Fluid-pressure and analogous brake systems – Multiple systems – Fluid pressure and electric
Reexamination Certificate
1999-03-19
2001-04-10
Butler, Douglas C. (Department: 3613)
Fluid-pressure and analogous brake systems
Multiple systems
Fluid pressure and electric
C310S328000, C303S020000, C303S155000, C303S113100, C303S115100, C188S370000, C188S072100, C188S158000
Reexamination Certificate
active
06213564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to anti-lock brake systems. More particularly, the present invention relates to brake systems (principally anti-locking automotive brake systems) in which a braking force is applied by an electrically actuated piezoelectric element.
2. Description of the Prior Art
Prior automobile braking systems typically comprise a toot pedal to which the driver applies a force, means for multiplying the foot pedal force, and means for transmitting this force to one or more stationary brake pads which press (either directly or indirectly) against the rotating wheel(s). When the brake friction pads press against the turning wheels (or, more specifically, against the brake drums or brake discs associated with the wheels), the rotation of the wheels slows down due to the friction between the brake friction pads and the wheels. The amount of braking friction at each wheel is typically proportional to the force each brake friction pad applies to the respective wheels.
In the prior art, transmission of the (multiplied) foot pedal forces to the brake pads is typically accomplished by mechanical means. Such prior systems include hydraulic, pneumatic, lever-action, and other power transmission systems, as well as combinations of such mechanisms.
A problem with prior mechanical foot pedal-to-brake pad force transmission systems is that they are subject to wearing out (particularly at linkage connections, at seals, and at other areas where parts move against each other). Another problem with prior mechanical foot pedal-to-brake pad force transmission systems comprising hydraulic or pneumatic lines, is that they are susceptible to failure by leaking hydraulic fluid or air. Another problem with prior mechanical foot pedal-to-brake pad force transmission systems comprising hydraulic or pneumatic lines is that they are relatively expensive, heavy, cumbersome, and take up a lot of space.
When an automobile driver applies pressure to the brake pedal in a vehicle initially moving at a uniform rate of speed, the wheels tend to slow down relative to the ground. As the brake pressure applied is increased, the braking forces on the vehicle increase in response to the driver's commands. The braking forces on the vehicle will continue to increase with increased brake pedal pressure until the maximum braking capability of the particular tire and road surface is reached. Upon further application of the brake input, the rotational speed of a wheel becomes lower than the vehicle speed, and “slippage” occurs between the tire and the driving surface. This slippage, when severe, can lead to lock-up of a wheel and skidding of the vehicle. When wheel slip occurs between the wheel and the road surface, the ability of the vehicle to follow driver's commands is reduced, and the braking distance is considerably increased.
Anti-lock Braking Systems have been proposed to minimize or eliminate slippage between the wheel and the road surface. Anti-lock Braking Systems (ABS) are feedback control systems that attempt to maintain controlled braking under all operating conditions. This is typically accomplished by prior braking systems by controlling the slip at each wheel so as to obtain optimum braking forces within the limits of the tire-road combination.
Although various ABS schemes have been proposed in the prior art, virtually all prior ABS designs use the same fundamental elements, namely: a foot pedal which is operated by the vehicle's driver; a booster/master cylinder/modulator which increases the braking force deliverable to the wheels; mechanical (hydraulic or pneumatic) transmission lines; a wheel speed sensor; control circuitry to automatically adjust the braking force in response to the signals from the wheel speed sensor; and wheel brakes.
In ABS systems, signals from the wheel speed sensor are sent to control circuitry which determines from these signals whether tire/road slippage is imminent (or has occurred). The control circuitry then sends an electrical signal to the booster/master cylinder/modulator, which typically causes a mechanical valve to open within the booster/master cylinder/modulator, thereby reducing pressure in the mechanical (hydraulic or pneumatic) transmission lines. The reduction of pressure in the transmission lines causes each of the wheel brakes connected thereto to release from engagement with the respective wheels.
One method by which a wheel slip condition is identified in an ABS system is by comparing the speed of each wheel (as measured by the wheel speed sensor) to a reference speed (either a standard value or a measured value). The reference speed is an estimate of the true vehicle speed based on current and previous measured values of the individual wheel speeds. If the speed of a wheel is significantly less than the reference speed, then the wheel is deemed by the control circuitry to be excessively slipping. The control circuitry then reduces the pressure at the master cylinder/booster/modulator actuating the brake in order to reduce brake torque. The reduction of brake torque causes a reduction of the braking force, which then causes a reduction of the slip in the wheel. After a period of constant braking pressure following the pressure reduction, the pressure actuating the brake is increased until wheel slip occurs again. The cycle of decreasing the brake pressure, maintaining constant brake pressure, and then increasing brake pressure is repeated until excessive slip no longer occurs.
In other ABS systems, determination of the slip condition is based on the sensed peripheral speed of the wheel, and the computed acceleration or deceleration of the wheel. Acceleration and deceleration (with a negative value) are derivatives of the velocity of the wheel that are computed by the control circuitry. Based on the changing curve of the peripheral speed of the wheel, a velocity the vehicle body is also obtained (computed). From the velocity of the vehicle body and the peripheral velocity of the wheel the slip ratio is calculated. From the wheel acceleration (or deceleration) and the slip ratio, the control circuitry determines whether the state of the vehicle corresponds to a condition of excessive slipping (or in a state of slip recovery). In other words, when the wheel slip ratio exceeds a certain fixed (previously computed) value or when wheel acceleration (wheel speed) decreases beyond a certain fixed value, the control circuitry reduces brake pressure.
The control circuitry also determines when there is a state of recovery from slippage. When the wheel slip ratio decreases beyond a certain fixed value and wheel acceleration (wheel speed) acquires a tendency to increase brake pressure is increased. Thus, when the control circuitry determines that tire/road slippage is no longer imminent, the same electrical-mechanical control circuitry causes the full pressure in the mechanical transmission lines to recover (corresponding to the amount of displacement of the foot pedal) and the brakes to re-engage. In prior ABS designs this described sensing and engagement-disengagement sequence of the wheels brakes typically takes place many times per second.
All of the problems discussed above with respect to prior automobile braking systems also apply to prior ABS designs.
Another problem with prior ABS designs is that, due to the time delay between electrical signal input from the controller to the booster/master cylinder/modulator and the actual engagement or disengagement of the wheel brake from the wheel, the efficiency and precision of the ABS system (and therefore the degree of control afforded the vehicle's driver) are each inherently limited.
Another problem with prior ABS designs is that because the controller-generated electrical signal causes mechanical transmission line pressure changes which affect at least two (and in most cases four) wheels, it is not usually possible or practical to individually and independently control the braking at each wheel.
SUMMARY OF THE INVENTION
Accordingly, it
Bolduc David J.
Butler Douglas C.
Clark Stephen E.
Face International Corp
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