How to Measure Strains with a Data Acquisition System?
2019-07-31 | 7 min read
What Is a Strain Gauge?
Strain gauge sensor measurement is common in mechanical design applications. It measures tensile, compression, and torsional forces. Mechanical designers create appropriate body structure and frames to withstand operating stress and physical abuse to their products to ensure safe usage. For example, if a mechanical engineer designs a bicycle frame, it has to withstand the weight of the bicyclist and the stresses on the frame and joints through the riding experience, whether that is on the road or on rough terrain.
A strain gauge sensor comes in handy for obtaining physical strain measurements on mechanical prototypes. The sensor is typically the size of a fingernail. You can easily attach it to the surface of a body structure for testing.
Benefits of Wheatstone Bridge vs. Direct Measurements
Strain gauge sensors are normally resistive elements. Axial or traverse strains will cause resistance change (∆R). Strain measurement is the ratio of ∆R over its unstrained resistance of R.
In instrumentation measurement, you can easily measure ∆R/R. However, most test engineers prefer a Wheatstone bridge circuit, as shown in Figure 2.
Here are the reasons test engineers prefer a Wheatstone bridge circuit over direct resistance measurements:
- It is more accurate in measuring resistances than a direct two-wire measurement. The output voltage (VC – VD) of the Wheatstone bridge circuit is expressed in millivolts output per volt input. It is linear in relationship and has low error across the output voltage range.
- It is capable of measuring very low resistances. The ∆R can be very low. A Wheatstone bridge circuit allows you to measure down to the milli-ohms range.
- Temperature compensation capability is possible using a Wheatstone bridge. Strain gauge sensors are tiny resistive elements that are susceptible to environmental temperature change. Using a half-bridge or full-bridge configuration will minimize the external temperature effects. Strain gauges complementing each other on the bridge will experience the same temperature effects and cancel each other out with the differential voltage output (VC – VD).
- Measurement sensitivity increases when deploying multiple strain gauges on two or four arms of the Wheatstone bridge. The placement of the strain gauges and the type of Wheatstone bridge configuration used can double or quadruple the measurement sensitivity.
Strain Gauge Measurements with a DAQ Data Logger
A modern data acquisition (DAQ) system such as Keysight’s DAQ970A provides Wheatstone bridge configuration and direct resistive measurement methods. However, the more popular method is the Wheatstone bridge configuration because of the previously mentioned benefits. Figure 3 shows the quarter-bridge configuration with one of the four arms connected with a strain gauge. When used with a DAQ data logger, the voltage output (VC – VD) connects to one of the DAQ’s differential voltage input channels.
The DAQ970A system has strain calculations of the Wheatstone quarter bridge built-in. The configuration setup is easy: enter the sensing type as a quarter-bridge configuration, the gauge factor, and the excitation voltage. Gauge factor is the ratio of the fractional change in resistance to the fractional change in length (strain) along the axis of the gauge. The excitation voltage is the voltage applied to the bridge by an external source. Figure 4 shows the DAQ’s trend chart monitoring display mode of a strain gauge in a quarter-bridge configuration.
DAQ systems come with various Wheatstone bridge configuration types to simplify a test user’s setup configuration. Choosing the correct type of configuration helps a test user optimize measurement accuracy and sensitivity of the strain gauges.
Keysight’s DAQ970A allows a test user to choose from three types of Wheatstone bridge configurations: quarter-bridge (Figure 3), half-bridge (Figure 5), and full-bridge (Figure 6).
For half-bridge and full-bridge configurations, test users can select the orientation of the strain gauges in bending, Poisson, or bending Poisson positions. When choosing Poisson positions, test users have to enter the Poisson negative ratio value for the strain gauge. For example, in Figure 7, a test user can select
- strain gauge 1 and 3 to form a half-bridge bending configuration
- strain gauge 1 and 2 to form a half-bridge Poisson configuration
- all four strain gauges to form a bending Poisson configuration
Test users and product designers want remote PC application software that can monitor the strain gauge measurements, log the data continuously over a long period, and possibly set an alarm limit if the strain gauge data measurement crosses a certain threshold. Such remote PC application software is available in the market. Figure 8 shows a snapshot of Keysight’s BenchVue software, which meets all the requirements mentioned above.
Strain measurements using Wheatstone bridge configurations are popular because of benefits such as measurement accuracy, low resistance measurement capability, less susceptibility to temperature changes, and better measurement sensitivity based on the bridge configuration.
A modern DAQ data logger makes strain measurements easy. The DAQ system has Wheatstone bridge configurations and strain calculations built-in. You can easily configure a large number of strain sensors in minutes. PC application software for remote monitoring and data logging is a convenient tool for product designers to ensure extensive coverage of product testing and characterization.