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 23. May 2017 10:33, 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

14 of 21 people found this article useful

Did you find this article useful?

Other Articles

Wireless Power Transfer and Microwave Energy Harvesting

Wireless Power Transfer and Microwave Energy Harvesting
Although wireless energy transfer is an old idea (Tesla patented one design in 1901), technological advances and the rise of portable devices have made it relevant again for different applications, such as wireless charging and energy harvesting. This webinar will consist of two parts. The first part is a review of low frequency power transfer in two categories: short range-inductive charging and powering of electronic devices and electric vehicles, and mid-range power transfer through coupled resonant circuits. We will give several examples and highlight the role of simulation in the design of transfer systems. The tools in CST STUDIO SUITE® used for the design and optimization of these devices will be demonstrated, including the use of hybrid circuit-EM co-simulation to optimize matching circuits to improve link robustness. The second part of the webinar is concerned with wireless energy transfer over longer distances, where the far-field transfer of RF energy may be used. We make a distinction between harvesting RF energy from signals present in the ambient and transferring RF energy by intentionally transmitting RF signals. After a discussion of the power densities, we will continue with (intentional) RF energy transfer for powering sensors to be used in Smart Buildings. The blocks of a far field RF energy transfer system will be discussed: Transmit antenna (and maximum allowed transmit power), propagation channel and rectifying antenna or rectenna. The components of the rectenna: rectifier, dc-dc boost converter and antenna, will then be discussed. Several examples will be shown. Read full article..

Design and simulation of a novel slow-wave structure for THz applications

Design and simulation of a novel slow-wave structure for THz applications
The development of slow-wave structures for generation in the terahertz regime is a popular ongoing area of research. This article presents the design and simulation of a novel 'quasi-parallel-plate' slow-wave structure to be used in Backward-Wave Oscillators (BWOs) operating at this regime. Read full article..

Microstrip Patch Array Design

Microstrip Patch Array Design
This article explains the design process for a planar microstrip patch array for WLAN frequencies using the circuit and full-wave 3D solvers and optimization tools in CST STUDIO SUITE®. The goal in this case is to design an array with high directivity, low cost and low sidelobes, exhibiting a good impedance matching in the frequency range 5.18 – 5.85 GHz. The same approach can also be used to design other types of array by using a different radiator or array layout. Read full article..

Design of Circularly-Polarized Patch Antennas using CST MICROWAVE STUDIO®

Design of Circularly-Polarized Patch Antennas using CST MICROWAVE STUDIO®
An RFID Reader Antenna with the following specifications was designed: - Frequency: 908.5 - 914 MHz (In Korea) - VSWR: less than 2 with 50-ohm impedance - Polarization: circular - Axial ratio: less than 3 dB @ 908.5 - 914MHz - Gain: 6 dBi @ 1W transmitted power - Size and weight: as small as possible This article is published with the permission and courtesy of Prof. Bierng-Chearl Ahn and his colleagues at Chungbuk University, Korea. Read full article..

Applications of 3D Electromagnetic Modeling in Magnetic Recording: ESD and Signal Integrity

Applications of 3D Electromagnetic Modeling in Magnetic Recording: ESD and Signal Integrity Document type
John Contreras and Al Wallash, Hitachi Global Storage Technologies, presentation at the 5th North American Userforum, 2008. Read full article..
Back Back  

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