Abstract:
A method and program product are associated with a brake mechanism. The brake mechanism includes a brake operable to restrict movement of a vehicle and an actuator coupled to the brake. The actuator is operable to selectively apply and release the brake. The mechanism further includes a controller coupled to the actuator. A method, implemented in the controller and program code, initializes the brake mechanism and calibrates and/or performs diagnostics on the brake mechanism.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to brake system, and more particularly, to method, apparatus, and program product for controlling a parking brake.  
       BACKGROUND OF THE INVENTION  
       [0002]     Most vehicle designs incorporate parking brakes. Typical parking brake configurations continuously employ regular drum brakes on a rear wheel. Parking brakes commonly rely on simple mechanical linkage to engage the brakes. The driver may simply pull a lever which is coupled to a brake cable which actuates the brakes. To release the brake, a button is pressed while lifting and releasing the lever. For these types of parking brakes, there may be a relatively large amount of “play” in the brake cable, i.e., a relatively large range of motion of the lever and brake cable may be required in order to supply sufficient braking force to retain the vehicle in place. This is generally satisfactory, since the drive may simply lift the lever until sufficient force has been applied.  
         [0003]     However, in some systems, the parking brake is engaged electronically. The driver may simply depress a pedal, lever, button or other suitable means, which sends a signal to a controller or actuator which engages the brake.  
         [0004]     In these type of systems, since the brake is automatically actuated, it is important to know when a target force is being applied to the wheel(s), such that the vehicle is retained in its current position. Some systems accomplish this by using a force sensor which measure the force being applied by the brake. The brake or brake actuator may therefore be controlled using closed loop forced feedback.  
         [0005]     However, such sensors add cost to the system. And harsh environmental factors, such as temperature variation and moisture, reduce the reliability and accuracy of the sensors. Additional circuitry may be used to compensate for the drift and sensitivity variations caused by the factors, however, this again adds cost and complexity to the system.  
         [0006]     The present invention is aimed at one or more of the problems identified above.  
       SUMMARY OF THE INVENTION  
       [0007]     In a first aspect of the present invention, a method for calibrating a brake mechanism having a brake coupled to an actuator is provided. The actuator includes a motor and is controlled through rotations of the motor. The motor includes the steps of initializing the brake mechanism, applying a predetermined power level to the actuator, establishing motor stall and responsively determining a reference motor position, and establishing a home motor position as a function of the second position and a predetermined constant.  
         [0008]     In a second aspect of the present invention, a brake mechanism, is provided. The brake mechanism includes a brake operable to restrict movement of a vehicle and an actuator coupled to the brake. The actuator is operable to selectively apply and release the brake. The mechanism further includes a controller coupled to the actuator. The controller is operable to initialize the brake mechanism and apply a predetermined power level to the actuator, to establish motor stall and responsively determine a reference motor position, and to establish a home motor position as a function of the second position and a predetermined constant.  
         [0009]     In a third aspect of the present invention, a program product for calibrating a brake mechanism having a brake coupled to an actuator is provided. The actuator includes a motor and is controlled through rotations of the motor. The program product includes program code means for initializing the brake mechanism, for applying a predetermined power level to the actuator, for establishing motor stall and responsively determining a reference motor position, and for establishing a home motor position as a function of the second position and a predetermined constant.  
         [0010]     In a fourth aspect of the present invention, a method for providing diagnostics for a brake mechanism having a brake coupled to an actuator is provided. The actuator has a motor and is controlled through rotations of the motor. The method includes the steps of establishing a current motor position, incrementing power to the motor to achieve a target position, and determining the power required to move motor to the target position when the motor has reached the target position. The method also includes the step of determining if the required power is outside of a predetermined range.  
         [0011]     In a fifth aspect of the present invention, a brake mechanism is provided. The brake mechanism includes a brake operable to restrict movement of a vehicle and an actuator, having a motor, coupled to the brake. The actuator is operable to selectively apply and release the brake. The mechanism also includes a controller coupled to the actuator and being operable to establish a current motor position, increment power to the motor to achieve a target position, and determine power required to incrementally move the motor has and to determine if the required power is outside of a predetermined range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
         [0013]      FIG. 1  is a block diagram that illustrates a brake system environment consistent with the principles of the present invention.  
         [0014]      FIG. 2  is a graph representing forces incident on the actuator of  FIG. 1  versus displacement of the actuator.  
         [0015]      FIG. 3A  is a first portion of a flowchart that embodies steps suited for implementation within the brake system environment of  FIG. 1 .  
         [0016]      FIG. 3B  is a second portion of the flowchart of  FIG. 3A .  
         [0017]      FIG. 4  is a second graph representing forces incident on the actuator of  FIG. 1  versus distance of the actuator.  
         [0018]      FIG. 5A  is a first portion of a second flowchart that embodies steps suited for implementation within the brake system environment of  FIG. 1 .  
         [0019]      FIG. 5B  is a second portion of the flowchart of  FIG. 5A . 
     
    
     DETAILED DESCRIPTION  
       [0020]     The block diagram of  FIG. 1  illustrates a brake mechanism  10  that is consistent with the principles of the present invention. The brake system  10  employs position control functions to regulate the actuation and release of a brake  20 , such as a parking brake. Generally, a controller  12  may execute a combined load/position algorithm configured to control the movement of an actuator  14 . The actuator  14  is coupled to the brake  20  and may be configured to selectively actuate and release brake  20  in response to a command or command signals. The brake  20  is operable to restrict movement of a vehicle (not shown). As such, the travel of the actuator  14  causes a force to be transferred to the brake  20 .  
         [0021]     The algorithm controlling the movement of the actuator  14  includes calibration and diagnostic routines which may account for variations within the brake mechanism  10  and for determining when a target load is being applied by the brake  20 .  
         [0022]     In one embodiment, the brake  20  is a disc brake which is directly coupled to the actuator  14 . As such, the travel of the actuator  14  causes a force to be transferred directly to the brake  20 .  
         [0023]     In another embodiment, the brake  20  is a drum brake. The actuator  14  is connected to a brake cable (not shown). The brake cable, in turn, is coupled to the brake  20  via a brake lever (not shown). In one embodiment, the brake lever is operable to actuate the drum brakes/calipers of, for example, the rear brake  20  of a vehicle (not shown). The brake is operable to restrict movement of the vehicle. As such, the travel of the actuator  14  causes a force to be transferred to the brake lever via the cable.  
         [0024]     An operator may initiate actuation or release of the brake  20  through actuation of a control device  22 , such as a button and/or lever. The control device  22  may transmit an actuation and/or release signal to the controller  12 . The controller  12  may include a computer, central processing unit, microprocessor or other suitable control device.  
         [0025]     In general, the controller  12 , in response to the actuation and/or release signal, may initiate processing of a position feedback control program (or program product) resident in the controller  12 . The program may instruct the controller  12  to transmit a command to a motor in the actuator  14 . In addition to the motor, the actuator  14  may incorporate a position sensor, a power screw and a gear set (not shown) for gaining mechanical advantage. In response to the command, the actuator  14  may travel in directions along an axis of the actuator  14 . Alternatively, it will be appreciated that movement of the actuator  14  may occur in any direction corresponding to an increase or decrease of brake force. This movement of the actuator  14  is accomplished according to a position feedback control program.  
         [0026]     In the illustrated embodiment, the position feedback control program requires is based on a home position, i.e., the zero force or drag position, at which no force is exerted by the brake  20 . In one aspect of the present invention, the controller  12  implements a calibration routine under power-up, e.g., when the vehicle&#39;s engine is started. The calibration routine is aimed at determining the zero force or drag position of the actuator  14 . In the illustrated embodiment, this zero force position is defined in terms of a (rotary) motor position within the actuator  14 . For example, the rotary motor position may be defined in terms of turns (counts) of the motor.  
         [0027]     With reference to  FIG. 2 , an exemplary force/displacement curve for illustrating operation of the calibration routine is shown. The zero force position is labeled X 2 . The two forces, F REF  and F 1 , are within the linear force/displacement region  24  of the actuator  14 . F REF  and F 1  are predetermined values. In one embodiment, F REF  is defined as the force at which the motor will stall, i.e., rotational velocity equal to zero and F 1  is the defined as approximately the force required to hold the vehicle on a 20% grade.  
         [0028]     Generally, the calibration routine determines the home motor position (X 2 ) as a position or count of the motor by establishing the motor position at which the actuator  14  exerts a force equal to F REF  and then, using the known nominal characteristics of the actuator  14 , establishing X 2 .  
         [0029]     With particular reference to  FIGS. 3A and 3B , a method  26  for calibrating and providing start-up diagnostics for the brake mechanism  10  according to an embodiment of the present invention is shown. In a first step  26 , the brake mechanism  10  is initialized and an initial position (X 0 ), i.e., count, of the motor is established.  
         [0030]     Next, a predetermined power level is applied to the actuator  14 . In the illustrated embodiment, the actuator  14  is controlled via a pulse width modulated (PWM) signal in a conventional manner. The actual power applied to the actuator  14  will be controlled by the duty cycle of the PWM signal and the supply or bus voltage. Thus, in a second step  30 , the bus voltage is measured. Based on the measured bus voltage, an open loop power value, i.e., PWM duty cycle, is calculated in a third step  32  to achieve the predetermined power level. In a fourth step  34 , the power is applied to the actuator  14  through application of the PWM signal to the motor.  
         [0031]     Then, motor stall is established. In a first decision block  36 , if the motor has stalled, i.e., rotation velocity equals zero (as established via the position sensor), then control proceeds to a fifth step  40 . If motor stall has not been established control proceeds back to the first decision block  36  via sixth step  38 .  
         [0032]     Once motor stall has been established, the reference position X 1  is determined in the fifth step  40 .  
         [0033]     In another aspect of the present invention, the method  26  may perform a diagnostic as a function of X 1  to determine if the brake mechanism  10  has a retained load. In a second decision block  40 , the difference between the initial position (X 0 ) and the reference motor position (X 1 ) is compared with a predetermined minimum value (min). If the difference is less than or equal to the predetermined minimum value, then a signal may be generated in a seventh step  44 , e.g., a flag may be set and/or an indicator light may be turned on. The signal may be indicative of a retained load.  
         [0034]     In an eighth step  46 , the home motor position is established as a function of the second position (X 1 ) and a predetermined constant (A 0 ) by the equation: 
 
 X   2   =X   1   −A   0 . 
 
         [0035]     The predetermined constant, A 0 , is determined as a function of the nominal characteristics of the brake mechanism  10  and is expressed in turns or counts of the motor.  
         [0036]     In one embodiment of the present invention, in a ninth step  49  a predetermined number of home position values may be stored, e.g., in a stack, and averaged to determine an average home position value. This average home position value may be used in the position feedback control algorithm used to control the parking brake mechanism  10  in response to user actuation of the control device  22 . After the home position (X 2 ) has calculated, another diagnostic test may be performed. In a tenth step  50 , the controller  12  switches to closed loop position control. In an eleventh step  52 , a target position or target motor position (X 3 ), which in the illustrated embodiment corresponds to F 1 , is determined as a function of at least one of the reference position and the home motor position and a second predetermined constant (A 1 ). For example, the target position, X 3 , may be determined by the equation: 
        X 3 =X 1 +A 1 , where A 1  is a determined as a function of the nominal characteristics of the brake mechanism  10 .        
 
         [0038]     In a twelfth step  54 , command signals are generated to the motor to move to the target position. As discussed above, in the illustrated embodiment, the command signals are PWM signals. In a third decision block  56 , if the target position has not been achieved, control returns to the twelfth step  54 . Otherwise, control proceeds to a thirteenth step  60 .  
         [0039]     During the loop defined by the twelfth step  54  and the third decision block  56 , the command signals, i.e., the PWM signal levels required to move the motor from X 3 , are monitored, and if excessive, an error signal is generated.  
         [0040]     Once, the target position has been reached, commands signals are generated to the motor to move to the home position, X 2  in the thirteenth step  60 . In a fourth decision block  62 , if the home position has not been achieved, control returns to the thirteenth step  60 . Otherwise, control proceeds to a fourteenth step  64 . In the fourteenth step  64 , a confirmation signal is generated.  
         [0041]     In another aspect of the present invention, a steady-state diagnostic algorithm may be provided. With reference  FIGS. 5A and 5B , the steady-state diagnostic algorithm or method  66  is implemented only when the brake mechanism  10  is in a steady-state condition.  
         [0042]     In a decision block  68 , if a steady-state condition does not exist the method  66  proceeds to a first step  70  and returns to the normal operating mode. In one embodiment of the present invention, a steady-state condition is defined by either a zero position error or zero velocity of the motor.  
         [0043]     If the steady-state condition exists, the method  66  proceeds to a second decision block  72 . With reference to  FIG. 4 , the current motor position is established using the position sensor and, if the current position (X N ) of the motor/actuator  14  is within the linear operating range  24  of the actuator  14 , then the method proceeds to a second step  74 . F′ 1  is the force corresponding to the X N  on the force/position curve. Otherwise, the method  66  proceeds to the first step  70 .  
         [0044]     In the second step  74 , the bus or supply voltage to the motor is measured and, in a second step, the command signals to the motor are incremented to increase power to the motor to achieve a target position (X N+M ). In one embodiment, as discussed above, the command signals are in the form of PWM signals.  
         [0045]     In one embodiment, the target position X N+M  is calculated using the equation: X N+M =X N +M, where M is a number of turns of the motor, e.g., one.  
         [0046]     In a fourth step  78 , the duty cycle of the PWM command signals are monitored. In a third decision block  80 , if the commanded or target position has not been reached, then control returns to the third step  76 . Otherwise, the method  66  proceeds to a fifth step  82 . In the fifth step  82 , the power required to move motor to the target position is calculated based on the change in the duty cycle of the PWM command signals.  
         [0047]     In a fourth decision block  82 , if the calculated required power (to move from X N  to X N+M ) is within a predetermined power range, then control proceeds to a sixth step  86 . If the calculated required power is within the predetermined power range, i.e., is acceptable, this may be indicative of an acceptable home position (see above), acceptable efficiency within the brake mechanism  10 .  
         [0048]     If the required power is outside the predetermined power range, then the method proceeds to either of seventh step  88  or an eighth step  90 . In the seventh step  88 , the required power is below the predetermined power range which may be indicative of an improper home position (see above) or other actuator non-compliance. In the eighth step  90 , the required power is above the predetermined power range which may be indicative of a low efficiency in the brake mechanism  10  and/or decreased actuator compliance, due, for example, to reduced brake pad thickness.  
         [0049]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims.