Understanding touch control technologies
By Hal Philipp, Quantum Research Group -- 9/25/2007
Today, consumer electronic products are sold as much on their design and appearance as they are on functionality. The packaging design of such products is often an integral part of the manufacturer's brand. The Apple iPod is a classic example with its clean white lines and innovative touch controls. In this environment, traditional mechanical switches are very restrictive, both in terms of appearance and the complex mechanical arrangements needed to accommodate them. The use of mechanical switches in consumer products is therefore in decline and a variety of touch control technologies is replacing them. Such touch controls include resistive membrane switches, piezoelectric switches, and touch controls based on capacitive sensing. This article gives a brief overview of the main touch technologies and considers which are likely to dominate in future and why.
Cheaper than mechanical switches, capable of being tightly sealed, and versatile in appearance, resistive membrane switches have been widely adopted since the 1970s. They consist of a flexible top layer, an insulating spacer and a lower substrate. Graphics are applied to the top layer upper surface, and a conductive pattern of silver or carbon conductive ink, is printed onto the lower surface. A matching conductive pattern is printed onto the lower substrate. The conductive layers are pressed together through holes in the spacer to create a contact. To create tactile feedback, metal or plastic domes placed beneath the overlay can be used to provide a 'click' when switching takes place and embossing on the top layer can be used to guide users' fingers to the 'sweet spot' of each switch. However, membrane switches have a number of disadvantages. Firstly, they are not true touch switches. Physical travel is needed to make a contact—0.1 to 0.5mm for a flat panel keypad or 0.5 to 1.2mm for a tactile type - and they need physical force to operate - typically between 0.5N (Newton) and 3N for a flat panel. Adding tactile feedback takes this up to between 1.5N and 5N. These factors limit the rigidity and thickness of overlays, speed of operation and ease of use. Of course, physical movement creates wear too, which means that the feel of the keys varies over time and more frequently used keys on a panel develop a different feel from those that are used less often. Membrane switches therefore have limited life and, even during the lifetime of an electronic product, can become increasingly difficult to operate.
Piezoelectric switches and panels
salt, tourmaline, and manufactured ceramics such as Barium Titanate and Lead Zirconate Titanates (PZT), their crystalline structures produce a voltage and electrical charge proportional to the pressure. Physical movement to produce a usable switching voltage or charge is typically between 1µm and 10µm. In fact, it is applied force, rather than physical movement that generates an output from the piezoelectric element. The switching element uses a piezoelectric 'pill'. The overlay—the part that the user sees—is printed, stamped or embossed with the required graphic design and operating information. A punched insulating layer, into which the piezoelectric pills are inserted, is sandwiched between two layers of conductive foil that constitute the switch contacts and the whole assembly is mounted on a carrier plate as shown in Figure 1. Fast control keyboards must operate with an applied force of less than 1N and piezoelectric pills some 200 microns in thickness will generate about 1VDC with 1N force. Piezoelectric inks have replaced the pills in some designs in order to reduce assembly costs, but at the expense of an increase in applied pressure to produce sufficient voltage or charge so that a switching action can be detected. The voltage output from a piezoelectric element increases with pressure in a linear fashion and the output voltage is dependent upon ambient temperature, operating force and speed, and both the thickness and type of material used for the overlay. This host of variables requires relatively complex electronics to take account of wide variations in both physical operation and environmental conditions under which the switches may be required to function. The complex construction is expensive when compared with other keyboard technologies and has severely limited the use of piezoelectric touch controls in consumer electronic and electrical products.
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Capacitive sensors—simple idea, complex implementation
Capacitive buttons and keys come in two basic types: those that use a mechanical key to active them, as shown in Figure 2, and those that rely on proximity or touch. Key-operated switches are of relatively complex construction, involve mechanical movement and present challenges in making them mechanically robust. Despite this, they are sometimes used in PC keyboards. The upper plate consists of plastic membrane onto which a conductive film has been printed to create the upper electrodes. The lower plate is a printed circuit board with conductive tracks that form the lower electrodes of the capacitive elements.
Touch controls eliminate mechanical movement and rely on the operator's finger to affect the charge level on an electrode or capacitor. The sense electrode can be placed behind any insulating layer, typically glass or plastic, so it's easy to achieve an environmentally sealed touch pad. Despite these clear advantages, adoption of this attractive technology has, however, been limited by a variety of technical challenges.
Firstly, touch sensing involves measuring or detecting changes in capacitance or charge levels. The degree of change that indicates a touch has taken place has to be programmed into a microcontroller. In other words, the system has to be calibrated. However, changes in charge levels can occur due to a variety of external influences. Electrostatic discharge and electro-magnetic interference can cause false triggering, and temperature changes affect calibration. Build-up of contaminants or moisture on the surface of the keypad can affect operation and it is difficult to produce keypads with keys of differing shapes and sizes, something that's desirable when equipment makers want to make their products more aesthetically attractive than competing products. These problems can be overcome through various electronic and mechanical compensation mechanisms, but at a cost that rules out the use of traditional capacitive sensing for cost-sensitive consumer applications.
Charge-transfer sensing—the capacitive sensor re-invented
A relatively new technology, charge-transfer sensing, promises to overcome the technical problems associated 
with traditional capacitive sensors. Based on the principle of conservation of charge, an elementary principle of physics, charge transfer sensing can be used for touch or proximity sensing. In other words, the technique can sense a finger approaching a control panel and be calibrated to operate even before it touches the surface. Devices are available that automatically re-calibrate every time they are switched on, that incorporate automatic drift compensation to take account of changing environmental conditions or ageing, and that can differentiate between intentional and unintentional touch in many instances. Charge-transfer sensing devices have very good EMC performance, something that is increasingly important in today's RF-rich environments. A single device can be used for individual touch buttons, panels of keys, sliders or even rotary controls. Some even combine these functions within a single chip. Most important of all, control panels using charge-transfer sensing are simple and economical to produce so applications for the technology are growing daily. A few examples are shown in Figure 3. Charge-transfer sensing is already widely used in domestic appliances such as cookers and food blenders. It is also found on the control panels of MP3 players, LCD monitors and personal computers. New applications are being developed in cellular phones, hand-held remote controls and pointing devices, and new classes of touch screen.
Author information
Hal Philipp is the CEO of Quantum Research Group Ltd.
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