Due to the explosive nature of an airbag system, testing its components requires careful attention to detail. In the case of the airbag inflator, you must take care not to trigger the device while testing it. On the other hand, testing a complete airbag system requires that you do trigger the system so you can observe the airbag inflate. Because testing destroys the airbag in a controlled explosion, you must ensure the setup is safe and that you gather all the data you need when the airbag deploys. You don't get a second chance to inspect a particular airbag, and running extra tests—which destroy additional airbags— becomes expensive.

Although you can test every component in an airbag inflator using automated test stations (see "Don't blow it," p. 7), you obviously can't test the deployment of every airbag in a production run. After you deploy an airbag in a test, you can't remanufacture it and ship it.

To help ensure product quality, airbag-system manufacturers test samples taken from production lots. Automakers also may have suppliers perform lot-acceptance testing (LAT) as part of their purchase-contract obligations. Although the number of samples per lot varies, typical tests involve one airbag, randomly selected from a production lot.

Fig 1 Tests of an airbag system place the unit in a realistic position in a special test chamber. Courtesy of Microsys.

Airbag test systems include equipment that triggers the initiator and measures the response time as well as inspection equipment that reveals how the bag reacted during deployment. A commercial airbag-deployment inspection system may cost as much as $200,000 and typically includes:

• a deployment chamber, equipped with the appropriate interlocks for operator safety,
• a ventilation system that removes dust and toxic residue after a test,
• a PC that controls the test and analyzes test data,
• one or two high-speed cameras that record the airbag deployment, and
• data-acquisition hardware that records test parameters. As an option, the test system also may include sensors that measure chamber temperature and internal air pressure.

When running a test, a technician installs a sample airbag system in a fixture (Fig. 1) housed in a deployment chamber. The technician then closes the deployment chamber, selects the appropriate test sequence, and runs the test. The test system then programs and triggers the cameras and "fires" the airbag.

Inflator provides gas

Each airbag contains an inflator that provides the gas that inflates an airbag on command from electronic sensors. The inflator incorporates one or two initiators that start a rapid chemical reaction—a controlled explosion—that actually generates the gas. Initiators can generate the gas at different rates, depending on the deceleration detected during a crash by other electronic systems.

Fig 2 Three images show the deployment of an airbag as it inflates. Courtesy of Microsys.

The signal that fires an initiator is typically a 3-ms to 5-ms square wave, with an amplitude of 1.75 A. To ensure an initiator fires properly, the test system measures the voltage across the initiator's ignitor wire. As the wire heats, its resistance varies, and when the initiator fires, the explosion opens the wire and the current drops to zero. In an airbag system that uses two initiators, the test system will sequence the firings according to the manufacturer's specifications.

Once the airbag has deployed, the test system downloads the video data from the cameras. For production tests, this data includes approximately 100 video frames taken at 1000 frames/s during the first 1/10 of a second as the airbag inflates.

An experienced technician, or possibly an engineer, will analyze the video frames to ensure the airbag deployed properly. Fig. 2 shows three images from a typical inflation sequence, as acquired by a side-view camera.

The technician or engineer will step through the frames, or view them in slow motion, noting any impediment to the proper deployment. If the airbag included dual initiators, the technician will look for signs that firing the second initiator caused the airbag to inflate at a different rate. In most automated test systems, the ability to synchronize the video frames with the acquired data makes it easy to correlate observations with electrical or other test data.

If an airbag fails to inflate properly, the technician can send the test data to a design engineer or quality engineer for further analysis. When diagnosing the failure, the engineer will look for things that may have mechanically impeded the deployment, such as housing materials in the airbag system that didn't fully swing out of the way. He or she will also look at the initiator's current waveforms and the images to determine if that component worked properly. Only after looking at this information can the engineer decide if a production lot requires further testing.

Although this inspection method has proven effective, you can expect to see better inspection methods in the near future. Instead of relying on a technician or engineer to check images, test software will automatically analyze images to determine the characteristics of airbag deployment. This software will apply image-analysis techniques similar to those currently used to analyze how crash-test dummies move in a collision. In effect, this software takes the technician out of the inspection process and improves the repeatability and accuracy of the inspection test results.

For further reading

"Testing Dual Airbag Inflators and Modules with the Model 2790 SourceMeter Switch System," Application note 2378, Keithley Instruments, Cleveland, OH, 2002.

"Why Test Airbags?" Case study, Microsys Technologies, Mississauga, ON, Canada, 2002. http://www.micro-sys.com.

 

Author Information:

Dan Romanchik has a BSEE from the University of Detroit, 12 years of experience in test engineering, and 13 years of experience writing about test technology. You can contact him by e-mail at dan@danromanchik.com.

 

Don't Blow It   The inflator that produces the gas needed to fill an airbag contains two initiators. Each initiator works separately—one operates during a slow-speed collision and the other during a high-speed collision. Each initiator includes a bridgewire that, when heated, sets off an explosive reaction that generates gas that inflates an airbag. A "primer" material that coats each bridgewire ignites when current above a specific level passes through the wire. The resistance of this wire is typically 2 Ω. Modern airbag systems use two initiators, so each inflator comes with four leads (Fig. A). Fig A Four-wire measurements help ensure teh accuracy of electrical tests of an airbag inflator assembly. An inflator test system must measure the following: Bridgewire continuity . You test the continuity of a bridgewire by sourcing a current through the bridgewire and measuring the voltage across it. To avoid igniting the device under test, use a test current of 50 mA or less, which generates a voltage of 100 mV or less across the bridgewire. Because the resulting voltage is small, consider making a four-wire measurement to ensure accuracy. Insulation resistance and isolation. Typically, a metal container houses the airbag inflator. To ensure the inflator will operate properly, the initiators must be electrically isolated from the housing. To verify this isolation, you measure the insulation resistance between the bridgewire and the metal housing. This specification ranges from 10 MV to 100 MV. To measure the insulation resistance, apply a voltage between the housing and the bridgewire and measure the leakage current. With a test voltage of 500 V, you should measure a leakage current of 50 µA or less. Shorting clip resistance. Each inflator includes a temporary shorting clip across the initiators' terminals; this prevents the inflator from igniting during shipping or installation. Measuring the proper resistance of each shorting clip—typically 10 mV to 100 mV—ensures the clips make good electrical connection and removes the fear of accidental ignition during shipping and handling. A test system will measure the clip's resistance several times. First, it gets measured at the beginning of the test sequence, before the insulation-resistance test, to ensure the inflator will not ignite when the test voltage is applied. Finally, the test system measures the resistance of the clip to ensure it is back in place. (The test system must mechanically remove the shorting clip to measure the continuity of the bridgewire.) In addition to the three measurements above, the test system must also check the continuity between two connections and the metal housing. A continuity check from one housing contact to the other ensures a good contact for the insulation-resistance test. If no proper connection exists, the test system will measure the insulation resistance of an open circuit, not the true insulation resistance of the inflator. Read the complete article online at Test & Measurement World Online