CST – Computer Simulation Technology

Electromagnetic Circuit Co-Simulation of a Touchscreen Capacitance Sensor System

Electromagnetic field simulation is an extremely useful tool in the design and analysis of devices based on the measurement of capacitance to establish the presence and/or position of objects. An example of this is a touchscreen device which is covered in this article.

For such simulations, the CST EM STUDIO® (CST EMS) electrostatic solver can automatically extract the capacitance matrix for arbitrary complex electrode systems. With the aid of parametric analysis, the matrix may be generated for a large number of finger positions above the electrodes. This capacitance matrix is seamlessly transfered to the integrated CST DESIGN STUDIO™ (CST DS) circuit simulation module to allow the detection circuits to be simulated and optimized.



Figure 1: Electrode potential definition

The starting point for the simulation of such a sensor is the geometrical construction of the model. This can be achieved by either constructing the sensor electrode system using the simple, but powerful geometric modeler built into the CST STUDIO SUITE® GUI. Alternatively, for complex sensor electrode arrays, the CST® EDA import interface may be used. Supported formats include Cadence Allegro...

®, Mentor Graphics®, Expedition®, ODB++ amongst others.

The effect of a finger placed above the sensor array is a critical requirement in the simulation. 3D CAD models may also be incorporated into the model. In this case, a 3D finger model was imported via the SAT interface.

The next step entails the definition of the electrode potentials in the system. The definition of potential groups in CST EMS allows a straightforward but general workflow. The potentials are shown in Figure 1 for a simple touchscreen sensor which were defined using this feature.



Figure 2: Effect of the finger on the electric field in the electrode system

A plot of the electric field for a particular finger position is shown in figure 2. This result forms the basis of a simulation of the touch screen sensor. The equivalent capacitance is available for each simulation carried out for a particular finger position. This is extended by parametric analysis allowing the user to extract the capacitance as a function of finger position which may vary in 3 dimensions i.e. horizontally and vertically. Once the parametric analysis is complete, the data is automatically transferred to the CST DS circuit simulator for transient analysis.



Figure 3: CST DS Schematic of the capacitive matrix sensor

Figure 3 shows the CST DS schematic in which a standard GPIO is used to generate a dedicated number of pulses to capacitors which generates a voltage on them. After a certain number of pulses the transfered charge is discharged by a series resistor while an analogue comparator indicates when GND is reached. This discharge time is measured by a timer and used for correlation. The difference in time is used to detect whether a finger is present or not.



Figure 4: Typical result of captured time showing detected finger position at cx2y2

Figure 4 shows a typical result whereby an indication of the position of the finger is given by the discharge times. The range of possible positions is extremely wide and depends on the parametric set defined by the user. Furthermore, the complexity of the detector circuit may be increased accordingly. Optimization on the field and/or system level may be easily carried out in the integrated optimizer.

Simulation offers an insight into the behaviour of a device that cannot be achieved in a test environment. Another benefit is that the number of prototypes may be significantly reduced and accelerates the development process. In addition, unwanted effects and disturbances in existing equipment may also be efficiently and cost-effectively investigated.

References

[1] http://www.atmel.com/products/touchsolutions/bsw/qmatrix.aspx

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