Abstract:
A method for detecting an onset of clamping force in a system including the steps of introducing a vibration into the system, monitoring the vibration, and identifying a relative change in the vibration.

Description:
[0001]     This invention was created in the performance of a cooperative research and development agreement with the Department of the Air Force (Contract No. F33615-01-2-5804/CRADA 01-156-PR-01). Thus, the government of the United States may have certain rights to the invention. 
     
    
     BACKGROUND  
       [0002]     The present invention is directed to a force switch and, more particularly, to a vibration force switch for detecting the onset of a clamping force.  
         [0003]      FIG. 1  illustrates a typical electric park brake (or electric caliper) assembly  10 , such as the type described in U.S. Pat. No. 6,550,598, the entire contents of which are incorporated herein by reference. The brake assembly  10  includes a caliper housing  12  having a ball screw assembly (not shown) slidably received therein. The ball screw assembly is adapted to translate a rotational force supplied by a motor (not shown) into linear movement of a piston  14 . The piston  14  may also be advanced by an increase of hydraulic fluid pressure within the caliper housing  12 .  
         [0004]     The piston  14  typically is aligned with a first brake pad  16  such that linear advancement of the piston  14  causes linear advancement of the first brake pad  16  towards a rotor  18  (i.e., a brake disk). A second, fixed brake pad  20  is typically provided on an opposite side of the rotor  18  such that the rotor is positioned between the two brake pads  16 ,  20 .  
         [0005]     As the first brake pad  16  is advanced towards the rotor  18  (i.e., in the direction of arrow A), the brake pad  16  engages the rotor  18  such that the rotor is clamped between the two brake pads  16 ,  20 . The clamping of the rotor  18  prevents the rotor from rotating about its axis, thereby supplying a braking force to an associated vehicle wheel. At this point it should be apparent that the braking force applied to the rotor  18  increases as the brake pad  16  (and piston  14 ) continue to advance in the direction of arrow A.  
         [0006]     The brake assembly  10  may be modeled as a spring according to Hooke&#39;s Law for Springs and therefore the following equation 1 applies: 
 
F=kX  (1) 
 
 where F is the braking force, k is the spring constant for the assembly  10  and X is the relative distance the assembly  10  has been displaced from equilibrium. Thus, the amount of braking force applied to the rotor  18  may be determined based on the distance the piston  14  and first brake pad  16  have traveled (i.e., X in equation 1) after the initial application of braking force (i.e., the point at which the first brake pad  16  initially engages or touches the rotor  18  to clamp the rotor  18  between the two brake pads  16 ,  20 ). 
 
         [0007]     Accordingly, there is a need for an apparatus and method for detecting the onset of force in a system such as the brake assembly  10 .  
       SUMMARY  
       [0008]     One aspect of the present invention is a method for detecting an onset of clamping force in a system. The method includes the steps of introducing a vibration into the system, monitoring the vibration, and identifying a relative change in the vibration.  
         [0009]     Another aspect of the present invention is a braking unit. The braking unit includes a caliper housing, a brake pad adapted for movement relative to the caliper housing to engage a rotor and a vibration-generating device connected to the caliper housing and adapted to introduce a vibration into the caliper housing, wherein the caliper housing vibrates at a first amplitude when the brake pad is not in contact with the rotor and a second amplitude when the brake pad is in contact with the rotor.  
         [0010]     A third aspect of the present invention is a clamping apparatus. The clamping apparatus includes at least one clamping member adapted to clamp an object and a vibration-generating device connected to the clamping member and adapted to introduce a vibration into the clamping member, wherein the clamping member vibrates at a first amplitude when the object has not been clamped and a second amplitude when the object has been clamped.  
         [0011]     Other embodiments, objects and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a front elevational view of a typical electric brake assembly;  
         [0013]      FIG. 2  is a front elevational view of a brake assembly in a non-engaged position according to one aspet of the present invention;  
         [0014]      FIG. 3  is a front elevational view of the brake assembly of  FIG. 1  in an engaged position; and  
         [0015]      FIG. 4  is a graphical illustration of a vibration according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     As shown in  FIGS. 2 and 3 , an electric brake assembly, generally designated  100 , may include a caliper housing  12 , a piston  14  and two brake pads  16 ,  20  for engaging a rotor  18 . A vibration-generating device  22  is connected to the caliper housing  12 . The word “connected” should not be limited to a physical connection, but rather should be understood to mean that the vibration-generating device  22  is positioned with sufficient proximity such that the vibration-generating device  22  is capable of introducing a vibration into the housing  12 .  
         [0017]     According to one aspect, the vibration-generating device  22  may be a piezo-electric device such as an ultrasound transceiver. Alternatively, an ultrasound transducer and a separate receiver may be used. The receiver may be positioned adjacent to or in the vicinity of the ultrasound transducer. According to a second aspect, the vibration-generating device  22  may be a motor having a natural vibrating frequency when operating. Those skilled in the art will appreciate that any device capable of generating a vibration within the caliper housing  12  may be used as the vibration-generating device  22  according to the present invention.  
         [0018]     The vibration-generating device  22  may be positioned at various locations relative to, on or within the caliper housing  12 . In one embodiment, the device  22  may be positioned to minimize exposure to heat generated by the assembly  100 . In a second aspect, the device  22  may be positioned such that the excitation of the device  22  is parallel with the direction of arrow A (see  FIG. 3 ). According to a third aspect, the device  22  is attached (e.g., via epoxy) to the inner lid (not shown) of the caliper housing  12 .  
         [0019]     Referring to  FIG. 4 , the vibration-generating device  22  may be actuated to introduce a vibration into the system (i.e., brake assembly  100 ,  FIG. 2 ) and is continuously monitored by a receiver  24 , such as an ultrasound transceiver, during the advancement of the piston  14 . The vibration may be continuous throughout the advancement of piston  14  and brake pad  16 . According to one aspect, the receiver may include a band pass filter to eliminate noise and anomalous disruptions, thereby providing a more consistent vibration curve (see  FIG. 4 ). However, at the point that the pads  16 ,  20  first contact the rotor  18  (i.e., the onset of force), designated point B in  FIG. 4 , the addition of the mass of the rotor  18  to the system dampens the vibration (i.e., there is a characteristic change in the vibration). The damping may be detected as a relative decrease in the amplitude of vibration (see  FIG. 4 ) or as a relative change in the natural frequency of the overall system. The point B at which the relative change in vibration occurs corresponds to the relative position of the piston  14  when the pads  16 ,  20  first contact the rotor  18 .  
         [0020]     An braking control unit  26  may be provided to receive vibration signals from the receiver  24  and monitor the vibration of the system. The braking control unit  26  may determine the point B that corresponds to the pads  16 ,  20  contacting rotor  18 . Furthermore, the braking control unit  26  may generate control signals for controlling the brake assembly  100  based on the vibration signals. Once point B has been determined, the braking control unit  26  may determine the amount of braking force applied to the rotor  18  based on the position of the piston  14  relative to point B. Alternatively, the braking control unit  26  may include a band pass filter, rather than the receiver  24 .  
         [0021]     Although the invention is shown and described with respect to certain embodiments, equivalents and modifications will occur to those skilled in the art upon reading and understanding the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the claims.