Once a switch or button is needed in a human-machine interface (MMI), system designers have to face the question of which technology to choose to accomplish this task. In many applications, especially in price-sensitive consumer products, tablet (or quasi-tablet) switches and keypads / keyboards have replaced traditional mechanical switches. The technologies used include resistive membrane switch panels, piezoelectric switch panels, and touch panels based on capacitive sensing. This article will briefly introduce the typical structure, advantages and disadvantages of these technical solutions, and then analyze the emerging emerging charge transfer sensing technologies. This technology can solve many of the problems inherent in other technologies, and its cost is also attractive for mass-produced consumer applications.
Membrane Switch The simplest and cheapest resistive membrane switch consists of a flexible top layer, an insulating spacer, and a substrate layer under it. The outer surface of the top layer is usually printed with graphics or text, and the lower surface is coated with a conductive pattern, usually printed with silver or carbon conductive ink. The underlying substrate layer is also coated with a matching conductive pattern. When the two conductive layers are pressed together through the holes in the separator, it is equivalent to turning on the switch. The entire assembly is glued together. When the user needs tactile feedback, a metal or plastic dome member can be placed behind the component to create a “click” feel when the switch is pressed, and the surface of the component can be rolled to guide the user’s fingertips To the center of each button or switch. While it is cheaper than a mechanical switch, it can be tightly sealed, and the graphics printed on its surface can have many variations.
Membrane switches also have many disadvantages. First of all, to make effective contact requires a relatively large physical force. For a simple flat-panel membrane switch, the magnitude of this force is usually between 0.5N (Newton) and 3N, while for a tactile type it should be between 1.5N and 5N. In addition, a certain physical movement distance is required to bring the switches together. For a tablet keypad, this distance is 0.1 to 0.5 mm, and for a touch-sensitive type, 0.5 to 1.2 mm. The combination of these two factors places strict restrictions on the stiffness and thickness of the cover chosen for the upper part of the membrane switch. At the same time, it also limits the operating speed of the keyboard and the ease with which the user can use it. In addition, due to wear caused by mechanical movement, the tactile sensation of the keys will gradually decrease over time. This results in different forces and angles required for different keys to ensure reliable contact.
Piezo switches offer several advantages over resistive membrane switches. Piezoelectric effect is a characteristic of some specific crystal materials, including natural quartz crystals, potassium tartrate crystals, tourmaline, and artificial ceramic materials such as barium titanate and lead zirconate titanate (PZT). When mechanical pressure is applied to these materials, their lattice structure generates voltages and charges that are proportional to the pressure. (Conversely, if a certain electric field is applied to it, the deformation of the lattice structure will cause a change in the size of the material.) This switch requires only negligible physical movement, and usually a usable switch is produced between 1 μm and 10 μm Voltage or charge. In fact, it is the simple application of external force rather than physical movement that produces the output of the switching element. This switching element uses a piezoelectric chip. The surface layer, that is, the part seen by the user, can print, cover or press out the required information. The piezoelectric chip is inserted into a stamped insulation layer (sleeve), which is then sandwiched between two conductive sheets that make up the switch contact. Finally, the entire assembly is supported by a carrier plate. The high-speed control keyboard requires less than 1N of force when working. Industrial switches require a force of 3N to 5N. The thickness of the chip used in the piezoelectric keyboard is usually about 200 microns, and its output is about 1VDC when a force of 1N is applied.
In recent years, piezoelectric inks have replaced the position of piezoelectric wafers in some designs, mainly to reduce assembly costs, but the corresponding price is that a greater force must be applied to generate a voltage sufficient to detect the switching action . When the applied pressure is increased, the output voltage of the piezoelectric unit increases linearly. The specific value of the output voltage depends on the ambient temperature, force and speed, as well as the thickness and type of the covering material. With so many variables, complex electronic systems are needed to handle the effects of the wide range of changes in environmental conditions and physical operations that require normal operation of the switch. This complex structure makes it expensive compared to other keyboard technologies, but the advantages of piezo switches are obvious when metal covers must be used for aesthetic or safety reasons.
Capacitive sensors Capacitive buttons and switches are divided into two basic types: types that require mechanical keys to trigger, as shown in Figure 2, and types that only require proximity or touch. The key-triggering structure is relatively complex and includes mechanical moving parts, but how to make its mechanical structure more robust is the current challenge. Nevertheless, it has been widely used on PC keyboards. The upper half is made of a plastic film printed with a conductive film as the upper electrode, and the lower half is a printed circuit board with a conductive line as the lower electrode of the capacitor unit. The touch or proximity keyboard omits mechanical moving parts and instead uses the operator’s finger to affect the level of charge on the electrode or capacitor. Its sensing electrodes can be placed behind any insulating layer (usually glass or plastic), and it is easy to make a keyboard that is sealed from the surrounding environment. However, the adoption of this attractive technology can also cause some technical challenges.
The first is that touch sensing requires measuring or detecting the charge or charge level on the capacitor. The degree of change that indicates that a touch has occurred must be programmed into the microcontroller. In other words, the system must be calibrated. The problem is that changes in charge levels can be caused by many external influences. Electrostatic discharge and electromagnetic interference can cause malfunctions, and changes in temperature can affect calibration. Accumulation of moisture or other contaminants on the surface can affect its accuracy and repeatability. Finally, it is difficult to manufacture keyboards with keys of different shapes and sizes, and this is often an element that electronic device manufacturers are eagerly seeking to beautify their product shapes and increase market competitiveness. However, overcoming these points through various mechanical or electronic compensation methods makes the traditional capacitive sensing technology costly, so it is not suitable for many applications, especially cost-sensitive consumer appliances.
The emerging charge transfer sensing technology, while overcoming all of the above issues, is also extremely attractive in price to companies that mass-produce consumer products. Explanation of Charge Transfer Technology Charge transfer sensing is a technology based on a basic physical law, the law of conservation of charge, also known as switched capacitor or QT (where Q refers to charge and T refers to transfer) technology. The QT sensor is essentially a microcontroller that is programmed to charge a sensing pad with an unknown capacitance to a known potential. The sensing pad can be any conductive object, from pads on the PCB to optically transparent indium tin oxide (ITO) areas coated under or above the display screen. Finally, the charge on the sensing disc is transferred to the measurement circuit. Through one or more charge-transfer cycles, the capacitance of the sensing disk can be measured. The charge-transfer-acquisition process is completed in burst mode through the switching of the MOSFET transistor controlled by the microprocessor. The extra capacitance caused by an object such as a finger can affect the flow of charge and be detected. By using intelligent signal processing, decision logic is very reliable. For example, a decision filter is used that needs to detect many successful samples before a touch is stored. This can eliminate malfunctions caused by electrostatic burrs or accidental momentary touch or approach. Another feature is Adjacent Key Suppression (AKS). By using an iterative technique that repeatedly measures the signal corresponding to each key, it is compared to find the one with the largest change, and finally the largest The signal change determines the key selected by the user. In the following time, as long as the signal of this key is above a certain threshold, AKS will ignore or suppress the signal of all other keys. This prevents misoperation of adjacent keys, which is especially important for small control panels such as handheld remotes.
QT sensing ICs are available in single- or multi-key, matrix keyboard, touch slide control bar, touch wheel (such as iPod), touch screen and other applications, and combinations of these applications. In multi-key applications, the sensitivity of each key can be set independently. This helps to adopt different key sizes and shapes to suit functional and aesthetic requirements. As a means of distinguishing from other similar products, the shape design of electronic and electrical products is increasingly important, especially for consumer products, and QT sensing technology provides unparalleled flexibility in this regard. QT technology also solves the electromagnetic compatibility problems that plague traditional capacitive sensors. The QT sensor uses a sparse, random charging method that uses spread-spectrum modulation and adds a long delay between bursts of pulses. A single pulse is only 5% or less of the interval between bursts. The advantages of this spread spectrum method include: lower interference between sensors, reduced RF emissions and sensitivity, and lower power consumption. Other advantages of charge sensing technology include that QT devices have been programmed with an automatic drift compensation mechanism to cope with slow changes in signals due to time lapse or changes in environmental conditions. This overcomes a common problem with traditional capacitive sensors.
Unlike traditional capacitive sensors, QT technology has several orders of magnitude in dynamic range, and QT sensors do not require coils, oscillators, RF components, dedicated cables, RC networks, or many discrete components. As an engineering solution, it is simple, reliable, sophisticated and inexpensive. The number of external components is very small. Application of Charge Transfer Sensing Technology The current and potential applications of charge transfer sensing technology are increasing every day. This technology has been widely used in household appliances such as cookers and food mixers, and also in MP3 players, LCD displays and PCs. New applications such as cellular phones, handheld remote controls, pointing devices, and new touch screens are also being developed.
Post time: Nov-28-2019