Elmomc SimplIQ Digital Servo Drives-Bell Getting Started Manual de usuario descargar pdf (Pagina 64)

The SimplIQ for Steppers Getting Started & Tuning and Commissioning Guide

MAN-BELGS (Ver. 1.1)

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

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

. 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.

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-

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Elmomc SimplIQ Digital Servo Drives-Bell Getting Started Manual de usuario Pagina 64