Abstract:
An assembly for actuating a parking brake comprises a screw, a nut mounted for translation on the screw, and a cable connected between the brake and the nut. A compliant element is mounted on the screw, and is adapted to expand axially with a change in tension in the cable.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to brakes for motor vehicles, and more particularly to an apparatus for sensing variation in applied load of a parking brake actuator. 
     BACKGROUND OF THE INVENTION 
     Most vehicle designs incorporate parking brakes. Typical parking brake configurations employ regular drum brakes on a rear wheel with a simple mechanical linkage to engage brake shoes to their respective drums. For instance, an actuator may pull a steel brake cable taut in response to an operator depressing a pedal, lever or button. The resultant tension on the cable is transferred to the end of a brake lever. Other cables may draw the brake shoes firmly against the drums in response to rotation of the lever. A release knob or button causes the actuator to return to its initial position, reintroducing slack into the cables and, consequently, disengaging the brake shoes. 
     The reliability and integrity of brake performance depends largely upon the manner in which the actuator releases tension in the brake cable. For instance, should the actuator release too much cable, over-release will result. This condition requires the actuator to reel in excessive cable slack prior to a next application, translating into system delays and damage. Conversely, too little release of the cable may result in a brake drag. Such a condition occurs when the brake pads do not fully release. Brake drag retards vehicle performance while causing overheating and damage to brake pads and other system components. 
     To reduce the occurrence of over-release and drag, some brake manufacturers incorporate systems that use either position or periodic force adjustments to regulate cable release. In particular, one type of position adjustment is made by a position adjuster proximate to the actuator that periodically retracts cable slack to compensate for lining wear and stretching. Another type of position adjustment is made by a position-based release mechanism that consistently moves the actuator a preset distance that ideally corresponds to an optimum brake release point. The preset distance needs to be manually adjusted at service intervals to compensate for system wear factors. In practice, even if serviced regularly, cable stretching and other aging factors cause release points to substantially migrate in between adjustments, allowing the problems of over-release and brake drag. In addition, some users do not service the braking system regularly. 
     Closed loop position feedback is known in other types of powered mechanisms. In particular, improved position adjustment is achieved with linear position feedback from a sensor (e.g. linear variable displacement transducer (LVDT), linear resistive potentiometers, and optical sensors). However, these sensors are expensive and prone to reliability limitations due to wear and contaminants. 
     SUMMARY OF THE INVENTION 
     The present invention is an assembly for actuating a parking brake. The assembly comprises a screw, a nut mounted for translation on the screw, and a cable connected between the brake and the nut. A compliant element is mounted on the screw, and is adapted to expand axially with a change in tension in the cable. 
     Accordingly, it is an object of the present invention to provide a parking brake actuator of the type described above which reliably senses any change in steady state position of the nut. 
     Another object of the present invention is to provide a parking brake actuator of the type described above which can be electronically operated. 
     These and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of a parking brake actuator according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows one embodiment of a parking brake actuator  10  according to the present invention including a motor  12 . The motor  12  is preferably a direct current motor, and drives a gear train  14  of any suitable configuration to rotate a screw  16 . A nut  18  is threadingly engaged with the screw  16  such that the nut translates linearly as the screw rotates. The nut  18  is fixed to a tube  20 , which in turn is pivotably attached to a force balancer  22 . A pair of cables  24  and  26 , one for each of the rear wheel brakes, is attached to the force balancer  22 . A single cable configuration is also acceptable and functions identically except the force equalization is performed external to the apparatus in FIG.  1 . 
     A force transducer or other suitable force sensor measures the tension in cables  24  and  26 . The force may be measured in terms of motor current or by any other conventional standard. The force transducer relays the measured force back to a controller  28 , which operates the motor  12  until a preset load registers at the transducer. For instance, the motor  12  may continue to operate until zero pounds of force is indicated. Further details of this arrangement are described in U.S. patent application Ser. No. 09/930,890 filed Aug. 16, 2001, entitled “Multiple Hall Effect Position Sensor” and hereby incorporated by reference. 
     Once the preset load is achieved, a control program advantageously instructs the controller  28  to transition from operation within a force control mode to a position control mode. A position sensor is provided to furnish information to the controller  28  to reliably release the parking brake without brake damage due to brake drag from under release, and to avoid undue vibration and damage to the parking brake system due to over-release. In a preferred embodiment, the position sensor takes the form of a slide potentiometer shown schematically at  30  provided proximate the nut  18 . 
     In conjunction with the position sensor, a compliant spring element  32  detects any change in the desired steady load on the cables  24  and  26 . The compliant element  32  may be a bellville washer, and is disposed around the screw  16  between a pair of flat washers  34  and  36 . The flat washer  34  in turn bears against a relatively stationary shoulder  38 , while the flat washer  36  bears against a thrust bearing  40  and a retaining nut  42  to fix the flat washer  36  relative to the screw  16 . In effect, the compliant element  32  extends to take up any slack in the cables  24  and  26 . 
     The position x p  of the nut  18  as measured by the potentiometer  30  is equal to the deflection x s  of the compliant element  32  subtracted from the position x n  of the nut relative to the screw. Because the nut  18  is fixed relative to the screw  16 , x s  is equal to the inverse of x p . Also because the nut  18  is fixed, any drop in cable tension T is equal to the spring rate k s  of the compliant element multiplied by the change in position x s  of the compliant element. Combining these two equations, the change in the absolute position x p  of the nut is equal to the inverse of the change in cable tension divided by the spring rate k s  of the compliant element. Knowing the spring rate of the compliant element thus allows the system to determine the change in cable force from a change in potentiometer output. If the change in cable force is greater than a predetermined value the actuator can reapply or release as the measured change in force dictates. In practice, the sensitivity and usable range are determined by the stiffness and active length of the compliant element, and the compliance of the loading system including the brake cables and wheel brake mechanisms. 
     While the embodiment of the invention disclosed herein is presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. For instance, slots may be provided in the periphery of the plunger to serve as the anti-rotation feature, or a permanent magnet with annular pole pieces and a non-magnetic housing may be substituted for the like parts described above. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.