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Dual Loop Feedback Information and Configuration

Primary Software: Driver Software>>NI-Motion/FlexMotion
Primary Software Version: 4.5
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
What is dual loop feedback and when/how would I use it?

Solution:
Motion control systems often use gears to increase output torque, increase resolution, or convert rotary motion to linear motion. When gears are used a backlash can be introduced into the system. Backlash is when there is a delay between the motion on the load side and the motor side, which can cause a loss of position accuracy and system instability.

Using two position sensors, also known as encoders, for an axis can help solve the problems caused by backlash. One sensor will be on the load side and the other will be on the motor side, before the gears. The sensor on the load side is referred to as primary feedback and is used for position feedback. If the encoder connected to the load is used to close the velocity loop the backlash in the system will lead to a delayed response in terms of correcting the velocity. The sensor on the motor side is used to correct this and is referred to as secondary feedback.

To set up a dual loop feedback system use the following steps:
  • In Measurement and Automation Explorer(MAX), under the Axis Configurations settings, set your type of feedback to Dual Loop Encoder
    *Note: Analog feedback cannot be used in this system
  • The encoders will be Encoder 1 and 3, by default
    1. Attach the primary (load) sensor to Encoder 1
    2. Attach the secondary (motor) sensor to Encoder 3
  • Still in MAX, under the Control Loop Settings tab. Set the Derivative Gain (Kd) to zero. You will you use the Velocity Gain (Kv) instead.
  • Make moves as in any other system
For more information on dual loop feedback, refer to page 3-9 of the NI-Motion User Manual.

Related Links: NI-Motion User Manual

Attachments:


dual loop.JPG - dual loop.JPGdualloop.JPG - dualloop.JPG

Report Date: 02/08/2007
Last Updated: 01/09/2008
Document ID: 467JO44M

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