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.

Electrode potential definition
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 worflow. The potentials are shown in Figure 1 for a simple touchscreen sensor which were defined using this feature.

Effect of the finger on the electric field in the electrode system
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.

CST DS Schematic of the capacitive matrix sensor
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.

Typical result of captured time showing detected finger position at cx2y2
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.

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

CST Article "Electromagnetic circuit Co-Simulation of a touchscreen capacitance sensor system"
last modified 19. May 2016 6:32
printed 30. Apr 2017 12:57, Article ID 905

All rights reserved.
Without prior written permission of CST, no part of this publication may be reproduced by any method, be stored or transferred into an electronic data processing system, neither mechanical or by any other method.


14 of 21 people found this article useful

Did you find this article useful?

Other Articles

3D EM simulation of mixed analog / digital multilayer PCB

3D EM simulation of mixed analog / digital multilayer PCB
This article describes the use of CST MICROWAVE STUDIO® (CST MWS) to solve a coupling problem in a mixed analog – digital multilayer PCB card. Courtesy and permission of Alvarion, Ltd, Tel-Aviv, Israel. This example gives an insight into the usefulness of simulation of problems that cannot be investigated easily via measurement and allows the engineer to carry out virtual experiments as demonstrated here with the cutting of the signal trace. Experiments may show the presence of a particular problem but not its location. Even when the problem has been located, further prototypes and experiments are costly and time-consuming. CST MWS offers a straightforward workflow for the set-up and simulation of such problems via its advanced user-interface and EDA interfaces. Read full article..

Combined 3D electromagnetic and spin response simulation of MRI systems

Combined 3D electromagnetic and spin response simulation of MRI systems Document type
Modern MRI systems are highly complex devices, and the interaction between the body and MRI coils introduces additional challenges into the design process. The body, with its complicated heterogeneous interior, causes major disturbance to the homogeneity of the magnetic fields, while energy absorbed by the body can cause harmful heating. Experimental measurement of these effects is often impossible, but simulation with 3D body models can help the engineer identify risks to the patient and suggest ways to reduce them. Read full article..

Optical Device Simulation Plasmonics and Nanophotonics

Optical Device Simulation Plasmonics and Nanophotonics Document type
Optical devices are already key components in many areas, such as communications, remote sensing, or medical applications, and their role will only increase in the future. Simulating such devices helps in optimizing their efficiency and in reducing cost of design and development. With its powerful solvers and user-friendly interface, CST® STUDIO SUITE® offers a unique platform for handling such challenges. Read full article..

Light Trapping in Thin-Film Silicon Solar Cells with periodic Nano-Structures

Light Trapping in Thin-Film Silicon Solar Cells with periodic Nano-Structures
This article summarises the simulation study conducted with CST MICROWAVE STUDIO® (CST MWS) of thin-film silicon solar cells with nano-structured interfaces. The good agreement between the experimental data and solar cell simulations shows the reliability and versatility of the performed FIT simulations to investigate nano-optics of thin-film solar cell devices in 3 dimensions. This article is presented with the courtesy and permission of Hasse, C. and Stiebig, H. , Forschungszentrum Juelich who gave a presentation of their work at the CST European User group Meeting at Boppard, Germany, 9-10th March 2006. Read full article..

eASIC Reduces Multi-Level Package Design Times with CST MICROWAVE STUDIO

eASIC Reduces Multi-Level Package Design Times with CST MICROWAVE STUDIO Document type
eASIC is a fabless semiconductor company specializing in Single Mask Adaptable ASIC™ (application-specific integrated circuits). They produce custom integrated circuits for a wide range of applications, designed with the specific needs of the customers in mind. Supporting customers effectively, especially during the development stage of a new product, requires a rapid, cost-effective design and manufacturing process. Read full article..
Back Back  

Your session has expired. Redirecting you to the login page...