Rotor diagnostics
Reading rotor imbalance from the ground
Vibration says a rotor is out of balance without saying why. Measuring pitch and clearance optically from the ground puts all three blades in one frame.
Two failure modes, one symptom
| Cause | What differs between blades | Typical origin | Remedy |
|---|---|---|---|
| Mass imbalance | Weight distribution | Manufacturing tolerance, ingress water, added repair material, ice | Locate and correct the mass distribution |
| Aerodynamic imbalance | Effective pitch angle, at equal mass | Pitch setting or the calibration behind it | Correct the pitch setting or its calibration |
A rotor that is out of balance produces vibration at the rotational frequency, and that vibration is usually how the problem announces itself. The difficulty is that the same symptom has two quite different causes.
Mass imbalance means the blades differ in weight distribution — from manufacturing tolerance, from water that has found its way inside, from a repair that added material, or from ice. Aerodynamic imbalance means the blades are set at different effective pitch angles, so they do not produce equal thrust even though their masses match.
The distinction matters because the remedies are unrelated. One is resolved by finding and correcting a mass distribution; the other by correcting pitch setting or the calibration behind it. A measurement that only reports vibration amplitude leaves the choice between them to inference.
Why measure from the ground
Pitch difference between blades is a geometric property, and geometry is observable from outside the machine. That is the premise of a ground-based optical measurement: a camera on the ground, a portable processing host, and calculation of blade pitch and clearance from what it sees.
Three consequences follow. No climb is required, which removes the access planning and the working-at-height exposure that a nacelle measurement carries. No installation inside the turbine is needed, so the measurement can be made without preparing the machine. And all blades are observed in the same reference frame by the same instrument, which is precisely what makes them comparable — a per-blade measurement taken separately would carry its own setup error into every comparison.
The documented WIT-RB20 workflow also states a self-powered service duration of six hours, which is a practical figure for a campaign that moves between turbines without site power.
Reading a 0-20 m range
The published fields state a clearance accuracy of 20 cm over a clearance measurement range of 0 to 20 m.
That range describes the measurement envelope — the span of clearance values the documented measurement is specified across. It is not a statement about how far from the tower the equipment is set up, which is a site-geometry question governed by the documented setup and the sight lines a particular location allows.
Reading the accuracy figure requires the same care as any other. Twenty centimetres bounds the clearance measurement under the conditions the product document states. What the campaign usually cares about is not an absolute clearance but the difference between blades, and a consistent instrument observing three blades in one frame is well suited to a comparison even where absolute accuracy is the more conservative figure.
Capture-quality warnings earn their place
The documented workflow includes live progress and capture-quality warnings during the measurement, and this deserves more attention than a feature list usually gives it.
An optical measurement of a rotating machine depends on conditions the operator does not control: light, visibility, viewing angle, and whether the rotor is turning steadily enough for the capture to be valid. A campaign that discovers a capture was unusable after leaving the site has to return to it, and the cost of returning is almost always larger than the measurement itself.
Feedback during capture converts that from a repeat visit into a repeat attempt. For fleet work this is often the difference between a schedule that holds and one that does not.
What the output supports
The documented workflow ends in report export after calculation. What that output is good for is worth being precise about.
It supports comparison between blades on one turbine, which is the question that separates aerodynamic from mass imbalance. It supports comparison over time, if the same turbine is measured again after an intervention, which is how a correction is confirmed rather than assumed. And it supports comparison across a fleet, which is how a systematic pitch calibration issue becomes visible as something other than a series of unrelated single-turbine problems.
What it does not do is replace a vibration analysis. The two answer different questions, and a diagnosis that uses only one of them is choosing between the two failure modes rather than distinguishing between them.
Evidence register
Sources and limitations
The distinction this page turns on is not a vendor framing: it is written into the vibration standard for this machine class.
- ISO 20816-21:2025 — Mechanical vibration — Measurement and evaluation of machine vibration — Part 21: Horizontal axis wind turbines, applicable above 200 kW rated generator output. It states that evaluating the unbalance of a slowly turning rotor requires measurement and analysis considering both mass and aerodynamic unbalance, and it is explicit that its recommended evaluation zones for continuous-load operation are in most cases not suitable for the early detection of faults. Unlike other parts of the series it treats vibration excited by wind rather than generated by the machine, and requires low-frequency vibration to be included because of blade and tower flexibility at low rotor speeds.
- VDI 3834 Part 1 — referenced by ISO 20816-21 (Annex B) for assessing the influence of rotor unbalance on vibration.
- IEC 61400-1:2019 — design requirements covering the mechanical systems and support structures the vibration acts on.
Limitation: the standards above define how vibration is measured and evaluated. They do not establish detection capability for any particular ground-based instrument, and nothing here states an accuracy or a detection threshold for one. Whether a given measurement can separate the two imbalance types is a property of that measurement's method and procedure, not of the standard.
See the source and editorial policy for how supplied product manuals, external method sources and claim limitations are kept separate.
Decision support
Frequently asked questions
What causes rotor imbalance on a wind turbine?
Broadly two things. Mass imbalance, where blades differ in weight distribution through manufacture, water ingress, repair or icing. And aerodynamic imbalance, where blades are set at different effective pitch angles so they do not produce equal thrust. Both raise vibration, and vibration alone does not say which one is present.
Why measure from the ground rather than in the nacelle?
Pitch differences between blades are a geometric property visible from outside the turbine. Measuring them from the ground avoids a climb, avoids interrupting operation for installation, and observes all blades in the same reference frame, which is what makes them comparable to each other.
What does the 0-20 m clearance range apply to?
It is the range over which the documented clearance measurement is specified, with the stated accuracy of 20 cm. It describes the measurement envelope, not the distance from which the equipment is set up. Site geometry and the documented setup are separate questions to confirm for a specific turbine.
Does the report replace a vibration analysis?
No. It answers a different question. A vibration measurement tells you something is out of balance; a pitch and clearance measurement helps distinguish whether the geometry differs between blades. They are complementary, and using one where the other is needed is how a mass problem gets treated as a pitch problem.
Engineering review
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