This article is part of the TechXchange: Delving into EMI, EMC and Noise
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.
SAE J1113/28 notes that vehicle electrical/electronic systems may be affected when immersed in an electromagnetic field generated by sources such as radio and TV broadcast stations, radar and communication sites, mobile transmitters, cellular phones, and so forth. Other SAE standards for evaluating EMC include:
- Performance Levels and Methods of Measurement of Electromagnetic Compatibility of Vehicles, Boats (up to 15 m), and Machines (16.6 Hz to 18 GHz), J551/1 and
- Function Performance Status Classification for EMC Immunity Testing, J1812
The SAE is certainly not the only organization involved in standards for automotive applications. The Automotive Electronics Council (AEC) Component Technical Committee also has several documents that detail requirements for ESD, stress and IC latch-up testing. Also, the Institute of Electrical and Electronic Engineers (IEEE) EMC Society has ten technical committees that deal with EMC from management, measurement and environment aspects to electromagnetic interference control and high power electromagnetics.
As part of the board of directors of the IEEE EMC Society, Kimball Williams says the EMC Society makes a very strong effort in its standards committees to provide liaison and communication efforts with other technical organizations that are dealing with electronics and potentially with EMC in electronics. Kimball is the EMC Society’s liaison for SAE and CISPR/A (International Electrotechnical Commission) from the radio interference side. [See sidebar: Relating National and International Standards.]
Component and Vehicle Level Testing for EMI/EMC
With all of the standards for EMC, getting started can be daunting task. Jastech EMC Consulting, LLC brings a systems engineering approach to EMC. James Muccioli, EMC consultant at Jastech, says,“The first thing you’re going to do is bound your system.” Once the limits of how far to test are established, the requirements need to be researched.
”Each car company typically has vehicle level tests that they run. Then there are system level requirements, module level requirements, component and even down to the IC level requirements for EMC,” says Muccioli. EMC is like any other feature explains Muccioli. If you expect good EMC performance, it needs to be taken into account up front in the design process. “Once you understand what your environment is from both immunity and an emission standpoint, then you can start designing your electronics module or your system around that,” he says.
Muccioli cautions that some types of logic react differently than other types. In addition, EMC considerations impact components, software, and layout including wiring and packaging and more. EMC validation at the earliest prototype level may avoid problems later.
When all the systems are assembled into the vehicle, EMI/EMC chambers for testing complete vehicles are quite common. In fact, General Motors has four large anechoic chambers that are a hundred feet on a side. “They can put an 18-wheel vehicle in there and run it on the rolls,” says Williams.
Analysis Tools
Identifying potential EMC problems should be part of the system analysis performed on the design. One frequently used technique is Failure Mode and Effects Analysis (FMEA). The FMEA is such a common automotive tool that there is even an SAE spec for it. SAE has J1739 for Potential Failure Mode and Effects Analysis in Design (Design FMEA), Potential Failure Mode and Effects Analysis in Manufacturing and Assembly Processes (Process FMEA) and even one for non-automotive applications, ARP5580. Automotive suppliers also use the Automotive Industry Action Group (AIAG) FMEA methodologies.
A design FMEA analyzes the impact on the system design to determine how potential failure modes affect the system operation. This analysis is used to prioritize failure modes based on the risk to system operation, safety and/or customer satisfaction. Both software and hardware are considered in the FMEA that can be performed at any level from the component up to the complete system.
“Nowadays you link that [the FMEA] with design of experiments,” explains Bogden. With an initial FMEA, design of experiments helps organize the testing approach including analyzing a particular area for focus. Other well-known and frequently used analysis tools include Taguchi Method, Design for Six Sigma, Reliability Block Diagrams (RBDs) and Fault Tree Analysis (FTA).
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Going to wide open throttle is a failure mode that is identified in failure analysis. The failure can be detected in software and there would be different layers to it he says. With the system recognizing the wide open throttle condition and the actuator not responding to a command to go back to closed throttle, different steps can be taken. First of all, the power to the device can be turned off through the software. Then mechanically the unit would return to a predetermined position. However, if the unit is jammed and can’t move, the next step is to cut the fuel to the engine -- also through the software. This will definitely cause the vehicle to stop. “It’s better to stop than going wide open throttle until you hit something,” says Bogden. Beside a strategy for the control system, the driver should also almost instinctively know what to do to stop his/her vehicle. Unfortunately, most drivers are not prepared for this situation.
The Impact of EMI on Sensors, Microcontrollers and Other Circuitry
The car companies do a lot of work to characterize their environment. Component level, module level and system level tests are done at a higher level than a vehicle would experience and those tests overlap. “The component and system level tests are made for developmental tests,” says Muccioli. “The vehicle test is just to validate.” Multiple tests covering the same fields and overlapping environments produce a high confidence level in low EMC at the vehicle level.
Muccioli has performed testing to establish the correlation of EMC problems and solutions. “We got correlation from an emission standpoint from the chip level, to the module level and ultimately when you put the module into the vehicle showing that if you have a noisy microprocessor you can see it all the way to the vehicle level,” he says. “Also, if you quiet it down, you see the quiet down to the vehicle level.”
EMC testing bombards components with rather strong direct radiation across the spectrum – stronger than what the component would experience when it is mounted in a vehicle says Bogden. “When the semiconductor guys test their parts they can get them to misbehave but they are applying such a strong field directly to the part, it’s probably not what I would consider realistic in the real world,” he says. This level of testing discovers the device’s level of immunity and ensures that it is sufficiently robust.
Changes are occurring in advanced semiconductor technology that impact EMC. The IEEE EMC Society cites a number of hardware trends that can cause increased EMC problems at the device level including:
- The ongoing miniaturization of devices, resulting in smaller gaps in their geometries;
- Lower power consumption, with smaller voltage swings or quieter signals inside the chips;
- Higher clock speeds that are well above 3GHz; and
- Wider bandwidth ports, including WiFi and microwave.
At the same time the society has indentified electromagnetic compatibility trends that include:
- An increasing number of radiators,
- An increasing number of receptors,
- Increasing susceptibility of those receptors,
- Increasing demand for communications, particularly wireless
These trends have increased the need for EMC and those engineers who specialize in this field.
Establishing Expertise
Williams has a recommendation for companies without an internal background for EMI/EMC that have recently identified a need for this expertise. “As in any organization, the number one thing they want to do is hire good people to start with and then support them to be active in participating in their relevant professional community and working with the relevant standards community. If they do that, they are probably going to be well guided in their work,” he says.
Engaging an experienced consulting service such as Jastech EMC Consulting is another way to get expertise immediately and on a periodic basis. The fulltime experts can get a design started so that subsequent EMC problems are minimal. “EMC is not a black magic,” concludes Muccioli. “You have got to follow a process and if you follow a process, you should be able to catch a majority, if not all of the anomalies.”
Relating National and International Standards
The process for establishing national and international standards is very straightforward according to Kimball Williams, past president IEEE EMC Society and currently on the board of directors. “Unfortunately, most people don’t even know it exists,” he laments. Companies frequently develop internal standards for their products because they need to confirm that the product will survive in the real world. Subsequently, the company will contribute to standards at the national committee level. In the automotive industry, that means joining one of the committees under the SAE, since the SAE is the arm of the American National Standards Institute (ANSI) which has been chartered for following and documenting automotive standards for the U.S.
The national committees worldwide join together and contribute to the international standards – all meet together at either the International Electrotechnical Commission (IEC) or the International Standards Organization (ISO). “Right now, for EMC work, the IEC is handling all of the emissions and ISO is handling all of the immunity,” says Williams. The national committees contribute and provide the experts for the international standards development organization and achieve consensus at the international level. Any company that wants to use a national or international standard simply obtains the standard -- some are free, others have a nominal fee and some are a bit more expensive. A company can chose to implement either the national or international standard. “Most contemporary companies that have products that cross borders are tending to move toward the international standards,” says Williams.
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