In a rotating travelling-wave motor, the annular stator is excited by piezoelectric actuators to produce bending vibrations in the form of circulating waves. The material surface points of the stator move along elliptical paths. At maximum deflection, these points have a velocity component in the circumferential direction, which sets the press-fitted rotor in rotation. The vibration is excited at a resonance frequency of the stator in the ultrasonic range. This results in vibration amplitudes of up to several micrometres, which are converted into continuous rotational motion via frictional coupling. Typical performance figures for modern travelling-wave motors include a torque of up to 2 Nm and speeds of around 100 rpm.

This operating principle gives the drives a number of favourable characteristics, making them advantageous alternatives to conventional electric drives in numerous applications. Of particular interest are their simple mechanical design and their ability to generate high torques and power outputs even at low speeds, which often eliminates the need for an additional gearbox. Further advantages include high holding torque when the power supply is switched off, high dynamics and high precision in fine positioning.

The precise generation of translational and linear motion is a key task in modern technology. Classic examples of applications include the positioning axes of manufacturing machines. In devices used in the consumer goods industry, such as CD players, it is also necessary to move trays linearly or to position the read and write heads of storage media.

Vibration drives can generate translational motion directly without a gearbox. This offers the following advantages over the conventional rotary motor-gearbox solution:

• no gearbox losses, resulting in high overall efficiency,
• fewer components,
• compact design,
• no backlash or play,
• good dynamics,
• position resolution in the range of a few nanometres.

Translational piezoelectric vibration actuators have also been commercially available since around the mid-1990s. To date, forces of up to 40 N and travel speeds of up to 300 mm/s can be achieved. Whilst this opens up a wide range of potential applications, this potential has not yet been fully exploited. Furthermore, the performance specifications are insufficient for many other tasks.

As part of the work carried out by our team, we first investigated the types of operation and designs of linear vibration drives that are theoretically feasible and practically viable. In this context, the ‘rower’ principle – whereby forces are applied at two opposite points on an actuator – was identified as promising. Using several prototypes, various designs were characterised in terms of their performance. Optimised drives are being designed, built and applied in a range of applications.