Abstract:
An electromechanical cable actuator assembly is disclosed, the actuator having a motor, a gear assembly coupled to the motor, a spring-loaded return assembly coupled to the gear assembly being configured to apply a force to the gear assembly to return the electromechanical cable assembly to a first position; and an electronic motor control circuit coupled to the motor. The electronic motor control circuit includes a drive circuit configured to drive the motor in a first direction against the force exerted by the spring assembly and a braking circuit configured to slow the rate of return of the cable assembly to the first position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/144,016, filed on Jun. 3, 2005, which claims the benefit of U.S. Patent Application No. 60/548,324, filed on Feb. 27, 2004, the specification of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to a system for imparting a force on a cable; more particularly the present invention pertains to a system for imparting a force on a cable using a motor, a gear train, and a pulley. The pulley receives rotating force from the motor through the gear train and, when the pulley rotates, force is placed on the cable. The force causes the cable to move a predetermined distance. 
       BACKGROUND OF THE INVENTION 
       [0003]    Changing consumer demands for passenger vehicles in the United States have encouraged automobile manufacturers to build multiple-use or utility vehicles that are suitable to carry both passengers and/or cargo. One of the keys to the adaptability of such utility vehicles to carry either passengers and/or cargo is the invention of complex seating systems which enable individual seats to fold, to flip, to collapse—which movements enable movement into and eventual storage of the seats in recesses built into the vehicle. 
         [0004]    Complex seating systems require complex mechanical motion control mechanisms. These complex mechanical motion control mechanisms employ latches, levers and cables to govern seat movement and placement. As demand for newer and more complex seating arrangements increases, the need has arisen to provide electromechanical actuators when latches or other mechanical locking mechanisms are to be released from a remote location, or when additional force is needed, or extended cable travel is required. 
         [0005]    Electromechanical actuators used in vehicle seating systems are subject to a variety of design constraints. Specifically, such vehicle-mounted electromechanical actuators must be small enough to be mounted unobtrusively within a vehicle, they must place a minimal energy demand on the electrical power system of a passenger vehicle, they must be capable of rapidly handling high loads, and they must operate quietly. 
         [0006]    While a variety of systems have been used to transform the energy of a motor into linear force on a cable; there remains a need in the art for a vehicle-mounted system that combines speed with high load capacity to quietly impart a force on a cable to effect a predetermined movement of the cable in fractions of a second while minimizing the power demand on a vehicle&#39;s electrical system. 
       SUMMARY OF THE INVENTION 
       [0007]    The disclosed vehicle-mounted electromechanical cable actuator assembly combines speed and quiet operation with a high load capacity to impart force on a cable to effect a predetermined movement of the cable in fractions of a second while minimizing the demands on a vehicle&#39;s electrical system. 
         [0008]    The electromechanical cable actuator assembly of the present invention for exerting a force on a cable includes a motor whose electrical energy requirements are compatible with ability of a typical 12-volt electrical power system of a passenger vehicle to provide the needed electrical energy. Connected to the output shaft of the motor are a series of speed reduction and torque increasing gear sets which eventually cause a pulley to rotate. The rotation of the pulley imparts a force on a cable which is wound around the pulley. 
         [0009]    The series of gear sets includes a face gear and spur gear set which engages a gear mounted to the output shaft of the motor. Engaging the face gear and spur gear set is an intermediate set of two spur gears. The intermediate set of two spur gears engages an arcuate or partial spur gear attached to a pulley. 
         [0010]    The electromechanical cable actuator of the present invention also includes a spring driven backdrive collocated with the pulley. After the pulley has completed its rotation, the backdrive returns the pulley to its starting position. 
         [0011]    In a first sense, an electromechanical cable actuator assembly includes a motor having a first output shaft with a first gear mounted thereon, a gear assembly coupled to the first gear, a spring-loaded return assembly coupled to the gear assembly being configured to apply a force to the gear assembly to return the electromechanical cable assembly to a first position, and an electronic motor control circuit coupled to the motor. The electronic motor control circuit includes a drive circuit configured to drive the motor in a first direction against the force exerted by the spring assembly, and a braking circuit configured to slow the rate of return of the cable assembly to the first position. 
         [0012]    In a second sense, a control circuit for an electromechanical cable actuator assembly having a motor, a gear assembly coupled to the motor and a spring-loaded return assembly coupled to the gear assembly being configured to apply a force to the gear assembly to return the electromechanical cable assembly to a first position includes a drive circuit configured to drive the motor in a first direction against the force exerted by the spring assembly, and one or more means for slowing the rate of return of the cable assembly to the first position. 
         [0013]    In a third sense, a method for slowing the rate of return of an electromechanical cable actuator assembly having a motor, a gear assembly coupled to the motor and a spring-loaded return assembly coupled to the gear assembly being configured to apply a force to the gear assembly to return the electromechanical cable assembly to a first position includes limiting a voltage generated by the motor as the spring-loaded return assembly forces to the gear assembly to return the electromechanical cable assembly to the first position. 
         [0014]    There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
         [0015]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
         [0016]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    A better understanding of the electromechanical cable actuator assembly of the present invention may be had by reference to the drawing figures wherein: 
           [0018]      FIG. 1  is a perspective view of an assembled electromechanical cable actuator assembly according to the present invention. 
           [0019]      FIG. 2  is an exploded perspective view of the electromechanical cable actuator assembly shown in  FIG. 1 . 
           [0020]      FIG. 3  is a side elevational view of the electromechanical cable actuator with the housing portions removed to illustrate the operation of the gear train. 
           [0021]      FIG. 4  is a perspective view of the electromechanical cable actuator with portions  20  removed to illustrate the operation of the spring driven backdrive functions. 
           [0022]      FIG. 5A  is a flowchart of the logic embodied in the electronic control of the disclosed invention for 1 cycle per switch activation. 
           [0023]      FIG. 5B  is a flowchart similar to that shown in  FIG. 5A  for 2 cycles per switch activation. 
           [0024]      FIG. 6  depicts a block diagram on an electronic control assembly and motor. 
           [0025]      FIG. 7A  depicts a first electronic braking circuit. 
           [0026]      FIG. 7B  depicts a second electronic braking circuit. 
           [0027]      FIG. 7C  depicts a third electronic braking circuit. 
           [0028]      FIG. 7D  depicts a fourth electronic braking circuit. 
           [0029]      FIG. 7E  depicts a fifth electronic braking circuit. 
           [0030]      FIG. 7F  depicts a sixth electronic braking circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. 
         [0032]    As will be seen in  FIGS. 1 ,  2 ,  3 , and  4 , the electromechanical cable actuator  10  of the present invention is a self-contained device whose size is conducive to allowing its installation in a vehicle. To meet the requirements of automobile manufacturers, the disclosed electromechanical cable actuator  10  must be operable using the available electrical power provided by the electrical system typically found on a passenger vehicle. Specifically, the disclosed electromechanical actuator  10  must operate at a low voltage (specifically, 12 volts in most U.S. passenger vehicles) and have a small current draw (typically, 5 amps maximum). Yet, at the same time, the electromechanical actuator  10  must be able to impart a sufficient level of force on a cable to rapidly operate the mechanical locking portions of various different types of complex vehicle seating systems. Thus, when electrical power is supplied to the motor  30  through either the closing of a switch or by a remote device, the rotational force of the motor  30  will be quickly translated into a sufficient amount of linear force on a cable so that seating system (not shown) which is attached to the cable  44  will be unlocked from its locking system and thereby be allowed to properly fold, flip, or collapse. In addition to being small in size, the electromechanical cable actuator  10  must be easy to manufacture, low in cost, quiet in operation, simple to install, and readily connectable to the electrical system of a passenger vehicle. 
         [0033]    As will be seen in  FIGS. 1 and 2 , the disclosed electromechanical cable actuator  10  includes a housing assembly  20 , which includes a lower housing assembly  22 , an upper housing assembly  24 , a housing  26  for the motor, and a power connection  28 . 
         [0034]    In the expanded view,  FIG. 2 , with the motor housing  26  moved away and the lower housing assembly  22  separated from the upper housing assembly  24 , the construction of the electromechanical cable actuator  10  may be better understood by those of ordinary skill in the art. 
         [0035]    As previously indicated, the motor  30  is used to drive the electromechanical cable actuator  10  of the present invention. The motor  30  is enclosed by a motor housing  26 , which includes a portion  27  for the insertion of a circuit board to control the operation of the motor  30  and limit current draw. The output of the motor  30  is rotating output shaft  32 . A small gear  34  is fitted to the output shaft  32 . The small gear  34  turns with the output shaft  32 . 
         [0036]    The motor assembly  30  is held in place by mounting screws  36  which pass through a motor mounting face  38  formed as part of the lower housing assembly  22 . The lower housing assembly  22  includes multiple holes  23  on its perimeter through which mounting bolts may pass to affix the electromechanical cable actuator  10  of the present invention to a mounting point on a vehicle (not shown). Also, as may be seen in  FIG. 4 , located on the lower housing assembly  22  are a cable hole  40  and a cable guide  42 . Cable hole  40  and cable guide  42  permit the cable  44  portion of the present invention to exit the electromechanical cable actuator  10 . Also included on the lower housing  22  assembly are a plurality of small holes  45  through which fasteners  46  may be placed for attaching the lower housing assembly  22  to the upper housing assembly  24 . 
         [0037]    The portion of the lower housing assembly  22  closest to the motor  30  includes first a well  48  in which the rotatable pulley assembly  80  is mounted. The opposite end of the lower housing assembly  22  includes a second well  49  for the mounting of the intermediate spur gear assembly  50 . 
         [0038]    Extending upwardly from the bottom of the lower housing assembly  22  is a first shaft  52  on which the intermediate spur gear assembly portion  50  of the present invention is mounted. Also extending upwardly from the bottom of the lower housing assembly is a second shaft  54  on which the rotatable pulley assembly  80  and face gear and spur gear assembly  64  are mounted. 
         [0039]    The upper housing assembly  24  includes an arcuate portion  25  which fits over the motor mounting face  38  on the lower housing assembly  22 . At the distal end of the upper housing assembly is a flanged portion  55  which fits over the lower housing assembly  22 . The flanged portion  55  encloses the intermediate spur gear assembly  50 . Between the arcuate portion  25  and the flanged portion  55  is an intermediate portion  56 . The intermediate portion  56  includes a first seat  58  for mounting the top of the first shaft  52  and a second seat  60  for mounting the top of the second shaft  54 . Also included in the upper housing assembly  24  are a plurality of holes  62  through which fasteners  46  may pass to attach the upper housing assembly  24  to the lower housing assembly  22 . 
         [0040]    Positioned just under the upper housing assembly  24  is a face gear and spur gear assembly  64 . The face gear and spur gear assembly  64  is turned by engagement of its teeth  63  with the small gear  34  affixed to the output shaft  32  of the motor  30 . Because the face gear  65  and the spur gear  66  in the face gear and spur gear assembly  64  are made as one piece, when the face gear  65  turns, the spur gear  66  will also turn. The face gear and spur gear assembly  64  are mounted by engagement of the central hole  68  formed therein and the top portion of the second shaft  54 . 
         [0041]    Just beneath the face gear and spur gear assembly  64  is the intermediate spur gear assembly  50 . The intermediate spur gear assembly  50  includes an upper large gear  71  whose teeth  73  engage the spur gear  66  of the face gear and spur gear assembly  64 . Affixed to the underside of the intermediate spur gear assembly  50  is a small spur gear  75 . Because the upper large spur gear  71  and the small lower spur gear  75  are formed as a single piece, when the large spur gear  71  turns, the lower spur gear  75  will also turn. Formed in the middle of the intermediate spur gear assembly is a hole  76  which enables the intermediate spur gear assembly  50  to be mounted on the first shaft  52 . 
         [0042]    Located under the intermediate spur gear assembly  50  is the rotatable pulley assembly  80 . The rotatable pulley assembly  80  includes a central hole  82  so that it may be mounted on the first shaft  52 . At the edge of the rotatable pulley assembly  80  is a spur gear section  84  which may be turned by engagement of its teeth  85  on the small spur gear  75  of the intermediate spur gear assembly  50 . 
         [0043]    Further included in the rotatable pulley assembly  80  is a spring return  86 . The spring return  86  is depicted in  FIG. 4 . When the rotatable pulley assembly  80  is turned, energy is stored in a coil spying  86 . The stored energy, when released from the coil spring  86 , will restore the cable  44  to its original position. 
         [0044]    In an alternate embodiment, the circuit board contained in the lower housing assembly  22  will include electronics which both limit the current draw and actuate the motor  30  for brief intervals when the energy in the spring  86  is released. The operation of the motor  30  for brief intervals both slows down and quiets the movement of the cable  44  to its start position. 
         [0045]    Operation 
         [0046]    The electromechanical cable actuator  10  of the present invention operates by first applying power to the motor assembly  30 . The output shaft  32  of the motor assembly  30  is then caused to turn. Because a gear  34  is attached to the output shaft  32  of the motor assembly  30 , the turning gear  34  which engages the teeth  63  of the face gear portion  65  of the face gear and spur gear assembly  64  will cause the face gear and spur gear assembly  64  to rotate. The engagement of the teeth  63  of the spur gear portion  66  of the face gear and spur gear assembly  64  with the teeth  73  of the large spur gear portion  71  of the intermediate spur gear assembly  50  will cause the small spur gear  75  to turn. This turning of the small spur gear  75  will cause the rotatable pulley assembly  80  to turn. Because the cable is affixed to the rotatable pulley assembly  80 , the force on the cable  44  will cause it to move. This movement is of sufficient length and force to unlock a locking mechanism or provide the initiation of movement which will enable the seats in a vehicle to be properly positioned, as desired by the driver of the vehicle. 
         [0047]    In the preferred embodiment of the invention, the cable travel was set to be approximately 34 mm. However, by modifying the various ratios and the size of the parts, it has been found that a cable travel of about 30 mm to about 55 mm falls within the capability of the disclosed invention. 
         [0048]    In the preferred embodiment of the present invention, it has been found that a sufficient cable load to release commonly used latches is obtainable. By slight adjustments to the size of the various components, it will be understood by those of ordinary skill in the art that a force on the cable may range from about 350 newtons to about 600 newtons. 
         [0049]    To assure that the electrical system of a passenger vehicle is not overloaded by the electromechanical cable actuator  10  of the present invention, it has been found that a motor  30  that provides a torque 140 N-mm to about 200 N-mm, whose current draw is about 5 amps in a 12-volt system, is preferable. To achieve the desired speed of cable operation, it has been found that a motor whose operating speed is from about 1,500 rpm to about 3,500 rpm is satisfactory. The time for the cable to travel through the predetermined travel length ranges from about 0.5 secs. to about 1.5 secs. 
         [0050]    In the preferred embodiment, the drive train provides a gear ratio of about 109:1. It will be understood by those of ordinary skill in the art that the disclosed system will enable a speed reduction in the range of about 100:1 to 125:1. 
         [0051]    The operation of the system is controlled pursuant to the flowcharts at  FIG. 5A  and  FIG. 5B .  FIG. 5A  demonstrates the operation of a single cycle of the system per activation of an activation switch.  FIG. 5B  is similar to  FIG. 5A  but shows two cycles of the system per activation of an activation switch. 
         [0052]    As may be seen in  FIG. 5A  and  FIG. 5B , both flowcharts include an initial group of steps A which set up the imbedded logic in the system before the activation of the activation switch is sensed. The steps in Group A begin with an initialization and watchdog enablement step  102 . Once completed, the switch interrupt function is enabled  104  and the watchdog time is cleared  106 . To conserve energy, system is then directed into a low power mode  108 . 
         [0053]    The activation  110  of the activation switch begins those steps designated as Group B. This triggers disabling the switch interrupt function  112 . If the activation switch was only activated for a short period of time, as could happen if the switch were inadvertently bumped, the logic step  114  returns the system to step  104 . If the designated period of time is exceeded, in the preferred embodiment 25 ms, the watchdog timer is cleared  116  and the motor  30  is activated  118  for a designated period of time. A current limit step  118  assures that the maximum designated current draw has not been exceeded. If an excess current draw is sensed, the motor  30  is turned off in step  122 . If the current draw is not exceeded, the time of operation of the motor  30  is measured and compared with a preset time in step  124 . If the motor is turned off, there is a built-in delay  126  where the speed of the backdrive is controlled. 
         [0054]    In the backdrive situation, energy stored in the return spring  86  causes the motor  30  to turn. The rotational force of the spring  86  therefore causes the motor  30  to act like a generator and produce electrical energy. Bi-directional diodes are used to limit the voltage that motor  30  can produce when acting as a generator. The interruption of motor operation and the use of the bidirectional diodes facilitate return of the cable  44  to its starting position at a near-constant rate and a significant reduction in the operating noise of the electromechanical actuator  10 . 
         [0055]    In  FIG. 5B , those of ordinary skill in the art will notice that an additional step  128  has been added which determines whether or not the motor  30  has cycled twice. If the motor  30  has cycled only once, then the motor  30  is caused to cycle again. If the motor  30  has cycled twice, then the logic flow goes to the top of the flowchart. 
         [0056]    There is thereby provided by the present invention an electromechanical cable actuator which is suitable for use in a passenger vehicle. The disclosed electromechanical cable actuator will provide the necessary forces and operate with the necessary speed and reliability to perform a large variety of functions in vehicles in addition to simply operating complex seating system mechanisms. 
         [0057]    The Electronic Control Circuitry 
         [0058]      FIG. 6  depicts a block diagram on an electronic control assembly  600  and motor  602 . As shown in  FIG. 6 , the electronic control assembly  600  includes a controller  610 , an electronic motor driver  630  suitable for driving the motor  602  and having an integral current sensor (not shown), a braking circuit  640  in parallel with capacitor C 1  (used to reduce electromagnetic noise), terminals  660  and buffer  650 . The controller  610  includes a central processing unit (CPU)  612  having a memory (not shown), a digital output  616  leading to the motor driver  630 , an analog-to-digital converter (ADC)  618  receiving a current sense feedback signal from the motor driver  630  and an input  620  for receiving a buffered switch signal provided by buffer  650  and a number of timers  614 . 
         [0059]    Although the exemplary controller  610  of  FIG. 6  uses a bussed architecture, it should be appreciated that any other architecture, such as discrete electronic circuit designs, state machines, programmable logic (e.g., FPGAs) and so on, may be used as is well known to those of ordinary skill in the art. 
         [0060]    In operation, the controller  610  can first initialize various portions of its peripherals  614 - 620  in order to perform various input/output operations and timing operations, e.g., watchdog timer operations, described above. 
         [0061]    Upon receiving a switch control signal via a terminal  660  and buffer  650 , the controller  610  can then activate motor driver  630  according to the proscribed times, sequences and conditions discussed above with variances to be expected from embodiment to embodiment. For example, in operation the controller  610  can activate the motor driver  630  for up to a few seconds, or cut driver operation early if the current sense feedback signal from driver  630  (which provides an indication of the output current of the driver  630 ) indicates the motor is consuming an excessive current indicative that the motor reached a mechanical stop, stalled or otherwise malfunctioned. 
         [0062]    As the actuator assembly associated with motor  602  is later forced back to its initial position, the motor  602  will tend to act like a generator. Consistent with most motors/generators, the motor  602  will tend to produce a voltage across its terminals as a function of motor speed and/or tend to provide an available current as a function of the torque/force acting upon the motor  602 . Accordingly, it can be appreciated that motor speed may be manipulated more directly by using a voltage control approach, or motor speed may be manipulated less directly via a force/torque control approach by controlling current absorption of motor current. 
         [0063]      FIG. 7A  depicts a first braking circuit  640 A in conjunction with motor  602 . As shown in  FIG. 7A , the first braking circuit  640 A consists of a single silicon diode D 1  having a voltage drop of about 0.7V to 0.9V. While a silicon diode is used in the present embodiment, a variety of other diodes, such as Shottkey diodes, germanium diodes and so on, alternatively can be used. Further, more than one diode may be placed in series to increase the voltage drop with each diode being all the same type or a mixture of types. Returning to  FIG. 7A , as diode D 1  will generally limit the voltage across motor to diode voltage V 1 , the braking circuit  640 A of  FIG. 7A  can be considered a voltage control approach. 
         [0064]      FIG. 7B  depicts a second braking circuit  640 B in conjunction with motor  602 . As shown in  FIG. 7B , the second braking circuit  640 B includes a Shotkey diode D 2  in series with a Zener diode D 3 . As Zener diodes can be crafted to have reverse-bias breakdown voltages ranging from a few volts to tens of volts, the second braking circuit  640 B can allow for a wide variety of controlled speeds with a selection of diode. For example, by using a Zener diode having a 3.6V breakdown voltage, reverse voltage V 23  can be limited to about 4 volts. Similarly, by using a Zener diode having a 4.6V breakdown voltage, reverse voltage V 23  can be limited to about 5 volts. 
         [0065]      FIG. 7C  depicts a third braking circuit  640 C similar to that of  FIG. 7B  but having a resistor R 1  substituted for Zener diode D 3 . While the present braking circuit  640 C may not be able to control voltage so precisely as the previously shown braking circuits (and tends to appear a bit more of a torque control device), braking circuit  640 C may provide a marginally less expensive circuit as compared to the circuitry of  FIG. 7B . 
         [0066]      FIG. 7D  depicts a fourth braking circuit  640 D similar to that of  FIG. 7A  but employing a transistor S 1  and resistor R 1  in lieu of a diode. While the present braking circuit  640 D might be expected to be more expensive than the braking circuit  640 A of  FIG. 7A , braking circuit  640 D nonetheless provides a viable and useful alternative embodiment. 
         [0067]      FIG. 7E  depicts a fifth braking circuit  640 E similar to that of  FIG. 7B  but employing a transistor S 2  and resistors R 2  and R 3  in lieu of Zener diode D 3 . Again while the present braking circuit  640 E might be expected to be more expensive than the braking circuit  640 B of  FIG. 7B , braking circuit  640 E nonetheless provides a viable and useful alternative embodiment. 
         [0068]      FIG. 7F  depicts a sixth braking circuit  640 F utilizing a controllable switch S 3  in series with an optional resistor R 5 . By sensing the voltage across the motor  602  or current through R 5 , and modulating the switch S 3  using a controller of some form, braking circuit  640 F can be used to control the voltage across motor  602 , control current (and thus torque) or control some combination thereof. However, it might be appreciated that the sixth braking circuit  640 F might also operate without any sensing, i.e., by simply engaging switch S 1  (either fully on or using a pulse width modulation (PWM) approach) whenever braking is desired and presumably when the motor is not being driven. 
         [0069]    As a further embodiment of note, it should be appreciated that, instead of using a braking circuit, such as any of those shown in  FIGS. 7A-7F , to slow the rate of actuator return, the controller  610  can provide a braking function by applying a partial drive signal to the motor, such as a pulse width modulated (PWM) signal having a duty cycle sufficient to slow, but not stop, the motor  602 . such a scheme may either wholly replace an independent braking circuit or be used as a means of supplementing am independent braking circuit. Of course, such drive signal might be expected to require more energy than the braking approaches discussed above, but may reduce component count as a benefit. 
         [0070]    Conclusion 
         [0071]    While the disclosed invention has been described in terms of its preferred and alternate embodiments, those of ordinary skill in the art will understand that numerous other embodiments of the present invention may become apparent while reading of the foregoing disclosure. Such other embodiments shall be included within the scope and meaning of the appended claims. 
         [0072]    The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.