Piezo legs lead march to miniaturisation

A miniature linear motor, not much bigger than an insect, has been developed which mimics the walk of an ant and can lift 1,000 times its own weight. The motor – Piezo Legs – was developed by Swedish firm PiezoMotor and is machined from a single solid piece and has an excellent power-to-weight ratio. The motor has pairs of elements arranged rather like the legs of an ant. By switching the power supply from one side of the pair to the other a walking like action can be simulated. Careful timing between the different pairs ensures there is sufficient contact with the ground for stability. As far as industrial applications go, Piezo Legs is most likely to be used turned over on its back and fixed into position. A rod is then rested on top of the legs, which when activated will move the rod back and forth to create linear motion comparable to conventional actuators. Piezoceramic materials are like tiny bimetallic strips in that they bend when an electric current is applied to them. Like a bimetallic strip, they consist of two or more layers of different ceramic materials bonded together and fixed at one end. Applying a current causes expansion of the layers, each by a different amount, which causes the free end of the piezo element to deflect. In practice, a piezo element does not consist of just two layers, but many, often up to a hundred, interleaved with layers of conductor material. These are only a few molecules thick down and laid down one at a time in a clean room production environment. Experience shows that in use they suffer from virtually no fatigue stress and so will never wear out. Conventional electric motors can be miniaturised but only to a point. And the smaller they get the more difficult they are to assemble. Piezotechnology is able to produce motors and actuators only a few millimetres in size. The demonstration units being used in the UK (by Unimatic Engineers based in London) are 20mm long, 5mm wide, 8mm high and weigh 20g. Unimatic told Eureka that the units are larger than usual to allow people to actually see them. Typical units are 5mm x 1mm x 2mm. The legs on the demo unit take 10,000 nanometre steps per second with positioning resolution of the same magnitude. Even allowing for a generous signal resolution time interval, the company claimed that position accuracy should be in the region of three to four step lengths, ie. about 10 nanometres. The thrust of the motor is around 10N, giving a power density unattainable by traditional electromechanical motors. The operating temperature envelope is –20°C to 70°C. The control electronics for Piezo Legs is virtually identical to those required for a conventional brushless DC servomotor. Unimatic's demo unit runs on a 4-48V supply. Each motor is custom-designed for its application: the power required dictates the size of the motor; the number of pairs of legs is increased to achieve greater stability; and the length of the legs determines their stroke or stride and their breadth their speed of actuation. NOTE:The piezo electric effect was discovered 125 years ago by Jacques Curie in La Rochelle, France, by working with local salt crystals. Originally, the intention was to create electricity by hammering the salt to change its atomic configuration, but eventually it was decided to reverse the process and apply electricity to create motion. Piezo is already used in a number of applications such as high performance speakers and ultrasound imaging. Modern diesel engines also use high pressure piezo-controlled fuel injectors to achieve low carbon emissions by using absolutely no excess fuel to go less than fully combusted. Super fast inkjet printers and the ignition spark on gas burners are also piezo-based.