Method for detecting decrease in tyre air-pressure and apparatus used therefor

Abstract: An apparatus for detecting decrease in internal pressure of a tyre based on rotational information obtainable irom tyres attached to a four-wheeled vehicle equipped with a limited slip differential device (6). The apparatus includes a shift amount calculating means for correcting a turning radius or a reciprocal of the turning radius of driving wheels (W 1 ,W 2 ) by using a shift amount which is a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels (W 1 ,W 2 ) and following wheels (W 3 ,W 4 ). There can be calculated accurate turning radius for the driving wheels which is not affected by the LSD. Consequently, decrease in internal pressure can be properly judged also in a vehicle that is comprised with a LSD.

Description: The present invention relates to a method for detecting decrease in tyre air-pressure and an apparatus used therefor. More particularly, the present invention relates to a method for detecting decrease in tyre air-pressure and an apparatus used therefor capable of properly judging that decrease in internal pressure has occurred in any tyres attached to a vehicle equipped with a limited slip differential device (LSD) and generating an alarm accordingly. It is conventionally known that the dynamic load radius of a tyre becomes smaller when the air-pressure of the tyre is decreased, and that the rotational velocity or rotational angular velocity becomes faster than that of a tyre having a normal air-pressure. For instance, there are proposed methods for detecting decrease in internal pressure based on relative differences of rotational angular velocities of tyres (see, for instance, Japanese Unexamined Patent Publication No 100620/1998, Japanese Unexamined Patent Publication No 66714/1997, Japanese Unexamined Patent Publication No 164720/1996 and Japanese Unexamined Patent Publication No 145654/1996). Among these techniques, apparatus A as recited in Japanese Unexamined Patent Publication No 100620.1998 is so arranged that in case it is judged that a wheel of decreased pressure is a following wheel when the vehicle is running on a corner, a turning radius is introduced from a rotational angular velocity of the driving wheels, acceleration in a lateral direction (hereinafter called "lateral G") at the time of running on a corner is calculated from the turning radius, and a judgement value is calculated on the basis of a lateral G including no errors by using a specified threshold. However, among recent vehicles, there are some which are equipped with a limited slip differential device in the differential gear to improve running performance at the time of, for instance, cornering. The mechanism of the limited slip differential device is so arranged that the differential effect is restricted until the differential torque exceeds a set value, and the driving wheels are rotated at identical velocities on the right and left. Therefore, in a vehicle equipped with such a mechanism, the rotational angular velocity at the outer side which in turn becomes slower than the essential rotational angular velocity, (i.e. the non-limited angular velocity), due to the influence of the limited slip differential device so that the turning radius is calculated to be larger than it actually is. Consequently, errors occur in judgement values so that a decrease in internal pressure cannot be properly judged. Document EP-A-0 970 823, which represents state of the art according to Art. 54(3) and (4) EPC, discloses an apparatus and a method for differentiating summer tyres and winter tyres based on rotational information that can be obtained from tyres attached to a four-wheeled vehicle equipped with a limited slip differential device, to be able to detect a deflated tyre in case of summer tyres and winter tyres, respectively, mounted on the vehicle. A judgement value which indicates whether a tyre is deflated or not is derived by means of the rotational speed of each tyre and the turning radius of the vehicle. A correction of the judgement value is performed with respect to a lateral acceleration or a driving force during cornering. An object of the present invention is to provide a method for detecting decrease in tyre air-pressure and an apparatus used therefor capable of properly judging decrease in internal pressure of a tyre in a vehicle equipped with a limited slip differential device. According to the present invention, there is provided an apparatus for detecting decrease in the internal pressure of a tyre based on rotational information obtainable from tyres attached to a four-wheeled vehicle equipped with a limited slip differential device including a shift amount calculating means which corrects a turning radius or a reciprocal of the turning radius of driving wheels by using a shift amount which is a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels and following wheels. In accordance with the present invention, there is further provided a method for detecting decrease in the internal pressure of a tyre based on rotational information obtainable from tyres attached to a four-wheeled vehicle equipped with a limited slip differential device including the steps of introducing a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels and following wheels, performing correction by using the difference between turning radiuses or the difference between reciprocals of turning radiuses as a shift amount for the turning radiuses or reciprocals of the turning radiuses of the driving wheels, and thereafter judging decrease in internal pressure of a tyre. Further aspects of the invention will be apparent from the following description, by way of example only, of an embodiment in conjunction with the attached diagrammatic drawings, in which: Figure 1 is a block diagram showing an apparatus for detecting decrease in tyre air-pressure according to the present invention; Figure 2 is a block diagram showing electric arrangements of the apparatus for judging decrease in tyre air pressure of Figure 1; Figure 3 is a view showing a relationship between reciprocals of turning radiuses and lateral G which are respectively obtained from following wheels and driving wheels of a vehicle equipped with a LSD; and Figure 4 is a flow chart. As shown in Figure 1, the apparatus for detecting decrease in air-pressure is designed to judge whether the air-pressure of any of the four wheels W 1. W 2 , W 3 and W 4 which are driving wheels and following wheels attached to a four-wheeled vehicle has decreased or not, and comprises ordinary wheel velocity sensors 1 which are respectively provided in connection with each of the tyres W 1 . W 2 , W 3 and W 4 as well as a control unit 2. Each wheel velocity sensor 1 detects rotational information for each tyre such as number of rotations, rotational velocity and rotational angular velocity. Outputs of the wheel velocity sensors 1 are supplied to the control unit 2. To the control unit 2, there are connected a display 3 comprising a liquid crystal display device, a plasma display device, or CRT for notifying a tyre W 1 . W 2 , W 3 and W 4 of which the air-pressure has decreased, and a switch SW4 which can be operated by the driver. As shown in Figure 2, the control unit 2 comprises an I/O interface 2a required for sending/receiving signals to/from an external device, a CPU 2b which functions as a centre of calculation, a ROM (memory device for read out only) 2c which stores a control operation program for the CPU 2b, and a plurality of RAMs (isokinetic random access memory devices) 2d into which data are temporarily written and from which data are read out when the CPU 2b performs control operations. The four-wheeled vehicle according to the present embodiment is a FF (front engine-front drive) vehicle, and is equipped with a limited slip differential device 7 associated with the differential gear 6 driving shafts 8. Note that 5 denotes a following or non-driven shaft. In this embodiment, each wheel velocity sensor 1 serves as a rotational information detecting means, and the control unit 2 includes shift amount calculating means, lateral G calculating means, and judging means. The shift amount calculating means stores as a shift amount a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels and following wheels, and performs correction. The judging means performs correction by using the difference between turning radiuses or the difference between reciprocals of turning radiuses as a shift amount for the turning radiuses or reciprocals of the turning radiuses of the driving wheels, and thereafter judges decrease in internal pressure of a tyre. The shift amount calculating means and judging means in the present invention will now be explained in the case where a difference between two diagonal sums is defined to be a judgement value D. That is, the judgement value D can be obtained from the following equation (1). D = F11 + F14 2 - F12 + F13 2 F11 + F12 + F13 + F14 4 Here, F1(X): rotational angular velocity of wheel (tyre) X : 1 = front left tyre, 2 = front right tyre, 3 = rear left tyre, 4 = rear right tyre. In the conventional apparatus A, errors might be included in the rotational angular velocity F1(x), depending on the size of velocity V of the vehicle (V1(X)/4), a first turning radius R of a vehicle, and acceleration in forward and rear directions FRA of each of the wheels. Thus, the first turning radius Ro of the vehicle can be calculated from the following equations (2) and (3) by performing correction of influences of load shift to a turning radius Ra of the vehicle which has been calculated based on the velocity V1 3 , V1 4 of the following wheels W 3 , W 4 . Ra = TW 2 x V1 3 + V1 4 V1 3 - V1 4 Ro = Ra x (1 + 2 x Po x V TW x 9.8 Here, TW denotes a track width which is the distance between the following wheels W 3 and W 4 , V is the speed of the vehicle, and Po a constant which is preliminarily stored in the ROM 2c. It is then judged whether the rotational angular velocity F1(X) which has been calculated for a judging period ΔT is to be rejected or not, while the first turning radius Ro which is one of the parameters used for rejection is calculated on the basis of the velocity V1 3 , V1 4 of the following wheels W 3 , W 4 . However, in case the air-pressure is decreased in either one of the following wheels W 3 , W 4 , the calculated first turning radius Ro includes errors so that no correct value can be obtained. Due to this fact, an estimation process is then performed on the basis of a ratio between each of the velocities V 1 , V 2 of the driving wheels W 1 , W 2 , a ratio between each of the velocities V 3 , V 4 of the following wheels W 3 , W 4 , and a difference between these ratios in order to confirm whether the first turning radius Ro of the vehicle is correct or not. In this estimation process, it is judged whether it has been estimated that the air-pressure of either of the following wheels W 3 , W 4 has decreases or not. In case it is judged that it has been estimated that the air-pressure of either of the following wheels W 3 , W 4 has decreased, it is judged that errors are included in the previously obtained first turning radius Ro, and a second turning radius R is obtained on the basis of the velocities V1 1 , V1 2 of the driving wheels W 1 , W 2 , similarly for the equation (2) which is a method for calculating the first turning radius Ro. With this arrangement, it is enabled to obtain a correct turning radius of the vehicle which is not affected by decreases in air-pressure. Based on the correct second turning radius R, it is then judged whether the rotational angular velocity F1(X) is to be rejected or not. Since lateral G of the vehicle which is proportional to the speed V of the vehicle and is in reverse proportion to the second turning radius R of the vehicle will be a large factor, the lateral G of the vehicle is calculated on the basis of the following equation (4) which utilises the second turning radius R of the vehicle. Lateral G = V 2 / (Rx9.8) The reason for introducing 9.8 as a denominator is for performing g calculation of lateral G. Thereafter, it is judged whether the calculated lateral G of the vehicle satisfies the following equation (5) or not. | lateral G | > GTH (e.g. GTH = 0.4 g) In the case where this equation (5) has been satisfied, it is judged that errors are included in the rotational angular velocity F1(X), and the rotational angular velocity F1(X) is rejected. On the other hand, in the case where the above equation (5) is not satisfied, it is judged that no errors are included in the rotational angular velocity F1(X), and a judgement value D is calculated based on equation (1). In a vehicle equipped with a limited slip differential device, the mechanism functions to restrict differential wheel speeds until the differential torque in a turn is held slower then the rotational angular velocity of the outer side wheel on a vehicle which is not equipped with a limited slip differential device. As a result the turning radius is calculated to be larger than it actually is. Thus, the present invention is characterised by performing correction of the turning radius which is calculated to be larger than it actually is when obtaining the turning radius from the driving wheels, so that a correct turning radius is obtained and utilised. That is, in the present embodiment, by using a vehicle which is equipped with a limited slip differential device and to which tyres (having a size of 225/45ZR17) each preliminarily made to assume a normal air-pressure (2.6 kgf/cm 2 ) are attached (hereinafter called "vehicle equipped with LSD"), turning movements to the left are performed at a radius of 70R at respective speeds of 20 km/h, 40 km/h, 60 km/h and 80 km/h. Reciprocals of turning radiuses which have been respectively obtained from the following wheels and driving wheels based on wheel velocity data are as shown in Figure 3. Note that the number of points for measurement at each of the velocities is not less than 30. As is evident from Figure 3, the reciprocal 1/R of the turning radius R which has been introduced from the driving wheel WFD is shifted by 0.004 from the reciprocal 1/Ro of the turning radius Ro which has been introduced from the follower wheel WRD. Thus, by defining this value of 0.004 as a shift amount (constant) α and adding this to the reciprocal 1/R of the turning radius R which has been introduced from the driving wheel WFD, the value can be made to approach the reciprocal 1/Ro of the turning radius Ro of the following wheel. Therefore, the following steps as shown in Figure 4 are performed in the present embodiment: after performing calculation of wheel velocities and specification of the position of a wheel having a tyre at lower pressures (Steps S1, S2), an estimation (judgement) is made whether the air-pressure of the following wheels has decreased or not (Step S3). In the case where the air-pressure of the following wheels has not decreased, a turning radius is introduced from the following wheels (Step S4), and in the case where the air-pressure of the following wheels has decreased, a reciprocal of a turning radius is introduced from the driving wheels (Step S5). It is then judged whether the turn is a right turn or not (Step S6), and if it is a right turn, 0.004 is decremented as the shift amount (constant) α (Step S7), and if it is not a right turn, 0.004 is incremented as the shift amount (constant) α (Step S8). After introducing an average speed (Step S9), a lateral G is calculated from the average speed and the reciprocal of the turning radius of the driving wheels (Step S10). Based on these calculated values, it is judged whether the rotational angular velocity is to be rejected or not (Step S11). In the case nothing is to be rejected (Step S12), the judgement value is calculated (Step S13). Correction of the judgement value is performed for eliminating influences of variable factors including lateral G or slip rate (Step S14) to consequently perform judgement processes whether an alarm is to be generated or not (Step S15). It should be noted that the slip rates are preliminarily calculated and stored in the RAM. In this manner, by performing correction of the reciprocal of the turning radius calculated from the driving wheels by using a shift amount (constant), decrease in internal pressure of a tyre can be properly judged even in a vehicle which is equipped with a limited slip differential device. It should be noted that the shift amount (constant) differs from vehicle to vehicle, and this amount can be obtained at the time of performing tuning. Further, while the difference between reciprocals of turning radiuses of driving wheels and following wheels is set to be the shift correction amount in the present embodiment, the present invention is not limited to this, and it is also possible to define the shift correction amount to be a difference between turning radiuses of driving wheels and following wheels. As explained so far, in the case of obtaining a turning radius from driving wheels of an actually running vehicle which is equipped with a limited slip differential device (LSD), the difference between turning radiuses or difference between reciprocals of turning radiuses calculated from following wheels and driving wheels which have been obtained with the tyres being at normal pressure is set to be a shift amount used for correction, thereby calculating an accurate turning radius for the driving wheels which is not affected by the LSD. Consequently, decrease in internal pressure can be properly judged also in a vehicle that is fitted with a LSD.

Claims: An apparatus for detecting decrease in internal pressure of a tyre based on rotational information obtainable from tyres attached to a four-wheeled vehicle equipped with a limited slip differential device (6) including a shift amount calculating means which corrects a turning radius or a reciprocal of the turning radius of driving wheels (W 1 ,W 2 ) by using a shift amount which is a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels (W 1 ,W 2 ) and following wheels (W 3 ,W 4 ). A method for detecting decrease in internal pressure of a tyre based on rotational information obtainable from tyres attached to a four-wheeled vehicle equipped with a limited slip differential device (6) including the steps of introducing a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels (W 1 ,W 2 ) and following wheels (W 3 ,W 4 ), performing correction by using the difference between turning radiuses or the difference between reciprocals of turning radiuses as a shift amount for the turning radiuses or reciprocals of the turning radiuses of the driving wheels, and thereafter judging decrease in internal pressure of a tyre.

Inventors: OSHIRO YUJI, OSHIRO, YUJI
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO RUBBER IND, SUMITOMO ELECTRIC INDUSTRIES, LTD., SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1999
Main classification: B60C23
IPC codes: B60C23