The freezing cold, dust, moisture – Human Machine Interfaces often have to withstand extremely harsh environmental conditions. Protection against vandalism and hygiene requirements also play an important role in some areas of application.
Whether in the rough daily routine of construction machinery, in food production or as a public terminal in the city – Human Machine Interfaces are often exposed to extreme environmental conditions. By using qualified materials and appropriately designed constructions, and choosing suitable operating technologies, HMIs can work reliably even in the most adverse conditions.
Replacing mechanical switches
Traditional mechanical buttons, which are operated by actual touch, are robust components, but germs or impurities can easily settle on mechanical and moving parts. Moreover, they cannot be installed flush in plants – an additional gateway for dust and the like. A resistant and hygienic alternative is provided by capacitive buttons.
They can be integrated flush into any system and have no mechanical elements where dirt particles or germs can settle. Thus, sealed, smooth and easy-to-clean user interfaces are possible, preventing the ingress of moisture or dust.
Sealed keyboards
Elastomer keyboards offer a high-quality, tactile feel with an underlying switch layer. They are characterised by high weather resistance in harsh environments, and moisture and chemical resistance. Elastomer keyboards are used when three-dimensional buttons are needed in environments with high moisture, such as in maritime or defence applications.
In the food, laboratory and medical sectors, the membrane keyboard plays an important role due to its closed surface, resistance to chemicals and its imperviousness to liquids and dust. The classic membrane keyboard consists of about eight layers, of which only the top layer, the “front foil,” is visible in the later application. The actual switching element, the snap disc, is placed between the different layers and is thus well protected against dust and moisture. Integrated acrylic inlays also prevent deformation of the snap disc.
Easy-to-clean touch panels
In harsh application areas, touch panels should be equipped with scratch-resistant and shock-insensitive surfaces and components. They must withstand high stresses from jet water, steam and dirt, and be sealed against harmful gases. The entire surface of the input system must be resistant to oil, solvents and aggressive chemicals. Touch panels should meet the IP67 protection class and have a continuous, closed front structure without dirt edges. This facilitates cleaning, which is of great importance in the food industry, for instance. The robustness against mechanical stresses can be ensured by choosing hardened glasses. In conjunction with a laminated composite glass pane on the back, for example, vandal-proof touch panels for public applications can be realised. Capacitive PCAP touch technology meets both industrial and medical requirements. It is resistant to water and saltwater, electromagnetic radiation and chemicals. Moreover, it can also be operated with gloves and can be equipped with antimicrobial surfaces.
Antimicrobial coatings
With antimicrobial coatings, HMIs are also equipped for particularly high hygiene requirements. Generally, a distinction is made between active and passive materials. Passive coatings prevent microbial colonisation through the surface structure alone. The so-called Lotus effect inhibits the adhesion of microorganisms to the material surface. In contrast, active antimicrobial materials contain biocidal components that attack microorganisms at the cell wall, in metabolism or in the genome. To meet high hygiene standards, it is recommended to provide switch handles and user interface for touch applications with coatings with Log reduction values of 5.25 (disinfection) to 6.05 (sterilisation).
The inside matters too
Vibrations, extreme temperatures and chemicals affect not only the surface of a Human Machine Interface. HMI assemblies generally consist of a multitude of layers or components. Adhesives and sealants ensure that thermal and electrical contacts remain consistently connected, possible damage from overheating is prevented and HMIs are protected from short circuits, contamination ingress or mechanical failure due to vibrations. For example, display layer bonds must have adequate peel adhesion, shear strength and impact resistance under harsh environmental conditions. Furthermore, vibration and shock absorbers and seals are indispensable components of HMI design. Thin and lightweight foams absorb vibrations and shocks very effectively and are flexible enough to fit into predefined housing dimensions and consider temperature change swelling and contractions.
All these materials must not lose their function in harsh conditions, for example by contracting, drying out or outgassing at extreme temperatures. Accordingly, careful attention should be paid to the materials for adhesives, seals and co. when designing an HMI for harsh environmental conditions.
