Clearance monitoring
Measuring clearance on a moving target
A clearance accuracy figure bounds the ranging measurement, not the capture of the minimum. The tip's closest approach is brief, so acquisition rate decides.
Why clearance is measured continuously
Blade-to-tower clearance is not a fixed dimension. It varies with wind speed, with the loading on each blade, with pitch and yaw state, with air density and with the blade's own condition over time. A number established at commissioning describes that day, not the asset.
That is why the question is normally posed as monitoring rather than measurement. What an operator wants is not a single figure but a distribution over time — how close the minimum approaches have been, whether they are trending, and whether a particular operating condition produces outliers.
Radar fusion and video, compared
| Approach | Range measured by | Published accuracy field | Fails when |
|---|---|---|---|
| WIT-RC30 (radar + camera fusion) | 80 GHz FMCW radar return, camera for reviewable context | 50 cm, range ≥130 m, acquisition ≥25 Hz | The radar return degrades; conditions outside the documented envelope |
| WIT-VC20 (video) | Computer-vision clearance calculation on an optical scene | 50 cm, IP65 enclosure, historical video and structured data retained | The optical scene degrades — visibility, contamination, illumination |
Two documented approaches appear in this catalogue and they solve the problem differently.
The WIT-RC30 combines an 80 GHz FMCW radar with a camera and processes both as a fused measurement. Radar supplies the range and does not depend on illumination; the camera supplies the visual context that makes an event reviewable. Its published fields state a measurement range of ≥130 m, an accuracy of 50 cm and an acquisition rate of ≥25 Hz, each with the conditions carried in the product document.
The WIT-VC20 is video-based, using high-definition imaging with night illumination and computer-vision clearance calculation. Its published accuracy field is also 50 cm, and it documents an IP65 enclosure rating along with retrieval of historical video and structured data.
The two are not ranked. They differ in what they depend on — one on a radar return, one on an optical scene — and that difference is what a site's conditions should decide.
Accuracy against a moving target
A stated accuracy figure is a property of the ranging measurement under documented conditions. It is easy to read as though it were the accuracy of the clearance result, and those are not the same thing.
A blade tip is a large, fast, non-cooperative target. It presents a changing aspect, its closest approach to the tower lasts a fraction of a second, and it repeats several times a minute. The measurement of interest is the minimum of a curve that the system samples rather than observes continuously.
So two error sources sit in the final number. One is the ranging accuracy the specification states. The other is whether the system sampled near enough to the true minimum to represent it. The second is not addressed by the accuracy figure at all.
Why acquisition rate is not a detail
This is why the acquisition rate belongs beside the accuracy figure rather than below it. At a documented ≥25 Hz, samples arrive frequently enough that the interval around the closest approach contains several of them, which is what makes the sampled minimum a reasonable estimate of the real one.
The general principle is worth holding onto when comparing any two systems: a highly accurate measurement taken at the wrong instant does not describe the minimum clearance, and a modest accuracy sampled densely through the approach may describe it better. Neither figure is meaningful alone.
What an event record has to carry
A clearance system that reports only a live value solves a monitoring problem and leaves an evidence problem. Decisions about a turbine are usually made weeks or months after the event that prompted them.
A retained event that can still be acted on later carries four things: the measured range, the visual context that lets a person confirm what was seen, the time, and the operating state around it. The RVBC-3000 documents retention of video and point-cloud event records; the VBTCMS-2000 documents retrieval of historical video and structured data. What exactly is retained, and for how long, is a product-document question worth settling before commissioning rather than after the first event of interest.
Where it lands in the control system
A clearance monitor is only useful if what it produces reaches something that acts. Both documented systems expose industrial interfaces — Modbus and CANopen are listed for the RVBC-3000 — and the practical questions are the same ones any added sensor raises.
Which controller consumes the value, and at what rate does it need it? Is the system expected to raise an alarm, to inform a curtailment decision, or only to record? What does the controller do when the monitor itself is unavailable? And where do the retained records live, given that they are the part with a retention period attached?
Environmental exclusions are stated in each product document. They matter here because a monitor's behaviour outside its documented envelope is exactly what a control strategy has to account for, and it is not something a specification headline will tell you.
Evidence register
Sources and limitations
Product values on this page are supplied documentation, restated. The external sources below cover the measurement problem itself and are independent of this catalogue.
- Helming et al., Wind Energy Science 8, 421 (2023) — open-access primary study measuring rotor blade deformation and tower–blade tip clearance on a 3.4 MW turbine (128 m hub height) with a terrestrial laser scanner at 190–200 m, scanning at 30 Hz. The authors report a spread of roughly 1.5 m between minimum and maximum measured clearance and state that their analysis was possible only in partial-load operation; full-load analysis would need SCADA or external wind data.
- IEC 61400-1:2019 — design requirements for the structural integrity of wind turbines, covering control and protection functions, mechanical systems and support structures.
- IEC 61400-5:2020 — the blade-specific part of the same series.
Limitation: the cited study measured one turbine with one instrument under partial load; it establishes that clearance varies measurably with load, not a performance figure for any monitoring product. Neither IEC part is reproduced here, and no minimum permissible clearance is stated on this page — that value belongs to the turbine's own design documentation. The 50 cm accuracy fields are supplied product values under the conditions their documents state.
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 does a stated clearance accuracy figure actually bound?
It bounds the measurement under the conditions the product document states, against a defined target. A blade tip is a large, fast, non-cooperative target whose closest approach lasts a fraction of a second, so an accuracy figure has to be read together with the acquisition rate — a precise measurement taken at the wrong instant does not describe the minimum clearance.
Why fuse radar with a camera instead of using one sensor?
They fail differently. Radar ranges through conditions that defeat optical sensing and does not depend on illumination; a camera supplies the visual context that makes a recorded event reviewable by a person. Fusing them means the range measurement and the evidence a human can audit come from the same event chain rather than from two unrelated systems.
Does a higher acquisition rate always give a better minimum-clearance figure?
It reduces the chance of sampling either side of the closest approach, which is the main systematic error in this measurement. It does not improve the underlying ranging accuracy, and it is bounded by what the documented processing chain supports. Both figures are worth reading together rather than optimising one.
What has to be retained for a clearance event to be useful later?
A measured value alone is difficult to act on months later. A retained event that carries the range measurement, the visual context, the time, and the operating state around it can be reviewed, compared against later events and used in a decision. Which of these are retained is a product-document question and should be confirmed before commissioning.
Engineering review
Match the monitoring approach to the evidence you need
Send the turbine platform, the site conditions and the record your process has to keep, and the documented configurations can be compared against them.