The essential
A slip ring passes signals between a rotating part and the stationary world through a ring and brush interface. Its rotor turns with the part and carries the sensor wires; its stator hands the signals to the data acquisition system. An end-of-shaft slip ring mounts on the free end of a shaft; a tubular one slides over a shaft that has none.
What does a slip ring do?
It transfers signals and power between stationary and rotating parts, on several channels at once. The analog signal passes through the contacts with low electrical noise and unlimited bandwidth. Assemblies have been built with up to 240 channels, for centrifugal compressor tests at NASA.
- Typical measurements: temperature, drive shaft strain, torque, stress, vibration, acceleration.
End of shaft or tubular?
The shaft decides. An end-of-shaft slip ring needs a free shaft end; a tubular one slides over the shaft.
- End of shaft: mounts on the end of a rotating shaft. For wheels, gearboxes, engines and compressors. Up to 60 connections as standard, and many models run up to 12 000 rpm.
- Tubular: the shaft passes through its bore. Mostly for automotive drive shaft measurements. Generally up to 8 connections.
- Custom slip rings are designed when neither fits.
Which sensors connect to it?
Thermocouples and strain gauges are the most common. Slip rings also carry digital signals: Gigabit Ethernet has passed through an SR36M spinning at 900 rpm.
- Thermocouples: fitted to spinning parts such as brake rotors or wheels, and routed to a slip ring on the outside of the wheel. Keep the assembly at a uniform temperature, so that a temperature gradient across the rings does not enter the measurement.
- Strain gauges: wired as a full Wheatstone bridge with all four arms on the rotating side, the rings sit outside the bridge and do not change its balance. Peak ring noise then stays under 0.1 % of the data signal.
When is an amplifier needed?
When the signal is low or the environment is noisy. An amplifier on the rotating side turns the sensor signal into a high-level voltage before it crosses the rings, which reduces the errors from temperature gradients across the slip ring and from interference.
- Electrically noisy environments
- Low strain levels
- High speeds
- Vibration
What makes it an instrumentation slip ring?
The rings and brushes are made of precious metals. This keeps the variation of contact resistance low enough for low-level instrumentation signals such as strain gauge bridges and thermocouples. Weatherproof versions, such as those of the B-Series, take water, mud, snow and dust.
To remember
- A free shaft end: end of shaft. No free end: tubular.
- Full strain gauge bridge on the rotor: the rings stay outside the bridge. Thermocouples need a uniform temperature or an amplifier on the rotor.
- Amplify on the rotor when the signal is low or the environment noisy.