Comparing the results of non-encapsulated PICMA® stack actuators driven with a sine wave signal with those driven by a square wave signal (Table 2) shows that 2 to 3 times higher frequencies are achievable with sinusoidal operation. This is not surprising since the harmonic content in the square waves that contributes to the additional losses is missing from the pure sine wave operation. The strong dependence of the power loss on the drive voltage amplitude (~V²) can also be seen from the measured values in Table 2.
In addition, the different loss factors between pure unipolar control and semi-bipolar control is shown for identical amplitudes at 0V to 120 V and -15V to 105V. The closer the operation gets to the polarity reversal point, the more it effects the reorientations and dielectric hysteresis, increasing the effective loss factor (see Fig. 4b), and limiting the achievable frequency.
Air cooling allows 4-5 times higher frequencies compared to the confined condition without free convection. A further increase of nearly one order of magnitude is possible by liquid cooling of the encapsulated PICMA® actuators filled with heat conductive paste (Fig. 9). While air cooling allows frequencies up to 1kHz, 3.3kHz can be reached using liquid cooling. Even at amplitudes of 100Vpp (equivalent to 35μm displacement). the temperature only rises to 80°C maintaining a significant safety margin to the maximum allowable temperature of 150°C. The upper frequency limit in the experiment was determined by the bandwidth of the E-617KDYN amplifier (see Fig. 7b).
The high repetition frequencies allow very high cycle numbers to be reached within a short time interval. In life-time tests conducted on five samples of bare and encapsulated 5x5x36mm³ PICMA® stacks each, it was demonstrated that the actuators can run 1010 (10 billion) cycles without failures.
Tests conducted with encapsulated PICMA® stack actuators, filled with heat conductive paste, and operated with active liquid cooling demonstrated that continuous operation at full nominal displacement of 35µm is feasible with frequencies up to 3,300Hz, for sinusoidal drive signals. Higher frequencies could not be tested due to the power and bandwidth limitations of the E-617KDYN amplifier that was used in the experiments. For the best results in OEM applications, flexure piezo mechanisms, with integrated guidance, preloaded and motion amplifier, as well as position and temperature feedback, are recommended. The high performance of these readily available actuators can extend the limits of applications, such as high throughput micro dispensing, active vibration cancellation, and fast tool servos.
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