On Sun, 30 Aug 2020 13:22:20 +0100, John Rumm
wrote:

On 30/08/2020 10:49, T i m wrote:

Adrian was concerned about safety and we all know that the simpler a
solution the lower the chance of failure (MTBF).


The way one normally deals with safety critical software systems, is by
working very hard to avoid any of the software being safety critical in
the first place.

Oh, agreed, however, a simple firmware based system might be
inherently 'safer' than an OS / PC based system in the first place.

Typically this is done with hardware interlocks, since
they can usually be rigorously "proven" much more easily.

Yup, or like having thermal fuses in on the extruders and heated beds
of our 3D printers. ;-)

So for example with a kiln, you could have a max temperature sensors
that is hardwired into your safety relay/contactor, so should the temp
ever not be indicated as safe by all the sensors it trips out power to
the heaters. (software can obviously finesse that and log and report the
event etc)

Understood.

Given the code for an Arduino is held in NV RAM or prom/ whatever, I
would see it as less vulnerable to corruption than a fully 'live',
re/writeable OS sitting on any form of PC. I'm assuming it would be
easy to include an electronic watchdog timer on both an RPi and
Arduino but nothing (outside of external protection, as you highlight)
would work against a shorted heater driver / relay. [1]

Cheers, T i m

[1] Along those lines I've dug out a Playstation PSU I bought a while
ago (recommended by someone here, Theo possibly?) that I'm looking to
replace the current frame PSU I have in the 3D printer because it has
a built in ATX type low lever 'Enable' function that could be driven
by the controlling PC / RPi and so add *another* level of protection.

'If extruder / bed go over x or y DegC then cut power to printer'.

(Where the values are ideally below the levels of the thermal fuses
but above the peak printing temperatures). ;-)