User Manual
E727T0005, valid for E-727
BRO, 2019-06-28
Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Page 37 / 240
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email
Feedforward
In closed-loop operation, for positioning systems with piezo actuators the main component
regarding the tracking error (tracking error = commanded position – real position) is the phase
shift between the commanded position and the real position. By adding a feedforward signal to
the control algorithm this phase shift can be reduced.
For every axis, an input signal channel of the controller can be selected as feedforward signal using
the
Feed Forward Input Channel Index
parameter (ID 0x07030900; S1 in the figure below). This
way, an external preshaping sequence can be used to compensate for a known error, for example.
If no input signal channel is selected, the target signal is used as feedforward signal (value of
parameter 0x07030900 is zero).
The feedforward signal is fed in before the notch filters to avoid exciting the resonances of the
system.
Figure 15: Control structure for position control with feedforward signal
For every axis, the usage of the feedforward signal can be configured using the
Feedforward Gain
parameter (ID 0x07030600).
Notch Filters
The E-727 provides two notch filters per axis. The corrections by a notch filter only take place in
closed-loop operation by default, but can also be enabled for open-loop operation. The
appropriate frequency component is reduced in the control value to compensate for undesired
resonances in the mechanics.
The transfer function of a notch filter is as follows:
Where
G(s) is the transfer function of the notch filter
k is the bandwidth of the notch filter
s is the input signal
ω is the angular frequency, with ω = 2*π*f0, where f0 is the notch filter frequency in Hz
r is the notch rejection