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JG VEL
JP #B
EN
This system requires the position of the motor to be proportional to the joystick angle. Furthermore,
the ratio between the two positions must be programmable. For example, if the control ratio is 5:1, it
implies that when the joystick voltage is 5 Volts, corresponding to 1028 counts, the required motor
position must be 5120 counts. The variable V3 changes the position ratio.
Instruction
#A
V3=5
DP0
JG0
BGX
#B
V1=@AN[1]
V2=V1*V3
V4=V2-_TPX-_TEX
V5=V4*20
JG V5
JP #B
EN
The continuous dual loop, enabled by the DV1 function is an effective way to compensate for
backlash. In some cases, however, when the backlash magnitude is large, it may be difficult to
stabilize the system. In those cases, it may be easier to use the sampled dual loop method described
below.
This design example addresses the basic problems of backlash in motion control systems. The
objective is to control the position of a linear slide precisely. The slide is to be controlled by a rotary
motor, which is coupled to the slide by a leadscrew. Such a leadscrew has a backlash of 4 micron, and
the required position accuracy is for 0.5 micron.
The basic dilemma is where to mount the sensor. If you use a rotary sensor, you get a 4 micron
backlash error. On the other hand, if you use a linear encoder, the backlash in the feedback loop will
cause oscillations due to instability.
An alternative approach is the dual-loop, where we use two sensors, rotary and linear. The rotary
sensor assures stability (because the position loop is closed before the backlash) whereas the linear
sensor provides accurate load position information. The operation principle is to drive the motor to a
given rotary position near the final point. Once there, the load position is read to find the position
error and the controller commands the motor to move to a new rotary position which eliminates the
position error.
Since the required accuracy is 0.5 micron, the resolution of the linear sensor should preferably be
twice finer. A linear sensor with a resolution of 0.25 micron allows a position error of +/-2 counts.
122 • Chapter 7 Application Programming
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Update jog speed to value of variable VEL
Loop back to label, #B
End
Position Control by Joystick
Interpretation
Label
Initial position ratio
Define the starting position
Set motor in jog mode as zero
Start
Read analog input
Compute the desired position
Find the following error
Compute a proportional speed
Change the speed
Repeat the process
End
Backlash Compensation by Sampled Dual-Loop
DMC-1000
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