Rotary Position Sensors

Rotary position sensors provide electrical outputs proportional to shaft rotation. They measure multiturn motion and angular displacement of less than one turn. Rotary position sensors use many different sensing technologies. Potentiometers detect variable resistance and measure the angle of rotation as the shaft turns. Hall effect sensors develop an electrical potential across an axis perpendicular to an applied current flow. Permanent magnets mounted to the rotating sensor shaft supply a magnetic field. Variable inductance sensors include a component on the rotating sensor shaft that induces magnetic field effects on multiple stationary coils. Resolvers consist of a rotating coil and two stationary coils that are 90° apart. They measure the currents generated in the stationary coils and compare them, providing precise measurements of the rotor position. Synchros are variable transformers that are similar to resolvers, but that contain three stationary coils 120° apart. The coil on the rotating shaft induces a current in the other two coils that can be used to provide angular position outputs. Rotary variable differential transformer (RVDT) signal conditioners translate the rotary movement of a ferromagnetic armature into an AC voltage that is proportional to the armature position. Capacitive displacement transducers are non-contact devices that use differential capacitance for high resolution angular measurement. Other unlisted, specialized, and proprietary sensing technologies are also available.

Selecting rotary position sensors requires an analysis of device categories, operating specifications, and angular measurements. Suppliers typically divide products into the following categories: sensors, sensor elements, instruments, gauges, and recorders. Sensors, a category that includes transducers, provide packaging and signal conditioning. By contrast, bare sensor elements or chips do not. Instruments or meters are self-contained rotary position sensors that produce local outputs and include signal processing or analysis. Gauges or indicators are sensing devices with integrated dials or simple readouts. Recorders and totalizers measure values or accumulate totals. They may also provide statistical functions. Operating specifications for rotary position sensors include temperature range, shock rating, and vibration rating. Angular measurements include maximum measured angular stroke, mechanical range, and accuracy.  

Rotary position sensors differ in terms of electrical outputs. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Analog voltages are simple, usually linear functions. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several types of digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Other digital outputs for rotary position sensors include transistor-transistor logic (TTL) signals. Outputs that change the state of a switch or alarm are also available.

• How To Substantially Reduce Encoder Cost While Gaining Functionality With Multi-Turn Rotary Position Sensors (.pdf)
  Many applications require rotation counters that can measure angles greater than 360º. However the low-cost 10-turn potentiometers most design engineers are familiar with can't always meet user...
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• Position Sensors and Position-Sensing Instruments
  Position sensors include wide range of sensors, switches and technologies that are used to determine the placement, speed or movement, direction, and location of a given target. Linear position...
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• Touchless Rotary Sensors White Paper (.pdf)
  A new generation of touchless position sensors solves a number of new problems while simplifying existing on-line control applications by providing design engineers with an opportunity to reduce...
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Part Numbers for Rotary Position Sensors