How Is A Low-cost Touch Control System Designed?

Views: 323     Author: Reshine Display     Publish Time: 2024-02-20      Origin: Site


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How Is A Low-cost Touch Control System Designed?

Capacitive touchscreens are no longer considered a novelty. As millions of smartphones and tablets are shipped, their adoption is increasing. However, now that they are prevalent, people are less willing to pay a greater price for the technology.

To retain profit margins in this competitive climate, OEMs must lower device costs, and the touchscreen module is one of the most expensive components of touchscreen-powered devices. Designers can lower system costs by employing appropriate panel stack-ups and patterns, displays, materials, routing, and controllers.

1. Cover Lens and Touchscreen Sensor

A standard touchscreen system consists of a projected capacitive touchscreen sensor laminated to a protective cover lens, a bonded flexible printed circuit (FPC) with the touchscreen controller attached to it, and a display. The FPC connects the touchscreen controller to the host processor. The display sits beneath the touchscreen sensor and is typically separated by an air gap or directly laminated.

A standard capacitive touchscreen system consists of a projected capacitive touchscreen sensor laminated to a protective cover lens, a bonded FPC with the touchscreen controller attached to it, and a display.

The cover lens is the touchscreen system's topmost physical layer. The cost varies greatly depending on the material used (glass or polymethylmethacrylate, or PMMA), special coatings (oleophobic, hydrophobic), decorative ink, and the number of drill holes for cameras or sensors. PMMA, a less expensive, lighter, and shatter-resistant alternative to the more durable and optically transmissive glass, can cut expenses by up to 50%.

However, PMMA sensors may exhibit reduced signal sensitivity. PMMA is also more flexible than glass, which can cause panel bending when a finger or other object presses down with substantial force. Panel bending might result in erroneous and inaccurate touch reports. However, a glass substrate or an extra shield layer in the touchscreen sensor can prevent panel bending. As a result, the material of the cover lens must be carefully addressed while stacking touchscreen sensors.

2. Modern touch goods employ a variety of sensor stack-ups

Touchscreen sensors have complex architectures. They are created by sputtering indium tin oxide (ITO) onto a glass or PET substrate and then etching a proprietary design into the ITO. Each sensor layer's patterns and structures are specific to the system's design requirements.

Standard touchscreen designs often employ two-layer ITO touchscreen sensors, such as the MH3 and diamond sensors shown in Figure 2, to provide high accuracy, linearity, and multi-touch performance. Substrates used in two-layer sensor designs include glass and polyethylene terephthalate (PET). PET is less expensive and provides superior display noise immunity, but it will result in modest optical clarity reduction. Ultimately, the most efficient way to reduce touchscreen sensor prices is to limit the number of stack-up layers.

By incorporating single-layer sensors, system designers can reduce sensor prices by up to 50%. Fewer layers—substrate, ITO, and optically clear adhesive (OCA)—help touch panel vendors reduce material and tooling costs. Handling fewer layers also increases manufacturing yield. Low-cost single-layer touchscreens are made with a single PET substrate with a simplified proprietary pattern that has good optical transmissivity.

Single-layer sensors have reduced accuracy and linearity and a limited number of supported finger touches. These low-cost single-layer sensor systems are excellent for entry-level smartphones and feature phones.

System designers who have previously used resistive touchscreens or no touchscreens may find this stack-up option ideal for their design and budget requirements. Single-layer capacitive touchscreens have several advantages over resistive touchscreens, including superior optical clarity, lower power consumption, increased longevity, and a better user experience.

Single-layer multi-touch technologies, such as Cypress's SLIM (Single-Layer Independent Multi-touch), can save up to 40% of the cost of dual-layer sensors. Single-layer sensors offer significantly lower performance but excel at accommodating the tiniest form factors. Single-layer multi-touch sensors can also enable thin border or borderless touchscreen sensors, allowing the touchscreen's active area to expand. Designers looking to reduce both cost and thickness can investigate single-layer sensors.

Smaller screen sizes are far more inexpensive. The size of the active area affects touchscreen expenses. System designers must evaluate all options for optimizing panel design and selection.

3. FPC Design

Another way to reduce device expenses is through FPC design. The FPC connects the touchscreen panel's sensory input/output (I/Os) to the touchscreen controller, which in turn connects to the host processor.

FPCs may be active or passive. In active FPCs, the touchscreen controller and any other external components, such as resistors and capacitors, are located on the FPC itself. Passive FPCs contain simple routing lines and a touchscreen controller, with external components installed on a printed circuit board (PCB).

Whether active or passive, FPCs can be routed in a variety of ways. The routine's efficiency and versatility make it easier to integrate other hardware components. However, keep in mind that the number of routing levels increases costs. Thoughtful routing on a single layer will assist in reducing FPC expenses. Single-layer routing has significant benefits for signal integrity and compact FPC architecture.

4. Displays

In a touchscreen system, the projected capacitive touchscreen sensor is located on top of the display. Displays are naturally noisy, therefore display noise can directly pair with the touchscreen sensor. This lowers touch sensitivity and causes erroneous touch activation. Good design decisions can help to reduce display noise while also improving performance and cost-effectiveness. Related product: 10.1 inch PCAP capacitive touch screen.

Displays are naturally loud. Noise from displays can capacitively pair with touchscreens, reducing touch performance.

To reduce display noise, the industry has historically added an ITO "shield" layer between the display and the touchscreen sensor. Although effective, the shield layer increases the cost and thickness of the touchscreen module. To isolate the display from the touchscreen sensor, another option is to use a small air gap, often between 0.2 mm and 0.5 mm.

An air gap is less expensive than a shield layer, but it increases the width of the touchscreen module, which is growing unpopular among OEMs who want to make sleeker, thinner handsets. The choice of display will be a more crucial design consideration.

Thin-film transistor (TFT) LCDs remain the most popular displays for mobile phones and tablets today, and they are available in two varieties: DC common voltage (DCVCOM) and a common voltage (ACVCOM). The difference is the mechanism for driving the common electrode layer (VCOM). Active-matrix organic LED (AMOLED) displays are also becoming increasingly popular in high-end gadgets, thanks to their broad viewing angles, enhanced brightness and contrast, lower power consumption, and reduced thickness.

AMOLEDs have very low display noise and are among the quietest screens, but they are pricey.

DCVCOM displays are also often silent and pricey. ACVCOM, on the other hand, produces a lot of noise while remaining very inexpensive. The display choice is heavily influenced by the device's intended use by end users. The target application will determine the hardware and performance that are appropriate for its clients.

5. Touchscreen controller

Although less expensive than the display or touchscreen panel, the touchscreen controller has the greatest impact on touchscreen system performance. The touchscreen controller uses capacitive sensing and processing technology to detect finger touches and movements and communicate their location and behavior to the host CPU.

When a finger is placed on a projected capacitive touchscreen, the touchscreen controller detects the change in capacitance and translates it to digital values. This digital conversion is then processed by the touchscreen controller's powerful touch resolution algorithms before being sent to the host CPU with touch coordinates and other necessary data.

Noise-sensitive signals provide a significant technical issue for touchscreens. Controllers with high-quality analog front ends, built-in noise-handling capabilities, and complex processing algorithms are required. With touch becoming the preferred user interface for many consumer electronic gadgets, the quality of touchscreen controllers will have a direct impact on the end-user experience. Choosing the correct touchscreen controller is critical for obtaining performance and cost benefits.

A controller with a high signal-to-noise ratio (SNR) and good noise handling can compensate for signal strength loss caused by noise sources like a cheaper PMMA cover lens or a noisy ACVCOM display. To help optimize the performance of low-cost and multi-touch single-layer sensors, touchscreen controllers must provide compatible processing algorithms. Furthermore, the economic advantages of single-layer FPC routing can only be realized if the touchscreen controller pinout supports a flexible routing design.

Touchscreen controllers can also lower system expenses by supporting additional functionalities. For example, most touchscreen controllers read water on a touchscreen as a finger touch since their capacitance signatures are similar. To address this issue, touchscreen panel producers might apply a costly layer of hydrophobic coating to the cover lens.

When water drops land on the cover lens, the coating helps to break them apart into tiny droplets, preventing them from registering as touches. Nonetheless, a touchscreen controller that natively supports water rejection via its hardware and firmware capabilities may detect and reject water on the touchscreen using built-in algorithms, saving the OEM additional coating costs.

6. Summary

Savvy designers who understand the touchscreen system and its main components can dramatically cut costs by making sensible design and selection decisions for the cover lens, sensor material, and stack-up, display type, and FPC routing. A creative and high-performance touchscreen controller can reduce costs without sacrificing performance, ensuring that the end product sells in the first place.

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