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TECHNICAL FIELD 
     The present invention relates to post driving methods and portable apparatus to perform such functions. 
     BACKGROUND ART 
     For centuries, posts have been driven into the ground by applying an essentially downward force at or near the top of a post. Humans may apply intermittent, brute force with sledgehammers or similar instruments. Machines may apply such force continuously or intermittently so long as the total energy imparted is sufficient to embed the post to a desired depth while not damaging the post. Pistons, actuated mechanically or hydraulically, may lack portability, be unwieldy, or cause post damage. 
     SUMMARY OF THE INVENTION 
     Various embodiments of the present invention solve problems associated with the prior art. Embodiments afford, for example, easy handling by an operator and a hammering force intended to decrease the chance of post damage. 
     Accordingly, a portable apparatus for driving a post into the ground is provided in an embodiment of the present invention. The apparatus has a base which is removably attachable to the post and a hammer module which is tethered to the base. The hammer module is so tethered to permit vertically reciprocating motion relative to the base and has a face for transmitting force to the top of the post. The base and the hammer module constitute an assembly which includes a drive arrangement to cause vertical reciprocation of the hammer module. In an embodiment, the assembly includes a frame, a motor mounted to the frame, an axle coupled to the motor, and a cam mounted on the first axle. In another embodiment of the invention, the assembly also includes a second axle having a second cam mounted on it. The axles and the motor have axes of rotation which are parallel to a reference axis. The second axle is coupled to the motor to rotate in a direction opposite to the direction of the first axle. Centroids of the cams are, alternately, simultaneously above and simultaneously below the axes of rotation of the first and second axle, so as to cause the hammer module to reciprocate vertically. 
     In accordance with a preferred embodiment, the hammer module has two ends and is elongate essentially along a long axis that is generally horizontal and transverse to the axes of rotation with the face proximate a working end of the hammer module. The base has a body and an arm that is pivotally attachable to a pivoting end of the hammer module, so that it is the working end which reciprocates vertically. The apparatus may also include a damping mechanism coupled between the base and the working end. A constraint upon vertical reciprocation may, in this way, be provided. 
     A further embodiment of the present invention is a method for driving a post into the ground including providing an apparatus in accord with a previously described embodiment, positioning it so that its face engages the top of the post, and powering the drive arrangement of the apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal view of a post driver according to an embodiment of the invention. 
     FIG. 2 is a top cross-sectional view of a hammer module and associated drive arrangement according to an embodiment of the invention. In this view, cam centroids are positioned between the axles. 
     FIG. 3 is a top cross-sectional view of a hammer module and associated drive arrangement according to the embodiment of FIG.  2 . In this view, the axles have been rotated 180° from the position shown in FIG.  2 . 
     FIG. 4 is an exploded view of components of the drive arrangement for a hammer module according to an embodiment of the invention. Three cams are included in this embodiment as are in FIGS. 2 and 3. 
     FIG. 5 is a plan view of a cam according to an embodiment of the invention. 
     FIG. 6 is a side cross-sectional view of a drive arrangement with the cams in a position in which each of the cam centroids is simultaneously above the axes of rotation of the axles upon which the cams are mounted. 
     FIG. 7 is a top view of a latching mechanism, in accordance with an embodiment of the invention. 
     FIG. 8 is a top view of an operator handle, in accordance with an embodiment of the invention. 
     FIG. 9 is a longitudinal view of the opposite side of the post driver of FIG.  1 . 
     FIG. 10 is a longitudinal view, schematically representing the elements of a portable post driver according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 10 is a longitudinal view, schematically representing the elements of a portable post driver  10  according to an embodiment of the invention. This embodiment includes an assembly having a base  13 , shown attached to post  100  at attachment point  65 . Further, the base  13  is shown tethered to a hammer module  61  at tethering point  64 . The assembly also includes an associated drive arrangement  60  which may or may not be disposed within hammer module  61 . Drive arrangement  60 , in operation, causes hammer module  61  to reciprocate vertically, as shown by arrow R. The reciprocating motion of the module  61  causes an essentially downward hammering force to be transmitted via module face  62  to a top  63  of post  100 . The portable post driver  10  is intended to efficiently drive post  100  into the ground while a user experiences minimal vibration or other discomfort while holding on to the base  13  during operation. The amount of hammering force imparted by the reciprocating action is intended to be sufficient to move the post  100  into the ground without damaging it. The base  13  may include a latch or other fastener for attachment to post  100  as well as a handle for convenience of the user. The user may, during operation, apply some downward force upon the handle to provide any required guidance or alignment of the driver  10  to insure that the downward portion of arrow R essentially coincides with a desired post-driving direction. The module  61  is tethered to the base  13  so that, in an embodiment, a module face  62  may engage the top  63  of the post  100  and impart the hammering force. The associated drive arrangement  60  may, for example, have a motor which spins a cam upon an axle to provide power to initiate and maintain reciprocating motion of the face  62 . The arrangement  60 , alternatively, may be based upon electrical, electromechanical, magnetic, or other mechanisms known in the art which will provide this type of reciprocating motion. 
     FIG. 1 shows a preferred embodiment of a portable post driver  10  in accordance with the present invention. An elongate frame  11  having a working end  110  and a pivoting end  111  defines a structural portion of a hammer module. Working end  110  is caused to vertically reciprocate (shown as arrow O) in, essentially, the post driving direction (shown by arrow  1 ). A face, which in this embodiment is a striker plate  12 , is coupled to the frame  11  at or near the working end  110  and is oriented so that reciprocation of working end  110  causes it to strike the top  101  of a post  100 , thereby imparting an essentially downward hammering force upon post  100 . Striker plate  12  is, preferably, made from hardened metal capable of withstanding repeated impacts with tops of metal posts. 
     Base  13 , in this embodiment, includes a body  130  and an arm  131 . Body  130  is oriented essentially parallel with the length of post  100  and is removably attached to post  100 . Latch  14 , when engaged with clasp  15 , adjustably secures post  100  to body  130 . A top view of a latching mechanism, in accordance with an embodiment of the invention, is shown in FIG.  7 . Here, end  140  of latch  14  is pivotally attached to body  130  at pivot point  70 . End  141  of latch  14  is rotated in the general direction of arrow L to fit around the periphery of a post  100  so that end  141  is adjacent to body  130  at attachment point  71 . Clasp  15  may then be used to secure end  141  to body  130 . Embodiments of a latching mechanism include, within the scope of the invention, components sized and shaped to accommodate a variety of styles and cross-sections of post which are known in the art. Mechanical clasps as shown may be substituted with other fasteners known in the art within the spirit of the present invention. 
     An operator handle  16  having a grip  160  is secured to a side of body  130  adjacent clasp  15  and circumferentially opposite to latch  14 . Operator handle  16  is further supported upon body  130  with handle member  161  attached to operator handle  16  on one member end. Handle member  161  is, itself, secured to body  130  by bracket  162 . A top view of an operator handle  16 , in accordance with an embodiment of the invention, is shown in FIG.  8 . The handle  16  shown is designed for two-handed use; however, other configurations may be used within the spirit of the present invention. In addition, a weight (not shown) may be coupled with handle  16  to further reduce undesirable vibration during operation of post driver  10 . 
     Arm  131  is pivotally coupled, near arm end  1310 , to the pivoting end  111  of frame  11  at joint  19 . Carrier handle  132  is attached to arm  131  near arm end  1310  to afford a convenient way for a user to carry portable post driver  10  when it is detached from a post  100 . Opposite arm end  1311  is attached to body  130 . Vertical reciprocation of working end  110  is generated when motor  17  is activated. An operator accomplishes motor activation and deactivation by turning switch  163  on or off. Switch  163  is shown positioned adjacent to operator handle  16  for convenient operation. Power cable  164 , mounted on body  13 , provides power communication between switch  163  and motor  17 . Motor  17  may, for example, be a 12 volt DC motor powerable by a suitable battery or other 12 volt DC source. Solenoid  166  is shown electrically coupled between switch  163  and motor  17 , in line with power cable  164 . 
     Springs  18  are attached between body  130  and working end  110  proximate to striker plate  12 . Springs  18  provide another base-hammer module connection in addition to the pivoting connection at joint  19 . Springs  18 , in the embodiment shown, serve to dampen and constrain the amount of reciprocation of working end  110 . The springs  18  may also assist, when tuned with the reciprocating forces and accounting for the rigidity of the pivoting connection at joint  19 , to more efficiently return the striker plate  12  to top  101 . The effectiveness of the resultant hammering force and the speed of post driving may be enhanced by efficient use of springs  18  having appropriately advantageous spring constants and damping characteristics. In the specific embodiment of FIG. 1, springs  18  are an assemblage including upper springs  181  and lower springs  182  which are separated by dividing plate  183 . The inventors have preferredly used upper springs  181  of about 1 inch diameter and about 1.5 inch length and lower springs  182  of about 1 inch diameter and about 4 inch length. Upper springs  181  are preferredly rated at about three times the force constant as lower springs  182  (for example, upper springs  181  may be rated at about 600 lb/in and lower springs  182  rated at about 200 lb/in). 
     FIGS. 2 and 3 are top cross-sectional views of an embodiment of a hammer module and an associated drive arrangement. A cut-away of frame  11  illustrates the mechanically coupled components which provide, when operational, reciprocating motion of working end  110 . Other mechanical, electromechanical, or magnetic drive arrangements which will produce reciprocating motion of working end  110  are within the spirit of the present invention. 
     Motor  17  is mounted to frame  11  proximate pivoting end  111 . When motor  17  is activated, motor shaft  200  rotates. Belt  201  mechanically couples rotary motion of motor shaft  200  with that of a first axle shaft  202  of a first axle  20 . First gear  25 , mounted inapposite to belt  201  on first axle shaft  202 , is coupled with a second gear  26 , mounted on second axle  21  so that second axle  21  rotates in an opposite direction from first axle  20 . Reference axis  2  is, herein, defined such that the axes of rotation of first axle  20 , second axle  21 , and motor  17  are parallel to axis  2 . Two first axle cams  27  are eccentrically mounted upon first axle  20 ; one second axle cam  28  is eccentrically mounted upon second axle  21 . Cams  27  and  28  are particularly mounted at positions along their respective axle lengths so that the cams  27  and  28  are perfectly free to rotate. Two first cams  27  may each have a different thickness than second cam  28 . Cams  27  and  28  may, as shown in FIGS. 4 and 5, be indexed about their respective axles  20  and  21  by a key way  52 . Tapped holes  53  are provided for mounting first cams  27  and second cam  28  to axles  20  and  21  with set screws (item  40  in FIG.  4 ). Cams  27  and  28  are, in this embodiment, bell-shaped. FIG. 5 shows the position of cam centroid  51 . 
     Cam centroids  51  are, when cams  27  and  28  are mounted on their respective axles, not collinear with the axes of rotation  2  of first axle  20  and second axle  21 . FIG. 2 shows a relative axle position in which the centroids  51  of cams  27  and  28  are laterally positioned between first axle  20  and second axle  21 . FIG. 3 shows a relative axle position following a 180° rotation from the position of FIG.  2 . Movement of centroids  51  is such that they alternate between being in a first position where the centroids  51  are simultaneously above and a second position in which the centroids are simultaneously below their respective axles&#39; axes of rotation causes the hammer module to reciprocate vertically. FIG. 6 illustrates cams  27  and  28  in the first position. Note, that the first and second positions occur at relative axle positions 90° out of phase from the positions shown in FIGS. 2 and 3. Note, also, that when the centroids  51  are in the first position and are simultaneously above the axles, force balance calculations made upon the hammering module would predict that a peak in the downward hammering force occurs in the first position. Thus, when the cam centroids  51  are at their highest position relative to the essentially vertical post driving direction, O, the force to move the working end  110  downward is at a maximum and the module will impact post  100 . Movement of centroids  51  to their lowest relative position will force the working end  110  upward. Upward motion of working end  110  will be constrained by springs  18  which will further, as the springs  18  are stretched, provide some additional downward force during the following rotational cycle of axles  20  and  21 . Spring compression on the next half cycle may provide additional upward force to that provided by centroid movement. 
     FIG. 4 is an exploded view of first axle  20 , second axle  21 , cams  27  and  28 , and associated mounting hardware known in the art. Flanges  41 , grommets  42 , sealers  43 , are assembled using bolts  44 . The relative positions of cams  27  and  28  are indexed as previously described. 
     FIG. 9 illustrates the side of the post driver opposite the side shown in FIG.  1 . Gear  25 , driven by motor  17 , causes gear  26 , and axle  21  to rotate in the opposite direction to motor  17  and axle  20 . Frame  11  is elongated along axis  9 . In this embodiment, striker plate  12  is located between first axle  20  and second axle  21  measured along axis  9 . 
     An optimum running speed for motor  17  is, approximately, 2600 rpm. The inventors have used, in an embodiment, a gear ratio of about 2:1 resulting in a speed of axles  20  and  21  of about 1300 rpm. First cams  27  used weighed about 1.4 lb each; while second cam  28  was about twice the weight of each first cam  27 . As in FIGS. 2 and 3, the second cam  28  used had about twice the overall thickness of each first cam  27 . 
     Although the invention has been described with reference to several preferred embodiments, it will be understood by one of ordinary skill in the art that various modifications can be made without departing from the spirit and the scope of the invention, as set forth in the claims hereinbelow.

Summary:
A portable apparatus for driving a post into the ground is provided in an embodiment. The apparatus has a base removably attachable to the post as well as a hammer module tethered to the base to permit vertically reciprocating motion of the module relative to the base and post. The apparatus may be motor driven with rotating, eccentrically mounted cams providing the mechanism for creating a hammering force through reciprocating motion. The hammer module has a face which imparts the hammering force to the top of the post. A method for using such an apparatus is provided in a further embodiment.