Introduction To Open And Closed Loop Operation - THORLABS K-Cube Piezo Controller User Manual

5.3 Introduction to Open and Closed Loop Operation

The KPZ101 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
potentiometer, one of the external inputs, the GUI or a combination of these three.
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
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.
Closed loop operation is only possible if an external position signal is available to
control the piezo cube. For the Thorlabs range of piezo actuators, the KSG101 strain
gauge reader is a fully self contained unit that is designed to work with the KPZ101 in
closed loop mode, providing the feedback signal necessary for closed loop operation.
5.3.1 Description of Open Loop Mode
Fig. 5.2 shows a simplified block diagram of the piezo controller in open loop mode.
Hub
Off
Hub Channel 1
Hub Channel 2
EXT SMA
Pot
DSP
USB
Fig. 5.2 Open Loop Schematic Diagram
DAC HV
K-Cube Piezo Driver
for more details.
SMB
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