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K-Cube Piezo Driver
The APT User utility will be used throughout the rest of this tutorial to interface with
the piezo controller.
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 combi nation 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 certai n amount of
displacement.
The need for a closed loop mode of o peration arises from the fact that the
displacement produced by the piezo actuator is not a lin ear function of th e driving
voltage. All piezo actuators show a ce rtain 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 th e system. Thi s 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 pi ezo controller. Rather than outputting a constant
voltage, the p iezo controller continuously monitors the po sition 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 avai lable 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.
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