Constant-resistance signal conditioner for dynamic strain measurement

A dynamic strain measurement circuit includes a constant resistance conditioner. The constant resistance conditioner compensates for changes in lead wire resistance due to temperature. The conditioner includes adjustable resistance connected between a voltage supply and the strain gage. A potential difference is detected in the circuit and compared to a reference potential. Differences between the reference potential and the detected potential produce an error signal which is used to drive the control of the resistance value of the adjustable resistance in the measurement circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to a dynamic strain measurement circuit, and 
more particularly, to a circuit design for maintaining a constant output 
from the circuit despite changes in lead-wire resistance. 
Strain gages are typically used to measure dynamic stress on an object. One 
typical field in which a strain gage is used is in the monitoring of 
dynamic stress on turbine and compressor blades in turbo jet engine 
testing. Usually these strain gages are single active arm strain gages. 
The lead wires connected to these strain gages are typically of a small 
diameter with a high resistance. 
Changes in temperature can effect changes in the effective resistance of 
the lead wires. This change is significant when compared to the change in 
gage resistance from the strain. Because there is no direct access to the 
gage itself, it is not possible to use three and four wire resistance 
measurement techniques to detect or compensate for these resistance 
fluctuations. Furthermore, it is not possible to connect the signal 
conditioner guard shield to a source of common mode voltage. 
It has been known to provide one of two different types of signal 
conditioners: a) a conditioner having a constant voltage excitation 
supply; and b) a conditioner having a constant current excitation supply. 
The constant voltage type conditioner has excellent common-mode rejection 
of unwanted signals. However, such a conditioner is sensitive to changes 
to the resistance of the lead wires going to the strain gage. Constant 
current conditioners are immune to such lead wire resistance changes 
However, because of their inherently unbalanced output impedances, such 
constant current conditioners typically have very poor common-mode 
rejection. In addition, because of their wide active band width, constant 
current conditioners are typically noisier than constant voltage 
conditioners. 
Thus, the problem exists of providing common mode rejection and a 
minimization of the impact of changes in lead wire resistance. 
SUMMARY OF THE INVENTION 
The present invention solves the above described problem by providing a 
constant voltage supply and compensation for changes in lead wire 
resistance. 
In an embodiment of the present invention, adjustable resistance is added 
to the strain measurement circuit loop that already includes the strain 
gage and the constant voltage supply. A voltage difference between two 
points in the measurement loop is compared to a reference voltage. To the 
extent that there are differences between the measured voltage and the 
reference voltage, an error signal is produced. This error signal is used 
to drive changes in the adjustable resistances. Thus, as the resistance of 
the lead wires increases with temperature, the control resistances can be 
adjusted to decrease and consequently maintain the same resistance 
throughout the dynamic strain measurement circuit. The compensation for 
changes in lead wire resistance is slowed so as to avoid canceling out a 
rapid resistance change in the circuit that is due to changes of the 
strain gage itself as it is subjected to strain. 
In a more detailed embodiment of the present invention, the adjustable 
resistances are light responsive resistances whose resistance is adjusted 
by adjusting current through an adjacent lamp or light emitting diode. It 
is also possible that the adjustable resistances can take the form of a 
motor driven potentiometer where the motor is driven in response to the 
error signal. Also it is possible that the adjustable resistances can take 
the form of field effect transistors where the control signal can adjust 
the gate voltage and thus effect the channel width of the field effect 
transistor.

DETAILED DESCRIPTION 
In FIG. 1, R.sub.gage corresponds to the resistance of the strain gage in a 
dynamic strain measurement device of the present invention Resistances 
R.sub.lead10 and R.sub.lead11 correspond to the effective resistance of 
the lead wires 10 and 11 connected to the strain gage. Adjustable 
resistances R1 and R2 are connected to resistances R.sub.lead10 and 
R.sub.lead11 respectively. Adjustable resistance R1 is connected to the 
constant voltage supply 15 which has associated with it ballast 
resistances RB1 and RB2. Similarly, adjustable resistance R2 is also 
connected to the constant voltage supply. 
The voltage difference between points A and B is detected by an 
instrumentation amplifier 110 whose output constitutes a voltage signal 
which is indicative of the strain detected by the strain gage R.sub.gage. 
Capacitors C1 and C2 serve to couple only the dynamic or a-c component of 
the strain signal at points A and B into amplifier 110. The resistance 
compensation circuit of the present invention is connected to the same 
potential points as the instrumentation amplifier. 
The compensation circuit 111 includes an instrumentation amplifier 112 
having its "+" terminal connected to point A of the dynamic strain 
measurement circuit and its "-" terminal connected to point B of the 
dynamic strain measurement circuit. The instrumentation amplifier 112 
produces a voltage difference signal as an output, where the output is 
indicative of a potential difference between point A and point B in the 
dynamic strain measurement circuit The output of the instrumentation 
amplifier is provided to the "+" terminal of an error amplifier 113. A 
reference voltage is supplied to the "-" terminal of that same error 
amplifier. The output of the error amplifier 113 is an error signal 
indicative of a difference between the expected voltage difference at 
points A and B, and the reference voltage V.sub.ref. The output of the 
error amplifier is provided to a low pass filter. The low pass filter 114 
then provides its output to a buffer amplifier 115. The buffer amplifier 
115 produces an output signal which controls the resistance value of the 
adjustable resistances R1 and R2. As the resistance of the lead wires 
increases over time due to temperature increases, the voltage or potential 
difference between points A and B increases and becomes larger than the 
reference voltage V.sub.ref. The difference signal is used as a control 
signal to reduce the resistances of the adjustable resistances R1 and R2 
so as to provide constant resistance throughout the measurement circuit. 
This thereby avoids any influence of lead wire resistance changes upon 
strain detection. 
The complete circuit of the present invention is illustrated in FIG. 2. In 
this diagram, the control mechanism for adjusting the resistance of the 
adjustable resistors is shown in greater detail. In this diagram, a 31.2 
volt battery source V.sub.B, which can comprise five batteries in series, 
supplies excitation to the strain gage SG. Current flows through 
adjustable resistances R1 and R2 as well as through the ballast 
resistances RB1 and RB2. Furthermore, there is an effective resistance 
associated with each of the lead wires 21 and 22. These resistances are 
represented by resistances RL1 and RL2. Amplifier 25 detects the voltage 
difference between points A and B of lines 21 and 22. The detected voltage 
difference is a signal output representative of the strain detected by the 
dynamic strain gage SG. The same voltage is also applied to amplifier 212 
which also produces a voltage difference signal. Under normal operation, 
without detection of strain, the voltage between points A and B will be 
12.6 volts. This voltage is sensed by the lower instrumentation amplifier 
212, which converts it to a single-ended voltage at point C of 
approximately 4.2 volts. A reference voltage is divided down from a 9.2 
volt battery and is reference voltage V.sub.ref at point D. The voltage at 
point C is supplied to the "+" terminal of operational amplifier (op amp) 
213. The voltage at point D is supplied to the "-" terminal of the op amp 
213. That amplifier compares the voltages appearing at its "+" and "-" 
terminals and generates an error signal based on the difference between 
the voltages at points C and D. This error signal is provided as an input 
to a current amplifier 214. The current amplifier drives LEDs 30, 31, 32, 
and 33. LEDs 30 and 31 are paired with the light responsive resistances 
R30 and R31 respectively. Light emitting diodes 32 and 33 are paired with 
light responsive resistances R32 and R33 respectively. If the lead 
resistances RL1 and RL2 increase, the voltage between points A and B will 
increase. This increased voltage is detected by the amplifier 212 and 
compared to the reference voltage V.sub.ref at amplifier 213. The 
resulting increased error signal drives more current through the LEDs. As 
a consequence, the corresponding resistances are reduced in response to 
the increase of light from the LEDs. The resistances decrease until the 
voltage between points A and B is again 12.6 volts, thus correcting for 
lead resistance change. 
The capacitance C101 connected between pins 1 and 8 of amplifier 213 is 
provided to slow the response of the compensation signal. By slowing the 
response, the compensation circuit does not compensate for or cancel out a 
voltage change at points A and B which is due to a strain detection by the 
strain gage. 
This embodiment accommodates values of lead resistances from 0 to 100 ohms 
in each lead and provides a current of approximately 30 milliamperes 
through the gage. Light-emitting diodes/photocell devices such as those 
used in the present invention are available from EG&G Vactec. 
The circuit of the present invention compensates for the lead resistance of 
a dynamic strain measurement circuit. This avoids negative effects from 
changes of lead resistance with temperature while still providing good 
common-mode rejection and signal-to-noise ratio. 
The adjustable resistances can be motor driven potentiometers or field 
effect transistors in place of the LED/photocell pairs which are 
illustrated in the embodiment of FIG. 2. In those other configurations, 
the control signal produced by the error amplifier 113 or 213 would, with 
current amplifier 114 or 214, control the motor to change the slide of the 
potentiometer or the gate of the field effect transistors. Other current 
controlled adjustable resistances can also be employed in place of the 
resistances R1 and R2 of FIG. 2.