Anti-Skid Braking System

The present invention relates to anti-skid devices and especially to an anti-skid brake for vehicle wheels which utilizes the road-wheel torque to power the wheel brake so that the resisting torque applied by the brake cannot exceed the torque generated by the road on the wheel of the vehicle.

It has been known for some time that, under most road conditions, skidding does not occur suddenly as a result of an instantaneous switch from the coefficient of rolling friction to the coefficient of sliding friction at the wheel-road contact and a simultaneous switch in the brake from a coefficient of sliding friction to a coefficient of static friction. The change from rolling contact to skidding is a gradual, but rapid transition where the coefficient of friction varies with slippage while the brake switches from sliding friction to static friction at the instant the wheel reaches 100% slippage.

Many anti-skid devices sense wheel rotation, vehicle acceleration, or brake pressure to detect a skid condition. Even though some prior art devices measure torque to detect slippage, most of these systems use electronic circuits to pump or pulse the brake pressure causing the tire-road contact to alternate between 100% skidding and free rolling conditions. These prior art systems improve control during braking, but have a number of disadvantages including: (a) not providing optimum stopping conditions, but instead increase stopping distance by operating in the high percentage skid and free roll areas, thus preventing only sustained full skids; (b) their effectiveness is influenced by road conditions since any given wheel rotation rate, vehicle acceleration, or brake pressure could occur at any point in the range of skid; and finally, (c) the systems are complicated by complex electronic-hydraulic circuitry resulting in high manufacturing, installation and maintenance costs in addition to low reliability.

There are also a number of anti-skid devices which sense wheel-road torque and use this to operate servo valves. These similarly have disadvantages such as the brake pressure not being limited at the optimum stopping condition, but merely modified at the 100% skid area, thus permitting possible 100% skids.

The present invention, on the other hand, relates to a combination of devices which are directed toward preventing skidding of a vehicle on any road by adapting the braking force to the existing road wheel friction condition. Thus, the invention will sense the torque applied by the road to the wheel and use this wheel torque to power and control the brake, thus providing a limit on the braking capacity which is dependent on the existing road conditions.

By using the difference in pressure between the pilot brake and the main brake to control and power the brake action, the need for a servo valve is eliminated. The present invention is an improvement over the invention in my prior U.S. Pat. Nos. 3,872,952 and 3,923,345.

An anti-skid brake mechanism is provided having a fluid drive system for applying fluid pressure to a wheel main brake. A torque measuring pilot brake is provided for actuating the fluid drive system by sensing and measuring a torque created by the road-wheel contact. A converter system is provided for converting the torque measured by the pilot brake into hydraulic pressure. The converter also compares the pilot brake pressure to the main brake pressure and modifies the main brake pressure so that the combined braking effort of the pilot brake and the main brake does not exceed the capacity of the road-wheel interface to maintain maximum friction for existing conditions. The main brake driving system is powered by the hydraulic pressure generated in the converter.

Other objects, features and advantages of the present invention will be apparent from the description and the drawings, in which:

The FIGURE is a schematic sectional view of a preferred embodiment of the present invention having a combination of pilot brake, main brake and converter assembly.

Referring now to the FIGURE, an anti-skid brake includes a pilot brake assembly A affixed to a specially designed converter assembly B and is hydraulically connected to and operated by the hydraulic pressure generated in a conventional master cylinder 2. A specially designed telescoping compound power converter assembly B consists of two hydraulic cylinders and pistons, one end of which is affixed to the stationary wheel mount 8 and the other end attached to the pilot brake assembly A. The pilot brake assembly is also connected to one of the two cylinders in the converter assembly.

A main wheel brake assembly C is affixed to the stationary wheel mount 8 by means of spring 19 and is hydraulically connected to the other cylinder of the compound power converter assembly B. A rotatable brake disc 18 is rigidly affixed to and rotated with the vehicle wheel in the same manner as conventional disc brakes.

Again referring to the FIGURE, the operator presses on the brake pedal 1 generating a hydraulic pressure P.sub.1 in the master cylinder 2. The pressure P.sub.1 is limited to a predetermined maximum pressure by relief valve 20 and by accumulator 21. The hydraulic pressure P.sub.1 is transmitted through the hydraulic line 3 to the pilot brake cylinder 5 and to the cylinder 9 in the converter assembly B.

The pressure P.sub.1 acting on the area A.sub.1 of the pilot piston 4 causes the pilot brake pad 15 to be pressed into contact with the moving brake disc 18 resulting in a drag or braking force F.sub.1 which is equal to 2P.sub.1 A.sub.1 .mu.1, where .mu..sub.1 is the coefficient of friction between the brake pad 15 and the disc 18. The force F.sub.1 causes the pilot piston assembly A to move in the direction of rotation of the disc 18 thereby compressing spring 10 and energizing seal 12 to prevent passage of fluid from cylinder 7 to cylinder 9, spring 10 and generating a pressure P.sub.2 in the fluid in cylinder 7. The fluid pressure P.sub.2 is communicated through passage 17 to main brake cylinder 13 where it acts against area A.sub.2 of the main brake piston 14, which in turn presses the main brake pad 16 into contact with the brake disc 18 causing a second braking force F.sub.2 which is equal to 2P.sub.2 A.sub.2 .mu..sub.1. Note that the relationship between F.sub.1 and the transfer assembly B is such that a second equation for F.sub.1 exists such that:

where F.sub.s is the resisting force of spring 10.

Since the equation for decelerating a wheel may be expressed as:

Where:

m=mass of load on wheel;

a=the deceleration of the mass;

R.sub.w =the radius of the wheel;

R.sub.B =the effective radius of the brake assembly; and

F.sub.1 & F.sub.2 are previously described.

It can be easily seen that one equation of the present invention is: E1 ? ##STR1## which shows that the controlled deceleration of a vehicle upon which the brake is installed is directly proportional to the control pressure P.sub.1 as long as a/g is less than .mu..sub.R where: g is the universal gravitational constant and .mu..sub.R is the maximum possible coefficient of friction between the wheel and the road for the existing road conditions.

In the case of a hard or panic stop where maximum P.sub.1 is generated, a/g is equal to .mu..sub.R but skidding is prevented by the fact that overpressuring of P.sub.1 actually causes P.sub.2 to decrease, since P.sub.1 A.sub.y opposes P.sub.2 A.sub.x so that the total braking effect actually remains at the optimum point, regardless of road conditions. This can easily be seen if W.mu..sub.R is substituted for ma in the previous equation which is then rearranged as follows: ##EQU1## where W equals the weight supported by the wheel. It is evident that when a/g is equal to .mu..sub.R, any change in P.sub.1 causes P.sub.2 to change inversely proportional to P.sub.1.

When the brake is applied on any surface, the brake torque T.sub.B is proportional only to P.sub.1 since all other factors in the equation are constant. However, the brake torque T.sub.B is equal to and limited by the road torque which is equal to .mu..sub.R WR.sub.w. As long as .mu..sub.R does not decrease appreciably, it is not possible for the wheel to skid. If a sudden large decrease in .mu..sub.R is experienced, such as entering a wet spot or ice with the brakes applied, the brake may momentarily skid. The skid results in a decrease in the road A torque T.sub.B and the forces F.sub.1 and F.sub.2. Since both F.sub.1 and F.sub.2 were acting to compress the main spring 19, the main spring 19 is now partially relieved and spring 19 expands, moving the entire main brake assembly C and disc 18 in a direction opposite to the original. Depending on the relationship between areas A.sub.x and A.sub.y, the pressure in the main brake pressure P.sub.2 will be reduced unlocking the entire brake. As soon as the brake is unlocked, the disc 18 and wheel will start to turn in the original direction as a result of the new road force F.sub.R which is now made up of .mu..sub.R where .mu..sub.R is a lower value than the original. The brake immediately reapplies itself and adjusts to the new road conditions, except now the limiting F.sub.R is a much lower value and will prevent skidding on the new, low .mu..sub.R road surface. The FIGURE shows only one half of the pilot brake and main brake calipers. However, in all equations the caliper halves not illustrated are accounted for by multiplying the areas A.sub.1 and A.sub.2 by two. A compensator 22 is included in the main brake circuit which allows the piston 6 to fully retract in cylinder 7 and uncover seal 11 to recharge the main brake circuit in the event of fluid leakage.

It should be clear that the present invention is not to be construed as limited to the particular forms disclosed herein, since these are to be considered as illustrative rather than restrictive.