EUREKA MARCH 2002 COVER FEATURE STORY

Piezo actuators turn it on in two dimensions

Tom Shelley reports on low cost piezoelectric actuators that can work valves, power linear and rotary stepper drives, and position in 2D



Low cost, planar, piezoelectric bi-morphs, first revealed in Eureka's August 1998 edition, have now reached the point where they can be used to turn rotary gas valves, drive linear or rotary stepper motors and position to points in a 2D plane 100 times per second.

Conceived as a means of driving low cost and ultra compact circuit breakers, packaged single and double element devices are to be launched next month. Derived valve actuators and 2D positioners, presently demonstrable on the lab bench, will follow close on their heels.

Applications range from circuit breakers, where they are already well established, to drivers for door locks, car headlamps actuators and possible micro crawling and walking robots.

The basic idea of using a tuning fork shaped piece of metal strip, coated with piezoelectric material, is now well established. Anchoring the foot of one leg and allowing the other to move allows the achievement of much larger linear movements than a simple beam. When the devices were first announced, inventor and PBT technology director Simon Powell said they were criticised because of the high voltages required to drive them, the fragility of then existing products and their cost. Having perfected the bi-morphs for circuit breakers, his team has turned them into a range of suitably packaged and protected actuators with integrated circuitry powered by 9V batteries. Already available to order, they will be launched officially at the forthcoming Hanover Fair as the 'Servocell' range. Currently available in single or double actuator element versions, mounted on driving PCBs or separate, at a volume sale price of around 20 each, a linearised version capable of straight up and down movements is also on offer. And a version with closed loop feedback control to attain a positioning accuracy of 30 microns will be announced at the show, priced around 25 each for volume sales.

Producing a linear movement with a piezoelectric actuator is no mean task. Powell says that lead zirconate titanate, the favoured material, is highly non linear and shows a strong temperature dependence in its performance as well as hysteresis and creep. But these drawbacks can be overcome by using compensatory electronics.

"We can't sell them under the 'plug and play' label since somebody else has this, but that is exactly what they are," he says. His development team is known within PBT and elsewhere by the acronym 'pitgoadi' which stands for 'plan it, then go out and do it', One example of this proactive approach is a demonstration piezoelectric door lock. Other possible applications include robotic grippers, dosing pumps, machine tool interlocks and weaving machines.

The linearised actuator can switch position at up to 20Hz and others at up to 50Hz. Forces are up to 0.5N and movements are several mm. Two actuators side by side can be used to drive a friction motor, one raising and lowering the axis of a bell crank, the other operating the bell crank arm.

The 2D positioner is made from an initially flat, single, tuning fork shaped piece of strip in ingenious fashion by mimicking the Japanese art of origami. One set of folds turns the strip into two tuning forks, but with the strips positioned vertically above each other instead of side by side. A second fold places the two forks at 90 degrees to each other. One end of one fork is anchored, while the other end of the other fork is free to move.

The free end can then be moved within an area several mm square at up to 100Hz. Sine and cosine signals applied to the piezo material on the two forks can be used to make the joined corner move in a circle. This can be used to drive a motor shaft through a miniature crank. It delivers a lateral force of up to 0.25N, but this is more than enough for applications such as positioning the end of an optical fibre. Power consumption is about 1mJ to achieve a single, full-scale deflection.

Another application for both 1D and 2D actuators is the control of pneumatic and gas valve orifices. PBT is already able to demonstrate a proportional valve with a 1mm orifice, working at mains pressure, which has the added advantage that it retains its position with power off. In this design two piezo actuators are used to move a cam. Should the device be required to fail to a closed condition, this can be achieved by storing charge in a capacitor sufficient to achieve closure. Alternative designs, based on stepper motors or solenoids, have to incorporate a spring return. This requires significantly stronger actuation with increased power consumption. The PBT team has also developed a 5mm orifice valve, operated by similar technology. We have been asked not to disclose either what or who this is for, but we can say that the application called for a new kind of valve, in addition to the new actuator, in order to eliminate O rings and so reduce friction.

Further applications are limited only by the imagination. For example, six sets of three 2D actuators would be sufficient to power a six legged crawling or walking robot. Such devices have for some time been postulated for inspection inside small spaces, internal surgery in humans, and military applications - which could give a new meaning to the word 'bugged'. Planar actuators could be stacked together to produce increased forces, and actuator and cam movements could also be used for car ventilation system control.

All the above developments are covered by patent applications and patents granted. (More information at www.pitgoadi.co.uk )

Design Pointers

Single and double elements piezoelectric bi-morphs are available as programmable, packaged units running off 9V power supplies. Strokes are up to several mm, forces are up to 0.5N, and response frequencies up to 50Hz. Positioning accuracy with closed loop feedback is 30 microns

Pneumatic and gas valves with piezoelectric stepper drives can be made to open and close orifices up to 5mm across

2D positioning can also be undertaken with a new device over an area several mm square. Response is up to 100 Hz and the device can be used to drive a motor shaft. Energy consumption is about 1mJ per full scale deflection

BACK