Older torque sensors, such as rotary transformers or slip-ring-style torque sensors, have a
strain gauge output - typically millivolt per volt. With this output, you need a
strain gauge amplifier to power and condition the signal, which is then turned into a usable high-level output.
Telemetry torque sensors are typically
self-amplified and do not require any type of strain gauge conditioning. They can come with a variety of high-level output types: typically a +/-10vdc or a frequency output.
Your calibration data includes a
shunt cal value, which performs a number of functions. The shunt cal value is derived from a resistor in parallel with one leg of the wheatstone bridge. When a switch is closed, it shunts the resistive value across that leg. Typically, you’ll get somewhere between 50 percent and 80 percent of full-scale output as the shunt value.
➤ Tip 19: If you have a self-amplified torque sensor, the
shunt cal value can also check the
functionality of the amplifier.
The shunt value can also check for
output changes. It’s derived during the calibration process from the full-scale output of the torque sensor. So if there’s a change in the torque sensor’s full-scale output, it will be seen by a change in the shunt cal value.
Depending upon the accuracy of the shunt network, a shunt cal value can help with
accurately spanning your instrumentation to the output of the torque sensor (i.e. 5vdc = 100Nm). Another way of spanning your instrumentation and torque sensor would be using full-scale output value from the calibration data sheet. Each torque sensor should come with a “
Test Protocol”. This should include linearity and hysteresis, shunt value, and the output at specific torque levels.
➤ Tip 20: Instead of using the shunt value to span your instrumentation, you can use the
full-scale output of the torque sensor, which you can find on your calibration data sheet.
If you choose not to use the full-scale output or the shunt value, you can
span your instrumentation by performing a
dead-weight calibration with a lever arm and calibrated weights. However, this can difficult to do as it forces you to break the driveline, lock down the shaft and add a calibration arm. The correct number of calibrated weights are needed along with a person to hang the weights, which has the potential to cause injury. The hanging of weights can also bend the arm and put a parasitic load on the torque sensor, causing errors in reading. Plus, the arm, weights and instrumentation generally need to be calibrated once a year.
➤ Tip 21: Use a
reaction torque sensor and hydraulics in place of the lever arm and the calibrated weights. It’s an easier way to perform a calibration versus using a lever arm and weights.
For more information on torque sensors and to learn the proper ways to measure torque in your test environment,
download HBM's torque reference book on the HBM website.