Abstract:
A method for braking force distribution control for a vehicle with a direct four wheel drive mode, which does not become an obstacle to an effective braking force and disconcert the driver&#39;s feeling. This method relates to a braking force distribution control for a vehicle during direct four wheel drive mode and under the condition that the anti-skid control is inhibited, which does not operate pressure reduction control or the amount of pressure reduction.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to braking force distribution control for a four wheel drive vehicle capable of conducting anti-skid control. 
     2. Description of Related Art 
     Conventionally, for a vehicle with a direct four wheel drive mode, when a brake pedal is stepped on to increase the hydraulic pressure of front wheels but decreasing the hydraulic pressure of rear wheels because of the braking force distribution control, a torque T 1  acts on the front wheels to stop the wheel rotation while the counter torque acts on a front wheel axle and a rear wheel axle, thereby causing vibration in the drive train as shown in FIG.  7 . 
     This vibration is considerably large and the continuous vibration delays the braking effectiveness resulting in a longer braking distance. This is disconcerting to the driver, and gives an adverse effect on the drive train including differential-related parts and an auto free hub. 
     SUMMARY AND OBJECT OF THE INVENTION 
     It is an object of this invention to provide a method for braking force distribution control for a vehicle with a direct four wheel drive mode, which does not become an obstacle to an effective braking force and disconcerts the driver&#39;s feeling. 
     With this invention a method for braking force distribution control for a four wheel drive vehicle is provided. The vehicle uses a direct four wheel drive mode for operating anti-skid control and braking force distribution control. The vehicle has a hydraulic unit comprising a main hydraulic circuit connected between a master cylinder and an inlet valve via a wheel cylinder and an auxiliary hydraulic circuit connected between the wheel cylinder and an auxiliary reservoir via an outlet valve. A wheel speed sensor determines the respective wheel speed, and an electronic control device controls the hydraulic unit, wherein the braking force distribution control for a vehicle during the direct four wheel drive mode, and under the condition that the anti-skid control is inhibited, does not operate pressure reduction control or limit the amount of pressure reduction. 
     With this invention, a method for braking force distribution control for a four wheel drive vehicle is provided. The vehicle uses a direct four wheel drive mode for operating anti-skid control and braking force distribution control. The vehicle has a hydraulic unit comprising a main hydraulic circuit connected between a master cylinder and an inlet valve via a wheel cylinder and an auxiliary hydraulic circuit connected between the wheel cylinder and an auxiliary reservoir via an outlet valve. A wheel speed sensor determines the respective wheel speed. An electronic control device controls the hydraulic unit, wherein the braking force distribution control during the direct four wheel drive mode and under the condition that the anti-skid control is inhibited operates pressure increase in the brake hydraulic pressure, or normal brake hydraulic pressure increase if vehicle vibration is detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic view of the brake control device; 
     FIG. 2 is a view explaining the braking force distribution control when the pressure reduction is being inhibited; 
     FIG. 3 is a flowchart of the braking force distribution control restricting the pressure reduction; 
     FIG. 4 is a flowchart of the braking force distribution control restricting the amount of the pressure reduction; 
     FIG. 5 is a flowchart of the braking force distribution control when the wheel vibration is detected; 
     FIG. 6 is a flowchart of another braking force distribution control when the wheel vibration is detected; and 
     FIG. 7 is a view explaining the wheel vibration while in the direct four wheel drive mode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following sections, an embodiment of this invention is explained with reference to the drawings. 
     A four wheel drive vehicle has a various driving modes, and a direct four wheel drive mode is one of them. Under the four wheel drive mode, a front wheel axle and a rear wheel axle are mechanically connected (directly geared), i.e., a rigid four wheel drive. For example, a four wheel drive mode for a part-time four wheel derive vehicle is considered to be categorized in the rigid four wheel derive. Therefore, if the counter torque acts on the front wheel axle and the rear wheel axle, vibration is caused on the wheels. 
     In a brake hydraulic device for a motor vehicle, hydraulic pressure generated in a hydraulic unit  20  is supplied to the respective wheel cylinder  14  for the respective front and rear wheels as shown in FIG. 1 so as to conduct brake control on a left front wheel  1 , a right front wheel  2 , a left rear wheel  3 , and a right rear wheel  4 . For example, regarding the brake control such as anti-lock brake control system (ABS), braking force distribution control system, and vehicle behavior control system, an electronic control device  30  controls the hydraulic unit  20  based on signals from a wheel speed sensor  31  and a brake switch  32  in order to provide an appropriate and preferable brake control. Here, the electronic control device  30  may be comprised of special hardware and general computing members such as microcomputers. 
     FIG. 2 shows one example of the hydraulic unit  20  which is equipped with a hydraulic circuit for brake control such as the anti-lock brake control system (ABS), the braking force distribution control system, and the vehicle behavior control system. Furthermore, FIG. 2 is an example of an X-piping, wherein the left front wheel  1  and the right rear wheel  4  are connected to a first hydraulic circuit  21  at one side and the right front wheel  2  and the left rear wheel  3  are connected to an independent second hydraulic circuit  22  at the other side. 
     The hydraulic unit  20  comprises a main hydraulic circuit  41  connecting a master cylinder  12  and a wheel cylinder  14  via an inlet valve  23 , an auxiliary hydraulic circuit  42  connecting the wheel cylinder  14 , and a return hydraulic circuit  43  returning the hydraulic pressure from the auxiliary reservoir  27  to the main hydraulic circuit  41  via the pump  25 , thereby controlling to open or close the inlet valve  23  and the outlet valve  24  for conducting the predetermined brake control on the respective wheel. Further, the main reservoir  13  reserves the brake fluid, a check valve  28  prevents the reverse flow, and a motor  26  drives and controls the pump  25 . 
     Braking operation is explained next. 
     The electronic control device  30  determines the brake control. For example in FIG  3 , the control device  30  uses signals from sensors such as a wheel speed sensor  31 . First, the electronic control device  30  is initialized and determines the wheel speed based on the signals from the wheel speed sensor  31 . The deceleration of right and left front wheels are determined based on the information regarding the right and left wheel speed. Brake hydraulic pressure in the wheel cylinder of right and left front wheels is determined based on signals from the hydraulic pressure measurement sensor or the information from the hydraulic control of the hydraulic unit. An estimated vehicle speed and an estimate vehicle deceleration are determined based on the information from the respective wheel speed. A type of brake control mode is determined. If an anti-lock control mode is selected, the anti-lock brake control is conducted, but if the anti-lock brake control mode is not selected, normal brake control is conducted. 
     In normal braking, if a brake pedal  11  is stepped on, the brake hydraulic is generated in the master cylinder  12 , and the inlet valve  23  is open while the outlet valve is closed. Then, the brake hydraulic pressure generated in the master cylinder  12  is directly supplied to the wheel cylinder  14 , thereby effectuating the brake on the wheels  1 ,  2 ,  3 ,  4 . 
     For example, the electronic control device  30  conducts anti-lock brake control by controlling opening and closing of the inlet valve  23  and the outlet valve  24  and controlling the driving of the pump  25 . 
     Anti-lock brake control controls the brake hydraulic pressure by repeating a pressure increase mode, a pressure hold mode, and a pressure decrease mode in every cycle. For example the pressure increase mode in one cycle conducts open-close control on the inlet valve  23 , closes the outlet valve  24 , and increases the hydraulic pressure in the wheel cylinder  14 . 
     If there is a failure found in the hydraulic unit or the electronic control device because of the failure found in a wheel speed sensor, the failure found in a pump motor, the failure found in the inlet valve or the outlet valve, or the failure due to an excessively high or low line source, the anti-lock brake control is stopped. 
     Braking force distribution control is explained next. 
     For example in FIG. 3, inhibiting the pressure reduction under the braking force distribution control is stated with determining braking force distribution control parameters in a Step  11  (S 11 ), and whether braking force distribution control is initiated is determined based on the parameters in a Step  12  (S 12 ). For example, the parameters can be the front wheel axle deceleration and the rear wheel axle deceleration. If the braking force distribution control is initiated, the braking force distribution control mode is judged in a Step  13 . Then, whether to stop the braking force distribution control is determined in a Step  14  (S 14 ). If the braking force distribution control is to be stopped or is not to be initiated, the normal brake pressure increase is conducted in the Step  1  (S 1 ). 
     If the braking force distribution control is not stopped, either the pressure increase mode, the pressure hold mode, or the pressure decrease mode is selected depending upon the vehicle behavior and the condition. That is, whether the pressure increase mode is in motion or not is determined in a Step  15  (S 15 ), and if the pressure increase mode is in motion, pressure is increased in the Step  2  (S 2 ). However, if the pressure increase mode is not in motion, whether the pressure hold mode is in motion in the Step  3  (S 3 ) is determined, and if the pressure hold mode is in motion, the hydraulic pressure is held in the Step  3  (S 3 ). However, if the pressure hold mode is not in motion, whether the anti-lock brake control is being inhibited is determined in a Step  17  (S 17 ), and if the anti-lock brake control is being inhibited, the hydraulic pressure is held in the Step  3  (S 4 ). However, the anti-lock brake control is not being inhibited, the hydraulic pressure is reduced in the Step  4  (S 4 ). 
     Accordingly, in the case when the anti-lock brake control is being inhibited, the hydraulic pressure is not to be reduced but to be held, and there is no counter torque acted on the front wheel axle and the rear wheel axle directly geared, thereby eliminating the possibility of causing the problematic vibration. 
     For example, the braking force distribution control restricting the amount of pressure reduction is shown in FIG.  4 . The steps to be described here are covered by the description of the FIG. 3 above; therefore, the explanation of the same steps are omitted here. 
     When whether the anti-lock brake control is being inhibited or not is determined in the S 17 , if the anti-lock brake control is being inhibited, whether the amount of the pressure reduction is larger than a predetermined amount is determined in a Step  21  (S 21 ). If the amount of the pressure reduction is larger than the predetermined amount, the hydraulic pressure is to be held in the S 3 . 
     If the anti-lock brake control is not being inhibited in the S 17  or the amount of the pressure reduction is smaller than the predetermined amount in the S 21 , the hydraulic pressure is reduced in the S 4 . 
     If the anti-lock brake control is being inhibited and the amount of the pressure reduction is larger than the predetermined amount, the hydraulic pressure is to be held to precisely prevent the causation of the vibration. 
     For example, FIG. 5 shows when the braking force distribution control recognizes the wheel vibration. Here, FIG.  3  and FIG. 4 already explained the inhibition of the pressure decrease and the restriction of the amount of the pressure reduction, and the same steps there and the explanation of which will be omitted. 
     If, in the Step  14  (S 14 ), it is determined that the braking force distribution control is not being stopped, whether the anti-lock brake control is being inhibited and there is wheel vibration are determined in the Step  15  (S 15 ). If there is wheel vibration, the hydraulic pressure is increased, while if there is no wheel vibration, the steps described above referring to FIG.  3  and FIG. 4, where the pressure reduction is inhibited and the amount of the pressure reduction is restricted, are conducted. Here, whether there is the wheel vibration or not is checked by the level of acceleration of the wheels, vibration frequency, and so on. 
     For example, FIG. 6 shows when another braking force distribution control recognizes the wheel vibration. In the S 15  of the braking force distribution control shown in FIG. 5, if the anti-lock brake control is being inhibited and there is wheel vibration, normal brake pressure increase is conducted in S 15 . 
     Accordingly, if the anti-lock brake control is being inhibited and there is wheel vibration, the vibration is prevented by increasing the hydraulic pressure. 
     This invention has the following advantages. 
     This invention provides braking force distribution control which prevents a various problems occur in a direct four wheel drive mode, such as the wheel vibration caused during the braking force distribution control, disconcerting feeling due to the vibration, and a failure in the drive train (e.g., deferential-related section, and auto free hub). 
     This invention provides stable braking control without an extra or special switch for the recognition of the direct four wheel drive mode and provides stable braking control even if the switch fails. 
     It is readily apparent that the above-described embodiments have the advantage of wide commercial utility. It should be understood that the specific form of the invention hereinabove described is intended to be representative only, as certain modifications within the scope of these teachings will be apparent to those skilled in the art.