Re: Diagnostics

Don Y <blockedofcourse@foo.invalid> wrote:

On 10/18/2024 8:53 PM, Waldek Hebisch wrote:

One of the FETs that controls the shifting of the automatic
transmission as failed open.  How do you detect that /and recover
from it/?

 
Detecting such thing looks easy.  Recovery is tricky, because
if you have spare FET and activate it there is good chance that
it will fail due to the same reason that the first FET failed.
OTOH, if you have propely designed circuit around the FET,
disturbance strong enough to kill the FET is likely to kill
the controller too.

 
The immediate goal is to *detect* that a problem exists.
If you can't detect, then attempting to recover is a moot point.

In a car you have signals from wheels and engine, you can use
those to compute transmission ratio and check is it is expected
one.  Or simply have extra inputs which mountor FET output.

 

The camera/LIDAR that the self-drive feature uses is providing
incorrect data...  etc.

 
Use 3 (or more) and voting.  Of course, this increases cost and one
have to judge if increase of cost is worth increase in safety

 
As well as the reliability of the additional "voting logic".
If not a set of binary signals, determining what the *correct*
signal may be can be problematic.

Matching images is now a stanard technology.  And in this case
"voting logic" is likely to be software and main trouble are
possible bugs.

(in self-driving car using multiple sensors looks like no-brainer,
but if this is just an assist to increase driver comfort then
result may be different).

 
It is different only in the sense of liability and exposure to
loss.  I am not assigning values to those consequences but,
rather, looking to address the issue of run-time testing, in
general.

I doubt in general solutions.  Various parts of your system
may have enough common features to allow single strategy
in your system.  But it is unlikely to generalize to other
systems.  To put it differently, there are probabilites
of various events and associated costs.  Even if you
refuse to quantify probabilites and costs your design
decisions (assuming they are rational) will give some
estimate of them.

Even if NONE of the failures can result in injury or loss,
it is unlikely that a user WANTS to have a defective product.
If the user is technically unable to determine when the
product is "at fault" (vs. his own misunderstanding of how it
is *supposed* to work), then those failures contribute to
the users' frustrations with the product.
 

There are innumerable failures that can occur to compromise
the "system" and no *easy*/inexpensive/reliable way to detect
and recover from *all* of them.

 
Sure.  But for common failures or serious failures having non-negligible
pobability redundancy may offer cheap way to increase reliability.
 

For critical functions a car could have 3 processors with
voting circuitry.  With separate chips this would be more expensive
than single processor, but increase of cost probably would be
negligible compared to cost of the whole car.  And when integrated
on a single chip cost difference would be tiny.
>
IIUC car controller may "reboot" during a ride.  Intead of
rebooting it could handle work to a backup controller.

>
How do you know the circuitry (and other mechanisms) that
implement this hand-over are operational?

 
It does not matter if handover _always_ works.  What matter is
if system with handover has lower chance of failure than
system without handover.  Having statistics of actual failures
(which I do not have but manufacturers should have) and
after some testing one can estimate failure probablity of
different designs and possibly decide to use handover.

 
Again, I am not interested in "recovery" as that varies with
the application and risk assessment.  What I want to concentrate
on is reliably *detecting* faults before they lead to product
failures.
 
I contend that the hardware in many devices has that capability
(to some extent) but that it is underutilized; that the issue
of detecting faults *after* POST is one that doesn't see much
attention.  The likely thinking being that POST will flag it the
next time the device is restarted.
 
And, that's not acceptable in long-running devices.

Well, you write that you do not try to build high reliablity
device.  However device which correctly operates for years
without interruption is considered "high availability" device
which is a king of high reliablity.  And techniques for high
reliablity seem appropiate here.

It is VERY difficult to design reliable systems.  I am not
attempting that.  Rather, I am trying to address the fact that
the reassurances POST (and, at the user's perogative, BIST)
are not guaranteed when a device runs "for long periods of time".

 
You may have tests essentially as part of normal operation.

 
I suspect most folks have designed devices with UARTs.  And,
having written a driver for it, have noted that framing, parity
and overrun errors are possible.
 
Ask yourself how many of those systems ever *use* that information!
Is there even a means of propagating it up out of the driver?

Well, I always use no parity transmission mode.  Standard way is
to use checksums and acknowledgments.  That way you know if
transmission is working correctly.  What extra info you expect
from looking at detailed error info from UART?

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                              Waldek Hebisch