Yaskawa SGDB - Manual (Page 20)

FOR FIRST-TIME USERS OF AC SERVOS
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(c) Performance of servomotor
A servomotor must have “instantaneous power” so that it can start as soon as a
start reference is received.
The term “power rating (kW/s)” is used to represent instantaneous power.
It refers to the electric power (kW) that a servomotor generates per second.
The greater the power rating, the more powerful the servomotor.
(3) Detector
A servo system requires a position or speed detector. It uses an encoder mounted on
a servomotor for this purpose.
Encoders are divided into the following two types:
(a) Incremental Encoder
An incremental encoder is a pulse generator, which generates a certain number
of pulses per revolution (e.g., 2,000 pulses per revolution). If this encoder is con-
nected to the mechanical system and one pulse is defined as a certain length
(e.g., 0.001 mm), it can be used as a position detector.
However, this encoder does not detect an absolute position and merely outputs a
pulse train. Zero point return operation must be performed before positioning.
The following figure illustrates the operation principle of a pulse generator:
Phase A pulse train
Phase B pulse train
Slit
Center of
revolution
Fixed slit
Light-emitting
element
Light-receiving
element
Rotary
disc
Rotary slit
Phase A
Phase B
Phase Z
(b) Absolute encoder
An absolute encoder is designed to detect an absolute angle of rotation as well as
to perform the general functions of an incremental encoder. With an absolute en-
coder, therefore, it is possible to create a system that does not require zero point
return operation at the beginning of each operation.
• Difference between an absolute and incremental encoder:
An absolute encoder will keep track of the motor shaft position even if system
power is lost and some motion occurs during that period of time. The incremental
encoder is incapable of the above.
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