Electromagnetic compatibility (EMC) is a topic that few mechanical engineers get excited about. They point to the electronics engineers when the system fails the legal testing process. However, the theme has so many mechanical aspects that mechanical engineers cannot do without the EMC basics.

What do you do when you want to ensure that the electronics in a housing do not overheat? Right, you make a nice hole in the casing so that the heat can get out. Easy. From a purely mechanical perspective, there is little wrong with that approach. Electronical engineers will be less happy with the solution since there is a huge risk that such a hole transmits all the electromagnetic radiation and the device will no longer pass the compatibility tests that are required by law before an electronic product release.

Marcel van Doorn: “The challenge with EMC is that mechanical and electronical engineers often don’t speak each other’s language.”

Marcel van Doorn, teacher at High Tech Institute and retired at the beginning of this year after a long career at Philips, has often seen things go wrong. “Mechanical engineers never heard anything about electromagnetic compatibility during their education. Radiation from antennas is usually completely new knowledge for them. As a result, they do not realize who their design choices affect electromagnetic compatibility. Electronical engineers have that wisdom, but communication between the two disciplines regularly goes awry. Drawings are thrown over the fence without much explanation and then things are bound to be misunderstood. Regularly, you hear about electronic devices or installations that are disturbed by electromagnetic radiation from nearby mobile phones. Think of robot arms or scooters that tilt, screens that become unreadable, or communication connections that are broken.”

Although EMC is the domain of the electronical engineer, Van Doorn emphasizes that it is also the responsibility of their mechanical counterparts, precisely because many things have to be solved in mechanics. “The challenge is that they often don’t speak each other’s language.” Therefore, Van Doorn has trimmed down the extensive EMC course for electronical engineers to a one-day, hands-on refresher course, which can be followed at the High Tech Institute, especially for mechanical engineers.

 

In harmony

Back to basics, what is electromagnetic compatibility? “It’s a positive word,” says Van Doorn. “After all, it means that devices are compatible with each other, that they continue to function properly in close proximity. That is the goal you pursue. If they are in harmony with each other, one device will not disturb the other. Mobile communications and security services should not be affected by it either, and vice versa.”

“When you used to go to a hospital, you were often asked to switch off your phone,” he continues. “To take no chances, cell phones had to be turned off so that heart monitoring systems, among other things, continued to run normally. Virtually no one did – and does – so now the EMC requirements in the medical world have become much stricter.”

 

No slit, but holes

What exactly is wrong with the aforementioned hole in the electronic housing? “Because of EMC considerations, electronics are often put in a casing,” answers Van Doorn. “This way, you create a Faraday cage from which no electromagnetic field can escape. If you make holes in the housing for cooling or to allow cables to run through, you breach that shielding.” Whether that also poses a problem, depends on the frequencies in the system. “If such a slit is resonant for the wavelength, the radiation simply flies out. It may be difficult to imagine, but then you have created an effective antenna.”

The solution is relatively simple: do not make a slit, but instead go for a series of small holes that together have the same surface area. As a result, the heat can escape to a sufficient extent, but the electromagnetic radiation cannot.

“Once you know it, it’s simple.”

Now that the frequencies in electronics are increasing, from MHz to GHz and higher, the wavelengths are getting smaller and the design correspondingly more challenging. “A frequency of 1 GHz means a wavelength of about thirty centimeters”, Van Doorn calculates. “The rule of thumb is that if you want to reduce the radiation emission level by a factor of ten, the hole in the casing should be no more than one twentieth of the wavelength. In this case, one and a half centimeters. At 10 GHz you already go to 1.5 mm.”

You can apply the same simple calculation to other situations. “An electronic engineer often tells his mechanical colleague that the printed circuit board must be grounded,” says Van Doorn. “In the design, he must then include a connection to the chassis. At frequencies of 1 GHz, that wire again not be longer than one and a half centimeters. So the old-fashioned, robust design has to become more and more refined.”

“In addition, the ground wire and other cabling cannot be everywhere,” warns Van Doorn. “The fields emitted by the electronics board can couple precisely with those cables, which often results in a much more efficient antenna than the traces on the PCB. So position the cable alongside the printed circuit board, and certainly not above it. Once you know it, it’s simple.”

 

'Hear it and you forget it, see it and you remember it, do it and you understand it.'

Dropping pennies

Electronics should tell their mechanics colleagues about things like this, but in practice many development companies lack that communication. The result is that a device does not pass the EMC tests and an expensive redesign is required. The aim of the High Tech Institute training ‘EMC for mechatronic engineers’ is therefore to make mechanics aware of the issues, to teach them the EMC language and to give them a number of simple tools with which to solve EMC problems.

In his training courses, Van Doorn follows the principle of Confucius: “Hear it and you forget it, see it and you remember it, do it and you understand it.” Van Doorn: “Of course I can give an extensive theoretical discourse on all aspects of EMC, but that goes in one ear and out the other. As a teacher, it is important that you make the link between simple theory and practice. During my career I have collected many demos in which all the principles are explained in a simple way. With a spectrum analyzer you can then see, for example, that a large slit emits much more than a pattern of small holes. Because of that very important, practical side, I did not think it was wise to give this course online in corona time. You have to be able to feel it, to get hands-on with the theory.”

Van Doorn encourages students to bring their own product. “During the lessons, we discuss these and in almost all cases there are a lot of points for improvement.” It is really nice if the course is given as in-house training, Van Doorn has experienced. “Then the mechanical and electronical engineers gather around their device and there is plenty of discussion. Suddenly you hear the pennies dropping everywhere.”

Van Doorn notes that there is more and more deliberations between different disciplines. “Through trial and error, companies have become wiser. I do see an improvement there, but things still go wrong very regularly, even between the different sub-disciplines in an electronics department. The major benefit of the course must be that mechanics are aware of the challenges in EMC, that they ask the right questions of their electronics colleagues, and that they close the door before the horse has bolted.”

Mechanics really don’t need to become EMC experts for this, Van Doorn emphasizes. “With a refresher course of one day you can overcome a lot of problems. It doesn’t take much time and it certainly will pay off. So, managers of mechanics departments, send your people and avoid expensive redesigns.”

This article is written by Alexander Pil, tech editor of Mechatronica&Machinebouw.
Marcel van Doorn also teaches the training 'EMC design techniques'