Analog Devices AN536 Application Notes

AN-536

a

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • World Wide Web Site: http://www.analog.com

Dimensional Gaging Measurements with Model 3B17

Linear Variable Displacement Transducers (LVDTs) are
common in gaging systems. Two LVDTs can be used to
measure the thickness or taper of an object. For a thick­ness measurement, 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. When the object to
be measured is placed between the two LVDTs, the dis­placement of both LVDTs are added together and then
the computer or control system will subtract this total
from the known distance between the LVDTs. Taper or
slope measurements are done by positioning the two
LVDTs at the same level at a known distance apart. If the
object to be measured is on the same level as the LVDTs,
both LVDTs would have the same reading. The differ­ence between the displacements of the LVDTs divided
by the known difference in position will yield the slope.

(A – B)1 + (A – B)

–
B
+

LVDT 2

+
A

–

–
B
+

LVDT 1

+
A

–

Figure 1. Additive LVDT Connections

+EXC

2

A – B (SIGNAL)

A

–EXC

3B17

In a gaging application where many measurements are
made, and thus many LVDTs are used, the cost for
a signal conditioner for each transducer can be high.
One Analog Devices’ 3B17 LVDT signal conditioner can
be configured to handle two inputs for both the differen­tial and additive applications, thus cutting the signal condi­tioning costs in half. Figure 1 shows two LVDTs connected
together with the input to the signal conditioning being the
sum of both LVDTs. The LVDTs should have matched
gains. Both LVDTs will receive the same excitation
signal up to a maximum of 20 mA. The demodulation
synch signal internal to the signal conditioner is two
times the voltage at Pin 3 of the conditioner (labeled A)
minus the voltage at Pin 2 (labeled A – B). This signal
should not vary more than +50%. Numerically this
indicates:

|

V

| > |(

A – B

)1 + (

A – B

A

1

)2|

MODEL 3B17 LVDT INPUT SIGNAL CONDITIONING
MODULE

The 3B17 is a nonisolated signal conditioning module
for LVDT interfacing. It provides a programmable, very
stable, low distortion ac excitation voltage for the trans­ducer and accepts a 20 mV rms to 5 V rms input signal
from the LVDT. All gain and span adjustments are ac­complished with screwdriver attachments on the top of
the module. The interpretation of the LVDT output volt­age is done with a synchronous demodulator. This de­modulator converts the ac output of the LVDT into a dc
voltage. It automatically compensates for any phase
error between primary and secondaries of the LVDT,
and eliminates the need for a phase adjustment. It also
rejects any residual quadrature or null voltage providing

accurate, linear ±10 volt and 4 mA–20 mA outputs.

AN-536

+EXC

A – B

–

B

+
+

A

–

–EXC

P2

V/I

I

SPAN

I

PROTECTED

PRECISION

P1

1

2

3

4

OSCILLATOR

INPUT

PROTECTION

PROTECTION

RANGING CARD

(IF USED)

AMP

V

SPAN

A + B

DEMOD

V

OUT

ZERO

JUMPERS

V

ZERO

OUTPUT

PROTECTION

3B17

I

OUT

I

OUT

RETURN

LOOP V+

LOOP COM

COM

V

OUT

+15V

PWR COM

–15V

1

R

2

L

Figure 2. Block Diagram of Model 3B17

INSIDE THE 3B17

The 3B17 accepts inputs from 4-wire, 5-wire and 6-wire
LVDTs or RVDTs. A block diagram of Model 3B17 is
shown in Figure 2.

The 3B17 provides an ac excitation of 1 V rms to 5 V rms
at frequencies ranging from 1 kHz to 10 kHz on input
screw terminals 1 and 4. The ac excitation is limited to a
20 mA rms load; this sets a lower circuit primary imped-

ance of 50 Ω for a 1 V excitation and 250 Ω for a 5 V

excitation. If the primary impedance of the LVDT/RVDT

is below 50 Ω, the impedance of the LVDT/RVDT can be

increased by increasing the excitation frequency. Input
protection of up to 130 V rms is provided for the excita­tion and input circuitry. The signal is amplified to pro­vide the high level voltage output.

The 3B17 is designed to compensate for error terms
typically found in LVDT or RVDT applications, such as

quadrature voltages (voltages that are 90° out-of-phase

to the output signal) and null voltages. Quadrature and
null voltages can appear at the differential output of the
LVDT/RVDT; they can be caused by interwinding capaci­tance and winding asymmetries or a fixed-phase shift
from the primary to the secondary of the LVDT/RVDT.
The two secondary windings are identified as A and B,
with the normal output being A – B. (Refer to Figure 1).
The 3B17 generates the function A + B (a voltage that is
in phase with the secondaries and nearly invariant with
the core replacement) by manipulating the A and A – B
outputs. Since A + B is much larger than the quadrature
voltage, A + B can drive the demodulator, eliminating
the need to manually trim the phase of the demodulator.
In addition, the 3B17 automatically rejects any residual
quadrature voltages.

You can adjust the voltage output over ±5 V from the

center setting of the LVDT/RVDT. After you adjust the
voltage output, you can independently adjust the 130 V

ac protected current output over a ±5% span range for

zero and span.

OTHER LVDT CONNECTIONS

A 6-wire LVDT is shown connected to the 3B17 in Figure

2. 4-wire and 5-wire connections are shown in Figures
3a, 3b, 4a and 4b.

1

2

3

4

1

2

3

4

a. b.

Figure 3. Four-Wire LVDT Connections

11

2

3

4

2

3

4

a. b.

Figure 4. Five-Wire LVDT Connections

–2–