Abstract:
A touchpad stylus includes a housing, a stylus member axially moveable in the housing, and an energy-absorbing cushion member interposed between a rear extremity of the stylus member and an internal cavity of the housing, the cushion member lightly forwardly biasing the stylus member relative to the housing. The stylus member has a smoothly spherical tip extremity, and the device avoids impact damage to fragile outer membranes of existing touchpad devices because the full mass of the housing is isolated from impact with the touchpad, the stylus member itself having very significantly less mass than an aggregate mass of the complete assembly.

Description:
RELATED APPLICATION  
       [0001]     The present application is a continuation of U.S. application Ser. No. 10/186,323, filed Jun. 27, 2002, now U.S. Pat. No. ______, entitled “Touchpad Stylus Having Isolated Low-Mass Contact Element,” the contents of which are all incorporated by reference herein in their entirety. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to electronic pointing devices and writing implements, and more particularly to styli for use with graphic tablets such as touchpads.  
         [0003]     In the art of touchpads, it is conventional to provide a thin, flexible membrane as an outer member to be contacted by a stylus that is held by a user of the touchpad. A particularly vexing problem with this arrangement is that the flexible membrane, in order to permit high-resolution pointing and/or writing, is typically very thin and delicate, being particularly subject to damage in the event that the user subjects the touchpad to significant impacting contact by the stylus, which necessarily also has a fine point for operating the touchpad at high resolution.  
         [0004]     It is known to use a conventional ball point pen cartridge as a stylus point, when it is desired to produce an inked record (such as on a paper strip temporarily covering the touchpad). When the cartridge is in good condition the rolling action of the ball avoids sliding contact wear of the membrane and/or the cartridge. However, when ink is not desired, an empty cartridge generally does not provide this advantage because the ball normally fails to roll. More importantly, the touchpads in use are still subject to excessive damage even when the stylus incorporates a ball pen cartridge.  
         [0005]     In my previously-filed patent application (U.S. patent application Ser. No. 10/186,323, now U.S. Pat. No. ______), I disclosed a touchpad stylus which provides smoothly increasing axial displacement of the stylus member of up to approximately 0.05 inches in response to axial loads ranging from approximately 0 to up to approximately 200 grams. This touchpad stylus was based on the general belief that the “normal” pressure applied by a signator is typically less than about 200 grams. Recent test work, however, has revealed that a number of individuals occasionally press down against a touchpad with increased force, such as between 200 grams and 500 grams and occasionally between 500 grams and about 1000 grams.  
         [0006]     Accordingly, there is a need for a touchpad stylus that does not suggest outer membranes of touchpads to excessive damage even when forces of between about 200 grams and about 1000 grams are applied.  
       SUMMARY  
       [0007]     The present invention meets this need by providing a stylus device that eliminates or greatly reduces the damage to the thin outer membranes of touch pads in normal use. It has been discovered that the bulk of damage to touchpads in use occurs at the initial impact of the stylus against the membrane. The stylus device of the present invention avoids this problem by limiting forces of impact when the stylus is brought into contact with the touchpad or similar device.  
         [0008]     In one aspect of the invention, the stylus device includes an elongate stylus housing; a stylus body having a tip portion for contacting the touchpad device, the body being supported by the housing for axial movement along the longitudinal axis; a stop for limiting forward movement of the stylus body relative to the housing; and biasing means for forwardly biasing the body with a force being limited to a predetermined maximum axial bias over a predetermined minimum travel distance of rearward movement relative to a rest position of the stylus body, the minimum travel distance being not less than 0.05 inch, the maximum axial bias being not more than approximately 1000 gm. Preferably the stylus body has a mass that is less than 10 percent of an aggregate mass of the stylus device for significantly limiting impact loading of the touchpad device upon initial contact by the stylus body. Preferably the mass of the stylus body is not greater than 0.5 gm.; more preferably the mass of the stylus body is not greater than only approximately 0.2 gm. Notwithstanding the above, the stylus body preferably has a mass that is less than only approximately 5 percent of an aggregate mass of the stylus device. As used herein, the term “stylus body” is inclusive of the above-recited tip portion and any rigid elements rigidly assembled therewith.  
         [0009]     The biasing means can include a resilient cushion member interposed between a forwardly facing cavity surface of the housing and a rear extremity of the stylus body.  
         [0010]     The rear extremity the stylus member can form an engagement surface having a progressively increasing cross-sectional engagement area over an axial distance not less than the minimum travel distance, the cushion member having a contact area facing the stylus member, the contact area being greater than a maximum of the engagement area. The engagement surface can include a face portion and a tapered portion, the face portion being generally planar and oriented approximately perpendicular to a longitudinal axis of the housing. The tapered portion of the engagement surface can be approximately conical. The face portion of the engagement surface can have an area that is approximately 25 percent of the maximum of the engagement area, and the contact area of the cushion member is preferably at least approximately 4 times the maximum of the engagement area for enhanced cushioning by the cushion member. Preferably the cushion member also has an axial depth that is not less than double the minimum travel distance. Further, the maximum of the engagement area is preferably not more than approximately 0.01 square inch.  
         [0011]     Preferably the minimum travel distance of the stylus body relative to the housing is at least approximately 0.05 inch and the maximum axial bias is not more than approximately 1000 gm.  
         [0012]     The stop can include a resilient stop member interposed between a housing stop surface of the housing and a stylus stop surface of the stylus body. The stylus body can have a cylindrical stem portion, with the stylus stop surface forming a generally annular enlargement of the stylus body, and the housing stop surface also being generally annular and facing the stylus stop surface, the stylus body protruding through the stop member. The stop member can include a polymeric O-ring.  
         [0013]     The stylus body can include a polymeric material that preferably has a frictional coefficient not greater than 0.2, as measured dry and with the material contacting steel. More preferably, the coefficient of friction is not more than approximately 0.1. The polymeric material of the stylus body can be an acetal resin. Preferably the tip portion of the stylus body forms a contact surface having a radius of curvature that is not less than 0.5 mm for limiting localized stress at locations of contact on the touchpad device. More preferably, the radius of curvature of the contact surface is at least approximately 1.0 mm.  
         [0014]     The stylus device can be provided combination with the touchpad device, the touchpad device having an active surface and being responsive to force locally applied against the active surface, and further comprising a flexible sheet material removably covering the active surface to provide a replaceable writing surface to be contacted by the tip portion of the stylus. Preferably the flexible sheet material has a permanently applied adhesive coating for affixing the sheet material to the touchpad device.  
     
    
     DRAWINGS  
       [0015]     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where:  
         [0016]      FIG. 1  is a perspective view of touchpad stylus according to the present invention, the stylus being positioned against a touchpad for use therewith;  
         [0017]      FIG. 2  is a lateral sectional view of the stylus of  FIG. 1 ;  
         [0018]      FIG. 3  is a detail sectional view within region  3  of  FIG. 2 ;  
         [0019]      FIG. 4  is a detail view of a stylus head portion appearing in  FIG. 3 ;  
         [0020]      FIG. 5  is a sectional view as in  FIG. 3 , showing the stylus in a loaded condition;  
         [0021]      FIG. 6  is a graph of a progressively increasing biasing force of the stylus of  FIG. 1  against rearward movement of a stylus member of the stylus relative to a housing thereof; and  
         [0022]      FIG. 7  is a detail view as in  FIG. 3 , showing an alternative configuration of the stylus head portion.  
     
    
     DESCRIPTION  
       [0023]     The present invention is directed to a stylus for use with touchpads and similar devices wherein a stylus point or tip is positioned and/or moved on a surface in electronic pointing operations or the production of graphical data. With reference to  FIGS. 1-6  of the drawings, a touchpad stylus  10  includes a stylus housing  12  and a stylus member  14  having a tip extremity  16  that is adapted for contact with an outer membrane  18  of a touchpad  20 , the membrane  18  covering a pad member  19 . It will be understood that details of the touchpad  20  are outside the scope of the present invention, other than that typical touchpads include counterparts of the membrane  18 , being a somewhat delicate member that is subject to damage when used with many styli of the prior art. As further shown in  FIG. 1  and described below, the touchpad  20  can be provided with an optional thin replaceable sheet member  21  covering the outer membrane  18 .  
         [0024]     The stylus housing  12  has a rear or body portion  22  and a threadingly connected front or nose portion  24  which together form a conveniently hand-held device that is generally symmetrical about a longitudinal axis  25 , the nose portion having a passage  26  formed therethrough for slidingly receiving a cylindrical stem or shank portion  28  of the stylus member  14  in concentric relation to the axis  25 , the tip extremity  16  projecting forwardly of the nose portion  24 . The stylus member  14  also has an enlarged head portion  30  that forms a forwardly facing stylus stop surface  32  extending outwardly from a rear extremity of the shank portion  28 , the nose portion  24  of the housing  12  also having a rearwardly facing housing stop surface  34 . A resilient O-ring  36  is supported on the shank portion  28  of the stylus member, being interposed between the stylus and housing stop surfaces  32  and  34  for cushioned limitation of forward travel of the stylus member  14  relative to the housing  12 . The tip extremity  16  is formed having a smooth convex profile, a preferred profile being of spherical radius R. The radius R is sufficiently large to appropriately limit localized compressive stress and shear loading of the outer membrane  18  and/or the optional sheet member  21 , while permitting a desired degree of visual access to portions of the touchpad  20  to which the tip extremity  16  is being applied. For this purpose, the radius R can range from relatively small (0.2 mm for example) to relatively large (0.75 mm for example); however, as a result of experimental testing it has been found that a range of from 0.25 mm to 0.5 mm is preferred, being equivalent to a spherical tip diameter of from 0.5 mm to 1.0 mm.  
         [0025]     Preferably the stylus member  14  is formed of a material having relatively low density yet being sufficiently hard to retain its shape in normal use, the material also having a very low coefficient of friction with the outer membrane  18  of the touchpad  20  and/or the sheet member  21  optionally applied thereto for minimal wear of those materials. Further, the stylus member (or at least that portion including the tip extremity  16 ) is also formed of a material having high wear resistance for preserving a desired shape of the tip extremity  16 . Commercially available materials particularly suited for use in the stylus member  14  include formulations of DELRIN® acetal resin that are available from a variety of sources. Of particular interest are Delrin 500, 550, and 507, having a coefficient of friction of between approximately 0.1 and 0.3, both static &amp; dynamic as measured dry, on steel. Similarly, Delrin AF has a coefficient of friction between approximately 0.05 and 0.15, and Delrin 570X has a coefficient of friction of approximately 0.2. Another formulation *?* of Delrin acetal resin has a coefficient of friction of approximately 0.25 and a wear factor of 216, which compares favorably with an alternative material, nylon, having a friction coefficient of 0.35 and a wear factor of 72. Accordingly, it is preferred that the material of the stylus member be or include a polymer having a coefficient of friction not greater than 0.2, which is believed to be easily obtainable by selection from available stock. Further, a most preferred material for the stylus member  14  is Delrin AF, selected for having a friction coefficient of not greater than approximately 0.1.  
         [0026]     The housing body portion  22  has a forwardly facing cushion cavity  38  formed therein for supporting a cushion member  40  that is contacted by an engagement surface  42  of the stylus member head portion  30 . As shown in  FIG. 3 , an unloaded position of the stylus member  14  relative to the housing  12  is preferably characterized by substantially no axial clearance and no axial (compressive) loading of the head portion  30  and the O-ring  36  between the cushion member  40  and the housing stop surface  34 , the cushion member  40  having a substantially planar front surface in its unloaded condition in the exemplary configuration of  FIGS. 1-6 . Suitable materials for the cushion member include low density silicon foam, a preferred such material being commercially available as No. 86235K42 EXOBLOC™ silicone foam from McMaster Carr of Los Angeles, Calif. Thus foam has a UL-94-HF-1V-0 rating, a density of from 9 to 19 pounds per cubic foot, and requires only approximately 2 to 3 psi for compression to 25 percent of a relaxed thickness. In the preferred exemplary configuration of the stylus  10 , the cushion member  40  has a thickness of approximately 0.125 inch and an outside diameter of 0.200 inch, the cushion cavity  38  having the same dimensions.  
         [0027]     The engagement surface  42  of the head portion  30  is also preferably configured for producing, in combination with the cushion member  40 , a desired force-displacement profile of the stylus member  14 . Toward that end, the engagement surface  42  has a first portion  42 A that is preferably barely in contact with the cushion member  40  when the stylus member  14  is in its advanced, unloaded position shown in  FIG. 3  (and by broken lines in  FIG. 5 ), and a second portion  42 B for contacting the cushion member when the stylus member is displaced rearwardly as shown by solid lines in  FIG. 5 . As shown In  FIG. 6 , preferred configurations of the touchpad stylus  10  provide for smoothly increasing axial displacement of the stylus member  14  of up to approximately 0.05 inches in response to axial loads ranging from approximately zero up to approximately 1000 grams. Typically, preferred configurations of the touchpad stylus  10  provide for smoothly increasing axial displacements of the stylus member  14  of up to approximately 0.05 inches in response to axial loads ranging from approximately 0 up to approximately 500 grams, more typically ranging from approximately 0 up to approximately 200 grams, and most typically ranging from approximately 0 up to approximately 100 grams. In the illustrated exemplary configuration, the first portion  42 A of the engagement surface is substantially planar, the second portion being generally frusto-conical for producing a mildly increasing force-displacement profile that smoothly becomes a more rapidly increasing profile as the stylus member  14  is displaced toward the retracted position shown by the solid lines in  FIG. 5 . This behavior is approximated by the upper logarithmic scale in  FIG. 5 . More particularly, and as shown in  FIG. 4 , the planar first portion  42 A of the engagement surface  42  has a diameter D 1 , the second engagement surface portion  42 B sloping outwardly and forwardly toward a diameter D 2 , the diameter D 2  being coincidently an overall outside diameter of the stylus member head portion  30 .  
         [0028]     In the preferred exemplary configuration, the head portion  30  of the stylus member  14  is formed with the diameter D 1  of the first engagement surface portion being approximately 0.050 inch, the limiting diameter D 2  of the second engagement surface portion being approximately 0.104 inch (the stylus stop surface  32  radially projecting approximately 0.010 outwardly from an approximate diameter of 0.094 inch of the stylus member shank portion  28 . The limiting diameter D 2  of the engagement surface second portion  42 B is preferably located forwardly of the first portion  42 A by a distance equal to or only slightly greater than the full axial travel distance of the stylus member  14  between the positions shown in  FIGS. 3 and 5 , as best shown in  FIG. 5 . Thus in this described configuration the engagement surface  42  contacts progressively increasing area portions of the cushion member  40  as the stylus member  14  is displaced rearwardly.  
         [0029]     It will be appreciated that the stylus member  14  can be very low in mass as compared with an aggregate mass of the touchpad stylus  10 . For example, approximating the volume of the stylus member as being equivalent to a diameter of 0.1 inch and a length of 1 inch, the material having a density of 1.6 relative to water, the approximate mass is only 
 
0.1**2 (in)*1.0 (in)*2.54 (cm/in)**3*1.6*π/4=0.2053 gm. 
 
 This contrasts favorably with an estimated aggregate mass of at least about 4 gm for a complete stylus of sufficient size to be comfortably grasped and controlled by a user thereof, the mass ratio being approximately 20. It is contemplated that satisfactory operation of the touchpad stylus  10  is obtained with the mass of the stylus member being not greater than 0.5 gm, although the mass is preferably not greater than the approximately 0.2 gm of the above-described preferred example. 
 
         [0030]     With further reference to  FIG. 7 , an alternative configuration of the stylus member head portion, designated  30 ′ has a cylindrical counterpart of the engagement surface second portion, designated  42 B′, for producing a nearly linear force-displacement curve as approximated by the lower linear scale in  FIG. 6 . As shown in  FIG. 7 , the diameter D 2  is only slightly greater than the diameter D!, the difference being due to a slight rounding of the engagement surface  42  at the outer margin of the first engagement surface portion  42 A that is provided for preventing cutting of the pad member  40  when being deformed by the head portion  30 .  
         [0031]     As further shown in  FIG. 1 , the sheet member  21  that is optionally applied to the touchpad outer membrane  18  has a permanently applied adhesive coating  44 , the coating  44  being formulated for securely affixing the sheet member  21  to the outer membrane  18  of the touchpad  20 , yet permitting removal of the sheet member without damage to the membrane  18  and without leaving any significant portion of adhesive material on the membrane. A material suitable for use as the sheet member  21  and having the adhesive coating  44  is commercially available as WRITEWRIGHT™ from Fellowes of Itasca, Ill. This material also has a slightly textured writing surface opposite the adhesive coating  44  for facilitating marking thereof by a suitable writing instrument.  
         [0032]     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, other shapes and sizes of the engagement surface  42  are contemplated as well as different formulations and geometry of the cushion member as variations of the configurations described above being within the ordinary skill in the art and providing, minimum stylus member travel distances of 0.025 inch or greater with the maximum axial bias not exceeding approximately 200 grams. Also, a coil compression spring or other type of spring member can be substituted for the cushion member  40 , and the stylus member  18  can be provided with a rolling-ball at the tip extremity  16 . Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.