Abstract:
Keyboards with a construction which isolates the interior of the device from water and other deleterious substances in the ambient surroundings. The keyboard also has the capability of transmitting to a second device, upon activation of a key, a coded, preferably infrared (IR) signal uniquely corresponding to the key which is activated.

Description:
RELATED APPLICATION 
     This application is a continuation of Provisional application 60/115,006, filed on Jan. 6, 1999. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to keyboards and, more particularly, to novel, improved keyboards so constructed that water and other foreign substances cannot penetrate to the keyboard interior. 
     BACKGROUND OF THE INVENTION 
     Disclosed in copending U.S. patent application No. 09/046,947 are appliances with a mode of operation which allows the appliance user to log onto the Internet. Particularly in the composition of e-mail messages, a keyboard type input device is advantageous, if not essential, to make e-mail messaging practical. 
     Appliances of the character described in the preceding paragraph are designed so that they can be used in environments—such as a kitchen—where water and other foreign substances are present. For a keyboard to be usable in such environments, exclusion of water and other foreign substances from the keyboard interior is needed. Otherwise water and/or other foreign substances might penetrate to the interior of the device and damage, if not totally ruin, internal components of the device. 
     SUMMARY OF THE INVENTION 
     There have now been invented and disclosed herein certain new and novel keyboards which have the requisite impermeability to foreign material desired for applications where exposure to water and/or other foreign matter can be expected. 
     These novel keyboards have a frame and a printed circuit board (PCB) mounted to the frame. An elastomeric keypad with character and function keys overlies the PCB. Pressing a key completes a corresponding electrical circuit on the PCB; the result is the transmission of an appropriate signal to the appliance or other unit serviced by the keyboard. 
     A base and a battery cover complete the configuration of the keyboard and isolate its internal components from the surrounding environment. 
     An integral peripheral portion of the elastomeric keypad seals the gap between the keypad and the base of the keyboard unit. Isolation of the keyboard interior from the ambient surroundings is completed by two seals or gaskets. One gasket seals the gap between the base and the battery cover. The second gasket keeps water and/or other foreign material from leaking through an opening in the battery cover. This opening accommodates a fastener which removably secures the battery cover in place. 
     The resistance of the keyboard to penetration by foreign substances is materially enhanced by virtue of there being no openings for keys through which foreign substances might reach the interior of the keyboard. This elimination of key-accommodating openings also contributes to cost effectiveness by eliminating the costs of sealing those openings. 
     In addition to their impenetrability, the novel keyboards disclosed herein have the advantage of being simple, rigid, lightweight, and manufacturable at low cost. 
     The modest cost of the keyboards disclosed herein is in part attributable to electronics which translate a keystroke into a coded signal that controls the operation of an appliance or other unit which the keyboard serves. This circuitry includes a column and row matrix of switches matched to the keyboard keys, an array of shift registers, and a microcontroller. When a key is activated, the corresponding switch is closed, and the microcontroller acquires the information that uniquely identifies a particular key (by column and row). From this information, the microcontroller and ancillary components generate a train of digital pulses uniquely identifying the key that was activated. 
     The encoded signals are preferably transmitted to the associated appliance at an infrared (IR) frequency. This eliminates the need for a connecting cable and the disadvantages appurtenant to the use of a connecting cable. Also, the use of a signal in the IR range makes the orientation of the keyboard relative to the unit it serves much less critical. 
     Infrared (IR) transmitters are preferably located at both ends and at the back side of the keyboard. This makes the keyboard omni-directional; that is, the combination of an IR signal and multiple transmitters means that the signal can be transmitted to the appliance (or other unit) the keyboard serves generally irrespective of the orientation of the keyboard relative to the appliance. 
     Another novel feature of the keyboards disclosed herein is a set of communicating channels formed in the reverse (inner) side of the elastomeric keypad. When a key is depressed, the pressure in the confined space between the key and the PCB may increase to the extent that the keypad shifts relative to the PCB, making the keyboard inoperable. The channels in the keypad eliminate this problem by increasing the volume of the space into which air can flow, thereby making the key activation-associated pressure build-up so small as to be inconsequential. 
     If deemed necessary, shifting of the elastomeric keypad relative to the PCB can be also (or instead) eliminated by positively anchoring the keypad to the PCB. 
     Another important advantage of the invention, especially from the practical point-of-view, is that the external configuration of keyboards embodying its principles are free from nooks and crannies and consequently easy to clean. In fact, to this end, the keyboard can be immersed in water (or any other liquid cleaning medium), if desired. 
     Yet another important advantage of keyboards as disclosed herein is the elastomeric keypad is so configured that: (a) the character and functional keys are dished and guide a user&#39;s fingers into the correct positions on the keys, and (b) the keys provide tactile evidence to the user of a key having been activated. 
     This keyboard has the advantage over input devices such as the remote controls disclosed in the above cited &#39;947 application that e-mail messages can be easily composed. That is a much slower and cumbersome task when a remote control is employed as, when using that device, a cursor has to be moved to a character and the character then clicked on to select the character. Also, a remote control with a full complement of letter, number, and reference keys would be perhaps impracticably large and bulky. 
    
    
     The important features, objects, and advantages of the present invention will be apparent to the reader from the foregoing and the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of an impermeable (waterproof) keyboard constructed in accordance with and embodying the principles of the present invention; 
     FIG. 2 is a top view of the FIG. 1 keyboard. This figure shows a set of character and function keys preferred if the keyboard is to be used to control an appliance of the character disclosed in the above-cited &#39;947 application; 
     FIG. 3 shows the relationship between FIGS. 3A and 3B which, taken together, constitute an exploded view of the FIG. 1 keyboard; 
     FIG. 4 is an exploded view of a keyboard frame and a keyboard base, looking at the lower sides of those components; 
     FIG. 5 is a transverse section through the FIG. 1 keyboard, taken generally along line  5 — 5  of FIG. 2; 
     FIG. 6 is a fragmentary section showing how an elastomeric keypad of the FIG. 1 keyboard can be anchored and kept from shifting relative to a complementary PCB component of the keyboard; 
     FIG. 7 is a fragmentary section showing yet another technique for anchoring the elastomeric keypad of the FIG. 1 keyboard to the PCB and thereby keeping it from shifting; 
     FIG. 8 is a bottom view of the elastomeric keypad of the FIG. 1 keyboard; this figure shows a set of communicating channels formed in the keypad to keep pressures built up when a key is depressed at an inconsequential level; this keeps the keypad from shifting relative to the associated and complementary PCB; 
     FIG. 9 shows the relationship between FIGS. 9A and 9B which, taken together, schematically show the electrical contacts of the key-associated switches; those are composed of contacts on the bottoms of the keys (FIG. 5) and associated traces on the PCB of the FIG. 1 keyboard; also shown in FIGS. 9A and 9B are leads connecting the traces to other components of the keyboard&#39;s operating system; 
     FIG. 10 shows the relationship between FIGS. 10A and 10B which, taken together, constitute a schematic diagram of the operating system components employed in the FIG. 1 keyboard to generate and transmit coded, digital, infrared signals as the keys of the FIG. 1 keyboard are pressed; and 
     FIG. 11 is a flow diagram showing how a microcontroller incorporated in the keyboard detects a key that is pressed and causes the IR transmitter components of the keyboard to transmit an IR signal uniquely identifying the activated key. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIG. 1 depicts a keyboard  20  constructed in accord with, and embodying, the principles of the present invention. The major mechanical components of keyboard  20 , perhaps best shown in FIGS. 3A,  3 B, and  5 , include: an elastomeric keypad  22 ; a complementary PCB  24 ; a frame  26  on which PCB  24  is mounted; a base  28  which cooperates with keypad  22  and frame  26  to enclose PCB  24 ; LED&#39;s  30 ,  32 , and  34  (see FIG. 10A) which function as infrared transmitters; a battery access cover  44 ; and a gasket  46  which seals the gap between base  28  and battery access cover  44 . 
     Elastomeric keypad  22  includes both character keys and function keys (see FIG.  1 A). The character keys—collectively identified by reference character  48 —may be those constituting a conventional QWERTY key set. The function keys include conventional Tab, Caps Lock, Shift, Control, Alt, Escape, Delete, Space, Enter, and Backspace keys  49  . . .  64  as well as an On/Off key  66 . Additional function keys of the illustrated, exemplary keypad  22  include mode switches  68 ,  70 ,  72 , and  74  and a key block  76  which includes security/monitor, talk, and unlock buttons  78 ,  80 , and  82 . Mode keys  68 ,  70 ,  72 , and  74  are employed to operate an appliance of the character disclosed in the &#39;947 application in a selected one of its APPLIANCE, TV, INTERNET, and CD modes. The security/monitor, talk, and unlock buttons  78 ,  80 , and  82  in set  76  are employed with an appliance of the same general character to enter a SECURITY mode of operation. The previously disclosed unit functions in each of the foregoing modes is described in the following table: 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Mode 
                 Function 
               
               
                   
               
             
             
               
                 ENTERTAINMENT 
                 Allows the user to watch television 
               
               
                 CD 
                 Allows the user to play a conventional audio 
               
               
                   
                 (or audio/visual) disk; in this mode the user 
               
               
                   
                 can also play a disk of the character described 
               
               
                   
                 in U.S. Pat. Nos. 5,724,102 and 5,801,784 
               
               
                   
                 to retrieve lessons on a particular subject or 
               
               
                   
                 information on performing a task from the 
               
               
                   
                 disk. 
               
               
                 APPLIANCE 
                 Gives the integrated module control over one 
               
               
                   
                 or more appliances - as examples only, a 
               
               
                   
                 kitchen range, refrigerator, central heating 
               
               
                   
                 system, air conditioner, or central vacuuming 
               
               
                   
                 system. Also, diagnostic information on the 
               
               
                   
                 controlled appliance(s) may be gathered and 
               
               
                   
                 made available on the display screen of the 
               
               
                   
                 appliance and at a service or repair facility. 
               
               
                 SECURITY/MONITOR 
                 Allows one to visually identify and converse 
               
               
                   
                 with a person at an entry way, and to unlock a 
               
               
                   
                 door at the entry way, all from the location of 
               
               
                   
                 the appliance; the appliance can also be used in 
               
               
                   
                 this mode to monitor an area remote from the 
               
               
                   
                 appliance - a baby&#39;s bedroom for example. 
               
               
                 INTERNET 
                 Allows a user to log onto and browse the 
               
               
                   
                 Internet, send and receive e-mail messages, and 
               
               
                   
                 to take advantage of the other opportunities 
               
               
                   
                 which the Internet provides. 
               
               
                   
               
             
          
         
       
     
     Also found on keypad  22  is a set of navigation keys collectively identified by reference character  84 . 
     The functions of these keys are described in above-cited application Ser. No. 09/046,947 except for REV and FWD keys  90  and  92  and PAUSE key  98 . These keys are employed in a CD mode of operation of the appliance controlled by keyboard  20 . They function in the same manner as conventional CD controls with the same designations. 
     Also found on keyboard  20  is a group  116  of control keys. These are VOLUME UP and VOLUME DOWN keys  118  and  120 , MUTE key  122 , and CHANNEL UP and CHANNEL DOWN keys  124  and  126 . The functions of these keys are also described in application Ser. No. 09/046,947. 
     As best shown in FIG. 5, the tops of the character and function keys just described have a concave, or dished, upper surface with the upper surface  128  of key  130  being representative. This promotes an advantageous “touch and feel” character of the keys, making it easier for a keyboard operator to position his fingers on a wanted key. The touch and feel characteristics of keyboard  20  are further promoted by constructing keypad  22  so that the several character and function keys emulate the familiar feel of a conventional keyboard, which indicates to the user that a key has been depressed far enough for activation. User friendliness may also promoted by color coding the keys to their functions. 
     As mentioned above, user friendliness can be promoted by color coding the keys of keyboard  20 . A representative color scheme follows: The ON/OFF button is bright green, the mode selection buttons (TV, Internet, CD, Monitor, etc.) are light gray, the navigation buttons are dark gray and the MUTE button is yellow. This brings the user&#39;s eye to the ON/OFF button immediately upon approaching the keyboard. While the unit is in use the user&#39;s eye will be drawn to the mode selection and navigation buttons. If the user is interrupted or otherwise wants the sound turned off, the MUTE button can be easily found because it stands out with its bright yellow color. 
     Turning now to FIGS. 2 and 5, an electrical switch contact is mounted on the inner or bottom side of each character and function key on keyboard  20 . The contact  134  of switch S 42  on the bottom of character key  130  (“g”) is representative. 
     Located on the top or upper surface  136  of PCB  22  is a complementary set  137  of contacts (or traces) (see FIGS. 9A and 9B) with those traces of switch S 42  identified by reference characters  138  and  140  in FIG. 9A being typical. When a key is depressed and its contact comes into engagement with the complementary traces on PCB  22 , a coded electrical signal is generated by the circuitry shown in FIGS. 10A and 10B. This signal is unique to the character or function which has been selected. The LED transmitters  142 ,  144 , and  146  (see FIG. 10A) operating in the infrared range transmit the coded signals to the appliance or apparatus identified by reference character  147  in FIG. 1 to which commands can be inputted by activating keys of keyboard  20 . These LED&#39;s are mounted in cavities  148 ,  150 , and  152  behind windows  154 ,  156 , and  158  in the base  28  of keyboard  20 . 
     One LED  142  propagates infrared energy through end wall  160  of keyboard base  28 , a second LED  144  propagates energy through the opposite end wall  162  of the base, and the third LED  146  is located midway between the two end walls of the base in back wall  164 . The same signal is propagated from each of the three LED&#39;s  142 ,  144 , and  146 . Consequently, and because the signals are in the IR range, a usable signal will be transmitted to the keyboard-controlled device essentially without regard to the angle through which the keyboard might be rotated relative to the infrared radiation receiver of the controlled device. 
     Pressing a character or function key of keypad  22  results in an increase of the air pressure in the space between that key and PCB  24  (a representative one of these spaces is identified in FIG. 5 by reference character  166 ). Experience has shown that this overpressure can be high enough to shift the keypad relative to the PCB  24 . This is detrimental and can result in erratic electrical contact and in degradation of the touch and feel characteristics of the keyboard. In keyboard  20 , this problem is simply and elegantly solved by forming a set of channels collectively identified by reference character  170  in FIG. 8 on the lower side  172  of keypad  22 . These channels provide fluid communication between the keypad—PCB interspaces (collectively identified by reference character  174 ). Consequently, when a character or function key is depressed, air in the space below that key can flow into the adjacent and succeeding interspaces, increasing the volume into which the air can flow and limiting any increase in pressure to a level well below that at which keypad  22  might shift. 
     Optionally, or in addition, the shifting of keypad  22  relative to PCB  24  can be prevented or enhanced by the retainer arrangement shown in FIG.  6 . Arrow headed, integral plugs  178  are formed on the lower sides of keypad segments which are located between the various keys of keyboard  20  and physically contact the PCB (see reference character  179  in FIG.  5 ). These plugs are designed to fit through complementary apertures  180  in PCB  24 . The head  182  of the plug then positively locks the PCB-contacting segment  179  from which it depends to the PCB. 
     Alternatively, the keypad can be adhesively attached to the PCB. This arrangement is shown in FIG. 7 in which the adhesive is identified by reference character  186 . 
     Referring now to FIGS. 3A,  4 , and  5 , the frame  26  keyboard of  20  includes a flat plate  188  of the same size and configuration as PCB  24 . Rigidity is added to plate  188  by integral stiffeners. In the herein disclosed embodiments of the invention, the stiffeners are an integral peripheral rim  190  and internal, also integral ribs collectively identified by reference character  192 . 
     Also incorporated in frame  26  are integral battery holders  194 ,  196 ,  198 , and  200  (see FIG.  4 ). The battery holders are aligned along the rear edge  204  of the frame and include the customary contacts engageable by the positive and negative terminals of the batteries loaded into these battery holders. One of these batteries is shown in phantom lines in FIG.  5  and identified by reference character  202 . 
     PCB  24  is fastened to frame  26  by screws which extend into internally threaded fittings. These fittings are located in apertures formed in the peripheral rim  190  and internal ribs  192  of frame  26 . The screws are collectively identified by reference character  209  in FIG.  3 A. Two of the inserts are identified by reference characters  210  and  212  in FIG. 5, and reference characters  214  and  216  identify the apertures in which the fittings are located. 
     As is best shown in FIG. 5, keypad  22  has an integral, depending flange  220  which extends completely around the keypad. This flange has an L-shaped configuration, which defines an inwardly directed recess  222 . After PCB  24  and frame  26  have been fastened together, the resulting PCB/frame assembly  224  is assembled to keypad  22  with the edges of assembly  224  in recess  22  and the segments  179  of the keypad in contact with the keypad, PCB/frame unit  201  (FIG.  3 A). Upon the subsequent assembly of keyboard base  28 , the PCB  24 , the components mounted on the PCB, the batteries such as battery  202 , battery holders  194  . . .  200 , and the contacts such as  134  on the bottoms of the: (a) character keys in QWERTY key set  48  and (b) function keys  49  . . .  74 , and  78  . . .  114 , and  118  . . .  126  are isolated from the ambient surroundings in a manner that keeps water and other foreign materials from reaching these electrical components. 
     Referring now to FIGS. 3B,  4 , and  5 , the base  28  of keyboard  22  includes a flat plate  226 , an integral outer wall,  228 , and an also integral inner wall  230 . Rigidity is provided by ribs which are integral with the plate  226  and two side walls  228  and  230  of base  28 . These ribs are collectively identified by reference character  232  in FIG.  3 B. 
     The outer and inner walls  228  and  230  of base  28  taper from the rear side  164  of keyboard  22  to the front side  236  of the keyboard. This results in keypad  22  being inclined for ease of use. 
     There is an elongated slot  238  in the plate  226  of base  28  near the rear side  164  of keyboard  20 . Battery holders  194  . . .  200  extend downwardly through slot  236 , furnishing the access to the battery holders needed for installation and replacement of the batteries. 
     Keyboard base  28  is fastened by screws  240  to the assembly  224  of PCB  24  and frame  26  after this assembly has been installed in the depending flange segment  220  of keypad  22 . These screws are threaded into complementary fittings  242  with the latter being located in the peripheral rim  190  and ribs  192  of keyboard frame  26 . 
     As screws  240  are tightened, the upper edge  246  of keyboard base outer wall  228  is pressed against the inwardly extending lip  248  at the bottom end of keypad flange  220 . This isolates the interior  250  of keyboard  20  and the components housed in that space from the ambient surroundings, preventing water and other foreign material from reaching the interior of keyboard  20  and perhaps damaging internal keyboard components. 
     Access to battery holders  194  . . .  200  is gained through an opening  252  (see FIG. 5) in the keyboard base  28 . Removable battery cover  44  spans this opening, isolating the interior  250  of the keyboard from the ambient surroundings. 
     As shown in FIGS. 3B and 5, battery cover  44  has a flat bottom  258  with an integral rib  260  extending around its periphery. This flange adds rigidity to the battery cover as do internal, also integral ribs collectively identified by reference characters  262 . 
     Battery cover  44  is secured in place toward the opposite ends of keyboard  20  by thumbscrews  264  and  266 . Only the structure associated with screw  264  will be described herein as the structure associated with both screws is the same. 
     Referring then particularly to FIG. 5, battery cover  44  has an integral boss  267  located at the intersection of intersecting internal ribs  262 . This boss has an internal recess  268  with a step  270 . Located directly above recess  268  in keyboard frame  26  as indicated by centerline arrow  272  is a stepped recess  274  formed in an integral boss  276  of keyboard frame  26 . In the stepped recess  274  is an internally threaded fitting  278 . 
     Also employed in the installation of battery cover  44  is an o-ring seal  280 . 
     To fasten battery cover  44  in place, o-ring seal  280  is installed in the aperture  268  through battery cover boss  267  and seated on the step  270  in that opening. Next, thumbscrew  264  is displaced through in seal  280  and through the passage  268  in boss  267  and then threaded into keyboard frame insert  278 , clamping battery cover  44 , seal  46 , and seal  280  in place. 
     Seal  280  keeps water and other foreign material from penetrating to the interior  250  of keyboard thumbscrew  264 . A second gap around the periphery  286  of the battery cover  44  is sealed by seal  46 . This seal is trapped between that cover and the edge  287  of the opening  252  in base  28 . 
     As is perhaps best shown in FIG. 4, integral, depending feet  288  are formed at the four comers of seal  46 . These feet support keyboard  20  from the surface  290  on which the keyboard is rested. These feet also keep keyboard  24  from sliding on that surface. 
     Referring next to FIGS. 9A and 9B, it was pointed out above that the activation of a user-activatable key of keyboard  20  (any one of the 87 keys  49  . . .  74 ,  78  . . .  114 , or  18  . . .  126 ) results in a switch associated only with that key being closed and that this switch closure results in a signal unique to the activated key being generated and transmitted to the device  147  served by keyboard  20  (see FIG.  1 ). These 87 switches are identified in FIG. 9A and 9B by reference characters S 1 -S 87 . 
     As was also briefly discussed above, each of these switches is made up of: (a) two traces on PCB  24  which are normally electrically isolated from each other but, when a key is activated, can be bridged by a third, key-mounted element (a contact) of the switch. The contact is located on the bottom of the key with which the switch is associated. 
     Switch S 42  with its contact  134  and traces  138  and  140  was described above. The other 86 switches have the same elements (two traces and a contact) as switch S 42 . 
     The trace  138  of switch S 42  and the equivalent traces of the other 86 switches are connected to leads identified in FIGS. 9A and 9B by reference characters: A 1 -A 3 , B 1 -B 3 , C 1 -C 3 , D 1 -D 3 , E 1 -E 3 , F 1 -F 3 , G 1 -G 3 , and H 1 -H 3 . Trace  140  and its cconnected to leads KB 0 , KB 1 , and KB 2 . 
     Leads A 1 -H 3  are generally referred to hereinafter as “column” leads and leads KB 0 , KB 1 , and KB 2  are referred to as “row leads.” The net result of these connections is to arrange switches S 01  through S 87  in a column and row matrix  300  in which the columns are represented by leads A 1  . . . H 3  and the rows by leads KB 01 , KB 02 , and KB 03 . 
     Column leads A 1  . . . H 4  are connected in groups of eight to shift registers  302 ,  304 ,  306 , and  308 ; and leads KB 0 , KB 1 , and KB 2  are connected to the input side of a programmable microcontroller  310 . 
     The closing of representative switch S 42  by pressing character key  130  causes lead KB 2  to go to ground. This “wakes up” microcontroller  310  which scans the shift registers  302  . . .  308 , looking for the column lead (F 2 ) also grounded by the closing of switch S 42 . Once the grounded lead F 2  is found, the microcontroller  310  has specifically identified the switch S 42  that was closed by column (F 2 ) and line (KB 2 ). This results in the microcontroller so turning transistor  312  (FIG. 10A) on and off as to cause IR emitters  142 ,  144 , and  146  to transmit the same coded, digital signal uniquely identifying the switch S 42  and, consequently, the key  130  that was closed by the keyboard user. The device  147  served by keyboard  20  consequently receives and acts upon the command selected by activating key  130 . 
     The steps involved in generating and outputting a coded digital signal from keyboard  20  when a key is pressed are shown in the logic diagram of FIG.  11 . 
     Turning then to that figure, controller  310  sends a signal to line A 1  to set shift registers  302  . . .  308 . The controller next checks the state of matrix row lines KB 0 , KB 1 , and KB 2 . 
     If none of these three lines is low, controller  310  sends a clock pulse to shift registers  302  . . .  308 , which respond by shifting one bit to the right. As shown by line  312  in FIG. 11, the cycle is then repeated. 
     If controller  310  finds that one of the row lines KB 0 , KB 1 , and KB 2  has gone low, it compares the KB lines with the eight bit locations in each of the shift registers  302 ,  304 ,  306 , and  308 , thereby determining which key of keyboard  20  was pressed or otherwise activated. Next, the digital code for the activated key is determined by controller  310  from a look-up table (not shown) and loaded into FIFO memory. From memory, this code is sent to IR transmitters  142 ,  144 , and  146  by turning transistor  314  on and off as discussed above. 
     The FIG. 10 operating system functions in much the manner as just described when a combination of keys—for example, a letter key and a shift-key are pressed. In this case, the digital IR signal transmitted to module  147  is that for the upper case version of the user-selected letter. 
     The invention may be embodied in many forms without departing from the spirit or essential characteristics of the invention. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather then by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.