Fundamentals ... (cont.)
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Position Servo-Control (Closed-Loop Operation)
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Position servo-control eliminates nonlinear behavior of PZT ceramics and is the key to highly repeatable nanometric motion.
PI offers the largest selection of closed-loop piezo mechanisms and control electronics worldwide. The advantages of position servo-control are:
- Very good linearity, stability, repeatability and accuracy
- Automatic compensation for varying loads or forces
- Virtually infinite stiffness (within load limits)
- Elimination of hysteresis and creep effects
PI closed-loop PZT actuators and systems are equipped with position measuring systems providing sub-nanometer resolution and bandwidths up to 10 kHz. A servo-controller (digital or analog) determines the output voltage to the PZT by comparing a reference signal (commanded position) to the actual sensor position signal.
PI closed-loop piezo actuators provide sub-nanometer resolution, repeatability and linearity to 0.003%.
For maximum accuracy, it is necessary to mount the sensor as close as possible to the part whose position is to be controlled. PI offers piezo actuators with integrated sensors as well as external sensors.
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Heat Generation in a PZT in Dynamic Operation |
As mentioned before, PZTs are reactive loads and therefore require charge and discharge currents that increase with operating frequency. The thermal active power, P (apparent power X power factor, cos j), generated in the actuator during harmonic excitation can be estimated with the following equation:
(Equation 23)
Heat generation in a PZT.
Where:
P = power converted to heat [W]
tan ∂ = dielectric loss factor (» power factor, cos j, for small angles d and j)
f = operating frequency [Hz]
C = PZT actuator capacitance [F]
Up-p = peak-peak drive voltage [V]
For the description of the loss power, we use the loss factor tan d instead of the power factor cos j, because it is the more common parameter for characterizing dielectric materials. For standard actuator PZT ceramics the loss factor is on the order of 1 to 2% (small-signal conditions only). In large-signal conditions however, 8 to 12% of the electrical power pumped into the actuator is converted to heat (varies with frequency, temperature, amplitude etc.). Therefore, the maximum operating temperature can limit the PZT dynamics. For large amplitudes and high frequencies, cooling measures may be necessary. A temperature sensor mounted on the ceramics is suggested for monitoring purposes.
In addition, a new generation of amplifiers employing energy recovery technology has been developed for high-power applications. Fig. 26 shows the block diagram of such an amplifier. Instead of dissipating the reactive power at the heat sinks, only the active power used by the piezo actuator has to be delivered. The energy not used in the actuator is returned to the amplifier and reused as supply voltage in a step-up transformation process. The combination of low-loss, high-energy PZT ceramics and amplifiers with energy recovery are the key to new high-level dynamic piezo actuator applications.
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Fig. 29. Closed-loop position control of a stage driven by a piezo actuator. For optimum performance, the sensor is mounted directly on the object to be positioned.
Fig. 28. Block diagram of a typical PI closed-loop PZT positioning system.
Fig. 27.Variety of digital piezo controllers
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