Driven slowly at the speed of sound
Original article date: November 1997
A new form of small, low speed, high torque drive is now available in the form of an ultrasonic micromotor.
The principle of ultrasonic motors has been known for some time, but it is only very recently that motors have been able to be made commercially. By harnessing the characteristics of piezoelectric materials, an ultrasonic wave is generated which can develop a rotating or linear motion, depending upon the configuration of the materials used.
The progressive wave ultrasonic motor does not use electromagnetic action, which is the basic driving principle of conventional motors. Instead, the stator is composed of an elastic body and the piezoelectric ceramics are bonded to it, polarised in the direction through the thickness. They alternately expand and contract as voltage is applied. This causes a bending oscillation at the surface of the elastic body.
The two electrodes of the piezoelectric ceramics are formed with a quarter wavelength difference. As the different voltage phases (sine and cosine waves) are supplied to each electrode, the resulting waves combine to form a progressive wave on the surface of the elastic body. When pressure from the spring is applied on the rotor, rotational motion is created in the opposite direction to the progressive wave, as a result of frictional force. Since contact pressure is utilised, the shaft is maintained when electricity is not applied.
Now, EMS, having specialist knowledge of small rotating machines and notably DC micromotors, has embraced the rotary possibilities of piezoelectric motors and has unveiled an ultrasonic micromotor.
The first of a planned range is the USR30, a 30mm diameter motor which is only 9mm deep, but which delivers up to 50Nm torque at speeds up to 250rpm. This combination of motor size, design and output power cannot be matched by conventional DC motors, says EMS. Later models will extend to a 60mm motor capable of 400Nm torque at just 100rpm. Furthermore, the motors can be made very inexpensively and inherently electrically very quiet, satisfying EMC considerations.
Although the principle of the ultrasonic motor relies on a friction take-up of the ultrasonic wave motion, the motor is still rated to give an operating life of up to 2000h continuous operation at rated torque and speed. The friction feature also produces excellent holding torque, eliminating the need for the expensive electromagnetic clutches and brakes needed by conventional motors.
Characteristics of the ultrasonic motor
| (1) Quick response and excellent control: |
| Turn off in approximately 1ms |
| Capable of switching rotational direction within 10-90ms |
| (2) High holding torque |
| Maintain stationary with more than rated torque when electricity is not applied |
| Useful as self-holding function (mechanical brake) |
| (3) Quiet rotation |
| Noise is less than 45dB at 10cm from back side of motor. |
| (4) High degree of freedom in designing |
| Freedom in shape of motor (single direction shaft, two-direction shaft, hollow shaft, etc.) |
| Freedom in function and shape of driver |
| (5) Very low magnetism |
| Does not generate any magnetic field, Motors composed of non-magnetic materials are available |
| Motors which have magnetism of less than 20 gamma and operate in high magnetic fields of 10 000G are already in use |
| (6) Low speed and high torque |
| Capable of directly driving a load without using speed-reduction gears, etc. |
| (7) Compact, light, simple configuration |
| Absence of winding wires allows thin, lightweight structure. |
| (8) Progressive wave drive |
| Utilising a progressive wave as the driving source does not create cogging. |
| (9) Applicable in a vacuum |
| Motors driving in a vacuum environment are available |
| (10) Dust-free |
Typical applications
| (A) Applications as a linear actuator take advantage of compactness, low-speed, high torque, quick response and high holding torque |
| Substitute for pneumatic or hydraulic cylinders |
| Accurate positioning for coordinate plotter |
| Lens or mirror drive in optical instruments |
| Actuator for robotic auto-handlers |
| Parts carriers |
| Operation of shutters or blinds |
| Actuator for VCRs or CD plauers |
| Head actuator for floppy disk drives and hard disk drives |
| Servomotor for pen recorders, plotters |
| Printer head drives |
| (B) Applications that take advantage of compactness, low speed, high torque, and quiet operation |
| Operating roll screens |
| Film rewind mechanisms in cameras |
| Paper feeding mechanisms for printers, faxes and copying machines |
| Feeding mechanisms for telephone cards |
| (C) Other applications |
| High intensity magnetic fields (eg magnetic resonance imaging) |
| Flow regulating valves |
| Constant flow pumps |
| Tube pumps |
| Chucks and clamps |
- EMS
- Contact: Jeff Mead
- Tel: 01734 817391
November 1997