Capacitive touch panels - principles of operation

The most popular touch technology today is capacitive solutions, known as CTP or P-CAP, and used in smartphones and advanced displays, among others.

There are several technologies available on the market that are used in the production of touch screens. Touch devices are common in public spaces and their users may encounter potentially many different solutions that look very similar on a daily basis. However, the modules used in, for example, car navigations, will differ significantly from those installed in ATMs in terms of their operation, resistance to damage, light transmittance and service life. The most popular touch technology today is that used in capacitive solutions - also known as CTP or P-CAP. It is used, among others, in smartphones and advanced displays.

Basics and division

In order to present the P-CAP touch technology in a cross-sectional view, one should start with understanding the basics of its operation. The functioning of capacitive screens is based on the detection of disturbances in an electrostatic field disturbed by contact with electrically conductive objects - such as fingers or a stylus. By taking advantage of the conductivity of the human body or a dedicated object when touching the screen, we can change the electrostatic capacity between the electrodes by capacitive coupling. The display controller will detect the change in the electrostatic field and its location.

However, CTP screens can also be divided into two subcategories: surface-capacitive and capacitive-projection.

Surface solutions ...

The first type of P-CAP touchscreen displays is the surface-capacitive type. One side of the screen is covered with a colorless conductive layer (e.g. indium tin oxide - ITO). The voltage-generating electrodes are only present in the four corners of the display, but thanks to the above-mentioned common conductive layer, it is possible to create a uniform electric potential over the entire surface of the screen. The effect of touching its other side is to introduce a disturbance by changing the electrical capacity of the system. The position of the contact point can be determined by measuring the voltage changes at the four corners of the module independently.

Surface-capacitive displays are quite cheap and simple to build - that's why they are often used in larger, less complicated devices. A solution involving multiple electrodes distributed over the entire surface of the screen would be impractical and expensive in this case. This type of module is therefore less precise and may also have difficulties with multi-touch operation.

… And projective

Capacitive-projection displays are mainly used in smartphones and tablets - unlike their surface counterparts, they use many electrodes. Under the glass protective layer of the module there is a conductive layer consisting of small electric wires distributed over its entire surface. Changing the electrostatic capacity of the electrodes takes place by touching the glass with a finger or a stylus - the device detects exactly where the point of contact has occurred, by analyzing the change in the electric field right next to the screen surface.

Thanks to this solution, it is possible to install the panel behind thick, even several millimeters hardened protective glass. At the same time, a high accuracy of the image can be maintained, and the display will also be able to detect touch in several places simultaneously. However, this is quite an expensive solution for larger panels. There is also a risk of errors related to the formation of high resistance - that is why capacitive projection screens are primarily used in smaller, more advanced devices.

Advantages and disadvantages of P-CAP touch screens

Standard, resistive touch displays are suitable for small, simple applications that require detection of a single point of touch at a time. In turn, capacitive solutions allow for multi-point touch detection, high sensitivity and precision in recognizing the contact position, as well as high resistance to damage and good screen visibility even in conditions of intense lighting.

The disadvantages of P-CAP touch displays may be the higher price compared to their resistive counterparts and the need to operate only with dedicated tools or bare fingers (or the use of gloves with the function of capacitive touch screens). These types of modules also become somewhat problematic to use when they come into direct contact with water and dirt.

Sample applications - not only smartphones

The first touchscreen with P-CAP technology was invented 10 years earlier than the display with the resistance system. However, capacitive solutions gained popularity only in 2007, when they were used by Apple in the first iPhone model. Since then, this technology has actually become a standard not only in mobile applications.

Capacitive screens will work well in applications where the final devices will be exposed to intensive use - these are solutions very resistant to mechanical damage and to electromagnetic interference. It is also possible to additionally protect the display by using various types of filters. Publicly available kiosks, HMI operator panels, complex industrial interfaces - these are just a few examples of the use of CTP screens.

The article was published courtesy of the Unisystem company

Source: https://tek.info.pl/