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'[PIC:] Baby Incubator Controller'
2003\08\11@061420 by Alan B. Pearce

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>At this poinmt I have one more question:
>
>If Microchip sells devices like a PIC controller, and does not
>offer any warranty for serious projects like an Incubator
>Controller for example .... where can I find a secure device to use ?

I think you will find that the chip supplier you select will require you to
use chips which are screened to MIL-STD procedures, or something similar, to
ensure the highest likelihood of chip reliability. They may then also
auditable requirements for static safe handling within your facility at all
stages of manufacture and test, and all this before signing off on allowing
their product to be used on any sort of life support system. Now you can
perhaps understand why medical equipment is so expensive.

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2003\08\11@063326 by Vasile Surducan

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face
For medical equipment used in life insurance, there is a tehnique called
redundancy. Every system is doubled on essential functions. Then the
producer does not need to offer a secure device. The secure is created by
the projectant itself. He is also in charge if the pacient dies...



On Mon, 11 Aug 2003, Alan B. Pearce wrote:

{Quote hidden}

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2003\08\11@065714 by Wouter van Ooijen

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> For medical equipment used in life insurance, there is a
> tehnique called
> redundancy.

But redundancy has its own problems. Take for instance a heating system:
it can fail in two modes: heading while it should not, or not heating
while it should. Using two heating systems will lower the rate of the
second failure mode, but it will raise the first! Clever tricks can
include serial/parallelling of both sensors and heaters.

Redundancy with voting introduces a single-point-of-failure in the
decision mechanism. But this can be usefull if the failure rate of that
mechanism is much lower than the rates of the voters.

Wouter van Ooijen

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consultancy, development, PICmicro products

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2003\08\11@071623 by Vasile Surducan

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On Mon, 11 Aug 2003, Wouter van Ooijen wrote:

> > For medical equipment used in life insurance, there is a
> > tehnique called
> > redundancy.
>
> But redundancy has its own problems. Take for instance a heating system:
> it can fail in two modes: heading while it should not, or not heating
> while it should. Using two heating systems will lower the rate of the
> second failure mode, but it will raise the first! Clever tricks can
> include serial/parallelling of both sensors and heaters.
>
 This is redundancy too :)
 But thinking at "low level" design and not at input-output parameters.
 Doubling the sensors, doubling the heater resistance and doubling the
control module.  Something wrong ? Then sensors are not measuring the same
temperature with an accepted error, or one (or both) controling modules
are not working well. Then an acustical warning signal for the nurse and
keep the two heating in series, these are dimensioned in such a manner
that not allowed increasing the temperature more than 25...
It's redundancy.



> Redundancy with voting introduces a single-point-of-failure in the
> decision mechanism. But this can be usefull if the failure rate of that
> mechanism is much lower than the rates of the voters.
>

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2003\08\11@073700 by Alan B. Pearce

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> For medical equipment used in life insurance, there is a
> tehnique called redundancy.

I think you would find that they would insist on you using high reliability
devices irrespective of any redundancy in the system. Having standard
reliability redundant devices does not automatically make the device more
reliable. You need to get the MTBF figure for all devices higher to get the
overall reliability up, and adding redundancy only adds more failure points.

As an aside on this, someone mentioned using an FPGA or similar arrangement
instead of using a micro. This is probably a good way to go if it is
possible to program a state machine or similar control system into such a
chip, for the application you have. In working out the reliability of items
for space electronics, the least reliable points are solder joints, so
anything you can do to minimise these will probably increase your
reliability.

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2003\08\11@075154 by Intosh, Ph.D.

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part 1 1342 bytes content-type:text/plain; charset="us-ascii"; format=flowedsource= http://www.piclist.com/piclist/2003/08/11/001051a.txt

One design element that no one mentioned is the "wet" heater.

In my consideration on this problem, I found it useful to think of a 4
dimensional space, with two position and two momentum vectors.  One is for
the humidity and one is for the dry heater.

Think of this like a car approaching another car on a highway.  The closer
the cars, the smaller the delta velocity to have minimum close rate without
collision.  A plot of gas feed against two dimensions, position and
relative velocity, gives the ideal feed rate.  A second plot for applied
braking, a third one for air bag deployment :-) can be envisioned.

In the incubator, the wet and dry heaters interact, so there is one
4-dimensional space, and not 2 2-dimensional spaces.  In some designs, a
lot of heat is stored in the wet heater before it is sensed.

And do have a safety cut off that works even if the PIC is removed and the
heaters are wired "on."

Just don't let Olin write the marketing piece, you'll be sued even if it
works flawlessly.

---
Aubrey  McIntosh
http://www.piclist.com/member/AM-vima-Y84
PIC/PICList FAQ: http://www.piclist.com


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part 2 229 bytes content-type:application/pgp-signature (decode)

2003\08\11@081057 by Mike Harrison

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On Mon, 11 Aug 2003 12:35:43 +0100, you wrote:

>> For medical equipment used in life insurance, there is a
>> tehnique called redundancy.
>
>I think you would find that they would insist on you using high reliability
>devices irrespective of any redundancy in the system. Having standard
>reliability redundant devices does not automatically make the device more
>reliable. You need to get the MTBF figure for all devices higher to get the
>overall reliability up, and adding redundancy only adds more failure points.
>
>As an aside on this, someone mentioned using an FPGA or similar arrangement
>instead of using a micro. This is probably a good way to go if it is
>possible to program a state machine or similar control system into such a
>chip, for the application you have. In working out the reliability of items
>for space electronics, the least reliable points are solder joints, so
>anything you can do to minimise these will probably increase your
>reliability.

Alternatively, programming a state machine into a micro is also a good way to ensure good
predictability of behavious, as all possible states can be considered and dealt with in an
easy-to-verify manner.  Things like range-checking of inpus will also be easier to do in a micro.

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2003\08\11@145518 by Robert Rolf

picon face
"Alan B. Pearce" wrote:
>
> >At this poinmt I have one more question:
> >
> >If Microchip sells devices like a PIC controller, and does not
> >offer any warranty for serious projects like an Incubator
> >Controller for example .... where can I find a secure device to use ?
>
> I think you will find that the chip supplier you select will require you to
> use chips which are screened to MIL-STD procedures, or something similar, to
> ensure the highest likelihood of chip reliability. They may then also
> auditable requirements for static safe handling within your facility at all
> stages of manufacture and test, and all this before signing off on allowing
> their product to be used on any sort of life support system. Now you can
> perhaps understand why medical equipment is so expensive.
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You mean because there are so many hungry (greedy?) lawyers/companies
out there?

We can purchase a magnetic base for use in microelectrode recording
apparatus for $200Can from a research supplies catalog, or we can
go down the street to a local machine shop supplier and get the SAME
unit for $30. Similar differentials apply to such things as stainless
steel bolts, washers, nuts, tweezers, stopwatches, etc.
What can possibly justify the 7x higher cost from the research
catalog? Surely the cost of warehousing/handling is not THAT much higher.

R

What do you call 10,000 lawyers at the bottom of a lake???




A good start....

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2003\08\12@145510 by Peter L. Peres

picon face
> Things like range-checking of inpus will also be = easier to do in a
> micro.

Why not go all the way and use a PC running Microsoft Windows to control
the unit in real time through the USB port ? At least the GUI would be
nice. Since someone is bound to sue you over this anyway, what difference
does it make ? (just kidding)

Peter

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