Patent Publication Number: US-9898103-B2

Title: Interacting tips for a digitizer stylus

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/839,985 filed on Aug. 30, 2015, which is a continuation of U.S. patent application Ser. No. 14/005,308 filed on Sep. 16, 2013, now U.S. Pat. No. 9,122,322 which is a National Phase of PCT Patent Application No. PCT/IL2012/050095 having International Filing Date of Mar. 15, 2012, which claims the benefit of priority under 35 USC § 119(e) of U.S. Provisional Patent Application No. 61/453,560 filed on Mar. 17, 2011. The contents of the above applications are all incorporated by reference as if fully set forth herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention, in some embodiments thereof, relates to a stylus for use with a digitizer sensor, and more particularly, but not exclusively, to interacting tips for a stylus. 
     BACKGROUND OF THE INVENTION 
     Touch technologies are commonly used as input devices for a variety of products. The usage of touch devices of various kinds is growing sharply due to the emergence of new mobile devices such as mobile phones, Personal Digital Assistants (PDA), tablet PCs, wireless flat panel displays (FPD) and other devices. Some of these devices are not connected to standard keyboards, mice or like input devices, which are deemed to limit their mobility. Instead there is a tendency to use touch input technologies of one kind or another, such as using a stylus and/or finger for user interaction. 
     Styluses are known in the art for use with a digitizer sensor. Position detection of the stylus provides input to a computing device associated with the digitizer sensor and is interpreted as user commands. Position detection is performed while the stylus tip is either touching and/or hovering over a detection surface of the digitizer sensor. Often, the digitizer sensor is integrated with a display screen and a position of the stylus over the screen is correlated with virtual information portrayed on the screen. 
     U.S. Patent Application Publication No. 2010/0155153 entitled “Digitizer, Stylus and Method of Synchronization Therewith,” describes a method for operating a digitizer with an autonomous asynchronous stylus. Typically the stylus is self-powered, e.g. battery operated. The method includes sampling outputs from a digitizer, detecting from the outputs at least one pulsed signal transmitted from an autonomous asynchronous stylus at a defined rate, determining a location of the stylus interaction with respect to the digitizer, and tracking stylus interaction with the digitizer over subsequent pulsed signals transmitted from the stylus. 
     U.S. Patent Application Publication No. 2007/0146351 entitled “Position Input Device and Computer System,” the content of which is incorporated herein by reference describes a computer system including a position pointing device for transmitting position signals. The position pointing device includes a built-in power supply unit, a plurality of signal transmitting units provided at a plurality of portions of the position pointing device, and a power control unit for controlling transmission power of each of a plurality of signal transmitting units. 
     U.S. Pat. No. 7,292,229 entitled “Transparent Digitizer” which is assigned to N-trig Ltd., the contents of which is incorporated herein by reference, describes a passive electro-magnetic stylus which is triggered to oscillate at a resonant frequency by an excitation coil surrounding a digitizer. The stylus operates in a number of different states including hovering, tip touching, right click mouse emulation, and erasing. The various states are identified by dynamically controlling the resonant frequency of the stylus so that the stylus resonates at a different frequency in each state. There is also described a stylus including both a front tip and a reverse tip, the reverse tip located at a second end of the stylus remote from the front tip. Both the front tip and the reverse tip are associated with a same resonant circuit, and an electric field concentration is created in a gap formed in the vicinity of each of the front tip and the reverse tip in response to triggering the stylus at the resonant frequency. A position of a tip of the stylus, e.g. the front tip and/or the reverse tip with respect to the digitizer sensor is determined based on signals sensed by the sensor. It is described that a detectable difference between output from the front and reverse tips is obtained by forming a gap in the vicinity of the front tip with a different geometry than that of a gap in the vicinity of the reverse tip. 
     U.S. Pat. No. 7,843,439, entitled “Touch Detection for a Digitizer” assigned to N-Trig Ltd., the contents of which is incorporated herein by reference, describes a detector for detecting both a stylus and touches by fingers or like body parts on a digitizer sensor. The detector typically includes a digitizer sensor with a grid of sensing conductive lines, a source of oscillating electrical energy at a predetermined frequency, and detection circuitry for detecting capacitive influence on the sensing conductive lines when the oscillating electrical energy is applied, the capacitive influence being interpreted as a touch. The detector is capable of detecting simultaneous occurrences of multiple styluses and/or finger touches. 
     U.S. Pat. No. 5,793,360 entitled “Digitizer Eraser System and Method,” the contents of which is incorporated herein by reference, describes writing/erasing digitizer pen including a resonance tuning circuit associated with a writing tip of the pen and a separate resonance tuning circuit associated with the erasing tip of the pen. A phase output of the resonance tuning circuit of the eraser tip is altered responsive to pressure applied on the eraser tip. A digitizer sensor detects the change in phase and identifies that input is being received from the eraser tip. The eraser tip is used to select and delete text, cells or other objects displayed on an associated display screen. 
     U.S. Pat. No. 5,576,502 entitled “Pointing Unit and Improved Stylus Pen,” the contents of which is incorporated herein by reference, describes an eraser unit or pointing unit for a second pen unit of a pressure sensitive stylus. The eraser unit includes a main body with a dome shaped cap that is inserted in an opening of a main body of the stylus. The cylindrical main body with the dome shaped cap is movable with respect to the main body of the stylus and recedes toward the main body of the stylus responsive to pressure applied on the cap. 
     SUMMARY OF THE INVENTION 
     According to an aspect of some embodiments of the present invention there is provided a stylus including a primary tip and/or writing tip on one end and a secondary tip on an opposite end of the stylus. According to some embodiments of the present invention, each of the writing tip and the secondary tip is capable of providing input to an associated digitizer and/or touch screen when brought in proximity to a sensing surface of the digitizer and/or touch screen. According to some embodiments of the present invention, the secondary tip includes power saving features. 
     According to an aspect of some embodiments of the present invention there is provided one or more secondary tips, each of which can be retrofitted on an existing stylus. In some exemplary embodiments, the secondary tip is an add-on device that can be selectively and/or repeatedly mounted and dismounted on a stylus by the user. Optionally, a kit including a plurality of secondary tips is provided for operation with a stylus, each of which is operable to provide dedicated functionality when mounted on the stylus for interacting with the digitizer. 
     According to an aspect of some embodiments of the present invention there is provided a stylus for use with a digitizer sensor, the stylus including a housing confined by a first and second end, a primary tip positioned at the first end of the housing and associated with a first transmitting element, the first transmitting element having a first diameter, a secondary tip positioned at the second end of the housing and associated with a second transmitting element, the second transmitting element having a second diameter that is larger than the first diameter, a transmitting unit for transmitting a first signal with a first amplitude via the first transmitting element and for transmitting a second signal with a second amplitude via the second transmitting element, wherein the first amplitude is at least twice the second amplitude, and a powering unit for powering transmission of the first and second signal. 
     Optionally, the first amplitude is at least five times the second amplitude. 
     Optionally, the second diameter is at least twice the first diameter. 
     Optionally, the second diameter is at least 3 mm. 
     Optionally, the second diameter is at least 5 mm. 
     Optionally, the second transmitting element is a conductive portion of the secondary tip. 
     Optionally, the secondary tip has a dome shape. 
     Optionally, the secondary tip is formed from conductive rubber. 
     Optionally, the secondary tip is coated with non-conductive material. 
     Optionally, the second signal is a modulated form of the first signal. 
     Optionally, modulation includes at least one of frequency and phase modulation. 
     Optionally, the first and second signals are burst signals and wherein modulation includes modulation of a repetition rate of the bursts. 
     Optionally, the stylus includes a tip position detecting unit operative for differentiating between a touch and hover operational state of the secondary tip of the stylus. 
     Optionally, the tip position detecting unit includes a contact switch that is activated by the secondary tip responsive to contact pressure applied on the secondary tip. 
     Optionally, the stylus includes a signal modulation circuitry operative to modulate a signal responsive to identifying the touch operation state of the secondary tip of the stylus. 
     Optionally, the stylus includes circuitry for blocking transmission of the first signal responsive to identifying a touch operational state of the secondary tip of the stylus. 
     Optionally, the stylus includes circuitry for blocking transmission of the second signal responsive to identifying a touch operational state of the primary tip of the stylus. 
     Optionally, the stylus includes circuitry for blocking transmission of the second signal responsive to identifying a hover operational state. 
     Optionally, the first and second signals are transmitted simultaneously. 
     Optionally, the first and second signals are burst signals that are transmitted at a pre-defined frequency. 
     Optionally, the stylus includes a first transmitting unit for transmitting the first signal and a second signal transmitting unit for transmitting the second signal, wherein the first and second transmitting units are operated independently. 
     Optionally, the stylus includes a first powering unit for powering transmission of the first signal and a second powering unit for powering transmission of the second signal, wherein the first and second powering units are independent. 
     Optionally, the secondary tip, the second powering unit and the second transmitting unit are housed in second housing selectively detachable from the housing. 
     According to an aspect of some embodiments of the present invention there is provided a stylus for use with a digitizer sensor, the stylus including a housing confined by a first and second end, a primary tip positioned at the first end of the housing and associated with a transmitting element, the transmitting element having a first diameter, a transmitting unit for transmitting a signal via the transmitting element, a powering unit for powering transmission of the signal via the transmitting element, and a secondary tip positioned at the second end of the housing, wherein the secondary tip is a passive tip formed from conductive material, the secondary tip having a diameter of at least 3 mm. 
     Optionally, the secondary tip is formed from conductive rubber. 
     Optionally, the secondary tip is movable with respect to the housing and recedes toward the housing responsive to contact pressure applied on the secondary tip. 
     Optionally, the stylus includes a contact switch that is activated in response to the secondary tip receding toward the housing by a defined distance. 
     Optionally, responsive to activation of the contact switch, the secondary tip is grounded via a low impedance connection. 
     Optionally, the stylus includes circuitry operable to switch, at a pre-defined frequency, between high and low impedance connection to GND responsive to activation of the contact switch. 
     Optionally, the secondary tip and the contact switch are housed in a separate housing that is selectively detachable from the housing of the stylus. 
     According to an aspect of some embodiments of the present invention there is provided interacting tip unit operable for providing input to a digitizer sensor, wherein the interactive tip unit comprises an interacting tip formed of conductive material having a dimension of at least 3 mm, the interacting tip unit adapted for retrofitting on one end of a stylus associated with the digitizer sensor. 
     Optionally, the interactive tip includes a housing, a transmitting unit for transmitting a signal via the interacting tip, and a powering unit for powering transmission of the signal. 
     Optionally, the interactive tip includes circuitry for modulating the signal responsive to detecting contact pressure applied on the interacting tip. 
     Optionally, the interacting tip is dome shaped. 
     Optionally, the interacting tip is formed from conductive rubber. 
     According to an aspect of some embodiments of the present invention there is provided a stylus kit including a stylus for use with a digitizer sensor, the stylus with a housing confined by a first and second end, a primary tip positioned at the first end of the housing and associated with a first transmitting element, the first transmitting element having a first diameter, a transmitting unit for transmitting a first signal with a first amplitude via the first transmitting, and a powering unit for powering transmission of the first signal, and a plurality of secondary tips units operable to be attached to the stylus, wherein each of the plurality of secondary tip units is operable to provide input to the digitizer sensor. 
     Optionally, each of the plurality of secondary tip units provides dedicated functionality selected from the group consisting of: erasing, drawing with a wide line, and drawing with a defined color. 
     Optionally, at least one of the plurality of secondary tip units is operable to transmit a signal. 
     Optionally, at least one of the plurality of secondary tip units comprises a tip position detecting unit for identifying touch and hover operations states of the secondary tip unit. 
     Optionally, at least one secondary tip unit from the plurality of secondary tip units is operable to transmit a signal with amplitude that is at most half the amplitude of the first signal transmitted by the first transmitting element. 
     Optionally, at least one secondary tip unit from the plurality of secondary tip units includes a tip with a diameter that is at least double a diameter of the first transmitting element. 
     According to an aspect of some embodiments of the present invention there is provided a stylus for use with a digitizer sensor, the stylus including a housing confined by a first and second end, a tip positioned at the first end of the housing and associated with a first transmitting element, a first conductive area substantially surrounding the tip and electrically isolated from the first transmitting element, a first conductive area substantially surrounding the tip and electrically isolated from the first transmitting element, wherein the first conductive area and the second conductive area are electrically isolated from each other, circuitry for transmitting a signal via the first transmitting element, wherein the circuitry is electrically connected at a first end to the transmitting element and at a second end to at least the first conductive area, and a switching element for selectively connecting the circuitry at the second end to the second conductive area. 
     Optionally, a tip pressure detecting unit associated with the tip for detecting pressure applied on the tip, wherein the switching element is operable to connect the circuitry at the second end to the second conductive area, responsive to a pre-defined detected pressure on the tip. 
     Implementation of the method and/or system of embodiments of the invention can involve performing or completing selected tasks manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of embodiments of the method and/or system of the invention, several selected tasks could be implemented by hardware, by software or by firmware or by a combination thereof using an operating system. 
     For example, hardware for performing selected tasks according to embodiments of the invention could be implemented as a chip or a circuit. As software, selected tasks according to embodiments of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In an exemplary embodiment of the invention, one or more tasks according to exemplary embodiments of method and/or system as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes a volatile memory for storing instructions and/or data and/or a non-volatile storage, for example, a magnetic hard-disk and/or removable media, for storing instructions and/or data. Optionally, a network connection is provided as well. A display and/or a user input device such as a keyboard or mouse are optionally provided as well. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. 
       In the drawings: 
         FIG. 1  is a simplified diagram of an exemplary stylus including a primary tip and an exemplary secondary tip that emits a signal derived from circuitry of the primary tip in accordance with some embodiments of the present invention; 
         FIG. 2  is a simplified diagram of an exemplary stylus including a primary tip and an exemplary secondary tip powered with a same power unit as that of the primary tip in accordance with some embodiments of the present invention; 
         FIG. 3  is a simplified diagram of an exemplary stylus including a primary tip and an exemplary secondary tip that operates independently from the primary tip in accordance with some embodiments of the present invention; 
         FIGS. 4A and 4B  are simplified diagrams of exemplary styluses, each including a primary tip through which a signal is transmitted and an exemplary secondary tip that is a conductive object in accordance with some embodiments of the present invention; 
         FIGS. 5A and 5B  are simplified diagrams of exemplary styluses retrofitted with exemplary secondary tip units in accordance with some embodiments of the present invention; 
         FIG. 6  is a simplified schematic diagram of a stylus with a primary tip and a plurality of exemplary secondary tips that can be selectively mounted on the stylus in accordance with some embodiments of the present invention; 
         FIG. 7  is a simplified schematic diagram of exemplary digitizer system for use with a stylus including a primary and secondary tip in accordance with some embodiments of the present invention; and 
         FIG. 8  is a simplified schematic diagram of an exemplary primary or secondary tip of a stylus in accordance with some embodiments of the present invention. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
     As used herein, the term tip, e.g. as in the phrases secondary tip, eraser tip, conductive tip, primary tip, writing tip, means any end of a stylus used to interact with a digitizer sensor and/or touch screen. It is noted that the term “tip” as stated herein does not require that the end be tapered or pointed. 
     The present invention, in some embodiments thereof, relates to a stylus for use with a digitizer sensor, and more particularly, but not exclusively, to interacting tips for a stylus. 
     According to some embodiments of the present invention, the functionality of a stylus is improved by including an additional interacting tip on an end opposite that of a writing tip of the stylus. According to some embodiments of the present invention, the secondary tip provides additional functionality such as for example erasing, marking, drawing wide lines and/or drawing with selected colors, or others. Optionally, a functionality of the secondary tip is selected by a user with a switch included on the stylus. Optionally, a functionally of the secondary tip is selected with a select feature displayed on an associated touch screen. Optionally, a secondary tip is associated with a specific functionality so that selection is not required. 
     In some known styluses that include both writing and an eraser tips, a signal is typically transmitted from both tips throughout operation of the stylus. Typically, transmission is from a conductive portion of a tip that operates as an antenna. Optionally, the conductive portion is coated with non-conductive material. Typically, transmission from both tips is required since each of the writing tip and eraser tip may operate in a hover mode as well as in a touch mode. During a hover mode of a stylus, a stylus is typically unaware of its orientation, e.g. unaware of which tip is interacting with the digitizer sensor. The present inventors have found that one of the potential draw backs of a battery operated stylus with two signal transmitting tips, e.g. a primary and secondary tip, is a significant increase in power required and/or significant reduction in battery life that may occur due to the additional continuous transmission that may be required from the secondary tip. Another potential drawback is the increase in the bill of materials associated with adding an additional signal transmitting tip to the stylus. 
     The present inventors have found that functionalities typically associated with an secondary tip of a stylus are functionalities that are used less often and/or for shorter periods of time as compared to functionalities provided by the writing tip and/or primary tip of the stylus, and that the accuracy required for many of these functionalities is typically lower than the accuracy required for functionalities assigned to the primary tip, e.g. the writing tip of the stylus. Based on these observations, the present inventors have found that it may be beneficial to improve battery life and/or reduce power requirement by transmitting a lower amplitude signal at the secondary tip. In some exemplary embodiment, amplitude of signal transmitted in the secondary tip is between 2-20 times less than that of the primary tip stylus. The present inventors have found that a same SNR on the digitizer, e.g. same and/or similar SNR obtained from the primary tip can be maintained with the secondary tip when transmitting a lower amplitude signal with a thicker antenna. According to some embodiments of the present invention, in order to comply with the lower power transmission, dimensions of a conductive portion of the secondary tip are increased so that detection of a lower amplitude transmission can be enhanced and/or so that the transmission power can be further reduced. Optionally, a diameter and/or at least one dimension of a conductive and/or transmitting portion of the secondary tip is 2-10 times larger than a diameter and/or corresponding dimension of a conductive and/or transmitting portion of the primary tip. 
     In some exemplary embodiments, performance of a stylus including two transmitting tips is further improved by blocking transmission of one of the two tips while the other tip is operating in a touch operation mode. Typically, each of the primary and secondary tips is associated with a pressure detecting mechanism for sensing contact pressure applied on the tip during a touch operational state. In response to determining a touch operation mode for one of the tips, transmission may be temporarily blocked for the other tip, and then resumed when the touch operational mode is terminated. Typically, a primary tip of the stylus is associated with a pressure sensing mechanism for sensing a threshold pressure applied on the tip associated with a touch operational mode and also for sensing different levels of pressure applied while writing. Optionally, a touch operational mode of the secondary tip is detected with a simpler mechanism such as a push button switch and/or contact switch that is activated when the tip recedes due to contact pressure and an extension of the tip engages the switch. 
     In some exemplary embodiments, a stylus includes a single oscillator for providing a time base for generating signals for both the primary tip and secondary tip of the stylus. Optionally, the signal transmitted by the secondary tip is different from the signal transmitted by the primary tip so that a digitizer can distinguish between input received by the primary tip and input received by the secondary tip. Optionally, the difference includes at least one of a modification in phase, frequency, amplitude, and pulse repetition rate. Optionally, at least one of amplitude shift keying (ASK), phase shift keying (PSK) and frequency shift keying (FSK) is used to modify the transmitted signal. In some exemplary embodiments, an additional modification is introduced in response to a detected touch operational state of the secondary tip. Optionally, a frequency of the signal is modified in response to a detected touch operational state. Optionally, the secondary tip only operates, e.g. transmits a signal, in a touch operational mode and does not transmit during a hover operational state. In some exemplary embodiments, both the primary and secondary tips use a same power source. Optionally, the secondary tip uses a dedicated power source. Optionally, the secondary tip uses a dedicated signal generator time base. 
     According to some embodiments of the present invention, a stylus includes a primary tip through which a signal is transmitted and a secondary tip that functions as a conductive object that provides input, responsive to capacitive coupling with the associated digitizer sensor, but through which no signal is transmitted. In some exemplary embodiments, the secondary tip is constructed from a conductive soft material, e.g. conductive rubber. 
     In some exemplary embodiments, the secondary tip is connected to earth ground (GND) via a low impedance connection. Optionally, the secondary tip includes circuitry to alternately couple the tip to GND via one of low and high impedance connection. Typically, output from the digitizer arising from the presence of a grounded conductive object is different than output obtained from the digitizer in the presence of a conductive object that is in a float state, e.g. high impedance to GND. Optionally, during one of a hover or touch operational state, the secondary tip is repeatedly switched between GND and float to provide pre-defined input to the digitizer sensor. Optionally, during a touch operational state, the secondary tip is connected to GND and during a hover operational state the secondary tip is repeatedly switched between GND and float. Optionally, low impedance can be eliminated by adding a insulating cover on the tip. 
     According to some embodiments of the present invention, the secondary tip is a standalone device that can be retrofitted on an existing stylus, writing utensil, and/or rod. Optionally, the secondary tip can be used as a standalone device. According to some embodiments of the present invention, there is provided a kit with a plurality of secondary tips, each providing a distinct input to an associated digitizer sensor, e.g. distinct frequencies and/or distinct geometries of the tip for providing different output on the digitizer due to capacitive coupling. Optionally, each tip is associated with a dedicated functionality. Optionally, transmission via the secondary tip is initiated responsive to attachment to an existing stylus, e.g. using a switch that is activated responsive to attachment to an existing stylus. Typically, the oscillator and/or a portion of the circuitry in the secondary tip are powered, e.g. operated continuously even while the tip is not transmitting. Alternatively, the secondary tip operates continuously, e.g. transmits continuously and does not require activation. Optionally, the secondary tip is only operated, e.g. only transmits a signal during a detected touch operational state of the tip. 
     Referring now to the drawings,  FIG. 1  shows a simplified diagram of an exemplary stylus including a primary tip and an exemplary secondary tip that emits a signal derived from circuitry of the primary tip in accordance with some embodiments of the present invention. According to some embodiments of the present invention, a stylus  110  includes a primary tip  20  emitting a signal  25  and a secondary tip  30  emitting a signal  35 , both during operation of stylus  110 . Optionally, signal  25  is transmitted by a first transmitting element associated with primary tip  20  and signal  35  is transmitted with a second transmitting element associated with secondary tip  30 . Typically, the transmitting elements are formed from conductive portions of the tips. 
     According to some embodiments of the present invention, a signal emitted and/or transmitted by primary tip  20  and secondary tip  30  is generated and/or transmitted by circuitry  40 , e.g. an application specific integrated circuit (ASIC), and is powered by a power unit  50  included in stylus  110 . Typically, circuitry  40  functions as a transmitting unit. Typically, power unit  50  includes one or more batteries and/or a super capacitor, e.g. lithium batteries and/or alkaline batteries. Optionally, the batteries are rechargeable. The power supply typically provides voltage of about 1.5V-3.5V, e.g. 1.5V, 3.2V, or other stable voltage generated by a power management solution, e.g. DC to DC converter. Optionally, a regulating converter is used to obtain stable voltage over a life time of the battery, e.g. to maintain a steady performance. 
     Typically circuitry  40  includes an oscillator, e.g. a multi-vibrator or crystal interface that generates a signal with frequency between about 2 KHz and 1000 KHz, e.g. 20-40 KHz. Optionally, burst signals are used for transmission, and circuitry  40  generates burst signals every 5-20 msec, e.g. every 7.5 msec or every 15 msec. In some embodiments, a signal with amplitude of up to 25V is transmitted. Typically, the signal generated by circuitry  40  is transmitted via a conductive portion of primary tip  20 . Optionally, circuitry for generating the signal is connected on one end to the conductive portion of primary tip  20  and on another end to a conductive portion  95  of housing  90 . Typically conductive portion  95  surrounds primary tip  20  so that an electric field concentric with a geometry of the tip is created between conductive portion of primary tip  20  and conductive portion  95  as described in more details for example in incorporated U.S. Pat. No. 7,292,229. Alternatively, the generated signal is transmitted via an antenna proximal to primary tip  20 , e.g. surrounding primary tip  20 , e.g. as when primary tip  20  is formed from non-conducting material. 
     According to some embodiments of the present invention, a writing tip of the stylus, e.g. primary tip  20  is associated with a pressure sensing unit  28  for monitoring pressure applied on primary tip  20 . Typically, pressure sensing unit  28  detects a switch between no or low pressures applied on the tip during a hover operational state and higher pressures applied during a touch operational state, and provides output indicating the current operational state of the primary tip. Optionally, primary tip  20  and/or pressure sensing unit  28  may be similar to the transmitting tip and/or to a pressure sensing unit described in U.S. Patent Application Publication No. 2010-0051356, entitled “Pressure Sensitive Stylus for a Digitizer,” the contents of which is hereby incorporated by reference. 
     In some exemplary embodiments, pressure sensing unit  28  provides input to circuitry  40  and a modulated signal is generated based on a pressure level detected by pressure sensing unit  28 . Optionally, repetition rate of the generated and/or transmitted bursts is altered, e.g. bursts are transmitted every 7.5 msec during a touch operational state and every 15 msec during a detected hover operational state. Optionally, a frequency of the transmitted oscillating signal is altered and/or the phase of the oscillating signal is altered. 
     According to some embodiments of the present invention, stylus  110  additionally includes a secondary tip and/or secondary end  30  through which a signal  35  is transmitted. According to some embodiments of the present invention, a time base for signal  35  is generated with an oscillator associated with circuitry  40  and modification and/or conditioning of signal  35  is performed with signal processor and/or circuitry  60  so that a signal transmitted by secondary tip  30  is distinguishable from a signal  25  transmitted from primary tip  20 . Optionally, phase and/or frequency of signal generated by circuitry  40  are modified. Optionally, circuitry  60  includes a frequency divider and modification is based on frequency dividing. Optionally, circuitry  60  includes functionality of a voltage controlled oscillator (VCO) and/or a multi-vibrator. Optionally, circuitry  60  functions as a transmitting unit. 
     Typically, the amplitude of signal  35  transmitted via secondary tip  30  is significantly lower than the amplitude of signal  25  transmitted via primary tip  20 . Optionally, the amplitude of signal  35  is at most half the amplitude of signal  25 . Optionally, amplitude of signal  35  is between 0.05-0.75 times the amplitude of signal  25 , e.g. 0.1-0.5 times the amplitude of signal  25 . In some exemplary embodiments, low signal to noise ratio (SNR) detection due to the lower power transmission is compensated for by using a larger diameter tip, e.g. diameter ‘D’ over which signal  35  is transmitted as compared to a diameter ‘d’ over which signal  25  is typically transmitted. Typically, secondary tip  30  has a rounded symmetrical shape, although other shapes may be adapted depending on the assigned functionality of the secondary tip. Optionally, secondary tip  30  has a hemispherical or dome shape. Optionally a 2-10 mm diameter tip is used for secondary tip  30 , e.g. a 6 mm diameter tip. Typically, a diameter of primary tip  20  is between 0.75-2 mm, e.g. 1 mm. 
     Typically, secondary tip  30  is movable with respect to housing  90  of the stylus and recedes toward housing  90  in response to pressure applied on the tip. Typically, an elastic element  38  associated with the secondary tip  30  provides a recoil force responsive to contact pressure applied on the tip. In some exemplary embodiments, a touch operational state of secondary tip  30  is detected based on output from a tip position detecting unit including a switch  65 . Optionally, in response to secondary tip  30  receding into housing  90 , contact between an element  32  rigidly connected to tip  30  is established and switch  65  is activated and/or changes its state. Optionally, switch  65  is replaced with another pressure sensing mechanism. Typically, switch  65  provides input to circuitry  60  for modifying signal  35  while switch  65  is activated. Typically, a state of switch  65  can indicate one of an ON or OFF state but does not detect a range of pressure as does pressure sensing unit  28 . Although, functionality of switch  65  is limited as compared to that of pressure sensing unit  28 , power consumption and bill of materials may be significantly lower. The present inventors have found that for functionalities typically associated with a secondary tip of a stylus, e.g. erasing and/or marking, a user will typically press down on the secondary tip with high pressure so that high sensitivity is not required to differentiate between a hover and touch operational state. 
     Optionally, phase and/or frequency of signal  35  are modified so that touch and hover operational states of secondary tip  30  can be differentiated during interaction with a digitizer sensor. Typically, a frequency emitted by secondary tip  30  is between 2 KHz-1000 KHz. Optionally, circuitry  60  includes a frequency divider for altering a frequency of a signal generated by circuitry  40 . Optionally, a lower frequency signal is used for a hover operational state which is the more dominant operational state, since the power consumption for lower frequency transmission on a constant capacitive load signal is lower than that of a higher frequency signal. Optionally, switch  65  and circuitry  60  are mounted on a PCB  62  associated with secondary tip  30 . 
     Typically during operation of stylus  110 , a signal  25  from primary tip  20  and signal  35  from secondary tip  30  are transmitted simultaneously. Optionally, switch  65  also provides input to circuitry  40  to block transmission through primary tip  20  while switch  65  is activated. Optionally and similarly, pressure sensing unit  28  provides input to circuitry  40  and/or  60  for blocking transmission to secondary tip  30  while a touch operational state is detected by pressure sensing unit  28 . 
     Reference is now made to  FIG. 2  showing a simplified diagram of an exemplary stylus including a primary tip and an exemplary secondary tip powered with a same power unit in accordance with some embodiments of the present invention. According to some embodiments of the present invention, a stylus  120  includes a primary tip  20  emitting a signal  25  and a secondary tip  30  emitting a signal  36 , both during operation of stylus  120 . According to some embodiments of the present invention, each of signal  25  from primary tip  20  and signal  36  from secondary tip  30  are generated and/or transmitted independently but are powered by a common powering unit  50 . Optionally, ASIC and/or circuit  40  generate a signal transmitted via primary tip  20 , and ASIC and/or circuitry  70  associated with secondary tip  30  independently generates a signal transmitted via secondary tip  30 . Optionally, circuitry  40  and circuitry  70  are in electrical communication and function as transmitting units for transmitting signals via tips  20  and  30  respectively. Optionally, transmission of signal  25  and signal  36  are synchronized. In some exemplary embodiments, each of primary tip  20  and secondary tip  30  are associated with a pressure sensing unit  28  for detecting different operational states of the tip, e.g. hover operational state and different pressure levels in a touch operational state. Alternatively, secondary tip  30  is associated with a push button switch and/or secondary tip  30  is not pressure sensitive. Typically, signals emitted by the primary tip  20  and secondary tip  30  are distinct. Typically, each of circuitry  40  and  70  are operable to receive input from its respective pressure sensing unit  28  and alter its output based on the received input. 
     Reference is now made to  FIG. 3  showing a simplified diagram of an exemplary stylus including a primary tip and an exemplary secondary tip that operates independently from the primary tip in accordance with some embodiments of the present invention. According to some embodiments of the present invention, a stylus  125  includes a primary tip  20  emitting a signal  25  and a secondary tip  30  emitting a signal  36 , both during operation of stylus  125 . In some exemplary embodiments, tip  30  is associated with dedicated circuitry  41 , e.g. an ASIC and a dedicated power unit  51 , e.g. one or more batteries. Optionally, tip  30  is pressure sensitive and is associated with switch  65  for sensing when tip  30  is depressed, e.g. is in a touch operational state. Optionally, circuitry  41 , switch  65  and power unit  51  are connected and/or mounted to a PCB  63 . In some exemplary embodiments, tip  30 , circuitry  41 , switch  65 , power unit  51  and PCB  63  are included in a secondary tip unit  98  that is operated independently from primary tip  20  and the rest of stylus  125 . 
     Reference is now made to  FIGS. 4A and 4B  showing simplified diagrams of two exemplary styluses including a primary tip through which a signal is transmitted and an exemplary secondary tip that is a conductive object in accordance with some embodiments of the present invention. According to some embodiments of the present invention, a stylus  130  and/or  135  includes a primary tip  20  emitting a signal  25  and a secondary tip unit  97  including a secondary tip  31 , e.g. a non-transmitting tip constructed from a conductive object that can provide input to a digitizer sensor in a passive manner, responsive to capacitively coupling with the sensor at distinct locations on the sensor. According to some embodiments of the present invention, secondary tip  31  is constructed with a relative wide diameter tip, e.g. large relative to diameter of primary tip  20 , to enhance its ability to provide input to the digitizer sensor based on capacitive coupling. Optionally secondary tip  31  is formed from a soft conductive metal. 
     In some exemplary embodiments, secondary tip  31  is directly connected to GND  69  by a low impedance connection, e.g. connected to a conductive portion of housing  91 . Connection to GND may be by direct connection or via switch  65 . Optionally, secondary tip  31  is electrically floating. Optionally, secondary tip is pressure sensitive and is alternately switched, e.g. with switch  65  between a GND state, e.g. low impedance connection to GND and a floated state, e.g. high impedance connection to GND responsive to a switch between a touch and hover operational state. In some exemplary embodiments, switch  65  is used to connect secondary tip  31  to a low impedance connection to GND  69  responsive to contact pressure applied on secondary tip  31 , e.g. during a touch operational state of secondary tip  31 . Optionally during a hover operational state, connection to GND is via high impedance connection. Optionally, switch  65  requires power for operation and is connected to power unit  50  and or to a dedicated power unit  51 , e.g. one or more batteries. 
     Alternatively, during a touch operational state of tip  31 , circuitry  67  provides for alternately switching between high and low impedance connection to GND, e.g. with switching mechanism  66 . Optionally, switching mechanism  66  provides for switching between high and low impedance connection to GND at a frequency between 10-500 KHz. Typically, a touch operational state is activated responsive to activation of switch  65 . Optionally, switch  65 , circuitry  67  and power unit  51  are mounted on PCB  63  associated with secondary tip  31 . 
     Reference is now made to  FIGS. 5A and 5B  showing simplified diagrams of styluses retrofitted with exemplary secondary tip units in accordance with some embodiments of the present invention. In some exemplary embodiments, secondary tip units  97  and/or  98  are independent add-on units that can be retrofitted on stylus  140  or stylus  145 , e.g. known styluses. In some exemplary embodiments, a cap  99  of stylus  140  is removed and secondary tip unit  98  is positioned in its place. Optionally, housing  91  and cap  99  include screw threads so that cap  99  is removed and replaced with secondary tip unit  98  by screwing cap  99  off housing  91  and subsequently screwing secondary tip unit  98  on housing  91  of stylus  140 . Alternatively, secondary tip unit  97  is used and replaces cap  99 . Optionally, signal transmission is initiated responsive to attachment of secondary tip unit  98  on housing  91  of stylus  140 , e.g. with a switch that is activated responsive to attaching secondary tip unit to housing  91 . Alternatively, secondary tip unit  98  operates continuously and does not require initiation for it to begin transmitting. 
     Referring now to  FIG. 5B , according to some embodiments of the present invention, secondary tip unit  97  includes an attaching mechanism  88  for fitting e.g. temporarily fitting secondary tip unit  97  over a stylus  145  and or other writing utensil and/or pointing device without requiring removal of a cap, e.g. cap  99 . Optionally, secondary tip unit  98  includes an attaching mechanism  88  and is fitted, e.g. temporarily fitted over a stylus  145 . Optionally, attaching mechanism  88  includes an elastic band and/or sleeve that can be friction fitted over housing  90  of stylus  145  or other object with a suitable diameter. 
     Reference is now made to  FIG. 6  showing a simplified schematic diagram of a stylus with a primary tip and exemplary secondary tips that can be selectively mounted on the stylus in accordance with some embodiments of the present invention. According to some embodiments of the present invention, there is provided a kit  200  including a plurality of secondary tip units each of which can be attached to a stylus  150 . In some exemplary embodiments, kit  200  includes a plurality, e.g. two or more secondary tip units each emitting a signal, e.g. secondary tip units  981 ,  982  and  983 . In some exemplary embodiments, kit  200  includes a plurality, e.g. two or more secondary tip units that provide a different capacitive coupling, e.g. secondary tip units  971 ,  972  and  973 . Optionally, kit  200  includes both signal emitting secondary tip units and non-signal emitting secondary tip units, e.g.  971 ,  972 ,  973 ,  981 ,  982  and  983 . According to some embodiments of the present invention, each of secondary tips  971 ,  972 ,  973 ,  981 ,  982  and  983  are assigned dedicated functionality. Optionally, each of secondary tip units  981 ,  982  and  983  include same and/or similar hardware but provide different frequency and/or repetition rates. In some exemplary embodiments, one of the secondary tips is assigned the functionality of an eraser. Optionally, one tip is assigned the functionality of a wider eraser and another tip is assigned the functionality of a narrow or thin eraser. In some exemplary embodiments, some of the secondary tips in kit  200  are assigned the functionality of drawing with a specific color and/or a specific line width. Optionally, functionality of each of the secondary tips in kit  200  can depend on an application being used. Optionally, a user may assign and/or select a functionality of one or more secondary tips, e.g. by selecting a virtual button on an associated display unit. Typically, each of the secondary tips in kit  200  provides output that can be distinguished from output provided by the other secondary tips in kit  200 . It is noted that although each of the secondary tip units are shown to include a switch  65  for sensing one of a touch or hover state of the tip, one or more of the tips can alternatively include a pressure sensing unit  28  and/or may not include pressure sensing ability, e.g. may not include switch  65  or pressure sensing unit  28 . Optionally, one or more of tips  30  or  31  are fixedly connected to secondary tip unit so that they are not moved in response to contact pressure applied on the tip. 
     Reference is now made to  FIG. 7  showing a simplified schematic drawing of exemplary digitizer system in which a stylus including a primary and secondary tip is used in accordance with some embodiments of the present invention. According to some embodiments of the present invention, a stylus  100  including both primary tip  20  and secondary tip  30  and/or a stylus  100  including both primary tip  20  and secondary tip  31  is used to interact with a digitizer sensor  512  of a digitizer system  500 . It is noted that stylus  100  generally refers to any one or more of stylus  110 ,  120 ,  125 ,  130 ,  135 ,  140 ,  145  and  150  described herein. 
     Digitizer system  500  may typically be suitable for any computing device that enables touch input between a user and the device, e.g., mobile, desktop, or tabletop computing devices that include, for example, flat panel display (FPD) screens. Examples of such devices include Tablet PCs, touch enabled lap-top computers, tabletop computer, PDAs or any hand held devices such as palm pilots, mobile phones, navigation systems or any other devices that facilitate electronic gaming. According to some embodiments of the present invention, digitizer system  500  comprises a sensor  512  including a patterned arrangement of conductive lines  518 , which is optionally transparent, and which is typically overlaid on an FPD. Typically sensor  512  is a grid based sensor including horizontal and vertical conductive lines. 
     According to some embodiments of the present disclosure, the grid generally comprises two sets of conductive lines  518 , wherein each set comprises spaced apart conductive lines, and wherein conductive lines of different sets cross each other at junctions, but no contact exists between conductive lines belonging to the two sets. In some embodiments, each of the two sets comprises substantially equally spaced apart parallel straight conductive lines, wherein the two sets are substantially orthogonal ones, although other arrangements can be designed. In some non-limiting embodiments, the parallel conductive lines are spaced at a distance of approximately 2-8 mm, e.g. 4 mm, depending on the size of the FPD and a desired resolution. The two sets can be formed on one or more layers, as long as two conductive lines of belonging to two sets are isolated from one another. Optionally, the conductive lines are formed as thin lines, rectangles, diamonds, a sequence of one or more rhombuses, or any other shape. 
     According to some embodiments of the present invention, a signal emitted by one of primary tip  20  and secondary tip  30  (and/or secondary tip  31 ) of a stylus  100  is detected by sensor  512 . Optionally, a signal transmitted by a stylus tip at a height of up to 4 cm above sensor  512  is detected by sensor  512 . Optionally, when one of primary tip  20  or secondary tip  30  (or secondary tip  31 ) of a stylus  100  is positioned in proximity to sensor  512 , output from the other tip is not detected by sensor  512 . Optionally, while primary tip  20  is being used to interact with digitizer sensor  512 , output from secondary tip  30  is not detected by digitizer sensor  512 . In some exemplary embodiments, secondary tip  31  provides input to digitizer sensor  512  responsive to capacitive coupling formed between secondary tip  30  and one or more conductive lines  518  of sensor  512 . Typically, input to digitizer sensor  512  responsive to capacitive coupling formed between secondary tip  30  and one or more conductive lines  518 , is detected responsive to a triggering pulse and/or signal applied to one or more of conductive lines  518  as is discussed in further detail herein below. 
     According to some embodiments of the present disclosure, circuitry associated with digitizer sensor  512  includes one or more ASICs  516  electrically connected to conductive lines or areas  518  by connection  532  and mounted on one or more PCBs  530 . Each ASIC  516  typically comprises circuitry to stimulate, sample, process and convert the sensor&#39;s output into a digital representation. Optionally, the digital output is forwarded to a digital unit  520 , e.g. digital ASIC unit for further digital processing and for determining locations of input to digitizer sensor  512 . Optionally, digital unit  520  is also mounted on PCB(s)  530 . 
     According to some embodiments of the invention, digital unit  20  receives the sampled data from ASIC  516 , reads the sampled data, processes it and determines and/or tracks position of physical objects, such as one or more of styluses  100 , one or more tokens  545 , one or more hands  546 , and/or one or more fingers  547 . In some exemplary embodiments of the present invention, hovering of an object, e.g. stylus  100  or hand  546  and/or one or more fingers, may also be detected and processed by digital unit  520 . Calculated position and/or tracking information is forwarded to the host computer  522  via interface  524 . Typically, digital unit  520  and/or digitizer system  500  is able to detect more than one simultaneous interactions with digitizer sensor  512 , e.g. more than one finger touches and/or stylus touches. 
     According to some embodiments of the present invention, digital unit  520  includes a processing functionality and/or block  521  for differentiating between input obtained from a variety of different tips of stylus  100 , e.g. primary tip  20 , secondary tip  30 , secondary tip  31  and also for differentiating between input obtained when a tip is touching or hovering over sensor  512 . According to some embodiments of the present invention, processing block  521  provides capability for differentiating between different types of secondary tips  30  and/or different types of secondary tips  31 . Optionally, processing block  521  differentiates between the different tips based on a frequency, amplitude, phase or pattern formed by input provided to digitizer sensor  512 . Optionally, during a course of interaction with digitizer system  500 , a secondary tip of a stylus  100  may be exchanged for an alternate tip providing alternate functionality. Typically, information identifying a specific tip in use and/or an operational state of that specific tip in use is forwarded to host  522 . Typically, information identifying a specific tip and/or an operational state of that specific tip is used to alter a display on an electronic screen associated with host  522 . 
     According to some embodiments, in order to detect hand  546 , finger  547 , token  545 , and/or secondary tip  31 , digital unit  520  produces and sends one or more triggering pulses to at least one of the conductive line  518  at a time, and measures the signal on the same conductive lines or on other conductive lines  518 , e.g. orthogonal conductive lines. Typically the triggering pulses and/or signals are analog pulses and/or signals. Optionally, the triggering pulses and/or signals are confined to one or more pre-defined frequencies, e.g. any sub-range also referred to as “frequency window” of about 20 to about 1000 KHz, within the frequencies of about 15 to about 200 KHz. 
     Typically in response to token  545  and/or secondary tip  31  (in an electric float state) positioned over and/or near a junction between two orthogonal conductive lines, a coupled signal at the junction is increased by about 0.5-20%, e.g. 5-10%. Optionally, the increase in the coupled signal is responsive to increased capacitive coupling between a triggered conductive line and passive conductive line, e.g. a line that is not triggered. Typically, a degree of change in the coupled signal is a function of material that is used to form token  545  and/or secondary tip  31  and/or dimensions of token  545  and/or secondary tip  31 , e.g. shape and size of the token and secondary tip  31 . Typically, while secondary tip  31  is connected to GND, a coupled signal at the junction is decreased. Optionally, by alternating between grounding and floating secondary tip  31 , an oscillating output and/or output with a pre-defined pattern is obtained by sensor  512 . Typically, in response to a hand or a finger over and/or near a junction between two orthogonal conductive lines, a coupled signal at the junction is reduced by about 1-30%, e.g. 3-5%. 
     Typically, output from the digitizer sensor is forwarded to a host  522  via an interface  524  and the output is further processed by an operating system of host  522  and/or any application executed thereby. 
     According to some embodiments of the present invention, digital unit  520  and ASIC  516  serve as the controller of the digitizer system and/or have functionality of a controller and/or processor. According to some embodiments of the present invention, digital unit  520  together with ASIC  516  includes memory and/or memory capability. Memory capability may include volatile and/or non-volatile memory, e.g., FLASH memory. 
     According to some embodiments of the invention, host  522  includes at least a memory unit and a processing unit to store and process information obtained from ASIC  516  and/or digital unit  520 . According to some embodiments of the present invention memory and processing functionality may be divided between any of host  522 , digital unit  520 , and/or ASIC  516  or may reside in only host  522 , digital unit  520  and/or there may be a separated unit connected to at least one of host  522 , and digital unit  520 . According to some embodiments of the present invention, one or more tables and/or databases may be stored to record data and/or outputs, e.g. images or patterned outputs of sensor  512 , sampled by ASIC  16  and/or calculated by digitizer unit  520 . In some exemplary embodiments, a database of data from sampled output signals may be stored. Data and/or signal values may be stored in volatile and nonvolatile memory. 
     Further details describing operation of digitizer system  500  can be found for example in incorporated U.S. Pat. No. 7,292,229 and in incorporated U.S. Pat. No. 7,843,439. It will be appreciated that the disclosed digitizer system is exemplary only and is not meant to limit the scope of the disclosure. Rather, the disclosed stylus can be operated with any suitable digitizer system or circuitry design associated with a digitizer sensor, including for example a combined single chip. 
     In the description below and in particular when referring to detected locations, the terms digitizer and sensor may be used interchangeably, and may typically refer to a sensing surface on which stylus  100  is used. 
     Reference is now made to  FIG. 8  showing a simplified schematic diagram of an exemplary primary and/or secondary tip of a stylus in accordance with some embodiments of the present invention. The present inventors have found that by enlarging a conductive area surrounding a stylus tip, an area over which a stylus transmits can be enlarged and an ability to detect a presence of the stylus may be improved. Optionally, this feature provides improved awareness of the digitizer to the stylus during a hover operational state. According to some embodiments of the present invention, a conductive portion of tip  201  is surrounded by more than one conductive ring and/or section, e.g. conductive rings  951  and  952 , and is electrically isolated from the conductive rings. Optionally, conductive rings  951  and  952  are coated on a housing  90  of a stylus  190  and/or are an integral part of housing  90 . Typically, housing  90  additionally includes an insulating section for electrically isolating an area between rings  951  and  952 . 
     According to some embodiments of the present invention, circuitry included in an ASIC  401 , for generating a transmission signal is connected at one end to a conductive portion of tip  201  and at another end to one or more conductive rings  951  and/or  952  by selective connection. In some exemplary embodiments, ASIC  401  is operable to selectively connect and disconnect to ring  952  with a switch  411 . Typically, by connecting to more than one conductive area, an effective area for transmission can be increased or decreased. Optionally, by connecting ring  952 , an effective conductive area provided by the rings is doubled and/or increased by at least 50%. 
     Optionally, selective connection is responsive to input provided by a pressure sensing unit  28 . Alternatively and/or additionally, selective connection is responsive to a user command, e.g. obtained by button selection. Optionally, for a stylus that provides two-way communication, selective connection is responsive to input provided by an associated digitizer sensor. 
     In some exemplary embodiments, during a hover operational state, ASIC  401  connects to both ring  951  and  952  during transmission to provider transmission over a wider area. Optionally, during a touch operational state connection to ring  952  is broken, and transmission is obtained between tip  201  and ring  951  so that a more localized transmission signal can be obtained, e.g. as is typically desired to improved location detection. Optionally, ASIC  401  additionally provides for generating a first amplitude signal while transmitting over a smaller area, e.g. while only one of conductive rings  951  and  952  are connected for transmission and for generating a second amplitude signal while transmitting over a larger area, e.g. while both conductive rings  951  and  952  are connected for transmission. Optionally, a lower amplitude signal is transmitted during a touch operational state since the proximity of the stylus to touch sensor improves reception of the transmitted signal. 
     The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”. 
     The term “consisting of” means “including and limited to”. 
     The term “consisting essentially of” means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. 
     Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.