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Determining PRECISE POSITION
Magnetostrictive measurement essential to curing a painful back ailment
By Design News Staff -- Design News, June 25, 2006
Accurate measurements in filling mechanisms and dispensing instruments are common in professional tools, patient therapy and business machines. However, the requirements for accuracy increase dramatically in a surgical procedure — especially when the measurement affects an implant.
Working with medical design firm Devicix, Disc Dynamics developed a precise surgical tool to create an artificial nucleus for a minimally invasive surgical procedure, called the Dascor Disc Arthroplasty System. The implant restores the proper gap height in patients with Degenerative Disc Disease by injecting a two-part polymer to replace a portion of surgically removed disc between the patient’s vertebrae. The technique requires a precise amount of material and depends on an accurate measurement.
The initial technique used a pneumatic process for the injection, but proved difficult to control. A newer design uses a motor driving ball screw, monitored by a linear position sensor, to deliver an accurate quantity of the polymer. Disc Dynamics’ engineers chose Temposonics’ C-Series linear position sensor for ease of connection to the mechanical system, as well as eliminating three potential problems: wear, drift-over-time and periodic calibrations.
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The extremely small magnetostrictive sensor is 36 mm with an 18 mm dead zone and 21 mm null zone and 4 mm sensor shaft diameter. The resulting overall length-to-active zone ratio enables smaller device designs in desktop-size medical equipment. The linear position sensor, combined with a pressure sensor, provide the feedback for closed loop control of the injection process to dispense exactly the amount determined from a CT scan.
In the system, the C-Series provides position feedback for calibrating its forward and rearward travel limits, eliminating the need for redundant limit switches. In addition, the resolution and repeatability of the absolute output of the C-Series sensor eliminated an encoder initially used in the design.
Contact: Jesse Russell, MTS Sensors Division, jesse.russell@mts.com
Tel: 919-677-2314.
