Temperature measurement is an important aspect of induction-heating control. Proper temperature ensures that the parts will meet the metallurgical parameters needed by the end user. This article discusses measuring temperature in induction-heating processes, with examples focusing on non-contact infrared temperature measurement.



Fig. 1. Induced current flow via magnetic fields

The Basics of Induction-Heating Systems

Induction heating uses high-frequency electrical current to heat materials that are electrically conductive. This method relies on “inducing” a current flow in a conductive workpiece through alternating magnetic fields. As the molecules are rapidly moving back and forth, they produce heat.

The basic components of an induction-heating system include a power supply, an induction coil and the workpiece. As current is sent through the coil, the coil generates a magnetic field. Then, as the workpiece is passed through the magnetic field, the part becomes heated (Fig. 1).

The degree of the object’s magnetism affects the speed of heating, and the amount of current and frequency in the coil’s magnetic field determines how hot the part will get and how deeply the heat will penetrate the object. Typically, surface-hardened processes are done at high frequencies, and deep-heating processes (such as heating billets for forging) are done at low frequencies in order to heat the entire billet.

The induction coil is copper tubing that has water running through it so that it is kept at a temperature well below 100°C. Thus the hottest part in the system is the part being heated.

Benefits of Induction Heating
  • Controllable – You can localize the heating to isolated areas of a workpiece depending on the coil design.
  • Non-Contact – Nothing touches the workpiece in order to heat it. This can be a critical factor in situations where the material being heated must avoid contaminants or where the part is moving.
  • Clean – Because heat is actually generated by the workpiece as opposed to applied to it, there are no open flames, gases or fumes (unless generated by the workpiece).
Typical Applications
The most widely used applications include high-temperature metalworking such as melting, welding, forging, brazing and extruding processes and surface-hardening techniques (such as hardening for gears, bearings, hand tools, saw blades, drive shafts, axles, etc.). It is also commonly applied to continuous processes such as annealing and production of wire, rods or tubing.

Other lower temperature and innovative induction-heating application examples include:
  • Temporary expansion of tight-fitting parts during assembly processes
  • Container lid safety-foil sealing
  • Paint drying
  • Food preparation
  • Surgical-tool sterilization


Fig. 2. Typical infrared thermometer components

Induction Heating Temperature-Control Methods

Several methods may be employed to control induction-heating process temperatures. The most crude and least precise methods are simple color observation of the workpiece. Other methods include the following.

Timers
With this method, the length of time that the workpiece stays within the coil sets the temperature. Though this method may work fine with some processes, it offers no indication of the actual temperature. If temperature is a critical parameter, the only way to determine the process time is to make several test runs, and this often means many scrapped parts to get the right time factor.

Power Detection
With this method, power input measurements to the coil assume the right temperature. As with timers, there will be a learning curve and scrap during startup processes, and temperature is never actually measured. This method also does not account for the fact that if the workpiece is placed in the coil in slightly different locations every time, temperature consistency and accuracy will vary.

Thermocouples
Thermocouples are often not a viable option because they require touching or placement very near the workpiece to be effective. In addition, because they are mostly metallic, they may be affected by the magnetic field and also become heated, thus presenting an incorrect temperature reading.

Infrared Temperature Sensors
Similar to how induction heaters can rapidly heat an object without needing to touch it, infrared temperature sensors can rapidly measure temperature of an object without needing to touch it. This offers a number of advantages:
  • Measure actual workpiece temperature with repeatable accuracy – The temperature of the piece being heated can be directly and precisely measured even at a distance.
  • Fast response to measure fast-moving parts – Quick response time and no contact requirement also means that moving parts are easily measured.
  • No contamination risk – With highly sensitive processes, non-contact means there is no risk of contaminating the workpiece.
  • Low process interference – They can be placed well outside the magnetic field and process environment, saving valuable configuration and setup time.
Every object in the world radiates infrared energy, and the amount of radiant energy emitted is proportional to its temperature. Infrared thermometers measure the intensity of the radiant energy and produce a signal proportional to the product temperature (Fig. 2).

A wide variety of infrared temperature-sensing options are available today, and selection and configuration can be complex. The workpiece material makeup, surface properties and particular induction-heating setup all must be understood as all may affect readings. Beyond this, integration with process-control equipment and data-collection schemes adds even more complexity.

Though not comprehensive, the following sections cover examples of infrared thermometer induction-heating applications for illustration purposes.

Induction-Heating and Infrared Temperature-Measurement Applications

The application of infrared temperature measurement serving induction-heating processes may take a variety of forms. Products available today range from portable, handheld devices to fixed-mount, dedicated-function spot thermometers and scanning and thermal-imaging systems. Outputs from these devices also vary from simple temperature data collection and alarming to comprehensive 2-D and 3-D imaging and complex process-control integration.

Though not a comprehensive list, the following are a few induction-heating examples involving fixed-mount infrared temperature devices intended to highlight the unique measurement characteristics, benefits and consideration factors.

From a fixed-placement infrared sensor installation perspective, production temperature measurement will typically occur under two scenarios.
  • Stationary Process – Measurement occurs as the item is in a stationary state (even if temporarily) while it is in the process of heating up to assure uniformity and that it has reached ideal temperature state.
  • Continuous Process – Measurement occurs as the item is moving – having already reached ideal temperature – to assure ideal temperature is maintained.
In either scenario, the temperature reading will be used to determine whether adjustment of heating time or power is required and whether the object is ready to move on to a next stage.

Fortunately, infrared temperature sensors can serve both scenarios well due to their inherent rapid-measurement capabilities. They come in a variety of forms, from spot thermometers to scanners and imaging cameras, depending on the production requirements.

Fig. 3. Billet heating using two sensors

Example 1: Stationary Billet Heating

Key Challenge – The entire billet has to be heated to the same temperature, thus the center of the billet must be the same as the exterior.

Heating of billets for forging, extrusion or brazing is a common application of induction heating. Billets – often several inches thick and several feet long – get heated in long induction-heating coils. These are equipped with infrared thermometers to assure that the billet reaches an appropriate temperature throughout before it can move on to a next step.

This process uses two sensors. One of them views the outside temperature and uses PID contol to hold this temperature at the desired set point. The second sensor views the center of the billet. With on/off control, it runs the process until the core is the same temperature as the outside. The billet is now ready for extrusion or forging (Fig. 3).

Fig. 4. Shows proper mounting of IR sensor to measure temperature of the part inside the induction coil

With this method of heating, the part is inserted in the coil and removed when it reaches the correct temperature. This requires that the part be measured while it is inside the coil, and therein lies a problem. The instrument has to look between the coils and often several inches of insulation to measure the target. There is usually not enough space to view the target without some type of interference, even if a hole is made for a viewing area (Fig. 4).

The problem is to get an opening large enough to be sure the cone of vision is unobstructed. Often a two-color instrument may be used to solve the problem because they are not affected by objects in the line of sight.

Fig. 5. Heat from induction coil welds seam on tubing

Example 2: Continuous Tube or Pipe Production

Key Challenges:
  • Measurement areas are continuously moving
  • Environmental obstructions (water and steam) interfere with the temperature viewing area
  • Limited sensor mounting space
With applications such as tube welding, heating of long bars or annealing of wire or cable, the workpiece runs continuously through the coil. With these processes, the infrared thermometer is typically placed just past the coil exit, where the workpiece should be at its hottest. With closed-loop control, the thermometer can monitor and regulate the power to the coil to assure consistent temperature.

With the tube-welding example, tubing starts as a flat strip. It is then rolled into the round shape and run through an induction coil, where the heat welds the seam to become one continuous piece. Placement of the infrared thermometer immediately after the induction coil assures that the tube temperature is at the optimum temperature for welding as the final rollers push the two edges of the seam together (Fig. 5). The two-color thermometer is chosen for this process due to the small size of the weld. Often another induction coil is located further down the line to anneal the tube. Again, an infrared thermometer can control the annealing process.

For this application, the tube is usually moving through the coil and immediately quenched with water. The problem is how to see the part as it exits the coil – there is a wall of water in the line of sight, and the thermometer cannot see through it.

Fig. 6. Induction coil reheats coated product resulting in coating reflow

Example 3: Coatings

Key Challenge – Changes to surface properties affect infrared temperature-measurement characteristics.

Another induction-heating application where temperature measurement may be critical is metal-coating processes such as galvanizing or powder- or paint-coating processes. With each of these applications, the goal is to assure peak adhesion or bonding of the coating to the base surface.

In the example of galvanizing and tin reflow lines, the coating itself is melted and applied to the surface. The product then passed through an induction coil to be reheated to reflow the coating. An infrared thermometer is typically aimed right at the exit of the coil, and it is configured to control the coil power and strip temperature (Fig. 6).

For powder coating, the metal part is heated before the coating is applied. The bare metal temperature is the critical parameter. The powdered coating is applied to the hot metal and cured with the residual heat in the metal part.

The painting process usually involves long or continuous products and induction heating is often used as a much faster and less-expensive alternative to extending an oven to increase the line speed or handle more product. The choice of instrument is dependent on the type of coating. The right wavelength instrument will ensure that emissivity will not change with the color or the thickness of the paint. The instrument is installed to measure the coating temperature at the exit of the coil, ensuring that the paint has reached the correct temperature for perfect bonding of the paint to the metal.

Infrared thermometers are produced utilizing many wavelengths and having wide temperature ranges. Table 1 will aid in selecting a proper wavelength for many typical applications.IH

For more information:Jeff Kresch is marketing director and Vern Lappe is vice president of customer/technical service for Ircon, Inc., 7300 N. Natchez Ave., Niles, IL 60714; tel: 847-967-5151; fax: 847-647-0948; e-mail: info@ircon.com; web: www.ircon.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: magnetic field, thermal imaging, infrared sensor, emissivity, blackbody, wavelength