Take a threaded nut, surround it with at least one pair of piezoelectric ceramic actuators, and run a threaded shaft through its middle. Apply phased current to each actuator to produce a small amount of strain. The nut squiggles, if you will. Vibrating at the resonant frequency of the system, the nut causes the threaded shaft to spin, moving it to and fro. This takes place without any magnetics, making the movement especially suitable for implantable drug pumps that risk being scrambled by MRI scans, says David Henderson, president of New Scale Technologies Inc., the company that recently received a patent on its Squiggle motor design.
Yet, the most promising application for the tiny motors (the smallest of which measures 3 mm in diameter) is focusing and zooming of mobile phone cameras. Their low power consumption and few parts (four total) make them attractive to an industry that's sensitive to size and costs, Henderson says. New Scale has set up a division specifically addressing this market and is developing a reference design.
The efficiency of the ceramic motors doesn't drop off with miniaturization, Henderson explains, something which plagues regular electromagnetic motors as their diameters reach a threshold of about 6 mm. Electromagnetic motors in this diminutive range devote more energy to developing heat than making motion, which strains batteries. Also, tiny electromagnetic motors have to spin pretty fast to develop much useable torque. That usually means a gear train is needed to manage the rpms.
The piezoelectric actuators are tuned to vibrate at the resonant frequency of the motor, Henderson says, which usually falls between 40 and 100 kHz. Vibration amplitude controls speed. Phase controls direction. The company is working with semiconductor manufacturers to produce a single chip ASIC drive and control electronics.
Resolution of the motor falls in the micrometer range, but sensors are necessary to produce repeatable steps, the company says. Either rotation or translation can be measured using inductive, Hall effect, or opto-interrupting sensing.
"Piezoelectric motors are considered key motors for the cell cam market," Henderson says, which is predicted to reach the 500 million mark within a few years. "With two motors per camera [zoom and focus] that's a billion motors," he adds.
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