Abstract:
A self-correcting wireless inertial navigation system and method employ a mobile unit having an inertial sensor and a transmitter connected to the output of the initial sensor for broadcasting an RF measurement signal. A base station has receivers responsive to the signal, an interferometer connected to the receivers and a processor programmed to obtain inertial measurements from the signal and to correct the measurements in accordance with phase difference triangulation information derived by the interferometer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a self-correcting wireless inertial navigation system and to a method of self-correcting wireless inertial navigation and is useful in particular, but not exclusively, for local position measurement, which is useful for many different applications, for example computer input, docking, robotic programming, and other measurement tasks.  
           [0003]    2. Description of the Related Art  
           [0004]    The Global Positioning System (GPS) has revolutionized position sensing for large-scale outdoor systems, such as shipping and land navigation. However, GPS technology cannot be directly applied to small scale position measurement because clocks cannot count fast enough to time short duration light pulses.  
           [0005]    Ultrasonic systems imitating the Global Positioning System have been investigated, but suffer from inaccuracies caused by moving air currents, echoes and acoustic noise. One prior art local position sensing system employs magnetic sensors and emitters to detect position for motion capture, and is usually used in film gaming and virtual reality applications.  
           [0006]    Other researchers have attempted to directly use phase information to track position. For example, one prior art system uses a coarse and a fine frequency measurement to determine the location of a transmitter as a function of wavelength. However, phase measurement alone cannot determine the world coordinates of a target. If the transmitter moves more than a single wavelength during a measurement iteration, the absolute position of the transmitter becomes uncertain.  
           [0007]    Inertial navigation systems have been investigated for local position measurement. However, inertial systems suffer from integrator drift and must be compensated or calibrated using a secondary measurement system.  
           [0008]    In U.S. Pat. No. 6,176,837, issued Jan. 23rd, 2001 to Eric M. Foxlin, there is disclosed a tracking system and method for tracking the position of a body which employ two sensor systems to obtain two types of measurements associated with motion of the body, one comprising acoustic measurement. An estimate of the orientation and position of the body is updated, based on, for example, inertial measurement, and the estimate is then updated based on, for example, acoustic ranging.  
           [0009]    It has also been proposed to effect motion tracking by a combination of inertial, ultrasonic and geomagnetic sensors for use, in particular, in high-end virtual reality and military applications.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    According to the present invention, the position of a mobile unit is determined by inertial sensing of the position, by phase difference triangulation measurement of the position and by employing the phase difference triangulation measurement to correct the inertial sensing of the position.  
           [0011]    Preferably, measurement of the position of a mobile unit by the inertial sensing is transmitted as an RF signal from the mobile unit to a base unit, and the RF signal is also employed for the phase difference triangulation measurement, which enables the present invention to be implemented in an elegant and inexpensive manner.  
           [0012]    Thus, phase information from the RF signal may be employed to correct the inertial position measurement for drift. The inertial sensing provides global position information, independent of a line of sight. The phase information from the phase difference triangulation measurement ensures that local error does not accumulate.  
           [0013]    When the communication system employed for transmitting and receiving the RF signal is also employed for the phase difference triangulation, the present system can be implemented in an elegant and inexpensive manner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will be more readily apparent from the following description of a preferred embodiment thereof given, by way of example, with reference to the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 shows a block diagram of a self-correcting wireless inertial navigation system embodying the present invention; and  
         [0016]    [0016]FIG. 2 shows a block diagram of parts of the system of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    In the accompanying drawings, there is shown a self-correcting wireless inertial navigation system which includes a base station  10  and a mobile unit  12 . The mobile unit  12  in includes an accelerometer  14 , which in the present embodiment of the invention is in implemented as an ADXL202 two-axis accelerometer manufactured by Analog Devices Inc., a microcontroller  16 , implemented as a PIC16 F876-20 microcontroller manufactured by Microchip Corp., and a transmitter  18  in the form of a TXM-900-HP II receiver board having an antenna  20  for broadcasting a measurement signal in the form of an RF signal.  
         [0018]    The base station  10  has three antennas  22   a,    22   b  and  22   c  for receiving the measurement signal. This antenna  22   a  is connected to an RF receiver  24 , implemented as an RXM-900-HP-II receiver board on an MDEV-900-HP-II evaluation board manufactured by Linx Technologies Inc. The two antennas  22   b  and  22   c  are connected to an interferometer/phase detector  26  in the form of an AD8302 RF/IF Gain and Phase Detector manufactured by Analog Devices Inc. and the antenna  22   a  is also connected to the interferometer/phase detector  26 . The receiver  24  and the interferometer/phase detector  26  are connected to a PIC16F876-20 microcontroller  28 , which outputs through a MAX233 serial driver  30 , manufactured by Maxim Integrated Products, to a Dell Optiflex GXPro Dual 200 MHz Pentium Pro personal computer  32 . A monitor  34  is provided for displaying the output of the personal computer  32 .  
         [0019]    In the operation of this system, the accelerometer  14 , acting as an inertial sensor, provides a pulse width modulated inertial sensor output signal for each axis to the microcontroller  16 , which supplies a corresponding frequency shift keyed signal to the transmitter  18  for broadcasting the inertial measurement data as the RF measurement signal from the antenna  20  of the mobile unit  12  to the antennas  22   a - c  of the base station  10 .  
         [0020]    At the base station  10 , the interferometer/phase detector  26  effects phase difference triangulation of the RF measurement signal from the antennas  22   b  and  22   c  and provides a corresponding output to the microcontroller  28 . The inertial measurement data, through the antenna  22   a  and the receiver  24 , is supplied directly to the microcontroller  28 .  
         [0021]    The personal computer  32  is configured to employ the phase difference triangulation output from the interferometer/phase detector  26  to correct the inertial measurement and to output corresponding data to the monitor  34 .  
         [0022]    The inertial measurement data is derived as follows:  
         [0023]    Acceleration measurements made by the accelerometer  14  are integrated twice to arrive at displacement. That is 
         Δx=∫∫xdt  (1) 
         [0024]    To obtain distance traveled during a single sampling interval of the accelerometer system 
         Δx=x i T  (2) 
         [0025]    where 
         Δ x=x   i   T+x   i−1   (3) 
         [0026]    and T is the sampling interval. That is, the distance traveled during time period T is the average velocity during T, time T.  
         [0027]    The position from a known starting point x 0  is therefore  
               x   i     =         ∑   j          Δ                   x   l         =     x   0               (   4   )                               
 
         [0028]    However, errors are also summed. Maximum error grows linearly with i as: 
         e i =ie max   (5) 
         [0029]    the error at iteration i is equal to the number of iterations multiplied by the maximum inertial measurement error. The maximum error is an amalgamation of sensor error, signal conditioning error and digitizer resolution. Generally, the sensor error dominates, and the other two error sources can be ignored.  
         [0030]    Because very accurate measurements over extended time periods are required, sensor drift due to error accumulation is a primary concern. The sensor error is fixed by the component manufacturers, so error can only be controlled by altering i, the number of iterations between land marking operations. Because a reasonably high refresh rate, and continuation of operations for an extended period of time are required, i cannot implicitly be changed. However, by employing a secondary measurement system, we can maintain i=1. The maximum error from the accelerometer will then be 
         e=max(e inertial ′e local )  (6) 
         [0031]    The error will be the greater of the inertial measurement error or the local measurement error. Because the inertial system is rezeroed at every iteration, i is fixed at 1.  
         [0032]    The correction of the inertial measurement by phase difference triangulation measurement is effected in accordance with the following equations, which show the manner of calculating the position of the mobile unit  12  based on the two receiving antennas  22   b  and  22   c,  both at a distance r from the origin. In the following calculation, λ is the carrier wavelength, φ is the measured phase angle, and d 1  and d 2  are the respective distances from the transmitter to each receiving antenna. 
         
       d 
       1 
       =kλ+m 
     
         
       d 
       2 
       =jλ+n 
       
         
           
             
               
                 d 
                 1 
               
               - 
               
                 d 
                 2 
               
             
             = 
             
               
                 
                   
                     ( 
                     
                       k 
                       - 
                       j 
                     
                     ) 
                   
                    
                   λ 
                 
                 + 
                 
                   
                     ( 
                     
                       m 
                       - 
                       n 
                     
                     ) 
                   
                   ± 
                   
                     ( 
                     
                       
                         m 
                         - 
                         n 
                       
                       λ 
                     
                     ) 
                   
                 
               
               = 
               
                 
                   φ 
                   π 
                 
                 ± 
                 
                   q 
                   2 
                 
               
             
           
         
                 
         
             
         
       
     
         [0033]    The first plus/minus is required because we do not measure which wave is leading. The second plus/minus is required to correct the phase to correspond from 0 to 360 degrees, even though phase only measures from 0 to 90 degrees.  
         [0034]    From the estimated position of the transmitter, based on the inertial measurement: 
           {tilde over (d)}   1 ={square root}{square root over (( {tilde over (x)})}+r ) 2   +{tilde over (y)}   2   
           {tilde over (d)}   2 ={square root}{square root over (( {tilde over (x)})}−r ) 2   +{tilde over (y)}   2           k   ~     =     floor                   (         d   ~     1     λ     )                                       j   ~     =     floor                   (         d   ~     2     λ     )                             
  {tilde over (m)}={tilde over (d)}   1   −{tilde over (k)}λ   
         
       ñ={tilde over (d)} 
       2 
       −{tilde over (j)}λ 
     
         [0035]    The minimum distance to (m, n) on the line  
       m   =       n   ±       φ                 λ     π       ±     q   2                             
 
         [0036]    along the perpendicular line 
           m =( {tilde over (m)}+ñ )− n   
         [0037]    is given by:  
       n   =           (       m   ~     +     n   ~       )     2     ±       φ                 λ       2                 π         ±     q   4                             
 
         [0038]    Back substitution yields m.  
         [0039]    By substituting the measured m and n in for the estimated m, and n, the new distances are calculated. 
         
       d 
       1 
       =kλ+m 
     
         
       d 
       2 
       =jλ+n 
     
         [0040]    The actual position is determined using triangulation.  
             (     x   +   r     )     2     +     y   2       =             d   1   2          
     (     x   -   r     )     2     +     y   2       =     d   2   2                             
 
         [0041]    Which can be solved as:  
       x   =         d   1   2     -     d   2   2         4      r               y   =     ±         d   2   2     -       (         d   1   2     -     d   2   2         4      r       )     2                                 
 
         [0042]    The new measured position is given by (x, y), where y is chosen as the solution which lies in the positive half plane