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Touch Panel Application


How touch panel has become a part of daily life

Touch panel (Figure 1) technologies are a key theme in current digital devices, including smart phones, slate devices like the iPad, the screens on the backs of digital cameras, the Nintendo DS, and Windows 7 devices. The term touch panel encompasses various technologies for sensing the touch of a finger or stylus. 

A touch panel is a piece of equipment that lets users interacts with a computer by touching the screen directly. Incorporating features into the monitor like sensors that detect touch actions makes it possible to issue instructions to a computer by having it sense the position of a finger or stylus. Essentially, it becomes a device fusing the two functions of display and input.

Today, the touchscreen has been transformed into a more user-friendly and environmentally robust replacement for the computer keyboard and mouse. Because of that, touch is changing the world. With a touchscreen, people with little or no computer experience can instantly work with complex software programs, without even being aware they're doing it. And computers can go to work in places where a keyboard or mouse would be too cumbersome, fragile, or impractical. Their characteristics (precision in position sensing, rapid response to input, durability, and installation costs) differ greatly depending on the methods used to sense touch input. Some typical touch-panel sensing methods are: Resistive film touch panels, Capacitive touch panels, Surface acoustic wave touch panel etc .. Below we will take a look on the capacitive touch panels.
 

Touch Panel
 
Figure 1 - Touch Panel

Capacitive touch panels: With this method, the point at which the touch occurs is identified using sensors to sense minor changes in electrical current generated by contact with a finger or changes in electrostatic capacity (load). Since the sensors react to the static electrical capacity of the human body when a finger approaches the screen, they also can be operated in a manner similar to moving a pointer within an area touched on screen. Two types of touch panels use this method: surface capacitive touch panels (Figure 2) and projective capacitive touch panels (Figure 3). The internal structures differ between the two types.
 

Surface capacitive touch panels
Figure 2 - Surface capacitive touch panels

Projective capacitive touch panels
Figure 3 - Projective capacitive touch panels
 

Description

A Capacitive touch panel is analyzed using the Electrostatic Module of EMS. The analysis focuses on determining the capacitance and the electric field strength.

After creating an Electric Conduction study in EMS, four important steps shall be followed: 1 - apply the proper material for all solid bodies, 2- apply the necessary boundary conditions, or the so called Loads/Restraints in EMS, 3 - mesh the entire model and 4- run the solver. Note that for the Electric Conduction analysis no air shall be modeled.

3D Model of the simulated Touch panel
Figure 4 - 3D Model of the simulated Touch panel

Materials

In the Electric Conduction analysis of EMS,  the main property we need is the relative permittivity of material (Table 1).

Table1 - Table of materials
 
Components / Bodies Material Relative permittivity
Inner Air Air 1
Outer Air Air 1
Touch Panel (18 bodies) Copper 1

Load and Restraint

 In this study, one part of the touch panel will be labeled as a ground and the other parts (17 parts) as  floating conductors.

Meshing

Meshing is a very crucial step in the design analysis. EMS estimates a global element size for the model taking into consideration its volume, surface area, and other geometric details. The size of the generated mesh (number of nodes and elements) depends on the geometry and dimensions of the model, element size, mesh tolerance, and mesh control. In the early stages of design analysis where approximate results may suffice, you can specify a larger element size for a faster solution. For a more accurate solution, a smaller element size may be required.
Mesh quality can be adjusted using Mesh Control , which can be applied on solid bodies and faces.

Results

After running the simulation of this example many results can be obtained. Electric Conduction module of EMS is used to compute and visualize : Electric Field (Figure 5), Displacement Field (Figure 6) and potential (Figure 7). A results table also generated contains the capacitance of the components mentioned as floating conductors (Table 2).

Capacitance matrix of touch panel
Figure 5 - Capacitance matrix of touch panel

Electric Field in touch panel
Figure 6 - Electric Field in touch panel
Displacement Field

Figure 7 - Displacement Field
Potential in a touch panel
Figure 8 - Potential in a touch panel

Conclusion

By creating many simulation scenarios in EMS for various design parameters, the characteristics of the touch panel can be easily optimized. The parameters of the models can easily be updated without having to worry about the underlying complexities of the model.