Most encoder-feedback systems have a single encoder mounted on the motor for velocity and/or position feedback. In some systems, however, a secondary (or auxiliary) encoder is placed directly on the load for dual-loop control.
Why would you need dual-loop control?
Auxiliary encoders are especially useful in applications with a load that's not directly driven by the motor. These include applications with elements like belts, ball screws, gears, and chains. In other words, most machines.
Every mechanical link in a system results in less rigidity. And when you lose rigidity, that means the motor position will not always be indicative of the load position. So if you try to close the position loop with the motor feedback only, you are going to get position errors.
By adding a second device to the system, you have a second position feedback loop. Your servo drive or controller can correct for the discrepancies.
If the load overshoots its target location, the motor can back off a little bit. If the load bounces back and falls short of the target location, the motor can rotate a little further.
Why not just put the encoder on the load?
So if the load position is what’s critical, then why not use a single encoder and put it on the load instead of the motor? Why do you need two loops?
Without feedback on the motor, backlash can leave the motor briefly unloaded. This would result in instability in the velocity loop, with the motor suddenly accelerating. Closing both loops allows you to get the smooth velocity control on the motor using the primary encoder and the smooth position control on the load using the auxiliary encoder.
- Auxiliary encoders are used to increase position accuracy and machine stability
ADVANCED Motion Controls' Capabilities
- Most DigiFlex® Performance™ and FlexPro servo drives accept Auxiliary Incremental Encoders and provide dual loop control. See all available models.