Abstract:
An electric retractor extracts and retracts a seat belt in response to tension in the seat belt. The electric retractor may include a spool rotatably attached to a retractor frame. A seat belt is wound on the spool. The spool is rotated by a motor via a worm gear system that permits limited axial motion of the worm, but generally prevents the motor from being back-driven by tension in the seat belt to prevent forced seat belt extraction. The gear system may also cut off power to the motor in the event of excessive seat belt tension to prevent further payout of the seat belt. A senses tension in the seat belt and activates the motor to retract or extract the seat belt from the retractor. An emergency control system may override the web guide control in response to abnormal vehicle dynamics to provide reversible pre-crash pretensioning and/or crash pretensioning.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to seat belt systems designed to protect the occupants of a vehicle during a collision. More specifically, the invention relates to an electric seat belt retractor system which is uniquely capable of protecting an occupant during normal use, pre-crash situations, and crash situations, and is capable of constantly and dynamically adjusting to the positioning of the occupant. 
   2. Description of Related Art 
   A “control system” may be defined as a system in which an operation is to be performed (or omitted) in a manner determined by measuring some characteristic of the system. Thus, efficient operation of the system can be maintained despite relatively unpredictable changes in the system. The present invention has application to a wide variety of control systems. 
   There are many applications in which it may be desirable to monitor and/or automatically adjust the tension in a flexible member. One such application is safety restraints for protecting vehicle occupants from impact. Such restraints are often known as “seat belts.” 
   Seat belts are known to increase the safety of occupants in motorized vehicles. Seat belt use is often cited as the most useful line of defense in reducing accident related injuries. Legislation requiring manufacturers to include seat belts in their vehicles has been in place for many years. More recently, laws have been enacted requiring consumers to use seat belts. 
   The benefits of seat belt use are numerous. In a collision, seat belts may prevent the occupant of a vehicle from striking the interior of the vehicle or other objects within the vehicle, including other occupants. Seat belts aid in keeping the occupant inside the vehicle during a roll-over or other accident situation to enhance the probability of survival and injury avoidance. Seat belts may also keep the driver behind the wheel and in control of the vehicle prior to an impending or potential collision, averting additional damage or injuries. Seat belts also enhance the effectiveness of other safety devices. For example, in a vehicle with airbags, a seat belt keeps the occupant in the seat so that the airbag can better protect the occupant. 
   Seat belts vary in their configuration, but one common type of seat belt is the three point safety harness. A three point safety harness includes a lap belt and a shoulder strap that cooperate to anchor an occupant on each side of his/her lap and at one shoulder. In one commonly employed three-point safety harness configuration, the seat belt webbing traverses the occupant&#39;s upper body in a diagonal fashion, passes through a latch plate, and then traverses the occupant&#39;s lap. The latch plate is fastened to a buckle, which is secured to the vehicle to restrain both the occupant&#39;s lower and upper body. One end of the webbing is typically anchored to the vehicle. The other end is secured by a seat belt retractor. 
   For convenience and due to variations in seat position and occupant size, three point safety harnesses are usually adjustable to provide proper safety and comfort. A seat belt retractor allows the safety harness to be adjustable and to lock the webbing in the event of an accident. Conventional seat belt retractors include webbing anchored at one end to a spool. Rotation of the spool is controlled for extraction and retraction of the webbing by a combination of various ratchet wheels, springs, lock dogs, pawls, gears, and the like. 
   Preferably, in a three-point safety harness, the shoulder strap rests lightly on the occupant&#39;s shoulder and allows the occupant&#39;s upper torso relatively free movement. However, many occupants fail to properly adjust the tension in the safety harness once the seat belt has been fastened. If too much slack is left in the shoulder strap portion of the webbing, the shoulder seat belt system may not properly protect the occupant. Therefore, seat belt retractors have been designed to automatically remove excess slack from the shoulder strap. Generally, this is done by providing a constant bias on the spool in the direction of webbing retraction. However, in actual application, seat belt systems usually contain substantial slack, often 120 mm or more. This is clearly not ideal in that the slack can defeat the effectiveness of the seat belt in a crash situation. 
   In addition, removal of slack can often cause the occupant discomfort. This discomfort may cause an occupant to use the seat belt improperly, for example, by placing the shoulder portion behind their upper torso, or by simply not using the seat belt. The safety features of the three-point safety harness are defeated when discomfort leads occupants to misuse or avoid using the system. 
   Generally, the difficulty with existing seat belt systems can be summarized as inability to adequately and dynamically adjust to the position of the occupant. An ideal seat belt system should be able to restrain an occupant comfortably during normal operation. This, however, requires constant adjustment because the occupant is constantly mobile, moving and reaching about the interior of the vehicle. Conventional systems are unable to freely and constantly adjust and often result in uncomfortable binding if the occupant makes a substantial move. 
   Furthermore, most conventional systems are unable to adequately respond to pre-crash situations. Most existing systems simply lock the belt in place, but are unable to draw the occupant back into position. Many such systems are unable to provide crash pretensioning to restrain the occupant during an actual crash, and to allow optimum interaction with airbags and other supplemental restraint systems. 
   Hence, conventional seat belt systems are lacking in a number of respects, and a need exists for enhanced seat belt systems that overcome the shortcomings of the prior art. More generally, there is a need for control systems capable of adjusting the available length of a flexible member depending on tension present within the member. Conventional control systems generally lack the ability to dynamically and accurately control the tension. There is a need for control systems capable of controlling tension, particularly in the presence of relatively unpredictable factors such as the motion of a vehicle passenger. 
   SUMMARY OF THE INVENTION 
   The present invention has been developed in response to the present state of the art, and in particular, in response to problems and needs in the art that have not yet been fully solved by currently available control systems. According to one implementation of a control system according to the invention, an electric seat belt retractor is controlled based on sensing tension in the seat belt to provide powered extraction and retraction of the seat belt. In addition, the electric seat belt retractor provides pre-crash pretensioning, crash pretensioning, and automatic locking, in addition to the existing functions of emergency locking, extraction, and retraction. 
   The present invention provides an electrical retractor which overcomes many of the limitations of the prior art. The present invention provides constant and dynamic extraction and retraction of the seat belt webbing in order to follow the occupant&#39;s motion about the vehicle. This provides substantially increased comfort to the occupant, while at the same time maintaining a high degree of safety and effectiveness. 
   In the event of a pre-crash situation, a sensor associated with the seat belt system retracts the seat belt webbing until the occupant is in a safe and secure position. This position is maintained in the event of a crash such that the occupant is properly positioned and safely restrained by the seat belt. As an added benefit, the occupant is restrained in a proper position to receive the added protective benefits of an airbag or supplemental restraint system. 
   In one embodiment, a web guide lever is employed together with a potentiometer or any angular, linear, or photoelectric sensor. Other types of sensors could also be employed such as linear variable displacement transducers (LVDT&#39;s), optical sensors, Hall Effect sensors, pressure sensors, piezoelectric or resistance-based load cells, and the like. 
   The present invention provides a previously unknown type of secondary electromechanical feedback. Servo control is used to greatly improve motor control compared to known motor implementations. More precisely, a sensor may be mounted to an axle of a web guide lever to supply exact information about webbing dynamics. This information allows the servo amplifier to adjust the direction and speed of the retractor motor in a precise manner. Thus, minimum web tension can be achieved in the seat belt system to assure ride comfort as well as rapid response to any fast or slow changes in the webbing configuration. 
   Thus, in summary, during normal use the system allows the seat belt webbing to be extracted and reduces the force when worn for improved occupant comfort. When the vehicle begins to reach its limits of adhesion due to increased lateral “g” force or excessive vehicle velocity, the motor drives the mechanism described below to retract the seat belt webbing with sufficient force to pull the occupant back more firmly into the seat and to attain a more favorable position for air bag deployment in the event of a crash. The mechanism described below also provides the structural “lock-up” function necessary to support belt loading and restrain the occupant during a crash. In addition, the system of the present invention provides a fail-safe mechanism whereby in the event of the loss of electrical power, the mechanism is locked in place to provide protection to the occupant. 
   The electric seat belt retractor of the present invention provides automatic extraction and retraction of the seat belt and other safety features using a unique mechanical configuration. The electric seat belt retractor of the present invention includes a gear assembly which may include a spool, a worm wheel, and a worm. The spool is rotatable about an axis within a retractor frame. A seat belt is connected to and wound around the spool. The worm wheel is coaxially connected to one end of the spool axis. The worm, connected to a drive shaft, operably engages the worm wheel. Rotation of the drive shaft rotates the worm which drives the worm wheel about the spool axis. The drive shaft is connected to the retractor frame such that the drive shaft is rotatable and axially slidable. 
   The electric seat belt retractor also includes a motor coupled to the drive shaft for rotating the drive shaft. The motor is electrically connected to a circuit which activates and deactivates the motor to extract and retract the seat belt in response to tension in a portion of the seat belt which extends from the spool. In response to a rapid extraction force applied to the seat belt, a torque is created in the worm wheel that forces the worm wheel to slide the worm and drive shaft together axially until the worm contacts a switch that cuts power to the motor and prevents further extraction. 
   The electric seat belt retractor may include a spring around the drive shaft between a connector for the drive shaft and the worm such that the spring biases the worm and drive shaft against axial movement toward the connector in response to torque created in the worm wheel when the occupant pulls against the seat belt. The torque is in the direction of seat belt extraction. The spring arrests translation of the worm when the spring bias becomes equal to the torque to keep the worm and drive shaft in operable engagement with the worm wheel. Preferably, once the worm contacts the switch, the worm remains engaged with the worm wheel to prevent further rotation of the worm wheel in the direction of seat belt extraction. 
   The electric seat belt retractor may also include a tension sensor in communication with the seat belt. As mentioned above, the tension sensor may be any type of sensor which provides the necessary characteristics for operation of the system. The tension sensor is in electrical communication with the circuit. In response to changes in tension in the seat belt, the tension sensor activates the motor to retract or extract the seat belt from the electric seat belt retractor. Preferably, the tension sensor comprises an arm pivotally connected near a belt opening of the retractor where the seat belt exits the retractor. The arm is sized and positioned to extend over the belt opening. The unattached end of the arm includes a webbing passage through which a portion of the seat belt passes. The arm pivots between substantially covering the belt opening and about a ninety degree angle with respect to the belt opening in response to changes in tension in the seat belt. Preferably, the arm is biased toward the belt opening by a spring. 
   When the seat belt of the present invention is used, the occupant pulls the seat belt to insert the latch plate into engagement with the buckle. This pull increases tension in the seat belt. The increased tension causes the arm to pivot outward from the belt opening, toward the ninety-degree position. In response, the tension sensor activates the motor to pay out seat belt webbing. Once the tension in the seat belt returns to a lower level, for example, due to release of the seat belt or engagement of the latch and the buckle, the reduced tension and the bias of the tension sensor towards the belt opening causes the tension sensor to activate the motor to retract the seat belt. As the seat belt is retracted, the tension in the seat belt increases. Increasing the tension causes the arm to pivot to form an angle of about forty-five degrees with the belt opening. Once the arm is positioned at about forty-five degrees, the tension sensor deactivates the motor. When the arm is positioned at about forty-five degrees, a comfortable amount of tension is present in the seat belt. 
   In certain embodiments, the electric seat belt retractor includes one or more systems which override the tension sensor in order to provide additional safety features. For example, an automatic locking system may activate or deactivate based on the number of rotations of the spool to prevent overextension of the seat belt. When the automatic locking system is activated, the tension sensor is overridden by the automatic locking system which activates the motor for retraction but prevents extraction. Similarly, an emergency control system may override the tension sensor to provide reversible pre-crash pretensioning and/or crash pretensioning in response to sensors that track certain vehicle dynamics such as pitch, yaw, panic braking, loss of traction, dramatic steering wheel movement, and the like. 
   In view of the foregoing, the electric seat belt retractor provides substantial advantages over conventional systems. The electric seat belt retractor senses the tension in the seat belt such that a constant bias in the direction of retraction is unnecessary. The worm wheel and sliding worm and drive shaft provide a safety lock which prevents unintentional extraction of the seat belt. Conventional locking pawls and ratchet wheels are unnecessary. Together with a simple emergency control unit and automatic locking system, the seat belt retractor provides enhanced seat belt take-up with comparatively fewer components. Stated more generally, the present invention provides enhanced structures and method for accurately controlling tension within a flexible member. 
   These and other features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the advantages and features of the invention are obtained, a more particular description of the invention summarized above will be rendered by reference to the appended drawings. Understanding that these drawings illustrate only selected embodiments of the invention and are not therefore to be considered limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1  is a perspective view of one embodiment of an electric seat belt retractor; 
       FIG. 2A  is a side view of a tension sensor of an electric seat belt retractor detecting maximum tension in the seat belt; 
       FIG. 2B  is a side view of a tension sensor of an electric seat belt retractor detecting minimal tension in the seat belt; 
       FIG. 3A  is a side view of a worm gear drive system used in one embodiment of an electric seat belt retractor which is locked due to torque in the worm wheel; 
       FIG. 3B  is a side view of a worm gear drive system used in one embodiment of an electric seat belt retractor during normal operation; 
       FIG. 4  is a functional block diagram for a circuit of one embodiment of an electric seat belt retractor that provides automatic locking functionality; 
       FIG. 5  is a perspective view of components integrated with an electric seat belt retractor to provide automatic locking; and 
       FIG. 6  is a functional block diagram for a circuit of one embodiment which provides pre-crash pretensioning and crash pretensioning in an emergency. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the invention are now described with reference to  FIGS. 1–6 , wherein like parts are designated by like numerals throughout. The members of the present invention, as generally described and illustrated in the Figures, may be constructed in a wide variety of configurations. Thus, the following more detailed description of the embodiments of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
   In this application, the phrases “connected to,” “coupled to,” and “in communication with” refer to any form of interaction between two or more entities, including mechanical, electrical, magnetic, electromagnetic, electromechanical and thermal interaction. The phrase “attached to” refers to a form of mechanical coupling that restricts relative translation or rotation between the attached objects. The phrases “pivotally attached to” and “slidably attached to” refer to forms of mechanical coupling that permit relative rotation or relative translation, respectively, while restricting other relative motion. 
   The phrase “directly attached to” refers to a form of attachment by which the attached items are either in direct contact, or are only separated by a single connector, adhesive, or other attachment mechanism. The term “abutting” refers to items that are in direct physical contact with each other, although the items may not be attached together. 
   The structures, methods, and principles of the present invention are applicable to a wide variety of control systems, and more specifically to systems in which the tension of a flexible member is to be dynamically adjusted or kept constant. The following disclosure focuses on automotive safety, and more specifically, on an enhanced seat belt system. The elements outlined below may be readily adapted to other control systems through the application of knowledge available in the art. 
   With reference to  FIG. 1 , there is illustrated a perspective view of an electric seat belt retractor  10 . The electric seat belt retractor  10  is shown in a configuration corresponding to installation of the retractor  10  on a B-pillar of a vehicle (not shown) for a shoulder seat belt system. The retractor  10  can be installed in other locations of a vehicle. For example, the retractor  10  may be installed beside a rear seat for use in a lap and/or a shoulder seat belt system. The retractor  10  may be used with a variety of lap, shoulder, and/or four or five point seat belt systems. 
   The electric seat belt retractor  10  includes a seat belt  12 . The seat belt  12  is seat belt webbing of about two inches in width and a length determined by factors such as the type of seat belt system using the retractor  10 , the position of the retractor  10  in the vehicle, and the size of the seat (not shown). Generally, one end of the seat belt is anchored to the vehicle outside the retractor  10 . As mentioned above, the remainder of the seat belt  12  may be threaded through a latch plate or buckle and a D-ring before being connected to the retractor  10 . 
   Generally, the retractor  10  adjusts the seat belt  12  by paying out or retracting the seat belt  12  as needed. A rewind spring (not shown) is generally used to pay out or retract the seat belt  12 . In the retractor  10 , excess seat belt webbing  12  is taken up by the rewind spring connected to a spool  14  around which the seat belt webbing  12  is wound to form a take-up mechanism. An axle  15  of the spool  14  rotates about an axis  16  secured within a retractor frame  18 . Preferably, the spool  14  rotates to pay out and retract the seat belt  12  in response to the needs of an occupant. 
   To rotate the spool  14 , an input gear is connected to the spool  14 . The input gear may take the form of a worm wheel  20  is connected coaxially to one end of the spool  14 . The worm wheel  20  operably engages an input gear, which may take the form of a worm  22 . Rotation of the worm  22  in one direction drives the worm wheel  20  to rotate the spool  14  to pay out the seat belt  12 . Rotation of the worm  22  in the other direction rotates the worm wheel  20  which rotates the spool  14  to retract the seat belt  12 . 
   The size and configuration of the worm wheel  20  and worm  22  may vary based on the seat belt system used with the retractor  10 . Using a worm wheel  20  and worm  22  to drive the spool  14  provides high torque for extraction and retraction of the seat belt  12 . In one embodiment, the gear ratio between the worm wheel  20  and worm  22  is 30:1. Alternatively, the number of teeth on the worm wheel  20  and worm  22  may be varied to provide different gear ratios. 
   In one embodiment, the worm  22  is fixed to a drive shaft  24 . Preferably, the worm  22  is connected such that rotation of the drive shaft  24  rotates the worm  22  and the worm  22  will not move laterally with respect to the drive shaft  24 . In certain embodiments, rather than being connected, the worm  22  and drive shaft  24  may be formed from a single piece of material. 
   The worm  22  may be connected to the drive shaft  24  using various mechanical connectors. For example, the worm  22  may be secured by one or more set screws  26 . Alternatively, pins (not shown) may pass through the shaft and engage the worm  22  to allow rotation and prevent lateral movement of the worm  22  with respect to the drive shaft  24 . Furthermore, the worm  22  may be welded to the drive shaft  24 . The worm  22  is positioned along the drive shaft  24  to operably engage the worm wheel  20 . 
   Preferably, the drive shaft  24  is connected by a first connector  28  and a second connector  30  to the retractor frame  18 . Alternatively, a single connector  28  may be used. The retractor frame  18  serves as a base  31  for the connector  28 ,  30 . The connectors  28 ,  30  are configured to secure the drive shaft  24  to the frame  18  but still allow the drive shaft  24  to rotate and slide laterally within the connectors  28 ,  30 . 
   In certain embodiments, the connectors  28 ,  30  are pillow blocks. The pillow blocks may be secured to the retractor frame  18  by screws or bolts. The pillow blocks may include bearings (not shown) between the drive shaft  24  and a race (not shown) of the pillow block. The bearings facilitate rotational and lateral movement of the drive shaft  24  within the pillow blocks. 
   Referring still to  FIG. 1 , the retractor  10  includes a motor  32 . The motor  32  is coupled to the drive shaft  24  to rotate the drive shaft  24  in either direction. Preferably, the motor  32  is a DC motor. The motor  32  is electrically coupled to a circuit (discussed in more detail below) which activates and deactivates the motor  32  in response to tension in a portion of the seat belt  12  which extends from the spool  14 . 
   Because the worm  22  and drive shaft  24  move together laterally, in order for the rotating worm  22  to drive the worm wheel  20 , the worm  22  is held laterally stationary. Preferably, the worm  22  is positioned on the drive shaft  24  such that the worm  22  abuts the second connector  30 . Thus, when the worm  22  rotates in the direction to retract the seat belt  12 , the second connector  30  prevents the worm  22  from screwing past the worm wheel  20  so that the worm  22  drives the worm wheel  20 . Similarly, when the worm  22  rotates in the direction for paying out the seat belt  12 , the first connector  28  may laterally hold the worm  22  and drive shaft  24  for the worm  22  to drive the worm wheel  20 . 
   In a preferred embodiment, the retractor  10  includes a compression spring  34  positioned around the drive shaft  24  in a space between the worm  22  and the first connector  28 . The spring  34  holds the worm  22  in operable engagement with the worm wheel  20  for driving the worm wheel  20  when paying out the seat belt  12 . 
   Generally, a worm  22  and worm wheel  20  gear system can not be “back-driven.” Therefore, a driving force for the system should operate to rotate the worm  22 , not the worm wheel  20 . If the worm wheel  20  experiences a torque (referred to herein as “back-drive torque”), teeth of the worm wheel press against the teeth of the worm  22  and, due to the difference in angles between the teeth on the worm wheel  20  and worm  22 , the driven worm wheel  20  moves the worm  22  laterally along its axis, instead of rotating. 
   This back-drive torque provides lock up for the retractor  10  to prevent rapid extraction of the seat belt  12  such as during an emergency. As illustrated in  FIG. 1 , the seat belt  12  is wound around the spool  14 , and the worm  22  engages the worm wheel  20  such that emergency extraction introduces a counter-clockwise torque on the worm wheel  20 . Alternatively, the seat belt  12 , spool  14 , worm  22  and worm wheel  20  may be arranged such that emergency extraction introduces a clockwise torque on the worm wheel  20 . 
   Preferably, a back-drive torque is introduced when the seat belt  12  is extracted by an external force. When the worm wheel  20  experiences the back-drive torque, the worm wheel  20  slides the worm  22  and drive shaft  24  towards the first connector  28 . The drive shaft  24  passes through the first connector  28  and the worm  22  contacts and compresses the spring  34  against the first connector  28 . Alternatively, a spring  34  may not be used and the worm  22  may contact the first connector  28  directly. The first connector  28  serves as a stop  36 . When the spring  34  is compressed, the first connector  28  prevents further lateral movement of the worm  22  and drive shaft  24 . When the worm  22  abuts the compressed spring  34  and stop  36 , the worm  22  preferably maintains engagement with the worm wheel  20 . Thus, the retractor  10  is locked to prevent further seat belt  12  extraction. 
   Generally, the back-drive force rotates the worm wheel  20  a minimal distance in a counter-clockwise direction before the spool  14  is locked. The spool  14  remains locked so long as the back-drive torque is greater than the bias force of the spring  34 . When the back-drive torque is released, or decreased below the bias force of the spring  34 , the spring  34  moves the worm  22  and drive shaft  24  back to a normal operating position with respect to the worm wheel  20  and the motor  32  is activated to retract or extract the seat belt  12  as needed. The retractor  10  is unlocked. 
   As used herein, “extraction force” refers to a force which causes seat belt  12  extraction at a rate greater than the extraction rate caused by regular use of the seat belt  12 . Generally, regular extraction forces are minimal and do not cause the worm  22  and drive shaft  24  to move laterally before the retractor  10  responds by driving the worm  22  in the direction to pay out the seat belt  12 . The extraction force, referred to herein, occurs when a vehicle experiences an accident or extreme conditions leading to a possible accident such as panic braking, swerving, or the like. 
   In certain embodiments, the retractor  10  includes a switch  38  in electrical communication with the electrical circuit (discussed below) which powers and operates the motor  32  of the retractor  10 . The switch  38  provides an additional safety feature to ensure that when spool  14  is locked, the motor  32  can not be activated in the direction of seat belt  12  extraction. The switch  38  may also serve as a sensor to detect when the retractor  10  has been locked in an emergency. 
   Preferably, the switch  38  is secured to the first connector  28  such that as the drive shaft  24  slides through a passage (not shown) in the connector  28 , the drive shaft  24  activates the switch  38 . The switch  38  may be a microswitch which is closed under normal conditions and opened by the lateral movement of the drive shaft  24 . When the switch  38  is closed, the circuit is provided with an operational flow of power for activating the motor  32 . When the switch  38  is open, the power flow is interrupted such that the motor  32  can not be activated to pay out the seat belt  12  and defeat the locking of the spool  14 . Once the retractor  10  unlocks, the spring  34  slides the worm  22  and drive shaft  24  back into normal operational position which causes the drive shaft  24  to close the switch  38  and restore power flow in the circuit to the motor  32 . 
   In addition to locking the spool  14 , the electric seat belt retractor  10  should pay out and retract the seat belt  12  based on the actions of the occupant. These actions may be determined by sensing the amount of tension present in the portion of the seat belt  12  which extends from the retractor  10 . For example, as an occupant buckles a latch plate to a buckle, the tension in the seat belt  12  increases. Once the seat belt  12  is buckled or unbuckled, the tension decreases. In addition, as an occupant moves their upper torso while buckled in the seat belt  12 , the tension in the seat belt changes once again. 
   Referring now to  FIG. 2A , the retractor  10  includes a tension sensor  40 . The tension sensor  40  measures tension in the seat belt  12  between the retractor  10  and the other end of the belt  12 . Based on the tension, the retractor  10  may be controlled to retract or extract the seat belt  12  as necessary. Preferably, the tension sensor  40  is connected to the retractor frame  18 . Alternatively, the tension sensor  40  may be positioned at other locations along the length of the seat belt  12  extending from the spool  14  of the retractor  10 . 
   Generally, the retractor frame  18  comprises a frame which is anchored to a vehicle. One side of the retractor frame  18  comprises the belt opening  42 . The belt opening  42  is where the seat belt  12  extends from the frame  18 . The seat belt  12  is extracted and retracted from the spool  14  through the belt opening  42 . The belt opening  42  may be of various sizes. For example, the belt opening  42  may comprise one whole side of the frame  18 . Alternatively, the belt opening  42  may be of a minimal size that still allows the seat belt  12  to be extracted and retracted. 
   In one embodiment, the tension sensor  40  serves as a web guide that orients and untwists the seat belt  12  before the seat belt  12  is wound around the spool  14 . In addition, the tension sensor  40  may serve as a door that opens and closes the belt opening  42  in response to the level of tension in the portion of the seat belt  12  extending from the retractor  10 . 
   The tension sensor  40  includes an arm  44  which extends over the belt opening  42 . The arm  44  is sized to substantially cover the belt opening  42 . The arm  44  is pivotally connected to the retractor frame  18  at one side of the belt opening  42 . The arm  44  may be connected by various pivoting mechanisms. For example, the pivot  46  may comprise an axle  47  (Seen in  FIG. 5 ) which passes through the arm  44  and is secured to opposite sides of the frame  18 . 
   The pivot  46  allows the arm  44  to pivot through an angle  48  measured between the arm  44  and a reference line  50  indicated generally by the belt opening  42 . When the arm  44  pivots to substantially cover the belt opening  42 , the angle  48  is about zero degrees. Generally, the pivot  46  allows the arm  44  to pivot freely to form an angle  48  between about zero degrees and about ninety degrees. Alternatively, based on the position and orientation of the tension sensor  40 , the angle  48  may range between about zero degrees and about one-hundred and eighty degrees. Of course different configurations may allow for still different angle ranges. 
   The unconnected end of the arm  44  includes a webbing passage  52 . The extended portion of the seat belt  12  is threaded through the webbing passage  52 . Preferably, the seat belt  12  is wound on the spool  14  such that the seat belt  12  exits the spool  14  and extends from one end of the arm  44  to the other end, the webbing passage  52 . In this manner, tension between where the seat belt  12  winds around the spool  14  and a portion of the seat belt  12  which is threaded through the webbing passage  52  causes the arm  44  to pivot about the pivot  46 . When high tension is present in the seat belt  12 , the arm  44  is extended away from the belt opening  42 , creating an angle  48  of about ninety degrees. When very low or minimal tension is present, the arm  44  substantially covers the belt opening  42  and creates an angle of about zero degrees. In certain embodiments, the arm  44  may include a torsional spring  53  (shown in  FIG. 5 ) which is loaded when tension in the seat belt  12  extends the arm  44 . The torsional spring  53  may bias the arm  44  towards the retractor frame  18  when the seat belt tension is minimal. 
   By sensing the tension, the tension sensor  40  is capable of controlling the retractor  10  to activate and/or deactivate the motor  32  to retract or extract the seat belt  12  as necessary. The tension sensor  40  is in electrical communication with an electrical circuit (See  FIG. 4 ) for powering and controlling the retractor  10 . Components for the electrical circuit may be secured to a circuit board  54  connected to the retractor frame  18 . The tension sensor  40  measures the tension in the seat belt  12  by measuring the position of the arm  44  with respect to the belt opening  42  and translating this position into voltage which is delivered to the motor  32 . 
   In one embodiment, a potentiometer  55  may be used to perform the translations. For example, an axle of a rotary potentiometer  55  may be coupled to the pivot  46  of the tension sensor  40  such that movement of the arm  44  moves a wiper in the potentiometer  55  to vary the level of power provided to the circuit. Preferably, the potentiometer  55  varies the level and polarity of voltage across the potentiometer  55  made available to the circuit. 
   In other embodiments of the invention, the sensor  40  need not be an angular sensor, but may rather be a linear sensor or some other type of sensors. Thus, in place of the arm  44 , an element that translates or moves in a manner different from angular or linear motion may be used. In fact, the sensor  40  need not have any moving elements, but may utilize a sensor that detects relative position, motion, or tension through the use of optical, magnetic, or other intangible effects. In place of the potentiometer  55 , a wide variety of sensors, including Hall effect probes, linear variable displacement transducers (LVDT&#39;s), magnetic readers, optical readers, piezoelectric or resistance-based load cells, and the like may be used. 
   Referring again to  FIG. 2A , when a predetermined level of tension exists in the seat belt  12 , the seat belt  12  moves the arm  44  to an extended position forming an angle  48  of about ninety degrees. The predetermined level of tension may be the amount of tension present when the seat belt  12  is buckled and in normal use. In one embodiment, with the arm  44  between about forty-five degrees and ninety degrees, the potentiometer  55  provides a positive voltage which activates the motor  32  to turn the worm  22  in the direction to pay out the seat belt  12 . Preferably, the voltage level increases as the arm  44  moves from about a forty-five degree angle  48  to about a ninety degree angle  48 . Thus, as the arm  44  moves towards the ninety degree angle  48 , the motor  32  speeds up proportionally until the maximum pay out speed for the motor  32  is reached. 
   Referring now to  FIG. 2B , when minimal or no tension exists in the seat belt  12 , the arm  44  moves towards the retractor frame  18 . As mentioned above, the arm  44  may be moved by gravity or a torsional spring  53  (See  FIG. 5 ). When the arm  44  forms an angle  48  of between about forty-five degrees and about zero degrees, the potentiometer  55  provides a negative voltage which activates the motor  32  to turn the worm  22  in the direction to retract the seat belt  12  onto the spool  14 . Similarly, the potentiometer  55  gradually provides more negative voltage as the angle  48  approaches zero, until the motor  32  reaches a maximum retraction speed. 
   Referring back to  FIG. 2A , as the motor  32  retracts the seat belt  12 , tension is again introduced into the portion of the seat belt passing through the tension sensor  40 . The tension causes the arm  44  to extend. As the arm  44  extends, the negative voltage decreases until the potentiometer  55  fails to provide either negative or positive voltage to the motor  32 . Thus, the circuit provides no power to the motor  32 . The motor  32  is deactivated. Preferably, during normal use, the motor  32  is deactivated when the tension sensor  40  forms about a forty-five degree angle  48 , as seen in  FIG. 1 . 
   The tension sensor  40  allows the retractor  10  to be controlled for paying out and retracting the seat belt  12  in response to the tension in the seat belt  12 . In addition, any slack introduced in the seat belt  12  by, for example, the occupant first buckling the seat belt  12  or moving his/her upper torso, is automatically removed based solely on the tension detected by the tension sensor  40 . Those of skill in the art recognize that the polarity of the voltage for paying out or retracting the seat belt  12  may be reversed from that described above. Furthermore, the angles  48  used to describe deactivation and activation of the motor  32  for retraction and extraction are illustrative. Of course, the tension sensor  40  may provide the activation voltages or no voltage when the arm  44  forms other angles  48  in response to tension in the seat belt  12 . For example, no voltage may be provided by the tension sensor  40  when a sixty degree angle  48  is formed. 
   In conventional retractors, slack in the seat belt  12  is constantly removed by a bias on the seat belt  12  in the direction of seat belt retraction. The bias is created by a coil spring in communication with the spool which is loaded when the seat belt is extracted and recoils once the seat belt is latched or released. The recoil of the coil spring creates a constant tension, or bias, in the seat belt in the direction of retraction. This constant bias can be uncomfortable for the occupant. 
   In contrast, the tension which raises the arm  44  to about forty-five degrees is tension which may be unnoticeable to the occupant. The amount of tension felt in the seat belt  12  when the arm  44  is at forty-five degrees, is affected by the effect of gravity on the arm  44  and any bias provided by a torsional spring  53  on the arm  44  at the pivot  46 . Therefore, the amount of tension in the seat belt  12  when the retractor  10  is deactivated may be adjusted by varying the bias of the torsional spring  53 , weight of the arm  44 , or orientation of the potentiometer  55  with respect to the pivot  46 . 
     FIG. 3A  illustrates a side view of one embodiment of a gear drive system  56  for the present invention. The basic operation of the gear drive system  56  is described above in relation to  FIG. 1 .  FIG. 3A  illustrates the gear drive system  56  in which back-drive torque may be introduced by rapid extraction of the seat belt  12 . However, the gear drive system  56  may be used in other applications which introduce a back-drive torque, for example to determine when a powered system is overloaded. 
   As discussed above, the gear drive system  56  includes a worm wheel  20 , drive shaft  24 , motor  32  and worm  22  fixed to the drive shaft  24 . The drive shaft  24  is secured by one or more connectors  28 ,  30 , such as pillow blocks, which allow rotational and axial movement of the drive shaft  24 . The connectors  28 ,  30  are connected to a base  31  such as a retractor frame  18 . 
   The worm wheel  20  is coupled to a load  58 . In the illustrated embodiment, the load  58  is the rotatable axle  15  of a seat belt retractor spool  14 . Operation of the gear drive system  56  in response to a back-drive torque (indicated by arrow  59 ) introduced by extraction of the seat belt  12  is discussed above. However, different loads  58  may be coupled to the worm wheel  20 . For example, a rack (not shown) for a power window system of a vehicle may be coupled to the worm wheel  20 . 
   If during operation of the motor  32  to move the load  58 , an overload condition exists, the gear drive system  56  automatically deactivates to prevent damage to system components and/or users. An overload condition, as used herein, refers to a condition in which the load  58  is impeded or abnormally accelerated in some manner contrary to normal movement. This overload condition creates a back-drive torque  59  in the worm wheel  20 . 
   For example, in response to the back-drive torque  59 , the worm  22  screws past the worm wheel  20  and moves the drive shaft  24  laterally. Lateral movement of the drive shaft  24  may be controlled by a compression spring  34 . If the overload condition creates a back-drive torque  59  greater than the bias of the spring  34 , the laterally moving drive shaft  24  may be used to stop the system. The sliding drive shaft  24  may activate a switch  38  to interrupt power to the motor  32  and deactivate the system  56 . The switch  38  may act as a sensor  60  for detecting an overload condition for the system  56 . 
   In  FIG. 3A , the gear drive system  56  is illustrated in an overload condition. The drive shaft  24  has moved axially in response to a back-drive torque created in the worm wheel  20 . The switch  38  has been activated and power to the motor  32  is interrupted stopping rotation of the drive shaft  24 . 
   Referring now to  FIG. 3B , if the overload condition is resolved, by reducing or removing the back-drive torque  59 , the switch  38  is deactivated to restore power to the motor  32 . The worm  22  and drive shaft  24  are returned to a normal position. The system  56  may then continue normal operation under control of the tension sensor  40 . 
   Referring now to  FIG. 4 , a functional diagram of an electrical circuit  62  for operating an electric retractor  10  is illustrated. The circuit  62  may comprise a variety of configurations. For example, the components may be connected in series, parallel or some combination of these. The circuit  62  may also include various electrical components which are well known and have been omitted for clarity. 
   The circuit  62  includes the switch  38 , the motor  32  and the tension sensor  40 . The tension sensor  40  is electrically coupled to a power source  64 . Preferably, the power source  64  is the same power source for the electrical system of a vehicle. In one embodiment, the tension sensor  40  includes a potentiometer  55  which regulates the magnitude and polarity of the voltage provided to the motor  32  based on tension in the seat belt  12 . If tension in the seat belt  12  causes the retractor  10  to lock, the drive shaft  24  activates the switch  38  which opens the circuit  62  and stops power flow to the motor  32 . If the tension is released and the drive shaft  24  returns to within normal ranges, the switch  38  is closed and power is restored to the motor  32 . 
   In certain embodiments, the electric retractor  10  comprises an automatic locking system  66 . The automatic locking system  66  is a system which retracts the seat belt  12  onto the spool  14  and does not permit the seat belt  12  to be extracted until the automatic locking system  66  is deactivated. With the automatic locking system  66  activated, an occupant is not able to extract additional seat belt webbing. 
   The automatic locking systems  66  may be used when fastening child safety seats using a regular seat belt system (lap or shoulder). The automatic locking system  66  is activated when a predetermined amount of seat belt webbing  12  has been extracted from the retractor  10 . This predetermined amount may be referred to as an activation threshold. Similarly, the automatic locking system  66  is deactivated when a predetermined amount (a deactivation threshold) of seat belt webbing  12  has been retracted onto the spool  14  of the retractor  10 . Generally, the activation threshold is defined as substantially all of the seat belt  12  being extracted and the deactivation threshold is defined as substantially all of the seat belt  12  being retracted. However, these thresholds may vary. 
   In one embodiment, the automatic locking system  66  includes a sensor  68  and counting module  70  which cooperate to determine when the activation and deactivation thresholds have been reached. The sensor  68  may be a rotational sensor  68  that detects revolutions of the spool  14 . For each rotation, a signal is provided to the counting module  70 . The counting module  70  includes an analog circuit that increments a count, for example, through the use of incrementally variable capacitance or resistance, for each rotation in the direction of seat belt extraction and decrements the count for each rotation in the direction of seat belt retractions. When the count reaches or exceeds a number corresponding to the activation threshold, the counting module  70  activates the automatic locking system  66 . When the count reaches or falls below a number corresponding to the deactivation threshold, the counting module  70  deactivates the automatic locking system  66 . 
   Once activated, the automatic locking system  66  includes well known electrical components for overriding the normal operation of the tension sensor  40 . The automatic locking system  66  then provides power to the motor  32  for retracting the seat belt  12  regardless of the level of tension measured by the tension sensor  40 . In certain embodiments, high tension measured by the tension sensor  40  may be used to deactivate the motor  32  and stop retracting the seat belt  12 . While activated, the automatic locking system  66  prevents powering of the motor  32  for extraction of the seat belt  12 . 
     FIG. 5  illustrates one embodiment of an automatic locking system  66  for use with the present invention. The rotational sensor  68  comprises an optical sensor  68  which is activated by a reflector  72  connected to a wheel  74 . Alternatively, various mechanical sensors may be used to detect revolutions of the spool  14 . The wheel  74  is connected to the spool  14  such that rotation of the spool  14  rotates the wheel  74 . The optical sensor  68  may include a pair of lasers, or sub-sensors (not shown), which allow the direction of rotation to be determined by identifying which sub-sensor was activated first. Activation of the sensor  68 , sends a signal to the counting module  70  which maintains a count as described above. 
   Preferably, a seat belt retractor  10  provides pre-crash pretensioning and pretensioning in response to sensors which determine that an accident is very likely to occur or that an accident has occurred. Pretensioning is the intentional retraction of the seat belt  12  into the retractor  10 , pre-crash pretensioning, in anticipation of an accident. Generally, pre-crash pretensioning occurs a few seconds prior to an accident. Pre-crash pretensioning is activated by one or more vehicle dynamics. A vehicle dynamic is a measurement of one or more characteristics of the operation of a vehicle. For example, vehicle dynamics may include measurements such as sudden braking, loss of traction, spinning of the vehicle, dramatic changes in the pitch and/or yaw of the vehicle, and other such dynamics of a vehicle. The vehicle dynamics may be affected by the speed of the vehicle, condition of the road, and the like. 
   Pre-crash pretensioning retracts the seat belt  12  onto the spool  14  to reduce the amount of slack in the seat belt  12 . Minimal slack improves the ability of the seat belt  12  to protect the occupant in an accident. In addition, pre-crash pretensioning alerts the occupant that the vehicle dynamics indicate an accident may occur. Such an alert may allow the occupant to take evasive actions such as steering corrections or braking to avoid an accident. Preferably, if the accident is avoided, the seat belt retractor  10  should extract a portion of the seat belt  12  to relieve the tension in the seat belt  12  introduced by the pre-crash pretensioning. 
   Accident pretensioning is also an intentional rapid retraction of the seat belt  12  into the retractor  10 . However, in contrast to pre-crash pretensioning, crash pretensioning is activated when a crash sensor is activated. Accident pretensioning occurs milliseconds into the accident. The purpose of crash pretensioning is to remove any excess slack and to assist in positioning the occupant in the seat such that other safety systems can effectively protect the occupant. For example, using the present invention, activating the worm  22  in the direction to retract the seat belt  12  may be done with such a high torque that the tension introduced into the seat belt  12  can re-position the upper torso of an occupant against the seat. Accident pretensioning may or may not be reversible once the accident event ends. 
   Referring now to  FIG. 6 , a functional block diagram illustrates a circuit  76  for controlling the electric retractor  10  in an emergency situation to provide pre-crash pretensioning and crash pretensioning. The circuit  76  includes the power source  64 , motor  32 , switch  38 , and tension sensor  40  discussed above. 
   In addition, the circuit  76  includes an emergency control system  78 . The emergency control system  78  may be very simple or complex. In certain embodiments, the emergency control system  78  may be analog and may be integrated with the circuit  76  and with other safety systems of a vehicle such as airbag systems. Alternatively, the emergency control system  78  may include a simple logic module. The emergency control system  78  overrides the tension sensor  40  and activates the motor  32  to retract the seat belt  12  and provide pre-crash pretensioning or pretensioning based on inputs from vehicle dynamics sensors  80 . 
   Preferably, the emergency control system  78  is in electrical communication with a plurality of vehicle dynamics sensors  80 , designated  80   a – 80   c , which may be positioned throughout a vehicle. Alternatively, the vehicle dynamics sensors  80  may be integrated with the emergency control system  78 . Generally, each sensor  80  measures a single vehicle dynamic. For example, one sensor  80  may comprise an accelerometer for measuring rapid deceleration. The vehicle dynamics sensors  80  may send signals continuously or when the dynamic is outside an acceptable threshold range. 
   In addition, the emergency control system  78  receives input from a crash sensor  82 . A crash sensor  82  is activated when the vehicle experiences an impact during an accident. Of course, other events in the initial stages of an accident may also trigger a crash sensor  82 . 
   Generally, the emergency control system  78  receives inputs from the sensors  80  and/or one or more crash sensors  82 . Based on these inputs and an algorithm, the emergency control system  78  determines whether pre-crash pretensioning or crash pretensioning should be activated. 
   If the vehicle dynamics sensors  80  send signals to the emergency control system  78  and the crash sensor  82  is not activated, pre-crash pretensioning is activated. If the vehicle dynamics sensors  80  stop sending signals or send signals that vehicle dynamics have returned to normal and the crash sensor  82  is not activated, the emergency control system  78  may stop overriding the tension sensor  40 . Then, because the pre-crash pretensioning put tension in the seat belt  12 , the tension sensor  40  activates the motor  32  to extract the seat belt  12  until the tension sensor  40  detects normal tension in the seat belt  12 . In this manner, the pre-crash pretensioning is reversible. 
   If the vehicle dynamics sensors  80  send signals to the emergency control system  78  and the crash sensor  82  is activated, the emergency control system  78  overrides the tension sensor  40  and activates the motor  32  to provide crash pretensioning. In certain embodiments, the emergency control system  78  may overpower the motor  32  such that a maximum retraction torque available from the motor  32  is used to retract the spool  14 . Overpowering the motor  32  may damage the motor  32 . However, safety of the occupant is most important and the motor  32  can be repaired or replaced if necessary. 
   In certain embodiments, the emergency control system  78  may provide pre-crash pretensioning initially followed by crash pretensioning once a crash sensor  82  is activated. Thus, pre-crash pretensioning and crash pretensioning may be provided in stages. The motor  32  allows for pre-crash pretensioning and crash pretensioning without expensive pyrotechnic pretensioners. In addition, the electric retractor  10  of the present invention allows for reversible pre-crash pretensioning in the event that an accident is avoided. 
   In summary, with reference generally to  FIGS. 1–6 , the present invention provides an electric retractor  10  for powered extraction and retraction of a seat belt  12 . The retractor  10  provides powered extraction and retraction using a simple worm gear drive system  56  (See  FIGS. 3A ,  3 B) which locks in response to a back-drive torque introduced by extraction forces due to rapid extraction of the seat belt  12 . 
   Powered extraction and retraction is controlled by a tension sensor  40  which proportionally activates the motor  32  to retract or extract the seat belt  12  based on the tension present in the seat belt  12 . The retractor  10  provides a relatively constant degree of tension on the seat belt while permitting relatively free occupant motion. Furthermore, the retractor  10  provides advanced features such as an automatic locking system  66 , reversible pre-crash pretensioning, and crash pretensioning using electronics and emergency control systems  78 . The retractor  10  provides the advanced features without pyrotechnic components and with fewer mechanical components than conventional retractors. 
   More broadly, the present invention provides enhanced structures and methods by which the tension in a flexible member may be accurately controlled. These enhanced structures and methods are applicable over a wide range of applications. 
   The present invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.