FMECA Data: Volume 8 (Circuit, Macro level)

AD_ward9

The FMECA for the circuit level (Volume 8 of the overall FMECA) and all circuit diagrams are in the zip file on http://www.deeplife.co.uk/or_submission.php. The file was too large to upload.

The HUD with the fancy features is part of a secondary system: only the simply HUD is considered here. See the review document for some further detail.

In line with the spirit of Open Revolution, we have publised both Submission A (non-compliant) and Submission B (which is regarded as meeting the needs for compliance until the next review stage).

The engineer producing Submission A was rather enthusiastic for Open Revolution and put certain things in which are considered bad practice, to highlight some of the design issues. The review team rejected those, so there are more differences than normal between Submission A and B.

Alex Deas
For the Deep Life Design Team (www.deeplife.co.uk)

jradomski

the link you gave does not work..
"no permission" error

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

AD_ward9

Odd, I get there straight away. I have just checked the permissions, and they are all world readable. I will ask our IT guys to look into it now. Thanks for letting me know.

Edit: Fixed. It seems the files have to be executable as well (.php is executed ). Please let me know if the problem is not resolved of if there are any other access issues. We did try to post these onto Rebreather World directly, but there were some issues with the setup of the RBW server.

Cheers
Alex

jradomski

works now.

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Faceless

Minor note on FMECA Top Level document:
The following link from
http://www.deeplife.co.uk/or_submission.php page
on the document
http://www.deeplife.co.uk/or_files/F..._V1_O51119.pdf
is returning 404 File Not found

AD_ward9

It is being updated.
V2 and V8 are available. We will try and get the updated V1 back on line asap with several of the other volumes at the same time.
Cheers
Alex

billy

Good morning Alex,

First of all thanks for all the information. You give us a lot of knowledge regarding safe critical designs

I would like to ask a couple of questions regarding the Electronics of OpenRevolution.

You have a mixed 5V/3.3V design. Why you have 2 different main voltages ?
Was difficult to do them all in 3.3V or is something that i am missing to fill the puzzle ?

Your prefered opamp seems to be the AD8628:
What was the key points for selecting it ? Beside what the pdf is saying.

For the Scrubber stick you selected the good old friend LM35.
LM35 has a voltage output. Wouldn't be easier to select a digital temperature sensor, or you don't think it will survive in the scrubber ?

You have parts from all the main companies Analog Devices, Maxim, National etc.
Is there a specific reason for not selecting a key partner ?
For example most of the time Maxim has everything you need to start a new project.
Is something else that i am missing ? Perchaps for the MTBF you want to achieve ?

Now on the clock design:
You seem that you don't trust ready made crystal modules. You prefer a crystal and the appropriate compoments
to create your clock. Why ?

Voltage supervisors for every different voltage (which is great). Did you see problems on the voltage lines, when the solenoid is firing ?

At last but not least.

a) How much is the cost of the pressure sensor
b) Where we can get samples

Thanks in advance

and sorry for the long post.

jasonvds

Bump!

AD_ward9

The existence of a 5V rail is due purely to the ADCs: we could not find decent Sigma Delta ADCs with enough inputs running off 3.3V. We use only Sigma Delta for this as they are perfectly linear by design, and have 24 bit resolution. The fact that the manufacturer does not have to step through ever single voltage on each input to test them also makes them cheaper than any other type of ADC - the silicon area is among the smallest too. The 3.3V Sigma Delta ADCs that are available have "issues" when tested, but sooner or later they will improve, and then the 5V rail will vanish.
Out of the thousands of op-amps available, for each application there is usually just one that is head and shoulders above all others. For example, for extremely low noise AC signals one particular Linear Technology op-amp takes the prize, but for near DC signals the AD8628 is par excellence, bar none. In a rebreather most signals are near DC, so voltage reference buffers and the CO2 sensor needs an op-amp that does not drift - the domain of the chopper amplifier. The temperature drift of 20nV/C worst case for the AD8628 is something no other op-amp comes close to. In fact, there was only a valve amp module in the 1950s that beat it: the AD8628 is otherwise an all time best for low noise performance with near DC signals. The AD8628 is not actually a chopper design, but is a chopper replacement, outperforming most choppers 10:1 on the relevant parameters.
We like old friends! We use the LM61 for the scrubber stick and the plug compatible LM35 for monitoring the temperature of the gas in the CO2 and He sensing path. We got there by trying all other avenues first: in the Rev A design, we used 0803 size NPT resistors, but during testing they were simply not rugged enough and meant fitting a fiddley heat sink, so we replaced them by digital sensors with an I2C interface - those failed the safety assessment because of their daisy chain and excess wiring, so Rev B build uses the LM35/61, as it tolerates multiple sensors failing and as their output goes via two MUXes into high precision ADCs which are there anyway, there is no advantage from digital output sensors. We oversample the sensors heavily, and noise is not a problem.
Maxim are not a favoured supplier: lead times and MOQs, as well as the performance of their parts simply are not good enough to replace the AD or Linear Tech parts we are using. Maxim's muxes are about the only thing that makes the relative pain of dealing with them worthwhile. For a low volume build, you are right that one supplier is preferable, but for higher volume build multiple suppliers are not a problem. We use all these suppliers for other projects, and the same manufacturer is involved, so it is not a problem to select the best part and treat the supply logistics as a very secondary item.
Actually, the opposite. We use SG-550CFs. We tested every component for helium susceptibility (pure He at 1400msw), and the results of that caused us to change clock components. We do not use a solenoid: solenoids have two failure states, neither being fail safe. The design uses a variable orifice valve, or rather two, so that if all the electronics dies (say, you ran a spike through the whole lot), then it still keeps you alive in the same manner as a KISS.

The power supplies are well regulated, and except for some cross talk between the injector motor and the Hall position sensor on the orifice, we do not see any correlation between noise on any signal and motor actuation. I will post some pictures later.
Commercial secrets Well, we have to have some ..

Thanks for your comments,

Alex

jradomski

I could not agree more.. I have used the ad8628 myself in several applications, and it shines above the rest..

The only negative is the 1ma per amplifier.. Analog Devices has a few other amps that perform reasonably close (and still substantially better than the competition) but with a substantially reduced power draw..


I also agree on MAXIM, they have some good parts but are an unreliable supplier.. I'll only use them when I find a part that they make thats significantly better than the competition.. I like their analog switches....

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.