As we pivot to a discussion about strain gauge simulation, let's quickly revisit the key points and links from our previous articles on sensor simulation and ECU validation.
Let’s look at another resistive-based sensor that can help monitor the strain a mechanical structure experiences during operation. Strain gauges have a variety of applications, such as during static and fatigue testing of aircraft wings, to help ensure their mechanical integrity. In this specific application, the load system applies force to flex a wing fitted with multiple strain gauge sensors monitored by a data acquisition system. In other use cases, strain sensors can monitor torque supplied by an engine or the strain experienced by bridge support systems.
Strain gauges work under the principle that the electrical conductance of a physical element changes as the shape of that element changes. In other words, as a metal conductor is stretched under stress, it will become thinner, and its resistance will increase. That change in resistance can be correlated with strain. A strain gauge sensor has four distinct elements, and the number of those active determines whether the strain gauge is quarter, half or full bridge (with full meaning all four elements are active).
Figure 1: A quarter bridge strain gauge with one active element.
As the strain gauge experiences a load and starts to stretch in the diagram above, the resistance of the active element changes in proportion to the applied load. When an excitation voltage is applied, the change in resistance can be accurately measured by a data acquisition system and converted to units of strain. As with RTDs and thermocouples, different strain gauges can be used. In addition to quarter, half and full, there are different nominal resistances for strain gauges, 350 ohms being a common bridge value.
Given the range of options in strain gauges, it can be equally important to be able to simulate them across a wide range of applications. Using Pickering’s PXI strain gauge series (model 40-265) as an example, what is shown is a partial block diagram of a strain gauge simulation channel.
Figure 2: A partial block diagram of a strain gauge simulation channel.
In this case, the bridge has one active element, a very accurate programmable resistor, using a network of resistors similar to our RTD simulator. The excitation signal can be provided by the internal PXI power rail or by an external/user-provided source via the module connector, and two additional connection points give the ECU access to the bridge output.
Note that only one element of the bridge simulates an active strain gauge, and one would naturally assume that this is a quarter bridge strain gauge simulator. However, the model 40-265 series is highly stable with temperature and has an output range adequate to simulate the characteristics of a full bridge, even though just one arm is adjustable.
The model 40-265 specification table below highlights the programmable resistor resolution and accuracy required to effectively simulate strain gauges.
Figure 3: Note the resolution and accuracy commonly required to simulate strain gauges.
As shown, Pickering offers five base models covering a range of bridge elements between 350 ohms and 3 kiloohms.