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BECKHOFF
3
Servo System Overview
This topic provides a quick lesson in servo system - an overview of what it is and how it works.
What is a Servo System?
A servo system essentially comprises an intelligent servo drive and a servo motor that operates with a PLC or
CNC to perform complex, specialized moves in one or more directions, or axes. These complex and specialized
moves, which are needed in the automation of industrial tasks, are collectively known as motion control.
Servo systems are applied in many different field for automation - in the motor industry, the petrol industry, the
textile industry, in packaging systems, warehousing systems and so on.
Closed Loop Servo Systems
In a servo system, feedback information - motor position and motor velocity is sent from the feedback unit of the
motor back to the servo amplifier. The servo amplifier analyses the feedback, makes adjustments as needed,
and generates new currents to bring the motor to the commanded velocity. This cycle constantly repeats itself in
a closed loop. A closed loop that controls the position of the shaft or load is called a position loop. A closed loop
that keeps the velocity of the motor on the commanded value is called a velocity loop.
Servo System Components
A servo system consists of:
Servo motor
Load
Feedback device
Servoamplifier
(servo drives)
Setup Software
07/03
A servo motor moves machinery in a single axis of motion.
Electrical motors are driven by magnetic fields. Motors have a stationary field generated
by the magnets of the motor and a rotating or movable field called stator winding or
armature. They operate on the principles of synchronous motors. All rotary motors have
some type of bearing that supports the rotor at each end.
Every motor has at least two magnetic motor poles, normally four or six. The servo
amplifier generates the current in the stator so that a controllable torque is available at the
shaft.
The servo motors turn (travel) in two directions - positive and negative. Two forms of
angular measurement are commonly used in motion control - degree measurement and
radian measurement, where 360 degrees constitute one revolution or 2p radians.
The servo amplifier operates with standard synchronous servo motors as well as with
direct drive motors (rotary or linear). For more information about these motors see the
motor manuals.
Motor Stabilizing
Stabilizing (tuning) the motor is a fundamental task in achieving best system performance.
To stabilize a motor, you must set up initial values for and adjust several motion param-
eters using DRIVE.EXE. These parameter settings compensate for the difference
between the actual motion and the commanded motion - getting the actual as close to the
commanded as possible, with minimal oscillation and noise. This difference is called
following error.
The load is the machinery and equipment that each motor drives. It is everything
connected to the output shaft of a motor, including the shaft itself. A motor must be appro-
priately sized to its load to ensure the motor is powerful enough to carry out your
automation tasks. A servo system delivers and converts motion to a load via one or more
of the following mechanical techniques:
Direct drive
motor connected to a rotating table
Screw drive
motor connected to a lead screw carrying a slide (moving table)
Rack and pinion
motor connected to a cogwheel that moves a rack
Belt and pulleys
motor connected to rollers that move conveyor belts or chains and sprockets
Every closed-loop servo system needs at least one device to return feedback information
from each motor (or load) to servo drive. Depending on the feedback device, feedback is
transmitted back to the servo drive in the form of digital signals or analog signals. Two
types of feedback devices are supported:
Encoder - returns analog or digital signals (optical)
Resolver - returns analog signals (magnetical)
The servo drives comprise a three-phase, power supply, and high-performance control
unit all housed in a single enclosure. The several control loops are realize totally digital in
the micro controller.
Servo System Overview
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