000 and PR 2000.The cursor W, which is moved along the rail C by the motor M through a friction drive,carries both the slidewire contact and the pointer, which moves along the scale S.

JANUARY 1954 AUTOMATIC POTENTIOMETER 195

Fig. 10. Automatic recording potentiometer/bridge type PH 2000, showing recordingmechanism. Top left: rack of amplifiers and supply units. Top right: in the openedcover of the instrument will be seen the slidewire, servo-motor and tachometer-generator,below which the recording mechanism is mounted.

In order to ensure the high degree of reliabilitydemanded of equipment intended for industrial use,all the components are loaded far below their ratedvalues; the use of components having only a limitedlife has been avoided.

Fig. 10 is a photograph of the instrument openedto show the interior. The front cover (at right)contains the slidewire, servo motor, tachometer-generator and recording apparatus, the electronicsection (rack E, fig. 9) being mounted in the body

Fig. Ll , Chassis with amplifier and supply units. All the leadsare taken to a central connecting strip, to facilitate testing.

of the unit (left). This rack is shown in detail injig. 11, whilst jig. 12 depicts one of the amplifiersremoved from the rack.

Fig. 12. One of the amplifier units. All the components arereadily accessible.

196 PHILIPS TECHNICAL REVIEW VOL. IS, No. 7

To facilitate maintenance, all the main compo-nents, moreover, can be checked without any diffi-culty in situ, as all the principal leads are mountèd ona central connecting strip on the rack (see fig. ll).The rack is connected to the other parts of thecircuit through a line of plug sockets at the rear.All the components of each of the units are immedia-tely accessible, once the chassis is removed from therack (fig. 12).

The slidewire

As previously mentioned, the measuring accuracyis determined by the precision of location of theslidewire contact and the effective value of Rn,as well as the precision of the other resistors and thereference voltage. It is relatively easy·to manufac-ture fixed resistors to a fine tolerance and with ahigh degree of stability, but with slidewires thisis not such a simple matter. The total resistance ofa slidewire can certainly be made quite constant,but it is' equally necessary that its absolute value,as well as its fractional variation as a function ofthe position of the slider shall be carefully pre-determined. (The maximum tolerance is 0.1 %.) Inthis way it is possible to change to a different mea-suring range without recalibrating the whole system,merely by changing the fixed resistors (unit B) andthe scale. The slidewire Rn can then also be changedwithout necessitating recalibration of the scale. Afixed resistor is permanently connected across theslidewire, such that' the equivalent resistance ofthe two is exactly 1000 n.

The cursor W is designed to carry the contacts oftwo slidewires simultaneously, thus making itpossible to build up more complex circuits if re-quired.

In order to secure a positional accuracy compati-ble with the measuring tolerance, the slidewiremust have a high degree of linearity and must havea sufficient number of turns. In this instrument itis made with 1300 turns over a length of 260 mm,and has a linearity within ± 0.1 %. This meansthat the adjustment is accurate to about 0.1 mm.

Bridge supply voltage

For resistance measurements the bridge may befed with alternating current at mains frequency.Sometimes, however, it may be necessary to employ. direct current, for example, when the resistance tobe measured is such a long way from the equipmentthat the capacitance of the leads would influencethe result (these capacitances can, of course, bebalanced out at the bridge, but it may be moreconvenient to avoid this complication by using

direct current). When D.e. is used, .the supplyvoltage is obtained from a rectifier with the neces-sary smoothing.

For D.e. voltage measurements, the bridge isused as a potentiometer. For this purpose thecurrent through Rn must be of a constant and knownvalue: it is therefore obtained froin a source ofvoltage that is stabilized by means of a gas-dischargetube, whose operating voltage remains extremely. constant throughout its life (type 85 A 1). Thecircuit is fed with a current of 1mA (fig. 13) througha number of resistors, one of which (Rv) is variable,to. enable the current to be adjusted to this value.

Fig. 13. Calibration of potentiometer. By means of the variableresistor Rv, the current for the potentiometer network is ad-justed to 1 mA. This is verified by comparing the voltageacross the resistor Rm (1018!l) with that of a Weston standardcell SE. For this operation the switch S is set to position 1.'Vith correctly adjusted current, the motor M does not operate.

Resistor Rm has a value of 1018 n, and in order to.adjust the current to the exact value, the voltageacross this resistor is compared with that of aWel?tonstandard cell (1.018V). This is effected by means 'of aswitch S (fig. 13). When this switch is set to position1, the difference between the voltage across Rm andthat of the standard cell SE is applied to the inputof the amplifier A. If these voltages are equal, novoltage is applied to A, and the mot~r M is inoperat-ive, but if a potential difference exists the motorrevolves. This' difference can be adjusted ~o zeroby 'means of Rv, the indication of this being that.the motor stops. The current is then at the correctvalue. Owing to the stability of the operatingvoltage of the tube 85 A 1, this calibration need not ~,be made very frequently (e.g. once a month).

The amplifier

As electronic amplification can be effected mostsimply with A.C. voltages, the amplifier used isconstructed as an A.C. amplifier. For the measure-ment of D.e. voltages, or for resistance measure-ments with the bridge operated on D.e., a ,converteris included between the_measuring element circuit

JANUARY 1954 AUTOMATIC POTENTIOMETER 197

Fig. 14. The scale and slidewire are assembled as a single unit, as shown in this photo-graph of the non-recording version of the instrument.

and the amplifier. This converter consists of a contactvibrator, the energiûng coil of which is connectedto a source of A.C. voltage at mains frequency. Forelectrostatic measurements, a vibrating capacitor isemployed. For both resistance and D.C. voltagemeasurements, therefore, the frequency of thevoltages to be amplified is 50 cis.

To ensure high sensitivity, the greatest possibleamplification is required, but in practice this islimited by the inevitable noise and pick-up; thereis of course no objcct in increasing the gain to thepoint where the amplified voltages of these disturb-ances would be sufficient to start the motor orsaturate the amplifier. The spurious voltagesoccurring in the first valve correspond to a voltageof 5 [J.V on the grid of this valve. For this reasonit is desirable to adjust the amplification to such

a value that only when a signal voltage of about10 [J.V is applied to the grid of the firstvalve, \\ ill the motor start to move. (This assumesthat there is no pick-up superimposed on thesignal voltage, which might saturate the amplifier).By using a transformer before the input valve, theminimum input voltage at which the motor willoperate is reduced to about 1 [J.V. The input trans-former also has the effect of reducing the input

~ impedance of the amplifier (without the trans-former, this would be about 1 MD; with transformerit is roughly 1 kD). As the motor positively rotatesat 100 V, the voltage gain required in the amplifierin then about 108•

When a vibrating capacitor is used, the inputimpedance can be very high, say 1012 0. Now, owingto this high impedance across the cathode and gridof the first valve, the spurious voltage producedin the amplifier - designed, it should be remember-

ed, for industrial use - IS considerably greaterthan the value mentioned above. For this reason, theamplification is so adjusted that the motor operatesonly on an input voltage of at least 300 [J.V.

The driving mechanism

For high accuracy it is essential that the positionof the pointer in relation to the scale should agreeexactly with that of the slidewire contact. Anybacklash or flexibility in the transmission betweenslider and pointer would at once introduce errors.Such sources of error are avoided by mounting thepointer directly on the slider; in addition, the scaleand the slidewire are rigidly mounted together(see fig. 14). The transmission between the motorand the cursor can thus in no way affect the accuracyof measurement.

The driving force must be at least high enoughto overcome all sources of friction, and 300 grammesis adequate in this respect. To protect the cursorfrom being driven too forcibly against the end ofof the scale by test signals beyond the actualmeasuring range, the potentiometer drive is of theslipping friction type.

The motor and generator

The motor 'is a two-phase induction motor withshort-circuit rotor, as shown in diagram in fig. IS.One of the windings (A) is fed from the outputamplifier, and the other (B), with a supply 900

different in phase, from the mains. When the pointof balance of the measuring circuit is passed, thevoltage delivered by the amplifier undergoes aphase shift of 1800 and the motor reverses. Tosecure accuracy and stability in operation, a motorwith a high starting torque is used.

198 PHILlPS TECHNICAL REVIEW VOL. 15, No. 7

Fig. IS. Construction of the induction motor. Windings Aare connected to the amplifier. Windiugs B are fed from themains in such a way that the currents flowing in A and Bdiffer in phase by 90°. An iron ring C distributes the magneticfields set up by A and B so as to approximate to a rotatingfield. D is the short-circuit rotor.

Fig. 16 shows the principle of the tachometergenerator. This also has two windings, one of which(A) is connected to the mains and the other (B)to the input of the amplifier section A2 (see figs 8and 9). The TOtOI'consists of an aluminium cylinder(C) which revolves round a stationary iron core (D).When the TOtOI'is brought into motion by the motor,a voltage is induced in winding B which is propor-tional to the speed of rotation. The frequency isthe mains frequency and is therefore independentof the speed.

Reversal of the direction of rotation results in aphase shift of 1800 in the voltage and, if the signof the supply voltage has been correctly chosen,the phase relationship between the voltage deliveredby the amplifier Al and that from the genel'atoTwill be such that the latter provides a dampinge.m.f. proportional to its speed. The ratio of thevoltage delivered by the generator when stationary

77313

Fig. 16. Construction of the tachometer generator. Windings Aare fed from the mains. When the aluminium rotor C is inmotion, the magnetic field due to eddy currents in the rotorgives rise to a voltage in the windings B porportional to thespeed of rotation. The output from the windings B is fed tothe input of the amplifier section A2 (see fig. 8).

to that at full speed (approx. 2000 r.p.m.) is in theregion of 1: 1000. With the rotor stationary, thevoltage induced in the winding B is therefore sosmall that it has a negligible effect on the result ofmeasurement.

Summary. This article contains a description of an instrumentfor measnring D.C. voltages, resistances and impedances, ofrobust construction and high driving power. In conjunctionwith a suitable transducer, this instrument is suitable for theindustrial measurement and recording of nou-electricalquantities such as temperature, pressure, displacement,humidity, etc. The action of the instrument is based on thenull principle which has many advantages over the direct-reading system, e.g. greater accuracy and higher sensitivity.The drawback of the null method, that an adjustment has tobe made for each reading, is eliminated by using a sorvo-rno torto effect the necessary adjustment automatically. The servo-motor, moreover, provides a considerable driving force; thesetting time for full-scale deOection is about 1 sec. A tacho-meter generator coupled to the motor shaft provides a voltagewhich critically damps the motion, to prevent oscillationabout the balance point (aperiodic damping).