Equipment Reliability Institute
ERI News - your reliability newsletter
August 2005 - volume 20


Wayne TustinHello, readers -

EMC specialist Bill Parker helps us understand electromagnetic interference filters on our first article. Then specialist John Riddle describes containing an extremely sensitive infrared camera at almost absolute zero temperature in high vacuum without using expendable refrigerants. Then I've written a few words about vibration and shock test fixtures, which Steve Brenner will be teaching at his upcoming fixture design course. Finally, Bob Renz shares more of his hard-won vibration testing "smarts."

We appreciate feedback -- send us your comments (both good and bad) on what we have assembled for this issue of ERI News.

Best wishes,
Wayne Tustin
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EMI Filters
by William H. Parker

I was recently called to provide electromagnetic compatibility consulting services to a new client. His business involves designing and manufacturing AC/DC electrical power conversion equipment. For a new and promising application, his product must meet the Part 15, Class A, electromagnetic emission limits of the Federal Communications Commission. These limits have been established to prevent such products from interfering with AM, FM, TV, and other established communications services.

Upon arrival at the client’s facility, I determined that the new product was emitting excessive radio frequency conducted emissions on its input AC power line, as well as on its output DC leads. As the client had previously determined, an AC input EMI powerline filter was needed, as well as a DC output powerline filter. As well as directly exceeding the conducted emissions limits of FCC, Part 15, the powerline conducted emissions were radiating from the power leads. That radiation exceeded the Part 15 radiated emissions limits. By successfully suppressing the conducted emissions, we could also solve the radiated emissions problem, provided there were no radiated emissions from apertures or seams in the metal enclosure of the new product.

Traditionally (and erroneously), the method often used to specify an EMI powerline filter is to match up the insertion loss performance of filters in a filter manufacturer’s catalog with the number of decibels that the conducted emissions exceed the conducted emissions limit. The fallacy of this approach is that the filter manufacturer measures and specifies his filter performance in a 50 ohm input/50 ohm output system, as specified by MIL-STD-220. This 50 ohm system is convenient, as radio frequency measurement equipment usually has 50 ohms input impedance. “Convenient” does not mean correct, however, as power line source and load impedances are usually NOT 50 ohms. The filter manufacturer’s 50 ohm insertion loss curves, then, are not performance-predictive; the performance of a filter in a 50 ohm insertion loss test circuit is not the same as its performance in an actual power line circuit.

Another shortcoming of the filter manufacturer’s insertion loss curves is that they do not differentiate between filter common mode performance and differential mode performance. Common mode interference voltage on a power line is that “noise” voltage from each power lead to local electrical ground; the common mode interference voltage from each lead to ground is identical in phase and amplitude. The differential mode interference voltage is that interference voltage between the power leads, with no reference to local ground.

As it turns out, different types of filter components are required to suppress common mode and differential mode interference. To solve common mode problems, common mode inductors and line-to-ground (“Y”) capacitors are needed. To solve differential mode problems, differential mode inductors and line-to-line (“X”) capacitors are needed. Part of my job as an EMC consultant, then, is to determine which powerline interference is common mode, and which is differential mode. Another factor is to establish appropriate safety margins between minimal and nominal required filter insertion loss versus frequency, to preclude manufactured equipment failure to meet electromagnetic emissions limits due to equipment or filter component tolerances. Also, the filter must be designed and built to comply with all other appropriate mechanical, environmental, and electrical specification requirements for its intended application.

In the particular case at hand, with two weeks of work at the client’s facility, lab measurements, detective work, CM/DM interference determination, and a small number of hand-built EMI filter prototypes, the client and I had produced optimized input and output EMI filters, which easily allowed the new equipment to pass the FCC Part 15, Class A, conducted and radiated emission limits, with reasonable safety margin, and minimal added cost, weight, or volume.

Bill is an ERI's specialist for EMC supporting, EMI testing, RF shielding, EMI detection and suppression fields.

For a listing of Bill's available short courses (EMC 501 and EMC 510), click here. For more information about Bill, please visit his page at ERI.

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A Very Low Temperature Enclosure
for an Infrared Camera
by John Riddle

Here is an interesting fixturing problem one of our customers brought us at TetraTek Products, Inc. How do you contain an extremely sensitive infrared camera while maintaining a background temperature of almost absolute zero in a high vacuum without the use of expendable refrigerants? The solution involved a number of unusual materials and techniques.

Compressor driven two-stage closed-cycle helium refrigeration systems, based on the Gifford-McMahon thermodynamic cycle, offer usable cooling at very low temperatures. Refrigeration systems of this type are available from the CTI Cryodyne Division of the Helix Technology Corporation, http://www. helixtechnology.com. One of the most useful features of this device is that there are actually two surfaces for heat exchange, one at liquid nitrogen temperatures (about 80o Kelvin or –315o F) and another at very dense helium gas temperatures (about 10o Kelvin or –442o F). Normally, this type of refrigeration system is employed in a high vacuum Cryopump to condense more volatile gasses on surfaces protected by a chevron baffle and an outer radiation shield and to adsorb gasses such as helium in a cryogenically cooled activated charcoal matrix. In our case, we decided to use the closed-cycle helium refrigeration system as the primary cooling means.

Our solution included:

  1. An electropolished stainless steel access door and optical inlet flange.
  2. A copper outer shroud door, gold plated on both sides to minimize thermal transfer in the infrared.
  3. A copper inner shroud door, gold plated on the outside with a black glazing on the inside.
  4. A copper inner shroud, gold plated with black glazing on the inside for high radiation heat transfer to the camera.
  5. A copper outer shroud, gold plated on both sides.
  6. An electropolished stainless vacuum pressure vessel with an interface port and dual fitted shroud adapters for attachment of the customer’s Camera.
  7. The Gifford-McMahon helium refrigeration system head.

The most difficult problem to solve was how to conduct the radiation energy impinging on the entire surface of the shroud to the cryoengine located on only one end while limiting the conduction heat transfer to the vacuum case.

The thermal shroud was fabricated from relatively thick oxygen free copper rolled and continuously welded into a solid integral structure. The entire inner and outer shrouds were gold plated to reduce thermal transfer and the door surfaces were machined and attached with soft Indium metal gaskets.

Each shroud was supported with custom TetraTek stainless steel vibration isolators. All hardware was gold plated and drilled to prevent trapped gasses from forming virtual leaks under high vacuum. The stainless steel case was continuously welded, examined for structural integrity, and leak checked using a mass spectrometer. The finished product achieved a temperature of 11o Kelvin on the inner shroud with the customer’s camera in place.

John Riddle, an ERI specialist, has been the President of TetraTek Products, Inc. for the last ten years. John can assist you with fixture, data acquisition, environmental test and process control consulting. For more information about John please visit his page at ERI.

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Vibration and Shock Test Fixtures
by Wayne Tustin

What is a fixture?
It’s an intermediate structure, bolted to and driven by a shaker (focus of this article) or shock test machine and some device under test (D.U.T.).

Figure 1 - Function of fixture

Let’s examine the moving parts of Figure 1, the shaker armature, the fixture and the D.U.T. (all of which, at low test frequencies act as just one mass, all moving together). But at higher test frequencies, the shaker armature behaves more like two masses joined by a spring, while the fixture and the D.U.T. act like numerous masses joined by numerous springs.

An important question, especially at higher test frequencies: where,

  • on the shaker table at A, Figure 2,
  • on the fixture at B, or
  • on the DUT at perhaps C or D,

shall we locate our control accelerometer?

Figure 2 - Where to put the Control Accelerometer

That’s an important decision. See Figure 3. The control accelerometer informs your computer that controls your test what’s happening out at your shaker. Where to attach the control accelerometer? We must decide at what “input” location(s) we want to control our test.

Another necessary decision: at what location(s) on the D.U.T. do we want to record responses?

If you are responsible for testing, be sure that your customer specified these locations. They tremendously affect test outcome.

Figure 3 - System Block Diagram

At Figure 4 we have a family of transmissibility (mechanical amplification) curves for an idealized, very simple, spring-mass system. An ideal fixture would behave in this way. On the magnified graph at right, a 3000 Hz fixture resonance is shown at frequency ratio 1, where the forcing frequency matches the natural frequency . (Real hardware responses are more complex.)

Perhaps we are testing up to 2,000 Hz, which falls at frequency ratio 0.67. Notice that our fixture has boosted shaker motion by half.

Figure 4 - Transmissibility

With our control accelerometer at “B” (Figure 2), that resonant buildup will have prompted our computer to reduce electrical drive to the shaker to about 0.67, in order to not overdrive the fixture and D.U.T. That’s one reason “B” is so popular. With control at “B”, our computer electrically corrects for our fixture’s mechanical resonance.

One fixture type
One popular fixture shape is the “ell” or “bookend” fixture. The fixture of Figure 5 suggests fixturing a car radio. “Input” is one of Figure 2’s many “B” locations. “Response” is one of Figure 2’s many “C” or “D” locations. Test results can change dramatically – depending upon which “input” and which “response” location you choose. These (as well as intensity and frequency) should be specified.

Figure 5 - Ell or Bookend Fixture

Shaker force limits
You will recall Newton’s F = MA, which we often transpose to

We are concerned with at least three M terms, so that

For a given shaker force F and its armature mass Marm, we want to minimize fixture mass Mfixt so we can maximize load mass MDUT. Right? Thus we’ll use aluminum or magnesium for our fixtures.

The foregoing is a small (probably under 1%) of what Steve Brenner will teach at Las Vegas February 20-22, 2006. This class is aimed at people who are responsible for the design or procurement of vibration and shock fixtures. Fundamentals of vibration and shock will be covered, as well as emphasis on modern design and construction techniques. Check the course's page to see full details.

Wayne Tustin, ERI's president, can be reached by e-mail or phone (805) 564-1260. Read more about Wayne at ERI's website.

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Test Lab Musings (part 9)
by Robert L. Renz

Install an hour meter your power amplifier to show total “On” hours. Install another hour meter on the shaker cooling air blower to track the shaker run time. Why? So you'll know how much the system has been powered and how much testing you've done.

Keep a log sheet for your accelerometers, cables, and charge amplifiers
to track their data by serial number, purchase date, price, repair history,
problems, and repairs. Why? So you'll know which products are most (and which are least) reliable. Use wire markers or similar to assign each cable
a serial number.

Establish a test lab log sheet to track test dates and test time – when its budget time, it helps to be able to document exactly how many hours your shaker was tied up with tests.

When you set up your vibration profile in your shaker controller, verify that the display covers your entire test range. Many displays end at 1995 Hz or so as a default, but you can usually convince them to extend above 2000 Hz. While you’re at it, verify that the file-save parameters are set up to save all the data you might need for the report.

Robert L. Renz of General Dynamics - Advanced Information Systems at Bloomington, Minnesota.

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Cooling of Electronic Assemblies - what you should learn


Packaging of today's electronic systems is being driven toward mechanical designs characterized by ever increasing power density levels. Therefore, incorporating thermal management techniques when the paper is "blank," becomes crucial in minimizing product development costs. Avoid expensive design iterations, production problems and field failures.

The Cooling of Electronic Assemblies course (October 24-26, 2005, Las Vegas, Nevada), with Joel Newberger, will show you how to develop reliable, cost-effective solutions to your toughest design problems. This course covers (1) practical methods of thermal design, (2) describes methodologies for quick, simple-to-use thermal design techniques for packaging electronic equipment into enclosures of all sizes, and (3) how to develop cost-effective solutions for actual hardware used in either commercial or military/aerospace applications. Electronic, electrical, mechanical, and other engineering disciplines responsible for design testing and cooling of electronic equipment will benefit from this seminar.

Take advantage of the early-bird US$100 discount when you register by by September 23! And for three or more participants from the same organization, take an extra US$100 discount each.

 
Random Vibration Text Book


As we told you last issue, Wayne’s minimal-mathematics, minimal-theory 4-color printed hardbound text "Random Vibration & Shock Testing, Measurement, Analysis & Calibration" (cover photo below) has been published. A CD-ROM with Wayne's video clips and animations is included.

This book is being used at ERI short courses in the USA and abroad. It's available for purchase at our website. For more detailed information about the book, please click here.
 
Santa Barbara, CA course coming up


Will you be visiting Southern California this month? Might a visit to ERI at Santa Barbara - the America’s Riviera®
(between mountains and seashore, about 1/4 of the way from Los Angeles to San Francisco) provide a suitable business purpose to qualify all or part of your trip as business expense? Consider participating August 24-26 in Wayne's basic vibration and shock course.
 
Is the data you gathered faulty?


Are hidden transfer functions in your measurement system limiting its performance? Have you looked closely enough?

One of the focus areas in this course is the need for the testing staff to be in total control of the transfer function of the measurement system at all times. Being in control here is part of the measurement engineer's job. The validity of the test data rests on, among other things, this system transfer function.

Would it matter if the data you gathered (or that was gathered for you, possibly by persons unknown, or on which the test specification or standard you must meet is based) was faulty? If you have any interest in data being faulty, then you'd better learn from Chuck Wright about all the ways data gathering can "go wrong". Chuck will be teaching "Applied Measurements Engineering" next October 17-19, in Las Vegas, Nevada.

It is difficult, often impossible, once data has been gathered, to identify it as "good" or "faulty". Learn what questions you should ask (of the data gatherers) before accepting their data. Did they take appropriate precautions "up front"? The additional efforts Chuck teaches can be thought of as investing in "data insurance".

Take advantage of the early-bird US$100 discount when you register by by September 16! And for three or more participants from the same organization, take an extra US$100 discount each.
 
Vibration course in Australia

Deepak Jariwala, an ERI specialist, will be teaching "Fundamentals of Random Vibration and Shock Testing" at Maribyrnong, Australia, on November 22-24 , 2005.

This course applies to builders and users of equipment demanding high reliability, whether or not subject, in service, to vibration and shock. This includes aerospace, land and sea vehicles. Increasingly, random vibration is used for HALT, ESS and HASS.

 
Are you a Newcomer?


Are you a newcomer to vibration and shock testing and measurement? To ESS, HALT & HASS?
ERI can help you "up the learning curve".

You may be the test technician or the test engineer operating test equipment (shaker, shock test machine, pneumatic hammers, accelerometers, etc.) or the lab manager. Nearly everyone in this sort of activity has "stumbled" into it. He/she had little if any prior practical background and possibly little theoretical background. Does this apply to you? That was Joe Youngman's situation. Joe, you may recall, is the protagonist in the ongoing series of TEST Engineering and Management articles that commenced in 2004. Joe had just graduated from Local University with a degree in Computer Engineering. Joe was fortunate, on his first job, in that he immediately acquired a mentor who could answer Joe's questions.

Or you may be part of a design team whose products are dynamically tested or screened. Or part of a production engineering team whose products are dynamically screened. Or part of quality control or reliability engineering, needing to answer questions about failures during test or screening.

In any event, you have many questions. Bring your questions to us. Note the upcoming "open" courses listed below, where your questions will be answered. Or consider training at your facility with a "live" ERI instructor. Or bring him to your facility via the Internet, or consider "distance learning" as described at our site.

You probably are budgeted a certain amount of annual training. Use it in the ways just suggested to get the answers you need to more effectively do your job. If you want to "dig out" your own answers, consider Wayne Tustin's 2005 text "Random Vibration & Shock Testing (…)".
 
Vibration and Shock courses coming up


Wayne will teach short courses in vibration testing, shock testing, measurement, analysis, calibration, HALT, ESS and HASS at the following locations:

August 24-26, 2005
Santa Barbara, California

September 27-29, 2005
Mississauga, Canada

October 4-6, 2005
Longmont, Colorado

December 6-8, 2005
Detroit, Michigan

2006 Courses

February 13-15, 2006, Las Vegas, Nevada

Perhaps you’d like to have customized training presented at your facility, for your designers and test specialists. Contact us.
 
"Cooling"
Message Board


Have you questions about
the "natural" or "artificial" cooling of your electronics packages? Direct your questions (or observations) to ERI's Electronics Cooling message board.
Sometimes the board moderator, Joel Newberger, himself, will assist you. Joel is ERI's electronics cooling specialist.
 
Announcements


Accelerated Testing
ERI's Wayne Tustin has agreed to chair a two-hour session on "Accelerated Testing" at the 2006 annual meeting of the IEST (Institute of Environmental Sciences & Technology), to be held at Phoenix, Arizona, May 7-10.  Wayne is seeking speakers.

Failure Analyst
Are you a failure analyst?  Could you teach a short course in failure analysis, relating to failures that occur during HALT and HASS?  Please e-mail Wayne.

HALT/HASS tutorial
Wayne will be presenting a HALT/HASS tutorial at SAVIAC's 76th Shock & Vibration Symposium, scheduled for the week of October 30 thru November 4, 2005, at the Royal Sonesta Hotel in New Orleans, LA.

Pickin' up Good Vibrations
Wayne has been featured in an article by Frank Nelson, published on 5/9/05 in the Business News section of the Santa Barbara News-Press.

Specialist Wanted
For short-term teaching and consulting assignments:
1. Strain-gage theory and practice
2. Structural dynamics
Reply to Wayne Tustin.

 
Climatics
Message Board


Have you questions about humidity (condensing or non-condensing)? Temperature? Altitude? Salt fog? Fungus? About measuring any of the "climatic" environments? Or about conducting "climatic" environmental tests?

Direct your questions (or observations) to ERI's Climatics message board. Sometimes the Climatics board moderator, Steve Brenner, himself, will
assist you. Steve is ERI's
climatics test specialist.
 
Could we host an ERI course?


Have you noticed that most ERI “open” or “public” short courses are held at military/naval/commercial sites, rather than at hotels?  Why? 
Well, partly to save rent, but mostly to facilitate visits to working areas.  Two examples: We like to visit vibration and shock labs to see (and demonstrate) shakers and shock test machines. We like to visit electronic development areas to see cooling and EMC activities. We recognize that hosting an ERI short course involves effort and we try to compensate by offering two no-charge participations.  Might this idea interest your organization?

 
Contact information


ERI - Equipment Reliability Institute
1520 Santa Rosa Ave.
Santa Barbara - CA - 93109
Tel: (805) 564-1260
Our fax number:
(805) 966-7875

Wayne Tustin tustin@equipment-
reliability.com

Webmaster webmaster@equipment
- reliability.com

Websites
http://www.equipment-
reliability.com

http://www.vibrationand
shock.com

Copyright © 2000-2005 Equipment Reliability Institute. All rights reserved.

 
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