AD598–Applications
PROVING RING-WEIGH SCALE
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
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.
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.
+
15V
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.
6.8µF
0.1µF
6.8µF
0.1µF
+VS
20
19
18
17
16
15
1
2
3
4
–VS
–15V
OFFSET 1
OFFSET 2
SIG REF
EXC 1
EXC 2
LEV 1
SIGNAL
REFERENCE
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.
RL
5
6
7
8
9
LEV 2
SIG OUT
FEEDBACK
OUT FILT
A1 FILT
1µF
VOUT
FREQ 1
FREQ 2
B1 FILT
B2 FILT
C1
634k
10k
0.015µF
14
13
12
C4
0.33µF
C3
C2
0.1µF
0.1µF
A2 FILT
10 VB
VA 11
AD598
VB
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.
VA
SCHAEVITZ HR050
LVDT
Figure 19. Proving Ring-Weigh Scale Circuit
FORCE/LOAD
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.
PROVING
RING
CORE
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.
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.
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.
–10–
REV. A
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