1. Field of the Invention:
This invention relates generally to a method and apparatus for determining the location of a point and more particularly to a method and apparatus for determining the coordinates of a contact point on a resistive type semianalog sensitive surface.
2. Discussion of Background:
There are known, such as shown by French Pat. No. 2,520,498 sensitive surfaces of the type consisting of a support, generally of a glass plate, on which are placed a grid consisting of two terminals between which a certain number of emitting resistant strips are connected in parallel. Also placed on the support are conductive detecting strips placed between the resistant strips and connected separately to a selecting switch so that the voltage of each conductive strip is sampled indidually in successsion. The emitting resistance strips and the conductive detecting strips, which can also have some resistance, most often consist of a thin conductive enamel deposited on the glass plate. When a conductive object, for example an ordinary finger, comes to rest on this sensitive surface, it forms an electrical connection between at least one detecting strip and at least one emitting strip of the grid. By scanning the various strips with a selector, it is possible to digitally determine the coordinates, which are determined by the number of the detecting strip where a voltage is detected, and an analog determination of the other coordinate by the value of this voltage.
Preferably, as described in said French patent application, said emitting strips are alternately polarized in opposite directions. For each direction of polarization, a certain number of measurements are made of the voltage of each detecting strip whose averages are computed separately, and the coordinates of the finger are deduced by comparing the averages obtained.
Although such a process makes it possible to solve a great number of problems presented by the use of sensitive surfaces of this type, some difficulties in practical use still remain, such as are listed below.
The electrical contact resistance of the skin at the end of the operator's finger is highly variable and most often large (50 k.OMEGA. to 1M.OMEGA.) and would allow only the passage of a current of very small intensity between the grid and the detecting strip. Therefore, only a detector with very high impedance can be used.
For obvious reasons of user safety, only very low excitation voltages can be used. The user's body conducts direct voltages through the fingertip not only from contact with another point of the circuit, but also by static electricity, and especially alternating voltages by an antenna effect with ambient spurious signals. The combination of these two effects makes it difficult to obtain a favorable signal/noise ratio, and consequently, precise measurements.
The variability of the useful signal in relation to a constant background noise leads to extremely variable relative variations over the extent of the surface, particularly in the direction of the analog measurement nearing zero.
Finally, scanning the various strips by the selector and taking various measurements can take considerable time, which is impractical for the user if standard filtering techniques are used.
The present invention aims at providing a method and apparatus that make it possible to overcome the above difficulties which are encountered in scanning of sensitive surfaces of this type, i.e., which meet all the requirements of safety, precision and speed.