Temperature Measurement for the Automotive Industry
Companies that manufacture parts for the automotive industry have to produce products that have very high quality. There can be no defective parts delivered to the factory, and, of course, the automotive companies want the lowest price, fastest delivery and a record of the quality tests.
|Fig. 2. Induction scanning of two shafts|
One of the parameters that require continuous monitoring during production is the temperature of the part. There are many methods to measure temperature during production. A very common sensor is the thermocouple, and they are used in a wide variety of furnaces and ovens. When used in a furnace, the thermocouple has one major disadvantage – it does not measure the actual temperature of the part, it measures the temperature of the thermocouple tip or the atmosphere in the oven.
A much better temperature sensor is the infrared thermometer. It has many advantages over the thermocouple. If installed properly, it measures the part temperature and not the environment in the furnace or oven. Infrared thermometers do not touch the part as it moves through the process, so they are ideal for measuring the temperature of moving targets without interfering with the actual process.
Infrared thermometers are used for forging, induction heating, induction scanning of axles, induction welding and annealing. They are also used on mesh-belt furnaces for the heat treating of automotive parts. When installed correctly, the infrared thermometer provides:
- Faster start-up of the process
- Accurate, repeatable process temperatures
- Less scrap
- Higher production rates
- Higher product quality and records for quality control
Infrared Thermometer Basics
Infrared thermometers are produced in three basic formats: spot instruments that measure the temperature of a specific spot on the hot target; thermal imagers that provide a complete image of the hot target; and line scanners that are used to measure the temperature of wide targets like steel in rolling mills. Scanners can provide a complete thermal image of the entire width and length of the hot strip as it is being processed. For the automotive industry, the two formats most often used are spot thermometers for production and thermal imagers used mainly for testing and quality control.
Spot thermometers are provided in two different measurement modes. The basic modes are called single-wavelength (or brightness) pyrometers, which work at one specific narrow spectral bandwidth like 0.7-1.0 microns. These thermometers are very popular for temperature measurement but have some disadvantages.
Single-wavelength spot thermometers are designed to measure a specific target on the product. For example, they may measure a spot that is 0.5 inches (12 mm) in diameter. The product being measured has to be larger than 0.5 inches (12 mm), and between the sensor and the hot target there can be no obstructions in the line of sight. In addition, the lens has to be kept clean. If the lens gets dirty, the instrument will indicate a lower temperature than the real temperature. Of course, there is the mysterious factor called emissivity, which causes a lot of confusion. (For more details on emissivity, please contact the writer.)
A very popular infrared thermometer used in the automotive industry is called a two-color, or ratio, thermometer. A two-color thermometer actually has two detectors that look at two different wavelengths for a single hot target. This instrument has several advantages over the single- wavelength thermometer:
- The target need not completely fill the optical spot size
- There can be obstructions in the line of sight, like the edges of induction coils
- Smoke, steam and dust in the line of sight will not affect the temperature reading
- A dirty lens will not cause the instrument to indicate an incorrect temperature
- This instrument has no emissivity control. It does have a setting called “E Slope,” but for the majority of the temperature applications the setting is 1.000.
The question becomes, how much signal can the two-color instrument lose because of not filling the spot, obstructions in the line of sight and dirty lens as well as smoke, steam and dust that are in the air? The two-color thermometer can sense the loss in signal. When all of the losses total up to greater than 95%, the instrument becomes invalid. It provides a relay output to indicate the excess loss in signal, and the outputs are driven to zero value, thus preventing any erroneous temperature readings. Because of all of these advantages, the two-color thermometer can work with many difficult applications and still provide an accurate temperature indication.
|Fig. 1. Billet heating for forging|
Applications for an Infrared Thermometer
There are hundreds of parts in an automobile that are produced by forging. The steel billet is usually heated in an induction heater and then placed in the die, where it is forged into the finished part. Temperature of the billet as it is being forged is very critical. If the billet is too cold, the risk of breaking the die is very high. If the part is too hot, then the metallurgical properties of the part have been compromised, and a defective part can be produced. Infrared thermometers can provide two functions. They can actually control the billet temperature by controlling the induction heater. A more common application is to measure the billet temperature as it enters or progresses toward the die. If the thermometer detects a billet temperature too high or low, it closes a relay, which removes the billet from the process and prevents die damage or defective parts. Figure 1 shows a thermometer measuring the temperature of the billet as it slides down a chute on its way to the die. Common forged parts include gears, camshafts, bearings and connecting rods just to mention a few.
|Fig. 3. Thermal image of an engine in a test cell|
Induction Heat Treating
In the automotive industry, induction heating is used to heat treat a wide variety of parts. Surface hardening of cam lobes, crankshaft journals, gear teeth and bearing races is very critical to quality. An especially difficult process for controlling temperature is induction scanning of axles and shafts. The induction coil moves up and down the shaft, and there is a water quench immediately after the coil that makes it almost impossible to see the hot part. With the use of a fiber-optic infrared instrument and a quartz rod extension tip, however, the fiber can be placed at the exit of the coil. The quench water keeps the fiber cool and clean, and a thin layer of water on the tip does not affect the accuracy of the temperature measurement.
The temperature, usually above 1500°F (800°C), is very critical to having a quality product. The infrared thermometer can be used to control the temperature and also provide an output for recording the values used for quality-control records. By using an infrared thermometer, every part is heated correctly, process times can be shortened (especially on the night shift when there is extra power to provide faster heat-up cycle times) and product quality is assured.
|Fig. 4. Thermal image of a tube weld|
The thermal imager is becoming a very valuable tool in this application. It has the advantage of providing the user with a complete and accurate thermal image of the entire part, and the part can be moving or standing. A good example would be in an engine test cell. The camera can point out areas that are being overheated. By using the special software, the operator can draw in areas of interest, often called AOI. The AOI can be a circle, box, line or a free-form area. Within the area, the instrument can measure the highest, the lowest or the average temperature. These values can be used to provide alarms to prevent damage or to indicate a process failure (Fig. 3).
Thermal imagers can also be used as part of process control. A common application that uses infrared thermometers is the production of stainless steel pipes for oil, gas and hydraulic lines as well as exhaust systems used in many automotive products. To produce the pipes, a continuous ribbon of flat stainless steel is run through a series of rollers that form the flat stock into a tube shape. At the exit of the rollers is an induction coil that heats the edges of the ribbon and actually welds them together without using a welding rod.
Normally, a two-color infrared thermometer is used to control the temperature of this process, but a thermal imager can provide an output for process control as well. In addition, it can show the entire heating of the tube to indicate how well the heating is progressing. Figure 4 shows a thermal image of tubing being welded with this process. By using this instrument the operator can ensure every weld is perfect, and by controlling the power and even the speed he can get the heating pattern desired for each size of tubing produced.
|Fig. 5. Sensor installation at exit of mesh-belt furnace|
Many automotive parts are heat treated using mesh-belt furnaces. The parts can be tempered, case hardened or austempered. The customer would like to measure the temperature of the product in the furnace. He can use thermocouples, but they only indicate the temperature of the atmosphere in the furnace, not the real product temperature. Infrared thermometers cannot be used because of reflection problems. The furnace walls and the heaters are hotter than the parts. Therefore, when the thermometer looks at the parts, it measures energy from the part as well as reflected energy from the hot furnace walls, which makes the instrument indicate temperatures that are too high.
For this type of furnace, the infrared thermometer is used to measure the parts as they exit the heating section but before they move into the quench. Figure 5 shows a typical installation of the sensor outside the oven. The sensor is aimed so that it sees across the belt and can measure any part that moves along it. Normally, a single-wavelength thermometer is used for this application. A single-wavelength thermometer is the desired choice rather than a two-color thermometer, which is affected by any stray reflected infrared energy.
This temperature measurement still leaves the question: What is happening inside the oven? A very unique instrument called a Datapaq data logger solves the problem. This is a digital data logger that is placed inside a special thermally protected container or barrier. The thermal barrier allows the data logger to travel through the oven during the actual heating process. With as many as 10 thermocouples connected and placed in locations across the width of the belt, a complete thermal picture can be seen of the entire inside of the furnace. In addition, as the data logger progresses through the furnace, a heating temperature profile is provided for the operator. As the data logger progresses through the furnace, it sends the temperatures via radio telemetry so they can be observed in real time.
The result is a record of the entire furnace, ensuring uniform heating anywhere. As a result, proper quality is assured because parts will be evenly heated to the correct temperature. This test is part of the requirements to meet the automotive specification of CQI-9 for temperature uniformity. Figure 6 shows a typical output of the thermocouples. Using software, a thermal image can be generated as well.
|Fig. 6. Temperature of parts moving through the oven|
Maintenance of Infrared Thermometers
Infrared thermometers are easy to maintain. Simple steps are as follows:
- Keep the sensing heads cool. Most can operate from 32-150ºF (0-65°C) without cooling. Do not over cool, as this can cause condensation inside the sensor and destroy it.
- Be sure to connect the sensor to a good electrical earth ground, especially when working with induction heaters.
- Keep the lens clean using an air purge.
- To meet ISO requirements, have the instrument calibrated on a certified black body annually.
Temperature measurement is a critical part of the heating process for automotive parts. If properly installed and maintained, the instruments can provide many cost savings in the form of higher quality, faster start-up, improved production rates and less scrap. IH
For more information: Contact Vern Lappe, director of technical services, Ircon/Raytek Corp, 1201 Shaffer Rd. Santa Cruz, California 95060; tel: 847-967-5151 (x202); fax: 847-647-0948; e-mail: firstname.lastname@example.org; web: www.ircon.com
Photos furnished by Inductoheat Inc. and Datapaq Inc.