Editor's Note: This is the first of a series of seven teardown articles exploring the electronic makeup of the Toyota Prius hybrid vehicle. All are featured at www.techonline.com/underthehood.
Airbags have become a fixture of automobiles. Federally mandated in U.S. cars sold after April 1,1989, airbags provide a valuable and effective supplemental restraint system when used in combination with your normal seat belts. For those who have experienced an airbag deployment, you know that it is a violent event and costly to replace, so great care is built into the design of the airbag control module (ACM) to insure it goes off when it needs to but doesn't deploy in a minor fender-bender. We manually fired off the airbag at the live ESC Prius Teardown to demonstrate the drama of an airbag event and because, well let's be honest, engineers just like to see things blow up sometimes. After getting a whiff of gunpowder and feeling the heat generated in deployment, we set off on looking at the electronics.
The Toyota Prius airbag system varies in complexity depending on whether optional side-impact airbag curtains are ordered. In the case of the primary driver side airbag located in the steering wheel, the ACM detects whether a collision of sufficient magnitude for deployment has occurred and also varies the degree of deployment to compensate for seating position. Those pulled up close to the wheel will not see the degree of deployment as those positioned further back.
Analog components form the critical front-end for the ACM, with accelerometers being the primary impact detection sensors. Two MEMS-based accelerometers made by Denso of Japan (#7Q 32 and #8T J8) are surface-mounted directly to the ACM board. The two MEMS components are placed orthogonal to one another, one in the Y-axis for front collision detection and the other oriented in the X axis and presumed to correspond to side-impact events. The sensors themselves are housed in a metal-lidded ceramic package and composed of two chips stacked on top of one another. The MEMS element appears to be a micromachined strain gauge which then connects to a second chip below for signal processing and communication with the rest of the ACM electronics.
(Click on image to enlarge)
A small module (Toyota #09H29) containing a sliding mass which shorts out contacts with sufficient deceleration is also present. While we are not certain of its function, it could serve as the first line of impact detection to trigger activity in the balance of the ACM circuitry or perhaps serve as a backup sensor to the MEMS devices.
A Renesas #HD6432695 32-bit CPU with memory monitors the MEMS outputs and performs the algorithmic computations to determine the fire/no-fire threshold and probably select the stage 1 or stage-2 deployment levels based on driver proximity. Most memory for the processor is resident on the Renesas chip and no discrete memory devices other than a small serial EEPROM are present.
From there it is back to what appears as a more mixed-signal interface with three Denso-custom parts made by STMicroelectronics. The #151821-1390 numbered component is duplicated twice, leading to a possible conclusion that one affects the driver-side air bag module and the second the passenger-side airbag. The second Denso/STMicro part (#151821-136) is visually similar to the first pair of parts at the die level, perhaps a slight variation to handle communications control between the ACM and other Prius modules over the car's communication busses. Given the custom nature of the parts, precise details become hard to determine and I'm guessing here " reader input always welcome if you have some other thoughts. There were other airbag sensing modules in the car, probably used for side-curtain functions, so consider this module as only part of the total solution in the fully-optioned Prius variant.
The ACM shown here was housed in yet another of the Prius' many cast metal module housings, using only compliant peripheral-leaded packages and an anti-corrosion varnish. As with many aspect of the Prius' constellation of electronics, the story on conservative design for safety and reliability of critical systems remains the same. In that vein, you will notice the two large electrolytic capacitors on the ACM. These too are probably a redundant safety element to assure enough local power for deployment in case the collision disrupts the normal 12-V source of energy.
David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)
ESC Prius teardown
Video: Tear down that Prius!
Tried and true design concepts drive Prius
Inside the Toyota Prius: Part 2 - The dashboard display system
Inside the Toyota Prius: Part 3 - Skid-control module plays it safe
Inside the Toyota Prius: Part 4 - Controllers keep dual motors humming
Inside the Toyota Prius: Part 5 - Inverter/converter is Prius' power broker
Inside the Toyota Prius: Part 6 - Navigation unit bridges automotive, CE
On Demand Seminar: The Toyota Prius exposed
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