Strain Gauge Input Products
By:
Wilkerson Instrument Co

STRAIN GAUGE INPUT

Strain gauge sensors are used in a variety of applications to measure applied strain on an object.

The strain gauge consists of a resistor that changes value with applied strain. The strain resistor may be applied singly, in pairs (half bridge), or four resistors connected in a bridge configuration.

The strain resistor is bonded with adhesive to an object that will be subjected to the strain force. The strain resistor will change value based upon the deformation of the mount.

Signal conditioning for these three configurations basically provide either an excitation current or voltage to the strain gauge and measures the change in voltage across the strain resistor(s) to create an output proportional to the applied strain.

Many products are manufactured and sold that utilize strain gauges as sensors to measure parameters such as weight or pressure.

This document deals with the processing of a strain gauge sensor that is fully defined by the manufacturer.

EXCITATION

Single resistor strain gauges are excited by a constant current and the voltage change across the gauge resistor is processed to provide an output proportional to the applied strain.

Half bridge and full bridge strain gauges are excited by a constant voltage source and the voltage change created by the unbalancing of the circuit due to applied strain is processed to provide an output proportional to applied strain.

Due to the low signal levels created by strain gauge, the excitation source must be stable and have a low noise output.

INPUT AMPLIFIER CONSIDERATIONS

The low level of the strain gauge output signal and the normally lengthy cable between the strain gauge and the signal conditioner dictates that the input amplifier have excellent stability and noise rejection capability. Further, the method of wiring between the sensor and conditioner is critical in keeping noise pickup to a minimum.

INPUT IMPEDANCE

The load presented by the conditioner to the signal source must be high enough to not load the source and create an excessive drop in the signal level.

Wiring between the source and the load has a finite resistance that also contributes to the drop in signal at the signal conditioner. In addition, the lead resistance will change with temperature and this change in resistance must be small in relation to the conditioner's input impedance so the input level will not change excessively with ambient temperature changes.

COMMON MODE REJECTION

If the same signal is put on both input terminals of a signal conditioner, there would be no output if the unit had perfect common mode rejection.

Common mode rejection is usually best at DC and deteriorates as the frequency of the input common mode signal increases.

Common mode rejection is valuable for voltage input conditioners when the two input leads pass sources of electrical noise that capacitively couple the noise to both leads as a common mode signal. The conditioner's rejection capability can be very effective in reducing the influence of this noise to acceptable levels.

A conditioner designed for high common mode rejection cost more than one with less rejection. The application and installation reasonably dictates the need or not.

Wiring practices between the signal source and conditioner input can have a great influence on the amount of noise at the input. Good wiring practices can eliminate the need for a high common mode rejection in the conditioner by reducing the common mode signal to an acceptable level.

STABILITY VERSUS AMBIENT TEMPERATURE

Every component used in the design and manufacture of a signal conditioner has a temperature coefficient that influences the stability of the output versus ambient temperature.

For best cost/performance ratio, the design should be done with the most accessible components. The factor that controls this parameter the most is input signal level.

As an example, assume a conditioner is designed to accept a 1 volt input and the drift on the output, for a 30 Deg C ambient temperature change, is 1% of the output. If the input could be changed to 2 volts, the output drift could be cut almost in half.

Depending on the actual input level, the first amplifier can vary very widely in cost (10 to 20 times).

SPEED OF RESPONSE OR BANDWIDTH

The speed at which a signal conditioner responds to input signal level changes is determined by its bandwidth. The common method of specifying bandwidth is to state the frequency at which the output level has dropped 3db(to 70.7%) of its DC value.

The response to a step change is also a common method of specifying the speed of response. If the specification is stated as "time constant of x seconds", it is assumed the response moves about 63% toward its final value. For a change to move 99% toward its final value requires about 5 time constants; 99.9% requires about 7 time constants.

Wider bandwidth requires better wiring practices to keep noise pickup to a minimum.

Lower bandwidths are effective in reducing noise by not allowing it to pass through the conditioner.

COST FACTORS OF STRAIN GAUGE INPUT SIGNAL CONDITIONER

  1. Absolute accuracy of measurement.
  2. Strain gauge configuration, single, half, or full bridge.
  3. Speed of response versus accuracy and stability.
  4. Stability versus ambient temperature.
  5. Environment (hazardous, humid, corrosive, etc.)
  6. Span of measurement. (Span is the difference between the zero scale signal and full scale signal.) The narrower the span the more costly it is to meet stability requirements.

Wilkerson Instrument Co.,Inc.
2915 Parkway Street
Lakeland, FL  33811
800-234-1343
www.wici.com