Designing Lead-Free Piezo Motors

PI R&D engineers investigated a different approach: rather than looking at the ceramic material alone, they redesigned the actuator structure around the characteristics of lead-free ceramics. The result is a four-layer ultrasonic actuator designed to improve thermal behavior while reducing the required drive voltage.

An ultrasonic piezo motor operates close to the mechanical resonance of its actuator. This makes changes in resonance frequency important. With lead-free bulk ceramics, heat generated during operation can shift the actuator's eigenfrequency. The drive electronics then have to compensate for a moving operating point. At high duty cycles, this can affect motion stability and make continuous operation more difficult.
The actuator developed in this work uses copper-doped potassium sodium niobate (KNN) ceramics.

Individual ceramic plates measure 25 × 11 × 1 mm. Four of these plates are incorporated into a composite ultrasonic actuator (PILine piezo-motor family).

The important part of the design is how those layers are assembled.

The four lead-free ceramic layers are bonded to cavity-embedded elastic elements made from copper, with an alumina pusher forming the contact element.

The elastic elements perform several functions. They provide the mechanical structure required by the actuator, improve thermal conduction, and incorporate cavities that help dissipate heat. Their geometry and clamping positions were also designed to minimize unwanted oscillation amplitudes and associated mechanical losses. The actuator was installed in a metal housing and preloaded against the runner bar of a U-551K005 test stage.

This is an important distinction from simply substituting a lead-free piezo ceramic into an existing ultrasonic motor. The ceramic, mechanical structure, thermal path, clamping arrangement, and drive conditions were considered together.

Impedance measurements were performed with the actuator mounted on the stage under a 10N preload. The measured eigenfrequency was 194.3kHz. The stage was then operated continuously using the controller's auto-frequency function. The search range was set from 192 to 196kHz while the controller tracked the optimum operating frequency. At closed-loop velocities of 20 and 40mm/s, the required motor output remained below 60% of the controller's maximum allowable voltage during ten minutes of continuous operation. The optimum operating frequency moved toward 196kHz, leading the researchers to investigate fixed-frequency operation as well.

Fixed-frequency testing provides a useful indication of whether thermal changes continue to move the operating point. At a closed-loop velocity of 20mm/s, frequencies from 196 to 198kHz were tested over ten minutes. The required voltage remained relatively constant after the initial operating period. The authors attribute this behavior to the improved thermal conductivity of the composite structure and the cavities between the layers.  

This addresses one of the main differences with lead-free ultrasonic motors. If temperature causes a large resonance shift, the motor and controller must continually compensate. Reducing that shift makes the actuator considerably easier to operate as part of a precision motion and positioning system.

Thermal stability alone is not sufficient. The actuator also has to provide useful mechanical output.

The researchers measured push/pull force using a rope-and-roller arrangement with dummy loads. The U-551K005 test stage delivered 4N against gravity at closed-loop velocities of 10, 20, 40, and 60 mm/s.

Tests were performed at 192 and 194kHz, with the required voltage remaining below 70% of the controller's maximum drive voltage.