The recent Toyota recall has brought a lot of attention to electromagnetic compatibility (EMC) and electromagnetic interference (EMI). This is a topic that many people know little about except those automotive experts that deal with it on a daily basis.  Since EMC impacts every vehicle electronic system, Auto Electronics contacted some knowledgeable experts to shed a little light on EMI/EMC. It’s been there long before the first digital electronics and the first microprocessors were introduced to vehicles in the 1970s.

Understanding the Impact of EMI

Dennis Bogden, retired director of Powertrain Electronics Engineering at General Motors, has had extensive experience in dealing with critical vehicle electronics systems.  Currently owner and chief engineer at Robotronics SP, a consulting firm specializing in the design of embedded electronic products, he still has to deal with EMC. Bogden points out that there are five different EMC domains: radiated immunity, radiated emissions, conducted immunity, conducted emissions and electrostatic discharge (ESD).

Wiring is part of the consideration for both conducted and radiated emissions. As a result, EMC testing done at a component level does not provide a complete picture. “You can find a component that works fine in its own component testing and then you hook up the antennas and the conduction to it and all of a sudden it has a problem,” says Bogden.  Worst case, shielded wiring may be required, but twisted pairs or just keeping certain wires away from each other may be the solution to a direct influence of EMC.

Software is an important consideration for some EMC problems. From an immunity point of view, the software is not impacted, but the right software can reduce EMI. A switch provides a simple example. With a switch, switch bounce and arcing can occur. A software routine that monitors the switch bounce and provides adequate settling time before making a reading is a common solution.

Microprocessor clock frequencies and software loops can pose unique problems that are normally easy to detect. “First of all, you have to select the microprocessor crystal frequency so that it doesn’t lay on top of other radio frequency bands,” says Bogden.  Software executing periodic timing loops can create problems, too. “You may find out that you are generating a periodic frequency that happens to create a problem someplace else,” says Bogden. “This is very rare, but I have run into it in the past,” he quickly adds. Changing the timing loop slightly is an easily implemented fix.  

Certain control techniques are known to produce problems. For example, pulse width modulation (PWM) control for a motor or solenoid. The PWM technique tends to generate both conducted and radiated waveforms. “If you run them at the right frequency, sometimes you can have a conducted path coming back through the wiring and that can affect some other device,” says Bogden. Alternatively, radiated EMI can occur as well. In either case, the solution is adjusting the frequency of the PWM signal in software to get outside of the problem area.

A Which Hunt

A product introduced without adequate EMC testing can cause a “which hunt” when customers complain. Which frequency or frequencies are causing the field problem?  Fortunately, there are several organizations and sufficient standards and documentation to prevent most problems before the customer is involved.

SAE’s Electromagnetic Compatibility (EMC) Standards committee has developed and periodically updates several EMC standards. “Most companies have their own EMC test procedures and limits, which tend to exceed these standards, especially for safety critical systems,” says Bogden.  SAE J1113 is an SAE Recommended Practice that establishes uniform laboratory measurement techniques for determining the susceptibility of electrical, electronic, and electromechanical ground-vehicle components to undesired electromagnetic sources. As shown in Table 1, the document has had numerous additions and revisions since it was first established in 1987. The most recently revised J1113/11 was approved on 6/25/07, but others are works in progress or being updated in an ongoing process.

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