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Piezomotors on surgical robot make big moves

Dec. 18, 2013
Researchers at Worcester Polytechnic Institute used nonmetallic piezomotor actuators from MICROMO to make a robot compatible with the gigantic magnetic fields inside MRI scanners.
Worcester Polytechnic Institute’s robot, shown here on the test bench with a test fixture inside an MRI machine, uses plastic parts and piezoelectric motors that let it operate in the strong magnetic fields of MRI machines.

A completely nonmetallic surgical robot, able to work on patients lying in MRI machines, is under development at Worcester Polytechnic Institute, Worcester, Mass. The robot can position a high-energy, ultrasound probe exactly at a tumor site.

MRI machines generate powerful magnetic fields that make it impossible to use ferrous objects like steel leadscrews, permanent-magnet motors, gearboxes, and actuators. So a team headed by WPI Professor Greg Fischer fashioned positioning devices from actuators based on a piezoelectric ceramic.
Piezoceramics expand or contract under the influence of an applied voltage. Though most piezomotors provide motion on the order of a few millimeters, the MRI robot produces up to about 100 mm of linear travel or a continuous 360° rotation by using Piezo LEGS linear motors from MICROMO, Clearwater, Fla.

The linear piezomotors are complete self-contained units that include a drive rod. They use multiple sets of bimorphic drive legs, each leg consisting of two piezoelectric layers cemented together so an applied voltage makes one layer contract, the other expand. Applying a voltage makes the legs flex slightly.

The MICROMO Piezo LEGS motor is a complete module containing a drive rod and piezoelectric ceramic bimorphic legs. The legs consist of two piezoelectric ceramics sandwiched together. Applying a voltage to the leg forces one side to contract, the other to expand. The resulting motion is used to move a drive rod forward or back.

Alternating pairs of legs "walk" a drive rod forward in nanometer steps at speeds as high as 15 mm/sec. One set of legs always touches the actuator, making the actuators inherently safe (providing braking when unpowered) with the motors provide a holding force of up to 10 N.The WPI robot consists of one module with X, Y, Z translation and two rotational modules that correspond to the arc angles of a head frame. Linear motors handle a needle insertion and rotary motors drive aluminum leadscrews via 3D-printed custom pulleys and rubber or fiberglass-reinforced timing belts.

Although the masses involved are small, the materials generte more friction than conventional designs, so they need high torque to produce motion.

Electrical noise can garble MRI images. Noise was potentially problematic because of the sophisticated drive waveforms with a lot of harmonics necessary to drive the piezoactuators. So  Fischer's team developed custom controls employing an FPGA to adjust the relative frequency and phase of the drive signal waveforms.

FPGA signals go to high-speed d/a converters and then to high-bandwidth, high-voltage linear amplifiers. External filters remove electrical noise. An encoder provides feedback for closed-loop position or velocity control.

Up to eight of these boards, one per axis, sit in a modular backplane inside a shielded enclosure inside the MRI scanner room with the robot. A custom cable harness connects the controller and robot.

Fischer eventually wants to devise a toolbox for developing MRI-compatible robots that would incorporate sensors, actuators, controllers, amplifiers, and communication protocols.

To demonstrate the approach, the team is creating variations of the MRI robot for uses in treating brain cancer and Parkinson’s disease, and for performing biopsies of prostate cancers.

About the Author

Leland Teschler

Lee Teschler served as Editor-in-Chief of Machine Design until 2014. He holds a B.S. Engineering from the University of Michigan; a B.S. Electrical Engineering from the University of Michigan; and an MBA from Cleveland State University. Prior to joining Penton, Lee worked as a Communications design engineer for the U.S. Government.

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