Updating the Automotive Air Bag Static Deployment Test System A test system designed for analysis of automobile air bag leading edge throw and velocity has been improved, with concomitant increases in system resolution and quantification of intermediate velocity. An additional feature of the updated system is a visual representation of the test process, provided by a high-speed video camera.   William Rosenbluth, Automotive Systems Analysis, Inc. (ASA) Figure 1. The principal components of the improved deployment test device are the illuminator, the checkerboard target detector, the deploy controller, and the data acquisition system. To evaluate air bag envelope dynamic characteristics vs. occupant position, it is often important to know air bag envelope leading edge throw (deployment distance out from the static position) and leading edge velocity. These performance characteristics are not usually included in manufacturer test data. Such evaluations, when done independently of vehicle crash tests, are called static deployments. The object of any static air bag deployment test system is to characterize air bag leading edge throw and leading edge velocity with the highest resolution economically feasible. A new air bag characterization system described in "A Test System for Recording Air Bag Deployment" [1] used photodetectors to locate air bag leading edge position at 1 in. intervals and sampled all 34 positions at 4000 sample-words/s. The test system has since been improved to achieve higher resolution and faster sampling rates. The improvements were undertaken to increase the system resolution and quantify any potential intermediate velocity peaks smoothed in the original data method. Intermediate velocity peaks were found, although the overall velocity profiles proved to be substantially comparable. Figure 2. The improved checkerboard target is fitted with phototransistors at each of the alternating 1 in.2 intersections and at 1/2 in. midpoints along the desired deployment axis (see illustrative bubble). When any position is illuminated, the phototransistors conduct, and keep that intersection's current contribution to positional diverted to a waste-sink, thus keeping the sector positional output low. When the illumination is masked, as by the deploying air bag envelope, the respective intersection's current contribution is allowed to flow to the current summing junction and then to the sector-sensing 200 resistor. In addition, many clients desired a visual representation of the test process. To achieve this, an Olympus MAC 1000s high-speed video camera has been added to the system to allow video documentation of the deployment tests. The improved test system still has photodetectors positioned on the deployment axis of a checkerboard target, but they are now placed at 0.5 in. intervals with a time resolution of 25,000 sample-words/s, using the same data acquisition hardware. The increase in sample rate was accomplished by changing the shadow location reporting scheme to use relative vs. absolute location addresses, thereby allowing the required A/D position monitors to drop from 34 to 3 channels (plus 3 channels for squib electrical firing data). The new scheme increases the position resolution by a factor of 2 and the time resolution by a factor of 6; the squib electrical firing data (current flow and timing) are still recorded with the same resolution as the positional data. Background of Vehicle Air Bag Technology Photo 1. In this detail of the phototransistor placement on the measurement axis, note that the original 1 in. devices and the improved 1/2 in. devices are both visible. In its Fourth Report to Congress on the Effectiveness of Occupant Protection Systems [2], the National Highway Traffic Safety Administration (NHTSA) estimates that, based on 11% effectiveness in all crashes, air bags have saved 2263 lives from 1987 through 1997. NHTSA also estimates a subset of crashes where injuries and even fatalities were caused by air bags. Passenger vehicle air bag systems are generally designed so that the folded fabric envelope is fully inflated before the occupant contacts it in a crash ride-down. The inflated envelope thus forms a benign air pillow or cushion--hence the term air bag. The air bag serves to assist and improve the safety characteristics of seatbelt equipped vehicles. After a crash, however, there are sometimes questions as to whether certain injuries are air bag induced vs. crash induced. The answers can often be found with a forensic investigation of markings and artifacts in the crash vehicle, EEPROM data in air bag and ABS computers, and an investigation of the dynamic characteristics of the air bag itself. Questions as to whether certain injuries are actually caused by the air bag or the traffic accident can arise when one or more of the following conditions can be identified and confirmed: Delayed deployment, allowing the occupant to hit the wheel/dash before the air bag is fired Punchout, when a passenger adjacent to the air bag door is struck by the air bag door flap Membrane loading, in which an occupant positioned just back from the air bag door is thrown about the passenger compartment by the force of the inflating envelope Bag slap, in which an occupant at a distance from the air bag door is struck by the leading edge of air bag fabric at a high velocity Figure 3. The new detector circuit operates by allowing the phototransistors to switch current flow between the waste-sink (the +12 Vref for each decade) and the current summing junction (the base of the 2N2907 for the respective decade). Leading edge velocity is unreasonably high at throw distances corresponding to expected occupant positions, allowing the envelope to cause bag slap injuries in expected occupant positions (as contrasted to out of position, OOP, conditions) Failing to make a no-fire decision, when the system could determine that the occupant was belted and the vehicle velocity was low enough so that there was more likelihood of an injury from the deploying air bag than from riding down the impact, while belted, without deploying the air bag Failing to deploy, when the vehicle velocity is high enough that there is unquestionable benefit from deploying the air bag, compared to riding down the impact, with and/or without seatbelts Premature deploy, when crash conditions are absent or under a reasonable threshold Data to assist with the evaluation of four of the eight injury conditions (punchout, membrane loading, bag slap, and leading edge velocity) are collected with the static deploy system described in this paper. These conditions as related to the design of air bag systems are further described in [3,4]. Other air bag static deployment tests are described in [5,6]. Figure 4. The sample output voltage data patterns (oscillograph display created by SnapMaster) are observed on the data acquisition computer screen immediately after the deploy event. Note that top trace (showing squib current and voltages) is shown with a time (X) axis of 50 ms (the first 50 ms of the event), and that axis timing is different from the lower traces (showing each of the positional sector voltage staircases) vs. time on the X axis. The X-axis scale is 100 ms (the first 100 ms of the event). The three staircases represent the envelope time progress in the three positional sectors, 0–11.5 in., 12–23.5 in., and 24.0–34.0 in. Overview of the Test System Figure 1 shows an overview of the entire test system [7], which consists of an illuminator, a checkerboard target embedded with sensors, a deploy controller, and a data acquisition (DA) system. The DA system consists of a PC, a Keithley MetraByte DAS-1820HC DA card, SnapMaster DA software, and the photodetectors on the checkerboard target. The latter units are phototransistors that switch current from a waste port into a current summing junction. They are positioned at and halfway between the checkerboard intersections along the deployment axis of interest. In addition to the phototransistors' state, the voltage across the air bag squib and the current through the air bag squib are recorded. The phototransistor response at every 0.5 in. of outward deployment is recorded at 25,000 sample-words/s using Snap-Master to drive the DA card. The recorded throw distance vs. time is used to calculate the air bag's velocity during deployment as a function of time. Photo 2. The passenger air bag under test is mounted in a fixture at the bottom of the checkerboard target. Note that the deployment scale (1 in. markers) is identified on the backboard and on the sideboard. The sideboard markers can also be seen on the high-speed video (Photo 4, 500 fps) and give a visual reality to the data collected. This video often helps our nontechnical clients appreciate the physical reality of air bag deployment. (A sample-word consists of many parameters recorded, six in this case, as a group each time the system clock triggers a sample-word capture. The sample-word rate directly defines the resolution that can be achieved for the process being monitored. Data acquisition advertising often uses the term "samples per second," which is a larger number equal to sample-word rates times the number of parameters in each word. Thus, 25,000 six-parameter sample-words/s = 150,000 samples/s.) Test System Operation The checkerboard target in Figure 1 is fitted with phototransistors at and halfway between each of the alternating square intersections along the desired deployment axis. (Practically speaking, the axis of interest is the axis from the envelope [air bag] cover to the occupant injury location. For drivers, this is approximately normal to the plane of the air bag cover. For passengers, this is ~17–20° upward from the top of the instrument panel cover.) The mounting cross-section scheme is identical to the original device (see Figure 2). However, there are now twice the number of sensors, and a comparison of the original and new positions is shown in Photo 1. Photo 3. The air bag under test whose deploy data are shown in Figures 4, 5, 6, and 7 is shown at its ~3/4 peak throw. Note the shadow just visible behind the envelope. The new detector circuit (see Figure 3) describes the essence of the new system. When illuminated, each intersection phototransistor dumps its current into the bias supply. When dark, each phototransistor allows its current to flow into the summing junction (emitter) of the sector-summing transistor (2N2907), and then into the collector to the 200 (omega) sense resistor. The voltage across the resistor is produced by the sum of the current contributions from the dark phototransistor intersections (… Idark * 200 (omega)) flowing through the transistor. The target is divided into three sectors (0–11.5 in., 12–23.5 in., and 24–34 in.), each having identical summing junctions and 200 (omega) sense resistors. In a static deployment test, before deployment the air bag is positioned at the bottom of the checkerboard target, and the entire target is illuminated with high-intensity light. Since the envelope remains folded and the target is illuminated, each phototransistor dumps its intersection current into the bias supply and thus keeps the voltage low across the 200 (omega) sense resistor. Upon firing, as the test air bag envelope proceeds to unfold, it physically intercepts the illuminator and its shadow darkens successive intersection transistors along its deploy axis. The phototransistors open when darkened, and the intersection currents are routed to accumulate through the respective 200 (omega) sense resistors. The result is a voltage staircase representing the leading edge position of the air bag envelope in time (with 1/2 in. resolution), sampled at 25,000 sps, i.e., one position sample every 0.04 ms. From these data we can derive both a position vs. time and a positional velocity. Photo 4. The air bag under test, whose deploy data are plotted in Figures 4, 5, 6, and 7, is shown in a video clip ~70 ms after electrical ignition. At this time it has assumed its most commonly full inflated state, well back from its 32 in. peak throw. Note the fidelity of the shadow on the backboard and the side scale visually showing the throw (in.). The sample voltage pattern shown in Figure 4 is generated by adding fixed current increments from the covered positions to a current summing junction. The summed current is then fed into a summing resistor. There are three summing resistors, one for each axis sector (0–11.5 in., 12–23.5 in., and 24–34 in.), and the voltage across each is monitored by a separate channel of the DAS 1802HC DA card. To more clearly display the data immediately after a deployment, summed voltage data are shown as three-sector voltage staircases. Note that the successive staircases (for a full-axis air bag envelope sweep) are seen at positionally successive times. Also note that the upper trace in Figure 4 showing squib current and voltages is plotted with a time (X) axis of 50 ms (the first 50 ms of the event), which is different from the lower trace showing positional voltage ladder data. The lower traces are shown with a time (X) axis of 100 ms (the first 100 ms of the event). This split-timing axis capability is inherent in SnapMaster and allows selective data resolution enhancement where desired. A comparison of potential error rates shows the new system to produce a 7.5 3 improvement over the original system for 95% confidence at 100 mph. Example Air Bag Deployment Photo 2 shows a passenger air bag under test mounted at the bottom of the checkerboard target. Note that the air bag is generally mounted so as to interfere with the illuminator beam at the bottom phototransistor location before firing. This provides a data origin locator in future analysis. Figure 5. An expanded scale version of the throw vs. time data shown in Figure 4 clearly shows the three sector voltage staircases generated across the 200 (omega) resistors in increasing time. Figure 6. The captured data are shown as two data traces vs. time, envelope throw (in.) vs. time (ms), and envelope velocity (mph) vs. time (ms). This representation is an easier way to comprehend the voltage staircases in Figures 4 and 5. Figure 7. An even more useful graph shows the data plotted in Figure 6 as envelope velocity (mph) vs. throw (in.). This form of data is especially useful when it is overlayed on a time-positional analysis of vehicle occupants in a vehicle collision impact. Note that the air bag normal-axis throw peaks at a greater distance than its final inflated state position at 0 mph. This a common characteristic of vehicle air bags, and is due to the folding pattern combined with the furling effect of the inflating cloth envelope. In Photo 3, the air bag under test is at its approximate 3/4 inflation. Note the air bag envelope shadow on the target checkerboard. The high-speed video frame excerpt in Photo 4 shows the deploying envelope shadow on the checkerboard target at full inflation (not maximum throw). Recall that Figure 4 shows the data traces (oscillograph display created by SnapMaster) observed at the operator console. The upper panel shows squib volts and amps vs. time (in milliseconds). Note that the firing pulse occurs at ~5 ms on the X-axis 50 ms scale. The lower panel shows the three successive-interference voltage ladder traces (shadow progression) for the air bag. The black line steps through positions 0–11.5 in.; the green line, through positions 12–23.5 in.; and the red line, through positions 24–34 in. The positional voltage ladder traces are shown with a time (X) axis of 100 ms (the 1st 100 ms of the event). Note that the maximum throw of 33 in. occurs at 27 ms on the time scale. Thus, the time from the firing pulse until the envelope leading edge first reaches its maximum throw is ~22 ms (27–5 ms). Figure 5, an expanded scale version of the throw vs. time data shown in Figure 4, clearly shows the voltage staircases generated across the 200 (omega) resistors. This oscillograph panel is provided on the computer screen immediately after test device deployment, allowing the operator to verify that a test unit deployment has been successfully recorded. SnapMaster automatically saves the data and allows the operator to print the display screen in color. The saved data are then processed to derive the charts shown in Figures 6 and 7 . Figure 6 shows two traces derived from the data points represented in Figure 4: Envelope deploy throw (in inches) vs. time (in milliseconds) Envelope velocity (mph) vs. time (in milliseconds) Figure 7 shows envelope velocity (mph) vs. throw (in inches). This plot is recursive, meaning that it is showing the air bag expanding and contracting during its inflation phase, eventually stabilizing at zero velocity. Consequently, it is moving up and down the phototransistor array in time, thus producing the circular data form at the end of the velocity vs. throw plot. Note that the air bag's normal-axis throw peaks at a distance greater than that of its fully inflated state at 0 mph. This common characteristic of vehicle air bags is due to the folding pattern combined with the furling effect of the inflating cloth envelope. The velocity vs. throw form of data presentation is especially useful when analyzing occurrences of injuries alleged to have been caused by air bag deployments. Figure 8 shows a velocity vs. throw overlay for deployments of two air bags with identical part numbers, one recorded with 1 in./ 4000 sample-words/s, and the other recorded with 1/2 in./25,000 sample-words/s. Given the inevitable variances between the two ballistic events, the reader can see that: The general velocity in the range of interest (18–28 in.) is substantially similar. The increased resolution with the 1/2 in./25,000 sample-words/s system represents a clear design improvement. Figure 8. Comparing the two deployments for identical P/N air bags, but with inevitable ballistic variances, we see that the general deploy characteristic in the critical 18–28 in. zone is substantially similar. This confirms that both data systems are accurate, but with the 25,000 sample-words/s system capturing local velocity variations missed because of the smoothing (averaging) effect of the lower sample rate system. Summary This article has addressed a way to improve a digital method of characterizing the ballistic process of air bag envelope throw and envelope leading edge velocity--data not otherwise available in the industry. The improved method provides accurate and useful air bag deployment data with resolution equal to or better than the precision obtained with high-speed video and graphical analysis techniques. SnapMaster is a registered trademark of HEM Data Corp, Southfield, Michigan 48076. Keithley MetraByte DAS- 1802HC is a product of Keithley Corp., Cleveland Ohio 44139. References 1. W. Rosenbluth. May 1998. "Analyzing Air Bag Deployment for Safety," Sensors, Vol. 15, No. 5:58-66. 2. "Effectiveness of Occupant Protection Systems and Their Use." May 1999. Fourth Report to Congress, National Highway Traffic Safety Administration, U.S. DOT. 3. D.S. Breed and V. Castelli. Feb.–March 1988. "Problems in Design and Engineering of Air Bag Systems," SAE Paper 880724. 4. O. Spiess et al. 24–27 Feb. 1997. "Development Methodology of an Airbag Integrated Steering Wheel in Order to Optimize Occupant Protection Balanced Against Out-of-Position Risks," SAE Paper 970777. 5. L.K. Sullivan and J.M. Kossar. Feb. 1992. "Air Bag Deployment Characteristics," NHTSA, Vehicle Research and Test Center, DOT HS 807 869. 6. M. Powell and A.K. Lund. Jan. 1995. "Leading Edge Deployment Speed of Production Air Bags," Insurance Institute for Highway Safety. 7. W. Rosenbluth. Test Fixture for Digital Measurement of Vehicle Air Bag Deployment Characteristics, U. S. Patent No. 5,850,085. William Rosenbluth is Principal Engineer, Automotive Systems Analysis, Inc. (ASA), 12015 Canter Lane, Reston, VA 20191-2129; 703-860-1766, fax 703-860-1799, bill.rosen bluth@wdn.com

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