1. Field of Invention
The present invention relates to touch sensitive screens or sensors. More particularly, the present invention relates to a capacitive touch screen that provides a control signal indicative of where the screen or sensor was touched by a fingertip or utensil.
2. Description of the Related Art
Touch screens are becoming more prevalent as data input interfaces as computers and other electronic devices become more ubiquitous. For example, touch screens may now be found in workshops, warehouses, manufacturing facilities, restaurants, on hand-held personal digital assistants, automatic teller machines, casino game-machines, and the like.
One type of touch screen, a capacitive touch screen, includes a capacitive sensing circuit with multiple sensor bars that each produce an electric field. Often, but not necessarily, the sensing circuit is laminated with an optically transparent protective substance, such as a glass pane. A touch in proximity to one or more of the sensor bars modulates the electric field and creates a signal. The signal is passed from the sensor bars to a controller using a network of lead lines that electrically connect the sensing circuit to the controller. The controller resolves the signal or signals to determine the location of the touch on the screen. The coordinates of the location may then be communicated to another processor for further processing, such as to a computer for entering an order of an item displayed and touched on the screen.
Touch screen designers are constantly faced with problems when designing touch screens and, more specifically, sensing circuits. The designers are constantly seeking an acceptable trade off between the number of sensor bars that are required to provide adequate resolution in the xe2x80x9cYxe2x80x9d direction, and the number of lead lines that are necessary to connect those sensor bars to the controller.
One existing solution to the problem is through the use of an asymmetrically addressed sensor circuit layout. In such systems, the lead lines on the edges of the screen are each connected to several sensor bars so that there can be many more sensor bars than lead lines. The connections of the lead lines to sensor bars is periodic in that every nth bar is connected to the same line. The repeating period is different on one side of the screen from the other (thus the asymmetric label) so that each sensor bar in the screen has a different right-side/left-side lead line connection combination than any other sensor bar on the screen. This unique addressing combination allows the control system to uniquely determine the sensor bar being touched.
The invention provides a technique for designing a sensor circuit for a capacitive touch screen that allows each lead line to be connected to multiple sensor bars of the sensing circuit and corresponding lead lines on each side of the screen connect to exactly the same sensor bars. In this aspect, the sensor bar-to-lead line connections on the left and right sides of the screen are symmetric. The lead line to sensor bar connections for the sensing circuit are arranged such that a touch on any sensor bar creates a signal pattern that is uniquely distinguishable from a touch on another sensor bar connected to the same lead line.
In one aspect, the sensor array includes a series of sensor bars, with each of the bars having a first end and a second end. The first and second ends of each sensor bar are connected to lead lines that electrically connect the sensor bars to a controller. Two or more sensor bars may be connected to the same lead line. However, by carefully laying out the sensor bar to lead line connections over the entire sensing circuit, a touch on any one sensor bar creates a signal pattern over a set of sensor bars (including the touched sensor bar) that is distinguishable from any other similarly-sized set of sensor bars. Thus, a touch can be resolved to a particular set of sensor bars, and then the relative strengths of the signals on each sensor bar in the set identifies more precisely where on the touch screen the touch occurred.
In another aspect, sets of sensor bars having non-unique membership sensor bar to lead line connections may be laid out by first identifying signal magnitude patterns that are likely and highly unlikely to result from possible real world touches. If a set of sensor bars having a first combination of bar-to-line connections is used, the same bar-to-line connections may be used in a different combination, so long as the different combination is highly unlikely to produce a similar signal pattern. In other words, the same set of bar-to-line connections can be reused in different sets of sensor bars, so long as the order of the bar-to-line combinations in each of the sets cannot create the same signal magnitude pattern as a touch elsewhere on the sensing circuit.