Abstract:
Embodiments of amplifiers and electrodes for capacitive styluses are disclosed. The amplifiers detect an input signal from a capacitive touch sensor, such as a touchscreen or touchpad, amplify and invert the signal, and emit the amplified and inverted signal to cause the capacitive touch sensor to detect a touch. The electrode sets, or tips, pair a sensing electrode and an emitting electrode, shielded from each other to limit interference, in a fine-point non-deforming configuration that improves usability over existing stylus tips.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage submission under 35 U.S.C 371 of PCT Application No. PCT/US2014/017004, filed Feb. 18, 2014, which claims the benefit of and priority to U.S. provisional patent application No. 61/765,788, filed Feb. 17, 2013, and U.S. provisional patent application No. 61/882,159, filed Sep. 25, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     This disclosure relates to a stylus for use with a capacitive touchscreen, and more specifically to a stylus using active electronics to interact with a capacitive touchscreen. 
     SUMMARY OF THE PRIOR ART 
     Styluses for use with capacitive touchscreens are known in the art. Most such styluses are passive, having a wide conductive tip that is electrically coupled to the stylus body, such that when the body is gripped by a user, the user is electrically coupled to the tip. This allows the capacitance of the user&#39;s body to be sensed by the touchscreen across a large enough area to simulate a fingertip touch. Touchscreens on many of the most popular devices today require such large touches and capacitances in order to function; contacts by smaller capacitances or across smaller contact regions are ignored by the devices&#39; firmware in order to reject capacitive noise, thereby helping to lower complexity and cost. 
     Precisely locating and “touching” points on a screen is aided by having a stylus with a small, non-deforming tip. Not only does a small tip allow the surrounding screen to be seen by the user, thereby helping the user to position the tip precisely, but also a non-deforming tip means that the firmware will have a consistent contact shape from which to determine the centroid. 
     Higher resolution touchscreens exist, but generally require a stylus that is specifically designed to interact with the given touchscreen so that the touchscreen can ignore other touches as noise. This eliminates the user&#39;s ability to use a fingertip to interact with the touchscreen, drastically reducing convenience and requiring that special hardware (the stylus) be developed and kept with the device. 
     Touchpad capacitive sensors are designed to require close proximity to avoid accidental touch detection, further limiting their capabilities. For example, custom hardware has been developed by some manufacturers that enables a stylus to be detected at some distance from the screen, thus allowing a touchscreen to display a cursor at an anticipated contact point. But this does not work for standard capacitive touchscreens which are designed to detect the capacitance of a user&#39;s fingertip; instead, special hardware for these touchscreens requires the use of a special stylus, thereby entirely preventing users from using their fingertips. 
     A stylus capable of interacting with a mutual capacitance touch device using a small, non-deformable tip is therefore desirable. 
     SUMMARY OF CERTAIN ASPECTS OF THE EMBODIMENTS 
     Embodiments are disclosed that use an amplifying electronic circuit, electrically coupled to a sensing electrode and an emitting electrode, to interact with a mutual-capacitance touchscreen circuit such that the touchscreen circuit and its firmware will detect what is treated as sufficient capacitance over a sufficient contact area to identify as a “touch”. 
     The embodiments use an electrode set having a sensing electrode and an emitting electrode, both coupled to a circuit that detects the capacitive flux signal generated by an electronic device and inverts, amplifies, and re-emits the signal. The electrode set has a shielding layer between the sensing electrode and the emitting electrode to substantially reduce mutual capacitance, feedback, and signal crosstalk between the two electrodes. The amplified inverted signal causes reduced charging of the capacitors between the rows and columns close to the stylus in the touchscreen circuit, which causes a mutual-capacitance touchscreen circuit to detect sufficient capacitive coupling over a large enough region to cause its firmware to register a “touch”. The circuit embodiments disclosed further amplify only those levels of the signal that are necessary to depress charging of the mutual capacitance circuit nearest the current touchpoint to avoid a “wavy line” effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a stylus with an embodiment of an anodized sensor/emitter tip assembly; 
         FIG. 2  is a drawing of a stylus with an embodiment of a laser direct structured (“LDS”) sensor/emitter tip assembly; 
         FIG. 3  is a circuit diagram of an inverter-amplifier using transistors; 
         FIG. 4  is a circuit diagram of an inverter-amplifier using transistors and having an op-amp final stage; 
         FIG. 5  is a circuit diagram of an inverter-amplifier using op-amps; 
         FIG. 6  is an orthogonal view of an anodized sensor/emitter tip assembly; 
         FIG. 7  is a cross-section view of an anodized sensor/emitter tip assembly along the plane of line A-A of  FIG. 6  and through the longitudinal axis; 
         FIG. 8  is an orthogonal view of an emitting electrode for an anodized sensor/emitter tip assembly; 
         FIG. 9  is an orthogonal view of a shield for an anodized sensor/emitter tip assembly; 
         FIG. 10  is an orthogonal view of a sensing electrode attached to or formed on a coaxial cable for an anodized sensor/emitter tip assembly; 
         FIG. 11  is an orthogonal view from the tip end along the longitudinal axis of an embodiment of a tip formed by LDS for an active stylus; 
         FIG. 12  is a perspective view of an embodiment of a tip formed by LDS for an active stylus; 
         FIG. 13  is an orthogonal side view of an embodiment of a tip formed by LDS for an active stylus; 
         FIG. 14  is an orthogonal side view of an embodiment of a tip formed by LDS for an active stylus; 
         FIG. 15  is a perspective view of an embodiment of a hybrid LDS-and-anodized tip for an active stylus; 
         FIG. 16  is a side view of an embodiment of a hybrid LDS-and-anodized tip for an active stylus; 
         FIG. 17  is a side view of an embodiment of a hybrid LDS-and-anodized tip for an active stylus; 
         FIG. 18  is a cross-sectional view of an embodiment of a hybrid LDS-and-anodized tip for an active stylus along the line A-A of  FIG. 17  and through the longitudinal axis of the tip; 
         FIG. 19  is a graph of an oscilloscope trace of voltage detected at the surface of a capacitive touchscreen; 
         FIG. 20  is an expanded view of a segment of the graph of an oscilloscope trace of voltage detected at the surface of a capacitive touchscreen; 
         FIG. 21  is a graph showing both an idealized input signal and an idealized output signal; 
         FIG. 22  is a perspective drawing of a force-sensing tip for an active stylus; 
         FIG. 23  is a top view of a force-sensing tip for an active stylus; 
         FIG. 24  is a side view of a force-sensing tip for an active stylus; 
         FIG. 25  is a cross-sectional view of a force-sensing tip for an active stylus along the plane A-A through the longitudinal axis of the force-sensing tip shown in  FIG. 23 ; 
         FIG. 26  is an exploded view of the components of the force-sensing tip for an active stylus of  FIG. 25 . 
         FIG. 27  is a circuit diagram of an inverter-amplifier circuit for an active stylus with a variable power level setting subcircuit; 
         FIG. 28  is a circuit diagram of an inverter-amplifier circuit for an active stylus with a fixed power level setting subcircuit; 
         FIG. 29  is a circuit diagram of an inverter-amplifier circuit for an active stylus with circuitry to respond to multiple capacitance lines; and 
         FIG. 30  is a drawing of a stylus with a force-sensing tip assembly approaching a touchscreen. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following detailed description of embodiments references the accompanying drawings that form a part hereof, and in which are shown by way of illustration various illustrative embodiments through which the invention may be practiced. The embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical changes may be made without departing from the spirit and scope of the invention. The detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention is defined solely by the appended claims. 
     Please refer to  FIG. 1 , an orthogonal view of an embodiment of an active stylus. Externally, the stylus  10  comprises a body  11 , a fairing  12 , and a tip  100 . A printed circuit board  20  and battery  30  housed inside the body  11  are shown as dashed outlines; the printed circuit board  20  may comprise all circuitry necessary to implement the various electronic functions of the stylus, including a battery charger, power supply for the amplifier circuit, and the amplifier circuit itself. Unlike a passive capacitive stylus, in which a stylus body serves to electrically couple a conductive tip to the user&#39;s hand, an active stylus does not necessarily need do this, and so may be made of either conductive or nonconductive materials, or a combination thereof. Here, the body  11  of the stylus  10  serves to hold the tip  11  and to contain active electronic circuitry  20  and a battery  30  for powering the circuitry.  FIG. 1  shows an embodiment of a tip  100  comprising an anodized sensor/emitter, as disclosed below (see  FIG. 6  through  FIG. 10  and related descriptions). 
       FIG. 2  shows an alternate arrangement of an embodiment of an active stylus using a tip  200  formed using laser direct structuring (LDS), as disclosed below (see  FIG. 11  through  FIG. 14  and related descriptions). The circuit board  20  is positioned such that an end of the circuit board  20  fits within a slot in the LDS tip  200 , thereby allowing the leads (not shown) of the electrodes of the LDS tip  200  to be soldered directly to the appropriate pads (not shown) for electrically coupling the electrodes of the LDS tip  200  (see below) to the circuit board  20 . 
       FIG. 30  shows an embodiment of an active stylus using a force-sensing tip  700  approaching a touchscreen  1 . 
     Please refer to  FIG. 19 , a graph showing the sequential scan of the column traces of a typical mutual-capacitance touchscreen and the capacitance generated between the touchscreen and the oscilloscope probe. An oscilloscope probe detects the column trace from four separate columns of the touchscreen. The detected voltages indicate that the probe is located between the second and third column, and is closer to the second column trace than to the third. To the far left, the oscilloscope probe detects only noise. In the segment labeled “Col A”, a first ITO trace for a first column is energized, and capacitance between the probe and the touchscreen builds, eventually reaching an equilibrium. In the segment labeled “Col B”, the second ITO trace for the second column is energized, and because the probe is closer to this trace, the detected voltage is significantly higher and the resulting mutual capacitance moves to equalize at a higher level, about four times the level of “Col A”. Note that for “Col B”, but not for the other columns or the noise, the voltage detected exceeds the input equivalent of the V threshold  voltage described below. In the segment labeled “Col C”, the third ITO trace corresponding to the third column is energized. The amount of energy received is only about twice as much as was received for “Col A”, and so the curvature of the slope is negative, as the capacitance gradually stabilizes at a lower level. In the segment labeled “Col D”, a fourth ITO trace corresponding to the fourth nearby column is energized. Only a small amount of voltage is detected by the probe, and the curvature is again negative. Subsequently, the detected voltage again reduces to the level of the noise from the system. The relative amounts of energy detected by the probe indicate that the oscilloscope&#39;s probe was located between Column B and Column C, and was closer to Column B. 
       FIG. 20  shows an expanded view of a very short part of the waveform that energizes the touchscreen&#39;s traces. The output waveform as generated is square, but the leading edges show rounding due to the capacitive charging of the circuit. The detected voltage ranges from about +50 mV to −50 mV, and the detected frequency is about 294.1 kHz. Note that the voltage detected exceeds the input equivalent of V threshold , as shown. 
     Please refer briefly to  FIG. 21 , a graph showing both an idealized signal from a prior-art touchscreen and the idealized resultant output signal of an embodiment of an active stylus. The idealized representation of the input waveform is representative of a short sample of one segment of the oscilloscope trace shown in  FIG. 19 . This expanded view of the input waveform shows a square wave of about 40 mV peak-to-peak (+−20 mV). The stylus detects this signal and responds by emitting an inverted amplified signal, the output waveform, also a square wave, of about 24V peak-to-peak (+−12V). An inverting amplifier circuit  300 ,  400 ,  500  takes the input voltage signal and outputs a function of that voltage signal, where the output voltage follows the following function (in C language syntax):
 
 V   out =( V   in   &lt;V   threshold  ?0 : −K*V   in )
 
     where K is a large constant. When the detected voltage is below a threshold value, no voltage is output; when the detected voltage is at or above a threshold value, the voltage is inverted and amplified to the saturation limit of the circuit, resulting in a series of square-wave pulses. Alternately, this output voltage may be level-shifted so that both positive and negative voltage square wave components are output; the function followed is along the lines of:
 
 V   out =( V   in   &lt;V   threshold    ?K*V   in   : −K*V   in )
 
     Because the input is a square wave, the output will likewise be a square wave, but inverted compared to the input signal, and at a significantly higher amplitude. The output signal frequency is inherently matched to the input signal frequency by the amplifier. Embodiments of the circuit may amplify the signal by a factor (K) of about 600 to 1500, resulting in an input signal having a threshold voltage of 20 mV being amplified to between 12V to 30V depending on the requirements of a particular implementation. Please note that the actual threshold-setting circuit may be implemented at any stage of the amplifier, but in the embodiments as shown it has been put in the final stage driver circuit. The input signal thus may be amplified and shifted repeatedly before an intermediate signal is compared against V threshold ; the input equivalent of V threshold  may thus be significantly different from the V threshold  that the intermediate signal is compared to within the circuit. 
     Referring now to  FIG. 3 , a circuit diagram showing an embodiment of the electronic circuit for an active stylus using transistors, the inverting amplifier circuit  300  has sensor connection block  301 , first-stage amplifier  310 , second stage amplifier  320 , driver  330  and emitter coupling block  340 . 
     The signal level output by the circuit may be in the range of from 18V to 30V. Below 18V, common capacitive touchscreens such as those on the Apple® iPad® may not be sufficiently affected to cause them to detect a touch, while above 30V the signal may cross-couple between the sensing electrode and emitting electrode, resulting in feedback, in addition to this causing excessive and unnecessary power use. 
     Focusing on the first-stage amplifier  310  of the circuit diagram of  FIG. 3 , a sensing electrode, such as sensing electrode  112  of  FIGS. 6-10  or sensing electrode  212  of  FIGS. 11-14 , is electrically coupled to a low frequency cancellation capacitor  311 , which eliminates low frequency noise such as electrical hum that might otherwise be overlaid upon the desired signal. Power to the circuit is supplied through a level shift resistor pair  312  and an output power resistor  313 ; the level shift resistor pair  312  serves to pull up the signal from a typical input level of 0V±20 mV to a level of 0.5V±20 mV. A power noise filtering capacitor  315  tied to the input power helps to eliminate noise generated by the power supply circuit (not shown). This level-shifted signal is fed into an amplifying transistor  314 , which inverts and amplifies the signal in conjunction with a further boost from the emitter bypass subcircuit  316  to about 0.5V±400 mV. The inverted and amplified signal is output from the inverter-amplifier block through a filtering capacitor  317  to again filter out low-frequency noise. 
     Input into the second stage amplifier  320  of the circuit diagram of  FIG. 3  is supplied through a current-limiting resistor  321 , which feeds into the second stage amplifier  320 . The second-stage amplifier  320  comprises a level shifting resistor pair  322 , a power supply resistor  323 , and an inverting amplifier transistor  324 . Input power to the power supply resistor  323  is coupled to filter capacitor  335  to filter out noise from the power supply. Output is filtered by capacitor  327  to remove low-frequency noise, and feeds into the driver circuit  330 . The second stage amplifier  320  amplifies the signal to about the range of 0-3 volts peak-to-peak. 
     The driver circuit  330  uses a level setting resistor pair  332  to eliminate signals below a threshold voltage level V threshold ; by selecting an appropriate level below which the inverted and amplified signal is eliminated, the output signal is emitted only when a signal from a nearby trace is detected, and so capacitive charging is suppressed only when appropriate. Failure to so limit the output signal results in a “wavy line problem”, wherein suppression is performed for other columns in addition to the nearest, causing the capacitive sensor to sense a touch across a wide area, which in turn results in the touch circuit hardware and firmware sending incorrect position data in touch events; the end result is that if a user attempts, for example, to draw a line across the screen, the line is not straight, but rather follows a wavy or sinusoidal path. Although the level setting resistor pair  332  is optional, it provides significant benefit to the circuit and to the end user by eliminating the wavy line problem through an extremely simple hardware solution rather than a complex software method or mixed hardware-software means. After filtering and level-setting, the signal is fed into the driver subcircuit  334 , which amplifies and inverts the signal a third time; power to the driver subcircuit  334  is noise-filtered by coupling the power through filter capacitor  335 . This may be done to a relatively high voltage level of between +18V to +30V by using a MOSFET, chosen for its power-handling capacity. Output from the circuit may be electrically coupled to an emitting electrode through an optional filter capacitor  337  to level-shift the signal and/or eliminate low-level noise; the emitting electrode may be for example emitting electrode  110  of tip  100  or emitting electrode  210  of tip  200  or emitting electrode  610  of tip  600 . 
     It should be noted that although not shown in the circuit diagram, the shield  111  or shield  211  is coupled to ground. 
     Referring now to  FIG. 4 , a circuit diagram showing an embodiment of the electronic circuit for an active stylus using an op-amp for the driver circuit, the inverting amplifier circuit  400  comprises a sensor connection block  301 , first-stage amplifier  310 , second-stage amplifier  320 , op-amp driver circuit  430 , and emitter coupling block  340 . By adjusting level shifting resistor pair  322  of the amplifier block  320 , and replacing driver circuit  330  with a more efficient op-amp driver circuit  430 , the inverting amplifier circuit  400  has significantly lower power consumption. To have sufficient output power while nevertheless maintaining high efficiency, an operational amplifier (or “op amp”) is used in the op-amp driver circuit  430 . A high-gain high-bandwidth amplifier quickly responds to changes in signal level; for example, an op amp with about 50 MHz bandwidth or response time is suitable for use. The first-stage amplifier  310  and second-stage amplifier  320  are used to feed an amplified signal to op-amp driver circuit  430  through current-limiting resistor  431 . Power supply to the second-stage amplifier  320  is noise-filtered by coupling it to filter capacitor  425 . Level shifting resistor pair  322 , power supply resistor  323 , and current-limiting resistor  431  are selected to deliver voltage to the inverting input of the op-amp  434  that will be just within the input limits of the op-amp  434 . The op-amp driver circuit  430  takes as input the output of the second-stage amplifier  320 , at about 3V, and amplifies it to a level sufficient to act upon a capacitive touch sensor. A voltage divider resistor pair  432 , in concert with filter capacitor  436 , is used to supply a clean, filtered voltage to the non-inverting input of the op-amp  434 ; this voltage is the threshold voltage V threshold , below which the input signal will be clipped. The threshold voltage in this embodiment may be calculated as:
 
 V   threshold   =V   ps   *R 21/( R 22 +R 21)
 
     or more generally as the power supply voltage, in this example circuit shown as +24V, times the resistance of the resistor coupled to ground and divided by the sum of the resistances of both resistors. By supplying a fixed positive voltage to the non-inverting input, and the signal voltage to the inverting input, the circuit is inverting. Output is fed back through feedback resistor  433  to stabilize the circuit. The signal output from op-amp  434  is electrically coupled to an emitting electrode such as emitting electrode  110  of tip  100  or emitting electrode  210  of tip  200  or emitting electrode  610  of tip  600 . 
     Referring now to  FIG. 5 , an inverting amplifier circuit  500  using two op-amp stages, the circuit  500  comprises a sensing electrode connection block  301 , a first stage inverting amplifier  510 , a second stage non-inverting driver  520 , and an emitter coupling block  340 . Input differential is electrically coupled to the first op-amp  514  through current limiting resistor  513 . Input signal is electrically coupled to the first op-amp  514  through filter capacitor  511  and level-setting resistor  512 . Feedback to stabilize the circuit is provided by coupling the op-amp output back to the signal input via feedback resistor  515 . Amplifying input power and ground are directly electrically coupled to the op-amp terminals. The output of the first op-amp  514  is inverted and amplified from input levels to a level sufficient to drive the second stage non-inverting driver  520 ; the output from the first stage inverting amplifier  510  is electrically coupled to the second stage non-inverting driver  520  through output filter capacitor  517  to cancel noise. The amplification of the circuit is described by the function:
 
 V   out   =−K*V   in   *R   feedback   /R   level  
 
     where R feedback  is set by feedback resistor  515  and R level  is set by level-setting resistor  512 , and K is a constant. In an example embodiment, an input voltage level was amplified from about 20 mV to about −3V, for a K of about 150. 
     Input to the second stage non-inverting driver  520 , through filter capacitor  517 , may optionally be coupled through a current-limiting resistor (not shown). 
     The input signal is coupled to the non-inverting input of the driver op-amp  524  through current limiting resistor  522 . The input signal is also coupled to the inverting input of the driver op-amp  524  through impedance matching resistor  521 , which along with input power is coupled to the inverting input of the driver op amp through power input resistor  523 . The driver op-amp  524  has its power terminals tied to the power supply voltage, in this example embodiment 24V, and ground respectively. Output from the driver op-amp  524  is coupled to the inverting input of driver op-amp  524  via feedback resistor  525 , and also to the emitter coupling block  340  either directly or optionally via a filter capacitor  527 . Without the filter capacitor  527 , output may be from 0V to +24V; with the filter capacitor  527 , output may be shifted to +−12V (24V peak-to-peak). The output voltage of the driver circuit  520  may be between 18V to 30V, and may be approximated by the function:
 
 V   output =(1+( R   feedback   /R   power ))* V   input  
 
     where V input  is the voltage measured after the filter capacitor  517  of the first-stage inverting amplifier  510 . The threshold voltage V threshold  is set by the choice of values for the impedance matching resistor  521 , power supply resistor  523 , and feedback resistor  525 . 
     Referring now to  FIG. 6 , an orthogonal view of an embodiment of an anodized sensor/emitter tip, in combination with  FIG. 7 , a cross-section view of an embodiment of a tip for an active stylus, the tip  100  comprises two electrodes, an emitting electrode  110  and a sensing electrode  112 , separated by a shielding layer  111 . The tip  100  may be substantially radially symmetrical around its longitudinal axis; in particular, the two electrodes and the shielding layer may be symmetrical around their respective longitudinal axes (which are congruent to the longitudinal axis of the tip  100  itself when assembled) in order to present a consistent capacitive signature to a touchscreen regardless of the rotational orientation at which the stylus is held during use. All three layers  110 ,  111 ,  112  are conductive, and so must be electrically insulated from each other. Some embodiments may use a pair of electrically insulating layers (not shown), for example of a nonconductive polymer, one between the shield  111  and the emitting electrode  110 , and one between the shield  111  and the sensing electrode  112 , to electrically isolate the three layers. Other embodiments may anodize one, two, or all three layers  110 ,  111 ,  112  so that the anodizing serves as electrical insulation, thereby eliminating the additional layers. One way of doing this is to anodize at least the shield  111  with a layer of anodizing (aluminum oxide) sufficient to prevent electrical conductivity between the layers; the emitting electrode  110  and sensing electrode  112  may optionally also be anodized. Alternately, the emitting electrode  110  and sensing electrode  112  are anodized, in which case the shield  111  may optionally also be anodized. Anodizing serves as insulation when sufficiently thick; a layer of anodizing (Type II or Type III) of thickness 0.0002 inches or greater is sufficient for the voltages involved. 
     As shown in  FIG. 8 ,  FIG. 9 , and  FIG. 10 , the emitting electrode  110  has an internal flange  110 F, a distal surface  110 D and a proximal edge  110 P, wherein the internal flange  110 F extends radially outward from the distal surface  110 D and the proximal edge  110 P is opposite to the distal surface  110 D and the shield  111  has an external flange  111 F having an external flange distal surface  111 D and external flange proximal surface  111 P at the proximal end of the shield  111 , wherein the external flange  111 F is located between and therefore electrically isolates the proximal edge  110 P of the emitting electrode  110  from a distal face  112 D of the sensing electrode  112 . 
     Referring again to  FIG. 11 , the sensing electrode  112  is formed of a conductive material such as a metal or metal alloy, conductive polymer, or conductive shell formed over a polymer. The proximal surface  112 P of the sensing tip  112  may be in the form of a spherical cap, paraboloid, one sheet of a hyperboloid of two sheets, or similar smoothly curved surface. In some embodiments, the sensing tip  112  is electrically connected to the center conductor  121  of a coaxial cable  120 ; the sensing tip  112  may be set closely against the dielectric  122 . In some embodiments, the sensing electrode  112  is formed of solder that is melted onto the center conductor  121  of the coaxial cable  120  and is then molded into an appropriate shape as listed above; in other embodiments, the sensing electrode is formed of a conductive metal such as copper, aluminum, steel, etc., and then soldered, brazed, welded, press-fitted, staked, or otherwise electrically and mechanically connected to the center conductor  121 . The shielding conductor  123  (which may be braided, foil, or both) of the coaxial cable  120  is electrically connected to the shielding layer  111  to extend the shielding layer at least past the emitting electrode  110  and in some embodiments to substantially near the PCB (not shown). The shielding layer  111  may have an outer diameter substantially similar to the outer diameter of the coaxial cable  120 , and an inner diameter substantially similar to the outer diameter of the coaxial cable&#39;s dielectric layer  122 . The emitting electrode  110  has a central axis hole of substantially the same diameter as the coaxial cable  120 , thereby allowing the emitting electrode  110  to slide over the outer sheath  124  of the coaxial cable  120 . The center conductor  121  of the coaxial cable  120  is electrically coupled to the printed circuit board (not shown), the shielding conductor  123  of the coaxial cable  120  is electrically coupled to ground on the printed circuit board (not shown), and the emitting electrode  110  is electrically coupled to the printed circuit board (not shown) with a wire (not shown). 
     Referring to  FIG. 8  in combination with  FIG. 7 , the emitting electrode  110  may be a right circular conical frustum having a cylindrical hole, axially aligned with the axis of the conic frustum and of a size to fit closely around the coaxial cable  120  and the shield  111 , through the center. An optional internal flange  110 F at the distal end may be used to align and stabilize the emitting electrode  110  in the fairing  12  when assembled. 
     Referring now to  FIG. 9  in combination with  FIG. 7 , the shield  111  may be a hollow cylinder having an external flange  111 F on one end, the external flange  111 F being wide enough to electrically isolate the emitting electrode  110  from the sensing electrode  112 , for example by having its widest diameter be as wide as the proximal end of the emitting electrode  110 . The shield  111  is formed of a conductive material, and in some embodiments is anodized aluminum. The shield  111  has a first segment in which the central hole is of a diameter to admit the dielectric  122  of the coaxial cable  120 , and a second segment in which the central hole is of a diameter to admit the shielding layer  123  of the coaxial cable. The shield  111  is electrically coupled to the shielding layer  123  of the coaxial cable  120 . 
     To assemble the anodized tip  100 , the coaxial cable may have its outer insulating layer  124  stripped, a lower portion of the shielding layer  123  stripped, and a short segment of the dielectric  122  removed, leaving a short segment of the center conductor  121  exposed. The shield  111  may then be slipped over the end of the coaxial cable  120  until the distal end of the shield  111  abuts the insulating layer  124  of the coaxial cable  120 , and the shielding layer  123  may then be electrically coupled to the shield  111 . The sensing tip  112  may then be attached to or formed upon the center conductor  121  such that the sensing tip&#39;s distal face  112 D abuts the external flange proximal surface  111 P of the external flange  111 F of the shield  111 . The emitting electrode  110  may then be slid down over the coaxial cable  120  to rest on the shield  111  such that the proximal edge  110 P of the emitting electrode  110  abuts the external flange distal surface  111 D of the external flange  111 F. The coaxial cable  120  may then be slid into the fairing  12  until the distal surface  110 D of the emitting electrode  110  abuts the fairing  12  and the internal flange  110 F rests inside a receiving portion of the fairing  12 . The center conductor  121  may then be connected to the sensing electrode pad (not shown) on the PCB (not shown), the shielding layer  122  may be connected to ground (not shown) on the PCB (not shown), and the emitting electrode  110  may be connected to the emitting electrode pad (not shown) on the PCB (not shown) using, for example, a wire (not shown). 
     Another embodiment of a stylus probe uses laser direct structuring (LDS) to form metallized tracks on a polymer substrate. Refer to  FIG. 11 , a perspective view of an embodiment of a probe for an active stylus, in combination with  FIG. 12 , a top view of said probe,  FIG. 13 , a side view of said probe from the left of line B-B of  FIG. 12 , and  FIG. 14 , a side view of said probe from the right of line B-B of  FIG. 12 . The probe substrate is injection-molded, its surface is laser etched along the paths and regions that will form the electrodes and their conductive paths, and electrodes are electrolessly plated onto the substrate along the laser-etched regions. The probe  200  has a body  201  formed of a polymer suitable for use with the LDS process. The body is a thin polymer shell, for example a conical frustum of a right circular cone capped by a spherical cap such that a vector tangent to the edge of the spherical cap is congruent to a vector lying in a surface of the frustum. The body  201  may have a flange  201 F with a snap  201 S for attachment into a fairing  12 , with the flange  201 F serving to align and stabilize the tip  200  in the fairing  12  when assembled. A slot  201 B in the distal end of the body  201  acts as a receiving point into which a PCB (not shown) may be inserted so that the pads  210 P,  211 P,  212 P of the electrodes can be soldered directly to the PCB. An emitting electrode  210  and emitting electrode trace  210 T ending in an emitting electrode pad  210 P, a shielding trace  211  and shielding trace pads  211 P (one on each end of the shielding trace  211 ), and a sensing electrode  212  and sensing electrode trace  212 T ending in a sensing electrode pad  212 P are then formed onto the body  201 . Nonconductive traces  291 ,  292  prevent electrical conductivity between the three traces, while the shielding trace  211  lies between and capacitively isolates the emitting electrode  210  from the sensing electrode  212  and sensing electrode trace  212 T; the emitting electrode trace  210 T is positioned so that it is effectively isolated from the other traces by distance. The emitting electrode trace  210 T is routed along a body cutout  201 C, and the shielding trace  211  and sensing electrode trace  212 T are routed along a body cutout  201 D, to provide physical space so that they do not rub against the fairing  12  when assembled, which could otherwise result in wear damage to the traces. Capacitive isolation may optionally be further increased by the use of a metallized insert (not shown) placed within the shell and electrically coupled to ground. 
     An optional protective cover (not shown) may be used to cover the body  201  to prevent wear to the electrodes, in particular the sensing electrode  212 , that might otherwise be caused by ordinary use, such as by rubbing the tip  200  against a capacitive touch device&#39;s surface. The protective cover may be made user-replaceable so that when the protective cover wears down, it can be readily swapped for a new one by the user, thus extending the lifespan of the tip  200  and hence the stylus. Alternately or additionally, the tip  200  may optionally be made user-replaceable, thereby allowing user replacement of worn probes and extension of the lifespan of the active stylus  10  as a whole. 
     Please refer now to  FIG. 15  through  FIG. 18 , four views (perspective, side, side, and cross-sectional) of an embodiment of a hybrid anodized-and-LDS tip. The hybrid tip  600  has a body  602 , a bearing  605 , an emitting electrode  610 , a shield  611 , and a sensing electrode  612 . The body  602  is composed of a polymer suitable for LDS, and the emitting electrode  610  and the emitting electrode trace  610 T are electrolessly plated upon the body  602 . The body  602  has a cylindrical hole through it along its longitudinal axis, into which a shield  611 , having a flange  611   f  on its proximal end, can be inserted. The shield  611  likewise has a cylindrical hole through its center aligned with its longitudinal axis, into which a bearing  605  may be inserted, the bearing being a press fit into the hole. The bearing may be a low-friction polymer such as nylon, HDPE, PTFE, or other suitable material with low friction, low conductivity, and long wear characteristics. The sensing electrode  612 , comprising sensing electrode shaft  612 S and sensing electrode tip  612 T, is slidably disposed within the bearing  605  and may be mechanically coupled to a pressure sensor (not shown) such as a diaphragm with one or more strain gauges or a spring and sensor to detect longitudinal depression of the sensing electrode  612  within a housing. The sensing electrode tip  612 T may be in the shape of a hemisphere or spherical cap or other smoothly curved surface, and may optionally be wider than the sensing electrode shaft  612 S. The body  602  may be formed in the shape of a right circular conic frustum abutting a right circular cylinder, the large base of the frustum being congruent to an end of the cylinder, with the emitting electrode  610  deposited on the side surface of the frustum and the emitting electrode trace  610 T being deposited as a thin stripe along the side of the cylindrical portion of the body  602 . The shield flange  611 F of the shield  611  has a maximum diameter that is at least as large as the larger of the largest diameter of the sensing electrode tip  611 T and the diameter of the proximal end of the emitting electrode  610 . 
     In use, the sensing electrode shaft  612 S, shield  611 , and emitting electrode trace  610 T are electrically coupled to their respective contact pads on a PCB  20  having an inverting amplifier circuit  300 ,  400 ,  500 . 
     Differentiating a stylus touch from a finger touch is also possible. Touchscreens typically scan their arrays at a refresh rate of 60 Hz. The circuitry of the active stylus can be modified to respond only to every alternate scan, far more frequent than the speed at which a finger could be moved back and forth. The result is a touch that appears and disappears 30 times per second. This imposes some deterioration on the smoothness with which a moving touch can be tracked, but touchscreen manufacturers can increase the refresh rate to compensate; for example, by doubling the refresh rate, this will give the same 60 Hz granularity as existing screens while still allowing a touch from a stylus to be readily distinguishable from a touch made by a fingertip. 
     Alternately, instead of responding to alternate scans, the stylus electronics can respond to alternate cycles within a given scan. As shown in  FIGS. 19 and 20 , each 60 Hz scan of the touchscreen rows and columns is performed with a much higher-frequency waveform that allows the touchscreen to check charge/discharge times repeatedly. Because the inverting-amplifier electronics are responding directly to the input signal, instead of using a signal generator, by adding a flip-flop or a similar switch, the electronics can be modified to ignore every second cycle. Furthermore, this may optionally be made configurable, or may be switched automatically by a pressure sensor so that the alternating-cycle response occurs only when the stylus is not physically touching the screen, thereby allowing both hover and touch to be differentiated. 
     Another alternative is for the stylus to change its response rate only during an initial contact period during the first few cycles after the stylus detects the capacitive flux of a touchscreen or touchpad at a level sufficient to ensure that the touchscreen or touchpad is being influenced by the stylus. This allows the touchscreen firmware and touch-detection software to detect which touch is being generated by a stylus, and to begin tracking that touch. The stylus may then respond to every cycle, thus ensuring smooth interaction and tracking of the touch. 
     Furthermore, these alternatives may be combined in a single implementation to provide the ability to differentiate between hover and touch. 
     Please refer now to  FIG. 22  through  FIG. 26 , five views (perspective, front, side, cross-sectional, and expanded, respectively) of an embodiment of a force-sensitive tip. The force-sensitive tip  700  has an emitting electrode  710 , a shield  711 , and a sensing electrode  712 . The emitting electrode  710  has internal threads  710 T and a shoulder  710 S so that the emitting electrode may be screwed onto a stylus body (not shown) with the shoulder abutting a front end edge of the stylus body. The emitting electrode  710  uses an ogive shaped outer surface  710 C to increase the diameter of the electrode nearer the contact with a touchscreen surface, thereby increasing capacitance between the outer surface  710 C and a touchscreen surface as a stylus with the tip  700  is brought near a touchscreen surface (as illustrated in  FIG. 30 ), allowing a reduction in the voltage used by the stylus, and hence reducing overall power needs of the active stylus  10 . The emitting electrode  710  further has a central hole  710 H through which the shield  711 , which contains and isolates the sensing electrode  712 , protrudes. The shield  711  in this embodiment does not have a flange; its diameter is about the same as the diameter of the sensing electrode tip  712 T&#39;s widest diameter. The shield  711  is made of a conductive material or materials such as a metal or a conductive polymer, and may be monolithic or made of a plurality of different materials; in some embodiments, the shield  711  is formed of anodized aluminum and has a copper ring  715  press-fit around its distal end  711 D to provide a place to form a reliable solder joint with a wire (not shown) while ensuring conductivity to the aluminum portion of the shield  711 ; in said embodiments, anodizing and surface oxidation of the shield  711  must be removed from the outside surface of the distal end  711 D prior to installation of the copper ring  715  in order to ensure maximum conductivity; the anodization is retained elsewhere on the shield to provide insulation between the shield  711  and the sensing electrode  712 , and between the shield  711  and the emitting electrode  710 . The shield  711  may be a slip fit within the central hole  710 H of the emitting electrode  710 , or the shield  711  may be of sufficiently smaller diameter than the central hole  710 H of the emitting electrode  710  to allow an optional slip-on polymer cap  731  to be placed over the shield  711  and sensing electrode  712 . The optional cap  731  provides a low-friction bearing between the shield  711  and the emitting electrode  710  and protects the touchscreen  1  from the tip  712 T of the sensing electrode  712 . The cap  731  may for example be made of PET, ETFE, PTFE, HDPE, nylon, or another low-friction long-wearing nonconducting polymer, and may be designed to be user-replaceable as it wears out either at its proximal face  731 P or along its sides on its bearing surface  731 S. The shield  711  may optionally further comprise a retaining ring  711 G (shown only in  FIG. 26 ) that the cap  731  can slip over in order to prevent the cap  731  from sliding off. The shield  711  comprises a flange  711 F which interacts with travel-limiting blocks (not shown) inside the chassis  11  of a stylus  10  to prevent overtravel in both forward (less force) and rearward (excessive force) directions. The shield  711  has a proximal conduit  711 A through its center aligned with its longitudinal axis, into which the sensing electrode  712  fits; the shield  711  also has a distal conduit  711 B into which a portion of the PCB (not shown) protrudes; the proximal conduit  711 A and distal conduit  711 B merge in a flared section  711 C, which may optionally be either tapered as shown or squared-off to provide another overtravel prevention stop. This allows the shield  711  to have a narrow tip, desirable for usability reasons, while also accommodating the PCB, which cannot be made too narrow without sacrificing strength. The sensing electrode  712 , comprising sensing electrode shaft  712 S and sensing electrode tip  712 T, is disposed within the shield  711  and is electrically coupled to the PCB (not shown) by spring  721 , which also serves to bias the assembly of the sensing electrode  712  and the shield  711  outward from the stylus  10 . The distal end of the spring  721 D is soldered to the PCB, while the proximal end of the spring  721 P rests in a well  712 W formed in the back of the sensing electrode  712 . The shield  711  may then be mechanically coupled to a force sensor (not shown) such as a gated photodetector and light source, or a diaphragm with one or more strain gauges, or a spring and sensor to detect longitudinal depression of the sensing electrode and shield assembly. The sensing electrode tip  712 T may be in the shape of a hemisphere or spherical cap or other smoothly curved surface, and may optionally be wider than the sensing electrode shaft  712 S. 
     In use, the sensing electrode  712 , shield  711 , and emitting electrode  710  are electrically coupled to their respective contact pads on a PCB  20  having an inverting amplifier circuit  300 ,  400 ,  500 ,  800 ,  801 ,  900 . 
     Referring now to  FIG. 27  and  FIG. 28 , two variants of a circuit for an active stylus, the circuit  800  or circuit  801  has a first amplification stage  810  and a second amplification stage  820 , a signal limiter  830  which may optionally be either an adjustable signal limiter  840  (as shown in  FIG. 27  for circuit  800 ) or a fixed signal limiter  841  (as shown in  FIG. 28  for circuit  801 ), and a driver stage  850 . 
     The first amplification stage  810  receives input from a sensing electrode such as sensing electrode  110 , sensing electrode  210 , or sensing electrode  710 , through the line labeled SIGNAL_IN. Please note, the sensing electrode and touchscreen glass effectively and inherently form a capacitor through which the initial signal passes before being sensed by the sensing electrode. Varistor  811  acts as a protection mechanism for the circuit in the event that the sensing electrode is connected to electrical current, for example, if a misbehaving child sticks the stylus tip into a wall outlet. The signal then passes through a high-pass filter  812  to amplifying transistor  813 . Power enters the circuit from the power supply and is filtered by filter capacitor  814 . Level-setting resistor pair  815  and output level resistor  816  bring the output of the amplifying transistor  813  back to the input of the amplifying transistor  813  to form a feedback loop, and output from the amplifying transistor  813  along with additional power is output to the second amplification stage  820 . The input signal that came from the sensing electrode is inverted and amplified from a peak level of about 20 mV to a peak level of about 200 mV. 
     The second amplification stage  820  receives its input signal from first amplification stage  810 . The input is filtered through high pass filter  822 , and is input into the amplifying transistor  823 . Power is supplied to the amplifying transistor  823  through output level resistor  826  after having been filtered by filter capacitor  824 . Power is fed into the circuit via level-setting resistor pair  825 . Output from the second stage amplifier is then sent to both the signal limiter  830  (which may be fixed signal limiter  841  or adjustable signal limiter  840 ) as well as the driver stage  850 . Output from the second amplification stage  820  is at a peak level of about 3V. 
     The signal limiter  830  is optional, and furthermore may be either adjustable signal limiter  840  as shown in circuit  800  or fixed signal limiter  841  as in circuit  801 . Its purpose is to reduce the output power level for certain types of devices with which an active stylus may be used. Certain touchscreens use higher power levels than other touchscreens, and so the output signal would be correspondingly higher unless the circuit  800  or circuit  801  has some way of reducing its output power to compensate. This is desirable both to reduce power requirements (saving battery life in a battery-powered stylus) and to reduce the “wavy line problem” where overdriving the output signal can cause the touchscreen hardware and firmware to move the detected contact point from where the active stylus touches to a different spot because the touchscreen hardware and firmware are being overwhelmed by too much signal. The fixed signal limiter  841  may be chosen either for cost reasons or because the most likely use of the stylus will be with certain known devices. The adjustable signal limiter  840  may be chosen either for development and testing or when the stylus is expected to be used with a large variety of touchscreens whose qualities are not known. Both work by providing a drain path for excess signal; when inhibited, the full signal is sent along from the second amplification stage  820  to the driver stage  850 , whereas when not inhibited, part of the signal is dumped to ground thereby reducing the power sent along to the driver stage  850 . 
     Fixed signal limiter  841  is simply a resistor  844 , a diode  845 , and a gain select line from a microcontroller unit (MCU) (not shown). The resistor  844  forms a divider circuit with resistor  826  of the second amplification stage  820 . When the gain select line is powered high, the diode  845  is reverse-biased and so the full power output from the second amplification stage  820  is sent to the driver stage  850 . When the gain select line is low, current leaks across the diode  845  and so drains part of the output signal power, thereby causing less power to go to the driver stage  850 . In some embodiments, the MCU is in communication with the touchscreen device, which indicates whether the power output should be reduced or should be left at full power, the communication for example being via Bluetooth RF communications hardware. 
     Adjustable signal limiter  840  comprises a digital resistor  842  in communication with an MCU (not shown) via two I2C standard format communications lines. The MCU (not shown) may receive information from any source to determine at what level to set the digital resistor  842 ; for example, the MCU may be linked via Bluetooth with software on a touchscreen device (in a production model), or may be set by test equipment (during development). Power is supplied to the digital resistor  842  from the power supply, said power being filtered with filter capacitor  843  so that the digital resistor  842  will behave in a stable manner. The amount of power dumped to ground depends on the ohm level to which digital resistor  842  is set by the MCU. The range is known to the designer, and so may be under as fine a control as is necessary to work with in development or to produce for consumer use. 
     Output from the second amplification stage  820 , as drained (or not drained) by signal limiter  830 , finally is sent to the driver stage  850 . Input is filtered by filter capacitor  851 , and level-setting resistor pair  852  regulates the input signal to driver MOSFET  853 . Power to the driver MOSFET  853  is filtered by filter capacitor  854  and set by level-setting resistor  855 . Output from driver MOSFET  853  is coupled back to its input via level-setting resistor pair  852  to form a feedback loop, and is also sent as the output from the driver stage  850  through filter capacitor  858  to an emitting tip such as any of emitting electrode  112 ,  212 ,  712 . A varistor  859  coupled between the output line and ground provides ESD protection to the circuit  800  or the circuit  801  in case of static electricity shock transferred through the output line from an emitting electrode. 
     Please note that the first amplifier stage  810  and second amplification stage  820 , as well as the signal limiter  830 , use a first ground (shown by the unlabeled ground), while the driver stage  850  and the final output filter  860  are connected to a second ground (labeled as GND_E). The two grounds are electrically coupled together through a high-ohm resistor. This improves isolation between the two stages to prevent the two stages from interfering with each other. 
     Please refer now to  FIG. 29 , a circuit for an active stylus. The circuit  900  comprises a first amplification stage  810  and second amplification stage  820  and driver stage  850  as described above for  FIG. 27  and  FIG. 28 . (Please note that values of some components may differ to generate different voltage levels as needed by the overall circuit.) 
     As in circuit  800  and circuit  801 , circuit  900  uses separate grounds electrically coupled by a high-ohmage resistor for the lower voltage and higher voltage subcircuits. Stages  810 ,  820 ,  850 ,  960 ,  970 , and  990  use a common ground, while the final driver stage  980  uses the second ground. 
     Output from the first amplification stage  810  is sent to the second amplification stage  820 . Output from the second amplification stage  820  is sent both to driver stage  850  and to comparator stage  970 . Output from the driver stage  850  is sent through filter capacitor  959  to a regulator stage  960 . Output from the comparator stage  970  is sent through a final driver stage  980 . Output from the regulator stage  960  as well as the final driver stage  980  are sent to output mixer stage  990  which recombines the two signals and outputs them via an emitter electrode. This allows the circuit  900  to adapt to a wide variety of input frequencies and signals from a wide variety of touchscreen devices. A microcontroller unit (MCU) (not shown) also receives the output from the driver stage  850  to determine the presence or absence of signal, to determine the output levels to generate, and to control power to the overall circuit; this saves on battery life. 
     Regulator stage  960 , which may also be a charge pump circuit or integrator circuit, receives its input from the driver stage  850 . The regulator stage  960  removes the oscillations from the signal; ideally the regulator stage  960  would output a constant voltage at the peak of the input signal. Due to losses, the regulator stage outputs about 80% to 85% of the input signal peak voltage; when the input signal peaks at 3V, the regulator stage  960  outputs about 2.5V. 
     The comparator stage  970  has a reference voltage generator  971 , supplying the output of reference voltage generator  971  to the non-inverting input of an op-amp  975 ; this input is supplied through a voltage divider  973  and is filtered by filter capacitor  974  to further stabilize it. The signal from the second amplification stage  820  is supplied to the inverting input of op-amp  975 . Feedback from the op-amp  975  output is linked back to the inverting input. Output from the comparator stage  970  is a square wave of the input frequency sensed by the sensing electrode (not shown) ranging from 0V to 3V. 
     Final driver stage  980  is an IC amplifier, pin  1  of which is always enabled. Power is supplied to pin  3 , the square wave output of comparator stage  970  is supplied to pin  2 , and output pins  5  and  6  are tied together, outputting a pure square wave matching the input frequency sensed by the sensing electrode (not shown) at between 24V to 30V. This square wave signal is supplied to the output mixer stage  990 . 
     Output mixer stage  990  is a MOSFET power transistor. The voltage level from the regulator stage  960  is applied to the MOSFET gate, the square wave pure frequency signal from the final driver  980  is applied to the MOSFET source, and output from the MOSFET drain is electrically coupled to a high voltage source, for example 30V, through a resistor and the resultant signal is sent through filter capacitor  991  to the emitter electrode; the resultant output signal has its baseline at the high voltage, and when an input signal is being detected it has a square waveform dipping from 30V to some intermediate voltage, said square waveform dipping as low as 0V when an input signal at full voltage is detected. An alternate embodiment has the voltage level from the regulator stage  960  applied to the MOSFET gate, the MOSFET source is connected to ground, and the MOSFET drain is electrically coupled to the final driver  980  output, with the resultant signal sent through a filter capacitor to the emitter electrode; the resultant output signal has its baseline at 0V, and emits a square waveform rising from 0V to some intermediate voltage, said square waveform rising as high as 30V when an input signal at full voltage is detected. Optionally, a varistor (not shown) may be connected after the capacitor  991  to ground to provide for ESD protection. 
     The embodiments described herein disclose improvements in the field of capacitive sensing, and more particularly, in styluses for interacting with touchscreen and touchpad sensors and other capacitive sensing devices.