Electromagnetic Interference (EMI)

By R. C. (Bob) Scott, P.E., Inveng LLC

I-ENG-A of Dallas


Electromagnetic Interference (EMI) was once an item of interest and concern for only scientists, electronic/electrical, and telephony engineers.卉 has now evolved into a major topic of concern, discussion, and news coverage for the worldwide populace.䨩nk of the plethora of recent Toyota problems and the coverage of such problems by USA Today, CNN, and other primary news providers. Talk about coming out of the darkness and into the light.嬥ctromagnetic Interference can involve Electro Static Discharge, Radio Frequency Interference, Magnetic Interference, and many types of Power Line perturbations.


In the late 1970s and early 1980s, all product personnel (including myself) had to get interested in EMI.稹?಩marily because the Federal Communication Commission (FCC) limited the amount of radio frequency energy a product could emit, either in the air (radiated) or on the power line (conducted).ᬳo, we were designing more and more products used in an uncontrolled environment.ᴠthe time, we correctly believed we would be able to protect our products from those emissions and off we went, oblivious to the ramifications to come.


For years we had heard the solid state physicists talk about ﬥ flowᮤ other theories that made transistors and integrated circuits work.祠proved those fellows wrong.鴠was 鴴le blue smoke.ɦ you let the 鴴le blue smokeﵴ of the case, the part ceased to work.篲se yet, 鴴le blue smoke鳠hard to catch.祴ting it all back in the part࣡se is virtually impossible (we never succeeded).


As time progressed, through various failures and successes, we learned a great deal about EMI and its effects on computers and related integrated circuits.毲 example:⡴her than dying, as initial computers did, a computerలimary mode of failure now is to hiccup.頣annot tell you how many times a week I must restart my notebook (several in any case) and it has not died yet.请ever, I have thought about expediting the process numerous times.


Modern automobiles are equipped with many computers in their controls (ignition, acceleration, cruising, braking, etc.).ᣴually, the correct reference for these computers is �dded controllersࠂut do not kid yourself - they are simply small computers used to obtain sensor information and take the necessary actions to control things.裂course, it is really important that they do not hiccup - especially in cases where the safety of life or the damage to property is concerned.馠you think about the situation, you certainly would not want a cash register to add $100 to your bill for no apparent reason, just because of a small hiccup.


We product engineers have become quite adept at designing circuits and systems that can be immune to some very high levels of EMI, as well as temperature, humidity and other elements of an environment.䨥re are many techniques we can employ and many tests we can perform to prove a system঵nctionality preventing hazards to life and property.


Even more interesting (an understatement), consider the case of commercial/military aircraft.㥶eral of the modern airplanes on which we all fly utilize what is referred to as 칠by wire.Ԩis term is used when there are a number of embedded controllers communicating with each other acquiring data and controlling the operation of the aircraft.詣cups are simply not allowed.鴒s hard to park on a cloud and fix things.⥳ides, hiccups occurring during flight can be catastrophic.�ern aircraft avoid occurrence of these nasty hiccups by heavily employing the principles of redundancy, voting, and other ᩬ safe䥣hniques.


It is not too difficult to design and build products that operate correctly.⵴ the engineers building and operating such products must constantly keep their thoughts and actions in the pro-active mindset required to prevent and/or correct interference induced failures.


Several years ago, we were contacted by a firm that was experiencing problems with a robotic unit they were producing.䨥 clientࣵstomer required the robotic unit to perform 120,000 cycles without a mistake.ᴠthat time, their unit could do no more than 10,000 cycles.祠suggested a suite of tests for both hardware and software that could identify their difficulties (there is usually more than one).䨥se tests encompassed things like the various elements of EMI, temperature and humidity, integrity of communications, software performance and so forth.䨥 robotic product used several embedded controllers with serial communications lines between the controllers.ᬳo, each controller interfaced with the sensors and actuators required for correct operation.毲tunately, the entire robot was enclosed with no public access, so there was no danger to life and property.桩lure, however, could be embarrassing.ᮤ, for an expensive product produced in quantity, failure can have an extremely negative impact on the manufacturerࢯttom line.


We began with Electro Static Discharge (ESD) testing.䨩s testing is the least expensive and easiest to perform.䨥 unit failed immediately, and at a very low level of ESD.祠discovered that the unit was constructed with various pieces of ungrounded metal.㯮struction of the unit did position these pieces of metal out of reach of the user, thereby not presenting a safety hazard.请ever, the ungrounded metal acted as an antenna and was radiating noise through the unit.ᦴer grounding the various pieces of metal, the unit was able to pass high levels in ESD (25 KV+ static air discharge at about 0.01 joules).


Continuing our testing, we moved on to Radio Frequency Interference (RFI).䵲ing these tests, the unit was subjected to high levels of radio waves from about 10,000 Hertz to multiple Giga Hertz impinging at various angles on the equipment.᧡in, the unit failed at fairly low levels.桲ious methods of shielding were employed to prevent the introduction of such frequency interference (noise) into the system and its various embedded controllers.


During the RFI testing, we also discovered that position sensing for the robotic elements sometimes ﳴ௳ition (i.e., the position information was incorrect).ᦴer further investigation, we found that this anomaly was occurring because the client, in their design, had allowed for, but not implemented, the use of quadrature for position sensing.祠were able to change a gate array implementing the quadrature position counting scheme.⹠now, the unit was beginning to perform fairly well.


Of course, the client by now was beginning to realize the advantage of performing such tests and really wanted to continue the testing.㯬 next came testing for power line perturbations.䨥re are many tests involved in this type of testing which introduces noise into the unit via conduction on the unit࣯nnection to AC mains.䨥 unit passed through these tests quite well, including the test for lightning (a pulse of 6,000 volts at 3,000 amps).


An examination of the communications between the embedded controllers and the protocol used for that communications link indicated that the scheme did not possess a robust error checking routine.䨥 scheme was, however, a standard in the client੮dustry.⥣ause all the communications lines were contained within the unit and additional protection from noise introduction was provided by the earlier changes, we were able to prove the unit operated well in spite of the error checking issue.


Because construction of the unit incorporated a controlled environment, temperature and humidity testing was performed with no failures, as we anticipated.


After all tests were completed, and associated corrective actions implemented, a tremendous improvement was noted in the operation of the units tested.䨥 last I knew (a year or so ago), the units were at a count of 250,000 cycles and still going without failure, twice the customer௲iginal requirement.㯬 with all the hoopla concerning EMI and its associated problems, we proved, through proper design, testing and corrective actions, those problems can be averted.鴠can be done.


Electromagnetic Interference

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