Feedback devices
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.
Contents
Shapes
Feedback devices come mainly in two shapes:
- Rotary
- Linear
Shape of device doesn't change their operation principle.
Feedback device types
Encoder
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.
Resolver a.k.a. synchro
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.
- More info: Wikipedia:Synchro
- How resolver works: Youtube
Tachometer
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
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
| 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, 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
- Motor types
- Resolution of motors
- Extensive article about different types of feedback devices: http://www.optoresolver.com/help/glossary.htm
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