8. Closed-Loop Systems
Because of the nearly unlimited resolution of the motion, piezoelectrical actuators are excellently suited for high precision positioning in the µm range to the nm range. But because of hysteresis the correlation between the applied voltage and the actuator’s motion is not perfectly matched.
There are some applications in practice where a high resolution of the motion is necessary, but the absolute positioning accuracy is not. The classic example is the problem of fiber positioning. The light of one fiber has to be coupled most efficiently into a second fiber, the knowledge of the absolute position of the fiber is not important.
Another example: If it is possible to return after each positioning event (transaction) to the 0 voltage position, the hysteresis does not affect the event (see also chapter 3.8.).
Of course some applications demand a high positioning repeatability. This can be reached by combining piezoelectrical actuators with a measurement system. Because of their high dynamics, piezoelectrical actuators are well suited for a closed-loop system with a measurement system.
piezosystem jena uses a variety of measurement systems. With strain gauges it is possible to reach a position accuracy of 0.1 – 0.2%. Better results can be reached with special inductive and capacitive sensors.
You have to be careful working with a measurement system! Each measurement system always measures the motion at the place were the measurement system is located.
Variations between the measurement system and the point which should positioned (such as temperature effects), cannot be detected by the system.
piezosystem jena has developed piezo-elements with a measurement system and we have also developed complete electronic controllers with integrated closed-loop control. The closed-loop is controlled by a PI or PID regulation circuit; the actual position measured is shown on the display. (Figure 8.1.)
Figure 8.1. principle of the closed-loop control
Often it is not possible to widen a piezoelectrical element with a measurement system. Therefore it is important to investigate carefully if an integrated measurement system is really needed. A closed-loop system with sensors is more complicated and expensive than a piezoelectrical system without a measurement system.
If measurement is used in a closed-loop system, the full range of the motion will be smaller by about 10 – 20% to preserve the dynamic of the closed loop regulation.