Group Design

I guess they just like to feel they are doing "something"

They are doing something. Redesign does not mean starting over. Every time a second rev of a design is produced it is a redesign.

What will be interesting is if they can get a working design that can work in all modes of operation. Most projects don't even attempt that. That will be a design worth reworking to meet approvals.

There seem to be lot more replies to this thread as soon as an opportunity to offer something negative was presented. That says a lot about this group I would say... or maybe just people in general.

I was starting to think I was posting in s.e.d.

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  Rick C. 

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There is presently a decision being made about various methods of detecting end of travel for the back stroke of the motor. One of the selection crit eria is "simplicity", 1 to 3. I don't see that as an objective value, rath er a subjective rating of what they think of each approach. Cost is rated from 1 to 3 without any real consideration of a dollar (or pound) figure. The three methods that use a detector each are rated 1 on cost (because the re is some) and two methods are rated 3 on cost because they use timing (gu essing) or motor current on reaching a mechanical stop.

So do Hall effect sensors have any real issues such as magnetic interferenc e? They work off of a magnetic field, but not a rapidly varying field (the motor output is single digit RPM in use) so will local magnetic effects im pact it, like the motor current? The sensor will probably be 9 inches away . Is a magnet built into Hall effect sensors or is that required to be add ed?

Another choice is an optical sensor. They list as a disadvantage optical i nterference! That seems rather unlikely in a closed box especially. There are openings, but not large. For some reason this one is listed as poores t in simplicity. Seems simple to me.

They list circuit boards as being needed for those two and a micro switch. I know these can be mounted on cables, but is there a reason why that woul d be a bad idea for reliability? I recall in my very early days in enginee ring seeing front panels with switches and LEDs being wired up by hand via ribbon cable and heat shrink tubing. That was equipment for a NASA ground station, forty years ago. Likely they would not accept that now?

There is also the option of the motor shaft encoder, but that is significan tly more expensive and I recall there was a strong dislike of the $15 price . I suggested they include a couple of the options in the circuit board an d after development is done leave out the one you don't use. They seem to think they can make all the decisions without knowing for sure how this wil l need to be designed.

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  Rick C. 

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But not something that will be useful to anyone who actually needs a ventilator.

The entire design of the Puritan Bennett 560 current-model approved ventilator from Medtronic was released under a permissive license way back in March. Why does your group think they can (or should?) do better.

You get criticism most of the because you spend most of your time prevaricating. Change your default mode of response to match that of any vaguely reasonable person and you'll find discussion much more productive.

CH

Sorry, that's just your BS. Nothing to reply to.

Yes, I've looked at that design and it is very complex and uses a EOL CPU. They have a sensing circuit to detect power going to the piezo sounder. Not sure that sort of robustness is required.

Thank you for your input. I look forward to your next reply. Oh, and please trim your posts.

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  Rick C. 

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[...]

I don't think you got mu point here. I just commented on your (friends) statement '"prove" your design will not hurt the patient in any way'. That is simply not true. There is always a risk/benefit and as long as the benefits outweigh the risks, some risk may be deemed acceptable. This presentation highlights how important risk/benefit is in the new MDR (EU medical deviceregulation).

And this goes for all medical devices, from complex (like ventilators or MRI scanners) to simple. The scalpel being an example of the latter, not a comparison to the ventilator.

I'm not familiar with FCC, but if it's anything like EU CE marking to get (non medical) electronic devices approved for sale, you might be in for a shock when it comes to electronic medical devices, especially those that include software.

Like has been said multiple times. You should start (preparation for) the approval process before/during the design, not after.

And any idea who this manufacturing partner will be? It will need to be a partner already familiar with medical devices, preferrably with ventilators. And you have to come with something they could not have done themselves.

[...]

One hand, these problems are only minor and can be solved by any capable electronics engineer. On the other this sort of thing would get me worrying about the skills of the team. But if their skills are in medical and regulations, an electronics engineer would be a good supplement to the team.

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Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail) 

The whole earth is in jail and we're plotting this incredible jailbreak. 

This sensing circuit is most probably a risk control that has been implemented as the result of a design risk analysis. The sounder is probably used for a medical alarm, which puts a lot of requirements on performance of the sounder circuit.

You can only be sure that sort of robustness is desired if you perform a design risk analysis.

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Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail) 

Riffle West Virginia is so small that the Boy Scout had to double as the 

parts that need copper area for heat sinking. They are using a linear regulator to drop 12+ volts

to 5V and 3.3V. The back lights for the LCDs draw 300 mA! So the 5V regulator gets quite warm,

You should be concerned. The whole thing about safety critical kit is that it should be conservatively designed, with all parts running well within their ratings. If the hw bod is happy with a regulator running at 70c, he must be clueless. Sorry, but it needs to be said.

If you care about your professional career and any litigation issues, you should either have enough influence on the project direction to stop design errors like that, or just walk away from it...

Chris

se but there are several

It was a prototype board before I came on board. I am involved in the desi gn review of the next rev and I will be pushing for a design review of the layout as well. Not sure how well that will be received.

The project leader has indicated numerous times that he wants to get the bo ard rolling in spite of us not having good requirements. He brought someon e on board to help with that, but this is very different from what I am acc ustomed to. My experience with requirements analysis was on DOD projects a nd it had lots of formalities with each requirement being linked to a test.

We just had a meeting today with the guy who has written a systems level re quirements document. He has indicated that he will be breaking requirement s down further for the software, but not for the hardware. I asked why and time was cited, not enough time to do that. What???

I don't have any specific duties at the moment. I might participate in a d esign review of the schematic and layout, but I won't be doing any further design work.

I just think it is crazy to try to push this design to completion without k nowing what it is supposed to do in detail. There seems to be an obsession with turning the machine off even though it has battery backup which would be depleted in an hour if left connected or in a couple of months if not. There is an alarm for when the machine is unplugged and running off batter y. So what should turning the machine off do exactly? I can't get an answ er, but we need to complete the board design.

Yeah, we are nearing the end of this road.

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  Rick C. 

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Hmm, interesting. Most linear regulators I'm familiar with are rated to

125 C junction temperature, and some higher. What safety or regulatory issues have you run into that were caused by running some part 55C below its rated temperature?

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 

ll

near

o the 5V

That was the case temp via an IR gun. So the die was hotter. I would not approve a design with a part running so hot. I believe the failure rate go es up quite quickly with temp. The real point is this is because the layou t was not correct using thermal breaks around the thermal pad!!! Easy enou

Did you not read the original post about this?

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  Rick C. 

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I've designed a fair bit of stuff to work in ambients higher than that, from remote satcom stuff to downhole instruments running at 175C.

The amateur vent stuff is all March 2020 at this point, so I'm interested in the broader context.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 

all

linear

So the 5V

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not approve a design with a part running so hot. I believe the failure rat e goes up quite quickly with temp. The real point is this is because the l ayout was not correct using thermal breaks around the thermal pad!!! Easy

So??? Higher temps means shorter life. Is that a false statement?

There is also the issue of heating up things adjacent to the hot part and a

t hot.

In other words, there is no reason to have such an abortion for a design th at it gets so hot. Why is this an issue for you? You are not making any s ense at all.

What broader context???

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  Rick C. 

  ---- Get 1,000 miles of free Supercharging 

Okay, so you've got zilch. No worries.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 

It's ok, a group of Afghan schoolgirls is on top of it and seems to be doing a more serious job:

That's not the point and reliability is affected by temperature. It may not matter for a short lived down hole item where failure has no safety implications, but not good enough otherwise. The constant temperature cycling will affect wire bonds and reliability. Cheap tacky design, using a linear regulator to go from 12v to 5, then 3.3, considerable current, where a point of use switcher would be a far more efficient and probably more reliable solution as well Wasted power needs a bigger backup battery etc...

Chris

Not yours, I gather, but it is mine. I'm not being argumentative, I just want to know if there's good reliability data out there that I should know about. Opinions can be useful too, if they're at least based on detailed anecdotes.

So we're told by MIL-HDBK-217, based on a famously flawed methodology (removing parts always improves reliability), invalid statistical assumptions (Arrhenius scaling of failure rates over large temperature ranges), and data from the 1960s iirc. Is there actual data to support that with modern components?

I know it isn't what the cool kids do, but that doesn't make it unreliable or cause it to fail safety inspections AFAICT. Medical may be different, but it would pass UL with no issues. What regulations nwould you expect it to fall foul of?

Cheers

Phil Hobbs

>

--
Dr Philip C D Hobbs 
Principal Consultant 

. w

Efficiency may be personally important to an engineer who wants to overdesi gn everything, but this device has a 2 amp motor running off the 15 volts a nd it is all line powered, so no need to squeeze every last joule out of th e design. We are measuring some low level signals that could easily be dis rupted by the switching currents. So adding that type of a noise source wo uld only be done if actually required.

Oddly enough most of the power used in the electronics other than the motor are in the LED back lights for the low power LCD displays. I have thought of using a switcher for them, but even at the 250 mA power level it's just not worth the hassle of dealing with the noise.

Hmmm... maybe that is a bigger impact than anyone here is thinking. The d isplay is on all the time. The motor only runs at high power about half th e time. So it's really more like 1 amp. Now the ratio is more like 4:1. We don't have good data on the motor current, so no one has paid attention to the battery time.

There are so many holes in this net. lol

BTW, does anyone understand what Phil Hobbs is going on about??? Why does he talk in code?

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  Rick C. 

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A 7805 at 70C is not too hot, nothing like that. I'd be worried of any nearby electrolytic capacitors, I have seen these fail if put too close to some hot part. Once on a product of mine - though the board was "too hot" only for one cap of a particular series, other units have worked for well over 5 years 24/7. And once I repaired our TV set (some 15+ years ago, a CRT one), it had an electrolytic cap next to a "red" hot

7805. I have *never* seen a 78xx regulator fail because of overheating and I have designed in quite a few. One can have them run hot within reason, just keep in mind that they have temperature protection and if you overheat one it will protect itself by more or less cutting off the power at its output. Another thing to keep in mind is to not forget to put a diode in parallel if the output voltage is > say 5-6V; they do die if the input goes 0V or so with the output capacitor charged.

Dimiter

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