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Position sensing, the electromagnetic way
By Design News Staff -- Design News, February 2, 1998
Determining position and orientation with Polhemus' FASTRAK system uses three orthogonal coils in a transmitter to generate a reference frame. Signals from these coils excite a smaller set of similarly-arranged passive sensor coils in a probe device. Digital signal processing then determines the orientation of each sensor coil relative to the transmitters. Data from the three sensor coils is then combined to give six-degrees-of-freedom coordinates of the sensor package. By knowing the position and orientation of the sensor package inside the probe, the location of the probe tip and the angular orientation of the probe is determined.
With TracerCAD's "pen" probe, according to Greg Pratt, not only is the probe tip position used but the slope of the pen as well. This allows direct gaging of surfaces rather than using more individual data points, which is more time consuming. "We use the entire probe as a shaping mechanism measuring points and flats," says Pratt.
Polhemus says accuracy of the system is 0.03 inch RMS with a resolution of 0.0002 inches/inch. Bill Panepinto, sales and marketing vice president, adds, "We are always looking to better these by software enhancements, closer calibration, and manufacturing technology improvements, such as closer coil tolerances for a more perfect geometric winding." Future developments: an RF link from the sensors to the system electronics unit for less cumbersome, cable-free measurement; increases in transmitter power to widen the area of coverage; and very small sensor packages to allow invasive medical measurements via placement on the tip of an endoscopic-type probe or even a unit that may be swallowed by the patient.
Other applications for the FASTRAK system include:
- Golf swing analyses using from four to eight sensors.
- In orthopedics for monitoring range of motion of patients
undergoing physical therapy.
- Motion picture character-animation uses include scanning character
and object models and giving them motion characteristics.
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