Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018 387
The control system is responsible for scaling Motion Counts into equivalent
Motor Feedback Counts. In this case, because the motor is directly coupled to the
load, one rotation of the shearing drum translates to one revolution of the motor
feedback device. Assuming that the motor feedback device is a typical optical
encoder with 4000 Feedback Counts/rev, a displacement of 300,000 Motion
Counts would be scaled by the control system to be 4000 Feedback Counts.
Rotary Shear Application with Gearbox
Instead of the motor directly driving the rotary shear drum, in this application
example the motor is driving the shearing drum through a 3:1 gearbox. Since the
Motion Unit is tied to the load, for example, Load Rev, the Motion Resolution,
Conversion Constant, and Rotary Unwind determined in the example above
apply equally well to this application.
The scaling of Motion Counts to Motor Feedback Counts, however, is not the
same due to the presence of the gearbox. In this case, one revolution of the
shearing mechanism translates to three revolutions of the motor. Nevertheless, the
control system has a count scaling feature that performs this scaling automatically.
This is done by selecting a Rotary Transmission as the Load Type, and setting the
Transmission Ratio Output to 1, and the Transmission Ratio Input to 3. In this
way, 300,000 Motion Counts per Load Rev is scaled exactly to 12,000 Motor
Feedback Counts, or three Motor Revs.
Rotary Motor with Gearbox/Ball-Screw Application
Because this is a linear application, Motion Resolution would be expressed as
Motion Counts per Load millimeter m or Load inch, in this case, say millimeter.
The motor feedback would, however, be rotary and resolution expressed in Motor
Feedback Counts per Motor Rev, in our case 4000 Feedback Counts per Motor
Rev. The default Motion Resolution is 1,000,000 Motion Counts per millimeter
and if the Position Unit is, say, centimeters, the Conversion Constant would be
10,000,000 Motion Counts per centimeter. This would be automatically
calculated by entering a Position Scaling of 1 centimeter per 10 millimeters.
Given that our application uses a 4:1 gearbox and a 5 mm pitch ball-screw, 5 mm
of ball-screw travel translates to 4 revolutions of the motor, or 16,000 Feedback
Counts. Again, the control system's count-scaling feature performs this scaling
automatically. This is done by selecting Linear Actuator as the Load Type, setting
the Transmission Ratio Output to 1, and the Transmission Ratio Input to 4,
setting the Actuator Type to be "Screw", and the Lead to 5 mm/rev. In this way,
5,000,000 Motion Counts, or 5 mm worth of screw displacement, is scaled exactly
to 16,000 Motor Feedback Counts, or exactly 4 Motor Revs.
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