Galil Motion Control CDS-3310, User Manual

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Chapter 6 Programming Motion CDS-3310
sets their initial values.
The positions of the auxiliary encoders may be interrogated with the command, DE?. For example
DE ?,,?
returns the value of the A and C auxiliary encoders.
The auxiliary encoder position may be assigned to variables with the instructions
V1= _DEA
The command, TD a,b,c,d, returns the current position of the auxiliary encoder.
The command, DV a,b,c,d, configures the auxilliary encoder to be used for backlash compensation.
Backlash Compensation
There are two methods for backlash compensation using the auxiliary encoders:
1. Continuous dual loop
2. Sampled dual loop
To illustrate the problem, consider a situation in which the coupling between the motor and the load
has a backlash. To compensate for the backlash, position encoders are mounted on both the motor and
the load.
The continuous dual loop combines the two feedback signals to achieve stability. This method
requires careful system tuning, and depends on the magnitude of the backlash. However, once
successful, this method compensates for the backlash continuously.
The second method, the sampled dual loop, reads the load encoder only at the end point and performs a
correction. This method is independent of the size of the backlash. However, it is effective only in
point-to-point motion systems which require position accuracy only at the endpoint.
Example
Continuous Dual Loop
Connect the load encoder to the main encoder port and connect the motor encoder to the dual encoder
port. The dual loop method splits the filter function between the two encoders. It applies the KP
(proportional) and KI (integral) terms to the position error, based on the load encoder, and applies the
KD (derivative) term to the motor encoder. This method results in a stable system.
The dual loop method is activated with the instruction DV (Dual Velocity), where
DV 1,1,1,1
activates the dual loop for the four axes and
DV 0,0,0,0
disables the dual loop.
Note that the dual loop compensation depends on the backlash magnitude, and in extreme cases will
not stabilize the loop. The proposed compensation procedure is to start with KP=0, KI=0 and to
maximize the value of KD under the condition DV1. Once KD is found, increase KP gradually to a
maximum value, and finally, increase KI, if necessary.