Mitsubishi Electric MELSEC iQ-F Series User Manual page 10

FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
3.2 Servo amplifier and servo motor
The servo amplifier controls the movement quantity and the speed according to the commands given by the
positioning controller. The servo motor then transmits rotation to the drive mechanism after receiving signals
from the servo amplifier.
3.2.1 Positioning control in accordance with command pulse
In accordance with speed and position command pulses from the positioning controller, PWM (pulse width
modulation) control is performed by the main circuit of the servo amplifier in order to drive the motor. The
rotation speed and the rotation quantity are fed back to the amplifier from the encoder attached to the servo
motor.
3.2.2 Deviation counter function
The difference between the command pulses and the feedback pulses counted by the deviation counter in the
servo amplifier is called accumulated pulses.
While the machine is operating at a constant speed, the accumulated pulse quantity is almost constant.
During acceleration and deceleration, the accumulated pulse quantity changes more dramatically.
When the accumulated pulse quantity becomes equivalent to or less than a specified quantity (in-position set
value) after command pulses have stopped, the servo amplifier outputs the positioning complete signal.
The servo motor continues operation even after that. Then, when the accumulated pulse quantity becomes 0,
the servo motor stops.
The time after the servo motor outputs the positioning complete signal until it stops is called the stop settling
time.
Speed
Accumulated
pulses
Stop settling time
3.2.3 Servo lock function
The servo motor is controlled so that the accumulated pulse quantity counted in the deviation counter
becomes 0.
For example, if an external force for forward rotation is applied on the servo motor, the servo motor performs
the reverse rotation operation to eliminate the accumulated pulses.
Accumulated pulses in deviation counter
3.2.4 Regenerative brake function
During deceleration, because the servo motor rotates by the load inertia of the drive mechanism, it functions
as a generator and electric power returns to the servo amplifier.
The regenerative resistor absorbs this electric power and functions as a brake (called a regenerative brake.)
A regenerative brake is required to prevent regenerative over voltage in the servo amplifier when the load
inertia is large and operations are frequently performed.
The regenerative resistor is required when the regenerative power generation quantity during deceleration
exceeds the allowable regenerative electric power of the servo amplifier.
Command speed
Motor speed
The accumulated pulse quantity is 0, and
positioning is completed.
Minus pulses
Plus pulses
0 (zero)
3 Components of Positioning Control and Their Roles
Time
Servo motor
Reverse rotation operation
Forward rotation operation
Stop
3.2 Servo amplifier and servo motor
1
2
3
4
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