Abstract:
An apparatus for positioning a pull type farm implement has an elongated tongue being drawn by a motor vehicle having a vehicle hitch on the rear end thereof configured for attachment to the tongue, the apparatus including a hydraulic hitch mechanism configured to have one end portion attached to the vehicle hitch and an opposite end portion configured to be attached to the tongue of the implement in a non-pivoting manner, the hitch mechanism having at least one hydraulic cylinder for selectively adjusting the lateral position of the front end of the tongue. An optional GPS is operable to detect the position of the tool bar relative to the desired path and repeatedly generate position signals and apparatus for controlling the hydraulic hitch mechanism to adjust the lateral position of the tongue so that the implement follows the desired path of movement in response to the GPS signals.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to guidance control systems for implements that trail a motor vehicle. 
     In the farming industry, guidance systems for controlling the position of trailing implements, particularly those which are mounted to the three point hitch of a motor vehicle, have been developed and marketed for many years. The design of some of the types of systems has limited their effectiveness and accuracy with regard to positioning the implement. Guidance systems that have exhibited reliable and accurate operation are those manufactured and marketed by Sunco of North Platte and Madrid, Nebr. 
     The Sunco systems are marketed under the trademark AcuraTrak® have exhibited superior operating characteristics for guiding many types of trailing implements that are pulled by farm tractors. While some guidance systems are designed to be more concerned with controlling the tractor itself, the AcuraTrak system is designed to control the lateral position of an implement relative to the tractor that is pulling it, with the focus being on proper positioning of the implement along a desired path as the implement is pulled through a field during operation. The currently marketed Acura Track guidance systems operate in a manner based on principles that are set forth in U.S. Pat. Nos. 5,240,079, 5,150,849 and Re. 34,080, and are generally applicable to certain aspects of the present invention. All of these patents are specifically incorporated by reference herein. 
     The guidance systems disclosed in those patents utilize a wand mechanism associated with the implement which is dragged along the ground and is a common means for sensing the relative position of the implement relative to the rows. The wand is mounted on the implement in a manner that it is positioned in the middle of the distance between two adjacent rows. The midpoint between two rows is usually the lowest point between the rows, since a cultivating operation tends to move dirt from the middle toward the plants, for example. The wand is angularly moveable so that its angular position can change depending upon whether the implement is oriented in the center between adjacent rows or is offset from the center. Thus, if the implement is moving to one side of center to the other, the angular position of the wand will change and provide corrective signals for controlling the guidance system to alter the lateral position of the implement relative to the rows. 
     It should be easily understood that if an operator is cultivating a crop that has already reached a stand, or is planting or marking out rows, or the like, the difficulty does not reside in the operator being able to keep the tractor between the rows without the tractor wheels crushing the crop, but rather keeping the implement from getting too close to the crop which could, in the case of cultivation, result in damage to the roots of the crop by the cultivator blades. In the case of marking out rows, it is obviously desirable to have uniform rows. 
     With the advent of global positioning systems (GPS), it has been the desire, if not the goal of many in the agricultural industry to use GPS systems as the navigation tool to control tractors during a planting or other farming operation to make and/or follow rows that are properly aligned in a field and relative to other rows. Improvements in the GPS signals now enables positioning within a tolerance of an inch or less during a farming operation, and also enables accurate mapping of paths that a vehicle should travel. 
     While such guidance systems as described above are effective, particularly when coupled with GPS systems and are connected to a three-point hitch of a tractor or other type of motor vehicle, there are still significant challenges to existing guidance systems that operate in connection with an implement with an elongated tongue having a forward end that is pivotally connected to a drawbar, and the rear end fixedly connected to the body of the implement and preferably to the tool bar. Such an implement will be referred to herein as a “tongued implement”. 
     Providing guidance of such tongued implements has been attempted by forcing the drawbar laterally relative to the tractor, but such forced lateral movement simply does not work well, particularly on sloped land where gravity creates side forces that dramatically affect the operation of the tractor and drawn implement combination. 
     While other prior solutions included U.S. Pat. No. 5,025,866 which had a tongue driven by a hydraulic cylinder at the rear end of the tongue where it is pivotably connected to the tool bar and the angle is changed by operation of a hydraulic cylinder, that solution is impractical for extremely large implements that may cover 20 to 30 rows of crops in a single swath, because of the extremely large forces that would generated by implements of such size on a pivotable connection between the tongue and the tool bar. 
     The difficulty that is experienced in the operation of tongued implements being drawn by a tractor is described in connection with  FIGS. 1 and 2  where a tractor, indicated generally at  20 , is pulling an implement, indicated generally at  22 , along a path  24  that has a down slope to the right as indicated by the arrow, such as would be the case on a side hill. The down slope is defined to be where the tractor  20  is located in  FIG. 1  and extends only to the position slightly ahead of the tool bar  32 , with the implement  22  being on level ground. 
     The tractor has rear wheels  26 , front wheels  27  and a drawbar  28  that is connected to the tractor at  30  and is kept from pivoting around connection  30  by conventional hitch structure so that it is oriented along the center line of the tractor as indicated. The implement  22  has a tool bar  32  to which eight planter row units or other implements  34  are attached, with the implement riding on a frame structure (not shown) to which wheels  36  are mounted. The implement  22  has a tongue  38  rigidly attached to the tool bar  32  and is preferably supported by braces  40  to insure that the 90° angle between the tongue and the tool bar is maintained during operation. The front end of the tongue is pivotally connected to the drawbar  28  at  42  which defines the pull point for the implement. 
     As shown in  FIG. 1 , because of the slope that was encountered during forward movement, the tractor rear wheels  26  have started sliding to the right or down the hill, which moves the pull point to the right pulling the implement  22  downhill before the implement gets to the slope. As the tractor rear wheel  26  slides to the right, the pull point  42  is pushed to the right and as shown in  FIG. 2 , the implement remains off track, i.e., the wheels  36  do not follow the track  24  as long as the rear wheels  26  of the tractor  20  are off track. 
     As previously indicated, prior ineffective guidance attempts included forcing the drawbar laterally from side to side. This is shown in  FIG. 3  where the drawbar  28  has been pivoted around the connection  30  to one side by 15 inches. Thus, the pull point defined by the connection  42  is 15 inches to the right of the center of the tractor which has the tendency to pull the rear of the tractor off the path and significantly increases the side load on the tractor which results in substantial inefficiency. 
     Such movement of the drawbar from side to side in the manner shown in  FIG. 3  is known to be ineffective in steering the implement and such drawbar steering simply does not work as is evidenced by the fact that there are few commercial products in the marketplace that attempt to utilize such operation for providing guidance. It is important that the pull point be maintained generally in the center of the tractor to have effective and efficient operation. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to apparatus for positioning a pull type farm implement which has an elongated tongue fixedly attached to an elongated transverse tool bar while being drawn by a motor vehicle moving generally along a desired path, the vehicle having a vehicle hitch on the rear end thereof configured for attachment to the front end portion of the tongue, the apparatus comprising a hydraulic hitch mechanism having an elongated main frame, the mechanism being configured to have one end portion attached to the vehicle hitch and an opposite end portion configured to be attached to the tongue of the implement in a non-pivoting manner, the hitch mechanism having at least one hydraulic cylinder attached to the main frame for selectively adjusting the lateral position of the front end of the tongue relative to the vehicle hitch in a manner whereby the pull point of the motor vehicle is relatively unchanged, a global positioning system (GPS), the GPS including a receiver and an antenna, the antenna being mounted on the implement, the GPS being operable to detect the position of the tool bar relative to the desired path as the vehicle moves forwardly generally along the desired path and repeatedly generate signals that are indicative of the detected position; and means for controlling the hydraulic hitch mechanism to adjust the lateral position of the front end of the tongue so that the implement follows the desired path of movement in response to the GPS signals. 
     Embodiments of the invention are also directed to a method of maintaining a centered pull point of a motor vehicle with a vehicle hitch for pulling a farm implement having a tongue fixedly and non-rotationally attached to a transverse tool bar when the rear end of the motor vehicle pulling the farm implement becomes laterally offset relative to a desired path the motor vehicle is intending to traverse, comprising the steps of providing a hydraulic hitch mechanism having an elongated main frame, the mechanism being configured to have one end portion pivotably attached to the vehicle hitch and an opposite end portion configured to be pivotably attached to a tongue mount that is attached to the tongue of the implement in a non-pivoting manner, the hydraulic hitch mechanism moving the front end of the tongue in a direction opposite the direction that created the lateral offset by an amount generally equal to the amount of the offset. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a greatly simplified top view of a tractor pulling a tongued implement wherein the back of the tractor is sliding down a slope to the right and departs from the desired path; 
         FIG. 2  is a view similar to  FIG. 1  illustrating the tractor and the implement being off of the desired path; 
         FIG. 3  is a view similar to  FIG. 2 , but illustrating the implement being off of the desired path as a result of swinging the drawbar to a laterally offset position; 
         FIGS. 4-9  illustrate a sequence of positions of a tractor and trailing tongued implement together with the guidance system of the present invention providing correcting action during operation; 
         FIG. 10  is a perspective view of the hydraulic hitch mechanism embodying the present invention; 
         FIG. 11  is a side view of the mechanism shown in  FIG. 10 ; 
         FIG. 12  is a front view of the mechanism shown in  FIG. 10 ; 
         FIG. 13  is a top view of the mechanism shown in  FIG. 10 ; 
         FIG. 14  is a perspective view of a portion of the U-joint that is a component of the mechanism shown in  FIG. 10 ; and 
         FIG. 15  is a perspective view similar to  FIG. 14 , but showing the entire U-joint mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     Broadly stated, a guidance control system and apparatus for farm tractors in combination with a trailing implement of the type which has a tongue rigidly attached to the body of the implement, which is preferably a tool bar as shown and described herein. As previously indicated, such an implement is referred to herein as a “tongued implement”. It is contemplated that the guidance control system could be used with vehicles other than farm tractors and could have general applicability to any such vehicle that would utilize a vehicle hitch to which some type of tongued implement could be attached where side to side adjustability for controlling the path of movement of the implement is desired. 
     However, the system is particularly useful in guiding implements that trail a tractor including but not limited to those used in row crop farming. The preferred embodiment of the system disclosed herein is particularly configured for use with a 2-point vehicle hitch, in conjunction with one or more coulter discs. 
     The preferred embodiment of the present invention is directed to a system which is configured to operate to have a tongued implement being pulled by a vehicle accurately follow a desired path that is defined in some manner and stored in a memory of a GPS system. The preferred embodiment has a GPS antenna that is mounted either to the forward end of the implement being drawn, such as on the tool bar of many implements and it is the operation of the GPS system to generate correction signals when the implement veers off of the desired path. It should be understood that while GPS control is preferred, the system can provide guidance for implements that use the wand technology that has been described herein. 
     The preferred embodiment of the present invention effectively tracks the position of the implement along the desired path and makes corrections to the orientation of the implement relative to the tractor. Significantly, if the tractor is traveling along a path that is generally within the bounds of correction by the implement, the implement will keep moving along the desired path. 
     Referring to the drawings, and particularly  FIGS. 4-9 , which comprise a sequence of drawings indicating the operation of the present invention which is a system and apparatus for providing a guidance capability for a tongued implement that is being pulled by a motor vehicle such as a tractor. The reference numbers that were used in  FIGS. 1-3  are also employed in  FIGS. 4-9  where they refer to common elements. However, elements given reference numbers  50  and above are not found in the prior art representation shown in  FIGS. 1-3 , but are present in the discussion and illustration of the embodiments of the present invention that are shown in these drawings. Thus, the tractor  20  has a two-point hitch which includes side arms  50  and  52  which are respectively connected to the bottom of the tractor approximately at points  54  and  56  and are shown converging together by the dotted lines at a point  58  which represents the pull point of the tractor. 
     By the nature of the design of the preferred embodiment, the pull point stays in line with the center of the tractor. It is important that the implement guidance not influence the tractor navigation. If the tractor has a separate guidance system, it is desirable that it be “decoupled” from the implement guidance system, and if that is done, both guidance systems benefit and can operate at their full potential. In the preferred embodiment, and as will be described, the hydraulically controlled cylinders operate to adjust the tongue of the implement, which is displaced or separated by a considerable distance from the tractor. This tends to decouple the implement guidance system in that the pull point is not appreciably changed and does not affect the tractor navigation. 
     In this regard, and referring to the prior art representation of  FIG. 3 , if the drawbar  28  is swung 15 inches to the right of center, then the angle θ of the drawbar relative to the center line of the tractor is 19.67°. Empirically, the side load P S  is equal to the implement load P L ×Tan θ or 0.357 P L . The total load P T  is equal to P L /Cos 19.56° which equals P L /0.942 or 1.062P L . In contrast, an offset of 15 inches using the preferred embodiment of the present invention produces an angle θ of 5.26° which results in a total load P T  equal to 1.004 P L  (see  FIG. 4 ). This means that the side load has not changed nearly at all compared to 1.062. The preferred embodiment of the present invention provides effective guidance control for a pull type hitch and maintains the pull point substantially in the center of the tractor even when a lateral correction of 15″ is being made. 
     Returning to  FIG. 4 , a hydraulic hitch mechanism, indicated generally at  60 , is attached to the arms  50  and  52  as well as to the tongue  38 ′ which is similar to the tongue  38  shown in  FIGS. 1 and 3  except that it has a flat end portion for connection to a rear end portion of the hydraulic hitch mechanism  60 . 
     With that introduction and before describing the sequence of operations shown in  FIGS. 4-9 , reference is made to the hydraulic hitch mechanism shown in  FIGS. 10-15 . Referring initially to the perspective view of  FIG. 10 , the hydraulic hitch mechanism  60  has a vehicle mount, indicated generally at  62 , a main frame, indicated generally at  64 , and a tongue mount, indicated generally at  66 . The vehicle mount  62  is configured to be attached to a two-point hitch of a farm tractor and the opposite end of the hydraulic hitch mechanism has the tongue mount  66  for attachment to the tongue  38 ′ shown in  FIG. 4 . 
     The vehicle mount  62  has a two-point mount frame  68  that has pin brackets, indicated generally at  70 , at the outer end thereof, each of which has a cylindrical pin  72  that are configured to engage the conventional receiving ends of the two-point side arms  50  and  52  of the tractor. It should be understood that the entire hydraulic hitch mechanism may be pivoted around the pins  72  that are fitted into the receiving ends of the arms  50  and  52 . While the mount frame  68  is shown to be a three interconnected sections  74 ,  76  and  78 , that are angled relative to one another, the two-point mount frame  68  also has a rear support frame comprised of side sections  80  and  82  and a bridging section  84 . The sections  74  through  84  are preferably made of 4″×4″×⅜″ thick steel tube stock that are welded together as well as welded to the pin brackets which are also substantial and preferably made with approximately ⅜″ steel plate side members  86  and upper and lower square steel tube stock  88 . The sections  74  through  84  define a pocket, indicated generally at  90 , in which a U-joint  92  is mounted (see  FIGS. 14 and 15 ). The vehicle mount also has a front lateral stiffener linkage, indicated generally at  93 , which is connected to the center section  76  preferably by welding. The linkage  93  has connection portions  94  which connect to center section  76  and to a first link  96  which in turn is connected to a second link  98  in a manner whereby each of the links are pivotable about a pin  100  and  102 . Similarly, the link  98  is pivotally connected to a drawbar plate  104  by a pin  106 . 
     It is important to the operation of the hydraulic hitch mechanism that the vehicle mount  62  not move laterally relative to the tractor. It is for this reason that the lateral stiffener linkage  93  is provided and it is bolted to the underside of the tractor by bolts that extend through slots  108  in the drawbar plate  104 . The linkage  93  has a relatively wide width to provide the necessary stiffness to the hydraulic hitch mechanism  60  and the pivotable linkage  93  provides the necessary stiffness in a lateral direction but enables the mechanism  60  to pivot around the pins  72  in the two-point hitch arms  50  and  52 . 
     Main frame  64  is manufactured from rectangular steel tube that is preferably approximately 3″×6″×⅜″ steel stock which is preferably welded together. It comprises a top strut  110 , a middle strut  112  and a lower strut  114 . The top strut  110  has an inclined front portion  116  and the lower strut has an inclined rear portion  118 . Vertical struts  120  and  122  are provided between adjacent horizontal struts as shown and a stiffener plate  124  preferably made of at least ¼″ steel stock is welded between the upper strut  110  including the incline portion  116 , the vertical strut  122  and the middle horizontal strut  112 . Similarly, a triangular stiffener plate  126  is provided between middle strut  112 , lower strut portion  118  and vertical strut  120 . The space between the middle strut  112  and lower strut  114  is sufficient so that it clears both the top and bottom of the frame section  84  enabling the main frame  64  to be pivoted in a generally horizontal direction by virtue of its connection to a vertical set of pins in the U-joint  92 . 
     As shown in  FIG. 10 , the mount frame section  76  has a cylindrical collar  128  extending therethrough in which a pin of the U-joint  92  is located. A similar collar is provided in the section  84  which enables the main frame  64  to pivot around the axis of the collar  128  during operation. 
     Turning to  FIGS. 14 and 15 , the U-joint  92  is illustrated and has two half sections  130  and  132  which are bolted together by bolts  134 . As shown in  FIG. 14 , each of the sections  130  and  132  are identical in structure and have semi-cylindrical recesses  136  for receiving vertical pins  138  and horizontal pins  140 . The pins have a truncated conical inner end  142  which enables them to be placed between the half sections  130  and  132  in almost, if not contacting relation. The thickness of the half sections  130  and  132  is preferably such that there is a small gap  144  at the interface so that when the bolts  134  are tightened, they firmly hold the pins  138  and  140  in position. The pin  140  has an outside diameter approximately equal to the inside diameter of the collar  128  so that rotation is possible. The main frame  64  preferably has similar collars in the middle strut  112  and lower strut  114  for receiving the vertical pins  138 . 
     As best shown in  FIGS. 10 and 15 , the axes of the pins  72 ,  138  and  140  and all extend through a common point. The advantages of the design is that the three pivot points share a common center axis. Thus each joint pin has only the actual stress load applied and is not compounded by being off center. The vertical U-joint pins  138  will only have the implement pulling force, and no shear stress due to being off center as in other designs. The horizontal U-joint pins  140  will only have the tongue weight. The two point pins  72  will have the tongue weight when the implement is raised, and pulling stress when the implement is engaged with the ground. There is no extra stresses introduced, as there would be if the pins were offset. With offset pivot points the distance of the offset creates a magnitude of leverage stress besides the normal load stress on the pivot pins. 
     With regard to the tongue mount  66 , it preferably has an interface plate  150  that is preferably flat and has a substantial surface area with a number of apertures  152  for bolting the interface plate to a similar tongue interface plate  154  that is preferably welded to the tongue  38 ′ as shown in  FIG. 4 . The tongue mount  66  also has upper and lower flanges  156  that have an aperture for receiving a pivot pin (not shown) but located within hollow cylinder portion  158  that is welded between the upper strut  110  and the middle strut  112 . The tongue mount  66  also has a pair of spaced side flanges  160  that are welded on opposite sides of the cylindrical section  158  and have an aperture for receiving a pin  162  (see  FIG. 11 ) which interconnects a piston  164  of a hydraulic cylinder  166 . While the cylinder  166  shown in  FIG. 10  is located on the left side as viewed in that drawing, a similar cylinder  167  is provided on the opposite side of the main frame  64 . The rear end of the cylinder is similarly connected by a pin  168  that is mounted between a pair of rear flanges  170  that are welded to the main frame  64 . 
     The left hydraulic cylinder  166  is connected to a control valve  172  by lines  174  and  176  whereas the right hydraulic cylinder  167  is connected to the valve by lines  178  and  180 . A source of hydraulic pressure preferably coming from the tractor to which the hydraulic hitch mechanism is attached is supplied through lines  182 . It should be understood that when the left cylinder  166  is operated to extend the piston  164 , the tongue mount  66  will rotate in a clockwise direction as viewed from the top and such rotation will also result in the piston of the cylinder  167  being retracted by a comparable amount. Thus, operation of the cylinders  166  and  167  cause the tongue mount  66  to rotate in either direction, which when connected to the tongue  38 ′ causes the tongue to be laterally moved one way or the other in an amount controlled by signals from the GPS system to maintain the implement moving along the desired path as will be further explained. An angular sensor  182  is connected by an arm  184  to the tongue mount  66  at  186 . The sensor  182  is preferably a Hall effect device, although other types of angular sensing mechanisms may be used. 
     Returning now to  FIG. 4  which shows the hydraulic hitch mechanism  60  on the tractor and connected to the implement  22 , the implement is also provided with a pair of stabilizing Coulter discs  190  which anchor the implement from sliding laterally during operation, so that when the tongue is moved relative to the tractor by the hydraulic hitch mechanism, the position of the tongue basically pivots around the Coulter discs  190  as is desired. The GPS antenna  192  is mounted preferably at or forwardly of the tool bar  32  and centered so that it is coextensive with the center of the implement draft which is identified at location  194 . 
     As a result of the GPS guidance capability, it is evident from  FIG. 4  that the rear wheels  26  are off the desired track  24 , i.e., they are to the right as shown in the drawing while at this particular point in time, the front wheels  27  are on track. Also, because of the operation of the guidance system causing the hydraulic hitch mechanism to be in the position shown, the implement  22  is still on track inasmuch as the wheels are centered along the lines  24  which is indicative of the implement being on track. It is also important to note that the line through the center of the implement draft and through the tongue  38 ′ extends forwardly to the pull point  58  which means that the tractor does not experience any significant side loading which is important for efficient operation of the tractor as well as the guidance system. 
     Referring now to  FIG. 5 , the front wheels  27  are turned left for the purpose of moving the tractor back on track and while this is happening, the guidance system maintains the implement on track as is indicated. The implement maintains its on track operation and side forces are not experienced by the tractor because the pull point is still substantially centered on the line of the tongue. 
     Referring to  FIG. 6 , the front wheels  27  have now traveled to the left of the correct track  24  while the rear wheels  26  have now returned to be on track and the hydraulic hitch mechanism  60  has adjusted to maintain the implement on track during this portion of the correction. 
     In  FIG. 7 , with the tractor rear wheels on track, the driver turns the front wheels  27  to the right so that the front wheels will also be on track and the hydraulic hitch mechanism  60  makes the angular adjustment so that the implement  22  continues to stay on track while the tractor straightens out. In  FIG. 8 , the front wheels have returned to be on track as are the rear wheels and the hydraulic hitch mechanism is substantially straight. In the final drawing of the sequence shown in  FIG. 9 , the driver straightens out the front wheels  27  and the front wheels, rear wheels and the implement are all aligned in perfect running orientation. 
     With regard to the GPS system that is to be used in the system of the present invention, it should be understood that the present invention utilizes a GPS system, rather than having invented it. There are several types of GPS systems that have been developed and marketed and which have been employed in implementations relating to agricultural applications. Some of the systems that have been utilized to date include the RTK (Real Time Kinematics) system that uses a base station and GPS to typically provide a ±1″ accuracy. Other systems that may be used include the DGPS HP (Differential GPS High Performance) system that typically provides accuracy to ±4″. This version utilizes GPS satellite signals for basic location information and requires at least an OmniSTAR HP satellite correction signal subscription for each system. Other systems include a DGPS (Differential GPS) system that typically provides accuracy to approximately ±10″. This version utilizes GPS satellite signals for basic location information and requires at least an OmniSTAR VBS satellite correction signal subscription for each system or the pre-WAAS signal which is differentiated by the source of correction signal. 
     It is preferred that the system used with the present invention be the most accurate possible. Because a cultivator, for example, can cut plants from rows as a result of a deviation of only a few inches, the RTK system which has a ±1″ accuracy is preferred over the other above-described systems. To the extent that other systems exist which are more accurate than ±1″, they are preferred for the reason that such close tolerances can be very important with regard to the successful operation of a guidance system in certain applications. 
     Since the present invention controls the implement along the preferred paths, a GPS antenna and control module  192  is preferably located on the implement and more particularly on the center of the tool bar  32  of the implement  30 . However, it should be understood that it may be located away from center provided offsets are calculated as may be necessary. It may also be beneficial to have the GPS antenna  192  mounted on a structure that is forward of the tool bar by a few inches or more so that some lead time in the position of the path is gained, which would enable timely correction of the path of the implement. 
     Although not shown, it should be understood that the antenna and control module  192  are shown to be at the same location, they may be separated such as locating the control module within the cab of the tractor. The antenna must be associated with the tool bar to detect or monitor its position. Communication between the control module and the antenna  192  can be accomplished by conductors extending between the two or by a wireless communication link, such as a Bluetooth or other type of communication link. 
     While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
     Various features of the invention are set forth in the following claims.