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    Home : Application Briefs : Physical Parameters Can be Modified Directly in Comprehensive Touch Screen Model

Physical Parameters Can Be Modified Directly in Comprehensive Touch Screen Model

Application Briefs
The Challenge
The challenge of this project is to create a model of a capacitive touch screen, including physical modeling of a user’s finger, the touch screen materials, and the touch-detection circuitry. A capacitive touch screen works by measuring the change in capacitance between two conductors caused by a nearby finger. In the touch screen, two transparent conductors, which run in perpendicular directions, are separated by the screen. These conductors act as a parallel-plate capacitor where they overlap; the capacitance is determined by numerous physical parameters. For example, the conductor width, length, resistivity, and separation, along with the screen material properties and thickness, all affect the coupling between the conductors.

To operate a single pixel of the touch screen, a particular frequency is transmitted along one conductor and received by the other. The capacitive coupling between the conductors causes a particular phase shift in the received signal. When a finger is close enough to where the conductors overlap, it changes the capacitance between the conductors because the finger appears to be grounded. This causes an additional phase shift, which is detected by a simple demodulator circuit and converted into a DC signal. When the DC signal exceeds a given threshold, a “touch” is detected by the pixel. This process occurs for all of the touch screen pixels to determine which locations are being touched.

The traditional approach to modeling a touch screen uses a purely circuit-based design that uses components whose values must be precomputed in another environment. Changes in the design parameters would require the recalculation and re-entry of these and other values into the circuit model before subsequent simulations; this process makes experimentation and fine-tuning of the model difficult and time-consuming. For example, the coupling capacitance between the transmitting and receiving conductors would have to be calculated and fed into the circuit simulation ahead of time. Similarly, the effect of a grounded finger would have to be incorporated as some time-dependent capacitance whose time-domain waveform is predetermined and loaded into the simulator. Also, the demodulator, which is essentially the multiplication of two sine waves, would have to be implemented using a full demodulator circuit as opposed to simply multiplying two signals.

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The Solution

  Figure 1 – The resulting touch screen model. The component, Touchscreen1, contains the equivalent circuit model of the
  touch screen as determined by the given physical parameters and the distance from the finger to the screen surface.
  The right-hand portion of the schematic is a demultiplexer circuit that outputs a voltage related to the finger distance.

To address these shortcomings, a model of a touch screen pixel was created in MapleSim. The top level of the model is shown in Figure 1. The physical design parameters were easily incorporated directly into the design, meaning that the circuit representation of the transmitting and receiving conductors were specified in terms of the underlying physical parameters. Now, those parameters can be changed quickly and easily without having to adjust the individual circuit parameters. In the model, the fingertip and touch screen are represented in space, which allows their relative positions to be visualized easily using the animation capabilities in MapleSim. The distance between them is fed into a variable capacitor that mimics the real-world touch screen behavior. The demodulator was created simply by multiplying the transmitted and received sinusoids plus a phase shift and determining the DC component of the result. When the DC component exceeds a specified value, the pixel is “on,” which is indicated in the visualization as a green square, as shown in Figure 2.

For this project, the multiphysics capabilities in MapleSim helped to quickly create a touch screen design environment that allowed an engineer work with real parameters and not just calculated values. Furthermore, the visualization allowed the engineer to see the results of his or her touch screen design--not just as a waveform but as a finger actually touching a screen--and the resulting detection circuit output. In conclusion, with MapleSim, a complex multi-domain problem was modeled in a natural way, allowing a compact and intuitive solution to be created.

Figure 2 – The finger, shown in red, is not close enough to activate the touch screen pixel in a). In b), it is now close enough to cause the detector to register a “touch” which is indicated as a green box in the figure.