Digital RF technology leads the way to more accurate noncontact measurement of torque.
Honeywell Sensing and Control
Golden Valley, Minn.
If you haven’t taken a good look lately at torque-measurement technology, it may surprise you. Particularly noteworthy is the transfer of signals from the sensor. Original designs for rotating- torque sensors used contact mechanisms like slip rings and brushes. The technology soon moved to noncontact methods using Torque telemetry goes digital Digital RF technology leads the way to more accurate noncontact measurement of torque. transformers and optics.
However, the sensor itself still needs bearing contact. Analog-based telemetry was tried, but initial technology was bulky and costly to implement. Today, digital-based telemetry is economical and sophisticated. Its advance sparks ideas for torque measurement in a multitude of industries.
Modern torque sensors have specially designed structures that deform in a predictable and repeatable manner with the application of torque. Strain gages mounted on the structure in a four-arm Wheatstone-bridge configuration translate the degree of deformation into a voltage signal.
Of course, the torque sensor still had to get the voltage signal from the rotating structure to the stationary readout. As stated earlier, slip rings were the only signal transfer method available for many years. But they suffered from several limitations and shortcomings. Heat generation from friction limited rotational speed to the slow end. Rings and pickup brushes needed regular maintenance for wear and tear. Frequency response across the slip rings was low as was the signal-to-noise ratio, especially at low signal levels.
As testing demanded faster rotational speeds and longer test cycles, the rotary transformer soon became the signal transfer method of choice. The noncontact design of rotary transformers allows higher operating speeds and longer operational test cycles with less maintenance. However, it doesn’t stand up in harsh installations and needs more expensive accarrier- based instrumentation that limits the response of the system.
As demands for greater accuracy grew, so did the need for a totally noncontact system. Analog telemetry was the first attempt to meet that need.
Analog wireless-telemetry systems couple a sensor with a builtin radio transmitter module and stationary receiver. Sensor and transmitter power comes from onboard batteries or can be inductively coupled in. The amplified signal from the sensor frequency modulates (FM) a radio signal in the transmitter module. The transmitted FM wave is picked up by a hoop antenna attached to the receiver. The receiver decodes the FM signal converting it to an analog output voltage that’s scaled to show torque readings on a display. The system can also route the signal to external data-acquisition systems for later analysis.
Inductive-power applications don’t need batteries. A rotating antenna captures ac power from the inductive supply and converts it to the regulated dc voltage needed to power the sensor and transmitter.
Because the transmitter module uses FM, the signal isn’t prone to static and other impulse noise found around high-power equipment. The radio frequency varies or deviates from a fixed-center frequency in proportion to the level of torque. The amount of frequency deviation is usually 200 kHz or less. Multiple transmitters with different center frequencies let multiple strain or temperature measurements take place on the same rotating device. However, each transmitter needs its own dedicated receiver tuned to that transmitter center frequency.
Analog telemetry systems can be bulky and costly. The additional receivers needed for multiple channels can cause space issues. Installation, alignment, and setups can be tedious and time consuming. The antenna, straps, and battery power had reputations for failing. Thus the systems needed occasional maintenance, adjustments, and replacements. The conversion to digital telemetry systems solved many of these problems.
Modern digital telemetry torque measuring systems came from advancements in microchips and surface-mount technologies. The miniaturized circuitry offers better performance, flexibility, and application scope while reducing time, product size, initial technology costs, and total cost of ownership. Digital telemetry systems condition signals and digitize directly on the rotating part. Data transfers have a 3-kHz bandwidth and transmit as digital data. The use of multiple microprocessors helps optimize data management and control.
Digital systems are designed with clear upgrade paths for custom software and development, open system architectures, and flexible mechanical formats. An excess of computing performance capacity supports communication standards such as Ethernet, USB, and similar protocols while still allowing access to the speed and torque measuring element. The typical electronics system has a rotating signalconditioning module and antenna, a stationary receiver, a signal-processing module, and is powered by an inductive power source similar to an analog system.
The rotating signal-conditioning module sends regulated dc excitation to the strain gages and amplifies the low-voltage signal. The signal is then converted into digital data and modulated onto a radio frequency. The stationary receiver passes the signal to the signal-processing module to decode the digital data. The micro-processors in the module transfer the data to the various digital and analog outputs. Usually the digital output is RS-232, while analog outputs can be voltage, current, or frequency based. The signal-processing module is typically powered by a low-current 12 or 24-Vdc power source.
Digital-telemetry growth portends new features and endless mechanical possibilities. Microchip and surface-mount technology can physically integrate rotor electronics and telemetry into one element. Fully digital systems without switches or potentiometers offer independent input and output scaling options determined by the user. This lets technicians tailor standard systems to the test conditions needed at the time. For example, gravity and buoyancy compensation can be built into calibration routines. High-speed Ethernet links let Web pages send setup and configuration data. Likewise, network connections can deliver data readings for torque, speed, power, and temperatures, accomplishing both send and receive tasks without the need for special PC software.
Honeywell Sensing and Control, sensing.honeywell.com