Abstract:
In varying embodiments, the fault tolerant linear actuator of the present invention is a new and improved linear actuator with fault tolerance and positional control that may incorporate velocity summing, force summing, or a combination of the two. In one embodiment, the invention offers a velocity summing arrangement with a differential gear between two prime movers driving a cage, which then drives a linear spindle screw transmission. Other embodiments feature two prime movers driving separate linear spindle screw transmissions, one internal and one external, in a totally concentric and compact integrated module.

Description:
[0001]    This application claims priority to U.S. Provisional Patent Application Serial No. 60/386,661, filed Jun. 5, 2002. 
     
    
       [0002] The U.S. Government may own certain rights in this invention pursuant to the terms of the U.S. Department of Energy grant number DE-FG04-94EW37966. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates generally to electromechanical actuators, and specifically to a linear actuator having improved fault tolerance and positional control.  
           [0004]    A number of approaches have been developed to manipulate the linear position of an object or device through the use of an actuator. Linear actuators are pervasive where the movement of very large loads is required. Linear actuation has traditionally been met by the use of hydraulic and pneumatic cylinders. Electromagnetic actuators are known, however, to provide increased performance in many aspects as compared to either hydraulic or pneumatic cylinders.  
           [0005]    One drawback to the use of electromagnetic actuators is a certain degree of increased complexity, giving rise to increased concern over the reliability of such devices. Accordingly, certain electromagnetic linear actuators have incorporated fail-safe mechanisms of one type or another. As an example, U.S. Pat. No. 4,289,996 discloses a powered linear actuator having dual closed loop servo motor systems driving a screw jack. The dual motors drive the screw jack through differential gearing and each has an armature lock which functions automatically if a motor circuit fails thereby enabling the other motor to continue driving the actuator alone. Potentiometer feedback is applied to dual error amplifiers or polarized relays that compare the feedback position signal with the input command signal and drive separate motor energization channels.  
           [0006]    U.S. Pat. No. 5,865,272 discloses a linear actuator having an output shaft having a pair of driven wheels mounted thereon. One of the driven wheels is rotatably mounted in a fixed plane and has a drive nut for an associated thread on the output shaft. The other drive wheel is rotatably fixed to the output shaft. An input shaft is in a side-by-side relationship with the output shaft and adapted to be rotated by a suitable power source. The input shaft provides a drive wheel for each of the driven wheels, with the ratio between each drive and driven wheel set being chosen to rotate the driven wheels at different speeds in the same rotational direction and thereby produce a controlled axial movement of the output shaft in a direction depending upon the relative rotation of the driven wheels. A fail-safe arrangement is provided in the form of a clutch between the drive wheels of the input shaft, a back-drive for the output shaft, and biasing means for affecting a back-drive.  
           [0007]    U.S. Pat. No. 5,957,798 discloses an electromechanical actuator having a linear output for moving an external load, the actuator having at least two drive motors, a synchronizer connected to the outputs of the drive motors, a differential mechanism combining the outputs of the drive motors, and a quick release mechanism connected to the differential mechanism and the actuator output. The quick release mechanism releases support of the external actuator load in response to an internal actuator jam and maintains support of the external actuator load in response to an external actuator overload.  
           [0008]    U.S. Pat. No. 6,158,295 discloses a linear actuator including a housing, a spindle rotatable in both directions, a threaded nut driving a piston rod, and a motor capable of driving the spindle through a transmission. A disengagement unit is arranged in the transmission for interrupting the connection between the motor and the spindle in case of operational failure, such as overloading of the spindle. The disengagement unit comprises a braking device adjustable with respect to the actuator housing to cooperate with a coupling device for control of the rotational speed of the spindle when it is disengaged from the motor.  
           [0009]    Although each of these designs provides certain advantages, none of these designs provides a fully fault-tolerant linear actuation solution totally suitable for use in applications where life or safety is at risk. Each of these designs has its drawbacks, as will be appreciated by those of skill in the art. For example, as noted above, in any application in which a mechanical device, such as an actuator, is employed to perform a function, there is the potential and the risk of failure of the mechanical device and attendant loss of functionality. In certain situations, such failure may have only minor consequences. Wherever actuators are employed in applications in which life or safety are at risk, however, the consequences are much more severe. In high-stakes applications, such as the control of an aircraft control surface, disengagement of the actuator from the applied load is simply not an acceptable approach. Similarly, locking up the actuator with a brake would generally not be an acceptable approach in such an application. Accordingly, there is an unmet need to prevent sudden or catastrophic failure in the linear actuators employed.  
           [0010]    Although electromechanical solutions offer definite advantages over the lower-technology hydraulic and pneumatic solutions often used in traditional linear actuation applications, the rugged simplicity of the fluid cylinder has made it tough to beat from a cost and reliability standpoint. Further, it is known that single point failures frequently occur in electromagnetic linear actuators. Where a linear actuator is susceptible to loss of function from a single point failure, the actuator could completely fail to operate in the event of such a failure. As noted above, this is an unacceptable situation in many applications.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention solves the problems associated with current linear actuators. For example, in various embodiments, the systems of the present invention overcome the risk of failure by incorporating features enabling them to continue to operate under a partial or total fault on one side of a dual system. Thus, the present invention provides, in certain embodiments, fault tolerant duality in a compact, concentric, fully integrated module. This compactness and integration does not exist in any existing designs.  
           [0012]    In accordance with one aspect of the present invention, a fault tolerant linear actuator is provided that incorporate velocity summing, force summing, or a combination of the two. In one embodiment, the invention offers a velocity summing arrangement with a differential gear between two prime movers driving a cage, which then drives a linear spindle screw transmission. This embodiment is reconfigurable, but since it has only one transmission, it does not eliminate all possible single point failures. A second embodiment features two prime movers driving separate linear spindle screw transmissions (one internal and one external) in a totally concentric and compact integrated module. This system has no single point failures, which is desirable where failure would result in loss of life or high cost. A third embodiment uses two rotary actuators driving acme screws in place of the linear spindle screw transmission to make a very rugged high force system. A fourth embodiment is a force summing linear actuator based on a dual set of linear spindle screw drives summing forces through two clutches at the output attachment plate. A fifth embodiment uses an intermediate gear train between the input prime movers and the output spindle screws in order to better balance the torque/speed ratios and to enable a significantly higher motor speed than in the second embodiment. This two-stage reduction also allows for a significant reduction in the weight of the actuator.  
           [0013]    The development of certain technologies makes it possible for the, electromechanical actuators of the present invention to surpass the performance of prior known designs in essentially every aspect of performance. As an example, the commercial availability of the roller spindle screw transmission is a significant step forward in performance. As another example, the development of modern highly-integrated circuits allows for increases in performance and reductions in cost at the same time. Using these and other technologies, the present invention not only offers high load capacity, it also offers very long life, high precision, and high velocity in a compact configuration and the potential for a high level of actuator intelligence.  
           [0014]    Intelligence within the actuator itself makes it possible to balance operational priorities (speed, load, precision, smoothness, etc.) in real time. Intelligence within the actuator permits the system of the present invention to be highly fault tolerant. This fault tolerance depends on a full awareness of all the performance capabilities of the actuator in real time. This awareness requires access to a wide spectrum of sensors, each generating data quantifying performance criteria used to judge the actuator&#39;s operation. Depending on the application, these performance criteria may be prioritized to meet in-situ operational goals. Here, the principal goal is to maintain operation under a fault. Depending on the operational requirements, the output of a faulty prime mover in an actuator may be quantified and used as a basis to temporarily raise the performance of the one or more fully-operational prime movers in order to make up for the loss of performance from the faulty prime mover.  
           [0015]    Alternately, the faulty prime mover may be taken completely out of service by braking it and “limping home” using the remaining prime movers.  
           [0016]    The teachings of the present invention may be employed in any application in which there is the potential for loss of life, a need to preserve a long mission in harsh environments without possibility of repair, or a potential for high cost resulting from sudden failure. This layered control should combine to give more precise operation under significant load disturbances.  
           [0017]    Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention, together with other important aspects thereof upon reading the detailed description that follows in conjunction with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    For more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying FIGURES.  
         [0019]    [0019]FIG. 1 depicts an isometric cutaway view of a velocity-summing fault-tolerant linear actuator according to one embodiment of the present invention;  
         [0020]    [0020]FIG. 2 depicts an isometric cutaway view of a velocity-summing fault-tolerant linear actuator according to a second embodiment of the present invention;  
         [0021]    [0021]FIG. 3 depicts an isometric cutaway view of a dual fault-tolerant linear module based on a combination of rotary actuators;  
         [0022]    [0022]FIG. 4 depicts an isometric cutaway view of a force-summing fault-tolerant linear actuator; and  
         [0023]    [0023]FIG. 5 depicts an isometric view of a velocity-summing fault-tolerant linear actuator with two-stage transmissions. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Although making and using various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many inventive concepts that may be embodied in a wide variety of contexts. The specific aspects and embodiments discussed herein are merely illustrative of ways to make and use the invention, and do not limit the scope of the invention.  
         [0025]    [0025]FIG. 1 depicts an isometric cutaway view of a velocity summing fault tolerant linear actuator  100  according to one embodiment of the present invention. Actuator  100  provides a dual set of prime movers  102  and  104  operating through a differential gearset  106 , which then drives a cage  108  containing two sets of spindle screw drives  110  and  112  operating on a single linear output screw  114 . Fault tolerant linear actuator  100  is fault tolerant up to the differential gearset  106 , e.g., either prime mover  102  or  104  may be disabled (e.g., braked) and the remaining prime mover  102  or  104  may still operate.  
         [0026]    The trunnion  116  as part of the outer shell  118  provides one attachment to the environment with the other attachment being on the linear output screw  114 . The dual prime movers  102  and  104  are arranged in a symmetrical layout. Prime mover  102  incorporates field coil cylinder  120  and armature  122 . Prime mover  104  incorporates field coil cylinder  124  and armature  126 . Prime movers  102  and  104  are mounted on rotary needle bearings  128  and  130 , respectively, and drive multiple central differential planetary gears  132  mounted on bearings  134  in planetary cage  136  supported by planetary cage needle bearing  138 .  
         [0027]    The planetary cage  136  also contains the planetary screws  140  and  142  supported by thrust bearings  144  and  146 . The planetary cage  136  as a unit is supported by principal thrust bearings  148  and  150  in the outer shell  118  of the actuator  100 . The end plates  152  and  154  of the actuator  100  are fixed to the shell with machine bolts  156 .  
         [0028]    Depending on the application, actuator  100  may be designed to provide varying types of service, e.g., light, medium, or heavy-duty service. Actuator  100  is dynamically reconfigurable in real time. Should one prime mover (e.g.,  102  or  104 ) lose torque capacity past a certain limit, the remaining prime mover (e.g.,  104  or  102 ) may be instantaneously raised to greater than 100% of its normal torque capacity to maintain the normal level of performance for the actuator  100 . Sensor systems, operational criteria, and performance histories may then be used to monitor the performance of actuator  100  relative to its reduced performance envelope.  
         [0029]    [0029]FIG. 2 depicts a velocity summing linear fault tolerant actuator  200  having no single point failures. Fault tolerant actuator  200  incorporates a pair of rotary prime movers  202  ( 1 ) and  204  ( 2 ), that may either be, e.g., BDCM or SRM-type, motors, driving a pair of linear spindle screw transmissions  206  and  208  acting on an external screw shaft  210  and an internal screw cylinder  212 . Fault-tolerant actuator  200  incorporates an inner motion frame  214  that travels along both the external screw shaft  210  and the internal screw cylinder  212 . Inner motion frame  214  also contains the two rotary prime movers  202  and  204  and their associated planetary screws  216  and  218 . Inner motion frame  214  is prevented from rotation on these screws with the use of linear cross-roller bearings  220  and  222 . The length and placement of these cross-roller bearings  220  and  222  will be dependent on the stroke requirements of the application.  
         [0030]    As seen in FIG. 2, external screw shaft  210  functions as the output shaft for the fault-tolerant actuator  200  while the outer shell  226 , which contains the internal screw cylinder  212 , also incorporates the input attachment  228 . Linear cross roller bearings  220  and  222  prevent the inner motion frame  214  from rotating relative to the external screw shaft  210  and the internal screw cylinder  212 . Field  234  and armature  236  of the prime mover  202  supported by bearings  238  and  240  drive the linear planetary screws  216  in spindle bearings  244  in spindle cage  246  supported by principal thrust bearings  248 . Field  250  and armature  252  of the prime mover supported by bearings  254  drive the linear planetary screws  218  in spindle bearings  240  in spindle cage  258  supported by principal thrust bearings  260 .  
         [0031]    Note that only one set of the linear cross roller bearings  220  and  222  is necessary to constrain the rotary motion of the inner frame  214 . Bearing  230  is more effective in resisting the torque load on the inner frame  214  because of the larger diameter and higher torsional stiffness of the outer cylinder shell  226 .  
         [0032]    The linear fault-tolerant actuator  200  of FIG. 2 is not only fault tolerant in velocity summing between two independent prime movers  202  and  204 , but also exhibits no single point failures between its two linear screw transmissions. This is a velocity summing concept with reconfiguration of the prime mover velocities in real time. The design in FIG. 2 has considerable merit for applications requiring compactness, greater simplicity, higher ruggedness, and partial fault tolerance in the electrical prime movers and their electronic control subsystems.  
         [0033]    Many applications require a combination of low output velocity and high output force. Also, desirable properties of small size, high stiffness, and low cost usually accompany this type of application. FIG. 3 depicts a linear actuator module  300  that uses two externally-threaded rotary actuators  302  and  304  to drive two internally-threaded cylinders  306  and  308  in series. In certain embodiments, module  300  may be designed to generate high force at relatively low cost. Although not necessarily optimized for applications requiring high linear velocities or rapid response to input commands, module  300  may be optimized to generate high force in a rigid, yet small package. In certain embodiments, three or more linear actuators (e.g.,  302 - 304 ) may be combined to create an even more fault-tolerant linear actuator module  300 .  
         [0034]    In module  300  there is one external rectangular cylinder  310  attached to the actuator reference frame  312 . Actuator reference frame  312  anchors each of the (externally-threaded) internal rotary actuator modules  302  and  304 . In certain embodiments, the two internally-threaded rectangular cylinders  306  and  308  use linear cross roller bearings  314  and  316  for precision and stiff operation relative to the external rectangular cylinder  310 . Other embodiments may employ sleeve-type bearings for the same function.  
         [0035]    Module  300  may be employed in very low cost applications, such as in automobiles or in very low weight applications, as found in the deployment of large flaps on aircraft. In a manufacturing cell, module  300  may also be used in fixturing. Combined with high precision small motion actuators, module  300  is useful for application where both very high force and high precision are required.  
         [0036]    The threaded interface between the externally-threaded rotary actuators  302  and  304  and the internally-threaded rectangular cylinders  306  and  308  may vary by application. For example, certain embodiments employ acme screw thread. Acme screw mechanisms are low in cost, resistant to shock and oscillatory forces, tolerant of contamination, and reliable for extended service at low velocities. Acme threads will, however, generate more friction than alternate transmissions such as the ball screw or the spindle screw.  
         [0037]    [0037]FIG. 4 depicts a linear fault tolerant actuator  400  having no single point failures. This is achieved by creating dual force paths in a single envelope wherein either of the force paths (prime mover and transmission) may be removed from service by a clutch release or similar mechanism in the event of failure.  
         [0038]    [0038]FIG. 4 depicts an isometric cutaway of a dual force path linear actuator  400 . The system uses a pair of planetary roller screws  402  and  404  driven by separate prime movers  406  and  408 , all in a concentric configuration. Prime mover  406  drives planetary roller screws  402 , which in turn drive a roller screw shaft  414  with external threads. Prime mover  408  drives planetary screws  404  that drive a roller screw cylinder  420  with internal threads. The roller screw shaft  414  and the roller screw cylinder  420  are attached at one end to an output cylinder  422 .  
         [0039]    The roller screw cylinder  420  is separated from the output cylinder  422  by the outer clutch  424 , while the roller screw shaft  414  is separated from the output cylinder  422  by the inner clutch  426 . Should either of prime movers  406  or  408  fail, the associated clutch  424  or  426  may be energized to take that prime mover  406  or  408  out of service. This system ensures that operation would continue even under a major fault in one of the force pathways. In certain embodiments, a single force path may have the capacity to double its normal output for a short period of time to compensate for the failed subsystem, in order to prevent any major system failure.  
         [0040]    Roller screw shaft  414  and outer shell, along with the roller screw cylinder  420 , are connected through clutches  424  and  426  to the output cylinder  422 , by means of end cap screws  428 . Nut  430  connects the screw shaft  414  to the plate  432 , which holds inner clutch  426 .  
         [0041]    As noted above, there are two separate prime movers  406  and  408  within linear actuator  400 . Field  434  and armature  436  on support bearings  442  drive planetary screws  402  supported by spindle bearings  444 . Spindle bearings  444  transfer forces through the planetary screw cage  446  to principal thrust bearing  448  to the inner motor frame  450  holding the motor fields, which is attached to the input attachment cylinder  452  through end cap screws  428 .  
         [0042]    The second prime mover  408  incorporates field  438  and armature  440  on support bearings  454  driving planetary screws  404  through support bearings  456 . Support bearings  456  act through the planet cage  458  by means of thrust bearings  460 . Hence, each prime mover-transmission combination independently creates a driving force on the output cylinder  422 .  
         [0043]    Constructed as shown in FIG. 4 and described above, linear actuator  400  eliminates the risk of total actuator failure brought on by any single point failure. Failures associated with threat to life, a significant economic loss, or the continuation of a long duration mission all suggest the need for continued operation even under a fault such as a lost prime mover, transmission, communication link, sensor, or power supply. Achievement of this goal requires the inclusion at least two fully independent pathways to drive the output. In the past, this meant that two separate linear actuators were arranged side-by-side and set up with separate control loops.  
         [0044]    Although the inclusion of a separate actuation mechanism provides for a degree of fault tolerance, such a combination is complex, space-inefficient and heavy. Such a design also introduces a level of functional uncertainty that designers find unattractive. Redundancy, which sets aside one part of a dual system while the other one operates is a waste of both resources and priorities (weight, volume, cost, etc.).  
         [0045]    In the embodiment shown in FIG. 4, all resources are employed at all times, maximizing output performance and accepting a reduced performance reserve in the event of a partial fault.  
         [0046]    A fifth embodiment of the present invention is shown in FIG. 5 and generally designated  500 . Actuator  500  is made up of two completely independent subsystems  502  and  504  to provide operation even under a complete failure of one of the subsystems.  
         [0047]    The two actuator subsystems  502  and  504  of actuator  500  are geometric inverses of each other. Spindle screw set  522  drives a small diameter screw shaft  534  with external threads, while spindle screw set  538  drives a large diameter screw cylinder  540  with internal threads. Spindle screw set  538  may be at a diameter three times greater than spindle screw set  522 , which would, of course, require an angular velocity reduction of three-to-one in order to maintain the same contact linear velocity at the screw threads. This reduction also reduces the stored kinetic energy in the rotating parts.  
         [0048]    Subsystem  502  is driven by prime mover  508 . Subsystem  502  is guided and supported by cage  512 , which holds planet gears  514  in planet bearings  516 . Planet gears  514  mesh with bull gear  518  and sun gear  520 , that drive sun gear  520  attached to the spindle screw set  522  supported by spindle nut support bearings  524 . The principal cross roller bearing  526  separates the sun gear  520  from the bull gear  518  and transfers the actuator load from the spindle set  522  to the actuator carriage at the bull gear  518 . End caps  528 ,  530 ,  532  are used to assemble subsystem  502 .  
         [0049]    Subsystem  504  may be described in the same manner as subsystem  502 , except that it is the geometric inverse of subsystem  502 . In operation, axial loads pass from the actuator screw shaft  534  to spindle screw set  522  through principal cross roller bearing  526  to the actuator carriage  554  and then through principal cross roller bearing  536  on to spindle screw set  538  out to the outer shell  540  of the actuator  500 . The anti-rotation splines  542  and tangs  544  prevent the carriage from rotating in the actuator  500 . Seals  546  and  548  prevent the escape of the lubricant from the actuator  500 . A utility coil volume  550  is provided between the actuator carriage and the end-cap  552  of the outer cylinder shell  540  for the supply of power, communications, and lubricant to the moving carriage.  
         [0050]    In special applications, the need for low weight is critical. This may achieved, for example, by using high RPM prime movers. There becomes a mismatch between this high RPM and the low speed/high force needed at the output shaft. To make this combination feasible, an intermediate gear reduction must occur between the motors and the linear spindle screw transmissions.  
         [0051]    In normal prime mover applications, a prime mover maximum angular velocity between at least about 3,000 and 4,000 RPM is generally considered ideal. For extremely low-weight applications, maximum prime mover angular velocities between at least about 15,000 and 30,000 RPM may be required. Such designs may output five to ten times more horsepower for the same weight of the prime mover. In order to multiply the motor torque, a first stage gear reducer, such as an epicyclic gear train, is inserted between the prime movers and the associated linear spindle screw transmission in order to balance the input and output speeds, as well as the forces involved. This first stage reduction allows for design optimization of both the prime movers and the linear spindle screw transmission.  
         [0052]    Structurally, the strength of actuator  500  is entirely dependent on the load carrying capacity of the spindle screw sets  522  and  538  and the two principal cross roller bearings  526  and  536 . Subsystem  502 , which includes spindle screw set  522 , crossroller bearing  520 , gear transmission  514  and prime mover  508 , is completely independent of subsystem  508 , but they occupy a common moving carriage, which transfers the load from the actuator screw shaft to the outer cylinder screw shell.  
         [0053]    Because the spindle screw sets  522  and  538  create a turning resistance due to friction, an anti-rotation spline  542  is built into the right side of the actuator screw shaft  534 , in order to prevent rotation of the carriage  554 . In one embodiment, it is likely that spindle screw sets  522  and  538  will be of the same length to carry the same load.  
         [0054]    In another embodiment, the lead on spindle screw set  522  is at least about 0.2 in./rev. given a desired output speed of at least about 3.5 in./sec., an angular velocity of at least about 1050 RPM would be demanded of prime mover  508 . The intermediate gear transmission ratio for subsystem  502  would have to be at least about 14.3 to 1. The equivalent desired speed for spindle set  538  would be at least about 300 RPM.  
         [0055]    In yet another embodiment, the lead of the internal cylinder screw  550  is at least about 0.7 in./rev. Given a maximum angular speed of at least about 30,000 RPM for second prime mover  556 , the intermediate gear transmission ratio of subsystem  504  would be at least about 100-to-1. The low speeds in the spindle screws  522  and  538  will be very helpful in extending the life of these critical parts in actuator  500 .  
         [0056]    Nonetheless, the high rotational speed requirements place considerable demands on the intermediate gear transmissions. First, the exceptionally high angular velocities will store considerable kinetic energy. For epicyclic gears, this requires that the planets be as small as possible.  
         [0057]    In certain additional embodiments, the subsystems  502  and  504  may operate in opposite directions in order to better balance the friction turning torques on the moving carriage  554 .  
         [0058]    Owing to the use of roller screws, subsystems  502  and  504  are naturally non-backdrivable. Depending on the pitch of the screw threads and the application, there still may be a need to put in place brakes on each of the armatures to prevent the system from walking under oscillating external loads.  
         [0059]    In certain other embodiments of actuator  500 , each subsystem  502  and  504  provides one-half of the total stroke length. Accordingly, actuator  500  may always return to the neutral position and operate in only one-half its useful range, with one side completely incapacitated. Alternately, a partially failed side could “limp” home to the center of its range, and then be locked in place, so that the remaining operable side could provide fifty percent of the range capacity about the center position.  
         [0060]    It should be mentioned that in some applications, it would be useful to provide for consistent lubrication of the actuator. For example, a low viscosity oil under pressure may be used to provide a misted atmosphere inside the actuator volume. The lubricant could be recirculated in a closed circuit and may also be cooled if the duty cycle demands that heat be removed from the system. This, then, requires at least about two seals: a first seal between a smooth surface on the carriage  554  and the outer cylinder shell  540  and a second seal between a smooth surface on the actuator screw shaft  534  and an extension of the actuator carriage  540 . The other end of actuator  500  is sealed by an end cap  552  on the outer cylinder shell  540 .  
         [0061]    Additional objects, advantages and novel features of the invention as set forth in the description that follows, will be apparent to one skilled in the art after reading the foregoing detailed description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instruments and combinations particularly pointed out here.