Torque Measurement Techniques – A Timeline
As shown from the diagram below, slip ring technology was the earliest method of transmitting power to and signal from a rotating shaft to a stationary housing. As it was inherently limited by lower speeds and plagued by brush-ring wear, frequent maintenance was required. These shortcomings opened the door for the development of rotary transformers. While untainted by slip ring technology shortcomings, they had some of their own. Less tolerance to thrust and bending moment loads in addition to requiring some sophisticated AC carrier excitation and signal conditioning, led manufacturers to find yet another way to overcome these hindrances. The new comer was in the form of analog telemetry system, first introduced in the late 70’s by Accurex Corp. This system included an on-board signal conditioner/radio transmitter that modulated the strain gage bridge output to a radio frequency signal. A hoop antenna picked up the transmitted signal and fed it to a data acquisition system. The preferred form of excitation of this system was yet another on-board battery pack. These systems were limited to frequency responses around the 1000 Hz in addition to being cumbersome and difficult to set their hoop antennas. The newest kid on the block was the digital telemetry system. By the late 80’s microprocessor and surface mount technologies has already made tremendous strides leading to the miniaturization of circuitries and the development of several families of micro-chips. Manufacturers making use of these developments in digital telemetry introduced several look-alikes that were almost identical in form and in inherent limitations as well. A signal processing module tethered to the rotating sensor by no more than 0.25” also acted as the radio receiver. All the other needed intelligence was mounted on board the sensor. Some of these inherent limitations were; antennas were limited to a few select sizes, cables from the signal conditioning receiver to data acquisition systems were also limited to certain lengths, proximity to steel structures affect readings accuracy, and emitted emi (electro-magnetic interference) were sometimes more than allowed by government. FCC had to stop one such model and fine its manufacturer.
SensorData’s BT400 series is by large today’s state of the art in digital telemetry. It uses the well established Blue Tooth platform, which makes it immune to either emi or emf. Its transceiver can be placed anywhere from 0.25”- 30.00’ instead of the tethered 0.25” of available models, and is not affected by proximity to steel structures.