Control Properties / Definitions - Siemens SINAMICS G130 Engineering Manual

6.2.2

Control properties / definitions

Criteria for assessing
control quality
Rise time
Characteristic angular
frequency -3 dB
Ripple
Accuracy
Definition of key criteria for assessing control quality
Explanations, definitons
The rise time is the period which elapses between an abrupt change in a setpoint and the moment the actual
value first reaches the tolerance band (2 %) around the setpoint.
The dead time is the period which elapses between the abrupt change in the setpoint and the moment the actual
value begins to increase. The dead time is partially determined by the read-in, processing and output cycles of the
digital closed-loop control. Where the dead time constitutes a significant proportion of the rise time, it must be
separately identified.
The limit frequency is a measure of the dynamic response of a closed-loop control. A pure sinusoidal setpoint is
input to calculate the limit frequency; no part of the control loop must reach the limit. The actual value is measured
under steady-state conditions and the ratio between the amplitudes of actual value and setpoint is recorded.
-3 dB limit frequency: Frequency at which the absolute value of the actual value drops by 3 dB (to 71 %) for the
first time. The closed-loop control can manage frequencies up to this value and remain stable.
The ripple is the undesirable characteristic of the actual value which is superimposed on the mean value (useful
signal). Oscillating torque is another term used in relation to torque. Typical oscillating torques are caused by
motor slot grids, by limited encoder resolution or by the limited resolution of the voltage control of the IGBT power
unit. The torque ripple is also reflected in the speed ripple as being indirectly proportional to the mass inertia of
the drive.
Accuracy is a measure of the magnitude of the average, repeatable deviation between the actual value and
setpoint under nominal conditions. Deviations between the actual value and setpoint are caused by internal inac-
curacies in the measuring and control systems. External disturbances, such as temperature or speed, are not
included in the accuracy assessment. The closed-loop and open-loop controls should be optimized with respect
to the relevant variable.
Engineering Information
SINAMICS Engineering Manual – November 2015
SINAMICS S120
337/528
Ó Siemens AG