24
6250 Servo Controller User Guide
Position
Response Types
The following table lists, describes, and illustrates the six basic types of position responses.
The primary difference among these responses is due to damping, which is the suppression
(or cancellation) of oscillation.
R e s p o n s e
Description
Profile (position/time)
Unstable
Instability causes the position to
oscillate in a exponentially diverging
fashion.
Position
Time
Over-damped
A highly damped, or
over-damped,
system gives a smooth but slower
response.
Position
Time
Under-damped
A slightly damped, or
under-damped,
system gives a slightly oscillatory
response.
Position
Time
Critically damped
A critically-damped response is the
most desirable because it optimizes
the trade-off between damping and
speed of response.
Position
Time
Oscillatory
An oscillatory response is
characterized by a sustained position
oscillation of equal amplitude.
Position
Time
Chattering
Chattering is a high-frequency, low-
amplitude oscillation which is usually
audible.
Position
Time
Performance
Measurements
When we investigate the plot of the position response versus time, there are a few
measurements that you can make to quantitatively assess the performance of the servo:
❏
Overshoot—the measurement of the maximum magnitude that the actual position exceeds the
position setpoint. It is usually measured in terms of the percentage of the setpoint value.
❏
Rise Time—the time it takes the actual position to pass the setpoint.
❏
Settling Time—the time between when the commanded position reaches the setpoint and the
actual position settles within a certain percentage of the position setpoint. (Note the settling
time definition here is different from that of a control engineering text book, but the goal of the
performance measurement is still intact.)
These three measurements are made before or shortly after the motor stops moving. When the
motor is moving to reach and settle to the setpoint, we call such period of time the
transient. When the motor is not moving, it is defined as in steady-state.
A typical stable position response plot in preset mode (
MCØ
) is shown below.
Setpoint
Commanded
Position
Actual
Position
Setpoint
Target Zone Mode
Settling Band
Rise Time
Transient
Settling Time
Overshoot
Steady State
Position Error
Steady State
Time
Position
