Navigation method for vehicles with electronic compass

A method for determining the north direction or the travel direction of a vehicle having an electronic compass is proposed by means of which field disturbances at the magnetometer of the compass, occurring during the navigation drive, are measured and evaluated for avoiding angle errors on the direction indication of the compass. For this purpose, the measurement values (Px, y) of the magnetometer are continuously checked by an evaluating circuit of the navigation system and with a deviation (.DELTA.x, y) of several successive measurement values past a predetermined tolerance range (A) of the locus diagram and a simultaneous angle change of the earth's magnetic field vector, an intervention is effected in the direction indication. In this connection, according to the invention, the rate of change (.DELTA..zeta./t) of the direction indicated by the compass is limited to a predetermined value (B/C), this limit value (B/C) being weighted in dependence on the duration (t1, t2) of the measurement value deviation and the limit of the rate of the indicated direction change (.DELTA..zeta./t) (FIG. 4).

BACKGROUND OF THE INVENTION 
The invention is based on a method for determining the north direction or 
the travel direction of a vehicle having an electronic compass including a 
magnometer fixedly arranged in the vehicle and an evaluating circuit which 
cyclically receives the measurement values of the magnetic field vector 
effective at and measured by the magnetometer and processes the 
measurement values to check a predetermined elliptical locus diagram and 
to determine parameters of the travel direction and to correct the locus 
diagram during a deviation of several successive measurement values from a 
predetermined value of the locus diagram. 
Such a method for correcting the elliptical locus diagram determined for 
the magnetic field vector effective at the magnetometer is known from 
German Offenlegungsschrift 3,509,548. In this document, it is proposed to 
check the values of the magnetic field, measured by the magnetometer, with 
respect to a deviation from the determined locus diagram and in each case 
then to perform a correction of the locus diagram whenever deviations of 
the measurement values from the locus diagram exceed a particular amount 
several times. The disadvantageous factor in this prior art method is that 
the angle errors in the direction indication, occurring due to magnetic 
field disturbances, are corrected only very inadequately and too slowly by 
this method. 
In a heading measuring device known from German Patent Specification 
2,651,678, the magnetic field sensor is caused to switch to a further 
sensor for correcting angle errors during the occurrence of magnetic field 
disturbances, if the values measured by the magnetic field sensor deviate 
from a nominal value by a predetermined amount. In this prior art 
arrangement, it is disadvantageous that an additional sensor is needed and 
at least partially is influenced by the earth's magnetic field and thus by 
external field disturbances. 
SUMMARY OF THE INVENTION 
The present invention has the object of suppressing or limiting navigation 
errors due to interfering fields or interfering field changes acting on 
the magnetometer with angle disturbances occurring for a short time or 
longer term, to enable the direction to be indicated as accurately as 
possible during the disturbances and thereafter. 
The method according to the invention, characterized in that, with a 
deviation of the measurement values from the elliptical locus diagram (0) 
and with a simultaneous angle change of the earth's magnetic field vector, 
the rate of change of the direction indicated by the compass is limited to 
a predetermined value and that the limiting of the rate of change of the 
direction indication is weighted in dependence on the duration of the 
measurement value deviation the characterizing the advantage of the that, 
when interfering fields or interfering field changes occur, the correction 
influence is weighted in dependence on the duration of the deviation of 
the measurement value from the nominal value to suppress or limit changes 
in the direction indication. 
Advantageous further developments and improvements of the features 
specified in the main claim are possible by means of the measures listed 
in the sub-claims. For this purpose, it is particularly suitable to 
suppress a change in the direction indication completely with a short-time 
deviation of the measurement values from the elliptical circle diagram 
past the predetermined permissible amount. With a longer-duration 
deviation of the measurement values from the nominal value of the 
elliptical circle diagram past a predetermined amount, the rate of change 
of the direction indication is advantageously limited to a maximum value 
depending on the vehicle speed. In this arrangement, the maximum values of 
the change in direction, dependent on the vehicle speed, are suitably 
determined by means of the limit of the permissible transverse 
acceleration of the vehicle and these values are stored in a data memory 
of the evaluating circuit. At the end of the inadmissible measurement 
deviation, the direction of the earth's field then determined via the 
circle diagram is suitably used directly and without correction again for 
determining the direction indication.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a compound navigation system for vehicles by means of which 
the driver can find the desired destination in strange surroundings by 
specifying the direction of the destination and the linear distance 
(homing system). It consists of an input and output unit 10, a 
microprocessor 11 having a data memory 12, a position transmitter 13 and a 
travel direction transmitter 14. The microprocessor 11 is usually 
contained, together with the data memory 12, in the input and output unit 
10. Pulse transmitters of a tachogenerator or corresponding transmitters 
at the vehicle wheels, which may already be present in the vehicle can be 
used as position transmitter 13 A magnetometer or a magnetic field meter 
are used as travel direction transmitter 14. The microprocessor 11 
processes the signals emitted by the position transmitter 13 and by the 
magnetometer 14 and it executes the control and input commands enabled by 
the operating keys of the unit 10. Furthermore, it controls the output of 
data and direction arrows on a liquid crystal display (LCD) 15 of the unit 
10. Numerical values can be changed in the direction of `higher` and 
`lower` on the LCD 15 by means of a toggle key 16. The numerical value 
currently displayed on the LCD 15 is in each case stored by operating an 
acknowledgement key 17 and output on a display 18 in the lower area of the 
LCD 15. A function selection key 19 is used for switching the navigation 
system over within a menu offered in accordance with the inscription 25 on 
the left-hand edge area of the unit 10, the information items displayed in 
each case on the LCD 15 being identified by an arrow 20 on the LCD 15 at 
the level of the inscription 25. A further key switch 21 is used for 
switching the compound navigation system on and off. A 7-segment display 
22 in the upper LCD area is used for identifying various preset 
destinations. A compass rose 23 of the LCD 15, having 16 different 
invisible arrow segments, is used for direction information, the activated 
direction arrow 24 representing either the north direction, the travel 
direction or the direction of the travelling destination. 
The magnetometer 14 arranged fixedly in the vehicle has two probes which 
are offset by 90.degree. with respect to one another, are arranged in the 
plane of travelling of the vehicle to measure the components of the 
magnetic field effective at the magnetometer 14 and cyclically output the 
measured values in the form of electric measurement values into the 
evaluating circuit 11 for determining the travel direction or the north 
direction. As described in greater detail in German Offenlegungsschrift 
3,509,548, the displacement of the centre point of the elliptical locus 
diagram of the magnetic field vector from the origin of the coordinates 
thereby forms a direction-independent hard-magnetic interfering field 
vector H.sub.H according to FIG. 2. Further parameters of the elliptical 
locus diagram 0 represented in FIG. 2 are the major and the minor semiaxes 
a, b and the rotation about the angle .beta. with respect to the system of 
coordinates x, y. During the calibration of the navigation system, these 
ellipse parameters are determined in known manner by means of a circle 
drive of the vehicle and are stored in the data memory 12. During the 
subsequent navigation drives, the measurement values Px, y supplied by the 
magnetometer 14 are then continuously acquired for determining the earth's 
magnetic field vector H.sub.E and the angle .phi. between the earth 
magnetic field vector H.sub.E and the travel direction (x-axis). 
Due to a limited measurement value accuracy, the manner will now be 
explained, in accordance with FIG. 2 in conjunction with FIG. 4, in which 
a magnetic field disturbance is detected and processed. Within a 
predetermined band width or permissible range A, the measurement values 
P), y may deviate outwards or inwards from the elliptical locus drive 0 
without triggering correction measures. If, however, the deviation 
.DELTA.x, y of several successive measurement values Px, y from the 
elliptical locus diagram 0 exceeds the predetermined amount A, this 
deviation is detected as magnetic disturbance in the navigation system. 
The evaluating circuit subsequently checks on the basis of the direction 
angle .phi. continuously stored in an intermediate part of the data memory 
12, whether angle change .DELTA..phi./t of the earth's magnetic field 
vector H.sub.E is present at the same time. If this is not the case, the 
direction angle is retained unchanged and the measurement value deviation 
.DELTA.x, .DELTA.y is further processed for correcting the locus curve 0 
by a preferably weighted averaging of the measurement values. 
If, however, an angle change .DELTA..phi./t is found at the same time at 
the evaluating circuit, the rate of change of the direction indicated by 
the compass is limited to a predetermined value, this value being weighted 
with the duration of the measurement value deviation. During a short-time 
deviation of the measurement values Px, y from the elliptical locus 
diagram 0 past the predetermined range A, a change in the direction 
indication .DELTA..phi./t is completely suppressed by the evaluating 
circuit. With a longer-duration deviation of the measurement values Px, y 
from the elliptical locus diagram 0 past the predetermined range A, the 
rate of change of the direction indication .DELTA..phi./t is limited to a 
maximum value which depends on the travelling speed of the vehicle. 
In FIG. 3, the curve B represents the maximum values of the permissible 
change in direction .DELTA..phi./t which depend on the vehicle speed and 
which are obtained from the permissible transverse acceleration of the 
vehicle. These values can first be determined on a dry road and with good 
tires on the vehicle and stored in table form in the data memory 12. 
However, it is much simpler and more suitable to replace the table by a 
value for the permissible transverse acceleration which is between 0.2 and 
0.4 g depending on the type of vehicle. The speed measured in each case 
can then be used for continuously calculating the limit value on the curve 
B [.DELTA..phi./t.sub.max =f (v)]. The maximum steering wheel angle of the 
vehicle forms a further maximum limit of the angle change .DELTA..phi./t. 
This limit, which is proportional to the vehicle speed, is represented by 
the straight line C in FIG. 3. It can also be stored in the data memory 
12. However, the limit value of the curve B is also suitably calculated 
continuously from the vehicle speed in this case. The greater limit value 
on curve A or B is then suppressed by a logic circuit and the smaller 
limit value is compared with the actual direction change .DELTA..phi./t. 
FIG. 4 will now be used for explaining the manner in which changes in the 
direction indication during the occurrence of magnetic field disturbances 
are corrected. FIG. 4a shows that in the first part of a navigation drive, 
the measurement values supplied by the magnetometer 14 still exhibit a 
deviation .DELTA.x, y which is within the permissible deviation range A. 
FIG. 4b shows that the angle change .DELTA..phi./t during this time is 
also below the permissible maximum limit which is obtained from the 
characteristics B and C according to FIG. 3 and is, for example, about 
10.degree. /sec at a speed of v=40 km/h. In FIG. 4c, the dot-dashed line u 
shows that the measurement values are being continuously monitored by the 
evaluating circuit and the locus diagram 0 is being recalibrated. With the 
occurrence of a short-time disturbance 1 over a time t1 of about 2 
seconds, any change in the direction indication .DELTA..phi./t is now 
suppressed and the direction last indicated is retained unchanged as can 
be seen from FIG. 4b. According to FIG. 4 c, a correction signal K is now 
generated and the measurement value deviation .DELTA.x, y is averaged by 
means of a corresponding program loop of the microprocessor 11 and this 
value is used to correct the locus diagram 0 with a time-dependent 
weighting, which is less (&lt;1) than the weighting during the recalibration 
of a permissible deviation. At the end of the disturbance 1, the change in 
the direction indication .DELTA..phi./t is enabled again. 
Since the vehicle direction can have continuously changed during the 
magnetic field disturbance which occurred for a short time, this direction 
change must now be updated on the display. In this connection, however, 
the maximum values according to FIG. 3 should not be exceeded. For this 
purpose, it is proposed to continue the rate of change .DELTA..phi./t of 
the direction indication 24 with the speed-dependent maximum value B and C 
until the actual direction is indicated or the actual angle change of the 
travel direction has fallen below the maximum value B/C of the rate of 
change .DELTA..phi./t for the direction indication. This updating can be 
seen in FIG. 4b. 
When a longer-duration disturbance 2 of more than two seconds occurs, the 
indication of an angle change .DELTA..phi./t is again first suppressed and 
the indicated direction is retained. After that, an angle change 
.DELTA..phi./t is permitted again, but it is limited to the 
travelling-speed-dependent maximum values B/C according to FIG. 4b. In 
addition, a signal for averaging the measurement value deviations 
.DELTA.x, y is generated during the time t2 of the disturbance 2 and the 
locus diagram 0 is correspondingly dynamically recalibrated. During this 
process, a smaller weighting is first suitably provided and an increasing 
weighting with a longer disturbance. In this manner, magnetic field 
changes in the vehicle due to connection or disconnection of electric 
loads can also be well compensated. 
With the end of the measurement value deviations during the disturbance 2, 
the direction of the earth's magnetic field determined directly from the 
compass is now, however, processed without correction for determining the 
direction indication and this newly determined travel direction or north 
direction is then displayed without a readjustment limit, preferably 
delayed in time by several measurement cycles. Subsequently, any change in 
the direction indication is then determined by the evaluating circuit in 
the usual manner via the direction of the earth's magnetic field and is 
displayed. In this connection, the measurement values or their deviation 
from the locus diagram 0 is simultaneously checked at particular time 
intervals. During this process, a dynamic correction of the locus diagram 
0 can also be performed in such a manner that even relatively small 
measurement value deviations are continuously utilized for correcting the 
locus diagram 0 by averaging. To avoid residual errors occuring due to a 
continuous averaging after relatively long disturbances, the averaging is 
then begun again.