GB2321681A

GB2321681A - Electronic braking system for vehicles

Info

Publication number: GB2321681A
Application number: GB9702156A
Authority: GB
Inventors: Mark Leighton Howell; Alan Leslie Harris; Simon David Stevens
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1997-02-03
Filing date: 1997-02-03
Publication date: 1998-08-05
Anticipated expiration: 2017-02-03
Also published as: GB2321681B; DE69813774D1; WO1998033689A1; ES2193511T3; GB9702156D0; JP2001509753A; EP0956225B1; EP0956225A1; US6234585B1; DE69813774T2

Abstract

An electronic braking system for a vehicle, comprises an electronic controller, responsive to electronic control signals generated at the brake pedal, for controlling fluid supply from a power source to the brakes, wherein the brake pressure is raised to a first predetermined level (jump-in) at a prescribed level of initial pedal travel and is released at a second predetermined level (reverse jump-in), the second level being lower than the first. The jump-in brake pressure level can be arranged to be variable with vehicle speed. In some cases, the brakes are arranged to be pre-filled to a low pressure at an early stage in the pedal travel and then maintained at that low level until jump-in is triggered.

Description

DESCRIPTION ELECTRONIC BRAKING SYSTEM FOR VEHICLES The present invention is concerned with electronically controlled braking systems for vehicles, commonly referred to as "Brake-by-Wire (BBw) systems.

In order to provide a responsive "feel to the driver, conventional braking systems usually corporate brake pressure booster having a feature known as "jump-in". "Jump-in" involves the use of engine generated power, in the orm of vacuum, to apply the brakes to a predetermined pressure as soon as the driver moves the brake pedal far enough to operate the control valves, controlling the supply of fluid to the brakes. This operation is illustrated in Figure 1 of the attached drawings, where the pressure jumpe to 10 bar at a pedal effort of 20 units. The pedal is relieved of the reaction loads during this stage of the operation, so that pedal effort, which would otherwise need to be increased beyond the valve-operating threshold in fixed proportion to the brake pressure, remains constant until the jump-in pressure is attained. This is typically between 3bar - 5bar, but may be as high as lObar, and it seems to the driver as though the necessary pedal travel has occurred without any change in applied effort - hence tile feeling of responsiveness. As soon as the required pressure is reached, the reaction force from the master cylinder returns the booster to a balanced state so that further pressure increase is controlled directly by the driver's additional pedal effort. Unwanted speed dependent variations in, e.g. lining friction, can make high levels of jump-in unacceptable at low speeds, for example during parking, forcing compromise with regard to the chosen characteristics.

A further feature of vacuum boosters is that they exhibit considerable hysteresis1 so that during brake release the jump-in action is reversed at lower brake pressures than the original jump-in, and is not clearly felt by the driver. It is thereby possible to provide sensitive control of low-pressures for check braking and especially for creep, e.g. on an inclined driveway or (in a car with automatic transmission) in. heavy stop-go traffic.

Brake-by-Hire (BBW) systems suffer less hysteresis1 and can achieve more rapid pressure changes than can a vacuum boosted system. These can enhance the general feeling of responsiveness1 but they pose special difficulties during jump-in, which requires mor.e subtle control of pressure rise at the beginning of braking than would a smooth characteristic.

Particular problems associated with the application of "jump-in" to Brake-by-Wire systems are: 1. The reverse jump-in action disturbs smooth control during normal check-braking since the operating pressure is much less affected by hysteresis.

2. The enhanced control precision highlights the effects of brake lining-p variation, causing further compromise.

3. Overshoot control is difficult during a rapid increase to a low pressure because of the brakecaliper's non-linear volumetric displacement chararte-istics.

in accordance with the present invention there is provided an electronic braking system for a vehicle wherein control of the vehicle brakes is achieved by the use of electronic control signals generated at a brake pedal in response to a driver's braking demand and an electronic controller which is adapted to control the supply of fluid under pressure from a power source to the brakes in accordance with said electronic signals corresponding to the driver' demand, the system being adapted to raise the brake pressure to a first predetermined level (jump-in) at a prescribed level of initia]. brake pedal travel and to release the brake pressure (reverse jump-in) at a second predetermined level, which is lower than that at jumpin.

Advantageously, the jump-in brake pressure level is arranged to be variable with vehicle speed. For example, the level of jump-in brake pressure can be arranged to be reduced at lower vehicle speeds and increased at higher vehicle speeds.

In some embodiments, the brakes are arranged to be pr-filled to a low pressure at an early scage in the pedal travel and then maintained at that low level until jump-in is triggered, whereupon the braking pressure is raised to the jump-in level.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 illustrates a typical application of "jumpin" within the brake pressure vs. pedal effort characteristic; Fig. 2 shows a typical characteristic of the variation of brake. pressure with pedal travel; mig. 3 illustrate6 the operation of a system in accordance with the present invention where reverse jump-in occurs at a demand pressure lower than that of the jump-in; Fig. illustrates the operation of a further system in accordance with the present invention where a low-pressure pre-fill is applied at an early stage in the brake travel; Fig. S illustrates the non-linear relationship between displacement volume of The brake calipers and applied brake pressure; Fig. 6 is a typical curve of jump-in pressure vs.

vehicle seed where the jump-in pressure equals the pre-fill pressure below 2kpm; Fig. 7 is a curve of brake pressure vs. pedal travel in the case of no jump-in bering arranged; Fig. 8 is a curve of brake pressure demand vs.

pedal travel, with jump-in at okph and a pre-fill pressure of 2bar; Fig. 9 is a curve of brake pressure demand vs.

pedal travel, with jump-in over 5kph and a prc-fill pressure of 2bar; and Figs. lOa and 10b together constitute a flow diagram of one possible embodiment of a system incorporating the present invention.

As mentioned hereinbefore, and as illustrated in Figure 1, conventional vacuum boosters are responsive to an initial pedal effort for the control of the jumpin feature. However, with Brake-by-Wire systems, pedal effort is difficult to monitor directly using the technology available to present BBW systems1 and the alternative of master-cylinder pressure is often not available at the beginning of the brake application due to the travel needed to close the master cylinder cutoff valve. Thus, the early stages of braking are controlled by monitoring pedal travel, in the knowledge of the designed or stored relationship of effort to travel. Figure 2 shows the nominal relationship between pedal travel and brake pressure, and since the latter is a linear function or effort (ref Fig. 1), it also represents the form of the travel/effort rc-lationship. Travel s therefore a convenient way to control jump-in in 3BW systems because, as indicated above, effort changes during jump-in are intended to be minimum and therefor not easily observed.

In a system in accordance with the present invention, the first problem identified above is overcome by arranging for the reverse of the jump-in pressure to occur at a predetermined demand pressure, lower than that of the jump-in (see Fig. 3), thus replicating with the precision and consistency of logic the haphazard, manufacturing-tolerance-depe dant function of the vacuum booster. In the illustration of Figure 3, jump-in occurs to a pressure of lObar at a pedal effort 20N whereas reverse jump-in occurs at a demand pressure of only about 1.5bar. Is should be noted that the lower demand pressure at which reverse jump-in occurs need not be positive and could be zero bar.

The use of logic by BEW systems to control pressures enables the jump-in level to be varied with vehicle speed. Typically, it could be reduced at low specks, and, possibly, increased at very high speeds, thus enabling better compromises to be reached across th whole speed range. Figure 4 shows a normal jump-in value of lObar and a low-speed value of Sbar, but these figures will be dependent upon vehicle characteristics and manufacturer's preference.

Inherent within the concept of jump-in is the ned to increase pressure rapidly from zero to the jump-in levcl. This is a significant problem for a closed-loop control system suc as BSW because the brake calipers have very on-linear characteristics between, say, zero and 2bar due to the running clearances (see Figure 5).

Furthermore, it is possible that in practical systems powered actuatior such as BBW may deliberate be partnered with calipers having enlarged running clearances in order to obtain improvements in disc life and braking comfort without incurring the penalty of long pedal travel. Wide vale openings needed to fill the calipers quickly will cause unacceptable overshoe: unless the control hardware and sortware can resold in a rapid and stable manner. This problem becomes much easier if the filling process can take place more slowly. In the solution proposed here and shown also in Figure 4, the brakes are filled to a low pressure, e.g. 2.0bar, at a relatively early stage in the pedal travel, and maintained at that pressure until the jumpin is triggered. During normal usage, this allows better control of brake-fill because the error signal (between demand and actual brake pressure) remain3 small, such Lhat wide openings are avoided. The low pre-fill pressure will not be noticed by the driver, but is sufficient to avoid the need for extreme valve openings during the subsequent jump-in, to the benefit of smooth control. Naturally, wide openings will become unavoidable in the event of a very rapid brake application, but in this situation overshoot in the low-pressure region is unlikely to be a problem because the driver will usually be seeking medium, or even emergency pressure levels.

Figure 6 shows one possible characteristic of jump-in pressure vs. vehicle speed, the jump-in pressure being equal to the pre-fill pressure of 2bar below 2kph.

Figure 7 shows a typical conventional curve of brake pressure demand vs. pedal travel in a case where jump-in is not used. Fig. 8 shows how the curve of Fig. 7 is modified if jump-in occurs at Okph and a prefill of 2bar occurs at a pedal travel of Smm. Fig 9 shows how the curve of Fig. 8 is modified if jump-in occurs at St1h and there is again a pre-fill of 2bar occurring at a pedal travel of Smm.

Figures lQa and lOb show a flow diagram of one possible system performing in accordance with the present invention. The various steps carried out by the latter system are as follows:- 10 Start (brake pedal released) 1 Read pedal travel 14 Calculate new brake pressure demand 16 Read vehicle speed 18 Calculate jump-in pressure 2.0 New pressure demand > 0? 22 New pressure demand > jump-in pressure? 24 Brake pressure demand = 0 2 G Brake pressure demand = pre-fill pressure 28 Read pedal travel 30 Calculate new brake pressure demand 32 Read vehicle speed 34 Calculate reverse jump-in pressure 36 Brake pressure demand > reverse jump-in pressure? 35 Brake pressure demand = new brake pressure demand 40 Brake pressure demand = 0.

Claims

C-nAMNS

1. An electronic braking system for a vehicle where control of the vehicle brakes is achieved by the use of electronic control signals generated at a brake pedal in response to a driver's braking demand and an electronic controller which is adapted to control the supply of fluid under pressure from a power source to the brakes in accordance with said electronic signals corresponding to the driver's demand, the system being adapted to raise the brake pressure to a first predetermined level (jump-in) at a prescribed level of initial brake pedal travel and to release the brake pressure (reverse jump-in) at a second predetermined level, which is lower than that at jumpin.

2. An electronic braking system as claimed in claim 1, wherein the jump-in brake pressure level is arranged to be variable with vehicle speed.

3. An electronic braking system as claimed in claim 2, wherein the level of jump-in brake pressure is arranged to be reduced at lower vehicle speeds and increased at higher vehicle speeds.

4. An electronic braking system as claimed in claim 1, 2 or 3, wherein the brakes are arranged to be pre-ìl3.ed to a low pressure at an early 3cage in the pedal travel and then maintained at that low level until jump-in is triggered, whereupon the braking pressure is raised to the jump-in level.

5. An electronic braking system substantially as hereinbefore described with reference to Figures 3,4,6,S,9,10a and lob of the accompanying drawings.