Abstract:
A method for estimating a force exerted by a first body onto a second body including the steps of providing a motor having at least one detectable motor signal, wherein the motor is adapted to advance the first body into engagement with the second body, determining a first value for the motor signal prior to the first body engaging the second body, determining a second value for the motor signal after the first body engages the second body, and generating a force value based upon a comparison of the second value to the first value.

Description:
[0001]     The low force level detection system and method 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 low force level detection system and method. 
     
    
     BACKGROUND  
       [0002]     The low force level detection system and method relates to systems and methods for detecting force and, more particularly, systems and methods for detecting low force levels in electromechanical brake systems.  
         [0003]     Electromechanical brake systems typically include a housing, a rotor, brake pads, an actuator/caliper and a motor. The actuator is adapted to drive the brake pads into engagement with the rotor, thereby clamping the rotor between the pads. The motor drives the actuator into engagement with the brake pads and the rotor. Therefore, the amount of force applied to the rotor by the brake pads is a function of the distance that the actuator is advanced by the motor.  
         [0004]     Prior art systems attempt to detect the position of the actuator at the onset of force (i.e., the point of initial contact). Then, assuming that the brake system can be modeled as a spring, the braking force may be calculated using Hooke&#39;s law: 
 
 F   0   =k ( x   0   −x   i )  (Eq. 1) 
 
 wherein F 0  is the braking force, k is the spring function of the system, x i  is the position of the actuator at the onset of force and x 0  is the position of the actuator at a subsequent time. 
 
         [0005]     Such prior art systems attempt to detect the actual contact point. However, at the onset of force, the signal to noise level of the distinguishable signals is small and difficult to use. Therefore, such systems often encounter difficulty identifying the point at which contact is made and/or force is applied, thereby giving rise to inaccurate measurements of braking force.  
         [0006]     Accordingly, there is a need for an improved system and method for estimating the contact force between two bodies.  
       SUMMARY  
       [0007]     In one aspect, a method for estimating a force exerted by a first body onto a second body is provided. The method may include the steps of providing a motor having at least one detectable motor signal, wherein the motor is adapted to advance the first body into engagement with the second body, determining a first value for the motor signal prior to the first body engaging the second body, determining a second value for the motor signal after the first body engages the second body, and generating a force value based upon a comparison of the second value to the first value.  
         [0008]     In another aspect, a brake system is provided. The brake system may include a rotor, an actuator aligned to engage the rotor, a motor adapted to advance the actuator into engagement with the rotor, the motor generating at least one detectable motor signal, at least one sensor positioned to monitor the motor signal, and a processor in communication with the sensor, the processor being adapted to determine a first value of the motor signal prior to the actuator engaging the rotor and a second value of the motor signal after the actuator engages the rotor and generating a force value based upon a comparison of the second value to the first value.  
         [0009]     In another aspect, a method for estimating a clamping force between brake pads and a rotor is provided, wherein the brake pads are urged into engagement with the rotor by a motor having at least one detectable motor signal. The method may include the steps of monitoring a motor signal of the motor, the motor signal including at least one of a motor speed, a motor current, a commutation time between motor position pulses and estimates thereof, determining a first value for the motor signal prior to the brake pads engaging the rotor, determining a second value for the motor signal after the brake pads engage the rotor, and determining a clamping force value based upon a comparison of the second value to the first value.  
         [0010]     Other aspects of the low force level detection system and method will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1A  is a schematic illustration of an electromechanical brake system according to one aspect of the low force level detection system and method;  
         [0012]      FIG. 1B  is a schematic illustration of the electromechanical brake system of  FIG. 1A  with the actuator at a second position;  
         [0013]      FIG. 1C  is a schematic illustration of the electromechanical brake system  FIG. 1A  with the actuator at a third position; and  
         [0014]      FIG. 2  is a graphical illustration of the low force level detection system and method showing clamping force and current plotted against position of the actuator. 
     
    
     DETAILED DESCRIPTION  
       [0015]     As shown in  FIG. 1A , the disclosed low force level detection system and method is embodied in an electromechanical brake system, generally designated  10 . Brake system  10  may include a housing  12 , a motor  14  (e.g., an electric motor), a processor  15 , a sensor  17 , an actuator  16 , two brake pads  18 ,  20 , a rotor  22 . The motor  14  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 actuator  16 , thereby urging the actuator  16  linearly into engagement with the brake pads  18 ,  20 . As the actuator  16  engages the brake pads  18 ,  20 , the brake pads  18 ,  20  clamp the rotor  22  and supply a braking force to the rotor  22 , as shown in  FIG. 1B .  
         [0016]     As shown in  FIG. 2 , as the actuator  16  moves from its initial position P i  (see also  FIG. 1A ) to the clamping position P c  (see also  FIG. 1B ), the motor  14  may be generally in a no-load state (i.e., there is no clamping force exerted by the motor  14 ). In the no-load state, the no-load motor speed ω NL  (motor speed is not shown in  FIG. 2 ) and the no-load motor current I NL  may remain relatively constant when a constant voltage is applied across the motor  14 . However, as the actuator  16  contacts the brake pads  18 ,  20  and initiates clamping of the rotor  22  (i.e., at position P c ), there may be a decrease in motor speed ω and a corresponding increase in motor current I.  FIG. 2  graphically illustrates an example of the clamping force F relative to the motor current I when a constant voltage is applied across the motor  14 , wherein there is a sudden increase in motor current I as the clamping force F begins to increase (i.e., beyond position P c ).  
         [0017]     Accordingly, the following motor and actuator equations may be used to model the brake system  10 : 
 
 Jω=K   T   I−T   L   −μω−T   C   (Eq. 2) 
 
 LI=V−RI−K   E ω  (Eq. 3) 
 
 wherein J is the inertia of the motor  14 , ball screw and gear train, ω is the motor speed, K T  is the motor torque constant, I is the motor current, T L  is the load torque, μ is the dynamic friction in the motor  14 , ball screw and gear train, T C  is the cogging torque, L is the motor inductance, V is the source voltage, R is the motor resistance and K E  is the EMF constant. 
 
         [0018]     Assuming no load torque (i.e., T L =0) when the motor  14  is in the no-load state, Eqs. 2 and 3 may be solved to yield:  
               ω   NL     =           K   T     ⁢   V     -     RT   C           μ   ⁢           ⁢   R     +       K   T     ⁢     K   E                   (     Eq   .           ⁢   4     )                 I   NL     =         μ   ⁢           ⁢   V     +       K   E     ⁢     T   C             μ   ⁢           ⁢   R     +       K   T     ⁢     K   E                   (     Eq   .           ⁢   5     )             
 
 Then, assuming T L =T F0  at a motor speed of ω F0  or a motor current of I F0 , Eqs. 2 and 3 may be solve to yield:  
               ω     F   ⁢           ⁢   0       =           K   T     ⁢   V     -     R   ⁡     (       T   C     +     T     F   ⁢           ⁢   0         )             μ   ⁢           ⁢   R     +       K   T     ⁢     K   E                   (     Eq   .           ⁢   6     )                 I     F   ⁢           ⁢   0       =         μ   ⁢           ⁢   V     +       K   E     ⁡     (       T   C     +     T     F   ⁢           ⁢   0         )             μ   ⁢           ⁢   R     +       K   T     ⁢     K   E                   (     Eq   .           ⁢   7     )             
 
 Combining Eqs. 4 and 6 yields:  
                 ω     F   ⁢           ⁢   0         ω   NL       =             K   T     ⁢   V     -     R   ⁡     (       T   C     +     T     F   ⁢           ⁢   0         )               K   T     ⁢   V     -       T   C     ⁢   R         =     1   -         T     F   ⁢           ⁢   0       ⁢   R           K   T     ⁢   V     -       T   C     ⁢   R                     (     Eq   .           ⁢   8     )             
 
 and combining Eqs. 5 and 7 yields:  
                 I     F   ⁢           ⁢   0         I   NL       =           μ   ⁢           ⁢   V     +       K   E     ⁡     (       T   C     +     T     F   ⁢           ⁢   0         )             μ   ⁢           ⁢   V     +       K   E     ⁢     T   C           =     1   +         K   E     ⁢     T     F   ⁢           ⁢   0             μ   ⁢           ⁢   V     +       K   E     ⁢     T   C                       (     Eq   .           ⁢   9     )             
 
 wherein ω F0  is the motor speed corresponding to clamping force F 0 , I F0  is the motor current corresponding to clamping force F 0  and T F0  is the load torque at clamping force F 0 . 
 
         [0019]     Solving Eqs. 6 and 9 for T F0  yields the following equations:  
               T     F   ⁢           ⁢   0       =         1   -       ω     F   ⁢           ⁢   0         ω   NL         R     ⁢     (         K   T     ⁢   V     -     RT   C       )               (     Eq   .           ⁢   10     )                 T     F   ⁢           ⁢   0       =             I     F   ⁢           ⁢   0         I   NL       -   1       K   E       ⁢     (       μ   ⁢           ⁢   V     +       K   E     ⁢     T   C         )               (     Eq   .           ⁢   11     )             
 
 wherein, in one aspect, K T , μ, T C , V, R and K E  may be presumed to be relatively constant. 
 
         [0020]     Thus, the load torque T F0  at clamping force F 0  may be determined by measuring the motor speed ω F0  relative to the no-load motor speed ω NL  and using Eq. 10 or, alternatively, by measuring the motor current I F0  relative to the no-load motor current I NL  and using Eq. 11.  
         [0021]     Furthermore, the load torque T F0  may be related to the clamping force F 0  as follows: 
 
 T   F0   =F   0   G   (Eq. 12) 
 
 wherein G is the gain. The gain G may be a function of the screw pitch, the gear reduction and/or the efficiency of the actuator. However, the gain G may be generally constant at relatively low clamping forces F 0 . 
 
         [0022]     Therefore, according to one aspect, the clamping force F 0  may be determined as follows:  
               F   0     =       κ   1     ⁡     (     1   -       ω     F   0         ω   NL         )               (     Eq   .           ⁢   13     )             or                           F   0     =       κ   2     ⁡     (         I     F   ⁢           ⁢   0         I   NL       -   1     )               (     Eq   .           ⁢   14     )             
 
 wherein κ 1  and κ 2  are constants. In one aspect, constants κ 1  and/or κ 2  may be determined graphically and/or by experimental data. In another aspect, constants κ 1  and/or κ 2  may be calculated by determining the various values of K T , μ, T C , V, R, K E  and G. 
 
         [0023]     At this point those skilled in the art will appreciate that various motor signals may be used according to the low force level detection system and method. For example, commutation time between motor position pulses and estimates of motor signals may be used.  
         [0024]     Accordingly, the clamping force F 0  applied to the rotor  22  by the brake pads  18 ,  20  and the actuator  16  may be determined by measuring a motor signal value (e.g., motor speed or motor current) relative to the motor signal value at a no-load state using the sensor  17  such that the processor  15  may correlate the measured value into a clamping force value (i.e., a low force level).  
         [0025]     In another aspect, the low force level detection system and method may provide a technique for estimating brake pad wear and/or the thickness T X  of the brake pads  18 ,  20  at some subsequent time after use.  
         [0026]     The brake system  10  may be provided with new or full brake pads  18 ,  20 , wherein both brake pads  18 ,  20  and all linings have an initial thickness T i . In one aspect, each individual brake pad  18 ,  20  may be presumed to have a thickness of about ½ of the total pad thickness T i  (i.e., the initial thickness of each pad may be ½T i ). In another aspect, each brake pad  18 ,  20  may be presumed to wear generally equally.  
         [0027]     Referring to  FIGS. 1A and 1B , when the system  10  is provided with new or full brake pads  18 ,  20 , the actuator  16  may have an initial position P i  (i.e., a fully retracted position) and a clamping position P C  (i.e., the position where the actuator  16  and brake pads  18 ,  20  initially begin to clamp the rotor  22 ). The actuator may be moved to the initial position P i  by fully backdriving the actuator  16 .  
         [0028]     The nominal distance D i  traveled by the actuator  16  from the initial position P i  to the clamping position P C  when the brake pads  18 ,  20  are new or full may be determined as follows: 
 
 D   i   =P   C   −P   i   (Eq. 15) 
 
         [0029]     As shown in  FIG. 1C , after use and associated wear of the brake pads  18 ,  20 , the actuator  16  must travel to position P X  to initiate clamping. Accordingly, the thickness T X  of the brake pads  18 ,  20  at some subsequent time after use may be determined as follows: 
 
 T   X   =T   i −[( P   X   −P   i )− D   i ]  (Eq. 16) 
 
 wherein, the total pad wear may be determined as follows: 
 
Total Pad Wear= T   i   −T   X   (Eq. 17) 
 
 In one aspect, each pad  18 ,  20  may be presumed to have a thickness of about ½T X  at some subsequent time after use. 
 
         [0030]     The positions P C , P X  of the actuator  16  at the onset of clamping may be determined using any known techniques. In one aspect, positions P C , P X  of the actuator  16  may be determined by monitoring motor signals, as discussed above. However, those skilled in the art will appreciate that any technique capable of determining positions P C , P X  may be used.  
         [0031]     For example, the distance between the fully backdriven position of an actuator and the onset of clamping of a new pair of brake pads (total thickness of 20 mm) may be about 30 mm. After several months of use, the distance between the fully backdriven position of the actuator and the onset of clamping may be about 34 mm. Therefore, applying Eq. 16, the resulting pad thickness may be estimated to be about 16 mm (i.e., 20 mm−[(34 mm)−(30 mm)]), wherein each brake pad may be about 8 mm thick.  
         [0032]     Although the low force level detection system and method is shown and described with respect to certain aspects, modifications may occur to those skilled in the art upon reading the specification. The low force level detection system and method includes all such modifications and is limited only by the scope of the claims.