Is capacitive touch screen okay?

- May 11, 2020-

Capacitive touch screen technology uses the current sensing of the human body to work. The capacitive touch screen is composed of a composite glass screen. The interlayer and inner surface of the glass screen are coated with an ITO layer. The outer surface layer is made of a thin layer of silica glass protective layer. The interlayer ITO coating is used as the working surface. Four corners lead out four Electrode, inner ITO as shielding layer to ensure a good working environment. When the finger touches on the metal layer, due to the electric field of the human body, the user and the touch screen surface form a coupling capacitor. The capacitor acts as a direct conductor, and the finger draws a small current from the contact point. This current flows out from the electrodes on the four corners of the touch screen, and the intensity of the outflow current is proportional to the distance between the four poles. The controller calculates the proportion of the four currents accurately to get the position of the touch point.


The capacitive touch solution can meet the needs of most devices with touch screen methods such as PCB, capacitive and single-layer indium tin oxide (ITO). When choosing the smartest and safest solution, the size and Factors such as power consumption are also critical.


"Is the capacitive touch screen okay?"


From industrial, automotive, medical devices to smart phones and tablets and other common terminal products, the shadow of capacitive sensing technology can be found. The reason why capacitive touch technology can be quickly popularized is that it can easily enhance the user experience of the device, allowing manufacturers to switch from traditional switches to more attractive touch functions.


Capacitive sensing technology also helps reduce the number of mechanical components of the device, thereby extending the life of the device and reducing its size. As long as the combination of these characteristics is properly designed, calibrated, and controlled, the attractiveness of products with capacitive sensing can be doubled.


Capacitive sensing technology is also widely used for touch buttons and slider functions, especially in consumer, commercial and industrial applications, but the most common target applications are touch pads and touch screens. Designing sensors that are both low-cost, responsive, and energy-saving to operate stably in complex environments is already a common demand in the current market. However, there are some requirements that are still very challenging for engineers.


touchpad


For the user interface, the most basic touch sensing application is the familiar projected capacitive touch technology touchpad. These designs consist of a matrix of rows and columns of conductive material layers between glass plates. Applying a voltage to this grid will generate an electric field that can be measured at each intersection. When a conductive object, such as a human finger, approaches and touches the PCT panel, it changes the electric field at the contact point and creates a capacitance difference.


PCT technology can be implemented in two ways: self-capacitance touchpad and mutual capacitance touchpad.


The self-capacitance design is on the printed circuit board (PCB), surrounded by grounded copper foil. In this way, each sensor on the PCB will form a parasitic capacitance with the surrounding grounded copper foil and the electric field line on the top of the sensor. When the finger approaches, additional capacitance is introduced, causing the electric field to distort. The disadvantage of this design is that it can only detect one touch at a time. Therefore, although it is a very economical model, it is only suitable for devices with limited space behind the screen.


However, the mutual capacitance sensing method refers to the capacitance existing between any two charged objects), which can realize multi-touch detection, and is very suitable for complex device designs equipped with large displays. When the finger touches, the presence of the finger is judged by the reduction of the two capacitors. Most importantly, each intersection has its own unique mutual capacitance, which can be tracked independently.


For mutual capacitance touchpads, the presence of a finger will cause the capacitance to decrease. For self-capacitance touchpads, the presence of a finger causes the capacitance to increase, thereby determining the position of the finger.


Touch screen


Multiple capacitive touch panels can be combined to form a touch screen or touch panel, used to determine the position of one or more fingers on a single glass plate. Multi-touch technology has been widely used in devices with limited space such as mobile phones, tablet computers, and high-end wearable devices, and can be divided into three major categories of applications such as PCB, capacitive, and single-layer indium tin oxide touch panels.


PCB touch panel: low cost, low power consumption, but difficult to manufacture


PCB touch panels are basically two or more PCB self-capacitive touch panels placed near the display. For prototype construction and commercial equipment without space constraints, the use of low-cost standard PCB processes is ideal. When designing touch buttons for PCB touch panels, size is usually the key parameter to consider. However, the shape and pad pitch (distance between buttons) should also be taken into consideration in order to minimize error detection.


Capacitive touch panel: more flexible, but less use cases


The capacitive touch panel has two vertically stacked highly conductive materials—ITO conductive layers, one layer for columns and one column for rows. The key feature of this design is that each intersection has its own unique mutual capacitance, which can be independently tracked by the touch controller.


Capacitive touch panels are suitable for many applications because they can provide multi-touch and are easy to configure to support two or more touchpads. In addition, its ultra-thin module design is ideal for applications with larger screen sizes.


However, these designs are not without disadvantages-the two layers of ITO required for the conductive layer are very expensive. In addition, the power consumption of the capacitive touch panel is also very high. The high sleep current of the controller results in higher power consumption, which is not suitable for wearable products pursuing simplicity.


Single-layer ITO touch panel: low cost, low power consumption and easy to construct


The single-layer ITO touch panel method provides multiple advantages of the capacitive touch panel at a lower cost. The biggest difference is that the number of touch pads is preset, so it cannot be changed flexibly like a capacitive touch panel. The predefined traits are extremely beneficial for the size and arrangement of controller computing resources. From a manufacturing point of view, this method is very similar to a capacitive touch panel, but the capacitive touch panel uses only a single ITO layer.


Before determining the most suitable mode for our application, we need to weigh all the advantages and disadvantages of the design. Overall, the design and functional requirements of most devices can be easily solved by capacitive touch solutions, but to decide which one is the smartest and safest for a specific use case, such as size and power consumption Other factors are also important.