Technical article: Resistive touch sensors on the rise. The Renaissance of Handle Input.
Item no. 1862
Description
| Introduction | |
| As early as the beginning of the 1970s, the first usable resistive touch sensors that could be operated with the index finger were developed in the USA, initially as a keyboard substitute and then also transparently placed in front of CRTs (image-generating cathode ray tubes). At that time, they were in competition with input via light pens. The main disadvantage of the dominant light pen was that it had to be held by an "annoying" cable that often became brittle. And why should one not be able to point directly at the screen with the finger and trigger an input without the stylus? The main technical advantage of the light pen was that it was largely independent of changes in the position and size of the image on the screen. In this respect, this technology is similar to the mouse technology that dominates the stationary use of computers today. However, the difference is that in contrast to touch technology, which acts directly on the screen - when selecting an object with the mouse - the screen is not partially covered by a manipulating hand and thus becomes unclear. This advantage of mouse technology has, among other things, initially led to the widespread disappearance of the (light) pen as an input medium. However, in public, non-supervised applications such as ATMs or portable devices such as PDAs, the use of the mouse is practically impossible due to its susceptibility to interference. Here, the disadvantage of covering the screen when entering data with a finger or even a stylus is accepted because the advantages of the interactive, largely intuitive operating options outweigh the disadvantages. Today, resistive touch sensors trimmed for durability enable precise interaction on graphic surfaces with the help of a stylus, especially on portable devices. | |
| Touch technology problems with CRT displays | |
| Successful use of the touch screen requires the most precise coordination possible between the location determined by the touch on the touch sensor and the image lying behind it. Here, the two main problems, namely the instability of image generation with CRTs on the one hand and the real achievable resolution and linearity of the touch sensors on the other hand, stood in the way of the widespread use of touchscreens. Significant advances in CRTs brought about sensor surfaces in the picture tube that allow the image size to be regulated. Now, when inputting via touch screens, the main metrological task is essentially only to determine the absolute coordinates of the touch position as distortion-free as possible. In the case of structures that do not require such high resolution, such as the operation of at least finger-key-sized control fields on a touchscreen keyboard, key-sized discrete buttons are used. Since the buttons and the graphic elements of the underlying image are mechanically rigidly co-ordinated with each other, the data technical effort required to interrogate this keyboard is relatively low. Thus, a 12" screen (30 cm diagonal) can accommodate more than 100 firmly defined buttons that can be operated like a discrete keyboard. The key legends are either printed on a paper template that is replaced when changes are made, or variable key field layouts are displayed on a screen with a graphic interface. | |
| Considerably more effort is required if the graphic template under the touch sensor is to consist of arbitrarily designed objects that have to be selected precisely via the touch. In this case, it is essential to determine the touch position of the finger (stylus) from analogue changing measured values on the touch sensor. The interactive operation of the computer can be carried out almost completely intuitively via the now highly developed graphical user interfaces with a matching input medium acting directly on the screen. Such further development is crucial for the successful, practical use of these technologies. | |
| Various touch sensor technologies | |
| A wide variety of sensor techniques have been developed to suit the external conditions of use. In a very robust arrangement, for example, ultrasound is passed through the glass tube surface from several sides and the point of finger contact is deduced from the measured reflection and attenuation. In another stable glass touch sensor, the distribution of forces created by pressing on several support points located at the edge is used to determine the location of the touch. Measurement signal drift, e.g. due to temperature fluctuations and other environmental influences, is minimised with appropriate circuitry and programming. This also applies to touch sensors, which use the capacitive change caused by the pointing finger to determine the location. Other methods circumvent the signal drift problems and observe the space directly above the screen surface by means of a scanning "infrared light curtain" generated by numerous IR LEDs arranged all around. The finger position is determined by evaluating the shadows cast on the IR sensors, which are also arranged all around. In all these arrangements, the robustness of the sensor units is an advantage, as is required, for example, for use in ATMs against vandalism. | |
| Conditions for portable use | |
| However, due to the mechanical effort, the weight and the high operating energy, the aforementioned systems are not suitable for use in portable, smaller devices. Since the portable device is normally operated by the owner with a great personal interest in maintaining functionality, protection against vandalism plays practically only a subordinate role. Here, low weight, high position resolution, high transparency and low power consumption are among the decisive factors. Above all, a long service life of the touch sensor is extremely important for the now very popular, precise operation with a stylus (pen). The breakthrough for touchscreen technology in portable applications was initiated with the advent of the extremely light and space-saving graphic LC displays. Because of the inertia of the changes to the pixels of the LCD, the use of a light pen is no longer necessary at all. Mouse technology dominates in stationary office computers with LCD monitors. For machine control and portable devices, the great advantages of safe operation via graphic interfaces are also urgently needed and used. However, the use of the mouse is rather annoying and prone to interference. Here, the completely flat LC screens in combination with modern, foil-based, resistive touch sensors - with their possibility of being operated via fingers or stylus - have their preferred fields of application as so-called touch screens. | |
| Resitive touch sensors: The 4-wire technology | |
| In the simpler 4-wire technology, the supporting glass pane has two bar-like electrodes in the x-direction, between which an electrically homogeneous, transparent resistive conductive layer is vapour-deposited and contacted. On top of this, held at a small, insulating distance by small spacers, is a thinner flexible foil, which also has bar electrodes and a resistance layer in the y-direction. If, for example, a measuring voltage is applied to the x pair of electrodes of the glass plate, an electrical potential distribution is created on the resistance surface. If the touch surface is now pressed, a local electrical contact is created between the glass plate and the foil and taps the electrical potential prevailing here, which is now conducted via the y-electrodes of the foil to an A/D converter and digitised there. In the next moment, the tapping is reversed. The measuring voltage is applied to the y-electrodes of the flexible foil. This now generates a potential distribution in the y-direction, which can be picked up and digitised at the x-electrodes of the supporting glass plate. This "scanning of the touch position" is done continuously via the touch-screen µ-controller, so that when a pen is used, graphic inputs such as the recognition of handwriting and the capture of signatures are also possible. Long-term disadvantages of this 4-wire technology are mainly to be seen in the operation with a pen, which deforms and possibly injures the cover foil, so that the potential distribution in this foil shifts and its homogeneity is destroyed. Therefore, the service life of the 4-wire touch sensor is usually limited to approx. 1 million local touches. | |
| Durable touch sensors through 7-wire technology | |
| A significant increase in service life is achieved by using the conductive layer on the flexible foil only to sense the potential at the point of contact. Furthermore, various potential distributions are used to determine the location. Several electrodes applied to the edges of the supporting glass plate are used for generation, which are also used for sensing the correction of reference voltages serving the measurement. Such long-life touch sensors with a self-correcting 7-wire technology are offered by GeBE Computer und Peripherie GmbH in Germering in various sizes between 5.7" and 17" - also customised - together with the appropriate controller. With these 7-wire touch sensors, the manufacturer Fujitsu has succeeded in achieving a tenfold increase in life expectancy compared to 4-wire technology. The resolution in cooperation with the available touch sensor controller goes down to about 0.1 mm. The high transparency of the sensors, which are provided with optically anti-reflective layers in all reflective interfaces, should also be emphasised. The risk of injury with a stylus is defused by an additional hard coding on the cover foil. Small hand-held devices today have touch sensors that are not only located above the LC display, but also form areas with so-called fixed soft keys. This makes it possible to completely dispense with input via discrete keys on these devices at low cost. |
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