Specifying HMI Enclosures: Mounting, Sealing and Operator Ergonomics

An HMI is the one piece of industrial equipment that an operator touches hundreds of times a day, so its specification has an ergonomic half and an environmental half. Both are usually written late, and both are decided in the enclosure. This guide sets out the questions to answer before the housing is drawn.

Start with the Operator, Not the Box

An HMI that is hard to use will be used incorrectly, and that is a safety issue rather than a comfort one.

Choosing the Mounting System

• Panel-mounted. Simplest and cheapest; the cut-out, the gasket and the rear clamps are the whole interface. Best when the position is fixed and the panel is at the right height.
• Wall or bracket-mounted. A complete housing fixed to a structure; suits positions the panel cannot reach.
• Pedestal-mounted. Rigid and suitable for a standing workstation, with the cable run inside the column.
• Arm-mounted. Allows the screen to be brought to the work. Needs a stiff joint and a light housing.
• Overhead or ceiling-mounted. Used where floor space is scarce; needs a defined cable route and a service method.

Sealing Classes and What They Cover

• Specify the class for the installed assembly, not for the housing alone.
• Specify the front and the rear separately where only one face is exposed.
• Remember that a class obtained in a laboratory is obtained with the entries made good; a gland fitted on site can lose it.
• Use a continuous gasket and enough fixings for even compression.
• Where wash-down applies, design for a directed jet and for the chemicals used.
• Where outdoor exposure applies, add UV and temperature to the gasket specification.

Viewing Angle, Height and Reach

• Set the screen centre so the operator reads it without bending the neck.
• Tilt the face towards the normal eye position, and avoid a surface that reflects overhead lighting.
• Keep the reach envelope in mind: controls that are frequently used should be within easy reach, and emergency controls should be reachable without stepping.
• Where several operators use the station, provide adjustment or agree a compromise height.
• Allow for the fact that a screen in direct sunlight is often unreadable regardless of its specification, and consider a hood or a different position.

Connector and Service Access

• Provide connector cut-outs with room behind them for the plug and the cable bend.
• Provide a strain relief at the entry, so the connector is not the load path.
• Make the rear of the housing accessible or the housing removable for service.
• Allow for USB, Ethernet and power as a minimum, and add the site-specific connections.
• Keep the service fasteners reachable once the housing is mounted.

Cleaning, Wash-Down and Hygiene

• Specify the cleaning method: wipe, spray, hose or pressure jet.
• Choose a gasket material compatible with the cleaning chemicals.
• Avoid crevices and recesses that trap material in hygienic applications.
• Use stainless where corrosion from cleaning agents is a risk.
• Provide a smooth, cleanable surface, and avoid exposed threads where hygiene matters.

Environment, Vibration and EMC

• State the ambient temperature range, including the effect of direct sun behind glass.
• State the vibration environment, because it affects the mount more than the housing.
• State any EMC requirement, since a metal housing is part of the screening and its joints must stay conductive.
• Consider impact where the HMI is in a trafficked area, and specify the housing accordingly.
• Where the HMI sits near coolant or swarf, protect the front face and the connectors.

What to Put in the Brief

1. Operator position, standing or seated, and the working height. 2. Mounting method and the mount's interface drawing. 3. Protection class for the front and the rear. 4. Cleaning method and chemicals. 5. Display drawing, connector list and depth behind the face. 6. Environment: temperature, vibration, impact, EMC. 7. Finish, colour and identification.

Our Approach to HMI Specifications

• We fabricate the housing, the bezel, the mounting bracket and the cut-outs to the component drawings and the mount's interface.
• We keep the sealing face flat and the gasket line continuous, and we fit the fixings needed for even compression.
• We will say when a proposed mounting position or a requested sealing class does not fit the housing being discussed.
• We do not supply the display, the computer, the mounting arm or the seals, and we do not certify the machine's safety functions.
• Every quotation states which operations are completed in our plant.

Send the operator position, the display drawing and the environment, and the housing can be specified around the person who will actually use it.

Frequently Asked Questions