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Infrared touchscreens are again becoming an alternative in HMI development. A modified approach does away with familiar drawbacks, like thick frames and restricted functionality in sunlight. Multitouch and gesture recognition are now possible in the meantime.
Infrared technology for touchscreens has a long history. The principle is simple: a light grid is mounted on the X and Y axes of a screen. If this light grid is interrupted, any penetrating object is detected and its position can be evaluated. This technology is simple and robust, and has the advantage of being separate from the display. It is used where displays are exposed to extreme environmental conditions, such as ambient temperature or vandalism. Examples are ticket vending machines and ATMs. There you can concentrate on protection of the display surface and make it meet requirements, e.g. mount a thick pane of protective glass that can be replaced in case of damage.
There are some disadvantages however. The diagonal increases the number of IR emitters and detector diodes as well as current consumption. Bright ambient light, such as direct sunlight at some times of the day, superimposes on the useful signal and dazzles the photodiodes receiving the signal. Since the touchscreen is mounted behind the front panel, the display lies relatively deep in the housing, making the edge areas difficult to read when viewed at an oblique angle. A new approach in the now familiar technology avoids these disadvantages, offering a modern operating concept at the same time.
Infrared laser with reflection
The technology, known as zForce, places the transmitter and receiver side by side in a striped enclosure that only needs to be mounted on one side of the display. It does not evaluate the interruption of a light curtain but reflection of the emitted light by an object within sight. Recognition of multifinger functions and gestures is done by the built-in controller. This technology offers several advantages over PCAP touchscreens; operation does not have to be carried out with a conductive object, and it is insensitive to electromagnetic fields and bright ambient light.
System integration an applications
With infrared touchscreens the sensor is connected to the housing, but not necessarily glued to the display. Figure 3 shows a cross-section through the construction. The display surface is located behind the front panel, touch sensor and protective glass far inside the device. To view the entire contents of the display, the user must stand in a straight line in front of the device. There are several options for integrating the zForce sensor. It can be mounted either flush with the housing or on the outside of the housing (Figure 4). Deposits such as dust and water above the display cannot impair its function. The display moves closer to the front of the device. For notebook users there are ready made strips that can be upgraded to a touchscreen notebook.
zForce technology is ideal for use in harsh indoor and outdoor environments where other touch principles fail. It can also be used to retrofit existing systems. The wide temperature range makes it easy to use in industrial environments. It can be operated with any object that reflects light, including protective gloves, credit cards and pens. Operation is no problem even with wet or dirty hands or long fingernails. The touch sensor is mounted outside the display, so this can be protected from environmental conditions without regard to the touch screen. The touch sensor is insensitive to electromagnetic interference, so it can be used in commercial vehicles, agricultural equipment and machines with large electric drives. The functional principle allows the display surface to be shielded against eavesdropping, and not to be disturbed by radiation in a sensitive environment. Of course, the protection also works inwards: the display opening as an entrance gate for electrical interference signals can be sealed. As a "smart sensor", e.g. on a worktop made of wood or stone, the sensor can also operate without a display. The costs scale well with the size of the screen, since in contrast to the IR touchscreen only one dimension has to be covered. Even this is not a must: with a selective touch area for example, the on-screen menu of a large screen in a lower corner can be operated without the touch sensor having to cover the entire width of the screen.
Configuration as proximity sensor
Depending on orientation of the laser, zForce can be used as a touchscreen, light barrier or proximity sensor. Figure 5 shows the differences. With alignment rotated by 90°, the sensor serves as a unidimensional user interface. In contrast to conventional proximity sensors, which evaluate the signal strength as an indicator for the position of an object, here the combination of transmitter and receiver signal determines the position of an object.
Touchscreen without touch
While replacement of conventional touchscreens is obvious, zForce technology finds further areas of application. It is ideal for the monitors of imaging processes in medical technology. Image quality such as contrast, coating, antireflection coating and parallax are retained because the optical properties of the display surface are not affected. This is not the only advantage of not having to touch the surface with sterile gloves, but also in public places where elevators are called, vending machines operated and toilets flushed — germs have no chance to spread! Even in the food industry or a restaurant kitchen devices can be operated with dirty fingers without touching the surface and making the screen underneath unreadable.
Used as a light barrier the technology can detect the presence of (unwanted) objects and the system can take appropriate action. As a proximity sensor it can also be installed in moving objects to avoid collision with the surroundings such as in suction or mowing robots.
The built-in controller presents itself as a USB HID (human interface device), so it works straight away with the operating system of a corresponding host and replaces or complements the mouse functions as single or multi-touch. Alternatively, it provides information via I2C.
Comparison with other touch technologies
Compared to other technologies, zForce performs well. In particular the lack of coupling to the underlying display enables applications that cannot be implemented with other touchscreens. Figure 6 shows the sensor in side view.
The touch sensor itself is resistant to the usual chemicals used for cleaning or sterilization in medical environments. Vandalism, usually directed against the display, is made ineffective there with a suitable windscreen. Since the functional principle is not based on electromagnetic fields but on "invisible" light, the sensor neither emits electromagnetic radiation nor is its function impaired by fields or interference pulses. Filters prevent the sensor from seeing ambient light from the sun or strong light sources. Service life is long regardless of the number of operations.
While the argument for PCAP is the flat, flush-fitting surface, the zForce sensor scores with its suitability for special applications. It offers multitouch functions with the recognition of several fingers and gestures, and can be operated with all media, be it thick gloves, credit cards or fingernails. This feature can also be exploited to enable non-contact operation in sterile areas or with dirty hands. In terms of special design, the touch sensor can also disappear completely into a surface and be used for hidden operation. Figure 7 compares the technologies.
Although the majority of touchscreens today are based on the PCAP principle, there are applications for which it is less suitable. zForce technology offers the advantage of separate mounting; the display surface is not influenced by the touchscreen. This allows it to be designed according to other criteria: for rugged use, for operation with any objects where actually touching the screen is not desired, or where image quality must not be influenced by an additional layer over the display.