Abstract:
The present invention includes a system for removing drift from a head-worn gyro, which utilizes forward and rearward looking IR sensors, which are used to determine if gyro output signals, which indicate downward or upward rotation of the head, are correct.

Description:
FIELD OF THE INVENTION 
     The present invention relates to head tracking systems, and more especially to improved systems and methods for wearable head tracking systems for use while walking, including walking on uneven terrain in an uncontrolled environment, such as outdoors. 
     BACKGROUND OF THE INVENTION 
     In the past, Micro-Electro-Mechanical System (MEMS) gyroscopes (gyro) have been widely available in the industry, but with their unavoidable drift, often provide insufficient performance for use for head tracking systems, unless some calibrating or corrective augmentation of the MEMS is employed. 
     Head tracking has been known to be used for gyroscopic or inertial measurement augmentation. Such augmentation has been done with optical, ultrasonic, and magnetic techniques, each with various deficiencies. 
     One common deficiency is that they often are limited to use in controlled environments, such as an aircraft cockpit. When an open and/or uncontrolled environment exists, such as with a person walking outdoors, the necessary hardware to perform precise head tracking is often expensive, heavy and power hungry. With these deficiencies, prior art head tracking systems may have been believed to be unsuitable for augmentation of inertial measurement units for many head tracking systems. 
     The present invention overcomes one or more of these problems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a system and method for head tracking while walking. 
     It is a feature of the present invention to include a range finder augmentation of a MEMS gyro output. 
     It is an advantage of the present invention to provide for a low cost system for correcting gyro drift. 
     Accordingly, the present invention is a method of removing drift from a body-worn gyro comprising the steps of: 
     Providing a body-worn gyro which outputs gyro rotation rate signals; 
     Monitoring a forward looking distance to the ground in front of a person wearing said body-worn gyro; 
     Monitoring a rearward looking distance to the ground behind a person wearing said body-worn gyro; 
     Augmenting said gyro rotation rate signals when:
         said gyro rotation rate signals indicate that a head of a person has rotated to a more downward looking orientation;   said forward looking distance has not decreased; and   said rearward looking distance has not increased.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a typical flat terrain environment for a head tracking system. 
         FIG. 2  shows a typical continuously sloped terrain environment for a head tracking system. 
         FIG. 3  shows a typical flat terrain environment with an obstacle for a head tracking system. 
         FIG. 4  shows a typical non-continuously sloped terrain environment for a head tracking system. 
         FIG. 5  is a block diagram of a representative electronic system utilizing the head tracking system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Now referring to the Figures, wherein like numerals refer to like matter throughout, and more particularly referring to  FIG. 1 , there is shown a helmet  10 , which is shown in two configurations, as if it were worn by a single person walking (not shown). In configuration A, head worn structure  10  is shown at angle, which herein shall be described as forward looking. In configuration B, the angle of head worn structure  10  is described here as downward looking. In configuration A, the distance from head worn structure forward illuminating infra-red distance measuring apparatus  12  to the ground along line  1 AF is roughly the same as the distance from head worn structure rearward illuminating infra-red distance measuring apparatus  14  to the ground along line  1 AR. It should be noted that, throughout this description, infra-measuring sensors are described because they are believed to provide practical cost effective performance. However, it should be understood that other systems for measuring distance could be used and, in some cases, such as laser range finders, may provide superior accuracy, albeit at a higher cost. Ultrasonic and other range finders could be used as well in some applications. In configuration B, the head worn structure is downward looking and the head worn structure forward illuminating infra-red distance measuring apparatus  12  points along a line  1  BF, which is shorter than  1 AF, and the line  1  BR from head worn structure rearward illuminating infra-red distance measuring apparatus  14  in configuration B is longer than the corresponding line  1 AR in configuration A. From this information, the system of the present invention infers that a downward looking rotation of the head worn structure  10  has occurred from configuration A to configuration B. A corollary of this is that, if the distance measuring apparatuses do report any change from configuration A to configuration B, it can be inferred that the head worn structure  10  is angularly stationary about the axis perpendicular to the head worn structure forward illuminating infra-red distance measuring apparatus  12  and head worn structure rearward illuminating infra-red distance measuring apparatus  14 . This is helpful in calibrating out drift from a gyroscope, i.e., if a gyro is reporting out a rotation and the head worn structure forward illuminating infra-red distance measuring apparatus  12  and head worn structure rearward illuminating infra-red distance measuring apparatus  14  do not report any change, then the rotation from the gyro is presumed to be drift (false report of rotation) and such output from the gyro will be ignored, or commensurately tagged for adjustment. 
     Now referring to  FIG. 2 , there is shown an example of the present invention in operation on an inclined plane. In configuration A, the length of the line  2 AF and  2 AR are not the same length with respect to each other, as was the case in configuration A of  FIG. 1 . In configuration B of  FIG. 2 , the line  2 BF and the line  2 BR are about the same in length. This demonstrates that it is not the absolute length of the line which is determinative of rotation but the rate of change of the value. In  FIG. 2  going from configuration A to configuration B, a downward looking rotation is detected just as in  FIG. 1 , irrespective of the fact that  FIG. 2  depicts an inclined plane, i.e., the system and method of the present invention have been shown to work on both a flat surface and an inclined plane. 
     Now referring to  FIG. 3 , there is shown a configuration A with a line  3 AF from head worn structure forward illuminating infra-red distance measuring apparatus  12  and a line  3 AR from head worn structure rearward illuminating infra-red distance measuring apparatus  14 , which are approximately the same length. In configuration C, an obstacle  300  is shown and the distance  3 CF from head worn structure forward illuminating infra-red distance measuring apparatus  12  to the obstacle  300  is now shorter than  3 AF. However, line  3 CR from head worn structure rearward illuminating infra-red distance measuring apparatus  14  to the ground remains the same length as  3 AR in configuration A. Under this situation, the present invention will react to the obstacle with a conclusion that there has not been rotation. Since  3 CR is unchanged from  3 AR, any change in  3 CF compared to  3 AF will be ignored as not arising out of a rotation. This is because, as shown in  FIG. 1 , when there is rotation and there is change in a forward looking distance, there will also be a change in the rearward looking distance. 
     Now referring to  FIG. 4 , there is shown a configuration D where the ground is not continuously sloped. Indeed, there is a point of inclination  400  where the ground begins to go uphill. Configuration D is treated the same as configuration C of  FIG. 3 . As the head worn structure  10  moves closer to the point of inclination  400 , the line  4 DF gets progressively shorter while the line  4 DR remains the same. The system of the present invention does not report rotation unless both the head worn structure forward illuminating infra-red distance measuring apparatus  12  and the head worn structure rearward illuminating infra-red distance measuring apparatus  14  report distance change, which are used to calculate head worn structure rotation, and those calculated head worn structure rotation changes are equal and opposite for the forward sensor rotation and the rear sensor rotation. 
     Now referring to  FIG. 5 , there is shown a detailed view of an exemplary head worn structure  10 , which includes a head worn structure forward illuminating infra-red distance measuring apparatus  12  and a head worn structure rearward illuminating infra-red distance measuring apparatus  14 , which both output a signal which is representative of a distance measurement to an IR comparator  500  or any hardware, software or combination which performs the function of monitoring changes in the measured distances, a MEMS gyro  502 , which could be substituted with other inertial measurement units with known trade-offs and a controller  504 . Also shown is a MEMS/IR interface  506 , which could be independent as shown, or included within either  502  or  504 . This location ( 502 ,  504 , or  506 ) of the place of final utilization of the data from head worn structure forward illuminating infra-red distance measuring apparatus  12  and head worn structure rearward illuminating infra-red distance measuring apparatus  14  is a matter of designers choice and is expected to vary, depending on design details for differing applications. In  FIG. 5 , the combined dashed and dotted line shows an embodiment where no independent MEMS/IR interface  506  is present. When there is a MEMS/IR interface  506 , the data may follow along the dotted lines. 
     In one embodiment, controller  504  is a microprocessor or controller for a head tracking system, which may be coupled to a display, radio or other useful device. In another embodiment of the present invention, controller  504  could be coupled to a heads-up display disposed on the head worn structure  10  and displaying information, such as location of items of interest which cannot be readily seen due to darkness, obstructions, smoke etc. 
     The above description has been focused on  2 D head worn structure forward illuminating infra-red distance measuring apparatus  12  and head worn structure rearward illuminating infra-red distance measuring apparatus  14  rotating around the same axis of rotation to detect forward and rearward head tilting. The present invention can be a  3 D system where additional sensors could be deployed for measuring side to side head movements with similar logic and structure. 
     The invention has been described herein as a head tracking system because it is believed that such an application is well suited for the present invention. It should, however, be understood that other applications which are not for head tracking are contemplated, such as turn by turn navigation and compass display and others. 
     The above description is intended to describe preferred or exemplary embodiments of the present invention. It should be understood that the present invention is not intended to be limited by the inclusion of details in this description. Instead, the Applicant intends that the present invention will include variations of the claimed invention without departing from the spirit and scope of the invention.