Joined: 14 Dec 2003
|Posted: Sat Mar 06, 2010 9:06 pm Post subject: Toyota's unintended acceleration?
|From Popular Mechanics
Anatomy of Toyota's Problem Pedal: Mechanic's Diary
What's the real problem behind Toyota's unintended acceleration? Is it
simply a sticky pedal, or is the trouble more fundamental? PM senior
automotive editor Mike Allen delves into modern car tech, explaining why
widespread theories about electrical throttle problems and electromagnetic
interference are misguided.
By Mike Allen
Published on: March 3, 2010
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) s 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
n 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
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.
|Posted: Thu Jan 24, 2013 3:32 am Post subject:
|Toyota is not known for cover ups, but if they do one and the truth is they have made a defect, their high quality rep will be in the toilet. It will be better for them to come clean and fix it if there is a real issue.
||All times are GMT - 7 Hours
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