By Sandhya Mallikarjun
An all-digital sensing method avoids the issues associated with the analog approach (Figure 3 below). The digital method detects changes in sensor capacitance by making the capacitance part of an RC-delay line.
A simple full-digital time-to-digital converter (TDC) measures this delay line against a reference RC-delay line that outputs changes in impedance. The effect of parasitic capacitance on the RC delays is eliminated by compensation at power-up.
When a finger touches the sensor pad, the capacitance increases, which increases the RC delay time. This introduces a change in impedance, which is then compared with the calibrated impedance to determine a touch condition. The sensing resolution can be easily improved by adjusting the resistance of the RC-delay lines.
Whether a touch-sensor controller uses the analog or digital detection methods, the controller can interface to a microcontroller using a simple SPI or I2C interface. Typically the MCU is in master mode and the touch-sensor controller is in slave mode for data exchanges.
If the MCU lacks such serial interfaces, software emulation of an appropriate serial interface can be used, but this approach adds memory and performance overhead. Recently touch-sensor controllers have been integrated on a single chip with microcontrollers.
Touch-sensitive controls now provide flexible, reliable and cost-effective alternatives to traditional mechanical buttons, sliders, rotary wheels, and switches.
The latest touch sensors enable designers to unleash their creativity in developing intuitive interfaces with the ability to hide or illuminate buttons and "morph" touchpad patterns. Table 2 below shows variations of sensor geometries and applications.
Proximity touch control offers an attractive alternative for simple interfaces that require just one or two buttons. The proximity sensor can easily integrate into the final product design and provide long-term advantages such as low power consumption and long life span.
A metal door handle is an ideal proximity sensor application. The extremely sensitive sensor detects the presence of a hand approaching the door handle. Then power can be applied to security hardware requiring much higher power. As part of a car's alarm system, every proximity detection can be logged and the owner notified (perhaps via cell phone) that someone is repeatedly trying the door handle of the car.
When used with a metal object 10mm2 and a 1mm cover thickness, a proximity sensor can detect the approach of a hand at distances as great as 2 inches. In addition to door handles, proximity touch applications include appliances, MP3 players, remote controls and mobile phones.
Sophisticated LCD Touch-Screen Solutions
At the opposite end of the spectrum from simple proximity sensors are sophisticated touch-sensitive LCDs that give a high-end feel to many "must-have" products.
Most notably, Apple products such as iPods and iPhones have put consumers' expectations into a steep, upward gradient. Similar touch-screen technology can enhance everything from GPS units and universal remote controls to digital picture frames and internet-connected refrigerators and washing machines.
Compact devices such as phones and GPS units can use a flexible touch-sensor PCB to overlay the device's display. In these applications, the capacitive touch module has transparent sensor pads and traces that can be implemented using an indium tin oxide (ITO) layer on a glass or plastic panel (Figure 4 below).
Displays on the latest premium appliances include sensors to detect a touch that tells the display to turn on. When touch is no longer sensed, the displays turn off to leave a clean, sleek appearance that has the added benefit of keeping power consumption to a minimum.
With no moving parts and easy conformity to curved surfaces, touch-sensor switches can be ideal for automotive applications. To adapt touch technology to these applications, auto manufacturers need automotive-grade and wide-temperature-qualified touch-sensor controllers at lower cost.
The key is to minimize the total cost of implementing the touch-sensor solution. At the right price, touch sensors will enable automotive design engineers to implement innovative interface features.
Nintendo's Wii uses 3D positioning sensing. In the world of computer-aided design, one of the latest innovations is the 3D mouse, which allows engineers to control their designs more intuitively by moving the mouse in three-space.
Also, Microsoft is now demonstrating its own vision of future user interfaces, in the form of Microsoft Surface. This interface uses similar technology to the iPod Touch to recognize multiple points of contact as well as actual objects (such as a paintbrush) and interact with the contact appropriately and intuitively.
Sandhya Mallikarjun is a Staff Systems/Applications Engineer working in the Embedded Platform Solutions Business Group of Fujitsu Microelectronics America, Inc., where she has been for more than 8 years. She holds a BSEE from Gulbarga University, India and an MBA is from Univ. of Phoenix.