Not one to shrink from a technically intriguing challenge, EE Times, Embedded Systems Design and Techonline partnered with Munro and Associates and Portelligent and bought a brand-new, loaded Toyota Prius and summarily rendered it useless on the show floor to see how its electronic systems were implemented.

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Stay tuned for the Under The Hood special for details on the hybrid gas/electric power train control systems, power delivery/recovery electronics and infotainment/navigation systems. For now, we take a look at the skid control module (SCM).

The SCM works to control and correct traction loss. Working in concert with the Prius brake control system--which drives hydraulic friction braking and optimizes the use of regenerative braking as a fuel-saving measure--the SCM provides the smarts to tell each wheel what to do, when. The module communicates with the drive train control electronics to modulate delivered power. Primary inputs to the SCM include the individual wheel-speed sensors and the hydraulic pressure at the wheels' brake cylinder. Other SCM inputs are yaw, deceleration-rate and steering-angle sensors.

Most of us at some point have gotten overzealous with the accelerator on a slick road, compromising traction. The SCM here compares the rotation speed of the front (drive) wheels with that of the rear (undriven) wheels. This enables it to detect simple wheel spin and back off on applied power to achieve speed coherence between the front and rear wheels.

Another task for the SCM is dealing with—and preventing—wheel lockup during panic stops. Antilock braking systems are standard on virtually all cars these days, and the SCM adds ABS chores to its list. The module monitors wheel speed to be sure each wheel is hauling down to a stop at the same rate. When the SCM detects a wheel (or wheels) locking up, it instructs the brake control system to suspend further braking, to minimize skid and maximize braking effectiveness.

Overall traction and stability control are the last elements of the SCM's function. The module comes into play in controlling understeer (which results in the car's "pushing through" an intended turn) and oversteer (which causes back-end spinout). By monitoring steering wheel angle, relative wheel speeds, yaw and possibly lateral g-forces, the SCM can detect when front-wheel traction loss (understeer) or rear-wheel traction loss (oversteer) is imminent during turns. Again, the SCM works with the braking system to direct the applied braking forces carefully at each wheel.

Implementation of the SCM is conservative, as is the case for most of the mission-critical electronics we found in our Prius teardown. A meaty cast housing in the cabin and under the dash holds a single pc board populated with peripherally leaded IC packages and spacious component layout. The SCM exhibits central symmetry--corresponding, I suspect, to the safety-minded control split of front-right/left-rear and front-left/right-rear wheels. With redundancy implemented in this way, a partial failure still leaves one front wheel and one back wheel available for an SCM-driven control loop.

Analog circuits are key to the SCM implementation, since input signal conditioning and output braking actuator drive are inherently nondigital tasks. The computational brains behind the SCM come from a Toyota-branded Toshiba TMP1984FDFG 32-bit microprocessor and a Mitsubishi M30620 16-bit microprocessor, the latter a design with mask- works indicating a 1995 copyright date. Again, conservative design seems to be the guiding principle.

Beyond the number crunching, however, Toyota turned to custom devices for the mixed-signal interfaces. The Toyota DA023 and DA034 are analog control or conditioning devices, based on a peek at the bare chips. No merchant-market foundry markings were seen on the dice, but each chip sported a little Toyota logo, located next to the part number.

While the backside of the circuit board is mostly void of components, the large control currents within the analog parts require attention to thermal management. The DA023 and DA034 chips, replicated on the left and right sides, use a thermal pad on the SCM board reverse side that carries heat away from the chips and makes contact into the SCM casing for keeping things cool.

In sum, the Prius' complex stability control relies on conservative processor choices and custom-crafted analog circuits.

David Carey is president of Portelligent (www.teardown.com). The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.

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