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.