Apparatus and Method for Determining the Position of a Driven Coil Within a Grid of Spaced Conductors

This invention relates to a method and apparatus for determining the position of a coil with respect to a grid of spaced conductors and more particularly to such instruments which are relatively simple and low cost.

Various high resolution apparatus for translating a position of a movable instrument, such as a pointer or pen, into electrical signals for transmission to a local or remote utilization device are well known in the art.

U.S. Pat. No. 4,210,775, assigned to Talos Systems, Inc., pertains to a digitizer in which an instrument, such as a pen having a coil disposed about a nosepiece located immediately above the writing tip of the pen, is utilized in conjunction with a tablet comprising a grid of conductors, including a group of parallel spaced conductors oriented in an X direction and a group of parallel spaced conductors oriented in a Y direction. The pen tip is moved along a worksheet disposed on a support surface containing the grid of conductors. An oscillator applies a signal of predetermined constant frequency and amplitude to the coil. The coil is inductively coupled to the conductors of the grid, thereby inducing signals in the conductors. In accordance with electromagnetic theory, the magnitude and phase of the signals induced in the respective grid conductors depend on the location of the conductors with respect to the pen tip. The grid conductors in each group are sequentially scanned to sequentially couple the induced signals to a differential amplifier by means of multiplexing circuitry. The multiplexing circuitry selects the respective grid conductors in response to an address decoder. The address decoder is driven by a scan counter. The scan counter is incremented by a signal produced by a divider circuit driven by the oscillator. A position counter is incremented in response to the oscillator as the X and Y groups of grid conductors are sequentially scanned. A phase sensitive detector responsive to the output of the amplifier produces a positive signal having a step-like envelope as grid conductors approaching one side of the pen tip are sequentially scanned. The phase sensitive detector also produces a negative signal having a step-like envelope as conductors on the opposite side of the pen tip are sequentially scanned in a direction away from the pen tip. The signal produced by the phase sensitive detector is filtered to produce a characteristic wave shape signal having a positive peak as the scanning approaches the pen tip. The filtered signal falls steeply across the zero axis to a negative peak as the scanning passes under the pen tip, and decreases as the scanning continues in the direction away from the pen tip. The filtered signal is differentiated and compared to a predetermined threshold level to produce a stop pulse. The stop pulse is utilized to disable the position counter. At this point the contents of the position counter represent the location of the pen tip with respect to the X group of grid conductors and are loaded into an output register. The contents of the output register are then outputted to a utilization device. The scan counter and the position counter are reset and the conductors of the Y group of grid conductors are scanned in a similar manner to produce a digital position number representing location of the pen tip with respect to the Y group of conductors.

Other patents which employ multiplexers to sequentially scan the respective X and Y conductors of groups of parallel spaced conductors and assigned to Talos Systems, Inc. include U.S. Pat. Nos. 4,185,165 and 4,260,852. While all of these patents which employ multiplexers to sequentially scan the grid conductors produce desirable results in a satisfactory manner, the equipment to accomplish these results are relatively complex and costly.

Accordingly, it is an object of this invention to provide a simplified and lower cost digitizer.

Briefly described and in accordance with one embodiment thereof, the invention provides a digitizing system and method for converting a position of an instrument, such as a pen or cursor, to an electrical signal representing a position of the instrument with respect to a grid of conductors. The instrument generally includes a coil for coupling electrical signals to a grid of conductors including a group of parallel spaced conductors oriented in an X direction and a group of parallel spaced conductors oriented in a Y direction. For one embodiment of the invention sixty-four parallel spaced conductors are arranged in both the X and Y directions with a constant spacing between conductors. Typically, the conductors are deposited on a printed circuit board with the conductors in the X direction deposited on one side of the printed circuit board and the conductors in the Y direction deposited on the other side of the printed circuit board; however, alternatively, wires or other conductors may be embedded or disposed upon some other non-conducting surface.

The pen or cursor which operates in conjunction with the grid conductors comprises a coil of wire wound on a non-metallic form. An oscillator applies a signal to the coil of a predetermined frequency, typically 100 Khz. Typically, the coil operates at 12 volts at 150 milliamps. The more current, the larger the signal induced in the conductors of the grid, however, heating problems limit the amount of current which can be applied to the coil. The coil is inductively coupled to the conductors of the grid, thereby inducing signals in the conductors. In accordance with electromagnetic theory, the magnetic and polarity of the signals induced in the respective grid conductors depends upon the location of the conductors with respect to the coil.

A multiplexing circuit is employed to sample the signal from the center conductor of the X direction array of conductors. By determining whether the signal is of a positive or negative polarity, the location of the coil with respect to the grid conductors is known, that is, whether the coil lies above or below the sampled center conductor. The multiplier then samples the signal from the center conductor of the half section (upper or lower) in which the coil is located, and again, by ascertaining whether the signal is positive or negative, a determination is made in which quarter section the coil is located. Additional readings are taken in the same fashion until it is ascertained that the coil lies between two adjacent X direction conductors.

If the coil is exactly equidistant between the two conductors, the magnitude of signal induced in each conductor will be equal and opposite in polarity, and therefore, it is known that the coil is, in fact, precisely in the middle of the two conductors. When the coil is not equidistant between the two conductors, the magnitude of the signal from the two conductors will increase or decrease. That is, one will increase, the other will decrease.

The exact coil position is then determined by forming a ratio of the induced signal from the two conductors and comparing this ratio with values stored in a look-up table. Since ratios between the two signals are employed rather than using absolute values, any variation in signal strength from the coil to the conductors will have no effect on the measurement. Also, the measurements will be frequency and phase insensitive.

A like set of measurements is then made sampling signals from the Y direction array of conductors to precisely locate the coil with respect to the Y direction array of conductors.

The above-mentioned and other features and objects of the invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an illustration of the grid conductors of a digitizer illustrating the manner in which the location of a coil contained within the grid is ascertained;

FIGS. 2 and 3 are circuit diagrams of a digitizer processor in accordance with the principles of this invention; and

FIG. 4 is a diagram of a circuit for sampling the signals from the grid conductors of FIG. 1 to apply the signals therefrom to the circuit of FIGS. 2 and 3.

Referring now to FIG. 1 of the drawings, there is illustrated thereby, schematically, the grid conductors of a tablet. This tablet typically contains sixty-four parallel conductors (numbered 1-64) arranged in the X direction and sixty-four parallel conductors (numbered 65-128) arranged in the Y direction. Assuming the location of a coil at position 10 during one moment in time, the function of the digitizer is to determine that location precisely. In accordance with this invention, an ac signal is applied to the coil and is inductively coupled to the conductors. X conductors on one side of the coil will have a positive signal induced therein, while X conductors on the opposite side of the coil will have a negative signal induced therein. Thus, by noting the polarity of the signal induced in a particular conductor being sampled, it is readily ascertained that the coil is either above or below such conductor.

In accordance with the principles of this invention, the signal inductively coupled from the coil to a conductor in the middle of the tablet is sampled, for example, conductor 32. When the signal on that conductor is sampled, it is determined by the polarity of the signal that the coil is, in fact, above the conductor 32 and within the area containing the conductors 1-32. Accordingly, it is not necessary to sample any signals induced in conductors 33-64. The next sampling is taken at a conductor in the center of the upper half of the tablet such as conductor 16. By noting the polarity of the signal from conductor 16, it is ascertained that the coil is, in fact, above conductor 16, and therefore, in the upper quarter of the tablet, namely somewhere between conductors 1 through 16. The next sampling is taken at, for example, conductor 8 and, by denoting the polarity it is ascertained that the coil lies within an area containing conductors 1-8. The next sampling occasions a measurement to be made of the signal on conductor 4 and by the polarity of the signal, it is determined the coil is, in fact, above conductor 4 and located somewhere between conductors 1 through 4. A measurement is then made of the signal at conductor 2 and by noting a different polarity from those previously measured, it is ascertained that, in fact, the coil is located below conductor 2. The equipment then samples conductor 3 and judging from the polarity of the sampled signal, it is ascertained that the coil is above conductor 3. Thus, with only six samples the course location of the coil (within two conductors) has been ascertained. This is contrasted to sampling all sixty-four conductors as in other systems.

Accordingly, it is now known that the coil lies between conductors 2 and 3. If the signals measured at conductors 2 and 3 are equal, then it is known that the coil is equally distant from conductors 2 and 3. However, if the signals are not equal then a ratio is taken of the signals obtained from the sampling of conductors 2 and 3, and this ratio is compared in a look-up table with values stored therein to determine the precise location of the coil. Since ratios are employed rather than absolute values of the signals sampled from conductors 2 and 3, the system is amplitude, frequency and phase insensitive. After determining the position of the coil with respect to conductors 1 through 64, the same procedure is carried out with respect to the Y direction conductors 65 through 128 to locate the coil precisely in the other direction.

The circuitry for carrying out this procedure is set forth in FIGS. 2, 3 and 4. FIGS. 2 and 3 include all of the circuitry necessary except for the conductor sampling means which is employed to sample the necessary ones of the conductors 1 through 128, and this is shown in FIG. 4 of the drawings. The conductor sampling means connects conductors of the tablet to the circuitry of FIGS. 2 and 3.

In the example set forth above, the signals from conductors 64, 32, 16, 8, 4, 2 and 3 are successively applied to an input 12 of the digitizer circuit of FIGS. 2 and 3. Each of these signals is amplified and filtered at circuit 14 in order to provide proper operating signal levels for the system. The output from amplifier and filter circuit 14 is applied to a synchronous detector 16 to convert the input ac signal to a dc signal. The output from synchronous detector 16 is applied to an amplifier and smoothing filter 18. Circuit 18 smoothes any ripples in the output from the synchronous detector 16 and further amplifies the dc signal.

The output from the amplifier and smoothing filter circuit 18 is applied to an analog-to-digital converter 20 which converts the dc voltage output from circuit 18 to a binary number. This binary number is applied to a microprocessor 22. The binary number is utilized in the microprocessor 22 to determine which conductor of the tablet to sample next in accordance with the stored program therein. The output from microprocessor 22 at pins 27 through 33 are the addresses of the conductors of the tablet to be sampled and these are applied to the sampling circuit of FIG. 4. These are the addresses of the conductors to be connected to the input 12. The program stored in the microprocessor determines which conductor of the tablet to sample next as well as developing the ratios of the specific conductors to be compared to the values in the look-up table to determine the position of the coil in the X direction. Signals are also applied to input 12 from respective Y conductors to determine location of the coil in the Y direction as well. Connections from pins 1, 6, 8, 10-19, 35 and 39 of microprocessor 22 are coupled to a UART 24 to communicate with auxiliary equipment such as a computer or terminal. The outputs from UART 24 are taken from TTL to RS232C via interfaces 25 and 27.

An output 28 from the microprocessor is a 400 KHz signal which is applied to a divider 30 to divide the signal by four to provide a 100 KHz signal. This signal is applied via line 32 and transistor 34 to synchronous detector 16 as a reference input thereto. Transistor 34 converts and level shifts the signal from divider 30. The 100 KHz signal is also applied to a push-pull amplifier 36 with the outputs therefrom being applied to the coil to drive the coil in push-pull.

The circuit for sampling the sixty-four X (numbered 1-64) and the 64 Y (numbered 65-128) conductors is shown in FIG. 4. The circuit includes two one-of-eight selectors 40 and 42 and sixteen switching circuits, eight for switching to or sampling appropriate X conductors and eight for switching to or sampling appropriate Y conductors, only two of each being shown, for simplicity of explanation.

A switching circuit 44 connects a selected one of the X conductors 1-8 to the input 12 of the circuit of FIGS. 2 and 3 via a line 46 and a switching circuit 48 connects a selected one of the X conductors 49-64 to the same input via line 46. Similar switching circuits (not shown) are employed to connect selected ones of the conductors 9-48 to the input 12.

Switching circuit 50 is employed to connect a selected one of the Y conductors 65-72 to input 12 via line 46 and switching circuit 52 is used to connect a selected one of the Y conductors 121-128 to input 12 via line 46. Other switching circuits (not shown) are employed to connect selected ones of the conductors 73-120 to the input 12.

The inputs to the sampling circuit of FIG. 4 are received from the output of microprocessor 22 along lines 54, 56, 58, 60, 62, 64, 66 and 68. The input along line 54 indicates that Y conductors are to be sampled and is thus applied to the one-of-eight selector 42 for the Y conductors. The input at line 62 denotes that X conductors are to be sampled and is accordingly coupled to the one-of-eight selector 40 for the X conductors.

The one-of-eight selector 40 determines which of the eight X conductor switching circuits, including switching circuits 44 and 48, is to be enabled while the one-of-eight selector 42 selects the appropriate one of the eight switching circuits for the Y conductors, including switching circuits 50 and 52. This selection of an appropriate switching circuit is made by the selectors 40 and 42 in accordance with address command received from microprocessor 22 along lines 56, 58 and 60 which is coupled to both of the selectors 40 and 42.

The other inputs to the sampling circuit from the microprocessor is the input on lines 64, 66 and 68. These inputs are applied to all of the switching circuits including switching circuits 44, 48, 50 and 52, and selects which of the eight conductors coupled to a particular switching circuit is to be coupled to the output line 46.

The computer program for operating microprocessor 22 is written in assembly language and is as follows: ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6## ##SPC7## ##SPC8## ##SPC9##

While I have described above the principles of my invention in conjunction with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of my invention as set forth in the accompanying claims.