Optoelectronic sensors equipped with lasers and red-light technology are indispensable across various industrial automation sectors. These sensors usually require a clean, dust-free, and dry environment to function optimally. However, there are numerous applications where optical sensors prove to be the most suitable choice despite the presence of harsh and dirty conditions. Notably, steel mills, from raw steel production to various metallurgical processes, including casting and hot-rolling, frequently employ optical sensors. In addition to the challenging dust and debris factors, extreme heat is often a characteristic of steel and metal production.
Optical Allrounders
One of the standout features of photoelectric sensors, when compared to other sensor types, is their extended sensing range. Moreover, these sensors possess the unique capability to detect objects irrespective of their material. While inductive sensors work best at short distances and are ideal for detecting metal objects, capacitive sensors can identify different materials from a greater distance. However, capacitive sensors are susceptible to interference from residual media such as liquids or moisture, and they can even be triggered by a simple touch with a human finger.
Beyond their widespread applications in various industries, optical sensors also play a crucial role in safety-critical functions.
Operating Sensors in High-Temperature Environments
In the steel production sector, many stages of the manufacturing process are marked by elevated ambient temperatures. Liquid steel and iron can reach temperatures ranging from 1400°C (2552°F) to 1536°C (2797°F). During continuous casting and hot-rolling, material temperatures can range from 650°C (1202°F) to 1250°C (2282°F). The temperature can often be determined by referring to steel color charts.
Optical sensors are typically rated for operation within ambient temperatures of 55 to 60°C (131-140°F). This maximum temperature is primarily limited by the specifications of the optical components, particularly laser diodes.
One might naturally assume that operating photoelectric sensors in such severe conditions is ill-advised. However, there are methods available to ensure that optical sensors function reliably even in these demanding environments.
Detecting Glowing Metals
It is essential to ensure that the target object’s material temperature remains below 600-700°C. If the goal is to reliably detect red-hot or white glowing steel parts with temperatures exceeding 700°C (1292°F), standard laser- or red-light sensors are not suitable. Red-hot steel emits light at the same wavelength used by optoelectronic sensors, which can disrupt their function. In such applications, sensors based on infrared light technology, such as pyrometers or infrared temperature sensors, are the appropriate choice.
Ensuring the Reliability of Optoelectronic Sensors in Harsh Environments
There are several strategies to guarantee the reliability and longevity of optoelectronic sensors under exceptionally harsh conditions:
Increased Distance from the Target Object: One significant advantage of photoelectric sensors is their extended range, which allows them to be mounted at a certain distance from the target object. This setup reduces the impact of heat emitted by the detected object, often maintaining the sensor at room temperature. However, in extremely hot regions, such as steel production, ambient temperatures can reach up to 50°C (122°F).
Use of Optical Glass Fibers: For applications requiring precision in detecting relatively small objects, a maximum installation distance might be limited. In such cases, chemical-resistant glass fibers are a suitable solution, capable of withstanding temperatures of up to 250°C (482°F). These prefabricated fiber optic assemblies can be easily attached to the sensor, allowing the sensor itself to be placed in a cooler and protected location.
Cooling of Sensors: Water cooling systems used in steel manufacturing can be problematic. They are costly, require instrumentation air or clean cooling water, and pose safety risks. As a result, careful consideration should be given to the installation of photoelectric sensors:
- Determine the distance from the hot object to the sensor.
- Assess the maximum temperature at this location.
- Consider the duration of extended exposure to high temperatures during normal operation and breakdown.
Protective Housings: It’s essential to protect optoelectronic sensors against temporary overheating in applications characterized by intermittent high temperatures. One effective solution is the use of protective enclosures.
- Protective Aluminum Housing: A durable die-casted aluminum housing acts as mechanical protection for the sensor, safeguarding it against extreme temperatures. The housing features a glass or PMMA window that preserves the sensor’s optical components.
- Air-Blower for Dust Control: An optional air-blower helps prevent dust buildup on the sensor, enhancing its performance and longevity.
Water Cooling: In cases where prolonged exposure to extreme temperatures is anticipated, additional cooling with water can be employed. By using a small amount of water, standard sensors can operate continuously at temperatures of up to 160°C. The water effectively stabilizes the sensor’s temperature and can even reduce it during overheating. Moreover, the water cooling can be activated only when an emergency arises, thus ensuring the sensor’s protection.
The main features of the protective housing include protection against heat, dust, and moisture, an IP67 rating (IP69K with cable sleeve), a durable die-casted aluminum housing, a glass or PMMA screen, an optional air-blower for dust control, quick and easy sensor mounting and replacement, a cable gland for protective sleeve, optional water cooling (for continuous operation up to 160°C/320°F), and ATEX-Approval for the glass version (Zone 22). Two sizes of protective housings are available, the BOS/BOD23K and the BOS50K
By implementing these strategies, optoelectronic sensors can continue to operate effectively in even the harshest of environments, overcoming extreme temperatures and ensuring reliability and longevity in demanding industrial settings.
No comment