The dangers of faulty car airbags recently have become all too clear. The product-liability issues associated with airbags and the largest, most costly automotive recall in history make it essential to characterize them thermally at high speeds and with high levels of sensitivity and accuracy.
When designing an airbag system, researchers must be able to study, during the deployment, the thermal results of the complex interactions between the various components such as crash sensor(s), the electronic controller unit, propellant, inflator/ignitor and bag design.
Thermal characterization of fast-moving components using traditional methods such as mounting contact-temperature sensors—thermocouples, thermistors, or RTDs—is often impossible. Non-contact techniques such as spot pyrometers and infrared (IR) cameras with thermal detectors typically aren’t fast enough to stop motion in order to characterize temperature with sufficient accuracy and sensitivity.
Fortunately, the latest generation of high-speed infrared (IR) cameras with quantum detectors can translate infrared radiation into data faster than earlier generations, which were equipped with uncooled microbolometer (thermal) detectors. But before investing in new characterization equipment, it’s critical to understand the strengths and weaknesses of various aspects of camera design.
High-speed thermal images of airbag deployments, captured with a FLIR SC6800-series MWIR science-grade IR camera, demonstrate the ability to capture airbag images quickly and accurately. This capability is essential to understanding an documenting deployment interactions (see the video at: www.flir.com/HighSpeedIR).
Cooled quantum detectors are key
Traditional IR cameras were designed to react to incident radiant energy: infrared radiation heats the pixels and creates a change in resistance that is used to calculate temperature. Although they offer high durability and portability at a relatively low price, standard IR cameras have limitations for high-speed R&D applications.
According to Markus Tarin, CEO of MoviTherm (a company that uses quantum-detector IR cameras to measure temperatures on fast-moving objects), a traditional uncooled microbolometer camera has a fixed integration time of 8–12 ms.
“This means that the object image is smeared over multiple frames,” he explained. “High-speed quantum cameras have adjustable integration times, allowing the camera to freeze the frame and perform accurate temperature measurement of fast-occurring events. By being able to capture details by a factor of 100 times better, quantum detectors allow our technicians to see the impossible.”
Although more expensive than uncooled microbolometer thermal cameras, high-speed IR cameras with cooled quantum detectors are capable of capturing high-resolution images at 1000 frames per second. These quantum detectors typically are made of indium antimonide (InSb), indium gallium arsenide (InGaAs), or Strained Layer Superlattices (SLS) and are photovoltaic. This means the detector’s crystalline structure absorbs photons that elevate its electrons to a higher energy state, which changes the conductivity of the material.
These cameras essentially are counting photons, which allows for upgraded sensitivity and the ability to detect temperature differences of less than 18 milliKelvins, or .018°C.
In addition to improved sensitivity, quantum detectors also react quickly to temperature changes, with a time constant on the microsecond timescale. This combination of short exposure times and high frame rates makes quantum detectors ideal for stopping motion on high-speed targets for accurate temperature measurement, as well as proper characterization of how thermal temperatures rise over time on fast-heating targets.
Dr. Robert Madding, President of RPM Energy Assoc., noted that quantum detectors work by photons of the proper energy impinging onto the detector. “They add their energy to electrons in the semiconductor, elevating them above the detector energy bandgap into the conduction band,” he said. “This can be measured as a change in detector voltage or current, depending on detector design. This can occur very fast.”
For high-speed automotive applications, look for infrared cameras that offer high frame rates at high resolution. FLIR’s X6900sc high-speed MWIR camera, for example, captures full 640 x 512 images at 1000 frames per second (fps).
Factors other than frame rates affect an IR camera’s suitability for characterizing rapid temperature changes on fast-moving components:
• Integration time (how long the camera collects data for each frame).
• The temperature of the target can have an impact on integration time (snap shot speed) of the camera. Hotter targets emit more radiant infrared energy, thus more photons; colder targets emit fewer photons. Look for cameras with enough sensitivity to measure colder targets at fast frame rates.
• The number of A/D converters or channels available. High-speed IR cameras typically have 16 or more channels; low-performance cameras typically have just four.
• High-speed pixel processing. Check for processing speeds (pixel clock rates) of 200 megapixels/sec or higher.
• Use of a next-generation Read Out Integrating Circuit (ROIC). Earlier ROICs were non-linear at low well fills, which caused the camera’s Non-Uniformity Correction to break down, resulting in poor imagery and questionable temperature-measurement accuracy at high speeds on colder targets. Newer ROIC designs are linear-to-low well fill.
• The ability to synchronize and trigger to external events, such as an airbag deployment. A separate triggering system better allows for synchronizing recordings by strictly controlling the integration start time and the frame rate.
• Sensitivity to subtle temperature changes simplifies detecting small hot spots. Cooled IR cameras can detect changes as small as 0.02°C; uncooled cameras have a sensitivity of around 0.03°C. Although that difference may sound insignificant, it represents more than a 30% improvement in sensitivity.
By including thermal imaging during the design and testing phases of airbag innovation, R&D teams can more readily identify weak points and improve overall safety. But the type of camera and its features can have an impact on imaging success.
Choosing a cooled thermal camera with the highest available speed, sensitivity and integration times allows researchers to track temperature shifts accurately over the airbag ignition period. These cameras will also provide crisply detailed stop-motion frames, so researchers can closely examine each stage of inflation and identify the exact moment a problem begins.
Author Chris Bainter is Americas Business Development Director - Science Segment, at FLIR Systems, Inc. The Wilsonville, OR-based company's name comes from the acronym for forward-looking infrared imaging systems. FLIR was founded in 1978 with airborne IR systems.