THORLABS Kinesis PFM450E, Руководство пользователя

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Rev E Mar 2022
Page 22
Chapter 4 PC Operation - Tutorial
4.3 Introduction to Open and Closed Loop Operation
The PPC001 Precision Piezo Controller has two distinct modes of operation: open
and closed loop mode. In open loop mode, the output voltage is controlled directly,
using a joystick, the external BNC input, the GUI or a combination of these. The output
voltage is proportional to the controlling source. This voltage controls the expansion
of the piezo actuator, turning the voltage into a certain amount of displacement.
The need for a closed loop mode of operation arises from the fact that the
displacement produced by the piezo actuator is not a linear function of the driving
voltage. All piezo actuators show a certain amount of nonlinearity, hysteresis,
temperature dependence and aging - see Appendix D for more details.
In precision positioning applications this can seriously limit the accuracy and
repeatability of the system. This problem can however be eliminated when closed
loop control is used. Closed loop mode assumes that the piezo actuator is fitted with
a position sensor, usually a strain gauge, that is capable of reading the actual position
and this signal is fed back to the piezo controller. Rather than outputting a constant
voltage, the piezo controller continuously monitors the position signal and, if
necessary, adjusts the output voltage to maintain the required position.
4.4 Tuning
Note
The PPC001 controller and PFM450 focus mount are a matched pair that have been
calibrated together for maximum accuracy. On power up, the controller checks that
it is connected to the PFM450 with which it was calibrated. If there is a mis-match,
the controller sends an error message to the server. Operating a controller with an
unmatched stage is possible, but the position readings will not be accurate.
Mismatched units can be returned to the factory for re-calibration, contact tech
support for more details.
Caution
Default PID values have already been optimized and stored within the stage/
controller pair, and these are loaded on power up.
If problems are encountered (e.g. stability of the closed loop position control)
the position PID parameters can be adjusted to tune the stage for the given
application. Normally, only minor adjustment of the Proportional, Integral and
Derivative parameters should be necessary, and some trial and error will be
required before the ideal settings for a specific application are achieved.
In general, adjust the notch filter settings to achieve stable performance, then
make minor adjustment to the PID parameters to optimize the performance.
See Section 5.2.3. for more details on these parameters.