Calibration apparatus of angular velocity sensor in self-contained navigational system

A calibration apparatus of an angular-velocity detection sensor for detecting a heading of a vehicle which is used in a self-contained navigational system, comprising an angular-velocity detection unit for calculating an angular velocity from an output signal of the angular-velocity detection sensor; an integration unit for calculating a current heading of the vehicle by integrating the angular velocity and by adding the integrated angular velocity to the previous heading of the vehicle; a location and heading detection unit which estimates a location of the vehicle from the current heading calculated by the integration unit, a heading obtained by a magnetic sensor and from a distance obtained by a distance sensor and which compares the estimated location with road network data obtained from a road map memory to obtain an estimated heading of the vehicle; and a heading correction unit for calculating a difference between the heading of the integration unit and the heading of the location and heading detection unit and integrating the difference over a predetermined distance to obtain an error per unit distance, for calculating the difference or the error per unit distance, as a correction value for correcting the current heading of the integration unit, and for feeding the correction value back to the intergration unit. The current heading of the integration unit is corrected by subtracting the correction value from the current heading of the integration unit.

FIELD OF THE INVENTION 
The present invention relates to a calibration apparatus of an 
angular-velocity sensor (rate gyro) which is used in a self-contained 
navigational system as one of heading sensors. 
DESCRIPTION OF THE PRIOR ART 
A variety of automatic vehicle navigational systems have been developed and 
used to provide information about the actual location of a vehicle as it 
moves over streets. For example, one general approach to such navigational 
systems is known as "dead reckoning", in which the vehicle is tracked by 
advancing a "dead reckoned position" from measured distances and courses 
or headings. A system based upon dead reckoning principles may, for 
example, detect the distance travelled and heading of the vehicle using 
distance and heading sensors on the vehicle. 
The heading sensor used in dead reckoning may comprise a magnetic sensor or 
turning-angle sensor. In the magnetic sensor, a reading error of more than 
10.degree. tends to occur due to the distortion of earth's magnetism and 
influence of external magnetic fields, and spurious magnetic fields 
associated with the vehicle have also to be compensated. In the 
turning-angle sensor, there is used a differential odmeter for detecting a 
difference of rotation between the left and right wheels. Since the 
turning-angle sensor detects the rotation of tires, it has its 
disadvantage in that there occur errors caused by a change or distortion 
in tire radius, slips and the like. 
In order to improve the drawbacks, it has been proposed to replace the 
above described sensors with angular-velocity sensors such as optical 
fiber gyros, vibration gyros and coma-type gyros which detect directly 
rotational angular velocities. For example, in the optical fiber gyro, 
laser light is transmitted through an optical fiber ring in the clockwise 
direction and anticlockwise direction. If the optical fiber ring is 
rotated, then the lights in both directions become different in time to 
pass through the optical path. This time difference is detected as a phase 
difference. 
The vibration gyro is one wherein a rotational angular velocity is detected 
by detecting a change of the vibration distribution of a particle 
vibrating in a container, since the particle disposed in rotational 
coordinates is subjected to Coriolis force. 
The errors involved in the angular velocity detected by the 
angular-velocity sensor are relatively small. However, if an error is 
involved in an initial heading used in obtaining the angular velocity by 
integrating the sensor output, this error will be fixed thereafter. That 
is to say, even if the detected angular velocity is accurate, the 
distinctive features of the angular-velocity sensor cannot be utilized if 
the initial heading is inaccurate. On the other hand, if the vehicle 
heading is corrected to match with the heading obtained from the magnetic 
sensor, it will be subjected to the distortion of earth's magnetic field 
in that place. In addition, in a method wherein a vehicle is stopped on a 
road to match with the road heading on a map, stopping the vehicle is 
troublesome, and an accurate calibration cannot be performed since the 
method depends upon the position of the vehicle stopped on the road. 
Moreover, there are some cases where an error occurs in the absolute value 
of an angular velocity detected, because of insufficient adjustment and 
deterioration of the signal processing systems of the angular-velocity 
sensor. That is, the scale factor of the angular-velocity sensor 
fluctuates. The error caused by this fluctuation in the scale factor does 
not appear when the vehicle moves in a straight line, but it appears each 
time the vehicle turns. Therefore, if the vehicle travels a large number 
of curves in the road, errors are then accumulated and interfere with dead 
reckoning. 
Accordingly, it is an object of the present invention to provide an 
improved calibration apparatus which is capable of automatically 
calibrating the output of an angular-velocity sensor which is used in a 
self-contained navigational system using map matching. 
SUMMARY OF THE INVENTION 
In order to achieve the above object, as shown in FIG. 1, a calibration 
apparatus of an angular-velocity detection sensor for detecting a heading 
of a vehicle, which sensor is used in a self-contained navigational 
system, comprises angular-velocity detection means 1 for calculating an 
angular velocity from an output signal of the angular-velocity detection 
sensor 15, and integration means 2 for calculating a current heading of 
the vehicle by integrating the angular velocity and by adding the 
integrated angular velocity to the previous heading of the vehicle. The 
calibration apparatus of angular-velocity detection sensor further 
comprises location and heading detection means 3 which estimates a 
location of the vehicle from the current heading calculated by the 
integration means, a heading obtained by a magnetic sensor and from a 
distance obtained by a distance sensor. The location and heading detecting 
means 3 then compares the estimated location with road network data 
obtained from a road map memory to obtain an estimated heading of the 
vehicle. In addition, there is provided heading correction means 4 for 
calculating a difference between the heading of the integration means and 
the heading of the location and heading detection means and integrating 
the difference over a predetermined distance to obtain an error per unit 
distance, for calculating the difference or the error per unit distance, 
as a correction value for correcting the current heading of the 
integration means, and for feeding the correction value back to the 
integration means. 
According to the calibration apparatus of the angular-velocity sensor, the 
output of the angular-velocity detection sensor 15 is detected at all 
times and the angular velocity d.theta.r/dt is calculated by the 
angular-velocity detection means 1. The angular velocity d.theta.r/dt is 
then integrated and the integrated angular velocity is added to the 
previous heading of the vehicle by the integration means 2 to calculate a 
current heading .theta.r of the vehicle. 
The current heading .theta.r of the vehicle is supplied as base data of map 
matching to the location and heading detection means 3. The heading output 
.theta.e obtained from the location and heading detection means 3 with the 
aid of the map matching is inputted to the heading correction means 4, 
together with the current heading .theta.r obtained from the integration 
means 2. 
The heading correction means 4 calculates the difference between the 
heading output .theta.r of the integration means 2 and the heading output 
.theta.e of the location and heading detection means 3. If necessary, this 
difference is accumulated over the distances travelled by the vehicle. The 
reason that the difference is accumulated is that the accuracy of data is 
increased, since errors resulting from various factors are involved in the 
difference not accumulated. 
Thereafter, the correction value .delta..theta. for correcting the heading 
output .theta.r of the integration means 2 is calculated from the 
difference or an accumulated value of the difference, and fed back to the 
integration means 2. 
Since the integration means 2 corrects the current heading .theta.r with 
the correction value .delta..theta. and the corrected heading is supplied 
as base data of the map matching to the location and heading detection 
means 3, the accuracy of the map matching can be enhanced. 
In addition, as shown in FIG. 2, in a self-contained navigational system 
wherein the location and heading of the vehicle on road are detected with 
the aid of location and heading detection means 3 by map matching, the 
calibration apparatus of the angular-velocity sensor according to the 
present invention comprises: 
an angular-velocity detection sensor 15 for detecting a heading of the 
vehicle; 
angular-velocity detection means 1 for calculating an angular velocity from 
an output signal of the angular-velocity detection sensor 15; 
integration means 2 for calculating a current heading of the vehicle by 
integrating the angular velocity and by adding the integrated angular 
velocity to the previous heading of the vehicle; and 
heading correction means 7 which, when it has detected a turning of the 
vehicle, calculates a first change .delta..theta.r of a heading output 
.theta.r obtained from the integration means 2 and a second change 
.delta..theta.e of a heading output .theta.e obtained from the location 
and heading detection means 3, and which calculates a correction value 
.delta..phi. for correcting the angular velocity of the angular-velocity 
detection means 1, with the aid of the first and second changes 
.delta..theta.r and .delta..theta.e or accumulated values of the changes, 
and which feeds the correction value .delta..phi. back to the 
angular-velocity detection means 1. 
The calibration apparatus shown in FIG. 2 is similar to that of FIG. 1 
except the heading correction means 7. 
When the heading correction means 7 has detected a turning of the vehicle, 
it calculates the first change .delta..theta.r (amount of rotation) of the 
heading output .theta.r and the second change .delta..theta.e of the 
heading output .theta.e before and after the turning. If necessary, these 
first and second changes are accumulated over the distances travelled by 
the vehicle. The reason that the changes are accumulated is that the 
accuracy of data is increased, since errors resulting from various factors 
are involved in the change not accumulated. 
The correction value .delta..phi. for correcting the angular velocity of 
the angular-velocity detection means 1 is then calculated with the aid of 
the first and second changes .delta..theta.r and .delta..theta.e or 
accumulated values of the changes, and fed back to the angular-velocity 
detection means 1. 
The angular-velocity detection means 1 corrects the scale factor of the 
angular velocity with the correction value .delta..theta., and the 
corrected angular velocity is supplied to the integration means 2. Since 
the output of the integration means 2 is supplied as base data of the map 
matching to the location and heading detection means 3, the accuracy of 
the map matching can be enhanced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, there is shown a self-contained navigational system 
into which a calibration apparatus of an angular-velocity sensor according 
to the present invention may be incorporated. The self-contained 
navigational system comprises a console 11, a display 12, a magnetic 
sensor 13, a distance sensor 14, an angular-velocity sensor 15, a road map 
memory 10 for storing map data and the like, and a memory drive 16 for 
reading data from the road map memory 10. The self-contained navigational 
system further comprises a location and heading detecting apparatus 17 
which evaluates the probability of existence of a vehicle on each road and 
calculates the location and heading of the vehicle from the evaluation. 
The self-contained navigational system further comprises a navigation 
controller 18 which retrieves and reads a predetermined range of a road 
map, drives the display 18 and performs various arithmetic controls such 
as controls of the location and heading detecting apparatus 17. 
The above described console 11 has a keyboard (not shown) which allows a 
vehicle operator to start and stop this self-contained navigational 
system, and to move a cursor on the picture screen of the display 12 and 
to scroll the road map displayed on the picture screen. 
The road map memory 10 comprises a mass storage medium memory such as a 
CDROM, magnetic tape and like. In the road map memory 10, the road map is 
divided into mesh blocks, and data (map data) of route junctions (nodes) 
to detect a vehicle location at the unit of each mesh block stored. This 
road map data is used for graphic display and route calculation. 
The display 12 comprises a CRT (cathode Ray Tube) and a crystalline panel, 
and displays textual and line graphic information such as road maps, 
vehicle location, recommended routes, vehicle heading and distance to a 
destination. 
The magnetic sensor 13 detects an absolute angle of the heading of the 
vehicle as it moves over streets. 
The angular-velocity sensor 15 comprises an optical fiber gyro, but it may 
comprise a vibration gyro or a mechanical type gyro. 
The distance sensor 14 is used to detect distances travelled by the 
vehicle. For example, the distance sensor 14 can constitute a vehicle 
speed sensor which senses the speed of the vehicle, or one or more wheel 
sensors which sense the rotation of the wheels of the vehicle. 
The navigation controller 18 receives information about vehicle locations 
from the location and heading detecting apparatus 17, and displays on the 
map the current location and destination of the vehicle. More 
specifically, the navigation controller 18 is constituted by a 
microcomputer (not shown), a graphic data processor (not shown) and a 
picture image processing memory (not shown), and performs the display of 
menus, retrieval of maps, switching of contraction scale, zoom scroll, 
display of the current location and heading of a vehicle, display of a 
destination or guide spots, and display of the heading and distance to an 
destination. 
The location and heading detecting apparatus 17 integrates the distances 
detected by the distance sensor 14 and the headings detected by the 
magnetic sensor 13 and angular-velocity sensor 15, and calculates the 
location of the vehicle by comparing the integrated data with the map data 
that has been read by the memory drive 16, and outputs the location and 
heading of the vehicle. 
FIG. 4 shows the structure of the calibration apparatus of the 
angular-velocity sensor 15. 
The calibration apparatus comprises angular-velocity detection means 21 for 
detecting the rotational angular velocity of the angular-velocity sensor 
15, and integration means 22 for integrating the angular velocity each 
unit sampling times to calculate a heading angle and for adding the 
heading angle to the heading obtained at the previous sampling time to 
calculate a heading .theta.r at the current time. 
The calibration apparatus further comprises location and heading detection 
means 23, heading-error-amount count means 24, initial-heading set means 
25, and drift calibration means 26. 
The location and heading detection means 23 calculates repeatedly the 
similarity between the map data and the estimate location obtained from 
the heading output of the magnetic sensor 13 and distance output of the 
distance sensor 14, and determines a most probable vehicle location and 
heading on the road by map matching 
The heading-error-amount count means 24 compares the heading output 
.theta.r from the integration means 22 with the estimated heading output 
.theta.e (or heading of the estimated road) from the location and heading 
detection means 23, at intervals of unit travel distances .delta.l shown 
in FIG. 6. If the accumulation of the difference between the heading 
output .theta.r and the estimated heading output .theta.e is above a 
predetermined value, then the heading-error-amount count means 24 feeds a 
heading correcting output back to the integration means 22. 
The initial-heading set means 25 sets an initial heading .theta.o of a 
vehicle with the aid of the magnetic sensor output or heading of the road, 
when the vehicle starts travelling. 
The drift calibration means 26 calibrates an angular-velocity drift error 
of the angular-velocity sensor 15. 
The calibrating operation of the angular-velocity sensor 15 constructed as 
shown in FIG. 4 will hereinafter be described with respect to FIG. 5. 
The output d.theta.r/dt of the angular-velocity detection means 21 is 
calculated in step S1, and integrated in step S2 at intervals of 
predetermined clock times .delta.t. The integrated output is added to the 
previous heading .theta.r' (initial heading is .theta.o) to obtain the 
following present heading .theta.r: 
##EQU1## 
Next, in step S3, there is obtained the difference between the heading 
.theta.e(li) obtained every travel distances li (i=1, 2, . . . , and n) 
shown in FIG. 6 by the location and heading detection means 23 and the 
heading .theta.r(li) that has been obtained in the step 2, and in step S4, 
the difference is accumulated by the following equation: 
##EQU2## 
The accumulation (.delta..theta.j(n)) is continued until it becomes a 
predetermined value. If the accumulation (.delta..theta.j(n)) becomes 
above the predetermined value, it is then stopped. 
In step S5, it is determined if .delta..theta.j(n) is above the 
predetermined value. If yes, an error to one interval of travel distance 
.delta.1 is calculated by the following equation: 
##EQU3## 
The error is subtracted from the heading .theta.r that has been obtained 
by the integration means 22. This heading (.theta.r-.delta..theta.j(n)/n) 
is supplied as .theta.r to the location and heading detection means 23, 
and the map matching is continued. If, on the other hand, 
.delta..theta.j(n) is not above the predetermined value, the step S5 then 
returns back to the step S1, and the above described accumulation is 
repeated. 
Thus, if the accumulation of the difference (error) between the output of 
the angular-velocity sensor 15 and the heading obtained by the map 
matching becomes the predetermined value, then by subtracting the error 
from the heading obtained by the integration means 22, the 
angular-velocity sensor 15 can be calibrated so that a correct heading is 
obtained at all times. 
FIG. 7 shows a second embodiment of the calibration apparatus of the 
angular-velocity sensor according to the present invention. 
As shown in FIG. 7, this embodiment is identical to the embodiment of FIG. 
4, except that a rotational angle error-ratio count means 27 is provided 
instead of the heading-error-amount count means 24 and that the output of 
the rotational angle error-ratio count means 27 is fed back to the 
angular-velocity detection means 21. 
If the rotational angle error-ratio count means 27 detects that the vehicle 
heading is rotated above a predetermined angle, then it calculates the 
following ratio: 
##EQU4## 
where .vertline..delta..theta.r.vertline. is an absolute value of the 
rotational angle .delta..theta.r calculated from the heading output 
.theta.r of the integration means 22, and 
.vertline..delta..theta.e.vertline. is an absolute value of the rotational 
angle .delta..theta.r calculated from the heading output .theta.e of the 
location and heading detection means 23. The above described ratio is 
accumulated each time the angle above a predetermined value is detected. 
If the accumulated ratio becomes above the predetermined value, then it is 
fed back to the angular-velocity detection means 21 as a scale factor 
correcting output. 
The correcting operation of the scale factor of the angular-velocity sensor 
15 will hereinafter be described with respect to FIG. 8. 
The output d.theta.r/dt of the angular-velocity detection means 21 is 
calculated in step S11, and integrated in step S12 at intervals of 
predetermined clock times .delta.t. The integrated output is added to the 
previous heading .theta.r' (initial heading is .theta.o) to calculate the 
following present heading .theta.r: 
##EQU5## 
Next, in step S13, rotational angles .theta.r(t-1) and .theta.e(t-1) at the 
time the beginning of one curve in a road has been detected are obtained. 
The beginning of the curve can be detected, for example, by detecting that 
the output d.theta.r/dt of the angular-velocity detection means 21 or the 
differentiated value d.sup.2 .theta.r/dt.sup.2 is above a threshold. 
Rotational angles .theta.r(t) and .theta.e(t) at the time the end of the 
curve has been detected are also obtained. The end of the curve can be 
detected, for example, by detecting that the output d.theta.r/dt of the 
angular-velocity detection means 21 or the differentiated value d.sup.2 
.theta.r/dt.sup.2 is below the threshold. The difference between 
.theta.r(t) and .theta.r(t-1) and the difference between .theta.e(t) and 
.theta.e(t-1) are obtained as follows: 
EQU .delta..theta.r=.theta.r(t)-.theta.r(t-1) 
EQU .delta..theta.e=.theta.e(t)-.theta.e(t-1) 
If .delta..theta.r and .delta..theta.e are above a predetermined value, 
then the ratio of the difference between the absolute values is calculated 
by the following equation: 
##EQU6## 
It is noted that the absolute values are used in the calculation of the 
above described ratio, in order that the fluctuations of the scale factors 
are prevented from being cancelled with each other at the left and right 
curves. 
In step S14, the calculation of the ratio is repeated each time the vehicle 
travels each curve, and accumulated by the following equation: 
##EQU7## 
In step S15, it is determined if .delta..theta.(m) is above a predetermined 
value. If yes, the step S15 advances to step 16, in which the scale factor 
is corrected. The scale factor is corrected by dividing d.theta.r/dt by 
the following equation: 
##EQU8## 
where .delta..phi.(m)/m is an average value of .delta..phi.(m) to one 
curve. The scale factor may also be corrected by subtracting a value 
obtained by multiplying d.theta.r/dt by .delta..phi.(m)/m, from 
d.theta.r/dt. When .delta..phi.(m)/m is far below 1, the result of the 
former computation is nearly the same as that of the latter computation. 
Therefore, even if the scale factor of the angular-velocity sensor 15 
fluctuates, the fluctuation can be automatically calibrated by matching 
with the rotational angle obtained from the map matching each time the 
vehicles travels a curve in a road. 
The present invention is not limited to the above described first and 
second embodiments. For example, if the first and second embodiments are 
used together, the effect of the calibration is further enhanced. 
While certain representative embodiments and details have been shown for 
the purpose of illustrating the invention, it will be apparent to those 
skilled in this art that various changes and modifications may be made 
therein without departing from the scope of the invention.