Difference correcting method for posture determining instrument and motion measuring instrument

• US 20060161363A1
• Filed: 02/16/2004
• Published: 07/20/2006
• Est. Priority Date: 02/14/2003
• Status: Active Grant

• Current Assignee: Akebono Brake Industry Company Limited (Akebono Brake Corporation), Ryosuke Shibasaki
• Original Assignee: Akebono Brake Industry Company Limited (Akebono Brake Corporation), Ryosuke Shibasaki
• Inventors: Konishi, Yusuke, Kanasugi, Hiroshi, Shibasaki, Ryosuke, Yoshida, Nobuyuki
• Granted Patent: US 7,233,872 B2
• Time in Patent Office: Days
• US Class Current: 702/94
• CPC Class Codes: A61B 2560/0223 of calibration, e.g. protocols for calibrating sensors
• CPC Class Codes: A61B 2562/0219 Inertial sensors, e.g. accelerometers, gyroscopes, tilt switches
• CPC Class Codes: A61B 5/11 Measuring movement of the entire body or parts thereof, e.g. head or hand tremor, mobility of a limb for measuring pulse A61B5/02 ;A61B5/1038 takes precedence; motion detection to correct for motion artifacts in physiological signals A61B5/721
• CPC Class Codes: A61B 5/1116 Determining posture transitions
• CPC Class Codes: A61B 5/6814 Head
• CPC Class Codes: A61B 5/6824 Arm or wrist
• CPC Class Codes: A61B 5/6828 Leg
• CPC Class Codes: G01C 19/5607 using vibrating tuning forks double-ended tuning forks using planar vibrating masses suspended at opposite ends G01C19/5719

Abstract

The error of a sensor in motion capture system is corrected. An acceleration sensor determines the direction of gravity (G1) in a still initial state. The direction G1 will never change always if a drift dose not occur. However, actually if an object to which an acceleration sensor is attached moves along a coordinate axis, the direction gradually changes due to a drift with time. According to the invention, after a given time has passed, the acceleration sensor is stopped to determine the direction of gravity. Referring to the data on this determination, the true direction of gravity to be judged is compared with the direction of gravity G1 influenced by the drift. The difference between them is assumed to be an error due to a drift, and the error is subtracted from the determined value to correct the error of the measurement value. Similarly to the direction of gravity, assuming that there is an error in any direction in the coordinate system, the errors are corrected, and more accurate motion determination is possible.

Claims

1. A difference correcting method for a posture determining instrument equipped with at least an acceleration sensor to determine a posture and/or a position in a predetermined coordinate system, characterized by comprising:

determining a direction of gravity in an initial still state by the acceleration sensor;

moving the initial direction of gravity associatively with a motion of the determining instrument in the coordinate system to calculate a direction of gravity after the motion;

setting the acceleration sensor in a still state to measure a direction of gravity after a passage of optional time;

comparing the measured direction of gravity of the still state with the direction of gravity after the motion in the coordinate system;

regarding a difference therebetween as an error; and

correcting a measured value in the coordinate system based on the error.

6. A method according to claim 1 or 5, characterized in that a magnetic direction sensor is provided, an initial magnetic direction is determined by the magnetic direction sensor, the initial magnetic direction is changed associatively with the motion of the moving object, a magnetic direction is then measured by the magnetic direction sensor after a passage of optional time, an error is measured based on a difference between the after-motion magnetic direction and the magnetic direction after the passage of the optional time, and the error is corrected associatively with error measurement in a direction of gravity by the acceleration sensor.

2. A motion determining instrument of a moving object, equipped with an acceleration sensor and an angular velocity sensor to measure a posture and/or a position in a specific coordinate system according to output values from the sensors, and to determine a motion of the moving object to which the sensors are attached, characterized by comprising:

after-motion gravitational direction measuring means for determining a direction of gravity in an initial still state by the acceleration sensor, and moving the initial direction of gravity associatively with a motion of a determining instrument in the coordinate system to measure a direction of gravity after the motion;

still time gravitational direction measuring means for measuring a direction of gravity in a still state of the acceleration sensor after a passage of optional time;

difference determining means for comparing the direction of gravity of the still state measured by the still time gravitational direction measuring means with the direction of gravity after the motion measured by the after-motion gravitational direction measuring means to determine a difference therebetween; and

correcting means for correcting measuring data for specifying a motion or a position of the moving object according to the difference obtained by the difference determining means.

3. A motion determining instrument according to claim 2, characterized in that the acceleration sensor is a 3-axis acceleration sensor, and the angular velocity sensor is a 3-axis angular velocity sensor.

4. A method of attaching a motion determining instrument to a human body, the motion determining instrument being equipped with an acceleration sensor and an angular velocity sensor to measure a posture and/or a position in a specific coordinate system according to outputs from the sensors, and to determine a motion of the moving object to which the sensors are attached, characterized by comprising determining an attaching angle of the sensor to a limb of the human body and a relative positional relation between an external coordinate system which becomes a reference for human body motion measurement and the sensor based on a relative position between a direction of gravity obtained from the acceleration sensor and a specific posture of the human body which becomes a reference.

5. A moving object motion measuring method for a 3-axis direction motion measuring instrument equipped with a 3-axis acceleration sensor and a 3-axis angular velocity sensor, characterized by comprising:

specifying a relative positional relation between a frame constituting the object or a frame equivalent portion and a sensor coordinate system to attach the sensors to the moving object;

taking an initial posture in which a positional relation with a world coordinate system is known, initializing the instrument, and determining an initial direction of gravity with respect to the sensor coordinate system by the acceleration sensor based on initial posture information;

using a relative positional relation among a direction of the rotated sensor coordinate system, the angular velocity sensor, and the frame to determine a direction of the frame, setting an original point as a start point, determining a position of a joint which is an adjacent frame connection portion based on a direction and a length of each frame, determining a position of an adjacent joint based on the determined joint position and the direction and length of the frame to measure how the frame has moved, and simultaneously rotating, since the initial direction of gravity is relatively rotated when the angular velocity sensor is rotated associatively with a motion of the moving object, the initial direction of gravity in the sensor coordinate system associatively with rotation of the sensor to calculate a direction of gravity G1; and

measuring a direction of gravity G2 by the 3-axis acceleration sensor when the moving object is set in a still state, comparing the after-motion direction of gravity G1 after a passage of predetermined time with the still-time direction of gravity G2, determining an error based on a difference therebetween, determining an error changing parameter based on the error information according to time-change characteristics of a drift error of the angular velocity, sensor, and correcting motion measuring data of the moving object thereafter according to the error changing parameter.