Page 113 - Mechanical Engineers' Handbook (Volume 2)

102 Bridge Transducers

As a final note, remember that the resistance of semiconductor bridge transducers is
strongly a function of temperature. When using shunt or series calibration techniques on
semiconductor bridges, ambient-temperature changes should be taken into account.

7 RESISTANCE BRIDGE TRANSDUCER MEASUREMENT
SYSTEM CONSIDERATIONS
7.1 Bridge Excitation
When amplifiers and power supplies were formally designed around vacuum tubes, com-
ponent drift was a problem in bridge transducer measurement systems. Alternating-current
power supplies in bridge circuits eliminated many of these problems by operating at fre-
quencies above dc. Most bridge transducer power supplies today are dc. When comparing
dc supplies with ac, the following advantages are associated with dc:
1. Simpler circuitry
2. Wider resultant instrumentation system frequency response
3. No cable capacitive or inductive effects due to the excitation
4. Simpler shunt calibration and bridge balance circuitry

Independent of type of supply, the power level selected has to take account of all
variables which affect the measurement. These include gage resistance, gage grid area, ther-
mal conductivity of flexure to which gage is mounted, flexure mass, ambient test temperature,
whether used on a static or dynamic test, accuracy requirements, and long- or short-term
measurement. These variables account for the fact that a strain gage is a resistance which
has to dissipate heat when current passes through it. Most of the heat is conducted away
from the gage grid to the transducer flexure. The result of inadequate heat conduction is
gage drift.
For transient measurements, a steady transducer zero reference is not as important as
for static measurements. Bridge power can be significantly elevated to increase measurement
system signal-to-noise ratio.
The following specifications define key performance parameters of dc output instru-
mentation power supplies. Input supply can be either ac or dc.
1. Warmup Time. The time necessary for the power supply to deliver nominal output
voltage at full-rated load. It is usually specified over the range of operating temper-
atures.
2. Line Regulation. The change in steady-state dc output voltage resulting from an input
voltage change over the specified range.
3. Load Regulation. The change in steady-state dc output voltage resulting from a full-
range load change.
4. Efficiency. The ratio of the output power to the input power.
5. Load Transient Recovery. The time required for the output dc voltage to recover and
stay within a specified band following a step change in load.
6. Periodic and Random Deviation. The ac ripple and the noise of the dc output voltage
over a specified bandwidth with all other parameters held constant.
7. Stability (Drift). The deviation in the dc output voltage from dc to an upper limit
which coincides with the lower limit as specified above in 6.