1.4 The Standards as Vocabulary, Not Subject
about 6 minutes · 2 figures, 1 interactive · quiz of 5 · Slides
After this section you can: name what a standard gives the lab and what it leaves to the lab, and say what to do when the standard and the machine disagree.
The standards are where the words come from: grms, tolerance, control channel, duration, category, method. A technician who can read the standards can read any test plan. Nobody needs to memorize them.
Three of them account for most of what a lab meets. MIL-STD-810 is the military environmental standard, and tailoring is its whole point; Method 514 (a method is the standard's numbered chapter for one environment) covers vibration, Method 516 covers shock, and the standard's Part One says in plain words that the default curves are for when no data exists. RTCA DO-160 covers airborne equipment; its Section 8 is vibration, with categories (classes set by where the box sits on the aircraft). NASA's General Environmental Verification Standard (GEVS) covers spacecraft components and is the usual source of their qualification and acceptance curves. Other industries have their own: automotive, rail, shipboard, seismic. The shape is always the same.
Every standard gives the lab four things: tolerances on the profile (how far the control channel may depart from the profile before the run is out of tolerance), a rule for where control is measured, a duration, and a list of what the report must contain. The standards agree on more than people think, and they differ on exactly the things that decide a test: the width of the tolerance band, where the control accelerometer goes, and how long the test runs.
The controller reads the control channel, usually an accelerometer on the fixture at the article's interface, compares its spectrum with the profile, and adjusts the drive to the amplifier until they match. A standard says whether control is one accelerometer or several averaged.
Every standard leaves the same things to the lab: the fixture, the mounting, the sensor placement, the notch and limit decisions, and what to do when the run does not go to plan. A notch lowers the profile over a narrow band, usually at a resonance, by agreement with the article's owner. A limit puts a ceiling on a response channel, and the controller lowers the drive when that channel reaches it. The fixture and the mounting are the rest of this volume; the sensor placement, the notch and limit decisions and the run that does not go to plan are Volume 2's subject.
Decide. The standard tells you what to prove and how to document it. It does not run the test. When the standard and the machine disagree, when the shaker cannot deliver the profile or the fixture cannot hold the tolerance (usually because it resonates in that band) or the schedule cannot afford the duration, the standard does not change. The plan does, in writing, before the run, with the customer's name on the deviation. The deviation names the requirement paragraph it departs from and says which band, where, and by how much, the way Figure 1.7 marks the band against the tolerance; it gives the reason and what the departure does to what the test proves. The customer's name on it is the name of whoever owns the requirement, whether that is an outside customer, a prime contractor or the lab's own program office; how a deviation is written is Volume 2's subject. The machine's arithmetic in Chapter 10 finds most of these before the article arrives; the low-level run finds the rest. When it is the low-level run that finds one, the test holds at low level: the shortfall goes in the log, the test engineer takes it to the customer, and nothing goes to full level until the plan changes.
The profile with its tolerance band, and the run
Amber: the profile. Blue: the as-run control trace. Gray: the tolerance band, a limit on the curve.
The same run under three standards' tolerance bands
Toggle the bands. Solid, dashed and dotted bands are three different standards judging one run.
| Tolerance band | Control channel rule | Duration rule | Report contents | |
|---|---|---|---|---|
| MIL-STD-810 | a band on the curve plus a limit on the overall level; wider above a named frequency | named control location; tailoring may move it | derived from exposure with time compression; tailorable | tailoring record, deviations, data |
| RTCA DO-160 | a band on the curve by category | at the fixture interface | fixed per category and test type | category, curve, deviations |
| NASA GEVS | a band on the curve plus overall level | at the mounting interface, average of control channels allowed | fixed per test type | levels, durations, signature comparisons |
Words, not numbers. The numbers are in the standards and change by revision; the shape of what each asks for does not.
Common Questions
How wide is a tolerance band, in numbers?
The chapter gives the shape and not the numbers, because the numbers are the standard's and they change from one revision to the next. Figure 1.8 sets the three standards side by side in words and names the revisions it was read from. The band that applies is the one in the revision of the standard the requirement cites, read from the standard itself.
When the customer's requirement and a standard disagree, or the plan cites no standard, which governs?
The requirement does, because it is the document that names the standard in the first place (1.5, The requirement); a standard governs a test only through the requirement that cites it. If two standards would give different verdicts on the same run, the one the requirement cites is the one that applies. If the plan cites no standard, the lab asks before it schedules, the same as for any other gap in the requirement.
Can I add a notch or a limit during the run on my own judgment?
Not on your own. A notch changes the profile, and it is set by agreement with the article's owner; the limits are named in the procedure before the run (1.5, The procedure). A change that comes up during the run is a decision for whoever the procedure gives that authority to, and it is recorded when it happens. Who holds that authority, and how a run that does not go to plan is handled, is Volume 2's subject.
Does the standard's list of report contents become my report template?
The standard's list is the floor, not the whole report. The report also has to carry the requirement and its definition of pass, the procedure as run, the result against that definition, and every deviation with who decided it and why (1.5, The report). Reading and writing a report are Volume 2's subject.
Quiz: Try It
Open book. 5 questions drawn from this section's pool. 4 correct passes the quiz. Nothing is recorded here; in the course your passing attempt opens the next section.