Abstract:
Transparent tablet for evaluation of motor responses furnishes the coordinates of the contact points of at least one conductive probe tip on tablet. The tablet is overlaid on its outer face by a resistive transparent film, such as tin dioxide. By the provision of a current-generating device in first and in second directions, voltage gradients are set up along the resistive transparent film in the first and second directions. The voltages picked up by the conductive probe tip at the point of contact are proportional to the distance of the point to the edges of the tablet along each of the directions. The numerical values of the point coordinates are furnished by the conversion to digital form, by an A/D converter, of the voltage.

Description:
BACKGROUND OF THE INVENTION 
   The present invention refers to a device that furnishes the spatial and temporal coordinates of the points of the point in a transparent plate that is touched by the tip of a probe. Such a device is particularly useful in the evaluation of motor performance, alternative or augmented communication systems, map-making, support systems for special teaching techniques, etc. 
   The integration between several sources of sensory information and the production of motor responses is a physiological function required in many common situations, with applications in the research of motor behavior in normal and handicapped individuals. 
   One line of research of said function is based on the touch, with probe tips, of targets of different shapes, dimensions, distances and positions, said targets being either static or dynamic. Static targets comprise figures printed or drawn in sheets of paper or cardboard, while dynamic targets can be shown in video monitors or projection devices. The act of touching can be triggered by visual or auditory stimuli, so as to allow the detection of correlations between the subject&#39;s responses to standardized stimuli. Said investigations are carried out having in mind the clinical analysis and remedial action in dysfunction-bearing populations. However, said research has been hindered by the lack of available specific and sophisticated tools that—owing to their high cost—are seldom available in areas with poor socio-economic conditions. 
   In view of the preceding, the invention has the main objective of providing a low-cost device that can be employed either with static or dynamic targets, and suitable for connection to a personal computer. 
   SUMMARY OF THE INVENTION 
   The above object is achieved by the invention by means of a device comprising a transparent glass plate overlaid on its outer surface by a transparent resistive film provided with means that produce sheet electric currents in a first and in a second directions, said currents having attributes that allow the identification of said currents, as well as at least one conductive probe, the determination of the coordinates of the point of contact between the tip of said probe and said surface being achieved by the measurement of the voltages picked up by said tip in each of said directions, said voltage values being digitized by an analogic/digital converter. 
   According to the invention, said identifying attributes are provided by the multiplexation of said currents. 
   According to another feature of the invention, the identification of the currents in a first direction and in a second direction is provided by the time switching of said currents, this is to say, the currents are applied to the transparent resistive sheet alternately and sequentially in a first and in a second directions. 
   According to another feature of the invention, the first current is an AC current with a first frequency and the second current is an AC current with a second frequency, said first and second currents being simultaneously applied to said transparent sheet. 
   According to yet another feature of the invention, said glass plate has a rectangular shape and said directions are ortogonal to each other. 
   The above mentioned features of the invention shall be better understood by the following description of several illustrative embodiments that follow, taken in connection to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows, by means of a block diagram, the main component parts of the proposed invention. 
       FIG. 2  shows the electrical connections relating to the transparent resistive film that overlays the glass plate. 
       FIG. 3  shows the block diagram of a system provided with two probes, according to the invention. 
       FIG. 4  shows a second embodiment of the invention. 
       FIG. 5  shows the structure of the data packets forwarded to the computer, corresponding to one- and two-probe devices. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawing of  FIG. 1 , the object of the invention consists of a tablet comprising a glass plate  10  overlaid with a transparent resistive film such as tin dioxide, and at least one conducting probe  21  connected by a flexible cable  22  to the A/D converter  24 , the output of which is connected to a microprocessor  20  which processes the information collected by the probe tip. Said processing performed by block  27  comprises formatting the information concerning the X and Y coordinates at the moment that said tip touched the tablet. Said processing additionally comprises the filtering of transients and noise, as well as repeated or null readings, which can be implemented by software stored in microprocessor  20 . 
   The data is formatted according to the application, because the system allows the emulation of a standard mouse, among other features. In this case the rate of transmission is 1200 bauds, the mouse function being activated by means of double press (i.e., double click) and drag, for example. In another embodiment, the probe operates as a mouse, in which case it will be provided with a pushbutton which will be connected to the microprocessor input through a second wire. 
   When the application requires the X/Y position data in real-time, the transmitted packet comprises a start byte  61 , two bytes  62 ,  63  containing the X value, one separating byte  64  and two bytes  65 ,  66  for the Y coordinate, such as depicted in  FIG. 5-   a . When two probes are used, an additional byte  67  is included, informing which probe the data refers to, followed by a separation byte  68 , as shown in  FIG. 5-   b.    
   These packets are processed by the data communication block  28 , which may be a USART or UART operating in the duplex mode, that communicates with the microcomputer by means of a serial interface  29  and 9-way cable, infrared or radio link, etc. 
   According to  FIG. 2 , the tablet comprises a rectangular glass plate overlaid with a resistive conducting transparent film such as, for example, tin dioxide, having a surface resistance in the order of 300 . . . 1000 ohms per square, in the embodiment described herein. According to the invention, a DC voltage is applied between vertical edges  31 ,  32  of the resistive film and, in the following half-cycle, the same voltage is applied between horizontal edges  36 ,  37 . This operation is carried out by driver block  25 , that generates two symmetrical square waves  38  and  39  with opposing phases, the former being forwarded by line  25   a  to a first and second set of parallel-connected diodes, said first set comprising diodes  41  having their cathodes connected to upper edge  36  of said rectangular resistive film and said second set comprising diodes  34  having their anodes connected to the right-hand edge  32  of said rectangular resistive film. Opposed-polarity square wave  39  is forwarded to a third and fourth diode sets through line  25   b , said third diode set comprising diodes  42  having their anodes connected to lower edge  37  of said rectangular resistive film and the fourth diode set comprising diodes  23  having their cathodes connected to the left-hand edge of said rectangular resistive film. All diodes are uniformly distributed along the edges to which they are connected, the distance between them determining the precision of the measured touch-point coordinates. 
   According to the invention, when a voltage difference is applied between opposing edges, a laminar current is set up between said edges, the magnitude of which depends on the value of said voltage as well as on the film resistance. Considering, for instance, the half-period t 2 -t 3  when wave  38  is negative and wave  39  is positive, diode sets  23  and  34  are directly polarized and conducting, while diode sets  41  and  42  are inversely polarized and cut-off. In this case, left-hand edge  31  of the tablet has a positive potential relative to right-hand edge  32 , and current lines  43  are set between said edges. Therefore, the voltage measured at any point along the X direction will be in proportion to the distance of said point to said edges: right on the left-hand edge  31  the voltage value will be equal the square-wave voltage  39  less the conduction voltage drop through the diodes. On the right-hand edge  32  the voltage will be equal to the lower square-wave voltage  38  plus the conduction voltage drop through the diodes, while in-between it will have an intermediate magnitude. Each time the probe tip touches the tablet surface, the voltage at the contact point is read by the A/D converter  24 . 
   In the following half-cycle t 3 -t 4 , the polarities of said square-waves are inverted, and diode sets  41  and  42  will conduct, the resulting current flux being directed from upper edge  26  to the lower edge, the voltage measured at each point of the tablet surface being proportional to the distance of said point to said edges. 
   According to  FIGS. 1 and 2 , microcontroller  20  is provided with an internal clock  26  which synchronizes operation of driver  25  as well as of A/D converter  24 . The square-wave frequency is matched to the operating speed of said A/D converter, typically in the order of 1 kHz. When two probes are used as shown in  FIG. 3 , said clock also synchronizes the switching between readings of probes  21  and  51  by means of the auxiliary switch  23  which has two inputs connected to cables  22  and  52 . Thus, the X and Y coordinate readings of probe  21  are followed by the X and Y coordinate readings of probe  51 . 
   The probe tips can be metal or a non-metallic conducting material. Soft or flexible conducting materials, such as graphite, are desirable due to reduction of friction against the conducting film surface and diminishing wear of the latter. The tip size determines the resolution in X and Y coordinate measurement, which is usually in the order of 1 millimetre. 
   A second embodiment of the invention is shown in FIG.  4 . Instead of switching a DC voltage, two AC signals are simultaneously applied between opposing edges of the tablet. A first signal with frequency f 1 , supplied by generator  53 , is applied between the side edges while a second signal having frequency f 2 , supplied by generator  54 , is applied between the upper and lower edges. Therefore, two sets of current lines will coexist in said resistive film: a first set in the X direction with frequency f 1 , and a second set of current lines in the Y direction with frequency f 2 . 
   When probe tip  21  touched the resistive film surface, cable  22  will transmit a signal with two frequencies; the same will happen with probe tip  51  and cable  52 . Said cables are connected to the two inputs of a switching device  65 , which is controlled by the microprocessor clock  26 , said switching device transmitting the signals picked up by probe tip  21  interspersed with the signals picked up by probe tip  51  to its output port. Said signals are fed to pass-band filters  55  and  58 , respectively tuned to frequencies fl (correspondent to the X axis) and f 2  (correspondent to the Y axis), the rectification and filtering of said signals by blocks  56  and  57  furnishing DC voltages which are proportional to the position of each probe tip along the X and Y axes. A second switching device  23  forwards, in alternation, said voltages to the A/D converter  24 , the remaining processing of the information following the steps already described. 
   Although the preceding description was based in two illustrative embodiments, it will be understood that modifications can be introduced while remaining within the bounds of the invention. For instance, a different material can be substituted for the tin dioxide in the transparent film that overlays the glass plate, as long as this material exhibits the necessary features of transparency and uniformity. 
   Considering that the tablet of the invention is transparent, it can be employed in a large variety of applications, ranging from digitizing table, spatial keyboard, signature digitalization, mouse, and so on. 
   Therefore, the invention is defined by the following claims.