Abstract:
A brake control apparatus is provided with a simulator for applying a biasing force to a manually operated braking member of a vehicle in response to braking operation. A sensor detects conditions of the vehicle including a braking condition of the vehicle. A first biasing device applies a first biasing force to the manually operated braking member in response to braking operation, and a second biasing device applies a second biasing force, as well. Only one of the first and second biasing devices applies the biasing force to the manually operated braking member when a predetermined condition is detected by the sensor, while both of the first and second biasing devices apply the biasing forces in the normal braking operation. The second biasing device may include a cylinder having a bore defined therein, and a piston slidably received in the bore to define a chamber filled with brake fluid, an elastic member for biasing the piston to expand the chamber, and a reservoir communicated with the chamber of the cylinder for storing the brake fluid drained from the cylinder, and controlled by a controller in response to outputs of the sensor.

Description:
This application claims benefit of Provisional No. 60/066,739 filed Nov. 21, 1997. 
     This application claims priority under 35 U.S.C. Sec. 119 to No. 9-192684 filed in Japan on Jul. 17, 1997, the entire content of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a brake control apparatus for controlling a braking force applied to wheels of a vehicle, and more particularly to the brake control apparatus having a stroke simulating function for providing dummy braking load to a vehicle driver. 
     2. Description of the Related Arts 
     An apparatus for providing dummy braking load to a vehicle driver in his braking operation is known as a stroke simulator. For example, German patent publication (Offenlegungsshrift) No. 1961039 discloses an apparatus having the stroke simulating function. In that publication, in order to avoid difficulty in forward movement of the brake pedal in the braking operation, a spring is directly mounted on the brake pedal, and a solenoid valve for use in a braking force control device is controlled to increase or decrease the brake pressure in a wheel brake cylinder of the vehicle in response to the depressing force of the brake pedal, to apply the braking force to each wheel, with the dummy braking load applied to the brake pedal. Also, Japanese Patent Laid-open Publication No. 63-64858 discloses an apparatus which has a solenoid valve disposed between a master cylinder and a wheel brake cylinder mounted on a wheel for selectively changing the communication between the master cylinder and the wheel brake cylinder, and the communication between the master cylinder and an absorbing member for consuming the brake fluid. 
     According to the one having the spring mounted on the brake pedal as disclosed in the above-identified publication, only a fixed braking load may be provided by the biasing force of the spring mounted on the brake pedal. With respect to the above-described apparatus for controlling the solenoid valve to change the communication between the master cylinder and the wheel brake cylinder, or the communication between the master cylinder and the absorbing member, if the solenoid valve is activated, the master cylinder is communicated with the wheel brake cylinder, so that only a fixed braking load may be provided by means of the absorbing member. Thus, any braking loads corresponding to brake feelings determined on the basis of the vehicle conditions and the driver&#39;s taste can not be provided. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a brake control apparatus having a stroke simulator for providing dummy braking loads corresponding to various brake feelings in accordance with conditions of a vehicle and a vehicle driver&#39;s taste. 
     To accomplish the above and other objects, a brake control apparatus is provided with a simulation device for applying a biasing force to a manually operated braking member of a vehicle in response to braking operation of the manually operated braking member. In this apparatus, a detection device is provided for detecting conditions of the vehicle including a braking condition of the vehicle. A first biasing device is provided for applying a first biasing force to the manually operated braking member in response to braking operation of the manually operated braking member, and a second biasing device is provided for applying a second biasing force to the manually operated braking member in response to braking operation of the manually operated braking member. Only one of the first and second biasing devices is adapted to apply the biasing force to the manually operated braking member in response to braking operation of the manually operated braking member, when a predetermined condition is detected by the detection device. 
     Preferably, the second biasing device comprises a cylinder device including a cylinder having a bore defined therein, and a piston slidably received in the bore to define a chamber filled with brake fluid, an elastic device for biasing the piston to expand the chamber, and a reservoir device communicated with the chamber of the cylinder device for storing the brake fluid drained from the cylinder device. 
     The brake control apparatus may further include a valve device disposed between the reservoir device and the cylinder device, and a controller for controlling the valve device in response to output of the detection device. The controller is preferably adapted to activate the valve device to apply the second biasing force to the manually operated braking member in response to braking operation of the manually operated braking member, when the predetermined condition is detected by the detection device. 
     The brake control apparatus according to the present invention may be embodied as follows: 
     First, the apparatus includes a sensor which is provided for detecting conditions of the vehicle including a braking condition of the vehicle, a first elastic member and a second elastic member which are connected in series with the manually operated braking member to be moved thereby. In this apparatus, only one of the first and second elastic members is adapted to provide a biasing force to the manually operated braking member, when a predetermined condition is detected by the sensor. Accordingly, the brake feeling can be changed under the predetermined condition. 
     Secondary, in addition to the above components, the apparatus further includes a cylinder in which brake fluid is filled and at least one of the first and second elastic members is accommodated, a piston slidably received in the cylinder, a reservoir for draining the brake fluid from the cylinder, a solenoid valve which is disposed between the reservoir and the cylinder, and a controller which is provided for controlling the valve in response to outputs of the sensor. The controller is adapted to activate the valve to provide a biasing force to the manually operated braking member by only one of the first and second elastic members, when the predetermined condition is detected by the sensor. Accordingly, the manually operated braking member is biased by both of the first and second elastic members in a normal braking operation, while it is biased only by one of the first and second elastic members under the predetermined condition, so that the brake feeling can be provided in accordance with the vehicle conditions and the vehicle driver&#39;s taste. 
     In the apparatus, the first elastic member may be accommodated in the piston to minimize the apparatus. 
     Thirdly, the apparatus includes a sensor which is provided for detecting conditions of the vehicle including a braking condition of the vehicle, a first elastic member which is connected with the manually operated braking member to be moved thereby, and a master cylinder which is connected with the first elastic member, in series. A first absorbing device is provided for storing the brake fluid from the master cylinder, and provided with a second elastic member which returns the brake fluid to the master cylinder. A solenoid valve is disposed between the first absorbing device and the master cylinder, and a controller is provided for controlling the valve in response to outputs of the sensor. The controller is adapted to activate the valve to provide a biasing force to the manually operated braking member only by the first elastic member, when the predetermined condition is detected by the sensor. Accordingly, the manually operated braking member is biased by both of the first and second elastic members in a normal braking operation, while it is biased only by the first elastic member under the predetermined condition, so that the brake feeling can be provided in accordance with the driver&#39;s taste with a simple structure using a conventional master cylinder. 
     Fourthly, the apparatus includes a sensor which is provided for detecting conditions of the vehicle including a braking condition of the vehicle, a cylinder in which a first elastic members connected with the manually operated braking member to be moved thereby is accommodated, a piston slidably received in the cylinder, a reservoir for draining the brake fluid from the cylinder, a solenoid valve which is disposed between the reservoir and the cylinder, and a master cylinder which is connected with the first elastic member, in series. A first absorbing device is provided for storing the brake fluid from the master cylinder, and provided with a second elastic member which returns the brake fluid to the master cylinder, and a controller is provided for controlling the valve in response to outputs of the sensor. The controller is adapted to activate the valve to provide a biasing force to the manually operated braking member only by the second elastic member, when the predetermined condition is detected by the sensor. Accordingly, the manually operated braking member is biased by both of the first and second elastic members in a normal braking operation, while it is biased only by the second elastic member under the predetermined condition, so that the brake feeling can be provided in accordance with the vehicle conditions and the driver&#39;s taste. 
     Fifthly, the apparatus includes a sensor which is provided for detecting conditions of the vehicle including a braking condition of the vehicle, a master cylinder which is connected with the first elastic member, a first absorbing device for storing the brake fluid from the master cylinder and having therein a first elastic member for returning the brake fluid to the master cylinder, and a second absorbing device for storing the brake fluid from the master cylinder and having therein a second elastic member for returning the brake fluid to the master cylinder. A solenoid valve is disposed between the first absorbing device and the master cylinder, and a controller is provided for controlling the valve in response to outputs of the sensor. The controller is adapted to activate the valve to provide a biasing force to the manually operated braking member only by the second elastic member, when the predetermined condition is detected by the sensor. Accordingly, the manually operated braking member is biased by both of the first and second elastic members in a normal braking operation, while it is biased only by the second elastic member under the predetermined condition, so that the brake feeling can be provided in accordance with the driver&#39;s taste with a simple structure using a conventional master cylinder. 
     Sixthly, the apparatus includes a sensor which is provided for detecting conditions of the vehicle including a braking condition of the vehicle, a master cylinder which is connected with a manually operated braking member in series, and which has first and second pressure chambers, a first absorbing device for storing the brake fluid from the first pressure chamber and having therein a first elastic member for returning the brake fluid to the master cylinder, and a second absorbing device for storing the brake fluid from the second pressure chamber and having therein a second elastic member for returning the brake fluid to the master cylinder. A solenoid valve is disposed between the first absorbing device and the first pressure chamber, and a controller is provided for controlling the valve in response to outputs of the sensor. The controller is adapted to activate the valve to provide a biasing force to the manually operated braking member only by the second elastic member, when the predetermined condition is detected by the sensor. Accordingly, the manually operated braking member is biased by both of the first and second elastic members in a normal braking operation, while it is biased only by the second elastic member under the predetermined condition, so that the brake feeling can be provided in accordance with the driver&#39;s taste with a simple structure using a conventional master cylinder having dual pressure chambers. 
     With respect the sensor, preferably, a depressing force sensor may be employed for detecting a depressing force of the manually operated braking member. A stroke sensor may be employed for detecting a depressing speed of the manually operated braking member. A steering angle sensor may be employed for detecting a steering angle of the vehicle. A pressure sensor may be employed for detecting pressure applied to a wheel brake cylinder or a master cylinder. A wheel speed sensor may be employed for detecting a vehicle speed. Accordingly, the vehicle conditions including braking condition of the vehicle can be detected accurately. Furthermore, a mode switch may be provided for selectively energizing the solenoid valve, so that the brake feeling can be provided in accordance with the driver&#39;s taste. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above stated objects and following description will become readily apparent with reference to the accompanying drawings, wherein like reference numerals denote like elements, and in which: 
     FIG. 1 is a block diagram illustrating a stroke simulator for use in a brake control apparatus according to a first embodiment of the present invention; 
     FIG. 2 is a diagram showing a relationship between a depressing force and a brake pedal stroke in the above embodiment; 
     FIG. 3 is a block diagram illustrating a brake control apparatus according to the first embodiment of the present invention; 
     FIG. 4 is a flowchart showing a main routine of the brake control according to the present invention; 
     FIG. 5 is a flowchart showing a sub-routine for a first example of a stroke simulation executed in the main routine as shown in FIG. 4; 
     FIG. 6 is a diagram showing a relationship between a wheel speed and depressing force to the stroke simulator for use in the first example of the stroke simulation; 
     FIG. 7 is a flowchart showing a sub-routine for a second example of the stroke simulation executed in the main routine as shown in FIG. 4; 
     FIG. 8 is a diagram showing a relationship between a brake pedal speed and a stroke of the stroke simulator to be controlled for use in the second example of the stroke simulation; 
     FIG. 9 is a flowchart showing a sub-routine for a third example of the stroke simulation executed in the main routine as shown in FIG. 4; 
     FIG. 10 is a diagram showing a relationship between a road coefficient of friction and depressing force to the stroke simulator for use in the third example of the stroke simulation; 
     FIG. 11 is a flowchart showing a sub-routine for a fourth example of the stroke simulation executed in the main routine as shown in FIG. 4; 
     FIG. 12 is a diagram showing a relationship between a selected position of a mode switch and depressing force to the stroke simulator for use in the fourth example of the stroke simulation; 
     FIG. 13 is a flowchart showing a sub-routine for a fifth example of the stroke simulation executed in the main routine as shown in FIG. 4; 
     FIG. 14 is a block diagram illustrating a second embodiment of the stroke simulator for use in the brake control apparatus according to the present invention; 
     FIG. 15 is a block diagram illustrating a third embodiment of the stroke simulator for use in the brake control apparatus according to the present invention; 
     FIG. 16 is a block diagram illustrating a fourth embodiment of the stroke simulator for use in the brake control apparatus according to the present invention; 
     FIG. 17 is a block diagram illustrating a fifth embodiment of the stroke simulator for use in the brake control apparatus according to the present invention; and 
     FIG. 18 is a block diagram illustrating a sixth embodiment of the stroke simulator for use in the brake control apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is schematically illustrated a first embodiment of a stroke simulator  1  for use in a brake control apparatus according to the present invention. The stroke simulator  1  includes a cylinder  5 , a piston  6  slidably received therein, a first elastic member  4 , a second elastic member  8 , a solenoid valve  10 , a reservoir  9  for storing brake fluid, a controller  38  for controlling the solenoid valve  10 , and a sensor  20  which is electrically connected to the controller  38 . The sensor  20  represents various sensors such as a steering angle sensor for detecting a steering angle of a vehicle, a depressing force sensor for detecting depressing force applied to a brake pedal  2 , a stroke sensor for detecting depressed amount of the brake pedal or depressing speed thereof calculated on the basis of the depressed amount, pressure sensors for detecting pressures in wheel brake cylinders operatively mounted on the wheels, wheel speed sensors for detecting wheel speeds of the wheels, and etc. 
     The first elastic member  4  and the second elastic member  8  of springs are aligned with a rod  3 , which is connected to the brake pedal  2 . The second elastic member  8  is disposed in a pressure chamber  7  which is defined in the cylinder  5 , and the volume of which is varied in response to sliding movement of the piston  6 . The second elastic member  8  is adapted to bias the piston  6  to its initial position. The reservoir  9  under atmospheric pressure is provided for receiving the brake fluid drained from the pressure chamber  7  through an outlet port in the cylinder  5 . The solenoid valve  10  of a normally open type is disposed between the reservoir  9  and the pressure chamber  7  of the cylinder  5 , and controlled by the controller  38  on the basis of the signals output from the sensor  20 . According to this embodiment, therefore, the depressing force characteristic can be changed from a given position of the brake pedal stroke, as shown in FIG.  2 . 
     The brake control apparatus having the stroke simulator  1  as constituted above will be explained hereinafter with reference to FIG. 3. A braking force control device  42  is provided for controlling the braking force applied to the wheels. The braking force control device  42  is disposed independently from the stroke simulator  1  according to the present embodiment, but it may be associated with the simulator  1 . In order to detect the vehicle condition or the braking condition accurately, various sensors are installed. For example, a steering angle sensor  21  is installed on a steering wheel  17 . A depressing force sensor  22  and a stroke sensor  23  are installed on the brake pedal  2 . A pressure sensor  24  is disposed to detect the pressure in the wheel brake cylinder  31 . Furthermore, a mode switch  26  is provided for changing the brake feeling in accordance with the driver&#39;s selection. The output signals of the sensors are fed to the controller  38 , which is adapted to energize the solenoid valve  10  to provide the stroke of the brake pedal  2  only by means of the biasing force of the first elastic member  4 , when the output signals from the controller  38  satisfy the predetermined conditions as described later in detail. The braking force control device  42  is adapted to supply the brake fluid to the wheel brake cylinders including a wheel brake cylinder  31  as shown in FIG. 3, thereby to apply the braking force to the wheels. The brake fluid is supplied to the wheel brake cylinder  31  from an accumulator  37  which accumulates the brake fluid discharged from a pump  34  driven by a motor  35 . The motor  35  is controlled in dependence upon the pressure of the brake fluid supplied to the wheel brake cylinder  31 , which is detected by a pressure sensor  39 . When the pressure of the brake fluid is decreased to be lower than a certain level, the pump  34  is driven by the motor  35  to introduce the brake fluid stored in a reservoir  36  and discharge the pressurized brake fluid to the accumulator  37 . In the case where the pressure of the brake fluid is to be increased, the pressurized brake fluid accumulated in the accumulator  37  is supplied to the wheel brake cylinder  31  by energizing a normally closed solenoid valve  32 . When the pressure in the wheel brake cylinder  31  is to be reduced, the brake fluid therein is drained into the reservoir  36  by energizing a normally closed solenoid valve  33 . The solenoid valves  32 ,  33  and the motor  35  for driving the pump  34  are controlled by the controller  38  in response to the signals input thereto from the mode switch  26  and sensors  21 ,  22 ,  23 ,  24 ,  25 ,  39  which detect the vehicle conditions and the braking condition. 
     According to the present embodiment as constituted above, a program routine for performing the brake control according to the stroke simulator  1  and the braking force control device  42  is executed in accordance with a flowchart as shown in FIG.  4 . Its program routine starts when an ignition switch (not shown) is turned on, and electric power is fed to the controller  38  from a battery (not shown). At the outset, initialization of the apparatus is made at Step  101  to clear various data stored in the controller  38  and input initial data into a random access memory, or RAM. Then, the program is set at Step  102  to repeat this main routine with a predetermined period. According to the present embodiment, the main routine is repeated in a 6-millisecond cycle, while the period is not limited to 6 milliseconds. At Step  103 , the signals output from the mode switch  26 , sensors  21 ,  22 ,  23 ,  24 ,  25  and etc. are input into the controller  38  through its input ports, and various data are read by a microcomputer (not shown) in the controller  38 . 
     Then, the program proceeds to Step  104  where stroke simulations are performed in accordance with one of the flowcharts as shown in FIGS. 5-13, as will be described later in detail. At Step  105 , a desired wheel cylinder pressure it set in accordance with a map which is provided in advance. The program further proceeds to Step  106  where it is determined whether a signal output from the pressure sensor  24 , i.e., an actual wheel cylinder pressure is equal to the desired wheel cylinder pressure obtained from the map at Step  105 , or not. If the result is affirmative, the program returns to Step  102 . If the result is negative, the program proceeds to Step  107 , where it is determined whether the actual wheel cylinder pressure is greater than the desired wheel cylinder pressure. If the actual wheel cylinder pressure is greater than the desired wheel cylinder pressure, the program proceeds to Step  108 , where the control valve for decreasing the pressure, i.e., pressure decrease solenoid valve  33 , is energized to be on. If the actual wheel cylinder pressure is equal to or smaller than the desired wheel cylinder pressure, the program proceeds to Step  109 , where the control valve for increasing the pressure, i.e., pressure increase solenoid valve  32 , is energized to be on. The solenoid valves  32 ,  33  are controlled to optimize their “on” period in accordance with a difference between the actual wheel cylinder pressure and the desired wheel cylinder pressure. Thus, Steps  102 - 109  are repeated to perform the braking force control. 
     Next, the stroke simulation executed at Step  104  in FIG. 4 will be explained with reference to five examples as follows. FIG. 5 shows a first example of the stroke simulation, where the simulation is determined on the basis of depression of the brake pedal  2 . That is, the stroke simulation is determined on the basis of a condition of a brake switch (not shown), which is turned on when the brake pedal  2  is depressed, and turned off when the brake pedal  2  is not depressed. The brake switch is disposed in the vicinity of the brake pedal  2  to output the on/off signal to the input port of the controller  38 . Unless the brake pedal  2  is depressed at Step  201 , the simulation is not performed. If the brake pedal  2  is depressed, the brake switch is turned on, so that the program proceeds to Step  202 . If the brake pedal  2  continues to be depressed, the program proceeds to Step  203 . At Step  202 , a depressing force to be controlled by the stroke simulator is calculated in accordance with a map as shown in FIG. 6, which is provided in advance for obtaining the depressing force to be controlled by the stroke simulator in response to the vehicle speeds calculated from the wheel speeds obtained by the signals output from the wheel speed sensor  25 . That is, a depressing force (Fp) to be controlled is obtained in response to the vehicle speed. Then, at Step  203 , the actual depressing force detected by the depressing force sensor  22  is compared with the depressing force (Fp) to be controlled. If the actual depressing force is greater than the depressing force (Fp) obtained from the map, the program proceeds to Step  204 , where the solenoid valve  10  is energized to be on. Otherwise, the program proceeds to Step  205 , where the solenoid valve  10  is not energized to be off. According to this routine, if the actual depressing force has become greater than the depressing force to be controlled, which is provided in response to the vehicle speed, the characteristic of the depressing force may be changed from a given position of the brake pedal stroke as shown in FIG. 2, so that an appropriate brake feeling can be provided in response to the vehicle speed. 
     FIG. 7 shows a second example of the stroke simulation, where it is determined at Step  301  whether the brake pedal  2  is depressed, or not. If it is determined at Step  301  that the brake pedal  2  is not depressed, the program proceeds to Step  309 , where an initial value (Sp0) is set for a stroke to be controlled (Sp), and returns to the main routine. On the contrary, if the brake pedal  2  is depressed, and the brake switch is turned on, the program proceeds to Step  302 , where a signal indicative of a pedal stroke output from the stroke sensor  23  is determined. At Step  302 , it is determined whether the pedal stroke detected by the actual sensor is greater than a predetermined value (Th1), or not. If the result is affirmative, the program proceeds to Step  303 . Otherwise, the program returns to the main routine. Then, at Step  303 , a stroke to be controlled by the stroke simulator is calculated for the present cycle, to provide the presently obtained value Sp(n) of the stroke to be controlled (Sp). This value is calculated in accordance with a map as shown in FIG. 8, which is provided in advance for obtaining the stroke to be controlled by the stroke simulator in response to a brake pedal speed which is calculated by dividing the variation of the signals fed from the stroke sensor  23  by the calculating period. Then, at Step  304 , it is determined whether the present value Sp(n) obtained at Step  303  is smaller than the previously obtained value Sp(n−1). If the result is affirmative, the present value Sp(n) is provided for the stroke to be controlled (Sp) at Step  305 , then the program proceeds to Step  306 . Therefore, the value of the stroke to be controlled (Sp) is used for the value Sp(n) in the next cycle. On the contrary, if the present value Sp(n) is equal to or greater than the previous value Sp(n−1), the program proceeds to Step  306 , where an actual stroke detected by the stroke sensor  23  is compared with the stroke to be controlled (Sp). If the actual stroke is greater than the stroke to be controlled (Sp), the program proceeds to Step  307 , where the solenoid valve  10  is energized to be on. Otherwise, the program proceeds to Step  308 , where the solenoid valve  10  is not energized to be off. According to this routine, if the actual pedal stroke has become greater than the stroke obtained from the brake pedal speed, the characteristic of the depressing force may be changed from a given position of the brake pedal stroke as shown in FIG. 2, so that an appropriate brake feeling can be provided in response to the brake pedal speed. 
     FIG. 9 shows a third example of the stroke simulation, where a coefficient of friction of the road (abbreviated as road μ) is determined at Step  401 . That is, the coefficient of friction of the road on which the vehicle is traveling is selected from one of a low-μ, mid-μ and high-μ, by estimating a vehicle speed on the basis of the output of the wheel speed sensor  25  provided for each wheel, and determining a level of drop of each wheel speed on the basis of the estimated vehicle speed. At Step  402 , the depressing force to be controlled by the stroke-simulator is calculated in accordance with a map as shown in FIG. 10, which is provided in advance for obtaining the depressing force to be controlled by the stroke simulator in accordance with the road μ selected at Step  401 . Then, at Step  403 , the actual depressing force detected by the depressing force sensor  22  is compared with the depressing force to be controlled (Fp) which is obtained from the map. If the actual depressing force is greater than the depressing force to be controlled (Fp), the program proceeds to Step  404 , where the solenoid valve  10  is energized to be on. Otherwise, the program proceeds to Step  405 , where the solenoid valve  10  is not energized to be off. According to this routine, if the actual depressing force has become greater than the stroke to be controlled, which is obtained from the road μ, the depressing force characteristic may be changed from a given position of the brake pedal stroke, as shown in FIG. 2 Therefore, an appropriate brake feeling can be provided in accordance with the road μ. 
     FIG. 11 shows a fourth example of the stroke simulation, wherein it is determined at Step  501  whether the brake pedal  2  is depressed, or not. If the brake pedal  2  is depressed, and the brake switch is turned on, then the program proceeds to Step  503 . If the brake pedal  2  is not depressed, the program proceeds to Step  502 , where the depressing force to be controlled by the stroke simulator is calculated in accordance with a map as shown in FIG. 12, which is provided in advance for setting the depressing force to be controlled by the stroke simulator in accordance with a position of the mode switch  26  for changing the braking characteristic. The mode switch  26  is a multistage changeover switch, e.g., 4 stages in this example, to change the brake feeling. Then, at Step  503 , the actual depressing force detected by the depressing force sensor  22  is compared with the depressing force to be controlled (Fp) which is obtained from the map. If the actual depressing force is greater than the depressing force to be controlled (Fp), the program proceeds to Step  504 , where the solenoid valve  10  is energized to be on. Otherwise, the program proceeds to Step  505 , where the solenoid valve  10  is not energized to be off. According to this routine, if the actual depressing force has become greater than the stroke obtained in accordance with the selected position of the mode switch  26 , the depressing force characteristic may be changed from a given position of the brake pedal stroke as shown in FIG.  2 . Therefore, an appropriate brake feeling can be provided in accordance with the vehicle driver&#39;s taste. 
     FIG. 13 shows a fifth example of the stroke simulation, wherein it is determined at Step  601  whether a brake pressure control, such as an anti-skid control, a stability control, or the like is being performed, or not. If the brake pressure control is being performed, the program proceeds to Step  602 , where the solenoid valve  10  is energized to be on. Otherwise, the program proceeds to Step  603 , where the solenoid valve  10  is not energized to be off. According to this routine, if the brake pressure control is being performed, the depressing force characteristic may be changed from a given position of the brake pedal stroke, as shown in FIG.  2 . Therefore, an appropriate brake feeling can be provided in the brake pressure control. 
     Next, other embodiments of the stroke simulator  1  will be explained with reference to FIGS. 14-18. FIG. 14 illustrates a second embodiment of the simulator  1 . The brake pedal  2  is connected to the rod  3 , which is connected to one end of the first elastic member  4 . A piston  61  formed with a recess  62  for receiving the other end of the first elastic member  4  is slidably fitted into a cylinder  51 . A pressure chamber  71  is defined between the piston  61  and the cylinder  51 . The second elastic member  8  is disposed in the pressure chamber  71  to be placed in series with the first elastic member  4 , and adapted to bias the piston  61  to its initial position where the brake pedal  2  is not depressed. 
     The pressure chamber  71  is communicated with the reservoir  9  through an output port, from which the brake fluid is discharged in response to forward movement of the piston  61 . 
     The solenoid valve  10  of a normally open type is disposed between the reservoir  9  and the output port of the cylinder  51 , and controlled by the controller  38  on the basis of the signals output from the sensor  20 . According to this embodiment with the first elastic member  4  accommodated in the recess  62  of the piston  61 , therefore, the stroke simulator  1  can be made small in its axial direction. In operation, if the brake pedal  2  is depressed when the solenoid valve  10  is off, the brake pedal  2  is moved against the biasing force of the first elastic member  4  and second elastic member  8 . When the solenoid valve  10  is turned on, the communication between the pressure chamber  71  and the reservoir  9  is shut off, so that the volume of the pressure chamber  71  is not varied. That is, the sliding movement of the piston  61  is stopped, and the brake pedal  2  is moved only against the biasing force of the first elastic member  4 . Therefore, the braking characteristic can be changed by energizing the solenoid valve  10  at a proper time when a predetermined condition is satisfied. 
     FIG. 15 illustrates a third embodiment of the simulator  1 , wherein the brake pedal  2  is connected to one end of the rod  3 , which is connected to one end of the first elastic member  4 . The other end of the rod  3  is connected to a master piston  13   a  of a master cylinder  13 . A pressure chamber  14  is defined in the master cylinder  13 , and its volume is varied in response to sliding movement of the master piston  13   a . A first absorbing device  11  is connected to the pressure chamber  14  to absorb the brake fluid discharged from the pressure chamber  14 , and return the brake fluid thereto by means of biasing force of a second elastic member  81 . That is, the first absorbing device  11  includes the second elastic member  81  which is accommodated in a housing of the device  11 , and which is adapted to bias the brake fluid to be discharged into the pressure chamber  14  of the master cylinder  13 . The normally open solenoid valve  10  is disposed between the master cylinder  13  and the first absorbing device  11 , and controlled by the controller  38  on the basis of the signals output from the sensor  20 . In operation, if the brake pedal  2  is depressed when the solenoid valve  10  is off, the brake pedal  2  is moved against substantially the total of biasing forces of the first elastic member  4  and second elastic member  81 , because a spring accommodated in the master cylinder  13  has a relatively small biasing force. When the solenoid valve  10  is turned on, the communication between the pressure chamber  14  and the first absorbing device  11  is shut off, so that the volume of the pressure chamber  14  of the master cylinder  13  is not varied. In this case, only the biasing force of the first elastic member  4  acts against depression of the brake pedal  2 . Therefore, the braking characteristic can be changed by energizing the solenoid valve  10  at a proper time when a predetermined condition is satisfied. 
     FIG. 16 illustrates a fourth embodiment of the simulator  1 , wherein the brake pedal  2  is connected to one end of the rod  3 , which is connected to a cylinder  51  and a master cylinder  13 , in series. One end of the rod  3  is connected to the piston  6  which is slidably received in the cylinder  51 , in which a first elastic member  41  is provided for biasing the brake pedal  2  to the initial position thereof. The other end of the rod  3  is connected to the master piston  13   a  in the master cylinder  13 . A pressure chamber  71  is defined in the cylinder  51 , and connected to the reservoir  9  to drain the brake fluid in the pressure chamber  71 . Then, the normally open solenoid valve  10  is disposed between the pressure chamber  71  and the reservoir  9 , and controlled by the controller  38  on the basis of the signals output from the sensor  20 . The first absorbing device  11  is connected to the pressure chamber  14  to absorb the brake fluid discharged from the pressure chamber  14 , and return the brake fluid thereto. The second elastic member  81  is disposed in the first absorbing device  11  to return the brake fluid therein to the master cylinder  13 . In operation, if the brake pedal  2  is depressed when the solenoid valve  10  is off, the brake pedal  2  is moved against substantially the total of biasing forces of the first elastic member  41  and second elastic member  81 , because the spring accommodated in the master cylinder  13  has a relatively small biasing force. When the solenoid valve  10  is turned on, the communication between the pressure chamber  71  and the reservoir  9  is shut off, so that the volume of the pressure chamber  71  of the cylinder  51  is not varied. In this case, only the biasing force of the second elastic member  81  acts against depression of the brake pedal  2 . Therefore, the braking characteristic can be changed by energizing the solenoid valve  10  at a proper time when a predetermined condition is satisfied. 
     FIG. 17 illustrates a fifth embodiment of the simulator  1 , wherein the brake pedal  2  is connected to one end of the rod  3 , the other end of which is connected to the master piston  13   a  in the master cylinder  13 . The first absorbing device  11  is connected to the pressure chamber  14  to absorb the brake fluid discharged from the pressure chamber  14 , and return the brake fluid thereto by means of the biasing force of the first elastic member  42 . In addition, a second absorbing device  16  is connected to the pressure chamber  14  to absorb the brake fluid discharged from the pressure chamber  14 . The second absorbing device  16  has a second elastic member  82  therein to return the brake fluid to the master cylinder  13 . The normally open solenoid valve  10  is disposed between the master cylinder  13  and the first absorbing device  11 , and controlled by the controller  38  on the basis of the signals output from the sensor  20 . In operation, if the brake pedal  2  is depressed when the solenoid valve  10  is off, the brake pedal  2  is moved against substantially the total of biasing forces of the first elastic member  42  and second elastic member  82 , because the spring accommodated in the master cylinder  13  has a relatively small biasing force. When the solenoid valve  10  is turned on, the communication between the pressure chamber  14  and the first absorbing device  11  is shut off, so that only the biasing force of the second elastic member  82  acts against depression of the brake pedal  2 . Therefore, the braking characteristic can be changed by energizing the solenoid valve  10  at a proper time when a predetermined condition is satisfied. 
     FIG. 18 illustrates a sixth embodiment of the simulator  1 , wherein a master cylinder  15  having a first pressure chamber  18  and a second pressure chamber  19  defined therein, and a first piston  15   a  and a second piston  15   b  slidably received in the first and second pressure chambers  18 ,  19 , respectively. The brake pedal  2  is connected to one end of the rod  3 , the other end of which is connected to the first piston  15   a . The first absorbing device  11  is connected to the first pressure chamber  18  to absorb the brake fluid discharged from the first pressure chamber  18 , and return the brake fluid thereto. A first elastic member  42  is disposed in the first absorbing device  11  to return the brake fluid therein to the master cylinder  13 . Also, the second absorbing device  16  is connected to the second pressure chamber  19  to absorb the brake fluid discharged from the second pressure chamber  19 , and return the brake fluid thereto. A second elastic member  82  is disposed in the second absorbing device  11  to return the brake fluid therein to the master cylinder  13 . Then, the normally open solenoid valve  10  is disposed between the first pressure chamber  18  of the master cylinder  15  and the first absorbing device  11 , and controlled by the controller  38  on the basis of the signals output from the sensor  20 . In operation, if the brake pedal  2  is depressed when the solenoid valve  10  is off, the brake pedal  2  is moved against substantially the total of biasing forces of the first elastic member  42  and second elastic member  82 , because both of the springs accommodated in the master cylinder  15  have a relatively small biasing force. When the solenoid valve  10  is turned on, the communication between the first pressure chamber  18  and the first absorbing device  11  is shut off, so that only the biasing force of the second elastic member  82  acts against depression of the brake pedal  2 . Therefore, the braking characteristic can be changed by energizing the solenoid valve  10  at a proper time when a predetermined condition is satisfied. 
     In the embodiments as shown in FIGS. 14-18, a check valve (not shown) may be provided for bypassing the solenoid valve  10 . The check valve  10  is adapted to prevent the brake pedal  2  from being delayed in its returning motion when the brake pedal  2  is released rapidly while the solenoid valve  10  is on, due to delayed response of the valve  10  when it ifs turned off. It should be apparent to one skilled in the art that the above-described embodiments are merely illustrative of but a few of the many possible specific embodiments of the present invention. Numerous and various other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention as defined in the following claims.