Page 122 - Mechanical Engineers' Handbook (Volume 2)

8 AC Impedance Bridge Transducers 111

Figure 33 Problems of phase sensing. (Adapted with permission from ‘‘Information as a ‘Noise Sup-
pression’ Method,’’ by Peter Stein, Stein Engineering Services, Inc., Phoeniz, AZ, LR/MSE Publ. 66,
1975.)

If a modulated signal emerges from a measuring system which is initially balanced (zero
output for zero input), phase sensing must be done in a phase-sensitive manner. The general
principle for all phase sensors is as follows:
If the system output is of the same sign at the same time as the time-varying supply
power, the measurand must have been positive. If the signs are opposite, the mea-
surand must have been negative.
Phase sensing is accomplished by a phase-sensitive demodulator. A reference signal is fed
from the bridge supply power to the phase-sensitive demodulator. This signal is compared
with the amplitude-modulated signal for phase determination. A half-wave rectifier with
transformer coupled reference and amplitude-modulated signals forms one basis for a de-
modulator.
After phase sensing, a low-pass filter is required to separate the non-self-generating
analog signal proportional to the measurand from the other frequencies which appear as
sidebands around harmonics of power supply frequency. Final selection of a power supply
frequency is a trade-off between the maximum frequency content in the measurand and
the low-pass filter roll-off characteristics. While a 10 1 ratio is typical, the power supply
frequency may vary between 3 and 20 times the maximum nonself-generating signal fre-
quency.
Note that introduction of an ac power supply requires a bridge-balancing network in-
corporating complex impedance in the balance controls.

8 AC IMPEDANCE BRIDGE TRANSDUCERS
Having discussed bridge transducers that use resistive sensing elements and ac power sup-
plies (amplitude modulation), a logical question is whether bridge transducer sensing ele-
ments can be capacitive or inductive. In practice, many bridge transducers do employ