AD598

REV. A

–12–

signal is summed with the signal from the output position

LVDT; this summed signal is integrated such that the output

position is now equal to the input position. This circuit is an

efficient means of implementing a mechanical servo-loop since

only three ICs are required.

This circuit is similar to the previous circuit (Figure 23) with

one exception: the previous circuit uses a potentiometer instead

of an LVDT to provide the input position signal. Replacing the

potentiometer with an LVDT offers two advantages. First, the

increased reliability and robustness of the LVDT can be ex-

ploited in applications where the position input sensor is located

in a hostile environment. Second, the mechanical motions of the

input and output LVDTs are guaranteed to be identical to

within the matching of their individual scale factors. These

particular advantages make this circuit ideal for application as a

hydraulic actuator controller.

DIFFERENTIAL GAGING

LVDTs are commonly used in gaging systems. Two LVDTs

can be used to measure the thickness or taper of an object. To

measure thickness, the LVDTs are placed on either side of the

object to be measured. The LVDTs are positioned such that

there is a known maximum distance between them in the fully

retracted position.

This circuit is both simple and inexpensive. It has the advantage

that two LVDTs may be driven from one AD598, but the disad-

vantage is that the scale factor of each LVDT may not match

exactly. This causes the workpiece thickness measurement to

vary depending upon its absolute position in the differential

gage head.

This circuit was designed to produce a

±10 V signal output

swing, composed of the sum of the two independent

±5 V

swings from each LVDT. The output voltage swing is set with

an 80.9 k

Ω resistor. The output voltage V

given by:

+









× 500 µA × R2.

The V/F converter is currently set up for unipolar operation.

The AD652 data sheet explains how to set up for bipolar opera-

tion. Note that when the LVDT core is centered, the output fre-

quency is zero. When the LVDT core is positioned off center,

and to one side, the frequency increases to a full-scale value.

To introduce bipolar operation to this circuit, an offset must be

introduced at the LVDT as shown in Step 10 of the design

procedures.

LOW COST SET-POINT CONTROLLER

A low cost set-point controller can be implemented with the cir-

cuit shown in Figure 23. Such a circuit could possibly be used

in automobile fuel control systems. The potentiometer, P1, is

attached to the gas pedal, and the LVDT is attached to the but-

terfly valve of the fuel injection system or carburetor. The posi-

tion of the butterfly valve is electronically controlled by the

position of the gas pedal, without mechanical linkage.

This circuit is a simple two IC closed loop servo-controller. It is

simple because the LVDT circuit is functioning as the loop inte-

grator. By putting a capacitor in the feedback path (normally oc-

cupied by R2), the output signal from the AD598 corresponds

to the time integral of the position being measured by the

LVDT. The LVDT position signal is summed with the offset

signal introduced by the potentiometer, P1. Since this sum is in-

tegrated, it must be forced to zero. Thus the LVDT position is

forced to follow the value of the input potentiometer, P1. The

output signal from the AD598 drives the LM675 power ampli-

fier, which in turn drives the solenoid.

This circuit has dual advantages of being both low cost and high

accuracy. The high accuracy results from avoiding the offset er-

rors normally associated with converting the LVDT signal to a

voltage and then subsequently integrating that voltage.

MECHANICAL FOLLOWER SERVO-LOOP

Figure 24 shows how two Schaevitz E100 LVDTs may be com-

bined with two AD598s in a mechanical follower servo-loop

configuration. One of the LVDTs provides the mechanical input

position signal, while the other LVDT mimics the motion.

The signal from the input position circuit is fed to the output as

a current so that voltage offset errors are avoided. This current

EXC 1

EXC 2

LEV 1

LEV 2

FREQ 1

FREQ 2

B1 FILT

B2 FILT

OFFSET 1

OFFSET 2

SIG REF

SIG OUT

FEEDBACK

OUT FILT

A1 FILT

A2 FILT

AD598

0.1

µF

1

2

3

4

5

6

7

8

9

10

11

12

13

14

16

15

17

18

19

20

V

B

V

A

+V

0.015

µF

0.1

µF

0.1

µF

0.33

µF

0.01

µF

1

µF

30k

50k

Ω

INPUT PI

INPUT

MECHANICAL

POSITION

OUTPUT

POSITION

SCHAEVITZ E 100

LVDT

100

Ω

10k

0.33

µF

1000pF

150k

0.1

µF

+V

MASS ON SPRING

620 N/m

33 GRAMS

0.068

µF

49.9k

4.99k

20k

47

µF

47

µF

33

µF

+25V

GND

POWER SUPPLY

+V

LM675

IN4740A

10V

GUARDIAN SOLENOID

12 VDC 2–INT–12D

62 CONE

Figure 23. Low Cost Set-Point Controller