Engineers interact with human-machine interfaces (HMI) every day. For example, the screen and keyboard of a programmable logic controller (PLC) is an HMI.

Valuable real-estate on the face of the controller is taken up by the screen itself, the key pad and graphics to illustrate what the keys do.

To design an HMI without any wasted space, and with enough space to be used efficiently, every part of the product needs be designed while keeping the others in mind.

Does your key assembly have enough space to vent air inside the housing? Are you using the best adhesives for the materials in your assembly? Are you efficiently using real estate or are you cramming everything together, as if you’re attempting to create the tiniest PLC ever?

All of these considerations, and many more, are vital to the creation of an efficient HMI.

Epec Engineered Technologies is one of many companies who serve Tier Ones and OEMs across North America for the production of HMI assemblies. To help their customers, the company is sharing some of their top design considerations for developing the right design the first time.

“There are a lot of tricks you can do to help eliminate not only material costs, but also design costs and assembly labor costs,” said James Long, Director of Engineering at Epec. “For example, if you have two circuit boards with three connectors and bring them down to one board with one connector, you save a lot of money and your assembly process is easier as well.”

#1 – Overall Height of Assembly

Slight miscalculations in the height of an assembly may not leave enough venting space for keys, or will leave too much. Both scenarios will result in key failure either due to debris entering the assembly, or the jamming of keys due to lack of sufficient space.

To avoid issues like these, Long recommends avoiding developing an HMI’s housing first.

“Some customers come to us needing a ballpark on how thick a key assembly needs to be after they’ve already created a pocket either in a metal or plastic enclosure,” Long said. The lesson to be learned here is not to jump the gun on your design. Plan everything ahead.

Epec recommends membrane switch thickness, without embossments, to typically be about 0.030” to 0.035”.

Non-embossed key assembly thickness, side view. (Image courtesy Epec Engineered Technologies.)

With embossments, this increases to 0.042” to 0.045”.

Embossed key assembly thickness, side view. (Image courtesy Epec Engineered Technologies.)

The space allowed in an efficient key assembly design is also important to the supporting subpanel layer, which should be the same size as the key membranes in both height and width.

“We can do the subpanel in several different ways,” Long said. “We can bring two technologies together and vertically integrate the assembly with a rigid PCB board where the front side is a standard membrane style switch, where we can use backside real estate for supporting electronics like a micro-controller or mounting a display, so you can get a dual purpose out of that functionality.”

It’s important to ensure that all subpanel cut-outs be 0.030” larger than the membrane, unless specifically used in the mounting of hardware. Subpanel cut-outs should also be 0.060” larger than the window.

Graphic overlay specifications. (Image courtesy Epec Engineered Technologies.)

These guidelines help ensure that the subpanel is not visible after assembly.

#2 – Adhesive Selection

Understanding what materials to use in the overall HMI assembly will affect what adhesives benefit your design the most.

However, adhesives are not as simple as many may think. Low surface energy (LSE) and high surface energy (HSE) adhesives are unique in their reaction to various materials.

LSE adhesives are designed to bond to hard-to-stick surfaces including:

Plastics
Polypropylene
Polyethylene
Polystyrene
Thermoplastic polyolefin
EVA
PTFE
Powder coated paints
Oily metal surfaces

“A lot of times, people want to mount things to something like a pelican enclosure, a polypropylene exotic ABS type of material,” Long said. “The problem with that is, you can’t take a LSE adhesive and attach it to a pelican case because it will pretty much just come right off. That’s where you go into the more exotic HSE types of adhesives.”

HSE adhesives bond best to substrates including:

Steel
Aluminum
Polycarbonate
Polyimide
Polyester
PVC
Acrylic
Rigid polyurethane

HSE and LSE adhesives react to surfaces differently. (Image courtesy Epec Engineered Technologies.)

Epec recommends 3M’s 300LSE and 4952 VHB adhesives for LSE surfaces, and 3M’s 467MP and 468MP adhesives for HSE surfaces.

#3 – Switch Venting

Believe it or not, the ability for keys to vent air is a highly critical point of designing any HMI. Key pads are hermetically sealed, but there needs to be enough room within the assembly for air to slosh around.

Not allowing enough space for keys to properly vent or allowing too much space can make or break a key assembly.

“You don’t ever want to vent outside the assembly, because then you essentially have a vacuum cleaner,” Long explained. “When you’re pushing that button, it will suck dirt and dust into those venting channels and that’ll get into the key itself and cause key failure.”

Conversely, a lack of venting space can cause key failure due to loss of tactile feel.

“The venting gets really tricky, especially in smaller assemblies because you have nowhere to go with that vent,” Long added.

HMI switch vent channels. (Image courtesy Epec Engineered Technologies.)

#4 – Tactile Operator Feedback

On the topic of keys, defining tactile operator feedback can at first seem difficult, but it all comes down to the sensation of pressing the keys on an HMI’s key assembly.

Users register the effect of their input into a keypad as a sensation in the pressing of a button. This sensation comes from a degree of resistance in the key and a snapping or clicking sound or feeling in its depression.

Epec describes this as the snap action of a momentary metal snap dome in a tactile membrane switch. All of this is done without any mechanical parts.

Tactile feedback is determined by design factors including:

Material type
Key travel
Trip force
Diameter
Life cycles
Key shape (see illustration below)

Common examples of metal snap domes. (Image courtesy Epec Engineered Technologies.)

Long and his team at Epec first establish customer preferences for key designs.

“We send physical samples that we’ve built before, which we think will match what a customer is trying to do. I’ll usually send three different samples: one with low tactile force, one with medium and one with high. They’ll say what they like and we can continue the design.”

#5 – Rigid Printed Circuit Board (PCBs)

When designing a flexible HMI, polyester sheets may be at risk due to their inability to handle high soldering temperatures. Epec suggests replacing the polyester layer with a rigid PCB to solder components directly and eliminate intermittent or open circuits caused by external stresses.

Electronic components including LEDs, resistors, capacitors and more can be mounted to the rear side of a printed circuit.

Keypad interconnections to the application’s electronics are also expanded as connectors can be soldered on the rear surface of the PCB through cut-outs in the metal support plate.

HMI design with rear side of rigid PCB for added electronics components. (Image courtesy Epec Engineered Technologies.)

These are only some of the many application benefits of using a rigid PCB rather than a polyester layer.

“The rigid PCB design also gives us flexibility in regards to pitch and the type of components that can be added,” Long said. “On a standard polyester membrane we can only do them one mm pitch on any lead length, whereas on a printed and flex, we can do very fine pitch components and/or connectors.”

Lead Times for Designing a Human-Machine Interface

To optimize lead time for designing any HMI, it’s important to approach your supplier as early in the design process as possible.

If a Tier One has already developed half of the product, the supplier’s job is only made harder by having to design with limitations that didn’t consider all of the design variables inherent to an HMI’s electronics and interior assemblies.

Customers who may not have considered a more paced design schedule have created nightmare design scenarios for Long.

“If we go back to a rigid PCB and added real estate at the back of the board, a lot of times they’ll add secondary boards internally in these assemblies,” Long explained. “They’ll be run out of space and they don’t realize they can move all of those components to the back side of the user interface. This could have saved another board assembly and let me meet that overall envelope they’re trying to meet.”

Although approaching a supplier as earlier as possible is key, customers shouldn’t approach companies like Epec without any data, either. Know your goals, create a plan and then get a supplier on your team.

The above design tips and tricks are only a few aspects of what Epec recommends to keep in mind when designing an HMI. To learn more about Epec’s top 10 HMI design considerations, check out their ebook, Human-Machine Interface Design Considerations.

Epec Engineered Technologies has sponsored this post. It had no editorial input into this post. All opinions are mine. --Kagan Pittman