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Servo System Overview
3.1
Feedback Device
Servo motors are available with these feedback units:
l
l
l
In a closed-loop feedback system, the innermost loop is the commutation loop, which monitors the motor's rotor
and ensures that it keeps spinning. Outer loops are: Position loop, Velocity loop and Current loop
Velocity information and the velocity loop are derived from (are computed based on) position information.
The current loop is also known as a torque loop, since amplitude of the electrical current is directly proportional
to torque. Torque is force applied in an axis of rotation.
Resolvers
The servo amplifier can use single (two poles) or multi-speed (multiple poles) resolver feedback to calculate
primary position, velocity, and commutation information. A resolver can be thought of as a transformer whose
output is unique for any given shaft position (an absolute position feedback). The transformer is driven with a
sinewave reference signal. Two AC signals are returned from the resolver into the Sine and Cosine inputs. All
three of these sinewave signals are low-level and susceptible to noise.
Encoders
Encoders direct pulses of light, from a light source at the motor or load, to photo detectors through an encoded
disk. These light pulses are then converted into digital feedback information. There are two general types of
encoders - rotary and linear. Rotary (rotating disk) encoders are typically mounted to the motor shaft. Linear
encoders are typically mounted to the load.
3.2
The Motion Profile
Overview
Motion operations are universally embodied in a graph called the motion profile. Understanding and using
motion profiles to define your motion application is an important part of achieving best system performance.
The motion profile plots one or more motion operations and measures it against time.
Commanded motion
the motion that is supposed to happen ideally and precisely, without error, when the motor executes a velocity
or position command
Actual motion
the motion that really happens in the motor, when a velocity or position command is executed
Closing the Gap between Setpoint and Actual
Best system performance is achieved when you can stabilize or "dampen" the difference or "close the gap"
between the commanded motion and the actual motion. This difference is called following error. Stabilizing the
servo system means setting the relevant parameters in the servo amplifier, to get as close to the commanded
position as possible.
Basic Motion Profile Characteristics
Commanded and actual motion profile shapes have the following characteristics that are also universal to all
motion operations:
Profile Characteristic
Moving
In Position
10
RESOLVER
®
ENDAT
compatible Heidenhain encoder
®
HIPERFACE
compatible Stegmann encoder
Meaning
Moving refers to the execution of a motion instruction that makes the motor
move. A motion profile's moving portion represents most of the profile - the
motion itself. The motor is considered moving for as long as the motion controller
is commanding new positions. The point at which motion stops is known as the
target position.
When a motion command stops executing, and the motor slows to within a few
counts of its target position, the motor is considered to be stopped, or "In
Position." A range of positions, typically plotted in a motion profile, represents in
position. That is, In Position is signaled when the motor gets close enough to the
target position -- within its In-Position range that you have specified, via its
parameter. An In-Position signal is often used to make sure the motor stops
before the machinery continues its operation.
07/03
BECKHOFF
Setup Software
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