ATLANTA – Piezoelectric materials that allow a robot’s camera eye to replicate the muscle motion of a human eye could help make safer, more effective robotic tools for MRI-guided surgery and robotic rehabilitation.
The biologically inspired technology, developed by Joshua Schultz and Jun Ueda of the George W. Woodruff School of Mechanical Engineering at Georgia Institute of Technology, could lay the groundwork for investigating research questions in systems that possess a large number of active units operating together.
“For a robot to be truly bioinspired, it should possess actuation, or motion generators, with properties in common with the musculature of biological organisms,” said Schultz, a doctoral candidate under the direction of assistant professor Ueda. “The actuators developed in our lab embody many properties in common with biological muscle, especially a cellular structure.”
Muscles in the human eye are essentially controlled by neural impulses, Schultz said. The actuators under development will capture the performance and kinematics of the human eye.
Piezoelectric materials expand or contract when electricity is applied to them, providing a way to transform input signals into motion. This is the basic principle for piezoelectric actuators that have been used in various applications, but their use in robotics has been limited because of the piezoelectric ceramic’s minuscule displacement. The cellular actuator concept developed by the researchers connects many small actuator units in series or in parallel. Their lightweight, high-speed approach includes a single-degree-of-freedom camera positioner used to illustrate and understand the performance and control of the technology.
“Each musclelike actuator has a piezoelectric material and a nested hierarchical set of strain amplifying mechanisms,” Ueda said. “We are presenting a mathematical concept that can be used to predict the performance as well as select the required geometry of nested structures. We use the design of the camera positioning mechanism’s actuators to demonstrate the concepts.”
Their work shows mechanisms that can scale up the displacement of piezoelectric stacks to the range of the ocular positioning system. Previously, such stacks for this purpose were too small.
During their experiments, the researchers sought to resolve a previous obstacle. A cable-driven eye can produce the eye’s kinematics, but rigid servomotors make it impossible to test the hypothesis for the neurological basis for eye motion.
Although some measure of flexibility could be used in software with traditional actuators, it would depend largely on having a continuously variable control signal, and it could not show how flexibility could be maintained with quantized actuation corresponding to neural recruitment phenomena.
“Unlike traditional actuators, piezoelectric cellular actuators are governed by the working principles of muscles – namely, motion results by discretely activating, or recruiting, sets of active fibers, called motor units,” Ueda said.
“Motor units are linked by flexible tissue, which serves a twofold function. It combines the action potential of each motor unit and presents a compliant interface with the world, which is critical in unstructured environments,” he added.
The researchers presented a camera positioner driven by a novel cellular actuator technology, using a contractile ceramic to generate motion. They used 16 amplified piezoelectric stacks per side, which addressed the need for more layers of amplification. The units were placed inside a rhomboidal mechanism.
The work offers an analysis of the force-displacement trade-offs involved in the actuator design and shows how to find geometry that meets the requirement of the camera positioner, Schultz said.
“The goal of scaling up piezoelectric ceramic stacks holds great potential to more accurately replicate human eye motion than previous actuators,” he said. “Future work in this area will involve implantation of this technology on a multidegree-of-freedom device, applying open- and closed-loop control algorithms for positioning and analysis of co-contraction phenomena.”
Details of the research, funded by the National Science Foundation, were presented this summer at the IEEE International Conference on Biomedical Robotics and Biomechatronics in Rome.