Conventional piezoelectric materials, such as PZTs, have reasonably high electromechanical coupling over 70%, and excellent performance at room temperature. However, their coupling factor (converting electrical to mechanical energy and vice versa) drops substantially at cryogenic temperatures, as the extrinsic contributions (domain wall motions) are almost frozen out below 130 K.

A step in translating the slider inside the channel that makes up the motor (from left to right). The orange color shows activated stacks inside a related fixture. The clamping elements have two clamping surface areas, and they are removed from the wall when the related stack is activated. The middle element contracts when the stack is activated, thus achieving translation along the channel.

The solution to this problem is to use effective cryogenic piezoelectric stacks that are operational at extreme temperatures, and particularly at cryogenic temperatures, using a flextensional design that can amplify the stack displacement. Significant advances have been made in developing relaxor ferroelectric single crystal piezoelectric materials (such as PMN-PT single crystal) that have much higher electromechanical coupling (over 90%) at room temperature. The properties remain high at extremely low temperatures because their properties are based on the intrinsic/lattice contributions. The heat generated during operation is minimized in order to reduce the heat burden on the thermal control of the instrument or local environment. The motor also has the ability to self-brake while unpowered.

Using these flextensional actuators in an inchworm motor configuration, this design dissipates very little thermal energy with a simple control of driving voltage and/or frequency, and has high stroke that is only limited to the length of the travel guides. In addition, because the flexure frame and guides can be made from the same material with simple structures, unlike conventional electromagnetic motors, the relative thermal mismatch can be minimized. Also, the conceived motor can provide significant precision with minimum displacements in the single-digit nanometer range.

A slider is driven inside a channel that consists of an expanding and contracting flextensional actuator sandwiched between two clamping actuators. The slider is made to move by sequentially releasing the clamping surfaces of the two clamps and alternately contracting or expanding the middle element, depending on the desired moving direction, and which clamp is activated.

This work was done by Stewart Sherrit, Mircea Badescu, Yoseph Bar-Cohen, Xiaoqi Bao, and Hyeong Jae Lee of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49541

This Brief includes a Technical Support Package (TSP).
Piezoelectric Actuated Inchworm Motor (PAIM)

(reference NPO49541) is currently available for download from the TSP library.

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Motion Control & Automation Technology Magazine

This article first appeared in the April, 2015 issue of Motion Control & Automation Technology Magazine.

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