Vehicle heading detecting apparatus

Abstract: A vehicle heading correcting apparatus comprising a gyro (43), integrating means for integrating output data of said gyro (43), a global positioning system receiver (42), acquisition means for acquiring heading data of a vehicle supplied from said global positioning system receiver (42), first computing means for computing a first turning angle (ϑB - ϑA) of said vehicle based on two consecutive data of said heading data acquired by said acquisition means, second computing means for computing a second turning angle (Σø(t)) of said vehicle based on an integrated value of said output data of said gyro (43) obtained by said integrating means during a period of time (T) that said two consecutive data are acquired, offset computing means for computing a difference (Σø(t) - (ϑB -ϑA)) between said first and second turning angles and for computing an offset value (δ) of said output data of said gyro (43) by diving said difference (Σø(t) - (ϑB -ϑA)) by said period of time (T), and offset correcting means for correcting said output data of said gyro (43) with said offset value (δ) calculated by said offset computing means.

Description: FIELD OF THE INVENTION The present invention relates to an apparatus which detects the current heading of a vehicle with a gyro and a GPS (Global Positioning System) receiver. The invention also relates to an apparatus which detects the current location of such a vehicle. DESCRIPTION OF THE PRIOR ART As an apparatus for providing information about the current heading of a vehicle traveling streets, there has been proposed a vehicle heading detecting apparatus, which is provided with a gyro for detecting the heading of a vehicle and a processing unit for processing the output signal of the gyro and obtains the current heading data of the vehicle with a heading change amount (angular velocity) δ that occurs upon movement of the vehicle. Based on the angular velocity δ, the current heading  of the vehicle is calculated by the following equation:  = o + δ where o is a heading obtained in the last sampling. Based on the current heading  and a travel distance δL obtained, for example, from the output of a wheel sensor, the current location data (Px, Py) of the vehicle can be obtained by adding the east-west directional component δx (= δL x cos ) and south-north directional component δy (= δL x sin ) of the travel distance δL to the previous location data (Px', Py'). Therefore, the aforementioned heading detecting apparatus has been used for detecting the location of a vehicle. However, the gyro tends to generate an output (offset) due to the influence of temperatures or humidities, even when the gyro output should be zero during the time that the vehicle is in its stopped state or its straight traveling state. Since this offset output is accumulated with time, the heading of the vehicle that departs from an actual heading of the vehicle is to be detected. It is therefore necessary to obtain an accurate heading at all times and to correct a detected angular velocity with the accurate heading. Then, making use of the fact that only the offset occurs in the gyro output when the vehicle stands still, there has been proposed a method of subtracting the offset from a subsequent output-during travel. Many apparatuses using such a method have been proposed (e.g., Japanese Patent Publication JP-A-58 39360). However, the offset of the gyro tends to drift due to changes in temperatures and humidities independently of whether the vehicle is in its stopped state or traveling state. Therefore, even if the gyro output during travel is corrected with an offset value obtained during the stop of the vehicle, a value after the offset correction will contain errors because of the aforementioned drift. Fig. 8 shows how the output of the gyro drifts with time, t. It will be seen from the figure that an actual offset value Ω n departs from an estimated offset value Ω' measured at the time, to, during the stop of the vehicle. If the period of time from the start of the vehicle to the next stop of the vehicle is short, the drift of the gyro is small and can be corrected, but if the period of time from the start of the vehicle to the next stop of the vehicle is long, the drift of the gyro cannot be corrected and becomes larger and larger. In such a case, errors caused by the drift of the gyro are accumulated. As a method for providing information about the actual location of a vehicle traveling streets, there is known "dead reckoning", in which a distance sensor, a heading sensor (GPS receiver or gyro) and a processing unit for processing the distance and heading data acquired from the distance and heading sensors are employed and the current location data of the vehicle are acquired with an amount of distance change δL and an amount of heading change δ (in a case where a gyro is used) or a heading  (in a case where a GPS receiver is used). In dead reckoning, the east-west directional component δx (= δL x cos ) and south-north directional component δy (= δL x sin ) of the distance change amount δL that occurs as the vehicle moves along a street are calculated, and the current location output data (Px, Py) of the vehicle are acquired by adding the calculated components δx and δy to the previous location output data (Px', Py'). However, there is the drawback that the accumulation of errors occurs due to inherent limitations on the achievable accuracy of the heading sensor. That is, in a case where the heading of the vehicle is obtained by measuring Doppler shift which occurs by receiving waves from GPS satellites during travel, an error in the GPS heading is increased as the speed of the vehicle is decreased. When, on the other hand, the gyro is employed, it is known that an error in the gyro output data will appear frequently at the time that the heading change has become more than a predetermined value, at the time that the power source is turned on, at the time that the vehicle travels at very low speeds or at the time that it is detected that the vehicle is traveling on rough roads such as mountain roads. Unless compensation for the errors is made, the dead reckoned positions will become increasingly imprecise or inaccurate. Then, it has been proposed that both the gyro and the GPS receiver are used. If either the angular velocity data of the gyro or the heading data of the GPS receiver will be reduced in reliability, one data can be compensated for by the other data. That is to say, the current heading of the vehicle can be estimated by calculating Kalman filter gain taking into consideration the characteristic errors inherently contained in the outputs of the gyro and the GPS receiver, and by processing the GPS receiver heading output and the gyro heading output with a weight processing method which is based on the calculated Kalman filter gain. However, in this method, it is important how the characteristic error components contained in the gyro output and the GPS receiver output are evaluated. That is to say, the individual error components are evaluated by some method and if these components are set to constant values, the processing can be most easily performed. However, setting to the constant values is insufficient, and it is desirable to evaluate the error components accurately at real time by some method. In addition, since the offset that is contained in the angular velocity data of the gyro varies with time, it is necessary to take into consideration the error in the angular velocity data resulting from that variation. Furthermore, it is also necessary to take the scale factor (output gain) of the gyro into consideration because sometimes the scale factor departs from a standard value. The inventor of the present application has proposed a heading detecting apparatus (Japanese Patent Laid-open Publication JP-A-3188316), which measures the outputs of the gyro and the GPS receiver and processes them at real time and is capable of accurately estimating the current heading of a vehicle with the aid of the output higher in reliability between the both outputs. In this apparatus, only dispersion values that are contained in the final outputs of the gyro and the GPS receiver are measured, and individual error factors contained in the outputs of the gyro and the GPS receiver have not been taken into consideration. It is, accordingly, an important object of the present invention to provide an improved vehicle heading detecting apparatus which is capable of estimating the current heading of a vehicle accurately by individually analyzing and evaluating the error factors contained in the heading data of the GPS receiver and in the angular velocity data of the gyro and by determining the rate of use of the output data of the gyro and the GPS receiver. SUMMARY OF THE INVENTION In accordance with one important aspect of the present invention, there is provided a heading detecting apparatus comprising a global positioning system receiver for outputting heading data of a vehicle, and a gyro for outputting angular velocity data of the vehicle. The heading detecting apparatus, as shown in Fig. 9, further comprises first means (A), second means (B), and third means (C) connected to the global positioning system receiver. The first means (A) is connected to the gyro for calculating an error of an offset value that is contained in the angular velocity data of the gyro. The second means (B) is connected to the first means (A) for calculating an error that is contained in the angular velocity data of the gyro, on the basis of the error of the offset value calculated by the first means (A), a change rate of time of the error of the offset value multiplied by a predetermined time, and the angular velocity data of the gyro multiplied by an error of a scale factor of the gyro, The third means (C) is connected to the global positioning system receiver for calculating an error that is contained in the heading data of the global positioning system receiver and depends on at least a velocity of the vehicle. The heading detecting apparatus further comprises fourth means (D) and fifth means (E). The fourth means (D) is connected to the second means (B) and the third means (C) for calculating a Kalman filter gain by calculating a degree of reliability of the angular velocity data from the error of the angular velocity data of the gyro calculated by the second means (B) and a degree of reliability of the heading data from the error calculated by the third means (B). The fifth means (E) is connected to the fourth means (D) for calculating a current estimated heading of the vehicle by weight processing, on the basis of the Kalman filter gain, heading data obtained from the angular velocity data of the gyro and also the heading data outputted from the global positioning system receiver. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and advantages will become apparent from the following detailed description when read in conjunction with the accompanying drawings wherein: FIG. 1 is a graph showing how the output of a gyro drifts with time; FIG. 2 is a block diagram of a heading detecting apparatus of the present invention; FIG. 3 is a block diagram illustrating a location detecting apparatus into which one embodiment of the heading detecting apparatus is incorporated; and FIG. 4 is a flow chart illustrating how the heading of a vehicle is obtained according to the heading detecting apparatus of FIG. 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 illustrates a preferred embodiment of a heading detecting apparatus of the present invention which may be employed in a vehicle location detecting apparatus. The location detecting apparatus comprises a wheel sensor 141 which senses the number of rotations of the left and right wheels (not shown) respectively of a vehicle (this sensor is used as a distance sensor), a GPS receiver 142, and a gyro 143. The location detecting apparatus further comprises a road map memory 12 for storing road map data, a locator 11 which calculates an estimated heading of a vehicle in accordance with the output data sensed by the gyro 143 and GPS receiver 142 and also calculates the location of the vehicle with the aid of the data of the wheel sensor 141, a data memory 16 connected to the locator 11, a display 17 connected to the navigation controller 15 for displaying the current location of the vehicle, and a keyboard 18 connected to the navigation controller 15. The data memory 16 connected to the locator 11 has stored therein a heading error r(v) that is contained in the data of the GPS receiver 142, an error qo of a gyro offset value that is contained in the angular velocity data w(t) of the gyro 143, a gyro noise N, an error a of the scale factor of the gyro 143, etc. In the locator 11 described above, the number of rotations of the wheel is obtained by counting the number of the pulses outputted from the wheel sensor 141 with a counter (not shown), and travel distance output data per unit time are calculated by multiplying the count data of the counter by a predetermined constant number indicative of a distance per one count. Also, a relative change in the heading of the vehicle is obtained from the gyro 143. Then, based on the relative change and the absolute heading output data of the GPS receiver 142, the locator 11 calculates the heading output data of the vehicle, as will be described below. The road map memory 12 stores road map data of a predetermined area in advance and comprises a semiconductor memory, cassette tape, CD-ROM, IC memory, DAT or the like. The display 17 comprises a CRT display, crystalline liquid display or the like and displays a road map that the vehicle is traveling and a current location of the vehicle. The navigation controller 15 is constituted by a graphic processor, an image processing memory and the like, and generates instructions to perform a retrieval of the map on the display 17, switching of scale, scrolling, a display of the current location of the vehicle and the like. The data memory 16 stores a gyro offset error qo, an estimated change rate ε of the gyro offset error, a noise component N that is contained in the gyro output, a scale factor error α of the gyro (rate of a turning angle measured from the gyro output and an actual turning angle), and values for fixed numbers a and b in the GPS heading error, r(v) = av -1 + b. The angular velocity data w(t) of the gyro 143 and the heading data (t) of the GPS receiver 142 are sampled every constant time. If it is assumed that the time from the previous process from the current process is δt, the number of samplings will be proportional to the time δt. The angular velocity data w(t) of the gyro as the vehicle is in its stopped state is normally zero, but the gyro generates an output if an offset occurs in the gyro. In estimating this gyro offset value, a value that has been used during travel before the vehicle stops is to be used. Of course, the angular velocity data w(t) of the gyro during the stop of the vehicle can also be integrated and averaged. The error qo of the gyro offset value represents what extent of fluctuation the gyro offset value has, and is obtained by sampling a plurality of the angular velocity data w(t) of the gyro during stop and calculating a dispersion that is contained in the plurality of angular velocity data w(t) of the gyro. The estimated change rate ε of the gyro offset error is a value that is obtained from experience as a function of temperature. The vehicle heading detecting process by the apparatus constructed as described above will hereinafter be described in detail. During travel, the location of the vehicle is displayed on the display 17 in accordance with the individual sensor output data stored in the locator 11. Also, during the display, the data of each sensor are read in every constant time by break-in, and then the heading of the vehicle is updated. The vehicle heading detecting flow at the time of this break-in is shown in FIG. 4. It is noted that the break-in may also be made every constant distance that is obtained based on the output data indicative of the distances traveled by the vehicle. The above described constant time or distance is set, depending upon the type of the gyro and the functional performance of the GPS receiver. In step (1), the velocity v of the vehicle, the angular velocity data w(t) of the gyro 143 and the heading data (t) of the GPS receiver 142 are read in. Next, in step (2), the gyro offset value Ω, gyro offset error qo, change rate ε of the gyro offset error, noise component N, and the gyro scale factor error α are read out of the data memory 16. The step (2) advances to step (3), in which the fixed numbers a and b of the GPS heading error r(v) are read out of the data memory 16. When the heading of a vehicle is obtained from the heading data of the GPS receiver 142, the error in the heading is increased as the velocity of the vehicle is decreased, and the heading error of the vehicle becomes a monotonic decrease function of velocity. Therefore, the GPS heading error r(v) can be expressed as follows: r(v) = av -1 + b However, the GPS heading error is not limited to this function. For example, the GPS heading error can be expressed as follows: r(v) = a arctan(b/v) + c In addition, an error in the measurement of waves from the GPS as well as the velocity of the vehicle can be taken into consideration. The estimated heading of the vehicle is obtained based on each data read out of the data memory 6. or this purpose, the step (3) advances to step (4), in which the dispersion q 2 (t) of the angular velocity data w(t) of the gyro 143 is first calculated by the following equation: q 2 (t) = (qo + εT) 2 δt 2 + N 2 δt + (αδ) 2 where T is the time that has elapsed from the previous stop of the vehicle, qo is a gyro offset error (containing an error of quantization) which is a constant, εT is equal to the change rate of the gyro offset error multiplied by the elapsed time T and is an error resulting from a change (drift) of the gyro offset, and N 2 is a dispersion caused by noises. The reason that the errors qo and ET are added up is that these errors are not considered to be an independent phenomenon. If these errors are considered to be an independent phenomenon, then second power of qo and second power of εT will be added up (Japanese Patent Laid-open Publication JP-A-3279809). The reason that δt is not squared is that the noise error N is proportional to 1/2 power of the number of additions. δ is a turning angle of the vehicle obtained by a difference between the previous turning angle and the current turning angle, and αδ is a dispersion of the turning angle caused by the error of the scale factor. The value of α depends on the surrounding temperature and is determined every a certain temperature range as a gyro standard. Therefore, the value of a is used as it is. Next, the dispersion r 2 (t) of the heading data (t) of the GPS receiver 142 will be calculated by the following equation: r 2 (t) = (av -1 + b) 2 In the step (5), by using q 2 (t) and r 2 (t), an estimated heading  in which errors are taken into consideration is calculated by the following equation: (t) = K(t)(t) + (1 - K(t)))((t-1) + w(t)) where (t) is a current heading, (t-1) is the previous heading, w(t) and (t) are sensor output data that are used as the current heading is calculated, and K(t) is a Kalman gain which is a variable of 0 < K(t) < 1. By using the previous Kalman gain K(t-1), K(t) is obtained by the following equation: K(t) = (q 2 (t) + σ 2 (t-1))/(q 2 (t) + r 2 (t) + σ 2 (t-1)) The dispersion of the estimated heading is calculated by the following equation: σ(t) = (K(t)r 2 (t)) 1/2 As described above, the mean and dispersion of the estimated value of the gyro offset, the error of the change rate of that estimated value, the noise component, the error of the gyro scale factor, and the mean and dispersion of the error contained in the GPS receiver output have been calculated and stored. Then, when calculating the estimated heading of a vehicle, the dispersions that are contained in the output data of the gyro and GPS receiver are respectively calculated from the aforesaid stored data, and the estimated heading can be obtained based on the data that have been weighted. When the GPS heading data cannot be obtained for some reasons, K(t) is made 0 and the estimated heading  is expressed by the following equation: (t) = (t-1) +w(t) From this estimated heading and the distance data of the wheel sensor 141, the estimated location of the vehicle can be calculated. At this point, it is a matter of course that a map matching method may be used which compares an estimated location of a vehicle with road map data, evaluates a degree of correlation with respect to road map data, corrects the estimated location and displays the current location of the vehicle on roads (Japanese patent "kokai" publications JP-A-63-148115 and JP-A-64-53112). While the subject invention has been described with relation to the preferred embodiments, various modifications and adaptations thereof will now be apparent to those skilled in the art. For example, errors other than the aforementioned errors, such as an error of quantization caused by A/D conversion, can be taken into consideration.

Claims: A heading detecting apparatus comprising a global positioning system receiver (142) for outputting heading data of a vehicle, and a gyro (143) for outputting angular velocity data of said vehicle, said apparatus being characterized by further comprising: first means (A) connected to said gyro (143) for calculating an error of an offset value that is contained in said angular velocity data of said gyro (143); second means (B) connected to said first means (A) for calculating an error that is contained in said angular velocity data of said gyro (143), on the basis of said error of said offset value calculated by said first means (A), a change rate of time of said error of said offset value multiplied by a predetermined time, and said angular velocity data of said gyro (143) multiplied by an error of a scale factor of said gyro (143); third means (C) connected to said global positioning system receiver (142) for calculating an error that is contained in said heading data of said global positioning system receiver (142) and depends on at least a velocity of said vehicle; fourth means (D) connected to said second means (B) and said third means (C) for calculating a Kalman filter gain by calculating a degree of reliability of said angular velocity data from said error of said angular velocity data of said gyro (143) calculated by said second means (B) and a degree of reliability of said heading data from said error calculated by said third means (B); and fifth means (E) connected to said fourth means (D) for calculating a current estimated heading of said vehicle by weight processing, on the basis of said Kalman filter gain, heading data obtained from said angular velocity data of said gyro (143) and also said heading data outputted from said global positioning system receiver (142). A location detecting apparatus comprising sensor means (141) for sensing distances traveled by a vehicle, a global positioning system receiver (142) for outputting heading data of said vehicle, and a gyro (143) for outputting angular velocity data of said vehicle, said apparatus being characterized by further comprising: first means (A) connected to said gyro (143) for calculating an error of an offset value that is contained in said angular velocity data of said gyro (143); second means (B) connected to said first means (A) for calculating an error that is contained in said angular velocity data of said gyro (143), on the basis of said error of said offset value calculated by said first means (A), a change rate of time of said error of said offset value multiplied by a predetermined time, and said angular velocity data of said gyro (143) multiplied by an error of a scale factor of said gyro (143); third means (C) connected to said global positioning system receiver (142) for calculating an error that is contained in said heading data of said global positioning system receiver (142) and depends on at least a velocity of said vehicle; fourth means (D) connected to said second means (B) and said third means (C) for calculating a Kalman filter gain by calculating a degree of reliability of said angular velocity data from said error of said angular velocity data of said gyro (143) calculated by said second means (B) and a degree of reliability of said heading data from said error calculated by said third means (B); fifth means (E) connected to said fourth means (D) for calculating a current estimated heading of said vehicle by weight processing, on the basis of said Kalman filter gain, heading data obtained from said angular velocity data of said gyro (143) and also said heading data outputted form said global positioning system receiver (142); and location detecting means (11) connected to said sensor means (141), said global positioning system receiver (142) and said gyro (143), said location detecting means (11) detecting a current location of said vehicle based on said current estimated heading calculated by said fifth means (E) and on said distances obtained from said sensor means (141). A location detecting apparatus as set forth in claim 2, which comprises a data memory (16) connected to said location detecting means (11), a navigation controller (15) connected to said location detecting means (11), a road map memory (12) connected to said navigation controller (15), a display (17) connected to said navigation controller (15), and a keyboard (18) connected to said navigation controller (15).

Inventors: MORITA KAZUYA, MORITA, KAZUYA, Morita, Kazuya, c/o Osaka Works of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1993
Main classification: G01S5
IPC codes: G01C21, G01S5