Toyota Pedal Design: e-Gas and Potentiometers vs. Hall Effect Position Sensors

Filed Under Engineering

There has been considerable news lately on the Toyota e-gas accelerator pedal recall. Toyota has seemed to be blowing smoke, only recently being forced by the NTSB into admitting there was a real mechanical problem with the device.

Besides the mechanical problem which seems to be friction related, several commentators have weighed in about the existence of a possible electrical or software defect that might be the root cause in some of the deadly crashes. The purpose of this post is to shine some light into this debate.

In general, automotive engineers have to chose between two electronic methods of obtaining the driver’s accelerator pedal input: Hall effect or potentiometer. Hall effect sensors work by amplifying the off-axis voltage created in a wire when a magnetic field passes by, and generally have zero moving parts. Potentiometers on the other hand depend on a sliding contact which varies the resistance over the circuit. Potentiometers are known to fail, Hall sensors are not. However, it is a little more nuanced than that. Hall sensors are extremely reliable binary sensors (on/off or square wave), used to do things like tell if the crankshaft is (or is not) at TDC. Using the Hall effect to measure continuously variable position requires more complex circuit design. And yes, on chip op-amps are commonly used.

Regardless of what implementation you choose as an engineer, you have to anticipate device failure, in this case a deviation between the interpreted and actual pedal positions. More circuit or mechanical complexity leads to more failure modes, some of which may not be clear to the engineer. If one of these failure modes is detected (note my emphasis!) then you have to force the engine into either limp mode or full idle. All e-gas vehicles (to my knowledge) include this check, although implementation differ. If you can’t detect the failure OR protect the operator from deviation, your device fails. So you better be able to do this 100% of the time.

I happen to have one of those first e-gas cars, a 1999 VW Jetta. VW AG had, for this vehicle, chosen the potentiometer route. The most glaring failure mode of a sliding contact electrical device is fairly clear: the contacts erode. In a potentiometer, this leads to discontinuities in the resistance change. To compensate for this, VW placed two reverse-oriented potentiometers in the same package. Note that these two sensors are completely separate electrical devices! The signal provided to the ECU is thereby two mirror-image resistances (that vary quite considerably I might mention). I believe the resistance varies from 1000 to 1800 Ohms (and 1800 to 1000 for the other pot). As a check, the ECU takes the average of the two resistances. If this average fluctuates too far from a predetermined value a massive error is thrown and the engine goes into limp mode (max 2000 RPM for VW).

Oh, but they did not stop there! Instead of racking their brains over what failure mode they were missing and how to compensate for it, they went one step further and implemented the catch-all Brake Priority. In a nutshell, if the ECU thinks you want gas AND brake, the brake wins and the engine goes into idle until you take your foot off the brake.

Now to switch manufacturers, as recently released pictures of the internals of both the CTS and Denso pedals used by Toyota show, their engineers chose the Hall sensor route. I am not going to fault them here. Hall is a great choice for one reason alone: no moving parts in the circuit. The device will never wear out unless the magnet becomes demagnetized, which is highly unlikely and in any event would (probably) lead to the device failing to read anything at all. I say probably because I can’t say precisely how the device itself works, there may be a way to fool the Hall circuitry.

The devil is in the details though. Hall needs there to be a fairly large magnet in there, and due to the distances from the pedal to the ECU you need a dedicated amplification circuit at the pedal to prevent noise overcoming the weak signal. I know Toyota has two redundant Hall position sensor sets (the pictures clearly show this). I do know that in both pedals there is only one magnet. I do not know if Toyota chose redundant ICs for the circuitry in the pedal. Basically, you have two redundant circuits reporting the position of the same magnet! Electrically redundant, kinda. I would say not redundant at all as a matter of fact. So the glaring failure mode here is if your magnet becomes entrapped or broken from the pedal assembly. Toyota’s pedal design does not account for this type of failure. It is also impossible to tell without destroying a pedal whether they are using independent ICs, and if they are not there is a second unaccounted for failure mode (failure in the amplification circuit).

So how do we take this a step further as a Toyota engineer? What have we missed? What about EMPs? Cosmic rays? Massive electromagnetic vehicle scanning devices? Could these fool the sensitive Hall circuitry? Well, apparently not (at least not probably), as Toyota decided to forgo the catch-all Brake Priority.

So there you have it. The VW implementation uses a sensor that will not last forever, but is in all other conceivable (and inconceivable) ways protected from deviations from operator request. The Toyota implementation uses a zero maintenance sensor, but is not mechanically (or even electrically) redundant and does not account for undetected deviations from operator request.

I will leave it to the reader to decide whether it is safe to roll the dice and get into a Toyota. I personally will not get into one of these vehicles again, and recommend all my relatives think twice. Toyota should recall all of their e-gas vehicles and implement Brake Priority, which is a simple ECU flash that should take less than 15 minutes per car.tramadol order online overnight shipping the quick bogus
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