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AD598

LVDT Signal Conditioner

Analog Devices 

AD598–Applications

PROVING RING-WEIGH SCALE

Figure 20 shows an elastic member (steel proving ring) com-

bined with an LVDT to provide a means of measuring very

small loads. Figure 19 shows the electrical circuit details.

The advantage of using a Proving Ring in combination with an

LVDT is that no friction is involved between the core and the

coils of the LVDT. This means that weights can be measured

without confusion from frictional forces. This is especially im-

portant for very low full-scale weight applications.

+15V

6.8µF

0.1µF

6.8µF

0.1µF

–15V

C1

0.015µF

C2

0.1µF

VB

1 –VS

2 EXC 1

+VS 20

OFFSET 1 19

3 EXC 2

OFFSET 2 18

4 LEV 1

SIG REF 17

5 LEV 2

SIG OUT 16

6 FREQ 1 FEEDBACK 15

7 FREQ 2 OUT FILT 14

8 B1 FILT

A1 FILT 13

9 B2 FILT

A2 FILT 12

10 VB AD598 VA 11

1µF

634k 10k

C4

0.33µF

C3

0.1µF

SIGNAL

REFERENCE

RL

VOUT

VA

SCHAEVITZ HR050

LVDT

Figure 19. Proving Ring-Weigh Scale Circuit

FORCE/LOAD

CORE

PROVING

RING

LVDT

Figure 20. Proving Ring-Weigh Scale Cross Section

Although it is recognized that this type of measurement system

may best be applied to weigh very small weights, this circuit was

designed to give a full-scale output of 10 V for a 500 lb weight,

using a Morehouse Instruments model 5BT Proving Ring. The

LVDT is a Schaevitz type HR050 (± 50 mil full scale). Although

this LVDT provides ± 50 mil full scale, the value of R2 was cal-

culated for d = ± 30 mil and VOUT equal to 10 V as in Step 9 of

the design procedures.

The 1 µF capacitor provides extra filtering, which reduces noise

induced by mechanical vibrations. The other circuit values were

calculated in the usual manner using the design procedures.

This weigh-scale can be designed to measure tare weight simply

by putting in an offset voltage by selecting either R3 or R4 (as

shown in Figures 7 and 12). Tare weight is the weight of a con-

tainer that is deducted from the gross weight to obtain the net

weight.

The value of R3 or R4 can be calculated using one of two sepa-

rate methods. First, a potentiometer may be connected between

Pins 18 and 19 of the AD598, with the wiper connected to

–VSUPPLY. This gives a small offset of either polarity; and the

value can be calculated using Step 10 of the design procedures.

For a large offset in one direction, replace either R3 or R4 with

a potentiometer with its wiper connected to –VSUPPLY.

The resolution of this weigh-scale was checked by placing a 100

gram weight on the scale and observing the AD598 output sig-

nal deflection on an oscilloscope. The deflection was 4.8 mV.

The smallest signal deflection which could be measured on the

oscilloscope was 450 µV which corresponds to a 10 gram

weight. This 450 µV signal corresponds to an LVDT displace-

ment of 1.32 microinches which is equivalent to one tenth of the

wave length of blue light.

The Proving Ring used in this circuit has a temperature coeffi-

cient of 250 ppm/°C due to Young’s Modulus of steel. By put-

ting a resistor with a temperature coefficient in place of R2 it is

possible to temperature compensate the weigh-scale. Since the

steel of the Proving Ring gets softer at higher temperatures, the

deflection for a given force is larger, so a resistor with a negative

temperature coefficient is required.

SYNCHRONOUS OPERATION OF MULTIPLE LVDTS

In many applications, such as multiple gaging measurement, a

large number of LVDTs are used in close physical proximity. If

these LVDTs are operated at similar carrier frequencies, stray

magnetic coupling could cause beat notes to be generated. The

resulting beat notes would interfere with the accuracy of mea-

surements made under these conditions. To avoid this situation

all the LVDTs are operated synchronously.

The circuit shown in Figure 21 has one master oscillator and

any number of slaves. The master AD598 oscillator has its fre-

quency and amplitude programmed in the usual manner via R1

and C2 using Steps 6 and 7 in the design procedures. The slave

AD598s all have Pins 6 and 7 connected together to disable

their internal oscillators. Pins 4 and 5 of each slave are con-

nected to Pins 2 and 3 of the master via 15 kΩ resistors, thus

setting the amplitudes of the slaves equal to the amplitude of the

master. If a different amplitude is required the 15 kΩ resistor

values should be changed. Note that the amplitude scales lin-

early with the resistor value. The 15 kΩ value was selected be-

cause it matches the nominal value of resistors internal to the

circuit. Tolerances of 20% between the slave amplitudes arise

due to differing internal resistors values, but this does not affect

the operation of the circuit.

Note that each LVDT primary is driven from its own power am-

plifier and thus the thermal load is shared between the AD598s.

There is virtually no limit on the number of slaves in this circuit,

since each slave presents a 30 kΩ load to the master AD598

power amplifier. For a very large number of slaves (say 100 or

more) one may need to consider the maximum output current

drawn from the master AD598 power amplifier.

–10–

REV. A

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