CST

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 29. Mar 2017 9:13, Article ID 905
URL:

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.

Feedback

13 of 19 people found this article useful

Did you find this article useful?

Other Articles

Advanced Modelling and Measurement of Wideband Horn Antennas

Advanced Modelling and Measurement of Wideband Horn Antennas
This paper presents the excellent correlation of measurement results to the numerical calculations of a SATIMO dual-ridge horn antenna which include the directivity, boresight gain, and return loss vs. frequency. The structure is electrically large which lends itself, as a result of its efficient memory utilisation, to the CST MICROWAVE STUDIO® (CST MWS) Time Domain Solver. Broadband, high resolution gain results can be obtained within a single run with as many as 100 farfield monitors being defined. The results are presented with the courtesy and permission of SATIMO, Italy. Read full article..

Electromagnetic Simulation Applied to Automotive EMC Testing

Electromagnetic Simulation Applied to Automotive EMC Testing
Nearly every product that has some sort of electronic circuitry must be subjected to EMC testing prior to its commercialization. In the automotive industry, the large number of electronic modules, sensors, and cable harnesses, many in safety-critical systems, in modern vehicles demands strict standards for reliability and security under challenging conditions. Vehicles must comply with standards such as CISPR-25 and ISO 11451, and individual regulations from the automakers which, very often, are even more stringent. Read full article..

Plane Wave Interactions with a Dielectric Half-Space at 60 THz

Plane Wave Interactions with a Dielectric Half-Space at 60 THz
In this article, CST MICROWAVE STUDIO® is used to illuminate an infinite dielectric half-space with a uniform plane wave and the reflection and transmission quantities are obtained. This problem has an analytical solution which serves to validate the simulation. The same procedure is then applied to a more generalized geometry which lacks a known analytical solution. Read full article..

Investigating the Principles of a Dielectric Laser Accelerator

Investigating the Principles of a Dielectric Laser Accelerator
This article presents a principle investigation on dielectric laser accelerators via simulation. 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..
Back Back  

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