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Compact, Efficient Drive Circuit for a Piezoelectric Pump


Figure 2 illustrates the voltage and current waveforms at the terminals of the piezoelectric actuators. Suppose that C2 has been initially charged.At the beginning of the first half cycle, when a timing pulse occurs, the triac is turned on and current begins to flow from C2 through the triac and L2. When the magnitude of the current increases above the minimum hold current, the triac becomes latched on. The triac then continues to conduct until the current drops below the minimum hold current, which is essentially zero for the purpose of this application. The minimum hold current is reached and the triac turns off when the voltage on C2 reaches a negative peak near –450 V. The reversal of polarity is provided by L2. The precise magnitude of the peak reverse voltage is slightly less than |450 V| by an amount that depends on the energy lost in the various exchanges of energy involved in the reversal of polarity. While the triac remains off, C2 remains in its negatively charged state, in which only parasitic dielectric losses slowly reduce the magnitude of the voltage on C2.

Image At the beginning of the second half cycle, when the next timing pulse arrives, the process described in the preceding paragraph is repeated, except that the currents and voltages applied to C2 are reversed. Again, the difference between |450 V| and the magnitude of the peak voltage depends on the energy lost.

If the operation as described thus far were allowed to continue, the voltages would continue to decay and operation would halt after a number of cycles. To enable continuous operation, it is necessary to replenish the energy lost. This is accomplished as follows: After the completion of the polarity reversal of the second half cycle and before the arrival of the next timing pulse, the replenish-control subcircuit senses the triac cutoff and turns on Q1 to recharge C2 to 450 V. At the chosen 20-Hz pump frequency, the triac-off time is long enough to enable C2 to be recharged to 450 V by use of relatively low charging current, provided that the energy lost is relatively low.

Because recharging is done only during the positive half cycle, a dc offset is induced in C2; however, this offset is typically a small fraction of the peak drive voltage and does not adversely affect the operation of the piezoelectric actuators. The advantage of recharging only during positive half cycle is that the circuit can be less complex and contain fewer components than would be needed for recharging during both half cycles.

This work was done by Chris Matice of Stress Engineering Services Inc., and Frank E. Sager and Bill Robertson of Oceaneering Space Systems for Johnson Space Center. Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)} to Oceaneering Space Systems. Inquiries concerning licenses for its commercial development should be addressed to: Oceaneering Space Systems 16665 Space Center Blvd. Houston, TX 77058-2268 Refer to MSC-22887, volume and number of this NASA Tech Briefs issue, and the page number.