Rotary Sensing Technologies for Medical and Robotic Shaft Angle Sensing Applications
- Friday, 01 June 2012
Hall Effect Sensors
The current 3-axis rotary Hall effect sensors, like GMR technology, require that the magnet rotate directly above the Hall application-specific integrated circuit (ASIC) to accurately detect the changes in the magnetic field. These are typically end-of-shaft applications. To enable this technology in through-hole rotary applications, gears and a satellite magnet must be employed. One gear is placed on the shaft driving the second gear that houses the sensor’s magnet. This results in an indirect measurement of the true position of the steering shaft. Like the GMR sensor, Hall devices are a true non-contacting technology that lose their inherent benefit of contact-free position sensing when gears are employed. This gear train burdens the device with mechanical complexity. It now becomes a contacting solution with the mechanical play, audible gear noise, irregular torque, and hysteresis all contributing to reductions in absolute linearity.
PIHER's PST Technology
PIHER's new proprietary Hall-effect-based technology is called PST: Position Sensor Through-hole. PST provides simplified design characteristics in applications that require through-shaft, 360° absolute feedback. This patented non-contacting technology is designed specifically for through-hole sensing applications.
The key feature in this technology is the ability to sense true absolute 360° position of a shaft using only a ring magnet and one ASIC. This is desirable not only in shaft angle applications, but other rotary/pivot point sensing applications common in robotic arm or medical devices, where direct shaft sensing has been difficult to package.
Using a single bipolar ring and an ASIC, PIHER's PST technology requires that the ring magnet be permanently fastened to the rotating shaft. This ring magnet is magnetized in a simple bipolar North/South manner divided at 0° and 180°. The technology requires that the Hall effect sensing chip be placed anywhere around the 360° orbit of the ring magnet. The technology can be configured as a sector device using an arc magnet, which can be magnetized to provide full-scale output over the sector.
The Hall chip can be placed as close as 0.5 mm (air gap) from ring magnet in a typical 25-mm diameter application. The Hall sensing device is capable of all normal signal processing and error correction providing a stable analog, PWM, or Serial output (up to 14 bit resolution). A key feature of this technology is its tolerance for misalignment, both in terms of axial and radial play.
The proprietary feature of PST is its use of the application's existing bearing assemblies for the rotating shaft. The sensing element is in a fixed position, typically common to the bearing mountings. As the bearings approach their end of life tolerances and create play on the rotating shaft, the PST fixed sensing element accommodates this unstable shaft state, maintaining its original linearity specification. The inherent technology is immune to these unstable shaft conditions up to 0.5 mm of run out.
The density of these two magnetic fields allows for a consistent and repeatable sensing of the fields even when there are changes in radial and axial position. Phrased differently, even with excessive bearing play, the technology can still achieve highly accurate position feedback, typically in the ±1% range that is common in SAS applications.
This characteristic of radial and axial tolerance is a breakthrough in rotary angle sensing design in terms of simplicity and robustness. With only two key components, designers are liberated from the constraints of designing a robust and exotic bearing assembly that will outlast the rotational life of the part. A simple rotor that holds the ring magnet in a loosely concentric position now covers half the design task. Fixing the Hall chip to remain in the target sensing region of the ring magnet is the second task. Given total design control, the engineer can now design a device without a conventional rotor and housing bearing. This creates a larger tolerance in this area as an intentionally designed-in air gap or bearingless sensor, depending on the sealing requirements in the final application.
This technology was done by Piher Sensors and Controls S.A., Tudela, Spain. For more information, visit http://info.hotims.com/40434-185.
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