Managing electromagnetic interference

Stray signals from the increasing number of electronic components in cars and trucks can cause hard-to-identify, transient errors. Interference specialists at Orbel Corp. work to prevent such gremlins.

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September 22, 2015

You’re not crazy. Sometimes your car really does something crazy, and no matter how many times you take it to the mechanic, he’s not going to be able to identify the cause or even replicate the error. And such problems are only expected to get worse in the future.

The culprit, according to Orbel Corp. product specialist Ed Nakuchi, is electromagnetic interference (EMI), stray electrical or magnetic signals generation by electronic devices and electric motors. Such signals can disrupt the operations of other electronic components, causing random errors that may never occur again.

“In a lot of situations, you can have an issue where because it’s something that doesn’t leave an obvious mark, it can be there one instant and then gone on another,” Nakuchi says. “A lot of it’s based on ambient conditions – humidity, temperature, what devices are running. It is one of those things where the majority of time, you don’t know if it’s there or not there. Electrical noise, if it comes in and causes an issue, it’s there in the instance while it’s happening, but it can totally disappear, and you’ll never find it.”

Orbel President Ken Marino says automotive manufacturers and their suppliers have been approaching his company earlier in the design process, hoping to find ways of isolating EMI and shielding components from interference. Two trends are pushing the need to better understand EMI – the general increase in wireless noise in cars from smart phones, tablets, and other devices and the use of electronic controls for systems such as brakes and engines.

“With the brakes and throttle, they may not have any shielding issues at all, but it’s a braking system, so if there’s any chance of it being influenced by an outside EMI, designers have to consider things very carefully,” Marino says. “They have to protect their systems from everyone else’s systems.”
 

Noisier environment

Even ignoring new systems, such as General Motors’ in-car WiFi networks, cars have become extremely noisy, electromagnetically speaking. The modern car has significantly more electric motors than vehicles from a few years ago with manually cranked windows a relic of the past, and powertrain engineers replacing mechanically driven steering and braking systems with electric ones. Each of those motors can generate a magnetic field that could interfere with sensitive electronics.

“Everybody’s using WiFi, Bluetooth, and satellite communications. Everything is becoming wireless, so the problem is increasing exponentially,” Nakuchi says. “And it doesn’t take much EMI to throw a system off. Satellite communications for radio or GPS, you’re down to -120dBm which is very sensitive.”

Nakuchi’s suggested solution is shielding – typically putting a conductive metal cage around the most sensitive parts of systems that either generate EMI or are susceptible to it. He adds that shielding can be extremely effective and inexpensive, but designers need to consider shielding needs early in the design process. If they don’t solutions can get expensive very quickly.
 

Designing in EMI protection

Probably the biggest challenge in designing shielding into a component is knowing when it’s necessary, Nakuchi says. Because interfering signals can be so small, it’s not always clear which components need protection and which ones don’t.

“You can have a set of components that are on the low side of the probability curve for noise generation, but the next batch of the same parts can be on the high side,” Nakuchi says. “It’s not stable. It can vary based on components and conditions. Everything’s a probability. There’s a curve of how much noise these things can generate. Sometimes you get the right conditions to where you build the thing, and it’s working fine. Then later, for no apparent reason, it becomes susceptible to EMI.”

When that occurs, shielding suppliers can come in with cages to protect the device that’s failing. If designers kept EMI control in mind when crafting the component, Marino says fixes can be easy. If EMI was an afterthought, shielding becomes a challenge.

“When they’re designing these printed circuit boards (PCBs), we need so much room for our shields. The smaller you get the PCBs, the higher the density of parts, and we have to create a crazy-looking shield that costs a fortune,” Marino says.

He adds that simply designing such a shield is costly, but the process gets more expensive as producers figure out how to install the device and go into full production. Since EMI problems are often discovered late in the process, entire manufacturing lines can be idled with designers try to solve shielding problems.
 

Design challenges

Leaving a little bit of space, especially around the microprocessors that are most susceptible to interference, improves the chances of being able to use an inexpensive, easy-to-install shield, Marino explains.

However, leaving a little bit of extra space is easier said than done. Automotive designers are looking for any opening to shave weight in vehicles to improve fuel economy. No single PCB is particularly heavy, but if you make each one 10% larger to leave room for shielding that may or may not be needed, the pounds can add up.

Heat is also a concern. Tightly-packed PCBs, covered with capacitors and processors, can overheat, leading to component failure, so for thermal control, designers favor open designs with lots of venting.

“Higher power and faster speeds generate more thermal energy, so cooling becomes an issue,” Nakuchi explains. “Board-level shields tend to be vented, so it’s a balancing act between the thermal issues and EMI challenges. It’s a very thin line between balancing all of the challenges.”

Changing vehicle materials could also make shielding designs more difficult. The steel sheet metal in most conventional cars does a pretty good job of shielding internal electronics from outside noise, leaving engineers only the challenge of isolating signals inside the vehicle. That could change as automakers turn to carbon fiber reinforced plastics and other non-metallic materials for vehicle bodies. In addition, the lightest vehicles – the ones with the least metal shielding – tend to be hybrids or all-electric cars. Marino notes that electric drivetrain components can generate very large magnetic fields. So designers have more EMI to deal with and less natural shielding to contain it.

Marino adds that because the design challenges are so complex, the earlier that specialized companies can enter the design process, the more likely it is that manufacturers will find cost-effective solutions.

“It’s a hard conversation on the back end, and things are very expensive,” Marino says. “We had one client working on a brake system. They were convinced that they had no problems, but they were also convinced that they could have problems from other systems. That program started two years ago, when people were just starting to think about what components to use. It was a lot easier to talk about shielding solutions at that stage.”

 

Orbel Corp.
www.orbel.com

 

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.