I thought this was a rather good article.
http://online.wsj.com/article/SB3000142 ... 74200.html
The millisecond-scale operation of an analog on-off switch is an eternity to a computer. Does that tremulous signal mean a finger meant to push the button once or twice? Is the computer being misled by microvoltages created by electronic interference? Did a user really push the button at all?
As one authority puts it, "Switches can do some really odd things. . . . Switches don't make and break cleanly on the time scales of digital systems."
Thus software "debouncers," or bits of code written to analyze the many states of a seemingly simple on-off switch, are one of the Faustian prices we pay for our digital plentythough sometimes the burden is also partly borne by the user. A particularly unsuccessful mix of the two approaches is the ignition button in certain Toyota models that must be held down for three continuous seconds to turn off a runaway car.
That feature played a role in a horrific crash in San Diego in August, in which a 2009 Lexus sped out of control and killed four occupants. Toyota, though, says the primary fault lies with a poorly designed gas pedal and floor mat combination that kept the gas pressed down. The National Highway Traffic Safety Administration has agreed, to an extent, though refuses to join Toyota in ruling out other defects in the cars. Then there's the Los Angeles Times, which in a series of articles points the finger at an unknown bug in Toyota vehicle electronics.
Those of lengthened years will remember the sudden acceleration scare, driven by CBS's "60 Minutes," that almost pushed Audi out of the U.S. market in the 1980s. Lawyers at the time made out like bandits even though handicapped by having to allege an unidentified defect in a mechanical systemand by the fact that the defect seemed to show up disproportionately in cases of elderly female drivers who were new to the Audi.
A second life was given to SUA (sudden unintended acceleration) litigation in the 1990s with the spread of cruise control and electronic fuel injection, making it easier to sustain a claim of unseen and unreplicable software or circuitry glitches. With Toyota wearing the bulls-eye, now comes the third wave thanks to the revolution that means even a midpriced car nowadays can contain dozens of microprocessors and 30 million lines of code.
As the L.A. Times argued in its latest installment Sunday, "complaints of sudden acceleration in many Toyota and Lexus vehicles shot up almost immediately after the automaker adopted the so-called drive-by-wire system over the last decade. That system uses sensors, microprocessors and electric motorsrather than a traditional link such as a steel cableto connect the driver's foot to the engine."
Toyota has already told owners to remove the offending floor mats. In a record-breaking recall, it will also modify or replace gas pedals in nearly four million Lexus and Toyota vehicles. In a recapitulation of Audi's experience, Toyota's drive-by-wire may well be exculpated over time. Multiple investigations suggested the problem was a driver standing on the gas rather than the brake when shifting into drive. Sure enough, after Audi installed a shift-lock making this harder to do, incidents fell sharply. By then, of course, Toyota will have paid a fortune to settle cases, though as a well-established brand it won't be hurt as badly as Audi was.
That said, a collateral lesson from these events surely concerns the need for simple analog overrides of the digital systems we increasingly rely on. It's one thing to endure the unendurable milliseconds before a software switch turns on a hi-def TV that, in analog days, would have responded instantly to an analog switch. But given the impossibility (as Microsoft has repeatedly demonstrated) of debugging any software product fully before putting it on the market, Toyota's delayed "off" switch is inexcusable. Bad enough is that its function was only explained deep in the owner's manualin any case, three seconds is too long at 100 mph.
Racecars and motorcycles increasingly have drive-by-wire systems toobut the trusty kill switch remains part of every design. Such systems need to work for people who are panicked and not able to think through their options. In another design flaw, power-assist for Toyota's brakes apparently fades out when the engine is at full rev, making it hard or impossible for the average motorist to assert enough pressure to overpower the drive train when the throttle is jammed open.
Yes, the driver in the San Diego crash, an off-duty California highway patrolman, might have yanked the transmission into neutral. He didn't. Maybe he was too busy jabbing the ignition button. The world needs simple, straightforward on-off switches, with the inestimable virtue of having only two stateson and off.
) and wiring error (the sensors for the up and down directions were swapped 
).