Don’t expect Plug & Play

One of the first steps in the conception phase of a touch system is the evaluation and consideration of the appropriate touch technology for the application. Easy-to-operate touch panels have long been standard in modern industrial visualisation systems. Depending on the requirements of the individual applications, various technologies, such as infrared, acoustic, camera-based, resistive or capacitive solutions, are deployed.

Projected Capacitive Touch (PCT) technology made its break-through in the consumer market with high-quality tablets and mobile telephones, but thanks to the available software, PCT has now also triumphed in many industrial areas. PCT is, however, not always the first choice, and there are certainly application areas where the previously very popular resistive touch is still the best solution. Examples are applications that require operation not only with ‘normal’ fingers but also with thick textile gloves. However, in systems that permit PCT, the technology is unbeatable. It is characterised with properties that include: robustness; contact-free; wear-free sensor; attractive and high-quality design; various glass types (also printed) available as front, and; low-maintenance.

Many industrial applications still require a proven and cost-effective operating concept with resistive touch. Put simply, the screen is a mechanical switch with detection of the contact position. The resistive touch responds to pressure and can be glued simply onto an appropriate display. Because the plastic foil normally attached to the front side is easily scratched mechanically, no pointed or sharp objects may be used for activation. Special constructions with a front-side glass, scratch-resistant but easily breakable, are offered. The data is evaluated with a controller.

The oldest and simplest four-wire touch may suffer from temperature drifts. Furthermore, mechanical stress can cause micro-tears in the ITO (Indium Tin Oxide). Because these malfunctions are technology-dependent and irreversible, the touch ages and so requires a recalibration in many cases. The state-of-the-art five-wire touch permits a larger number of operating cycles compared with four-wire models and needs only to be calibrated once. The technology's high accuracy makes it popular for screen sizes above 38.1cm (15") or for medical and industrial systems with high requirement.

Infrared is one of the oldest touch technologies. It is based on a light grid that is interrupted with a finger or a light-impervious pen. The operating concept cannot be deployed in those areas which are subjected to deposits that could unintentionally interrupt the light grid, such as by dust in a woodworking workshop or snow flakes in winter. The use of infrared light makes the touch susceptible to external light, as caused by strongly fluctuating sunlight in case of a display system in a train carriage near the door window. The touch must be mounted mechanically very accurately and flat. Provided the appropriate measures are observed, IR touch is suitable for vandal-proof applications.

Acoustic touch technology is based on an ultrasound waveform whose propagation time in the glass substrate is measured. The damping caused by the contacting finger and the subsequent measurement of the propagation time allows the position of the contact point to be determined. The touch can be operated only with an elastic material, such as a finger or rubber eraser, but not with a metal pointer, etc. The acoustic touch is also suitable for vandal-proof applications because a thick protective glass is possible. Outdoor applications subject to water or insect attacks are excluded.

For the camera-based touch, two or more cameras check the touch surface. Suitable measures allow the operating object (finger or pen) to be triangulated either with its shadow or its reflection. The technology is normally deployed for larger display diagonal sizes. Possible faults occur through soiling of the light source or the camera.

The surface capacitive touch measures the capacitive change caused by finger contact on the surface normally in balance. Although the technology is suitable for systems with larger diagonal sizes, it does not permit any multitouch operation. To ensure the correct evaluation, any capacitive coupling (e.g. between a water seam and the metallic housing frame) must be prevented.

The triumph of PCT

PCT based on the effect of the electrical field of a capacitor caused by a finger or a suitable pen is being used increasingly in modern HMI systems. The capacitor matrix and its interpolation permit a precise position detection.

The PCT sensors penetrate different materials and permit the operation of the screen even through closed glass and plastic surfaces. The glass front can designed to be vandal-proof. In applications for which glass is not suitable because of shattering danger, polycarbonate or PMMA (polymethylmethacrylate, plexiglas or acrylic glass) can be deployed. Wear-free operation is possible without direct contact with the actual sensor. PCT systems are available as single- or multi-touch, supporting detection and discarding of defined areas, e.g. for palm rejection. Furthermore, the effect of draining or standing water can be partially suppressed.

Because state-of-the-art display systems ensure not only the functionality, but must also allow high artistic creativity in the device design, the high penetration of the PCT plays a significant role. The glass front can be customised with the glass type and the printing. State-of-the-art glass shaping technology allows perfect openings for switches, etc. A one-plate front e.g. for the easy disinfection in medical devices, can also be implemented.

The integration of a PCT is no trivial task - resistive touch-screens cannot be simply replaced with capacitive touch models. The capacitors of the projected capacitive touch react to all interfering elements that change the field (dielectricity). These include metal parts, such as housing front bezels, the display rear housing, mounting rails, threaded bolts, grounding strips and cables. Unless they have a minimum separation, switching electrical fields also affect the PCT. The perpetrators can be display inverters and cables, data cables for the display (LVDS cables), RFID readers, graphic cards or embedded computer modules.

Because the touch sensor acts like an antenna array for any interference, an environment with powerful electromagnetic radiated interference also impairs the function of the touch and of the controller. The filters integrated in the controller determine how well a touch system handles the interference.

There are also some restrictions on the operation of the PCT. Constantly changing glove thicknesses cause problems. Such an example is an operator panel installed at a skiing resort where the complete range from a very thick winter glove, a thin woolen glove and finger without glove can occur. The problem is less the material penetration, but rather the detection of the position from where a user senses reliably the contact with the front display. This varies for the various glove thicknesses and cannot be determined automatically. Incorrect operating actions can occur when the finger hovers over the front or no operation occurs despite contact. Liquids or gels on the front display are also a reason for touch errors. In particular, salty liquids, including blood, cause problem situations for the PCT.

Mechanical integration

All the above-mentioned items must be considered for the mechanical construction which will accept a PCT sensor and controller. The minimum separations of the touch sensor to conducting surfaces, such as the metal housing, the display rear, printed-circuit boards, data cables, etc., apply. The flex tail of the projected capacitive touch must also maintain separation to the conducting elements. In no situation may the PCT cable be routed near an LVDS or inverter cable. The flex tail is often attached only over the contacting width to the sensor surface and so mechanically very sensitive (Figure 4). It must not have any hard folds or bends, tension or shearing forces must be avoided.

PCT sensors are often glued to a glass plate which faces the user. The gluing must withstand mechanical effects, e.g. temperature fluctuations, vibration and shock loadings. The touch unit is often also mounted by being glued in the housing front bezel. For a rear-side gluing, the adhesion and the longevity of any glass printing must be guaranteed. The same is true for rear-side seals.

In many applications, ceramic paint is used to prevent paint flaking when subjected to mechanical stresses. The ceramic paint, however, also requires the thermal hardening of the glass. Chemical hardening cannot be combined with ceramic paints.

The fitting accuracy of a cover glass plate into the housing front bezel is determined not only by the tolerances required by the manufacturing process, but also by the thermal expansion coefficients. In the worst case, the material expansion at higher temperatures can cause glass breakage. Manufacturing tolerances often mean that mounting gaps cannot be avoided. In this case, consider whether an elastic joint seal can prevent a ‘dirt joint’.

Electrical integration

Electrical integration is also decisive for the successful construction of a PCT system. The grounding concept, in particular, influences the function and the interference immunity of the touch. To correctly connect the touch controller in the star-arranged grounding of the system, the controller should be connected to the display housing with a braided, low-inductive metal cable with end eyelets. One eyelet is fixed with a mounting screw of the controller and so connects the cable reliably to the controller's own ground cable. The other eyelet is screwed to a reliable grounding point of the display housing. It must be guaranteed that an equivalent connection of the display housing to the grounding star point of the device also actually makes a good ground connection.

An inadequate ground connection can cause impermissible fault operations in an EMC measuring chamber during a test of a previously reliably functioning PCT system. A differentiation is made here between incorrectly detected touch events in the un-operated state and incorrect events in the operated state. The simpler method tests the un-operated system, whereby no touch events may be recorded in the complete interference spectrum. The extended test for which the touch is operated during the interference radiation is much more complicated. During this test, no events that deviate from the operation may occur, e.g. a deviation of the touch coordinate, undetected events or the indication of events although no operation was made.

An adequate interference immunity is normally achieved with a substantiated and sometimes significantly improved ground connection of the various components. This can be achieved, for example, with a conductive gluing of copper foils between the conductive housing front bezel and the display housing. In practice, an experimental procedure is often unavoidable. Shielding measures on the supply cables (ferrite on the USB cable, power system filter, etc.) are also ways of preventing cable-dependent interferences.

One of the first steps in the conception phase of a touch system is the evaluation and consideration of the appropriate touch technology for the application. Every technology has its advantages and disadvantages. In many cases, for example, a state-of-the-art PCT sensory system is not the correct route to success, a resistive touch would suffice fully and allow an uncomplicated integration.

After the decision has been made for the integration of a PCT, the demands placed on this technology must be considered explicitly at the start of the mechanical and electrical design work; only when all specifications are observed can fully satisfactory operation be achieved. If the significant general conditions of a PCT system are violated, apparently random fault situations can occur whose subsequent analysis and rectification is very time-consuming and cost intensive. A correctly planned and implemented PCT system, however, offers high functionality, with secure and reliable operation over the complete service life of the HMI product.