Abstract:
A method and apparatus is provided by the invention whereby a base station communicates with and determines the position of multiple remote devices on a two-dimensional surface. In one embodiment the base station employs a single channel radio transmitter, an infrared detector, and an infrared projecting apparatus. The radio transmitter sends a continuous stream of addressed and time-multiplexed commands to the multiple remote devices. The multiple remote devices receive commands and time synchronization from the radio signal. The projecting apparatus is located above the two-dimensional surface and projects an alternating pair of orthogonal, sweeping, infrared, line-shaped, illumination areas upon it with deterministic timing. Each remote device detects the passage of the sweeping infrared line-shaped illumination areas over it. The measured timing is used to derive the two dimensional position aboard the remote device which is then communicated back to the base station by an infrared data link.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Application No. 60/521,249, filed Mar. 19, 2004, said Provisional Application being incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to the field of communicating with and the locating of multiple remote devices and more specifically to the field of communicating with and locating of multiple remote devices by using a base station that establishes two-way communication with multiple remote devices and determines their individual positions on a two-dimensional surface.  
       BACKGROUND OF THE INVENTION  
       [0003]     Typical positioning determining systems determined the position of objects in the order of feet and meters. For example, GPS is a system that locates objects geographically in longitude and latitude on a scale of tens of feet. There are currently no systems or technology available to allow a central controller to communicate with and determine the position of multiple remote devices in an economical manner and on a small scale, such as would be required for Robotic Gaming for example. The required positioning resolution is, therefore, finer than for many currently available technologies. At the same time, there are advanced position determination systems, however they do not use simple circuitry and widely available components and thus have a high cost.  
         [0004]     For a two dimensional platform, position determination systems such as grid-pad systems or touch systems, such as used for stylus or mouse location, are not readily expandable to the task of locating multiple remote devices disposed on the two dimensional platform. Unfortunately, optical systems involving video cameras and array processing are not economical for this application either.  
         [0005]     A need therefore exists to provide an economical position determination system for individually locating multiple remote devices on a two-dimensional surface.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore an object of the invention to provide a position determination system for locating multiple remote devices on a surface.  
         [0007]     It is a further object of the invention to provide such a position determination system that allows for the remote devices to communicate information back to a base station that is used for the position determination.  
         [0008]     In accordance with the invention there is provided an economical means of allowing a base station to communicate with and determine the position of multiple remote devices on a two-dimensional surface.  
         [0009]     In accordance with the invention there is provided a base station that transmits control and timing information on a single radio channel that is received simultaneously by multiple remote devices.  
         [0010]     In accordance with the invention the base station illuminates the two-dimensional surface from overhead with a first narrow, line-shaped pattern of light repetitively sweeping with predetermined timing across the two-dimensional surface in a direction perpendicular to its line shape.  
         [0011]     In accordance with the invention the base station illuminates the two-dimensional surface from overhead with a second narrow, line-shaped pattern of light perpendicular to the first line-shaped pattern of light and repetitively sweeping with predetermined timing alternate to the sweep of the first line-shaped pattern of light across the surface of the two-dimensional surface in a direction perpendicular to its own line shape.  
         [0012]     In accordance with the invention there is provided a means whereby remote devices detect the time of passage overhead of each of the sweeping line-shaped patterns of illumination, thereby determining their individual x and y location on the two dimensional surface.  
         [0013]     In accordance with the invention there is provided a means by which the remote devices communicate their measured locations to the base station.  
         [0014]     In accordance with the invention there is provided a method and apparatus, whereby a base station communicates with and determines the position of multiple remote devices on a two-dimensional surface. In one embodiment the base station employs a single channel radio transmitter, an infrared detector, and an infrared projecting apparatus. The radio transmitter sends a continuous stream of addressed and time-multiplexed commands to the multiple remote devices. The multiple remote devices receive commands and time synchronization from the radio signal. The projecting apparatus is located above the two-dimensional surface and projects an alternating pair of orthogonal, sweeping, infrared, line-shaped, illumination areas upon it with deterministic timing. Each remote device detects the passage of the sweeping infrared line-shaped illumination areas over it. The measured timing is used to derive the two dimensional position aboard the remote device which is then communicated back to the base station by an infrared, RF, light, sound, ultrasound, or other data link. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention, and together with the written description and claims, serve to explain the principles of the invention. In the drawings: Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which:  
         [0016]      FIG. 1  illustrates simultaneous radio communication signals from a base station to multiple remote devices, the infrared communications from the remote devices back to the base station, and one of two alternating, orthogonal, sweeping, planar fields of infrared illumination that intersects the two-dimensional surface to form a line-shaped illumination area;  
         [0017]      FIG. 2  illustrates a communication and positioning system comprising a base station and remote devices disposed on a two dimensional plane with a Cartesian coordinate system;  
         [0018]      FIG. 3  is a block diagram of the base station comprising an intelligent controller, a data encoder, an RF transmitter, a data decoder, and an infrared projecting apparatus;  
         [0019]      FIG. 4  illustrates two projection drums, the mechanical linkage between them, the motor, a drive assembly, and a position sensor;  
         [0020]      FIG. 5  illustrates the end view of a projection drum with two slits and an infrared emitter;  
         [0021]      FIG. 6  illustrates the geometrical optics of a projection drum for two superimposed positions of a slit at the extremes of a sweep;  
         [0022]      FIGS. 7   a  through  7   h  illustrate diagrams of one or more projected line-shaped areas of infrared illumination on the two-dimensional surface for various positions of a projection drum;  
         [0023]      FIG. 8  is a block diagram of a remote device comprising an RF receiver/detector, a clock recovery and data decoder, an intelligent controller, a data encoder, a carrier modulator, an infrared emitter, and an infrared detector;  
         [0024]      FIG. 9  illustrates a serial data stream and the resulting RF carrier signal transmitted by the base station;  
         [0025]      FIG. 10  illustrates the bit format used in each word of the detected Manchester data stream;  
         [0026]      FIG. 11  illustrates the format of a single frame consisting of 18 words;  
         [0027]      FIG. 12  illustrates the relationship between one 10-bit Manchester word and two synchronous NRZ words;  
         [0028]      FIG. 13  illustrates the relationship between one half of a frame and the eighteen synchronous NRZ words;  
         [0029]      FIG. 14  illustrates an example of a transmission on synchronous infrared NRZ channel  6 ;  
         [0030]      FIG. 15  illustrates a graph of the projected line position along the two-dimensional surface versus the angle of a projection drum; and,  
         [0031]      FIG. 16  illustrates a graph of the differential error in measurement versus the angle of a projection drum. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]      FIG. 1  illustrates a gaming platform  100  that utilizes a position determination system  101  in accordance with a preferred embodiment of the invention. The position determination system  101  includes a base station  29 , coupled with a RF antenna  27 , coupled with a infrared projection apparatus  34  and a base station optical receiver  26 , which is preferably in the form of an infrared detector for receiving of a infrared signal having, for example, an approximately 56 kHz carrier frequency. Of course, other light detectors and carrier frequencies can also be used.  
         [0033]     Referring to  FIG. 2 , the infrared projection apparatus  34  and the base station optical receiver  26  are disposed above a substantially planar two-dimensional surface  21  that can be delineated by a Cartesian coordinate system in the plane of the surface  21 . Planar or substantially planar for purposes of this invention means flat enough so that the remote devices can move and maneuver on the surface in an acceptable manner for the nature and characteristics of the particular remote devices being used. For example, if the remote devices include toy vehicles, the surface  21  can include toy-sized hills and valleys, obstacles, and the like, and still be considered planar, substantially planar, or two-dimensional according to the terminology used herein.  
         [0034]     The base station  29  is electrically coupled with both the base station optical receiver  26  and the infrared projection apparatus  34 , as indicated by the electrical connector  19 , although other signal transmission and power supply methods and apparatus can be used to provide power and to provide communications between the base station  29  and the infrared projection apparatus  34 , as is within the knowledge and capabilities of persons skilled in the art. Preferably, the base station  29  is not disposed above the planar two-dimensional surface  21  but is disposed at a side thereof.  
         [0035]     As shown in  FIGS. 1 and 2 , remote devices  25  are disposed on the substantially planar two-dimensional surface  21  and in view of the base station optical receiver  26  and the infrared projection apparatus  34 . The remote devices  25  can be, for example, self-propelled or otherwise moveable game pieces, and they can be either remote controlled or automatically or manually controlled. An X-axis  201  and a Y-axis  202 , which is orthogonal to the X-axis  201 , are used for defining the Cartesian coordinates for the planar two-dimensional surface  21 .  
         [0036]      FIG. 3  is a block diagram of the base station  29 , comprising a base station control circuit  31 , a data encoder  33 , a RF transmitter  35 , the base station optical receiver  26 , a data decoder  37  and the infrared projection apparatus  34 . The base station control circuit  31  is coupled with the data encoder  33 , which is coupled to the RF transmitter  35 , which is further coupled with an antenna  27 . The optical detector  26  is coupled with the data decoder  37 , which is further coupled with the base station control circuit  31 . The infrared projection apparatus  34  is also coupled with the base station control circuit  31 .  
         [0037]     The data encoder  33  is for modulating a radio frequency (RF) carrier with 100% AM modulation by keying the RF transmitter  35  ‘on’ and ‘off’. This is also sometimes referred to as on-off keying (OOK). This RF transmission from the base station  29  is then broadcast to all the remote devices  25  using the antenna  27 .  
         [0038]     The base station control circuit  31  includes a processor (CPU)  31  a and is used to generate the RF transmission from the base station  29  to all of the remote devices  25 . For the RF transmission from the base station  29 , the data encoder circuit  33  receives intended message data from the CPU  31   a  and converts the intended message to a Manchester pulse code modulated (PCM) serial data stream, which is then transmitted to all the remote devices  25  as the RF transmission from the base station  29 . As described herein below, Manchester coding combined with the specific data sequence for the RF transmission from the base station  29  allows for accurate timing synchronization for each of the remote devices  25 .  
         [0039]      FIG. 4  illustrates an exemplary embodiment of the infrared projection apparatus  34 . As shown in  FIG. 4 , the infrared projector apparatus  34  is comprised of two rotatable drums  52 ,  53  situated with their rotational axes  48 ,  49 , respectively, perpendicular to one-another, and parallel to the plane of the two-dimensional surface  21 . The two drums  52 ,  53  rotate in unison, preferably, but not necessarily, through a bevel gear mechanism  51 , and are driven by a motor  63 . The base station control circuit  31  synchronizes the rotation of the drums  52 ,  53  to be in lock step with a data frame rate of the Manchester PCM serial data stream. This synchronization of drums  52 ,  53  rotation is preferably accomplished through feedback provided by position sensor  58  and control of the speed of the motor  63 . The position sensor  58  senses the rotational position of one of the drums, e.g., of drum  53 , which can be done by any of a variety of optical or magnetic detectors or other devices that are readily available and well-known to persons skilled in the art. Since the rotational motion of the drums  52 ,  53  are the same size and shape and are tied together by the bevel gear arrangement  51 , only one such sensor  58  is needed to monitor the rotational positions of both drums.  
         [0040]     Drum  52  is disposed with its rotational axis  48  parallel to the Y-axis  202 , so a light beam  61   a  emanating from a light source  59   a  and shining through a slit  54  in the cylindrical drum wall  46  sweeps along, i.e., in the direction of, the X-axis  201 . Drum  53  is disposed parallel to the X axis  201 , so a light beam  61   b  emanating from a light source  59   b  inside drum  53  and shining through a slit  55  in the cylindrical drum wall  47  sweeps along, i.e., in the direction of, the Y axis  202 .  
         [0041]      FIG. 5  is a cross-sectional view of drum  52  taken along section line  5 - 5  in  FIG. 4 . Drum  53  is identical to drum  52  and thus the same applies thereto. Each drum  52 ,  53  includes modulated infrared light source  59   a ,  59   b , respectively, disposed inside the drum. Each infrared light source  59   a ,  59   b  is modulated with a continuous wave (CW) carrier frequency of approximately 40 kHz. Each drum  52 ,  53  has two slits, or longitudinal apertures, e.g., apertures  54 ,  56  in drum  52  and apertures  55 ,  57  in drum  53 . Each of these elongated apertures or slits  54 ,  56  and  55 ,  57  is oriented parallel to the axis of rotation  48 ,  49  of their respective drums  52 ,  53 . The two slits  54 ,  56  are disposed diametrically opposite each other in the cylindrical wall  46  of drum  52 , i.e., 180 degrees from one another about the circumference of the drum  52 . Likewise, the slits  55 ,  57  are disposed diametrically opposite each other in the cylindrical wall  47  of drum  53 . Light from the sources  59   a ,  59   b  shines through one of the elongated slits,  54 ,  56  and  55 ,  57 , in each drum,  52 ,  53 , respectively. Propagation of light from the light source  59   a  in drum  52  through one of the slits  54 ,  56  results in a substantially planar beam  61   a , which forms an illuminated sweep line  23  (shown in  FIG. 2 ) on the planar surface  21  parallel to the Y-axis  202 . As the drum  52  rotates in the direction indicated by the arrow  44  in  FIG. 4 , the illuminated sweep line  23  ( FIG. 2 ) moves or sweeps in the direction of the X-axis  201 , as indicated by arrow  62   a  in  FIG. 2 . Propagation of light from the light source  59   b  through one of the slits  55 ,  57  in drum  53  results in a substantially planar second beam  61   b , which forms an illuminated sweep line  24  ( FIG. 2 ) on the planar surface  21  parallel to the X-axis  201 . As the drum  53  rotates in the direction indicated by the arrow  45  in  FIG. 4 , the illuminated sweep line  24  ( FIG. 2 ) moves or sweeps in the direction of the Y-axis  202 , as indicated by arrow  62   b  in  FIG. 2 . Therefore, the projection of each of the planar light beams  61   a  and  61   b  upon the surface  21  in  FIG. 2  results in an elongated, substantially line-shaped area of illumination, i.e., lines of light or lines of illumination on the surface, sometimes also referred to as sweep lines  23 ,  24  on the surface  21 .  
         [0042]     Preferably, the infrared projecting apparatus is located at least several diameters of the drums  52 ,  53  above the two-dimensional surface  21 . The mechanism comprising the rotating drums and slits described above is just one example way of producing the sweep lines  23 ,  24  on the surface  21 . Optionally, the slit on each drum could be replaced with a cylindrical lens (not shown) designed to collimate the infrared light in the narrow dimension of each of the planar beams  61   a  and  61   b  in order to form the illumination lines  23 ,  24 . Persons skilled in the art can devise many other ways of doing so, such as with rotating or oscillating mirrors, cylindrical lenses, spatial light modulated arrays, and the like, any of which can be used to implement lines of light that sweep across the surface  21  for use in this invention.  
         [0043]      FIG. 6  illustrates the geometrical optics of the extremes for a single X sweep or a single Y sweep denoted by points A and B corresponding to drum  52  (and drum  53 ), with an angle of ±45 degrees. The diagram shows both extremes of the sweep in superposition for illustrative purposes only. It is preferred that only one of the planar beams  61   a ,  61   b  per axis  201  or  202  is projected onto the surface  21  at any given time. Therefore, only one of the sweep lines  23 ,  24  is on the surface  21  at a time. However, other implementations in which several sweep lines  23 ,  24  may be formed on the surface simultaneously can also be used to implement this invention. In the embodiment shown in  FIG. 4 , the drums  52 ,  53  are fixed in rotational relation to each other such that the first line-shaped area of illumination  23  preferably sweeps along the X axis  201 , and the second line-shaped area of illumination  24  preferably sweeps along the Y axis  202 , but at different times. For example, drum  52  and drum  53  may be set by the bevel gear  51  interconnection or by any other synchronizing at a 90-degree rotational off-set from each other so that a sweep line  23  of one drum  52  is just leaving the surface  21  as the sweep line  24  of the other drum  53  is just entering the surface  21  and vice versa.  
         [0044]      FIGS. 7   a  through  7   h  illustrate a sequence of images showing the preferred motion of the sweep lines  23 ,  24  across the two-dimensional surface  21 . The images are labeled according to the angular rotation of the respective drum  52  or  53 . For reference purposes,  FIG. 7   a , is labeled as ‘−30 degrees’ and thus it defines a set of coordinate axis and an origin for reference purposes for the two-dimensional surface  21 .  
         [0045]     Referring to  FIG. 7   a , at −30 degrees of drum rotation, sweep line  23  is parallel to the Y-axis and is at a negative value along the X-axis  201 . Referring to  FIG. 7   b , at 0 degrees of drum rotation, sweep line  23  is at position X=0, and as shown in  FIG. 7   c , sweep line  23  is at a positive X value along the X-axis  201 . As shown in  FIG. 7   d , at 45 degrees of drum rotation, sweep line  23  is at the most positive value of X for the X axis  201  and sweep line  24 , which is oriented parallel to the X axis  201 , is at the most positive value for the Y axis  202 . Referring to  FIG. 7   e , at 60 degrees of drum rotation, sweep line  23  is no longer illuminating the two-dimensional surface  21 , i.e., has moved off surface  21 , and sweep line  24  is at a positive value of Y for the Y-axis  202 . Referring to  FIG. 7   f , at 90 degrees of drum rotation, sweep line  24  is at position Y=0. Referring to  FIG. 7   g , at 120 degrees of drum rotation, sweep line  24  is at a negative value of Y for the Y-axis  202 . Referring to  FIG. 7   h , at 135 degrees of drum rotation, sweep line  24  is at the most negative value of Y for the Y-axis  202  and sweep line  23  is at the most negative value of X for the X-axis  201 . Because of the symmetry of the drums  52 ,  53  and their rotation in unison with each other, the sweep lines repeat scanning with every 180-degree rotation of each drum. As shown in  FIG. 7   a  through  7   h , first the sweep along the X-Axis  201  is performed in response to the rotation of drum  53  and then a sweep along the Y-Axis  202  is performed in response to the rotation of drum  52 .  
         [0046]     The length of the slits,  54 ,  55 ,  56 ,  57 , the geometry of their positions and the position of the infrared emitter,  59   a  and  59   b , determine a length of each sweep line,  23  or  24 , that is projected onto the two-dimensional surface  21 . As illustrated in  FIG. 7 , during the 90-degree interval of rotation, when one of the slits  54 ,  55 ,  56 ,  57  moves between points A and B, the geometry varies and thus the length of the sweep line,  23  or  24 , as it illuminates the two dimensional surface  21 , varies. As a design parameter, the length of the slits  54 ,  55 ,  56 ,  57  should be such as to guarantee the minimum length of the sweep line,  23  and  24 , that is at least equal to a side length of the two dimensional surface  21 . Preferably, for the preferred embodiment the two-dimensional surface is square and is illuminated with same-length sweep lines,  23  and  24 .  
         [0047]     Preferably, the rotation of the two drums,  52  and  53 , is phased with respect to each other, so that, for example, when slit  54  of drum  52  reaches point B in its rotation (+45 degrees), then slit  55  of drum  53  is at point A in its rotation (−45 degrees). This phasing, and the positioning of the slits  54 ,  55 ,  56 ,  57  on the drums,  52  and  53 , results in only one of the slits  54 ,  55 ,  56 ,  57  being between points A and B, not inclusive, on their respective drums  52 ,  53 . When one of the drums,  52  or  53 , is exactly at point A or B the other of the drums,  52  or  53 , is at point B or A, respectively.  
         [0048]     One application of the invention may be for use in an indoor location with the two-dimensional surface  21  having an area, for example, as large as 10 feet by 10 feet. Of course, the size of the area is variable and is related to a height of the infrared projection apparatus  34  in relation to the two dimensional surface  21 . However, because the infrared transmitters are modulated at approximately 40 Khz and 56 kHz, the invention can also be used outdoors.  
         [0049]      FIG. 8  illustrates a block diagram of an example circuitry for a remote device  25 . In this example, the remote device comprising a RF receiver circuit  41  coupled with a clock recovery and data decoder circuit  43 , which is then further coupled with a remote device control circuit  45 . The remote device control circuit  45  is coupled with a data encoder circuit  47 , which is coupled with a carrier modulator circuit  49  and an infrared emitter  42 . An infrared receiver  44  is also provided and coupled with the remote device control circuit  45 . Furthermore, the clock recovery and data decoder circuit  43  is coupled with the data encoder circuit  47 .  
         [0050]     The RF receiver  41  is for receiving of the RF transmission  12  ( FIG. 9 ) from the base station  29  ( FIG. 3 ), transmitted from antenna  27  ( FIG. 3 ). The infrared receiver  44  is for receiving of infrared light emitted from the infrared sources,  59   a  and  59   b  ( FIG. 4 ), at approximately a 40 kHz frequency. The infrared emitter  42  is for providing of infrared light to the base station optical detector  26  ( FIG. 3 ), at approximately a 56 kHz frequency. That said, the base station infrared receiver  26  is not sensitive to the carrier frequency of the infrared sources,  59   a  and  59   b.    
         [0051]     Preferably, each remote device  25  is pre-assigned its own unique address, such as address values from 0 to 15. A protocol employing both direct addressing and time slot addressing is used so that data between the base station  29  and each remote device  25  is uniquely transmittable therebetween.  
         [0052]      FIG. 9  illustrates a Manchester serial data stream  14  and the resulting RF transmission  12  that is transmitted by the base station  29  to all the remote devices  25  for reception by the RF receiver circuit  41 . A digital ‘1’ value  14   a  in the serial data stream  14  modulates the RF transmission  12  to fully ‘ON’  12   a  and a digital ‘0’ value in the serial data stream  14  modulates the RF transmission  12  to fully ‘OFF’, as shown.  
         [0053]     Each remote device  25  receives and decodes all of the information sent by the base station  29  in the RF transmission  12  from the base station  29  in a frame-by-frame manner. The specific data format used by the base station  29  to communicate with the remote devices  25  is described herein below. Of course, those of skill in the art are aware of many different signal communications protocols and could devise other acceptable formats. Therefore, this description presented herein is just one example and is not intended to limit the present invention to a specific format or protocol. Instead, it is intended to describe a preferred embodiment.  
         [0054]      FIG. 10  illustrates a bit format that is used for each word  14   f  of the Manchester serial data stream  14  that is provided as the RF transmission  12  from the base station  29 . The information is transmitted in a sequence of serial words, each comprised of 10 bits. Below is a table of the bit definitions in the Manchester serial data stream  14 :  
                                                                   10   9   8   7   6   5   4   3   2   1                   S   P   D7   D6   D5   D4   D3   D2   D1   D0                    
 Referring to this table, the column headings 1 through 10 are indicative of a bit position in the Manchester serial stream defined where 1 is the last bit transmitted. D 0 -D 7  represent an 8-bit byte comprising information sent within the Manchester serial stream. P is a parity bit used for error detection, and S is a start bit, which is always set to digital ‘1’ value. 
 
         [0055]     Referring to  FIG. 11 , a sequence of eighteen words  14   f  constitutes a single data frame  14   b , which is delineated by a sync word  14   c . These data frames  14   b  serve to establish a measurement interval, a timing reference, and a set of time division multiplexed communication channels for each of the remote devices. In the preferred embodiment, sixteen (16) channels are assigned for a maximum of sixteen remote devices  25 , although any other number of devices and associated channels could be used in this invention. The words  14   f  for a single frame  14   b  are defined as follows:  
                                                       Word (0)   Sync byte           Words (1-8)   Data for remote devices 0 to 7           Word (9)   Command byte           Word (10-18)   Data for remote devices 8 to 15                      
 
 The words  14   f  are sent repetitively without gaps such that bit transitions occur regularly in synchronization with a steady clock signal, generated within the base station control circuit  31 . Thus, a beginning of each frame, denoted by the sync word  14   c , occurs at a steady and predictable rate. Preferably, the duration of each bit is 696 microseconds, the duration of each word  14   f  is 6.96 milliseconds, and the duration of each data frame  14   b  is 125.28 milliseconds. The predictable and steady nature of the data stream enables a remote device  25  to generate an internal clock in lock step with the timing of the base station  29  and a local frame sync signal in step with that of the base station  29  to an accuracy of ±5 microseconds. 
 
         [0056]     Because the RF transmission  12  from the base station  29  is a Manchester PCM serial data stream, clock information is embedded therein. Thus, within each remote device  25 , the clock recovery and data decoder circuit  43  ( FIG. 8 ) derives an internal clock signal for the remote device  25  from the RF transmission  12  from the base station  29 , which allows for synchronization of the internal clock signal for the remote device  25  with the master clock signal of the base station  29 . Furthermore, the receipt of the RF transmission  12  from the base station  29  is used to synchronize to the word  14   f  boundaries and frame  14   b  boundaries by the remote device control circuit  45 . The synchronization to the frame  14   b  occurs as a result of the sync word  14   c . Upon receipt of the sync word  14   c , the remote device control circuit  45  synchronizes itself with the frame  14   b . Thus, each remote device  25  is operating with its internal clock and frame sync signal synchronized to the master clock signal.  
         [0057]     Within the remote device  25 , a locally generated frame sync signal and the locally generated clock signal are used for position determination for each remote device  25  within the two-dimensional planar surface  21 . The accuracy in timing of ±5 microseconds in the measurement of time introduces a negligible error for the positional determination.  
         [0058]     A first half  14   d  of the data frame  14   b , labeled ‘X sweep’, is used to derive an X position along the X-axis  201  for the remote device  25 . A second half  14   e  of the data frame  14   b , labeled ‘Y sweep’, is used to derive a Y position along the Y-axis  202  for the remote device  25 . The first half is defined from the beginning of the sync word  14   c  to the end of word 7. During this time, the line-shaped pattern of illumination  23  emitted from infrared source  59   a  ( FIG. 4 ), which is parallel to the Y-Axis  202 , sweeps in the direction  62   a  ( FIG. 2 ) from one end of the two-dimensional surface  21  ( FIG. 7   h ) to the other end of the two-dimensional surface  21  ( FIG. 7   d ) in the positive X-direction, as shown in  FIGS. 7   h ,  7   a ,  7   b ,  7   c  and  7   d.    
         [0059]     For Y position determination for the remote device  25 , the second half  14   e  of the data frame  14   b  is used. The second half is defined from the beginning of the command word to the end of word  15 . During this time, the line-shaped pattern of illumination  24  emitted from infrared source  59   b  ( FIG. 4 ), which is parallel to the X-Axis  201 , sweeps in the direction  62   b  ( FIG. 2 ) from one end of the two-dimensional surface  21  ( FIG. 7   d ) to the other end of the two-dimensional surface  21  ( FIG. 7   h ) in the negative Y-direction, as shown in  FIGS. 7   d ,  7   e ,  7   f ,  7   g  and  7   h.    
         [0060]     Each remote device  25  on the surface  21  receives the alternating sweep lines  23 ,  24  provided by infrared sources  59   a  and  59   b , respectively. Because each remote device  25  is synchronized with the master clock signal and the Manchester frame  14   b , it is aware of, i.e., detects, the occurrence of each X sweep and of each Y sweep via its infrared detector  44 . A software counter within the remote device control circuit  45  tracks time with respect to the beginning of the X-sweep and Y-sweep intervals. When the remote device  25  detects the passage of an X sweep line  23  or a Y sweep line  24 , the value of the software counter is recorded. This recorded value is nearly directly proportional to the position of the remote device on the two-dimensional surface  21 . Thus, when the infrared detector  44  becomes energized with light emitted from either of the infrared sources,  59   a  or  59   b , it triggers an event that causes the software counted to stop incrementing. The value of this software counter is then optically provided from the remote device  25 , using the infrared transmitter  42 , to the base station  29  for receipt by the infrared receiver  26  at the base station  29 . Because, each remote device  25  is assigned a unique channel, the base station  29  records this time information for the current remote device  25 . At the base station  29 , this time is recorded for use in positional determination along one of the axes for the remote device  25 .  
         [0061]     The remote device controller  45  increments an internal counter over a period of one half of a frame. The counter is reset at the beginning of each half frame as shown in  FIG. 11 . For the X-axis  201 , during the first half  14   d  of the data frame  14   b , the X sweep line  23  sweeps across the two-dimensional surface  21 . When the X sweep line  23  passes over a remote device  25  on the surface  21 , the infrared detector  44  provides a control signal to the remote device control circuit  45  of that remote device  25  so that it stores the value of the counter. For the Y axis  202 , during the second half  14   e  of the data frame  14   b , the Y sweep line  24  sweeps across the two-dimensional surface  21 . When the Y sweep line  23  passes over the remote device  25 , the infrared detector  44  provides a control signal to that remote device control circuit  45  so that it stores the value of the counter.  
         [0062]     Preferably, for example, after a sweep line sweeps along the X-Axis from one end to the other of the two-dimensional surface  21 , the time information that is proportional to the X position for the remote device  25  is provided to the base station  29  during the Y sweep period. In this alternating manner, each of the remote devices  25  provides a first time and second time to the base station  29  for use in X-Y coordinate position determination thereof in the time of one frame.  
         [0063]     Referring back to  FIG. 4 , preferably the rotational speed of the drums  52  and  53  is synchronized to the data frame rate of the Manchester signal such that the each of the drums  52  and  53  rotates 90 degrees during one-half of the data frame,  14   d  and  14   e . The phasing of the rotation of the drums  52 ,  53  is such that during a X sweep or Y sweep, one of the slits  54 ,  55 ,  56 ,  57  for one of the drums,  52  or  53 , moves from position A to position B as shown in  FIG. 6 . Preferably, the infrared light source  59   a  is positioned in relation to the rotational axis  48  of the drum  52  such that the substantially planar beam  61   a  sweeps a 60-degree swath for a 90-degree rotation of the drum  52  from position A to position B. In the same manner, it is preferred that the infrared light source  59   b  in drum  53  be positioned in relation to the rotational axis  49  of the drum  53  such that the beam  61   b  sweeps a 60-degree swath for a 90-degree rotation of the drum  53 . Referring to  FIG. 5 , this relationship defines the distance ‘r’ between the rotational axis  48  and the infrared light source  59   a  as r=0.5176 R, where R is the radius of the drum  52 .  
         [0064]     Preferably, for each revolution of one of the drum  52 , two sweeps of the substantially planar beam  61   a  between the points A and B occur, which means that two X sweeps of line  23  occur across the surface  21  for each revolution of drum  52 . Likewise, each revolution of drum  53  causes two Y sweeps of line  24  to occur across the surface  21 . The speed of the motor  63  is maintained so that each of the drums  52 ,  53  rotates one revolution over the period of two data frames  14   b.  In the preferred embodiment, the data frame period is 125.28 ms and so the drums  52 ,  53  rotate once every 250.56 ms corresponding to a rate of 3.991 revolutions per second, or 239.46 RPM. Of course, this invention could also be implemented with one slit per drum or with more than two slits per drum and setting the rotation speed of the drums accordingly, as is well within the capabilities of persons skilled in the art.  
         [0065]     The rotation of the two synchronized and phased drums,  52  and  53 , and their respective infrared emitters,  59   a  and  59   b , emitting infrared light through slits  54 ,  55 ,  56 ,  57 , results in the projection of alternating sweep lines,  23  and  24 , onto the two dimensional surface  21  in lock step with the frame rate ( FIG. 7 ). During the period of one data frame  14   b , one X line sweep and one Y line sweep occurs, preferably covering the entire two-dimensional surface  21 .  
         [0066]     Preferably, the RF receiver circuit  41  ( FIG. 8 ) is a single transistor super-regenerative receiver/detector followed by an alternating current (AC) amplifier. An AC coupled amplifier is preferably used because the Manchester serial data stream  12  contains no DC component. However, it should be understood that other types of RF receivers could be used.  
         [0067]     The infrared emitter  42  is used to transmit data from the remote device  25  back to the base station  29 . Data words generated by the remote device control circuit  45  are converted to a synchronous serial non-return to zero (NRZ) data stream  1401  by the data encoder circuit  47 . The NRZ stream  1401  modulates a carrier signal with on-off keying (OOK). Preferably, the carrier frequency and infrared wavelength for the infrared sources,  59   a  and  59   b , is chosen to be compatible with inexpensive, readily available infrared detectors for use as optical receiver  26 . The frame timing described above is used to simplify the infrared NRZ communication. The NRZ data bits are defined as follows:  
                                                                   10   9   8   7   6   5   4   3   2   1                   P   D8   D7   D6   D5   D4   D3   D2   D1   D0                  
 
 The colunm headings 1 through 10 above indicate the bit position in the NRZ serial stream  1401 , where 1 is the last bit transmitted. Bits D 0 -D 8  constitute a 9-bit data word while bit  10  is used for parity. 
 
         [0068]     The timing of the NRZ data word  1401  is shown in  FIG. 12 . This format is synchronized in time to the one ten bit Manchester word  14   f . However each 10-bit NRZ word  1401  transmits in half the time of each 10-bit Manchester word  14   f . The time synchronization simplifies the data format by allowing for the use of a synchronous NRZ serial format that contains no start or stop bits.  
         [0069]     Time data, for position determination, and optionally other data from the multiple remote devices  25  is time division multiplexed (TDM) to provide, for example, eighteen (18) independent channels of communication to the base station  29 . The eighteen channels are multiplexed within each half of a data frame  14   d ,  14   e  as shown in  FIG. 13 . This allows each remote device  25  to report a 9-bit word to the base station  29 , that has encoded therein time information for use in position determination, for each of the X sweep  14   d  and Y sweep  14   e  periods of a single data frame  14   b . For example,  FIG. 14  illustrates an example of infrared communications  1401  on channel  6  from a remote device  25  back to the base station  29 . Odd parity is used so that if transmissions are absent in a given time slot, a parity error will result. This parity result is indicative of absence or blockage of a remote device  25  assigned to that channel.  
         [0070]     In the preferred embodiment, rate of rotation of the drums,  52  and  53 , is kept constant within each frame  14   b . Because of the geometry of the system, the position of the sweep lines,  23  and  24 , does not progress across the two-dimensional surface  21  linearly with respect to time. The position of the sweep lines,  23  and  24 , on the two-dimensional surface  21  is shown in FIG.  15  as a function of drum angle θ (see  FIG. 5 ) for the drums,  52  and  53 . In this example, the infrared projecting apparatus  34  is located 72 inches above the two-dimensional surface  21 . In this case the absolute positional accuracy of a given axis is 0.685 inches RMS over a range of 83 inches (±41.5 inches). The two-dimensional RMS positioning error is 1.324 inches RMS over a square measurement area 83 inches on a side.  
         [0071]     The differential positioning error is shown in  FIG. 16 . Differential positioning error is the error, in percent, in measuring a slight positional translation at a given location of the remote device  25  in relation to the angle of the drum,  52  and  53 , in degrees. Quantitatively the differential positioning error in this case is 10.0% RMS.  
         [0072]     With respect to the preferred embodiment, a positional resolution of ±0.125 inches is achievable over a square area having 72 inches on each side for the two-dimensional surface  21 .  
         [0073]     Preferably the geometry of the drums and optics used to generate the sweeping line is similar to the geometry described in U.S. patent application Ser. No. 10/613,915 entitled “Method and Apparatus for Producing Ambulatory Motion,” incorporated herein by reference. In this patent application, optimum numerical ratios are given for this geometry that minimize linearity errors. Referring to  FIG. 5 , in the preferred embodiment it is possible to decrease the linearity error by increasing the distance “r” of the infrared source  59   a  and  59   b  from the rotational axis of the drum. As the distance r increases, it more closely approximates the optimum result as described in the aforementioned patent application. However, this comes at the expense of a reduction in the total sweep angle covered during the 90-degree rotation of a slit  54 ,  56  and  55 ,  57  from point A to point B as shown in  FIG. 6 .  
         [0074]     Preferably the wavelength and modulation frequency of the modulated infrared sources  59   a ,  59   b  and the frequency of the infrared receivers  26  and  44 , is chosen such as to allow the use of sensitive and inexpensive infrared modules.  
         [0075]     In a variation on the preferred embodiment, the speed of the motor  63  is adjusted through proportional control of the drive voltage as part of a phase-locked loop. The loop servos the motor  63  drive in order to match the position sensor  58  signal to the frame rate and phase. In another variation, the motor  63  is a stepper motor and its speed is inherently synchronized to the drive signals generated by the base station control circuit  31 .  
         [0076]     Optionally, a position sensor  58 , in the form of for example a photo interrupter optical switch, is disposed within the infrared projecting apparatus for providing a feedback signal to allow the motion of the sweep lines,  23  and  24 , to be synchronized to the processes of the base station control circuit  31 . In this case, the position sensor  58  provides an index signal indicating the drum  52 ,  53  is in a particular position. The RF data signal for transmission is then preferably synchronized to this index signal.  
         [0077]     Numerous other embodiments may be envisaged without departing from the spirit or scope of the invention.  
         [0078]     The foregoing description is considered as illustrative of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. Accordingly, resort may be made to all suitable modifications and equivalents that fall within the scope of the invention. The words “comprise,” “comprises,” “comprising,” “include,” “including,” and “includes” when used in this specification are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups thereof.