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(1) (2) Airbags: more than just a bag of wind Although widely used in the US, Japan & Europe, vehicle airbags have only recently become popular in Australia. Here’s a rundown on how they work. By JULIAN EDGAR For those who have not seen the publicity surrounding their Australian introduction, the airbag (or Supplementary Restraint System – SRS) is a cushion which inflates out of the centre of the steering wheel (or dashboard) in the event of a front-end accident. The idea is to cushion the impact and prevent (or at the very least significantly reduce) injuries to the head and chest area of the victim. A typical airbag system is that introduced by Holden in its VR Commodore. Fig.1 shows the layout of the device. In the Commo­dore, a single airbag is fitted on the driver’s side while in some other cars, a passenger-side airbag is also fitted. Side impact airbags are currently being trialled by some manufactur­ers. Bag inflation COVER AIRBAG INFLATOR STEERING WHEEL CLOCK SPRING COIL CRASH SENSOR FIG.1: THE MAJOR components in the VR Commodore airbag system. The airbag inflates out of the centre of the steering wheel. (Courtesy General Motors Holdens). 4  Silicon Chip The Holden airbag is constructed of silicone-coated nylon. It has a volume of 65 litres and is 700mm in diameter. When trig­gered, it inflates in just 30 milliseconds and the bag then deflates THE INTERNALS OF the current Bosch airbag trigger, as used in the Holden VR Commodore. The circuit board on the right has been folded out for this photo; normally it is stacked above the other board. FACING PAGE: (1) The Holden Commodore airbag (shown here deployed in a promotion­al photo for the Toyota Lexcen) has a volume of 65 litres & takes just 30 milliseconds to inflate. (2) Toyota’s new Tarago Ultima & GLX vehicles have a driver’s side airbag as standard equipment. This is what it looks like when fully inflated. in about 100 milliseconds as the driver impacts it (by way of comparison, a blink of an eye also typically takes about 100 milliseconds). The deflation speed is controlled by providing two 45mm vents in the bag, while the initial expansion rate of the bag is controlled by the use of two internal tethers, which stop the bag from head-butting you before you hit it! A sodium azide gas generator is what causes the airbag to inflate so rapidly. This airbag inflator – located within the hub of the steering wheel –is triggered by a crash sensor via a “clock spring coil”, a device that does away with the need for slip rings. This is used because the necessary reaction time of the airbag is so short that sliprings (like those used for the horn, for example) are not reliable enough – one contact might be momentarily lifted at the time of impact and so the airbag would not trigger at precisely the required moment. Fig.2 shows the relationship between vehicle deformation, driver movement and airbag inflation in the Commodore. Triggering the action of the airbag is an electronic sen­sor. Just consider for a moment the magnitude of the task facing the designers of this sensor. To begin with, the “ideal” sensor must discriminate between a crash and a parking bump or driving over a gutter. FIG.2 (BELOW): the sequence of events during a crash. (Courtesy General Motors Hold­ens). Impact The crash begins when the front of the bumper contacts the impacting object. In the next 15ms the crash sensor determines the severity of the collision & decides whether to deploy the airbag. Burst out The airbag housed in the centre of the steering wheel splits its covering pad in predetermined places & begins to inflate rapidly. Inflation The airbag is now fully inflated as the driver begins to move forward. The seatbelt progressively restricts the driver’s forward movement. Contact The driver’s head & chest contact the airbag & it immediately begins to deflate. The large area of the bag evenly distributes head & chest loads thereby significantly reducing the risk of severe injury. Support The driver sinks deep into the continually deflating airbag & upon reaching the limit of forward movement, begins to rebound. Rebound The driver continues to travel rearwards until making contact with the seat back & head restraint. February 1994  5 THE TWO PIEZO accelerometers are contained within the metal housing. Note the rigid attachment of the accelerometer module to the cast aluminium chassis. It must also be totally reliable, totally immune to false triggering, and it must be capable of firing the airbag even if the normal battery supply has been lost during the im­pact. Finally, it is also preferable if it can detect any inter­ nal faults in the system, either within the sensor itself or in the airbag inflator. so-called “ball in a tube” sensor. This elec­ tro-mechanical sensor consists of a glass tube, with a steel ball held in place at one end by a magnet. Two electrical contacts are located at the other end and the tube is filled with a gas damp­ing medium. If a crash occurs, the rapid deceleration of the ball over­ comes the attraction of the magnet. The ball thus rockets down to the other end of the tube and shorts the electrical contacts, thereby causing the airbag to inflate. This crude sensor is now rarely used. To be effective, it needed to be Old-style triggers A variety of sensors has been used over the years – none of which had the capabilities of the “ideal” sensor described above. The simplest is the VIGN LAMP TEST ACCELEROMETER 1 ACCELEROMETER 2 P SQUIBS CPU TEST VOLTAGE REGULATOR N WATCH DOG V VIGN DELAY ENERGY ANALOG INHIBIT EXTERNAL SWITCHES INHIBIT FIG.3: BLOCK DIAGRAM of the Bosch airbag trigger sensor. It uses a micro­ controller to monitor the outputs from two accelerometers & has various other circuits to prevent false triggering. The airbag is triggered by simultaneously switching on two output transistors. 6  Silicon Chip located towards the front of the vehicle, otherwise the cushioning affect provided by the vehicle’s body as it crushed delayed the triggering action. However, a frontal loca­ tion caused problems in terms of the vulnerable wiring needed to connect it to the airbag. Tuning the electro-mechanical sensor was also difficult. Electronic crash sensors were then brought into use. One Bosch sensor used a strain gauge attached to a pendulum which was suspended in a damping medium. A calculated acceleration of 4G (about the same as occurs during a frontal impact at 15km/h) was required for the sensor to fire the airbag. However, the unreli­ ability of this type of sensor meant that a device such as a mercury switch was usually placed in series with it to prevent the bag from activating under normal operating conditions. Generally, in this type of system, the mechanically inte­grating sensors were placed within the crush zone and worked in conjunction with a centrally-placed electronic sensor. The latest sensor The Bosch electronic sensor currently in use is much more sophisticated than either of the above sensors. It incorporates all of the characteristics of the “ideal” sensor mentioned above and also includes crash event data-logging and a serial data link. It is also fully programmable, allowing it to be calibrated for different vehicles. Fig.3 shows a block diagram of the sensor. As shown, the sensor uses two accelerometers which are based on piezoelectric transducers. The sensing element consists of two reverse polarized piezo oxide bars with two electrodes each. These are cemented together and form a bimorph element. During deceleration, one bar is compressed and the other stretched. Because the two bars are reverse polarised, the sum of their individual voltages appears between the two outer elec­trodes; ie, the signal is effectively doubled, thus giving good sensitivity. A low-pass filter with a cut-off frequency of 300Hz is used between the sensor and its amplifier. This filters out the 10kHz resonance peak of the sensor and avoids signal distortion when the output signal is sampled by the mi- FIG.4: A TYPICAL sensor output during a crash. The microcontrol­ler’s algorithm is used to derive the core deceleration from the high frequency variations. Time T0 is the start of the crash, T1 is the beginning of the airbag inflation, and T2 is when the airbag is deflating under the impact of the occupant. Heavy braking (just prior to wheel lock-up) in a road car develops only about 0.9G deceleration crocontroller. The sensor’s amplifier is built to work within the somewhat mind-boggling range of ±35G! A crash is detected by using a microcontroller to sample the sensor output, perform an analog/digital conversion, and then integrate this value with respect to time. If the derived value exceeds a certain threshold, the airbag will be fired. However, this integration is not sufficient to discriminate between all crashes. Oblique impacts, offset crash­es, centre-pole crashes and slow frontal barrier crashes all cause problems with this approach. Further data processing is therefore superimposed on the straight integration to improve crash discrimination. Two separate channels are used, with each accelerometer monitored. For the bag to be fired, an “interval watchdog” must receive triggering pulses from each of the two signal processing programs. If one program is not working properly, then the watch­ dog detects the missing triggering edge and inhibits the output stages. The other important role which the FIRST CRASH TESTS WITH TARGET VEHICLE ANALYSIS OF CRASH DATA ADJUSTMENT OF DEPLOYMENT ALGORITHM TO TARGET VEHICLE COMPUTER SIMULATION OF DEPLOYMENT REQUIRED FIRING TIMES ACHIEVED? N Y PROGRAM TEST - ECU CRASH TEST WITH TARGET VEHICLE. FINAL VERIFICATION FIG.5: TYPICAL airbag sensor calibration flow chart. Crash testing plays an important role. sensor must play is in predicting the deceleration that the car will experience during the inflation time of the airbag. If the airbag inflates too late, then the crash victim will already be in contact with the bag as it expands. This could lead to a situation where the victim could actually suffer an increase in acceleration – in the opposite direction! During a crash, there are high frequency variations in the deceleration superimposed on a ramping curve. Tests with dummies have shown that these high frequency variations have little effect on the dummy’s “health” – it’s the core signal of low frequency deceleration which is vital. The algorithm must there­fore smooth the accelerometer’s output to obtain the core signal and then predict the magnitude of this core signal during the period that the airbag is inflating. Fig.4 shows the modulated and core deceleration signals derived from the accelerometers. Output stage The sensor’s output stages to the airbag inflator – or “squib” – are shown in Fig.3 and use two power transistors to fire the airbag. At the start of a crash, the microcontroller sends a trigger enable signal and – after a small delay – the output stages are enabled. If February 1994  7 program and some of these would have provided data to calibrate the airbag sensor (among other things). Testing is also carried out to ensure that the airbag can not be triggered by a hammer-blow or by driving along a rough road. Any unexpected inflation of the airbag could cause the driver to crash. Fault codes & data logging VOLKSWAGEN BARRIER testing of an airbag. Note the seatbelt stretch. Bosch state that in any impact over about 40km/h, the driver will impact the steering wheel, even when wearing a seat­belt. the crash is of sufficient magni­tude, both power transistors are switched on to close the firing loop and inflate the airbag. The firing squib is constantly monitored for inappropriate electrical conditions (like squib resistance change) and the power transistors are tested each time the car is started by sequent­ial­ly switching them on for a short time. Power reserve If the main power supply to the sensor module is disrupted during a crash, an on-board “energy reserve 8  Silicon Chip capacitor” is used as the power source instead. This power source is also constantly moni­ tored for fault conditions. Sensor calibration Calibrating the sensor to suit a specific vehicle is vital. Actual crash testing of a car into a barrier is expensive and so computer modelling is extensively used to reduce the number of test crashes required. Fig.5 shows a typical sensor calibration flow chart. Holden crashed 45 cars into a concrete barrier as part of the VR Commodore development If a fault is detected by the module, either in the sensor itself or in the airbag inflator, a warning light is illuminated on the dashboard. A corresponding fault code is also stored in non-volatile memory. The non-volatile memory is also used to store information generated during the crash itself. Stored within the EEPROM are samples of the deceleration signals encountered during the crash, the time interval between the start of the crash and the deploy­ment of the bag, any errors detected before and during the crash, and the elapsed time since the warning light had last been switched on. A study of some of the G forces recorded in EEPROMs during actual crashes might reveal some sobering statistics and could help improve SC vehicle design. Acknowledgements Thanks to Robert Bosch Australia and General Motors Hold­ens for supplying the information used in compiling this article.