Touch screens are not a new concept. Since the arrival of the iPhone® multimedia device, touch technology has become extremely popular and has benefited from a flood of innovations. Where previously touch screens were merely designed to replace keyboards and mice, today they convey a completely new operating experience. Featuring greater interaction between the user and device, this new “touch” experience has been achieved by a combination of different technologies. This article provides an overview of recent advances in touch-screen technology.
Resistive technology
Touch screens with resistive technology have been in common use for many years. They are inexpensive to manufacture and easy to interface.
In resistive-technology sensing, electrodes are connected to two opposite sides of a glass plate with a transparent, conductive coating. When a voltage is applied to the electrodes, the plate acts like a potentiometer, enabling the measurement of a voltage which depends on the position of the contact point, Figure 1.
Once a similar, second plate is arranged in close proximity to the first with electrodes offset by 90°, and with the coatings facing each other, a 4-wire touch screen is achieved. If the top plate, which can also consist of transparent plastic, is deflected by a finger or pen so that the plates touch at the contact point, the x- and y-position of the pressure mark can be determined by comparing the voltages.
Figure 1: Functionality of a resistive touch screen.
because resistive touch screens are pressure sensitive, they can be operated using any pen or a finger. In contrast to most capacitive technologies, they can also be operated if the user is wearing gloves. Due to the mechanical contact system and the DC operation, they have a high electromagnetic interference (EMI) tolerance. However, only one contact pressure point can be registered at a time, and no multitouch systems are possible.
Modern controllers like the Maxim MAX11800 autonomously calculate the X/Y coordinates, thus relieving the host CPU of this task. This facilitates rapid scanning which, for example, enables a clear and legible signature to be captured.
Capacitive styles
Another technology, termed “projected capacitive,” is gaining in popularity. The premise for this technology is straightforward: transparent conductor paths made of indium tin oxide (ITO) are applied to a transparent carrier plate mounted behind a glass plate. A finger touching the glass plate affects the electrical capacitor field and enables the contact to be detected and measured. Two fundamentally different approaches are used to implement a projected capacitive design.
Self-capacitance
With this design, a clearly defined charge is applied to a conductor path. An approaching finger conducts part of the charge to ground. The changing voltage at the conductor path is analyzed by the touch controller. The ITO conductor paths are arranged in X and Y directions in a diamond pattern (Figure 2).
Figure 2: Functionality and design of a self-capacitance touch screen.
The exact position of the finger on the glass plate is determined by the controller chip through interpolation. Only one finger position can be recorded at a time, so real multitouch functionality is not possible.
The MAX11855 is a controller for a self-capacitive touch screen. It can control up to 31 ITO paths. Its superior sensitivity enables touch detection from gloved hands, and its noise immunity allows for thin and simple ITO constructions without the need for special shielding or a safety gap between it and the LCD display.
Intelligent control also makes pseudo-multitouch possible, e.g., zooming through the spread of two fingers. The integrated microcontroller enables the device to be used flexibly for touch screens of different dimensions as well as for buttons or slider controls.
Mutual capacitance
With this technology, an activating finger changes the coupling capacity of two crossing conductor paths. The controller monitors each line, one after another, and analyzes the voltage curves at the columns (Figure 3). This technology is capable of multitouch; several finger contacts can be detected simultaneously.
Figure 3. Functionality of a mutual-capacitance touch screen.
The geometry of the conductor paths is optimized for several applications. The MAX11871 touch-screen controller is designed for this technology and is the first controller on the market to allow a capacitive touch screen to be operated with gloved hands or pens. Another advantage is that the screen can be mounted behind a thick glass plate, so very robust systems can be built.
Haptics rising quickly as a driving force
A user’s operating experience is changing significantly because of the emerging use of haptics. Haptic feedback enables the operator to “feel” a device execute a function.
Currently, cell phones use a very rudimentary method for this tactile feedback. An integrated vibration motor, which is primarily used to signal an incoming call when the device is in silent mode, is activated for a short moment with every touch contact, and the whole cell phone briefly vibrates.
Future haptic systems will further refine this experience. In the newest designs, the touch-screen glass plate is moved by special actuators that operate magnetically, e.g., the new linear-resonant-actuators (LRA), or are piezo-based. Piezo devices are available in single-layer versions. which require a high voltage (up to 300V) or in a more-elaborate multilayer technology which can reduce the required voltage.
The MAX11835 haptic driver has been designed for single-layer, high-voltage piezo actuation. It is controlled directly by a touch-screen controller like any of the devices mentioned above. It enables a nearly latency-free operation, which is very important for the touch experience. Its waveform memory allows for individual movement patterns, which can further optimize the operating experience.
As an example, buttons can be sensed by fingertips moving over the display. Pressing the button then executes a function and creates a different waveform, which is detected differently by the finger. In this way touch-screen devices can be operated safely without the operator having to constantly look at the display to verify an action. This significantly increases personal safety for navigational systems or medical devices.
Figure 4; The function generator built into the MAX11835 reduces the delay for a fast and individual haptic experience
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