Place, mount and back up the control sensor and an independent monitor
Say why it is the most checked channel on the test
Where It Goes
Method 514.8: screw-mounted to the fixture near the interface, or where the specification was derived
Every instrumentation location described in the test plan
The fixture survey's modal map sets the point: where it stands for the mounting points, never on a nodal line
How It Is Mounted
On a stud, in a tapped hole designed into the fixture, greased, torqued and written down
The best sensor in the drawer: fresh certificate, band well inside the 5 percent fraction, range with headroom
Its cable on the shortest route off the fixture, no connector in the moving run that need not be there
Its sensitivity from its certificate, read back, and checked against the serial on the fixture
Single-Point Control: Two True Statements
Method 514.8 calls single-point control "not recommended": one failed or mis-scaled channel
In practice, single-point control is what most small-article tests use
One statement is about the motion, the other about the sensor
Three Forms of Backup
A second control-quality sensor beside the first, run as a monitor with abort limits and named to take over
Averaged control on two adjacent sensors, a backup only if each is watched on its own
The independent monitor
The Independent Monitor
Its own sensor, cable, conditioner and sensitivity entry, on a recorder that shares nothing with the controller
It records what the table did, apart from the system whose behavior may be the question
Not the secondary level check: that protects the machine; the monitor is evidence for the test
The Control Point on a Fixture
The control sensor off the nodal lines, near the interface
The backup in its own hole; the monitor on its own cable, conditioner and recorder
What each shares with the controller, and what it does not
Figure 1.8. The control point on a fixture, with the fixture's nodal lines from its modal map drawn on it and the control sensor placed off them near the interface. Beside the control sensor, in its own tapped hole, is the backup sensor on its own cable and input, and beside both is the independent monitor on its own cable, conditioner and recorder. The links each one shares with the controller, and the links it does not, are marked.
The Public Case: HESSI
A sine-burst run at minus 12 dB, a quarter of full level, delivered about 21 g, roughly ten times what was intended
The investigation traced it to the test equipment, not the spacecraft
An independent monitor beside the control sensor, on a separate recorder, showed what the shaker had done
The Sensor That Was Not the One in the Table
A limit channel on a bracket: a 10 mV/g sensor among 100 mV/g siblings, entered at 100
The limit never reached, no notch, the control plot in tolerance
The monitor on its own recorder read ten times more; the operator stopped at minus 12 dB
Decide
The control sensor stud-mounted at the point the survey and the requirement give, in the plan, proven first
A named backup on every test: a second sensor with abort limits and named to take over, or averaged control with each channel watched
Where the lab cannot afford to argue later: an independent monitor on its own conditioner and recorder, read in the first minute of every level
Which form is decided in the plan, scaled to the test's severity and the article's value
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