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Posted on 23 July 2019

Highly Miniaturized Piezoactor Drive

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Capacitive loads require customized drive circuits

Piezoelectric actuators have found their way into series production. The generation of their driving energy is a new task for power electronics

By Thomas Baumann and Dr. Eckart Hoene, Fraunhofer-Institut IZM

 

One key feature of piezoelectric actuators is their potential for miniaturization. Besides the known applications like ink jets new ones coming up. Micro dosing pumps for liquids as lubricants or chemical additives are examples or systems in the human body implanted for medicine dosing systems or actuators for surgical requirements. Typical for these applications is the need of voltages in the range of 100 V up to kVs with alternating polarities. Electrical the piezoactor acts like a capacitance and its degree of mechanical deviation is controlled by the voltage applied to it. Therefore the needed currents are rather low. Up to now the circuitry to generate these voltages was an order of magnitude bigger than the actuator.

“Fraunhofer Institute für Zuverlässigkeit und Mikrointegration” (IZM) in Berlin has developed a highly miniaturized DC/AC converter for driving piezoelectric actors. Triggered was the idea by the IZM in Munich, where a piezo micro pump was developed with a size of 7 mm x 7mm x 1.1mm. The volume of the complete pump, i.e. piezoactor, electronics and power supply should be below 10 mm x 10 mm x 10 mm.

Requirements for driving circuit:

In principle the driver is a DC/AC converter. The input voltage is 3,7 V to be used in mobile applications. The output voltage is an ac voltage. The amplitude is maximum 150 V and minimum -40 V. The output frequency is variable up to 80 Hz.

Realization of a DC/AC converter with a size of just 7 mm x 7 mm was possible by developing a new topology. It is a combination of a boost converter and a buck boost converter, see Figure 1.

Topology of driving a piezoelectric actor

Boost and Buck Boost converter use only one inductance and are driven separately. The boost converter boosts the load potential up during one half period, while the buck boost converter is disabled. The other way round, boosts the buck boost converter the load potential down during the second half period, while the boost converter is disabled. The two transistors T3 and T4 decouple the boost from the buck boost converter. The pulse pattern for controlling T1 and T2 is generated by a microcontroller. The software also controls the maximum and minimum output voltages. Figure 2 shows output voltage and input current of the prototype in action.

Measurement results of output voltage and input current of a prototype

The small amount of components for the circuit makes a realization of the electronic within a volume of 7 mm x 7 mm x 3.5 mm possible.

PCB of prototype next to a sugar cube

3D Cad picture of prototype driver for piezoactors

Advantages:

The Topology is able to provide an ac output voltage. The maximum and minimum output voltages as well as the converted power are dependant on the hardware configuration. The prototype has a volume of 7 mm x 7mm x 3,5 mm. Accommodation to higher power levels are realisable. The small amount of components guaranties optimal volume. A Patent is applied for the topology.

 

 

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