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Popular Mechanics on Toyota Accelerator Recall

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Old 11-06-2011, 08:06 PM   #1
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Popular Mechanics on Toyota Accelerator Recall

I found this article very interesting, kind of reinforces my theories about the system, but also backs up my faith in the system as well. I find it actually better than a cabled system. This is an older article, some may have read it, just wanted to share.

Taken from PopMech website:

http://www.popularmechanics.com/cars...dustry/4347704


Toyota has recalled millions of cars and trucks—4.2 million to replace floor mats that might impede throttle-pedal travel, and 2.4 million to install a shim behind the electronic pedal assembly. All of the affected pedal assemblies were made by Canadian supplier CTS. Toyota's boffins have documented a problem that can make a few of these pedals slow to return, and maybe even stick down. Problem solved.

But the media, Congress—and personal-injury lawyers—smell the blood in the water. Not to diminish the injuries and a few deaths attributable to these very real mechanical problems, but they're statistically only a very small blip, which may explain why Toyota took so long to identify the issue, especially when it has symptoms similar to the similarly documented floor mat recall. Plus, sudden unintended acceleration (SUA) is notoriously difficult to diagnose because, more often then not, the problem can't be repeated in front of a mechanic. Let's not forget the Audi SUA episode back in the '80s; the National Highway Traffic Safety Administration eventually concluded that there was no mechanical problem. The culprit, as hard as this is to admit, was most likely driver error. To put the issue into context, in the last decade, there were about 24,000 customer complaints about SUA involving almost every major automaker. The NHTSA investigated fewer than 50.

The issue now is whether there's a more insidious problem unrelated to the two recalls already extant. Specifically, whether there's some design flaw in the entire concept of electronic throttle control. Some are questioning whether electromagnetic interference from devices like cellphones could be contributing to the acceleration problems.

It used to be that there was a steel cable that ran from the pedal itself through the firewall and attached to the throttle blades that admitted air to the intake manifold. A sticking throttle could be the result of friction anywhere—in the pedal pivot, between the cable itself and its nylon-lined sheath, or in the carburetor or fuel-injection throttle blades. (Does anybody remember carburetors?). Modern cars, which make up the majority on the market today, use a throttle pedal assembly that is connected to the engine only electronically. Signals are carried over wires to the engine management computer, which in turn sends electrical impulses to the stepper motor that actually controls the throttle blades.

Sounds like there are plenty of places for gremlins to seize control of the works, right? And that's where pundits who don't really understand the architecture of throttle-by-wire systems go wrong. It's all in the engineering.

Let's start at bottom of it all—your foot, which moves the pedal fore and aft in relation to the firewall. Inside the pedal assembly is a spring to make it return as you lift off, a device to add a little friction that dampens the movement (Your foot would tire in short order if there wasn't some damping), and a transducer of some sort that turns the movement of the pedal into an electrical signal. That transducer is a simple device, invented in 1879 by Edwin Hall (not 1979; 1879). It consists of only a single slab of semiconductor with a few wires attached to its edge, one on each end and one in the middle. With a voltage applied to the end wires, it acts as a voltage divider. Placing a magnet near the sensor changes the magnetic lines of flux, which literally push the electrons away from the electrodes and changes the voltage at the center wire. The magnet, in the Toyota case, is on the pedal arm. As the pedal moves, it alters the voltage at the semiconductor and that's how the engine computer knows the position of the pedal. The benefit of Hall-effect sensors is that there's no mechanical connection to corrode, no internal resistance, and other electronics, such as amplifiers, aren't needed. You could make one on your kitchen table with a refrigerator magnet and some doorbell wire.

There are two discrete Hall-effect sensors in the Toyota/CTS pedal, which is common industry practice. Just to make sure the sensors aren't confused, they run on totally separate circuits back to the ECM, three wires each. They don't even share an electrical ground. Like many onboard automobile sensors, they are also completely isolated from the vehicle ground. To reduce the potential for interference or mistakes, they operate at different voltages. The first sensor, known as ACCEL POS #1, has a nominal voltage range from 0.5 volts to 1.1 volts at idle and 2.5 volts to 4.5 volts at wide-open-throttle (WOT). The second sensor, ACCEL POS #2, delivers from 1.2 volts to 2.0 volts at idle and 3.4 volts to 5.0 volts at WOT. Why such a wide range of permissible voltages? The engine computer (ECM) recalibrates the sensor regularly, every time you start the car and the ECM goes through its power-on self-test.

Both accelerator-pedal-position Hall-effect sensors have to agree fairly closely, or the ECM will go into its limp-home mode, which turns on the Check Engine light and sets a trouble code.

There's more. If Toyota's engine-management scheme is anything like that of most other car companies, firmware inside the ECM also monitors the airflow into the engine, the throttle blade position and engine rpm, and calculates backwards to what the throttle pedal position should be. Any discrepancy, and a trouble code is set, the Check Engine light on the dash goes on, and you're dialing the service manager to make an appointment.

Bottom line: The system is not only redundant, it's double-redundant. The signal lines from the pedal to the ECM are isolated. The voltages used in the system are DC voltages—any RF voltages introduced into the system, by, say, that microwave oven you have in the passenger seat, would be AC voltages, which the ECM's conditioned inputs would simply ignore. Neither your cellphone nor Johnny's PlayStation have the power to induce much confusion into the system.

These throttle-by-wire systems are very difficult to confuse—they're designed to be robust, and any conceivable failure is engineered to command not an open throttle but an error message.

So what to make of the unintended acceleration cases popping up by the dozens? Not the ones explainable by problem sticky pedals, but the ones documented by people who claim their vehicle ran away on its own, with no input, and resisted all attempts to stop it? Some can probably be explained as an attempt to get rid of a car consumers no longer desire. Some are probably the result of Audi 5000 Syndrome, where drivers simply lost track of their feet and depressed the gas instead of the brake. It's happened to me: Luckily I recognized the phenomenon and corrected before it went bang. Others may not have the presence of mind.

But the possibility that a vehicle could go from idling at a traffic light to terrific, uncalled-for and uncontrollable acceleration because the guy next to you at a traffic light answered his cellphone? Or some ghost in the machine or a hacker caused a software glitch that made your car run away and the brakes suddenly simultaneously fail? Not in the least bit likely. Toyota deserves a better deal than the media and Congress are giving it.
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