Vehicle in-situ locating apparatus

Abstract: A vehicle in-situ locating apparatus comprises a running distance sensor (101), a heading direction sensor (102) based on the terrestrial magnetism, and a map data base (109, 110) for identifying the road on which the vehicle is running. In implementing the map matching for collating running data with map data, the difference between the heading direction and the road direction on the map is calculated through the modifi­ cation so that it is underestimated on the basis of the angle with respect to the magnetic north for specific sections memorized as map data or for all sections, whereby the probability of erroneous matching is lowered and the vehicle is located accurately even in such places as bridges, elevated roadways, tunnels and slopes where the terrestrial magnetic azimuth deviates.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a vehicle in-situ locating apparatus useful for locating such a vehicle as an automobile which runs mainly on roads or a self-propelling vehicle which runs mainly on a predetermined course. DESCRIPTION OF THE PRIOR ART A known method of finding the current location of a running vehicle by the vehicle's driver uses a distance sensor and an azimuth sensor to determine the vehicle's location in terms of the relative position with respect to the position of departure. The azimuth sensor used generally is a terrestrial magnetism sensor because of its ability of finding the absolute azimuth and inexpensiveness. However, the use of any sensor suffers a progressively falling accuracy due to the cumulation of the error of sensor. On this account, the development of map matching is under way, by which the position data collected during the run is compared with the map data and the inferred vehicle location is modified thereby to nullify the cumulative error. However, when a vehicle equipped with a location apparatus based on the terrestrial magnetism sensor runs on such road sections as a steel bridge, elevated roadway, tunnel or slope, where the terrestrial magnetic azimuth deviates, an erroneous map matching takes place, which deteriorates the accuracy of location. As a method of preventing such improperty, there is a technique of detecting magnetized or terrestrial magnetism variation areas using a digital map, as proposed in Japanese Patent Unexamined Publication (JP-A) No. 62-144016. Fig. 5 shows the system configuration of this method. A right wheel 501 and a left wheel 502 have the provision of a right wheel sensor 503 and left wheel sensor 504, respectively, and their outputs are passed to a processor 506, which also receives the output of a terrestrial magnetism sensor 505. The processor performs map matching for the map data read out of a digital map memory 507. Recorded on the digital map are identifiers of terrestrial magnetism variation areas, and if the inferred location calculated by the processor 506 is within such area, the azimuth output provided by the terrestrial magnetism sensor is determined to be unreliable and the heading direction is calculated by using only the wheel sensors that serve as an angle sensor. Another approach to this problem is disclosed in JP-A-63295913 where data is stored which represents the variation of the terrestrial magnetic field in regions which contain anomalies in that field. Disadvantages of this approach are that it requires a considerable amount of mapping to be carried out and further requires the storage of a large amount of data. Another conventional technique is a method and apparatus for vehicle navigation, as proposed in U.S. Patent No. 4,796,191 or Japanese Patent Unexamined Publication (JP-A) No. 61-56910. This technique is based on the fact that the region where the vehicle is possibly situated extends on a two-dimensional map due to the sensor's cumulative error, and it deals with a profile of equal probability on a two-dimensional plane having equal probability of the presence of the vehicle and, if the equal probability profile crosses the center line of a road on the two-dimensional map, it judges that the vehicle is possibly running on that road. Thereafter, the system further implements the judgement for the continuity of the road and the collation of the running distance vs. heading direction graph with the distance vs. road direction graph and, if the collation with high coincidence results, moves the inferred location to that road. The system uses two kinds of azimuth sensors, and infers the sensor error from the degree of coincidence between the sensors thereby to determine the equal probability profile. The system also has a calibration table for calibrating the heading direction detection means, the output of the heading sensor being calibrated in accordance with that table before the difference between the road direction from the map and the measured heading is calculated. Among the foregoing conventional techniques, the former system necessitates an angular sensor for the relief of the terrestrial magnetism sensor in a terrestrial magnetism variation area imparted by the map data, and it involves a problem of considerable azimuth deviation if the terrestrial magnetism variation area extends longer and the heading of vehicle varies progressively while the azimuth sensor is replaced with the angular sensor. The latter system is devised to cope with the above situation without the need of memorizing terrestrial magnetism variation areas as map data. The main cause of growing azimuth error in using a terrestrial magnetism sensor for the azimuth sensor is the distortion of terrestrial magnetic field when the vehicle is running on a steel bridge, elevated roadway or tunnel. This conventional system uses a pair of wheel sensors and a yaw rate sensor in unison with the terrestrial magnetism sensor so as to correct the anticipated value of azimuth sensor error on the basis of the degree of inconsistency among these sensors. Therefore it needs two kinds of azimuth sensors, and even with these sensors the anticipated error can be inaccurate. Namely, in case of combining a terrestrial magnetism sensor and a pair of wheel sensors, the wheel sensors have a growing error relative to the distance, resulting in a relatively large detection error. An experience revealed that when a vehicle was running in a road section which was gradually converted to an elevated roadway, the inconsistency of detected azimuth from the result of the terrestrial magnetism sensor was underestimated even in the locality of distorted terrestrial magnetism, and the system failed to find a road in the expected azimuth and located the vehicle in a parking place instead of a road. In the case of combining a terrestrial magnetism sensor and a yaw rate sensor which developed a growing error with time, the degree of inconsistence of the detected azimuth with the terrestrial magnetism sensor was not estimated properly when the vehicle was running slowly due to a traffic jam or the like on an elevated roadway or in a tunnel even though the zero-point adjustment was conducted at stops of the vehicle, and the system underestimated the error of terrestrial magnetism sensor during a run in a distorted terrestrial magnetism area, failing in the map matching and then resulting in a degraded locating accuracy. Accordingly, problems to be solved include the need of another sensor besides the terrestrial magnetism sensor for constituting a complete azimuth sensor, and, even in this case, the failure of map matching when the vehicle is running on an elevated roadway or in a tunnel where the distortion of terrestrial magnetism is most pronounced, which results in a degraded locating accuracy. SUMMARY OF THE INVENTION An object of this invention is to locate a vehicle accurately by using a distance sensor, terrestrial magnetism sensor, map data, and the map matching technique, even in such road sections as steel bridges, elevated roadways, tunnels and slops where the terrestrial magnetism deviates. In order to achieve the above objective, the inventive apparatus, which comprises a running distance sensor, a terrestrial magnetic azimuth sensor, a map memory means, an inferred location calculation means, an azimuth difference calculation means, and a map matching calculation means, is designed to evaluate the difference between the road direction and heading direction taking account of a modification dependent upon the angle between the magnetic north and road direction based on a pre-set formula or pre-set conversion table. According to this invention there is provided a vehicle locating apparatus comprising: running distance measuring means (101) which measures the running distance of a vehicle; heading direction detecting means (102) which uses at least one terrestrial magnetism sensor and detects the heading direction of the vehicle; inferred location calculation means (103) which calculates the relative movement value of the vehicle from the heading direction detected by said heading direction detection means and the running distance measured by said running distance measuring means, and calculates the inferred location of the vehicle by accumulating the relative movement value on the coordinates of the previous inferred location; map memory means (104) which stores road data; azimuth difference calculation means (105) which evaluates the difference (B) between the road direction (R) stored in said map memory means and the direction (M) detected by said heading direction detecting means; map matching calculation means (106) which judges, based on the azimuth difference, road data stored in said map memory means and data of the inferred vehicle location provided by said inferred location calculation means, as to whether the vehicle is running on a road on the map and, if it is judged that the vehicle is running on the road, corrects the inferred location calculated by said inferred location calculation means to the road; and output means (107) which delivers the inferred location provided by said inferred location calculation means; characterized in that the apparatus further comprises a modification value calculation means (108) which calculates an allowable value (Q) of the deviation of terrestrial magnetic azimuth for a given road direction for at least a first cause of terrestrial magnetism variation in accordance with the formulae: P = (R-D-K1 + 360) mod 180 Q = min (P x K2/(90-K1)-K2,K2) + min (K2 x (180-K1-P)/K1,K2) -K2 where: Q is said allowable value of the azimuth difference; R is said road direction; D is an angle between north and magnetic north; K1 is a first parameter; and K2 is a second parameter; and and further in that said azimuth difference calculation means calculates said difference (B) in accordance with the formula: B = dif (M,R) where: B is said azimuth difference between M and R; M is said detected heading direction; R is said road direction; and dif (A,B) denotes a signed angular difference between A and B; and said azimuth difference calculation means (105) also implements a modification to said difference (B) to calculate said azimuth difference (A), which modification is, if said difference (B) is of the same sign as the allowable value (Q), in accordance with the formula: A = max (abs(B) - abs(Q),O) x sign (B); whereas if said difference (B) is of a different sign to the allowable value (Q), then the modification is in accordance with the formula: A = B where: A is the modified difference between M and R, based on said comparison with Q; B is said azimuth difference between M and R; sign (A) is a function equal to 1 when A is positive, -1 when A is negative, or 0 when A is zero. Preferably, in comparing the heading direction with the road direction for map matching, the presence or absence of constructions made of ferromagnetic material and slopes along the road is examined in accordance with road section identifiers in the map data thereby to underestimate the azimuth difference using the formula or conversion table based on the experience, whereby an outstanding vehicle location apparatus capable of accurately locating the vehicle, even during a run on such road sections as elevated roadways, steel bridges, tunnels and slopes, is accomplished without adding an angular sensor. Since the need of modification for the azimuth difference is minimal, it is possible to accomplish a vehicle location apparatus which is as nearly superior as the above-mentioned one without memorizing such special map data as road section identifiers. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the vehicle location apparatus embodying the present invention; Figs. 2 and 3 are diagrams used to explain the operation of the embodiment; Fig. 4 is a flowchart showing in brief the processing of the embodiment; and Fig. 5 is a block diagram showing the conventional system arrangement. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a block diagram of an embodiment of this invention. Indicated by 101 is a running distance measuring means, and it measures the running distance of the vehicle in such a manner of counting the number of revolutions of a wheel. 102 is a heading direction detecting means, and it detects the heading direction of the vehicle using a terrestrial magnetism sensor. 103 is an inferred location calculation means, and it implements the calculation for advancing the vehicle's previous inferred location by the amount of a running distance measured by the running distance measuring means 101 in a direction detected by the heading direction detection means 102 thereby to determine a new inferred location of the vehicle. 104 is a map memory means, and it stores road data including coordinates and directions of lines that approximate roads. 109 is a magnetic north memory means, and it stores the angle of magnetic north with respect to the true north. In case the angular difference between the true north and magnetic north is small, the true north may be treated as the magnetic north practically, with the magnetic north memory means being omitted. 110 is a terrestrial magnetism variation section memory means, and it has a record of such sections as steel bridges, elevated roadways, tunnels and slopes where the terrestrial magnetism deviates due to the distortion of magnetic field. The data of the magnetic north and terrestrial magnetism variation sections may be stored as part of map data in the map memory means. 108 is a modification value calculation means, and it uses the magnetic north data read out of the magnetic north memory means to calculate the allowable value of the deviation of terrestrial magnetic azimuth as a modification value for road directions of terrestrial magnetism variation sections among roads read out of the map memory means. 105 is an azimuth difference calculation means, and it calculates the difference between the previous heading direction detected by the heading direction detecting means and the road direction read out of the map memory means by implementing the modification in accordance with the modification value provided by the modification value calculation means. 106 is a map matching means, and it judges as to whether the trace of previous inferred locations can be matched with the map on the basis of the azimuth difference and, if possible, implements the map matching by correcting the inferred location provided by the inferred location calculation means. 107 is an output means, and it delivers the resulting inferred location of the vehicle. The output means may be a display unit which simply displays the inferred location, or may be a display unit which displays the inferred location on the map. Alternatively, the output means may be a communication unit for sending the inferred location data, or it may be an audio output unit. The components 103, 105, 106 and 108 may be integrated by use of a microprocessor or the like. The terrestrial magnetism variation section memory means 110 may be a ferromagnetic body section memory means which indicates the presence of constructions made of such ferromagnetic material as iron along such road sections as steel bridges, elevated roadways and tunnels, or it may be a sloping section memory means which indicates the gradients of roads. Alternatively, the azimuth difference may be calculated invariably through the calculation of modification value on the basis of the inference of a ferromagnetic body section or sloping section, instead of having such a terrestrial magnetism variation area memory means as 110. Figs. 2 and 3 are diagrams used to explain the present invention. Azimuth detection based on the terrestrial magnetism sensor suffers error most frequently due to constructions made of ferromagnetic material (generally iron) existing along the road, such as an elevated roadway, steel bridge and tunnel. The terrestrial magnetism sensor equipped on a vehicle also develops a deviation of detected azimuth due to the slope of road. A first cause of the terrestrial magnetism variation is the presence of constructions of iron along the road. There is the regularity in the distortion of terrestrial magnetism, and according to the experiment it has a trend as shown in Fig. 2, in which the horizontal axis represents the azimuth of a road where iron constructions exist and the vertical axis represents the error of detected azimuth. The figure reveals such detection error as that a road closely pointing the magnetic north is detected to point more nearer the magnetic north, and a road closely pointing the magnetic south is detected to point more nearer the magnetic south. Accordingly, by memorizing that the road sections in concern are ferromagnetic body sections in the ferromagnetic body section memory, it is possible to eliminate the dropout of judgement for the azimuth difference by evaluating the difference between the heading direction and road direction on condition that there exist iron constructions. Namely, in calculating the difference between the heading direction and road direction with the terrestrial magnetism sensor, if the azimuth difference is zero then it is treated as zero. On the other hand, in the presence of an azimuth difference, if it is within the range of error depending on the angle with the magnetic north shown in the graph of Fig. 2, the azimuth difference is regarded to be zero, or if it is outside the range, the excessive angle is regarded to be the azimuth difference. The graph of Fig. 2 may be formed in a conversion table, or it may be expressed in a formula. In the latter case, the azimuth difference modified by the angle between the road direction or heading direction and the magnetic north is calculated as follows. For a heading direction of M detected by the terrestrial magnetism sensor and a road direction of R, the azimuth difference A is calculated as follows. P = (R-D-K1+360) mod 180 Q = min (P·K2/(90-K1) - K2, K2) + min(K2·(180-K1-P)/K1, K2) - K2 B = dif (M, R) (Case 1) For sign(B)·sign(Q) ≧ 0 A = max(abs(B) - abs(Q), 0)·sign(B) (Case 2) For sign(B)·sign(Q) < 0 A = B The azimuth is measured in degrees counterclockwise from the north, with the declination, i.e., a differential angle of the magnetic north from the north, being D. The heading direction M provided by the terrestrial magnetism sensor is assumed to be rendered a simple modification by offsetting the declination component. K1 and K2 have values based on the experiment as shown in Fig. 2. dif(A, B) denotes an angular difference (signed) between A and B. sign(A) is a function taking when A is positive, -1 when it is negative, or 0 when it is zero. abs(A) denotes the absolute value of A. In this manner, the azimuth difference modified by the angle from the magnetic north is calculated. Although the case where ferromagnetic body section identifies are memorized has been described, this technique has the exertion of effectiveness even without the provision of such special map data. In this case, the azimuth difference is calculated on assumption that all road sections are ferromagnetic body sections. The range of modification, which varies depending on the road direction, is ±20° at most, and the probability of the existence of roads having such awkward angular difference is low, allowing the technique to have its full exertion of effectiveness. Even in the presence of a situation as encountered on a ramp of highway, the heading direction varies soon, and it can be treated by holding the map matching or implementing concurrent processings of map matching and comparison for multiple candidates of road. A second cause of terrestrial magnetism variation is the slope of road. In this case, due to the inclination of terrestrial magnetic vector, the variation of terrestrial magnetism differs depending on the direction of slope of a road. Fig. 3 shows the relation between the direction of slope and azimuth error (counterclockwise) for the case of a constant slope. When the information provided by the slope section memory means has revealed that the road section read out of the map memory means is a slope section, a possible variation angle of terrestrial magnetic azimuth is calculated from the direction of the ascending or descending slope. As in the preceding case of terrestrial magnetism variation cause, if the difference between the road direction and terrestrial magnetic azimuth is within the calculated angle, the azimuth error is treated as zero, or if it is outside the range, a modified azimuth difference is calculated by the angle from the magnetic north, with the excessive angle being set as an azimuth error. In case the angle of slope is known quantitatively, more accurate modification is possible, while even the ascendance or descendance of slope is unknown, it is sufficient to calculate the angle by underestimating the azimuth difference in consideration of both polarities of gradient. The modification value of this case, which varies depending on the road direction, is plus or minus several degrees at most, and it is effective enough and is virtually small enough to be free from a misjudgement of road. Fig. 4 is a brief flowchart showing an embodiment of this invention. The process of map matching may rely on the conventional method such as the one proposed as an apparatus and method of vehicle navigation in U.S. Patent No. 4,796,191 or Japanese Patent Unexamined Publication (JP-A) No. 61-56910. The map matching process includes the calculation of the difference between the heading direction and road direction, and in this case the modification by the angle from the magnetic north is conducted against the above-mentioned first or second terrestrial magnetism variation cause for the road which is being examined whether it is the place where the vehicle is running. This modification may be conducted on the basis of the information provided b the terrestrial magnetism variation section memory means, ferromagnetic body section memory means, or sloping section memory means, or alternatively it may be conducted at all times. The following describes this method in brief. Step 301 enters the running distance provided by the running distance sensor, and step 302 enters the heading direction provided by the terrestrial magnetism sensor or the like. Step 303 implements the inferred location calculation in which the position moved from the previous inferred location by the value of the running distance is calculated to determine a new inferred location. Step 304 evaluates the difference between the heading direction and road direction through the modification based on the magnetic north described above, and selects a candidate road for matching. Step 305 implements the judgement of continuity to test whether the candidate road connects with the previously matched road. Step 306 collates the running distance vs. heading direction graph with the distance vs. road direction graph and, if the collation reveals high coincidence, selects the road as the object of matching. The calculation of coincidence also includes the modification based on the magnetic north described above. Only if decision step 307 reveals the presence of an object road for matching, step 308 moves the inferred location onto the matching object road. Finally, step 309 delivers the inferred location, and the sequence returns to step 301. In case of low coincidence, the matching process is suspended based on the judgement that the vehicle is running in a place other than a road, such as a parking place. An experience revealed a high probability of failing this judgement when the azimuth difference was great due to a distorted terrestrial magnetism or the like. However, by conducting the modification based on the angle from the magnetic north against the first or second terrestrial magnetism variation cause described previously, the probability of erroneous judgement is lowered and the vehicle can be located accurately. Although in the foregoing embodiment the modification calculation based on the magnetic north is carried out for the road direction read out of the map memory mans, the difference between the heading direction and road direction, which is the item of interest, may be evaluated by implementing the calculation of modification value for the heading direction provided by the terrestrial magnetism sensor, and the completely same effectiveness is accomplished.

Claims: A vehicle locating apparatus comprising: running distance measuring means (101) which measures the running distance of a vehicle; heading direction detecting means (102) which uses at least one terrestrial magnetism sensor and detects the heading direction of the vehicle; inferred location calculation means (103) which calculates the relative movement value of the vehicle from the heading direction detected by said heading direction detection means and the running distance measured by said running distance measuring means, and calculates the inferred location of the vehicle by accumulating the relative movement value on the coordinates of the previous inferred location; map memory means (104) which stores road data; azimuth difference calculation means (105) which evaluates the difference (B) between the road direction (R) stored in said map memory means and the direction (M) detected by said heading direction detecting means; map matching calculation means (106) which judges, based on the azimuth difference, road data stored in said map memory means and data of the inferred vehicle location provided by said inferred location calculation means, as to whether the vehicle is running on a road on the map and, if it is judged that the vehicle is running on the road, corrects the inferred location calculated by said inferred location calculation means to the road; and output means (107) which delivers the inferred location provided by said inferred location calculation means; characterized in that the apparatus further comprises a modification value calculation means (108) which calculates an allowable value (Q) of the deviation of terrestrial magnetic azimuth for a given road direction for at least a first cause of terrestrial magnetism variation in accordance with the formulae: P = (R-D-K1 + 360) mod 180 Q = min (P x K2/(90-K1)-K2,K2) + min (K2 x (180-K1-P)/K1,K2) -K2 where: Q is said allowable value of the azimuth difference; R is said road direction; D is an angle between north and magnetic north; K1 is a first parameter; and K2 is a second parameter; and and further in that, said azimuth difference calculation means calculates said difference (B) in accordance with the formula: B = dif (M,R) where: B is said azimuth difference between M and R; M is said detected heading direction; R is said road direction; and dif (A,B) denotes a signed angular difference between A and B; and said azimuth difference calculation means (105) also implements a modification to said difference (B) to calculate said azimuth difference (A), which modification is, if said difference (B) is of the same sign as the allowable value (Q), in accordance with the formula: A = max (abs(B) - abs(Q),O) x sign (B); whereas if said difference (B) is of a different sign to the allowable value (Q), then the modification is in accordance with the formula: A = B where: A is the modified difference between M and R, based on said comparison with Q; B is said azimuth difference between M and R; sign (A) is a function equal to 1 when A is positive, -1 when A is negative, or 0 when A is zero. A vehicle locating apparatus according to claim 1, wherein said formula for calculating said allowable value (Q) is obtained from a conversion table implementing said formula. A vehicle locating apparatus according any preceding claim, wherein said map memory means (104) stores road data and section identifiers which represent features of arbitrary sections of the road. A vehicle locating apparatus according to claim 3 wherein said section identifiers indicate the presence or absence of ferromagnetic bodies along roads, and wherein said azimuth difference calculation means calculates the azimuth difference (A), the difference (B) being modified using said formula or conversion table in response to the angle between the road direction or heading direction and the magnetic north and to the presence or absence of a ferromagnetic body. A vehicle locating apparatus according to claim 3, wherein said section identifiers indicate gradients of roads, and wherein said azimuth difference calculation means calculates the azimuth difference (A), the difference (B) being modified using said formula or conversion table in response to the angle between the road direction or heading direction and the magnetic north and to the gradient of the road.

Inventors: UEYAMA YOSHIKI, YASUI NOBUHIKO, UEYAMA, YOSHIKI, YASUI, NOBUHIKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: G01C21
IPC codes: G01C21