Abstract:
A control system for a disc brake adjuster mechanism includes a controller and an electric adjuster motor for operable connection to an adjuster mechanism for driving a friction lining towards and away from a disc brake rotor to maintain a predetermined running clearance between the brake rotor and the friction lining when a brake is not applied. The system further includes a brake displacement sensor, and the controller is programmed to determine the brake displacement at which a predetermined load on the friction lining is achieved due to the contact with the disc brake rotor. The predetermined load is determined from a parameter of the electric adjuster motor or adjuster mechanism. The system is programmed to measure the brake displacement during brake release.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims priority to United Kingdom Patent Application 0324243.5 filed on Oct. 16, 2003. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a control system and a control method for a disc brake, in particular an adjuster mechanism of a disc brake. 
   It is known to provide an electric motor to control the running clearance of friction linings relative to a brake rotor based upon signals from sensors that monitor the clearance take-up movement and the brake actuation stroke. The known systems tend to mimic the mechanical operation of a conventional brake clearance control device, known as an “automatic adjuster.” In such adjusters, a clutch having some degree of lost motion is provided where the level of free motion is equivalent to the maximum allowable running clearance. If the friction linings wear such that the running clearance is greater than the maximum allowable running clearance, the free running clearance is “taken-up” upon operation of the brake, and the further additional free movement, caused by the excess lining clearance, causes the clutch to rotate. The rotation moves the backstop or datum position for the return of the friction lining, thus progressively advancing the friction lining towards the brake rotor as the friction lining wears. When the friction lining contacts the brake rotor, the increased load in the system causes the clutch to slip, preventing further unwanted adjustment and/or overloading of the automatic adjuster. Such mechanical automatic adjusters are well known in the art. 
   In the brake of the present invention, it is important to reduce weight, the power consumption (whether electrical or pneumatic) and material costs. Unfortunately, in a conventional brake having an automatic adjuster of the mechanical kind or even an electric adjuster that mimics the mechanical operation, the strength of the mechanisms of the adjuster have to be extremely high. This is because the actual brake adjustment occurs only while the brake is being applied rather than during brake release. It is common in “sliding caliper” brakes for the load to be applied directly on only one side of the brake rotor. The caliper frame slides to apply a load to the other side of the brake rotor. The load is present before both friction linings fully contact the brake rotor. Therefore, the adjuster drive train has to be capable of driving through the load. 
   European Patent Application 0995923 (Meritor Automotive, Inc.) teaches a pressure sensor disposed at an input end of an operating shaft (“op-shaft”) of a disc brake to determine when operation of the brake occurs and the running clearance is taken-up. The position of the op-shaft when the running clearance has been taken up is measured such that, upon brake release, the electric motor driven adjuster mechanism may move the datum position for the return of the friction lining to maintain a constant running clearance as the friction lining wears. 
   The present invention seeks to overcome, or at least mitigate, the problems of the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention provides a control system for a disc brake adjuster mechanism. The system includes a controller and an electric adjuster motor for operable connection to the adjuster mechanism for driving a friction lining towards and away from a disc brake rotor to maintain a predetermined running clearance between the disc brake rotor and the friction lining when a brake is not applied. The system further includes a brake displacement sensor. The controller is programmed to determine the brake displacement at which a predetermined load on the friction lining is achieved due to the contact with the disc brake rotor. The predetermined load is determined from a parameter of the electric adjuster motor or the adjuster mechanism, and the system is programmed to measure the brake displacement during brake release. 
   The present invention also provides a method of determining the displacement of a brake at which a predetermined load on a friction lining is achieved. The brake includes a brake actuator, a friction lining, a brake rotor, a brake displacement sensor, a controller and an adjuster mechanism including an electric adjuster motor. The method includes the steps of producing a signal to drive the electric adjuster motor while there is a force acting between the brake rotor and the friction lining during release of the brake and monitoring a parameter of the electric adjuster motor or the adjuster mechanism to determine whether a predetermined load on the friction lining is achieved. The method also includes the step of determining the brake displacement at which the load is achieved. 
   The present invention also provides a control system for a disc brake adjuster mechanism. The system includes a controller and an electric adjuster motor for operable connection to the disc brake adjuster mechanism for driving a friction lining towards and away from a disc brake rotor to maintain a predetermined running clearance between the brake rotor and the friction lining when a brake is not applied. The system further includes a brake displacement measurer. The controller is programmed to determine the brake displacement at which a predetermined load on the friction lining is achieved due to the contact with the brake rotor. The predetermined load is determined from a parameter of the electric adjuster motor or the adjuster mechanism. The system further includes a separate actuator operable to apply the brake to retard rotation of the brake rotor. 
   These and other features of the present invention will be best understood from the following specification and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a partially sectioned plan view of one embodiment of a brake in accordance with the present invention; 
       FIG. 2  is an enlarged detail of  FIG. 1  showing a motor and gearbox installation; 
       FIG. 3  is a cross-sectional view along the line  3 — 3  of  FIG. 1 ; 
       FIG. 4  is a schematic diagram illustrating the electronic components of the control system; 
       FIGS. 5 and 6  are flowcharts showing an adjustment method according to one embodiment of the present invention; and 
       FIG. 6  is a flowchart showing an adjustment method according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a brake  8  including a caliper housing  10  that straddles a disc or rotor  12  mounted on an axle of the vehicle to be braked (not shown). The brake  8  is actuated by mechanical movement of an input actuator  15 , such as an air cylinder (shown in  FIG. 3 ). Such actuators are well known in the field of brake actuation. The input actuator  15  cooperates with an outer end of the operating shaft or ‘op-shaft’  14  of the brake  8 . An inner end of the op-shaft  14  is carried in a bearing attached to a lower end of an inner housing part  16 . The inner end of the op-shaft  14  has a pocket positioned eccentrically to the axis of rotation of the op-shaft  14  which, upon rotation, causes a reaction force to be transmitted to rollers  20 . The rollers  20  in turn transmit the applied load to a pair of spaced inner tappet members  22 . The inner tappet members  22  are threadedly engaged with associated outer tappet members  24 , which apply the input load from the input actuator  15  to the rear of an inner friction lining  26 , thus pressing the friction material of the inner friction lining  26  into frictional engagement with the disc or rotor  12 . 
   A reaction force is generated through the frictional engagement between the disc or rotor  12  and the inner friction lining  26  that is fed back through the inner tappet members  22 , the outer tappet members  24 , the rollers  20  and the op-shaft  14  supported by the inner housing part  16 . The inner housing part  16  is secured to an outer housing part  28  by bridging bolts  30  and  32 . Thus, the applied force that is generated by movement of the op-shaft  14  is ultimately transmitted by a reaction means to the outer housing part  28 , which in turn presses the outer friction lining  34  into frictional engagement with the disc or rotor  12 . Therefore, upon movement of the op-shaft  14 , the disc or rotor  12  is clamped between the inner friction lining  26  and the outer friction lining  34  to generate a braking force to brake the vehicle under control of the applied input movement. 
   As shown in  FIG. 1 , the brake  8  also includes an electric motor  40  that is adapted to drive via a reduction gearbox  42  a part of the telescopic tappet assembly, shown here by way of example as a multi-stage planetary gearbox. Upon rotation, the telescopic tappet assembly increases or reduces the overall length of the tappet assembly (which includes the inner tappet members  22  and the outer tappet members  24 ) in accordance with the direction of rotation of the electric motor  40 . The extension or contraction of the tappet assembly adjusts the rest position of the brake applying member, and therefore the clearance available between the friction linings  26  and  34  and the disc or rotor  12 . The electric motor  40 , the reduction gearbox  42 , the inner tappet members  22  and the outer tappet members  24  together constitute an adjuster mechanism of the brake  8 . 
   A rotary encoder  44 , that is driven from a part of the tappet assembly that moves upon adjustment, produces a signal which is arranged to be processed in an Electronic Control Unit (ECU)  80 . The output from the rotary encoder  44  is accumulated to measure the total position, and therefore total movement, of the adjustment mechanism. The output is proportional to the actual wear condition of the friction linings  26  and  34 . 
   Once it is determined that the brakes  8  have been released, the obtained clearance data is used by the ECU  80  determines whether an adjustment of the clearance is required. If an adjustment is required, then the electric motor  40  is driven to the new position. As shown in  FIG. 2 , the electric motor  40  output drives through the cycloid reduction gearbox  42  and onto a gear form  48  associated with the inner tappet members  22 . The inner tappet members  22  are threadedly engaged with the outer tappet members  24 , which are fixed against rotation. Rotation of the inner tappet members  22  cause the overall tappet assembly to either extend or contract. The torque required to drive the tappet assembly to produce the above-mentioned effect is substantially lower when the tappet assembly is not under any substantial axial loading because the friction level is drastically reduced between the inner tappet members  22  and the outer tappet members  24 . The torque required to produce the adjustment movement is substantially small with respect to the torque required had the brakes been applied, and therefore the reduction gearbox  42  and the tappet assembly drives can now be produced from a material that is substantially lighter. 
     FIG. 4  schematically illustrates the electrical components of the control system. The ECU  80  receives signals from the rotary encoder  44  and an op-shaft stroke sensor  82  and (in the method of the second embodiment) an air pressure sensor  84  (shown in broken lines). The ECU  80  may signal the driving of the electric motor  40  and may receive signals from the electric motor  40  or elsewhere on a motor drive circuit regarding the amount of current passing therethrough. The op-shaft stroke sensor  82  may be any suitable type of contacting sensor or non-contacting sensor. 
     FIGS. 5 and 6  show one embodiment of the method of operation of the control system in flowcharts. The method operates as follows: 
   The operation starts at step  100 , and the system begins by monitoring the output of the op-shaft stroke sensor  82  at predetermined intervals at step  102 . At step  104 , the ECU  80  determines whether the signal from the op-shaft stroke sensor  82  has reached a threshold value that indicates that the brake  8  has been applied. If the brake  8  has been applied, the ECU  80  signals the electric motor  40  to lengthen the brake tappet assembly at step  106 . The ECU  80  then begins to monitor the current flowing through the electric motor  40  at predetermined intervals and at step  110  senses when the current increases above a predetermined threshold value (which is indicative of the electric motor  40  stalling). Once this occurs, the ECU  80  then signals for the electric motor  40  drive to cease so that the electric motor  40  is no longer seeking to extend the tappet assembly. At step  114 , the ECU  80  stores the stroke sensor output in a memory “SSON” and at step  116  subtracts the stroke sensor zero offset value “SSF” (i.e., a stroke sensor reading when the op-shaft  14  is in a released rest position) from SSON. This value is then stored in memory “SSC”. At step  118 , the value SSC is then subtracted from a stored nominal clearance value “SSN” (i.e., the desired clearance value of the disc or rotor  12  to the friction lining (with the brake released) to equate to the amount of adjustment required to restore the clearance to the desired nominal clearance. This value is stored in memory “SSA” before the sequence of steps stops at  120 . 
   Referring now to  FIG. 6 , the sequence of steps starts at  122  with the brake  8  being applied. Again, the ECU  80  monitors the shaft stroke sensor  82  at predetermined intervals at step  124  and determines whether the brake  8  has been released in response to the shaft stroke sensor  82  providing a predetermined signal. Once this has occurred, the ECU  80  signals the electric motor  40  drive to commence at step  128  to lengthen the tappet assembly. The ECU  80  also monitors the extension of the tappet assembly via the rotary encoder  44  until the tappet assembly has been extended by a value equivalent to SSA. The ECU  80  then signals the electric motor  40  drive to cease and the adjustment cycle stops at step  136 . 
     FIG. 7  illustrates a control system and adjustment method according to a second embodiment of the present invention in which both the measurement of the required amount of adjustment and the lengthening of the tappet assembly occurs during the release of the brake  8 . 
   The sequence starts at step  138  and begins by monitoring the stroke sensor output at predetermined intervals at step  140 . To determine when the brake  8  is released, the ECU  80  monitors the air pressure in the input actuator  15  or pneumatic actuator using the air pressure sensor  84  at step  144  and at step  146  determines that the brake  8  is being released once the air pressure has fallen below a predetermined level at a predetermined rate. At step  148 , the ECU  80  signals the electric motor  40  to shorten the tappet assembly. However, at this point, the electric motor  40  torque is insufficient to overcome the friction induced by the force passing through the tappet assembly to the friction lining, causing the electric motor  40  to stall. The ECU  80  monitors the motor current at predetermined intervals at step  150  so it can determine when the current through the electric motor  40  has dropped to a predetermined threshold value that indicates that the torque of the electric motor  40  is sufficient to drive the tappet assembly. At the point that the electric motor  40  drive starts, the ECU  80  stores the stroke sensor output in memory SSOFF at step  154  before subtracting SSF (i.e., a stroke sensor reading when the op-shaft  14  is in a released rest position) from SSOFF at step  156  to give a value SSC which is stored in the memory. 
   To give the total amount by which the tappet assembly should be adjusted, SSC is subtracted from a stored nominal clearance value SSN (e.g., 0.25 mm). The result is stored in memory SSA and equates to the amount by which the inner tappet members  22  and the outer tappet members  24  must be extended to return the brake  8  to the correct running clearance. To do this, the ECU  80  signals the electric motor  40  to drive to lengthen the tappet assembly and monitors the position of the rotary encoder  44  at predetermined intervals until the ECU  80  determines that the tappet assembly has extended by amount SSA. Once the inside tappet members  22  and the outer tappet members  24  have extended by this amount, the ECU  80  signals the electric motor  40  to cease driving, and the adjustment procedure stops at step  168 . 
   One advantage of this adjustment method is that the stroke sensor outputs SSON and SSOFF are measured at the end of the brake  8  application cycle when the brake disc or rotor  12  and the friction linings  26  and  34  may be heated and have therefore expanded. Thus, the danger of “over adjustment” on the basis of values measured when the disc or rotor  12  and the friction linings  26  and  34  are cold does not arise, and the brake  8  clearance is correctly set for the brakes when hot. 
   It should be appreciated that as an alternative to measuring the electric motor  40  current, the point at which the tappet assembly comes out of contact with the inner friction linings may be determined directly or indirectly from rotation of the electric motor  40 , the gears  46 , or the inner tappet members  22 . 
   The ECU  80  may be programmed to only carry out the adjustment intermittently (e.g., every tenth brake application). Rather than seeking to correct the clearance fully each time it is determined that adjustment is needed, the ECU  80  may be programmed to signal the electric motor  40  to drive a fixed increment for each brake application so the correct adjustment is only achieved after more than one brake application. 
   The output from the tappet driven wear-out sensor or rotary encoder  44  may be recorded or accumulated to provide a signal indicative of the worn condition of the friction linings. 
   Should the signal from the wear-out sensor or rotary encoder  44  determine that a friction lining change is required, an alarm or other indication can then be issued. Re-adjustment of the brake  8  or retraction of the brake applying members is then instigated through use of an electrical or electronic switch (not shown). Once activated, the system determines whether the vehicle is in a correct condition to allow the brake to be ‘opened’, i.e., stationary. If this condition is satisfied, then the electric motor  40  is energized to cause the brake-applying members to retract from the disc or rotor  12 . As the brake  8  is no longer in correct adjustment, a flag is set to indicate to the ECU  80  that an adjustment is required. 
   After re-assembly of the brake  8 , the out-of-adjustment flag is recognized, and the brake  8  is re-adjusted. 
   It should also be appreciated that the same principles may be applied to other forms of the brake  8  that are not of the sliding caliper type. The invention may also be applied to electromechanical brakes in which an electric motor replaces the air actuator. In brakes of this type, the electric motor may carry out adjustments to maintain the correct running clearance, as well as supply a braking force. 
   The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.