Friction makes fast moves to low cost

Tom Shelley reports on small ultrasonic motors with a very large commercial potential British designed ultrasonic motors are potentially of exceptionally low cost and robust, being made up of only three parts. Motors have been prototyped, 25mm and 2mm in diameter, with potential applications in products ranging from toys to lab-on-a-chip precision dispensing micropumps. The motor has been developed by Professor Roger Whatmore, Dr Stephen Wilson and Joni Leinvuo, members of the Nanotechnology Group at Cranfield University. Like other piezoelectric friction motors, the rotor of the 'Flextensional' motor is ratcheted round. The motor holds position when not powered and has zero backlash. Drive is from a PZT disk, which excites an edge supported diaphragm that amplifies the vibrations and acts on elastic fins on the rotor, which is held against the stator by a preload spring. In theory, the stator requires no axle to run through its centre. The motor could thus be used in situations requiring a very thin motor structure, or where it is desirable to separate the electric drive from the rotating element. There are many possible variants of the basic design. The stator for a silicon 2mm version is formed using a combination of thick film PZT technology under development at Cranfield and Deep Reactive Ion Etching (DRIE). A two stage DRIE process is used to create a silicon membrane that supports raised teeth and an axle structure. The rotor is made of nickel. In another 2mm design, the stator is a square plate, 2 x 2 x 016mm thick. The piezoelectric material is Ferroperm PZ26. The amplifier diaphragm is made of stainless steel, 40 microns thick and 180 microns high. The rotor is soft copper, 38 microns thick and 2mm in diameter. The drive signal is 0 to 10V at 224kHz. Pre-load force is 0.1g. Measured speed at 10V was 1300rpm, (drivable in three modes) The 2mm motors can accelerate to almost full speed within 0.5ms and stop within 0.1ms due to the thin, light rotor and high torque. On the basis of angular response, the developers believe the motors could achieve positioning resolutions in closed loop of a few hundredths of an arcsecond, which translate to a few nanometres at the periphery of the motor. The 25mm prototypes produce 10mNm torque, and rotate at 1 to 200rpm. Step distance is 1 to 5 microns at 60 t0 80kHz. Applications are seen in toys for children, disposable cameras and mobile phones, where they might be used to drive buzzers and to autofocus and zoom lenses. In cars, they could be used for mirror, gauge and vent positioning. They could also be used to drive the pointers on analogue dial watches, and to act as dispensing micropumps for medical dosing applications and lab-on-a-chip systems. Collaborators were: BAE Systems, Morgan Matroc, Renishaw, Cetehor (France), Oxley Developments, Mr. Roger Feneley, NPL, Cedrat Recherche (France) and KTH Sweden. The design has been patented and the project EPSRC grant funded for commercialisation. Professor Whatmore also gratefully acknowledges the financial support of the Royal Academy of Engineering. Nanotechnology Group at Cranfield University Pointers * Motors are flat, and made out of only three basic parts, making them potentially both robust and very low cost * In a 2mm design, a drive signal of 0V at 224kHz produces a measured speed of 1300rpm. * The 25mm prototypes produce 10mNm torque, and rotate at 1 to 200rpm. Step distance is 1 to 5 microns at 60 t0 80kHz.