ANTISKID CONTROLLER

Abstract: The invention provides an anti-skid controller capable of correctly executing ABS control by accurately estimating a road surface µ without using additional sensors. Each wheel speed is detected, and acceleration of the vehicle body in a case where a wheel speed difference between front and rear wheels exceeds a specified value is determined. In this case, by using a change rate within a specified period of time with respect to a wheel speed of one in which wheel speed changing is small as the acceleration, a road surface µ corresponding to the acceleration is determined. Corrections regarding turning, bad road travelling, road surface changing, etc. can be carried out, by determining a lateral G based on a speed difference between left and right wheels.

Description: The present invention relates to a vehicular antiskid controller in accordance with the preamble of claim 1, and in particular to a controller wherein vehicular braking performance and stabilizing performance are improved by switching various control references to values suited to friction coefficients without specially using an additional sensor such as a G sensor. Frictional coupling in a vehicle between a tire and a road surface is important for safe travelling of the vehicle. This is because for any dynamic process (accelerating the vehicle by operating an accelerating pedal and decelerating the vehicle by operating a brake pedal, etc.) it is necessary to transmit between the tire and the road surface a force corresponding thereto. In this case, the frictional coupling between the tire and the road surface depends on a specific slipping of a wheel, and when the specific slipping exceeds a specified range, the frictional coupling is weakened and the travelling of the vehicle becomes unstable. Controllers for preventing a specific slipping rate of a vehicle from exceeding a specified range at times of decelerating or accelerating a vehicle are known. In these controllers, a wheel speed and acceleration of an individual wheel revolution of a wheel are detected by means of a wheel sensor, and based on values, thus obtained and other measured values, a wheel specific slipping rate etc., are calculated in an electronic evaluation controller. There has been devised an antiskid controller wherein braking pressure is reduced by a suitable controller when wheel slipping exceeds a specified range and braking pressure is increased when a wheel speed is restored and the wheel slipping becomes a suitable value, so that a braking distance is shortened by obtaining a vehicle braking force as strong as possible. In the above-described antiskid controller, if a level of the frictional coupling between the wheel tire and the road surface is known prior to starting of control, a control suited to the level of coupling between the tire and the road surface can be carried out from the start of the control. That is, in a case where the level of the coupling between the tire and the road surface differs from the known values, in other words road surface friction coefficient becomes different, control of brake liquid pressure accompanying with movements of wheels may be different from wheel speed dropping timings between front and rear wheels. Thus, for instance as described in JP-A-4 345 562, there has been proposed a method for executing antiskid control by obtaining a wheel speed difference between the front and rear wheels of the vehicle, estimating whether a level of friction on the road surface is low (hereinafter called low µ road) or high (high µ road) based on a relationship between the wheel speed difference and the duration of a state in which the difference exists and using a result of the estimation. In such an antiskid control method, however, only whether the road surface on which the vehicle travels is the high µ road or the low µ road is determined based on the relationship between a speed difference between front and rear wheels and the duration of the state having the speed difference, and not on specific road surface friction coefficients corresponding to each of road surface situations changing continuously. Therefore, for example, only determination of low µ roads and high µ roads can be realized, and a medium µ road surface including a wet road, a compressed snow road or the like must simply be determined as either the high µ road surface or as the low µ road surface. If the vehicle is controlled based on this kind of determination, vehicle behavior may become unstable. A vehicle body speed is used as a reference for determining the above-described wheel slipping ratio. If the vehicle body speed is not correctly calculated, a starting time of the antiskid control may not be accurately determined or control of brake fluid pressure may not be accurately executed. Thus, an optimum braking force cannot be obtained. In this kind of antiskid controller, usually the vehicle body speed of a two-wheel-drive vehicle is estimated from the wheel speeds of four wheels, and if braking hydraulic pressure is reduced by means of ABS control after starting braking the vehicle, the reduced speed of a rolling wheel is restored to the vicinity of the actual vehicle body speed. Therefore, an estimated vehicle body speed calculated from the wheel speeds of four wheels is almost equal to the actual vehicle body speed, and thus, nearly accurate ABS control performance can be obtained. In a case where more accurate control is to be executed even in ABS for the two-wheel-drive vehicle, however, an attempt has been made to estimate an accurate vehicle body speed by adding an acceleration sensor for detecting acceleration and deceleration of a vehicle body or dividing road surface friction coefficients into categories by using the method described in the above-described publication. On the other hand, in the case of a four-wheel-drive vehicle, four wheels are simultaneously driven by means of mechanical coupling of a differential gear and greatly affected by engine torque and transmission torque. Thus, even though braking hydraulic pressure is reduced by means of ABS control after a start of braking, a wheel speed may not be restored to the vicinity of the real vehicle body speed as in the case of the two-wheel-drive vehicle. Rather, an estimated vehicle body speed shifts away from the real vehicle body speed and, particularly on a road surface where a surface friction coefficient (road surface µ) is extremely low (extremely low µ road) such as on an ice road and the like, a difference between both speeds becomes greater. In this case, a reference speed and the like for reducing pressure are set so low that slipping of wheels happens more. Given this situation, in the case of a four-wheel-drive vehicle, the road surface µ is estimated by means of an additional sensor such as a G sensor for detecting deceleration of the vehicle body. Then, based on this road surface µ, optimizing ABS control is carried out by switching the estimated vehicle body speed and various control references to values suited to the road surface µ. Therefore, it is apparent that the additional sensor such as the G sensor for detecting deceleration of the vehicle speed is essential to ABS for the four-wheel-drive vehicle compared with other systems of the two-wheel-drive vehicle. The system is specialized for the four-wheel-drive vehicle, not only causing an increase in cost but also necessitating a different control from that for the two-wheel-drive vehicle, and a common system for all kinds of vehicles cannot be available. An anti-skid controller in accordance with the preamble of claim 1 is known from US-A-5 065 327. The present invention is made in view of these problems and it is an object of the invention to provide an antiskid controller wherein, without using an additional sensor such as an acceleration sensor (or a G sensor), a road surface µ can be correctly estimated in accordance with changing conditions of road surfaces and antiskid control can be accurately executed by using the estimated value. This object is solved by the measures indicated in claim 1. Further advantageous modifications of the present invention are subject matter of the dependent claims. In more detail, in accordance with the subject matter of claim 1, in addition to calculation of forward and backward direction acceleration, lateral direction acceleration of the vehicle is calculated based on speeds of right and left wheels. Then, the deceleration limit gradient is calculated based on the forward and backward direction acceleration and the lateral direction acceleration. At the time of turning and braking of the vehicle, etc., frictional coupling between each wheel and the road surface is divided into a friction force for decelerating the vehicle and a cornering force (lateral direction inertial force) for turning the vehicle. Therefore, from the forward and backward direction acceleration and the lateral direction acceleration equivalent to these forces, the deceleration limit gradient really corresponding to the friction coupling force between the wheel and the road surface can be obtained. By obtaining the vehicle body speed in accordance with the deceleration limit gradient thus obtained, the vehicle body speed, one of the antiskid control references, can be accurately obtained without using a load sensor such as a G sensor or the like. Therefore, by executing braking pressure control with the vehicle body speed as reference, braking efficiency can be increased and stability can be obtained. The basic effect of the controller defined in claim 2 is the same as in the case of the previously described controller, and it is specified that a period for obtaining the forward and backward direction acceleration and lateral direction acceleration is during travelling of the vehicle or application of a brake. With the controller thus structured, forward and backward direction acceleration and lateral direction acceleration is obtained only in a state where almost equivalent braking torque is applied as a ratio to a set load. Therefore, an accurate deceleration limit gradient can be obtained from the forward and backward direction acceleration properly corresponding to a road surface friction coefficient of the vehicle and the lateral direction acceleration corresponding to the turning state. In accordance with the subject matter of claim 4, it cannot be necessarily true that the calculated forward and backward direction acceleration accurately corresponds to the road surface friction coefficient. It can be true, however, that as long as braking pressure control is in operation, the changing ratio of the wheel speed calculated when slipping of wheels is a problem corresponds to that of the wheel speed at least equal to or higher than a µ peak. In this case, there is not such a big problem, for an inclination of a deceleration limit gradient is greater and the vehicle is in a direction for further deceleration. By executing the corrections defined in claims 5 to 8, it is possible to properly deal with a situation where there are changes in a state of a road surface for travelling. An antiskid controller of the present invention can be applied to not only four-wheel-drive vehicles but also two-wheel-drive vehicles such as FF, FR and the like, and by utilizing techniques according to the present invention, performance can be further improved. In the drawings: FIG. 1 is a block diagram showing an antiskid controller for elucidating the subject matter of the present invention; FIG. 2 is a flow chart for elucidating the subject matter of the present invention; FIG. 3 is a view illustrating a motion equation of wheels; FIG. 4 is a graph showing braking force distribution lines of wheels in a vehicle; FIG. 5 is a graph showing integrated values obtained by integrating wheel speed differences between front and rear wheels in a vehicle; FIG. 6 is a view illustrating a concept of the invention for estimating a road surface friction coefficient; FIG. 7 is a timing chart showing an operational state of ABS control; FIG. 8 is a flow chart showing a main routine in a first embodiment; FIG. 9 is a flow chart showing an example of forward and backward G estimating means in the first embodiment; FIG. 10 is a flow chart showing an example of vehicle turning determining means in the first embodiment; FIG. 11 is a flow chart showing an example of lateral G estimating means in the first embodiment; FIG. 12 is a flow chart showing an example of bad road state determining means in the first embodiment; FIG. 13 is a flow chart showing an example of ice road determining means in the first embodiment; FIG. 14 is a flow chart showing an example of road surface changing determining means in the first embodiment; FIG. 15 is a flow chart showing an example of road surface µ correction determining means in the first embodiment; FIG. 16 is a time chart at a time of executing forward and backward G estimation processing in FIG. 10; FIG. 17 is a time chart at a time of executing lateral G estimation processing based on vehicle turning determination in FIGs. 11 and 12; FIG. 18 is a time chart at a time of executing bad road state determination processing in FIG. 12; FIG. 19 is a time chart at a time of executing ice road travelling determining processing in FIG. 13; FIG. 20 is a time chart at a time of executing high µ road surface changing determination processing in FIG. 14; FIG. 21 is a time chart at a time of executing low µ road surface changing determination processing in FIG. 14; FIG. 22 is a time chart at a time of executing processing for determining correction to a side of a high road surface µ in FIG. 15; and FIG. 23 is a time chart at a time of executing processing for determining correction to a side of a low road surface µ in FIG. 15. FIG. 1 is a block diagram showing a hydraulic circuit structure in an ABS controller for elucidating the subject matter of the present invention. In FIG. 1, a brake pedal 20 is connected to a master cylinder 28 via a vacuum booster 21. Thus, hydraulic pressure is generated in the master cylinder 28 by pressing the brake pedal 20, the hydraulic pressure is supplied to wheel cylinders 31, 32, 33 and 34 provided on respective wheels (left front wheel FL, right front wheel FR, left rear wheel RL and right rear wheel RR). The master cylinder 28 is provided with two pressure chambers (not shown) in which similar brake hydraulic pressure is generated and supplying pipes 40 and 50 are connected to these pressure chambers. The supplying pipe 40 is branched into communicating pipes 41 and 42. The communicating pipe 41 is connected, via a solenoid valve 60a, to a brake pipe 43 communicating with the wheel cylinder 31. Similarly, the communicating pipe 42 is connected, via a solenoid valve 60c, to a brake pipe 44 connected with the wheel cylinder 34. As in the case of the supplying pipe 40, the supplying pipe 50 is branched into communicating pipes 51 and 52. The communicating pipe 51 is connected, via a solenoid valve 60b, to a brake pipe 53 communicating with the wheel cylinder 32. Similarly, the communicating pipe 52 is connected, via a solenoid valve 60d, to a brake pipe 54 communicating with the wheel cylinder 33. In the brake pipes 54 and 44 connected to the wheel cylinders 33 and 34, known proportioning valves 59 and 49 are installed. The proportioning valves 59 and 49 are for bringing the distribution of a braking force among the front and rear wheels FL to RR closer to an ideal state by controlling a brake hydraulic pressure supplied to the rear wheels RL and RR. Solenoid pickup type wheel speed sensors 71, 72, 73 and 74 are provided on the wheels FL to RR and signals thereof are input to an electronic control circuit ECU. The electronic control circuit ECU outputs driving signals to the solenoid valves 60a to 60d so as to control brake hydraulic pressure applied to the wheel cylinders 31 to 34 based on input speeds of the wheels FL to RR. The solenoid valves 60a, 60b, 60c and 60d connect, in an A position of a 3-port 3-position type solenoid in FIG. 1, the communicating pipes 41, 42, 51 and 52 respectively with the brake pipes 43, 44, 53 and 54, and in a B position the valves cut off all communications among the communicating pipes 41, 42, 51 and 52, the brake pipes 43, 44, 53 and 54 and branch pipes 47, 48, 57 and 58. In a C position, the brake pipes 43, 44, 53 and 54 respectively communicate with the branch pipes 47,48, 57 and 58. The branch pipes 47 and 48 are connected to a discharge pipe 81, while the branch pipes 57 and 58 are connected to a discharge pipe 91. The discharge pipes 81 and 91 are connected respectively to reservoirs 93a and 93b. The reservoirs 93a and 93b are for temporarily storing brake fluid discharged from the wheel cylinders 31 to 34 when the solenoid valves 60a to 60d are in the C position. Thus, brake hydraulic pressure in the wheel cylinders 31 to 34 can be increased when the solenoid valves 60a to 60d are in the A position, brake hydraulic pressure can be held when the solenoid values 60a to 60d are in the B position and brake hydraulic pressure can be decreased when the solenoid values 60a to 60d are in the C position. The solenoid valves 60a and 60b function as braking force adjusting means. Pumps 99a and 99b are for pumping up brake fluid stored in the reservoirs 93a and 93b and returning the brake fluid to the side of master cylinder 28. Check valves 97a, 98a, 97b and 98b are for preventing brake fluid pumped up from the reservoirs 93a and 93b from flowing back. A stop switch 10 is for detecting whether a driver presses the brake pedal 20 or not. Next, explanation will be made of ABS control with reference to FIGs. 2 to 7. FIG. 2 is a flow chart showing a main routine of the ABS controller. This processing is started from a step 10 when an ignition switch (not shown) is switched ON. After the processing is started, first in step 11 initial setting of each flag and counter is carried out. In following step 12, wheel speed signals from the wheel speed sensors 71 to 74 are input to a central arithmetic processing unit ECU 100. In step 13, based on signal inputs from the wheel speed sensors 71 to 74 a wheel speed and acceleration of a wheel (any one of FL to RR) of the controlled wheel are calculated. In step 14, a speed difference between the front and rear wheels is calculated. For the calculation of a front and rear wheel speed difference, a method for calculating a difference in a right side and in a left side independently, for instance, between FR wheels and RR wheels and between FL wheels and RL wheels is employed. A method for calculating an average value, for instance (FR wheel + FL wheel)/ 2 - (RR wheel + RL wheel)/ 2 may be employed. Here, in consideration of a wheel dropping phenomenon, it is apparent from a wheel motion equation (equation 1), as shown in FIG. 3, that a wheel speed dropping state ω of each wheel depends on a braking force TB. (Equation 1) ω = (µ * r * W - TB) / 1 Herein, ω: wheel dropping state (wheel angular velocity) µ: road surface friction coefficient r: wheel radius W: wheel load TB: braking force I: inertia moment of wheel As shown in FIG. 4, it is clear that the actual braking force is higher for the front wheels than for the rear wheels from a braking force distribution line by the P valve. Thus, it is expected from the equation 1 that a timing for a front wheel dropping is generally faster than that for the rear wheel dropping. In following step 15, integrating calculation for periodically and sequentially integrating the front and rear wheel speed differences obtained in step 14 is carried out. Here, as shown in FIG. 5, absolute values of the front and rear wheel speed differences are integrated. By means of the integrating calculation, effects of noise components in the speeds of the front and rear wheels can be reduced. For example, as shown at a point A in FIG. 5, if the rear wheel speed drops due to noises in a state where dropping in the front wheel speed is faster than that in the rear wheel speed, a speed difference between the front and rear wheels is restored to a normal travelling state and thus, it may be mistakenly determined that dropping in the speed is solved. However, by periodically and sequentially integrating front and rear wheel speed differences, effects of the noise error components can be reduced, making it possible to continuously monitor states of front and rear wheel speed differences. In step 16, whether an integrated value calculated in step 15 is above or below a specified value within an integrating period is determined. That is, whether the speed difference between the front and rear wheels is really occurring or not within this integrating period is determined. Here, if the integrated value exceeds the specified value, that is, a speed difference between the front and rear wheels exists, then processing moves to step 17. If it is determined that the integrated value is below the specified value, that is, speeds between the front and rear wheels are the same, processing moves to step 30. In step 17, it is determined whether it is a timing for calculating in step 18 an average changing ratio of values obtained by integrating wheel speed differences. Specifically, whether a preset specified period passes or not is determined. The specified period is set longer than a period in which the integration of wheel speed differences is carried out in step 15. In step 17, when it is determined that the specified period of time passes, processing moves to step 18. If it is determined that the specified period of time does not pass yet, processing moves to step 22, and conventional processing to determine starting of ABS control is carried out but road surface friction coefficient estimating processing is not executed. Here, as a method of determining the timing for calculating the changing ratio of the integrated values of wheel speeds, it may be possible to employ a method which determines whether the integrated value of speed differences between the front and rear wheels exceeds the specified amount or not without depending on a passage of time after a previous calculating timing. In step 18, an average changing ratio within a specified period of time with respect to the integrated values calculated in step 15 is calculated. For example, integral calculation is performed at every 5 ms and calculation of a changing ratio of the integrated value is performed at every 50 ms. This is for the reason that if the average changing ratio is calculated by using a time shorter or the same as a period in which wheel speed differences area integrated, the average changing ratio will be a speed difference between the front and rear wheels. In step 19, whether or not to estimate a road surface friction coefficient prior to starting of ABS control is determined depending on the size of the average changing ratio of the values obtained by integrating wheel speed differences in step 18. If the average changing ratio is below a preset specified value, as shown in FIG. 6, dropping states of the front and rear wheels are in a wheel movement stable area prior to a µ - peak in which frictional coupling between a road surface and a tire is largest. Since a road surface friction coefficient can be accurately estimated in this case, processing moves to step 20. On the other hand, if the average changing ratio exceeds the specified value, the dropping state of the front wheels (or the rear wheels) is suddenly increased and the front and rear wheels are in a wheel movement unstable area exceeding the µ - peak in which friction coupling between the road surface and the tire is largest. When accurate estimation of the road surface friction coefficient is impossible, processing moves to step 22 so that the starting of the ABS control determination processing is performed. Here, in step 22, the road surface friction coefficient calculated previously in step 20 corresponds to a friction coefficient immediately before the µ - peak between the wheel and the road surface for travelling. In step 20, if the changing ratio is below the specified value in step 19, estimating a road surface friction coefficient can be accurately carried out and calculation is carried out to estimate the road surface friction coefficient. Here, as a method of calculating an estimated value of the road surface friction coefficient, as described above, in the front and rear wheels with dropped wheel speeds, an average changing gradient of a wheel speed in which a change is smaller is employed, that is, in step 20, a method of using an average deceleration gradient as an estimated road surface friction coefficient is employed. That is, within the preset specified period of time used in steps 17 and 18, the average deceleration gradient of the wheel speed between a wheel of the front and rear wheels in which a speed change is smaller is calculated. Specifically, for instance, within the specified period of time, by using the period in which the difference in wheel speeds is calculated in step 14, the wheel speed of either the front or rear wheels in which a speed change is smaller in the period is picked up. Then, a linear equation approximately passing the picked up point is calculated and an inclination of the equation is used as the average deceleration gradient. High vehicle body deceleration occurs on a dried road surface when the braking force is applied to the wheels. In this case, due to the vehicle body deceleration load movement to the front wheels may increase and speeds of the rear wheels may drop faster than the front wheels. In this case, however, the front wheel speeds in which speed dropping is smaller because of load movement of the vehicle body and high friction coupling between the road surface and the wheels correspond to the wheel speed of wheels in which speed dropping is smaller in the above-described period. Thus, the average deceleration gradient is calculated by using the front wheel speed. It may be possible simply to employ as the average deceleration gradient a value obtained by dividing a difference between a value at the specified passage of time after the wheel speed is calculated in step 13 and a value at the specified passage of time after a current wheel speed is calculated by a specified period of time. Next, explanation will be made on a way of estimating road surface friction coefficients. Road surface friction coefficients can be estimated by using a graph showing a characteristic in the vicinity of the µ peak and a µ - s characteristics graph in FIG. 6. More specifically, the moment that the speed difference between the front and rear wheels suddenly increases, that is, immediately before a changing ratio of the integrated value within the specified period of time with respect to the speed difference between the front and rear wheels exceeds the specified value, the wheel in which a speed drops faster than the other exceeds the µ - peak in which friction coupling between the road surface and the tire is largest and in an area in which sudden dropping occurs. On the other hand, the remaining wheels are in an area immediately before the µ - peak. Thus, a deceleration gradient of the remaining wheel, that is, one in which a change in a wheel speed is smaller corresponds to vehicle body deceleration VB. A method of estimating road surface friction coefficients is expressed by an equation 2 using a road surface friction coefficient µ and vehicle body deceleration VB. (Equation 2) µF * WF + µR * WR = - ( VB / g ) * WB (Equation 3) µ ( WF + WR )= - ( VB / g ) * WB Herein, µF: front wheel road surface friction coefficient WF: front wheel load µR: rear wheel road surface friction coefficient WR: rear wheel load VB: vehicle body deceleration WB: vehicle weight g: gravitational acceleration Generally values µ of the front and rear wheels are equal (µF = µR = µ), and thus the equation 2 is expressed by the equation 3, a relationship of road surface friction coefficient µ ∞ vehicle body deceleration VB is established resulting in road surface friction coefficient ≒ vehicle body deceleration. In this way, road surface friction coefficients can be accurately estimated. Next, explanation will be made of a case where in step 16, it is determined that the integrated value is within a specified range in the integration period, that is, the front and rear wheels keep the same speed. In this case, the speeds of all the four wheels drop when the vehicle is in travelling state at a constant speed or a braking force is applied. A speed dropping phenomenon of the four wheels when a brake is applied on the vehicle is practically inconceivable on a high µ road in which friction coupling between the wheel and the road surface is strong and wheel speed dropping is slow because of the load movement. However, on a low µ road, speeds of the four wheels may simultaneously drop because the friction coupling between the wheel and the road surface is weak. In other words, as shown in steps 30 and 31, a low µ road is determined when almost no differences in speeds between the front and rear wheels are found for over a specified period of time in a state where a braking force is applied to the vehicle. This means that in step 30, whether an integrated value of speed differences between the front and rear wheels exceeds a preset specified value or not is determined. That is, whether a state in which differences in speeds between the front and rear wheels do not occur is continued for a specified period of time or not is determined. When it is determined that the integrated value does not exceed a specified value within a specified period of time, processing moves to step 31, and when the determination is otherwise, processing moves to step 22. In step 31, an average speed gradient of wheel speeds within a specified period of time is determined. That is, whether a wheel speed really drops or not is determined, and when the average speed gradient over a specified value is detected, it is determined that a braking force has been applied to the vehicle. Here, when the average speed gradient is determined to be over the specified value, processing moves to step 32, and when the gradient is below the specified value, processing moves to step 22 since the gradient is considered not as a low friction coefficient road surface. In step 32, determining that there have been produced almost no differences in speeds between the front and rear wheels for over the specified period in a state where speeds of all the wheels drops, the present road surface for travelling is determined to be a low friction coefficient road surface. Then, the information is sent to step 21 as information of ABS control reference. In following step 21, by means of the road surface friction coefficient estimated in step 20, various ABS control references (deceleration gradient of the estimated vehicle body speed, control start slipping reference value of each wheel, pressure decrease and increase after starting of ABS control, etc.) are switched from a state prior to ABS control. In step 22, it is determined whether or not to start ABS control, considering various ABS control references have been switched in step 21. In step 23, solenoid driving is executed in accordance with a determined result of starting ABS control in step 22. Therefore, by estimating the road surface friction coefficient before starting ABS control, control according to the road surface friction coefficient, that is, a situation on a road surface, which is conventionally impossible, can be properly executed from an initial cycle of starting ABS control. Moreover, it is possible to prevent, in advance, situations where a vehicle body becomes extremely unstable because of nonstarting of ABS control, a braking force greatly decreases and the like. Conventionally, for instance, as shown in FIG. 7, since the estimated vehicle body speed far different from the real vehicle body speed is used, control reference is largely different from real situations of the wheels and the road surface, and therefore, the time for staring ABS control is unstable. This will be explained based on the following equation for generally estimating the vehicle body speed: (Equation 4) VSB (n) = MED ( Vw0, VSB( n - 1 ) - αdown * t, VSB ( n - 1 ) - αup * t ) VSB: estimated vehicle body speed MED: intermediate value employed Vw0: selected wheel speed αdown: wheel deceleration upper limit constant αup: wheel acceleration upper limit constant t: calculation period n: number of calculating times By employing the equation 4, for instance, in step 22, an estimated vehicle body speed as a reference for determining a time of starting ABS control is calculated. If, in this case, a wheel acceleration upper limit constant αup and a wheel deceleration upper limit constant αdown are preset specified values, an estimated vehicle body speed is determined without consideration given to a friction coefficient of the road surface for current travelling. However, by means of a technique devised by the present invention, the friction coefficient of the road surface can be estimated beforehand, so that an estimated vehicle body speed can be brought closer to reality based on the road surface friction coefficient. This is for the reason that by calculating the estimated road surface friction coefficient and changing settings of a wheel acceleration upper limit αup and the wheel deceleration upper limit αdown, the estimated vehicle body speed can be more accurately obtained. Friction coupling between a wheel and a road surface for travelling, that is, a slipping situation between the wheel and the road surface, can also be accurately grasped. Therefore, ABS control is started in an optimum slipping situation of the wheel, and stable movement of the vehicle can be obtained. The following modifications can be made: In step 20 in the flow chart in FIG. 2, the road surface friction coefficient is estimated by using the speed of a wheel of the front and rear wheels in which a speed change is smaller. However, a vehicle body speed of the vehicle from each wheel speed may be calculated by using the equation 4 and a road surface friction coefficient from an average deceleration gradient of the vehicle body speed within a specified period of time can be estimated. That is, in step 14, a difference between the vehicle body speed and the speed of the front wheels and the difference between the vehicle body speed and the speed of the rear wheels are calculated. In step 15, absolute values of differences between values of differences between the vehicle body speed and the speed of the front wheels and values of differences between the vehicle body speed and the speed of the rear wheels are obtained, and the absolute values are periodically and sequentially integrated for a specified period of time. By using the integrated values, whether calculating a changing ratio of integrated values and estimating a road surface friction coefficient in steps 18 and 19 should be carried out or not is determined. In step 20, if it is determined that a road surface friction coefficient can be accurately estimated in step 19, the road surface friction coefficient is estimated. In this case, the average deceleration gradient of a vehicle body speed within the specified period of time corresponds to a vehicle body speed deceleration VB in the equation 2. Thus, based on a relationship of a road surface friction coefficient µ ∝ VB, a road surface friction coefficient between the road surface for travelling and the wheel can be accurately estimated by means of the average changing deceleration gradient of the vehicle body speed within the specified period of time. A speed of the vehicle is always a value in line with reality by correcting the wheel deceleration lower limit constant number µdown and a wheel acceleration upper limit αup in the equation 4 each time when a road surface friction coefficient is estimated. In step 18 in the flow chart in FIG. 2, the average changing ratio within the specified period of time with respect to integrated values of speed differences between the front and rear wheels is calculated. However, it may be possible to employ a method wherein a time is obtained each time an integrated value of wheel speed differences between the front and rear wheels reaches a preset specified amount and an average deceleration gradient during this period is used as a changing ratio. Furthermore, in step 20 in the flow chart in FIG. 2, the amount of load movement to the front wheels increases and the speed of the rear wheels drop earlier than that of the front wheel on a dried road in which high vehicle body deceleration occurs. In this case, with respect to the load movement amount, corrected loads of the front and rear wheels WF' and WR' for the wheel load W in the equation 1 for estimating a speed dropping situation of each wheel may be used. A load movement amount ΔW by vehicle body deceleration is obtained by an equation 5 and loads of the front and rear wheels WF' and WR' when a brake is applied are obtained by equations 6 and 7. (Equation 5) ΔW = { WB * ( H / L ) * ( - VB ) } / g (Equation 6) WF' = WF0 + ΔW (Equation 7) WR ' = WR0 - ΔW ΔW: vehicle load movement amount H: height of center of gravity L: wheel base WF0: front wheel static load WR0: rear wheel static load From the above equations 5 to 7, the load movement amount at the time of applying a brake depends on a vehicle body deceleration VB. Therefore, the real speed dropping state of the wheel at the time of applying the brake is determined by a balance between the braking force and the vehicle body speed (load movement amount) produced by braking. Next, a first embodiment of the invention will be explained. As mentioned above, an antiskid controller is described in which deceleration of the vehicle on a point in which a speed difference between the front and rear wheels at the time of applying a brake suddenly increases is used as estimated µ of a road surface, and starting of brake pressure control and control thereafter are executed according to the road surface µ. Thus, for instance, even in a four-wheel-drive vehicle, ABS control without using a G sensor can be performed. However, a technique mentioned above is for estimating the road surface µ from a changing state of a speed difference between the front and rear wheels when a brake is applied to the vehicle, and therefore the road surface µ cannot be estimated extremely accurately when the vehicle is in a turning state in which a speed difference is generated not by braking or when a road surface situation is bad. It is unlikely that a speed difference between the front and rear wheels to be is generated at the time of applying a soft brake on a road surface such as on an ice road in which a road surface µ is extremely low. When the integrated value of speed differences between the front and rear wheels within the specified period of time does not exceed the specified value, it is simply determined as a low road surface µ. Furthermore, during application of braking pressure, changing of a road surface (changing from a dried road to a compressed-snow road) and switching and correcting an estimated road surface µ on a mixed road surface (part of a compressed-snow road is ice road, etc.) may be necessary. Therefore, in the first embodiment, compared with the antiskid controller mentioned above, an antiskid controller with higher performance in which a road surface µ can be properly corrected according to changes in vehicle travelling conditions such as turning, road surface changing and the like and antiskid control suited to a road surface for travelling can be performed based on the estimated and corrected road surface µ is explained. A structure of the hydraulic circuit in the antiskid controller in the first embodiment will not be explained here, because it is the same as the one in FIG. 1. The structure of the hydraulic circuit in the first embodiment is not limited to the one in FIG. 1 but other antiskid controller hydraulic circuits may be applied. Next, ABS control executed in the first embodiment thus structured will be described with reference to FIGs. 8 to 23. FIG. 8 is a flow chart showing a main routine of an ABS controller. When an ignition switch (not shown) is switched ON, processing is started from step 100. After processing starts, first in step 101 initial setting of various flags and counters is performed. In following step 102, wheel speed signals of wheel speed sensors 71 to 74 are input to a central arithmetic processing unit ECU 100. In step 104, based on signals input from the wheel speed sensors 71 to 74, a wheel speed Vw and wheel acceleration Gw of a wheel to be controlled (any one of FL to RR) are calculated. Then, in step 106, based on each wheel speed Vw obtained in step 104, an estimated vehicle body speed Vb is calculated. By the processing, for instance, with a largest speed Vwmax of wheel speeds VwFL to VwRR as a reference speed, determination is made on whether the largest speed Vwmax is within a range from a speed obtained by subtracting a deceleration limit Vdown obtained by a function f1(µ) of an estimated road surface friction coefficient µ (hereinafter road surface µ) from a previously calculated estimated vehicle body speed Vb(n - 1) to one obtained by adding acceleration limit Vup obtained by a function f2(µ) of an estimated road surface µ to Vb(n - 1). If the largest speed Vwmax is found to be within the range from the speed of adding the acceleration limit Vup to one of subtracting the deceleration limit Vdown, the largest speed Vwmax is set as an estimated vehicle body speed Vb, if the largest speed Vwmax exceeds the speed of adding the acceleration limit Vup, the speed of adding the acceleration limit Vup is set as an estimated vehicle body speed Vb, and if the largest speed Vwmax is below the speed of subtracting the deceleration limit Vdown, the speed of subtracting the deceleration limit Vdown is set as the estimated vehicle body speed. As for the reference speed, other than the largest speed Vwmax an intermediate value obtained by adding a weight to each wheel speed Vw may be used. In step 106, together with calculation of an estimated vehicle body speed Vb, an estimated vehicle body deceleration Gb is calculated. For calculating the estimated vehicle body deceleration Gb, a method of calculating a changing gradient of an estimated vehicle body speed Vb per unit time can be used. In step 108, whether ABS control is currently being executed or not is determined. In step 110 and thereafter, a processing for estimating a road surface µ before ABS control is started is executed. On the other hand, during ABS control, switching and correcting of an estimated road surface µ are executed in step 136 and thereafter. First, a processing prior to starting ABS control will be explained. In step 110, a speed difference ΔVx between the front and rear wheels is calculated. For this calculation of a front and rear wheel speed difference, a method of independently calculating differences on a left side and a right side, specifically between FL wheel - RL wheel and between FR wheel - RR wheel are used. Also, a method of calculating an average value, such as ( FL wheel + FR wheel ) / 2 - ( RL wheel + RR wheel ) / 2 and the like may be used. Likewise, in step 112, a speed difference ΔVy between left and right wheels is calculated. For this calculation of a left and right wheel speed difference, a method of independently calculating differences on a front side and a right side, specifically FL wheel - FR wheel and RL wheel - RR wheel are used. Also, as in the case of the above-described front and rear wheel speed difference, a method of calculating an average value such as ( FL wheel + RL wheel ) / 2-( FR wheel + RR wheel ) / 2 and the like may be used. Next, by means of forward and backward G estimating means, deceleration in forward and backward directions of the vehicle, i.e., forward and backward G, corresponding to a largest road surface µ (µ peak) on a road surface on which the vehicle travels is estimated (step 200). By means of vehicle turning determining means whether the vehicle is in a turning state or not is determined (step 300). The forward and backward G estimating means in step 200 and the vehicle turning determination in step 300 will be described in detail later. In step 114, based on a result of determining the turning state of the vehicle in step 300, how to set acceleration in a lateral direction of the vehicle (lateral G) is determined. That is, when the vehicle is found to be in a straight advance state, lateral G is determined to be zero in step 11 and processing moves to step 118. On the other hand, when the vehicle is found not to be in a straight advance state, lateral G of the vehicle is estimated by means of lateral G estimating means and processing moves to step 118 (step 400). The lateral G estimating means in step 400 will be described in detail later. In step 118, a value obtained by synthesizing a forward and backward G estimated value calculated in step 200 and a lateral G estimated value set in step 400 or in step 116 is calculated. The value obtained by synthesizing the forward and backward G estimated value and the lateral G estimated value is used as a road surface µ estimated value. Next, by means of bad road state determining means, whether a currently travelling road surface is bad or not is determined (step 500). The bad road state determining means in step 500 will be described in detail later. In step 120, depending on a result of determining the bad road state in step 500, whether or not to switch a road surface µ estimated value is determined. In the case of a bad road state, processing moves to step 122 and the road surface µ estimated value is changed to a bad road µ value µbad. The bad road µ value µbad is a settable maximum value in order to apply a sufficient brake to the vehicle. If a bad road is not determined in step 120, no change is made to the estimated road surface µ value and processing moves to step 600. In step 600, by means of ice road travelling determining means, whether the vehicle is currently travelling on an ice road or not is determined. If the vehicle is found to be travelling on an ice road, processing moves to step 126 and the road surface µ estimated value is changed to an ice road µ value µice. Here, the ice road µ value µice must be set small to secure sufficient stability of the vehicle on the ice road on which the vehicle is found to be travelling in step 600. In step 124, if the vehicle is found not to be travelling on an ice road, no change is made to the road surface µ estimated value and processing moves to step 128. When processing reaches step 128, a road surface µ estimated value before starting ABS control has been established for the first time. In step 128, whether a timing for starting ABS control exists or not is determined. Conditions for starting ABS control are that, for instance, a stop switch 10 is ON, a wheel speed Vw is smaller than an estimated vehicle body speed Vb by a specified speed and wheel acceleration Gw is smaller than a preset reference deceleration. Then, processing moves to step 130 and these will be described later. When ABS control is determined to be in operation in step 108, processing moves to step 136 and a speed difference ΔVw between an estimated vehicle body speed Vb and each wheel speed Vw is calculated. By means of calculating a speed difference between the estimated vehicle body speed Vb and the wheel speed Vw, a slipping state of the vehicle can be grasped. Next, by means of road surface changing determining means, road surface changing during ABS control is determined (step 700). The determination of road surface changing in step 700 will be described in detail later. In step 138, if changing to a high µ road is determined by the road surface changing means in step 700, processing moves to step 140, a road surface µ estimated value is changed to a high µ value µHi and processing moves to step 800. Here, the high µ value µHi is set at a relatively high value so as to secure at least a sufficient brake applied to the vehicle on a road surface on which changing to the high µ road is determined in step 700. If changing to the high µ road is not determined in step 138, processing moves to step 142 and whether changing to a low µ road has been determined or not is determined in step 700. If changing to a low µ is determined, processing moves to step 144, a road surface µ estimated value is changed to low µ value µLo and processing moves to step 800. Here, the low µ value µLo is set at a relatively small value so as to secure at least sufficient stability of the vehicle during control on a low µ road on a road surface on which changing to a low µ road is determined in step 700. If changing to the low µ road is not determined in step 142, changing of a road surface µ estimated value is not made and processing moves to step 800. In step 800, by road surface µ correction determining means, whether the road surface µ estimated value is changed to a low side or a high side is determined. The road surface µ correction determining processing in step 800 will be described in detail later. In step 146, whether correction of the road surface µ to the low side is determined or not is determined in step 800, and if the correction to a low side is determined, processing moves to step 148, the road surface µ estimated value is lowered by a correction amount µ2 and processing moves to step 154. The lowering correction value µ2 in step 148 is set so as to enable determining lowering correction of the road surface µ in step 800. If correction of the road surface to the low side is not determined in step 146, processing moves to step 150 and whether correction of the road surface µ to a high side has been determined or not is determined in step 800. If the correction to a high side is determined, processing moves to step 152, the road surface µ estimated value is increased by a correction amount µl and processing moves to step 154. The increased correction value µl in step 152 is set to enable determining the increasing correction of the road surface µ in step 800. If correction of the road surface µ to a high side is not determined in step 150, correction of the road surface µ estimated value is not executed and processing moves directly to step 154. When processing reaches the step 154, a road surface µ estimated value during ABS control is established for the first time. In step 154, based on each wheel speed Vw of FL wheel to RR wheel and each wheel acceleration Gw, which state, increasing / holding / reducing, is to be selected for brake hydraulic pressure for each wheel is determined. After a control mode is selected in step 154, processing moves to step 130. Processing in step 130 is a step executed both prior to starting ABS control and during control. In step 130, a deceleration limit Vdown utilized for calculation of an estimated vehicle body speed Vb (step 106) is obtained according to a function f1(µ) of the road surface µ estimated value. Similarly, in step 132, an acceleration limit Vup is obtained according to a function f2(µ) of the road surface µ estimated value. Lastly, in step 134 solenoid driving is executed according to the result of determining ABS control start in step 128 or the control mode selected during ABS control in step 154, and processing moves to step 102 again. Next, main processings in the flow chart in FIG. 8 is explained. Processing contents of the forward and backward G estimating means are shown in FIG. 9. In FIG. 9, in step 202, integral calculation of speed differences ΔVx between the front and rear wheels calculated in step 110 is executed and in step 204, the integral calculation is continued until the time Tchk2 being a timing for calculating forward and backward G is passed. If passing of the time Tchk2 is determined in step 204, processing moves to step 206 and whether the integrated value ΣΔVx of ΔVx is over a value D1 for determining vehicle body forward and backward G is determined. If the integrated value ΣΔVx of ΔVx is below D1, since a speed difference between the front and rear wheels at the time of application of a brake to the vehicle is not increased suddenly is determined, processing moves to step 208 and a vehicle body deceleration (forward and backward G) Gx is used as a value of estimated vehicle body deceleration Gb calculated in step 106. Then, moving to step 210, the integrated value ΣΔVx of ΔVx is cleared to 0 and processing moves back to the main routine. On the other hand, if the integrated value ΣΔVx of ΔVx is over the value D1 for determining the vehicle body forward and backward G, since the speed difference between the front and rear wheels is greater, vehicle body forward and backward G is established and the vehicle body deceleration Gx is not renewed, moving to step 210 to execute similar processing. A series of operations of the forward and backward G estimation means are shown in FIG. 16. That is, the integrated value ΣΔVx of ΔVx is calculated every determining time Tchk 2 and whether the integrated value ΣΔVx is above a value D1 for determining vehicle body forward and backward G is determined. If the value is below D1, vehicle body deceleration Gx is a value of estimated vehicle body deceleration Gb. On the other hand, if the value is above D1, a previous value is maintained without renewing vehicle body deceleration Gx, and thus, estimation of vehicle body forward and backward G at the time of applying a brake to the vehicle can be accurately carried out. That is, in the vehicle, in consideration of the stability of the vehicle at the time of applying the brake, a slight difference in braking torque given to the front and rear wheels at the time of braking is set in order to prevent the rear wheels from reaching the locking state earlier than the front. Thus, a difference in timings between the front and rear wheels for sudden reduction in wheel speeds after exceeding the µ peak in the µ - s characteristic is produced. However, since the time difference is produced when a difference in braking torque given to the front and rear wheels is small, the other wheels are controlled almost in the vicinity of the µ peak when sudden reduction in a speed of one wheel occurs by exceeding the µ - peak. Therefore, by taking a timing for sudden increase of the front and rear wheel speed difference ΔVx and detecting wheel deceleration on a high speed side (close to vehicle body deceleration) at this time, deceleration corresponding to the µ peak on the road surface for travelling can be obtained because of correlation between the road surface µ and vehicle body deceleration. In the flow chart in FIG. 9, by comparing an integrated value ΣΔVx of front and rear wheel speed differences ΔVx and the determining value D1, sudden increase of the speed difference ΔVx between the front and rear wheels is detected. Sudden reduction in speeds of the other wheels against that of one wheel can be detected by means of, for instance, a difference in deceleration between the front and rear wheels. Processing contents of determining the vehicle turning state (step 300) are shown in FIG. 10. In FIG. 10, in step 302 integral calculation of front and rear wheel speed differences ΔVx calculated in step 110 is executed. In step 304, integral calculation of left and right wheel speed differences ΔVy calculated in step 112 is executed, and in step 306, the above-described integral calculation is continued until the timing Tchk3 for calculating lateral G is passed. If passing of Tchk3 is determined in step 306, processing moves to step 308 and whether an integrated value ΣΔVy of left and right wheel speed differences ΔVy is above a left and right wheel speed difference determining value D2 or not is determined. If the integrated value ΣΔVy of left and right wheel speed differences ΔVy is above the determining value D2, a left and right wheel speed difference is produced, and processing moves to step 310 and whether an integrated value ΣΔVx of Δx is above a front and rear wheel speed difference determining value D3 or not is determined. If the integrated value ΣΔVx of Δx is below the determining value D3, that is, the left and right wheel speed difference is large and the front and rear wheel speed difference is small, it is determined that the vehicle is turning the processing moves to step 312 and an average value ΣΔVyave per unit time of an integrated value ΣΔVy of ΔVy is calculated to use this for calculation of vehicle body lateral G in step 400. In step 314, the vehicle is in a turning state is determined beforehand and processing moves to step 324. In steps 324 and 326, the integrated value ΣΔVx of ΔVx and the integrated value ΣΔVy of ΔVy are cleared to zero and processing returns to the main routine. On the other hand, if the integrated value ΣΔVx of ΣVx is found to be over the front and rear wheel speed difference determining value D3 in step 310, that is, both left and right wheel speed difference and front and rear wheel speed difference are large, processing moves to step 316 and whether the turning state of the vehicle is determined or not is confirmed. If the vehicle is in the turning state, an unstable state of the vehicle is determined in step 318. This is for the reason that if a big increase exists in the front and rear wheel difference when the vehicle is in the turning state, the vehicle can be determined to be in such unstable states as drifting, spinning and the like. Then, moving to step 324 the above-described processing is executed. On the other hand, if nonturning of the vehicle is determined in step 316, the state of the vehicle currently determined is kept, and processing moves to step 324 to execute the similar processing. If an integrated value ΣΔVy of ΔVy is found to be below the left and right: wheel speed difference determining value D2 in step 308, it is determined that no left and right wheel speed difference is produced. The processing moves to step 320 and whether the integrated value ΣΔVx of ΔVx is over the front and rear wheel straight advance determining value D3 or not is determined. If this is below D3, that is, both left and right wheel speed difference and front and rear wheel speed difference are small, processing moves to step 322 and whether the vehicle is in a straight advance state or not is determined. On the other hand, if the integrated value of ΔVx is found to be above the determining value D3 in step 320, processing moves to step 316 and the above-described processing is executed. A series of operations of the vehicle turning determining means are shown, and as shown in FIG. 17, for each lateral G calculation timing Tchk3, the integrated value ΣΔVx of ΔVx and the integrated value ΣΔVy of ΔVy are calculated. By comparing the integrated values with the determining values D3 and D2, it is determined whether the state of the vehicle is in straight advance, turning or unstable. More specifically, by calculating integrated values ΣΔVx and ΣΔVy within the specified period Tchk 3 with respect to front and rear wheel speed differences ΔVx and left and right wheel speed differences ΔVy, momentary effects of noise components are determined, accurately determining a straight advance, turning or unstable state of the vehicle. When a previous state of the vehicle is turning, an unstable state of the vehicle is determined only when a front and rear wheel speed difference ΔVx and a left and right wheel speed difference ΔVy respectively exceed the determining values. Thus, determining the vehicle being in the unstable state by mistake can be prevented. Processing contents of the vehicle lateral G estimating means (step 400) are shown in FIG. 11. In FIG. 11, if it is determined that lateral G calculation timing Tchk 3 has passed in synchronization with vehicle turning determining processing in step 402, processing moves to step 404. In step 404, whether the vehicle is determined to be in the turning state or not is determined, and if it is determined to be in the turning state, processing moves to step 406 and left and right wheel speed sum ΔVz is calculated. Then, in step 408, vehicle lateral direction acceleration (lateral G) Gy is calculated. This Gy calculation can be calculated by, for instance, the following equation 8: (Equation 8) Gy = (ΣΔVyave•ΔVz)/2T Here, ΣΔVyave is an average value per unit time of integrated values ΣΔVy of ΔVy calculated in step 312 and T is a tread width of the vehicle. After processing is executed in step 408, processing returns to the main routine. On the other hand, if the vehicle is found to be not in the turning state but in the unstable state in step 404, moving to step 410, the largest lateral acceleration Gymax is set as the vehicle lateral GGy and processing returns to the main routine. It is preferable that as the largest possible value obtainable as a vehicle state, the largest lateral acceleration Gymax is set at one capable of applying a sufficient braking force even when the vehicle is in the drifting or the spinning state. A series of operations of the lateral G estimating means are shown and as shown in FIG. 17, according to determination of a state of the vehicle (straight advance, turning or unstable), the value of Gy is changed or calculated. As described above, in the lateral G routine, lateral G of the vehicle is calculated only when the vehicle is determined to be in the turning state. Thus, it is possible to surely prevent erroneous estimation of lateral G when the vehicle is in the straight advance state is prevented. When the vehicle is in the unstable state, since lateral G is set at the largest lateral acceleration Gymax, it is possible to control the vehicle to the stable side. After deceleration Gx in forward and backward directions of the vehicle and acceleration Gy in the lateral direction are obtained as shown in FIG. 17, synthesized G of the vehicle, that is, a road surface µ estimated value, is calculated as shown in the Figure. Specifically, a size of the synthesized G is obtained by means of vector calculation of deceleration Gx in forward and backward directions of the vehicle and acceleration Gy in the lateral direction. Processing contents of the bad road surface state determining means (step 500) are shown in FIG. 12. In FIG. 12, in step 502, wheel acceleration Gw and bad road determining reference acceleration G1 are compared, and if Gw is below G1, processing moves to step 510 and a count determining flag Fgup is set to "0". This determining flag Fgup is set to "1" when wheel acceleration Gw exceeds reference acceleration G1, this "1" is held as long as this state continues, and when Gw is below G1, this flag is reset to "0". This determining flag Fgup is utilized by a bad road counter Cgup for counting the number of times that Gw exceeds G1. On the other hand, if wheel acceleration Gw is found to be above the bad road determining reference acceleration G1 in step 502, in step 504, whether the count determining flag Fgup is "1" or not is determined. If the determining flag becomes "1", counting of the bad road counter Cgup is not executed, and only when this is "0" counting thereof is executed and then in step 508 the count determining flag Fgup is set to "1". In step 512, whether bad road determining timing Tchk 5 passes or not is determined. If not yet passed, processing returns to the main routine. If Tchk5 passes, in step 514, whether a value Cgup obtained by counting the number of times that wheel acceleration Gw exceeds bad road determining reference acceleration G1 within Tchk5 is above the bad road determining count C1 or not is determined. If this is more than the bad road determining count C1, processing moves to step 516, the bad road determining flag Fbad is set to "1" and processing returns to the main routine. On the other hand, if Cgup is found to be below C1 in step 514, in step 518 the bad road flag Fbas is reset to "0" and processing returns to the main routine. A series of operations of the bad road determining means are shown in FIG. 18. In FIG. 18, for every determining timing Tchk5, resetting of the count determining flag Fgup is executed depending on whether wheel acceleration Gw exceeds bad road determining reference acceleration G1. By means of the count determining flag Fgup, counting up of the bad road counter Cgup is executed, and when a count reaches over the bad road determining count C1, determining the bad road state the bad road determining flag Fbad is set. Here, if the bad road state is determined, the estimated road surface µ is changed to the bad road µ value µ bad. This bad road µ value µbad is preferably a settable largest road surface µ value. However, a method of changing the value depending on a magnitude of wheel acceleration Gw or a counted value of the bad road counter Cgup may be used. Processing contents of ice road travelling determination (step 600) are shown in FIG. 13. In the ice road travelling determination, by resetting an ice road determining flag Fice to "0" at every start of processing, the estimated vehicle body speed Vb can be changed to a ice road estimated vehicle body speed Vice each time ice road travelling is determined before ABS control and stability of the vehicle on the ice road can be secured. In FIG. 13, in step 602, the ice road determining flag Fice is reset to "0" and in step 604, the ice road estimated vehicle body speed Vice is calculated. The ice road estimated vehicle body speed Vice can be an estimated vehicle body speed occurring on a road surface µ corresponding to an ice road. Thus, by changing the estimated vehicle body speed Vb to the ice road estimated vehicle body speed Vice when ice road travelling is determined, the estimated vehicle body speed Vb is brought close to the real vehicle body speed and stability of the vehicle can be sufficiently secured. In step 606, by counting down of the ice road determining counter Cice, an ice road travelling determining timing is determined. In step 608, depending on the counted value of the ice road determining counter Cice, ice road travelling determination is executed. That is, since the ice road determining counter is counted down in the step 606, Cice < 0 and processing moves to step 610. In step 610, a characteristic of ice road travelling is monitored. Specifically, if a state where estimated vehicle body deceleration Gb gradually increases with ups and downs is detected, processing moves to step 612 and an instruction output for reducing pressure during time T1 is set. This is for the reason that though a characteristic of the ice road is determined in step 610, by monitoring a restored state of a wheel after pressure reduction ice road travelling can be further accurately determined. Therefore, determination of ice road travelling may only be performed in step 610. In step 614, by setting an ice road determining count C6 on the ice road determining counter Cice, a timing for determining a restored state of a wheel after pressure reduction of time T1 can be determined. The ice road determining count C6 must be a value allowing sufficient wheel restoring after pressure reduction of time T1. After processing in step 614, processing returns to the main routine. On the other hand, if a characteristic of the ice road is not determined in step 610, processing directly returns to the main routine. After the ice road determining count C is set on the ice road determining counter Cice in step 614, processing moves again from the main routine to step 606, to count down the ice road determining counter Cice, and if the ice road determining counter Cice > 0 in step 608, this is not the timing for determining a restored state of a wheel after pressure reduction and thus processing returns to the main routine. The above processing is repeated, and if it is determined, in step 608, that the ice road determining counter Cice = 0, this is the timing for determining a restored state of a wheel after pressure reduction and then processing moves to step 616. In step 616, the restored state of the wheel after pressure reduction is determined depending on the magnitude of wheel acceleration Gw, and if the largest value of wheel acceleration Gw from after pressure reduction to the timing for determining the restored state of the wheel is below ice road determining acceleration G6, processing moves to step 618. That is, if a restoring rate of the wheel is small even when braking pressure is reduced, extremely low µ road such as the ice road and the like can be determined, and in this case, the estimated vehicle body speed Vb is changed to the ice road estimated vehicle body speed Vice produced in step 604, and in step 620 the ice road determining flag Fice is set to "1" and processing returns to the main routine. On the other hand, if the largest value of wheel acceleration Gw is found to be above the ice road determining acceleration G in step 616, the road surface in which the restoring rate of the wheel after pressure reduction is large, that is, not an extremely low µ road surface such as the ice road and the like, is determined and processing returns to the main routine. A series of operations of the ice road travelling . determining means are shown in FIG. 19. When a characteristic of ice road travelling is determined by estimated vehicle body deceleration Gb, hydraulic pressure is reduced for time T1, the ice road determining count C6 is set on the ice road determining counter Cice and restoring state of the wheel after pressure reduction is monitored. If wheel acceleration Gw reaches above the ice road determining acceleration G6 during the period of time from counting down of the ice road determining counter Cice to reaching 0, with a timing of Cice = 0 the ice road determining flag Fice is set to "1", that is, the ice road travelling state is determined. By changing an estimated vehicle body speed Vb to the ice road estimated vehicle body speed Vice determined beforehand, stability of the vehicle is secured. Furthermore, since the estimated road surface µ has been determined to be the ice road travelling, by changing the estimated road surface µ to the ice road surface µ value µice, determination of brake pressure control start thereafter is executed. The ice road surface µ value µice may be changed depending on the magnitude of restoration G of wheel acceleration Gw after pressure reduction of time T1. Processing contents of the road surface change determining means (step 700) during brake pressure control are shown in FIG. 14. In FIG. 14, in step 702, a high µ road change determining flag Fhij is set to "0". This Fhij is set to "1" when a high µ road changing is determined, and after estimated road surface µ is changed, this is reset to "0" in step 702. Thus, during brake pressure control determination of high µ road changing can be executed by a number of times. Similarly, in a step 704, a low µ road change determining flag Floj is set to "0". In step 706, a high µ determining counter Cjmp is counted down. Then, in step 708, depending on the counted value of the high µ determining counter Cjmp, whether to execute high µ road change determining processing from step 710 or a low µ road change determining processing from step 730 and thereafter is determined. First, usually, since the high µ determining counter Cjmp has been counted down in step 706, Cjmp < 0 and thus processing moves to step 710. In step 710, whether currently executed control is in increased pressure control or not is determined. In the case of the increased pressure control, processing moves to step 712 and whether the specified increased pressure output is completed is determined. Here, as a control state in which specified increased pressure is generated, it is considered that an optimum hydraulic level may not be reached even. though the increased pressure is continuously generated when hydraulic level is higher than a current hydraulic level. Specifically, even when a hydraulic level has almost reached a level of starting brake pressure control by means of increased pressure output, a slipping level of a wheel is not increased enough to allow pressure reduction. That is, if changing to the high µ road is made during brake pressure control on the low µ road in which the hydraulic level is low, hydraulic control is necessary at the high hydraulic level. However, since the increased pressure output is set on the basis of the low hydraulic level, pressure must be reduced even after the increased pressure outputs are all completed and thus a wheel slipping state does not increase. Therefore, if output of specified increased pressure is determined in step 712, since changing to the high µ road is conceivable, processing moves to step 714 and increased pressure for confirming high µ road changing is output for time T2. By setting the confirming increased pressure time T2 larger than a rising gradient of increased pressure at the time of specified pressure output, the high µ road changing can be surely determined. In step 716, a high µ determining count C7 is seton the high µ determining counter Cjmp and thereby a speed falling state of the wheel after increasing pressure is output can be monitored. Then, in steps 744 and 746, a pressure reduction timer Trel for determining low µ road changing and the integrated value ΣΔVw of differences ΔVw between the estimated vehicle body speed Vb and the wheel speed Vw calculated in step 136 are cleared to zero and processing returns to the main routine. On the other hand, if completed specified increased pressure output is not determined in step 712, directly moving to step 744, similar processing is executed. Next, explanation will be made of a case where increased pressure for confirming high µ road changing is output in step 714, that is, determination of high µ road changing is executed. After a high µ determining count C7 is set on the high µ determining counter Cjmp in step 716, processing moves from the main routine again to step 706, Cjmp is counted down, and if Cjmp ≥ 0 is determined in step 708, processing moves to step 718 in order to monitor a wheel speed falling state caused by output of increased pressure for confirming high µ road changing. In step 718, a wheel falling state after increasing pressure is detected on the basis of the magnitude of the wheel acceleration Gw. That is, if wheel acceleration Gw from after pressure increase to a timing for determining the wheel falling state is below change determining acceleration G7, processing moves to step 720, determining prohibit flag Fnoc is set to "1" and processing moves to step 722. By executing the determination, it can be determined that the current road surface is not a high µ road. That is, in the step 714, by outputting increased pressure for confirming high µ road changing having a gradient larger than arising gradient of increased pressure by specified pressure increase, since the wheel speed always falls if the road surface is medium / low µ road, erroneous determination of high µ road changing can be prevented and accurate determination can be executed. On the other hand, if wheel acceleration Gw is found to be above the change determining acceleration G7, processing directly moves to step 722. In step 722, a timing for determining high µ road changing is determined to be at the time of the high µ determining counter Cjmp = 0, and, then moving to step 724, processing is executed. At the time of Cjmp ≠ 0, returning to the main routine a wheel falling state is monitored until reaching the determining timing. In step 724, if the determination prohibit flag Fnoc is "0", that is, wheel falling due to output of increased pressure for confirming high µ road changing, is not confirmed, processing moves to step 726. In step 726, high µ road changing is produced, the high µ road change determining flag Fhij is set to "1", and in step 728, the determination prohibit flag Fnoc is reset to "0" and processing returns to the main routine. On the other hand, if in step 724, the determination prohibit flag Fnoc is found to be set to "1", that is, since a road surface is medium / low µ road is confirmed, when a wheel falling state is determined by output of increased pressure for confirming high µ road changing, a change to a high µ road is not determined and similar processing is executed after moving to step 728. A series of operations of the high µ road change determining means are shown in FIG. 20. As shown in FIG. 20, when output of specified increased pressure is completed, increased pressure for confirming a change having a gradient larger than a rising gradient of increased pressure output at the time of specified pressure increase is output for time T2, the high µ determining count C7 is set on the high µ determining counter Cjmp and the wheel falling state after output of increased pressure is monitored. If wheel acceleration Gw is declined below the change determining acceleration G7 during the period of time when the high µ determining counter Cjmp is counted down and reaches 0, the determination prohibit flag Fnoc remains at "0". Thus, with a timing of Cjmp = 0, the high µ road change determining flag Fhij is set to "1", that is, changing to a high µ road is determined. When changing to the high µ road is determined, the estimated road surface µ is changed to the high µ value µHi and ABS control is executed. Here, a method of changing the high µ value µHi depending on the magnitude of wheel acceleration Gw indicating the wheel falling state after output of increased pressure may be used. Next, a processing in which determination of changing to the low µ road is executed is explained. If current control is other than increase pressure control in step 710, processing moves to step 730 and the high µ determining counter Cjmp is cleared and processing moves to step 732. In step 732, whether or not current control is in reduced pressure control is determined. If the current control is not the reduced pressure control, processing moves directly to step 744 to execute the above-described processing. If reduced pressure control is determined at the step 732, moving to step 734 pressure reduction timer Trel for determining a change to the low µ road is counted up. Then, in step 736, the integrated value ΣΔVw of speed differences ΔVw between the estimated vehicle body speed Vb and the wheel speed Vw calculated in step 136 is calculated, and processing moves to step 738. In step 738, whether or not the integrated value ΣΔVw of ΔVw is above an determined slipping amount S7 is determined. That is, depending on the magnitude of the ΣΔVw, a wheel falling state (slipping) is determined. When ΣΔVw is below S7, change to a low µ road is not determined since wheel falling (slipping) is small and processing directly returns to the main routine. On the other hand, when a slipping amount is above S7 and wheel falling (slipping ) is large, processing moves to step 740, and whether or not a time of pressure reduction timer Trel for determining a change to the low µ road is above an determining time T7 is determined. When the pressure reduction timer Trel is below T7, since restoration of the wheel after pressure reduction is started even when the wheel falling is produced, change to the low µ road is not determined and processing directly returns to the main routine. On the other hand, if it is determined that pressure reduction time is over the determining time T7 and it has been taking time for the wheel to be restored after pressure reduction is started, understanding that this is a change to a low µ road processing moves to step 742 and a low µ road change determining flag Floj is set to "1". Then, in step 744, the above-described processing is executed. That is, when a change to the low µ road is made, by determining that the wheel speed falls greater than the estimated vehicle body speed Vb and that it takes a long time for restoring the wheel speed even when pressure reduction is continued, a change to the low µ road is determined. On the high µ road, since restoration of wheel is fast by means of pressure reduction even when the wheel speed falls, pressure reduction time is made short. Since falling itself is small, a total slipping amount of the wheel is made small and conditions for determining the change to the low µ road are not met. A series of operations of the low µ road change determining means are shown in FIG. 21. As shown in FIG. 21, when the wheel speed falls and pressure reduction is started, the pressure reduction timer Trel is counted up and at the same time, the integrated value ΣΔVw of speed differences ΔVw between the estimated vehicle body speed Vb and the wheel speed Vw is calculated. By means of the integrated value ΣΔVw, the wheel falling state is expressed as a physical amount. When the pressure reduction timer Trel is over the determining time T7 and the integrated value ΣΔVw of ΔVw is above the determined slipping amount S7, the low µ road change determining flag Floj is set to "1". That is, a change to the low µ road is determined. The estimated road surface µ is changed to the low µ value µLo when the change to the low µ road is determined and ABS control is executed. Here, a method of changing the low µ value µLo on the basis of the integrated value ΣΔVw of ΔVw and by means of the pressure reduction timer Trel may be used. Processing contents of the road surface µ correction determining means (step 800) during brake pressure control are shown in FIG. 15. In FIG. 15, in step 802, the road surface µ increase determining flag Fpls is set to "0". The flag Fpls is set to "1" when correction of the road surface µ to the side making the road surface µ high, and after correction of the estimated road surface µ is executed, the flag Fpls is reset to "0" in step 802. Thus, during brake pressure control, correction of the road surface µ to the side of the high road surface µ is determined by any number of times. Likewise, in step 804, a road surface µ reduction determining flag Fmus is set to "0". Then, in step 806, whether currently executed control is pressure increase control or not is determined. If the pressure increase control is carrying out, processing moves to step 808 and whether or not the estimated vehicle body deceleration Gb calculated in step 106 is almost equal to deceleration limit Vdown referred to at the time of estimated vehicle body speed Vb calculation is determined. If the estimated vehicle body deceleration Gb is almost equal to the deceleration limit Vdown, in step 812, the increase determining counter Cpls is counted up and processing moves to step 812. That is, the determining means for correcting the road surface µ to the high side is based on the characteristic that the wheel speed does not fall enough to produce pressure reduction though pressure increase control continues and the estimated vehicle body speed Vb continues to be created with the deceleration limit Vdown. On the other hand, if currently executed control is not found to be pressure increase control in step 806, processing moves to step 822 and, if currently executed control is pressure reduction control, the reduction determining counter Cmus in step 832 and the increase determining counter Cpls in step 834 are simultaneously cleared and processing returns to the main routine. If the estimated vehicle body deceleration Gb is different from Vdown in step 808 or if currently executed control is not pressure reduction control in step 822, processing moves to step 824 and road surface µ lowering determination is executed (the determining processing will be described later). In step 812, whether the increase determining counter Cpls is over an increase determination counted value C8 or not is determined. If the Cpls is below C8, since a timing for correcting the road surface µ to the high side is not determined, the reduction determining counter Cmus is cleared to zero in step 820 and processing returns to the main routine. On the other hand, if the increase determining counter Cpls is found to be above the increase determination counted value C8 in step 812, a timing for correcting the road surface µ to a high side is not determined, and the estimated vehicle body speed Vb is changed to a largest wheel speed Vwmax in step 814. In step 816, the road surface µ increase determining flag Fpls is set to "1". Then, in steps 818 and 820, the increase determining counter Cpls and the reduction determining counter Cmus are cleared to zero and processing returns to the main routine. A series of operations of the determining means for correcting the road surface µ to the high side are shown in FIG. 22. As shown in FIG. 22, when current hydraulic pressure rises because of pressure increase control, if the state where the estimated vehicle body speed Vb continues to be produced by the deceleration limit Vdown is counted to the increase determining counted value C8 by the increase determining counter Cpls, the road surface µ increase determining flag Fpls is set to "1", that is, to correcting the road surface to a high side is needed. As for the estimated road surface µ, if necessity of correcting the road surface µ to the high side is determined, an increase correction value µ1 is added to a current road surface µ value. At the same time, by changing the estimated vehicle body speed Vb to the largest wheel speed Vwmax, a deviation from a real vehicle body speed can be corrected. This is for the reason that when the estimated road surface µ is smaller than the real road surface µ, the deceleration limit Vd obtained by the function F1(µ) of the estimated road surface µ is made small and as a result, the estimated vehicle body speed Vb created by the deceleration limit Vdown is created far different from the real vehicle body speed. Next, the determining means for correcting the road surface µ to the low side is explained. In step 824, when the estimated vehicle body speed Vb is not produced with the deceleration limit Vdown or when currently executed control is not found to be pressure reduction control in step 822, whether the vehicle body deceleration Gb is almost equal to the acceleration limit Vup referred to at the time of calculating the estimated vehicle body speed Vb. If Gb ≠ V, that is, the wheel speed is larger than Vup, it is determined that the estimated vehicle body speed is lower than the real vehicle body speed. Moving to the processing of step 826, the reduction determining counter Cmus is counted up and processing moves to step 828. In step 828, whether the reduction determining counter Cmus is above a reduction determination counted value C9 or not is determined. If the Cmus is approximately equal to C9, a timing for correcting the road surface µ to the low side is not determined and processing moves to step 834, the increase determining counter Cpls is cleared to zero and processing returns to the main routine. On the other hand, if the reduction determining counter Cmus is found to be above the reduction determination counted value C9, a timing for correcting the road surface µ to a side making the road surface µlow is determined, and a road surface µ lowering flag Fmus is set to "1" in step 830. Then, in steps 832 and 834, the reduction determining counter Cmus and the increase determining counter Cpls are cleared to zero and processing returns to the main routine. If GB ≠ Vup is not determined in step 824, since the estimated vehicle body speed is approximately equal to the real vehicle body speed, processing moves to step 832 and similar processing is executed so that processing returns to the main routine. A series of operations of the determining means for correcting the road surface µ to the low side are shown in FIG. 23. As shown in FIG. 23, the state where the estimated vehicle body speed Vb continues to be created with the acceleration limit Vup continues to be counted to the reduction determination counted value C9 by the reduction determining counter Cmus when current hydraulic pressure is a pressure holding or increase control after the pressure reduction control, the road surface µ reduction determining flag Fmus is set to "1", that is, necessity of correcting the road surface µ to the low side is determined. As for the estimated road surface µ, when correction of the road surface µ to the low side is determined, by subtracting the reduction correction value µ2 from the current road surface µ value correction of the road surface µ according to the road surface change is possible. From the foregoing descriptions, in the ABS controller of the first embodiment, the road surface µ from an output from the wheel speed sensor and the output from brake pressure control can be estimated. In the first embodiment, the estimated value of the road surface µ by various combinations of blocks, such as the forward and backward G estimating means + the road surface change determining means and the like can be determined. As described above, in an antiskid controller of the present invention, road surfaces µ corresponding to continuous changes of road surfaces for travelling can be accurately estimated without any additional sensors such as a G sensor or the like, and using the estimated road surface µ, accurate antiskid control can be executed.

Claims: An anti-skid controller, comprising: wheel speed detecting means (104) for detecting a wheel speed of each wheel (FL, FR, RL, RR) of a vehicle; and brake pressure control means for controlling brake pressure of a controlled wheel in order to prevent an excess slipping state of the controlled wheel based on a comparison between a vehicle body speed and a wheel speed of the controlled wheel, characterized by : forward and backward direction acceleration calculating means (200) for calculating acceleration in forward and backward directions corresponding to a road surface friction coefficient based on changes of relative wheel speeds between front and rear wheels (FL, FR, RL, RR) detected by the wheel speed detecting means (104); lateral direction acceleration calculating means (400) for calculating acceleration in a lateral direction of the vehicle based on wheel speeds of left and right wheels (FL, FR, RL, RR) detected by the wheel speed detecting means (104); deceleration limit gradient calculating means (130) for calculating a deceleration limit gradient of the vehicle based on the forward and backward direction acceleration and the lateral acceleration; and vehicle body speed calculating means (106) for calculating the vehicle body speed of the vehicle by using at least the deceleration limit gradient. An anti-skid controller according to claim 1, characterized in that the calculation of the forward and backward direction acceleration calculating means (200) is based on changes of relative wheel speeds of front and rear wheels (FL, FR, RL,RR) detected by the wheel speed detecting means (104) in a state before starting of brake pressure control after braking torque is applied to the front and rear wheels ((FL, FR, RL, RR)) of the vehicle, and the calculation of the lateral direction acceleration calculating means (400) is based on wheel speeds of left and right wheels (FL, FR, RL, RR) detected by the wheel speed detecting means (104) in a state before starting of brake pressure control after braking torque is applied to the front and rear wheels (FL, FR, RL, RR) of the vehicle. An anti-skid controller according to claim 1 or 2, characterized in that the forward and backward direction acceleration calculating means (200) calculates, when braking torque is applied to the front and rear wheels (FL, FR, RL, RR) of the vehicle, acceleration in forward and backward directions corresponding to a road surface friction coefficient based on a changing rate of wheel speeds on a high speed side in a case a change exceeding a specified value is produced in a difference in wheel speeds between the front and rear wheels (FL, FR, RL, RR). An anti-skid controller according to claim 3, characterized in that the forward and backward direction acceleration calculating means (200) calculates, in a case no changes exceeding a specified value is produced in a difference in wheel speeds between the front and rear wheels (FL, FR, RL, RR) and brake pressure control is started for the front or the rear wheels (FL, FR, RL, RR), and acceleration in forward and backward directions corresponds to a road surface friction coefficient based on a change rate of wheel speeds of the front or the rear wheels (FL, FR, RL, RR) when the brake pressure control is started. An anti-skid controller according to claim 1 or 2, characterized by : determining means (500) for determining whether a road surface for travelling is bad or not based on change conditions in wheel speed; and correcting means (800) for correcting, when the determining means (500) determines that the road surface for travelling is bad, the deceleration limit gradient calculated by the deceleration limit gradient calculating means (130) to handle the bad road. An anti-skid controller according to claim 5, characterized in that the correcting means (800) corrects the deceleration limit gradient calculated by the deceleration limit gradient calculating means (130) to a bad road deceleration limit gradient value specified beforehand in order to handle a bad road. An anti-skid controller according to claim 1 or 2, characterized by : determining means (142) for determining whether a road surface for travelling is an extremely low friction road surface or not based on a changing condition of a wheel speed when at least braking torque changes; and correcting means (800) for correcting, when the determining means (142) determines that the road traveled by the vehicle is an extremely low friction road surface, the deceleration limit gradient calculated by the deceleration limit gradient calculating means (130) in order to handle the extremely low friction road surface. An anti-skid controller according to claim 7, characterized in that the correcting means (800) corrects a deceleration limit gradient calculated by the deceleration limit gradient calculating means (130) to a bad road deceleration limit gradient specified beforehand in order to handle the extremely low friction road surface. An anti-skid controller according to claim 2, characterized by : determining means for determining, based on a change of a wheel speed with respect to an increase or decrease pressure of brake pressure after starting of anti-skid control, whether or not the deceleration limit gradient calculated by the deceleration limit gradient calculating means (130) is suitable for the road surface for travelling; and correcting means (800) during control for correcting the deceleration limit gradient when the determining means determines that the deceleration limit gradient is not suitable. An anti-skid controller according to claim 9, characterized in that the determining means determines, in a case where a falling level of a wheel speed is below a reference value even after brake pressure is increased over a specified amount, that the deceleration limit gradient is not suitable. An anti-skid controller according to claim 9, characterized in that the determining means determines, in a case where a falling level of a wheel speed is over a reference value even after brake pressure is decreased, that the deceleration limit gradient is not suitable. An anti-skid controller according to claim 9, characterized in that the determining means determines, in a case where brake pressures applied to a plurality of wheels are simultaneously increased in anti-skid control based on a wheel speed of each wheel (FL, FR, RL, RR), that the deceleration limit gradient is not suitable. An anti-skid controller according to claim 9, characterized in that the determining means determines, in a case where a wheel speed reaches the vehicle body speed or higher, that the deceleration limit gradient is not suitable. An anti-skid controller according to claim 1 or 2, characterized in that the forward and backward direction acceleration calculating means (200) calculates, in a case where an integrated value obtained by periodically and sequentially integrating wheel speed differences between the front and rear wheels (FL, FR, RL, RR) reaches a specified value or higher after braking torque is applied to the front and rear wheels (FL, FR, RL, RR) of the vehicle, acceleration in forward and backward directions corresponding to a road surface friction coefficient based on a change rate of a wheel speed on a high speed side.

Inventors: MASAKI SHOICHI, MATSUURA MASAHIRO, OCHI TSUYOSHI, TERAO HIDENORI, MASAKI, SHOICHI, MATSUURA, MASAHIRO, OCHI, TSUYOSHI, TERAO, HIDENORI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1995
Main classification: B60T8
IPC codes: B60T8