26
HA0378T Rev C Feb 2019
Chapter 4
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 the GUI, a joystick, the external BNC input, 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 E 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 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 LPS710 piezo actuated stage are a matched pair
that have been calibrated together for maximum accuracy. On power up, the
controller checks that it is connected to the LPS710 unit with which it was
calibrated. If there is a mis-match, the controller sends an error message to the
APTServer. 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 5.2.2. for more details on these parameters.
