WO2013165466A1

WO2013165466A1 - Stylus and stylus circuitry for capacitive touch screens

Info

Publication number: WO2013165466A1
Application number: PCT/US2012/067897
Authority: WO
Inventors: John Conrad MANN
Original assignee: Jcm Electronic Stylus Llc
Priority date: 2012-04-29
Filing date: 2012-12-05
Publication date: 2013-11-07
Also published as: EP2845083A1; CN104285198A; EP2845083A4; CN104285198B

Abstract

Active stylus circuitry for a capacitive touch screen includes an inverting charge integrator circuit and an inverting amplifier. To reduce power consumption of the active stylus circuitry, a touchscreen sensing circuit senses an increase in a driveline voltage at the capacitive touchscreen and, upon detection of the increase, connects a power supply to the inverting charge integrator and the inverting amplifier. An automatic gain control circuit may be implemented to adjust gain of the active stylus circuitry depending on the sensitivity of the capacitive touchscreen. A dual-tip active/passive stylus is disclosed in which charge flowing through the passive tip during use is measured to set a gain for the active stylus tip circuitry.

Description

STYLUS AND STYLUS CIRCUITRY FOR CAPACITIVE TOUCH SCREENS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional application Serial No.

61/639,951 filed April 29, 2012 and U.S. non-provisional application Serial No. 13/607,051 filed September 7, 2012, the disclosures of which are incorporated by reference.

BACKGROUND

[0002] Embodiments of the present invention pertain to stylus technology, and in particular circuitry enabling interaction between a stylus and a capacitive touch screen. One manufacturer of capacitive touch screens and stylus technology includes SYNAPTICS, INC. having its Corporate Headquarters at 3120 Scott Blvd., Santa Clara, CA 95054.

[0003] U.S. Patent No. 8,125,469 titled "Passive stylus for capacitive sensors" discloses in its Abstract a passive stylus for capacitive sensors comprising a tip and a shaft. The tip is configured to couple electrically with a capacitive sensing device and to couple physically and electrically with the stylus shaft. The tip comprises a contact surface, a support region, and a flexible region. The contact surface is configured to contact a device surface associated with the capacitive sensing device. The flexible region is disposed between the contact surface and the support region. The flexible region comprises a hardness gradient. The support region is configured to provide structural support to the flexible region.

[0004] U.S. Patent No. 5,942,733 titled "Stylus input capacitive touchpad sensor" discloses in its Abstract a capacitive touch pad comprising a substrate material, such as a PC board type laminate material, having a plurality of first parallel conductive traces running in a first (X) direction disposed on a first face thereof, and a plurality of second parallel conductive traces running in a second (Y) direction, usually orthogonal to the first direction, disposed on an opposed second face thereof. A layer of pressure-conductive material is disposed over one of the faces of the substrate. A protective layer with a conductive coating on its back surface is disposed over the top surface of the pressure-conductive material to protect it. In an alternate embodiment, a capacitive touch sensor comprises a rigid substrate material having a conducting material disposed on one face thereof. A layer of pressure-conductive material is disposed over the conductive material on the substrate. A flexible material, having a plurality of first parallel conductive traces running in a first (X) direction disposed on a first face thereof, and a plurality of second parallel conductive traces running in a second (Y) direction disposed on an opposed second face thereof is disposed over the layer of pressure-conductive material. A protective layer is disposed over the top surface of the pressure conductive material to protect it. In yet another embodiment, an air gap is used in place of the layer of pressure-conductive material and the upper layers are supported by a frame at the periphery of the touchpad.

[0005] U.S. Patent No. 5,488,204 titled "Paintbrush stylus for capacitive touch sensor pad" discloses in its Abstract a proximity sensor system including a touch-sensor pad with a sensor matrix array having a characteristic capacitance on horizontal and vertical conductors connected to sensor pads. The capacitance changes as a function of the proximity of an object or objects to the sensor matrix. The change in capacitance of each node in both the X and Y directions of the matrix due to the approach of an object is converted to a set of voltages in the X and Y directions. These voltages are processed by circuitry to develop electrical signals representative of the centroid of the profile of the object, i.e, its position in the X and Y dimensions. Noise reduction and background level setting techniques inherently available in the architecture are employed. A conductive paintbrush-type stylus is used to produce paint-like strokes on a display associated with the touch-sensor pad.

[0006] U.S. Patent No. 7,612,767 titled "Trackpad pen for use with computer touchpad" discloses in its Abstract a pen or stylus for use with a finger activated computer touchpad that uses capacitively coupled voltage signals to simulate the capacitive effect of a finger on the touchpad. In addition, the pen has buttons that can be utilized to capacitively couple control signals to the touchpad that are interpreted by application software as specific user-defined inputs. The pen has a conductive tip that is placed into contact with the touchpad. By biasing the touchpad electrodes with a properly timed voltage signal, the pen alters the charging time of the electrodes in the touchpad. This alteration in charging time is interpreted by the touchpad as a change in capacitance due to the presence of a user's finger. Thus, the pen can be used with touchpads that were designed to only detect finger movements.

[0007] U.S. Patent Application Publication No. 2010/0225614 titled "Stylus Device Adapted

For Use With A Capacitive Touch Panel" discloses in its Abstract a stylus device adapted for use with a capacitive touch panel, that includes a main body having a handle portion, and a transparent touch portion connected to the handle portion, adapted to be placed on the capacitive touch panel and having a flat touch surface. A transparent conductive membrane is formed on the touch portion and the handle portion, and covers the touch surface of the touch portion so that the transparent conductive membrane connects electrically a user's hand when the handle portion of the main body is held by the user's hand.

[0008] Embodiments of the present invention provide an alternative to known stylus technology for capacitive touch screen interfaces.

SUMMARY

[0009] Embodiments of the present invention include a stylus for a capacitive touch screen, and stylus circuitry. According to one embodiment, the stylus includes a conductive tip for providing capacitive coupling with a capacitive touch screen. An inverting charge integrator is connected to the conductive tip for providing an output signal proportional to a charge induced at the conductive tip. An inverting amplifier generates an amplified signal proportional to the signal output from the inverting charge integrator. A conductive contact provides electrical or capacitive coupling between the amplified signal and an exterior of the stylus. A power supply circuit powers the inverting charge integrator and the inverting amplifier.

[0010] According to another embodiment, an inverting transimpedance amplifier is connected to the conductive tip for generating an output signal proportional to a current induced at the conductive tip. An inverting integrator provides an output voltage proportional to an integrated output signal of the inverting transimpedance amplifier.

[0011] Another embodiment comprises a stylus for a capacitive touch screen. The stylus includes an elongated barrel for enclosing a stylus circuit. The stylus circuit includes an integrator circuit connected to a conductive tip of the stylus for generating an output signal proportional to a charge induced at the conductive tip, and an amplifier circuit for providing an output voltage proportional to the output signal from the integrator circuit. They stylus also includes a conductive contact for providing electrical or capacitive coupling between the output voltage from the amplifier circuit and a surface of the stylus. In an alternative configuration, an integrator circuit is connected to a conductive tip of the stylus for generating an output signal proportional to a charge induced at the conductive tip. An amplifier circuit provides an output voltage proportional to the output signal from the integrator circuit that is then coupled with a surface of the stylus.

[0012] Another embodiment includes a stylus for a capacitive touch screen comprising circuitry for determining an amount of charge induced at a stylus tip by a positive or negative going drive line transition in a capacitive touch screen. They stylus also includes circuitry for outputting to an exterior of the stylus a positive or negative going voltage change that is proportional to the determined amount of charge. In this configuration, a positive going drive line transition results in a positive going output voltage change, and a negative going drive line transition results in a negative going output voltage change.

[0013] Yet another embodiment includes a method for charging a tip of a stylus for a capacitive touch screen. The method includes determining an amount of charge induced at a stylus tip by a positive or negative going drive line transition in a capacitive touch screen, and outputting to an exterior of the stylus a positive or negative going voltage change that is proportional to the determined amount of charge. According to the method, a positive going drive line transition results in a positive going output voltage change, and a negative going drive line transition results in a negative going output voltage change.

[0014] The inverting charge integrator and/or the inverting amplifier may include one or more operational amplifiers. These components and/or their functions may also be integrated. Shielding may be provided for isolating components of the circuit such as the connection between the tip and the inverting charge integrator. The shielding may be connected to a ground of the stylus circuit. The shielding may take the form of a conductive enclosure having a circular cross-section and extending along an axis of at least a portion of the stylus, a conductive wrapping around at least a portion of the stylus circuit, a substantially continuous conducting layer of a circuit board for the stylus circuit, and/or one or more conducting planes above and/or below a plane of the stylus circuit.

[0015] The circuitry may include a power supply having one or more batteries. The batteries may be rechargeable. The power supply may include a DC-to-DC converter for increasing voltage supplied to the circuitry. An input ground and an output ground of the DC-to-DC converter may be common. In other embodiments, the DC-to-DC converter may include regulator circuitry utilizing feedback for regulating an output voltage. An optical isolator may be utilized for communicating a level of the output voltage to a feedback input to the converter. [0016] The tip of the stylus may take a variety of forms such as a ball point or other fine point of contact. The tip may comprise a metal and/or a conductive polymer. The tip may have a cross-section projected onto the touch screen of less than 3.5 millimeters.

[0017] These and other embodiments of the invention, as recited in the claims, are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is an example cross section of a stylus assembly for a capacitive touch screen in accordance with one embodiment of the present invention.

[0019] Figures 2-4 illustrate example alternative stylus circuitry schematics that may be included within circuitry module illustrated in Figure 1.

[0020] Figure 5 is a flow diagram describing an example operation of the circuitry disclosed in Figure 4.

[0021] Figure 6 illustrates an example capacitive circuit formed between a capacitive touch screen, stylus assembly, and the body of a person holding the stylus assembly.

[0022] Figure 7 is a flow chart illustrating an example operation of the stylus circuitry of

Figure 2 as implemented in the stylus assembly of Figure 1 within the capacitive circuit illustrated in Figure 6.

[0023] Figures 8-10 illustrate alternative power supply configurations for the stylus circuitry.

[0024] Figures 11 a- 11 c illustrate alternative shielding configurations for the stylus circuitry.

[0025] Figure 12 illustrates an example power-saving circuit.

[0026] Figures 13a-c illustrate oscilloscope captures of capacitive touchscreen drive line voltages at three different time scales.

[0027] Figure 14 illustrates a dual-tip stylus having automatic gain control. [0028] Figure 15 illustrates an example circuit for automatically setting a gain of the stylus amplification circuitry.

DETAILED DESCRIPTION

[0029] Detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0030] One example embodiment of the present invention includes a two-terminal stylus.

One terminal includes the stylus tip, the other terminal includes the stylus body. Viewed as a two- terminal device, the electronic behavior of the stylus may be described as a negative capacitor. The stylus presents a negative capacitive impedance to a circuit into which it is incorporated.

[0031] For example, the impedance of an ordinary passive capacitor is:

whereas the impedance of a negative capacitor is:

_/ω(— C) (x)C where ω = angular frequency, j= V— Ϊ, and C>0.

[0032] The two terminal stylus may exhibit negative capacitive impedance over a broad range of frequencies, including the frequencies involved in the operation of capacitive touchscreens.

[0033] Figure 1 is an example cross section of a stylus assembly 10 for a capacitive touch screen in accordance with one embodiment of the present invention. Stylus assembly 10 includes a barrel 12. Barrel 12 may be manufactured of electrically conductive material such as metal, an electrically conductive polymer, or other material capable of capacitively coupling changes in voltage at connector 20 (or other suitable connection) with the body of a person holding stylus assembly 10.

[0034] Battery 16 may comprise any battery form, and any number of batteries depending on the desired stylus shape and desired voltage level. In this example, a AAA alkaline penlight battery is used. Battery 16 may be rechargeable. A battery charge circuit and charge status LED (not shown) may be included in assembly 10. As discussed in detail below, battery 16 provides power to circuitry module 14 for operating the stylus assembly.

[0035] Switch 18 may be implemented to complete or interrupt the power supply from the battery 16 to the circuitry module 14. Switch 18 may take any form depending on the particular configuration of the stylus and the desired design. Switch 18 may be a traditional penlight pushbutton switch as shown in Figure 1.

[0036] Contact 22 provides electrical connection between the positive pole of battery 16 and circuitry module 14. Spring contact 28 provides electrical connection between the negative pole of battery 16 and switch 18, which is then connected to the power supply ground and stylus circuitry output (not shown) of circuitry module 14 via the conductive housing of the stylus. Of course, other battery connections and switch configurations may be implemented depending on the particular geometric configuration of the stylus.

[0037] Stylus assembly 10 includes tip 26 having electrical connectivity 24 to stylus circuitry

14. Tip 26 may comprise a variety of configurations including but not limited to a ball point, a fine tip or other low contact area geometry. Tip 26 may comprise a metal, an electrically-conductive polymer, or a combination of both. Tip 26 may be coated with TEFLON or other suitable material to prevent scratching the surface of the touch screen. Tips having a radius of curvature as low as 0.4mm have been confirmed to work with embodiments of the invention described herein. Tips with a smaller radius of curvature may also be implemented.

[0038] Figures 2-4 illustrate example stylus circuitry schematics that may be included within circuitry module 14 illustrated in Figure 1. Other circuit configurations may be implemented within the scope of the invention as recited in the claims. The circuitry may comply with the following transfer function: Where V_out is the voltage output of the circuit connected to the conductive body of the stylus assembly 12, A is the amplifier gain of the circuit with dimensions of inverse farads, and I (in) is the input current at tip 26 induced from the capacitive coupling tip of 26 with the capacitive touch screen during a drive line transition.

[0039] Example schematic 40 illustrated in Figure 2 includes tip input 42 connected to an inverting charge integrator comprising operational amplifier 44 and capacitor 46. Operational amplifier 44 may be, for example, MICREL Part No. MIC921. In this example, capacitor 46 may be in the range of 2-10 pF. The output of the inverting charge integrator is input into an inverting amplifier comprising operational amplifier 50, resistor 52 and resistor 48. In this example, resistor 52 is in the order of 5-20 kQ and resistor 48 is in the range of 1 -5 hfl. The ratio of resistor 52 to resistor 48 defines the gain of the inverting amplifier. This ratio may be adjusted to generate an appropriate output voltage for operation of the stylus with a plurality of different capacitive touch screen configurations. The gain may also be adjusted to reduce or eliminate oscillation. Operational amplifier 50 may be, for example, LINEAR TECHNOLOGIES Part No. LT 1354 having output in the range of +/- 10 volts. Power supplied to operational amplifiers 44 and 50 is not shown.

[0040] Output of inverting operational amplifier 50 may be connected to the body or external surface of stylus barrel 12 via connector 20 illustrated in Figure 1. Other connections between stylus circuitry output and the stylus barrel exterior 12 may be implemented. For example, a direct contact may be established between the circuitry module exterior (to which the circuitry output may be connected) and the interior of barrel 12. Alternatively, a portion of the circuitry module 14 connected to the circuitry output may be threaded for physically and electrically attaching to the stylus housing. Other contact configurations may be implemented.

[0041] Waveform 56 shows a current spike generated at tip 42 resulting from a positive transition of drive line voltage at the touch screen (not shown). Inverting charge integrator 44 outputs a downward voltage transition 58 that is proportional to the charge induced at the tip 42. Inverting amplifier 50 outputs an amplified positive voltage transition 60 to the conductive external surface of the barrel 12 that is proportional to the input voltage drop 58. [0042] Circuit 40 may require shielding to prevent oscillation caused by detection at inverting input of operational amplifier 44 of the voltage output to stylus body 12 via contact 20 illustrated in Figure 1. Example alternatives for shielding circuit 40 are described below.

[0043] Figure 3 illustrates an alternative schematic 70 for circuitry module 14 illustrated in

Figure 1. Example schematic 70 includes tip input 72 connected to the input of an inverting transimpedance amplifier 74. The output of transimpedance amplifier 74 is connected to an inverting integrator 70 composed of resistor 86, operational amplifier 76, and capacitor 71. Output of inverting integrator operational amplifier 76 is connected to the body or external surface of stylus barrel 12 via connector 20 illustrated in Figure 1.

[0044] Waveform 80 shows a current spike generated at tip 72 resulting from a positive transition of drive line voltage at the touch screen (not shown). Inverting transimpedance amplifier 74 outputs a negative voltage spike 82 that is proportional to the current induced at the tip 42. Inverting integrator 76 outputs an amplified positive voltage transition 84 to the conductive external surface of the barrel 12 that is proportional to the negative voltage spike 82.

[0045] Circuit 70 may require shielding to prevent oscillation caused by detection at inverting input of operational amplifier 74 of the voltage output to stylus body 12 via contact 20 illustrated in Figure 1. Example alternatives for shielding circuit 70 are described below.

[0046] Figure 4 illustrates an alternative schematic 90 for circuitry module 14 illustrated in

Figure 1. Circuit 90 is similar to circuit 40 illustrated in Figure 2, with the addition of sample/hold elements 98 and 102, edge detector 108 and timing/sequence circuit 110. The introduction of sample/hold elements 98 and 102 may reduce or eliminate oscillation caused by detection at inverting input of operational amplifier 74 of the voltage output to stylus body 12, thus reducing or eliminating the need for shielding of circuit 90.

[0047] Figure 5 is a flow diagram describing an example operation 120 of circuit 90. The operation of circuit 90 is not limited to the particular process illustrated in Figure 5. Modifications to the circuit 90 and process 120 may be made to best- fit a particular implementation. At initial step 122, switches A, B and C are open awaiting detection by edge detector 108 of a drive line transition at touch screen. Upon detection of a drive line transition at edge detector 108 at step 124, timing/sequence circuit 110 closes switch B at step 126 supplying voltage to sample/hold element 98. Timing/sequence circuit then opens switch B at step 128 to isolate sample/hold element 98 from inverting charge integrator 94. Timing/sequence circuit 110 closes switch A at step 130. Timing/sequence circuit then closes and opens switch C at steps 132 and 134. Timing/sequence circuit 110 then opens switch A at step 136, and returns the process to step 124 to detect another drive line transition.

[0048] Figure 6 illustrates an example capacitive circuit formed between a capacitive touch screen 150, stylus assembly 152 and the body of a person 154 holding the stylus assembly 152. Touch screen 150 may include a body and ground plane 156, a drive and sense electrode plane 158 and top glass 160. A relatively small capacitance exists between stylus tip 26 and touch screen electrodes 158 (e.g. -0.6 pF) in comparison to the relatively large capacitance that exists between the human body 154 and the touch screen ground plane 156 (e.g. -100 pF). The connection between the human body 154 and the ground plane may therefore be viewed as an AC short in the capacitive circuit illustrated in Figure 6. As a result, the body 154 may be at the same potential as the ground plane 156.

[0049] Figure 7 is a flow chart illustrating an example operation 170 of the stylus circuitry of

Figure 2 as implemented in the stylus assembly of Figure 1 within the capacitive circuit illustrated in Figure 6. At step 172, one or more touch pad drive lines transition from low (e.g. 0 volts) to high (e.g. 3.7 volts). At step 174, a charge is induced at the tip 26 of the stylus assembly. At step 176, the charge integrator outputs a negative voltage proportional to the charge induced at the tip 26. At step 178, the amplifier outputs an amplified positive voltage proportional to the charge induced at the tip 26. At step 180, the amplified voltage charges the human body 154 via contact 20 in electrical communication with stylus exterior 12 gripped by a user's hand. Because the body 154 is at the same potential as the ground plane 156, negative charge is supplied from the body through the stylus circuitry to the stylus tip 26 at step 182. At step 184 the negative charge supplied to tip from the body 154 is detected by one or more of the touch screen sense lines.

[0050] In an alternative configuration to the schematics shown above, a charge integrator may be implemented without a separate inverting amplifier, to generate an appropriate output voltage of the proper polarity. Alternative circuit configurations for stylus circuitry may also include a current integrating amplifier connected to the stylus tip whose output may input to a voltage controlled oscillator which generates pulses proportional to the voltage output by the current integrating amplifier. The output of the voltage controlled oscillator is a pulse train having a frequency proportional to the input voltage of the voltage controlled oscillator. The output of the voltage controlled oscillator is input into an RC low pass filter, having an output connected to the exterior of the stylus barrel.

[0051] Other analog implementations having discrete components generally complying with the transfer function and/or functionality described above. Other alternative configurations may include a digital processor for performing one or more of the functions described above, and having analog-to-digital and digital-to-analog converters for the processor input and output, respectively. A digital implementation may include appropriate discrete circuit components for power supply and interfacing with the stylus tip and barrel.

[0052] Figure 8 illustrates an example isolating power supply circuit 200 for stylus circuitry described in Figures 2-4. Other power supply circuits may be implemented. In this example, one or more batteries 202 supply DC voltage (e.g. 1.2 - 3.7 volts) to a DC/DC switcher IC 204. DC/DC switcher IC may be, for example, LINEAR TECHNOLOGIES Part No. LT1615. The Vcc and SW outputs of DC/DC switcher IC 204 are connected to transformer 206 for generating, AC current, +/- 12 volts in this example. Transformer 206 may be, for example, COOPER BUSSMAN Part No. SDQ12-100-R. The AC output of transformer 206 is input into rectifier 208 which, in turn, outputs +/- 12 VDC. The diodes comprising rectifier 208 may be, for example, DIODES, INC. Part No. ZHCS400.

[0053] Feedback may be provided to DC/DC switcher 204 to ensure constant DC voltage at the output of rectifier 208. In this embodiment, a photo isolator 210 is implemented to provide electrical isolation between the power supply output by rectifier 208 and the battery power supply 202. Photo isolator 201 may be, for example IXYS CORP., Part No. CPC1001N. Because the output of the stylus circuitry disclosed in Figures 2-4 and 6-7 is connected to the stylus body, the isolating configuration shown in Figure 8 electrically isolates the battery potential from the stylus output.

[0054] Figure 9 illustrates an alternative power supply configuration in which the negative input 224 to DC/DC switcher 222 is electrically connected to the circuit ground 226 of stylus circuitry 228. This configuration electrically isolates the output 230 of the stylus circuitry

(connected to the stylus body) from the potential created by the battery cell(s) 232. [0055] Figure 10 illustrates an alternative power supply configuration in which no components are necessary to increase DC voltage of the battery cell(s). In this configuration, one or more batteries 252 and 254 are arranged to provide sufficient potential, e.g. +/- 12 VDC, for operating stylus circuitry 256.

[0056] The circuit functionality described herein may be realized in a number of different technological ways. An analog approach may be utilize operational amplifiers of moderate (e.g. up to 40 MHz) gain-bandwidth product and moderate (e.g. up to 400 V/microsecond) output slew rate. To reduce cost of manufacture and lower power consumption, a custom analog Application Specific Integrated Circuit (ASIC) may be implemented. The functionality of the stylus may be implemented in a simple ASIC incorporating one or two dozen relatively low f_t (e.g. -500 MHz) transistors and integrated passives. Alternatively, a Configurable Analog Array or Field Programmable Analog Array may be implemented. A stylus design incorporating analog and digital elements may also be implemented in a single ASIC, or a combined functionality (analog + digital) field programmable component.

[0057] Negative capacitive impedance may also be achieved using a powered mechanical or micromechanical system or a powered pneumatic or hydraulic system, using the known mathematical correspondences between these types of systems. The input and output stages of a mechanical system could be springs and/or parallel flexible conductive plates, functioning in dual mechanical (a metal spring is also an inductor, two parallel flexible conductive plates also form a capacitor) and electrical modes.

[0058] Figures 11a through 11c are end-view cross-sections of the stylus assembly 10 illustrating various configurations for shielding the input to the stylus circuitry described in Figures 2 and 3 from the output of the circuitry to prevent oscillation (the sample/hold configuration disclosed in Figure 4 may not require shielding). In Figure 11a, a tubular shielding 400 connected to stylus circuit ground is located concentrically within and spaced between circuit board 405 and stylus barrel 402. In Figure 1 lb, one or more PCB ground layers or power layers 404 of the stylus circuit board 406 are utilized to shield the stylus circuitry and prevent oscillation. In Figure 11c, shielding is provided in a parallel plane 408 and/or 410 spaced above and/or below stylus circuitry 407. Other shielding configurations may be implemented for shielding the input circuitry from the circuitry output to prevent oscillation. [0059] Some capacitive touchscreen devices may not monitor all points on the touchscreen surface at all times. Rather, the device may scan the surface for a finger or fingers in raster scanning fashion. Thus, the device may only monitor a given rectangular patch. For example, the device may monitor at particular location plus or minus two or three drive lines for a short time, and then monitor other portions of the touchscreen at other times. With this touchscreen configuration, a stylus may be powered down while the touchscreen is looking elsewhere. The stylus may include a means of detecting when the raster scanning is near its location.

[0060] According to one example embodiment, the stylus may include a micropower analog comparator to sense drive line transitions in the tip's area. When the first transitions are detected, the analog comparator may activate a monostable multivibrator which enables power to the stylus circuitry described above for the time period that the sense pads of the touchscreen near that location are being monitored.

[0061] Figure 12 shows an example circuit schematic 500 for performing the previously- described power saving function. Analog comparator 508 receives a threshold voltage input (Input A) and tip input 502 (Input B). When tip Input B matches or exceeds threshold Input A, comparator 508 drives input 509 of OR gate 510. OR gate 510 also receives input 506 from the output of finger emulation circuitry 516. The output of OR gate 510 drives the trigger of retriggerable monostable multivibrator 512. Output of retriggerable monostable multivibrator drives switches 504 and 520 closed. Switch 504 electrically connects tip input 502 to the finger emulation circuitry 516, emodiments of which are described herein. Switch(es) 520 provide power 514 to the finger emulation circuitry 516. The output of the finger emulation circuitry 516 is communicated to a portion of the stylus body, as described above.

[0062] Figure 13a illustrates sensed drive line voltages at a given location on an iPad3 touchscreen occurring approximately every 17 milliseconds. Figure 13b shows that the driveline voltages at the given location on the touchscreen have a duration of approximately 2 milliseconds. Figure 13c shows the first 25 microseconds of the drive line voltage increase illustrated in Figure 13b.

[0063] In one embodiment, the comparator 508 would have a threshold input configured to trigger the monostable multivibrator 510 and power the finger emulation circuitry 506 when the sensed drive line voltages were initially increasing as illustrated in Figure 13c. In this example, the average power consumption of the stylus circuitry 506 will be approximately 2 msec / 17 msec = 12% of what the power consumption would be if that circuitry was powered up all the time. The touchscreen scanning timings for other touchpads may differ, but configurations that perform raster scanning may utilize this power saving approach.

[0064] It is recognized that different capacitive touchscreens may have different sensitivities.

According to one embodiment, the amplification of the stylus circuitry can be varied depending on the sensitivity of the particular touchscreen the stylus is intended to operate with. A given amplification setting may work with a range of different touchscreens. Setting a variable gain may be done manually and/or automatically.

[0065] Amplifiers, in general, can oscillate if provided enough feedback from the output to the input. The feedback path of interest is the capacitance of the human body holding the stylus to the bulk of the tablet computer, in series with the capacitance of the bulk of the tablet computer to the input tip of the stylus. It is desirable to operate the stylus in the region where the closed loop gain, composed of the feedback gain times the internal gain, is less than that which will produce oscillation (less than 1).

[0066] Preferably, the closed loop gain is slightly below that which will cause oscillation of the stylus-human-tablet system. A satisfactory manual way of setting the gain of the stylus would be to place the stylus tip against the touchscreen and operate a potentiometer to turn up the gain until oscillation occurs, then reduce the gain slightly so oscillation ceases. In one approach, the user may slide the stylus tip around the touchscreen at a given gain setting to confirm that no oscillation occurs. For example, the various internal metal components of the tablet computer may change the coupling to the stylus tip depending on location on the screen. This manual setting may be performed with the tablet turned on or turned off. The coupling capacitance from the bulk of the tablet to the stylus tip may be similar in each case.

[0067] The process of adjusting the internal gain of the stylus circuitry to an amount below that which produces oscillation of the system could be automated. In one example, a digitally controlled potentiometer may be configured to vary the gain in an incremental and systematic manner. An analog comparator monitoring the output voltage of the stylus may be implemented to detect oscillation. If the loop is not oscillating, the output voltage of the comparator will be zero.

Logic may be implemented to incrementally vary the gain setting higher and higher until oscillation is detected. At this point, the gain setting is reduced to a setting before oscillation was detected. The logic to have the digitally variable gain setting element slowly turned higher until oscillation is detected and then backed off a bit may be implemented in a microprocessor having software to perform the described functions, or with a CPLD finite state machine. Detecting oscillation may require that the tablet computer be turned off, so no touchscreen drive line transitions are detected. Additionally, a user may want to slide the stylus tip around different areas of the touch screen to allow detection of oscillation at any high-gain locations.

[0068] The manually or automatically determined gain value may be stored in memory so that the user would not need to repeat the variable gain setting operation unless the stylus was to be used with a different model of tablet computer.

[0069] Figure 14 illustrates a dual-tip stylus configuration 600 including a fine tip 602 at one end and a larger passive conductive elastomer 612 at the other end. The fine tip 602 may include similar data collection capability (e.g., tip charge and voltage detection) as the passive end 612. A microprocessor 614 may be implemented to process the charge and voltage data from each end and use the information to set the gain of the fine tip circuitry to the optimum level. In practice, a user may calibrate the stylus for a give capacitive touchscreen. In a calibration mode (which may be entered using switches 608), the user would first use the large passive tip 612 with the touchscreen to allow the stylus circuitry described herein to gather data about the touchscreen. The user would then then switch to the fine tip 602 to enable the circuitry to gather voltage and charge information for that tip. A microprocessor may then perform a calculation to set the gain of the fine tip circuitry 606. The stylus would then be ready for use. The stylus would store the gain setting information when powered off.

[0070] Figure 15 illustrates an example circuit configuration 700 for automatic gain control.

In this example, the circuit is implemented in a dual-tip stylus configuration, such as that shown in Figure 14. Other automatic gain setting approaches may be implemented. In a calibration mode entered at microcontroller 706 by switch(es) 708, passive tip 714 may be connected 720 in a voltage sense mode or charge sense mode. In the charge sense mode, integrator 712 measures charge at passive tip 714 and presents a relative voltage to analog-to-digital converter (ADC) 708. Digital output of ADC 708 representing passive tip charge is provided to microcontroller 728. In a voltage sense mode, tip 714 is connected to ADC 708, which presents digital input to microcontroller 706 representing passive tip voltage.

[0071] Fine tip 718 may also be configured at switch 728 in a charge sense mode or a voltage sense mode. In the charge sense mode, tip 718 is connected to integrator 716 which in turn is connected to ADC 704 for presenting a digital representation of fine tip charge to microcontroller 706. In the voltage sense mode, tip 718 is connected to ADC 704 for presenting a digital representation of tip voltage to microcontroller 706. Stylus body 724 may be variably connected at switch 722 between fine tip charge sense and ground for large tip voltage/charge sense and fine tip voltage sense.

[0072] Upon detecting the relative charge and voltage at the active fine tip 718 at the passive tip 714, microcontroller 706 may determine an appropriate gain setting for variable gain amplifier 716 to prevent oscillation of the finger emulation circuitry. Microcontroller 706 operates digital potentiometer 702 to control the gain of variable gain amplifier 716.

[0073] In an alternative configuration, the stylus may include circuitry for sensing voltages at the stylus tip that are generated by the touchscreen. The ratio of charge sensed at the tip to voltage sensed at the tip would be roughly equal to the coupling capacitance between touchscreen drive line and conductive elastomer tip.

[0074] Alternative embodiments may be implemented to achieve the desired function of negative capacitive impedance. One approach includes an analog integrating stage feeding a traditional successive approximation register type analog to digital converter (SAR A/D). The output of the DAC aspect of the SAR A/D would be the stylus output. Signal smoothing may be accomplished using a RC low pass filter of appropriate time constant. The reference voltage of the DAC component of the SAR A/D converter could be varied to set the overall gain of the circuit.

[0075] The negative capacitive impedance function could also be implemented with discrete transistors, resistors, and capacitors. Such a circuit may include a JFET input stage with a relatively large capacitance from the JFET gate to stylus circuit ground, followed by a voltage gain stage and finally an emitter follower. The emitter connection of the emitter follower would be the stylus output, connected to the stylus' conductive body. [0076] In an alternative embodiment, a hybrid analog-digital implementation may be implemented which utilizes capacitors and operational amplifiers to stage the charge sensed at the stylus tip to the stylus output. This approach includes an edge-triggered amplifying charge -input voltage-output analog shift register. This approach does not behave as a negative capacitive impedance. It does, however, drive the stylus body positive with respect to the stylus tip, for a positive-going touchscreen drive line transition. This approach may minimize oscillation.

[0077] Because the electrostatic signature of a fine tip stylus on the surface of a touchscreen may be more localized than that of a fingertip, the centroid-fmding hardware and firmware of the capacitive touchscreen device may deliver coordinates to the operating system which are slightly erroneous. For a given touchscreen, the coordinate errors may be consistent, repeatable and measurable. Coordinate errors may be corrected by, for example, implementing a coordinate correction table. For each known (x, y) coordinate on the touchscreen surface, the position (χ', y') read out from the touchscreen to the operating system may be captured. The errors x-x' and y-y' may be tabulated and used to generate a correction table which would accept (χ', y') as the input and the physically correct (x, y) as the output.

[0078] Depending on the amount of data, another approach includes translating the coordinate into a mathematical formula. This approach relies on the assumption that the sensing array of the touchscreen is a repeating unit cell in X and Y directions, resulting in periodic errors. A periodic system may be analyzed and characterized using Fourier Analysis. In this fashion, a large offset data table could be reduced to a small number of coefficients of (integer) harmonic number, amplitude, and phase to make up a correction formula. This formula could be incorporated into the touchscreen device operating system, or applications running on the touchscreen device.

[0079] The overall functionality of the stylus may be enhanced by include sensing, actuator and indicator capabilities. Data sensed by the stylus may be sent to the tablet computer with which the stylus was being used. Indicators on the stylus may be activated by commands from the tablet computer. Actuators on the stylus body, such as push switches, could command the tablet computer or application running to perform a variety of functions. Data transmission between the stylus and the tablet computer for purposes of information exchange, indication, and control may be accomplished using a variety of techniques including but not limited to RF (e.g. Bluetooth), IR, ultrasound, and near field communication technology. Traditional mouse button functionality (e.g. left-click, right-click, scroll, etc.) may be implemented on the stylus. A transceiver located at the stylus may communicate the user inputs to a transceiver associated with the touchscreen, such as a short-range wireless transceiver of a tablet computer.

[0080] In another configuration the stylus may include one or more pressure sensors to detect higher and lower pressures between the touchscreen and the tip as a user presses harder and softer on the stylus during use. Variations in pressure may be communicated to the tablet computer indicating, for example, variations in the width of brush or pen strokes. Variations in tip pressure may also be utilized as an input to gaming and other applications unrelated to drawing or writing.

[0081] In yet another configuration, the stylus may include a 1-axis to 3 -axis (translational) accelerometer, or a full 6-axis (translational and rotational) accelerometer. The accelerometer may include an integrated circuit which employs micromechanical structures for the sensing of movements. Acceleration information may be communicated to the tablet computer as an input to a variety of applications, including writing, drawing, music and gaming applications,

[0082] The stylus may also include haptic technology for providing tactile feed to a person holding the stylus. For example, the stylus could shake or vibrate slightly when menu items on the touchscreen are selected by the user. Haptic feedback may be utilized in gaming and other applications executed on the tablet computer. The stylus may include other indicators configured to generate light or sound.

[0083] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.

Claims

WHAT IS CLAIMED IS:

1. A circuit for a capacitive touch screen stylus, the circuit comprising:

a conductive tip to provide capacitive coupling with a capacitive touch screen;

an inverting charge integrator connected to the conductive tip to provide an output signal proportional to a charge induced at the conductive tip;

an inverting amplifier to generate an amplified signal proportional to the signal output from the inverting charge integrator; and

a touchscreen sensing circuit to sense an increase in a driveline voltage at the capacitive touchscreen and, upon detection of the increase, connect a power supply to the inverting charge integrator and the inverting amplifier.

2. The circuit of claim 1 wherein the touchscreen sensing circuit includes a comparator to compare the driveline voltage to a threshold voltage.

3. The circuit of claim 2 wherein the touchscreen sensing circuit includes a monostable multivibrator to connect power to the inverting charge integrator and the inverting amplifier in response to a signal output from the comparator indicating that the driveline voltage exceeds the threshold voltage.

4. The circuit of claim 1 wherein a positive transition in potential at the tip results in a positive transition in voltage at the conductive contact.

5. The circuit of claim 1 wherein a negative transition in potential at the tip results in a negative transition in voltage at the conductive contact.

6. The circuit of claim 1 wherein the inverting charge integrator and the inverting amplifier comprise one or more operational amplifiers.

7. The circuit of claim 1 wherein the conductive tip is a ball point having a diameter of less than 3.5 millimeters.

8. A circuit for a capacitive touch screen stylus, the circuit comprising: a conductive tip to provide capacitive coupling with a capacitive touch screen;

an inverting transimpedance amplifier connected to the conductive tip to generate an output signal proportional to a current induced at the conductive tip;

an inverting integrator to provide an output voltage proportional to an integrated output signal of the inverting transimpedance amplifier;

a conductive contact to provide electrical or capacitive coupling between the output voltage from the inverting integrator to an exterior portion of the stylus; and

9. The circuit of claim 8 wherein the touchscreen sensing circuit includes a comparator to compare the driveline voltage to a threshold voltage.

10. The circuit of claim 9 wherein the touchscreen sensing circuit includes a monostable multivibrator to connect power to the inverting charge integrator and the inverting amplifier in response to a signal output from the comparator indicating that the driveline voltage exceeds the threshold voltage.

11. The circuit of claim 8 wherein a positive transition in potential at the tip results in a positive transition in voltage at the conductive contact.

12. The circuit of claim 8 wherein a negative transition in potential at the tip results in a negative transition in voltage at the conductive contact.

13. The circuit of claim 1 wherein the inverting charge integrator and the inverting amplifier comprise one or more operational amplifiers.

14. The circuit of claim 1 wherein the conductive tip is a ball point having a diameter of less than 3.5 millimeters.

15. A circuit for a capacitive touch screen stylus, the circuit comprising: a conductive tip to provide capacitive coupling with a capacitive touch screen;

a gain control circuit to automatically detect an oscillation of the inverting amplifier and adjust a gain of the inverting amplifier based on the oscillation detection.

16. A circuit for a capacitive touch screen stylus, the circuit comprising:

a conductive tip to provide capacitive coupling with a capacitive touch screen;

17. A capacitive touchscreen stylus comprising:

a passive tip and an active tip, the passive tip having a touchscreen contact that is larger than the active tip;

a charge detection circuit to measure an amount of charge communicated to or from the passive tip; and

a gain setting circuit to set a gain for the active tip based on the amount of charge communicated to or from the passive tip determined by the charge detection circuit.

18. The stylus of claim 17 additionally comprising one or more input devices and a transceiver for communicating input received at the one or more input devices to a capacitive touchscreen device.

19. The stylus of claim 18 wherein the input device includes a press-button.

20. The stylus of claim 18 wherein the input device includes an accelerometer.

21. The stylus of claim 17 additionally comprising one or more output devices and a receiver for receiving signals from a capacitive device for generating output at the output devices.

22. The stylus of claim 21 wherein the output devices comprise one or more LEDs.

23. The stylus of claim 21 wherein the output devices comprise a haptic device.

24. The stylus of claim 21 wherein the output includes sound.

25. A stylus for a capacitive touch screen, the stylus comprising:

an conductive stylus tip;

an elongated stylus body having a conductive surface;

a negative capacitor circuit in electrical communication with the stylus tip and the stylus body; and

a circuit for applying power to the negative capacitor circuit.

26. The stylus of claim 25 wherein the negative capacitor circuit includes:

a variable gain amplifier; and

a circuit for variably setting the gain of the variable gain amplifier to prevent oscillation of the negative capacitor circuit.

27. The stylus of claim 25 wherein the negative capacitor circuit includes a microcontroller.

28. The stylus of claim 25 in combination with a capacitive touchscreen device, the capacitive touchscreen device programmed and configured to offset a set of detected stylus coordinates to correct for error between the set of detected stylus coordinates and an actual coordinate location for the stylus on the capacitive touchscreen.