Group Design

Nowadays, mostly they will have AC power from a generator. But not always. But the point was that batteries and long-term storage don't mix.

Even if the floor is not dirt, it's probably only a carpet away from dirt. Anyway, field hospitals are often dusty.

So, can I clean the using using a 100 psi garden hose jet nozzle, to clean all those nooks and crannies out?

True, but areas with large diurnal temperature variation do manage to have condensation - things are wet in the AM. My example is Virginia near Washington DC and Baltimore, MD (where I lived for a while).

Requirements. Hmm. Good point. They seem to be evolving as we speak. Which isn't necessarily bad.

I guess the problem we are having is that the full (perhaps draft) requirements list has not yet been published, so we are reduced to guessing what is and is not to be accomplished.

Joe Gwinn

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Joe Gwinn
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rapid deployment... or something. There is presently a decision being made about various methods of detecting end of travel for the back stroke of th e motor. One of the selection criteria is "simplicity", rated 1 to 3. I d on't see that as an objective value, rather a subjective rating of what the y think of each approach. Cost is rated from 1 to 3 without any real consi deration of a dollar (or pound since this is a UK project) figure. The thr ee methods that use a detector each are rated 1 on cost (because there is s ome) and two methods are rated 3 on cost because they use timing (guessing) or motor current on reaching a mechanical stop.

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with a battery inside it will need to be plugged in every two months or so to top off the charge. Dusty storage environs are another concern I suppos e even if they are used in a hospital like environment.

k they don't have dirt floors. Separate storage from use.

Sorry, I don't see this device being designed for use in outer space either . Being used in dusty rooms is not one of the requirements. I don't set t he requirements, you would need to talk to someone else about that.

c interference? They work off of a magnetic field, but not a rapidly varyi ng field (the motor output is single digit RPM in use) so will local magnet ic effects impact it, like the motor current (brushed DC)? The sensor will probably be 9 inches away. Is a magnet built into Hall effect sensors or is that required to be added?

age optical interference! That seems rather unlikely in a closed box espec ially. There are openings, but not large. For some reason this one is lis ted as poorest in simplicity. Seems simple to me and reliable.

icro switch. I know these can be mounted on cables, but is there a reason why that would be a bad idea for reliability? I recall in my very early da ys in engineering 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 n ow?

is significantly 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 cir cuit board and 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 will need to be used really.

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out the front with clearance in the metal case. The bag protrudes both sid es again, with clearance in the metal case. There is a smaller hole for th e power jack, again with clearance.

ine has openings for cooling. This is like that.

First you ask about sterilization, now you want to sterilize with a garden hose??? Ok, we have reached loony town center, everybody off.

ealed, but that will need to be specified.

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high humidity condensation can form, but even then, interrupting a beam is a pretty high contrast ratio. Dust is another matter.

You mean exactly where I am typing this post in a cozy, dry chair that has not seen any moisture other than my own perspiration? What are you talking about???

ual. Sealing in a plastic bag will prevent dust. Storage in a hospital fo r immediate use on need would either be plugged in or would be rotated and charged once a month or two. Are you telling me they don't keep equipment plugged in when not in use? It all has batteries and if you pull it into s ervice with a flat battery it has no backup in a power failure. Hospitals have been to this rodeo before. They know how to maintain equipment. We j ust need to identify the issues in the manual.

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ially more if other needs arise.

. The shaft encoder is sealed and won't dust up, is non-mechanical, so won 't wear any faster than the motor.

me. The shaft encoder is not the part that will detect a critical failure .

Are you asking for the full set of requirements? Why? Do you want to join the team? I'd be happy to hook you up. You can start by signing up at sl ack.com, helpfulengineering The project is project-open-vent-Bristol

Welcome aboard!

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  Rick C. 

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Ricketty C

Well, you asked for help, but it was difficult to nail down what was needed. A link would suffice.

Joe Gwinn

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Joe Gwinn

oin the team? I'd be happy to hook you up. You can start by signing up at slack.com, helpfulengineering The project is project-open-vent-Bristol

I asked for advice and insight and I thank everyone for their inputs. This was a big help. We had a video chat today and it looks like the reed sens or is the most interesting approach. For some reason there can be bias abo ut solutions. Rather than investigating details there is assumption that t he only solutions are what people have personally seen before. The team le ader was talking about a Hall effect device for under a dollar which I beli eve would have to be soldered to a circuit board rather than paying a few b ucks more for an encapsulated device with a flange mount and wires.

In reality the hard part of the whole thing is terminating the wire end wit h a connector. The durn crimp tools cost a fortune relative to the connect ors. At least one brand can be found with a connector on the wires, but no ne of these parts are stocked in very large quantities and we would have to qualify multiple part numbers so that a large production run would not req uire factory order delays. Seems there are nearly universally crappy data sheets on these things. Many don't provide even a basic explanation of the part number and often they only provide info on the sensitivity in terms o f AT (amp-turns) rather than giving a number for proximity of a given magne t. Little fuse gives distance, but as an "average" number rather than a gu aranteed pull in distance and a drop out distance when used with their reco mmended magnet that looks just like the sensor. We can use their AT number s as reference points. The really odd thing is they give the pull in AT as a range. I guess it's important to know the soonest it may activate as we ll as the latest. Still, I have no idea how to translate that into a dista nce for the magnet. Experimentation will be required.

Typical numbers are 10-25 pull in and 4 AT drop out. That vendor quotes th

dropping out requires 2 AT max.

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  Rick C. 

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Ricketty C

Ahh. That's never happened in Engineering before....

The wires from a reed relay are typically solid Kovar (so it can be sealed into glass), and this kind of wire is difficult to crimp, because it's solid wire, and because it's Kovar. The standard ways to connect to those leads is soft soldering or spot welding (in high production). If you really must put stranded copper leads onto a reed relay, solder a brass tube to the reed relay leads ad solder or crimp the brass tube to the stranded copper lead, and protect the whole thing with heat shrink tubing.

The reason that the datasheets are vague is that the magnetic sensitivity levels are not well constrained, so one designs the magnetic curcuit to ensure that the flux level change will be sufficient to ensure that all compliant reeds work, and always will work.

The reed switch manufacturers will have app notes giving examples of proven magnetic circuit designs. The original manufacturer was Hamlin, if memory serves. They had very good app notes. Littlefuse owns them now, and Mouser carries them.

Joe Gwinn

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Joe Gwinn

o join the team? I'd be happy to hook you up. You can start by signing up at slack.com, helpfulengineering The project is project-open-vent-Bristo l

his was a big help. We had a video chat today and it looks like the reed s ensor is the most interesting approach. For some reason there can be bias about solutions. Rather than investigating details there is assumption tha t the only solutions are what people have personally seen before. The team leader was talking about a Hall effect device for under a dollar which I b elieve would have to be soldered to a circuit board rather than paying a fe w bucks more for an encapsulated device with a flange mount and wires.

Sorry, you didn't include the smiley so I can't tell if this is sarcasm whi ch I'm pretty sure it is.

with a connector. The durn crimp tools cost a fortune relative to the conn ectors. At least one brand can be found with a connector on the wires, but none of these parts are stocked in very large quantities and we would have to qualify multiple part numbers so that a large production run would not require factory order delays.

Again, I'm not sure what you are talking about. Every part I've looked at provides connections of stripped and tinned, copper wire or without the tin ning.

No one said anything about a reed relay BTW. If I said "relay" it was a to tal brain cramp. This is a magnetic reed position sensor.

Many don't provide even a basic explanation of the part number and often th ey only provide info on the sensitivity in terms of AT (amp-turns) rather t han giving a number for proximity of a given magnet. Little fuse gives dis tance, but as an "average" number rather than a guaranteed pull in distance and a drop out distance when used with their recommended magnet that looks just like the sensor. We can use their AT numbers as reference points. T he really odd thing is they give the pull in AT as a range. I guess it's

ill, I have no idea how to translate that into a distance for the magnet. Experimentation will be required.

the test equipment as having a tolerance of ?2 AT. So to be certain of dropping out requires 2 AT max.

I think you fail to understand the issue. The sensors have very clear spec s. The problem is the specs are in terms that are of little value in desig ning the sensor into the application unless you do some rather awkward math . One company provides the magnet which means they can spec the gap, but t hey only offer an "average" gap distance and no drop out. Another specifie s a drop out distance, but not a pull in at all.

Littelfuse is the company that specs pull in distance as an "average" and n o drop out. They also don't seem to be very much stocked. As to the tech literature, it may exist somewhere, but they've made it very hard to find. When I try to view "Application Notes" I get "Error establishing a databas e connection". Their selection guide is not a PDF document, but some bizar re online thing. Absurd.

I'm guessing the inventory issue is not unique to Littelfuse and is due to the fact that any large users are pretty good at dealing directly with the factory.

Whatever. Companies like these make me want to design software.

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  Rick C. 

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Reply to
Ricketty C

It isn't.

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identifies the alloy as a 52% nickel alloy with iron, chosen for its magnetic properties. Kovar is nickel-cobalt alloy chosen for its thermal expansion properties.

The dry reed relay does depend on two glass-to-metal seals, but my understanding is that the glass is chosen to match the alloy. so it probably isn't Pyrex borosilicate glass.

Which is to say a dry reed switch which responds to a change in magnetic field. Low field - open. High field - closed. There's hysteresis - the reed will remain closed at a lower field than the one required to get it to close.

It doesn't. The dry reed relay was a Bell Labs development - invented and patented there in 1936. I've no idea who ended up manufacturing the first reed capsules - there were seven licensees in the US, and Hamlin was one of them, and more in Europe.

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Bill Sloman, Sydney
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Bill Sloman

Irony.

Then, these are connectorized reed switch capsules. And so I don't understand the problem then.

I meant a reed switch; these are widely used for sensing magnetic fields.

I do understand this - you were complaining that the spec ranges were so wide, too wide to design to. And I gave you the standard solution to exactly this problem.

The Hamlin app notes were very good, but I don't see them at Littlefuse. Sad.

Google yielded this in the first page:

.

Huge variety, most users use a few standard designs.

Well, I'd be interested to see the software that can overcome a malfunctioning position sender and deduce the correct location.

Joe Gwinn

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Joe Gwinn

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