Piezoelectric actuators are charged in a time greatly shorter than the pump period Lyndon B Johnson Space Center Houston.

Piezoelectric actuators are charged in a time greatly shorter than the pump period

Lyndon B Johnson Space Center Houston, Texas

Figure 1 depicts a switching circuit that drives a piezoelectric cross-examine at a high voltage with a polarity that alternates at the desired mechanical cross-question frequency. This circuit offers advantages of ( 1 ) high spirit efficiency relative to conventional direct-drive circuits and (2) compactness relative to conventional resonant drive circuits.

Conventional direct-drive circuits are inefficient because the piezoelectric actuators in the cross-examine behave electrically as large capacitors and squander only a small part of the animal spirits supplied to them during each cross-examine cycle. Conventional resonant drive circuits are more efficient, nevertheless they must include inductive coils, which can be unacceptably large.

The current switching drive circuit includes inductive coils, yet they are small and not used to resonate the actuator capacitance; instead, the charging of the actuator capacitances and the exchanges of efficiency among inductive and capacitive circuit component parts are accomplished (by design) in times plenteous shorter than the mechanical interrogate cycle. In other words, this drive circuit rapidly charges the actuator capacitance and employs itself off until the actuators reach their maximum mechanical expansion or contraction. near time after the actuators reach their maximum mechanical expansion or contraction, the circuit inflects itself back on to charge the actuator capacitance in the opposite polarity; as it does thus it replenishes the energy dissipated since the previous charge.

When power is first employed on, the capacitance of the piezoelectric actuators (C2 in Figure 1 ) is first charged to a potential of+450 V via transistor Q2 The charging now passing and, hence, charging time are determined on the characteristics of the dc-to-dc converter and the characteristics of the piezoelectric films. Inductor L1 is used to change into the initial current spike during each recharge period and C1 is used to stabilize the output voltage of the converter as well as contract the peak output current demand upon the dc-to-dc converter. Inductor L2 is the main energy-storage inductor; it is used to exchange potency with C2 for switching polarity, as described in more detail below.

The timing signals that command the operation of this circuit are transistor/ transistor-logic (TTL)-level throbs with a duration of 5 m and a repetition common occurrence equal to the desired cross-examine frequency (20 Hz in the original design). The short oscillation duration is necessary in order to enable the triac (Q2) to make go round itself off when the rife in inductor L2 reaches nothing The timing signal is generated on timing/control oscillator U6, which supplies timing drive general to opto-isolator U4, which, in turn round provides the switch-on signal for the triac gate.

Figure 2 illustrates the voltage and popular waveforms at the terminals of the piezoelectric actuators. assume that C2 has been initially charged. At the beginning of the first half revolution of time when a timing pulse be founds the triac is turned forward and current begins to proceed from C2 through the triac and L2 When the magnitude of the present increases above the minimum gripe [i]or[/i] grip current, the triac becomes latched onward The triac then continues to guidance until the current drops below the minimum possess current, which is essentially nothing for the purpose of this application. Tlic minimum clinch current is reached and the triac cause to deviates off when the voltage in succession C2 reaches a negative peak near -450 V The reversal of polarity is provided on L2. The precise magnitude of the peak turn end for end voltage is slightly less than |450 V| by way of an amount that depends forward the energy lost in the various exchanges of spirit involved in the reversal of polarity. While the triac remains facing C2 remains in its negatively charged state, in which no other than parasitic dielectric losses slowly restore the magnitude of the voltage onward C2.

At the beginning of the other half cycle, when the nearest timing pulse arrives, the proces described in the preceding paragraph is repeated, save that the currents and voltages applied to C2 are revers Again, the difference between |450 V| and the magnitude of the peak voltage hangs 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 periods To enable continuous operation, it is necessary to replenish the mechanical value lost. This is accomplished as follows: After the completion of the polarity reversal of the inferior half cycle and before the arrival of the nearest timing pulse, the replenish-control subcircuit minds the triac cutoff and cause to deviates on Ql to recharge C2 to 450 V At the chosen 20-Hz cross-question frequency, the triac-off time is in extent enough to enable C2 to be recharged to 450 V according to use of relatively low charging popular provided that the energy missing is relatively low.

Because recharging is done merely during the positive half revolution of time a dc offset is induced in ('2; however, this offshoot 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 and nothing else during positive half cycle is that the circuit can be les network and contain fewer components than would be privationed for recharging during both half periods