1. How do static torque sensors work?
Static torque sensors (also known as static torque meters or reaction torque meters) measure stationary torque, i.e. without rotation. The sensor consists of a stationary shaft and housing. The sensor’s shaft is mounted in-line such that the torque passes through the shaft, placing it under mechanical strain. The strain is measured by strain gauges (explained in the next paragraph) bonded to the shaft, which output a small voltage signal proportional to the mechanical strain. The sensors calibration sheet provides the relationship between strain and torque. The working principal is identical to that of dynamic torque sensors only that the sensor’s shaft cannot rotate. This makes static torque sensors cheaper than dynamic torque sensors because no bearings or method of passing voltage between a rotating shaft and stationary housing are required.
A strain gauge is a thin laminated foil pattern used to measure mechanical strain. Stretching the strain gauge causes its length to increase and its cross sectional area to decrease, thereby increasing its electrical resistance. By measuring the change in electrical resistance, it is possible to determine the amount by which the strain gauge has been stretched. Most torque sensors include 4 strain gauges, configured in a full bridge Wheatstone bridge formation, providing temperature compensation, common noise cancellation and a high signal to noise ratio.
2. General characteristics of static torque sensors
Static torque sensors are accurate, but high cost sensors. They are capable of measuring torque magnitudes, up to about 1,000,000 Nm, which is a larger maximum torque than can be measured with dynamic torque sensors. Most static torque sensors are mounted using a flange, though other options are available. Static torque sensors provide a mv/V output signal, though often include a built-in amplifier that outputs a 0-10 V signal. Static torque sensors generally have ingress protection of IP65, though IP67 rated models are also available.
3. Input and output signals
Static torque sensors require fixed 10-30 Vdc supply voltages. The strain gauge elements are arranged in a full bridge Wheatstone bridge formation, providing temperature compensation, common noise cancellation and a high signal to noise ratio. The sensor’s output signal takes the form of millivolts per volt of supply voltage. Many torque sensors include built-in amplifiers to amplify the signal and convert it to a standard form e.g. 0-10 V or 4-20 mA. If the torque sensor does not include a built-in amplifier then either the process controller must contain one, or an external amplifier must be used.
4. Applications of static torque sensors
Similarly to dynamic torque sensors, static torque sensors are used for measuring the torque of rotating machine shafts and motors. The difference between them is that dynamic sensors are mounted on the rotating shaft whereas static sensors are stationary (e.g. mounted to a motor housing) and measure the reaction force. They provide a good solution for applications with a torque of over 300,000 Nm or speed of over 50,000 RPM because dynamic torque sensors are not suitable for these conditions. Furthermore, they are used in applications requiring a very long lifetime, due to their lack of moving parts.
5. Typical specification
|Measurement range||0-1,000,000 Nm|
|Ambient temperature||-40 to 110 °C|
|Supply voltage||10-30 Vdc|
|Output voltage||4-20 mA, 0-10 V, mV/V|
|Ingress protection||IP65 typical, IP67 available|
|Passive / active||Active|
|Contact / non-contact||Contact|
6. Purchasing tips
- Mounting: The are several shaft mounting configurations available, the most common being flanges. The choice of configuration depends largely on the configuration of your existing shaft.
- Built-in amplifier: The lowest cost sensors output a non-amplified mV/V signal. However, this signal must be amplified either by the process controller or an external amplifier. Many sensors include built-in voltage amplifiers and therefore provide 0-10 V output signals.
- Built-in display: There are static torque sensors available with small built-in screens displaying the measured torque in real time, without the need to connect the sensor to a process controller or data logger.
7. Advantages of static torque sensors
Static torque sensors:
- Are highly reliable, have good ingress protection, a long lifetime and a lower cost (compared to dynamics torque sensors).
- Have a high measurement accuracy.
- Have very good temperature compensation owing to their full bride Wheatstone bridge.
- Do not consume any of the shaft power.
8. Disadvantages of static torque sensors
Static torque sensors:
- Supporting a heavy motor by a static torque sensor (so that it carries the reaction force) leads to a bending moment which is recorded up by the sensor. Even after adjusting the zero offset, this will result in reduced accuracy.
- Can not be fitted to rotating shafts.
- May require custom made mounts to be manufactured for connecting to the back of a motor.
9. Application tips
- Calibration: It is advisable to occasionally re-calibrate torque sensors. In most cases, calibration can be performed by securing one end of the sensor’s shaft and attaching a lever of known length to the other end. By applying a known weight to the lever, it is possible to map the voltage – torque relation of the sensor.
- Measuring dynamic torque with a static sensor: Static torque sensors can be used to measure the torque of rotating systems. To do so, mount the sensor in a manner that the reaction force to the torque travels through the sensor. For example, mount the sensor to the motor body. Note that supporting a heavy motor by a static torque sensor leads to a bending moment which is recorded up by the sensor. Even after adjusting the zero offset, this can reduce accuracy.