Position Resolving System

The present invention relates to position resolving systems such as electronic tablets or digitizers particularly applicable to portable or mobile computers, and more particularly to a tablet or digitizer system utilizing a cordless pen that may be used with the LCD display of a portable or mobile computer. The system of the present invention is a tablet or digitizer connectable to a host computer, such as a portable or mobile computer, for providing position information to the host computer for the accurate location of the position of an instrument. The system provides a means for locating a pen, cursor, or mouse in relation to a reference point on a surface, and digitizes such information for a variety of uses. Such information is supplied to a host computer or host system and may be utilized in the display of the information. The system of the present invention is also applicable to positional determination useful in pen based computer control systems and is appropriate for application to pointing devices and systems incorporated in pen-based computers. The information relating to the instrument location will typically be displayed in a conventional display unit associated with a host computer; however, portable or lap-top computers utilizing LCD displays are particularly adaptable to the system of the present invention.

As used herein, the term "instrument" refers to a pen, pointing device, mouse or cursor, the position of which on a predetermined surface is to be determined. The surface may take a variety of forms familiar to those skilled in the art; however, the present system is particularly applicable to the utilization of a glass surface on a LCD display to create what is presently referred to as electronic paper. That is, the instrument is utilized to "write" on the glass surface which presents a LCD display to the user at the glass surface upon which he or she is writing. The pen or cursor may be utilized to radiate a signal downward onto the tablet and a plurality of orthogonally positioned grid conductors and a detection system are used within the tablet to ascertain the location of the source of the signals and thus the position of the instrument. The utilization of such instruments has usually entailed the use of an electric wire connection from the instrument to the tablet or the host computer; however, cordless instruments have recently become available and provide numerous advantages. If the cordless instrument is to be a source of signals for the determination of the instrument position on the tablet, then the instrument must contain a means for directing such signals onto the tablet. One technique that has been suggested in the prior art is the generation of signals within the tablet to be directed outwardly of the tablet and intercepted and redirected by the instrument back onto the tablet. While this type of system provides for the utilization of a cordless pen or instrument, the power required to transmit the signal to the pen for its subsequent retransmission to the tablet as well as the complexity of the system are distinct disadvantages. The power requirements are particularly disadvantageous when such system is to be utilized in portable applications demanding minimum power drain. Another technique that may be employed in the utilization of a cordless instrument or pen is the provision of a source of power within the instrument itself. A significant problem that is inherent in the latter cordless instrument designs is the need for a power supply such as a battery to be carried within the instrument itself. Obviously, if the instrument is a pen, the power supply must be physically small and the system within the pen must not require more power than can reasonably be expected for a significant lifetime of the battery supply.

The present invention utilizes a self-powered cordless pen to generate an electromagnetic signal directed into the grid of the tablet. The use of a self-powered cordless pen presents many substantial advantages over the above mentioned bi-directional signal system. That is, the instrument of the present invention permits greater signal levels to be sensed in the tablet, permits unidirectional transmission and thus significantly reduces the complexity required for the positional determination of the instrument. Systems incorporating cordless pen designs for use with magnetic tablets have in the past required coherency in the detection of grid signals emanating from the pen. That is, the prior art has required a coherent reference so that the phase of the signals being sensed on the grid lines in the tablet can be detected. This coherent detection is necessary for the system to indicate the direction of the pen from a magnetic signal null, or zero condition, location. To provide this coherent signal detection, the prior art, needing a reference signal for coherent detection, has resorted to the utilization of reference clock channels or complicated schemes for the generation or regeneration of clock reference signals to provide the necessary coherency.

The present invention eliminates the need for such clock channels and instead relies exclusively upon signal magnitude; the phase of the signals radiating from the cordless pen is not detected and not required and therefore it is not sensitive to phase shift errors. The elimination of reference clock channels or reference signal channels reduces the system of the present invention to the requirement of only a single channel. The single channel system is substantially less complex and provides the substantial advantages of reduced cost, power consumption, and increased applicability to environments requiring low power consumption such as portable or lap top computer systems. The present invention employs a particular grid pattern and a unique processing of the information of the non-coherent amplitude detected signals on the conductors to provide information that would normally be provided by signal phase detection to indicate pen direction relative to a particular grid conductor. The system of the present invention has thus eliminated the need for synchronous demodulators and replaced such demodulation techniques with simple amplitude detection. The simplicity afforded by this system yields a less expensive but reliable and light weight tablet that consumes minimum power and is particularly adaptable to applications requiring portability and mobility.

A variety of sensing or sampling techniques are utilized in the prior art to detect the presence of a pen or instrument in the proximity of a grid line and to determine the distance from the nearest grid or conductor to the instrument. These variety of techniques are intended obviously to present a precise location of the instrument on the tablet surface; these prior art techniques are frequently very complicated and incorporate complex tablet and grid configurations. In addition to accurately depicting the position of the instrument on the tablet, it is important that this position information reliably follow any change in the position of the instrument. If the instrument moves rapidly, such as the movement of a pen during handwriting, the positional routine followed by the prior art may experience difficulty making rapid adjustments to the fast moving pen or instrument. To compensate for the requirement to follow the fast

moving instrument, prior art techniques have imposed further complexities on the grid and detection system that increase cost, consume more power and decrease the quality of the position information (more "jitter"). For example, to follow a fast moving instrument, prior art techniques frequently have employed fixed or single formula routines and large line or element spacings in order to minimize the number of grid lines or elements that are scanned or addressed to resolve position and to increase the range over which each scan can resolve position. The present invention overcomes these prior art difficulties by providing both rapid coverage to accommodate fast moving instruments and accuracy routines for precision in the location of the instrument. The routines thus followed by the system of the present invention are adaptive and automatically accommodate a fast moving instrument while preserving the accuracy afforded by a slower, but more accurate, positioning routine.

When pens or instruments are radiating electromagnetic signals onto an arrangement of grid conductors, the signal pattern induced in the grid conductors disproportionally changes as the instrument approaches the edge of the tablet. This disproportionality results in an erroneous indication of pen position; however, this error may be reduced (or simply ignored) by increasing the size of the tablet and restricting the active area (that area of the tablet which may be used) to that portion of the tablet sufficiently distant from the edges of the grid. However, such prior art techniques render the use of such tablets in environments such as portable computer applications impractical since in such environments the tablet must be as small as possible and any prohibited area adjacent the edges of the tablet detracts from the available size of the tablet. The present invention utilizes a compensation technique to minimize edge error through the utilization of appropriately selecting a search routine and physically positioning grid conductors in a predetermined manner.

It is therefore an object of the present invention to provide a tablet for ascertaining the position of a radiating instrument wherein the system incorporates minimum components.

It is another object of the present invention to provide a graphic tablet system utilizing a cordless pen.

It is another object of the present invention to provide a graphic tablet system utilizing a pen for radiating electromagnetic energy onto the tablet wherein signals received by the tablet are amplitude detected only.

It is another object of the present invention to provide a single channel graphic tablet system utilizing a cordless pen.

It is still another object of the present invention to provide an improved graphic tablet system incorporating the utilization of a cordless pen and a simplified design using only a single channel and without clock reference signals.

It is another object of the present invention to provide a graphic tablet utilizing a cordless pen and non-coherent amplitude detection.

It is another object of the present invention to provide a graphic tablet incorporating a cordless pen wherein the pen down signal is generated and sensed exclusively through amplitude keying.

It is still another object of the present invention to provide a graphic tablet system utilizing decision directed routines for rapid acquisition of pen position and providing high accuracy or resolution.

It is still another object of the present invention to provide automatically implemented selective routines including routines that provide substantial linearity.

It is still another object of the present invention to provide a means for minimizing edge effects; that is, the deleterious effects caused by the proximity of the tablet edge when the pen is close thereto.

It is another object of the present invention to provide a graphic tablet system having high noise immunity.

It is still another object of the present invention to provide a graphic tablet system having very low power requirements and therefore particularly useable in portable and mobile computer applications.

It is still another object of the present invention to provide a graphic tablet system that employs decision directed routines that minimize the number of grid elements that need to be addressed.

Briefly, in accordance with the embodiment chosen for illustration, the present invention utilizes a tablet incorporating a plurality of orthogonally related grid loops, each of which is connectable to a AM detector through amplifying and bandpass stages. A single select frequency is utilized and the amplitude of the signal is compared to a predetermined threshold. The pen incorporates a current controlled oscillator radiating the single frequency and utilizes a means for generating either full power or reduced power. The signal from the pen is non-coherently detected and no clock signal is utilized.

The resulting signals on the grid loops are processed using a decision directed technique for selecting routines appropriate for the conditions at the time of signal sensing. The process of routine selection is automatic and accommodates rapid pen movement while maintaining accuracy.

The present invention may more readily be described by reference to the accompanying drawings in which:

FIG. 1 is a simplified block diagram of a graphic tablet system incorporating the teachings of the present invention;

FIG. 2 is a simplified schematic representation of grid loops, useful in the explanation of the operation of the system of the present invention;

FIG. 3 is a simplified schematic representation of grid loops positioned in accordance with the teachings of the present invention;

FIG. 4 is a functional block diagram useful in the description of operations performed on the amplitude signal derived from the grid loops of the tablet;

FIG. 5 is a schematic block diagram showing the control processor and associated functional elements;

FIG. 6 is a functional block diagram of a cordless instrument or pen for use in the system of the present invention;

FIG. 7 is a waveform of the electromagnetic radiation emanating from the cordless pen of FIG. 6;

FIG. 8 is a flow chart useful in the description of operation of the system of the present invention;

FIG. 9 is an illustration of selected S-curves useful in explaining the operation of the system of the present invention; and

FIG. 10 is a schematic representation of instrument or pen signals useful in describing the phenomena of pen tilt as it affects the system of the present invention.

Referring to FIG. 1, a simplified schematic block diagram is shown indicating a tablet 10 constructed in accordance with the teachings of the present invention having X and Y grid lines 11 mounted in the tablet to receive electromagnetic radiation from a radiating instrument or pen 12 whose X and Y position is to be determined relative to the tablet. While the term "electromagnetic" is used throughout this description, it will be understood that in a more precise technical sense the system of the present invention is responsive to magnetic radiation. While the signals are generated through the utilization of an oscillating coil, the signal is nevertheless detected at a position close to the source of the signal and therefore, operates under near field theories. While it is common to utilize the term electromagnetic radiation, and while that term will be used herein, a more precise term would be magnetic radiation. It should be noted that the prior art has consistently referred to such radiation as electromagnetic; therefore, to conform with that convention, the present description will refer to the radiation as electromagnetic. Multiplexing line selectors 13 are mounted on the tablet for selectively connecting predetermined ones of the grid conductors to a grid selection and signal detection system 14. The grid of the present invention incorporates a plurality of X and Y grid lines arranged orthogonally as is customary in the art; however, as shown in FIG. 2, the grid lines of the present invention are formed into vertical and horizontal two sided loops, each side of which forms a grid line and which loop includes an end conductor connecting the two grid lines. For example loop 15 includes grid lines 16 and 17 connected by end conductor 18. Similarly, conductors 20 and 21 are connected by end conductor 22 to form grid loop 23. To facilitate the description of the grid system, only three grid loops 15, 23 and 25 are shown in FIG. 2, each of which is formed by two grid conductors interconnected by an end conductor. In the embodiment chosen for illustration the grid loops are single ended having one end thereof grounded and the opposite end applied to a terminal for access by the system during operation. It will be understood by those skilled in the art that the grid loops may utilize double ended techniques to provide a double ended input for amplification. When using double ended techniques, the center of the loop may or may not be grounded; however, grounding the center of the loop reduces undesired electrostatic pickup.

An electromagnetic signal emanating from an instrument such as a pen will present a maximum signal to the loop when positioned midway between the loop sides. As the point of the instrument moves from this loop midpoint, the signal present on the grid loop decreases. Successive grid loops are overlapped as shown in FIG. 2 and are spaced apart and have a loop width to optimize electromagnetic signal reception and the determination of instrument position. Each loop is shown comprising a single turn; it will be recognized that each loop may have several turns. The particular spacing of, and the width of, the grid loops and grid conductors will depend on numerous factors, including the overall size of the tablet, the distance the plane of the grid is from the pen or pointing device, the accuracy and tilt performance that is desired of the tablet, the number of loop selection multiplexers allowed by the system capacity, and the routines used for position estimation.

Grid loop size and spacing is also determined in relation to the coil diameter of the instrument or pen and the operating height of the coil over the grid. Specific pen coil size and the spacing of grid loops as well as grid width is chosen to obtain as much gain as possible while maintaining linearity in sensed data. In the embodiment chosen for illustration, and utilizing an operating frequency of 455 Khz, a grid conductor spacing of 0.26 inches has been chosen while a loop width of 0.91 inches has been chosen. The choice of the above frequency provides the advantage that it is below the AM band while being higher than display monitor frequencies or other common noise source frequencies.

The electromagnetic signal from the pen or instrument will induce corresponding signals in the respective grid loops as described; however, if the instrument approaches the edge of the tablet, it may be seen in FIG. 2 that the end conductors are also responsible for contributing to part of the signal in each loop. Since these end conductors are not equidistant from the instrument, the signal strength appearing on the respective grid loops will vary in a manner not predictably related to the instrument or pen position. This "edge effect" is sometimes minimized in the prior art by limiting the active area, or area of use, of the tablet to prevent the instrument or pen from approaching the physical edge of the tablet grid network. Thus, the active area boundary 26 as shown in FIG. 2 could be moved in the direction of arrow 27 to ensure that the pen or instrument remains sufficiently distant from the end conductors to minimize such edge effects. Of course, reducing the available active area in this manner means that the tablet would have to be made larger to accommodate an active of a given desired size. The system of the present invention minimizes such edge effects by strategic placement of the grid loop ends as shown in FIG. 3. In addition to the strategic placement of the grid loop ends, the utilization in the present invention of independent grid elements reduces the edge effects in comparison to those prior art systems that use a continuous pattern of interconnected grid elements.

Referring to FIG. 3, the grid loop ends are arranged with respect to adjacent grid loop ends to compensate for the edge effects caused by the instrument approaching the active area boundary 26 of the tablet working or active area. The grid edge placement takes advantage of both sides of the tablet board such that the grid loop ends remain substantially equidistant from the instrument and its electromagnetic radiation as the instrument approaches the edge of the active area of the tablet. To provide this substantially equidistant positioning, selected ones of the grid loop ends are routed to the opposite surface of the grid board such that each grid loop end conductor is essentially coincident in plan view with adjacent grid loop end conductors. Thus, it may be seen that grid conductor 16 is connected to grid conductor 17 through end conductor 18 to form grid loop 15. However, conductor 20 is connected to its corresponding grid conductor 21 in the adjacent grid loop through a conductor 22 on the opposite surface of the tablet 10. That is, the conductor 20 is connected via a plated through-hole 28 to the opposite side of the tablet and thence through conductor 22 on such opposite side to another plated through-hole 29 which in turn is connected to the opposing conductor 21 of the second grid loop. Similarly, successive adjacent grid loops have their respective loop sides interconnected by end conductors that may be positioned on the opposite side of the tablet. In this manner, the distances between or among the end conductors of adjacent grid loops, when viewed from the top of the grid or tablet, is greatly minimized. Thus, the grid loop end conductors are essentially coincident in plan view with adjacent grid loop end conductors to provide substantially equidistant positioning from an instrument or pen at or near the edge of the active area of the tablet.

The minimizing of the edge effects permits the expansion of the active area of the tablet to more efficiently utilize the available surface area; this latter feature becomes very important in applications where size of the tablet is critical as in applications requiring portability (e.g. lap top computer systems).

The electromagnetic signal from the instrument or pen induces a signal in the respective grid loops in the proximity of the instrument. The present invention utilizes non-coherent detection of signal amplitude on the respective grid conductors to ascertain which grid loop provides the greatest amplitude signal. This information is then utilized in subsequent differential non-coherent detection techniques to ascertain the position of the instrument in relation to a positional reference, such as a grid conductor or loop address, on the tablet. The non-coherently detected amplitude signal present on each grid loop is compared to a threshold amplitude level to provide information relating to the proximity of the instrument to the tablet surface. The detection of a signal of sufficient amplitude provides information to the system indicating that the system may now proceed with a selected method or routine to positionally locate the loop having the maximum signal amplitude thereon.

The instrument or pen utilized in the present system will be described more completely hereinafter; however, the pen is battery operated and contains a current controlled oscillator for producing directed electromagnetic radiation into the tablet surface. The pen has an on and an off condition as well as a pen up and a pen down condition. When the pen is on, but not in contact with the tablet surface, the amplitude of the electromagnetic signal is reduced; when the pen makes contact with the surface of the tablet a pen down switch is actuated to increase the output of the oscillator to maximum. In this manner, power is conserved when the pen is not actually in contact with the tablet while the system is nevertheless alerted. Further, the positioning system and its components may begin operating in contemplation of actual contact of the instrument or pen with the tablet surface. Such anticipatory operation minimizes delays in the determination of pen position and also assists in the processing of the position information when a pen is moved rapidly over the tablet surface and may actually and inadvertently be raised during the transition (in those instances when the pen instrument is being moved rapidly such as by a swift stroke). When the detected amplitudes on any X and any Y conductor exceed the predetermined minimum threshold amplitude, the sampling of the grid loops is undertaken; the result of the sampling is a digital address of the instrument position.

Referring now to FIG. 4, a functional block diagram is shown tracing the operations performed on the amplitude signal derived from a selected grid loop. The signal is applied to terminal 30 and fed through a series of gain and bandpass filter stages 31, 32 and 33. The signal is thus amplified and frequency components outside the selected bandpass are blocked. The individual gain and bandpass filter stages may be of conventional design; however, stability and low power consumption are very important. In a portable application, consideration should be given to the implementation of a circuit that has a gain that is insensitive to battery voltage variations and that has a high gain-frequency product versus power relationship to conserve power. For these reasons, discrete transistor video-type circuits are preferred over operational amplifier integrated circuits. The system of the present invention uses conventional discrete operational amplifiers. The final stage is provided with a differential output to be supplied to a low threshold, low offset, linear AM detector 34. This AM detector is a full wave detector for improved signal-to-noise ratio. As used herein, the term "non-coherent amplitude detection" means the detection of relevant signals without regard to the phase of such signals. That is, phase information is irrelevant and non-coherent amplitude detection specifically excludes the consideration of any phase information that may be present in the signal. Sample schematic output waveforms from the gain and bandpass filter stages are shown at 35 as applied to the detector. The AM detector 34 produces a full wave rectified signal as shown at 36, the mean value of which is representative of the signal amplitude on the selected grid loop. A DC-to-pulse-width converter 40 yields a gated pulse width signal 42 that is converted in a pulse width analog-to-digital converter 45 to convert the signal into digital format for application to a microprocessor for further handling as will be described more fully hereinafter. The pulse width converter 40 and analog-to-digital converter 45 may also take a variety of forms, all of which are well known in the art. A conventional dual slope implementation of an analog-to-digital converter may be used in the converting of the signal shown at 36 into digital format. It has been found desirable to align the entire system by reading a predetermined test signal having desired amplitude or amplitudes; in this manner, the system is appropriately calibrated for the reception of grid loop amplitude signals when received in normal operation.

The signal operations circuits of the functional blocks of FIG. 4 are of conventional design and may take any of several well known forms. However, it is important to recognize that the entire signal handling technique has occurred without coherent detection and with reliance upon only amplitude of the sensed signals on the grid loops.

Referring now to FIG. 5, the digitized signal representing sensed signal amplitude on the selected grid loop is applied to a microcontroller or microprocessor 50. The microprocessor is a commercially available microprocessor that has been selected for high power efficiency using low power CMOS technology that will maintain its present status at zero clock frequency. The processor chosen incorporates an internal eight bit counter to read part of the twelve bit digital input, the higher bits being read with an external counter. The latter technique being used to keep the input to the internal serial counter under 1 Mhz input capability. The processor provides control output to select the respective multiplexer lines for addressing selected grid loops in accordance with any one of the selected routines incorporated in the present system. It has been found that commercially available microprocessors such as a NEC 78C10 are satisfactory choices for use in the system of the present invention. The microcontroller or microprocessor 50 is connected to grid selecting multiplexers 13 of FIG. 1 for selectively gating the grid loops of the tablet to the input of the gain and bandpass filter stages of FIG. 4. The digitized amplitude resulting from the selection of a grid loop supplied to the microprocessor completes one step of a selected routine as will be described herein. The program for operating the tablet is stored in CMOS EPROM 52 while address latch 12 demultiplexes the microcontroller bus in order to address the EPROM. Of course, these parts are not needed when the internal ROM of the microcontroller contains the operational program. The system is coupled to a host computer through a conventional RS232 coupling 53 which supplies power to the system and receives and transmits information to and from the host system in a conventional manner.

In those environments where the host system may itself be battery powered, such as portable or lap top computers, it is important that the system conserve power. Thus, the system includes a CMOS timer 54 connected to the microprocessor; the timer periodically "activates" the microprocessor system to have the latter briefly scan the grid loops to observe whether the pen or pointing device is radiating an electromagnetic signal and if so, whether the pen is near enough to the surface of the tablet for the tablet to acquire, track and then resolve the position of the pointing device. If the pen or pointing device is not sufficiently close to the surface of the tablet, or if no signal is being radiated by the pen, then the microprocessor restarts the timer and goes into a "sleep" mode where it is consuming a very minimal amount of power. Due to the low percentage of the time that the microprocessor is active in this sleep/activate cycle, the average power consumed over this cycle remains very low. Of course, if the host computer system is off, the processor and the system of the present invention remain off. The waveform 56 is illustrative of this timing sequence. The portion 57 of the waveform indicates the "off" or "sleep" condition of the processor. At 58 the processor is turned on to permit the system to seek a suitable pen proximity signal; failing the detection of such signal, the system is shut off and returns to the sleep mode 59. As long as the host system is on, the processor will continue to cycle between the seek and sleep modes. Upon detection of a pen proximity signal, the system will switch on and remain on as shown at 55 until commanded to return to the sleep/seek modes. Thus, the timer insures that the microprocessor operates only a short period of time unless the presence of an appropriate amplitude pen signal is detected, digitized, and provided to the microprocessor. The application of such pen signal to the microprocessor insures that the processor remains on independently of the timer.

Referring now to FIG. 6, the cordless pen or pointing device utilized in the system of the present invention is shown in functional block diagram form. The pen is described in more detail in copending application Ser. No. 07/771,369 filed Oct. 1, 1991 and assigned to the Assignee of the present invention, which application is hereby incorporated herein. The pen includes a current controlled oscillator 60 that supplies a radiating oscillator coil 61 to emit electromagnetic radiation at a single frequency. A power supply, such as battery 62, is connected to the oscillator through a power on/off flip-flop circuit 63; a timer 64 is connected to the power source 62 and to the flip-flop 63. Battery voltage is also applied through the operation of a normally closed switch 65 to a Schmitt trigger 67, the output of which is connected to the flip-flop 63 and the oscillator 60. The normally closed switch 65 is the pen down switch that is activated when the point of the pen contacts the surface of the tablet. In its quiescent state, the pen is entirely off and no current other than normal leakage current flows. When the normally closed switch is open, battery voltage is applied to the timer and the Schmitt trigger and the oscillator is powered to one hundred percent of its rated power output.

When in this "pen down" condition, the amplitude of the signal detected by the grid is above the threshold of the required detected signal amplitude to instigate the pen location procedure of the system. If the pen is raised from the surface of the tablet, and the switch returns to its normally closed position, the timer maintains power to the oscillator for a limited time and the oscillator continues to oscillate but at reduced power, preferably about 40% of maximum. If the pen is close to, or in proximity to, the surface of the tablet (not touching the tablet . . . e.g. 0.5 inches) the system can continue to attempt to locate the pen and if the received pen signal is sufficiently strong, the system can resolve pen position. This ability to search and locate pen position when the pen is not in a pen down position permits the locating system to "prime" itself to react more quickly and accurately when the pen is replaced on the surface of the tablet. The system is able to react more quickly and provide information concerning pen position more promptly then if the pen were to be completely shut off during this period of time. The ability of the system to operate on receipt of signals when the pen is in proximity of but not in contact with the tablet surface provides substantial advantages over a system requiring pen down signals to be received before processing begins. The latter techniques are found in conductive or pressure sensitive surface systems found in the prior art. Having position data supplied to the system input before the pen down signal is received permits the system to operate in selected routines with system filters in operation; the system including filters does not therefore need to instantly go from a zero input condition to a full data value condition at the time of pen down. That is, the filters would have to rapidly slew to their eventual data value thus creating errors in the filter output. In the present system, the filters do not see a change in data from a pre-pen down condition to a post-pen down condition so that the filtering of the data proceeds without creating errors. Thus, the pen down signal results exclusively from amplitude keying and is instantaneous; there is no necessity for waiting for a particular window of transmission time as in several prior art systems nor is there an imposed time delay for encoded messages as other prior art designs require.

When the pen is again placed against the tablet surface and the normally closed switch is opened, the amplitude of the radiated signal from the pen returns to its former high level. To prevent the increase in the pen signal from creating errors within the system, the receipt of the pen down signal caused by the increased amplitude of radiation from the pen results in the generation of a pen down status that is maintained in the system for a minimum period of time. This period of time is approximately the time required to scan selected grid loops, or one scan time, and in the embodiment chosen for illustration will be approximately six milliseconds. Thus, any pen down signal resulting from the opening of the normally closed switch within the pen creates a pen down status that is maintained for a minimum of six milliseconds. In this manner, the increased signal amplitude detected by the grid can be verified by the system as a valid pen down signal and data read during this time may be discarded as will be described later.

A representative waveform is shown in FIG. 7 relating to the radiated pen signal. When the pen is off, no signal is generated as shown at the left end of the waveform of FIG. 7. When the pen is placed against the tablet surface, and the normally closed pen down switch is opened, the pen down signal results in the one hundred percent rated radiating power output of the oscillator as shown at 70 in FIG. 7. When the pen is raised from the surface of the tablet and the normally closed pen down switch again closes, the pen oscillator is not turned off but is switched to a reduced power output as shown at 71 in FIG. 7. This reduced amplitude output of the oscillator will continue for a predetermined time and then cease as the timer switches the pen off. However, if before that predetermined time expires the pen is again lowered into contact with the tablet to open the normally closed pen down switch, the oscillator output is again raised to its one hundred percent rated output as shown at 72 in FIG. 7. Again, when the pen is raised the output of the oscillator is reduced as shown at 73 and continues for a predetermined time after which the timer causes the pen to shut off.

It should be noted that the pen down signal created by the opening of the normally closed switch 65 results in amplitude keying; that is, no other event occurs in the system to tell the system that a pen down condition exists. It should also be noted that the pen down signal created by the pen down condition does not require additional circuitry or time; the same circuitry and system used to resolve instrument position is also used to generate and indicate the pen down condition. While the amplitude keying described herein presents significant advantages, it may be possible to combine amplitude keying with a frequency shift. That is, in the case of a mouse, the amplitude could be used to indicate mouse lift-off (lower signal amplitude when lifted) in order to stop data output and a frequency shift then used to indicate or transmit the mouse button status. Thus, in the system of the present invention, the change in amplitude of the electromagnetic radiated signal is the only keying event with the result that the signal is provided instantaneously without the need for encoding or waiting for a timing window. The instantaneous increase in signal amplitude can result in an induced error in the data read during the scan of the tablet surface during which the amplitude change occurred. The data reading error thus caused should not be utilized in the determination or attempted determination of pen position and should be disregarded to prevent erroneous or ambiguous signals from affecting indicated pen position. To accommodate the effects of amplitude change during a scan in which a pen down occurs, the system of the present invention internally generates a pen down status and maintains additional scan to confirm that a pen down condition is present and to provide positional data; the data gathered during the scan that included the pen down event is ignored. This missing data is reconstructed by averaging the data of the previous or prior pen down scan to the data from the next valid scan. It is important to note that as used in this paragraph, the term "scan" does not mean the sweeping of all X and Y conductors to ascertain amplitudes thereon; rather, it is intended to mean the selective sensing of predetermined grid loops in the vicinity of the pen position as calculated in accordance with a selected routine. The system of the present invention "scans" the entire tablet only when in the acquisition mode searching for the presence of a loop amplitude above a predetermined threshold to ascertain the presence of the pen or the proximity of the pen. Once this latter sensing has been accomplished, no more scanning of the complete tablet surface is undertaken, and all subsequent "scanning" is limited to only selected loops as will be described later.

The electromagnetic radiation from the oscillator coil penetrates the grid loops on the tablet. When the pen or pointing device is in the pen down condition, and generating maximum amplitude radiation, the sensed signal in the grid will be above the predetermined threshold level and the sensed signals on the grids are processed in accordance with a selected routine. It may be noted that the pen down signal, or lack of such pen down signal, can be used for purposes other than the determination of instrument or pen position. For example, when the pen is not in contact with the surface and no pen down signal has been generated, system filtering can be selected to permit the detection of the pen signal (not pen down signal--but signal being generated by the pen before the down condition) in the presence of more noise. This latter technique could be useful if a very low power output condition were to be used with a pen to extend battery life. If it were desirable to sense the pen at greater heights or distances from the tablet surface, the filtering could be increased further with additional built-in hysteresis to reduce the positional variations with noise. Conversely, the pen down signal may be utilized to open the system filters to adapt the system for very high instrument speed.

The routine to be selected will depend on several conditions; for example, if the pen is not down but is in proximity of the tablet surface sufficient to provide detectable signal levels, a certain routine is selected. If the pen is moving rapidly over the tablet surface, or approaches to within a predetermined distance to a tablet edge, a particular routine is selected; further, if the pen is in the down condition and is moving normally, or is stopped, a further routine selection is made. As signals are induced in the grid loops, the signal amplitudes present on the respective grid loops are digitized, and processed to determine the position of the instrument or pen on the tablet. As mentioned previously, the combined requirements of speed and resolution in the determination of pen position frequently result in compromises to achieve an improvement in one at the expense of the other. To overcome these seemingly diametric requirements, great expense in the form of circuit complexity and high power requirements have been required in the prior art. To maintain the simplicity of fewer components, and thus lower power requirements, the present invention develops the digitized amplitude signals and processes them in a unique manner.

The present system first scans, in an acquisition mode, each of the X and Y loops to locate an X and a Y loop whose amplitude signals are the maximum of all X and Y loops, respectively, and whose amplitudes exceed an "in proximity" threshold. The present embodiment utilizes interleaving in its scan process; that is, a first X loop is scanned, then a first Y loop is scanned, then a second X loop is scanned, etc. In this manner, the entire tablet surface is scanned for the presence of a sensed electromagnetic signal from the pen whose digitized amplitude exceeds a predetermined threshold. While interleaving is not required, it is preferred to sequential scanning that scans all X loops and then all Y loops. The preference for interleaving is based in part on the reduction of the dynamic error that results from the utilization of sequential scanning. That is, in sequential scanning, the information or data relating to the X position of the sensed pen must be mated with corresponding positional information in the Y direction; however, since all of the X conductors must be sensed before any of the Y conductors are sensed, the Y position information may differ substantially in time of occurrence from the X position information. This time difference can be very disadvantageous if the pen is moving during this period of time such that the X and Y data are not matched in time. Such dynamic errors may frequently be seen in displays as slight curvatures in lines drawn at 45.degree. angles across the display surface. In contrast, the time differential between the reading of the X and Y data is greatly reduced by interleaving to thereby minimize such dynamic errors.

When an amplitude signal on an X loop and an amplitude on a Y loop have both exceeded the "in proximity" threshold amplitude, the system then abandons its scanning routine in the acquisition mode and enters the tracking mode to concentrate on only that location in the immediate vicinity of the perceived pen signal. Upon entering the tracking mode the present system samples the amplitudes of a subset of X loops and Y loops that are closest to the pen position The sequence of sampling this subset is interleaved X and Y; that is, first the signal amplitude on an X loop is sampled, then a signal amplitude is sampled on a Y loop, and then another X loop is sampled, etc. The selection of the routine to be utilized in determining the sequence of the sampling will be described later. The present system will abandon the tracking mode and revert to the acquisition mode if the X and Y loop pair having the largest signal amplitudes thereon within the subset of X and Y loops no longer have signal amplitudes that both exceed the "in-proximity" threshold. Exiting the tracking mode may be the result of any of several events including the movement of the pen out of proximity with respect to the tablet, the pen being turned off as a result of lack of activity (no recent pen down event within the timing period allotted for such event), or the pen was moved across the tablet at such a rate that the subset of X and Y loops used to track the pointing device was not updated quickly enough to extract tracking information. When the system is in the tracking mode, and as long as it remains in the tracking mode, the system of the present invention concentrates on the location of the pen signal to quickly resolve the precise location or address on the tablet and to track that position as it changes through pen movement. The efficiency of the system in accomplishing this feat is predicated on the minimization of the number of loops that are sensed or read and altering the selection of loops that are used to resolve the pen position depending on pen speed (rate of change of pen position) and, when the pen approaches the edge of the tablet, upon pen position.

When the presence of the pen is sensed through the above interleaved scan, the subsequent process for determining and maintaining positional information regarding the pen no longer requires "scanning" as used in the prior art wherein all grid conductors are sensed; rather, pen position is determined by decision directed routines that sense signal amplitudes on selected grid loops within an subset of loops.

The implementation of the selected routines enables the system of the present invention to accurately resolve pen position in accordance with a routine that is adapted to the particular conditions at the time the resolution effort is undertaken. The routines concentrate on the immediate vicinity of the pen or instrument by reducing the number of loops or conductors that are "scanned" or sensed to a small number; thus, rather than continuously scanning the entire tablet surface, the routines permit the inquiry concerning pen or instrument position to be restricted to a small window of X and Y loops. The routines sample the data present on the loop having a signal with the maximum amplitude thereon and on the adjacent loops. The determination of which loops are thereafter to be sampled depends on decisions predicated on differences in signal amplitudes between selected grid loops. The routines therefore sample data, determine where additional data is to be sampled based on the relationship of the data on the sampled loops, and continue to sample data on the respective loops within this window to resolve the instrument position. The routines include one for resolving the instrument position through the sampling of data on three loops; this routine resolves the instrument position and facilitates rapid tracking of the instrument by incrementing or decrementing the address of the loop with maximum amplitude signal thereon by two. This "three loop routine" permits tracking during rapid instrument movement and may also be used to track instrument movement even though the instrument may not be in contact with the tablet but is in proximity to the surface thereof. The "three loop routine" is also chosen for resolving instrument position when the instrument approaches within a predetermined distance of the tablet edge. Since this routine requires only the sampling of three loop signal amplitudes, the routine may conveniently be used close to the edge of the tablet.

Another routine resolves instrument position by utilizing the sampled data on four selected loops. The latter routine provides a uniquely accurate resolution of instrument position and is more linear and less sensitive to tilt errors as will be described more fully hereinafter. This "four loop routine" is appropriately selected when the address or index of the loop with the maximum signal thereon is to remain the same or be incremented or decremented by no more than one. The selection of the "three loop routine" or the "four loop routine" is based upon information derived from sensed data within the above mentioned window and is automatic. A description of the method of selection of the respective routines, and a description of the routines will now be undertaken. The following description will relate only to the X direction loops, it will be understood that corresponding comments and description would equally relate to Y direction loops.

It will be understood by those skilled in the art that the physical location of each loop on the tablet may be measured from a predetermined origin. That is, a specific location on the tablet surface may be considered the origin and the physical location of a pen or instrument on the tablet can be measured from that origin to derive a digitally encoded address to be supplied to a host computer and/or display. Further, in the embodiment of the present invention chosen for illustration, the physical distances between the loop centers has been divided into 240 positions; therefore, a pen position may be determined by reference to the fixed position of the origin, the distance from the origin to the loop with the largest amplitude signal (expressed in terms of "positions"), plus (or minus) the number of "positions" from the loop with the maximum signal thereon to the pen or instrument.

When the loop with the maximum reading thereon has been determined through either a previous full scan (scan of all X and Y grid loops) while in the acquisition mode or a partial scan (scan of only selected grid loops) when in the tracking mode, then the amplitude of the signal on each of the loops on either side of the max amplitude loop are sampled and a difference between these amplitudes is formed. Depending on whether the difference is greater or less than zero, the second loop removed from the loop with maximum amplitude on a selected side is sampled to form a difference between that amplitude and the amplitude of the loop adjacent to but on the opposite side of the loop with maximum amplitude. Again, depending on whether the latter difference is greater or less than zero, the system samples either the amplitude on a selected third loop removed from the loop with maximum amplitude or the amplitude of the unsampled second loop removed from the maximum loop. If the amplitude that is selected to be sampled is the amplitude on the third loop removed from the loop with maximum amplitude, then a decision is formed to change the index (address) of the loop with the maximum amplitude by either one or two positions. The determination of whether to change the indicated position of the pen one or two positions is a decision based upon using either four selected loop amplitudes or three selected loop amplitudes as will be explained more fully hereinafter. However, if the amplitude of the signal on the previously unsampled second loop removed from the maximum amplitude loop is sampled, then the position of the instrument or pen is calculated using four selected loop amplitudes, and the indicated position of the loop with the maximum amplitude remains the same.

This decision directed selection of routines may more readily be described by reference to the flow diagram of FIG. 8 wherein it may be seen that at the completion of a partial scan in the tracking mode, the tablet adjusts its scanning window to closer align, or track, the position of the pen or pointing device. It accomplishes this tracking function by updating the index or address of the loop with the maximum amplitude thereon, loop Xi. The system then begins a partial scan by again sampling the amplitudes of three loops about the new maximum amplitude loop address. That is, the amplitudes on loops X(i-1), Xi, and X(i+1), are sampled to form a difference X(i+1)-X(i-1). Assuming, for purposes of description, that this difference is less than 0, the system then samples the second loop removed from the maximum loop on a selected side; that is, the system samples the amplitude of the signal present on loop X(i-2) and then forms a difference between that amplitude and the amplitude of the signal on the loop adjacent to but on the opposite side of the loop with the maximum amplitude. That is, X(i+1)-X(i-2). If the difference thus formed is greater than 0, then the system samples the amplitude of the loop X(i+2) and the position of the pen is determined using the following routine: ##EQU1## where: P(X)=estimated pen position in the X direction from the origin

Xo=index, or address, of X loop centered on the chosen origin

Xi=index, or address, of X loop having maximum signal amplitude

K=constant equal to the number of "positions" between loop centers (in this example, K=240)

AX() signal amplitude on the X() loop

and the index, or address, of the loop with the maximum amplitude signal thereon remains the same. If the last mentioned difference, that is X(i+1)-X(i-2) is less than 0, the system samples the amplitude on the loop X(i-3) and subsequently makes a decision in both the determination of pen position and whether or not to decrement the index or position of the pen by one or two loops. That is, if the difference Xi-X(i-3) is less than 0, then the position of the pen is determined using the following routine: ##EQU2## and the index or address of the loop with maximum amplitude signal thereon is decremented by two. If however the difference Xi-X(i-3) is greater than 0, then the position of the pen is determined using the following routine: ##EQU3## and the index or address of the loop with the maximum amplitude signal thereon is decremented by one.

The above description of the selected routines was based upon the assumption, recited above, that the difference in amplitudes on the loops X(i+1)-X(i-1) was less than 0. If, however, that difference was greater than 0, the system would then sample the second loop removed from the maximum loop on a selected side; that is, the system would sample the amplitude of the signal present on loop X(i+2) and then forms a difference between that amplitude and the amplitude of the signal on the loop adjacent to but on the opposite side of the loop with the maximum amplitude. That is, X(i+2)-X(i-1). If the difference thus formed is less than 0 then the system samples the amplitude of the loop X(i-2) and the position of the pen is determined by using the following routine: ##EQU4## and the index, or address, of the loop with the maximum amplitude signal thereon remains the same. If the last mentioned difference, that is X(i+2)-X(i-1) is greater than 0, the system then samples the amplitude on the loop X(i+3) and subsequently makes a decision in both the determination of pen position and whether or not to increment the index or position of the pen by one or two. That is, if the difference X(i+3)-Xi is greater than 0, then the pen is determined using the following routine: ##EQU5## and the index or address of the loop with maximum amplitude signal thereon is incremented by two. If, however, the difference X(i+3)-Xi is less than 0, then the position of the pen is determined using the following routine: ##EQU6## and the index or address of the loop with the maximum signal amplitude thereon is incremented by one.

The routine chosen for resolving pen position will utilize either the amplitude present on three selected loops or the amplitude present on four selected loops. The selection of a routine to resolve position through the use of four loops presents the greater accuracy; however, the routine utilizing the amplitude information from three loops permits the resolution of the position of the pen or pointing device while the latter is close to the edge of the tablet or when the pen or pointing device is moving across the tablet at a very high rate of speed (e.g.: over forty inches per second). The benefit of the decision directing scanning described here is that it automatically and dynamically selects the position resolving routine that is best for the given operating conditions at the moment.

As mentioned previously in connection with the description of the pen signal, the signal strength transmitted by the pen will either be 100% available radiated power (pen down condition) or reduced radiated power (pen up position). Also as previously described, when the pen is on but is not in contact with the tablet, the tablet will nevertheless sense the pen if it is sufficiently close to the tablet surface (in proximity--approximately 0.5 inches). The tablet will also track and resolve the position of the pointing device or pen even though the device is not in contact with the tablet. In this manner, the system is "primed" and is tracking the pen using its fastest pen speed routine until a pen down condition occurs.

Since the more accurate position resolving routine requires the sensing of the amplitude on four selected grid loops, the routine may be incapable of accurate pen position resolving near the grid edge since there may be insufficient grid loops to select four grid loops for the routine. Thus, when the pen position is within a predetermined distance from the edge of the active area of the tablet, this system limits the selection of routines to the routine requiring only the sensing of signal amplitude on three selected grid loops.

It may be seen by the above description that the decision concerning choice of precise routine used in resolving the position of the pen, is controlled by the immediate conditions of pen speed, pen signal (pen down, pen up in proximity) and the pen position (i.e., when the pen is near the tablet edge). The decisions concerning the routine specifically followed in any particular instance are made simultaneously with pen operation; that is, the decisions are made automatically and instantaneously during operation. In other words, if the pen is stationary, the decision to select the appropriate routine is made and the position of that stationary pen is promptly and accurately resolved by reference to the signal amplitude on four selected grid loops. If the pen is moved, the position is resolved by a different routine by reference to the signal amplitude on a different four selected grid loops that, while it presents the same positional accuracy or resolution as in the case of the stationary pen, nevertheless will change the index or address of the loop with the maximum amplitude by one. In the event the pen is moved rapidly, such as when a stroke across the tablet is made with the pen or when the pen is moved rapidly as in handwriting, the speed of response of the system to provide an accurate indication of pen position becomes more important. In the latter case, a third routine is implemented, automatically and without further processing necessary to make such decision, wherein the position of the pen is resolved by reference to the signal amplitude on three selected loops and the index or address of the loop with the maximum amplitude signal thereon is incremented or decremented by two.

When the routine selected relies upon the amplitudes existing on four grid loops, the resulting positional information has been found to be closer to a perfectly linear input/output transfer characteristic than would be expected. That is, for a given change in position of the pen, a perfectly linear system would present the identical position change at its output. Actual systems however always incorporate some degree of non-linearity. In electromagnetic systems of the type described here, this non-linearity is frequently discussed in terms of indicated pen positions and the necessary correction at that indicated position to produce an output that is essentially precisely the actual position. A plot of true positions versus indicated position for two positional routines is shown in FIG. 9; this type of plot is frequently referred to as an "S" curve. The abscissa scale represents the indicated pen position relative to a selected origin in a given direction; the ordinate represents the correction necessary to render the signal derived by the system to precisely concur with actual pen position. By reference to FIG. 9 it may be seen that the correction required is zero at the two indicated position end points. As the pen is moved away from these indicated position end points, the magnitude of the correction that must be added to the indicated position to obtain the actual pen position increases until a positive or negative peak correction is required. This non-linearity should be minimized whenever the tablet is being used for precise drawing applications or where it is important to accurately reproduce handwriting. The S-curve 90 is illustrative of routines such as the above described routine wherein the position is resolved using the signal amplitudes on three selected grid loops. The inherent linearity of the routines utilizing amplitudes on four selected grid loops is shown by reference to curve 91 in FIG. 9. It may be seen that curve 91 is substantially "flatter" than curve 90 and that the necessary correction to the sensed pen position is substantially less.

The above described S-curves show that an anticipated positional error is induced and that this error is non-linear and varies in accordance with pen position. Another positional discrepancy or error enters the system when the angle that the radiating pen makes with the tablet surface is changed. That is, if the operator holds the radiating pen vertically or perpendicular with respect to the plane of the tablet, the resulting S-curves may take the form of those shown in the above described FIG. 9. However, if the pen is tilted to form an angle, for example 30.degree. from perpendicular with respect to the plane of the surface of the tablet, then a second positional error is added to the system. This tilt error may not be significant until the tilt of the pen reaches thirty or more degrees from perpendicular. The above described routine using the amplitude signals derived from four selected grid loops has also shown to be less affected by pen tilt. Referring now to FIG. 10, a plot of pen position errors caused by pen tilt is shown. The necessary correction, or error, is shown as the ordinate scale while the angle of tilt is shown on the abscissa scale. Curve 95 is a plot of a typical tilt error of a routine for determining pen position. It may be seen that as the pen is tilted more than 30.degree., the tilt induced error continues to increase. In contrast, curve 96 represents a plot of the tilt error resulting from the use of the previously described routines utilizing the amplitude signal on four selected grid loops. It may seen that the tilt error using four grid loops increases at a lower rate up through approximately 30.degree. pen tilt, and that thereafter the error actually decreases.

The significantly increased linearity resulting from the four loop routines is thus represented in improved S-curve correction values; further, this improved linearity is accompanied by substantially improved tilt error characteristics. These four loop routines thus permit significantly increased accuracy in the positional resolution of pen location and do so using only the amplitude signal information present on four grid loops.

The following is a print-out of an implementation of the system of the present invention showing the operation of the system in resolving instrument position and following selected routines to arrive at an estimated instrument address. ##SPC1##