Infrared Touch Screen re-discovered

Fachbeitrag MEDengineering International 2020

Infrared touchscreens?

These are the ugly touchscreens that cannot be operated in sunlight, with the thick frames that are familiar from ticket vending machines. Multitouch, gestures? They don't exist! But a new technology takes up the well-known principle of the light barrier and transfers it into the current HMI technology.

Infrared technology for touchscreens has a long history. The principle is simple: a light grid is mounted in the x- and y-direction of the screen. If the 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 temperatures or vandalism. Examples are ticket vending machines and ATMs. There you can concentrate on the protection of the display surface and make it meet the requirements, e.g. mount a thick pane of protective glass, which can be replaced in case of damage. However, there are some disadvantages. The diagonal size increases the number of IR emitters and detector diodes as well as the current consumption. Bright ambient light, such as the direct sunlight at some times of the day, superimposes on the useful signal and dazzles the photodiodes which receive the signal. As the touch screen is mounted behind the front panel, the display is located relatively deep in the housing, making the edge areas difficult to read when viewed at an oblique angle. A new approach of the well-known technology avoids these disadvantages and at the same time offers a modern operating concept.

Figure 1: Functional principle of the infrared touch screen
Figure 1: Functional principle of the infrared touch screen

Infrared laser with reflection

The technology, known as zForce, places the transmitter and receiver side by side in an oblong enclosure that only needs one side of the display to be mounted on. It does not detect the interruption of a light curtain, but the reflection of the emitted light by an object within sight. The recognition of multi-finger functions and gestures is done by the built-in controller. This technology offers several advantages over PCAP touchscreens; activation does not have to be carried out with a conductive object. It is insensitive to electromagnetic fields and bright ambient light.

Figure 2: Functional principle of the zForce touch screen
Figure 2: Functional principle of the zForce touch screen

System Integration

With infrared touchscreens, the sensor is mounted 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 (see Fig. 4). Debris such as dust and water above the display cannot impair its function. The display is brought closer to the front of the device.

Figure 3 + 4: Cross section of a/an infrared/zForce touchscreen system
Figure 3 (left): Cross section of an infrared touchscreen system; Figure 4 (right): Cross section through a zForce touchscreen system

Applications

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. Even with wet or dirty hands or long fingernails, operation is easy. Since the touch sensor is mounted outside the display, the display can be protected from environmental conditions without regard to the touch screen. The touch sensor is insensitive to electromagnetic interference. It can therefore 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 or 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 touch screen 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 can be operated in a lower corner without the touch sensor having to cover the entire width of the screen.

Other fields of application

Depending on the orientation of the laser, zForce can be used as a touch screen, light barrier or proximity sensor. Figure 5 shows the differences. With the alignment rotated by 90°, the sensor serves as a one-dimensional 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.

Touch screen without touching

Figure 5: Use as light barrier (left) and proximity sensor (right)
Figure 5: Use as light barrier (left) andproximity sensor (right)

While the replacement of conventional touchscreens is obvious, the zForce technology finds further areas of application. It is ideal for monitors of imaging processes in medical technology. The 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 restaurant kitchen, devices can be operated with „dirty“ fingers without touching the surface and making the screen underneath unreadable. When used as a light barrier, the technology can be used to 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 collisions with the environment, such as in vacuum cleaners or mowing robots.

Software Integration

The built-in controller presents itself as a USB HID (Human Interface Device) and therefore works immediately with the operating system of a corresponding host and replaces or complements the mouse functions as a single or multi-touch. Alternatively, it provides the information via I²C.

Comparison with other touch technologies

Compared to other technologies, zForce performs well. In particular, the lack of mounting to the underlying display enables applications that cannot be realized with other touchscreens. Fig. 6 shows the sensor in side view.

Optics

Since the technology works without covering the display, the display can be optimized here for the application. Optical parameters such as transparency, reflection and contrast are not influenced by zForce.

Integration

Since the direct assembly with the display is not necessary, a monitor can be retrofitted with a retrofit kit. For notebook users, there are ready-made modules that can upgrade it to a touchscreen notebook.

Sturdiness

The touch sensor itself is resistant to conventional chemicals used for cleaning or sterilization in medical environments. The vandalism, which is usually directed against the display, can be made ineffective with a suitable cover glass. 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 present or interference pulses. Due to optical filters, the sensor is not influenced by ambient light from the sun or strong light sources. The service life is high regardless of the number of operations.

Ergonomics

Figure 6: Sensor module, right opening for transmitter and receiver
Figure 6: Sensor module

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.

Bottom line

Although the majority of touchscreens today are based on the PCAP principle, there are applications for which it is less suitable. The zForce technology offers the advantage of separate mounting; the display surface is not influenced by the touch screen. This allows it to be designed according to other criteria: For the rough use, for the operation with any object, where an actual touch of the touch screen is not desired, or where the image quality must not be influenced by an additional layer on the display. Yes, not even a display is required! It may also be the worktop in the kitchen.

 

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