Abstract:
A sensor system that is attachable to a moving object to calculate a displacement of the object relative to a fixed surface includes three laser light sources. The sources are fixedly aligned to direct beams along three respective beam paths toward the surface. Reflections from the surface are then received by the sensor system and used to calculate a relative velocity between the sensor and surface. The velocity is then integrated to compute a displacement. These displacements are transmitted via wireless link to a receiving station which uses the displacements to track and locate the object.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to an on-board sensor for measuring the displacements of a moving object. More particularly, the present invention pertains to a sensor that can be attached to an individual to track the individual&#39;s movements. The present invention is particularly, but not exclusively, useful for determining the location of an individual such as a firefighter or soldier who has entered a structure or an area inside which GPS or other navigation systems no longer work.  
       BACKGROUND OF THE INVENTION  
       [0002]     Soldiers of the future will require accurate indoor navigation for situational awareness, for coordinating sweeps of buildings, and for rescue operations for downed troops. Firefighters and rescue workers have similar needs for indoor navigation. Every year approximately 100 firefighters die in the line of duty, approximately 20% of them from being lost or trapped. RIT teams (Rapid Insertion Teams) are sent in to rescue the downed firefighter. These teams would greatly benefit from knowledge of where in the building the downed firefighter is located and the path he took to get there.  
         [0003]     Radio navigation, however, is problematic in and around buildings. For example, GPS does not work very well indoors due to low signal penetration as well as propagation perturbations caused by the presence of structural steel and electrical wiring in buildings. In addition, inertial navigation techniques also fail to provide a complete solution to the problem of indoor navigation. Although some inertial systems may be capable of the accuracy needed over timelines of operational significance, these systems are far too bulky and expensive for use on individual soldiers or firefighters.  
         [0004]     The approach considered here, on the other hand, involves a sensor that tracks the indoor surfaces to continuously determine and update the location of a person wearing the sensor. This location information can then be sent to other soldiers or commanders for use on situational awareness displays via a wireless network. For rescue operations, the geolocation accuracy necessary to find a downed individual is typically calculated based on the radius of effectiveness of a man with outstretched arms and legs. This generally equates to a circular error probability (CEP) of approximately two meters. In the case of a fire, smoke is often so thick as to preclude any visual clues. In these instances, finding a downed firefighter may result only from actually touching him.  
         [0005]     In light of the above, it is an object of the present invention to provide systems and methods for accurately measuring the movements of an individual and processing the measurements to determine the current location of the individual. It is another object of the present invention to provide a system for accurately tracking the indoor movements of an individual by measuring the individual&#39;s velocity. It is yet another object of the present invention to provide a lightweight, wearable sensor for determining an individual&#39;s velocity and displacement. Yet another object of the present invention is to provide a velocity and displacement sensor which is easy to use, relatively simple to implement, and comparatively cost effective.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0006]     The present invention is directed to a sensor system for calculating a displacement of an object relative to a surface. For example, the sensor system can be attached to a movable object such as an individual to track the individual&#39;s movements within a building. Information regarding the individual&#39;s movements and location can then be relayed by wireless link to a receiving station which is typically located outside the building.  
         [0007]     In greater structural detail, a typical embodiment of the sensor system includes three laser light sources for generating and directing three laser light beams along three respective beam paths. For the sensor system, the three beam paths are maintained at a constant spatial relationship relative to each other. Moreover, the sensor system is attached to the object to direct each outgoing light beam toward a selected surface of the building such as the floor. There, at the surface, each beam is reflected back toward the sensor to produce an incoming beam on each beam path.  
         [0008]     To receive and process each incoming beam, the sensor system includes a means for coherently detecting the reflected light for each beam path. For example, a homodyne detection scheme uses a sample of the light sent and beats the reflected signal against the sample on a detector. Functionally, in the context of the present invention, for each beam path, a detector and mixer interact to measure 1) a respective distance between the sensor and surface along the beam path, and 2) a respective frequency difference between the incoming and outgoing light beams (i.e. Doppler shift). From these measurements, the x, y and z components of a displacement vector for the object relative to the surface can be calculated. In greater detail, the measured distances are processed to determine an orientation of the beam paths relative to the surface. With the orientation known, the measured frequency difference for each beam path can be converted into a velocity vector having x and y components that lie in the plane of the surface. The processor then integrates the velocity vector to compute x and y displacement vector coordinates. A component in the z direction (i.e. sensor height) can also be calculated using the distance measurements.  
         [0009]     In one application, the sensor system is configured for attachment to the outer portion of a boot at an attachment location near the top rear of the boot. For this application, three laser beams are directed downwardly toward the ground, with each beam path being inclined at an acute angle relative to another beam path. The sensor system is then initialized at a predetermined building location which, for example, can be an entrance to a building the individual is entering.  
         [0010]     Once initialized, the sensor system acquires and records data corresponding to the frequency differentials and sensor/surface distances for each beam path. Specifically, the data is passed to a processor which completes the computation described above to compute the components of a displacement vector. A wireless link is established between the sensor system and a receiving station which is typically located outside the building. The displacements can then be accumulated at the receiving station and used to maintain a real-time location for the individual inside the building. It is to be appreciated that selected portions of the processing, or all of the processing, can be accomplished by a processor that is attached either to the individual or is located at the receiving station. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0012]      FIG. 1  is a perspective view of a sensor system for determining displacements of a moving object relative to a fixed surface shown attached to a firemen&#39;s boot and positioned inside a building;  
         [0013]      FIG. 2  is a schematic layout of a sensor system showing electrical connections and beam paths for one of three lasers that are provided in the sensor system shown in  FIG. 1 ; and  
         [0014]      FIG. 3  is a schematic diagram illustrating a coordinate translation for translating range and Doppler measurements along the sensor system&#39;s beam axes to an inertial x-y coordinate system wherein the x and y dimensions lie in a plane defined by a fixed surface (e.g. a floor) over which the sensor system travels. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring to  FIG. 1 , a sensor system  10  for determining displacements of a moving object relative to a fixed surface is shown. For the application shown in  FIG. 1 , the sensor system  10  is shown attached to the outer portion of a firemen&#39;s boot  12  at a location near the top rear of the boot  12 . In addition to tracking firefighters, other applications of the sensor system  10  include, but are not limited to, tracking soldiers that have entered buildings, tracking the locations of vehicles inside or outside buildings, and in general, is applicable to tracking the location of moveable objects relative to a fixed surface.  
         [0016]     For the sensor system  10 , three laser beams  14   a - c  are directed downwardly toward a surface  16 , which in this case is the floor, along respective beam paths  18   a - c.    FIG. 1  further shows that each beam path  18   a - c  is inclined at an acute angle relative to the other beam paths  18   a - c  (i.e. the beam paths  18   a - c  are not parallel).  
         [0017]     Continuing with reference to  FIG. 1 , for the sensor system  10 , the three beam paths  18   a - c  are maintained at a constant spatial relationship relative to each other. Moreover,  FIG. 1  shows that each laser beam  14   a - c  is reflected at the surface  16  and a portion of each reflection is directed back toward the sensor  10  along the respective beam paths  18   a - c.  In functional overview, the sensor system  10  receives and processes each reflected beam  14   a - c  as the boot  12  moves relative to the surface  16  to calculate relative displacements between the boot  12  and surface  16 .  
         [0018]      FIG. 2  shows a portion of the sensor system  10  for beam path  18   a  in further detail. It is to be appreciated the similar components can be included for beam paths  18   b  and  18   c  shown in  FIG. 1 . As shown in  FIG. 2 , the sensor system  10  includes a laser source  20  which is configured to emit a laser beam  14   a  along beam path  18   a.  As described in further detail below, a splitter  22  is provided to direct a portion of the laser beam  14   a  to a mixer  24  along beam path  26 . As shown, the remaining portion of the beam  14   a  is directed from the splitter  22  to a modulator  27  which modulates the laser beam  14   a  with a continuous wave modulation. From the modulator  27 , the beam  14   a  passes through receive optics  28  and reaches the targeted surface  16  along beam path  18   a.  Reflections from the surface  16  then travel back along beam path  18   a  to the receive optics  28  where the reflections are diverted onto beam path  30 . For the sensor system  10 , the receive optics  28  can include, but is not limited to, one or more of the following optical components: splitters, filters, mirrors and lenses.  
         [0019]     Continuing with reference to  FIG. 2 , once on beam path  30 , light reflected from surface  16  is partitioned at splitter  32 , as shown. Specifically, one portion is directed from the splitter  32  to a phase measuring circuit  34  and the remaining portion passes through splitter  32  and is input into mixer  24 , as shown. At the phase measuring circuit  34 , the phase of the reflected modulated signal is measured relative to the CW modulation leaving the modulator  27 . The relative phase is then sent via cable  36  to the processor  38  to determine a distance between the sensor system  10  and the surface  16  along beam path  18   a.  In one implementation, a modulation signal wavelength is selected to be larger than the measured distance to obviate modulo 2π ambiguities. In an alternate embodiment (not shown), a pulsed laser beam can be used to determine the distance between the sensor system  10  and the surface  16  along beam path  18   a  in accordance with procedures and system components known in the pertinent art.  
         [0020]      FIG. 2  further shows that the mixer  24  is in optical communication with a photodetector  40  along beam path  42 . Functionally, the detector  40  and mixer  24  interact to measure a respective frequency difference between the reflected light on beam path  30  and the unreflected light on beam path  26 . (i.e. Doppler shift). In one setup, the light on paths  26  and  30  is combined by the mixer  24  and beat against the photodetector  40 . The output of the photodetector  40  is then forwarded to the processor  38  via cable  44  which determines the frequency difference between the light on paths  26  and  30 . As detailed further below, once the processor  38  has acquired 1) the frequency difference between the light on paths  26  and  30 , and 2) the distances between the sensor system  10  and the surface  16  along beam paths  18   a - c,  the processor  38  computes the displacements of the sensor system  10  relative to the surface  16 . These displacements can then be sent via cable  46  to a transmitter  48  which sends a wireless signal  50  that includes displacement information to one or more receiving stations  52 . In addition to, or in lieu of, displacement information, the processor  38  can calculate an actual position (e.g. coordinates in a coordinate system similar to GPS coordinates) for wireless transmission to a receiving station  52 . The receiving station  52  can be carried by another firefighter who is located inside or outside the building.  
         [0021]     Referring back to  FIG. 1 , for the sensor system  10 , the x, y and z components of a displacement vector for the movements of the boot  12  relative to the surface  16  can be calculated. More specifically, the measured distances between the sensor system  10  and surface  16  along the beam paths  18   a - c  are processed to determine an orientation of the sensor system  10  relative to the surface  16 . In greater detail, this task is conducted to assess the impact of errors in range and Doppler measurement by the sensor system  10  on position accuracy in the inertial frame (i.e. x and y coordinates along the floor). To accomplish this error assessment, the range and Doppler measurements are translated along the beam axes (i.e. beam paths  18   a - c ) in the sensor frame of reference to the inertial frame (i.e. x and y coordinates along the floor). In this way, the sensor system  10  measures the coordinate tilt by measuring the distance to the floor in the directions of the three beam paths  18   a - c.  The required coordinate change is given by computing a rotation matrix for the coordinate tilt.  
         [0022]      FIG. 3  illustrates the coordinate tilt. The triangle Ã {tilde over (B)} {tilde over (C)} represents the plane of the sole of a boot  12 . The vector {tilde over (p)} denotes an orthogonal vector from point O to the bottom of the boot  12 . For the illustration shown, the triangle A B C is considered to be in the plane of the floor. The vector p denotes an orthogonal vector from the point O to the bottom of the boot  12  in this case.  
         [0023]     From  FIG. 3  it can be seen that the frame rotation matrix is defined by the vector:  
             ω   =       p        p          ×       p   ~            p   ~                      (   1   )             
 
 With this equation, l 1 , l 2 , and l 3  can be used to denote the measured distances in the three directions. In addition, three vectors of unit length can be defined in the direction of {right arrow over (OA)}, {right arrow over (OB)}, and {right arrow over (OC)}. It can be further assumed that the length of {right arrow over (OA)}, {right arrow over (OB)}, and {right arrow over (OC)} is l 0 . It follows that the vectors of {right arrow over (OÃ, {right arrow over (O{tilde over (B)})}, and {right arrow over (O{tilde over (C)})} are given by:
 
 {right arrow over (OÃ= ( l   0   −l   1 ) u   1   , {right arrow over (O{tilde over (B)})}= ( l   0   −l   2 ) u   2   , {right arrow over (O{tilde over (C)})}= ( l   0   −l   3 ) u   3 .  (2)
 
 The orthogonal vector {tilde over (p)} can be computed as:
 
 {tilde over (p)}=ũ   1   +ã ( ũ   2   −ũ   1 )+ {tilde over (b)} ( ũ   3   −ũ   1 )  (3)
 
 where the symbol ũ k =(l k −l 0 )u k , k=1, 2, 3 is used for simplicity of notation. The coefficients ã and {tilde over (b)} can be computed from the conditions that {tilde over (p)} is orthogonal to (ũ 2 −ũ 1 ) and (ũ 3 −ũ 1 ). The vector p can be computed by replacing ũ k  with u k  in Eq. (2). The rotation vector ω is projected in the directions of the x, y, and z axes in the inertial frame to define the coordinate transformation matrix in the x, y, and z directions. 
 
         [0024]     With the orientation known, the measured frequency difference for each beam path  18   a - c  can be converted into a velocity vector having x and y components that lie in the plane of the surface  16 . The processor  38  shown in  FIG. 2  then integrates the velocity vector to compute x and y displacement vector coordinates. A displacement component in the z direction can be calculated directly from the distance measurements.  
         [0025]     While the particular displacement and velocity sensor as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.