Mitsubishi Electronics QD75D Outline design of positioning system, Positioning system using QD75

1 PRODUCT OUTLINE

1.1.4 Outline design of positioning system

MELSEC-Q

The outline of the positioning system operation and design, using the QD75, is shown below.

(1) Positioning system using QD75

PLC CPU		Positioning module			Drive unit			Servomotor
			QD75 Forward run
			pulse train			Speed
				Devia-	D/A	command	Servo
Program		Read, write, etc.		tion	converter		amplifier	M
				counter
			Reverse run
			Setting data pulse train
Peripheral						Interface
devices		Read, write, etc.
interface						Feedback pulse		PLG
	Read, write, etc.
GX Configurator-QP

Fig. 1.2 Outline of the operation of positioning system using QD75

(a)Positioning operation by the QD75

1)The QD75 output is a pulse train.

The pulse train output by the QD75 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 QD75 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 QD75.

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).

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