Airbag Controller Testing Application Note

General

As automotive air bags become commonplace, testing and exercising their controllers become more demanding. The need for a suitable testing capability becomes increasingly critical. This application note will give you ideas on how to apply the Thermotron Product Test System (PTS) to your air bag controller testing needs. The modular design, flexibility and reasonable cost are some of the reasons why the PTS is an excellent choice for this task.

Typical Air Bag Controller Product

For an air bag controller to operate properly, it must receive stimulus representing the conditions expected during operation or testing. Depending on the particular unit, this stimulus will take on the form of power, control signals, and communications. An air bag controller will typically output a warning lamp signal and a squib firing signal. The squib firing signal causes the inflation of the actual air bag.

Occasionally, test programs are loaded into the units under test. The test programs perform operations that may require stimulus that is different from normal operation. The output of the unit under test may also be different from normal operation when executing a test program.

General Testing Strategy

If the air bag controller is operating in normal (no test program) mode, measurements of unit power consumption, squib output voltage and firing current, and warning lamp output a may be performed. A general block diagram of such a configuration is shown in Figure 1. This configuration allows testing of "N" air bag control modules.

Providing Air Bag Controller Power

A PTS Power Supply Module (PSM-2) can provide control of product power supplies. As a result, power supplies can provide power in a manner that synchronizes with the rest of the testing. Simulation of high and low voltage conditions is possible.

If test programs are downloaded to the controller, more extensive testing may take place. Some of this testing may be the result of "self tests" that the units are performing. Both operational and "download" modes of testing can be accomplished with a single PTS system. Note that the PTS allows for synchronization of the testing with environmental conditions produced by a Thermotron chamber.

Providing Air Bag Controller Stimulus

Very few signals are typically required for operation of an air bag controller. A Digital Waveform Generator (DWG-08) can produce the required control signals. Note that a DWG-08 may also be used to provide hardware synchronization signals (triggers). When a DWG-08 is operated in this mode, it is referred to as a "master". Other DWG-08 units triggered by the master DWG-08 are referred to as "slaves".

One or more Automotive Bus Modules (ABM-05) may be used to provide serial communications to and from the air bag controller. The ABM-05 is capable providing five serial ports that meet the customer's standard data communication protocol.

Sensing Air Bag Controller Outputs

Typical air bag controllers have outputs for activating a malfunction warning light and activating one (driver) or more (driver and passenger) squibs. To monitor the warning lamp output, an Analog Input Module (AIM-32) is used. Additionally, the AIM-32 is used to monitor product current consumption. To monitor the squib firing signals, a High Frequency Multiplexer (HFM-08) is used in conjunction with a Digital Waveform Analyzer (DWA-04). This combination of PTS function modules provides the capability of monitoring squib voltage and current waveforms. The squib outputs can thus be checked against known good waveform profiles to verify proper timing and amplitude.

Load Boards

Load boards provide an interface between the PTS and the product(s). Additionally, load boards provide simulated loads, a convenient location for test points, diagnostic displays and configuration jumpers. For example, if an air bag controller expects to see a particular resistive value for a squib, a load board would provide this. The load board would also provide current measurement circuitry, and warning lamp loads. As load boards are removable, extensive changes to the products can be accommodated by creating a new load board.

PTS Profiles

Once the PTS equipment is set up, the PTS software must be configured. To configure the PTS software, profiles are defined for the function modules that are used. No programming is required. It is important to have a clearly defined idea as to the signal requirements for product stimulus, the product output signals, and the PTS to product configuration. For example, a profile may be created for the DWA-04 that specifies that the squib firing signal must occur at the proper time and be the correct amplitude. If the waveform does not reach the defined amplitude, a failure is reported for that unit. Another example profile would instruct the AIM to indicate a fault if the warning lamp output did not deactivate within the appropriate time after the ignition signal is applied.

Possible Air Bag Controller Tester Physical Configuration

A 19 inch rack is typically used for PTS equipment mounting. AC power is normally distributed with an outlet strip and excess heat is dissipated with a blower located at the top of the rack. Casters on the bottom of the rack will contribute to convenience. Figure 2 illustrates a possible PTS configuration.

Load boards are typically located toward the top the rack and the PTS base unit is located toward the bottom, near the power supply. The PTS PC is typically located at a height that allows for easy operator access.

Connections between the PTS rack and the products can be accomplished via cables that connect to the back of the PTS cabinet. These cables can connect directly to the air bag controllers, or to replaceable adapters. Note that is often desirable to locate PTS and product connections inside the rack (as opposed to the front of the rack). This increases the cabling protection, limits cable lengths and reduces the chance of tampering.

Conclusions

This application note demonstrates the possibility of creating a capable, cost effective, flexible method of testing air bag controllers with the Thermotron Industries PTS. This testing can be done in either production or laboratory environments. Contact Thermotron to have your testing needs analyzed and solved.