Equipment Reliability Institute
ERI News - your reliability newsletter
November, 2002 - volume 9

Welcome to the Fall 2002 issue of ERI News. Our first article is by David Douthit of Mesa, AZ. Dave recently taught "Printed Circuit and System Weaknesses caused by Contaminants and Moisture" for Daimler-Chrysler Electronics at Huntsville, AL .

Wayne's "EH, ED or RS?" refers to electrohydraulic (sometimes called servohydraulic), to electrodynamic (sometimes call electromagnetic) and to repretitive shock machines (often called "bangers"). Wayne teaches a minimal-math course concerning vibration and shock theory, measurements, analysis, calibration and testing. In recent years he has added HALT, ESS and HASS. Newcomers to these latter fields may be a bit confused by the variety of "shaking" equipment. Hence this article, which deals with EH units.

At "Questions our readers have asked...", our specialist Harry Schwab talks about Modal Testing, in response to student Enrico's question.

We hope that you enjoy reading this newsletter and that you participate in debates that our articles may trigger.

Best wishes,
Wayne Tustin

Printed Circuit and System Weaknesses caused by Contaminants and Moisture
by David Douthit

On Sept. 26th Dave Douthit spoke to the DoD DMSMS (Diminishing Manufacturing Sources and Material Shortages) COTS (Commercial Off The Shelf) briefing at Huntsville, Alabama. Here are his topics:

1. Not only are parts becoming obsolete, but so are

• manufacturing equipment,
• materials,
• manufacturing processes,
• design tools,
• test equipment and
• testing protocols.

2. The production life cycle for components is down to typically 9 months; it used to be 3 years.

3. The service life cycle target for equipment intended for office type environments is now 5 to 7 years.

4. High reliability electronic equipment OEMs do not have enough economic leverage to alter the vendors' manufacturing/design practices.

5. Waiting for field failure reports to determine if design changes are needed or if there are component problems will no longer work. Why? Because it takes 3 to 5 years to "field" new electronic designs. By that time the equipment is obsolete and for the most part the "lessons learned" do not apply to new equipment.

6. The new "nano" copper technology is much less robust and durable than older components.

7. Failure rates for COTS components in high reliability/harsh environments are increasing.

8. There are indications that the failure rates for certain failure modes will vary exponentially with design changes.

9. Uprating of components is risky at best and dangerous at worst. Discourage this practice!

10. More thorough prototype testing is required to determine the viability of designs and materials.

11. Unfortunately, much of the needed test equipment and protocols does not exist.

12. The decreasing reliability of COTS is beginning to have a negative impact on National Security!

Readers who are disturbed by these points may want to forward them to others.

Dave can be reached at douthit@equipment-reliability.com. He is available in both a teaching and a consulting capacity, to discuss these points further. Dave is also one of the co-authors of the book "Contamination of Electronic Assemblies" recently launched.

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EH, ED or RS?
by Wayne Tustin

Electrohydraulic (servohydraulic), electrodynamic or repetitive shock - which kind of excitation device should we purchase in order to perform vibration testing and/or screening? This article's purpose is to explain each type and to compare the strengths and weaknesses of each type. See Table 1.

Electrohydraulic (often called servohydraulic) shakers are valued for their long stroke and high force commencing at extremely low frequencies. Their ability to deliver complex and/or random vibration is the principal advantage over mechanical shakers. They are widely used for automotive and seismic testing. Per pound or newton of force, electrohydraulic shakers are less expensive than electrodynamic shakers.

Each shaker (Figure 1) consists of a piston (which drives the load) moving in a cylinder. The piston is forced by high-pressure oil under control of a servovalve, which is driven electrically.

Under control of the slave valve, high pressure oil is applied to one side and then the other of the main piston, forcing the main piston outward, then inward. Similarly, under control of the pilot valve, high pressure oil is applied to one side and then the other of the slave valve, forcing it one way, then the other. The actuator on the pilot valve resembles the "voice coil" of a loudspeaker and the driver coil of an ED shaker. Several feedback loops (only the sensors are shown) help to stabilize the several actions so that developed force resembles the input electrical control "drive" signal.

Figure 1 - Electrohydraulic (servohydraulic) shaker

Note also that force F = pressure P x piston area A. P is typically 3000 psi, so that a shaker with an A of 12 in2 would develop 36,000 pounds force F. Such a shaker might only weigh 300 pounds, less than some loads.

Potentially damaging shaker cylinder motion would exceed load motion. Such a shaker requires a reinforced concrete reaction mass weighing perhaps 300,000 pounds to push against. By contrast, an electrodynamic shaker which also develops about 36,000 pounds weighs perhaps 400,000 pounds, so that adding reaction mass is seldom necessary.

EH Shaker limitations
Shaker piston travel, or displacement, is limited by the cylinder length, perhaps 6 inches or 150 mm. (A human factors research project once used a unit with 21 feet or 6.5 metre stroke.) The longer the stroke, the longer the oil column. Since oil is somewhat compressible; oil column resonance imposes an upper frequency limit. 6 inch stroke shakers can achieve perhaps 200 Hz. 2 inch stroke (50 mm) shakers perhaps 500 Hz.

Acceleration A is limited by A = F/M, where force F is nominally Pressure x Area, as discussed earlier. Acceleration is usually stated in g multiples or in m/s2.

Velocity V is a further limitation. V is limited by the hydraulic fluid supply and/or by the valve to some number of gallons or liters per second. Velocity is stated in in/sec or m/s. Figure 2 shows the limits of a particular EH unit.

Figure 2 - D, V and A limits

Among the difficulties when using EH shakers:

1. Oil column resonance can lead to a peak in force. Above that frequency there is a large phase shift and then force decreases rapidly.
2. Non-linear oil flow in the servo valve creates distortion and difficult waveform control.
3. Lateral resonances in the actuator/rod assembly lead to poor durability.
4. Don't ask for instant "on" unless the shaker has been running recently. If the shaker is cold, warm it before use.
5. Oil mist can contaminate a clean room.
6. High pressure oil leaks are messy and dangerous

Figure 3 illustrates a compact EH shaker. One or more can be easily moved into tight quarters for applying vibratory force to structures or between structures. This enables designers to "check out" their assumptions of linearity and/or high damping. In practice, bolted or riveted joints produce frictional damping and "gap" discontinuities. Elastomeric isolators are less linear than assumed. Frequency response can change dramatically at high input levels.

Figure 3 - Compact EH shaker

The basic electrohydraulic shaker terminates in a threaded connection which connects to the load. However, some laboratories desire a "table" for attaching a number of relatively small items. They perhaps get the idea from the "tables" of electrodynamic shakers. See Figure 4. With idealized non-rocking, non-flexing, straight-line motion, accelerometers at locations 1 through 5 would develop identical signals. In practice, the signals differ markedly at certain frequencies. The table can rock (about A) and also can flex (something like an umbrella).

Figure 4 - Table for electrohydraulic shaker

Now that we have examined the basic electrohydraulic shaker itself, let's examine the entire complex system, Figure 5. Why so complicated? Mainly because the valve/shaker combination is far from linear. Without a great deal of control and correction, shaker force would not resemble the input signal*. A sine wave, for example, would be badly distorted.

*For automotive NVH (noise, vibration, harshness) testing, the input signal might come from magnetic tape or hard disk playback, having been recorded on a test track or streets or highway or off-highway. Alternately, recorded data from many recordings may have been combined into a spectrum entered into the computer that controls the vibration test.

Figure 5 - Electrohydraulic shaker system

This article will continue in the next two issues. In February 2003 Wayne will discuss ED shakers, while in May 2003 he will conclude with RS systems. 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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Questions our readers have asked...

This section of our newsletter was created for you, reader! Feel free to send questions or suggestions to the webmaster. One of our specialists will respond.

Q: My name is Enrico and I am a student. I'm working on modal testing at my university. There is no one to help me. So I have some questions for you:
1- What is modal participation factor?

2- I tested (modal test) a small cylindrical tank at empty condition and got natural frequency and mode shape. Now I want to get natural frequency of this tank in 2 states:
a-when fill by water?
b-under internal pressure?

A: (1) Whenever a structure is dynamically excited, it will respond with a combination of its first several modes and resonant frequencies. Depending upon the excitation, different modes are excited at different relative amplitudes. The modal participation factors for a specific situation define the relative amplitudes of the various modes to the applied input.

(2-a) The dynamic response characteristics of a cylindrical vessel vary greatly depending upon many factors. How deep is the fluid in the vessel? If the vessel is only partially full (any air space at all), what is its orientation with respect to gravity? What are the characteristics of the fluid (density, viscosity, etc.)? How do the slosh and standing wave characteristics of the fluid couple with the vessel's resonances?

(2-b) If the vessel is under internal pressure, what is the fluid which is pressurizing it? What are the characteristics of the fluid?

The questions which you have asked can only be answered by extensive analysis. I recommend using a good finite element program which can take into account fluid characteristics.

Harry Schwab is one of ERI's group of specialists. Click here to read more about Harry.

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The IEST free web-based session performed by Wayne Tustin, on October 15, was a great success. He had more than 100 people in the "virtual" audience! If you missed it visit the presentation posted on the web. For more information on how to participate on the next sessions on Dec 3rd and in January, click here.

John Starr, a ERI specialist, and Wayne had their article "It's Analysis that counts" recently published on the Evaluation Engineering magazine. The article addresses how modern software and computer power have resulted in dramatic reductions analysis costs.

For any design, HALT, ESS or HASS vibration requirement, circuit card life is limited by the failure distributions of a few components on that card. Starr teaches how to add "Point of Failure" analysis to the design process as a vital step in identifying any vibration-weak areas. Starr has 2 courses scheduled for 2003: Feb 17-19 and June 23-25.

While (1) Dave Douthit was teaching Daimler-Chrysler Huntsville, AL, engineers how contaminants and moisture can disrupt automotive electronics, (2) Steve Brenner was teaching Redstone Arsenal, AL, helicopter specialists about HALT, dynamic and climatic testing to MIL-STD-810-F and (3) Wayne Tustin was teaching Canadians in an "open" vibration and shock course at CRIQ (Centre de Recherche Industrielle du Quebec), in Montreal.

Steve has an open course on "Thermal and Random Vibration Stressing for HALT, ESS, HASS and COTS Testing" coming up on March 18, 2003 and Douthit has 2 new 2003 dates for his "Contaminants and Moisture can Disrupt your Electronics".

Vibration and Shock new book coming up!
Back in 1984 Bob Mercado and I wrote "Random Vibration in Perspective". We printed about 5,000 copies. I've seen it in many test labs.
Someone suggested that I look for that book on Amazon's used books section. Amazon has 5 copies, at prices ranging from $105.90 to $462.24. I consider that a compliment, considering that new the books were $100.
ERI will soon announce an update of this book, which we hope it will be ready for 2003 printing. We may offer a prepublication sale in 2002. Stay tuned!

- Wayne

73^rd Shock and Vibration Symposium
Many of our readers (especially those who deal with the U.S. Navy) are familiar with SAVIAC, the Shock and Vibration Information Analysis Center.
Joel Leifer and his associates are busy organizing the 73^rd Shock and Vibration Symposium to be held at Newport, RI, November 19-22. If you can attend, please look for Wayne Tustin. Details can be found on the SAVIAC website.

Seeking teacher
Someone once told me that the number of pressure sensors sold is 10X the number of accelerometers sold. Whatever the ratio, it is certainly greater than 1:1. ERI is seeking someone to teach occasional short courses about dynamic and static pressure measurements and calibration.

Have you ever thought about becoming a consultant? Nearly every engineer or scientist, at some time, has that idea.
Well - what is a consultant? A consultant acts as an advisor to industry groups or to a specific business. He tells them what to do. Basically, then, a consultant is someone who advises. Advises whom? There you (as a fledgling consultant) normally would have a problem. Finding clients. Especially your first client.
But not today. Today and perhaps tomorrow (but don't wait too long) you have your first client waiting. Who? Us, Equipment Reliability Institute.

Here is a question on which we will appreciate your advice: in what additional reliability-related subjects should ERI offer training and consulting? Have you expertise in that subject?

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

Web sites
http://www.equipment-
reliability.com

http://vibrationand
shock.com

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