1 Technical Field of the Invention
The present invention relates generally to an antiskid brake system for automotive vehicles designed to control the brake force applied to each wheel of the vehicle during deceleration thereof.
2 Background Art
Antiskid brake systems (ABSs) are well known as a brake control system which controls the brake pressure during deceleration of an automotive vehicle. The antiskid brake systems work to control brake forces acting on wheels based on the fact that an increase in slip ratio S will cause a rate of change in coefficient of adhesion xcexc between tire and road (it is in effect the coefficient of friction), as shown in FIG. 1(a), to change greatly following a target slip ratio Sm (e.g., 10%). In practice, the antiskid brake systems determine the timing where a brake pressure (i.e., a brake fluid pressure) applied to each wheel is increased or decreased, as shown in FIG. 1(b), based on the wheel speed Vw and the wheel acceleration Vwxe2x80x2 and regulate the brake pressure acting on each wheel.
The wheel acceleration Vw**xe2x80x2 of each wheel is usually given by the equation (1) below.
I/rVw**xe2x80x2=Fmaxxe2x88x92Kpad xc2x7Pb**xe2x80x83xe2x80x83(1)
where the suffix xe2x80x9c**xe2x80x9d expresses FR, FL, RR, and RL standing for a front right, a front left, a rear right, and a rear left wheel, respectively, I is the moment of inertia of each wheel, r is the radius of tire, Fmax is a maximum road-tire frictional force, Kpad is a constant, and Pb** is a wheel cylinder pressure.
The product Kpadxc2x7Pb** represents the brake force. As apparent from the equation (1), in a range 1, as shown in FIG. 1(a), where the slip ratio is smaller, an increase in brake force will cause the coefficient of friction to increase, so that the wheel acceleration Vw**xe2x80x2 hardly changes, but when the brake force exceeds the maximum frictional force Fmax (i.e.; in a range 2), it reflects directly on the wheel acceleration Vw**xe2x80x2.
The antiskid brake systems usually determine the speed of a vehicle body VB (referred to as a vehicle speed below) based on the greatest of four wheel speeds and regulate the brake pressure so as to bring a slip ratio, as determined by a difference between the vehicle speed VB and the wheel speed Vw**, into agreement with a target slip ratio (e.g., 10%) in which the brake force reaches a peak thereof. For instance, when the difference between the vehicle speed VB and the wheel speed Vw** is great, the antiskid brake systems decrease the brake pressure.
Usually, a maximum deceleration of the vehicle when being brought to rest depends upon the condition of a road surface. In a case of a high friction road such as an asphalt road, a maximum of 1.0 G is reached. The determination of whether the coefficient of friction exceeds the peak or not must, therefore, wait until a higher deceleration as much as 1.0 G is reached. This, however, results in an undesirable drop in wheel speed, leading to a great wheel slippage.
It is difficult to derive the vehicle speed from speeds of four wheels accurately. The slip ratio in which the coefficient of friction reaches the peak depends upon the type of road surface. It is, therefore, impossible to precisely determine the speed producing the slip ratio in which the brake force reaches a maximum value (i.e., a target speed). Specifically, a shift in the target speed may result in a delay of the brake control, so that the wheel speeds drop undesirably, thereby leading to a great wheel slippage.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an antiskid brake system capable of determining a wheel skid precisely without using parameters such as vehicle speed and wheel speeds as employed in conventional antiskid brake systems.
It is a further object of the invention to provide a brake condition determining device capable of determining a braking condition of a vehicle accurately.
According to one aspect of the invention, there is provided an antiskid brake system for a vehicle. The system includes: (a) a wheel speed determining circuit determining a speed of a wheel of the vehicle; (b) a parameter determining circuit determining a parameter reflecting on brake torque applied to the wheel of the vehicle; (c) a braking condition determining circuit comparing a phase of a change in speed of the wheel determined by the wheel speed determining circuit with a phase of a change in parameter determined by the parameter determining circuit to determine a phase delay of the change in speed of the wheel and determining a braking condition based on the phase delay; and (d) an antiskid brake controlling circuit performing antiskid brake control based on the braking condition determined by the braking condition determining circuit.
In the preferred mode of the invention, the braking condition determining circuit increases the parameter in the form of a pulse and determines the braking condition the change in speed of the wheel as a function of a step change in the parameter.
The braking condition determining circuit determines the braking condition based on a fact that the phase of the change in speed of the wheel is delayed from the phase of the change in parameter determined by the parameter determining circuit as a slip ratio increases.
The parameter determined by the parameter determining circuit is a pressure of hydraulic fluid supplied cyclically to a wheel cylinder of the vehicle in the form of a pulse. A cycle in which the pressure of hydraulic fluid is supplied to the wheel cylinder is lower than a cycle of resonant vibrations occurring at a suspension system of the vehicle depending upon a type of the vehicle.
The wheel speed determining circuit provides a signal indicative of the speed of the wheel to the braking condition determining circuit. A frequency higher than a frequency of resonant vibrations occurring in a suspension system of the vehicle is removed from the signal indicative of the speed of the wheel.
The change in speed of the wheel is one of a first parameter representing a difference between instant values of the speed of the wheel sampled at a given time interval cyclically and a second parameter representing a difference between instant values of the first parameter sampled at the given time interval cyclically.
The change in speed of the wheel may alternatively be one of a first parameter that is a first difference between instant values of the speed of the wheel sampled at a given time interval cyclically minus a mean value of the first differences sampled for a given period of time and a second parameter that is a second difference between instant values of the first parameter sampled at the given time interval cyclically minus the second differences sampled for a given period of time.
The change in parameter determined by the parameter determining circuit is a change in pressure of hydraulic fluid supplied to a wheel cylinder of the vehicle in a given period of time.
The change in parameter determined by the parameter determining circuit may alternatively be the change in pressure of hydraulic fluid supplied to a wheel cylinder of the vehicle in a given period of time minus a means values of the changes in pressure of the hydraulic fluid for a given period of time.
According to another aspect of the invention, there is provided a braking condition determining device for a vehicle which comprises: (a) a wheel speed determining circuit determining a speed of a wheel of the vehicle; (b) a parameter determining circuit determining a parameter reflecting on brake torque applied to the wheel of the vehicle; and (c) a braking condition determining circuit comparing a phase of a change in speed of the wheel determined by the wheel speed determining circuit with a phase of a change in parameter determined by the parameter determining circuit to determine a phase delay of the change in speed of the wheel and determining a braking condition of the wheel based on the phase delay.