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.
The resistive membrane switch
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
When physical pressure is applied to some crystalline materials,
including natural crystals of quartz, Rochelle 
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.
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.当前市面上的消费类电子产品对设计和外观的要求和对功能的要求一样重要，这类产品的封装设计通常成为制造商品牌的一部分。Apple的ipod就以其简洁的白色线条和创新的触摸屏控制而成为经典的案例。在这种情况下，传统的机械开关在外观和结构适配性方面作用有限，因此处于衰退阶段，而各种触摸屏技术开始取代它们的位置。这些触摸屏技术包括阻性薄膜开关，压电开关和基于容性检测的触摸控制。本文对这些主要技术做简要介绍，并分析各自前景。
阻性薄膜开关
由于比机械开关便宜，易于封装，外观多样，自1970年以来阻性薄膜开关获得了广泛应用。它由顶层、绝缘装置和基座组成。图形输入到顶层后由墨水显影到底层，点击会使得绝缘装置出现空洞形成感应。但是，薄膜开关有很多缺点，首先，它们不是真正的触摸开关，它需要物理接触和力道来操作。这些限制了涂覆层的刚度和厚度、操作速度以及易用性。当然，物理接触也会磨损键盘，导致薄膜开关寿命有限，会随时间而变得难于操作。
压电开关和面板
当对一些晶体物质，比如石英晶体、罗谢尔盐、电气石等施加压力时，它们的晶体结
容性传感器-简单的想法，复杂的实现
容性按键有两种基本类型：采用机械按键激活和依靠点击。虽然采用机械方式的构造相对复杂，需要有稳健的机械移动，但有些也用在PC键盘中。 点击控制没有了机械移动，通过操作者的指头来改变电极或电容的电荷等级。传感电极可以放在任何绝缘层的后面，通常是玻璃或塑料，这样就可以很容易实现封装的触摸面板。但是尽管有这么多的优点，对这项技术的实现却有很多技术挑战。首先，点击传感需要测量和检测电容的电荷或者电荷等级，表征一次点击的电荷等级必须编程进入微处理器。换句话讲，系统必须可以校准。但是，一系列的外部影响会导致电荷等级的变化，静电释放和电磁干扰都会引起错误的触发，温度变化会影响校准。装配污染和湿度会影响操作，按键大小和形状也很难差异化，这会影响到最终产品的美观。虽然这些问题通过一些电子和机械的补偿方案可以克服，但是对于对成本敏感的消费类应用来讲这种传统的容性传感并不经济实用。
电荷迁移传感-改良后的容性传感
新的电荷迁移传感有希望解决传统电容传感的技术问题。基于电荷守恒原理，电荷迁移可以被用来实现点击传感。或者说，这种技术可以感测到指头要接近控制面板，甚至在接触到面板前就可以调整操作。设备在开机时可以自动的进行自校准，其中对漂移的自动补偿会考虑到环境条件的变化、老化以及有意识和无意识触碰的不同。电荷迁移传感设备有很好的EMC性能，这对现在多射频设备来讲尤为重要。一个设备就可以实现触摸、键盘、滑动甚至滚动控制，有些此类技术还包含有微芯片。最重要的是，采用此项技术的控制面板很简单经济，所以它的应用也越来越多。从家电到MP3播放器、手机等等领域都可以看到它的身影。
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