Patent Publication Number: US-8991278-B2

Title: Overforce protection mechanism

Description:
BACKGROUND 
     1. Field 
     Embodiments of the invention relate to the field of yieldable connecting rods; and more specifically, to automatic release mechanisms for connecting rods. 
     2. Background 
     Minimally invasive surgery (MIS) (e.g., endoscopy, laparoscopy, thoracoscopy, cystoscopy, and the like) allows a patient to be operated upon through small incisions by using elongated surgical instruments introduced to an internal surgical site. Generally, a cannula is inserted through the incision to provide an access port for the surgical instruments. The surgical site often comprises a body cavity, such as the patient&#39;s abdomen. The body cavity may optionally be distended using a clear fluid such as an insufflation gas. In traditional minimally invasive surgery, the surgeon manipulates the tissues by using hand-actuated end effectors of the elongated surgical instruments while viewing the surgical site on a video monitor. 
     The elongated surgical instruments will generally have an end effector in the form of a surgical tool such as a forceps, a scissors, a clamp, a needle grasper, or the like at one end of an elongate tube. The surgical tool is generally coupled to the elongate tube by one or more articulated sections to control the position and/or orientation of the surgical tool. An actuator that provides the actuating forces to control the articulated section is coupled to the other end of the elongate tube. A means of coupling the actuator forces to the articulated section runs through the elongate tube. Two actuators may be provided to control two articulated sections, such as an “arm” that positions the surgical tool and a “wrist” the orients and manipulates the surgical tool, with means for coupling both actuator forces running through the elongate tube. 
     It may desirable that the elongate tube be somewhat flexible to allow the surgical instrument to adapt to the geometry of the surgical access path. In some cases, the articulated sections provide access to a surgical site that is not directly in line with the surgical access port. It may be desirable to use cables as the means of coupling the actuator forces to the articulated sections because of the flexibility they provide and because of the ability of a cable to transmit a significant force, a substantial distance, through a small cross-section. However, a cable is only able to safely transmit a limited force. Thus it is generally necessary to provide a means for limiting the amount of force applied to the cable. 
     In a surgical application, the cable may be driven through an input range of motion at an input end by an actuator. The input range of motion is intended to drive an end effector, such as a surgical tool or articulated joint, through a corresponding output range of motion. However, the end effector may be prevented from moving, such as by contacting a solid obstruction. Thus the end effector may hold the output end of the cable in a fixed position, which may be at the end of its range of motion, while the actuator attempts to move the input end of the cable through its full range of motion. This will result in breakage of the cable without a protective mechanism. 
     Backdrivability, the ability of the mechanical system to move the input axis from the output axis, is one possible protective mechanism. However, a cable driven output lacks backdrivability because forces cannot be reliably transmitted by pushing on a cable. Without backdrivability, elastic components in series to the actuator output may be added as a protective mechanism. It is difficult to have enough elasticity and enough output force simultaneously. 
     A cable of small diameter, such as would be used to transmit motive forces to the end effectors of a laparoscopic surgical instrument, needs to be able to transmit forces that are close to the safe working limit of the cable. Thus, a protective mechanism for the cable must allow forces to be transmitted up to the protective limit and then prevent the forces from increasing significantly thereafter while allowing a full range of input motion. 
     In view of the above, it would be desirable to provide an improved apparatus and method for limiting forces applied to cables that keeps the cable at or below its load limit with the output end held at an end of its range of motion while the input end moves through its full range of motion. 
     SUMMARY 
     A overload protection mechanism protects a driven load, such as a driven lever. An overload lever is pivotally coupled to a first part of the driven load. The overload lever has a first end that receives an applied force and an opposing second end. A zero length spring mechanism is coupled to a second part of the driven load spaced apart from the first part and to the second end of the overload lever. The zero length spring mechanism urges the second end of the overload lever toward the second part of the driven load with a force that is substantially proportional to the distance between the second end of the overload lever and the second part of the driven load. A stop mechanism is coupled to the zero length spring mechanism to maintain a minimum distance between the second end of the overload lever and the second part of the driven load. 
     Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention by way of example and not limitation. In the drawings, in which like reference numerals indicate similar elements: 
         FIG. 1  is a simplified perspective view of a robotic surgical system with a robotically controlled surgical instrument inserted through a port in a patient&#39;s abdomen. 
         FIG. 2  is a perspective view of an overload protected cable driving mechanism. 
         FIG. 3  is a perspective view of an embodiment of a “zero length” spring. 
         FIG. 4  is a side view of a cable driving lever from the cable driving mechanism shown in  FIG. 2  with the cable driving lever in a level position for analyzing forces applied to the driven cable. 
         FIG. 5  is a schematic force diagram of the cable driving lever shown in  FIG. 4 . 
         FIG. 6  is a schematic force diagram of the spring overload protection portion of the cable driving lever shown in  FIG. 4 . 
         FIG. 7  is a side view of the cable driving lever shown in  FIG. 4  with the cable driving lever at the first end of its range of travel while the coupler link has moved through its range of travel to the opposite end of the range. 
         FIG. 8  is a schematic diagram of an embodiment of the invention using first class levers for the driving lever arm and the overload lever. 
         FIG. 9  is a schematic diagram of an embodiment of the invention using a first class lever for the driving lever arm and a second class lever for the overload lever. 
         FIG. 10  is a schematic diagram of an embodiment of the invention using a third class lever for the driving lever arm and a first class lever for the overload lever. 
         FIG. 11  is a schematic diagram of an embodiment of the invention using a third class lever for the driving lever arm and a second class lever for the overload lever. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known devices, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. 
     In the following description, reference is made to the accompanying drawings, which illustrate several embodiments of the present invention. It is understood that other embodiments may be utilized, and mechanical compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of the present disclosure. The following detailed description is not to be taken in a limiting sense, and the scope of the embodiments of the present invention is defined only by the claims of the issued patent. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like may be used herein for ease of description to describe one element&#39;s or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising” specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. 
       FIG. 1  is a simplified perspective view of a robotic surgical system  100 , in accordance with embodiments of the present invention. The system  100  includes a support assembly  110  mounted to or near an operating table supporting a patient&#39;s body  122 . The support assembly  110  supports one or more surgical instruments  120  that operate on a surgical site within the patient&#39;s body  122 . 
     The term “instrument” is used herein to describe a device configured to be inserted into a patient&#39;s body and used to carry out surgical procedures. The instrument includes a surgical tool, such as a forceps, a needle driver, a shears, a bipolar cauterizer, a tissue stabilizer or retractor, a clip applier, an anastomosis device, an imaging device (e.g., an endoscope or ultrasound probe), and the like. Some instruments used with embodiments of the invention further provide an articulated support for the surgical tool so that the position and orientation of the surgical tool can be manipulated. 
     The simplified perspective view of the system  100  shows only a single instrument  120  to allow aspects of the invention to be more clearly seen. A functional robotic surgical system would further include a vision system that enables the operator to view the surgical site from outside the patient&#39;s body  122 . The vision system can include a video monitor for displaying images received by an optical device provided at a distal end of one of the surgical instruments  120 . The optical device can include a lens coupled to an optical fiber which carries the detected images to an imaging sensor (e.g., a CCD or CMOS sensor) outside of the patient&#39;s body  122 . Alternatively, the imaging sensor may be provided at the distal end of the surgical instrument  120 , and the signals produced by the sensor are transmitted along a lead or wirelessly for display on the monitor. An illustrative monitor is the stereoscopic display on the surgeon&#39;s cart in the da Vinci® Surgical System, marketed by Intuitive Surgical, Inc., of Sunnyvale Calif. 
     A functional robotic surgical system would further include a control system for controlling the insertion and articulation of the surgical instruments  120 . This control may be effectuated in a variety of ways, depending on the degree of control desired, the size of the surgical assembly, and other factors. In some embodiments, the control system includes one or more manually operated input devices, such as a joystick, exoskeletal glove, or the like. These input devices control servo motors which, in turn, control the articulation of the surgical assembly. The forces generated by the servo motors are transferred via drivetrain mechanisms, which transmit the forces from the servo motors generated outside the patient&#39;s body  122  through an intermediate portion of the elongate surgical instrument  120  to a portion of the surgical instrument inside the patient&#39;s body  122  distal from the servo motor. Persons familiar with telemanipulative, teleoperative, and telepresence surgery will know of systems such as the da Vinci® Surgical System and the Zeus® system originally manufactured by Computer Motion, Inc. and various illustrative components of such systems. 
     The surgical instrument  120  is shown inserted through an entry guide cannula  124 , e.g., a single port in the patient&#39;s abdomen. A functional robotic surgical system may provide an entry guide manipulator (not shown; in one illustrative aspect the entry guide manipulator is part of the support system  110 ) and an instrument manipulator  130 . The entry guide  124  is mounted onto the entry guide manipulator  130 , which includes a robotic positioning system for positioning the distal end of the entry guide  124  at the desired target surgical site. The robotic positioning system may be provided in a variety of forms, such as a serial link arm having multiple degrees of freedom (e.g., six degrees of freedom) or a jointed arm that provides a remote center of motion (due to either hardware or software constraints) and which is positioned by a setup joint mounted onto a base. Alternatively, the entry guide manipulator may be manually maneuvered so as to position the entry guide  124  in the desired location. In some telesurgical embodiments, the input devices that control the manipulator(s) may be provided at a location remote from the patient (outside the room in which the patient is placed). The input signals from the input devices are then transmitted to the control system, which, in turn, manipulates the manipulators  130  in response to those signals. The instrument manipulator may be coupled to the entry guide manipulator such that the instrument manipulator  130  moves in conjunction with the entry guide  124 . 
     The surgical instrument  120  is detachably connected to the robotic instrument manipulator  130 . The robotic manipulator includes a coupler  132  to transfer controller motion from the robotic manipulator to the surgical instrument  120 . The instrument manipulator  130  may provide a number of controller motions which the surgical instrument  120  may translate into a variety of movements of the end effector on the surgical instrument such that the input provided by a surgeon through the control system is translated into a corresponding action by the surgical instrument. 
       FIG. 2  is a perspective view of a cable driving mechanism that is used in the surgical instrument  120 . Forces applied on an input gimbal plate  200  drive attached cables  222 ,  224 ,  226 . The input gimbal plate  200  is coupled to three lever arms  212 ,  214 ,  216  by three coupler links  202 ,  204 ,  206 . Each lever arm  212  is supported by a pivot  208  between a first end  207  and a second end  209  of the lever arm. A first end  203  of each of the coupler links  202  is pivotally coupled to an overload protection mechanism  230  on each of the lever arms  212 . A second end  201  of each of the coupler links  202  is pivotally coupled to the input gimbal plate  200 , such as by a ball and socket connection. The second ends of the coupler links are not collinear so that any change in the position of the input gimbal plate  200  will move at least one of the coupler links  202 ,  204 ,  206 . Movement of the coupler links is transmitted by the cables  222 ,  224 ,  226  to control, position, and/or orient any of a variety of surgical devices such as forceps, a needle driver, a cautery device, a cutting tool, an imaging device (e.g., an endoscope or ultrasound probe), or a combined device that includes a combination of two or more various tools and imaging devices. 
     Each coupler link  202  applies a force to the first end  207  of the lever arm  212 . The lever arm transfers that force to the cable  222  coupled to the second end  209  of the lever arm with multiplication of the force and displacement according to the well understood principles of levers. The coupler link  202  is coupled to the first end  207  of the lever arm  212  through an overload lever  232 . The overload lever is supported by a pivot point  238 . A first end  203  of the overload lever  232  is pivotally coupled to the coupler link  202 . An opposing second end  236  of the overload lever  232  is coupled to a pivot  240  on the first end  207  of the lever arm  212  by a preloaded spring  230  that urges the second end of the overload lever toward the first end of the lever arm. A stop  234  limits the travel of the second end of the overload lever toward the first end of the lever arm. 
     If the force applied to the first end  203  of the overload lever  232 , with the force multiplication of the overload lever, is less than the force required to overcome the force of the preloaded spring  230  urging the second end  236  of the overload lever toward the first end of the coupler link, then the overload lever provides a solid pivotal connection between the first end  203  of the coupler link  202  and the lever arm  212 . When the force applied to the first end  203  of the overload lever  232  reaches the force required to overcome the force of the preloaded spring  230 , the overload lever will begin to rotate, in a clockwise direction for the embodiment illustrated, limiting the amount of force the coupler link  202  can apply to the lever arm  212 . 
       FIG. 3  is a perspective view of an embodiment of a so-called “zero length” spring  230  that couples the second end  236  of the overload lever  232  to the first end  240  of the lever arm  212 . The “zero length” spring operates substantially as an ideal tension spring having ends connected to second end  236  of the overload lever and the first end  240  of the lever arm  212 . An ideal spring provides a force that is proportional to the distance between its ends  236 ,  240 . Thus, the ideal spring provides a zero force when it has a zero length. It will be appreciated that a real tension spring cannot have a zero length and that it will provide a zero force at some finite length. A so-called “zero length” spring is a spring mechanism that provides a force that is proportional to the distance between its ends, displacement, and which would provide a zero force if it had a zero length. In other words, the slope of a line that plots force against displacement passes through the origin of zero force at zero displacement. A “zero length” spring need not actually be capable of providing a spring having an effective length of zero. 
     The “zero length” spring shown in  FIG. 3  includes a first end cap  302  that is pivotally coupled to the first end  240  of the lever arm  212 . A pair of compression springs  304  are supported at a first end by the first end cap  302 . A slider  300  passes through the first end cap  302  and the compression springs  304 . A second end cap  306  supports a second end of the compression springs  304 . The second end cap  306  is coupled to the slider  300 . Thus the pair of compression springs  304  are captured on slider and held in compression between the first end cap  302  and the second end cap  306 . As the end  236  of the slider  300  is drawn away from the pivotal support  240  of the first end cap  302 , the second end cap  306  compresses the pair of compression springs  304 . This provides a spring force urging the end  236  of the slider  300  toward the pivotal support  240  of the first end cap  302 . The initial compression of the pair of compression springs  304  is chosen so that the assembly operates substantially as a “zero length” spring. 
     The overload protection mechanism will now be analyzed with reference to  FIGS. 4-6 .  FIG. 4  is a side view of a cable driving lever from the cable driving mechanism shown in  FIG. 2  with the cable driving lever arm  212  in a level position for analyzing forces applied to the driven cable  222 . The cable driving lever arm  212  and the coupler link  202  are at a first end of their range of travel. The stop portion  234  of the first end cap  302  has been removed to allow the “zero length” spring to be seen more clearly. The forces applied to the driven cable  222  will be proportional to the forces applied to the lever arm  212  as determined by the geometry of the lever arm. Limiting the forces applied to the lever arm  212  is therefore sufficient for limiting the forces applied to the driven cable  222 . 
     The forces applied to the first end  203  of the overload lever  232  by the coupler link  202  are balanced by the forces applied to the second end  236  of the overload lever by the “zero length” spring  230 . Once the preload forces of the spring  230  are overcome, the overload lever  232  will begin to rotate and limit the amount of force that is applied to the lever arm  212 . 
       FIG. 5  is a schematic diagram showing the forces generated by the components shown in  FIG. 4 . The force applied by the coupler link  202  is supported by the overload lever pivot  238  and the force therefore creates a rotational moment that is equal to the vertically applied force F times the distance/from the center of rotation to the point of application for the load times the sine of the angle θ between the load arm and a vertical reference as suggested by the rotational vector (Fl sin θ) at the right of  FIG. 5 . The rotational moment created by the applied force is counterbalanced by a moment created by the “zero length” spring  230  as suggested by the rotational vector at the left of  FIG. 5 . 
     Referring to  FIG. 4 , the portion of the “zero length” spring  230  that extends between the second end  236  of the overload lever  232  and the pivot  240  on the first end  207  of the lever arm  212  acts as a tension spring with a spring constant K. Therefore we may analyze the forces applied by the “zero length” spring  230  with reference to the triangle formed by the imaginary lines shown as triangle ovw. The center of the overload lever pivot  238  is represented as point o, the center of the connection between the second end  236  of the overload lever  232  and the spring  230  as point v, and the center of the connection between the pivot  240  on the first end  207  of the lever arm  212  and the spring as point w. 
     For the overload lever  232  to be in equilibrium, the moment M o  about the point o needs to be zero. From  FIGS. 5 and 6  we can determine the equation for the moment M o  about the point o as:
 
 M   o   =Fl  sin θ− K ( x−x   o ) t= 0
 
where K is the spring constant of the real springs  304 , x o  is the initial length of the effective tension spring formed by the “zero length” spring  230 , and x is the length of the effective spring  234 . The effective tension spring is the portion  234  of the spring  230  that extends from the second end  236  of the overload lever  232  (point v), and the pivot  240  on the first end  207  of the lever arm  212  (point w) and it is configured as a zero length spring. The spring force of the real springs  304  is configured so that the real springs provide a spring force that is substantially proportional to the distance between the ends of the effective spring  234  along the line x.
 
     The spring force acting through the effective spring  234  creates a moment about the center of the overload lever  232  by acting on an effective moment arm which has the length t of a line from the center of the shaft o normal to the line vw that represents the portion  234  of spring  230  that acts as a zero length tension spring. Hence, K(x−x o )t is the moment force created by the spring that counterbalances the moment created by the applied force  202 . Since this is a zero length spring, x o =0. 
     Rearranging the terms of the equation we have
 
 Fl  sin θ= Kxt  
 
     With the overload lever  232  at an angle theta (θ) to a vertical reference we can construct a right triangle oyv where the portion of the overload lever  232  between the pivot o  238  and the sprint connection v  236  forms the hypotenuse with a length a. The base of triangle oyv has a length of a sin θ. Using the similarity of triangle wvy to triangle wzo:
 
 t/b=a  sin θ/ x  
 
Rearranging the equation to solve for t:
 
 t=ab  sin θ/ x  
 
Substituting for t in the moment balance equation:
 
 Fl  sin θ= Kxab  sin θ/ x  
 
 Fl=Kxab/x  
 
 Fl=Kab  
 
Rearranging the terms to solve for the force F needed to rotate the overload lever  232 , we have:
 
 F=Kab/l  
 
     Thus, the equation for the force F indicates that the force is constant and independent of the angle theta θ of the link. Therefore, once the force applied to the overload lever  232  reaches Kx i  where x i  is the initial preload length of the effective tension spring because of the stop  234  that prevents the overload lever from rotating to the point where it is completely unloaded, the vertically applied force necessary to rotate the overload lever will remain substantially constant. 
       FIG. 7  is a side view of the cable driving lever shown in  FIG. 4  with the cable driving lever arm  212  still at the first end of its range of travel while the coupler link  202  has moved through its range of travel to the opposite end of the range. The rotation of the overload lever  232  limits the forces applied to the lever arm  212  and hence the forces applied to the driven cable  222 . 
     It will be noted that the length of the lever arm between the end  203  of the coupler link  202  that connects to the overload lever and the pivot point  208  of the lever arm  212  changes as the overload lever  232  rotates, which causes some variation in the forces applied to the driven cable  222  over the range of motion of the overload lever. 
     It will be further noted that the overload lever  232  may be used in configurations where the force applied to the overload lever is not applied in a direction that is parallel to the line that connects the center of the overload lever pivot  238  (point o) and the center of the pivot  240  (point w) that connects the zero length spring to the cable driving lever arm. This will cause variations in the force applied to the driven load as the configuration deviates from the configuration analyzed above. However, the described overload mechanism will still allow the force input to move through its range of motion with the driven output held in a fixed position and limit the force applied to the driven output to a substantially constant value. For example, a typical configuration of the type illustrated can limit the force applied to the driven output to within about ±25% of a nominal value as the direction of the force input varies by about 10 degrees from the ideal direction. 
     The embodiments described above and the corresponding illustrations show the use first class levers for the driving lever arm and the overload lever. First class levers have a fulcrum point that is between the applied force and the driven load. The invention may also be practiced using second or third class levers for either of the driving lever arm or the overload lever or both. Second class levers have the driven load between the fulcrum and the applied force. Third class levers have the applied force between the fulcrum and the driven load. 
       FIG. 8  is a schematic diagram of an embodiment of the invention using first class levers for the driving lever arm  802  and the overload lever  800 . The driving lever arm  802  is supported by a fulcrum  814  that is between the applied force  812  and the driven load  816 . The applied force  812  acts on the driving lever arm  802  through the overload lever  800 . The overload lever  800  is supported by a fulcrum  810  that is supported by the driving lever arm  802 . The overload lever fulcrum is between the applied force  812  and the load of the zero length spring  806 . The zero length spring  806  is coupled to a point  808  on the driving lever arm  802 . The other end of the zero length spring  806  is coupled to the overload lever  800  to urge rotation of the overload lever in opposition to the applied force  812 . The overload lever fulcrum  814  is between the applied force  812  and the load of the zero length spring  806 . The stop  804  limits the rotation of the overload lever  800  to provide a preload force that must be overcome before the overload lever rotates in response to the applied force  812  to prevent an overloading force being delivered to the driven load  816 . When the applied load is less than the preload force, the overload lever  800  and the driving lever arm  802  move together as a rigid lever. Thus the lever provides a stiff force transmission unless the preload force is exceeded. 
       FIG. 9  is a schematic diagram of an embodiment of the invention using a first class lever for the driving lever arm  902  and a second class lever for the overload lever  900 . The driving lever arm  902  is supported by a fulcrum  914  that is between the applied force  912  and the driven load  916 . The applied force  912  acts on the driving lever arm  902  through the overload lever  900 . The overload lever  900  is supported by a fulcrum  910  that is supported by the driving lever arm  902 . The overload lever fulcrum is between the applied force  912  and the load of the zero length spring  906 . The zero length spring  906  is coupled to a point  908  on the driving lever arm  902 . The other end of the zero length spring  906  is coupled to the overload lever  900  to urge rotation of the overload lever in opposition to the applied force  912 . The overload lever fulcrum  914  is to one side of the applied force  912  and the load of the zero length spring  906 . The stop  904  limits the rotation of the overload lever  900  to provide a preload force that must be overcome before the overload lever rotates in response to the applied force  912  to prevent an overloading force being delivered to the driven load  916 . 
       FIG. 10  is a schematic diagram of an embodiment of the invention using a third class lever for the driving lever arm  1002  and a first class lever for the overload lever  1000 . The driving lever arm  1002  is supported by a fulcrum  1014  that is to one side of the applied force  1012  and the driven load  1016 . The applied force  1012  acts on the driving lever arm  1002  through the overload lever  1000 . The overload lever  1000  is supported by a fulcrum  1010  that is supported by the driving lever arm  1002 . The overload lever fulcrum is between the applied force  1012  and the load of the zero length spring  1006 . The zero length spring  1006  is coupled to a point  1008  on the driving lever arm  1002 . The other end of the zero length spring  1006  is coupled to the overload lever  1000  to urge rotation of the overload lever in opposition to the applied force  1012 . The overload lever fulcrum  1014  is between the applied force  1012  and the load of the zero length spring  1006 . The stop  1004  limits the rotation of the overload lever  1000  to provide a preload force that must be overcome before the overload lever rotates in response to the applied force  1012  to prevent an overloading force being delivered to the driven load  1016 . 
       FIG. 11  is a schematic diagram of an embodiment of the invention using a third class lever for the driving lever arm  1102  and a second class lever for the overload lever  1100 . The driving lever arm  1102  is supported by a fulcrum  1114  that is to one side of the applied force  1112  and the driven load  1116 . The applied force  1112  acts on the driving lever arm  1102  through the overload lever  1100 . The overload lever  1100  is supported by a fulcrum  1110  that is supported by the driving lever arm  1102 . The overload lever fulcrum is between the applied force  1112  and the load of the zero length spring  1106 . The zero length spring  1106  is coupled to a point  1108  on the driving lever arm  1102 . The other end of the zero length spring  1106  is coupled to the overload lever  1100  to urge rotation of the overload lever in opposition to the applied force  1112 . The overload lever fulcrum  1114  is to one side of the applied force  1112  and the load of the zero length spring  1106 . The stop  1104  limits the rotation of the overload lever  1100  to provide a preload force that must be overcome before the overload lever rotates in response to the applied force  1112  to prevent an overloading force being delivered to the driven load  1116 . 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. The description is thus to be regarded as illustrative instead of limiting.