Rotary Sensing Technologies for Medical and Robotic Shaft Angle Sensing Applications
- Friday, 01 June 2012
This technical brief provides an overview of existing types of rotary sensor technologies, and compares the strengths and weaknesses of each type for use as through-hole shaft angle sensing devices.
Today’s sensor technologies for robotic and medical applications include many devices that have evolved from industrial applications. Because of this general migration, they are not ideally suited to the requirements in new robotic and medical products. Specifically, conventional contacting and non-contacting sensing solutions do not offer the customization, small package size(s), and environmental and durability requirements that are so necessary in these custom applications.
Shaft angle sensors have matured in the industrial marketplace over the past 10 years, especially with the increasing penetration of absolute position sensors, which broadcast the true position (shaft angle) at power on. These sensors do not need supplemental electronics or initial motion to trigger an index pulse to determine true position. Their suitability in medical and robotic applications has met with mixed success. When these devices are selected for use in medical equipment or robots, the inherent weaknesses of each technology becomes more pronounced. Some examples include:
• Existing sensor package is too large
to fit in the medical device or robotic
• Durability of the industrial device is not matched to the medical or robotic specification
• The industrial specified device offers only incremental position information
• The industrial device is ‘over spec’d’ and cannot meet cost targets in these new designs
Potentiometers are the most mature technology under consideration and have two key weaknesses. When used in a shaft angle sensing (SAS) applications, the termination points of the resistive track do not allow 360° of position feedback. The practical limit is approximately 340°. This is due to the need for terminating points at the ends of the resistive tracks, creating a ‘dead zone’ (no feedback) in this region. This can be overcome with gear reduction (achieving 360° of mechanical travel and feedback). However, this adds substantially to the design in terms of cost, part count, and mechanical hysteresis.
Second, the electrical contact has to maintain very tight dimensional repeatability over its entire mechanical travel. Large mechanical tolerances between these two create unstable conditions for the electrical contact, resulting in noise, excessive wear, or open circuit conditions.
Incremental and Absolute Encoders
Incremental devices are the simplest form of an optical encoder — a device that “increments” from one pulse to another. This technology requires that the counting be done upstream in the external electronics. Incremental devices cannot keep their true position when the power is turned off. When the power is turned back on, the device must be rotated to an index pulse to determine its true position.
Both types of optical encoders have three key design advantages: 1) the ability to resolve to very high resolutions within one rotation; 2) very fast response times that make them ideal in high-speed feedback applications; and 3) they are relatively immune to axial play on the shaft. The code disc can float up or down, and the available light hitting the photodiode remains in position. However, these optical devices have very little tolerance for radial play as the available light moves off of the target (photo-receptive) area of the diode. For low-speed SAS applications, the available resolutions are far greater than are typically necessary for shaft angle position feedback. Lastly, optical devices cannot tolerate dust or dirt on either the light source or the photodiode. This attribute is a strong consideration over the lifetime of a device when plastic bearing features are employed.
GMR Technology Applied in Shaft Angle Sensors
Current Giant Magnetic Resistance (GMR) technology mandates that the magnet rotate centered above the multi-layered sensing element (typically an Integrated Circuit) to detect the orientation of the magnetic field. To enable this technology in through-hole rotary applications, gears and a satellite magnet(s) must be employed. One gear is placed around the shaft that drives a separate gear holding the sensor’s magnet. This results in an indirect measurement. Moreover, when the GMR sensor (a true non-contacting technology) is used with these gears, it becomes a ‘contacting’ solution with the inherent disadvantages and complexities that gear trains imply: such as mechanical play, audible gear noise during mechanical life, irregular torque, hysteresis; many of these characteristics contribute to reductions in absolute linearity.