Efficient orientation estimation system using magnetic, angular rate, and gravity sensors
First Claim
1. An efficient orientation estimation system using magnetic, angular rate, and gravity sensors, comprisinga three axis magnetometer configured to be attached to an object, wherein said three axis magnetometer generates magnetic field data represented in a sensor reference frame;
- a three axis rate gyroscope configured to be attached to said object, wherein said three axis rate gyroscope generates angular velocity data represented in said sensor reference frame;
a three axis accelerometer configured to be attached to said object, wherein said three axis accelerometer generates acceleration data represented in said sensor reference frame, wherein said acceleration data includes a gravity measurement;
at least one processor comprising a tangible memory medium, coupled to said three axis accelerometer, to said three axis rate gyroscope, and to said three axis magnetometer, and configured todefine an Earth reference frame for an area in which said object is located, comprisinga unit vector ĵ
pointing vertically upwards and opposite to a direction of gravity;
a unit vector {circumflex over (k)} in a horizontal plane perpendicular to said unit vector ĵ
, and pointing in a direction opposite to a horizontal component of Earth'"'"'s magnetic field in said area;
a unit vector î
perpendicular to said unit vector ĵ and
perpendicular to said unit vector {circumflex over (k)};
wherein said unit vector î
, said unit vector ĵ
, and said unit vector {circumflex over (k)} form a right-handed reference frame;
define a sensor orientation as a rotation that rotates a vector in said sensor reference frame into a corresponding vector in said Earth reference frame;
set said sensor orientation to an initial sensor orientation;
iteratively calculate an updated value of said sensor orientation asobtain said angular velocity data from said three axis rate gyroscope;
obtain said acceleration data from said three axis accelerometer;
normalize said acceleration data to form a normalized acceleration having its vector length equal to one;
obtain said magnetic field data from said three axis magnetometer;
generate an Earth to sensor rotation as an inverse of said sensor orientation;
rotate said unit vector ĵ
by said Earth to sensor rotation, to form a gravity prediction in said sensor reference frame;
apply said Earth to sensor rotation to a negative of said unit vector {circumflex over (k)}, to form a horizontal magnetic field prediction in said sensor reference frame;
project said magnetic field data onto said normalized acceleration, to form a vertical magnetic component;
subtract said vertical magnetic component from said magnetic field data, to form a horizontal magnetic component;
normalize said horizontal magnetic component to form a normalized horizontal magnetic component;
calculate a vector cross product of said normalized acceleration with said gravity prediction, to form a gravity errorcalculate a vector cross product of said normalized horizontal magnetic component with said horizontal magnetic field prediction, to form a magnetic field error;
calculate an orientation derivative based onsaid angular velocity,said gravity error, andsaid magnetic field error;
integrate said orientation derivative to form said updated value of said sensor orientation.
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Abstract
A system that efficiently estimates an object'"'"'s orientation using magnetic, angular rate, and gravity sensors. The object may be for example a virtual reality headset or a user in a virtual reality environment. Magnetic and gravity data are used to correct errors that accumulate from integrating angular velocity. Unlike systems that use Kalman filter approaches, embodiments of the system apply a simple, highly efficient technique to generate magnetic and gravity error vectors; these error vectors are added directly to the angular velocity prior to integration. Error calculations are performed in the sensor reference frame rather than in the Earth reference frame. Magnetic error correction uses only the horizontal component of the magnetic field, which is efficiently calculated by subtracting off the projection of the magnetic field onto the measured gravity vector. Sensors and processors for calculating orientation may be integrated into a low-latency virtual reality display system.
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Citations
12 Claims
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1. An efficient orientation estimation system using magnetic, angular rate, and gravity sensors, comprising
a three axis magnetometer configured to be attached to an object, wherein said three axis magnetometer generates magnetic field data represented in a sensor reference frame; -
a three axis rate gyroscope configured to be attached to said object, wherein said three axis rate gyroscope generates angular velocity data represented in said sensor reference frame; a three axis accelerometer configured to be attached to said object, wherein said three axis accelerometer generates acceleration data represented in said sensor reference frame, wherein said acceleration data includes a gravity measurement; at least one processor comprising a tangible memory medium, coupled to said three axis accelerometer, to said three axis rate gyroscope, and to said three axis magnetometer, and configured to define an Earth reference frame for an area in which said object is located, comprising a unit vector ĵ
pointing vertically upwards and opposite to a direction of gravity;a unit vector {circumflex over (k)} in a horizontal plane perpendicular to said unit vector ĵ
, and pointing in a direction opposite to a horizontal component of Earth'"'"'s magnetic field in said area;a unit vector î
perpendicular to said unit vector ĵ and
perpendicular to said unit vector {circumflex over (k)};wherein said unit vector î
, said unit vector ĵ
, and said unit vector {circumflex over (k)} form a right-handed reference frame;define a sensor orientation as a rotation that rotates a vector in said sensor reference frame into a corresponding vector in said Earth reference frame; set said sensor orientation to an initial sensor orientation; iteratively calculate an updated value of said sensor orientation as obtain said angular velocity data from said three axis rate gyroscope; obtain said acceleration data from said three axis accelerometer; normalize said acceleration data to form a normalized acceleration having its vector length equal to one; obtain said magnetic field data from said three axis magnetometer; generate an Earth to sensor rotation as an inverse of said sensor orientation; rotate said unit vector ĵ
by said Earth to sensor rotation, to form a gravity prediction in said sensor reference frame;apply said Earth to sensor rotation to a negative of said unit vector {circumflex over (k)}, to form a horizontal magnetic field prediction in said sensor reference frame; project said magnetic field data onto said normalized acceleration, to form a vertical magnetic component; subtract said vertical magnetic component from said magnetic field data, to form a horizontal magnetic component; normalize said horizontal magnetic component to form a normalized horizontal magnetic component; calculate a vector cross product of said normalized acceleration with said gravity prediction, to form a gravity error calculate a vector cross product of said normalized horizontal magnetic component with said horizontal magnetic field prediction, to form a magnetic field error; calculate an orientation derivative based on said angular velocity, said gravity error, and said magnetic field error; integrate said orientation derivative to form said updated value of said sensor orientation. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. An efficient orientation estimation system using magnetic, angular rate, and gravity sensors, comprising
a three axis magnetometer configured to be attached to one or more body parts of a user, wherein said three axis magnetometer generates magnetic field data represented in a sensor reference frame; -
a three axis rate gyroscope configured to be attached to said one or more body parts of said user, wherein said three axis rate gyroscope generates angular velocity data represented in said sensor reference frame; a three axis accelerometer configured to be attached to said one or more body parts of said user, wherein said three axis accelerometer generates acceleration data represented in said sensor reference frame, wherein said acceleration data includes a gravity measurement; at least one processor comprising a tangible memory medium, coupled to said three axis accelerometer, to said three axis rate gyroscope, and to said three axis magnetometer, and configured to define an Earth reference frame for an area wherein said user is located, comprising a unit vector ĵ
pointing vertically upwards and opposite to a direction of gravity;a unit vector {circumflex over (k)} in a horizontal plane perpendicular to said unit vector ĵ
, and pointing in a direction opposite to a horizontal component of Earth'"'"'s magnetic field in said area;a unit vector î
perpendicular to said unit vector ĵ and
perpendicular to said unit vector {circumflex over (k)};wherein said unit vector î
, said unit vector ĵ
, and said unit vector {circumflex over (k)} form a right-handed reference frame;define a sensor orientation quaternion having unit length, which rotates a vector in said sensor reference frame into a corresponding vector in said Earth reference frame; set said sensor orientation quaternion to an initial sensor orientation quaternion; iteratively calculate an updated value of said sensor orientation quaternion as obtain said angular velocity data from said three axis rate gyroscope; obtain said acceleration data from said three axis accelerometer; normalize said acceleration data to form a normalized acceleration having its vector length equal to one; obtain said magnetic field data from said three axis magnetometer; generate an Earth to sensor rotation as a multiplicative inverse of said sensor orientation quaternion; rotate said unit vector ĵ
by said Earth to sensor rotation, to form a gravity prediction in said sensor reference frame;apply said Earth to sensor rotation to a negative of said unit vector {circumflex over (k)}, to form a horizontal magnetic field prediction in said sensor frame; project said magnetic field data onto said normalized acceleration, to form a vertical magnetic component; subtract said vertical magnetic component from said magnetic field data, to form a horizontal magnetic component; normalize said horizontal magnetic component to form a normalized horizontal magnetic component; calculate a vector cross product of said normalized acceleration with said gravity prediction, to form a gravity error calculate a vector cross product of said normalized horizontal magnetic component with said horizontal magnetic field prediction, to form a magnetic field error; calculate an orientation derivative as one half of said sensor orientation quaternion multiplied by a sum of said angular velocity, said gravity error multiplied by a gravity error proportional gain, said magnetic field error multiplied by a magnetic field proportional gain, a cumulative sum of said gravity error multiplied by a gravity error integral gain, a cumulative sum of said magnetic field error multiplied by a magnetic field error integral gain, a derivative of said gravity error multiplied by a gravity error derivative gain, and, a derivative of said magnetic field error multiplied by a magnetic field error derivative gain; integrate said orientation derivative to form said updated value of said sensor orientation quaternion; at least one display viewable by said user; a 3D model of a scene; a scene renderer coupled to said at least one display, said at least one processor, and said 3D model, wherein said scene renderer receives said sensor orientation quaternion from said at least one processor; calculates one or more rendering virtual camera poses, based on said sensor orientation quaternion; calculates one or more 2D projections of said 3D model, based on said one or more rendering virtual camera poses; transmits said one or more 2D projections to said at least one display; an image warper coupled to said at least one display, said scene renderer, and said at least one processor, wherein said image warper receives said one or more rendering virtual camera poses from said scene renderer; receives said sensor orientation quaternion from said at least one processor; calculates a change in pose between said one or more virtual camera poses and said sensor orientation; generates a rerendering approximation of said one or more 2D projections on said at least one display based on said change in pose; modifies one or more pixels of said at least one display based on said rerendering approximation; wherein said rerendering approximation comprises approximating said change in pose as a rotation around a unit vector {circumflex over (ω
)} comprising {circumflex over (ω
)}y and {circumflex over (ω
)}x, by an angle Δ
θ
;calculating a spatial translation vector ({circumflex over (ω
)}yΔ
θ
, −
{circumflex over (ω
)}xΔ
θ
);calculating a scaling factor to convert spatial distances to pixels based on pixel dimensions and fields of view of said one or more 2D projections; calculating a pixel translation vector by scaling said spatial translation vector by said scaling factor; translating one or more pixels of said one or more 2D projections by said pixel translation vector.
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Specification