Navigation device and direction detection method therefor

The navigation device includes vehicle condition discriminating unit for discriminating whether the vehicle is stopped or not, angular velocity detecting unit for detecting change of direction of the vehicle and producing angular velocity data on the basis of the detected change of direction, absolute direction detecting unit for detecting geomagnetism and producing absolute direction data on the basis of the detected geomagnetism, offset processing unit for producing corrected angular velocity data from the angular velocity data and a first reference value, and offset reset unit for renewing the first reference value. The offset reset unit renews the first reference value with the angular velocity data which is obtained when the vehicle is discriminated to be stopped, a change rate of the angular velocity data is within a predetermined second reference value and a change rate of the absolute direction data is within a predetermined third reference value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a navigation device which supports driving of a 
vehicle such as a car or the like by displaying and informing current 
position and travel direction of the vehicle, and more particularly to an 
offset processing of the navigation device for correcting an angular 
velocity data measured by an angular velocity sensor such as a gyro. 
2. Description of Prior Art 
A navigation device installed in a vehicle such as a car or the like has 
been required to precisely measure current position and travel direction 
of the vehicle and then accurately and rapidly display the current 
position, the travel direction of the vehicle and other information for 
satisfying user's demands together with a map information corresponding to 
the current position for assistance of a driver. 
One type of navigation devices calculates the current position and travel 
direction of the vehicle from electric waves transmitted from a GPS 
(Global Positioning System) satellite (hereinafter referred to as "GPS 
measurement method"). Another type of navigation device calculates them 
from data of a direction sensor such as a gyro, a geomagnetic sensor or 
the like and a travel sensor for measuring a travel distance or the like, 
each installed in the vehicle (hereinafter referred to as "stand-alone 
type measurement method"). The GPS measurement method has such various 
advantages that it is not required to set in advance a vehicle position on 
a map and that the measurement error of the vehicle position is remarkably 
small and accordingly high reliability can be achieved. Therefore, the GPS 
measurement method is broadly used. However, the GPS measurement method 
can not used when the vehicle travels through a shade of buildings, woods 
or in a tunnel. On the other hand, the stand-alone type measurement method 
has such disadvantage that the measured data is not always accurate 
because of the accumulated error, influences from variations of 
temperature or condition in the vehicle, and bad influence on the 
geomagnetic sensor resulting from traveling on iron bridges. Accordingly, 
the two types of measurement methods are used in combination in order to 
make up for the above deficiencies with each other. 
Since output data of an angular velocity sensor used for the stand-alone 
type measurement, such as a gyro, varies according to the variation of 
temperature or humidity, the output data is corrected by using an offset 
value which is occasionally reset (renewed). 
However, the above angular velocity correction method can not accurately 
detect the change of the direction when the vehicle is rotated by a 
turntable or the like in a parking zone, for example. This is because, in 
such a case, since the vehicle is stopped even though the turntable is 
rotated, the reset (renewal) of the offset value is erroneously executed. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a navigation device which can 
accurately detect the change of the direction of the vehicle even when the 
vehicle is stopped. 
According to one aspect of the invention, there is provided a navigation 
device including vehicle condition discriminating unit for discriminating 
whether the vehicle is stopped or not, angular velocity detecting unit for 
detecting change of direction of the vehicle and producing angular 
velocity data on the basis of the detected change of direction, absolute 
direction detecting unit for detecting geomagnetism and producing absolute 
direction data on the basis of the detected geomagnetism, offset 
processing unit for producing corrected angular velocity data from the 
angular velocity data and a first reference value, and offset reset unit 
for renewing the first reference value. 
The offset reset unit renews the first reference value with the angular 
velocity data which is obtained when the vehicle is discriminated to be 
stopped, a change rate of the angular velocity data is within a 
predetermined second reference value and a change rate of the absolute 
direction data is within a predetermined third reference value. 
According to another aspect of the present invention, there is provided a 
method of detecting traveling direction data obtained by a navigation 
device which detects angular velocity indicating the traveling direction 
of a vehicle, the method comprising the steps of, discriminating whether 
the vehicle is stopped or not, detecting change of direction of the 
vehicle and producing angular velocity data on the basis of the detected 
change of direction, detecting geomagnetism and producing absolute 
direction data on the basis of the detected geomagnetism, producing 
corrected angular velocity data from the angular velocity data and a first 
reference value, and renewing the first reference value with the angular 
velocity data which is obtained when the vehicle is discriminated to be 
stopped, a change rate of the angular velocity data is within a 
predetermined second reference value and a change rate of the absolute 
direction data is within a predetermined third reference value. 
According to the present invention, the change of the direction can be 
accurately detected, and a navigation device reliable in direction 
detection function can be provided.

DETAILED DESCRIPTION OF THE INVENTION 
A preferred embodiment according to the present invention will now be 
described below with reference to the accompanying drawings. 
FIG. 1 is a block diagram showing an embodiment of a navigation device for 
a vehicle according to the present invention. As shown in FIG. 1, the 
navigation device includes a geomagnetic sensor 1 for outputting a 
direction data of the vehicle on the basis of geomagnetism (earth 
magnetism), a gyro 2 for detecting an angular velocity variation of the 
vehicle, a travel distance sensor 3 for discriminating whether the vehicle 
is running or stopped by the speed of the vehicle and measuring a travel 
distance of the vehicle, a GPS measurement device 4 for receiving electric 
waves from a plurality of GPS satellites to calculate and output data of 
latitude, longitude, altitude and travel direction of the vehicle on the 
basis of the received signals, a system controller 5 for executing various 
processings such as calculation, control, etc., an input device 14 for 
inputting various instructions into the system controller 5, and a display 
15 such as a liquid crystal display, a CRT or the like. 
The system controller 5 includes an interface 6 for receiving the output 
signals from the geometric sensor 1, the travel distance sensor 3 and the 
GPS measurement device 4 and subjecting these output signals to A/D 
conversion, a CPU (Central Processing Unit) 7 for carrying out various 
calculations and controls in accordance with processing programs, a ROM 
(Read Only Memory) 8 in which various processing programs and other 
necessary information are beforehand stored, a RAM (Random Access Memory) 
9 in and from which information required to execute the programs are 
written and read, a recording medium 10 such as a CD-ROM, an IC card or 
the like in which digitalized map information, etc. are recorded, a buffer 
memory 11 such as V-RAM (video RAM) or the like for temporarily storing, as 
a frame buffer, an image information to be directly displayed, a graphic 
controller 12 for inputting graphic data such as map information into the 
buffer memory 11 and outputting the data stored in the buffer memory 11 as 
image information in response to the instruction from the CPU 7, and a 
display control circuit 13 for receiving the image information output from 
the graphic controller 12 and controlling an image display operation of the 
display 15 on the basis of the received image information. 
When the navigation device thus constructed is activated, the system 
controller 5 reads out information for access of the map display 
information and a display information for the vehicle position mark, etc. 
from the recording medium 10, and then stores these information into the 
RAM 9. Subsequently, the system controller 5 reads out latitude/longitude 
data corresponding to vehicle position information and travel direction 
information of the vehicle through the GPS positioning device 4, and reads 
out the map information corresponding to the obtained vehicle position from 
the recording medium 10. The read-out map information is fed to the graphic 
controller 12 to be displayed as the current position map on the display 
15. In addition, the system controller 5 displays the vehicle position 
mark on the map on the basis of the vehicle position information and the 
travel direction information. Subsequently, the vehicle position 
information and the travel direction information are periodically obtained 
from the GPS measurement device 4 to renew the display position, direction 
of the vehicle position mark, and the display map if necessary- Further, 
the output data from the geomagnetic sensor 1, the angular velocity sensor 
2 and the travel distance sensor 3 are periodically read out and then 
subjected to a predetermined calculation to produce the vehicle position 
and the travel direction of the vehicle. After comparing the thus 
calculated information with the information obtained from the GPS 
measurement device 4, a correction is carried out to reduce the deviation 
(error) between these information. In the detection of the traveling 
direction of the vehicle from the output data of the geomagnetic sensor 1, 
the gyro 2 and the travel distance sensor 3, the system controller 5 
calculates the traveling direction of the vehicle based on the changes of 
the angular velocity data which is obtained in the angular velocity 
detection process shown in FIG. 2. 
FIG. 2 shows the angular velocity detection process performed by the system 
controller 5 which include the offset reset operation. 
In the angular velocity detection process, first, the system controller 5 
obtains the output data .omega. from the gyro 2 and the travel distance 
data from the travel distance sensor 3 in steps S1 and S2. Then it is 
discriminated that whether the vehicle is stopped or not based on the data 
obtained from the travel distance sensor 3. If the discrimination in step 
S3 results in YES, the rate of change of the angular velocity .omega., 
i.e., the acceleration d.omega./dt is calculated and it is discriminated 
that the acceleration d.omega./dt is within (smaller than) a predetermined 
value k or not in step S4. If the discrimination in step S4 results in YES, 
the absolute direction data .theta..sub.m is obtained from the geomagnetic 
sensor 1, the change rate d.theta..sub.m /dt of the absolute direction 
data .theta..sub.m is calculated from the data previously obtained, and it 
is discriminated that the change rate d.theta..sub.m /dt of the absolute 
direction data .theta..sub.m is within a predetermined value k.sub.m in 
step S5. If the discrimination in step S5 results in YES, the reference 
output value (offset value) .omega..sub.0 of the gyro 2 is reset to be the 
angular velocity value .omega.(.omega..sub.0 =.omega.) at that time in step 
S6. If the offset value is thus reset in step S6, the corrected angular 
velocity value .omega..sub.c (=.omega.-.omega..sub.0) obtained by the 
offset processing in step S7 becomes equal to zero. 
If at least one of the discriminations in steps S3 to S5 results in NO, and 
the offset processing in step S7 is executed without resetting the offset 
value .omega..sub.0 and the corrected angular velocity value .omega..sub.c 
is calculated using the previous offset value .omega..sub.0. 
Next, the angular velocity detection is described in more detail with 
reference to FIGS. 3A to 3D. FIG. 3A shows the change of the angular 
velocity data .omega. output from the gyro 2 when the vehicle is rotated 
by 180 degrees by turntable or the like. IN FIG. 3A, the square measure of 
the oblique line area corresponds to the rotation degree of 180 degrees, 
and the rotation speed of the turn table is substantially constant except 
at the beginning and the ending of the rotation. FIG. 3B shows the change 
rate d.omega./dt of the angular velocity data .omega.. FIG. 3C shows the 
relative change of the direction obtained from the absolute direction data 
.theta..sub.m output from the geomagnetic sensor 1 when the vehicle is 
rotated by 180 degrees in the same manner. FIG. 3D shows the change rate 
d.theta..sub.m /dt of the absolute direction data .theta..sub.0 at that 
time. When the vehicle is rotated by 180 degrees, the change rate 
d.theta..sub.m /dt of the absolute direction data .theta..sub.m becomes 
out of the predetermined value k.sub.m during the rotation, as shown in 
FIG. 3D. 
If only the change rate d.omega./dt of the angular velocity data .omega., 
shown in FIG. 3B, is monitored and the offset value is reset in accordance 
with the monitor result, the change of the direction can not detected 
because the change rate d.omega./dt obtained from the gyro 2 only changes 
at the beginning and the ending of the rotation and the change rate 
d.omega./dt becomes within the predetermined value k in the intermediate 
of the rotation. Accordingly, the navigation device according to the 
present invention monitors not only the change rate d.omega./dt but also 
the change rate d.theta..sub.m /dt of the absolute direction data 
.theta..sub.m. Therefore, the offset value is not reset and the change of 
the direction can be detected because the change of rate d.theta..sub.m 
/dt obtained from the geomagnetic sensor 1 is thus monitored. Further, 
since the predetermined value k.sub.m determined to be a change rate when 
the direction of the vehicle is not changed (fixed), the offset reset is 
executed only when the change rate d.theta..sub.m /dt is within the 
predetermined value k.sub.m, i.e., only when the direction of the vehicle 
does not changed and the corrected angular velocity obtained by the offset 
processing is detected when the direction of the vehicle is changed. 
As described above, according to the present invention, the change of the 
direction can be accurately detected, and a navigation device reliable in 
direction detection can be provided.