Resistive Strain Gauges

Strain Gauges measure the forces applied to objects (such as beams) by measuring the how its dimensions change, they are a common method of implementing Load Cells. Resistors are firmly attached to the object so that as it moves the resistor film is stretched or compressed which in turn changes the value of the resistance. The resistors used are specially created for this purpose to ensure that dimensional changes change the resistor and are typically designed to accentuate the effect by using a long thin element in a pattern to maximise the change in a certain direction. A typical example of movement to be measured might be a beam (for example part of crane) that supports a weight hanging from its end. The weight causes the beam to be deflected downwards, so the top surface of the beam is stretched and the bottom surface is compressed.

The change in resistance of the resistor is typically very small (of the order of 0.1%). The resistor is often made of materials that exhibit relatively high temperature coefficients - the properties that make the resistor dimensionally sensitive also tend to make it temperature sensitive. Both of these properties encourage the use of a bridge circuit measure the resistance in order to deduce the strain. If each arm of the bridge is made up of strain sensitive material and none of the resistors are under strain then providing the temperature coefficients match and the resistors are accurate in value the bridge will remain balanced.

Of the 4 resistors in the bridge one, two or four may be attached to the object under stress. Where 2 or 4 resistors measure stress half are placed in a position where a force causes compression and the other half where the same force causes stretching. In the case of a beam for example the resistors could be placed on top and underneath the beam. Using 2 or 4 resistors in this way causes the bridge output voltage to change by a greater amount than if just one resistor is under stress.

The bridge network is excited by a voltage applied across the opposite corners, the voltage can be AC or DC. AC voltages can have the advantage that low DC outputs are not adversely effected by DC offsets, but the most commonly used voltage source is DC since it is more readily available and easy to measure.

Strain Gauge simulation can be a difficult task to do accurately and well, many users have resorted to in house designed solutions for simulation, but there is a better way.

Strain Gauge Simulation

Pickering Interfaces has applied the same methodology as used in other precision resistor modules to create the 40-265 Strain Gauge Simulator.

The functional diagram of the 40-265 Strain Gauge Simulator is shown.

Diagram of 40-265 Strain Gauge Simulator

Each channel of the 40-265 implements a bridge circuit using three high stability fixed resistors (for example 350 ohms) and a fourth variable resistor that can be varied by ±2% from its nominal value in the case of the 350 ohms version with a resolution of better than 2 mohms (0.002 ohms). The fine resolution available allows the user to simulate small strains while the range of the allows the simulation of high load with significant bridge imbalance.

The value of the resistance needed to balance the bridge circuit, the Balance Resistance, is stored in the module for use in user created application programs. The variable resistor value is set by a resistance value instruction in ohms.

In normal use the strain gauge measuring device supplies an Excitation Voltage (AC or DC) to the Excitation Port for that channel. The Internal Excitation Source is disconnected by relays which prevent the external and internal sources being applied at the same time.

The output from the bridge is measured by the Strain Gauge measuring device on the Bridge Output connection.With the variable resistor set to the Balance Resistance value the bridge differential output will be 0V. A strain is simulated by modifying the variable resistor so a voltage appears on the bridge output and is detected by the Strain Gauge measuring device connected to the bridge output. In a typical strain gauge measurement device the device will supply the Excitation Voltage and measure the output voltage, expressing the result as a ratio - making the result independent of the level of the Excitation Voltage.

The resistor values used in the bridge in user systems can have a variety of values. The measuring circuit used to monitor the output from the strain gauge is generally a high impedance load so often the actual value of resistor used in the strain gauge may not be critical. For example if the strain gauge being emulated is higher resistance then adding series resistance on the outputs will raise the output resistance. The excitation source however will see a greater load since it is driving (in the example used) a 350Ohm bridge.

Pickering Interfaces can offer the 40-265 with bridge values other than 350 Ohms, if the bridge value required is not on the 40-265 data sheet you can request its creation by contacting Pickering Interfaces.

Self Test and Calibration Support

To make it easier to manage this precision module the 40-265 provides some hardware support through a Calibration Port.

The Calibration Port can be used in a number of ways, some of which are more useful to Pickering Interfaces than to a user. The calibration port can be used to connect the strain gauge parts to an external DMM. With an external DMM a 4 terminal resistance measurement can be performed to check the accuracy of a resistor or the entire bridge. For users this is most commonly for verification purposes for the entire strain gauge bridge. Any of the 6 channels can be presented to the DMM, so it is a very convenient way of checking each channel. Where more than one 40-265 is used in a chassis they can also be connected together and the user can select any the variable resistors from any of the channels on the modules.

The Calibration Port can also be used to find the Balance Resistance. In this mode the bridge is connected and an internal excitation source is supplied (the external source on the Excite connections is disconnected). The Calibration Port connects to the bridge output so a DMM on the calibration Port can be used to measure the bridge output form the selected channel. Adjusting the variable resistor the value of the Balance Resistance can be found.

Summary

The 40-265 is a cost effective way of emulating a strain gauge. It can be connected to a Strain Gauge measuring circuit having an integrated Excitation Voltage. Six channels are supported in a single PXI module and the Calibration Port provides an easy way of checking and calibrating the operation of the module. It is far more cost effective module than in house methods of emulating the strain gauge and has a far higher performance level than alternative methods.

A video presentation showing how to use this card from LabView is available here

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