Estimating road friction coefficient

Abstract: A road surface friction coefficient estimating method and apparatus in an anti-lock control device for preventing locking of vehicle wheels during braking. The a road surface friction coefficient is estimated by comparing a deceleration of an estimated vehicle speed with a predetermined threshold value. The predetermined threshold value has different road surface friction coefficient values corresponding to different vehicle deceleration values. The braking hydraulic pressure is then repeatedly varied in accordance with signals indicative of the estimated road surface friction coefficient, signals indicative of the estimated vehicle speed, and one of the signals indicative of the slowest rear wheel speed and the front wheel speeds.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method and an apparatus for estimating a road surface friction coefficient according to the preambles of claims 1 and 3, respectively. Such a method is known from EP-A-0 327 946 and comprises the steps of: setting an estimated vehicle speed based on wheel speeds of a running vehicle; setting at least one threshold value representing predetermined road surface friction coefficients; calculating a deceleration of said estimated vehicle speed; estimating said road surface friction coefficeint by comparing said deceleration with said threshold value and obtaining an estimated road surface coefficient; and repeatedly varying hydraulic braking pressure in accordance with electric signals indicative of a value of said estimated road surface friction coefficient. Generally, in an anti-lock control apparatus for automotive vehicles, to maintain steering ability and running stability during braking, a control unit including a micro-computer controls hydraulic brake pressure. This brake pressure control also reduces braking distance. During operation of the anti-lock control apparatus, a control mode for hydraulic brake pressure is determined based on an electric signal indicative of a wheel speed. This wheel speed is detected by a wheel speed sensor. According to the control mode, a hold valve, which is a normally opened solenoid valve, and a decay valve, which is a normally closed solenoid valve, are selectively opened and closed to increase, hold constant and decrease the brake hydraulic pressure. In such anti-lock control, in many cases the control is performed by the steps of setting a target slip rate for braking, detecting a difference between an actual wheel speed (hereinafter referred as the wheel speed) and a target speed which is a wheel speed calculated from the target slip rate, and determining the required increase or decrease of the hydraulic brake pressure based on this detected difference and acceleration/deceleration of the wheel speed. Therefore, the wheel speed is rapidly converged to the target speed. However, since the wheel speed varies due to slight changes on a road surface, the abovementioned anti-lock control suffers from problems such that the decrease and increase of the hydraulic brake pressure is undesirably repeated many times due to the effect of the change on the road surface. As a result, much time is required to converge the wheel speed to the target speed. Also, the period in which a braking force is insufficient is extended because the hydraulic brake pressure is not rapidly and suitably increased. Thus, the braking distance increases and the deceleration of the vehicle is insufficient. It is the object of the invention to improve a method and apparatus according to the preambles of claims 1 and 3 respectively, in such that braking distance and the time required for stopping the vehicle are further reduced. This object is achieved by the characterizing features of claims 1 and 3, respectively. The method according to the present invention is characterized by the steps of: setting a plurality of pressure increase rates for increasing said braking hydraulic; and selecting one of said plurality of pressure increase rates in accordance with the value of said estimated road surface coefficient. SUMMARY OF THE INVENTION Further embodiments of the invention are indicated in the appended sub-claims. In the following, the present invention is described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing a three-channel anti-lock control apparatus according to the present invention; Fig. 2 is a flow chart showing the steps for determining a pressure increase rate; Fig. 3 is an explanatory diagram for a method of judging a road surface friction coefficient µ; and Fig. 4 is a diagram showing a control state in the anti-lock control according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a block diagram showing a three-channel anti-lock control apparatus according to the present invention. Output signals provided by wheel sensors 1 to 4 are transmitted to calculation circuits 5 to 8, respectively. These output signals are indicative of the wheel speeds as detected by wheel speed sensors 1 to 4, respectively. The right front wheel speed signal Vw1 and the left front wheel speed signal Vw2 are directly transmitted as first and second system speed signals Vs1 and Vs2 to first and second control logic circuits 9 and 10, respectively. A lower rear wheel speed is selected between the left rear wheel speed signal Vw3 and the right rear wheel speed signal Vw4 in a low-selection circuit 11. This lower rear wheel speed is transmitted as a third system speed signal Vs3 to a third control logic circuit 12. Each of the control logic circuits 9, 10 and 12 treats the respective system speed signals Vs1, Vs2 or Vs3 as a controlled object. In accordance with the respective system speed signals Vs1, Vs2 and Vs3 which are indicative of the wheel speeds, each of the control logic circuits controls the opening and closing of the hold valve HV and the decay valve DV. The signals representing the wheel speeds Vw1 to Vw4, respectively, are also transmitted to an estimated vehicle speed calculation means 13. In the estimated wheel speed calculation means 13, the highest wheel speed is selected from among the four wheel speeds Vw1 to Vw4 and an estimated vehicle speed Vv is obtained with a limitation of acceleration/deceleration of ± 1G which is the limitation of the wheel with the highest speed. The estimated vehicle speed Vv is output to the control logic circuits 9, 10 and 12. The estimated vehicle speed Vv obtained by the calculation circuit 13 is also provided to a road surface friction coefficient µ judgment circuit 14. Signals indicative of the road surface friction coefficient µ as determined by the judgment circuit 14 are output to the control logic circuits 9, 10 and 12. In accordance with these signals, each of the control logic circuits 9, 10 and 12 determines a pressure increase rate from a point at which the hydraulic brake pressure starts to increase. Fig. 2 shows a flow chart indicating a routine in which the pressure increase rate is determined in accordance with the signals indicative of the road surface friction coefficient µ provided by the road surface friction coefficient µ judgement circuit 14 (hereinafter judgment circuit 14). The flow chart shown in Fig. 2 will now be explained with reference to Figs. 3 and 4. In Step S1, an amount of speed decrease ΔV of the estimated vehicle speed Vv for a predetermined time period ΔT is calculated by the judgment circuit 14. In Step S2, a deceleration VvG (= ΔV/ΔT) of the estimated vehicle speed Vv is also calculated by the judgment circuit 14. In Step S3, judgment circuit 14 determines whether or not the deceleration VvG is equal to or greater than the predetermined first reference deceleration VvG1. If it is determined in Step S3 that the deceleration VvG is equal to or greater than the first reference deceleration VvG1 (VvG ≧ VvG1), in Step S4, the road surface friction coefficient µ is determined to be a high value and the judgment circuit 14 provides signals indicative thereof. In Step S5, the control logic circuits 9, 10 and 12 compare the road surface friction coefficient µ signals with the estimated vehicle speed signals Vv and the system speed signals Vs1, vs2 and Vs3, respectively. The control logic circuits 9, 10 and 12, in accordance with the outcome of the above comparison, control the opening and closing of the hold valves HV and decay valves DV and thus perform the pressure increase by the shortest pressure increase cycle t1. This pressure increase cycle t1 is predetermined for the high valued road surface friction coefficient µ (shown in Fig. 4). In the embodiment of the present invention, as is apparent from Fig. 4, one pressure increase cycle is defined by one unit comprising a combination of a pressure increase period and a pressure hold period. During a pressure increase cycle, the hydraulic break pressure is increased in steps by repetition of the pressure increase period and pressure hold period. The pressure increase cycle is varied such that the pressure hold period is changed while the pressure increase period is kept constant. If in Step S3 the judgment circuit 14 determines that the deceleration VvG is less than the first reference deceleration VvG1 (VvG < VvG1), then in Step S6, the judgment circuit 14 determines whether or not the deceleration VvG is equal to or greater than a predetermined second reference deceleration VvG2. This second reference deceleration VvG2 is less than the first reference VvG1 (VvG2 < VvG1). If the judgment circuit 14 determines in Step S6 that the deceleration VvG is equal to or greater than the second reference deceleration VvG2 (VvG ≧ VvG2), then in Step S7 the judgment circuit 14 determines the road surface friction coefficient µ to be a middle valued road surface friction coefficient µ. In Step S8, the control logic circuits 9, 10 and 12 compare the road surface friction coefficient µ signals with the estimated vehicle speed signals Vv and the system speed signals Vs1, Vs2 and Vs3, respectively. The control logic circuits 9, 10 and 12, in accordance with the outcome of the above comparison, control the opening and closing of the hold valves HV and decay valves DV and thus perform the pressure increase by a pressure increase cycle t2 (t2 > t1). This pressure increase cycle t2 is predetermined for the middle valued road surface friction coefficient µ (shown in Fig. 4). On the other hand, if the judgment circuit 14 determines in Step S6 that the deceleration VvG is less than the second reference deceleration VvG2 (VvG < VvG2), in Step S9, the judgment circuit 14 determines the road surface friction coefficient µ to be low valued. In Step S10, the control logic circuits 9, 10 and 12 compare the road surface friction coefficient µ signals with the estimated vehicle speed signals Vv and the system speed signals Vs1, Vs2 or Vs3, respectively. The control logic circuits 9, 10 and 12, in accordance with the outcome of the above comparison, control the opening and closing of the hold valves HV and decay valves DV and thus perform the pressure increase by a pressure increase cycle t3 (t3 > t2). This pressure increase cycle t3 is predetermined for the low valued road surface friction coefficient µ (shown in Fig 4). Accordingly, based on a selected one of the pressure increase cycles t1, t2 and t3, a pressure increase rate is determined for the wheels which are anti-lock controlled. As is apparent from the above explanation, a plurality of pressure increase rates are set in advance according to various road surface friction coefficients µ. The friction coefficients µ are determined on the basis of an amount of vehicle speed deceleration occurring during braking of the vehicle and one of the pressure increase rates is selected based on this determination. Then, the hydraulic brake pressure is increased by the selected pressure increase rate. As a result, it is possible not only to prevent excessive pressure increase of the hydraulic brake pressure on a road surface having a low friction coefficient µ but also to rapidly cope with the circumstance when the road surface friction coefficient µ changes from a low to high value during braking. Therefore, braking distance is reduced. Because the road surface friction coefficient µ is determined on the basis of vehicle deceleration, no problem exists whereby the pressure increase rate is not suitable for the road surface due to the fact that the wheel speed variably changes with slight changes of a road surface. Accordingly, it is possible to reduce the time to converge the wheel speed to the target speed and therefore provide more stable anti-lock control.

Claims: A method of estimating a road surface friction coefficient µ for a running vehice being braked, comprising the steps of: a) setting an estimated vehicle speed (Vv) based on wheel speeds (Vw) of said running vehicle; b) setting at least one threshold value (VvG1,VvG2) representing predetermined road surface friction coefficients; c) calculating a deceleration (VvG) of said estimated vehicle speed (Vv); d) estimating said road surface friction coefficient(µ) by comparing said deceleration (VvG) with said threshold value (VvG1,VvG2) and obtaining an estimated road surface coefficient (µ); and e) repeatedly varying hydraulic braking pressure in accordance with electric signals indicative of a value of said estimated road surface friction coefficient (µ); characterized by the steps of f) setting a plurality of pressure increase rates (t1,t2,t3) for increasing said braking hydraulic pressure; and g) selecting one of said plurality of pressure increase rates (t1,t2,t3) in accordance with the value of said estimated road surface friction coefficient (µ). A method as claimed in claim 1 characterized in that each of said plurality of pressure increase rates (t1,t2,t3) is defined by a plurality of first time periods in which said hydraulic braking pressure is increased and a plurality of second time periods in which said hydraulic braking pressure is held constant, said first and second time periods occurring alternately in succession An apparatus for estimating a road surface friction coefficient (µ) for a running vehicle being braked, comprising: a) first means (13) for receiving a plurality of wheel speed signals (Vw1,Vw2,Vw3,Vw4), for determining an estimated vehicle speed (Vv) based on said wheel speed signals and for outputting a first control signal indicating said estimated vehicle speed; b) second means (14) for receiving said first control signal, for calculating a deceleration (VvG) based on said first control signal, for setting at least one threshold value (VvG1,VvG2) representing predetermined road surface friction coefficients and for outputting a second control signal indicating an estimated road surface friction coefficient (µ), said estimated road surface friction coefficient (µ) being obtained by comparing said deceleration (VvG) with said at least one threshold value (VvG1,VvG2); and c) third means (9,10,11,12,) for receiving said plurality of wheel speed signals (Vw1,Vw2,Vw3,Vw4), said first control signal and said second control signal and for outputting a plurality of third control signals for repeatedly varying the hydraulic braking pressure, characterized in that d) said third means (9,10,11,12) sets a plurality of pressure increase rates (t1,t2,t3) for increasing said hydraulic braking pressure; and e) said third means (9,10,11,12) selects one of said plurality of pressure increase rates (t1,t2,t3) in accordance with a value of said estimated road surface friction coefficient (µ). An apparatus as claimed in claim 3, characterized in that said third means (9,10,11,12) sets each of said plurality of pressure increase rates (t1,t2,t3) by defining a plurality of first time periods in which said hydraulic braking pressure is increased and a plurality of second time periods in which said hydraulic braking pressure is held constant, said first and second time periods occurring alternately in succession.

Inventors: OKUBO SATOMI, OKUBO, SATOMI
Applicants: AKEBONO BRAKE IND, AKEBONO RES & DEV CENTRE, AKEBONO BRAKE INDUSTRY CO., LTD., AKEBONO RESEARCH AND DEVELOPMENT CENTRE LTD.
Application year: 1991
Main classification: B60T8
IPC codes: B60T8