The present invention relates generally to vehicular braking systems and more particularly to vehicle braking systems having anti-skid or antilock features.
Many known anti-skid devices simulate a driver induced anti-skid technique by cyclically increasing and decreasing the braking force exerted on the wheels so that a slipping wheel having a tendency to lock is permitted to re-accelerate back to a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into the brake cylinder, causing a lowering and raising the pressure of the fluid supplied to operate the brake cylinder associated with the wheels of a vehicle. In such a conventional antilock braking system, the controlled wheel occasionally begins slipping too much and operates for short times with a comparatively large amount of slip, this means a level of slip high enough to effect or reduce lateral forces available for steering and vehicle stability. In addition, excessive slip is frequently associated with reduced braking effectiveness, increased tire wear and shock loading on suspension components, difficulty in steering control, and is generally disturbing to the vehicle occupants.
While such cycling causes momentary reduction in braking effectiveness as well as reduced stability and steerability, and other undesirable effects, it is useful in allowing re-setting of the calculated vehicle velocity. Many antilock braking systems are invoked when a calculated wheel speed differs sufficiently from a sensed wheel speed. In the typical system, wheel speeds are used to determine slip by comparison to a computed vehicle velocity either directly or indirectly. Without a constant re-checking of the computed vehicle velocity, errors will accumulate and cause serious degradation leading to substantially reduced braking and/or reduced lateral force.
It is desirable to provide a system that is immune to the problem of accumulated errors and which acts as a continuous process with the corrective action taken being proportional to the deviation from a desired performance. Such a process is relatively easy to control using conventional feedback methods such as Proportional-Integral-Differential (PID) controllers. With such a system, a differential correction is readily included allowing the system to anticipate future conditions by reacting to the rate of change of the error condition. Also, an integral term can reduce the steady state error.
Some antilock braking systems also operate on a so-called pump-back principle where the same hydraulic fluid is returned to the brake pad actuators subsequent to an anti-skid event while others operate on a replenish principle where the reapply or build fluid comes from a separate source, frequently a hydraulic accumulator. The latter requires only a moderate sized pump while the pump-back type systems require a more expensive pump capable of supplying the maximum instantaneous flow rate.
It is desirable to provide a simple, quiet replenish type system utilizing a low cost accelerometer which system maintains maximum braking force near the peak of the mu-slip curve at all times.