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.
Sampled Dual Loop - Example
In this example, we consider a linear slide which is run by a rotary motor via a lead screw. Since the lead screw has
a backlash, it is necessary to use a linear encoder to monitor the position of the slide. For stability reasons, it is best
to use a rotary encoder on the motor.
Connect the rotary encoder to the X-axis and connect the linear encoder to the auxiliary encoder of X. Assume that
the required motion distance is one inch, and that this corresponds to 40,000 counts of the rotary encoder and 10,000
counts of the linear encoder.
The design approach is to drive the motor a distance, which corresponds to 40,000 rotary counts. Once the motion is
complete, the controller monitors the position of the linear encoder and performs position corrections.
This is done by the following program.
INSTRUCTION INTERPRETATION
#DUALOOP Label
CE 0 Configure encoder
DE0 Set initial value
PR 40000 Main move
BGX Start motion
#Correct Correction loop
AMX Wait for motion completion
V1=10000-_DEX Find linear encoder error
V2=-_TEX/4+V1 Compensate for motor error
JP#END,@ABS[V2]<2 Exit if error is small
PR V2*4 Correction move
BGX Start correction
JP#CORRECT Repeat
#END
EN
Motion Smoothing
The DMC-40x0 controller allows the smoothing of the velocity profile to reduce the mechanical vibration of the
system.
Trapezoidal velocity profiles have acceleration rates which change abruptly from zero to maximum value. The
discontinuous acceleration results in jerk which causes vibration. The smoothing of the acceleration profile leads to
a continuous acceleration profile and reduces the mechanical shock and vibration.
Using the IT Command:
When operating with servo motors, motion smoothing can be accomplished with the IT command.
This command filters the acceleration and deceleration functions to produce a smooth velocity profile.
The resulting velocity profile, has continuous acceleration and results in reduced mechanical
vibrations.
The smoothing function is specified by the following commands:
IT x,y,z,w Independent time constant
DMC-40x0 User Manual Chapter 6 Programming Motion • 122
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