Basic PrinciplesIn industrial plants, temperature plays an important role as an indicator of the condition of a process, product or piece of machinery. Precise temperature monitoring improves product quality and increases throughput. It also minimizes downtime, since production processes can proceed uninterrupted under optimal conditions.
Metals-processing operations use noncontact IR sensors to obtain accurate temperature measurements in a wide range of automation applications. IR instruments measure temperature according to Planck’s Law of black-body radiation, which states every object emits radiant energy, and the intensity of this radiation is a function of the object’s temperature. The sensor simply measures the intensity of radiation, thereby measuring an object’s temperature.
An IR thermometer can be compared to the human eye. The lens of the eye represents the optics through which the radiation (flow of photons) from the object reaches the photosensitive layer (retina) via the atmosphere. This is converted into a signal that is sent to the brain after being compensated for ambient temperature variation (Fig. 1).
Every form of matter – with a temperature above absolute zero – emits infrared radiation according to its temperature. This phenomenon, known as “characteristic radiation,” is caused by the internal mechanical movement of molecules. The intensity of this movement depends on the temperature of the object. Since the molecule movement represents charge displacement, electromagnetic (EM) radiation (photon particles) is emitted. These photons move at the speed of light and behave according to the known optical principles. They can be deflected, focused with a lens or reflected from reflective surfaces.
Noncontact and Other Considerations
The infrared-sensor configuration enables temperature measurement from a distance without contact with the object to be measured. As such, an IR device is useful for measuring temperature under circumstances where thermocouples or other probe-type sensors cannot be used or do not produce accurate results. These include applications where the object to be measured is moving; where the object is surrounded by an EM field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where a fast response is required.
Critical considerations for employing IR thermometers include:
- Field of view (target size and distance)
- Type of surface being measured (emissivity considerations)
- Spectral response (for atmospheric effects or transmission through surfaces)
- Temperature range and mounting (handheld portable or fixed mount)
- Response time
- Viewing port or window applications
- Desired signal processing
Technology AdvantagesTraditionally, heat treaters have depended on two elements - power and time - to harden, form or mold metal into high-quality products. By adding a crucial third element to control temperature and quality, they can ensure consistent and dependable production.
Combining noncontact infrared temperature measurement and data-acquisition software, IR sensor systems control a set-point temperature. The temperature of each part is read and recorded by the sensor as the software produces an accurate production record. Upon reaching the set-point temperature, the induction heater is automatically shut off, maintaining the correct temperature (Fig. 2).
Today’s advanced infrared sensors take temperature measurement a step further. Simultaneous analog and digital outputs allow temperature data to be integrated into a closed-loop control system and simultaneously output for remote temperature monitoring and analysis.
Smart IR sensors, with digital electronics and two-way communications, can be configured remotely from the safety of the control room – especially important for metals with changing emissivities. The result: increased functionality and greater control.
For induction heat-treating operations, the advantages of using noncontact IR temperature measurement include:
- Improved process control
- Increased productivity
- Higher throughput
- Reduced energy costs
- Less downtime
- Higher-quality products
- Lower scrap rate
- Improved maintenance
- Easy data recording
Recent InnovationsSensor manufacturers have pioneered new advancements in noncontact infrared thermometers, line scanners and imaging systems, addressing a host of application challenges. Today’s IR sensor technology provides better accuracy, higher reliability and greater ease of use than ever before.
Infrared thermometers can now be used to monitor temperatures of dynamic processes quickly and efficiently. Unlike other measurement devices, they measure the temperature of the product directly - allowing users to quickly adjust process parameters to optimize product quality. IR instruments also increase production efficiency and improve yields by enabling smaller units of measurement and a greater accumulation of temperature data. Temperature measurements can be made of a large area or a small spot.
Recent innovations in IR thermometer design include:
Integrated Sighting Techniques
Instrument designers have developed sensor platforms, providing integrated through-the-lens target sighting, plus either laser or video sighting. This combined approach ensures correct aiming and target location under the most adverse conditions (Fig. 4).
IR sensors may also incorporate simultaneous real-time video monitoring and automated image recording and storage to help users obtain valuable new process information. Operators can quickly and easily take snapshots of the process and include temperature and time/date information in their documentation.
Modern IR thermometers offer twice the optical resolution of earlier, bulky sensor models, extending their performance in demanding process-control applications and allowing direct replacement of contact probes.
Newer IR sensors utilize a miniature sensing head and separate electronics, achieving up to 22:1 optical resolution and operating in temperatures approaching 200°C (392°F) without any cooling. This design allows accurate measurement of very small spot sizes in confined spaces and difficult ambient conditions. The sensors are small enough to be installed just about anywhere, and when housed in a stainless steel enclosure, can withstand harsh industrial processes.
Innovations in IR sensor electronics have also improved signal processing capabilities – including emissivity, sample and hold, peak hold, valley hold and averaging functions. With some systems, these variables can be adjusted from a remote user interface for added convenience (Fig. 5).
Remote-Controlled, Variable Target Focusing
IR thermometers are also available with motorized, remote-controlled variable target focusing for fast and accurate adjustment of the focus of measurement targets, either manually at the rear of the instrument or remotely via an RS232/RS485 PC connection, where adjustments can be seen real-time through video.
With remote-controlled, variable target focusing, engineers can fine-tune the sensor’s measurement target focus from the safety of their own office and continuously observe and record temperature variations in their process in order to take immediate corrective action. The variable target focusing capability is particularly useful for large, multiple-sensor installations, where sensors are periodically replaced or the distance to the measurement object changes.
Suppliers are further improving the versatility of infrared temperature measurement by supplying systems with field-calibration software, allowing users to calibrate sensors on-site. Plus, IR systems offer different means for physical connection, including quick-disconnect connectors and terminal connections; different wavelengths for high- and low-temperature measurement; and a choice of milliamp, millivolt and thermocouple signals.
Instrumentation designers have responded to emissivity issues associated with IR sensors by developing short-wavelength units that minimize errors due to the uncertainty of emissivity in low-temperature applications like annealing. These devices are not as sensitive to changes in emissivity on the target material as conventional, high-temperature sensors. As such, they provide more accurate readings across varying targets at varying temperatures (See Fig. 6).
IR temperature-measurement systems with automatic emissivity-correction mode enable manufacturers to set up predefined recipes to accommodate frequent product changes. By quickly identifying thermal irregularities within the measurement target, they allow the user to improve product quality and uniformity, reduce scrap and improve operating efficiency. If a fault or defect occurs, the system can trigger an alarm to allow for corrective action.
Enhanced Data-Acquisition SoftwareMetals processors are finding that noncontact infrared technology employing enhanced data-acquisition software can help streamline production and increase product throughput. When using an IR temperature-measurement system for induction heating, for example, operators can pick a part number from an existing temperature set-point list and automatically record each peak temperature value.
ConclusionToday’s noncontact infrared temperature-measurement technology can help improve the reliability and efficiency of the most demanding metals-processing operations. End users save thousands of dollars – and hundreds of man-hours – by implementing advanced IR temperature-measurement systems.IH
For more information:Frank Schneider is worldwide product manager for all point sensor products for Raytek Corporation, 1201 Shaffer Road, Santa Cruz, CA 95061; tel: 831-458-1175; fax: 831-458-1239; email: Frank. Schneider@raytek.com; web: www.raytek.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: IR temperature measurement, characteristic radiation, IR sensor, electromagnetic radiation, emissivity
SIDEBAR 1 - Choosing the Right ThermometerInduction heating systems provide a popular method for localized heating of selected areas within a part, or they can be used to heat the entire mass of a part. Induction systems, therefore, come in many shapes and sizes. The selection of an optimum infrared thermometer for use on an induction heating system is highly dependent upon the system geometry and the geometry of the part within the coil.
Induction heating systems generate heat by creating an alternating electrical field around the object to be heated. An alternating electrical eddy current is induced within the part, thereby creating the heat. The system efficiency and consistency depends upon several variables, including the inductive coupling between the induction coil and the part, the alignment of the part within the coil, the power delivered to the coil, the conduction of heat within the part and ambient conditions, including air movement (Fig. 3).
A short-wavelength, single-wavelength IR thermometer is recommended for those applications where a metal is to be heated to a low or moderate temperature and for which the emissivity variation is low or moderate. Typical examples of this type of application include preheating or drying parts at temperatures below 600°F (315°C).
A dual-wavelength IR thermometer is recommended for those applications where the target emissivity is likely to vary or when it will be difficult to align the sensor to a small heated area. This includes a wide range of tempering and annealing processes. Because of the ability to compensate for temperature gradients, smoke and scale, the dual-wavelength sensor is particularly popular for the measurement of mild-steel parts when any of these conditions occur.
SIDEBAR 2 - Typical ApplicationNoncontact infrared thermometers are designed for use in metals-processing plants where monitoring and controlling temperature is critical to productivity and product quality.
Temperature readings show whether processes are operating within their proper ranges, whether a reheater is too cold or too hot, whether a stand needs adjusting or how much cooling should be applied. Each stage can be accurately monitored so the final part retains correct metallurgical properties as it travels through the production process.
In a zone annealing operation, for example, induction heating is performed in an environment where temperatures can reach 1200°F (650°C) and beyond. The annealing process must be performed without damaging very fragile parts. In the case of electronic components, this could include the crimp barrel of a pin and socket contact.
During zone annealing, set-point temperatures are often determined by the type of metal or alloy used for production as well as other physical characteristics of the part. Zone annealing becomes increasingly difficult, however, when dealing with small-diameter parts. Heating, temperature detection and release of the part into the quench container must be performed rapidly to ensure proper annealing.
A growing number of manufacturers have discovered infrared temperature-measurement technology improves the performance of their induction heat-treating processes. In the past, production operations relied on timers to control induction-heating sequences, but this method can be inconsistent from part to part. Operators often have difficulty synchronizing feeding and heat cycles, resulting in mis-annealing, over-annealing or incomplete annealing.
An IR sensor system optimizes zone annealing processes by enabling accurate temperature detection down to a very small target size - and at a greater measurement distance. Operators can pick a part number from an existing temperature set-point list, with each peak temperature value recorded to an automatically created folder. This solution improves process efficiency by eliminating sorting and allowing faster cycle times, which enhances control of the annealing zone and increases productivity.