Using Photoelectric Sensors in High Ambient Temperatures

Photoelectric sensors with laser and red-light are widely used in all areas of industrial automation. A clean, dust-free and dry environment is usually essential for the proper operation of photoeyes, however, they can be the best choice in many dirty and harsh applications. Examples of this are raw steel production in steel mills and further metallurgical processes down to casting and hot-rolling.

Cutting of billets at casting – Photo: M.Münzl
Cutting of billets at casting – Photo: M.Münzl

Photoelectric sensors are especially useful in these environments thanks to their long sensing distance and their ability to detect objects independent of their material.

Most photoelectric sensors are approved to work in ambient temperatures of 55 to 60 °C. The maximum temperature range of these sensors is most often limited by the specifications of the optical components of the sensor, like the laser-diodes, but by taking certain precautions photoelectric sensors can provide optimal use in much hotter applications.

Maximize the distance
In steel production many parts of the process are accompanied by high ambient temperatures. Liquid steel and iron have temperatures from 1400 to 1536 °C. Material temperature during continuous casting and hot-rolling are lower but still between 650 and 1250°C.

The impact of heat emission on the sensors can be reduced significantly by placing the sensor as far from the target object as possible, something you can’t do with inductive sensors which have a short range. Very often the remote mounting will allow the sensor to operate at room temperature.

If you intend to detect quite small objects with high precision, the maximum distance for the installation might be limited. For this purpose chemical resistant glass fibers are suitable and can handle temperatures up to 250 °C. These pre-fabricated fiber optic assemblies can be easily attached to the sensor. The sensor itself can be mounted on a cooler and protected place.

Detect Glowing Metals
If you want to reliably detect red-hot or white glowing steel parts with temperatures beyond 700 °C, you won’t be able to use standard laser or red-light sensors. Red-hot steel emits light at the same wavelength that it is used by photoelectric sensors. This can interfere with the function of the sensor. In such applications you need to use sensors which operate based on infrared light.

Add Protection

Sensor enclosure and protective cable sleeve
Sensor enclosure and protective cable sleeve

At many locations in the steel production process, the extensive heat is only temporary. In a hot rolling mill, a slab runs through a rougher mill multiple times before it continues to a multi-stage finishing mill stand to be rolled to the final thickness. After that the metal strip runs into the coiler to be winded up.
This process runs in sequence, and the glowing material is only present at each stage of production for a short time. Until a new slab runs out of the reheating furnace, temperatures normalize.

Standard sensors can work in these conditions, but you do run the risk of even temporary temperature hikes causing sensor failure and then dreaded downtime. To protect photoelectric sensors against temporary overheating, you can use a protective enclosure. These can provide mechanical and thermal protection to the sensors which often have plastic bodies. Additional protection can be achieved when a heat resistant sleeve is used around the cable.

Photoelectric sensors do have their limits and are not suitable for all applications, even when precautions are taken. Ask yourself these questions when deciding if they can be the right solution for your high temperature applications.

  • Which distance between the hot object and sensor can be realized?
  • What is the maximum temperature at this location?
  • How long will the sensor be exposed to the highest heat levels during normal operation and at breakdown?

Measuring Distance: Should I Use Light or Sound?

Clear or transparent sensing targets can be a challenge but not an insurmountable one. Applications can detect or measure the amount of clear or transparent film on a roll or the level of a clear or transparent media, either liquid or solid.  The question for these applications becomes, do I use light or sound as a solution?

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An application that measures the diameter of a roll of clear labels.

In an application that requires the measurement of the diameter of a roll of clear labels, there are a number of factors that need to be considered.  Are the labels and the backing clear?  Will the label transparency and the background transparency change?  Will the labels have printing on them?  All of these possibilities will affect which sensor should be used. Users should also ask how accurate or how much resolution is required.

Faced with this application, using ultrasonic sensors may be a better choice because of their ability to see targets regardless of color, possible printing on the label, transparency and surface texture or sheen.  Some or all of these variables could affect the performance of a photoelectric sensor.

Ultrasonic sensors emit a burst of short high frequency sound waves that propagate in a cone shape towards the target.  When the sound waves strike the target, they are bounced back to the sensor. The sensor then calculates the distance based on the time span from when the sound was emitted until the sound was received.

In some instances, and depending on the resolution required, a time of flight sensor may solve the above application. Time of Flight (TOF) sensors emit a pulsed light toward the target object. The light is then reflected back to the receiver. The elapsed time it takes for the light to return to the receiver is measured, thus determining the distance to the target. In this case, the surface finish and transparency may not be an issue.

Imagine trying to detect a clear piece of plastic going over a roll.  The photoelectric sensor could detect it either in a diffuse mode or with a retroreflective sensor designed for clear glass detection.  But what if the plastic characteristics can change frequently or if the surface flutters.  Again, the ultrasonic sensor may be a better choice and also may not require set up any time the material changes.

So what’s the best solution?  In the end, test the application with the worst case scenario.  A wide variety of sensors are available to solve these difficult applications, including photoelectric or ultrasonic. Both sensors have continuous analog and discrete outs.  For more information visit www.balluff.com.