Output signal processing unit of a fluxgate sensor
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a navigation system of a vehicle providing geographical position information to a driver during driving. More particularly, this invention relates to an improvement of an output signal processing unit of a fluxgate sensor used in the navigation system, which determines a moving direction of the vehicle. 
     2. Description of the Related Art 
     Generally, a conventional navigation system of a vehicle includes an output signal processing unit of a fluxgate sensor, which measures an angle of a moving direction relative to magnetic north in order to determine a geometrical position of a vehicle during driving. The output signal processing unit of a fluxgate sensor has a fluxgate sensor that obtains components of terrestrial magnetism both in the X and Y directions, where for example the moving direction is denoted as the X-axis and the direction perpendicular to it is denoted as the Y-axis, and measures momentarily the direction of the vehicle. The geometrical positions of the vehicle XX and YY are obtained from X and Y directions and velocity v of a velocity sensor of the vehicle, which are displayed on a map display, and thereby used in order to provided geographical position information of the vehicle to a driver. 
     Further, the above output signal processing unit of a fluxgate sensor includes a low pass filter removing a noise signal with a high frequency component due to variations etc. of the magnetic flux caused by a change of magnetic environment generated by driving the vehicle in the periphery of the flux. This low pass filter allows an output signal of the fluxgate sensor to be processed smoothly so that detecting accuracy of terrestrial magnetism is enhanced. Therefore, the above noise signal with a high frequency component does not cause the geographical positions to generate useless variations in the display and prevents the driver from making a mistake. 
     In the above output signal processing unit of a fluxgate sensor of the prior art, however, when the vehicle turns quickly, a high frequency component appears in the output of the fluxgate sensor, and the above low pass filter causes a time delay because it smoothly processes the high frequency component in the output of the fluxgate sensor. Consequently, a problem arises in that the detecting accuracy of direction in terrestrial magnetism declines, the following performance of the geometrical position becomes poor and the driver can easily make a mistake. Conversely the following performance becomes better when the low pass filter is not used, while the noise signal of the high frequency component can not be removed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above problem by providing an output signal processing unit of a fluxgate sensor, which performs better when a vehicle turns quickly. 
     FIG. 1 is a view showing a principle of an embodiment of the present invention. With this object in mind, the essence of the invention resides in an output signal processing unit of a fluxgate sensor comprising a low pass filter 2 for smoothing an output of the fluxgate sensor, wherein cutoff frequency of the low pass filters becomes higher or lower depending on the increase or the decrease of the angular velocity of rotation in turning the vehicle. 
     With the output signal processing unit of a fluxgate sensor, when the vehicle is going straight ahead, the angular velocity of rotation ω of the rate sensor 3 is zero so that the cutoff frequency of the low pass filter is smoothed to remove the high frequency noise. When the vehicle turns, the cutoff frequency of the low pass filter 2 becomes high depending on the increase of the angular velocity of rotation of the rate sensor 3, so that the frequency component of the output of the fluxgate sensor 1 is not smoothed, which improves the following performance without a time lag. Then, the high frequency component noise also passes through the low pass filter 2, but is smaller than the output signal of the fluxgate sensor 1 and the passing time of the noise is very short, preventing a driver from making a mistake. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described with reference to embodiments thereof, and to the accompanying drawings. Note, in the drawings, like parts are designated throughout by the same reference characters. 
     FIG. 1 is a view showing a principle of the embodiment of the present invention; 
     FIG. 2 is a view showing an output signal processing unit of a fluxgate sensor according to the embodiment of the present invention; 
     FIG. 3 is a view showing characteristics of P  n  determining a multiplying facter; 
     FIG. 4 is a view showing amplitude characteristics of a low pass filter of FIG. 2; and 
     FIG. 5 is a view showing a wave shape of an output signal in the low pass filter according to the embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of an output signal processing unit of a fluxgate sensor according to the present invention will be discussed with reference to the accompanying drawings. 
     FIG. 2 is a view showing the output signal unit of the fluxgate sensor according to the embodiment of the present invention. The overall construction of the preferred embodiment of the output signal processing unit of a fluxgate sensor will be discussed with reference to FIG. 2. The output signal processing unit of a fluxgate sensor includes: a fluxgate sensor 1, which detects components of terrestrial magnetism both in the X and Y directions where for example the moving direction is denoted as the X-axis and the direction perpendicular to it is denoted as the Y-axis, and measures momentarily the directions of the vehicle; a low pass filter 2 for removing a high frequency component of the output of the fluxgate sensor; a rate sensor 3 (i.e., an angular velocity sensor) of, for example, a piezo-electric oscillating gyro-sensor that detects an angular velocity ω of rotation of the vehicle while turning and causes a cut off frequency of the low pass filter 2 to increase or decrease corresponding to the increase or decrease of the angular velocity ω of the rotation respectively; an A / D (Analog to Digital) converter 4 for converting analog signals x, y of the fluxgate sensor 1 into digital signals x n  ,y  n  ; an A / D converter 5 of the rate sensor 3 for converting an analog signal ω into a digital signal ω n , a velocity sensor 6 which detects a velocity signal v of the velocity; a position data forming part 7, which forms geometrical position data xx and yy of the vehicle from the output signals x  n  &#39; and y  n  &#39; of the fluxgate sensor 1 in which the high frequency components are removed by the low pass filter and from the velocity signal v of the velocity sensor 6; a display 8 for displaying the geometrical position data xx and yy processed by the position data forming part 7 in a map drawn up beforehand. 
     Discussion will be given herebelow for the low pass filter 2. As shown in FIG. 2, the filter 2 includes two low pass filters 2-1 and 2-2 corresponding to the digital signals x  n  y  n  respectively from the fluxgate sensor 1. The low pass filter 2-1 includes a variable multiplier 21 that inputs the X n  component signal from the A / D converter 4, a delay element 22 for delaying the x n  component signal by one sampling period, a variable multiplier 23 that inputs a delay signal from the delay element 22, and an adder 24 for adding the outputs of the variable multipliers, 21 and 23 respectively to output the added signal X&#39; n  the position data forming part 7. The explanation of the low pass filter 2-2 is omitted, but is the same as that of the above low pass filter 2-1. 
     Next, an operation of the output signal processing unit of a fluxgate sensor will be discussed herebelow. The vehicle is provided with an X coordinate along the moving direction of the vehicle and a Y coordinate perpendicular thereto. The vehicle is also provided with the fluxgate sensor so as to be able to measure magnetic flux density B  x  in the X direction and magnetic flux density B  Y  in the Y direction. Expressing the magnetic flux density of terrestrial magnetism as B, the fluxgate sensor 1 generates an electrical signal X proportional to the magnetic flux density B X  as an x directional component and an electrical signal y proportional to the magnetic flux density B Y  as a Y directional component. The analog signals x and y from the fluxgate sensor 1 are converted into the digital signals X n  and y n ) respectively. In the low pass filters 2-1 and 2-2, the factors of the multipliers 21 and 25 are set to α=P n  /P m ) n  respectively and the factors of the multipliers 23 and 27 are set to β=(P m  i n -P n )/P min . Since the inputs of the low pass filters 2-1 and 2-2 are x n  and y n , their outputs x n  &#39; and y n  &#39; are ##EQU1## where P n  =min (P  min  , k·ω n  ), ω n  is an angular velocity of rotation of the rate sensor 3, and k takes a constant value. 
     FIG. 3 is a view showing characteristics of P determining a multiplying factor. As shown in the Figure, when the vehicle is not turning, the angular velocity ω n  of rotation is in a range of ω n  =0 to a given value of ω n  O . 
     Further in the case of 0≦ω n  ≦ω n  O , P  n  =P  m  i n =k·ω n  O, and in the case of ω n  ≧ω n  O , P n  =k·ω n  . In such a way the present invention allows the multiplication factors of the multipliers 21, 25 and 23, 27 to be variable depending on the output ω n  of the rate sensor 3. 
     Generally an amplitude |A(ω n  ·T) | of X  n  &#39; and |A(ω n  ·T )|= ##EQU2## 
     FIG. 4 is a view showing amplitude characteristics of a low pass filter of FIG. 2. The horizontal axis shown in the Figure indicates the angular velocity of rotation ω n  =2πf  n  and the vertical axis indicates the amplitude |A(ω n  T ) | where f  n  denotes frequency. When ω n  ·T=0, |A(O)|=|α+β|=1.0 for any of α and β . On the other hand when ω n  ·T=π, |A( π) |=|α-β|=|2P n  -P  m  i n |=|2k·ω n  -P  m  i n |=|2k·ω n  -k·ω n  O |. When α and β change depending on ω n  , the curve shown in the FIG. 4 moves upward and downward. Further when 0≦ω n  ≦ ω n  O, |A(π) |=|2k ω n  -P  m  i n |=k·ω n  O . In the present invention the angular velocity of rotation ω n  is given by the rate sensor 3 so that α and β of the low pass filter are controlled. As shown in the Figure, when 0≦ n  ≦ω n  O, |A(O)| becomes smaller by 3 dB at a cutoff frequency f  c  1 . If a cutoff frequency is f  c  2 when ω n  &gt;ω n  O an inequality of f  c  2 &gt;f  c  1 holds. That is to say, when the vehicle does not turn, the cutoff frequency in the low pass filter 2 becomes f  c  1 but changes from f  c  1 to f  c  2 and increases with the turning of the vehicle. After turning, the cutoff frequency changes from f  c  Z to f  c  1 and decreases. In such a way, the cutoff frequency of the low pass filter 2 is controlled with the angular velocity of rotation ω n  by means of the rate sensor 3. 
     FIG. 5 is a view showing a wave shape of an output signal in the low pass filter according to the embodiment. In a wave shape shown in the Figure, letting a digital signal x &#39;. denote an analog signal x  n  , the vehicle is going straight ahead or turns to a small degree to the time t 1  so that the angular velocity of rotation ω of the rate sensor 3 exists in 0≦ω n  ≦ω n  O where the cutoff frequency of the low pass filter 2-1 is little more than f  c  1 . Thereby, a high frequency noise is removed to make the output signal x &#39; smooth. When the vehicle turns from the time t 1  to the time t 2 , the output signal x&#39; changes from x&#39; 1  to x&#39; 2  and the angular velocity in rotation ω n  of the rate sensor 2-1 becomes ω n  &gt;ω n  O . Therefore the cutoff frequency of the low pass filter 2-1 changes from f  c  1 to f  c  2 to make a leading edge sharp as shown in the Figure. After the time t 2  the angular velocity of rotation ω  n  of the rate sensor 3 is zero, the cutoff frequency changes from f  c  2 2 to f  c  1 to remove the high frequency component so that the output signal x&#39; is smoothed. A dotted line shown in the Figure indicates a changing state of the output signal x &#39; in which the multiplying factors of the prior art low pass filter is kept to f  c  1. Therefore the low pass filter according to the present invention removes the high frequency noise while going straight ahead and also improves the following performance for turning of the vehicle compared with the prior art. Before the turning of the vehicle finishes, higher mode frequency components are superimposed for a short time but the change of the output x&#39; itself is so great that a driver is hardly influenced. Similarly, the output signal y&#39; may be explained. 
     Next another embodiment of a multiplying factor of a multiplier in the low pass filters 2-1 and 2-2 will be discussed. In the embodiment, the multiplying factors of the multipliers 21 and 25, and 23 and 27 of FIG. 2 are set to 
     
         α=(P.sub.n +δ) /P.sub.m i n . β=(P.sub.m i n -P .sub.n -δ) / P .sub.m i n , 
    
     where P  n  =min (P  m  i n -δ, k·ω n  ) , δ=a given value This is provided to prevent data from holding a constant value since x  n-  x n-1  and y  n  -y n-1  in said embodiment when ω n  =0(going straight ahead). 
     Thus letting an angle between the magnetic north and the moving direction of the vehicle be θ, x&#39; n  and y&#39; n  processed by the low pass filters 2-1 and 2-2 are processed by the position data forming part 7 so as to be ##EQU3## 
     In order to obtain the geometrical position xx and yy of the vehicle, this result and the velocity of the vehicle V are processed as 
     
         xx=∫V·cos θdt 
    
     
         yy=∫V·sin θdt . 
    
     This geometrical position xx and yy is displayed as a moving position of the vehicle in the map of the display 8 to be improved regarding the following performance in the turning of the vehicle according to the invention. 
     As mentioned before, according to the invention, the cutoff frequency of the low pass filter is increased or decreased depending on the increase or decrease of the angular velocity of rotation from the rate sensor so that it is possible to remove the high frequency component of the fluxgate sensor while going straight ahead and improve the following performance during turning.