Like most mature industries, the automotive industry is highly competitive. Customers demand quality, security, and economy. Competition requires increasingly fast times to market for new designs. Combining customer and competitive demands creates a dilemma: How to design and build the best product as fast as possible.
Experience has shown that efficiencies come to mature markets in the form of advanced automation and judicious use of new technologies. The automobile industry and its supplier ecosystem use advanced engineering across many disciplines: chemistry and thermodynamics govern internal combustion; mechanical engineering governs metal fatigue and stiffness of chassis components; and tribology describes brake and tire heating. Using thermographic techniques, automotive researchers have studied engine cooling, air conditioning, and heat dissipation of high-power electronic components. Regardless of the scientific discipline, there is a common, unifying characteristic to all these systems and components — thermal energy is critical to successful manufacture and operation. Heat degrades automotive parts and enables steel, aluminium, and plastic manufacturing processes. Heat also is produced by electronic components and is an indicator of efficient operation.
Infrared in Automotive
Automotive components and systems can be tested, measured, and evaluated using a variety of data types, depending on the test subject. The most common parameters, apart from geometrical and electrical characteristics, are pressure-and temperature-related measurements, with temperature being the most widely used in manufacturing. This has led to the increase in infrared (IR) imaging and thermographic test instrumentation, as well as discreet IR temperature measurements for monitoring manufacturing equipment and testing new designs and end-product quality.
Traditionally, thermocouples are used to make discreet temperature measurements, but these point-measurement systems can neither measure without contact, nor provide a full-field thermal image that provides temperature data over time for small, often geometrically complex and moving products. Thermographic imagers with focal plane array (FPA) sensors can provide wide-area, objective temperature measurements. New design and material improvements also have significantly increased the spatial resolution, while improved calibration has improved the IR imagers’ ability to robustly and repeatedly generate objective temperatures across a wide temperature range. This article explores the latest generation of infrared detectors and how they are answering the needs of the automotive industry in particular.
It is difficult to precisely date each step in the evolution of IR imaging, but one of the most defining parameters relates to spatial resolution, often defined as the space between the centers of two pixels on the FPA, or the FPA’s pitch. Since 1998, the most sensitive IR array sensors — indium antimonide (InSb) and mercury-cadmium-telluride (MCT) detectors — have had a 30-μm pitch. Table 1 shows the progress made by IR sensor producers during the past eight years and the development of third-generation sensors with 15-μm pitch. This performance has enabled IR camera manufacturers to, through the use of optics and electronic filtering, produce IR cameras with an effective spatial resolution down to 4 μm.
Thermographic cameras are used in five broad application categories within the automotive industry: maintenance, process control, R&D, quality control, and stress analysis. Maintenance applications typically involve the evaluation of manufacturing equipment to predict possible failures in production plants - mainly on electrical facilities. It is the most active application within the automotive industry because of the relatively low performance requirements for the IR cameras, often supported by uncooled, low-cost microbolometer- or ferroelectric-based (IR) cameras. Preventive or predictive maintenance applications are performed by maintenance technicians, which means that the cameras need to be easy to use, handheld, and can operate for extended periods on a battery for portability (see Figure 1).
Other applications of quantitative thermography in automotive industry typically use high-performance infrared imagers with cooled InSb and MCT technologies. These applications usually are described as “fast” and/or “high-resolution” thermography, and deal with thermal characterization, non-destructive testing (NDT), and stress analysis.
With each new year, automobiles consume more electricity as consumer demand for electronics and improved performance escalates. Some of these electrical parts govern safety-critical systems, and therefore, have to be tested carefully during the design process, often by the Tier 2 or 3 automotive part suppliers, prior to shipping to the final assembly plant. One example is an electronic monitoring system that reduces CO2 production by an internal combustion engine.
This automotive component manufacturer used a Titanium IR camera from Cedip Infrared Systems with G1 and G3 lenses to evaluate the part design by monitoring the thermal generation of the part as electric current was increased to 200 Amps within <200 ms. The magnifying germanium lenses (G1 and G3) allowed the Titanium IR camera to measure and record transient events at 4-μm resolution on the small electronic part, identifying hot spots in the component and possible design flaws.
The first step of measurement is to evaluate local emissivity. Using Altair software Cedip researchers are able to create a full-field map of emissivity, which can be applied either to real-time images or recorded video. The resulting corrected temperature map has been used by automotive researchers to redesign components based on hot spots and thermal gradients.
Designing new and more effective automotive brakes also depends on careful temperature measurements. Braking effectiveness is linked to pad and disc temperatures, both overall temperature as well as temperature gradient across the brake pad surface, to avoid stress and permanent deformations. The difficulty of making such temperature measurements is due to the wide range of temperatures encountered (20°C to 900°C) and disk brake rotation speed.
Using Titanium IR camera systems, automotive engineers can meet both requirements thanks to three special features. The camera has very precise external trigger modes to handle image acquisition from fast-moving targets. Titanium also combines four temperature ranges to extend traditional IR sensing ranges from 300-400°C, upwards to temperature bandwidths of 1000°C. Finally, although the Titanium is sensitive to a broad band if IR radiation — 1.5 to 5.1 μm — using bandpass filters at 2 μm reduces the camera’s sensitivity to changes in emissivity, yielding a more accurate temperature reading.
Another example of thermal imaging in automotive design and manufacture is tires. Tire tests are different from rotating brake tests. The temperature range for tire testing is less than brake testing, falling within 20°C to 80°C. Temporal synchronization for this application is more stringent. Integration time, which is the time of accumulating photons for measurement, should be as short as possible to avoid smearing. It is widely understood that long-wave infrared (LWIR, 8-12 μm) cameras have shorter integration time at 20°C than mid-wave infrared (MWIR, 2-5 μm) cameras. The Titanium 530 IR camera uses a LWIR cooled MCT detector to provide a thermal image with very low thermal resolution (NEDT= 25 mK) and integration time less than 50 μs. The camera’s Hypercal mode allows users to give priority to anti-smearing, similar to the fastshutter “sports” setting on many digital cameras. The system automatically adapts its calibration curve and its Non Uniformity Correction table to compromise between acquisition time and objective temperature measurement. Finally, Titanium IR camera systems also can record external analog signals (speed, pressure, temperature, etc.), helping users build a more in-depth picture of thermal behavior.
Stress and Fatigue Analysis
For most of the past decade, the automotive industry also has used IR imaging systems as part of their stress analysis and metal fatigue evaluation program. Both applications use a lock-in thermography technique to make energy measurements that compliment visible imagebased correlation methods for stress analysis. Often, these two approaches are seen as competitive; however, that ignores the inherent differences in the spatial and energy measurements generated by visible and IR imaging systems. Lock-in thermography measures minute changes in temperature through Fourier analysis produced by the thermoelastic effect in stressed materials.
Improvements in spatial resolution, thermal resolution, size, cost, and energy consumption are widening thermography applications in the automotive industry. At the same time, greater understanding by IR camera makers of automotive applications is leading to new calibration, filtering tools, lock-in analysis, and peripheral I/O, making thermography systems more effective for real-world automotive applications.