Abstract:
A method for controlling an electric brake system having a piston moveable by an electric motor is provided. The method includes the steps of storing data for correlating a position of the piston to a braking force applied by the piston, estimating a braking force associated with a specific position of the piston based upon the stored data, generating a control signal based upon the estimated braking force, determining a second braking force associated with the specific position and updating the stored data based upon a difference between the estimated braking force and the second braking force.

Description:
“This invention was made with Government support under Agreement F33615-03-3-2308 awarded by the Air Force Research Laboratory—Wright Patterson AFB, Ohio. The Government has certain rights in the invention.” 
   The subject matter described herein was created during the performance of a cooperative research and development agreement with the Department of the Air Force (Contract No. F33615-03-2308 P00002). Therefore, the government of the United States may have certain rights to the claimed subject matter. 

   BACKGROUND 
   The present application is directed to control systems and, more particularly, to adaptive control systems for electric brake systems and the like. 
   Electric brake systems have been developed for use in the automotive, aerospace and aeronautical industries to control the speed, stability and operation of various vehicles and devices. Electric brake systems, commonly referred to as brake-by-wire systems, have be used in combination with, or in place of, conventional hydraulic brake systems. 
   A typical electric brake system includes an electric motor adapted to advance a piston into engagement with brake pads and/or a rotor, thereby generating a braking force. The amount of braking force generated typically is a function of the distal advancement of the piston. Therefore, the braking force may be controlled by controlling the operation of the electric motor. 
   The braking force applied by an electric brake system typically is controlled by monitoring the force exerted by the piston and/or the position of the piston and controlling the electric motor based upon the force and/or position signals to achieve the desired braking result. The force may be monitored directly using force gauges or the like, thereby providing a direct indication of the braking force. The position of the piston may be monitored using various sensors such that the displacement of the piston may be converted into a force signal by, for example, modeling the brake system as a spring and multiplying the piston displacement by a spring constant. 
   Using the direct force signal may be advantageous because it is the command typically generated by the system level control software. However, the force signal typically has a high signal to noise ratio and therefore may require significant filtering, resulting in a slower response time and reduced performance. Therefore, it may be preferable to used the position signal for controlling the brake system. 
   Attempts have been made to determine the braking force based upon the piston position input using a static look-up table that estimates the force based upon the position input. However, such systems do not account for the dynamics within the brake unit and the inevitable wear of the components of the systems (e.g., the brake pads) and therefore may provide inaccurate results. 
   Accordingly, there is a need for an adaptive control system for controlling the braking force in an electric brake system based upon a piston position signal. 
   SUMMARY 
   In one aspect, a method for controlling an electric brake system having a piston moveable by an electric motor is provided and includes the steps of storing data for correlating a position of the piston to a braking force applied by the piston, estimating a braking force associated with a specific position of the piston based upon the stored data, generating a control signal based upon the estimated braking force, determining a second braking force associated with the specific position and updating the stored data based upon a difference between the estimated braking force and the second braking force 
   In another aspect, a method for controlling an electric brake system having a piston moveable by an electric motor is provided and includes the steps of storing a look-up table in a database, the look-up table including at least two data points correlating a position of the piston to a braking force applied by the piston, determining a specific position of the piston, estimating a braking force associated with the specific position of the piston based upon the data points stored in the look-up table, generating a control signal based upon the estimated braking force, determining a second braking force associated with the specific position and updating at least one of the data points in the look-up table based upon a difference between the estimated braking force and the second braking force. 
   In another aspect, a control system for an electric brake system is provided and includes a piston, an electric motor operatively connected to the piston, wherein rotation of the motor is translated into advancement of the piston, a controller in communication with the motor, the controller including data values adapted to correlate a position of the piston into an associated braking force, a position sensor for monitoring the position of the piston, the position sensor being adapted to generate a position signal and communicate the position signal to the controller and a second sensor adapted to determine a second braking force value and communicate the second braking force value to the controller, wherein the controller is adapted to determine a first braking force value based upon the position signal and the stored data and generate a control signal based upon the first braking force value, and wherein the controller is adapted to update the stored data based upon a difference between the first braking force and the second braking force. 
   Other aspects will become apparent from the following description, the accompanying drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of an electric brake system including an adaptive control system; 
       FIG. 2  is a block diagram of the electric brake system of  FIG. 1 ; 
       FIG. 3  is a block diagram of one aspect of the adaptive control system of  FIG. 1 ; 
       FIG. 4  is a graphical illustration of a membership function according to one aspect of the adaptive control system of  FIG. 3 ; 
       FIG. 5  is a graphical illustration of a membership function according to a second aspect of the adaptive control system of  FIG. 3 ; and 
       FIG. 6  is a graphical illustration of the operation of the adaptive control system of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , an electric brake system, generally designated  10 , may include a caliper housing  12 , an electric motor  14 , an actuator or piston  16 , two brake pads  18 ,  20  and a rotor  22 . The brake system  10  may include a ball screw assembly and a gear train (not shown) that may translate the rotational force of the motor  14  into distal advancement of the piston  16 , thereby urging the piston  16  linearly into engagement with the brake pads  18 ,  20  to apply a braking force to the rotor  22 . 
   As shown in  FIGS. 1 and 2 , a controller  24  may be in communication with the brake system  10  for controlling the advancement and retraction of the piston  16 , thereby controlling the resulting braking force applied to the rotor  22 . In one aspect, the controller  24  may include a processor (not shown), such as a computer processor, and may be in communication with a force sensor  26  and a position sensor  28 . 
   The force sensor  26  may directly monitor the braking force applied to the rotor  22  (see line  27 ) and may communicate force signals to the controller  24  by, for example, communication line  30 . The position sensor  28  may monitor the position of the piston  16  relative to the housing  12 , the brake pads  18 ,  20  and/or the rotor  22  (see line  29 ) and may communicate position signals to the controller  24  by, for example, communication line  32 . 
   In one aspect, the controller  24  may generate a control signal for controlling the operation of the motor  14  based upon the signals received from the sensors  26 ,  28  and may communicate a control signal to the brake system  10  by way of communication line  34 . The control signal may be generated according to the adaptive control system described in greater detail below. The control signal may be communicated directly to the motor  14  or to any appropriate portion of the brake system  10 . 
   Those skilled in the art will appreciate that the communication of signals and commands, as described herein, may be performed over physical communication lines (e.g., wires) or wirelessly. Furthermore, the communication of signals may be performed within a single device or between multiple devices. 
   As shown in  FIG. 3 , one aspect of the adaptive control system, generally designated  40 , may include a database or adaptive table  42 , an adaptation gain block  44 , a robustification deadzone block  46  and a summing block  48 . The system  40  additionally may include two inputs and one output: the position signal input  50 , the force signal input  52  and the estimated force output  54 . The estimated force output  54  may be used by the controller  24  for generating a control signal (see line  34  in  FIG. 2 ) for controlling the brake system  10 . 
   The adaptive table  42  may receive the position signal input  50  from the position sensor  28  and may correlate the position signal, using a look-up table, equation or the like, into the estimated force output  54 . The adaptive table  42  may include a discrete number of data points such that output values may be obtained by interpolating between the data points. 
   The estimated force may be communicated to the summing block  48  by line  56  and the summing block  48  may determine a difference value (i.e., an error signal) between the estimated force output  54  and the force signal input  52 . For example, the error signal may be generally equal to the force signal input  52  minus the estimated force output  54 . 
   The force signal input  52  may be based upon signals received from the force sensor  26 . For example, the force sensor  26  may be piezo device or the like. However, those skilled in the art will appreciate that the force signal input  52  may be based upon any available force measurement or an estimate of force, such as a high precision force estimate. For example, the force signal input  52  may be derived from measurements of the motor speed or motor current, as described in U.S. Ser. No. 11/235,392 filed on Sep. 26, 2005, the entire contents of which are incorporated herein by reference. 
   The output of the summing block  48  may be passed to the deadzone block  46  by line  58 . The deadzone block  46  may be provided to filter error signals that are less then a predetermined minimum threshold value prior to communicating the error signals to the adaptation gain block  44  (e.g., by way of line  60 ) or directly to the adaptive table  42 . For example, if the absolute value of the error signal is less than N, wherein N is the predetermined minimum threshold value, then the output of the deadzone block  46  may be zero, or some other value. If the absolute value of the error signal is greater than or equal to N, then the error signal may be passed along unchanged. 
   The system  40  may include a gain block  44  which may apply a gain G to the error signal and may communicate the modified error signal (i.e., (Error Signal)*G) to the adaptive table  42 , by way of line  62 , as a second input to the adaptive table  42 . The gain G may be selected based upon the desired properties of the adaptive table  42 . Foe example, the gain G may be selected based upon the desired speed at with the adaptive table  42  is to be updated. In one aspect, the gain G may be a numeric value less than 1, such as, for example, 0.1 or 0.2. 
   In one aspect, the adaptive table  42  may update the data values (or other correlating values) based upon the error signal. In another aspect, the data values may be updated based upon the modified error signal. 
   Accordingly, in one aspect, the data values in the adaptive table  42  may be updated each time the error signal is greater than or equal to N. Alternatively, the data values may be updated each time the error signal is not zero. 
   For clarity, reference will be made to the data values provided at Table 1, which includes six discrete input values (i.e., position values), see block  50 , having six corresponding output values (i.e., estimated force values), see block  54 : 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Position Signal Input (units) 
               Estimated Force Output (units) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               0 
               5 
             
             
                 
               0.1 
               10 
             
             
                 
               0.2 
               15 
             
             
                 
               0.3 
               20 
             
             
                 
               0.4 
               25 
             
             
                 
               0.5 
               30 
             
             
                 
                 
             
           
        
       
     
   
   The position values may be based upon the displacement of the piston and may have various units, such as inches, millimeters or the like. The estimated force values may be estimates of braking force that correspond to values of piston displace and may have various units, such as Newtons, dynes or the like. Furthermore, those skilled in the art will appreciate that the adaptive table  42  may have any number of data points and the data points may be scattered or organized in various ways. 
   In one aspect, as shown in  FIGS. 4 and 5 , a plurality of membership functions (i.e., “f(input)”) may be provided, wherein each membership function may correspond to a discrete input value from the adaptive table  42 . The membership functions may be linear and/or finite, as shown in  FIG. 4 , or non-linear and/or infinite, as shown in  FIG. 5 . For example, functions  69 ,  79  may correspond to input value 0, functions  70 ,  80  may correspond to input value 0.1, functions  72 ,  82  may correspond to input value 0.2, functions  74 ,  84  may correspond to input value 0.3, functions  76 ,  86  may correspond to input value 0.4 and functions  78 ,  88  may correspond to input value 0.5. The membership functions may provide an indication of how each discrete data point in the adaptive table  42  is effected by the error signal. 
   Accordingly, in one aspect, for each non-zero signal passed from the deadzone block  46 , the output values in the adaptive table  42  may be updated by multiplying the membership function for the corresponding input value by the error signal or the modified error signal and adding the product to the previous (i.e., not updated) output value, as shown by Eq. 1:
 
Output(input)′=Output(input)+ f (input)*(Error Signal)*Gain  (Eq. 1)
 
wherein Output(input) is the output value corresponding to a specific input value before the most recent update and Output(input)′ is the updated output value.
 
   Applying Eq. 1 to the six data points in Table 1, the following Eqs. 2-7 may be obtained:
 
Output(0)′=Output(0)+ f (0)*(Error Signal)*Gain  (Eq. 2)
 
Output(0.1)′=Output(0.1)+ f (0.1)*(Error Signal)*Gain  (Eq. 3)
 
Output(0.2)′=Output(0.2)+ f (0.2)*(Error Signal)*Gain  (Eq. 4)
 
Output(0.3)′=Output(0.3)+ f (0.3)*(Error Signal)*Gain  (Eq. 5)
 
Output(0.4)′=Output(0.4)+ f (0.4)*(Error Signal)*Gain  (Eq. 6)
 
Output(0.5)′=Output(0.5)+ f (0.5)*(Error Signal)*Gain  (Eq. 7)
 
wherein Eqs. 2-7 accurately update the discrete table values based upon interpolated values between the discrete table values and the error signal.
 
   EXAMPLE 1 
   Using the data values provided at Table 1, a position input value of 0.16 may correspond to an estimated force value of about 13. In one aspect, the estimated force value may be determined using an interpolation technique between input value 0.1 and input value 0.2. 
   Using the membership functions provided at  FIG. 4  and assuming, for example, the actual force input value is 14, the gain G is 1, and the minimum threshold value N is 0.5, the updated output values may determined as follows:
 
Output(0)′=5+(0%)*(14−13)*(1)=5
 
Output(0.1)′=10+(40%)*(14−13)*(1)=10.4
 
Output(0.2)′=15+(60%)*(14−13)*(1)=15.6
 
Output(0.3)′=20+(0%)*(14−13)*(1)=20
 
Output(0.4)′=25+(0%)*(14−13)*(1)=25
 
Output(0.5)′=30+(0%)*(14−13)*(1)=30
 
 FIG. 6  provides a graphical illustration of the original data values of Table 1 plotted against the updated data values.
 
   EXAMPLE 2 
   Using the data values provided at Table 1, a position input value of 0.32 may correspond to an estimated force value of about 21. In one aspect, the estimated force value may be determined using an interpolation technique between input value 0.3 and input value 0.4. 
   Using the membership functions provided at  FIG. 5  and assuming, for example, the actual force input value is 19, the gain G is 1, and the minimum threshold value N is 0.5, the updated output values may determined as follows:
 
Output(0)′=5+(0.5%)*(19−21)*(1)=4.99
 
Output(0.1)′=10+(3.5%)*(19−21)*(1)=9.93
 
Output(0.2)′=15+(5%)*(19−21)*(1)=14.90
 
Output(0.3)′=20+(81%)*(19−21)*(1)=18.38
 
Output(0.4)′=25+(6%)*(19−21)*(1)=24.88
 
Output(0.5)′=30+(4%)*(19−21)*(1)=29.92
 
   Accordingly, a system, method and apparatus are provided for updating a look-up table such that position signals may be converted into force signals with more accuracy, thereby improving the control of the electric brake system  10 . In one aspect, the adaptive table may be updated repeatedly during a brake apply state of the brake system  10 . 
   Although various aspects have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The public is hereby placed on notice that any patent that may issue on this application includes such modifications and is limited only by the scope of the claims.