Abstract:
A vehicle navigation system and method for propagating a position of a vehicle utilizing a multiaxis accelerometer determines the pitch and roll of the vehicle without utilizing gyros and utilizes the calculated pitch and roll to propagate the position of the vehicle with the acceleration signals from the multiaxis accelerometer. This occurs in one of three situations (in order of complexity): 1) when the vehicle is not moving; 2) when the vehicle is moving and other speed and/or heading information is available and 3) when the vehicle is moving and other speed and/or heading information is not available.

Description:
[0001]    This application claims priority from provisional application Ser. No. 60/187,552 filed Mar. 7, 2000. 
     
    
     
         [0002]    The present invention relates generally to vehicle navigation systems and more particularly to a vehicle navigation system propagating position with a multiaxis accelerometer.  
           [0003]    A known vehicle navigation system utilizes multiple sensors to propagate position of the vehicle relative to a database of roads. The sensors include gyros and accelerometers, including a multiaxis accelerometer. Generally, the multiaxis accelerometer includes three orthogonal accelerometers, each oriented along an axis of the vehicle. The first accelerometer is oriented along a longitudinal axis of the vehicle, a second accelerometer is oriented vertically in the vehicle and a third accelerometer is mounted orthogonally to the first two, along the lateral axis of the vehicle.  
           [0004]    Generally, acceleration of the vehicle along the longitudinal axis is measured by the first accelerometer and can be used to determine a current vehicle speed. Lateral acceleration, together with vehicle speed, can be used to determine a change in heading of the vehicle. Orthogonally mounted gyros in the vehicle are utilized to measure a change in pitch and change in heading of the vehicle.  
           [0005]    Information from the gyros is utilized to distinguish changes in pitch and roll from longitudinally and lateral acceleration, respectively. For example, if the vehicle is traveling on a road rolled slightly to the right, this will induce a signal in the lateral accelerometer which would indicate a slight left turn. This can be resolved by information from the roll gyro, which would confirm that the vehicle is not turning, but must be slightly rolled. Similarly, if the vehicle is traveling up a hill, gravity would induce some acceleration in the longitudinal accelerometer, which could be interpreted as acceleration by the vehicle in the longitudinal direction. This is also corrected by information from the pitch gyro confirming that the vehicle has changed pitch.  
           [0006]    The gyros add cost, size and weight to the vehicle navigation system. It is desirable to eliminate the gyros without sacrificing accuracy of the propagated position of the vehicle navigation system. Generally, this requires determining the pitch and roll of the vehicle without the use of gyros.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a vehicle navigation system and method for propagating a position of a vehicle utilizing a multiaxis accelerometer. The inventive navigation system and method determines the pitch and roll of the vehicle without utilizing gyros and utilizes the calculated pitch and roll to propagate the position of the vehicle with the acceleration signals from the multiaxis accelerometer. This occurs in one of three situations (in order of complexity): 1) when the vehicle is not moving; 2) when the vehicle is moving and other speed and/or heading information is available and 3) when the vehicle is moving and other speed and/or heading information is not available.  
           [0008]    The pitch and roll of the vehicle can first be determined when the vehicle is in a zero motion state. When the vehicle is in a zero motion state, the resultant acceleration vector from the multiaxis accelerometer should have a magnitude of 1G. The direction of the resultant acceleration vector should be straight down relative to the Earth. To the extent the resultant vector does not coincide with the vertical axis of the vehicle, the pitch and roll of the vehicle can be determined.  
           [0009]    Determining the pitch and roll of the vehicle while the vehicle is moving is more difficult. Generally, the navigation system must distinguish pitch of the vehicle from longitudinal acceleration of the vehicle and distinguish roll of the vehicle from lateral acceleration of the vehicle. Pitch and roll of the vehicle are determined by comparing information from the multiaxis acclerometer to other speed and/or heading information, such as GPS information (such as GPS speed and heading information) and/or analysis of map matching information. When GPS is available, GPS velocity (speed and heading) is accurate for speeds over 1.5 m/s. Map matching also provides accurate heading information. It is determined that lateral acceleration information from the multiaxis accelerometer represents vehicle roll rather than change in heading if the GPS velocity information and/or map matching information indicate that the heading has not changed. Similarly, GPS velocity information and map matching heading information may alternatively indicate that the vehicle is changing heading, rather than vehicle roll. In either event, if the availability of the GPS signal is subsequently lost, the multiaxis accelerometer can continue to propagate the vehicle position more accurately because it has determined the pitch and roll of the vehicle. Similarly, by comparing the GPS velocity information to the signal from the longitudinal accelerometer of the multiaxis accelerometer, the navigation system determines whether the vehicle is accelerating or is pitched.  
           [0010]    Pitch and roll (“attitude”) of the vehicle is also determined solely by the multiaxis accelerometer even when GPS velocity information and map matching information is not available. The pitch and roll of the vehicle is determined by the multiaxis accelerometers when gravity is substantially the only acceleration acting upon the multiaxis accelerometer. A vehicle could be moving, but is not changing heading, speed, pitch or roll. When the resultant vector from the orthogonal accelerometers and the multiaxis accelerometer is substantially 1G and is substantially constant for a period of time (1 to 5 seconds), the pitch and roll of the vehicle can be determined by comparison of the vehicle axes to the  1  G resultant vector. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
         [0012]    [0012]FIG. 1 is a schematic of the navigation system of the present invention installed in a vehicle;  
         [0013]    [0013]FIG. 2 is a logic chart for the attitude estimation logic for the navigation system of FIG. 1;  
         [0014]    [0014]FIG. 3 is a logic chart for the accelerometers and synthesized rate gyros of the navigation system of FIG. 1; and  
         [0015]    [0015]FIG. 4 is a flow chart for propagating position according to the present invention in the navigation system of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0016]    The navigation system  20  of the present invention is shown schematically in FIG. 1 installed in a vehicle  21 . The navigation system  20  includes an Operator Interface Module (“OIM”)  22  including input and output devices. The OIM  22  includes a display  24 , such as a high resolution LCD or flat panel display, and an audio speaker  26 . The OIM  22  also includes input devices  28 , preferably a plurality of buttons and directional keypad, but alternatively including a mouse, keyboard, keypad, remote device or microphone. Alternatively, the display  24  can be a touch screen display.  
         [0017]    The navigation system  20  further includes a computer module  30  connected to the OIM  22 . The computer module  30  includes a CPU  32  and storage device  34  connected to the CPU  32 . The storage device  34  may include a hard drive, CD ROM, DVD, RAM, ROM or other optically readable storage, magnetic storage or integrated circuit. The storage device  34  contains a database  36  including a map of all the roads in the area to be traveled by the vehicle  21  as well as the locations of potential destinations, such as addresses, hotels, restaurants, or previously stored locations. The software for the CPU  32 , including the graphical user interface, route guidance, operating system, position-determining software, etc may also be stored in storage device  34  or alternatively in ROM, RAM or flash memory.  
         [0018]    The computer module  30  preferably includes navigation sensors, such as a GPS receiver  38  and an inertial sensor, which is preferably a multi-axis accelerometer  40 . The computer module  30  may alternatively or additionally include one or more gyros  42 , a compass  44 , a wheel speed sensor  46  and altimeter  48 , all connected to the CPU  32 . Such position and motion determining devices (as well as others) are well known and are commercially available.  
         [0019]    The navigation system  20  propagates the position of the vehicle  21  relative to the map database  36 , i.e. relative to road segments and intersections. The navigation system  20  also determines the current location of the vehicle  21  in terms of latitude and longitude. Generally, the CPU  32  and position and motion determining devices determine the position of the vehicle  21  relative to the database  36  of roads utilizing dead reckoning, map-matching, etc. Further, as is known in navigation systems, the user can select a destination relative to the database  36  of roads utilizing the input device  28  and the display  24 . The navigation system  20  then calculates and displays a recommended route directing the driver of the vehicle  21  to the desired destination. Preferably, the navigation system  20  displays turn-by-turn instructions on display  24  and gives corresponding audible instructions on audio speaker  26 , guiding the driver to the desired destination.  
         [0020]    Preferably, the GPS receiver  38  provides speed and heading information to the CPU  32  utilizing the technique described in co-pending U.S. application Ser. No. 08/579,902, entitled “Improved Vehicle Navigation System and Method Using GPS Velocities,” which is hereby incorporated by reference in its entirety.  
         [0021]    Although the navigation system  20  of the present invention could include a variety of different combinations of different sensors and utilize a variety of different techniques for propagating the position of the vehicle  21 , the present invention relates specifically to the use of the multi-axis accelerometer  40 . It should be understood that some details would vary based upon the other sensors available and the specific techniques selected. For purposes of illustration, propagation of position using the multi-axis accelerometer  40  will be described with the benefit of the GPS receiver  38 , although other sensors could also be utilized.  
         [0022]    [0022]FIG. 2 is a flow chart for the attitude estimation logic. In step  60  qualified speed snaps are received from the GPS receiver  38  in accordance with the technique described in U.S. Pat. No. 6,029,111 entitled “Improved Vehicle Navigation System and Method Using GPS Velocities.” Generally, the speed snaps from the GPS receiver  38  are accurate for speeds over 1.5 m/s. In step  62 , the pitch error is computed according to the formula shown.  
         [0023]    In step  64 , a low pass filter is applied to the pitch angle error, preferably 3 Hz. In step  66 , pitch snaps are performed, snapping the pitch value. Similarly, with respect to calculation of roll angle error, in step  70 , qualified heading snaps are taken. Again, these heading snaps can be from GPS in accordance with the technique described in U.S. Pat. No. 6,029,111 or utilizing known map matching techniques. The roll error is then calculated in step  72  utilizing equations shown. A low pass filter is then applied to the roll error in step  74 , preferably 3 Hz. In step  76 , the roll snaps are performed.  
         [0024]    Concurrently, in step  80 , the linear accelerations are measured by the multiple axis accelerometer  40  (FIG. 1) from which linear accelerations relative to the vehicle frame are determined: A x  (forward acceleration), A y  (acceleration toward the right), and A z  (acceleration down).  
         [0025]    From these linear accelerations, pitch and roll are estimated in steps  82  and  84 , respectively, in the manner which will be described below. Low pass filters are applied to the pitch and roll estimates in steps  86  and  88 , respectively. Partial snaps are performed of the pitch and roll values in steps  90  and  92  respectively. The intelligent inertial monitor and snap controller  96  controls the snaps in steps  90  and  92  and the variable low pass filters  86 ,  88 .  
         [0026]    The synthesized pitch rate gyro  98  and a 90 second decay to zero  100  are input to the INS propagation of pitch  102 . The 90 second decay to zero  100  automatically gradually returns the calculated pitch back to zero in 90 seconds. The INS propagation of pitch  102  also receives pitch snaps from step  66  and partial pitch snaps from step  90 .  
         [0027]    A turn compensation adjustment  104  and a 10 second decay to zero  106  are input to the estimate of roll  108 , as are the roll snaps  106  and partial roll snaps  92 . The  10  second decay to zero  106  automatically gradually returns the calculated roll back to zero in ten seconds. The propagation of pitch  102  and estimate of roll  108  together give the estimate of attitude  110 .  
         [0028]    [0028]FIG. 3 is the logic flow chart for the accelerometers and synthesized rate gyros. Element  120  is the sensor suite orientation learn algorithm which calculates the rotation matrix C b   s  rotation matrix between the arbitrary sensor orientation and the body frame (forward, right, down). The rotation matrix converts the accelerometer signals  122  to the non-compensated car body frame linear acceleration signals in step  124 . The dead-reckoned speed and heading are input to a fizzy roll and yaw predictor  166 . Based upon the fuzzy roll and yaw protector  126 , C TURN  is computed in step  128 . C TURN  is a fast, dynamic compensation for yaw and roll access rotation during a turn. During a turn, roll is due to vehicle dynamics and yaw is due to vehicle side-slip. C TURN  is then applied to the non-compensated car body frame signal  124  to give the compensated car body frame acceleration  130 , which is oriented approximately flat against the road grade with x forward and y transverse even when turning, thus removing the effects of roll due to vehicle dynamics and yaw due to vehicle side slip.  
         [0029]    The compensated car body frame acceleration  130  can be converted to the navigation frame  132  by C n   b . The accelerometer bias estimate  134  is removed from the compensated car body frame acceleration signals  130  and gravity in the wander azimuth frame (forward right down with respect to gravity) is converted to the body frame by C b   w  and also removed from the acceleration signals. These linear acceleration signals (forward, right, down with respect to the vehicle body) are used to generate the linear forward acceleration A x , from which the speed of the vehicle can be determined. A new pitch rate ω Y  is calculated in step  140  as the linear acceleration downward A z  divided by the speed of the vehicle. Similarly, the yaw rate ω Z  is calculated as the negative lateral acceleration divided by the speed of the vehicle. ω Y  and ω Z  are the synthesized rate gyros which can determine pitch and yaw.  
         [0030]    The forward acceleration A x  and synthesized rate gyros ω Y  and ω Z  are used in the flow chart of FIG. 4, starting in step  150 . Low pass filtering is applied in step  152 . Preferably, a 10 Hz low pass filter is applied to the forward acceleration A x  while a 3 Hz low pass filter is applied to both the pitch and yaw rates ω Y  and ω Y  The new pitch is calculated in step  154  as the previously calculated pitch plus the pitch rate ω Y  times the elapsed time. Preferably, the pitch calculation is limited to plus or minus 10 degrees.  
         [0031]    In step  156 , the new heading is calculated as the previously calculated heading plus ω Z  time the elapsed time. In step  158 , the new speed is calculated as the old speed plus the forward acceleration A x , times the elapsed time.