Why a laser displacement sensor catches strip thickness drift before it winds into the coil

A finished coil of aluminium or steel strip hides any defects that may have come from the rolling line during manufacturing. Any thickness deviation in the middle of a coil cannot be inspected that length of off-spec strip is wound in with the good material. Therefore, the coil has to be unwound to find it.

There is also no reliable quality checking to catch the fault. It is only noticed when the customers use the product and found that it did not meet the standard.

A laser displacement sensor measures strip thickness continuously as the strip passes through the conveyor belt, without touching it, using the laser triangulation principle. Two laser sensors can be mounted facing each other across the strip. This configuration allows the calculation of live thickness value from distance readings that the mill operator can watch as a trend and act on while the drift is still a few metres long.

Where thickness drift on a rolling line comes from

The roll gap that gives the right thickness at the start of a process may not stay in its stable position. As the mill stands warm up, the roll gap changes with the thermal growth of the rolls and the housing.

Roll wear starts to accumulate through the process and alters the gap profile across the strip width. Additionally, the incoming coil may arrive at varying temperature, which changes how it behaves under the same rolling force.

None of these causes a sudden change in measurement. However, as each strip moves the exit thickness gradually, you would expect a gradual drift.

Offline sampling works like this. An operator cuts or marks a sample, measures it with a contact gauge such as a micrometer, offline calliper or a snap gauge, logs the result and returns to the production line. Between sampling, the strip keeps running and winding.

On a line running at 2 m/s, a ten-minute sampling interval is 1200 m of strip. If a drift starts just after one sample, most of that length is out of specification before the next sample confirms it.

Once that off-spec strip is inside the coil, it cannot be recovered cheaply. The off-spec section sits between two good sections and its exact position is unknown. In most cases, the affected length is written off rather than cut out.

The real cost of the measurement gap is the material wound into the coil while nobody was measuring.

How a laser displacement sensor measures without contact

A laser displacement sensor projects a small laser spot onto the strip surface. A receiving lens inside the sensor collects light scattered back from the spot and focuses it onto a detector array.

The laser and the receiver are set at a fixed angle, so the spot lands at a different point on the detector as the distance to the strip changes. The sensor electronics turn that spot position into a machine-readable distance value. The sensing element has no moving part and the sensors does not touch the strip, which is what allows it to run at line speed.

The measuring rate sets how fast the strip is sampled along its length. Divide the line speed by the measuring rate to get the spacing between readings. At 4 kHz and 2 m/s, the sensor reads the strip every 0.5 mm.

For this thickness control application, the sensor does not need that many data points. Therefore, we can take the average reading which steadies the value against surface texture and gives the operator a smooth trend.

Choosing between single sensor against a roller, or two sensors across the strip

There are two ways to turn distance readings into thickness on a strip line. Choosing between both methods depends on how the machine and roller configuration are set.

The single-sensor method can be applied when a roller that the strip wraps around is under tension, so the underside of the strip is held still against a surface at a known position. One sensor above the roller measures the distance to the top of the strip. The distance from the sensor to the bare roller surface is measured once with no strip present.

The strip thickness is the difference between the two. This works where the strip is in firm contact with the roller at the measurement point and the strip runs without vertical movement, because any runout or eccentricity in the roller appears in the reading as thickness.

The two-sensor method suits a free-running strip between rolls. One sensor sits above the strip and one below it on a rigid frame with a fixed gap between them. Each measures the distance to its own side of the strip, and the thickness is the frame gap minus the two readings.

This configuration accounts for vibration and bounce between the rolls as the strip move towards one sensor and it also move away from the other by the same amount, so the calculated thickness holds while the two individual readings swing.

The two conditions for this to work are alignment, so both spots sit at the same point on the strip from opposite sides, and time synchronisation, so both readings describe exact measurement at the same time.

What a metal surface does to a laser measurement

Rolled metal is a difficult target for an optical sensor due to its reflective surface, and the difficulty changes along the strip. A bright, freshly rolled surface reflects the laser spot like a mirror rather than scattering it, which means very little light returns to the receiver at the triangulation angle and the detector do not get enough diffusive light depending on the surface angle.

As the strip cools and oxidises, the surface dulls and light scatters more evenly, allowing for better measurement. Additional, oil films, scratches and rolling patterns also add local changes in brightness from one millimetre to the next.

The MicroEpsilon optoNCDT laser triangulation sensor addresses this challenge through its surface compensation feature. It allows the sensor to adjust its exposure reading by reading so the detector receives a usable signal from bright and dull surfaces alike, rather than a value that jumps as the reflectivity changes.

Otherwise, it also offers a laser line version that is specifically design for this measurement application. Instead of a round spot, the sensor projects a short line onto the surface and averages across it, which smooths out the speckle and local interference that a shiny or structured metal surface produces on a single spot. On our range, the optoNCDT 2300LL is the laser line triangulation sensor specified for metallic, rough, glossy, patchy and porous surfaces.

If the application demand measurement on hot strip, it is a separate problem. A red laser measuring a glowing surface competes with the infrared light the metal itself emits at elevated temperature. In this scenario, a blue laser sensor works better to provide accurate measurement as it operates at a shorter wavelength, away from the peak of that thermal emission, and holds its reading on red-hot glowing metal where a red laser sensor does not.

Choosing a laser displacement sensor for strip thickness

Which laser displacement sensor suits measurement of coil thickness at ambient temperature?

For a general thickness measurement application, the optoNCDT 1420 covers measuring ranges from 10 mm to 500 mm with ±0.1% FSO linearity and measures at up to 4 kHz. Where the strip surface is bright or patchy and a tighter tolerance is needed, the optoNCDT 2300LL covers 2 mm to 50 mm with ±0.08% FSO linearity, measures at up to 7.5 kHz with the laser line optic and anti-speckle feature.

Which sensor suits measurement on hot metals or fast-moving strip?

The optoNCDT 2300BL is the blue laser version of advanced laser triangulation sensors ILD2300, with measurement range from 2 mm to 50 mm with ±0.08% FSO linearity, an adjustable measuring rate to 49.14 kHz, and analogue, Ethernet, EtherCAT and RS422 output. It is specified for high-speed measurement of red-hot glowing materials.

How does the thickness value reach the mill control system?

An analogue output into the mill PLC gives a live thickness signal without changes to the control logic, and alarm limits set at the tolerance window flag a developing drift to the operator. A graphical digital panel meter can take both sensor inputs, do the two-sensor subtraction on board and display the thickness at the line with a relay alarm.

Not sure which laser displacement sensor suits your strip line?

The choice depends on four things: whether the strip is held on a roller or runs free at the measurement point, which decides between one sensor and two sensors; the strip speed and tolerance, which determine what sensors to use based on measuring speed and accuracy; the surface, which decides between a spot sensor with surface compensation, a laser line model and a blue laser model; and the mill control system, which decides which output signals you require.

If you are looking at continuous thickness measurement on strip, sheet or extruded product, have a chat with us about your application.

Our applications engineers can help you define the project scope, select the suitable sensors and assist with system integration and technical support.