Head tracking apparatus

A low cost head tracker for a virtual reality head set for determining the orientation of the head set relative to the earth's magnetic field includes a magnetic sensor responsive to the earth's magnetic field, and disposed on the head set and arranged with respect to a vertical axis of rotation of the head set to produce a displacement signal relative to the angular displacement of the head set with respect to a calibration orientation relative to the earth's magnetic field, and a signal processor connected to the magnetic sensor, and responsive to the electrical displacement signal for producing an output signal proportional to the orientation of the head set relative to the calibration orientation.

This invention relates generally to apparatus for determining the 
orientation of the head of the user of a computer display and more 
particularly to a low cost device for providing signals from which the 
orientation and tilt of a user's head can be determined, particularly for 
use in a virtual reality apparatus. 
Flux gate magnetometers are generally known. U.S. Pat. No. 3,840,726 
describes a system in which a flux gate magnetometer responsive to the 
earth's magnetic field is worn by a user, such as a foot soldier, to 
provide direction information to a dead reckoning computer that also 
includes an accelerometer. The system uses signals from the two sensors to 
determine distance and direction traveled by the user. The flux gate 
magnetometer portion of the system comprises a ting core upon which a 
drive winding and a first pair of output windings connected in series 
opposition are wound. A second pair of series opposition windings, 
orthogonal to the first pair, allows the magnetometer to produce signal 
outputs corresponding to the orientation of the earth's magnetic field 
vector H in two dimensions. A second magnetometer with a driving winding 
and one pair of output windings is oriented to provide a signal 
corresponding to the third direction. The system is designed solely to 
provide a signal that can be used to calculate the distance and direction 
in which the user travels. 
Flux gate sensors have also been used in vehicle compasses as shown in U.S. 
Pat. No. 4,425,717 and in sensors as described in U.S. Pat. No. 4,995,165. 
There is a need for apparatus that can be worn on the head of a user in a 
virtual reality system to determine the orientation of the user's head. 
U.S. Pat. Nos. 4,884,219 and 4,984,179 show a system in which a 
transmitter is used to generate a low frequency electric field, and 
sensors are mounted on a helmet to provide data both as to position and 
orientation of the helmet. U.S. Pat. No. 4,928,709 shows a device for 
measuring cervical range of motion that uses a conventional magnetic 
compass. 
U.S. Pat. No. 5,138,555 describes a helmet assembly having an unspecified 
sensing apparatus said to be coupled, for instance, by magnetic fields, in 
order that a signal representing positioning of the helmet assembly and 
the pilot's head be available to aiming apparatus. There is no detail. 
There is a need for an inexpensive, self contained head tracking system for 
use in combination with low cost virtual reality apparatus. Although flux 
gate magnetometers have been used for navigation and other purposes, they 
have not heretofore been recognized as a viable option for implementing a 
head tracking system that is self contained, requires no external 
transmitter, is low in cost, light weight, and reasonably accurate. 
Briefly stated and in accordance with a presently preferred aspect of this 
invention, a low cost head tracker for a virtual reality head set for 
determining the orientation of the head set relative to the earth's 
magnetic field includes a magnetic sensor responsive to the earth's 
magnetic field, and disposed on the head set and arranged with respect to 
a vertical axis of rotation of the head set to produce a displacement 
signal relative to the angular displacement of the head set with respect 
to a calibration orientation relative to the earth's magnetic field, and a 
signal processor connected to the magnetic sensor, and responsive to the 
electrical displacement signal for producing an output signal proportional 
to the orientation of the head set relative to the calibration 
orientation. 
In accordance with another aspect of this invention, the head tracker 
includes a second, tilt sensor on the head set arranged on an axis for 
measuring the angular tilt of the head set, the second sensor producing a 
signal proportional to the tilt of the head set. 
In accordance with another aspect of this invention, the signal processor 
combines the signals from the first and second sensors to produce an 
output signal proportional to the orientation of the helmet corrected for 
any error introduced in the displacement signal by any tilt of the head 
set. 
In accordance with a further aspect of this invention, the magnetic sensor 
comprises a magnetometer having a magnetic permeable core, a drive 
winding, and a first sense winding, and the signal processor is connected 
to the sense winding. 
In accordance with yet another aspect of this invention, the magnetometer 
comprises a second sense winding arranged in quadrature relative to the 
first sense winding, and the signal processor is connected to both the 
first and second sense windings. 
In accordance with a still further aspect of this invention, a signal 
processor for the head tracker includes a filter for passing high 
frequency components of this signals from the sense windings, an amplifier 
for amplifying the high frequency components, and a converter for 
converting the high frequency components to an output signal. 
In accordance with another aspect of the invention, the signal processor 
includes an oscillator connected to the drive winding and the converter. 
In accordance with yet another aspect of the invention, the converter 
comprises a detector synchronized with the oscillator. 
In accordance with a still further aspect of the invention, the signal 
processor comprises a filter connected to the converter for filtering the 
displacement signal. 
In accordance with yet a further aspect of the invention, the displacement 
signal is described by the equation V=M.sub.2 cos.theta.cos.phi.+M.sub.1 
sin.phi. where M.sub.1 is the vertical component of the earth's magnetic 
field, M.sub.2 is the horizontal component of the earth's magnetic field, 
.theta. is the orientation of the first sensor and .phi. is the 
orientation of the second, tilt sensor.

Referring now to FIG. 1, a head set for use in a virtual reality system or 
similar system is designated generally at 10. The head set includes a 
generally horizontally disposed main body 12 encircling the head 14 of the 
wearer, approximately at eye level. An arch shaped supporting portion 16 
preferably provided with a resilient pad 17 extends over the top of the 
head of the wearer to hold the body 12 in a fixed position. Preferably, 
the body carries one or two display subsystems 18, 20 positioned in front 
of the user's left and right eyes respectively so that they can be seen by 
the user when looking straight ahead, preferably in a binocular fashion so 
as to permit a three dimensional image to be displayed if desired. 
Preferably, the display subsystems are arranged close to the user's eyes 
so as to produce the illusion of a much larger display, such as a display 
that simulates a realistic situation such as a vertical aircraft 
simulation, game or the like. 
The displays themselves are relatively well known and as such form no part 
of this invention per se, except in combination with the other elements 
hereof. It will be understood that a cable, infrared or optical link or 
the like, between the head set and supporting external systems for 
generating the display, will need to be provided. A schematic 
representation of this portion of the invention is designated at 22. 
In use, the head set 10 is designed to remain fixed relative to the head of 
the wearer. That is, the relative position of the displays 18, 20 with 
respect to the wearer's eyes does not change. This simplifies the optical 
requirements on the display subsystem, but necessitates providing signals 
to the display processor that indicate any deviation of the user's head 
from a nominal calibration position. In low cost virtual reality systems 
of the type with which the present invention is especially useful, it is 
sufficient to provide signals representing the angular displacement of the 
user's head from an arbitrary calibration position. Absolute angular 
position is not needed. Preferably, the head tracker of this invention 
includes means for periodically calibrating the headset to a known 
calibration position. The calibration can be accomplished, for example, by 
instructing the user to look in a predetermined direction and press a key 
on a keyboard or a button on a controller, to establish the calibration 
position. Thereafter, relative displacements from the calibration position 
are sufficient to control the displays. 
The preferred embodiment of this invention includes as azimuth sensor 26 
and a tilt sensor 28. Preferably, sensor 26 is a flux gate magnetometer 
disposed on a central axis of rotation 32. Tilt sensor 28 which may be a 
magnetic sensor or a less expensive gravity operated sensor or any other 
sensor that can detect deviations from a horizontal position, is 
preferably disposed on the side of the helmet, most preferably on an axis 
extending through the head about which the head tilts. Depending upon the 
type of tilt sensor it may also be mounted in other positions, for 
example, on the top of the helmet together with the azimuth sensor. 
Together, azimuth sensor 26 and the tilt sensor 28 provide signals to the 
external system 22 that permit the orientation of the user's head to be 
determined on three axes. 
A block diagram of a flux gate magnetometer and the associated components 
for providing azimuth deviation signals from the magnetometer is shown at 
FIG. 2. A flux gate magnetometer 40 has a toroidal core 42 made from a 
high magnetic permeability material, such as ferrite. A drive winding 44 
is wound about the core and attached to a driver 46. The driver is driven 
by an oscillator 48 that produces a pulsating signal, preferably a square 
wave signal having a nominal frequency of 60 KHz. The frequency of the 
oscillator signal may vary over a relatively wide range, for example 
between about 1 KHz and 100 KHz. The driver 46 amplifies the oscillator 
signal to a level sufficient, when applied to winding 44, to repeatedly 
drive the magnetic core between a magnetically saturated state and an 
unsaturated linear state. 
First and second sense windings 52 and 54 are wound on the core 42. 
Preferably, the sense windings are arranged in quadrature so as to 
optimize the signal to noise ratio of the sensor. The sense windings may 
be wound over the drive winding. 
Each of the sense windings is connected to identical processing circuitry. 
We will describe only one of the processing circuits. Sense winding 52 is 
connected to a bandpass filter 60 which can be a high pass filter to 
reduce the cost. DC and low frequency components of the output of sense 
winding 52 are removed, and the remaining high frequency components are 
applied to amplifier 62, which amplifier preferably has adjustable gain. 
The output of amplifier 62 is an alternating sequence of positive and 
negative pulses, synchronized with the rising and failing edges of the 
square wave drive signal applied to the drive winding 44. The amplitude of 
the pulses is proportional to the angular position of the sensor. The 
pulses are applied to an input 64 of a sample and hold circuit 66. The 
sample and hold circuit has a gating input 68 that is driven by a timing 
circuit 70 that in turn is driven by oscillator 48. The output of 
amplifier 62 is sampled near the rising or falling edges of the square 
wave drive signal to provide a DC signal corresponding to the amplitude of 
the pulsating output signals produced by amplifier 62. The DC signal is 
connected to a filter 74 for removing any high frequency noise to product 
an output at terminal 76, which is a DC signal proportional to the angular 
orientation of the sensor. The elements of the second signal processing 
chain connected to winding 54 are designated by the same reference 
numerals primed and the functions are identical. 
The signal processor is shown in more detail in schematic diagram of FIG. 
3. A power supply decoupling circuit 80 provides power to the various 
circuit elements shown in FIG. 3, by means of connections that have been 
omitted for clarity. Oscillator 48 includes a counter timer whose 
frequency is determined by the combination of capacitor 84 and resistor 
86, and produces a squarewave output at terminal 88 that is connected to 
amplifier 46, which is configured in a common emitter configuration. The 
output of amplifier 46 is connected to a terminal 90 for connection to the 
flux gate magnetometer 40. 
The sense winding output of the magnetometer is connected to terminals 92 
and 94. Capacitor 96 forms a high pass filter connected to the sense 
winding, the output of which is connected to the adjustable gain amplifier 
stage 62, which includes an operational amplifier 102 and adjustable 
feedback resistor 104 for adjusting the gain. The output of amplifier 62 
is connected to input terminal 106 of a sample and hold detector 110. A 
trigger input 112 of detector 110 is connected to the output 114 of timing 
circuit 70, whose input 116 is connected to the output of oscillator 48. 
The output of sample and hold circuit 66 is connected to low pass filter 
74, whose output is connected to output terminal 120. The output of the 
identical second signal processing chain is connected to output terminal 
122. The output of timer 70 is connected to output terminal 124 for 
possible use by the signal processor. 
The manner in which the head tracker of this invention operates may be 
better understood by referring to FIGS. 4 and 5 which graphically 
illustrate the signals appearing at various stages of the head tracker. 
FIG. 4A shows the square wave output of oscillator 48 which, as amplified 
by driver 46, is applied to the drive winding 44 of magnetometer 40. FIG. 
44B shows the output from sense winding 52 as applied to the input of 
bandpass filter 60. The signal comprises a train of alternating positive 
and negative going pulses whose amplitude is proportional to the angular 
position of the sensor. 
The positive and negative going pulses are aligned with the leading and 
trailing edges of the square wave pulses applied to the drive winding. 
FIG. 4C shows the timing pulses applied to the sample and hold circuit 66 
by timing circuit 70. The timing pulses are aligned with the trailing 
edges of the square wave drive signal. 4D shows the output of filter 74 
which is a DC signal proportional to the angular orientation of the 
sensor. FIG. 5 shows the output signals appearing at terminals 120 and 122 
as a function of orientation of the sensor. Each of the signals varies 
sinusoidally with the output angular position with one signal leading the 
other by 90.degree.. This allows the angular orientation to be uniquely 
determined from the two output signals. 
The earth's magnetic field has a vertical component M1 and horizontal 
component M2. If the azimuth sensor is oriented with its access making an 
angle .theta. in the horizontal and a tilt angle .phi., then the output 
voltage V is described by V=M.sub.2 cos.theta.cos.phi.+M.sub.1 sin.phi.. 
The output is therefore a function of both the azimuth and tilt angles. As 
long as the user holds his head level, the tilt angle .phi. is 0, cos 
.phi. is 1, and sin .phi. is 0 and the equation becomes V=M.sub.2 
cos.theta., the tilt angle having no effect. If the tilt angle increases, 
the voltage deviates from a strictly sinusoidal function of the azimuth 
angle .theta.. If the tilt angle .phi. is measured independently, for 
example by fit sensor 28, then the azimuth .theta. can be determined even 
if the user tilts his head. 
Those skilled in the art will recognize that many modifications and changes 
may be made in the preferred embodiment without departing from the true 
spirit and scope of the invention which accordingly is intended to be 
defined solely by the appended claims.