Abstract:
A system and method for controlling the collisions between elements of an automotive rack and pinion steering apparatus and an end-of-travel stop is presented. The system comprises a sensor for sensing a set of dynamic variables of the rack and pinion steering apparatus and operative thereby to generate as output therefrom a first set of signals indicative of the set of dynamic variables; a controller responsive to the first set of signals and operative thereby to provide as output therefrom a modified torque assist command; and a motor drive assembly drive assembly responsive to the modified torque assist command and operative thereby to provide modified torque assistance to the rack and pinion steering apparatus. In another embodiment, the dynamic data is used to calculate a torque limit to be imposed upon the torque assist command whenever the steering system is close to end-of-stop. The two embodiments may also be superimposed. The invention reduces torque at end-of-travel impacts, and helps preserve the mechanical integrity of the power steering system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Nos. 60/154,612, filed on Sep. 17, 1999, and 60/154,683, also filed on Sep. 17, 1999, the disclosures of both of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     In electric power steering (EPS) systems some of the highest loads acting upon system components occur when—due to high handwheel rpm and thus high kinetic energy of the system—the system is brought to an abrupt halt at an end-of-travel stop. Such uncontrolled stops inflict high impact forces upon electrical and mechanical components causing high stress and possible failure thereof. Thus, it is desirable to control the speed (and thus kinetic energy) of the system components over the extent of their travel and at critical points, such as when the system approaches an end-of-travel stop. 
     SUMMARY OF THE INVENTION 
     A system for controlling the collisions between elements of an automotive rack and pinion steering apparatus at an end-of-travel stop is disclosed. The system comprises sensors for sensing a set of variables of the rack and pinion steering apparatus, namely an angular position of a steering wheel (or linear position of the rack), a rate of change thereof, and the angle (or linear distance for the rack) to an end-of-stop. From this information, a modification factor n may be calculated that is multiplied by a power steering torque assist command (TAC) to produce a modified torque assist command (MTAC). This modified command may represent a reduced torque assist or even a negative torque assist as required to prevent the steering system from striking the end-of-stop too hard. 
     In an alternative embodiment, the dynamic data is used to calculate a torque limit to be imposed upon the torque assist command whenever the steering system is close to an end-of-stop. The two embodiments may also be superimposed. 
     The invention reduces torque at end-of-travel impacts, and helps preserve the mechanical integrity of the power steering system. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a generalized diagrammatic representation of an automotive rack and pinion steering apparatus in communication with a sensor, a motor drive assembly and a controller. 
     FIG. 2 is a first generalized schematic representation of the signal flow between the automotive rack and pinion steering apparatus and the sensor, the motor drive assembly and the controller of FIG.  1 . 
     FIG. 3 is a second generalized schematic representation of the signal flow between the automotive rack and pinion steering apparatus and the sensor, the motor drive assembly and the controller of FIG.  1 . 
     FIG. 4 is a generalized schematic representation of the signal flow the control system. 
     FIG. 5 is a first exemplary graphical representation of the functional relationship between the error signal and a dimensionless number n. 
     FIG. 6 is a second exemplary graphical representation of the functional relationship between the error signal and a dimensionless number n. 
     FIG. 7 is a third exemplary graphical representation of the functional relationship between the error signal and a dimensionless number n. 
     FIG. 8 is a fourth exemplary graphical representation of the functional relationship between the error signal and a dimensionless number n. 
     FIG. 9 is a fifth exemplary graphical representation of the functional relationship between the error signal and a dimensionless number n. 
     FIG. 10 is a sixth exemplary graphical representation of the functional relationship between the error signal and a dimensionless number n. 
     FIG. 11 is a flow chart of the modification factor embodiment disclosed herein. 
     FIG. 12 is a graph showing exemplary assist torque limit curves for three different angular steering positions for the torque limit embodiment. 
     FIG. 13 is a flow diagram representing the algorithm used in the torque limit embodiment. 
     FIGS. 14A and 14B show scatter plots of rotations per minute squared versus pinion torque for torque-limit-protected and unprotected systems. 
     FIG. 15 is a flow diagram of an end of torque limit system that does not initially use an actual steering angular position signal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a generalized diagrammatic representation of an automotive rack and pinion steering apparatus  100  in communication with a sensor  200 , a motor drive assembly  300 , and a controller  400 . The rack and pinion steering apparatus  100  generally comprises a steering wheel  102  connected to a steering column  104 . The steering wheel  102  is subject to clockwise or counterclockwise steering commands from a driver. The steering column  104  is connected to a rack  108  through a pinion gear  106 . The rack  108  is connected to one or more roadwheels  112  through a steering linkage  110 . The driver thus is able to steer an automobile by directing the position of the roadwheel  112  through the rack and pinion steering apparatus  100 . 
     Referring to FIG. 2, a first generalized schematic representation of the signal flow between the automotive rack and pinion steering apparatus  100 , the sensor  200 , the motor drive assembly  300  and the controller  400  of FIG.  1 . The rack and pinion steering apparatus  100  is subject to a driver torque, T d , as a result of the driver steering commands; as well as a modified torque assistance, T m    302 , provided by the motor drive assembly  300 . The modified torque assistance, T m    302 , is denoted by a heavy arrow to indicate that this connection between the motor drive assembly  300  and the rack and pinion steering apparatus  100  is a mechanical link, rather than a communicative link. The rack and pinion steering apparatus  100  thereby provides roadwheel torque  116  to the roadwheels  112  for steering. The sensor  200  is operative to sense the torque, T c    202 , on the steering column  104  and the angular position, θ c    204 , of the steering column  104  and to provide measurements thereof to the controller  400 . The controller  400  is responsive to T c    202  and θ c    204  and operative thereby to provide as output therefrom a modified torque assist command MTAC  518 . The motor drive assembly  300  is thence responsive to the modified torque assist command MTAC  518  and provides the rack and pinion steering apparatus  100  with the aforesaid modified torque assistance, T m    302 . 
     Referring to FIG. 3, there is depicted a second generalized schematic representation of the signal flow between the automotive rack and pinion steering apparatus  100 , the sensor  200 , the motor drive assembly  300  and the controller  400  of FIG.  1 . The controller  400  comprises a steering assist subsystem  402  responsive to the torque, T c    202 , on the steering column  104  and the angular steering position, θ c    204 , and operative thereby to provide as output therefrom a torque assist command TAC  404  and the angular velocity, dθ c /dt=({dot over (θ)} c )=ω c    406 , of the steering column  104 . The torque assistance command, TAC  404 , is such that, absent the effect of an impact avoidance system  500 , a torque assistance, T a , is provided the driver at the steering column  104 . In FIG. 3 the impact avoidance system  500  is responsive to the torque assist command TAC  404 , the angular velocity, ω c  ({dot over (θ)} c )  406 , an the angular steering position signal, θ c    204  and operative thereby to provide as output therefrom a modified torque assist command, MTAC  518 . The modified torque assist command, MTAC  518 , is conveyed to the motor drive assembly  300  whereupon modified torque assistance, T m , is provided to the driver. 
     Impact Avoidance Systems 
     Two preferred embodiments of an impact management system are disclosed. The first calculates a modification factor, m  514 , to be multiplied by the torque assist command TAC  404  to provide the modified torque assist command, MTAC  518 . The other embodiment calculates a torque limit. If the torque assist command, TAC  404 , exceeds the torque limit, then the torque assist command is adjusted accordingly so as to produce the MTAC  518 . Both embodiments may be implemented simultaneously on the same system. 
     Modification Factor Embodiment 
     Reference will now be had to FIGS. 4 through 11, which refer to the modification factor embodiment of the invention. 
     Referring to FIG. 4, there is depicted a generalized schematic representing the signal flow of the impact avoidance system  500  of the invention. In FIG. 4, the angular steering position, θ c   204 , originating from the sensor  200 , is operated on by an absolute value operator  502  producing the absolute value, |θ c |  504 , of the angular steering position θ c    204 . An end-of-travel set point, θ cot    506  is provided. The end-of-travel θ cot    506  point is the value that the angular steering position, θ c    204  would be when the rack and pinion steering apparatus  100  has reached end-of-travel. The difference between |θ c  |  504  and θ cot    506  is calculated by a summing junction  508  yielding as output therefrom an error signal, E  510 . The error signal, E  510 , is indicative of the angular distance to the end-of-travel. The error signal, E  510 , and the angular velocity, ω c  ({dot over (θ)} c )  406  are thence provided to a database  512 . Based upon the error signal, E  510 , the angular velocity, ω c  ({dot over (θ)} c )  406  of the steering column  104 , and a predefined function, f i (E, θ), a number, n=f i  (E, θ),  514  having a value −1≦n≦+1 is generated as output from the database  512 . In the alternative, based upon the angular velocity, ω c , the database  512  is entered and a table is chosen whereby the number n  514 =f j (E) such that n has a value −1≦n≦+1. The nature of the f j (E) is made clear by exemplary functions shown in FIGS. 5 through 8 of the drawings. FIG. 5 is a linear function of E, with a predetermined slope, m 1 , from the origin to a value of n 1 =1. FIG. 6 is a linear function of E, with a predetermined slope, m 2 , and a predetermined y intercept, −b 1 , to a value of n 2 =1. FIG. 7 is a linear function of E, with a predetermined slope, m 3 , and a predetermined y intercept, −b 2 , to a value of n 3 =1, with a dead band, ΔE 1 . FIG. 8 is a nonlinear function of E increasing from the origin and asymptotically approaching n 4 =1. FIG. 9 is a nonlinear function of E increasing from a dead band value, ΔE 2 , and asymptotically approaching n 5 =1. FIG. 10 is a nonlinear function of E increasing from a y intercept, −b 3  and asymptotically approaching n 6 =1, with a dead band value, ΔE 3 . In either case, the number n  514  is multiplied in a multiplier  516  (FIG. 4) by the torque assist command TAC  404  to provide the modified torque assist command, MTAC  518 , which is then conveyed to the motor drive assembly  300 . The nature of the modified torque assist command, MTAC  518 , is such that if n&lt;0, the modified torque, T m , is such as to provide negative, or counter, torque assistance, T a , in some fractional amount, n·T a , and thus aid in avoidance of the aforesaid collision between elements of the rack and pinion steering apparatus  100  and an end-of-travel stop. If n=0 then the no torque assistance is provided, i.e., T a =T m =0. If n&gt;0 the modified torque, T m , is some positive fractional amount of the aforesaid torque assistance. Of course, n=1 indicates that T m =T a . 
     The motor drive assembly  300  thence provides the modified torque assistance, T m , at the steering column  104  (or rack  108 ) to aid in the avoidance of a collision between elements of the rack and pinion steering apparatus  100  and an end-of-travel stop. It will be appreciated by one skilled in the art that, though the angular steering position, θ c    204  and the angular velocity, ω c  ({dot over (θ)} c )  406  is referred to in this disclosure, it is also possible to utilize, for example, the linear position, L c , of the rack  108  instead. This is so in that θ c    204  and L c  differ only by a multiplicative constant, k, representative of the gear ratio between the steering column  104  and the rack  108 , i.e., θ c =k·L c  and ω c =k·dL c /dt. 
     Reference will now be had to FIG.  11 . Therein depicted is a flow chart  600  of the method of the invention. In box  602  the angular steering position, θ c ,  204  is measured. In box  604  the angular velocity ω c  ({dot over (θ)} c )  406  is measured. In box  606  the absolute value, |θ c |  504 , of the angular steering position θ c ,  204  is calculated. In box  608  a set point value θ cot    506  is provided, representing the end-of-travel of the steering system. In box  610  the difference between θ cot    506  and |θ c  |  504  is calculated yielding an error signal, E  510 . In box  612  a dimensionless number, n  514 , is generated from a database  512  based upon the error signal, E  510 , and the angular velocity ω c  ({dot over (θ)} c )  406 . In box  614  the dimensionless number, n  514 , is multiplied by the torque assist command, TAC  404 , yielding a modified torque assist command, MTAC  518 . In box  616  the modified torque assist command, MTAC  518 , is conveyed to the motor drive assembly  300  whereupon the nature of the modified torque assist command, MTAC  518 , is such that if n&lt;0, the modified torque, T m , is such as to tend to reverse or counter the aforesaid torque assistance, T a , by some fractional amount, n·T a , and thus aid in avoidance of the aforesaid collision between elements of the rack and pinion steering apparatus  100  and an end-of-travel stop. If n=0 then the no torque assistance is provided, i.e., T a =T m =0. If n&gt;0 the modified torque assistance, T m , is some fractional amount of the aforesaid torque assistance, T a , i.e., T m =n·T a . 
     Torque Limit Embodiment 
     Reference will now be had to FIGS. 12 through 15, which refer to the torque limit embodiment of the invention. In this embodiment, there is produced a torque limit on the steering assist motor that sets a maximum value for the assist motor torque. There may, however, be several different torque limiting systems operating simultaneously in the power steering system, the most common of which will be an upper torque assist limit that decreases with increasing vehicle speed or engine RPM, thereby preventing undesirable power steering “wobble” at high speeds. In the most preferred embodiment, the torque limit produced by this torque limit system will be used as a limiting torque value only if all of the other torque limiting systems produce a greater torque limit value. That is, the controller compares a plurality of torque limit values that are produced by the various torque limiting systems, and applies the lowest value to the motor. 
     Limitation of assist torque to reduce the energy in end-of-travel impacts is generally not desired until the steering system is approaching an end-of-travel, θ cot    506 . The torque assist torque command (TAC)  402  is therefore preferably limited only when the angular steering position, θ c    204 , passes a defined threshold angle off of its on-center position. The threshold angle is the angle from on-center beyond which actual motor torque is limited by the controller because of the end-of-travel impact assist torque limit. The invention uses a function dependent upon the actual, θ c    204 , or estimated position of the steering wheel to determine the assist torque limit, and therefore can be programmed to limit torque only when the steering wheel moves past the threshold angle. 
     The following equation is the equation used to determine the torque assist limit produced by the system in one embodiment of the invention: 
     
       
           TL=M (ω c −ω int ) 
       
     
     where M is the defined slope of the function, TL is the torque limit, ω c  is the angular velocity of the steering column, and ω int  is the angular velocity intercept, which is a value determined by the actual or estimated angular position of the steering wheel. 
     A representative graph of three torque limit equations is shown in FIG.  12 . Each line represents the equation used to determine assist torque limit for a particular angular steering position, θ c    204 . The Y-axis represents the torque limit (motor torque maximum), and the X-axis represents the speed of the system. The line corresponding to torque limit Position  1 , θ c1 , represents an angular steering position, θ c    204 , that is closer to center than either the line representing torque limit Position  2 , θ c2 , or torque limit Position  3 , θ c3 ,. As angular steering position, θ c    204 , approaches the end-of-travel, θ cot    506  the angular velocity intercept, ω int , of the function used to determine torque limit is decreased. The invention may utilize a table of defined angular velocity intercept values that correspond to various angular steering positions, θ c    204 . 
     Although the three torque limit position lines shown in FIG. 12 show torque limit values exceeding 3 Nm, in operation other torque limiting systems will generally signal the controller to limit assist torque to values below 3 Nm (as shown by the dashed line). Note that these torque limit values and ranges are merely illustrative and will, of course, vary from system to system in actual application. Notice that negative torque limit values are shown in FIG. 12 for steering rates ω c    406 , that exceed the angular velocity intercept value ω int . In this case, the controller can set a zero torque limit, which would entirely remove motor assist torque from the steering system or, optionally, the controller could have the motor generate a negative torque in opposition to the steering motion. 
     The slope M of the three lines in the example of FIG. 12 is approximately        M   =       -   0.0075            Nm     Rad   /   Sec       .                              
     This slope is one example of a defined slope that could be used in the invention, and is an example of a slope value that limits torque so that an approximately equivalent end-of-travel impact force occurs at all angular velocities, ω c  ({dot over (θ)} c )  406 . The slope can be set at a value that will yield approximately equivalent force during end-of-travel impacts among varying angular velocities, ω c  ({dot over (θ)} c )  406 , or any other value that protects the integrity of the mechanical components of the steering system. 
     Defining the slope value, M, based on desired impact range and the angular velocity intercept value, ω int , based on the angular steering position, θ c    204 , establishes the equation of the line, and allows the controller to use the equation appropriate for the angular steering position, θ c    204  to compute a maximum assist torque based on the angular velocity, ω c  ({dot over (θ)} c )  406 . The angular velocity intercept values, ω int , assigned to the angular steering positions, θ c    204 , may be assigned in a linear fashion. If the three angular steering positions shown in FIG. 12 are evenly spaced, for example, then their respective angular velocity intercepts, ω int , are decreasing linearly from the angular velocity intercept, ω int1 , assigned to position  1 , θ c1 , to the angular velocity intercept, ω int3 , assigned to position  3 , θ c3 . 
     The angular velocity intercepts, ω int , however, need not be linearly arranged with the angular steering position, θ c    204 . Angular velocity intercept values, ω int , can vary linearly, piecewise linearly, or nonlinearly, depending on the application. Regardless of their arrangement, however, angular velocity intercept values, ω int , over the range of angular steering positions θ c    204 , from on-center to the threshold position on either side of center will be large enough to prevent torque limits that reduce the assist torque. That is, the defined angular velocity intercepts, ω int , when used in the torque limit equation, will produce a torque limit that exceeds the actual motor torque as determined by the controller based on other inputs. Therefore, no limiting of torque from the end-of-travel impact management system will occur when the angular steering position, θ c    204 , is between on-center and the threshold positions. 
     An angular velocity intercept lookup table  513  can be included as part of the database  512  of FIG.  4 . Hence, both the modification factor embodiment and the torque limit embodiment disclosed herein may coexist simultaneously in the same power steering system structure as shown in FIGS. 1 through 4. The lookup table  513  stores defined angular velocity intercepts, ω int , for different angular steering positions, θ c    204 . The angular velocity intercept lookup table  513  may be physically located within the controller  400 , or may be in any location that is in communication with the controller  400 . Angular velocity intercept values, ω int , can also be calculated from defined formulas that use defined on-center, threshold, and end-of-travel angles, θ cot    506 , to interpolate angular velocity intercept values, ω int , for any angular steering position, θ c    204 . 
     After receiving data from the various input sources, the controller  400  determines the correct end-of-travel assist torque limit value, TL. The torque limit value, TL, will then be used to limit the controller output torque value, MTAC, if the controller  400  determines that the torque limit value, TL, is the smallest torque limit produced by the plurality of torque limiting systems. The torque limit value, TL, generated by the system will be used only if the output torque, value, MTAC  518  otherwise specified by the controller  400  for the motor is larger than the torque limit value, TL. 
     FIG. 13 shows the algorithm used by the system to compute and apply the torque limit value, TL, to the motor. In step  700  the algorithm is started. Step  700  will generally correspond to vehicle start up or electrical start up, 
     In step  701 , the angular steering position signal, θ c    204 , is input to the controller  400 . In step  702 , the angular steering position value θ c   204 , is used by the controller  400  to retrieve the matching angular velocity intercept value from the angular velocity intercept lookup table  513 . Alternatively, the angular steering position value θ c    204  can be used in a sub-routine function to interpolate angular velocity intercepts based on defined angular velocity intercept values for on-center, threshold, and end-of-travel positions. In step  703 , the angular velocity signal, ω c  ({dot over (θ)} c )  406 , is input to the controller  400 . In step  704 , the controller  400  uses the values obtained in step  701 - 703  and the equation for torque limit to compute the end-of-travel assist torque limit. In step  705 , the torque limit produced in step  704  is compared to the current output torque value, MTAC  518 , and the torque limits produced by the other torque limiting systems. If the torque limit value is greater than the current output torque value, MTAC  518 , or any of the torque limits of the other torque limiting systems, then flow proceeds to step  701  and the controller  400  checks for new sensor values. If, alternatively, the torque limit value produced in step  704  is less than the current output torque value, MTAC  518 , and all of the torque limits produced by the other torque limiting systems, then the current output torque value, MTAC  518 , is changed to the end-of-travel assist torque limit value, TL, in step  706 , and flow proceeds to step  701 . Optionally, the angular velocity value, ω c , can be taken before the angular steering position, θ c    204 , is determined in step  701  or before the angular velocity intercept, ω int , is retrieved in step  702 . 
     FIGS. 14A and 14B show illustrative torques on the pinion of the steering column for the left side and the right side for several end-of-travel impacts. The Y-axis represents the square of the angular velocity, ω c   2 , at the steering wheel. Because system inertia (I) is the same for each trial condition, rotations per minute squared values represent the relative kinetic energy of each trial. The X-axis represents the torque on the pinion gear in Newton meters. Five sample algorithms (series) are shown to reduce the end-of-travel impacts relative to an electric steering system without end-of-travel torque limit protection. 
     In another embodiment, the actual steering angular steering position, θ c    204 , is not known when the system is started. Because the angular velocity intercept value, ω int , is determined in relation to the angular steering position, θ c    204 , and no actual angular steering position is known at start, the system must assume an initial angular steering position. If the angular steering position corresponding to the end-of-travel angular velocity intercept value is used as the initial angular steering position value, the torque limit produced by the system will be maximally protective. If, however, the system changes the assumed position value to the actual value, θ c    204 , too suddenly, undesirable unevenness in steering assist could be created. To prevent this, the angular steering position value is “walked” towards its actual value, θ c    204 , and the torque limit produced by the system is gradually adjusted until it matches the value that would have been produced by a system that indicated actual angular steering position, θ c    204 , at start up. 
     FIG. 15 shows a preferred embodiment of a torque limit algorithm that could be used in an end-of-travel impact management system that does not use an absolute angular position signal, θ c    204 , immediately upon start up  700 . Steps that are identical to those in FIG. 13 are numbered identically in this drawing, while new steps are numbered in the  800 s for easy comparison. Step  801  is performed immediately upon start up  700  of the system, and entails setting an angular steering position estimate to a default value, which can be the end-of-travel value or any other value that provides the desired protection against end-of-travel impacts. Flow then proceeds to step  701 , where an angular position signal, θ c    204 , is input to the controller as in FIG.  13 . 
     If the angular position signal, θ c    204 , is found to be valid in step  803 , then flow proceeds to step  804 , where the controller  400  compares the actual position value, θ c    204 , input in step  701  to the angular steering position estimate value. The angular steering position estimate default value set in step  801  will be the angular steering position estimate value the first time flow reaches step  804 . If the angular steering position estimate value differs from the actual position value θ c    204 , by more than a defined amount, then flow proceeds to step  805  where the angular steering position estimate value is adjusted (gradually) toward the actual position, θ u    204 , at a specified rate, this rate being gradual enough so as not to generate a noticeable change from steering feel. After several passes, the difference between the angular steering position estimate and the actual position, θ c    204 , will fall below the threshold value defined in step  804 , and flow will proceed to step  810 , where the estimate value is set to the actual value, θ c    204 . Subsequent passes will flow through step  810  and the system will function without walking steps. If in step  803  the system determines the angular position signal is not valid, then the angular steering position estimate is ramped toward the default value set in step  801 . In either case, flow then proceeds to steps  702  through  706  as before and then returns to step  701 , where an angular position signal, θ c    204 , is again input. 
     Other embodiments include a formulation with a fixes torque limit equation having only one angular velocity intercept, ω int . In this embodiment, the angular steering position value, θ c    204 , input is scaled with a position-dependent constant. The net effect of such scaling is to produce torque limits, TL, that are similar to those produced in a system that uses multiple angular velocity intercepts, ω int . 
     Another embodiment uses a non-linear torque limit function. The torque limit function in this embodiment is designed to have a more negative slope, M, at greater angular velocity, ω c  ({dot over (θ)} c )  406 , values. As in the first embodiment, angular velocity intercept values, ω int , are defined for different angular steering positions, θ c    204 , and torque limit values, TL, are determined after angular velocity intercept values, ω int , are derived. This embodiment allows for nonlinear increases in torque limits, TL, at given angular steering position, θ c    204 ,as angular velocity values, ω c  ({dot over (θ)} c )  406 , increase. 
     Another embodiment uses a closed loop structure. In this embodiment, a target angular velocity corresponding to the desired amount of kinetic energy is computed as a function of angular steering position, θ c    204 . A look-up table can be used to define the target angular velocity values. Torque limits, TL, are decreased if the target angular velocity is less than the actual angular velocity, ω c  ({dot over (θ)} c )  406 , and increased if the target angular velocity is greater that the actual angular velocity, ω c    406 . An integrator coefficient is multiplied by the difference between actual, ω c  ({dot over (θ)} c )  406 , and target angular velocity to arrive at the desired torque limit, TL. 
     While referred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.