Technical Explanation for Servomotors and Servo Drives
3
Sensors Switches Safety Components Relays Control Components Automation Systems Motion / Drives
Energy Conservation Support /
Environment Measure Equipment
Power Supplies /
In Addition Others Common
Principles
Servo Operation and Configuration
A system built with servo drives and servomotors controls
motor operation in closed loop. The actual position, speed, or
torque of the servomotor is fed back to compare to the
command value and calculate the following errors between
them. Then the servo drive corrects the operation of the
servomotor in realtime using this error information to ensure
that the system can achieve the required performance. This
cycle of feedback, error detection, and correction is called
closed-loop control.
The control loop is processed by either of servo drive or
motion controller, or both depending on the required control.
The control loops for position, speed, and torque are
independently used to achieve the required operation.
Applications will not always require all three control loops. In
some applications, only the control loop for torque control will
be required. In other applications, current and speed for
speed control are required, and in still other applications,
three control loops for position control are required.
Servomotor
The most common types of industrial servomotors are those
based on brushless motors. The rotor has a powerful
permanent magnet. The stator is composed of multiple
conductor coils, and the rotor spins when the coils are
powered in the specified order. The movement of the rotor is
determined by the stator’s frequency, phase, polarity, and
current when the correct current is supplied to the stator coils
at the appropriate time.
Encoder
Servomotors are different from typical motors in that they
have encoders. This allows high-speed and high-precision
control according to the given position and speed commands.
Encoders are one of the hardware elements that form the core
of a servo system, and they generate speed and position
feedback. In many cases, the encoder is built into the
servomotor or attached to the servomotor. In certain
applications, the encoder is an independent unit that is
installed away from the servomotor. When the encoder is
installed in a remote location, it is used for related parameters
in addition to for control of servomotor operation.
Encoders are divided into two kinds.
• Incremental encoders
• Absolute encoders
Multi-turn absolute encoders are typically used for
servomotors.
Refer to the Technical Explanation for Rotary Encoders for
more information on encoders.
Servo Drive
Servo drives follow commands from the host controller and
control the output torque, rotation speed, or position of
motors.
The position, speed, or torque are controlled according to
inputs from a motion controller, feedback encoder, and the
servomotor itself, and the servo drive supplies the appropriate
amounts of power to the servomotor at the appropriate times.
The basic operating principle is the same as for an inverter, in
which the motor is operated by converting AC power to DC
power to be a certain frequency.
A servo drive also has the following functions.
• Communications with the motion controller
• Encoder feedback reading and realtime closed-loop control
adjustment
• I/O processing for safety components, mode inputs, and
operating status output signals
Speed control
Control
section
Oscillator
Error
counter
Motor
Encoder
Position
control
Position
command
Speed
command
Position feedback
Speed
feedback
Current
feedback
Torque control
Position
loop gain
Error/
speed
conversion
Frequency/
speed
conversion
Multiplication
Drive
Speed loop gain
Speed loop integral time constant
Motor shaft
Case
Encoder
Rotor
Permanent
magnet
Stator coils
Speed,
position,
or torque
Control circuits
PWM
Encoder
Motor
Smoothing
circuit section
Inverter
section
Converter
section
Servo drive
Fixed frequency
(50/60 Hz)
Required frequency
(0 to 400 Hz)