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7 Control Mode
7 Control Mode
The servo system consists of three major parts: servo drive, servo motor, and encoder.
Speed loop
control
×
M
Encoder
Motor
Current loop
control
Position loop
control
×
Speed
calculation
×
Speed
feedback
Position
feedback
Reference
input +
-
+
-
+
-
Current
feedback
Servo drive
Figure 7-1
Structure of a basic servo system
The servo drive, which is the core of a servo system, serves to perform accurate position, speed and
torque control on a servo motor. It supports four control modes, namely position control, speed control,
torque control, and hybrid (combination among position, speed and torque) control. Position control is
the most important mode of a servo system.
Descriptions of the control modes are as follows:
■
Position control
In the position control mode, the target position of a motor is determined by the sum of position
references, and the motor speed is determined by the position reference frequency. The servo
drive performs quick and accurate position and speed control through the encoder installed on the
motor or an external encoder (full closed-loop control). The position control mode mainly applies
to applications requiring positioning control, such as the manipulator, SMT machine, engraving and
milling machine (pulse sequence reference), and CNC machine tool.
■
Speed control
In the speed control mode, the servo drive performs quick and accurate speed control through
the speed reference sent through communication. The speed control mode mainly applies to
application requiring speed control or where a host controller is used for position control or the
commands sent from the host controller are used as the speed references for the servo drive, such
as the engraving and milling machine.
■
Torque control
In the torque control mode, the motor current is in linear relationship with the torque. Therefore,
torque control is implemented through current control. The output torque of the motor is controlled
by the torque reference sent through communication. The torque control mode mainly applies in
applications requiring strict tension control. For example, in winding/unwinding devices, torque
references are used to prevent the material from being affected by the change in the winding radius.