Rotary Torque Sensor
In this project, my team designed a Rotary Torque sensor for the Lab Dyno shown above. A strain gauge measured the shear stress due torque transferred from the big brushless DC motor to the small AC motor.
Sensor Research
Strain Gauge
During elastic deformation, strain gauges linearly change their resistance with increase in orthogonal tension and compression. Full-bridge configuration of Wheatstone bridge maximizes V_out for the largest difference between stress and strain. This measurement corresponds to high torque.
Strain Gauges were the go-to choice for our project due to high documentation and easy implementation.
Piezoelectric Crystal
Piezoelectric crystals are commonly used to measure vibration and impact force. Piezo elements produce an electric signal when mechanical stress is applied. These types of sensors are very popular in industrial applications like measuring cutting force on high-tolerance parts.
Piezoelectric elements based torque sensor was deemed infeasible due to high amount of drift, complex mounting mechanism and an unexpectedly long lead times for high quality piezo crystals.
Hall Effect Sensor
Proposed measuring solution involved a pair of neodymium magnets and a hall effect sensor located in between. Angular strain of coupling shaft is detected as a change of magnetic field strength at the sensor.
Hall effect sensor was the backup choice for the project. The alignment of neodymium magnets on the motor shaft was a tricky problem.
Data Transfer: Slip Ring
Slip ring is an electromechanical device that consists of rotating rings connected to stationary brushes. A slip ring transfers power/signals from rotating structure to a stationary structure or vice-versa. The strain measurements would be read from the strain gauge using a slip ring.
Preliminary Design
An HDPE rod with 0.5” diameter was chosen as the test shaft for the dyno. Torque would be transferred from the motor through four bolts and heat set inserts. Flanged rod gives room for connections at wider ends, while the smaller diameter measures larger strain.