The key terms of industrial metrology, briefly explained — for everyone who commissions a survey, accompanies one or has to read a measurement report.
What measurements are made with — and what the figures in a data sheet actually mean.
A portable coordinate measuring device that directs a laser beam at a reflector and follows it automatically. The spatial position of each probed point results from one distance and two angles. Unlike a measuring machine, the tracker is brought to the part, not the other way round.
The measuring principle by which the tracker determines the distance to the reflector without having to keep the beam unbroken. If the beam is interrupted, measurement can continue without returning to a home point — a decisive advantage in daily industrial operation with cranes, forklifts and moving personnel.
A precision steel sphere with a glass prism inside, whose centre point the tracker measures. The reflector is placed against the part or seated in a nest. Its spherical shape means the centre is always determined — the probing direction is irrelevant.
The statement of how far a measured value may deviate from the true value. For a laser tracker it is given as a formula, for example ±15 µm + 6 µm/m: a fixed portion plus a portion growing with distance. A blanket accuracy figure without reference to the measurement distance therefore says little.
How closely several measurements of the same point lie together. It is always better than the measurement uncertainty because systematic components cancel out. For the question of whether a plant has shifted since the last survey, it is the decisive figure.
A built-in sensor that detects the direction of gravity. This defines the vertical physically within the measuring system rather than deriving it by calculation — important wherever level, horizontality or inclination are checked, for instance on crane runways and foundations.
How coordinates become a basis for decisions.
The coordinate system to which all measured values refer. It is defined at the outset, for example from a main machine axis, an installation plane and a zero point. Without an unambiguous reference system, measured values are numbers without meaning — the choice of reference decides what a deviation actually means.
The comparison of the measured geometry with the required one, for example from a drawing or CAD model. The result is deviations per point or component, from which correction values are derived — that is, by how much and in which direction something must be moved.
A computational method that fits a measured point cloud into a nominal geometry as well as possible, so that the remaining deviations are as small and evenly distributed as possible. Useful when no single surface is decisive and the component as a whole is to be assessed.
Large plants can rarely be captured completely from a single station. The tracker is moved and tied back into the same coordinate system via common reference points. Network adjustment distributes the resulting residual discrepancies mathematically and reveals how reliable the linking is.
The traceable documentation of a survey: reference system, equipment, ambient conditions, measured values, deviations and correction values. It is the document with which one can argue towards a supplier, client or inspection body — and the basis for any later repeat survey.
The capture of a plant's as-built condition, in contrast to the planned one. It is the basis for conversions, retrofits and relocations, because plants that have grown over time almost never match the existing drawings.
The geometric quantities that matter in practice.
The permissible deviation of a surface or axis from its ideal shape (form) or from its nominal position relative to a datum (position). Flatness and roundness are form tolerances; parallelism and perpendicularity are position tolerances. Inspection follows an inspection plan, not intuition.
Three fundamental quantities of machine geometry. Flatness describes how far a surface deviates from an ideal plane; straightness the same for a line, such as a crane rail; parallelism the angular error between two axes or surfaces, for instance between two rolls.
Establishing the required spatial position of components relative to one another. Measurement is against a reference system; correction is by shims, adjusting screws or shifting. The difference to pure measurement: at the end there is not a value, but a plant that fits.
The reference axis to which all other components of a plant are aligned — for example the running direction of a press or the axis of a furnace. If it is defined before the foundation, later corrections to individual parts are minor interventions; if not, they become expensive.
The inspection quantities of a crane runway: gauge (rail spacing), level (vertical position), straightness (lateral course) and inclination (cross fall of the rail). Exceedances cause wear on wheels and rails and, in the extreme, a ban on operation.
A belt runs where the geometry sends it. If one roll is skewed to its neighbour by only tenths of a millimetre, the belt drifts sideways. The control system can compensate for a while — but only as long as it is not working at its limit.
What is meant on site and in a quotation.
A measurement proving the contractually or legally required condition — at handover of a plant, before commissioning or during a recurring inspection. The result is a document on the basis of which approval can be granted or refused.
Because the tracker captures position continuously, a deviation becomes visible while the adjustment is being made. Fitter and metrologist work at the same time: move, read, verify. A second appointment for checking is not needed.
The period after assembly in which a plant is gradually brought to correct operation — usually by adjusting, observing and adjusting again. If the geometry is known beforehand, this phase is considerably shortened or eliminated.
The systematic search for the cause of a malfunction rather than its effect. In practice this means measuring the geometry before replacing components. Frequently it turns out that a supposedly defective part is merely in the wrong position.
If something in a quotation or measurement report is unclear, just ask. A brief explanation costs nothing and avoids misunderstandings that become expensive later.