High power piezoelectric axial shockwave generation

Electrical pulse excitation 5The electrical equivalent circuitry describes a piezoceramiccomponent mainly as a capacitor. Byapplying a huge charge impulse to this PZTcapacitor(= generator stack), a mechanical shockwavewill be produced. Our experiment aimed forthe characterization of large volume, high dielectricstack actuators for a maximum mecha-nical pulseenergy output.A typical arrangement was:Piezo-generator capacitance: order of magnitude10 µFImpedance of charging circuitry during pulseignition: 1 ohmEquivalent RC time constant: 10 µsThis time constant is significantly shorter than thetransition time of the mechanical pulse.Applied peak voltage: up to +800 VPeak current: up to 800 AIt has to be taken into account, that by high fieldexcitation, the effective capacitance is remarkablyhigher than the above stated small signal value.This effective capacitance must be explicitly derivedfrom the electrical charging parameters.For shock generation, short-term a huge powerlevel is needed in the order of magnitude 0.5 Megawatts.It was produced by the following circuitry (seeFigure 7).The HighVoltagePulser HVP consists of a condensatorbank of parallel and seriel high voltage capacitorswith a total capacitance of typically 500 µRF>> piezo-shock-generator's capacitance.This storing capacitor is charged up to a level up to+800 V by a standard power supply (not shown inschematic).By an IGBT power transistor switch (2), the piezoshock-generatoris coupled to this electrical chargereservoir. Due to the very low resistance of thiscircuitry, a fast charging pulse of the piezo-elementoccurs.In terms of maximizing the shock wave energy, twooperating modes have been compared, differing inthe start conditions for the electrical pulse:It is well-known from common piezo-actuators, thata much higher mechanical energy output isachieved by the so-called semi-bipolar operationinstead of the unipolar mode. "Unipolar mode"means, the electrical pulse is applied to adischarged piezoelement (pre-pulse voltage level 0,voltage step 0V/U max V).A dramatic increase of the mechanical power outputof the shock generator is achieved by a pre-pulseconditioning of the piezo-shock-generator by connectingit to a negative voltage power supply withlevels down to -200 V. (switch position 1). To onepart, this is simple caused by the wider voltage stepapplied to the actuator (voltage step -200V/+U max ).10