Mitsubishi Electric Melsec FX Series Beginners Manual page 19

Positioning method type
Pulse count method
A position detector (such as a pulse encoder) is set up in a motor or rotation axis. The pulse number
generated from the position detector is counted by a high-speed counter. When the pulse number
reaches the preset value, the moving part stops.
In this method, because limit switches are not used, the stop position can be easily changed.
– Guideline of stopping precision: Approximately ±0.1 to 0.5 mm
(The stop precision shows a value in a case where the low speed is 10 to 100 mm/s.)
Fig. 1-9: Schematic drawing – Pulse count method
In speed control applications with inverters, stop precision is not very accurate. With the limit switch
method, a system operates without any feedback to the controller to indicate the location of the
workpiece. With the pulse count method, the speed can be changed and the stop command can be
executed at specific distances (at specific timings) according to the feedback from the pulse
generator connected to the motor. Both the limit switch method and the pulse count method,
however, are subject to a loss in stop precision due to the dispersion of distance that occurs for
workpieces at different speeds.
● When automatically stopping a moving part driven by a motor, stop the motor by a position signal
(using a limit switch or pulse count comparison). In general conditions, turn on the brake at the
same time.
FX Positioning Control Systems
Pulses are fed
back
PLG IM
Pulse generator
Programmable
PLC
controller
Inductive motor
Moving part
Inverter
INV
DC 0 to
10 V
High speed
High speed
Movement distance
counter unit
The Basics of Positioning Control
Ball screw
Low speed
120090da.eps
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