How do rotary torque sensors work?

There are two types of rotary torque sensors. Some rotary torque sensors, such as wireless models, are non-contacting.

Wireless, rotary torque sensors, which are based on radio telemetry, are reliable and easy to install. Wireless, rotary torque sensors are more expensive than contacting torque sensors, because wireless rotary torque sensors are non-contacting, meaning they do not require support bearings or mechanical contacting parts. As a result, maintenance is eliminated.

A stationary antenna induces power in a loop antenna on the rotating shaft. The power from the rotating shaft antenna is conditioned and excites the strain gauges. A shaft-mounted radio transmitter then sends the measurement signal back to the stationary antenna.

The telemetry antennas need to be somewhat flexible for ease of mechanical installation. Receivers should also have adjustments for peak coupling of the antenna for maximum induced power and received signal strength. The radio antenna gap is normally less than 3/4 inch.

Other rotary torque sensors are contacting. Slip rings are often used in contact-type torque sensors to apply power to and retrieve the signal from strain gauges mounted on the rotating shaft. However, slip rings are susceptible to wear. Maintaining an oil-free slip ring is not always easy in many industrial applications. Slip ring brushes, as well as, the support bearings that are internal to these torque sensors, eventually wear out.

In-line rotary torque transformers are ideal for measuring torque when transducers are mounted in-line with the rotating shaft. These consist of a strain gauge torque cell having a calibrated output and inductively coupled to the stationary windings on the assembly by the rotary transformer. The rotary transformer couples the strain gauges for power and signal return.

The rotary transformer works on the same principle as any conventional transformer except that either the primary or secondary coils rotate. The rotary transformer is simple, easy to use and is usually applied to smaller machine trains. Rotary transformers have some susceptibility to noise and require bearings for support, thus, maintenance is required. The act of mounting the in-line transducer also changes system dynamics and can mean the torque values themselves may change.