Mitsubishi Electric MELSEC-L LD75P User Manual page 523

(1) Movement amount per pulse
"[Pr.2] No. of pulses per rotation (Ap)", "[Pr.3] Movement amount per rotation (Al)", and "[Pr.4] Unit magnification
(Am)" are the items for determining how many rotations (equivalent to how many pulses) a motor should operate
to move a machine for movement amount set in a program.
The drive unit controls the motor with the pulse number.
The following shows the details of control of the LD75.
LD75
Command
CPU module,
value
GX Works2
*1 The upper limits of the command pulse frequency are as follows: LD75P: 200kpulse/s, LD75D: 4Mpulse/s
*2 For a drive unit without electronic gear function, or when not using electronic gear function, this value is 1 (single).
Consider a system with the motor connected to a ball screw, such as the figure above.
1 is set to the electronic gear of the drive unit.
The movement amount of the machine is in mm or inch unit. Set the command value in mm or inch units to the
LD75 for the CPU module program.
The motor is controlled by the drive unit in pulse number units. Consequently, since the command value in mm or
inch units is converted to pulse units, set Ap, Al, and Am so that the following relational expressions are satisfied.
• Number of pulses per rotation of motor = Ap
• Machine movement amount per rotation of motor = Al  Am
In this case, the machine movement amount for the command 1 pulse output from the LD75 is calculated by the
following expression.
The command frequency from the LD75 is limited by a ceiling. If the command frequency exceeds the upper limit, increase
the "Movement amount per pulse (A)" greater (N times) to decrease the command frequency.
In this case, the electronic gear on the drive unit must be increased by N times as well.
The command pulse from the LD75 is changes to 1/N times. Therefore, multiply it by N on the drive unit side to keep the
number of motor rotations.
Since the "movement amount per pulse (A)" is increased, the position accuracy (command resolution) for the command 1
pulse from the LD75 decreases.
Consider to decrease the command speed when the position accuracy is required.
Command
Drive unit
*1
pulses
Electronic
Ap
Al × Am gear
Movement amount per pulse (A) =
CHAPTER 12 CONTROL SUB FUNCTIONS
Motor
*2
Reduction gear
Rotating motor depending
on the number of pulses
Al × Am
Ap
Lead
Machine
Worm gear
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