Feedback devices

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Typical servo motor with encoder feedback

Feedback devices are mandatory part in closed loop systems and typically found in servo motor applications. In motion control FB devices are used to get position or velocity information from motor or machine.

Shapes[edit | edit source]

Feedback devices come mainly in two shapes:

  • Rotary
  • Linear

Shape of device doesn't change their operation principle.

Feedback device types[edit | edit source]

Encoder[edit | edit source]

Quadrature encoders are nowadays the most common feedback device type in servo motors. Quadrature encoders are incremental sensors so they require position zeroing/homing to get absolute position feedback.

Another type of encoder is absolute encoder. Absolute encoders typically output serial data of absolute position thus they may not need to be zeroed (in case of multiturn absolute encoders). The drawback of absolute sensors is typically higher price and lower compatibility & interchangeability. For supported absolute encoder devices and user guide, see Using serial data feedback device.

Resolver a.k.a. synchro[edit | edit source]

Resolver is an analog technology based on rotary transformer that can provide absolute position (single turn absolute, for multiturn absolute position zeroing is still needed). Resolvers are good for harsh conditions but they don't provide as high precision feedback data as encoders.

Tachometer[edit | edit source]

Tachometer is a small DC generator that outputs DC voltage proportional to rotation speed. It can be used as velocity feedback device but not as position sensor. Tachometers are often seen in dual-loop configurations combined with position sensor.

Hall sensors[edit | edit source]

Hall sensors are typically found in AC & BLDC motors only. Halls provide commutation information (drive current phase angle) for drive. Modern drives such as all GD drives don't require Hall sensors but can utilize them to make faster power-up possible. Hall sensors are too low resolution for position or high performance velocity control and that's why Hall sensors are typically combined with encoder.

Comparison[edit | edit source]

Device Durability Outputs available Absolute Position feedback precision Velocity feedback precision
Incremental encoder Medium Quadrature, Serial, SinCos No Medium to very high Medium to high
Absolute encoder Medium Serial (SSI, BiSS, EnDat etc), or parallel Gray signals Yes (single or multiturn) Medium to very high Medium to high
Resolver Very high Analog, 6 wires Yes (single turn) Medium Medium
Tachometer Medium to high Analog Yes (for velocity FB) N/A Medium to high
Hall Medium to high Digital outputs Yes (single turn) Very low Very low

See also[edit | edit source]

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