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Electronic Compass Design.

The Electronic Compass design was based on the 3DOF Sourceless Head Tracker   project described in the Virtual Reality Homebrewer's Handbook by Robin Hollands. Two Honeywell 91SS12-2 linear Hall effect magnetic sensors are mounted at right angles to each other. The outputs from the sensors are amplified using 1NA118P low power precision instrumentation amplifiers. Variable resistors enable the offset and gain of the amplifiers to be controlled.

In our design we have added a real magnetic compass. The LOHET sensors are placed at right angles to the compass needle and orientated north/south and east/west. The sensors thus sense the position of the needle instead of the earth's field. The needle produces a field which is approx. ten times greater than the earth's field and hence drift is reduced. We also use LOHETs which have been optimised for low drift.

The amplifier output voltages are sent via a two metre screened cable to the parallel port interface which consists of two ADC0804 analogue to digital convertors triggered for reading by control outputs from the port. Again preset resistors control the minimum and maximum voltages recognised by the A/D convertors. These are set at 0.5V and 4.0V. This range corresponds to 0-256 when the voltages are read by the parallel port as unsigned characters. The offset and gain controls are adjusted to produce a normal operating range of approximately 40-220 to allow for some drift which may then be automatically compensated for.

The full circuit diagram is shown below (to follow).

ęCopyright Hewlett-Packard 1997, 1998, 1999.
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