STRAIN GAUGE TEMPERATURE COMPENSATION SYSTEM

• US 3,665,756 A
• Filed: 08/01/1968
• Issued: 05/30/1972
• Est. Priority Date: 10/18/1965
• Status: Expired due to Term

• Current Assignee: John D Russell
• Original Assignee: John D Russell
• Inventors: Russell, John D.
• Primary Examiner(s): Queisser, Richard C.
• Assistant Examiner(s): Whalen, John
• Application Number:
  US04/765,725
• Time in Patent Office: 1,398 Days
• Field of Search: 73/885,141,359,362R 338/3-8
• US Class Current: 73/766
• CPC Class Codes: G01L 1/2281 for temperature variations

Abstract

This invention relates to a system for measuring strains in a member and providing an indication of such strains. The system includes at least one strain gauge connected in a bridge with other resistance members, preferably strain gauges, to compensate for temperature changes. The bridge is provided with characteristics to maintain a constant current from a source of constant voltage even when the member is subjected to a variable strain. The output voltage from the bridge provides an indication of such variations in strain. Preferably the resistors in the bridge constitute strain gauges which are connected to the member so that some of the strain gauges become stressed and others become strained when the member is stressed or strained. In one embodiment, one of the strain gauges becomes strained with changes in temperature but is not affected by stresses or strains in the member being measured. Additional resistors may also be included in the bridge to provide additional compensations for temperature changes.

Claims

1. In combination for performing the simultaneous and continuous measurement of strain and temperature on a test structure;

a resistance strain gauge having an active filament constituting a first resistor and having a second resistor responsive only to changes in temperature, the resistance strain gauge being attached to the test structure in such manner that when the test structure is placed under a test load the active filament of the resistance strain gauge is changed in resistivity;

bridge resistors combined with said first and second resistors of said strain gauge to form a Wheatstone bridge having pairs of input and output terminals;

a temperature compensation resistor in the Wheatstone bridge connected in series with the first resistor in the strain gauge, and physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating, said temperature compensation resistor being selected to achieve temperature compensation in the temperature range at which the strain gauges are to make readings;

a balancing resistor-physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating and connected in series with one of said bridge resistors;

a constant-voltage power supply electrically connected to the input terminals of the Wheatstone bridge;

a voltage-measuring device connected between the output terminals of the Wheatstone bridge to provide an indication of changes in the value of the first resistor as a result of strain; and

a current-measuring device electrically coupled between the power supply and the Wheatstone bridge to provide an indication of changes in temperature of the first and second resistors in the strain gauge.

2. In combination for performing strain measurement on a test structure:

a strain gauge having a first resistance and a second resistance, the gauge being attached to the test structure in such manner that when the test structure is placed under a test load the first resistor in the strain gauge is trained to provide a change in resistance value, the strain gauge also having a second resistor disposed in the strain gauge to be responsive only to changes in temperature;

a pair of additional resistors;

A Wheatstone bridge circuit formed from the first and second resistors and the pair of additional resistors and having first and second input terminals and first and second output terminals and having the first and second resistors of strain gauge coupled between the input terminals, the pair of additional resistors also being coupled between the first and second input terminals;

a temperature compensation resistor connected in series with the first resistor between the input terminals, said temperature compensation resistor being selected to provide temperature compensation in the temperature range at which the strain gauges are to making readings and physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating;

a balancing resistor connected in the Wheatstone bridge in series with one of said additional resistors, physically removed from the strain gauge to be unaffected by the temperature conditions in which the strain gauge is operating and selected to balance the Wheatstone bridge at a reference temperature in the temperature range;

a constant-voltage power supply having first and second output terminals electrically connected to the first and second input terminals of the Wheatstone bridge;

a current-measuring device responsive to the current flowing through the Wheatstone bridge to provide an indication of the temperature Compensation in the bridge; and

a voltage measuring device electrically connected between the first and second output terminals of the Wheatstone bridge to provide an indication of the strain in the first resistor of the strain gauge.

3. In combination for performing strain measurement on a test structure:

a plurality of resistance strain gauges attached to the test structure to elongate a first pair of the resistance strain gauges and compress a second pair of the resistance strain gauges when the first test structure is placed under a test load;

a Wheatstone bridge circuit having first and second input terminals and first and second output terminals and having individual ones of the strain gauges connected between individual ones of the input and output terminals;

a temperature compensation resistor connected in series with one of the strain gauges in one arm of said bridge, said temperature compensation resistor being selected to provide a compensation for temperature in the temperature range at which the strain gauges are to make readings and physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating;

a balancing resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating and connected in series with one of the bridge resistors in an adjacent arm of the bridge;

a constant-voltage power supply having first and second output terminals connected in series to the one pair of strain gauges and the temperature compensation resistor;

a current-measuring device connected in series with the power supply and the Wheatstone bridge; and

a voltage measuring device electrically connected between the first and second output terminals of the Wheatstone bridge.

4. The combination set forth in claim 3, including, a balancing resistor connected in series with the power supply and the other pair of the strain gauges to balance the operation of the bridge at a particular temperature.

5. In combination for performing strain measurement on a test structure:

a plurality of resistance strain gauges attached to the test structure in such manner that when the test structure is placed under a test load a first group of the resistance strain gauges are elongated while a second group of the resistance strain gauges are compressed;

a Wheatstone bridge circuit having first and second input terminals and first and second output terminals and having the resistance strain gauges coupled therebetween in such arrangement that a change in the resistance of the gauges due to changing temperature will change the resistance across the input terminals of the Wheatstone bridge while a change of resistance due to strain in the test structure will not;

yet a change of resistance due to temperature will leave the voltage across the Wheatstone bridge output terminals unchanged while a change of resistance due to strain in the test structure will change the voltage across the Wheatstone bridge output terminals;

a temperature compensating resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating and connected in series with one of the resistors in one arm of said bridge;

a balancing resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating and connected in series with one of said resistors in an adjacent arm of said bridge;

a constant-voltage power supply having first and second output terminals, the first output terminal of the constant-voltage power supply being electrically connected to the first input terminal of the Wheatstone bridge;

a current-measuring device electrically coupled between the second output terminal of the constant-voltage power supply and the second input terminal of the Wheatstone bridge; and

a voltage measUring device electrically connected between the output terminals of the Wheatstone bridge.

6. In combination:

a dual element resistance strain gauge having a center terminal and first and second end terminals and having an active filament and a dummy filament, said active and dummy filaments being connected together at the center terminal of the strain gauge, the free end of the active filament being connected to the first end terminal of the strain gauge and the free end of the dummy filament being connected to the second end terminal of the strain gauge, the active filament being responsive to changes in strain to provide corresponding changes in resistance value and the dummy filament being responsive to changes in temperature to provide corresponding changes in resistance value;

a pair of resistors connected in a bridge with the active and dummy filaments;

first means for applying a voltage across the first end terminal of the strain gauge and the second end terminal of the strain gauge;

second means for monitoring the voltage at the center terminal of the strain gauge to provide an indication of changes in strain of the active filament; and

a resistor physically removed from the strain gauge to be unaffected by the temperature conditions in the strain gauge, said resistor being connected in series with the active filament of the strain gauge and having a value to make the voltage monitored by the second means substantially free of temperature-induced erroneous indication of strain and a balancing resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating, and connected in series with one of said pair of resistors.

7. In combination for the measurement of strain at high temperatures:

a dual element resistance strain gauge having an active filament and a dummy filament and having a center terminal and first and second end terminals, both the active and dummy filaments being composed of an alloy of platinum and tungsten and said active and dummy filaments being connected together at the center terminal of the strain gauge, the active filament of the strain gauge being disposed in the strain gauge to be responsive to changes in strain by changes in resistance, the dummy filament being disposed in the strain gauge to be responsive to changes in temperature by changes in resistance, the free end of the active filament being connected to the first end terminal of the strain gauge and the free end of the dummy filament being connected to the second end terminal of the strain gauge;

a pair of additional resistors connected in a bridge circuit with the active filament and the dummy filament;

a temperature compensating resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating, and connected in series with the active filament;

a balancing resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating, and connected in series with one of said additional resistors;

first means for applying a voltage between the first end terminal of the strain gauge and the second end terminal of the strain gauge;

second means for monitoring the voltage at the center terminal of the strain gauge to determine the changes in resistance of the active filament of the strain gauge.

8. The combination set forth in claim 7 wherein the circuit means includes a temperature-compensating resistor connected in the bridge in series with the active filament and the dummy filament to receive the voltage from the first means.

9. The combination set forth in claim 8 wherein the circuit means further includes a balancing resistor connected in the bridge in series with the pair of additional resistors to balance the bridge.

10. In combination:

a dual element resistance strain gauge having an active filament and a dummy filament and having first and second end terminals and a center terminal, said active filament being disposed in the strain gauge to provide changes in resistance in accordance with changes in strain and the dummy filament being disposed in the strain gauge to provide changes in resistance in accordance with changes in temperature in the strain gauge, said active and dummy filaments being connected together at the center terminal of the strain gauge, the free end of the active filament being connected to the first end terminal of the strain gauge and the free end of the dummy filament being connected to the second end terminal of the strain gauge;

a pair of additional resistors connected to the active filament and the dummy filament to provide a bridge;

first means for applying a voltage across the first end terminal of the strain gauge and the second end terminal of the strain gauge;

second means for monitoring the voltage at the center terminal of the strain gauge;

a first resistor physically removed from the strain gauge to be unaffected by the temperature conditions in which the strain gauge is operating, said first resistor being connected in series with the active filament and the dummy filament of the strain gauge and the first means and having a value to make the voltage monitored by the second means substantially free of temperature-induced erroneous indication of strain; and

a second resistor physically removed from the strain gauge to be unaffected by the temperature conditions in which the strain gauge is operating, said second resistor being connected in series with the pair of additional resistors and the first means to provide the voltage monitored by the second means with a reference value before strain and temperature changes occur in such voltage and to balance the bridge at a reference temperature as a result of the inclusion of the first resistor.

11. The combination for measuring the strain imposed upon a test structure and the temperature of the test structure comprising:

a strain gauge attached to the test structure to become stressed in accordance with changes in stress and temperature of the test structure and having a first resistance member constructed to provide a change in resistance value in accordance with changes in both stress and temperature and disposed in the strain gauge to become stressed in accordance with strain imposed upon the strain gauge;

at least a second resistance member constructed to provide a change in resistance value in accordance with temperature only and disposed in contiguous relationship with the first resistance member;

a regulated power supply;

a bridge circuit including first and second input terminals and first and second output terminals;

means connecting the regulated power supply between the first and second input terminals of the bridge circuit;

means connecting the first and second resistance members between the first and second input terminals of the bridge circuit to obtain changes in the flow of current through the bridge circuit in accordance with changes in the temperature of the test structure and to obtain changes in the voltage between the first and second output terminals of the bridge circuit in accordance with changes in the stress imposed upon the test structure;

means for measuring the current flowing through the bridge circuit to provide an indication of the temperature of the test structure;

means connected between the first and second output terminals of the bridge circuit for measuring the voltage between such terminals to provide an indication of the stress imposed upon the structure and wherein the bridge circuit includes third and fourth resistance members each constructed to provide a change in resistance values in accordance with changes in stress and wherein the third and fourth resistance members are connected between the first and second input terminals of the bridge circuit and wherein the second, third and fourth resistance members are respectively Included in second, third and fourth strain gauges and wherein the second, third and fourth strain gauges are attached to the test structure to provide a stress in one direction upon the first and third strain gauges and to provide a stress in an opposite direction upon the second and fourth strain gauges a temperature compensating resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating and connected in series with said first resistance member; and

a balancing resistor, physically removed from the strain gauge, to be unaffected by the temperature conditions in which the strain gauge is operating and connected in series with one of said third or forth resistance members.