[Univ of Cambridge] [Dept of Engineering]

Stick-Slip Piezoelectric Actuators

T. Hack


Applications of piezoelectric actuators range from active reduction of seismic loads on buildings to the accurate positioning of optical systems in aerospace, automotive or other industries. Obviously actuator size, power and resolution vary depending upon the application.

This dissertation is concerned with stick-slip actuators that are capable of comparatively large motions and fine resolution. They have some advantages over existing actuators such as Inchworm motors and Picomotors. The stick-slip actuators that are investigated consist of two main parts: an outer holder and an inner sliding tube attached to a piezoelectric element that supports an inertial mass. The sliding tube is held in place by six ceramic balls whose contact forces with the tube are controlled by means of an adjustable spring. Friction between the balls and the tube is a key factor in their performance. To cause the sliding tube to move, the piezoelectric element is extended/contracted dynamically, according to an asymmetric waveform.

It is known that the performance of stick-slip actuators deteriorates when the frequency is increased, and also that the actuators move at different velocities for forward and reverse motion. This non-linear behaviour is explained by showing that the piezoelectric charge constant varies with the applied stress and is different in tension and compression. This variation can be minimised by preloading the piezoelectric element but this also has the effect of reducing the maximum achievable force.

The performance of three stick-slip actuators is investigated and it is found that the actuator based on the piezoelectric element with the largest extension and smallest capacitance performs best. The effects of introducing a second piezoelectric element that controls the friction force are investigated. Significant improvements are achieved both in terms of the velocity and of the actuator and the shape of the staircase motion, at some selected frequencies.

It is shown that the complex behaviour of a linear actuator can be modelled by using a single degree-of-freedom dynamic model which incorporates key non-linear features measured in the experiments.

A new type of stick-slip rotary actuator is investigated; angular resolution is as small as 0.2 arcsec and its angular velocity is large as 10 deg/s. Finally, a two degree of freedom actuator, that combines linear and rotary motion is presented.

Keywords: Stick-slip piezoelectric actuator, non-linear behaviour of PZT, stick-slip rotary actuator, 2d actuator, dynamic behaviour of PZT.

[Cambridge University | CUED | Structures Group | Geotechnical Group]

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