ElmoMC SimplIQ Digital Servo Drives-Bell Getting Started User Manual
Page 64
The SimplIQ for Steppers Getting Started & Tuning and Commissioning Guide
MAN-BELGS (Ver. 1.1)
64
A.5.4 Evaluating a Step Response – Rise Time, Settling
Time, and Overshoot.
A step response is the waveform (position or speed) the motor exhibits when its
reference command (position or speed) changes abruptly. Step responses are not
very practical in real-life motoring applications, as the reference commands are
nearly always acceleration limited and many times smoothed.
A step response is, however, good to reveal the detailed dynamic behavior of the
controller. The most popular step-response figures of merit are:
Rise time: The time since the reference has been changed until the value
(position or speed) covers 90% of the step.
Settling time: The time since the reference has been changed until the value
(position or speed) remains permanently within 3% of the step.
Overshoot: The percentage of the deviation to the other side while stabilizing
the step.
These figures of merit are shown in Figure 57.
0.011 0.012
0.046
0.050
0.074
0
200
400
600
800
1000
1200
1400
Target
Time
Figure 57: A step response: The rise time is about 0.01, the settling time is 0.074, and the
overshoot is about 30%.
The overshoot level, as well as the ratio between the rise time and the settling time,
reflect the gain and the phase margins
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. Gain or phase margin results that are too
low may result in a high step response overshoot (more than 40%) followed by an
undershoot and a long settling time. If the phase margin is too high, the settling time
is too long. These properties are depicted in Figure 58 below which is a simulation
of three design examples: One with reasonable margins, one with margins that are
too low and one with phase margins that are too high.
7
The gain margin is the factor in which the controller gain can be increased until loosing stability. The
phase margin is the difference of the open-loop phase from -180 degrees at the point where the open-