Say what to do when the standard and the machine disagree
The Three a Lab Meets Most
MIL-STD-810: military equipment; tailoring is its whole point; Method 514 vibration, Method 516 shock
RTCA DO-160: airborne equipment; Section 8 vibration, with categories
NASA GEVS: spacecraft components; the usual source of their qualification and acceptance curves
Automotive, rail, shipboard and seismic work have their own; the shape is the same
What Every Standard Gives the Lab
Tolerances on the profile
A rule for where control is measured
A duration
A list of what the report must contain
How the Controller Holds a Profile
The control channel: usually an accelerometer on the fixture at the article's interface
The controller compares its spectrum with the profile
It adjusts the drive to the amplifier until they match
The standard says whether control is one accelerometer or several averaged
Tolerance Is a Band and a Total
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.
Duration.
sequence step
the test
A long exposure at a low level became a short, hotter test. Where the control accelerometer sits decides which curve the tolerance is judged on; the controller holds that one point to the profile, and the rest of the control loop is Volume 2's subject. The three standards' bands are illustrative; the published tolerances for each standard replace them before this figure is final.
Figure 1.7. The profile from Figure 1.5 with its tolerance band, an as-run control trace under it, and the same trace judged under three standards. Same flight, same envelope, same run, different verdict by document. Below, a bar of the hours the article spends in that step in service against the minutes it spends on the shaker: the duration scaling of 1.3. VD
Three Standards Side by Side
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.
Figure 1.8. MIL-STD-810, RTCA DO-160 and NASA GEVS side by side on the four things that decide a test: tolerance band, control channel rule, duration rule and report contents. Read from MIL-STD-810H, RTCA DO-160G and GEVS revision B (GSFC-STD-7000B), the revisions current when this was written; the tolerances move between revisions, so check the one the requirement cites.
What Every Standard Leaves to the Lab
The fixture and the mounting
The sensor placement
The notch and limit decisions
What to do when the run does not go to plan
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, the standard does not change; the plan does
In writing, before the run, with the customer's name on the deviation
The deviation says which band, where, and by how much
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