Outline Design Of Positioning System - Mitsubishi Electric MELSEC-L LD75P User Manual

1.1.4

Outline design of positioning system

The outline of the positioning system operation and design, using the LD75, is shown below.
(1) Positioning system using LD75
CPU module
Read, write, etc.
Program
Peripheral
devices
Read, write, etc.
interface
Read, write, etc.
GX Works2
(a) Positioning operation by the LD75
1.
The LD75 output is a pulse train.
The pulse train output by the LD75 is counted by and stored in the deviation counter in the drive unit.
The D/A converter outputs an analog DC current proportionate to the count maintained by the deviation
counter (called "pulse droop"). The analog DC current serves as the servomotor speed control signal.
2.
The motor rotation is controlled by the speed control signal from the drive unit.
As the motor rotates, the pulse encoder (PLG) attached to the motor generates feedback pulses, the
frequency of which is proportionate to the rotation speed.
The feedback pulses are fed back to the drive unit and decrements the pulse droop, the pulse count
maintained by the deviation counter.
The motor keeps on rotating as the pulse droop is maintained at a certain level.
3.
When the LD75 terminates the output of a pulse train, the motor decelerates as the pulse droop
decreases and stops when the count drops to zero.
Thus, the motor rotation speed is proportionate to the pulse frequency, while the overall motor rotation
angle is proportionate to the total number of pulses output by the LD75.
Therefore, when a movement amount per pulse is given, the overall movement amount can be
determined by the number of pulses in the pulse train.
The pulse frequency, on the other hand, determines the motor rotation speed (feed speed).
Positioning module
LD75
Forward run
pulse train
Deviation
counter
Reverse run
Setting
pulse train
data
CHAPTER 1 PRODUCT OUTLINE
Drive unit
Speed
command
Servo
D/A
amplifier
converter
Interface
Feedback pulse
1
Servomotor
M
PLG
29