Capacitive Touch Screen
Capacitive Touch Screen is the control panel by measures the capacitance change caused in the touch by a conductive object (like a finger) to estimate the touch position.
Unlike resistive touch screen, which needs pressure to make the actual contact between two conductive layers inside, capacitive methods can obtain the capacitance change just by the light touch on the screen.
Consequently, it enables smooth and fast scrolling, high durability, and excellent optical performance.
As a result, the capacitive touch screen has more flexible material choices for substrates, for example, glasses and plastics, while resistive technologies require a flexible layer.
More about the comparison of different touch screen technology: “2 Popular Types of Touch Screens”
How capacitive touch screen work?
A layer of transparent metal conductive material (indium tin oxide: ITO) is coated on the surface of the substrate of the control panel.
When a conductive object touches, the capacitance of the touch position will be changed so that the touch position can be detected.
The capacitive touch screen is a multi-layer composite screen:
- The outer layer is usually silica glass due to its good transparency and hardness for protection.
- The inner surface of the glass and inner layer (layers) are each coated with metal conductive material (ITO), the key sensor layer with an electrode at each corner.
Since the electric field of the human body, when a finger approaches the touch screen, a coupling capacitance is formed in between, as shown in Figure 1.
In the high-frequency current environment, the capacitance becomes a direct conductor, the finger draws a small amount of current from the touch position. In parallel, the current flows out from the electrodes on each corner.
Because the current change of each electrode is proportional to the distance from the finger to each corner. The controller can obtain the position of the touch point by accurately calculating the ratio of the current change.
Classification of capacitive touch screen
The capacitive scheme is divided into surface-capacitive and projected-capacitive methods.
Given the working principle of their scheme, it has limited the touch function of each type, as shown in Figure 2.
Surface-capacitive touchscreen (SCT)
It consists of one conductive layer, of which four corners are connected to four perfectly synchronized alternative current (AC) voltage signals, as shown in Figure 3.
The touch position can be estimated from the current variation at four corner AC voltage sources caused by the fighter touch.
The process happens as follows:
- The finger touching the screen causes the current difference in four voltage sources.
- The voltage source is nearer to the touch point, where the current variation becomes larger due to the smaller resistive load.
- As a result, the touch locations are calculated from the ratio of the currents over four voltage sources.
Though it cannot deal with multiple touches, its high durability enables it widely integrated into automated teller machines (ATMs).
Projected-capacitive touchscreen (PCT)
Projected-capacitive touchscreens use two patterned conductive layers that are separated and crossed to each other in the shape of a matrix.
Horizontal and vertical patterns correspond to the position information of the touch event.
There are two approaches to creating and measuring the capacitance:
It measures the capacitance between the horizontal electrode and ground, and between the vertical electrode and ground, each row and column has its own capacitance, as shown in Figure 4(a).
Self-capacitance can respond quickly, but it has a problem locating the real touch position if two or more touches are loaded. The false identification is called ghost location.
Therefore, it usually applies in single-touch and gesture-touch, not multi-touch.
Another drawback of this approach is that the performance is directly affected by the humidity of fingers and the environment.
In the mutual-capacitance approach, the overlapped area of two sets of electrodes forms a capacitance.
The finger touch decreases the mutual capacitance due to the electric field loss by the finger placed between two electrodes.
It measures the capacitance of each overlapped area of horizontal and vertical patterns, as presented in Figure 4(b).
Since the measurement is individual, it can support multi-touch functions without limits on the number of fingers.
In addition, mutual-capacitance is not easily affected by the environment or finger.
With these benefits, it has become a popular touchscreen technology today.
Comparison in application
- Surface-capacitive touch panel is mostly used in public and industrial systems, such as industrial instruments, ATMs, and kiosk, which are outdoor or environment with strict temperature and humidity requirement. The cost is higher in comparison.
- Projected-capacitive touch panel has low drift issues and better transmittance, and it can achieve multiple touches. It is popular in consumer products with a small or medium-sized screen.
Solution of touch screen employed on display
In the basic structure of a capacitive touch display, the touch panel is bonded on top of the display to compose a touch display.
With the increasing market demand for thin and light screens, particularly in consumer electronic products, and the IC development of TDDI (Touch and Display Driver Integration), embedded touchscreen solutions have developed and emerged in the market.
Classification by solution
In general, the method to bond the touch panel to the display can divide into two solutions: add-on and embedded.
Add-on solutions: G+F/ G+F+F, G+G/ OGS
It is the traditional way, touch screen is a separate panel added on top of the display.
In light of the substrate used for the touch layer in the touch screen, the add-on solution can be further classified into “glass type” and “film type”.
Glass solutions include G+G (cover glass + glass with ITO/ITOs) and OGS (One Glass Solution).
OGS is an upgrade solution of G+G by directly applying the ITO on the cover glass. It saves the cost of one piece of glass and one time of bonding.
However, the drawback is that the strength of the glass decreases in the processing.
Film solutions include G+F (cover glass + film with ITO/ITOs) and G+F+F, in which ITO is coated on the film rather than the glass.
So it can be thinner and lighter than the G+G solution.
However, given that G+F only has one sensor layer (ITO film), it cannot achieve multi-touch function, but other solutions have no problem.
Embedded solutions: In-Cell, On-Cell
The touch layer embeds inside the display module.
There are two schemes: On-Cell and In-Cell.
On-Cell is to embed the touch layer between the color filter (CF) substrate and the polarizer.
In-Cell is to embed the touch layer behind the CF substrate and inside the cell layer, which is more complicated than On-Cell.
Comparison of features of different solutions
In general, the technology of add-on solutions is mature and simple and can implement at a low cost.
While the embedded solution can satisfy the market trend in thin and light screens, the technology is complicated and has higher requirements in IC support. It still needs time to achieve a high yield and lower the cost.
From the industry perspective, competing for the market share, touch panel manufacturers promote add-on solutions, while display panel manufacturers such as LG, Samsung, and Sharp push embedded solutions.
- Touch sensitivity: OGS > On-Cell > In-Cell
- Multi-touch: except for G+F, the others all are capable
- Screen transparency and visual effects: OGS＞ Embedded ＞GG＞GF＞GFF
- Light and thin: In-Cell＜OGS< On-Cell＜GF＜GFF＜GG (from thin to thick)
- Anti-impact/strength of the screen: On-Cell > OGS > In-Cell
- Maintenance cost: Embedded > OGS > GF/GFF/GG
- Cost/ technical difficulty: Embedded > Add-on
- Yield rate: Add-on > Embedded
Market value and development of capacitive touch screen
Finger touch is a more intuitive input interface to interact with displays and machines.
Feature advantages and values
In comparison with other touchscreen technologies, the below features have made the capacitive touch panel stand out in the application, particularly in consumer products, which pursue supreme experience and appearance.
- High accuracy: up to 99%
- High transmittance: 85%- 90%
- High sensitivity: sense force can be less than 2oz, response time less than 3ms
- High stability: the cursor does not drift after one calibration
- High durability/ long lifespan: each position can withstand more than tens of million touches
- Smooth multi-touch function: various options to archive the multi-touch
- Strong protection: cover glass can be high scratch-resistant (Mohs hardness 7H) and finished with good shielding from outer pollution, such as water, oil, fire, radiation, and static electricity
Development and challenges in the application
Since the 60s, originating from the US military, touchscreen applications have expanded to the mass consumption fields, and commercial and industrial fields.
In the current era of IoT, the demand for intelligent operations has rapidly increased, and more and more terminals have relied on touch displays to be input media.
For instance, in the vehicle industry, interior decoration is equipped with more and more touch displays for various control panels, and screens are getting broader and larger.
With this trend, the touch display market has entered a rapid development period of product diversification.
Although many advancements have been accomplished in touchscreen technologies, challenges still exist.
Integration with other technologies, and flexible and diversified applications are still trends and challenges for the future development of the touch screen industry.
If you need touch displays in your project, feel free to contact us, we have standard touch screen displays for your selection, and can provide a custom solution for your expectation from our engineering team.
The related article: “2 Popular Types of Touch Screens”