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.
Like the rotary transformer, the infrared (IR) torque sensor utilizes a contactless method of getting the torque signal from a rotating sensor back to the stationary world. Similarly using a rotary transformer coupling, power is transmitted to the rotating sensor. However, instead of being used to directly excite the strain gage bridge, it is used to power a circuit on the rotating sensor. The circuit provides excitation voltage to the sensor’s strain gage bridge, and digitizes the sensor’s output signal.
Figure 5: IR provides contactless sensing
This digital output signal is then transmitted, via infrared light, to stationary receiver diodes, where another circuit checks the digital signal for errors and converts it back to an analog voltage. Since the sensor’s output signal is digital, it is much less susceptible to noise from such sources as electric motors and magnetic fields. Unlike the rotary transformer system, an infrared transducer can be configured either with or without bearings for a true maintenance free, no wear, no drag sensor.
While more expensive than a simple slip ring, it offers several benefits. When configured without bearings, as a true non-contact measurement system, the wear items are eliminated, making it ideally suited for long term testing rigs. Most importantly, with the elimination of the bearings, operating speeds (rpm’s) go up dramatically, to 25,000 rpm and higher, even for high capacity units. For high speed applications this is often the best solution for a rotating torque transmission method.
Another approach to making the connection between a rotating sensor and the stationary world utilizes an FM transmitter. These transmitters are used to remotely connect any sensor, whether force or torque, to its remote data acquisition system by converting the sensor’s signal to a digital form and transmitting it to an FM receiver were it is converted back to an analog voltage.
Figure 6: FM connections work well over longer distances
For torque applications they are typically used for specialty, one of a kind sensors, such as when strain gages are applied directly to a component in a drive line. This could be a drive shaft or half shaft from a vehicle for example. The transmitter offers the benefits of being easy to install on the component as it is typically just clamped to the gaged shaft, and it is re-usable for multiple custom sensors. It does have the drawback of needing a source of power on the rotating sensor, typically a 9V battery, which makes it impractical for long term testing.
Understanding the nature of the torque to be measured, as well as what factors can alter that torque in the effort to measure it, will have a profound impact on the reliability of the data collected. In applications that require the measurement of dynamic torque, special care must be taken to measure the torque in the proper location, and to not effect the torque by dampening it with the measurement system.
Knowing the options available to make the connection to the rotating torque sensor can greatly affect the price of the sensor package. Sliprings are an economical solution, but have their limitations. More technically advanced solutions are available for more demanding applications, but will generally be more expensive. By thinking through the requirements and conditions of a particular application, the proper torque measurement system can be chosen the first time.
Ken Winczner, Sales Manager Sensor Developments Inc.