PATENT ABSTRACT
A force sensor particularly suited for use in an electronic stylus that senses the contact force on its nib for recording pen strokes and handwriting recognition. The sensor has a housing for a load bearing member for receiving an input force to be sensed and associated circuitry for converting the input force into an output signal indicative of the input force. The bearing member is movably mounted within the elongate body (up to 100 microns). The input force acting on the load bearing member is caused by contact on the nib. The load bearing member is biased against the direction of the input force. 
     The force sensor also has a light source and photo-detector for sensing levels of illumination from the light source. Associated circuitry converts a range of illumination levels sensed by the photo-detector into a range of output signals, so that the illumination level sensed by the photo-detector varies with movement of the load bearing member within the elongate body such that the output signal from the circuitry is indicative of the input force. Using an optical sensor avoids the need to use a delicate piezo-resistive sensor that requires careful tolerancing during production.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a Continuation-In-Part of U.S. application Ser. No. 11/155,556 filed Jun. 20, 2005, now issued as U.S. Pat. No. 7,590,311, which is a continuation of U.S. application Ser. No. 10/291,823 filed Nov. 12, 2002, now issued as U.S. Pat. No. 6,980,704, which is a continuation of U.S. application Ser. No. 09/575,174, filed on May 23, 2000, now issued as U.S. Pat. No. 6,870,966. 

   FIELD OF THE INVENTION 
   The present invention relates to the fields of interactive paper, printing systems, computer publishing, computer applications, human-computer interfaces using styli with force sensors and information appliances. 
   CO-PENDING APPLICATIONS 
   The following applications have been filed by the Applicant simultaneously with the present application: 
   
     
       
             
             
             
             
             
             
           
         
             
                 
             
           
           
             
               11/495,814 
               11/495,823 
               7,523,672 
               11/495,820 
               11/495,818 
               11/495,819 
             
             
               7,581,812 
               11/495,816 
               11/495,817 
             
             
                 
             
           
        
       
     
   
   The disclosures of these co-pending applications are incorporated herein by reference. The above applications have been identified by their filing docket number, which will be substituted with the corresponding application number, once assigned. 
   CROSS REFERENCES 
   Various methods, systems and apparatus relating to the present invention are disclosed in the following U.S. patents/patent applications filed by the applicant or assignee of the present invention: 
   
     
       
             
             
             
             
             
             
             
           
         
             
                 
             
           
           
             
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               10/943,878 
               10/943,849 
               7,043,096 
               11/071,267 
               11/144,840 
               11/155,556 
               11/155,557 
             
             
               11/193,481 
               11/193,435 
               11/193,482 
               11/193,479 
               11/255,941 
               11/281,671 
               11/298,474 
             
             
               7,055,739 
               09/575,129 
               6,830,196 
               6,832,717 
               09/721,862 
               10/473,747 
               10/120,441 
             
             
               6,843,420 
               10/291,718 
               6,789,731 
               7,057,608 
               6,766,944 
               6,766,945 
               10/291,715 
             
             
               10/291,559 
               10/291,660 
               10/531,734 
               10/409,864 
               10/309,358 
               10/537,159 
               10/410,484 
             
             
               7,077,333 
               6,983,878 
               10/786,631 
               10/853,782 
               10/893,372 
               6,929,186 
               6,994,264 
             
             
               7,017,826 
               7,014,123 
               10/971,051 
               10/971,145 
               10/971,146 
               7,017,823 
               7,025,276 
             
             
               10/990,459 
               11/059,684 
               11/074,802 
               7,334,739 
               10/492,169 
               10/492,152 
               10/492,168 
             
             
               10/492,161 
               10/492,154 
               10/502,575 
               10/531,229 
               10/683,151 
               10/531,733 
               10/683,040 
             
             
               10/510,391 
               10/510,392 
               10/919,261 
               10/778,090 
               6,957,768 
               09/575,162 
               09/575,172 
             
             
               09/575,170 
               09/575,171 
               09/575,161 
               6,982,701 
               6,982,703 
               10/291,538 
               6,786,397 
             
             
               6,947,027 
               6,975,299 
               10/291,714 
               7,048,178 
               10/291,541 
               6,839,053 
               7,015,900 
             
             
               7,010,147 
               10/291,713 
               6,914,593 
               10/291,546 
               6,938,826 
               10/913,340 
               10/940,668 
             
             
               6,992,662 
               11/039,897 
               11/074,800 
               11/074,782 
               11/074,777 
               11/075,917 
               11/102,698 
             
             
               11/102,843 
               11/202,112 
               11/442,114 
               6,454,482 
               6,808,330 
               6,527,365 
               6,474,773 
             
             
               6,550,997 
               10/181,496 
               6,957,923 
               10/309,185 
               10/949,288 
               10/962,400 
               10/969,121 
             
             
               11/185,722 
               11/181,754 
               11/203,180 
               11/124,158 
               11/124,196 
               11/124,199 
               11/124,162 
             
             
               11/124,202 
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               11/124,154 
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               11/124,192 
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               11/124,155 
             
             
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               11/124,201 
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               11/124,161 
               11/124,156 
               11/124,191 
               11/124,159 
             
             
               11/124,175 
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               11/124,170 
               11/124,187 
               11/124,189 
               11/124,190 
               11/124,180 
             
             
               11/124,193 
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               11/124,178 
               11/124,177 
               11/124,148 
               11/124,168 
               11/124,167 
             
             
               11/124,179 
               11/124,169 
               11/187,976 
               11/188,011 
               11/188,014 
               7,530,446 
               11/228,540 
             
             
               11/228,500 
               11/228,501 
               11/228,530 
               11/228,490 
               11/228,531 
               11/228,504 
               11/228,533 
             
             
               11/228,502 
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               11/228,482 
               11/228,505 
               11/228,497 
               11/228,487 
               11/228,529 
             
             
               11/228,484 
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               11/228,518 
               11/228,536 
               11/228,496 
               11/228,488 
               11/228,506 
             
             
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               11/228,528 
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               11/228,498 
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               11/228,493 
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               11/228,486 
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               11/228,477 
               11/228,485 
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               11/228,521 
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               11/228,532 
               11/228,513 
               11/228,503 
               11/228,480 
               11/228,535 
               11/228,478 
               11/228,479 
             
             
               6,238,115 
               6,386,535 
               6,398,344 
               6,612,240 
               6,752,549 
               6,805,049 
               6,971,313 
             
             
               6,899,480 
               6,860,664 
               6,925,935 
               6,966,636 
               7,024,995 
               10/636,245 
               6,926,455 
             
             
               7,056,038 
               6,869,172 
               7,021,843 
               6,988,845 
               6,964,533 
               6,981,809 
               11/060,804 
             
             
               11/065,146 
               11/155,544 
               11/203,241 
               11/206,805 
               11/281,421 
               11/281,422 
               11/482,981 
             
             
               7,571,906 
               11/482,982 
               11/482,983 
               11/482,984 
             
             
                 
             
           
        
       
     
   
   The disclosures of these applications and patents are incorporated herein by reference. Some of the above applications have been identified by their filing docket number, which will be substituted with the corresponding application number, once assigned. 
   BACKGROUND OF THE INVENTION 
   The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
   The Netpage pen is an electronic stylus with force sensing, optical sensing and Bluetooth communication assemblies. A significant number of electronic components need to be housed within the pen casing together with a battery large enough to provide a useful battery life. Despite this, the overall dimensions of the pen need to be small enough for a user to manipulate it as they would a normal pen. 
   The force sensor circuitry typically utilizes a piezo-electric element which demands precision assembly and careful tolerancing so that full deflection of the nib does not break the delicate crystal element. The considerations complicate the production of the pen particularly given the constricted space available within the stylus body. 
   SUMMARY OF THE INVENTION 
   Accordingly, this aspect provides a pen comprising: 
   an elongate chassis moulding; and, 
   a cartridge with a nib and an elongate body; wherein, 
   the cartridge is configured for insertion and removal from the elongate chassis moulding from a direction transverse to the longitudinal axis of the chassis moulding. 
   According to a closely related aspect, the present invention provides an ink cartridge for a pen, the ink cartridge comprising: 
   an elongate ink reservoir; and, 
   a writing nib in fluid communication with the ink reservoir; wherein, 
   the elongate ink reservoir has an enlarged transverse cross section along a portion of its length intermediate its ends. 
   By configuring the pen chassis and cartridge so that it can be inserted and removed from the side rather than through the ends, the capacity of the cartridge can be significantly increased. An enlarged section between the ends of the ink cartridge increases the capacity while allowing the relatively thin ends to be supported at the nib moulding and opposing end of the pen chassis. In a Netpage pen, inserting the cartridge from the side avoids the need to remove the force sensor when replacing the cartridge. Again, the thinner sections at each end of the cartridge allow it to engage a ball point nib supported in the nib moulding and directly engage the force sensor at the other end, while the enlarged middle portion increases the ink capacity. 
   Optionally, the cartridge is an ink cartridge and the elongate body houses an ink reservoir. Preferably, the pen is an electronic stylus with a force sensor assembly, and the cartridge is held in the stylus such that the nib is at one end of the elongate body and the other end of the elongate body engages the force sensor assembly. In some embodiments, the force sensor assembly has a load bearing member to receive an input force to be sensed and circuitry for converting the input force into an output signal indicative of the input force, the load bearing member abutting the opposite end of the elongate cartridge such that the input force comprises the axial component of the contact force on the nib transferred by the cartridge. 
   Preferably the elongate cartridge is biased against the load bearing member. In a further preferred form, the elongate cartridge has a flange surface proximate the nib end, and a biasing element between the flange surface and the chassis moulding biases the elongate cartridge against the load bearing member. Typically, this bias is between 0.1 Newtons and 0.2 Newtons (approx. 10 g-20 g). 
   Preferably, the circuitry is a piezoresistive bridge circuit. However, the circuitry could also be a capacitative or inductive force sensing circuit. In another option, the circuitry may be an optical force sensor. In a further preferred form, the load bearing member has a protrusion with round end for engagement with the cartridge. In another preferred form, the cartridge has a similar protrusion extending centrally from its end such that the distal end of the protrusion engages the rounded end of the protrusion from the load bearing member. In a particularly preferred form, the housing defines a recess for the circuitry, the rounded end of the protrusion from the load bearing member extends proud of the recess for engaging the cartridge. Preferably, a stop surface positioned around the opening to the recess engages the cartridge to limit elastic deformation of the force sensor assembly. 
   Typically, the force sensor assembly is configured to sense a maximum force of 5 Newtons (approx. 500 g). Preferaby, the load bearing member can move up to 100 microns relative to the chassis. 
   In a particularly preferred embodiment, the output signal from the circuitry support a hand writing recognition facility. 
   In a second aspect the present invention provides a force, sensor assembly comprising: 
   a load bearing member for receiving an input force; 
   a sensor circuit for converting the input force into a signal indicative of the input force; and, 
   a force transfer coupling for receiving an applied force and at least partially transferring it to the load bearing member as the input force; wherein, 
   the applied force and the input force are not co-linear. 
   According to a closely related aspect, the present invention provides an electronic stylus comprising: 
   an elongate body; 
   a nib extending from one end of the elongate body; 
   a load bearing member for receiving an input force; 
   a sensor circuit for converting the input force into a signal indicative of the input force; and, 
   a force transfer coupling for receiving an applied force caused by contact on the nib, and at least partially transferring it to the load bearing member as the input force; wherein during use, 
   the applied force and the input force are not co-linear. 
   With the use of a force transfer coupling, the sensor circuitry and load bearing member can remain fixed in the pen body while the ink cartridge is removed and replace. The force transfer coupling may need to be removed or shifted when the ink cartridge for the nib is being changed (assuming the stylus has a ball point nib) but there is less potential for damage to the force sensor. The deceleration shock from being bumped or dropped in its nib can break the sensor circuitry, which necessitates the replacement of the entire PCB. 
   Preferably, the force transfer coupling is an element configured for elastic deformation in a direction skew to the applied force. In a further preferred form, the element is a double bowed section that bows outwardly when axially compressed by the applied force. In a particularly preferred form, the load bearing member engages one of the bowed sections at its mid point such that the input force is perpendicular to the applied force. Preferably, the bowed section that does not contact the load bearing member is constrained against lateral displacement in order to stiffen the other bowed section. In some embodiments, the bowed sections have an arcuate lateral cross section to reduce contact friction with the load bearing member and the lateral constraint. Optionally, the load bearing member a rounded protrusion for contacting the bowed section of the force transfer coupling. 
   In some embodiments, the force transfer coupling is a hydraulic element that uses the applied force to create hydraulic pressure acting on the load bearing member. In a particularly preferred form, the hydraulic pressure acts such that the input force is perpendicular to the applied force. Preferably, the hydraulic fluid has low viscosity and low shear forces. In some embodiments, the hydraulic fluid is a silicon gel. Preferably the hydraulic fluid is contained in a reservoir at least partially defined by a flexible membrane such that the applied force acts on the hydraulic fluid via the flexible membrane. Optionally, the hydraulic fluid acts directly on the load bearing member. 
   In some preferred embodiments, the circuitry is a piezoresistive bridge circuit. Optionally, the nib of the electronic stylus is a ball point writing nib with a tubular ink cartridge extending from the nib toward the load bearing member such that the end of the cartridge opposite the nib transmits the applied force to the hydraulic coupling. Preferably, the output signal from the circuitry support a hand writing recognition facility. Preferably the circuitry is an integrated circuit (IC) mounted on a PCB (printed circuit board), the plane of the PCB being parallel to the longitudinal axis of the elongate body. 
   In some embodiments, the load bearing member can move up to 100 microns relative to the elongate body. Optionally, the input force is limited to a maximum of 5 Newtons. In a particularly preferred embodiment, the output signal from the circuitry support a hand writing recognition facility. 
   In a third aspect the present invention provides a force sensor assembly comprising: 
   a housing; 
   a load bearing member movably mounted in the housing for receiving an input force to be sensed, the load bearing member being biased against the direction of the input force; 
   a light source; 
   a photo-detector for sensing levels of illumination from the light source; and, 
   circuitry for converting a range of illumination levels sensed by the photo-detector into a range of output signals; wherein, 
   the illumination level sensed by the photo-detector varies with movement of the load bearing member within the housing such that the output signal from the circuitry is indicative of the input force. 
   According to a closely related aspect, the present invention provides an electronic stylus comprising: 
   an elongate body; 
   a nib extending from one end of the elongate body; 
   a load bearing member movably mounted to the elongate body for receiving an input force caused by contact on the nib, the load bearing member being biased against the direction of the input force; 
   a light source; 
   a photo-detector for sensing levels of illumination from the light source; and, 
   circuitry for converting a range of illumination levels sensed by the photo-detector into a range of output signals; wherein, 
   the illumination level sensed by the photo-detector varies with movement of the load bearing member within the elongate such that the output signal from the circuitry is indicative of the input force. 
   Using an optical force sensor is more robust than a piezo-resistive sensor. Installing an LED and photo-detector is less complex than the delicate requirements of a piezo-electric crystal. The full force deflection on the nib is relatively small, so the tolerancing in a piezo-resistive component needs to be high enough to prevent breakage. 
   Preferably, the light source is fixed to the housing for illuminating at least part of the load bearing member. Preferably, the photo-detector is mounted to the housing such that the load bearing member moves between the light source and the photo-detector. In a further preferred form, the load bearing member has an aperture through which light from the light source can illuminate the photo-detector, the aperture being positioned between the light source and the photo-detector at part of the load bearing member&#39;s travel within the housing. In a particularly preferred form, the load bearing member is biased with a spring, the spring having a spring constant equal to the maximum force the sensor is to sense, divided by the length in the direction of travel within the housing of the aperture. Optionally, the aperture is aligned with the light source and the photo-detector when the input force is the maximum force, and the load bearing member fully obscures the light source from the photo-detector when the input force is zero. 
   Conveniently, the light source is a LED. In some embodiments, the load bearing member has a maximum travel of 100 microns within the housing. In some embodiments, the nib of the electronic stylus is a ball point writing nib with a tubular ink cartridge extending from the nib toward the load bearing member such that the coupling is a detachable boot that fits over the end of the cartridge opposite the nib. 
   Typically, the force sensor is configured to sense a maximum force of 5 Newtons (approx. 500 g). In a particularly preferred embodiment, the output signal from the circuitry support a hand writing recognition facility. 
   In a fourth aspect the present invention provides a force sensor assembly comprising: 
   a load bearing member for receiving an input force to be sensed; 
   circuitry for converting the input force into an output signal indicative of the input force; 
   a coupling having an inner section for transmitting the input force to the load bearing member, an outer section for receiving an applied contact force and a collapsible section for allowing the outer section to move relative to the inner section when the contact force exceeds a threshold. 
   According to a closely related aspect, the present invention provides an electronic stylus comprising: 
   an elongate body; 
   a nib extending from one end of the elongate body; and, 
   a load bearing member mounted to the elongate body for receiving an input force caused by contact on the nib; 
   circuitry for converting the input force into an output signal indicative of the input force; 
   a coupling having an inner section for transmitting the input force to the load bearing member, an outer section for receiving the contact force on the nib and a collapsible section for allowing the outer section to move relative to the inner section when the contact force exceeds a threshold. 
   Inserting a collapsible section between the nib and the force sensor will allows static and dynamic contacts loads up to a predetermined threshold to be transmitted to the sensor. However, any loads that exceed the threshold, regardless of whether they are static or shock loads, will simply force the outer section of the coupling to collapse toward the inner section. The input force at the sensor remains at or below the threshold. 
   Preferably, the collapsible section has a deformable structure. In some embodiments, the deformable structure deforms plastically when the contact force exceeds a threshold. In one preferred embodiment, the deformable structure is a series of struts extending between the inner section and the outer section such that the struts buckle when the contact force exceeds their combined buckling loads. Optionally, the struts are inclined to the direction of the contact force in order to promote buckling at a lower threshold. In other embodiments, the deformable structure deforms elastically when the contact force exceeds a threshold. Preferably, the deformable structure has a pair of abutting slip surfaces biased against each other by a resilient member, such that the slip surfaces slide relative to each other if the input force exceeds the threshold created by friction between the slip surfaces. In a particularly preferred form, the resilient member is an elastic sleeve tightly fitted around the two components that respectively define the slip surfaces, the slip surfaces being inclined relative to the direction of the input force. 
   In a particularly preferred form, the coupling is biased against the load bearing member. Typically, this bias is between 0.1 Newtons and 0.2 Newtons (approx. 10 g-20 g). In some embodiments, the nib of the electronic stylus is a ball point writing nib with a tubular ink cartridge extending from the nib toward the load bearing member such that the coupling is a detachable boot that fits over the end of the cartridge opposite the nib. 
   Typically, the force sensor is configured to sense a maximum force of 5 Newtons (approx. 500 g). Preferaby, the load bearing member can move up to 100 microns relative to the housing. 
   In a particularly preferred embodiment, the output signal from the circuitry support a hand writing recognition facility. 
   In a fifth aspect the present invention provides a force sensor assembly comprising: 
   a housing; 
   a load bearing member for receiving an input force to be sensed; 
   circuitry for converting the input force into an output signal indicative of the input force; 
   a coupling for transmitting the input force to the load bearing member; and, 
   a compressible reservoir containing dilatant fluid mounted between the housing and the coupling to restrict the input force to the load bearing member caused by shock loading to the coupling. 
   According to a closely related aspect, the present invention provides an electronic stylus comprising: 
   an elongate body; 
   a nib extending from one end of the elongate body; and, 
   a load bearing member mounted to the elongate body for receiving an input force caused by contact on the nib; 
   circuitry for converting the input force into an output signal indicative of the input force; 
   a coupling for transmitting the input force to the load bearing member; and, 
   a compressible reservoir containing dilatant fluid mounted between the housing and the coupling to restrict the input force to the load bearing member caused by shock loading to the coupling. 
   A dilatant (or “shear thickening”) fluid is a non-Newtonian fluid whose viscosity increases with rate of shear. At a low shear rate the particles are able to slide past each other and the fluid behaves as a liquid. Above a critical shear rate friction between the particles predominates and the fluid behaves as a solid. 
   To prevent force sensor damage from an impulse (shock loading), an additional stop containing a dilatant fluid can be inserted between the element that couples the nib to the force sensor. The dilatant fluid can be contained in a sack formed from a flexible membrane. During normal operation of the pen the dilatant fluid sack acts as a liquid and deforms in response to movement of the cartridge, allowing normal forces to be transmitted from the cartridge to the force sensor. When a damaging impulse occurs, the dilatant fluid effectively hardens in response to the high shear rate, preventing movement of the cartridge and thereby protecting the force sensor. 
   Preferably, the compressible reservoir of dilatant fluid maintains a gap between the load bearing member and the coupling when the input force is not applied, and the compressible reservoir compresses to allow the coupling to directly engage the load bearing member with a steady application of the input force. In a further preferred form, the compressible reservoir is secured to the housing and the coupling, and the coupling is biased away from the housing to maintain the gap between the coupling and load bearing member when the input force is not applied. Preferably, the circuitry is a piezoresistive bridge circuit. However, the circuitry could also be a capacitative or inductive force sensing circuit. In another option, the circuitry may be an optical force sensor. In a further preferred form, the load bearing member has a protrusion with round end for engagement with the coupling. In another preferred form, the coupling has a similar protrusion extending centrally from a flange such that the distal end of the protrusion engages the rounded end of the protrusion from the load bearing member, and the compressible reservoir of dilatant fluid is positioned between the housing and the flange. In a particularly preferred form, the housing defines a recess for the circuitry, the rounded end of the protrusion from the load bearing member extends proud of the recess for engaging the coupling. Preferably, the compressible reservoir has an annular shape and is positioned around the opening to the recess and around the central protrusion from the flange of the coupling. 
   In a particularly preferred form, the coupling is biased against the load bearing member. Typically, this bias is between 0.1 Newtons and 0.2 Newtons (approx. 10 g-20 g). In some embodiments, the nib of the electronic stylus is a ball point writing nib with a tubular ink cartridge extending from the nib toward the load bearing member such that the coupling is a detachable boot that fits over the end of the cartridge opposite the nib. 
   Typically, the force sensor is configured to sense a maximum force of 5 Newtons (approx. 500 g). Preferaby, the load bearing member can move up to 100 microns relative to the housing. 
   In a particularly preferred embodiment, the output signal from the circuitry support a hand writing recognition facility. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  shows the structure of a complete tag; 
       FIG. 2  shows a symbol unit cell; 
       FIG. 3  shows nine symbol unit cells; 
       FIG. 4  shows the bit ordering in a symbol; 
       FIG. 5  shows a tag with all bits set; 
       FIG. 6  shows a tag group made up of four tag types; 
       FIG. 7  shows the continuous tiling of tag groups; 
       FIG. 8  shows the interleaving of codewords A, B, C &amp; D within a tag; 
       FIG. 9  shows a codeword layout; 
       FIG. 10  shows a tag and its eight immediate neighbours labelled with its corresponding bit index; 
       FIG. 11  shows a nib and elevation of the pen held by a user; 
       FIG. 12  shows the pen held by a user at a typical incline to a writing surface; 
       FIG. 13  is a lateral cross section through the pen; 
       FIG. 14A  is a bottom and nib end partial perspective of the pen; 
       FIG. 14B  is a bottom and nib end partial perspective with the fields of illumination and field of view of the sensor window shown in dotted outline; 
       FIG. 15  is a partial perspective of the USB cable and USB socket in the top end of the pen; 
       FIG. 16  is an exploded perspective of the pen components; 
       FIG. 17  is a longitudinal cross section of the pen; 
       FIG. 18  is a partial longitudinal cross section of the cap placed over the nib end of the pen; 
       FIG. 19  is an exploded perspective of the optics assembly; 
       FIG. 20  is an exploded perspective of the force sensor assembly; 
       FIG. 21  is an exploded perspective of the ink cartridge tube and nib engaging removal tool; 
       FIG. 22  is a partially sectioned perspective of a new ink cartridge engaging the nib end of the currently installed ink cartridge; 
       FIG. 23  is a partial perspective of the packaged force sensor on the main PCB; 
       FIG. 24  is a longitudinal cross section of the force sensor and main PCB shown in  FIG. 15 ; 
       FIG. 25  is an exploded perspective of the cap assembly; 
       FIG. 26  is a circuit diagram of the pen USB and power CCT&#39;s; 
       FIG. 27A  is a partial longitudinal cross section of the nib and barrel molding; 
       FIG. 27B  is a partial longitudinal cross section of the IR LED&#39;s and the barrel molding; 
       FIG. 28  is a ray trace of the pen optics adjacent a sketch of the ink cartridge; 
       FIG. 29  is a side elevation of the lens; 
       FIG. 30  is a side elevation of the nib and the field of view of the optical sensor; 
       FIG. 31  is an exploded perspective of the pad; 
       FIG. 32  is a longitudinal cross section of the pad with the pen inserted; 
       FIG. 33  is a schematic representation of the force sensor assembly; 
       FIG. 34  is a schematic representation of a top-loading ink cartridge and force sensor; 
       FIG. 35  is a schematic representation of a top loading ink cartridge into a pen with a retaining cavity for the pre-load spring; 
       FIG. 36  is a schematic representation of a double-bow right-angle force sensor coupling; 
       FIG. 37A  is a schematic representation of a hydraulic force sensor coupling; 
       FIG. 37  B is a longitudinal section of the hydraulic force sensor coupling shown in  FIG. 37A ; 
       FIG. 38  is schematic representation of an alternative configuration of the hydraulic force sensor coupling; 
       FIG. 39A  is a more detailed sketch of the hydraulic coupling between the cartridge and the force sensor; 
       FIG. 39B  is a section view taken along line  39 - 39  of  FIG. 39A ; 
       FIG. 40  is a schematic section view of the input force acting on the plunger and the detail of the force sensor mounting; 
       FIG. 41  is a schematic section view of an alternative force senor mounting without the input ball bearing; 
       FIG. 42  is a schematic section view of the force sensor chip deflection profile; 
       FIG. 43  is a schematic section view of the pressure sensor chip deflection profile; 
       FIG. 44  is a schematic section view of the force sensor using pressure sensor chip and hydraulic coupling; 
       FIG. 45  is a plot of sensed force versus time for an input impulse (tap) to the cartridge; 
       FIGS. 46A to 46C  are schematic section views of input mechanisms for the hydraulic coupling; 
       FIGS. 47A to 47C  are schematic section views of input mechanisms using a welded membrane; 
       FIG. 48  is schematic section view of the force sensor with a stop surface directly referenced to the back surface of the sensor chip; 
       FIG. 49  is a more detailed section view of the force sensor with its stop surface directly referenced to the back surface of the sensor chip; 
       FIG. 50  is schematic section view of a stop surface arrangement for the force input mechanism of the hydraulic coupling; 
       FIG. 51  is pen cartridge with collapsible element in an un-collapsed state; 
       FIG. 52  is pen cartridge with collapsible element in a collapsed state; 
       FIG. 53  is a stick friction collapsible element in un-collapsed state; 
       FIG. 54  is a stick friction collapsible element in collapsed state; 
       FIG. 55  is a sectioned perspective view of a stick friction collapsible element in an un-collapsed state; 
       FIG. 56A  is a plan view of an optical force sensor; 
       FIG. 56B  is an elevation of an optical force sensor; 
       FIG. 57  is a high-level block diagram of the operation of the optical force sensor; 
       FIG. 58  is a schematic section of a dilatant fluid o-ring to prevent impulse damage to force sensor; 
       FIG. 59  is a schematic section showing boot or cartridge with protrusion to accommodate thicker O-ring; 
       FIG. 60  is a block diagram of the pen electronics; 
       FIG. 61  show the charging and connection options for the pen and the pod; 
       FIGS. 62A to 62E  show the various components of the packaged force sensor; 
       FIG. 63  is a bottom perspective of the main PCB with the Bluetooth antenna shield removed; 
       FIG. 64  is a top perspective of the main PCB; 
       FIG. 65  is a bottom perspective of the chassis molding and elastomeric and cap; 
       FIG. 66A  is a perspective of the optics assembly lifted from the chassis molding; 
       FIG. 66B  is an enlarged partial perspective of the optics assembly seated in the chassis molding; 
       FIG. 67A  is a bottom perspective of the force sensor assembly partially installed in the chassis molding; 
       FIG. 67B  is a bottom perspective of the force sensing assembly installed in the chassis molding; 
       FIG. 68  is a bottom perspective of the battery and main PCB partially installed in the chassis molding; 
       FIG. 69  is a bottom perspective of the chassis molding with the base molding lifted clear; 
       FIGS. 70A and 70B  are enlarged partial perspectives showing the cold stake on the chassis molding being swaged and sealed to the base molding; 
       FIG. 71  is a bottom perspective of the product label being fixed to the base molding; 
       FIG. 72  is an enlarged partial perspective of the nib molding being inserted on the chassis molding; 
       FIG. 73  is a perspective of the tube molding being inserted over the chassis molding; 
       FIG. 74  is a perspective of the cap assembly being placed on the nib molding; 
       FIG. 75  is a diagram of the major power states of the pen; and, 
       FIG. 76  is a diagram of the operational states of the Bluetooth module. 
   

   DETAILED DESCRIPTION 
   As discussed above, the invention is well suited for incorporation in the Assignee&#39;s Netpage system. In light of this, the invention has been described as a component of a broader Netpage architecture. However, it will be readily appreciated that electronic styli have much broader application in many different fields. Accordingly, the present invention is not restricted to a Netpage context. 
   Netpage Surfaces Coding 
   Introduction 
   This section defines a surface coding used by the Netpage system (described in co-pending application as well as many of the other cross referenced documents listed above) to imbue otherwise passive surfaces with interactivity in conjunction with Netpage sensing devices (described below). 
   When interacting with a Netpage coded surface, a Netpage sensing device generates a digital ink stream which indicates both the identity of the surface region relative to which the sensing device is moving, and the absolute path of the sensing device within the region. 
   Surface Coding 
   The Netpage surface coding consists of a dense planar tiling of tags. Each tag encodes its own location in the plane. 
   Each tag also encodes, in conjunction with adjacent tags, an identifier of the region containing the tag. In the Netpage system, the region typically corresponds to the entire extent of the tagged surface, such as one side of a sheet of paper. 
   Each tag is represented by a pattern which contains two kinds of elements. The first kind of element is a target. 
   Targets allow a tag to be located in an image of a coded surface, and allow the perspective distortion of the tag to be inferred. The second kind of element is a macrodot. Each macrodot encodes the value of a bit by its presence or absence. 
   The pattern is represented on the coded surface in such a way as to allow it to be acquired by an optical imaging system, and in particular by an optical system with a narrowband response in the near-infrared. The pattern is typically printed onto the surface using a narrowband near-infrared ink. 
   Tag Structure 
     FIG. 1  shows the structure of a complete tag  200 . Each of the four black circles  202  is a target. The tag  200 , and the overall pattern, has four-fold rotational symmetry at the physical level. 
   Each square region represents a symbol  204 , and each symbol represents four bits of information. Each symbol  204  shown in the tag structure has a unique label  216 . Each label  216  has an alphabetic prefix and a numeric suffix. 
     FIG. 2  shows the structure of a symbol  204 . It contains four macrodots  206 , each of which represents the value of one bit by its presence (one) or absence (zero). 
   The macrodot  206  spacing is specified by the parameter S throughout this specification. It has a nominal value of 143 μm, based on 9 dots printed at a pitch of 1600 dots per inch. However, it is allowed to vary within defined bounds according to the capabilities of the device used to produce the pattern. 
     FIG. 3  shows an array  208  of nine adjacent symbols  204 . The macrodot  206  spacing is uniform both within and between symbols  208 . 
     FIG. 4  shows the ordering of the bits within a symbol  204 . 
   Bit zero  210  is the least significant within a symbol  204 ; bit three  212  is the most significant. Note that this ordering is relative to the orientation of the symbol  204 . The orientation of a particular symbol  204  within the tag  200  is indicated by the orientation of the label  216  of the symbol in the tag diagrams (see for example  FIG. 1 ). In general, the orientation of all symbols  204  within a particular segment of the tag  200  is the same, consistent with the bottom of the symbol being closest to the centre of the tag. 
   Only the macrodots  206  are part of the representation of a symbol  204  in the pattern. The square outline  214  of a symbol  204  is used in this specification to more clearly elucidate the structure of a tag  204 .  FIG. 5 , by way of illustration, shows the actual pattern of a tag  200  with every bit  206  set. Note that, in practice, every bit  206  of a tag  200  can never be set. 
   A macrodot  206  is nominally circular with a nominal diameter of (5/9)s. However, it is allowed to vary in size by ±10% according to the capabilities of the device used to produce the pattern. 
   A target  202  is nominally circular with a nominal diameter of (17/9)s. However, it is allowed to vary in size by ±10% according to the capabilities of the device used to produce the pattern. 
   The tag pattern is allowed to vary in scale by up to ±10% according to the capabilities of the device used to produce the pattern. Any deviation from the nominal scale is recorded in the tag data to allow accurate generation of position samples. 
   Tag Groups 
   Tags  200  are arranged into tag groups  218 . Each tag group contains four tags arranged in a square. Each tag  200  has one of four possible tag types, each of which is labelled according to its location within the tag group  218 . The tag type labels  220  are 00, 10, 01 and 11, as shown in  FIG. 6 . 
     FIG. 7  shows how tag groups are repeated in a continuous tiling of tags, or tag pattern  222 . The tiling guarantees the any set of four adjacent tags  200  contains one tag of each type  220 . 
   Codewords 
   The tag contains four complete codewords. The layout of the four codewords is shown in  FIG. 8 . Each codeword is of a punctured 2 4 -ary (8, 5) Reed-Solomon code. The codewords are labelled A, B, C and D. Fragments of each codeword are distributed throughout the tag  200 . 
   Two of the codewords are unique to the tag  200 . These are referred to as local codewords  224  and are labelled A and B. The tag  200  therefore encodes up to 40 bits of information unique to the tag. 
   The remaining two codewords are unique to a tag type, but common to all tags of the same type within a contiguous tiling of tags  222 . These are referred to as global codewords  226  and are labelled C and D, subscripted by tag type. A tag group  218  therefore encodes up to 160 bits of information common to all tag groups within a contiguous tiling of tags. 
   Reed-Solomon Encoding 
   Codewords are encoded using a punctured 2 4 -ary (8, 5) Reed-Solomon code. A 2 4 -ary (8, 5) Reed-Solomon code encodes 20 data bits (i.e. five 4-bit symbols) and 12 redundancy bits (i.e. three 4-bit symbols) in each codeword. Its error-detecting capacity is three symbols. Its error-correcting capacity is one symbol. 
     FIG. 9  shows a codeword  228  of eight symbols  204 , with five symbols encoding data coordinates  230  and three symbols encoding redundancy coordinates  232 . The codeword coordinates are indexed in coefficient order, and the data bit ordering follows the codeword bit ordering. 
   A punctured 2 4 -ary (8, 5) Reed-Solomon code is a 2 4 -ary (15, 5) Reed-Solomon code with seven redundancy coordinates removed. The removed coordinates are the most significant redundancy coordinates. 
   The code has the following primitive polynominal:
 
 p ( x )= x   4   +x+ 1  (EQ 1)
 
   The code has the following generator polynominal:
 
 g ( x )=( x+α )( x+α   2 ) . . . ( x+α   10 )  (EQ 2)
 
   For a detailed description of Reed-Solomon codes, refer to Wicker, S. B. and V. K. Bhargava, eds.,  Reed - Solomon Codes and Their Applications, IEEE Press,  1994, the contents of which are incorporated herein by reference. 
   The Tag Coordinate Space 
   The tag coordinate space has two orthogonal axes labelled x and y respectively. When the positive x axis points to the right, then the positive y axis points down. 
   The surface coding does not specify the location of the tag coordinate space origin on a particular tagged surface, nor the orientation of the tag coordinate space with respect to the surface. This information is application-specific. For example, if the tagged surface is a sheet of paper, then the application which prints the tags onto the paper may record the actual offset and orientation, and these can be used to normalise any digital ink subsequently captured in conjunction with the surface. 
   The position encoded in a tag is defined in units of tags. By convention, the position is taken to be the position of the centre of the target closest to the origin. 
   Tag Information Content 
   Table 1 defines the information fields embedded in the surface coding. Table 2 defines how these fields map to codewords. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Field definitions 
             
           
        
         
             
               field 
               width 
               description 
             
             
                 
             
           
        
         
             
               per codeword 
                 
                 
             
             
               codeword type 
               2 
               The type of the codeword, i.e. one of 
             
             
                 
                 
               A (b′00′), B (b′01′), C (b′10′) and D (b′11′). 
             
             
               per tag 
             
             
               tag type 
               2 
               The type 1  of the tag, i.e. one of 
             
             
                 
                 
               00 (b′00′), 01 (b′01′), 10 (b′10′) and 11 (b′11′). 
             
             
               x coordinate 
               13 
               The unsigned x coordinate of the tag 2 . 
             
             
               y coordinate 
               13 
               The unsigned y coordinate of the tag b . 
             
             
               active area flag 
               1 
               A flag indicating whether the tag is a member of 
             
             
                 
                 
               an active area. b′1′ indicates membership. 
             
             
               active area map 
               1 
               A flag indicating whether an active area map is 
             
             
               flag 
                 
               present. b′1′ indicates the presence of a map (see 
             
             
                 
                 
               next field). If the map is absent then the value of 
             
             
                 
                 
               each map entry is derived from the active area 
             
             
                 
                 
               flag (see previous field). 
             
             
               active area map 
               8 
               A map 3  of which of the tag&#39;s immediate eight 
             
             
                 
                 
               neighbours are members of an active area. b′1′ 
             
             
                 
                 
               indicates membership. 
             
             
               data fragment 
               8 
               A fragment of an embedded data stream. Only 
             
             
                 
                 
               present if the active area map is absent. 
             
             
               per tag group 
             
             
               encoding 
               8 
               The format of the encoding. 
             
             
               format 
                 
               0: the present encoding 
             
             
                 
                 
               Other values are TBA. 
             
             
               region flags 
               8 
               Flags controlling the interpretation and routing of 
             
             
                 
                 
               region-related information. 
             
             
                 
                 
               0: region ID is an EPC 
             
             
                 
                 
               1: region is linked 
             
             
                 
                 
               2: region is interactive 
             
             
                 
                 
               3: region is signed 
             
             
                 
                 
               4: region includes data 
             
             
                 
                 
               5: region relates to mobile application 
             
             
                 
                 
               Other bits are reserved and must be zero. 
             
             
               tag size 
               16 
               The difference between the actual tag size and the 
             
             
               adjustment 
                 
               nominal tag size 4 , in 10 nm units, in sign- 
             
             
                 
                 
               magnitude format. 
             
             
               region ID 
               96 
               The ID of the region containing the tags. 
             
             
               CRC 
               16 
               A CRC 5  of tag group data. 
             
             
               total 
               320 
             
             
                 
             
             
                 1 corresponds to the bottom two bits of the x and y coordinates of the tag 
             
             
                 2 allows a maximum coordinate value of approximately 14 m 
             
             
                 3 FIG. 29 indicates the bit ordering of the map 
             
             
                 4 the nominal tag size is 1.7145 mm (based on 1600 dpi, 9 dots per macrodot, and 12 macrodots per tag) 
             
             
                 5 CCITT CRC-16 [7] 
             
           
        
       
     
   
     FIG. 10  shows a tag  200  and its eight immediate neighbours, each labelled with its corresponding bit index in the active area map. An active area map indicates whether the corresponding tags are members of an active area. An active area is an area within which any captured input should be immediately forwarded to the corresponding Netpage server for interpretation. It also allows the Netpage sensing device to signal to the user that the input will have an immediate effect. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Mapping of fields to codewords 
             
           
        
         
             
               codeword 
               codeword bits 
               field 
               width 
               field bits 
             
             
                 
             
           
        
         
             
               A 
               1:0 
               codeword type (b′00′) 
               2 
               all 
             
             
                 
               10:2  
               x coordinate 
               9 
               12:4  
             
             
                 
               19:11 
               y coordinate 
               9 
               12:4  
             
             
               B 
               1:0 
               codeword type (b′01′) 
               2 
               all 
             
             
                 
                2 
               tag type 
               1 
               0 
             
             
                 
               5:2 
               x coordinate 
               4 
               3:0 
             
             
                 
                6 
               tag type 
               1 
               1 
             
             
                 
               9:6 
               y coordinate 
               4 
               3:0 
             
             
                 
               10 
               active area flag 
               1 
               all 
             
             
                 
               11 
               active area map flag 
               1 
               all 
             
             
                 
               19:12 
               active area map 
               8 
               all 
             
             
                 
               19:12 
               data fragment 
               8 
               all 
             
             
               C 00   
               1:0 
               codeword type (b′10′) 
               2 
               all 
             
             
                 
               9:2 
               encoding format 
               8 
               all 
             
             
                 
               17:10 
               region flags 
               8 
               all 
             
             
                 
               19:18 
               tag size adjustment 
               2 
               1:0 
             
             
               C 01   
               1:0 
               codeword type (b′10′) 
               2 
               all 
             
             
                 
               15:2  
               tag size adjustment 
               14 
               15:2  
             
             
                 
               19:16 
               region ID 
               4 
               3:0 
             
             
               C 10   
               1:0 
               codeword type (b′10′) 
               2 
               all 
             
             
                 
               19:2  
               region ID 
               18 
               21:4  
             
             
               C 11   
               1:0 
               codeword type (b′10′) 
               2 
               all 
             
             
                 
               19:2  
               region ID 
               18 
               39:22 
             
             
               D 00   
               1:0 
               codeword type (b′11′) 
               2 
               all 
             
             
                 
               19:2  
               region ID 
               18 
               57:40 
             
             
               D 01   
               1:0 
               codeword type (b′11′) 
               2 
               all 
             
             
                 
               19:2  
               region ID 
               18 
               75:58 
             
             
               D 10   
               1:0 
               codeword type (b′11′) 
               2 
               all 
             
             
                 
               19:2  
               region ID 
               18 
               93:76 
             
             
               D 11   
               1:0 
               codeword type (b′11′) 
               2 
               all 
             
             
                 
               3:2 
               region ID 
               2 
               95:94 
             
             
                 
               19:4  
               CRC 
               16 
               all 
             
             
                 
             
           
        
       
     
   
   Note that the tag type can be moved into a global codeword to maximise local codeword utilization. This in turn can allow larger coordinates and/or 16-bit data fragments (potentially configurably in conjunction with coordinate precision). However, this reduces the independence of position decoding from region ID decoding and has not been included in the specification at this time. 
   Embedded Data 
   If the “region includes data” flag in the region flags is set then the surface coding contains embedded data. The data is encoded in multiple contiguous tags&#39; data fragments, and is replicated in the surface coding as many times as it will fit. 
   The embedded data is encoded in such a way that a random and partial scan of the surface coding containing the embedded data can be sufficient to retrieve the entire data. The scanning system reassembles the data from retrieved fragments, and reports to the user when sufficient fragments have been retrieved without error. 
   As shown in Table 3, a 200-bit data block encodes 160 bits of data. The block data is encoded in the data fragments of A contiguous group of 25 tags arranged in a 5□5 square. A tag belongs to a block whose integer coordinate is the tag&#39;s coordinate divided by 5. Within each block the data is arranged into tags with increasing x coordinate within increasing y coordinate. 
   A data fragment may be missing from a block where an active area map is present. However, the missing data fragment is likely to be recoverable from another copy of the block. 
   Data of arbitrary size is encoded into a superblock consisting of a contiguous set of blocks arranged in a rectangle. The size of the superblock is encoded in each block. A block belongs to a superblock whose integer coordinate is the block&#39;s coordinate divided by the superblock size. Within each superblock the data is arranged into blocks with increasing x coordinate within increasing y coordinate. 
   The superblock is replicated in the surface coding as many times as it will fit, including partially along the edges of the surface coding. 
   The data encoded in the superblock may include more precise type information, more precise size information, and more extensive error detection and/or correction data. 
                                               TABLE 3                   Embedded data block            field   width   description                    data type   8   The type of the data in the superblock.               Values include:               0: type is controlled by region flags               1: MIME               Other values are TBA.       superblock width   8   The width of the superblock, in blocks.       superblock height   8   The height of the superblock, in blocks.       data   160   The block data.       CRC   16   A CRC 6  of the block data.       total   200                 6 CCITT CRC-16 [7]            
Cryptographic Signature of Region ID
 
   If the “region is signed” flag in the region flags is set then the surface coding contains a 160-bit cryptographic signature of the region ID. The signature is encoded in a one-block superblock. 
   In an online environment any signature fragment can be used, in conjunction with the region ID, to validate the signature. In an offline environment the entire signature can be recovered by reading multiple tags, and can then be validated using the corresponding public signature key. This is discussed in more detail in Netpage Surface Coding Security section of the cross reference co-pending application Ser. No. 11/193,479, which is entirely incorporated into the application with Ser. No. 11/193,481. 
   MIME Data 
   If the embedded data type is “MIME” then the superblock contains Multipurpose Internet Mail Extensions (MIME) data according to RFC 2045 (see Freed, N., and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME)—Part One: Format of Internet Message Bodies”, RFC 2045, November 1996), RFC 2046 (see Freed, N., and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME)—Part Two: Media Types”, RFC 2046, November 1996) and related RFCs. The MIME data consists of a header followed by a body. The header is encoded as a variable-length text string preceded by an 8-bit string length. The body is encoded as a variable-length type-specific octet stream preceded by a 16-bit size in big-endian format. 
   The basic top-level media types described in RFC 2046 include text, image, audio, video and application. RFC 2425 (see Howes, T., M. Smith and F. Dawson, “A MIME Content-Type for Directory Information”, RFC 2045, September 1998) and RFC 2426 (see Dawson, F., and T. Howes, “vCard MIME Directory Profile”, RFC 2046, September 1998) describe a text subtype for directory information suitable, for example, for encoding contact information which might appear on a business card. 
   Encoding and Printing Considerations 
   The Print Engine Controller (PEC) supports the encoding of two fixed (per-page) 2 4 -ary (15, 5) Reed-Solomon codewords and six variable (per-tag) 2 4 -ary (15, 5) Reed-Solomon codewords. Furthermore, PEC supports the rendering of tags via a rectangular unit cell whose layout is constant (per page) but whose variable codeword data may vary from one unit cell to the next. PEC does not allow unit cells to overlap in the direction of page movement. A unit cell compatible with PEC contains a single tag group consisting of four tags. The tag group contains a single A codeword unique to the tag group but replicated four times within the tag group, and four unique B codewords. These can be encoded using five of PEC&#39;s six supported variable codewords. The tag group also contains eight fixed C and D codewords. One of these can be encoded using the remaining one of PEC&#39;s variable codewords, two more can be encoded using PEC&#39;s two fixed codewords, and the remaining five can be encoded and pre-rendered into the Tag Format Structure (TFS) supplied to PEC. 
   PEC imposes a limit of 32 unique bit addresses per TFS row. The contents of the unit cell respect this limit. PEC also imposes a limit of 384 on the width of the TFS. The contents of the unit cell respect this limit. 
   Note that for a reasonable page size, the number of variable coordinate bits in the A codeword is modest, making encoding via a lookup table tractable. Encoding of the B codeword via a lookup table may also be possible. Note that since a Reed-Solomon code is systematic, only the redundancy data needs to appear in the lookup table. 
   Imaging and Decoding Considerations 
   The minimum imaging field of view required to guarantee acquisition of an entire tag has a diameter of 39.6 s (i.e. (2×(12+2))√{square root over (2)} s), allowing for arbitrary alignment between the surface coding and the field of view. Given a macrodot spacing of 143 μm, this gives a required field of view of 5.7 mm. 
   Table 4 gives pitch ranges achievable for the present surface coding for different sampling rates, assuming an image sensor size of 128 pixels. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               Pitch ranges achievable for present surface coding for different 
             
             
               sampling rates; dot pitch = 1600 dpi, macrodot pitch = 9 
             
             
               dots, viewing distance = 30 mm, nib-to-FOV separation = 1 
             
             
               mm, image sensor size = 128 pixels 
             
           
        
         
             
                 
               sampling 
                 
             
             
                 
               rate 
               pitch range 
             
             
                 
                 
             
           
        
         
             
                 
               2 
                 ~ 40 to 49 
             
             
                 
               2.5 
                 ~ 27 to 36 
             
             
                 
               3 
                 ~ 10 to 18 
             
             
                 
                 
             
           
        
       
     
   
   Given the present surface coding, the corresponding decoding sequence is as follows:
         locate targets of complete tag   infer perspective transform from targets   sample and decode any one of tag&#39;s four codewords   determine codeword type and hence tag orientation   sample and decode required local (A and B) codewords   codeword redundancy is only 12 bits, so only detect errors   on decode error flag bad position sample   determine tag x-y location, with reference to tag orientation   infer 3D tag transform from oriented targets   determine nib x-y location from tag x-y location and 3D transform   determine active area status of nib location with reference to active area map   generate local feedback based on nib active area status   determine tag type from A codeword   sample and decode required global (C and D) codewords (modulo window alignment, with reference to tag type)   although codeword redundancy is only 12 bits, correct errors; subsequent CRC verification will detect erroneous error correction   verify tag group data CRC   on decode error flag bad region ID sample   determine encoding type, and reject unknown encoding   determine region flags   determine region ID   encode region ID, nib x-y location, nib active area status in digital ink   route digital ink based on region flags       

   Note that region ID decoding need not occur at the same rate as position decoding. 
   Note that decoding of a codeword can be avoided if the codeword is found to be identical to an already-known good codeword. 
   Netpage Pen 
   Functional Overview 
   The Netpage pen is a motion-sensing writing instrument which works in conjunction with a tagged Netpage surface (see Netpage Surface Coding and Netpage Surface Coding Security sections above). The pen incorporates a conventional ballpoint pen cartridge for marking the surface, a motion sensor for simultaneously capturing the absolute path of the pen on the surface, an identity sensor for simultaneously identifying the surface, a force sensor for simultaneously measuring the force exerted on the nib, and a real-time clock for simultaneously measuring the passage of time. 
   While in contact with a tagged surface, as indicated by the force sensor, the pen continuously images the surface region adjacent to the nib, and decodes the nearest tag in its field of view to determine both the identity of the surface, its own instantaneous position on the surface and the pose of the pen. The pen thus generates a stream of timestamped position samples relative to a particular surface, and transmits this stream to a Netpage server (see Netpage Architecture section in co-pending application Ser. No. 11/193,479). The sample stream describes a series of strokes, and is conventionally referred to as digital ink (DInk). Each stroke is delimited by a pen down and a pen up event, as detected by the force sensor. 
   The pen samples its position at a sufficiently high rate (nominally 100 Hz) to allow a Netpage server to accurately reproduce hand-drawn strokes, recognise handwritten text, and verify hand-written signatures. 
   The Netpage pen also supports hover mode in interactive applications. In hover mode the pen is not in contact with the paper and may be some small distance above the surface of the paper (or tablet etc.). This allows the position of the pen, including its height and pose to be reported. In the case of an interactive application the hover mode behaviour can be used to move the cursor without marking the paper, or the distance of the nib from the coded surface could be used for tool behaviour control, for example an air brush function. 
   The pen includes a Bluetooth radio transceiver for transmitting digital ink via a relay device to a Netpage server. 
   When operating offline from a Netpage server the pen buffers captured digital ink in non-volatile memory. When operating online to a Netpage server the pen transmits digital ink in real time. 
   The pen is supplied with a docking cradle or “pod”. The pod contains a Bluetooth to USB relay. The pod is connected via a USB cable to a computer which provides communications support for local applications and access to Netpage services. 
   The pen is powered by a rechargeable battery. The battery is not accessible to or replaceable by the user. Power to charge the pen can be taken from the USB connection or from an external power adapter through the pod. The pen also has a power and USB-compatible data socket to allow it to be externally connected and powered while in use. 
   The pen cap serves the dual purpose of protecting the nib and the imaging optics when the cap is fitted and signalling the pen to leave a power-preserving state when uncapped. 
   Pen Form Factor 
   The overall weight (45 g), size and shape (159 mm×17 mm) of the Netpage pen fall within the conventional bounds of hand-held writing instruments. 
   Ergonomics and Layout 
     FIG. 11  shows a rounded triangular profile gives the pen  400  an ergonomically comfortable shape to grip and use the pen in the correct functional orientation. It is also a practical shape for accommodating the internal components. A normal pen-like grip naturally conforms to a triangular shape between thumb  402 , index finger  404  and middle finger  406 . 
   As shown in  FIG. 12 , a typical user writes with the pen  400  at a nominal pitch of about 30 degrees from the normal toward the hand  408  when held (positive angle) but seldom operates a pen at more than about 10 degrees of negative pitch (away from the hand). The range of pitch angles over which the pen  400  is able to image the pattern on the paper has been optimised for this asymmetric usage. The shape of the pen  400  helps to orient the pen correctly in the user&#39;s hand  408  and to discourage the user from using the pen “upside-down”. The pen functions “upside-down” but the allowable tilt angle range is reduced. 
   The cap  410  is designed to fit over the top end of the pen  400 , allowing it to be securely stowed while the pen is in use. Multi colour LEDs illuminate a status window  412  in the top edge (as in the apex of the rounded triangular cross section) of the pen  400  near its top end. The status window  412  remains un-obscured when the cap is stowed. A vibration motor is also included in the pen as a haptic feedback system (described in detail below). 
   As shown in  FIG. 13 , the grip portion of the pen has a hollow chassis molding  416  enclosed by a base molding  528  to house the other components. The ink cartridge  414  for the ball point nib (not shown) fits naturally into the apex  420  of the triangular cross section, placing it consistently with the user&#39;s grip. This in turn provides space for the main PCB  422  in the centre of the pen and for the battery  424  in the base of the pen. By referring to  FIG. 14   a , it can be seen that this also naturally places the tag-sensing optics  426  unobtrusively below the nib  418  (with respect to nominal pitch). The nib molding  428  of the pen  400  is swept back below the ink cartridge  414  to prevent contact between the nib molding  428  and the paper surface when the pen is operated at maximum pitch. 
   As best shown in  FIG. 14   b , the imaging field of view  430  emerges through a centrally positioned IR filter/window  432  below the nib  418 , and two near-infrared illumination LEDs  434 ,  436  emerge from the two bottom corners of the nib molding  428 . The use of two illumination LEDs  434 ,  436  ensures a more uniform illumination field  438 ,  440 . 
   As the pen is hand-held, it may be held at an angle that causes reflections from one of the LED&#39;s that are detrimental to the image sensor. By providing more than one LED, the LED causing the offending reflections can be extinguished. 
   Pen Feedback Indications 
     FIG. 17  is a longitudinal cross section through the centre-line if the pen  400  (with the cap  410  stowed on the end of the pen). The pen incorporates red and green LEDs  444  to indicate several states, using colours and intensity modulation. A light pipe  448  on the LEDs  444  transmit the signal to the status indicator window  412  in the tube molding  416 . These signal status information to the user including power-on, battery level, untransmitted digital ink, network connection on-line, fault or error with an action. 
   A vibration motor  446  is used to haptically convey information to the user for important verification functions during transactions. This system is used for important interactive indications that might be missed due to inattention to the LED indicators  444  or high levels of ambient light. The haptic system indicates to the user when:
         The pen wakes from standby mode   There is an error with an action   To acknowledge a transaction
 
Pod Feedback Indications
       

   Turning briefly to the recharging pod  450  shown in  FIGS. 31 and 32 , red and green LEDs  452  to indicate various states using colours and intensity modulation. The light from the LEDs is transmitted to the exterior of the pod via the polymer light pipe molding  454 . These signal status information to the user including charging state, and untransmitted digital ink by illuminating/pulsating one LEDs  452  at a time. 
   Features and Accessories 
   As shown in  FIG. 15 , the pen has a power and data socket  458  is located in the top end  456  of the pen, hidden and moisture-sealed behind an elastomeric end-cap  460 . The end-cap can be prised open to give access to the socket  458  and reset switch (at the bottom of recess  464 ) and remains open while the cable  462  is in use. The USB power and data cable  462  allows the pen to be used for periods that exceed the battery life. 
   The usual method of charging the pen  400  is via the charging pod  450  shown in  FIGS. 31 and 32 . As will be described in greater detail below, the pod  450  includes a Bluetooth transceiver connected by USB to a computer and several LEDs to indicate for charging status. The pod is compact to minimise its desktop footprint, and has a weighted base for stability. Data transfer occurs between the pen and the pod via a Bluetooth radio link. 
   Market Differentiation 
   Digital mobile products and quality pens are usually considered as personal items. This pen product is used by both genders from 5 years upwards for personal, educational and business use, so many markets have to be catered for. The pen design allows for substantial user customisation of the external appearance of the pen  400  and the pod  450  by having user changeable parts, namely the cap  410 , an outer tube molding  466  (best shown in  FIGS. 16 and 49 ) and the pod jacket  468  (best shown in  FIGS. 31 and 32 ). These parts are aquagraphic printed (a water based transfer system) to produce a variety of high quality graphic images and textures over all surfaces of these parts. These parts are accessories to the pen, allowing the user to change the appearance whenever they wish. A number of licensed images provide enhancers for the sale of accessories as an additional business model, similar to the practice with mobile phone covers. 
   Pen Mechanical Design 
   Parts and Assemblies 
   Referring to  FIG. 16 , the pen  400  has been designed as a high volume product and has four major sub-assemblies: 
   an optical assembly  470 ; 
   a force sensing assembly  474 ; 
   a cap assembly  472 ; and, 
   the main assembly  476 , which holds the main PCB  422  and battery  424 . 
   Wherever possible, moldings have been designed as line-of-draw to reduce cost and promote longevity in the tooling. 
   These assemblies and the other major parts can be identified in  FIG. 17 . As the form factor of the pen is to be as small as possible these parts are packed as closely as practical. The electrical components in the upper part of the pen, namely the force sensor assembly  474  and the vibration motor  446  all have sprung contacts ( 512  of  FIG. 24 and 480  of  FIG. 62A  respectively) directly mating with contact pads  482  and  484  respectively (see  FIG. 64 ) on the PCB  422 . This eliminates the need for connectors and also decouples these parts from putting any stress onto the main PCB. 
   Although certain individual molded parts are thin walled (0.8 to 1.2 mm) the combination of these moldings creates a strong structure. The pen is designed not to be user serviceable and therefore has a cold stake under the exterior label to prevent user entry. Non-conducting plastics moldings are used wherever possible to allow an omnidirectional beam pattern to be formed by the Bluetooth radio antenna  486  (see  FIG. 64 ). 
   Optics Assembly 
   The major components of the optical assembly are as shown in  FIGS. 18 and 19 . The axial alignment of the lens  488  to the image sensor  490  is toleranced to be better than 50 μm to minimise blur at the image. The barrel molding  492  is therefore has high precision with tight tolerancing. It has a molded-in aperture  494  near the image sensor  490 , which provides the location for the lens  488 . As the effect of thermal expansion is very small on a molding this size, it is not necessary to use a more expensive material. 
   The flex PCB  496  mounts two infrared LEDs  434  and  436 , a wire bonded Chip-on-Flex image sensor  490  and some chip capacitors  502 . The flex PCB  496  is 75 micron thick polyimide, which allows the two infrared LEDs  434  and  436  to be manipulated. Stiffeners are required in certain areas on the flex as backing for the attached components. The flex PCB  496  is laser cut to provide accuracy for mounting onto the barrel molding  492  and fine pitch connector alignment. 
   Force Sensing Assembly and Ink Cartridge 
     FIGS. 20 ,  23 ,  24  and  64  show the components and installation of the force sensing assembly. The force sensing assembly  474  is designed to accurately measure force put on the ink cartridge  414  during use. It is specified to sense between 0 and 500 grams force with enough fidelity to support handwriting recognition in the Netpage services. This captive assembly has two coaxial conductive metal tubes  498 , a retainer spring  504  and a packaged force sensor  500 . 
   Conductive Metal Tube 
   The conductive metal tubes  498  has an insert molded insulation layer  506  between two metal tubes (inner tube  508  and outer tube  510 ), which each have a sprung gold plated contact finger ( 512  and  514  respectively). Power for charging the battery is provided by two contacts  516  (see  FIG. 31 ) in the charging pod  450  and is conducted by these two tubes directly to recharging contacts  518  and  520  (see  FIG. 64 ) on the main PCB  422 , via a spring contact ( 512  and  514  respectively) on each tube. 
   When the pen cap assembly  472  is placed on the front of the pen  400 , a conductive elastomeric molding in the pen cap mates with the ends of both concentric tubes in the conductive metal tube part, completing the circuit and signalling the cap presence to the pen electronics (see  FIG. 18 ). 
   Force Sensor Operating Principles 
     FIG. 33  schematically illustrates the operation of the force sensing assembly  474 . The spring  700  applies a pre-load to the force sensor IC  526  (via a ball bearing  524 ) before the cartridge  414  is subject to any force at the nib  418 . The cartridge  414  itself is not pushed against the force sensor as it passes through the spring. Instead, the spring pushes a boot  702  against the force sensor, and the boot is coupled to the end of the cartridge. The boot  702  is a compromise between allowing easy manual insertion and removal of cartridge  414 , and ensuring the cartridge is held securely without travel. The use of a boot  702  also allows the inclusion of a stop surface  698 . The stop limits the travel of the boot  702  thereby protecting the spring  700  from overload. 
   Packaged Force Sensor 
     FIGS. 62A to 62E  are perspectives of the various components of the packaged force sensor  500 .  FIG. 62A  shows a steel ball  524  protruding from the front of a sensor IC (chip)  526 . The ball  524  is the point contact used to transmit force directly to the chip. Wire bonds  604  connect the chip  526  to the spring contacts  478 . The chip sits in the recess  564  formed in the rear molding  566  shown in  FIG. 62B . A pressure relief vent  584  in the base of the recess  564  allows air trapped by the chip  526  to escape. The front molding  606  shown in  FIG. 62C , has slots  608  in its underside for the sprung contacts  478  and a central aperture  610  to hold the ball  524 . Location details  612  mate with corresponding details in the coaxial conductive tubes  498  as shown in  FIG. 24 . 
   As there is only 10 microns full span movement in this system, the mounting of this assembly in the pen and use of axial preload is tightly toleranced. The force sensing assembly is mounted in the top of the pen so that it can only stress the pen chassis molding  416  (see  FIG. 16 ), and force will not be transmitted to the main PCB  422 . The force sensor is a push fit onto the end of the inner conductive metal tube  508  also trapping the retainer spring  504 , which makes a simple dedicated assembly  500 . 
   Retainer Spring 
   Turning to  FIGS. 20 and 24 , the retainer spring  504  is the equivalent to the boot  702  described in  FIG. 33 . It is a high precision stamping out of thin sheet metal with an insulating layer  708  at the point where it contacts the ball  524 . This inhibits electrical interference with the force sensor IC  526  caused by external electrostatic discharge via the ink cartridge  414 . The metal retainer spring  504  is formed into four gripping arms  530  and two spring arms  532 . A spent cartridge removal tool  534  is secured to the open end of the cartridge  414  with an interference fit. The gripping arms  530  grip a complementary external grip profile  704  on the removal tool  534 . The spring arms  532  extend beyond the end of gripping arms  530  to press against the stepped section  706  in the coaxial tube assembly  498 . This in turn pushes insulated base  708  against the ball  524  to put an accurate axial preload force of between 10 and 20 grams onto the force sensor. 
   Ink Cartridge 
   The pen ink cartridge  414  is best shown in  FIGS. 21A and 21B . Research shows that industry practice is for the ballpoint nib  418  to be made by one source and the metal tube  536  to be made by another, along with assembly and filling. There are no front loading standard ink cartridges that meet the design capacity and form factor requirements so a custom cartridge has been developed. This ink cartridge  414  has a 3 mm diameter tube  536  with a standard ballpoint nib inserted. The spent cartridge removal tool  534  is a custom end molding that caps the open end of the metal tube  536 . 
   The removal tool  536  contains an air vent  538  for ink flow, a location detail  540  and a co-molded elastomeric ring  542  around a recess  544  detail used for extracting the spent ink cartridge. The tool is levered down to engage the nib of the old cartridge and then drawn out through the nib end of the pen as shown in  FIG. 21B . The elastomer ring  542  reduces the possibility that a hard shock could damage the force sensor if the pen is dropped onto a hard surface. 
   The location detail  540  allows the ink cartridge  414  to accurately seat into the retainer spring  504  in the force sensing assembly  474  and to be preloaded against the force sensor  500 . The removal tool (apart from the co-molded elastomeric ring) is made out of a hard plastic such as acetal and can be molded in color to match the ink contents. 
   The ink capacity is 5 ml giving an expected write-out length comparable with standard ballpoint ink cartridges. This capacity means that refill cycles will be relatively infrequent during the lifetime of the pen. 
   Force Sensing Method 
   Pressing the nib  418  against a surface will transfer the force to the ball  524  via the gripping arms  530 . The force from the nib adds to the preload force from the spring arms  532 . The force sensor is a push fit into the end of the coaxial tube assembly  498  and both directly connect to the PCB with spring contacts ( 478  and  512  respectively). 
     FIG. 24  shows the limited space available for an axial force sensor, hence a packaged design is required as off-the-shelf items have no chance of fitting in this space envelope in the required configuration. 
   This force sensing arrangement detects the axial force applied to the cartridge  414 , which is the simplest and most accurate solution. There is negligible friction in the system as the cartridge contacts only on two points, one at either end of the conductive metal barrel  498 . The metal retainer spring  504  will produce an accurate preload force up to 20 grams onto the force sensor  500 . This is seen to be a reliable system over time, as the main parts are metal and therefore will not suffer from creep, wear or stiction during the lifetime of the pen. 
   This design also isolates the applied force by directing it onto the packaged force sensor, which pushes against the solid seat in the chassis molding  416  of the pen. This allows the force sensing assembly  474  to float above the main PCB  422  (so as not to put strain on it) whilst transmitting data via the spring contacts  478  at the base of the packaged force sensor  500 . The resulting assembly fits neatly into the pen chassis molding  416  and is easy to hand assemble. 
   Top/Side Loading Cartridge 
   As discussed above, the pen will require periodic replacement of the ink cartridge during its lifetime. While the front loading ink cartridge system is convenient for users, it can have some disadvantages. Front loading limits the capacity of the ink reservoir in the cartridge, since the diameter of the cartridge along its full length is limited to the minimum cartridge diameter, as dictated by the constraints of the pen nose. 
   The cartridge  414  must be pushed against the force sensor IC  526  (via the steel ball  524 ) by a pre-load spring  700  (see  FIG. 33 ). However, the cartridge  414  itself does not provide the face against which the spring pushes, since the cartridge must pass through the spring. This necessitates the boot  702  or retaining spring  504  discussed above. The boot is necessarily a compromise between allowing easy manual insertion and removal of cartridge, and ensuring the cartridge is held securely without travel. 
   A ‘top-loading’ cartridge, as illustrated in  FIG. 34 , can overcome these disadvantages. It will be appreciated that ‘top loading’ is a reference to insertion of the cartridge from a direction transverse to the longitudinal axis of the pen. Because of the other components within the pen, it is most convenient to insert the cartridge from the ‘top’ or apex  420  of the pen&#39;s substantially triangular cross section (see  FIG. 13 ). 
   The pre-load spring  700  can be placed toward the nib  418  of the cartridge  414 , thus providing a convenient mechanism for seating the cartridge against the force sensor ball  524  after insertion. A cartridge travel stop  712  is formed on the chassis molding  416  to prevent overloading the force sensor  526 . Since the cartridge itself provides the face against which the pre-load spring pushes, the boot is eliminated and the cartridge couples directly with the force sensor. 
   As the cartridge is no longer constrained to a single diameter along its full length, its central section can be wider and accommodate a much larger ink reservoir  710 . 
   The currently proposed pen design has an internal chassis  416  and an external tube molding  466 . The external molding  466  is user replaceable, allowing the user to customise the pen  400 . Removing the external molding  466  also provides the user with access to the pen&#39;s product label  652  (see  FIG. 71 ). Skilled workers in this field will appreciate that the chassis molding  416  and the base molding  528  could be modified to provide the user with access to a replaceable battery. 
   Referring again to  FIG. 34 , removing the external molding  466  (not shown) can also provide the user with access to the top-loading pen cartridge  414 . Once the external molding is removed, most of the length of the pen cartridge  414  is exposed. The user removes the cartridge by sliding it forwards against the pre-load spring  700  to extract its tail  718  from the force sensor aperture  720 , then tilting it upwards to free the tail  718  from the cartridge cavity  722 , and finally withdrawing the cartridge  710  from the pre-load spring  700  and cavity  722 . The user inserts a new cartridge by following the same procedure in reverse. 
   Since a top-loading cartridge can have a much greater capacity than a front-loading cartridge, it is not unreasonable to require the user to remove the external molding  466  to replace the cartridge  414 , since the user will have to replace a top-loading cartridge much less often than a front-loading cartridge. 
   Referring to  FIG. 35 , the pre-load spring  700  can be provided with its own cavity  716  and retaining ring  714  to make it easier to insert the cartridge  414 . 
   Force Re-Directing Couplings 
   The force sensor  526  is ideally mounted perpendicularly to the pen cartridge  414 , as illustrated in  FIG. 33 . This allows direct coupling between the pen cartridge and the force sensor. This coupling is somewhat independent of whether there is an intermediate boot  702  or not, as discussed above in relation to the side loading cartridge. To fit within the constrained space of the pen&#39;s tubular moulding  466 , it can be advantageous to mount the force sensor  526  in any desired position relative to the cartridge  414 . This involves re-directing at least part of the contact force being transferred along the cartridge  414 . 
   A suitable force sensor  526  for the pen is a silicon piezoresistive bridge force sensor, such as manufactured by Hokuriku (see Hokuriku,  Force Sensor HFD -500, http://www.hdk.co.jp/pdf/eng/e1381AA.pdf for details). The invention will be illustrated with reference to this force sensor. However it will be appreciated that many other force sensors are also suitable. 
   As shown in  FIG. 36 , the standard Hokuriku force sensor package measures 5.2 mm wide by 7.0 mm long (or 8.0 mm with leads) by about 3 mm thick. This thickness includes the ball  524 , which protrudes 150-200 microns. The headroom above the PCB  422  in the embodiment shown is just over 5 mm. The pen cartridge axis extends centrally through the boot  702  and is just under 3 mm above the PCB  422 . It is therefore possible to mount the standard Hokuriku force sensor package  526  on the PCB  422 , either longitudinally (see  FIGS. 37A and 37B ) or possibly laterally (see  FIG. 38 ), and provide an off-axis coupling mechanism between the pen cartridge  418  and the force sensor  526 . 
     FIG. 36  shows a force transfer element in the form of a double-bow coupling piece  726  between the cartridge  414  and the force sensor  526 . The lower, or force transfer bow  730  expands downwards when subject to force from the cartridge via the boot  702 . The force is transmitted through a right angle, providing the required coupling between the cartridge  414  and the force sensor  526  mounted on the PCB  422 . Each bow  728  and  730  is formed from a flexible sheet. The edges of each sheet are curved to minimize friction with the walls of the cavity. 
   The double-bow design acts as a centralizer, preventing the cartridge  414  from moving upwards when force is applied, and eliminating an area of friction. The top of the upper bow  728  can be pinned, if necessary, to eliminate another point of friction (or the cavity itself can provide a curved ridge contact). Friction between the force transfer bow  730  and the ball  524  of the force sensor  526  is small because the curvature of the ball minimizes the contact area. 
   The force sensor  526  mates with a recess in the chassis moulding  416  to form the cavity in which the double-bow coupling piece  726  operates. 
   The pen cartridge  414  or the boot  702  necessarily engages with the coupling piece  726  above the axis of the cartridge  414 , since it is impractical to align the two while efficiently utilizing the available space. However, because the ratio of the length of the cartridge to its diameter is large, negligible torsion is induced by this off-axis coupling. As discussed above, the centralizing function of the double-bow design minimizes friction. 
   The double bow coupling piece  726  can be thought of as having two spring constants. When unconstrained by the cavity, the double bow can act as a reasonably soft spring. It should be soft enough to guarantee that it expands to fill the cavity when subjected to the force of the preload spring. The softness will also be a function of the manufacturing tolerances of both the cavity and the double-bow coupling piece  726 . When the top bow  728  is constrained by the cavity, the double bow coupling piece  726  can act as a very stiff spring. It should be stiff enough to avoid resonant frequencies which overlap frequencies of interest in the real force signal. 
   The force sensor  526  shown in  FIGS. 20 ,  24  and  63 A to  63 E is mounted in the chassis moulding  416 , and makes electrical contact with the PCB  422  via a set of sprung leads. This prevents force being transmitted to solder joints between the force sensor  526  and the PCB  422 , and to the PCB itself 
   By contrast, in this aspect of the invention, the force sensor  526  is mounted flush with the PCB  422  and is therefore ideally soldered to it. Furthermore, the force sensor  526  must be securely attached to the chassis moulding  416  because it will be subject to a force pushing it away from the moulding. 
   To make this practical, the PCB  422  can be securely attached to the chassis moulding  416  via a set of clips formed in the chassis moulding  416  and extending below the PCB  422 . Pins can also be provided as part of the chassis moulding  416 , to penetrate and anchor the PCB  422 . The PCB  422  can then float within the tubular body  466 , with its main anchor point being in the centre of the pen, at the location of the force sensor  526 . 
   The embodiments shown in  FIGS. 37A to 50 , re-direct the force (at least partially) from the cartridge or boot  702 , to the sensor  526 , via a hydraulic coupling. As with the double bow coupling, this allows the force sensor to be positioned conveniently within the constraints of the pen body, and addresses other problems such as damage from the deceleration shock when the pen is tapped or dropped, and a relatively undamped transient response which limits the available sensor bandwidth. 
   The general layout of the design is shown in  FIGS. 37A ,  37 B and  38  using the Hokuriku HFD-500 force sensor discussed above in relation to the double bow coupling. As previously mentioned, other high range pressure sensors are also suitable. The sensor  526  can be used with or without the ball bearing  524 . The PCB  422  needs to float on its mounts so that the end stop behind the over-mould  734  brings all the axial pen force onto the pen chassis (not shown) rather than the surface-mount connection to the PCB  422 . 
     FIGS. 39A and 39B  show the hydraulic coupling in more detail. The ink cartridge  418  has a nib at its distal end and a boot  702  at the opposite end. The boot pushes a plunger  732  onto a membrane or gel surface  742  through an aperture in the over moulded package  734 . The increased pressure in the hydraulic fluid or gel  736  acts on the ball bearing  524  of the force sensor  526 . The output signal from the sensor  526  is transmitted directly to contacts on the PCB  422  via pins  740 . 
   The action of the input force F on the force sensor is schematically shown in  FIGS. 40 and 41 . It will be appreciated that these sketches are simplified and without the right-angle bend. The right angle in the fluid path has no effect on the fluid at low flow rates. 
     FIG. 40  represents the situation with an unmodified Hokuriku sensor  526 . The ball  524  acts as a piston, approximately, as its cross-sectional area normal to the direction of travel hardly changes. 
   Pressure throughout the fluid or gel  736 , (in the case of the Hokuriku sensor, silicone gel) is constant so:
 
 P=F/Ai=Fo/Ao,  
         Where   P is the pressure in the gel;   F is the input force;   Fo is sensed force;   Ai is the area of the plunger;   Ao is the projected surface area of the ball in plan view, or effective diaphragm size.   Thus
 
 Fo/F=Ao/Ai  
       

   This ratio of the output force to the applied force is here termed the Gearing Ratio (gr). Experimental results show that the Gearing Ratio for the Hokuriku sensor is 0.22. 
     FIG. 41  shows the Hokuriku sensor having been modified to remove the ball. The cavity of the sensor  526  is also filled with the fluid or gel  736  and the pressure acts directly on the sensor chip  526 , so the effective diaphragm size (Ao) is the top surface of the sensor chip  526 . 
   The difference between using a sensor with the ball bearing  524  and without the ball bearing, is that the top surface of the chip  526  does not act as a piston, but rather it deforms like a balloon. The force sensor chip is actually sensing a pressure instead of a force. Compare the typical force sensor deflection profile in  FIG. 42  to a typical pressure sensor deflection profile in  FIG. 43 . The deflection in the pressure sensor case will be less at the centre of the chip and it will be less sensitive, but simpler. This diaphragm diameter is also different from the first case and so will provide a different gearing ratio. A practical realisation of the sensor configured to respond to the pressure in the hydraulic coupling is shown in  FIG. 44 . It is important to vent the cavity  748  beneath the force sensor chip  526  with an aperture through the moulding  734 . 
   Any sensor chip  526  responsive to differential pressure can be used. However, high sensitivity are less preferred. The back of the chip must be open to the ambient air pressure. The range of pressures is in the order of atmospheres, so high-sensitivity sensor chips are less suitable, eg. 500 g force over a 4 mm 2  diaphragm (top surface of sensor chip) is 1.3 MPa=181 PSI=12 atm. 
   The fluid or gel  736  in the casing  734  should be incompressible. All bubbles should be removed, with a vacuum if necessary. The difference between various fluids is the sheer force and the resulting pressure head (loss) and loss of transmitted force.
 
 F in(effective)= F in− F sheer
 
 P effective= F in( eff )/ Ai−P head
 
   The pressure head loss is insignificant for silicone gel and it has proven to be a suitable for the requirements of the force sensor  526 . However other fluids or gels may be used and the issues to be considered when selecting a suitable fill for the casing are:
         i Lower viscosity decreases the strength of the chip  526  (or more correctly, the chip needs to be less rigid) and the easier it is to break.   ii Higher viscosity causes more hysteresis loss. The sensor signal should return-to-zero setting after release of the input force.   iii Secondary effects (resonant frequencies and standing waves) related to the effective elasticity of the coupling fill should be minimised.   iv Losses in the high frequencies can help to dampen the step/impulse response.       

   The elasticity of the boot, mounting, and writing surface all affect the self-resonant oscillations. A softer coupling (low stiffness) lowers the oscillation frequency, which is undesirable. Conversely, a stiffer coupling increases the deceleration force component of the pen-down action (for convenience, the pen down response is referred to as the “F 1 ” response). This F 1  response provides an unwanted artefact in the force signal and increases the risk of chip breakage.  FIG. 45  shows a typical tap response output signal that illustrates the F 1  response. 
   There are several possibilities for applying the input force to the hydraulic fluid or gel  736 . Three of the primary options are shown in  FIGS. 46A to 46C . 
   In  FIG. 46A , the input piston  752  forms a sliding fit with the aperture in the casing  734 . The piston is overly complicated for a microstructure and sealing the sides will cause friction—which is highly undesirable. 
   In  FIG. 46B , the input force F acts directly on an outwardly bulging membrane  754 . The diaphragm  754  is really only relevant to pressure sensors where the input is a liquid or gas. 
     FIG. 46C  shows a diaphragm  754  and plunger  752  combination. This mechanism can be made robust so that it is difficult to burst, the surface strength of the diaphragm  754  does not need to be so high that it interferes with force transmission and the exhaust of material around the sides of the plunger  752  can be restrained as it lowers the spring constant of the coupling and reduce the frequency response of the step/impulse function. Also, the exhausted material and wall expansion of the casing  734  (see  FIG. 41 ) increases the volume ratio (see N calculated below). Some increase is tolerable and in fact might be desirable for protecting the chip. 
   When designing the force input mechanism of  FIG. 46C , the relevant considerations are:
         i Shear force/piston effect   ii Strength: plunger collapses into the fluid   iii Gap provides a vent for the fluid to oscillate in and softens the coupling—undesirably lowering the oscillation frequency (see above).   iv Gap magnifies the volume ratio of the input piston relative to the output piston (perfect piston behaviour).       

   Volume ratio:
 
 N= ( X in× A in)/( X out× A out), where  X  is the axial displacement,
 
   N is approximately 20, if Xin at an input force of 500 g is approximately 400 microns. 
   Up to a point this axial magnification (Xin/Xout) is good as it means, in this case, that the 10 microns movement of the sensor diaphragm  754  might give a 0.2 mm cartridge movement. This allows a better end-stop protection mechanism (see  FIG. 37B ) to be used that does not have such critical tolerance requirements. 
   The surface of the diaphragm  754  can be:
         i Just the soft bulk material of the semi-cured silicone.   ii Silicone with a thin membrane   iii Silicone with say an epoxy (etc) painted over it.   iv The outer part of the fluid would be extra-hardened with a surface treatment.   v A welded film.       

   A thin membrane over silicone option is very fragile. The welded film can be too strong and already pre-strained, so most of the applied force is lost in stretching the film and not translated into fluid pressure. The welded film configuration is shown in  FIGS. 47A to 47C . In  FIG. 47A , the input force F i  is lost to F s  used for stretching the film  756 . In  FIG. 47B , the film  756  initially bulges outwardly so that the plunger  752  acts to reduce the film stretching and more of F i  is used to raise the fluid pressure. However, as shown in  FIG. 47C , the film  756  bulges, or exhausts around the sides of the plunger  752  when F i  and therefore plunger displacement, are relatively high. In this case, considerations i to iv discussed above become relevant. 
   End Stop for Direction Coupled Sensor 
   If a force re-directing coupling is not used, and the sensor is directly coupled to the cartridge or the boot (see  FIG. 33 ), the issue of overload damage to the sensor becomes a problem. The Hokuriku chip (referred to above) breaks at a static deflection of ˜50 microns at an applied force of 4.5 kg. Most of this deflection is in the moulded casing  734 , not the chip  526 . For example, at 500 g the 10 micron deflection is composed of no more than 2 microns in the chip  526 , the remaining 8 microns being in the moulded casing. 
   Static Overload Protection 
   To protect the chip  526  from static overload an end-stop that is set nominally at say 1 kg (equating to 16 microns deflection) would have to engage the casing somewhere between at 10 microns and 21 microns. Fabricating an end-stop to this accuracy is difficult. Firstly the end-stop has to be referenced with respect to the back of the moulded casing  734 , as the internal deflection of the chip  526  relative to the package is small. Tests confirm that an end-stop referenced to the front face does not protect the chip  526  as effectively. 
     FIGS. 48 and 49  show end stop arrangements  738  referenced to the back of the moulded casing. Testing has shown these arrangements to be successful at protecting the chip at large static loads without excessive interference in normal operation. The flange  758  should engage the end-stop  738  at all points within a very short range of travel of the boot  702 . This complicates the manufactures but an excessive engagement range can exceed the full scale operating range of the sensor  526 . 
     FIG. 49  is a more detailed sketch of the sensor and end stop in the pen context. Contact pressure on the nib  418  is directly transmitted up the cartridge  414  to the ball  524  of the sensor  526 . The end of the cartridge  414  is in the boot  702  which is pre-loaded against the ball  524  by the pre-load spring  700 . The end stop  738  takes the form of a cup-shaped element with a stop surface  712  at its top for engagement with the boot  702 . An optional layer  760  of material with a known spring constant can be positioned behind the sensor  526  for additional breakage protection. 
   Dynamic Load Protection 
   Shock loading is a problem for directly coupled force sensor as well as fluid coupled sensors. The fluid or gel transmits the deceleration shock just as well as the direct mechanical coupling. However, the membranes in the fluid couplings tend to break rather than the chips. Either failure would be irreparable in the Netpage pen shown in the figures, as there are no serviceable parts other than the removable cartridge and battery pack. 
   Fortunately, the “volume magnification” effect of the fluid coupling helps because it magnifies the failure threshold displacement. 
   As above:
 
 X in/ X out= N×A out/ A in= N×gr= displacement magnification
         where:   N=volume ratio   gr=gearing ratio   assuming no secondary effects.   So for the displacement magnification=10 (say)   Xin @500 g=10×10=100 microns       

   An end-stop fitted to prevent displacements of this dimension is more easily manufactured than one configured to stop 10 micron displacements. From  FIG. 50 , the ordinary worker will appreciate that a 100 micron gap between the flange  758  of the plunger  732  and the stop surface  712  of the end stop  738  is far easier than a 10 micron gap. 
   Deformable Force Sensor Coupling 
   For direct coupling between the pen cartridge (or boot) and the force sensor, the sensor is mounted so that the plane of the chip  526  is perpendicular to the axis of the cartridge  414  (see  FIG. 33 ). This coupling is somewhat independent of whether the assembly includes the boot  702  over the end of the cartridge  414 . 
   The force sensor  526  deflects in response to an applied force F. As discussed above, the sensor may break when the applied force exceeds the elastic limits of the sensor. 
   As shown in  FIG. 33 , the force sensor  526  may be recessed to prevent excessive deflection. However, even if the force sensor is protected from an excessive static force, an impulse may still be sufficient to break the sensor, such as when the pen is dropped on its nib  418 . 
   To prevent an impulse from breaking the force sensor, an element may be inserted between the nib and the force sensor that can collapse or grossly deform when the input force is above a safe threshold. The collapsible element is designed to absorb the energy of an impulse originating at the nib by collapsing, thus preventing the impulse from propagating to the force sensor. 
   The collapsible element may be designed to collapse permanently or temporarily. 
   If the collapsible element is designed to collapse permanently, then it is most usefully incorporated into the cartridge, since the cartridge is already designed to be replaceable by the user when the ink supply is exhausted or the nib is damaged. 
     FIG. 51  shows a pen cartridge  414  with an integral collapsible element  766 . The collapsible element  766  consists of a set of struts  770  joining two parts  762  and  764  of the cartridge  414 . The struts  770  transmit axial forces throughout the full dynamic range of the force sensor without substantial deformation, but are designed to have a buckling threshold, as shown in  FIG. 52 , when exposed to an excessive force or damaging impulse F. The outer section  764  of the cartridge  414  is permanently displaced along the longitudinal axis  768  toward the inner section  762  proximate the force sensor. To assist crumpling the struts  770  are set at an angle to the axis of the cartridge. 
   The example shows two struts, but additional struts can be used. 
   In a felt-tip pen cartridge or similar, the nib  418  itself can be used as the collapsible element. In a ballpoint pen cartridge  414  the housing surrounding the nib  418  can also be used as the collapsible element  766 . 
     FIG. 53  shows a temporarily collapsible element  766  suitable for insertion between the pen cartridge  414  and the force sensor. The collapsible element  766  consists of a pair of rods  762  and  764  held in an elastomeric sleeve  772 , with both rods meeting at a slip surface  776  inclined to the longitudinal axis  768 . 
   The element  766  transmits axial forces throughout the full dynamic range of the force sensor without slipping, but is designed to slip, as shown in  FIG. 54 , when exposed to an excessive force or damaging impulse F. The stick friction at the slip surface  776  and the force of the elastomeric sleeve  772  keeps the rods  762  and  764  from slipping except when exposed to an excessive force. 
   When the excessive force is removed the elastomeric sleeve  772  aligns the rods to restore the un-collapsed state of the collapsible element. Locating features  774  on both rods  762  and  764  prevent the sleeve  772  from moving away from the slip surface  776 . 
     FIG. 55  is a sectioned perspective of the stick friction collapsible element  766 , and shows the elastomeric sleeve  772  surrounding the mated rods  762  and  764 . 
   Optical Force Sensor 
   The Hokuriku force sensor discussed above is piezoresistive. Sensors of this type present several challenges. They necessitate a precision-assembly and the required form factor is not currently available in a standard part. Hence to prototype the part and tool up for volume production is costly. Furthermore, its full-force deflection is small, requiring careful tolerancing to prevent breakage. 
   To avoid these problems, this aspect of the invention provides an optical force sensor that uses the attenuation of an optical coupling between a light-emitting diode (LED) and a photodetector. 
     FIG. 33  shows a typical configuration of a force sensor  526  coupled with a pen cartridge  414  within a pen body  416 . The cartridge  414  is pre-loaded (spring  700 ) against the force sensor to eliminate travel before force sensing commences and to eliminate the need for fine tolerancing of the coupling between the force sensor and the cartridge (or the boot  702  which grips the cartridge  414 ). 
     FIGS. 56A and 56B  shows the optical force sensor. It consists of a rigid but movable core held within a rigid housing  416 . The end of the housing  416  has an opening (on the left) through which the end of the core  786  protrudes and engages with the pen cartridge (or boot) as shown in  FIG. 33 . The other end of the core engages with a spring  784 . 
   The other end of the housing  416  also has an opening (on the right) through which the other end of the core  786  protrudes to ensure the core remains centred in the housing. 
   The centre of the core has an aperture  782  which faces a LED  780  on one side and a photodetector  778  on the other. 
   As the cartridge  414  pushes against the core  786 , the core  786  pushes against the spring  784  and compresses it in proportion to the force applied to the cartridge  414 . As the core  786  moves in proportion to the applied force, the aperture  782  moves relative to the LED  780  and photodetector  778 . The amount of light detected by the photodetector  778  is therefore a function of the position of the core  786  and hence of the applied force. 
   The shape of the aperture  782  and the shape of the housing surrounding the LED  780  and the photodetector  778  determine how much light strikes the photodetector  778  as a function of the position of the core  782 . The amount of light is also affected by the beam profile of the LED, and this can be modified by using a collimating lens or a diffuser in front of the LED. 
   The force sensor has a desired dynamic range. The aperture  782  is positioned relative to the LED  780  and photodetector  778  so that when zero external force is applied close to zero light strikes the photodetector  778 . The spring  784  is chosen so that when maximum external force is applied the core  786  is displaced so that the aperture  782  aligns with the LED  780  and photodetector  778  and maximum light strikes the photodetector  778 . The aperture  782  is made wide enough so that transverse movement of the core  786  in the housing  416  does not affect light transmission. 
   If the maximum external force is F max  and the length of the aperture is a, then the required stiffness k of the spring is:
 
 k=F   max   /a   (EQ 1)
 
   During use of the pen, axial cartridge movement up to 100 microns is acceptable, and this imposes an upper limit on the length of the aperture. Although this would seem to impose severe mechanical tolerancing requirements on the length of the movable core, the length of the chamber which houses the core, and the length of the spring, this is not necessarily so. When the force sensor is assembled, the core does not need to be in contact with the spring. Instead, the external spring which pre-loads the pen cartridge against the force sensor can also be relied upon to pre-load the core against the force sensor spring. However, the aperture in the core has to be long enough to accommodate the full range of movement of the core. 
   The force is sampled at a rate that is determined by the expected frequency content of the force signal, the maximum allowed latency in detecting pen-down and pen-up events, and any requirement to low-pass filter the force signal to remove noise. 
     FIG. 57  shows a high-level block diagram of the force sensor. A force sensor controller  582  uses a pulse-width modulator (PWM)  788  to drive the LED  780  with a desired intensity. It uses an analog-to-digital converter (ADC)  790  to sample the photodetector (PD)  778  signal which represents the force signal. The PD  778  output current is converted to a voltage before being sampled by the ADC  790 . It is amplified by a programmable-gain amplifier (PGA)  792  and is typically also low-pass filtered. 
   The force sensor controller  582  can use the PWM  788  to cycle the LED  780  through a set of different intensities, and combine successive ADC  790  samples to obtain a higher-precision signal. In the limit case the ADC  790  can be a simple comparator. 
   The force sensor controller  582  can also operate in multiple modes. For example, when in pen-up mode it can simply be looking for a pen-down transition, while in pen-down mode it can be sampling the force signal with higher precision. A simple pen-down detection mode can help minimise power consumption. 
   The force sensor can be calibrated in the factory to determine the transfer function from applied force to photodetector output, and this can be used to determine gain and offset settings for the PGA  792 . The force sensor can also measure its zero-force signal when capped, and utilise an otherwise fixed transfer function. 
   Force Sensor Dilatant Fluid Stop 
   As previously discussed, direct coupling between the pen cartridge (or boot) and the force sensor, requires the sensor to be mounted so that the plane of the chip  526  is perpendicular to the axis of the cartridge  414  (see  FIG. 33 ). This coupling is somewhat independent of whether the assembly includes the boot  702  over the end of the cartridge  414 . 
   The force sensor  526  deflects in response to an applied force F. As discussed above, the sensor may break when the applied force exceeds the elastic limits of the sensor. 
   As shown in  FIG. 33 , the force sensor  526  may be recessed to prevent excessive deflection. However, even if the force sensor is protected from an excessive static force, an impulse may still be sufficient to break the sensor, such as when the pen is dropped on its nib  418 . 
   A dilatant (or “shear thickening”) fluid is a non-Newtonian fluid whose viscosity increases with rate of shear. Dilatant fluids are typically dispersions of solid particles in a liquid at a critical particle concentration which allows the particles to touch. At a low shear rate the particles are able to slide past each other and the fluid behaves as a liquid. Above a critical shear rate friction between the particles predominates and the fluid behaves as a solid. 
   Although the best-known dilatant fluid consists of a cornstarch dispersion in water, industrial dilatant fluids typically consist of polymer dispersions in alcohol or water (see for example U.S. Pat. No. 5,037,880 to Schmidt et al). 
   To prevent damage to the force sensor  526  from an impulse, an additional stop  798  containing a dilatant fluid  796  can be inserted between the boot  702  (or cartridge  414 ) and the force sensor  526 , as shown in  FIG. 58 . The dilatant fluid  796  can be contained in a sack  798  with a flexible membrane, formed into an o-ring to allow direct contact between the boot  702  (or cartridge) and the force sensor  526  through the hole in the middle. 
   During normal operation of the pen, the dilatant fluid o-ring acts as a liquid and deforms in response to movement of the cartridge  414 , allowing normal forces to be transmitted from the cartridge to the force sensor  526 . When a damaging impulse occurs, the dilatant fluid o-ring effectively hardens in response to the high shear rate, preventing movement of the cartridge and thereby protecting the force sensor. 
   The thickness of the o-ring does not need to be finely toleranced because the preload spring  700  preloads the cartridge  414  against the force sensor  526  largely independently of the o-ring. However, the ball  524  of the force sensor  526  needs to be sufficiently proud of the force sensor recess, formed by the surrounding stop  712 , to accommodate at least some dilatant fluid  796  between the boot  702  and the stop  712  when the force sensor is preloaded. 
   If the boot is provided with a pin  718 , as shown in  FIG. 59 , then a thicker o-ring can be accommodated. There is more displacement of the cartridge during a normal pen down event, but a thicker o-ring affords greater protection for the sensor  526 . 
   Cap Assembly 
   The pen cap assembly  472  consists of four moldings as shown in  FIG. 25 . These moldings combine to produce a pen cap which can be stowed on the top end of the pen  456  during operation. When capped, it provides a switch to the electronics to signal the capped state (described in ‘Cap Detection Circuit’ section below). A conductive elastomeric molding  522  inside the cap  410  functions as the cap switch when it connects the inner  512  and outer  514  metal tubes to short circuit them (see  FIG. 26 ). The conductive elastomeric molding  522  is pushed into a base recess in the cap molding  410 . It is held captive by the clip molding  544  which is offered into the cap and snaps in place. A metallised trim molding  546  snaps onto the cap molding  410  to complete the assembly  472 . The cap molding  410  is line-of-draw and has an aquagraphic print applied to it. The trim  546  can be metallised in reflective silver or gold type finishes as well as coloured plastics if required. 
   Pen Feedback Systems—Vibratory 
   The pen  400  has two sensory feedback systems. The first system is haptic, in the form of a vibration motor  446 . In most instances this is the primary user feedback system as it is in direct contact with the users hand  408  and the ‘shaking’ can be instantly felt and not ignored or missed. 
   Pen Feedback Systems—Visual 
   The second system is a visual indication in the form of an indicator window  412  in the tube molding  466  on the top apex  420  of the pen  400 . This window aligns with a light pipe  448  in the chassis molding  416 , which transmits light from red and green indicator LEDs  452  on the main PCB  422 . The indicator window  412  is positioned so that it is not covered by the user&#39;s hand  408  and it is also unobstructed when the cap  410  is stowed on the top end  456  of the pen. 
   Optical Design 
   The pen incorporates a fixed-focus narrowband infrared imaging system. It utilises a camera with a short exposure time, small aperture, and bright synchronised illumination to capture sharp images unaffected by defocus blur or motion blur. 
                                                   TABLE 5               Optical Specifications                                    Magnification   ~0.225                Focal length of lens   6.0   mm           Viewing distance   30.5   mm           Total track length   41.0   mm           Aperture diameter   0.8   mm           Depth of field   ~/6.5   mm 7             Exposure time   200   us           Wavelength   810   nm 8             Image sensor size   140 × 140   pixels           Pixel size   10   um           Pitch range 9     ~15 to 45   deg           Roll range   ~30 to 30   deg           Yaw range   0 to 360   deg           Minimum sampling rate   2.25   pixels per macrodot           Maximum pen velocity   0.5   m/s                         7 Allowing 70 um blur radius             8 Illumination and filter             9 Pitch, roll and yaw are relative to the axis of the pen.            
Pen Optics and Design Overview
 
   Cross sections showing the pen optics are provided in  FIGS. 27A and 27B . An image of the Netpage tags printed on a surface  548  adjacent to the nib  418  is focused by a lens  488  onto the active region of an image sensor  490 . A small aperture  494  ensures the available depth of field accommodates the required pitch and roll ranges of the pen  400 . 
   First and second LEDs  434  and  436  brightly illuminate the surface  549  within the field of view  430 . The spectral emission peak of the LEDs is matched to the spectral absorption peak of the infrared ink used to print Netpage tags to maximise contrast in captured images of tags. The brightness of the LEDs is matched to the small aperture size and short exposure time required to minimise defocus and motion blur. 
   A longpass IR filter  432  suppresses the response of the image sensor  490  to any coloured graphics or text spatially coincident with imaged tags and any ambient illumination below the cut-off wavelength of the filter  432 . The transmission of the filter  432  is matched to the spectral absorption peak of the infrared ink to maximise contrast in captured images of tags. The filter also acts as a robust physical window, preventing contaminants from entering the optical assembly  470 . 
   The Imaging System 
   A ray trace of the optic path is shown in  FIG. 28 . The image sensor  490  is a CMOS image sensor with an active region of 140 pixels squared. Each pixel is 10 μm squared, with a fill factor of 93%. Turning to  FIG. 29 , the lens  488  is shown in detail. The dimensions are:
         D=3 mm   R 1 =3.593 mm   R 2 =15.0 mm   X=0.8246 mm   Y=1.0 mm   Z=0.25 mm       

   This gives a focal length of 6.15 mm and transfers the image from the object plane (tagged surface  548 ) to the image plane (image sensor  490 ) with the correct sampling frequency to successfully decode all images over the specified pitch, roll and yaw ranges. The lens  488  is biconvex, with the most curved surface facing the image sensor. The minimum imaging field of view  430  required to guarantee acquisition of an entire tag has a diameter of 39.6 s (s=spacing between macrodots in the tag pattern) allowing for arbitrary alignment between the surface coding and the field of view. Given a macrodot spacing, s, of 143 μm, this gives a required field of view of 5.7 mm. 
   The required paraxial magnification of the optical system is defined by the minimum spatial sampling frequency of 2.25 pixels per macrodot for the fully specified tilt range of the pen  400 , for the image sensor  490  of 10 μm pixels. Thus, the imaging system employs a paraxial magnification of 0.225, the ratio of the diameter of the inverted image (1.28 mm) at the image sensor to the diameter of the field of view (5.7 mm) at the object plane, on an image sensor  490  of minimum 128×128 pixels. The image sensor  490  however is 140×140 pixels, in order to accommodate manufacturing tolerances. This allows up to +/−120 μm (12 pixels in each direction in the plane of the image sensor) of misalignment between the optical axis and the image sensor axis without losing any of the information in the field of view. 
   The lens  488  is made from Poly-methyl-methacrylate (PMMA), typically used for injection moulded optical components. PMMA is scratch resistant, and has a refractive index of 1.49, with 90% transmission at 810 nm. The lens is biconvex to assist moulding precision and features a mounting surface to precisely mate the lens with the optical barrel molding  492 . 
   A 0.8 mm diameter aperture  494  is used to provide the depth of field requirements of the design. 
   The specified tilt range of the pen is 15.0 to 45.0 degree pitch, with a roll range of 30.0 to 30.0 degrees. Tilting the pen through its specified range moves the tilted object plane up to 6.3 mm away from the focal plane. The specified aperture thus provides a corresponding depth of field /6.5 mm, with an acceptable blur radius at the image sensor of 16 μm. 
   Due to the geometry of the pen design, the pen operates correctly over a pitch range of 33.0 to 45.0 degrees. 
   Referring to  FIG. 30 , the optical axis  550  is pitched 0.8 degrees away from the nib axis  552 . The optical axis and the nib axis converge toward the paper surface  548 . With the nib axis  552  perpendicular to the paper, the distance A between the edge of the field of view  430  closest to the nib axis and the nib axis itself is 1.2 mm. 
   The longpass IR filter  432  is made of CR-39, a lightweight thermoset plastic heavily resistant to abrasion and chemicals such as acetone. Because of these properties, the filter also serves as a window. The filter is 1.5 mm thick, with a refractive index of 1.50. Each filter may be easily cut from a large sheet using a CO 2  laser cutter. 
   The Illumination System 
   The tagged surface  548  is illuminated by a pair of 3 mm diameter LEDs  434  and  436 . The LEDs emit 810 nm radiation with a divergence half intensity, half angle of /15 degrees in a 35 nm spectral band (FWHM), each with a power of approximately 45 mW per steradian. 
   Pod Design and Assembly 
                             TABLE 2               Pod Mechanical Specifications                                Size   h63 × w43 × d46 mm       Mass   50 g       Operating   −10~+55 C.       Temperature       Operating Relative   10-90%       Humidity       Storage   ~20 to +60 C. worst case       Temperature       Storage Relative   5-95%       Humidity       Shock and Vibration   Drop from 1 m onto a hard surface without damage.           Mechanical shock 600G, 2.5 ms, 6 axis.       Serviceability   Replaceable jacket (part of customisation kit).           No internal user serviceable parts - the case is not           user openable.       Power   USB: 500 mA.           External power adapter: 600 mA at 5.5VDC.                    
Pod Design
 
   The pen  400  is supplied with a USB tethered pod, which provides power to the pen and a Bluetooth transceiver for data transfer between the pen and the pod. Referring to  FIG. 31 , the pod  450  is a modular design and is comprised of several line of draw moldings. The pod tower molding  554  holds the pen at a 15 degree from vertical angle, which is both ergonomic from a pen stowing and extraction perspective, but also is inherently stable. 
   Pod Assembly 
   The assembly sequence for the pod  450  is as follows: 
   An elastomeric stop molding  556  is push fitted into the pod tower molding  554  to provide a positive stop for the pen when inserted into the pod. 
   The pod tower molding  554  has two metal contacts  516  pushed onto location ribs under the stop. These contacts  516  protrude into a void  558  where the nib molding  428  is seated as shown in  FIG. 32 . When a pen is present, they contact the coaxial metal barrels  498  around the ink cartridge  414 . These act as conductors to provide charge to the battery  424 . 
   The pod PCB  560  is offered up into the pod tower molding  554  and snapped into place. Sprung charging contacts  562  on the metal contact piece  516  align with power pads on the pod PCB  560  during assembly. The underside of the pod PCB  450  includes several arrays of red, green and blue LEDs  564  which indicate several charging states from empty to full. Blue is the default ‘charging’ and ‘pod empty’ status color and they are transmitted via a translucent elastomeric light pipe  566  as an illuminated arc around the pod base molding  568 . 
   Despite a reasonable centre of gravity with a pen inserted, a cast weight  570  sits in the base molding  568  to increase stability and lessen the chance of the pod  450  falling over when knocked. The base molding  568  screws into the tower molding  554  to hold the weight  570 , light pipe  566  and PCB  560  after the tethered USB/power cable  572  is connected to the pod PCB  560 . 
   Personalisation 
   In line with the market differentiation ability of the pen, the pod includes a pod jacket molding  468 . This user removable molding is printed with the same aquagrahic transfer pattern as the tube and cap moldings of the pen it is supplied with as a kit. 
   Therefore the pattern of the pen, cap and pod are three items that strongly identify an individual users pen and pod to avoid confusion where there are multiple products in the same environment. They also allow this product to become a personal statement for the user. 
   The pod jacket molding  468  can be supplied as an aftermarket accessory in any number of patterns and images with the cap assembly  472  and the tube molding  466  as discussed earlier. 
   Electronics Design 
                             TABLE 3               Electrical Specifications                                Processor   ARM7 (Atmel AT91FR40162) running at 80           MHz with 256 kB SRAM and 2 MB flash           memory       Digital ink storage   5 hours of writing       capacity       Bluetooth Compliance   1.2       USB Compliance   1.1       Battery standby time   12 hours (cap off), &gt;4 weeks (cap on)       Battery writing time   4 hours of cursive writing (81% pen down,           assuming easy offload of digital ink)       Battery charging time   2 hours       Battery Life   Typically 300 charging cycles or 2 years           (whichever occurs first) to 80% of initial capacity.       Battery Capacity/Type   ~340 mAh at 3.7 V, Lithium-ion Polymer (LiPo)                    
Pen Electronics Block Diagram
 
     FIG. 60  is a block diagram of the pen electronics. The electronics design for the pen is based around five main sections. These are:
         the main ARM7 microprocessor  574 ,   the image sensor and image processor  576 ,   the Bluetooth communications module  578 ,   the power management unit IC (PMU)  580  and   the force sensor microprocessor  582 .
 
ARM7 Microprocessor
       
   The pen uses an Atmel AT91FR40162 microprocessor (see Atmel,  AT 91  ARM Thumb Microcontrollers—AT 91 FR 40162  Preliminary , http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at 80 MHz. The AT91FR40162 incorporates an ARM7 microprocessor, 256 kBytes of on-chip single wait state SRAM and 2 MBytes of external flash memory in a stack chip package. 
   This microprocessor  574  forms the core of the pen  400 . Its duties include:
         setting up the Jupiter image sensor  584 ,   decoding images of Netpage coded impressions, with assistance from the image processing features of the image sensor  584 , for inclusion in the digital ink stream along with force sensor data received from the force sensor microprocessor  582 ,   setting up the power management IC (PMU)  580 ,   compressing and sending digital ink via the Bluetooth communications module  578 , and   programming the force sensor microprocessor  582 .       

   The ARM7 microprocessor  574  runs from an 80 MHz oscillator. It communicates with the Jupiter image sensor  576  using a Universal Synchronous Receiver Transmitter (USRT)  586  with a 40 MHz clock. The ARM7  574  communicates with the Bluetooth module  578  using a Universal Asynchronous Receiver Transmitter (UART)  588  running at 115.2 kbaud. Communications to the PMU  580  and the Force Sensor microProcessor (FSP)  582  are performed using a Low Speed Serial bus (LSS)  590 . The LSS is implemented in software and uses two of the microprocessor&#39;s general purpose IOs. 
   The ARM7 microprocessor  574  is programmed via its JTAG port. This is done when the microprocessor is on the main PCB  422  by probing bare pads  592  (see  FIG. 63 ) on the PCB. 
   Jupiter Image Sensor 
   The Jupiter Image Sensor  584  (see U.S. Ser. No. 10/778,056 listed in the cross referenced documents above) contains a monochrome sensor array, an analogue to digital converter (ADC), a frame store buffer, a simple image processor and a phase lock loop (PLL). In the pen, Jupiter uses the USRT&#39;s clock line and its internal PLL to generate all its clocking requirements. Images captured by the sensor array are stored in the frame store buffer. These images are decoded by the ARM7 microprocessor  574  with help from the Callisto image processor contained in Jupiter. 
   Jupiter controls the strobing of two infrared LEDs  434  and  436  at the same time as its image array is exposed. One or other of these two infrared LEDs may be turned off while the image array is exposed to prevent specular reflection off the paper that can occur at certain angles. 
   Bluetooth Communications Module 
   The pen uses a CSR BlueCore4-External device (see CSR,  BlueCore 4- External Data Sheet rev c,  6 Sep. 2004) as the Bluetooth controller  578 . It requires an external 8 Mbit flash memory device  594  to hold its program code. The BlueCore4 meets the Bluetooth v1.2 specification and is compliant to v0.9 of the Enhanced Data Rate (EDR) specification which allows communication at up to 3 Mbps. 
   A 2.45 GHz chip antenna  486  is used on the pen for the Bluetooth communications. 
   The BlueCore4 is capable of forming a UART to USB bridge. This is used to allow USB communications via data/power socket  458  at the top of the pen  456 . 
   Alternatives to Bluetooth include wireless LAN and PAN standards such as IEEE 802.11 (Wi-Fi) (see IEEE, 802.11  Wireless Local Area Networks , http://grouper.ieee.org/groups/802/11/index.html), IEEE 802.15 (see IEEE, 802.15  Working Group for WPAN , http://grouper.ieee.org/groups/802/15/index.html), ZigBee (see ZigBee Alliance, http://www.zigbee.org), and WirelessUSB Cypress (see  WirelessUSB LR  2.4- GHz DSSS Radio SoC,  http://www.cypress.com/cfuploads/img/products/cywusb6935.pdf), as well as mobile standards such as GSM (see GSM Association, http://www.gsmworld.com/index.shtml), GPRS/EDGE,  GPRS Platform,  http://www.gsmworld.com/technology/gprs/index.shtml), CDMA (see CDMA Development Group, http://www.cdg.org/, and Qualcomm, http://www.qualcomm.com), and UMTS (see 3rd Generation Partnership Project (3GPP), http://www.3gpp.org). 
   Power Management Chip 
   The pen uses an Austria Microsystems AS3603 PMU  580  (see Austria Microsystems,  AS 3603  Multi - Standard Power Management Unit Data Sheet v 2.0). The PMU is used for battery management, voltage generation, power up reset generation and driving indicator LEDs and the vibrator motor. 
   The PMU  580  communicates with the ARM7 microprocessor  574  via the LSS bus  590 . 
   The PMU uses one of two sources for charging the battery  424 . These are the power from the power and USB jack  458  at the top of the pen  456  (see  FIG. 15 ) and the power from the pod  450  via the two conductive tubes  498  (see  FIG. 24 ). The PMU charges the pen&#39;s lithium polymer battery  424  using trickle current, constant current and constant voltage modes with little intervention required by the ARM7 microprocessor  574 . The PMU also includes a fuel gauge which is used by the ARM7 microprocessor to determine how much battery capacity is left. 
   The PMU  580  generates the following separate voltages:
         3.0V from an LDO for the ARM7 IO voltage and the Jupiter IO and pixel voltages.   3.0V from an LDO for the force sensor and force sensor filter and amplifier (3.0V for the force sensor microprocessor is generated from an off chip LDO since the PMU contains no LDOs that can be left powered on).   3.0V from an LDO for the BlueCore4 Bluetooth device.   1.8V from a buck converter for the ARM7 core voltage.   1.85V from an LDO for the Jupiter core voltage.   5.2V from a charge pump for the infrared LED drive voltage.       

   At power up or reset of the PMU, the ARM7 IO voltage and 1.8V core voltage are available. The other voltage sources need to be powered on via commands from the ARM7  574  via the LSS bus  590 . 
   Indicator LEDs  444  and the vibrator motor  446  are driven from current sink outputs of the PMU  580 . 
   The PMU  580  can be put into ultra low power mode via a command over the LSS bus  590 . This powers down all of its external voltage sources. The pen enters this ultra low power mode when its cap assembly  472  is on. 
   When the cap  472  is removed or there is an RTC wake-up alarm, the PMU  580  receives a power on signal  596  from the force sensor microprocessor  582  and initiates a reset cycle. This holds the ARM7 microprocessor  574  in a reset state until all voltages are stable. A reset cycle can also be initiated by the ARM7  574  via a LSS bus message or by a reset switch  598  which is located at the top of the pen next to the USB and power jack  458  (see  FIG. 15 ). 
   Force Sensor Subsystem 
   The force sensor subsystem comprises a custom Hokuriku force sensor  500  (based on Hokuriku,  HFD -500  Force Sensor , http://www.hdk.co.jp/pdf/eng/e1381AA.pdf), an amplifier and low pass filter  600  implemented using op-amps and a force sensor microprocessor  582 . 
   The pen uses a Silicon Laboratories C8051F330 as the force sensor microprocessor  582  (see Silicon Laboratories,  C 8051 F 330/1  MCU Data Sheet, rev  1.1). The C8051F330 is an 8051 microprocessor with on chip flash memory, 10 bit ADC and 10 bit DAC. It contains an internal 24.5 MHz oscillator and also uses an external 32.768 kHz tuning fork. 
   The Hokuriku force sensor  500  is a silicon piezoresistive bridge sensor. An op-amp stage  600  amplifies and low pass (anti-alias) filters the force sensor output. This signal is then sampled by the force sensor microprocessor  582  at 5 kHz. 
   Alternatives to piezoresistive force sensing include capacitive and inductive force sensing (see Wacom, “Variable capacity condenser and pointer”, U.S. Patent Application 20010038384, filed 8 Nov. 2001, and Wacom,  Technology , http://www.wacom-components.com/english/tech.asp). 
   The force sensor microprocessor  582  performs further (digital) filtering of the force signal and produces the force sensor values for the digital ink stream. A frame sync signal from the Jupiter image sensor  576  is used to trigger the generation of each force sample for the digital ink stream. The temperature is measured via the force sensor microprocessor&#39;s  582  on chip temperature sensor and this is used to compensate for the temperature dependence of the force sensor and amplifier. The offset of the force signal is dynamically controlled by input of the microprocessor&#39;s DAC output into the amplifier stage  600 . 
   The force sensor microprocessor  582  communicates with the ARM7 microprocessor  574  via the LSS bus  590 . 
   There are two separate interrupt lines from the force sensor microprocessor  582  to the ARM7 microprocessor  574 . One is used to indicate that a force sensor sample is ready for reading and the other to indicate that a pen down/up event has occurred. 
   The force sensor microprocessor flash memory is programmed in-circuit by the ARM7 microprocessor  574 . 
   The force sensor microprocessor  582  also provides the real time clock functionality for the pen  400 . The RTC function is performed in one of the microprocessor&#39;s counter timers and runs from the external 32.768 kHz tuning fork. As a result, the force sensor microprocessor needs to remain on when the cap  472  is on and the ARM7  574  is powered down. Hence the force sensor microprocessor  582  uses a low power LDO separate from the PMU  580  as its power source. The real time clock functionality includes an interrupt which can be programmed to power up the ARM7  574 . 
   The cap switch  602  is monitored by the force sensor microprocessor  582 . When the cap assembly  472  is taken off (or there is a real time clock interrupt), the force sensor microprocessor  582  starts up the ARM7  572  by initiating a power on and reset cycle in the PMU  580 . 
   Pen Design 
   Electronics PCBs and Cables 
   There are two PCBs in the pen, the main PCB  422  ( FIG. 63 ) and the flex PCB  496  ( FIG. 19 ). The other separate components in the design are the battery  424 , the force sensor  500 , the vibrator motor  446  and the conductive tubes  498  ( FIG. 16 ) which function as the power connector to the pod  450  ( FIG. 31 ). 
   Main PCB 
     FIGS. 63 and 64  show top and bottom perspectives respectively of the main PCB  422 . The main PCB  422  is a 4-layer FR4 1.0 mm thick PCB with minimum trace width and separation of 100 microns. Via specification is 0.2 mm hole size in a 0.4 mm pad. The main PCB  422  is a rectangular board with dimensions 105 mm×11 mm. 
   The major components which are soldered to the main PCB are the Atmel ARM7 microprocessor  574 , the AMS PMU  580 , the Silicon Labs force sensor microprocessor  582 , the op-amps for force sensor conditioning amplifier  600  and the CSR Bluetooth chip  578  and its flash memory  594 , antenna  486  and shielding can  612 . 
   The force sensor  500 , the vibrator motor  446  and the coaxial conductive tubes  498  use sprung contacts to connect to pads on the main PCB  422 . All of these items are pushed down onto the main PCB  422  by the chassis molding  416  of the pen. 
   There are three connectors soldered onto the main PCB  422 ; the flex PCB connector  612 , the power and USB jack  458  at the top of the pen  456 , and the battery cable harness connector  616 . The cable harness to the battery is the only wired cable inside the pen. 
   Also soldered onto the main PCB  422  is the reset switch  598 . This is in the recess  464  shown in  FIG. 5 . 
   Flex PCB 
   The Jupiter image sensor  576  is mounted on the flex PCB  496  as shown in  FIG. 19 . As the critical positioning tolerance in the pen is between the optics  426  and the image sensor  490 , the flex PCB  496  allows the optical barrel molding  492  to be easily aligned to the image sensor  490 . By having a flexible connection between the image sensor and the main PCB  422 , the positioning tolerance of the main PCB is not critical for the correct alignment of the optics  426 . 
   The image sensor  490 , the two infrared LEDs  434  and  436 , and five discrete bypass capacitors  502  are mounted onto the flex PCB  496 . The flex is a 2-layer polyimide PCB, nominally 75 microns thick. The PCB is specified as flex on install only, as it is not required to move after assembly of the pen. Stiffener  612  is placed behind the discrete components  502  and behind the image sensor  490  in order to keep these sections of the PCB flat. Stiffener is also placed at the connection pads  620  to make it the correct thickness for the connector  614  the main PCB  422  (see  FIG. 28 ). The PCB design has been optimised for panel layout during manufacture by keeping it roughly rectangular in overall shape. 
   The flex PCB  496  extends from the main PCB, widening around the image sensor  490  and then has two arms  622  and  624  that travel alongside the optical barrel  492  to the two infrared LEDs  434  and  436 . These are soldered directly onto the arms  622  and  624  of flex PCB. The total length of the flex PCB is 41.5 mm and at its widest point it is 9.5 mm. 
   The image sensor  490  is mounted onto the flex PCB  496  using a chip on flex PCB (COF) approach. In this technology, the bare Jupiter die  628  is glued onto the flex PCB  496  and the pads on the die are wire-bonded onto target pads on the flex PCB. These target pads are located beside the die. The wire-bonds are then encapsulated to prevent corrosion. Two non-plated holes  626  in the flex PCB next to the die  628  are used to align the PCB to the optical barrel  492 . The optical barrel is then glued in place to provide a seal around the image sensor  470 . The horizontal positional tolerance between the centre of the optical path and the centre of the imaging area on the Jupiter die  628  is +/−50 microns. The vertical tolerance due to the thickness of the die, the thickness of the glue layer and the alignment of the optical barrel  492  to the front of the flex PCB  496  is +/−5 microns. In order to fit in the confined space at the front of the pen, the Jupiter die  628  is designed so that the pads required for connection in the Netpage pen are placed down opposite sides of the die. 
   Pod and External Cables 
   There are three main functions that are required by the pod and external cabling. They are:
         provide a charging voltage so that the pen can recharge its battery,   provide a relay mechanism for transferring stored digital ink to the Netpage server via its Bluetooth/USB adapter and   provide a relay mechanism for downloading new program code to the pen via its Bluetooth/USB adapter.
 
Pod
       

   Again referring to  FIGS. 31 and 32 , when the pen  400  is inserted into the pod  450 , power is provided by way of two sprung contacts  516  in the pod which connect to the two coaxial conductive tubes  498  that hold the ink cartridge tube  536  in the pen. The power for the pod  450  and the pen  400  charging is provided by USB bus power. 
   The pod has a tethered cable  572  which ends in two connectors. One is a USB “A” plug. The other is a 4-way jack socket. This 4-way jack socket is the same one present at the top of the pen (see socket  458  in  FIG. 15 ). When the 4-way jack is inserted into the pod&#39;s cable, it provides power for the pod and to the pen for charging. Otherwise, the power for the pod and the pen charging is provided by the USB bus power. 
   Three indicator LEDs  452  are present in the pod. They indicate the status of pen charging and communications. 
   Pod PCB 
   The pod PCB  560  contains a CSR BlueCore4-External device. This is the same type of Bluetooth device as used in the pen  400 . The BlueCore4 device functions as a USB to Bluetooth bridge. 
   Cabling 
   Three cables are provided with the pen. The first cable  572  is tethered to the pod. At the other end of the cable is a USB A connector and a 4-way jack socket. There are six wires going into the pod, the four USB wires and two from the 4-way jack socket. 
   The second cable is a USB cable  462  ( FIG. 15 ) with a USB A connector on one end and a 4-way jack on the other end. The 4-way jack can be connected to either the pod or the top of the pen. 
   The third cable is a plug pack power cable (not shown) which plugs into a power outlet at one end and has a 4-way jack on the other end. This 4-way jack can be connected to either the pod  450  or the top of the pen  456 . 
   Connection Options 
     FIG. 61  shows the main charging and connection options for the pen and pod:
         Option  1  shows a USB connection from a host  630  to the pod  450 . The pen  400  is in the pod  450 . The pod  450  and the pen  400  communicate via Bluetooth. The pod is powered by the USB bus power. The pen is charged from the USB bus power. As a result the maximum USB power of 500 mA must be available in order to charge the pen.   Option  2  shows a USB connection from the host  630  to the pod  450  and a plug pack  632  attached to the pod cable  572 . The pen  400  is in the pod  450 . The pod and the pen communicate via Bluetooth. The pod is powered by the plug pack. The pen is charged from the plug pack power.   Option  3  shows a USB connection from the host  630  to the pod  450  and a plug pack  632  attached to the pen  400 . The pen  400  is in the pod  450 . The pod and the pen communicate via Bluetooth. The pod is powered by the USB bus power. The pen is charged from the plug pack power.   Option  4  shows a plug pack  632  attached to the pod cable  572 . The pen  400  is in the pod  450 . There is no communication possible between the pod and the pen. The pod is powered by the plug pack. The pen is charged from the plug pack power.   Option  5  shows a USB connection from the host  630  to the pen  400 . The pen  400  is not in the pod  450 . The host  630  and the pen  400  communicate via USB, allowing a wired, non-RF communication link. The pen is charged from the USB bus power. As a result the maximum USB power of 500 mA must be available in order to charge the pen.   Option  6  shows the plug pack  632  attached to the pen  400 . The pen  400  is not in the pod  450 . The pen is charged from the plug pack power.   Other connection options are not shown. However, it should be kept in mind that the pod is powered via its 4-way jack connector (and not from the USB bus power) if there is a connector in this jack. Also, the pen is powered from its 4-way jack (and not from its pod connection) when there is a connector in this jack.
 
Battery and Power Consumption
       
   Referring to  FIG. 68 , the pen  400  contains a Lithium polymer battery  424  with a nominal capacity of 340 mAh. It&#39;s dimensions are 90.5 mm long×12 mm wide×4.5 mm thick. 
   Based on the pen design, Table 4 shows the current requirements for various pen and Bluetooth states. 
                                               TABLE 4                   Battery drain currents for all Pen states.                    Total               mA @       State   Notes   VBatt 1                      Pen Capped   Pen is off   0.110       Pen Active   Pen Down   92.7       Pen Hover-1   Pen up, trying to decoded tags   31.7       Pen Hover-2   Pen up, decoding tags   62.9       Pen Idle   Pen up, not trying to decode tags   28.8       Bluetooth Not Connected   Bluetooth IC off   0.0       Bluetooth Connection   Bluetooth connected in low power,   0.6       Timeout   no digital ink to download       Bluetooth Connected   Bluetooth connected in low power   4.1       (Sniff)   Sniff state       Bluetooth Connected   Bluetooth connected in high power   50.1       (Active)   Active state       Bluetooth Connecting   Bluetooth trying to connect Network   15.1           Access Point                 1 Sum of all current drains at battery. The Bluetooth currents can be concurrent with and additive to the Pen-state currents.            
Pen Usage Scenarios
 
   Some general usage scenarios are summarised here, showing the energy requirements needed to fulfil these scenarios. 
   Worst Case Scenario 
   Summary: The pen is used intensively for 4 hours (cursive writing) and will sit capped for one month (31 days), trying to offload stored digital ink. 
   The energy requirement for this scenario is 968 mAh. The nominal 340 mAh hour battery would achieve 35% of energy requirement for this scenario. 
   Single Working Week Case Scenario 
   Summary: The pen is used for cursive writing for a total of one hour a day for five days. and is capped for the remaining time. Total time for scenario is seven days. 
   The energy requirement for this scenario is 456 mAh. The nominal 340 mAh hour battery would achieve 75% of energy requirement for this scenario. 
   Single Working Week Not Capped During Working Hours Case Scenario 
   Summary: The pen is used for cursive writing for a total of one hour a day for five days. and is capped for the remaining time. Total time for scenario is seven days. 
   The energy requirement for this scenario is 1561 mAh. The nominal 340 mAh hour battery would achieve 22% of energy requirement for this scenario. 
   Software Design 
   Netpage Pen Software Overview 
   The Netpage pen software comprises that software running on microprocessors in the Netpage pen  400  and Netpage pod  450 . 
   The pen contains a number of microprocessors, as detailed in the Electronics Design section described above. The Netpage pen software includes software running on the Atmel ARM7 CPU  574  (hereafter CPU), the Force Sensor microprocessor  582 , and also software running in the VM on the CSR BlueCore Bluetooth module  578  (hereafter pen BlueCore). Each of these processors has an associated flash memory which stores the processor specific software, together with settings and other persistent data. The pen BlueCore  578  also runs firmware supplied by the module manufacturer, and this firmware is not considered a part of the Netpage pen software. 
   The pod  450  contains a CSR BlueCore Bluetooth module (hereafter pod BlueCore). The Netpage pen software also includes software running in the VM on the pod BlueCore. 
   As the Netpage pen  400  traverses a Netpage tagged surface  548 , a stream of correlated position and force samples are produced (see Netpage Overview above). This stream is referred to as DInk. Note that DInk may include samples with zero force (so called “Hover DInk”) produced when the Netpage pen is in proximity to, but not marking, a Netpage tagged surface. 
   The CPU component of the Netpage pen software is responsible for DInk capture, tag image processing and decoding (in conjunction with the Jupiter image sensor  576 ), storage and offload management, host communications, user feedback and software upgrade. It includes an operating system (RTOS) and relevant hardware drivers. In addition, it provides a manufacturing and maintenance mode for calibration, configuration or detailed (non-field) fault diagnosis. The Force Sensor microprocessor  582  component of the Netpage pen software is responsible for filtering and preparing force samples for the main CPU. The pen BlueCore VM software is responsible for bridging the CPU UART  588  interface to USB when the pen is operating in tethered mode. The pen BlueCore VM software is not used when the pen is operating in Bluetooth mode. 
   The pod BlueCore VM software is responsible for sensing when the pod  450  is charging a pen  400 , controlling the pod LEDs  452  appropriately, and communicating with the host PC via USB. 
   A more detailed description of the software modules is set out below. 
   The Netpage pen software is field upgradable, with the exception of the initial boot loader. The field upgradable portion does include the software running on the Force Sensor microprocessor  582 . Software upgrades are delivered to the pen via its normal communication mechanisms (Bluetooth or USB). After being received and validated, a new software image will be installed on the next shutdown/startup cycle when the pen contains no DInk pending offload. 
   Netpage System Overview 
   The Netpage pen software is designed to operate in conjunction with a larger software system, comprising Netpage relays and Netpage servers. The following is a brief overview of these systems in relation to the Netpage pen—a detailed discussion of the software for these systems and the specification of its interface to Netpage pen software is set out in the cross referenced documents. 
   Netpage relays are responsible for receiving DInk from pens, and transmitting that DInk to Netpage servers or local applications. The relay is a trusted service running on a device trusted by the pen (paired in Bluetooth terminology). The relay provides wide area networking services, bridging the gap between the pen and DInk consumers (such as Netpage servers or other applications). The primary relay device will be a desktop/laptop computer equipped with a Netpage pod. Bluetooth equipped mobile phones and PDAs can also be used as relays. Relays provide the pen with access to WAN services by bridging the Bluetooth connection to GPRS, WiFi or traditional wired LANs. 
   Netpage servers persist DInk permanently, and provide both application services for DInk based applications (such as handwriting recognition and form completion), and database functionality for persisted DInk (such as search, retrieval and reprinting). 
   Local applications may receive the DInk stream from the Netpage relay and use it for application specific purposes (such as for pointer replacement in image creation/manipulation applications). 
   Internal Design 
   The Netpage pen software is divided into a number of major modules:
         Image Processing   DInk storage and offload management   Host Communications   User Feedback   Power Management   Software Upgrade   Real Time Operating System   Hardware Drivers   Manufacturing and Maintenance mode   Force Sensor Microprocessor software   Pen BlueCore VM software   Pod BlueCore VM software       

   The remainder of this section gives a brief overview of these major software modules. 
   Image Processing 
   The position information in the DInk stream produced by traversing a Netpage tagged surface is produced by performing an analysis of tagged images captured by the Jupiter Image Sensor  576 . 
   The Image Processing module is responsible for analysing images captured by Jupiter, identifying and decoding tags, estimating the pose of the pen, and combining this information to obtain position samples. 
   DInk Storage and Offload Management 
   Any DInk which corresponds to physical marking of a Netpage tagged surface (e.g. excluding Hover DInk) must be reliably and transactionally recorded by the Netpage system to allow for accurate reproduction of the Netpage tagged surface. Ensuring such DInk is recorded is the responsibility of the DInk storage and offload management software. It persists DInk in flash memory on the Netpage pen, and arranges for offload of DInk to a Netpage server via a Netpage relay. This offload process is transactional—the pen software maintains its record of DInk until it can guarantee that DInk has been received and persisted by a Netpage server. 
   DInk may be streamed in real time to applications requiring real time response to DInk (for example applications which use the pen as a replacement for a mouse or table pointer, such as graphics editing applications). This may be normal DInk or Hover DInk (for applications supporting hover), and the ability of the Netpage pen software to stream DInk to such applications is orthogonal to the storage and offload requirements for persistent DInk. 
   Host Communications 
   The Netpage pen software communicates with the Netpage relay either through wireless Bluetooth communication, or through a wired USB connection. Bluetooth connectivity is provided by the pen BlueCore. USB connectivity is provided by using the Bluetooth module in “pass through” mode. 
   The Communications module of the software is responsible for reliably transmitting DInk from the DInk storage and offload management module to the relay. It also provides management functionality such as maintaining a persistent list of known, trusted relays, and allows pairing with devices according to user specification. The communications module includes third party software (namely the ABCSP stack, see CSR,  ABCSP Overview, AN 11) provided by CSR for communication with the pen BlueCore. Bluetooth communication is only performed with Bluetooth paired devices, and uses the Bluetooth encryption facilities to secure these communications. 
   User Feedback 
   The Netpage pen provides two LEDs (red and green) and a vibration motor for user feedback. The user feedback software module is responsible for converting signals from other software modules into user feedback using the provided mechanisms. 
   Power Management 
   The Netpage pen has a limited power budget, and its design allows for dynamic power saving in a number of ways. For example, the CPU can disable peripherals when they are not in use to save power, and the pen BlueCore can be placed into a deep sleep mode or powered down when it is not required. The CPU itself can be powered down when the pen is not performing higher functions. Indeed, the only always-on components are the Force Sensor microprocessor  582  and Power Management Chip  580  which can power on the CPU in response to external stimuli. The Power Management module  580  is responsible for analysing the current pen state and optimizing the power usage by switching off un-needed peripherals and other components as required. That is, this module intelligently manages the facilities offered by the Power Management module to provide optimal power usage given the required pen functionality. 
   Software Upgrade 
   The Netpage pen software is field upgradable, obtaining new software images via its Bluetooth or USB connections. The Software Upgrade module is responsible for managing the download of complete images via the Communications module, validating these images against included checksums, and arranging for the pen to boot from a revised image when it has been validated. 
   The Software Upgrade process happens largely concurrently with normal pen behaviour. The download of new images can happen concurrently with normal pen operation and DInk offload. However, the actual switch to boot from a new software image is only performed when no outstanding DInk remains to be offloaded. This simplifies management of the internal DInk formats, allowing them to be upgraded as necessary in new software loads. Existing pairing arrangements with relays are expected to survive software upgrade, although under some circumstances it may be necessary to repeat pairing operations. 
   It should also be noted that small parts of the Netpage pen software, such as basic boot logic, are not field upgradable. These parts of the software are minimal and tightly controlled. 
   Note that the Software Upgrade module also manages software images for the Force Sensor microprocessor. Images for the latter form a part of the Netpage pen software load, and the Software Upgrade module reprograms the Force Sensor microprocessor in the field when a new image contains revisions to the Force Sensor microprocessor software. 
   Real Time Operating System 
   The Netpage pen software includes a Real Time Operating System (RTOS) for efficient management of CPU resources. This allows optimal handling of concurrent DInk capture, persistence, and offload despite the latencies involved in image capture, flash manipulation, and communication resources. 
   The RTOS for the Netpage pen software is the uC/OS II RTOS from Micrium Systems (see Labrosse, J. L.,  MicroC OS II: The Real Time Kernel,  2 nd Edition , CMP Books, ISBN 1578201039). This part of the Netpage pen software is comprised largely of third party code supplied by Micrium, tailored and customized for the needs of the pen. 
   Hardware Drivers 
   The Netpage pen software includes hardware drivers for all peripherals (both internal to the CPU and external to it) required for operation of the Netpage pen  400 . This includes USRT  586 , UART  588  and LSS  590  drivers for external bus communication, as well as higher level drivers for managing the Jupiter Image Sensor  576 , the pen BlueCore  578 , the Force Sensor microprocessor  582 , the Power Management IC  580 , and other internal systems. 
   Manufacturing and Maintenance Mode 
   The Netpage pen  400  may be put into a special manufacturing and maintenance mode for factory calibration or detailed non-field failure analysis. A deployed pen will never enter manufacturing and maintenance mode. It is a configuration, diagnostic and rectification mode that is only expected to be used by Silverbrook engineers under controlled conditions. The mechanism for placing the Netpage pen software into maintenance mode is not described here. 
   Force Sensor Microprocessor Software 
   The Force Sensor microprocessor  582  is an independent CPU tasked with filtering and resampling the force data obtained from the Force Sensor  500  proper to produce a stream of force samples to be included into the DInk stream as recorded by the pen. It is also responsible for initiating a wakeup of the CPU  574  in response to a pen down, uncap, or timer event, in the case that the CPU has been switched off for power saving purposes. 
   Pen BlueCore VM Software 
   The pen BlueCore is capable of running a small amount of software in a virtual machine (VM). Such VM software is highly resource limited, but can access the Bluetooth functionality, the I/O ports, and a small number of GPIO pins on the pen BlueCore. A small part of the Netpage pen software will run on the pen BlueCore in order to manage bridging the CPU UART to the USB connection provided by the pen BlueCore. 
   Pod BlueCore VM Software 
   The Netpage pod  450  contains a CSR BlueCore Bluetooth module, but no general purpose microprocessor. The pod BlueCore runs Netpage pen software in its VM. This software is responsible for sensing when the pod  450  is charging a pen  400 , controlling the pod LEDs  452  to indicate charging and communications status, and managing the USB communication link between the pod BlueCore and the host PC. Note that BlueCore provides a split stack model for the Bluetooth network stack, and the majority of the Bluetooth network stack will in fact be running on the host PC (where it has considerably greater access to resources). 
   Pen Assembly Sequence 
   The various sub-assemblies and components are manually inserted into the pen chassis molding  416  (see  FIG. 65 ). There are no special tools required to insert any of the assemblies as there is extensive use of snap fits and bumps on moldings for location. The only assembly tool needed is a cold staking procedure required after a testing to seal the pen assembly. 
   The assembly sequence for the pen is as follows: 
   Pen Chassis Assembly 
   The elastomeric end cap  460  is fed through an aperture  634  at the end of the chassis molding  416  and a tab  636  pulled through to secure it in place. 
   Optics Assembly 
   The optics assembly sequence is as follows:
         The lens is offered up to the aperture stop in the barrel and adhered in place.   The infrared filter is pushed into place in the front of the barrel molding.   The flex with image sensor is offered up to the top of the barrel molding and accurately located onto two pins.   Epoxy is applied around the base of the barrel molding to bond the flex into place and seal the image sensor from light and particulate contaminants.
 
Optics Assembly Insertion
       

   As shown in  FIG. 66A , the optics assembly  470  with the unfolded flex PCB  496  protruding is inserted into the chassis moulding  416  and snapped into place. The IR LEDs  434  and  436  are then manipulated into cradles  638  either side of the barrel moulding  492  as shown in  FIG. 66B . 
   Force Sensing Assembly Insertion 
   As shown in  FIGS. 67A and 67B , the force sensing assembly  474  is fed through between the chassis moulding  416  and the optical barrel moulding  492 . The assembly  474  is pivoted down and the force sensor is secured in the correct orientation into the chassis moulding between ribs  640  and a support detail  642 . 
   The vibration motor  446  with elastomeric boot  644  is assembled into an aperture in the chassis  416 . The boot  644  has negative draft on the support detail  642 , which secures the motor  446  into the chassis  416  and orients it correctly. 
   A light pipe moulding  448  is placed into the chassis moulding  416  and is a force fit. 
   PCB and Battery Insertion 
   The end of the optics flex PCB  496  is offered into the flex connector  614  on the main PCB  422  and secured. 
   The main PCB  422  and LiPo battery  424  are then connected together as the socket is on the upper side of the PCB  422  and is not accessible when the board is in the chassis moulding  416 . The battery  424  has foam pads to protect the components on the lower side of the PCB and to inhibit movement of the battery when it is fully assembled. 
   Referring to  FIG. 69 , the main PCB  422  and battery  424  can now be swung into place in the chassis moulding  416 , with care being taken not to unduly stress the flex PCB  496 . 
     FIGS. 70A and 70B  shows a cold stake tool  646  sealing a cold stake pin  648  to an aperture  650  the base moulding  528 . The cold stake  648  is used to help locate the PCB  422  into the chassis moulding  416  and with gentle pressure the walls of the chassis  416  expand enough to allow snap fits to engage with the PCB and hold it securely. The PCB can still be extracted by flexing the chassis walls in the same manner if necessary. The battery can be tacked in place with adhesive tape if required. 
   The base moulding  528  is hinged onto the chassis moulding  416  and is fully located when the cold stake  648  appears in the aperture  650 . 
   Testing and Staking 
   At this point the assembly is complete enough to perform an optical and electronic diagnostic test. If any problems occur, the assembly can easily be stripped down again. 
   Once approved, a cold stake tool  646  is applied to the pin  648  from the chassis molding  416  swaging it over to hold the base molding  528  captive ( FIG. 70B ). This prevents any user access to internal parts. 
   Product Label 
     FIG. 71  shows a product label  652  being applied to the base molding  416 , which covers the cold stake  648 . This label carries all necessary product information for this class of digital mobile product. It is exposed when the customisable tube molding  466  (see  FIG. 73 ) is removed by the user. 
   NIB Molding Insertion 
   As shown in  FIG. 72 , the nib molding  428  is offered up to the pen assembly and is permanently snapped into place against the chassis  416  and the base moldings  528  to form a sealed pen unit. 
   Tube Molding Assembly 
   As shown in  FIG. 73 , the tube molding  466  is slid over the pen assembly. The tube  466  is a transparent molding drafted from the centre to allow for thin walls. An aquagraphic print is applied to the surface with a mask used to retain a window  412 , which looks through to the light pipe  448  in the pen during use. A location detail  656  on the chassis molding  416  provides positive feedback when the molding is pushed home. The user can remove the tube molding by holding the nib end and pulling without gaining access to the pen assembly. 
   Cap Insertion 
   The cap assembly is fitted onto the pen to complete the product as shown in  FIG. 74 . 
   Netpage Pen Major Power States 
     FIG. 75  shows the various power states that the pen can adopt, as well as the pen functions during those power states. 
   Capped 
   In the Capped state  656 , the Pen does not perform any capture cycles. 
   Corresponding Pen Bluetooth states are Connected, Connecting, Connection Timeout or Not Connected. 
   Hover 1   
   In the Hover 1  state  658 , the Pen is performing very low frequency capture cycles (of the order of 1 capture cycle per second). Each capture cycle is tested for a valid decode, which indicates that the user is attempting to use the Pen in hover mode. 
   Valid Pen Bluetooth states are Connected or Connecting. 
   Hover 2   
   In the Hover 2  state  660 , the Pen is performing capture cycles of a lower frequency than in the Active state  662  (of the order 50 capture cycles per second). Each capture cycle is tested for a valid decode, which indicates that the user is continuing to use the Pen in hover mode. After a certain number of failed decodes, the Pen is no longer considered to be in hover mode. 
   Valid Pen Bluetooth states are Connected or Connecting. 
   Idle 
   In the Idle state  664 , the Pen is not performing any capture cycles, however, the Pen is active in as much as it is able to start the first of a number of capture cycles within 5 ms of a pen down event. 
   Valid Pen Bluetooth states are Connected or Connecting. 
   Active 
   In the Active state  662 , the Pen is performing capture cycles at full rate (100 capture cycles per second). 
   Valid Pen Bluetooth states are Connected or Connecting. 
   Netpage Pen Bluetooth States 
     FIG. 76  shows Netpage Pen power states that are related to the Bluetooth wireless communications subsystem in order to respond to digital ink offload requirements. Additionally, the Pen can accept connections from devices in order to establish a Bluetooth Pairing. 
   Each of the possible Pen Bluetooth related states are described in the following sections. 
   Connected 
   In the Connected state  666  the primary task for the Pen is to offload any digital ink that may be present within Pen storage, or to stream digital ink as it is being captured. Whilst in the Connected state it should also be possible for other devices to discover and connect to the pen for the purposes of Bluetooth Pairing. 
   In order to reduce power consumption whilst connected, it is desirable to take advantage of the relatively low bandwidth requirements of digital ink transmission and periodically enter a Bluetooth low power mode. A useful low power mode will typically be Sniff mode, wherein the periodic Bluetooth activity required of the Pen is reduced based on the Sniff interval, with the Sniff interval being determined by the current bandwidth requirements of digital ink transmission. 
   Connecting 
   Whilst in the Connecting state  668 , the Pen attempts to establish a connection to one of a number of known NAPs (Network Access Points) either to offload digital ink stored within Pen memory, or in anticipation of a sequence of capture cycles. 
   Upon entry into the Connecting state  668 , the Pen attempts an Inquiry/Page of each device in round-robin fashion with a relatively high frequency. If the connection is unsuccessful, the frequency of Inquiry/Page is reduced successively in a number of steps in order to reduce overall power consumption. 
   An Inquiry can last for 10.24 s and is repeated at a random interval. Initially the Inquiry may be repeated on average at 5 s intervals for the first 3 attempts, followed by 30 s for the next 5 attempts and then 5 minute intervals for the next 10 attempts and 10 minute invervals for subsequent attempts. 
   Connection Timeout 
   In the Connection Timeout state  670 , the Pen maintains the current Bluetooth connection by entering a Bluetooth low power Sniff state with relatively long sniff interval (e.g. 2.56 seconds) for a period of at least 2 minutes before disconnecting. Re-establishment of the connection is not attempted, should the connection be dropped before 2 minutes have elapsed. 
   Not Connected 
   In the Not Connected state  672 , the Pen does not hold any digital ink in its internal memory, and is capped. There is no Bluetooth activity, and no Bluetooth connection exists. 
   Discoverable and Not Discoverable 
   The Pen is only discoverable  674  during the major states of Hover 1   658  and Idle  664 . The Pen periodically enters the inquiry scan and page scan states whilst in Hover 1   658  or Idle  664 , in order to respond to connection requests from other devices. 
   Cap Detection Circuit 
   Referring once again to  FIG. 26 , a cap detection circuit diagram is shown. As discussed above, the presence or absence of the cap assembly  472  on the nib molding  428  can directly determine the Pen power state and the Bluetooth state. The cap assembly  472  serves the dual purposes of protecting the nib  418  and the imaging optics  426  when the pen  400  is not in use, and signalling, via its removal or replacement, the pen to leave or enter a power-preserving state. 
   As described in the ‘Pod Assembly’ section above, the pen  400  has coaxial conductive tubes  498  that provide a set of external contacts—power contacts  678  and data contacts  680 . These mate with contacts  516  in the pod  450  to provide the pen with charging power and a USB connection. When placed over the nib molding  428 , the conductive elastomeric molding  522  short-circuits the pen&#39;s power contacts  678  to signal the presence of the cap. 
   The pen has three capping states:
         cap on   cap off, not in pod   cap off, in pod       

   In the cap on state, the CAP_ON signal  682  is high. The pen will be powered off, subject to other pending activities such as digital ink offload, as described above in the NetPage Pen Bluetooth States section. 
   In the cap off, not in pod state, the CAP_ON signal  682  is low. The pen will be powered on. 
   In the cap off, in pod state, the CAP_ON signal  682  is low. The pen will be powered on. 
   The CAP_ON signal  682  triggers transitions to and from the Capped state  656 , as described in the NetPage Pen Power States section above, via the power management unit  580  and the Amtel ARM7 microprocessor  574 . (see Pen Design section above). 
   The battery charger can use the VCHG signal  684  to charge the battery. The VCHG signal  684  can be connected to the USB VBUS voltage (nominally 5V) to allow the battery to be charged at up to 500 mA (based on the USB specification). The VCHG signal can also be connected to a higher voltage generated by boosting the USB VBUS voltage (maximum charging current would be lower than 500 mA). Alternatively, the VCHG signal can be connected to a different voltage, e.g. from a DC plug pack  632  (see Connection Options section) connected to the pod  450 . In this case, the pen is a self-powered USB device from the point of view of the USB host  630 . When the cap assembly  472  is removed, the CAP_ON signal  682  is pulled low via transistor Q 1   686 . The switching time of Q 1 , and hence the latency of cap removal detection, is a function of the stray capacitance of Q 1  and the value of resistor R 1   688 . A value of 1 Mohm results in a latency of about 0.5 ms. The cap removal detection latency must be balanced against the discharge rate of the battery in the capped state. A value of 1 Mohm yields a trivial discharge rate of 3 μA. Diode D 1   690  stops the battery being charged from the VCHG voltage  684  through R 1   688 . The external USB host  630  (see  FIG. 61 ) is connected to the USB device  692  in the pen  400  via the USB  694  and USB  696  signals. Although the circuit in  FIG. 26  is shown with reference to a four-wire USB interface, the cap detection function of the circuit only relates to the two-wire power interface, and the pen can have a two-pin external power interface rather than a four-pin external USB interface depending on product configuration. 
   The above description is purely illustrative and the skilled worker in this field will readily recognize many variations and modifications that do not depart from the spirit and scope of the broad inventive concept.