The Human Machine Interface (HMI) encompasses all the elements a person will touch, see, hear or use to interact with a machine. The task of the HMI developer is to build innovative, intuitive, and reliable interfaces that can withstand a variety of environments and machinery applications. A successful build requires an expert understanding of a complex design process. Human factors along with technical and commercial considerations must be addressed. This article provides an expert viewpoint on front panel materials and surface finishes used in HMI construction and reveals some of the key considerations to make the proper selection.
Front panels are typically fastened within a protective enclosure and then connected to the host system. (In smaller devices, they are often mounted directly in the equipment.) They must provide full protection and a controlled internal environment for electronic assemblies. In nearly all cases, it’s essential to develop a custom-made housing that perfectly encloses the assembly.
Factors that affect the choice of enclosure material and design include: environmental (exposure to water, solids, ingress, pollution, and cleaning agents; robustness (resistance to shock, impact, and vibration); and regulatory (material behavior when exposed to flames, smoke, and high temperatures).
Shock and vibration are additional causes of concern and can vary enormously depending on the situation. Consider a static machine compared to an off-road vehicle, or military equipment in the vicinity of an explosion. Environmental testing is therefore critical before selecting an enclosure. Vibration mounts and shock isolators are commonly used in HMI construction. Vibration mounts such as rubber washers dampen high-frequency, low-magnitude vibration, whereas shock isolators like cable mounts counteract high-magnitude, low-frequency motion. Many applications also demand internal isolators to provide shock absorption for equipment on the interior of an enclosure.
For protection against adverse interference or radiation, measures often include sprays (acrylic-based paints are used to create conductive, static-free surfaces with excellent characteristics), coated surfaces, which reduce electromagnetic or radio frequency interference (EMI/RFI), and copper foil, which protects against electromagnetic pulses.
While most enclosures are more than adequately strong, fatigue failure may be an issue with elements such as fastenings, hinges, and other elements that create an integrated structure from a set of components.
Proper fastening component selection is necessary to prevent major problems. PEM studs are widely used for mounting the front control panel assembly and custom-mounting hardware. They are “self-clinching,” which means the fastener cannot rotate in the material once it has been properly inserted. Once locked into place, they become non-movable and thus a permanent part of the host material. These studs are capable of strong, load-bearing threads in relatively thin sheets of metal.
Environmental Protection Standards
A system is only as strong as the weakest link. Therefore, it’s important the complete HMI construction is tested for environmental protection and not just the individual components. The International/Ingress Protection, or IP rating system, is widely used throughout Europe to indicate the level of protection against the intrusion of solids and liquids. The International Electrical Commission (IEC) directive 60529 implements the standard. In an IP number, the first digit represents protection against solids, and the second represents protection against liquids at various pressures.
Where water, fuel, cleaning solutions, fine dust, and other materials may come in contact with HMI control panels, the following IP numbers apply:
- IP 40: Protection against objects greater than 1 mm diameter (wires, tools, etc.) but not water.
- IP 60: Protection against dust but not water.
- IP 65: Protection against dust, water, and liquid jets.
- IP 67: Protection against dust and temporary water and liquid immersion.
In the U.S. and Canada, the National Electrical Manufacturers Association (NEMA) sets the standard for environmental sealing. NEMA standards meet or exceed the IP ratings listed, but the reverse cannot be assumed. The IP code only addresses requirements for protection against people and the ingress of solids and water. The NEMA types consider these (and numerous others), including corrosion resistance, effects of icing, gasket aging, and oil resistance.
IK Codes: Impact Protection
The European standard EN 62262 (the equivalent of international standard IEC 62262, 2002) classifies the degrees of protection provided by enclosures against external mechanical impacts. Any enclosure rated IK7 or above should be capable of resisting a free-fall hammer striking the enclosure between 2 to 20 joules.
German standard DIN 40050-9 created an additional rating for environmental sealing: IP69K for high-pressure, high-temperature wash-down applications. Devices must not only be dust tight (IP6X) but also able to withstand high-pressure and steam cleaning.
Popular materials include:
- Aluminum alloy, one of the most popular materials due to its high strength-to-weight ratio, excellent corrosion resistance, easy machining, and good conductivity.
- Stainless steel, which offers higher strength and hardness (even at thin panel thickness) and increased corrosion resistance, cryogenic toughness, and ductility over carbon steels.
- Composites, which are available in a variety of colors and finishes and are highly usable in flat front panels, particularly within machine controls.
- Carbon fiber, which offers an incredibly high strength-to-weight ratio and is commonly quoted as five times the strength of steel but only one third the weight.
- Fiberglass, a polymer composed of a plastic matrix reinforced by fine fibers of glass that is lightweight, extremely strong, and less brittle and expensive than carbon fiber.
- Plastic. While this material is ubiquitous, high-tooling costs make it only economic with larger quantities (typically more than 1,000 pieces), and environmental considerations, such as exposure to impact and harsh use, cleaning agents, UV rays, etc., can limit its applications.
A wide range of surface treatments are possible to decorate, protect, and expand the lifespan of front panel material. This enables manufacturers to customize according to the OEM while displaying and differentiating “their brand.”
Most paints contain resins, pigment reducers, and additives. Resins form a film and hold the pigment in place and determine the paint type. Reducers, including solvents and diluents, are used to control viscosity and volume. Additives are then used to add gloss, hardness, and other characteristics.
Powder coat is a pigment encapsulated in a powdered resin without the need for a solvent. The coating is typically applied electrostatically and then cured and hardened with heat. This treatment is preferred because it has a tough impact and weather-resistant finish, allows for color blending and special bleed effects, coats on thick without running or sagging, shows no difference between horizontal and vertical coated surfaces, and has gloss level capabilities that exceed 15 percent.
Commonly called electro-coating (e-coating), electro-deposition is considered an alternative to powder coating and is a process by which a metal object is submerged in a tank and an electric current is used to deposit paint on both the inside and outside cavities. Parts are then removed from the tank and heat cured. This treatment can cover even the most complex parts of assembled products according to specific performance requirements. Coatings generally have uniform thickness without porosity, and it is considered an environmentally friendly solution with no heavy metals or haps and low VOCs.
Anodizing harnesses the natural oxidizing process by increasing the layer thickness and corrosion resistance. An electrical current is passed from an anode (positive) to a cathode (negative). The object, which serves as the anode, is placed alongside cathodes in an anodizing tank containing dissolved chemicals. A hard layer of aluminum oxide forms on the object’s surface with the thickness being directly proportional to the time spent in the tank.
Mechanical finishing (for metals) commonly involves grinding, polishing, and brushing. These techniques can be used on any metal but are typically performed on aluminum and stainless steel. Abrasives are used when grinding and polishing to remove surface metal and create finishes like satin or mirror-polish. The final finish is determined by abrasive pressure, contact time, material feed rate, and wet or dry techniques.
The choice of materials and finishes will significantly affect the strength, durability, and suitability of the overall Human Machine Interface (HMI). Direct damage to the front panel can leave the entire system vulnerable, resulting in equipment down-time, production stoppages, additional maintenance, redundant labor, and other cost-affecting implications. To avoid these issues, it’s important to work with an HMI expert who can address all the technical and commercial considerations for a complex HMI development project.
Filed Under: Industrial automation