Non-touchscreen Human Machine Interfaces (HMIs) including membrane keypads, pushbutton stations, and emergency stop devices are widely used in harsh industrial environments as they offer tactile feedback and greater reliability in extreme operating conditions. However, exposure to extreme environmental conditions can cause the degradation of materials that may lead to failures affecting machine availability and safety. As these failures are due to material decisions rather than electronic failures, design decisions are extremely important in prolonging service life and ensuring the products can perform under these extreme conditions.
Extreme Temperatures
Both extreme high and low temperatures can cause problems for certain materials. In high temperatures, systems may experience the softening of plastics and premature ageing of membranes whilst extremely low temperatures may bring the challenge of brittle fractures of plastics and reduced membrane flexibility of polymers and sealing materials.
Moisture and Water Ingress
Moisture and water ingress represent the most common cause of HMI failures. Whilst Polyester membrane keypads can achieve high IP ratings, long-term exposure can create risks, such as the delamination of graphic overlays, water ingress through damaged overlays, and degradation of adhesive layers. Traditional mechanical switches operating in humid conditions can experience contact corrosion.
Chemical Resistance
Industrial environments often expose HMIs to oils, solvents, hydraulic fluids, acids and alkalis. Membrane keypads are widely used in these conditions as they provide a low profile and are cost effective but prolonged exposure to these substances can attack the polymers present in many keypads and push buttons, resulting in discolouration, stress cracking and loss of mechanical strength among other failures.
UV Exposure and Weathering
HMI interfaces in exposed industrial environments experience ultraviolet (UV) radiation and weathering which may significantly reduce service life. UV degradation is often gradual and may go unnoticed until there are signs of failure including loss of mechanical strength or discolouration. In many cases of membrane keypads, the overlay graphics and adhesive systems degrade before the switch technology itself reaches the end of its operational life.
Corrosion and Salt Exposure
Salt exposure presents one of the most aggressive environmental challenges for HMI systems because it not only affects external materials but can also accelerate degradation of internal electrical and mechanical components. Salt exposure can also create both chemical and electrical failure mechanisms, making material selection particularly important for long-term reliability.
Dust and Particulate Contamination
Dust and Particulate contamination present a significant challenge for HMI systems operating in industries such as mining, construction and agriculture. Dust causes gradual degradation and can lead to mechanical or electrical failure. The contaminants can penetrate poorly sealed interfaces, accumulate within mechanical assemblies and accelerate wear mechanisms throughout the keypads service life.
Solutions
Extreme environments present challenges for HMIs and Diamond HMI can support in choosing the right materials in designs. Changing the materials used is the most effective way to overcome many challenges. Silicone rubber keypads are a common solution across extreme conditions as their silicon-oxygen molecular structure provides excellent resistance to degradation, offering better thermal stability, better sealing performance and greater flexibility. Another common design solution is the use of Piezo switching technology, particularly with added stainless steel front panels. Piezo switching technology is a popular HMI design solution as they have a fully sealed metal operating surface which combats UV exposure, dust exposure, and water ingress. Equally, by Piezo technology having no moving mechanical contacts, many corrosion related failures can be eliminated.
Diamond HMI works with our customers to ensure help ensure designs can remain robust within extreme environments and are experienced in providing HMI systems specifically designed for harsh environments.