Recognize gesture direction
The most common type of touch we see now is Multi-Touch Gesture, which recognizes the direction of movement of two fingers when they touch, but cannot determine the exact location. It allows for operations like zooming, panning, and rotating. This multi-touch implementation is relatively simple, using an axis-based coordinate system. Dividing the ITO into X and Y axes allows sensing two touch operations, but sensing a touch and detecting its exact location are two different concepts. An XY-axis touchscreen can detect a second touch, but cannot determine its precise location. A single touch produces a single maximum value on each axis, thus determining the touch location. If a second finger touches the screen, there will be two maximum values on each axis. These two maximum values can be generated by two different sets of touches, making accurate judgment impossible. Some systems introduce timing-based judgment, assuming the two fingers are not placed simultaneously. However, there are always cases where they touch at the same time, in which case the system cannot guess. We can call points that are not actually touched "ghost points."
Identify finger position
Multi-TouchAll-Point is a popular topic recently. It can identify the exact location of touch points, eliminating the phenomenon of "ghost points." Multi-touch location recognition can be applied to the detection of any touch gesture, detecting simultaneous touches from all ten fingers of both hands, and also allowing other non-finger touch forms, such as palms, faces, fists, etc., even while wearing gloves. It is the most user-friendly human-computer interface method, well-suited for applications requiring simultaneous multi-handed operation, such as game control. Multi-TouchAll-Point scanning requires each row and column intersection to be scanned and detected individually; the number of scans is the product of the number of rows and columns. For example, a touchscreen consisting of 10 rows and 15 columns requires 25 scans using the axis coordinate method of Multi-TouchGesture, while the multi-touch location recognition method requires 150 scans.
Multi-TouchAll-Point uses mutual capacitance detection instead of self-capacitance. Self-capacitance detects changes in the capacitance (parasitic capacitance Cp) of each sensing unit. The presence of a finger increases the parasitic capacitance, thus indicating a touch. Mutual capacitance, on the other hand, detects changes in the mutual capacitance (coupling capacitance Cm) at the intersection of rows and columns. When rows and columns intersect, mutual capacitance is generated between them (including edge capacitance between row and column sensing units and coupling capacitance generated at the intersection). The presence of a finger decreases the mutual capacitance, thus indicating a touch and accurately determining the location of each touch point.
Touchscreen technology
Let me introduce touchscreens. Simply put, a touchscreen combines input and output, eliminating the need for mechanical buttons or sliders; the display screen serves as the human-machine interface.
The entire touchscreen module consists of an LCD, a touchscreen, a touchscreen controller, a main CPU, and an LCD controller. The touchscreen and touchscreen controller are the core components of the entire module, so we will focus on these two parts.
The structure of a touch screen generally consists of the following layers from top to bottom: 1. Surface cover; 2. Cover layer; 3. Mask layer & marking layer; 4. Optical adhesive; 5. First layer of sensing unit and substrate; 6. Optical adhesive; 7. Second layer of sensing unit and substrate; 8. Air layer or optical adhesive; 9. LCD display.
Surface shields are typically less than 100µm thick. All plastic overlays require a hard shield because touching the plastic surface with your fingers can scratch it. If the overlay is glass, a surface shield is not necessary, but the glass must be chemically strengthened or hardened. The surface shield needs to be optically matched with the overlay to prevent excessive light loss.
The cover layer can be 0-3mm thick. Not all touchscreens require a cover layer. The thinner the cover layer, the higher the signal-to-noise ratio and the better the sensing sensitivity. Commonly used materials include polycarbonate, acrylic glass, and glass.
The third layer consists of a mask layer and a label layer, with a thickness of approximately 100mm. The mask layer, located beneath the covering, conceals wiring and the edges of the LCD. Labeling text or icons are permitted in the design; however, the labels must be pressed fairly flat onto the ITO substrate, and the label material should be non-conductive.
The fourth layer is optical adhesive, with a thickness of approximately 25-200 mm. The thinner the optical adhesive, the better the signal-to-noise ratio. Optical adhesives with a high dielectric constant (er) can better sense the capacitance of the finger, thus achieving a higher signal-to-noise ratio. PSA pressure-sensitive adhesive is commonly used.
The fifth layer consists of the sensing unit and the substrate. The ITO coating is less than 100 nm thick. The ITO coating substrate can be 100 μm to 1 mm thick glass (IR ~1.52) or 25 mm to 300 mm thick PET film (IR ~1.65). Thicker ITO layers result in lower resistivity per unit area and better signal-to-noise ratio; thinner ITO layers result in better light transmittance. The substrate can be a thin film or glass. If the ITO is applied to the lower surface of a glass substrate, the glass substrate can serve as a surface cover.
The sixth layer is another optical adhesive. Compared with the previous optical adhesive layer, the thicker this optical adhesive layer is, the better the signal-to-noise ratio. This optical adhesive layer is usually used in combination with ACA-anisotropic conductive adhesive.
The seventh layer is also the sensing unit and substrate, and it uses the same material as the first substrate layer. Note that the thin film and glass should not be mixed. If ITO is on the upper surface of the substrate, a thicker substrate will result in a higher signal-to-noise ratio; if ITO is on the lower surface, a thinner substrate will result in a higher signal-to-noise ratio. Similarly, anisotropic conductive adhesive is required in the edge areas. Single-substrate processes are now available to simplify production and reduce costs.
The eighth layer is either air or optical adhesive. We know that air has a dielectric constant of 1, which reduces parasitic capacitance from the LCD's upper surface. Using optical adhesive makes the mounting more robust. Matching the optical parameters minimizes light loss; therefore, it's necessary to select an optical adhesive with the lowest possible dielectric constant and ensure a minimum distance of 250mm between the ITO sensing unit and the LCD's upper surface.
Finally, there's the LCD screen. For touchscreen design, it's a noise source, originating from the backlight and LCD pixel drive control signals. Passive dot matrix screens should generally be avoided, as they generate high-voltage signals on the front of the LCD. It's best to use active dot matrix screens with Vcom, which can provide a virtual ground or shielding function. If a passive dot matrix screen is absolutely necessary, an ITO shielding layer needs to be added to the touchscreen. This shielding layer must be grounded to eliminate the influence of parasitic capacitance CP.
Multi-touch screen controller
The multi-touch screen controller is the core of the touch screen module. This article takes Cypress's touch screen controller as an example.
Cypress's touchscreen controller is the Truetouch series, which is based on the widely used PSoC (System-on-a-Chip) technology. PSoC is a mixed-signal array that integrates programmable analog and digital peripherals and an MCU core, so the flexibility, programmability, and high integration of PSoC are also applicable to the Truetouch solution.
TrueTouch is a capacitive touchscreen solution. The structure of this type of touchscreen has already been introduced. There are many manufacturers and types of LCDs, as well as many types of sensing devices, including glass, thin films, and ITO, and even various models of ITO. TrueTouch is based on PSoC technology, so the flexibility of PSoC allows it to work well with a wide range of LCDs and ITO devices.
Why is Cypress's touchscreen controller named the Truetouch solution, or where does the "True" come from? Let's review the development of touchscreens. Initially, it was Single-touch—only one finger could touch or swipe. Later, Multi-touch gestures emerged—which could recognize the direction of two fingers, but couldn't determine their exact position, allowing for operations like zooming, panning, and rotating. Today, Cypress's Truetouch can achieve Multi-touch all-point, recognizing multiple fingers and determining their precise positions, making it true multi-touch. This is the origin of the name "True."
Truetouch's product family can be divided into three categories: single-touch, multi-touch gesture, and multi-touch all-point. Each category has various models, differing in screen size, scanning speed, communication method, memory size, and power consumption to meet different applications. The Truetouch series is based on PSoC technology, so these devices can be designed using the simple yet powerful PSoCdesigner software environment.
The value of the TrueTouch solution is mainly reflected in the following aspects: it maintains the inherent aesthetics, lightness, and thinness of touchscreens, allowing customers' products to stand out; it adopts capacitive touchscreen technology, eliminating the need for mechanical components and making it more durable; it boasts a complete series, from single-point touch to multi-point touch direction recognition and multi-point touch position recognition; based on PSoC technology, it is flexible and can be used with a wide range of LCDs and ITOs; all the value of PSoC is reflected in TrueTouch, such as flexibility, programmability, etc., which can shorten the development cycle, enable products to be launched quickly, and have high integration, allowing many peripheral components to be integrated into PSoC (i.e., TrueTouch products), which not only reduces system costs but also reduces overall power consumption and improves power efficiency.
Conclusion
This article introduces multi-touch technology, as well as touchscreens and touchscreen controllers. It can be said that touchscreens are the ultimate choice for human-machine interfaces. Whether it's single-touch, multi-touch for orientation recognition, or multi-touch for location recognition, they offer significant advantages in many applications, such as mobile phones, MP3 players, and GPS devices. These products inherently require small size and portability; how to enable these small products to perform more functions relies on the application of touchscreens.
