Drive by wire and the wish to survive the experience.

[David-HMSE Higham]

     Some general points/ideas.
     
     Sensor diagnostics - 
     Generally sensor failures are detected either hi and lo range, when 
     out of range a fail count is started. When this count equals a defined 
     limit then a sensor hard fault is set, and a default value is 
     substituted. If the sensor value goes back into range then a recovery 
     count is started, when this equals a limit then the sensor has 
     recovered, the hard fault is cleared and sensor value is used again. 
     Other features are usually incorporated such as limiting the number of 
     hard faults per mission and storing faults to E2PROM etc.
     
     DBW safety - 
     The 5v to wiper seems like a good idea, I will give that a try!
     
     On a DBW (Drive By Wire), the ECU would have calc'd the demanded 
     throttle position. If the TPS is deemed failed, then I assume the ECU 
     would enter some limp home mode (reduced power etc), and as stated 
     below, would uses MAF and RPM to estimate TPS for continued safety 
     checking. 
     
     Just to confirm DBW uses a Driver demand signal from the pedal, uses 
     this to calc throttle position set via a stepper motor, with TPS used 
     as feedback. (No throttle cable present.)??????
     
     Sensor Sampling - 
     Somebody inquired general sensor sampling rates. The sensors can be 
     slip into fast and slow sampling (posibly medium also.) Coolant temp 
     for example would be a slow sample >100ms where as MAF and MAP would 
     be much faster <10ms (could be speed dependant). It generally depands 
     on the control loop within which these sensors are used.
     
     Any comments?
     
     Cheers,
     
     Dave


______________________________ Reply Separator _________________________________
Subject: Re: Drive by wire and the wish to survice the experience.
Author:  "David A. Cooley" <n5xmt at bellsouth.net> at Internet-hel
Date:    31/12/98 15:48


At 03:36 PM 12/31/98 -0500, you wrote:
>It would kill both sides, but your control circuit would know 
>*immediately* that there was a failure. Any setup, with any >number 
of levels of redundancy, can *still* fail. The key point >then is 
that you be able to *know* it's failed. An ordinary TPS >setup gives 
you no such luxury; this slight change does, with >almost no extra 
hardware. There's nothing to stop you from running >dual (triple, 
quad, etc) TPSs as well, depending how much >redundancy you need, as 
opposed to failsafe, which is what I said >this trick does. (Though 
personally I'd consider an LVDT on the >throttle cable as backup 
instead, in my experience they're more >reliable.)
     
Actually, the TPS does have a way of knowing there is a failure...
In the cal, they program max hi and min low voltages that are valid... If 
the TPS input falls outside this range, it disables TPS, and estimates TPS 
from MAF or MAT and RPM etc.
Later,
Dave
     
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           David Cooley N5XMT           Internet: N5XMT at bellsouth.net
     Packet: N5XMT at KQ4LO.#INT.NC.USA.NA   T.A.P.R. Member #7068
       I am Pentium of Borg...division is futile...you will be approximated.
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