The most commonly used method to make this connection between rotating sensors and stationary electronics is the slipring. It consists of a set of conductive rings that rotate with the sensor, and a series of brushes that contact the rings and transmit the sensors’ signals.
Sliprings are an economical solution that performs well in a wide variety of applications. They are a relatively straightforward, time proven solution with only minor drawbacks in most applications. The brushes, and to a lesser extent the rings, are wear items with limited lives that don’t lend themselves to long term tests, or to applications that are not easy to service on a regular basis.
Figure 3: Sliprings are an economical solution
At low to moderate speeds, the electrical connection between the rings and brushes are relatively noise free, however at higher speeds noise will severely degrade their performance. The maximum rotational speed (rpm) for a slip ring is determined by the surface speed at the brush/ring interface. As a result, the maximum operating speed will be lower for larger, typically higher torque capacity sensors by virtue of the fact that the slip rings will have to be larger in diameter, and will therefore have a higher surface speed at a given rpm.
Typical max speeds will be in the 5,000 rpm range for a medium capacity torque sensor. Finally, the brush ring interface is a source of drag torque that can be a problem, especially for very low capacity measurements or applications were the driving torque will have trouble overcoming the brush drag.
In an effort to overcome some of the shortcomings of the slip ring, the rotary transformer system was devised. It uses a rotary transformer coupling to transmit power to the rotating sensor. An external instrument provides an AC excitation voltage to the strain gage bridge via the excitation transformer. The sensor's strain gage bridge then drives a second rotary transformer coil in order to get the torque signal off the rotating sensor. By eliminating the brushes and rings of the slip ring, the issue of wear is gone, making the rotary transformer system suitable for long term testing applications.
Figure 4: Rotary transformers improve performance
The parasitic drag torque caused by the brushes in a slip ring assembly is also eliminated. However, the need for bearings and the fragility of the transformer cores still limits the maximum rpm to levels only slightly better than the slip ring. The system is also susceptible to noise and errors induced by the alignment of the transformer primary-to-secondary coils. Because of the special requirements imposed by the rotary transformers, specialized signal conditioning is also required in order to produce a signal acceptable for most data acquisition systems, further adding to the systems cost that is already higher than a typical slip ring assembly.