Position sensing to soothe the aching back

May 10, 2007
A surgical tool for injecting polymer into vertebrae discs relies on a magnetostrictive sensor for absolute position feedback.

Visible in the upper right-hand corner of the Disc Dynamics surgical tool is the C-Series magnetostrictive sensor used to maintain constant pressure on the polymer flowing into the nucleus of the disc. The business end of the automated syringe deposits polymer as illustrated in the image depicting a step in the surgical process.

Medical design firm Devicix, Chaska, Minn., designed the linear sensor into a surgical tool developed by Disc Dynamics, Eden Prairie, Minn. The tool is basically a linear actuator that controls the plungers on a dual-cavity syringe. Each cavity contains one portion of a two-part polymer. During surgery, the magnetostrictive sensor feeds back position information about the plungers. A motor controller uses the feedback to keep polymer flowing at a predefined pressure into an expandable polyurethane balloon inserted into the disc after the nucleus has been removed. The injected polymer creates an implant that conforms to the shape and size of the disc space.

The tool is used in a minimally invasive treatment for degenerative-disc disease called Dascor. The flowable polymer cures to create a firm but pliable implant. The process takes only minutes and allows for short operating times.

Devicix used the magnetostrictive sensor, a C-Series Temposonic sensor from MTS Systems Corp. Sensors Div., Cary, N.C., as a way to get absolute position information without incorporating limit switches for a homing procedure. In operation, the output from the C-Series sensor gets compared with that from Hall-effect switches on the permanent-magnet motor driving the actuator. This comparison verifies that the position system is working properly.

The C-Series' modular architecture is built on a core sensor apparatus to which additional application-specific features can be added. This keeps down costs and helps fit the sensor into products with tight packaging constraints.

The magnetostrictive technology in Temposonic sensors uses a sonic strain pulse induced in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a movable permanent magnet that passes along the outside of the sensor. The other field comes from an "interrogation" pulse applied along the waveguide. The resulting strain pulse travels at ultrasonic speed along the waveguide and is detected at the head of the sensing element.

Onboard electronics determine the position of the magnet by measuring the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse. Using the elapsed time to determine position of the permanent magnet provides an absolute position reading that never needs recalibration or rehoming after a power loss.

MTS Sensors Div.,

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