DEVICE FOR DETERMINING A PRESSURE DROP IN A TIRE

Abstract: An apparatus for computing correction factors for correcting the output from a rotational frequency mechanism which is adapted to detect the rotational frequency of a tire fixed to a vehicle, and a slip percentage calculating apparatus and a tire pressure decrease detecting apparatus which utilize such a computing apparatus. In response to the operation of an initialization switch during an experimental linear coasting of a vehicle, the rotational frequency of a tire is detected. An initial correction factor is determined on the basis of this rotational frequency. This initial correction factor is stored in E 2 PROM. When the vehicle travels normally, the rotational frequency is initially corrected on the basis of the initial correction factor (at S2). The slip percentage is calculated on the basis of the initially corrected rotational frequency (at S3). A judgement value required to detect a decrease in the tire pressure is corrected on the basis of the slip ratio (at S9). Since an initial correction factor representative of only a relative difference between the effective rolling radii of the tires is obtained, the initial difference is eliminated accurately from the rotational frequency.

Description: The present invention relates to a tire pressure drop detecting device for detecting the drop in air pressure of a tire among tires mounted on a vehicle. The document EP-A-0 510 466 discloses a slip control device for a vehicle, wherein the wheel speeds are used for avoiding slip of the wheels in operation. In the conventional device, initial correction factors for the rotational speed values of the vehicle tires are determined from the rotational speed values which are detected when the vehicle is travelling linearly and is coasting. In this context, a slip factor is determined using the corrected wheel speeds. In recent years, as an example of a safety device of a four-wheel vehicle such as an automobile or a truck, devices for detecting the drop in air pressure of a tire have been developed and some of them have been put into practical use. An example of a method of detecting the drop in air pressure of a tire is a method utilizing the difference among the respective rotational frequencies F 1 , F 2 , F 3 , and F 4 of four tires W 1 , W 2 , W 3 , and W 4 provided in a vehicle. The tires W 1 and W 2 are right and left front tires. The tires W 3 and W 4 are right and left rear tires. In this detecting method, the rotational frequency F i of each of the tires W i (i = 1, 2, 3, 4) is detected for each predetermined sampling period on the basis of a signal outputted from a wheel speed sensor associated with the tire W i . The detected rotational frequencies F i are equal if all the effective rolling radii of the respective tires W i are the same, and the vehicle is linearly traveling. The effective rolling radius is the distance the tire W i freely rolling in a loaded state (rolling in a state where both the slip angle and the slip factor are zero) moves by one rotation divided by 2π. On the other hand, the effective rolling radius of each of the tires W i changes with changes in air pressure of the tire W i , for example. That is, when the air pressure of the tire W i drops, the effective rolling radius thereof is smaller than that at the time of normal internal pressure. Consequently, the rotational frequency F i of the tire W i whose air pressure drops is higher than that at the time of normal internal pressure. Therefore, the reduced pressure of the tire W i can be judged on the basis of the difference between the rotational frequencies F i . A judging equation used in detecting the drop in air pressure of the tire W i on the basis of the difference between the rotational frequencies F i is the following equation (1), for example (see Japanese Patent Laid-Open (KOKAI) No. 305011/1988, Japanese Patent Laid-Open (KOKAI) No. 212609/1992, etc.). D = F 1 + F 4 2 - F 2 + F 3 2 F 1 + F 2 + F 3 + F 4 4 x 100 If the effective rolling radii of the tires W i are the same, the respective rotational frequencies F i are the same (F 1 = F 2 = F 3 = F 4 ). Consequently, the judged value D is zero. Therefore, threshold values D TH1 and D TH2 (where D TH1 , D TH2 > 0) are set. When conditions indicated by the following expression (2) are satisfied, it is judged that there is a tire W i whose air pressure drops. When the conditions are not satisfied, it is judged that all the tires W i have normal internal pressure: D < - D TH1 or D > D TH2 The effective rolling radii of the actual tires W i include variations within the production standard occurring at the time of fabricating the tires W i (hereinafter referred to as an "an initial difference"). That is, even if the four tires W i have normal internal pressure, the effective rolling radii of the four tires W i differ due to the initial difference. Correspondingly, the rotational frequencies F i of the tires W i vary. As a result, the judged value D may be a value other than zero. Therefore, it is erroneously detected that the air pressure drops, although it does not drop. In order to detect the drop in air pressure with high precision, therefore, it is necessary to eliminate the effect of the initial difference from the detected rotational frequency F i . It is considered that as a technique for eliminating the effect of the initial difference from the rotational frequency F i , a technique disclosed in Japanese Patent Laid-Open (KOKAI) No. 271907/1992, for example, is applied. In the technique disclosed in this gazette, a vehicle is caused to linearly travel on trail at a predetermined speed, and any one of the rotational frequencies F i of the tires W i detected at that time is used as a basis, to find correction factors K i . When the rotational frequency F 1 of the tire W 1 is used as a basis, the correction factors K i are found by the following equations (3) to (6): K 1 = F 1 / F 1 K 2 = F 1 / F 2 K 3 = F 1 / F 3 K 4 = F 1 / F 4 The rotational frequencies F i detected at the time of normal traveling are respectively multiplexed by the correction factors K i . Consequently, the effect of the initial difference on the rotational frequency F i is eliminated to some extent. When the vehicle is a front engine front drive vehicle (FF vehicle) or a front engine rear drive vehicle (FR vehicle), each of the correction factors K 3 and K 4 expressed by the foregoing equations (5) and (6) is the ratio of the rotational frequencies of a driving tire and a following tire. On the other hand, driving torque or braking torque (hereinafter abbreviated to as "driving/braking torque") is applied to the driving tire at the time of traveling. The torque may cause the driving tire to slip. Therefore, the rotational frequency F i of the driving tire is generally expressed by the following equation (7). In the following equation (7), Rs is the slip factor, V is the speed of the vehicle, and r i is the effective rolling radius of the tire W i : F i = V 2πr i (1 + Rs) In the case of the FF vehicle, the tires W 1 and W 2 are driving tires, and the tires W 3 and W 4 are following tires, whereby the correction factor K 3 can be expressed by the following equation (8) from the foregoing equations (5) and (7): K 3 = F 1 F 3 = V 2πr 1 (1 + Rs) V 2πr 3 = r3 r1 (1 + Rs) The effect of the slip factor Rs is exerted on the correction factor K 3 expressed by the ratio of the rotational frequencies of the driving tire and the following tire. The same is true to the correction factor K 4 . More specifically, the slip factor Rs is expressed by the following equation (9) until the tire W i reaches the grip limit: Rs = 1 µ x 2T C X x W D x L 2 In the equation (9), µ is the coefficient of friction of a road surface, C X is the shear modulus of the tire W i , W D is the width of a grounding area, L is the length of the grounding area, and T is a driving/braking force. The driving/braking force T is approximately proportional to the square of the speed V of the vehicle at the time of constant-speed traveling. Consequently, the correction factors K 3 and K 4 include the effects of the coefficient of friction µ of the road surface and the speed V of the vehicle at the time of trial traveling. At the time of actual traveling, however, the vehicle travels on road surfaces having various coefficients of friction µ at various speeds V. Even if the rotational frequencies F i are corrected using the correction factors K 3 and K 4 found at the time of trial traveling, therefore, the rotational frequencies cannot be accurately corrected. Accordingly, it is difficult to eliminate the effect of the initial difference from the rotational frequency F i with high precision. When the vehicle travels around a corner and a curve (hereinafter represented by a "corner"), lateral acceleration is exerted on the vehicle. As a result, the load on the vehicle is moved toward the outside of the corner. Consequently, the effective rolling radius of the tire W i on the inside of the corner is increased, and the ground contact area thereof is relatively decreased. On the other hand, the effective rolling radius of the tire W i on the outside of the corner is decreased, and the ground contact area thereof is relatively increased. On the other hand, a driving force produced by the engine is almost equally applied to the tire W i on the inside of the corner and the tire W i on the outside of the corner by a differential gear. As a result, there arise variations in the slip factor Rs between the tire W i on the inside of the corner and the tire W i on the outside of the corner. Therefore, there arise variations in the rotational frequency F i between the tire W i on the inside of the corner and the tire W i on the outside of the corner. Even if all the tires W i have normal internal pressure, therefore, the variations in the rotational frequency F i are created by the variations in the slip factor Rs at the time of cornering. As a result, the judged value D includes an error corresponding to the variations in the slip factor Rs, whereby the reduced pressure may not be accurately judged. In order to judge the reduced pressure with high precision, therefore, the effect of the slip factor Rs must be eliminated. In order to eliminate the effect of the slip factor Rs, it is considered that a technique proposed in Japanese Patent Application No. 6-312123 previously filed by the present applicant is applied. In the proposed technique, the judged value D is corrected in the following manner. A variation component ΔD of the judged value D due to the variations in the slip factor Rs is proportional to a variation component ΔRs of the slip factor Rs. On the other hand, the variation component ΔRs of the slip factor Rs is proportional to lateral acceleration LA applied to the vehicle, and is inversely proportional to the turning radius R. The slip factor Rs shall be defined by the following equation (10) until the tire W i reaches the grip limit: Rs = 2T C X x W D x L 2 Furthermore, the driving/braking force T shall be proportional to the square of the speed V of the vehicle and front/rear acceleration FRA applied to the vehicle. The variation component ΔD of the judged value D can be expressed by the following equation (11) on the basis of the foregoing relationship, letting α1, α2 and α3 be constants of proportion: ΔD = LA x (α1 + α2 x V 2 + α3 x FRA) |R| The variation component ΔD expressed by the equation (11) is taken as a correction factor, and the correction factor is subtracted from the judged value D found by the foregoing equation (1). Consequently, the effect of the slip factor Rs on the judged value D is eliminated. In the proposed technique, the slip factor Rs is utilized upon being defined by the foregoing equation (10). However, the slip factor Rs is actually inversely proportional to the coefficient of friction µ of the road surface, as expressed by the foregoing equation (9). In the proposed technique, therefore, there are possibilities that the judged value D after the correction has a large error depending on the state of the road surface, and the effect of the slip factor Rs cannot necessarily be accurately eliminated from the judged value D. In the above-mentioned proposed technique, the relationship between the amount of the variation in the effective rolling radius of each of the front tires W 1 and W 2 and the amount of the variation in the effective rolling radius of each of the rear tires W 3 and W 4 out of the variations in the effective rolling radii of the tires Wi due to load movement at the time of cornering is not considered. This is based on the presumption that the amount of the variation in the effective rolling radius of each of the front tires W 1 and W 2 and the amount of the variation in the effective rolling radius of each of the rear tires W 3 and W 4 are the same. However, the front axle weight and the rear axle weight actually differ from each other. The front axle weight is the load exerted on the front axle on which the front tire is mounted. The rear axle is the load exerted on the rear axle on which the rear tire is mounted. For example, when the engine is set on the front side of the vehicle, the front axle weight is heavier than the rear axle weight. Therefore, the amount of the load movement on the front tire side and the amount of the load movement on the rear tire side at the time of cornering differ from each other. This is particularly significant in the case of an FF vehicle. As a result, the amount of the variation in the effective rolling radius of the front tire and the amount of the variation in the effective rolling radius of the rear tire differ from each other. In order to correct the judged value D with high precision, therefore, the difference in the amount of the load movement between the front tire and the rear tire at the time of cornering must be considered. The object underlying the present invention is to provide a tire pressure drop detecting device for detecting the drop in air pressure of tires mounted on a vehicle which is capable of considering differences between the amounts of load movement on the front tire side and the rear tire side at the time of cornering and making a corresponding correction so that the drop in air pressure of a tire can be detected with high precision. This object is solved in an advantageous manner by a tire pressure drop detecting device comprising the features of claim 1. Further developments of the tire pressure drop detecting device according to the invention are specified in the subclaims. In the tire pressure drop detecting device according to the invention, the initial correction factor is found on the basis of the rotational frequency of a tire detected when the vehicle is linearly coasting. At the time of linear coasting, no driving/braking torque is applied to a driving tire. That is, the slip factor of the driving tire can be considered to be zero. Consequently, the effect of the slip factor is not exerted on the found initial correction factor, whereby the initial correction factor can be found as a value faithfully representing the relative difference in effective rolling radius between the tires. If the initial correction factor found in this construction is used in correcting the rotational frequency, the effect of the initial difference can be eliminated from the rotational frequency with high precision irrespective of a friction coefficient of a road on which the vehicle travels. According to the present invention, the initial correction factor is thus found on the basis of the rotational frequency detected at the time of linear coasting on which the effect of the slip factor is hardly exerted. Consequently, it is possible to find the initial correction factor faithfully representing the relative difference in effective rolling radius between the tires. Therefore, the effect of the initial difference can be eliminated with high precision from the detected rotational frequency on the basis of the initial correction factor. Therefore, it is possible to prevent the initial difference from affecting detection of the drop in air pressure of the tire. As a result, it is possible to detect the drop in air pressure of the tire with high precision. The slip factor is inherently represented by the degree of the difference between the speed of the vehicle and the peripheral speed of the driving tire. On the other hand, the following tire hardly slips. Accordingly, the peripheral speed of the following tire can be considered to be equal to the speed of the vehicle. Consequently, the slip factor can be calculated on the basis of the rotational frequency of the driving tire and the rotational frequency of the following tire. Moreover, the difference between the rotational frequency of the driving tire and the rotational frequency of the following tire changes depending on the coefficient of friction of a road surface. Even if the vehicle travels on a road surface having any coefficient of friction, therefore, the slip factor can be accurately calculated on the basis of the rotational frequency of the driving tire and the rotational frequency of the following tire after the correction. As described in the foregoing, according to the present invention, the slip factor is found by a method faithfully conforming to the inherent definition of the slip factor. Consequently, the slip factor involves the effect of the coefficient of friction of the road surface. Therefore, irrespective of the friction coefficient at a road surface on which the vehicle travels, the slip factor can be accurately calculated. The effect of the slip factor on detection of the drop in air pressure of the tire can be eliminated with high precision. Therefore, it is possible to detect the drop in air pressure of the tire with high precision. In the device according to the invention, the judged value is corrected on the basis of the slip factor calculated by the slip factor calculating device and the lateral acceleration found by the lateral acceleration operating means. Consequently, the effect of the slip factor and the effect of load movement at the time of cornering can be eliminated from the judged value with high precision. As a result, it is possible to detect with high precision whether or not the air pressure of the tire drops. The judged value correcting means for correcting the judged value may be one for subtracting the amount of the variation in a judged value found on the basis of a predetermined operation expression determined in consideration of the difference between the front axle weight and the rear axle weight from the judged value found by the judged value operating means, to correct the judged value. In this construction, the amount of the variation in the judged value is found on the basis of the predetermined operation expression determined in consideration of the difference between the front axle weight and the rear axle weight. If the judged value is corrected using the amount of the variation in the judged value, therefore, it is possible to realize correction considering the difference between the amount of the load movement on the front tire side and the amount of the load movement on the rear tire side at the time of cornering. According to the present invention, therefore, the judged value is corrected by the amount of the variation in the judged value found on the basis of the predetermined operation expression determined in consideration of the difference between the front axle weight and the rear axle weight. Consequently, it is possible to make correction considering the difference between the amounts of the variations in the effective rolling radii of the front tire and the rear tire due to the relative difference between the front axle weight and the rear axle weight. Therefore, it is possible to detect the drop in air pressure of the tire with higher precision. An example of the predetermined operation expression for finding the amount of the variation in the judged value may be one representing the amount of the variation in the judged value by the sum of the product of a first factor and the lateral acceleration found by the lateral acceleration operating means and the product of a second factor, the lateral acceleration found by the lateral acceleration operating means and the slip factor calculated by the slip factor calculating device. In this case, an example of the first factor may be a factor determined by (a) sampling outputs of the judged value operating means and the lateral acceleration operating means in a case where the vehicle is caused to coast around a corner in a state where it is judged that the tires have normal internal pressure, and (b) finding as the first factor the slope of a linear expression approximating the relationship between the sampled judged value and the sampled lateral acceleration. When the vehicle is caused to coast, the slip factor can be considered to be zero. In this case, the judged value is represented by the product of the lateral acceleration and the first factor. Consequently, the slope in the relationship between the judged value and the lateral acceleration which are found when the vehicle is caused to coast around a corner is a first factor. The first factor is found by actually causing the vehicle to travel, whereby the actual state of the tire can be reflected on the first factor. Consequently, the actual state of the tire can be faithfully reflected on the correction of the judged value. Therefore, it is possible to detect the drop in air pressure of the tire with higher precision. An example of the second factor may be a factor determined by (a) sampling outputs of the judged value operating means, the lateral acceleration operating means and the slip factor calculating device in a case where the vehicle is caused to corner while applying driving torque to the tires in a state where it is judged that the tires have normal internal pressure, (b) approximating the relationship between a value obtained by subtracting the product of the first factor and the sampled lateral acceleration subtracted from the sampled judged value and the product of the sampled lateral acceleration and the slip factor, by a linear expression, and (c) finding the slope of the linear expression as the second factor. Also in this construction, the second factor is found by actually causing the vehicle to travel. Consequently, the actual state of the tire can be reflected on the second factor. Therefore, the actual state of the tire can be faithfully reflected on the correction of the judged value. Therefore, it is possible to detect the drop in air pressure of the tire with higher precision. In this case, the first factor may be a factor determined by finding the first factors corresponding to a plurality of types of tires and finding the average value thereof. Further, the second factor may be a factor determined by finding the second factors corresponding to the plurality of types of tires and finding the average value thereof. The amount of the variation in the judged value differs depending on the type of tires. For example, it differs between a studless tire and a summer tire (a normal tire), for example. When a tire which differs in type from the tire provided in the vehicle in a case where the first factor or the like is found is newly provided, it is expected that the correction of the judged value with high precision becomes difficult. In this construction, therefore, the first factor and the second factor are found for each type of tires. The average values of the plurality of first factors and the plurality of second factors which correspond to the found types of tires shall be respectively a final first factor and a final second factor. Regardless of the type of the tires provided in the vehicle, it is possible to make the correction with high precision. According to the present invention, the factors determined in consideration of the fact that a plurality of types of tires can be mounted are respectively taken as a final first factor and a final second factor. Regardless of the type of tires provided in the vehicle, therefore, it is possible to detect the drop in air pressure of the tire with high precision. Furthermore, the first factor may be a factor determined by finding the first factors corresponding to a plurality of types of load conditions and finding the average value thereof. Further, the second factor may be a factor determined by finding the second factors corresponding to a plurality of types of load conditions and finding the average value thereof. The judged value varies depending on the amount of the movement of the load on the vehicle, and the amount of the load movement differs depending on the total weight of the vehicle. That is, the judged value differs depending on load conditions such as the total weight of people on the vehicle and the live load. When the vehicle travels under load conditions different from the load conditions in a case where the first factor or the like is found, therefore, the correction of the judged value with high precision may, in some cases, be difficult. In this construction, therefore, the first factor and the second factor are found for each load condition, and the average values of the plurality of first factors and the plurality of second factors which correspond to the found conditions shall be respectively a final first factor and a final second factor. Even under any load conditions, therefore, it is possible to make the correction with high precision. As described in the foregoing, according to the present invention, the factors determined in consideration of the fact that the load conditions such as the total weight of people on the vehicle can differ are respectively taken as a final first factor and a final second factor. Even under any load conditions, therefore, it is possible to detect the drop in air pressure of the tire with high precision. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram showing the construction of a tire pressure drop detecting device to which one embodiment of the present invention is applied; Fig. 2 is a block diagram showing the electrical construction of the tire pressure drop detecting device; Fig. 3 is a flow chart for explaining the whole of processing for detecting the drop in air pressure of a tire; Fig. 4 is a diagram for explaining lateral acceleration exerted on a vehicle; Fig. 5 is a diagram for explaining a method of finding a factor A1; Fig. 6 is a diagram for explaining a method of finding a factor A2; Fig. 7 is a flow chart for explaining processing for operating an initial correction factor; Fig. 8 is a diagram showing a correspondence between the speed of a vehicle and the ratio of the rotational frequencies of a front tire and a rear tire at the time of linear coasting for finding an initial correction factor K 3 ; and Fig. 9 is a flow chart for explaining another embodiment of a method of finding an initial correction factor K 3 . PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail with reference to accompanying drawings. Fig. 1 is a schematic block diagram showing the construction of a tire pressure drop detecting device to which one embodiment of the present invention is applied. The tire pressure drop detecting device detects whether or not the air pressure of any of four tires W 1 , W 2 , W 3 , and W 4 provided in a four-wheel vehicle drops. The tires W 1 and W 2 respectively correspond to right and left front tires. The tires W 3 and W 4 respectively correspond to right and left rear tires. A wheel speed sensor 1 is provided in relation to each of the tires W 1 , W 2 , W 3 , and W 4 . An output of the wheel speed sensor 1 is fed to a control unit 2. A display 3 is connected to the control unit 2. The display 3 is for reporting the tire W i (i = 1, 2, 3, 4) whose air pressure drops, and may be constituted by a liquid crystal display device, a plasma display device, a CRT (Cathode-Ray Tube), or the like. An initializing switch 4 is also connected to the control unit 2. The initializing switch 4 is operated by a user in calculating initial correction factors K j (j = 1, 2, 3) for eliminating the effect of an initial difference between the tires W i . The initial difference means variations in effective rolling radius within the production standard occurring among the tires Wi. Fig. 2 is a block diagram showing the electrical construction of the tire pressure drop detecting device. The control unit 2 is constituted by a microcomputer including an I/O interface 2a, a CPU 2b, a ROM 2c, a RAM 2d, and an EEPROM (E 2 PROM) 2e. The I/O interface 2a is required to deliver signals to external devices such as the wheel speed sensors 1 and the initializing switch 4. The CPU 2b is for performing various operation processing in accordance with a control operation program stored in the ROM 2c. The RAM 2d is one to which data or the like is temporarily written when the CPU 2b performs a control operation and from which the written data or the like is read out. The EEPROM 2e is one for storing the initial correction factors K j . The wheel speed sensor 1 outputs a pulse signal corresponding to the speed of rotation of the tire W i (hereinafter referred to as "wheel speed pulses"). In the CPU 2b, the rotational frequency F i of each of the tires W i is calculated for each predetermined sampling period AT (sec) (for example, ΔT = 1) on the basis of the wheel speed pulses outputted from the wheel speed sensor 1. Fig. 3 is a flow chart for explaining processing for detecting the air pressure of a tire in the tire pressure drop detecting device. The control unit 2 performs this processing by the CPU 2b operating in accordance with the predetermined program stored in the ROM 2c. In the following description, it is presupposed that an objective vehicle is an FF vehicle. In this processing, the rotational frequency F i of each of the tires W i is first calculated on the basis of the wheel speed pulses outputted from the wheel speed sensor 1 (step S1). The tires W i are fabricated with the initial difference included therein, as described above. Consequently, the effective rolling radii of the respective tires W i are not necessarily the same even if all the tires W i have normal internal pressure. Therefore, the calculated rotational frequencies F i of the tires W i vary. On the other hand, judgment whether the air pressure of the tire W i drops is made on the assumption that the rotational frequencies F i of the respective tires W i are approximately equal in a case where all the tires W i have normal internal pressure. Consequently, the effect of the initial difference must be eliminated from the calculated rotational frequency F i . In order to eliminate the effect of the initial difference from the calculated rotational frequency F i , the rotational frequency F i is subjected to initial correction (step S2). Specifically, correction according to the following equations (12) to (15) is made: F1 1 = F 1 F1 2 = K 1 x F 2 F1 3 = K 3 x F 3 F1 4 = K 2 x K 3 x F 4 An initial correction factor K 1 is a factor for correcting the difference in effective rolling radius depending on the initial difference between the right and left front tires W 1 and W 2 . An initial correction factor K 2 is a factor for correcting the difference in effective rolling radius depending on the initial difference between the right and left rear tires W 3 and W 4 . An initial correction factor K 3 is a factor for correcting the difference in effective rolling radius depending on the initial difference between the front tire W 1 and the rear tires W 3 and W 4 . The initial correction factors K j are calculated in a case such as a case where the vehicle is first caused to travel, a case where the air pressure of the tire W i is supplemented, or a case where the tire W i is replaced, and are stored in the EEPROM 2e in the control unit 2. One feature of the present embodiment is in a method of finding the initial correction factors K j . Specifically, in the present embodiment, the initial correction factors K j faithfully representing only the difference in effective rolling radius between the tires W i depending on the initial difference are operated. The details will be described later. The cause of the variations between the rotational frequencies F i of the tires W i is not only the initial difference. For example, the difference in the slip factor Rs between the right and left driving tires W 1 and W 2 in a case where the vehicle travels around a corner is also one cause. For example, when the vehicle travels around a corner, lateral acceleration LA is exerted on the vehicle toward the outside of the corner, so that the load on the vehicle is moved toward the outside of the corner. As a result, the load applied to the tire on the inside of the corner is relatively decreased, while the load applied to the tire on the outside of the corner is relatively increased. Consequently, the ground contact area of the tire on the inside of the corner is relatively decreased, while the ground contact area of the tire on the outside of the corner is relatively increased. A driving force produced by the engine is almost equally applied to the tire on the inside of the corner and the tire on the outside of the corner by a differential gear. Consequently, there arises a difference in the slip factor Rs between the driving tires W 1 and W 2 . As a result, even if all the tires W i have normal internal pressure, there arise variations between the rotational frequency of the tire on the inside of the corner and the rotational frequency of the tire on the outside of the corner. In order to eliminate the effect of the slip factor Rs from the detection of the drop in air pressure, therefore, the slip factor Rs is calculated by the following equation (16): Rs = F1 1 + F1 2 F1 3 + F1 4 - 1 The slip factor Rs is represented by the degree of the difference between the speed V of the vehicle and the peripheral speed of each of the driving tires W 1 and W 2 . On the other hand, the following tires W 3 and E 4 hardly slip, whereby the peripheral speed of each of the following tires W 3 and W 4 may be considered to be equal to the speed V of the vehicle. If the ratio of the difference between the rotational frequencies F1 1 and F1 2 of the driving tires W 1 and W 2 and the rotational frequencies F1 3 and F1 4 of the following tires W 3 and W 4 to the rotational frequencies F1 3 and F1 4 of the following tires W 3 and W 4 which are used as a basis is found, it is possible to know to what extent the driving tires W 1 and W 2 slip. Accordingly, the slip factor Rs can be expressed by the following equation (17). It is clear that the foregoing equation (16) is derived if the equation (17) is changed: Rs = (F1 1 + F1 2 ) - (F1 3 + F1 4 ) F1 3 + F1 4 The slip factor Rs is inversely proportional to the coefficient of friction µ of a road surface, as expressed by the foregoing equation (9). That is, the higher the coefficient of friction µ is, the lower the slip factor Rs is, while the lower the coefficient of friction µ is, the higher the slip factor Rs is. As described in the foregoing, the slip factor Rs expressed by the foregoing equation (16) is found by a method faithfully conforming to the inherent definition of the slip factor. Consequently, the slip factor has already involved the effect of the coefficient of friction µ of the road surface. That is, if the coefficient of friction µ of the road surface is increased, the numerator in the foregoing equation (17) is decreased. Consequently, the slip factor Rs is decreased. On the other hand, if the coefficient of friction µ of the road surface is decreased, the numerator in the foregoing equation (17) is increased. Consequently, the slip factor Rs is increased. If the slip factor Rs is thus found by the foregoing equation (16), the accurate slip factor Rs can be found irrespective of the coefficient of friction µ of the road surface. The calculated slip factor Rs is stored once in the RAM 2d. The slip factor Rs is used in correcting the judged value D required to detect the drop in air pressure. The slip factor Rs expressed by the foregoing equation (16) is calculated, assuming a case where the objective vehicle is an FF vehicle, as described above. If the objective vehicle is an FR vehicle, the slip factor Rs is calculated by the following equation (18): Rs = F1 3 + F1 4 F1 1 + F1 2 - 1 The difference between the distances of the tire on the inside of the corner and the tire on the outside of the corner from the center of turning in a case where the vehicle corners is also one cause of the variations between the rotational frequencies F i of the tires W i . Further, the movement of the load on the vehicle is a cause of the variations between the rotational frequencies F i . When the vehicle turns in a counterclockwise direction, for example, the turning radii of the tires W 1 and W 3 on the inside of the corer are relatively small, while the turning radii of the tires W 2 and W 4 on the outside of the corner are relatively large. Consequently, the rotational frequencies F 1 and F 3 of the tires W 1 and W 3 on the inside of the corner are low, while the rotational frequencies F 2 and F 4 of the tires W 2 and W 4 on the outside of the corner are high. As a result, there necessarily arises a difference between the rotational frequencies of the right and left tires. Furthermore, when the vehicle turns in a counterclockwise direction, for example, the lateral acceleration LA is exerted on the center of gravity O of the vehicle, as shown in Fig. 4. The lateral acceleration LA is inversely proportional to the turning radius R and is proportional to the square of the speed V of the vehicle, and is exerted toward the outside of the corner (toward the right of the vehicle). As a result, the partial load on the vehicle which is proportional to the lateral acceleration LA is moved toward the outside of the corner from the inside of the corner. Correspondingly, the respective effective rolling radii of the following tires W 3 and W 4 vary by amounts expressed by the following expressions (19) and (20): (1 + p x LA) (1 - p x LA) p in the foregoing expressions is a factor proportional to the load on the vehicle. When the load on the vehicle is considered to be constant, the factor p can be considered as a constant. The effective rolling radius of the tire W i varies by the movement of the load on the vehicle, whereby there arise variations between the rotational frequencies of the right and left tires. As shown in Fig. 3, a turning radius R whose variation caused by the movement of the load on the vehicle has been eliminated is then calculated (step S4). More specifically, the speeds V1 3 and V1 4 of the following tires W 3 and W 4 are calculated by the following equations (21) and (22) on the basis of the rotational frequencies F1 3 and F1 4 after the initial correction. In the following equations (21) and (22), r is a constant corresponding to the effective rolling radius at the time of linear traveling, which is stored in the ROM 2c. V1 3 = 2 πr x F1 3 V1 4 = 2 πr x F1 4 The turning radius R' of the vehicle is then calculated by the following equation (23) on the basis of the calculated speeds V1 3 and V1 4 of the following tires W 3 and W 4 . In the following equation (23), Tw is the tread width, that is, the distance between the right wheel and the left wheel: R' = T w 2 x V1 4 + V1 3 V1 4 - V1 3 The calculated turning radius R' of the vehicle is subjected to such correction as to eliminate the effect of the movement of the load of the vehicle on the rotational frequency, as given by the following equation (24), to find a turning radius R after the correction. In the following equation (24), u 1 and u 2 are constants previously stored in the ROM 2c: R = R' x {u 1 + u 2 (V1 3 + V1 4 ) 2 } The following values are suitable as u 1 and u 2 : u 1 = 1 u 2 = 1 9.8 x β 2Tw where β = Q x H x α Tw x 100 Q the load on the vehicle H the height from the ground plane of the tire to the center of gravity of the vehicle α the rate of variation in the effective rolling radius with respect to the load The rotational frequencies F1 i found in the foregoing step S2 are corrected in order to eliminate variations caused by a difference between the distances of the tires W i from the center of turning on the basis of the found turning radius R of the vehicle (step S5). Specifically, rotational frequencies F2 1 to F2 4 after the correction are found by the following equations (25) to (28): F2 1 = |R| √ {(R - Tw/2) 2 + WB 2 } x F1 1 F2 2 = |R| √ {(R + Tw/2) 2 + WB 2 } x F1 2 F2 3 = |R| |R - Tw/2| x F1 3 F2 4 = |R| |R + Tw/2| x F1 4 Consequently, the rotational frequencies F2 i in which variation caused by a difference between the distances of the tire W i on the inside of the corner and the tire W i on the outside of the corner from the center line of turning (hereinafter referred to as "an inner and outer wheel difference") has been eliminated are acquired. In the foregoing equations (25) to (28), WB denotes the wheel base of the vehicle. The correction in the foregoing equations (25) to (28) is processing assuming a case where the objective vehicle is an FF vehicle, as described above. If the objective vehicle is an FR vehicle, the turning radius R' before the correction required to calculate the turning radius R is found on the basis of the speeds V1 1 and V1 2 of the front tires W 1 and W 2 which are following tires, after which the following equations (29) to (32) are applied: F2 1 = |R| |R - Tw/2| x F1 1 F2 2 = |R| |R + Tw/2| x F1 2 F2 3 = |R| √ {(R - Tw/2) 2 + WB 2 } x F1 3 F2 4 = |R| √ {(R + Tw/2) 2 + WB 2 } x F1 4 The rotational frequency F i may, in some cases, include an error depending on the turning radius R of the vehicle, the speed V of the vehicle, the lateral acceleration LA of the vehicle, and the front/rear acceleration FRA i of each of the tires W i . Specifically, when the turning radius R is relatively small, the tire W i may laterally slip, whereby it is highly possible that the calculated rotational frequency F i includes an error. When the speed V of the vehicle is significantly low, the detection precision of the wheel speed sensor 1 is significantly degraded, whereby it is highly possible that the calculated rotational frequency F i includes an error. Furthermore, when the lateral acceleration LA of the vehicle is relatively large, the tire W i may laterally slip, whereby it is highly possible that the calculated rotational frequency F i includes an error. Further, when the front/rear acceleration FRA i of each of the tires W i is relatively large, the effect of the slip of the tire W i due to rapid acceleration/rapid deceleration of the vehicle or the effect of a foot brake is considered, whereby it is highly possible that the calculated rotational frequency F i includes an error. When it is thus highly possible that the rotational frequency F i includes an error, it is preferable that the rotational frequency F i is rejected without being employed for detecting the drop in air pressure. The speed V of the vehicle, the lateral acceleration LA of the vehicle, and the front/rear acceleration FRA i of each of the tires W i are then calculated (step S6). More specifically, the speed V of the vehicle is calculated on the basis of the speed V2 i of each of the tires W i . The speed V2 i of each of the tires W i is calculated by the following equation (33): V2 i = 2πr x F2 i The speed V of the vehicle is calculated by the following equation (34) on the basis of the calculated speed V2 i of each of the tires W i : V = (V2 1 + V2 2 + V2 3 + V2 4 ) / 4 On the other hand, the lateral acceleration LA of the vehicle is calculated by the following equation (35) utilizing the calculated speed V of the vehicle: LA = V 2 / (R x 9.8) Furthermore, the front/rear acceleration FRA i of each of the tires W i is calculated by the following equation (36), letting BV2 i be the speed of the tire W i which is calculated at the time point of previous sampling period AT: FRA i = (V2 i - BV2 i ) / (ΔT x 9.8) 9.8 is inserted in the denominator in the foregoing equations (35) and (36) in order to convert the lateral acceleration LA and the front/rear acceleration FRA i into G (gravity acceleration) unit. Furthermore, the speed V of the vehicle, the lateral acceleration LA of the vehicle, and the front/rear acceleration FRA i of each of the tires W i may be found by being directly detected using sensors. It is judged whether or not the rotational frequency F i calculated at the time point of the current sampling is rejected on the basis of the turning radius R, the speed V of the vehicle, the front/rear acceleration FRA i of each of the tires W i , and the lateral acceleration LA of the vehicle (step S7). Specifically, when any one of the following four conditions (1) to (4) is met, the rotational frequencies F i are rejected: (1) |R| < R TH (for example, R TH = 30 (m)) (2) V < V TH (for example, V TH = 10 (km/h)) (3) MAX {|FRA i |} > A TH (for example, A TH = 0.1 (g) : g = 9.8 m/sec 2 ) (4) |LA| > G TH (for example, G TH = 0.4 (g)) When the rotational frequency F i is not rejected as a result of the judgment in the step S7, the judged value D is calculated by the following equation (37) on the basis of the rotational frequencies F2 i acquired in the step S5 (step S8): D = F2 1 + F2 4 2 - F2 2 + F2 3 2 F2 1 + F2 2 + F2 3 + F2 4 4 x 100 In calculating the speed V of the vehicle, the lateral acceleration LA of the vehicle and the front/rear acceleration FRA i of each of the tires W i in the step S6, the rotational frequencies F2 i which have been subjected to correction corresponding to the initial difference and the inner and outer wheel difference are used. On the other hand, the rotational frequency F i of the tire W i also varies depending on not only the initial difference and the inner and outer wheel difference but also the lateral acceleration LA applied to the vehicle and the slip factor Rs. Consequently, the effect of variation factors including the lateral acceleration LA applied to the vehicle and the slip factor Rs is exerted on the judged value D found in the step S8. The judged value D is then subjected to correction for eliminating the effect of the variation factors (step S9). Specifically, a corrected value C is found by the following equation (38). The corrected value C is subtracted from the judged value D, as expressed by the following equation (39). Consequently, a new judged value D' from which the effect of the variation factors has been eliminated is acquired: C = A1 x LA + A2 x LA x Rs D' = D - C In the foregoing equation (38), A1 and A2 are factors previously stored in the ROM 2c. The factors A1 and A2 are found upon trial traveling when it is known that each of the tires W i has normal internal pressure. The reason why the judged value D' is one whose effect of the variation factors has been eliminated will be described. At the time of cornering, the lateral acceleration LA is exerted on the vehicle toward the outside of the corner. Correspondingly, the load movement proportional to the lateral acceleration LA occurs in the vehicle. As a result, the load applied to each of the tires W i varies. On the other hand, the front axle weight and the rear axle weight of the vehicle generally differ from each other. Consequently, the amount of the load movement in the front tires W 1 and W 2 and the amount of the load movement in the rear tires W 3 and W 4 at the time of cornering differ from each other. As a result, the amount of the variation in the effective rolling radius of each of the front tires W 1 and W 2 and the amount of the variation in the effective rolling radius of each of the rear tires W 3 and W 4 during cornering differ from each other. Therefore, the judged value D varies. The amount of the variation in the effective rolling radius is proportional to the lateral acceleration LA. Consequently, the variation in the judged value D is also proportional to the lateral acceleration LA. Therefore, it is possible to correct the amount of the variation in the judged value D by an proportional expression of the lateral acceleration LA. If the load on each of the driving tires W i varies, the ground contact area thereof varies. Consequently, the slip factor of the driving tire W i on the inside of the corner and the slip factor of the driving tire W i on the outside of the corner differ from each other. As a result, there arises a difference between the rotational frequency F i of the driving tire W i on the inside of the corner and the rotational frequency F i of the driving tire W i on the outside of the corner. As a result, the judged value D varies. The slip factor Rs is the average of the slip factors of the right and left driving tires W i . The difference between the slip factor of the driving tire W i on the inside of the corner and the slip factor of the driving tire W i on the outside of the corner is proportional to the amount of the load movement during cornering. The amount of the load movement is proportional to the lateral acceleration LA. Accordingly, the amount of the variation in the judged value D is proportional to the product of the average slip factor Rs of the right and left driving tires W i and the lateral acceleration LA representing the degree of the difference in the slip factor Rs between the right and left driving tires W i . Accordingly, the amount of the variation in the judged value D can be corrected by a proportional expression of the product of the lateral acceleration LA and the slip factor Rs. Since the sum of the above-mentioned two variation factors is the amount of the variation in the judged value D during traveling, whereby the amount of the variation in the judged value D can be removed by the sum of the lateral acceleration LA and the proportional expression of the product of the lateral acceleration LA and the slip factor Rs. Furthermore, the corrected value C is found in consideration of the variation in the judged value D due to the difference between the front axle weight and the rear axle weight of the vehicle. Consequently, a relative difference between the front axle weight and the rear axle weight can be reflected on the correction of the judged value D. In other words, it is possible to make correction considering the difference between the amounts of the variation in the effective rolling radius of the front tire W i and the effective rolling radius of the rear tire W i due to the relative difference between the front axle weight and the rear axle weight. Therefore, it is possible to detect the drop in air pressure of the tire with higher precision. Methods of finding the factors A1 and A2 will be described in detail. (1) Method of finding factor A1 At the time of trail traveling, the vehicle is caused to coast around a corner. Specifically, the vehicle is caused to corner in a state where a speed change gear is set in a neutral position. At this time, the driving tires W 1 and W 2 inertially rotate. That is, the slip factor Rs ≒ 0. As a result, the second term in the right side of the foregoing equation(38) becomes zero. Consequently, the judged value D found during trail coasting is expressed by the following equation (40) from the foregoing equations (38) and (39): D = D' + C = D' + A1 x LA That is, the judged value D is expressed by a linear expression of the lateral acceleration LA applied to the vehicle. Therefore, the relationship between the lateral acceleration LA found during the trail traveling and the judged value D found during the trail traveling is approximated by the linear expression. Specifically, a graph respectively plotting the lateral acceleration LA and the judged value D in the horizontal axis and the vertical axis is prepared, as shown in Fig. 5. A method of least squares is then applied to the drawn graph. As a result, the following equation (41) is obtained: D = α x LA + β In the equation (41), α corresponds to A1. (2) Method of finding factor A2 At the time of trail traveling, the vehicle is caused to travel while applying driving torque thereto. A graph shown in Fig. 6 is prepared utilizing the lateral acceleration LA, the judged value D and the slip factor Rs which are found at this time. A method of least squares is then applied to the drawn graph. As a result, the following equation (42) is obtained: D - A1 x LA = γ x LA x Rs + δ In the equation (42), γ corresponds to A2. The factors A1 and A2 are thus acquired by actually traveling the vehicle. Consequently, the actual state of the tire W i can be faithfully reflected on the factors A1 and A2. Returning to Fig. 3, it is judged whether or not there is a tire W i whose air pressure drops on the basis of the judged value D' after the correction which is acquired in the step S9 (step S10). Specifically, it is judged whether or not the judged value D' satisfies the following expression (43). In the following expression (43), D TH1 = D TH2 = 0.1: D' < - D TH1 or D' > D TH2 As a result, when it is judged that the judged value D' satisfies the foregoing expression (43), it is judged that the air pressure of any one of the tires W i drops. On the other hand, when it is judged that the judged value D' does not satisfy the foregoing expression (43), it is judged that there is no tire W i whose air pressure drops. It is thus judged whether or not the air pressure of the tire W i drops while the vehicle is traveling. On the other hand, it is easier to understand for a driver to report a tire W i whose air pressure drops than to merely report that there is a tire W i whose air pressure drops. Therefore, the tire W i whose air pressure drops is specified. By using the judged value D' found by the foregoing equation (40), the following can be specified: If D' > 0, the reduced pressure tire is W 1 or W 4 . If D' < 0, the reduced pressure tire is W 2 or W 3 . Furthermore, in this case, if the vehicle is in a linearly traveling state, the following can be specified: If F2 1 > F2 2 , the reduced pressure tire is W 1 . If F2 1 < F2 2 , the reduced pressure tire is W 2 , If F2 3 > F2 4 , the reduced pressure tire is W 3 . If F2 3 < F2 4 , the reduced pressure tire is W 4 . If the tire W i whose air pressure drops is specified, the result is displayed upon being outputted to the display 3. The display 3 comprises indicator lamps respectively corresponding to the four tires W 1 , W 2 , W 3 , and W 4 , as shown in Fig. 2, for example. When it is judged that the air pressure of any one of the tires drops, the indicator lamp corresponding to the reduced pressure tire is turned on. Fig. 7 is a flow chart for explaining processing for finding the initial correction factors K j . Processing for operating the initial correction factors K j is performed by the control unit 2. The driver operates the initializing switch 4 after confirming that the vehicle is linearly coasting in finding the initial correction factors K j . The coasting is traveling in a state where the change speed gear provided in the vehicle is set to a neutral mode. In this case, no driving torque and braking torque are applied to a driving tire to which the driving force produced by the engine is transmitted. The initializing switch 4 is operated in such a state. Means capable of detecting the direction of travel of each of the tires W i , the set state of the change speed gear, and whether or not the foot brake is applied may be provided, to allow the operation of the initializing switch 4 only when each of the means satisfies each of predetermined conditions. The following are the predetermined conditions: the tire W i is approximately parallel to the direction of travel of the vehicle, the change speed gear is set to a neutral mode, and the foot brake is not applied, that is, the vehicle is linearly coasting. According to this construction, it is possible to prevent an operation of the initial correction factors K j from being executed, when the vehicle is not linearly coasting. The control unit 2 judges whether or not the initializing switch 4 is operated (step T1). As a result, if it is judged that the initializing switch 4 is operated, the initial correction factors K j are first cleared (step T2). Thereafter, processing in the subsequent steps T4 through T7 is repeatedly performed until I = n (steps T3 to T8). Specifically, the wheel speed pulses outputted from the wheel speed sensor 1 are accepted, and the rotational frequency F i of each of the tires W i is calculated on the basis of the accepted wheel speed pulses (step T4). Thereafter, the calculated rotational frequencies F i are stored as F(I, i) in the RAM 2d (step T5). I is incremented by "1" (step T6). Thereafter, the ratio F F of the rotational frequencies of the right and left front tires W 1 and W 2 and the ratio F R of the rotational frequencies of the right and left rear tires W 3 and W 4 are calculated, and the calculated ratios F F and F R are respectively cumulatively added (step T7). Specifically, the ratios F F and F R of the rotational frequencies are added to the previous results of addition F FA and F RA , to find new results of addition F FA and F RA . The ratios F F and F R of the rotational frequencies are calculated in accordance with the following equations (44) and (45): F F = F 1 / F 2 F R = F 3 / F 4 As a result of the foregoing, the results of addition F FA and F RA of n ratios F F and F R of rotational frequencies are acquired. The respective average values of the results of addition F FA and F RA are calculated by the following equations (46) and (47) (step T9). The calculated average values are respectively taken as initial correction factors K 1 and K 2 . K 1 = F FA / n K 2 = F RA / n When the initial correction factors K 1 and K 2 are calculated, an initial correction factor K 3 is then found. Specifically, processing in the steps T11 to T13 is repeatedly performed until I = n (steps T10 to T14). Specifically, the speed V(I) of the vehicle is first calculated by the following equation (48) on the basis of the rotational frequency F(I, i) of each of the tires W i stored in the RAM 2d in the step T5 and the initial correction factors K 1 and K 2 acquired as described above (step T11). Consequently, the speed V(I) of the vehicle at the time of each sampling time is calculated: V(I) = 2πr x F(I,1) + F(I,2) x K 1 + F(I,3) + F(I,4) x K 2 4 The ratio F FR (I) of the rotational frequencies of the front tire and the rear tire at the time of each sampling time is found by the following equation (49) on the basis of the rotational frequency F(I,i) of each of the tires W i stored in the RAM 2d in the step T5 and the initial correction factors K 1 and K 2 acquired as described above (step T12): F FR (I) = F(I,1) + F(I,2) x K 1 F(I,3) + F(I,4) x K 2 Thereafter, I is incremented by "1" (step T13). As a result of the foregoing, n correspondences between the speed V(I) of the vehicle and the ratio F FR (I) of the rotational frequencies are acquired. The correspondences are graphically represented, as shown in Fig. 8, for example. As apparent from Fig. 8, the ratio F FR (I) of the rotational frequencies is not constant but differs depending on the speed V(I) of the vehicle. This means that the driving tire slightly slips. Specifically, even when the vehicle is linearly coasting, the effect of the slip factor Rs is included in the rotational frequencies F(I,i). On the other hand, when the speed V(I) of the vehicle is zero, the driving tire does not slip. That is, the slip factor Rs = 0. Consequently, the ratio F FR (I) of the rotational frequencies corresponding to V(I) = 0 is hardly affected by the slip factor Rs. Therefore, the ratio F FR (I) of the rotational frequencies in the case of V(I) = 0 is presumed using a predetermined multiple regression technique (step T15). The presumed ratio F FR (I) of the rotational frequencies is taken as an initial correction factor K 3 , and is stored in the EEPROM 2e (step T16). As described in the foregoing, the initial correction factors K j shall be found at the time of linear coasting when the effect of the slip factor is not exerted on the rotational frequencies F i . Consequently, the initial correction factors K j can be found as ones faithfully representing only the relative differences in effective rolling radius between the tires W i without depending on the state of the road surface. Therefore, the rotational frequencies F1 i corrected using the initial correction factors K j can be ones whose initial differences have been eliminated with high precision. Although description has been made of one embodiment of the present invention, the present invention is not limited to the above-mentioned embodiment. Although in the above-mentioned embodiment, the initial correction factor K 3 is found using the multiple regression technique, the initial correction factor K 3 may be found by finding the average of the ratios of the rotational frequencies of the front and rear tires, for example. Specifically, the processing in the steps T10 to T15 shown in Fig. 7 may be replaced with processing in the steps U1 to U5 shown in Fig. 9. In Fig. 9, the processing in the steps U2 and U3 is repeatedly performed until I = n. The ratio F FR (I) of the rotational frequencies of the front tire and the rear tire is first found in accordance with the foregoing equation (49). Thereafter, the ratio F FR (I) of the rotational frequencies is cumulatively added (step U2). Specifically, the ratio F FR (I) of the rotational frequencies is added to the previous result of addition F FRA , to find a new result of addition F FRA . Thereafter, I is incremented by "1" (step U3). As a result of the foregoing, the results of addition F FRA of n ratios F FR (I) of the rotational frequencies is acquired. The average value of the results of addition F FRA is calculated by the following equation (50) (step U5). The calculated average value is taken as the initial correction factor K 3 : K 3 = F FRA / n According to this construction, the initial correction factor K 3 can be found more simply, as compared with that using the multiple regression technique. Therefore, it is possible to shorten the entire operating time of the initial correction factors K j . Although in the above-mentioned embodiment, the type of the tires W i is not referred to, the amount of the variation ΔD of the judged value D differs depending on the type of the tires W i . Specifically, the slip factor Rs differs if the shearing modulus C X differs even in a case where the driving/braking torque T applied to the tires W i and the coefficient of friction µ of the road surface are respectively the same, as expressed by the foregoing equation (9). Further, the amount of the variation in the effective rolling radius of each of the tires W i with respect to the load movement differs depending on the type of the tires W i . Consequently, it is preferable to correct the judged value D using a corrected value C which is determined with consideration that a plurality of types of tires can be mounted. Therefore, the factors A1 and A2 may be found in consideration of the fact that a plurality of types of tires can be mounted. Specifically, the factors A1 and A2 are found in the above-mentioned method in a state where four tires W i of the same type are mounted on the vehicle. Similarly, the factors A1 and A2 are found in a state where four tires W i of another type are mounted on the vehicle. The averages of the found factors A1 and A2 of the tires W i of each of the types is found. The found averages of the factors A1 and A2 are employed in calculating the corrected value C. The type of the tires W i can be generally classified into a studless tire (a winter tire) and a summer tire (a normal tire). The factors A1 and A2 are found with respect to each of the studless tire and the summer tire, and the averages thereof are employed in calculating the corrected value C. Consequently, it is possible to obtain the corrected value C evenly effective to any tires W i . Therefore, the drop in air pressure can be detected with high precision irrespective of the type of tires W i are mounted on the vehicle. Furthermore, the amount of the variation ΔD in the judged value D varies depending on not only the type of the tires W i but also the total weight of the vehicle. Specifically, the amount of the variation ΔD in the judged value D varies depending on load conditions such as the total weight of people on the vehicle and the live load. Consequently, it is preferable to correct the judged value D using the corrected value C considering that the total weight of the vehicle can differ. Therefore, the factors A1 and A2 may be found in consideration of the fact that the total weight of the vehicle can differ. Specifically, the factors A1 and A2 are found by changing the load conditions such as the total weight of people on the vehicle and the live load into a plurality of patterns which differ from each other. The averages of the found factors A1 and A2 are found, and the averages of the factors A1 and A2 are employed in calculating the corrected value C. Consequently, it is possible to obtain the corrected value C evenly effective under any load conditions. Therefore, the drop in air pressure can be detected with high precision under any load conditions. Furthermore, although in the above-mentioned embodiment, description is made of a case where the initial correction factors K j are used for the tire pressure drop detecting device, the initial correction factors K j can be also used for another system utilizing the rotational frequencies F i of tires W i , for example, an antilock brake system (ABS) and a vehicle navigation system. Although in the above-mentioned embodiment, description is made of a case where the slip factor Rs is used for the tire pressure drop detecting device, the slip factor Rs may be used for antilock brake control, for example. In addition thereto, various design changes can be made within the scope of technical matters as set forth in claims. INDUSTRIAL APPLICABILITY As described in the foregoing, the initial correction factor operating device, the slip factor calculating device, and the tire pressure drop detecting device according to the present invention are suitable for accurate detection that the air pressure of any one of tires mounted on a vehicle drops.

Claims: A tire pressure drop detecting device for detecting the drop in air pressure of a tire among tires mounted on a vehicle, comprising a slip factor calculating device including means adapted to accept the rotational frequency (Fi) of each tire in a case where the vehicle is linearly coasting, means adapted to find an initial correction factor (Kj) for eliminating an effect of a relative difference in the effective rolling radii depending on an initial difference between tires at the rotational frequency (Fi) on the basis of the accepted output, rotational frequency correcting means adapted to correct the rotational frequency (Fi) using the initial correction factor (Kj), and means adapted to calculate the slip factor (Rs) of a driving tire (Wi) on the basis of the rotational frequencies of a driving tire (Wi) and a following tire which are corrected by the rotational frequency correcting means, wherein the detecting device further comprises lateral acceleration operating means adapted to find a lateral acceleration (LA) exerted on the vehicle on the basis of the rotational frequency (Fi) of the tire corrected by the rotational frequency correcting means, judged value operating means adapted to substitute the rotational frequency (Fi) of a tire corrected by the rotational frequency correcting means in a predetermined operation expression to find a judged value (D), judged value correcting means adapted to correct the judged value (D) found by the judged value operating means on the basis of the slip factor (Rs) calculated by the slip factor calculating device and the lateral acceleration (LA) found by the lateral acceleration operating means, and means for performing processing adapted to detect the drop in air pressure of a tire (Wi) on the basis of the judged value (D) corrected by the judged value correcting means. The detecting device according to claim 1, wherein the judged value correcting means subtract an amount of the variation in a judged value (D) found on the basis of a predetermined operation expression determined in consideration of a difference between a front axle weight and a rear axle weight from the judged value (D) found by the judged value operating means, to correct the judged value (D). The detecting device according to claim 1 or 2, wherein the judged value correcting means are adapted to correct the judged value (D) by a sum of a product of a first factor (A1) and the lateral acceleration (LA) found by the lateral acceleration operating means, and a product of a second factor (A2), the lateral acceleration (LA) found by the lateral acceleration operating means and the slip factor (Rs) calculated by the slip factor calculating device. The detecting device according to claim 3, wherein the first factor (A1) is a factor set by a) sampling outputs of the judged value operating means and the lateral acceleration operating means in a case where the vehicle is caused to coast around a corner in a state where it is judged that the tires have a normal internal pressure, and b) finding as the first factor a slope of a linear expression (41) approximating a relationship between the sampled judged value (D) and the sampled lateral acceleration (LA). The detecting device according to claim 3 or 4, wherein the second factor (A2) is a factor set by a) sampling outputs of the judged value operating means, the lateral acceleration operating means and the slip factor calculating device in a case where the vehicle is caused coast around the corner while applying driving torque to the tires in a state where it is judged that the tires have a normal internal pressure, b) approximating a relationship between a value obtained by subtracting a product of the first factor (A1) and the sampled lateral acceleration (LA) from the sampled judged value (D) and a product of the sampled lateral acceleration (LA) and the slip factor (Rs), by a linear expression (42), and c) finding a slope of the linear expression as the second factor (A2). The detecting device according to any of claims 3 to 5, wherein the first factor (A1) is a factor set by finding the first factors corresponding to a plurality of types of tires and finding an average value thereof, and wherein the second factor (A2) is a factor set by finding the second factors corresponding to the plurality of types of tires and finding an average value thereof. The detecting device according to any of claims 3 to 5, wherein the first factor (A1) is a factor set by finding the first factors corresponding to a plurality of types of load conditions and finding an average value thereof, and wherein the second factor (A2) is a factor set by finding the second factors corresponding to the plurality of types of load conditions and finding an average value thereof.

Inventors: NAKAJIMA MIKAO, NAKAJIMA, MIKAO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO RUBBER IND, SUMITOMO ELECTRIC INDUSTRIES, LTD., SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1997
Main classification: G01P21
IPC codes: B60C23, B60T8, G01P21, G01M17, G01P3, G01P1