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Slides · 1.1

The Measurement Chain

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The Measurement Chain

  • Name each link from the motion at a point to the number on the screen
  • Say what each link can do to the number

Six Links, Then the Screen

  • Mount, sensor, cable, conditioner, filter, converter
  • Then the controller scales, computes and displays
  • The screen is the only place anybody looks
Motion at a point on the fixture or the articlethe chain to the controllerthe second chain, beside itMountreads high near its resonance;rattles when looseown mountSensorcarries the wrong sensitivity;reads cross-axis motion and base strainown sensorCablefilters a long run at high frequency;makes its own noise when it movesown cableConditioneradds a second gainand a low-frequency cutoffown conditionerFilterremoves what is out of band, and with itthe evidence of an overloadown filterConverterclips past its range;buries a small signal in its noiseown converterDisplayshows the units, detector and sensitivityit was given, never what happenedseparate recorderThe two chains share no box: no sensor, cable, conditioner, channel table or recorder in common.When they agree, every link in both has been checked against the other; when they disagree, the test stops.
Motion at a point on thefixture or the articleMountreads high near its resonance;rattles when looseSensorcarries the wrong sensitivity;reads cross-axis motion and base strainCablefilters a long run at high frequency;makes its own noise when it movesConditioneradds a second gainand a low-frequency cutoffFilterremoves what is out of band, and with itthe evidence of an overloadConverterclips past its range;buries a small signal in its noiseDisplayshows the units, detector and sensitivityit was given, never what happenedThe second chain, beside it:its own mount, sensor, cable,conditioner, filter, converterand a separate recorder.It shares no box with the first.When the two agree, every link inboth has been checked against theother; when they disagree, the test stops.
Figure 1.1. The measurement chain from the motion at a point to the number on the screen: mount, sensor, cable, conditioner, filter and converter, then the display, each drawn as a box with the one or two ways it lies written beneath it (the mount reads high near its resonance and rattles when loose; the sensor carries the wrong sensitivity and reads cross-axis motion and base strain; the cable filters and makes noise; the conditioner adds a second gain and a low-frequency cutoff; the filter hides an overload; the converter clips; the display shows the units, detector and sensitivity it was given). A second chain is drawn beside the first from the same point to a separate recorder, and shares no box with it.

The Mount and the Sensor

  • The mount is a spring: near its resonance the sensor reads high; loose, it rattles
  • The sensitivity the controller uses may not be the sensor's own
  • Cross-axis motion, a bending surface and temperature all make output
  • Range and low-frequency limit, fixed for most sensors on the day they were made

The Cable, the Conditioner, the Filter, the Converter

  • A long cable is a low-pass on a powered sensor; a moving cable makes noise
  • A shield in the wrong ground path puts the line frequency in
  • The conditioner's gain is a second number that has to agree with the first
  • The filter can hide an overload; the converter clips beyond its range

The Display Shows What Was Computed

  • Sensitivity, units, detector, averaging and resolution stand between the converter and the figure
  • A wrong one changes the figure without changing the motion

A Wrong Link Is Invisible on the Control Plot

  • The controller holds whatever arrives on the control channel to the profile
  • A sensor reading twenty percent low: the plot in tolerance, the article at twenty percent more
  • Nothing on the screen is out of place

Two Kinds of Error

  • Scale: the right shape at the wrong size, found only by comparison with something independent
  • Shape: a resonance in band, a cut low end, a saturated amplifier, hum, a rattle
  • Shape errors show, to someone who looks at the right plot

The Second Opinion

  • An independent channel beside the one that matters, recorded separately
  • Independent: its own sensor, mount, cable, conditioner and sensitivity entry, and where the stakes justify it, its own recorder
  • Two chains that agree have checked every link in both; two that disagree stop the test

Proof of a Chain

  • A known motion in at the sensor, the expected number out on the screen
  • Through every link and every setting the test will use
  • The control channel first: an error there is not an error in the data, it is the test

Decide

  • No channel is trusted until its chain is proven end to end
  • The control channel is proven first
  • Where the lab cannot afford to argue later: a second chain that shares no link with the control channel, read at every level

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