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Presence detection in high-temperature environments: Fiber optic sensors in the metal industry
Object detection in hot rolling mills, forging machines and annealing furnaces.
Table of Contents
In automated metalworking processes, the necessary presence detection of workpieces often places high demands on sensor technology, as it is required even where it is least accessible and hottest: directly on the glowing material between rolling stands, under the forging hammer, at the shear, or in the annealing furnace. These are conditions under which conventional optical or inductive sensors are no longer suitable.
Fiber optic sensors solve this problem by separating the sensitive electronics from the actual measuring point and exposing only a heat-resistant optical fiber to the high temperature range.
This article explains why fiber optic sensors are particularly suitable for high-temperature environments, why standard sensors reach their limits here, and shows typical applications in hot rolling mills, forging machines, cutting machines and annealing furnaces.
Why fiber optic sensors for high-temperature environments?
A fiber optic sensor transmits the light emitted by the light source integrated into the sensor – located some distance from the measuring point – to the actual measuring location and the received light signals back to its receiving and evaluation unit. Only the fiber end – consisting of glass fibers and heat-resistant metal – is actually located in the high-temperature zone; the actual, temperature-sensitive electronics can be mounted several meters away in a cooler, easily accessible location.
This offers several advantages in practice:
- The sensitive electronics are protected from heat, radiant heat and contamination.
- The fiber end is very compact and can be placed even in narrow spaces, such as between rolling mills.
- Fiber optics can be replaced separately if damaged, without having to replace the entire sensor.
- Transmission via light is insensitive to electromagnetic interference fields, such as those that occur during induction heating.
Why is standard sensor technology insufficient in these areas?
Several factors occur simultaneously in the metal industry, posing particular challenges for standard sensor technology:
- Intrinsic radiation of the material: Glowing metal itself emits intense infrared light, which many standard sensors interpret as interference or a false signal.
- High ambient temperature: Electronics in sensor housings that sit directly at the measuring point quickly exceed their permissible operating temperature in these areas.
- Scale, dust and cooling water vapor: Contamination of the optics by mill scale or steam from cooling systems
- Vibration and mechanical stress: Vibrations caused by rolling mills, presses, shears and transport systems
- Limited space: There is often no space for conventional sensor housings and electronics between rolling mills or under presses.
A sensor for these areas must therefore not only be heat-resistant, but also be able to cope with the material's own radiation and be compact enough to be mounted at all.
Applications in the metal industry
Hot rolling mills
Between the rolling stands, it is essential to reliably detect whether the material being rolled is in the expected position – for example, to release the next stand, to detect jams, or to report the position to switches. Due to the tight construction and proximity to molten material, high-performance fiber optic sensors are particularly suitable because they allow for a greater distance between the fiber heads and the molten material and, because of their small installation space, can also be installed in the often confined areas between the stands.
Forging machines
Before the actual forming process begins, it must be verified that a glowing blank is correctly positioned under the hammer or press before the working stroke is triggered. Reliable presence detection prevents wasted strokes and protects the tool and machine from damage caused by a mispositioned or missing blank.
Cutting machines and shears
Before cutting, it must be ensured that the bar stock or billet is actually in the shears and correctly positioned. Fiber optic sensors reliably detect the presence of material, even at high temperatures immediately after the rolling or forging process, thus preventing blank cuts or false triggering of the shears.
annealing furnaces
When material enters and exits annealing furnaces – such as bell, continuous, or chamber furnaces – its presence must be detected to precisely synchronize transport and furnace control. Since temperatures are particularly high directly at the furnace inlet or outlet, a fiber optic solution is often the only practical way to position the measuring point as close to the process as the application requires.
Continuous casting plants
Immediately after exiting the mold and along the cooling zones of the continuous casting machine, it is essential to verify that the strand is continuous and progressing through the expected position – for example, to enable subsequent roller conveyors, to detect strand breaks, or to synchronize with the cutting machine at the end of the casting strand. Directly at the mold and in the initial cooling zones, particularly intense radiant heat and sometimes considerable steam generation due to secondary cooling prevail – conditions under which conventional sensor electronics can hardly be operated continuously. Here, too, high-performance fiber optic sensors allow the actual electronic module to be placed outside this zone, with only the compact, heat-resistant fiber end positioned within acceptable proximity to the strand.
other areas
The same basic principle is also used in continuous casting plants, induction heating lines or transfer lines between individual forming steps: presence detection directly at the hot material, without sensitive electronics having to be directly exposed to the heat.
How fiber optic presence sensors work
Fiber optic sensors for presence detection work according to the same basic principles as classic photoelectric sensors, but transmit the sending and receiving signal via optical fibers:
- Light barrier principle: The transmitting and receiving fibers are arranged separately; the material interrupts the light beam between them; particularly suitable for materials with high or varying self-emission.
- Principle of reflection: The transmitting and receiving fibers are bundled in a common head; the signal is reflected by the material itself; this reaches its limits when the material emits high amounts of heat.
- Passive sensors: Passive sensors do not have their own light source, but rather analyze the self-emission of glowing objects to detect their presence. They are significantly cheaper and more robust than thermal cameras or temperature sensors; they are used especially for applications with high self-emission.
Methods in direct comparison
| Measuring principle | Presence is detected by: | Suitable for: | Features |
| Fiber optic light barrier | Light barrier interruption | high and varying self-emission, glowing metals | Measurement from two sides |
| Fiber optic reflex sensor | Evaluation Reflection | low self-emission, non-glowing metals | Measurement from one side |
| Fiber optic passive sensor | Evaluation of own emissions | high intrinsic emissions, glowing metals | Measurement from one side |
Advantages of fiber optic solutions
- Proven, robust measurement method: Simple, reliable measurement principle compared to alternative measurement techniques (such as thermal imaging cameras, temperature sensors) with significant cost advantages.
- Electronics protected: The actual evaluation electronics remain outside the high-temperature zone.
- Insensitive to electromagnetic interference: Advantageous when used near induction heating or drives.
- Compact design at the measuring point: Enables installation even in very confined spaces
- Lower follow-up costs in case of damage: Fiber optics and sensor can be replaced separately.
- Flexible installation: Fiber optic cables can also be routed through narrow or curved cable paths.
Selection criteria for fiber optic presence sensors
- Recording principle: Fiber optic light barrier, reflection, or passive sensors depending on the self-emission of the objects: Fiber optic passive sensors for glowing objects; light barriers for high and varying self-emission; reflection sensors for low self-emission
- Installation situation: Is measurement possible from only one side, or from both sides? Should the fiber optics be designed with axial or radial light emission (requiring less installation space)?
- Range or sensing range: How large must the measuring distance be to avoid exceeding the permissible temperature load of the fiber optic heads (i.e., how close to the objects being measured can they be mounted)?
- Length of fiber optic cable(s): How far from the measuring point must the sensor electronics be placed to avoid exceeding their permissible temperature load? What length of fiber optic cable, or fiber optic arms, does this entail, taking into account the cable run?
- Temperature resistance of fiber optics: Maximum continuous and peak temperature at the actual installation location
- Protective design of the fiber optics: If the fiber optics need to be protected against potential liquid ingress, a liquid-tight protective casing is required.
- Control interfaces: Binary or analog interface? Is an additional output signal for contamination monitoring required?
- Mechanical protection: Additional protective fittings against scale, splashing water or mechanical stress
- Cleaning needs: If necessary, purge air purge to protect the optics from dust and scale.
Response time: Required reaction speed for the respective machine control system
Why not simply make all the sensor electronics heat-resistant?
High-temperature electronics are generally significantly more complex and expensive, and are only available up to certain temperature limits. Furthermore, the required measuring distances rule out, for example, inductive high-temperature sensors from the outset.
Can fiber optic sensors reliably detect even intensely glowing material?
Yes, in particular fiber optic light barriers or passive sensors are specifically designed not to consider the self-radiation of glowing material as an interference signal, or even as an object to be detected.
Can fiber optic sensors also be used in very confined spaces, such as between rolling mills?
Yes, that is one of the main advantages: Since only the compact fiber end is located at the actual measuring point, the sensor can also be placed where there would be no room for a complete sensor housing.
How resistant are fiber ends to thermal and mechanical stress?
This depends on the specific fiber optic type: conventional fiber optics from Sensorik Austria are suitable for ambient temperatures up to 200°C, special types even above that.
The actual stress on the fiber optics can be influenced by the distance to the heat source. Therefore, fiber optic systems from Sensorik Austria offer particularly high light intensities to handle even greater measurement distances.
Fiber optics with protective lenses are also available to protect against scale or similar substances.
Fiber optic high-temperature sensors for presence detection from Sensorik Austria
Sensorik Austria develops optoGuard-HT fiber optic and classic presence sensors specifically for high-temperature areas of the metal industry – from hot rolling mills to forging machines and cutting machines to annealing furnaces.