Tiny Motors Make Big Moves

By Robert Repas

The plethora of shrinking devices spurred by new technologies need a special type of miniature electric motor that fits their small size. Instead of the typical magnet and coil construction, these motors use piezoceramic compounds that pull or push armatures into position using direct physical contact. Leading the charge in these miniature motors is PiezoMotor, a Swedish company allied with the Faulhaber Group in Germany and MicroMo Electronics in the U.S.

Two maxims about today’s technology are that it does more in a smaller package. Unfortunately, some things don’t scale well. For example, the power of a standard electric motor with magnets and coils drops off noticeably as it becomes smaller.

How it works
Piezomotors move due to piezoelectricity, a property of certain materials to generate an electric charge when placed under compression or tension loads. Jacques and Pierre Curie discovered that by compressing a crystalline material, such as Rochelle salt, they could create electricity. Of greater interest for motor applications is that the opposite is also true — an electric field placed over a piezocrystal changes the shape of the crystal. This ability to change shape is the basis for piezomotor technology.

Of course, Rochelle salt is no longer used. Instead, material scientists have

The motor shaft moves only nanometers for each step, but the motion can repeat thousands of times/ second. At that rate, the armature can actually move at linear speeds up to 100 mm/sec. Different models include designs for vacuum and nonmagnetic applications. Various sizes can handle pulling forces from one to several hundred Newtons. Moreover, the simple design supports mass production while still maintaining a high degree of precision.

Piezomotors are viable alternatives to standard dc motors, and in some cases may work better. Motion control in piezomotors can reach nanometer precision, a far greater resolution than available with dc motors. Dc motors become more expensive as they get smaller while piezomotors remain low cost in their size range. The direct linear drive offered by piezomotors removes the need for linear conversion of a dc motor’s rotary motion.

Though traditional miniature electric motors do a good job, there’s a limit to their practical precision, linear motion, and size reduction. Piezoelectric motors can shrink many designs now using conventional small electric motors. They can also be more precise, easier to control and adjust, lighter, and more reliable. For example, the Piezo Wave motor was originally developed for mobile phones. It’s now integrated into many applications including other handheld devices, medical technology equipment, electromechanical door locks, advanced toys, and cameras.

Of course the Piezo Wave’s small size is its obvious advantage. But the motor’s other qualities of robustness, motion dynamics, cost, power efficiency, and weight, makes it a contender for use in many products.

To help designers work with piezomotors, companies like MicroMo have demo kits available that contain the five elements of any piezomotor design: the piezomotor, a power supply, control electronics, a position sensor, and a demonstration mechanical interface. It’s astonishing what a small cube of ceramic material not much bigger than an ant can do.