Drive by wire and the wish to survice the experience.

[Chris Conlon]

Dave wrote:

> ...
>     I'm also 
>     contemplating drive by wire which has the potential for some 
>     interesting failure modes! 
>
> ...
>
>      I was thinking of using the Idle 
>     Switch to validate the TPS signal which used in the control loop along 
>     with pedal position. Call me cautious or something, but this does not 
>     seem enough to me! Cruise footbrake switch(s) not viable due to left 
>     footbraking if I'm feeling daring! Are there two pots. in the TPS, if 
>     not could I install a 2nd one?

One comment:

If the TPS is *just* a normal pot, and you're building all the
circutry yourself, you can easily get one level of failsafe by
doing things a little differently. Normally you put +5 (or
whatever) to the top of the pot, ground the bottom, and read Vs
from the wiper. Instead, put +5 to the wiper, and use both the
other terminals of the pot as the "top halves" of 2 separate
voltage dividers. (i.e. put a resistor from the top of the pot
to ground, and measure the voltage at that connection. Do
likewise with the bottom of the pot.) Now you have 2 separate
signals coming from one TPS, and thus a way to check A against
B. The drawback is the signals have the funky curve of that
type of voltage divider (you may need more bits in your A/D)
and the relationship between A and B is a little weird. But
almost any common pot failure mode (open, dead spot, short)
will give A/B signals that are obviously inconsistent, and
there's your extra safety margin.


> Any recommendations of A/D sampling freq. for the TPS? 

And one question: Bruce (I think) mentioned an OE app using 180Hz
sample rate on the TPS. Does anyone have an idea of the sample
rates used for other sensors, in any other application? Mainly
I'm thinking about MAP, but any trivia would be interesting.

   Chris C.