Control & Response Accelerometers
How control and response accelerometer locations, orientations, sensor selection and signal quality affect the validity and usefulness of random vibration test data.
What Is It?
Accelerometers are the primary measurement transducers in random vibration testing. Control accelerometers provide the feedback signal that the shaker controller uses to maintain the specified PSD. Response accelerometers measure the test article's response at critical locations. The placement, orientation, selection and signal quality of these sensors directly affect the validity and usefulness of the test data.
Why It Matters
The accelerometer data is the test evidence. If the control accelerometer is poorly placed, the shaker may not achieve the correct PSD at the test article. If the response accelerometers are poorly placed, the response data may not represent the critical locations. Poor accelerometer data undermines the test — the results may be unusable or, worse, misleading.
Accelerometer data is the test evidence. Poor placement, orientation or selection undermines the test. The control accelerometer determines what PSD the article actually sees; response accelerometers determine what response is measured.
Control Location
The control accelerometer location determines what PSD the shaker controller maintains. The ideal location is at the fixture-test article interface — this measures the actual input to the test article. If the control accelerometer is on the shaker table (below the fixture), it measures the shaker output, not the article input — fixture dynamics are not accounted for. If the control accelerometer is on the test article itself, it may be affected by article resonances.
| Control Location | What It Measures | Advantages | Disadvantages |
|---|---|---|---|
| Fixture-article interface | Actual input to article | Correct — article sees this | May be difficult to mount |
| Shaker table | Shaker output | Easy to mount | Does not account for fixture dynamics |
| On test article | Article response at that point | Shows article response | Affected by resonances — not a true input |
Response Channels
Response accelerometers are placed at critical locations on the test article to monitor the response during the test. The locations should be selected based on the pre-test analysis — locations predicted to have high response, locations near stress concentrations, and locations where damage is most likely. Multiple response channels provide a fuller picture of the article's dynamic behaviour.
- Place at critical locations identified by pre-test analysis
- Locations with predicted high response
- Locations near stress concentrations or likely damage sites
- Multiple channels provide fuller response picture
- Record response PSD and RMS at each channel throughout the test
Sensor Orientation
The accelerometer sensitive axis must be aligned with the excitation direction. A misaligned accelerometer measures a component of the response, not the full response. For triaxial accelerometers, the three axes should be aligned with the test axes. For single-axis accelerometers, the mounting must ensure the sensitive axis is parallel to the excitation direction.
Accelerometer sensitive axis must be aligned with the excitation direction. Misalignment measures a component, not the full response. Check alignment during installation.
Mass Loading
Accelerometers have mass — typically a few grams to tens of grams. For lightweight structures (thin panels, small components), the accelerometer mass can affect the structural response — the accelerometer acts as a mass-loaded point, changing the local dynamics. For lightweight structures, use the smallest possible accelerometer (sub-gram MEMS devices are available) or use non-contact measurement (laser vibrometer).
Mass loading check: m_accelerometer << m_effective,local Rule of thumb: m_acc < m_local / 100 If the accelerometer mass is a significant fraction of the local effective mass: → Use a smaller accelerometer → Use non-contact measurement (laser vibrometer) → Account for the mass loading in the analysis
Signal Quality
The signal quality from the accelerometers determines the usefulness of the data. Poor signal quality — noise, ground loops, cable vibration, poor mounting — produces unreliable data. Signal quality should be checked before the test by verifying the noise floor, checking for ground loops, ensuring cables are secured, and confirming that the mounting is solid.
- Noise floor — check before test; should be well below expected signal
- Ground loops — use isolated accelerometers or differential input
- Cable vibration — secure cables to prevent cable-induced noise
- Mounting — stud mount for high frequency; adhesive for temporary
- Calibration — verify accelerometer sensitivity before test
Number of Channels
The number of response channels depends on the test article complexity and the test objectives. For a simple article, 2-4 response channels may be sufficient. For a complex article, 10-20 or more channels may be needed to capture the response at all critical locations. The channel count should be planned during test design, based on the pre-test analysis.
Key Takeaways
- Control accelerometer should be at the fixture-article interface — measures the actual input
- Response accelerometers at critical locations identified by pre-test analysis
- Sensitive axis must be aligned with the excitation direction
- Mass loading matters for lightweight structures — use small or non-contact sensors
- Signal quality — check noise, grounding, cables, mounting and calibration before testing
Engineering judgement — what can change the conclusion
For Control & Response Accelerometers, the harmonised review should concentrate on the interaction between control strategy, sensor location and local structural response. A control channel can protect the fixture while allowing another location to exceed the intended environment. The engineering value comes from identifying the assumptions that can move the governing margin or failure mode, then testing those assumptions deliberately rather than adding complexity indiscriminately. Where simplified and high-fidelity methods coexist, the simpler method should be used as an independent trend or magnitude check so that agreement is based on physics rather than shared modelling assumptions.
Independent verification and evidence
Before Control & Response Accelerometers is used for a design or qualification decision, check sensor calibration/orientation, control versus response PSD, coherence, notching rationale, fixture modes and evidence that protected hardware still receives the required test environment. The evidence should be recorded against the actual acceptance quantity, including units, coordinate system, configuration and load state. Any extrapolation beyond the range of test data, handbook solutions or validated solver behaviour should be explicit, together with the sensitivity that demonstrates whether it matters to the final conclusion.