I talked to one of our Pepperl+Fuchs account managers after he spent an afternoon testing a sensor at a customer site that manufactures steel. Sensing hot steel has always been a challenge. Using an inductive prox sensor or mechanical limit switch is just not going to happen, because a slab of steel at 1400 °F will render them useless at less than an inch away. Having a puddle instead of a sensor doesn’t do anyone much good, right?
Using infrared (IR) radiation detectors is great… as long as the slab doesn’t stop on the line too close to the detector, as prolonged exposure to the high-temperature slab could melt the detector. But the main problem with them is their cost. IR detectors are usually housed in heavy-duty housings, are water-cooled, and cost thousands of dollars.
A regular photoelectric sensor can be problematic if the hot metal is radiating all kinds of its own light. When that happens, the reflected light from the sensor that makes it back to the sensor’s receiver gets drowned out by the “noise” of the metal’s radiated light, making it difficult to sense the metal.
This application is located at a hot-strip mill in the Midwest. Simply put, the process involves taking a slab of steel that is six- or seven-inches thick, reheating it, and running it through a series of rollers to change the shape and thickness of the slab to meet customer requirements.
Engineers at the mill were looking for a way to detect the presence of the strip or slab of steel at certain points of the process between the roughing mills, which start shaping the roll, and the finishing mills, which complete the process.
They tested our new VDM28-15 laser measurement sensor. Because it operates based on Pulse Ranging Technology (PRT), it emits extremely high bursts of light periodically rather than a continuous emission of lower light energy. Those high-energy bursts of light are emitted from the sensor to the steel and reflected back to the sensor at long distances, which reduces the ambient temperature at the sensor’s location. Using this technology, the VDM28-15 laser sensor reliably detected the hot slab of steel at temperatures of 600 °F to 1400 °F.
Another headache for hot steel detection is slag. Slag is the black scale that can appear on a section of hot metal as the metal corrodes. Sensors that reflect light off the steel see a drop in the reflected light signal with dark black slag, as it absorbs light, and the sensor sees less light energy reflected back to it. But the VDM28-15 was able to detect the black slag or hot metal both consistently—no difference in performance, due to its use of PRT.
Also, because the VDM28-15 has a laser light spot less than 0.5 inches in diameter, it can easily detect the thin edge of a slab of steel. Non-lasers, with their larger light beams, would be hard-pressed to do that.
The laser sensor was tested about six feet away from the metal, and the customer is going to install a metal shield in front of the sensor with an aperture of about 0.5 inch diameter for the laser beam to pass. A single VDM28-15 is a fraction of the cost of the IR detectors they were using. Instead of paying thousands of dollars, they’re looking at hundreds of dollars.
And, because this sensor is a distance measurement sensor, if they want, they can not only detect presence of the steel roll, but measure its thickness.
Thanks to the account manager, now I know a reliable way to detect hot steel at a fraction of the cost of conventional methods. After all, sensing 1400 °F metal is just something I couldn’t have really tested myself in the office.