ployment... or something. There is presently a decision being made about v arious methods of detecting end of travel for the back stroke of the motor. One of the selection criteria is "simplicity", rated 1 to 3. I don't see that as an objective value, rather 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 since this is a UK project) figure. The three metho ds that use a detector each are rated 1 on cost (because there is some) and two methods are rated 3 on cost because they use timing (guessing) or moto r current on reaching a mechanical stop.
erence?
They were invented for telephone exchanges, and were remarkably reliable (p articularly when compared with the open-contact relays they replaced).
If they stick closed, you've managed dump enough energy into the contacts a s it opened or closed to melt the iron alloy - this normally means having u sed them to switch a significant current running through a significant indu ctance to create a vacuum arc inside the capsule).
"Bad contacts" are higher resistance than specified. If you use them for lo ng enough, the contact resistance does rise, and it's not exactly the same from one contact closure to the next (which is why I went to a mercury-wett ed relay in the one application where I used one).
If dry reed relays don't work for you, you haven't used them carefully enou gh.
Should have been three. Pick the two outputs that agree and go with that.
The 'variable reluctance' may be one type, but my Ford has a permanent magnet at the coil, and senses the passage of a slot on the crankshaft by a little dynamo pulse. It's AC-only, so there's some wizardry involved in startup (like, a few dry cycles before it gets a spark timing correct). Being just a coil and a permanent magnet, you'd think it's reliable (but the coil can go intermittent, and once did). I had the joy of doing a tabletop demo for friends, of 'normal' sensor operation, dip in hot water, then completely inert until the internal thermal stresses relaxed and the wire end hit its contact. Waving a table knife past the sensor gave a nice little blip on a meter.
Not most but rather all wheel speed ABS sensors are Hall effect. That's because the wheel RPM must be measured down to near zero RPM so that it can detect when the brakes are locked and the vehicle is skidding. VR (variable reluctance) sensors do badly at low RPMs while Hall effect sensors can detect rotation at almost a dead stop (creep).
My 2001 Subaru Forester blew one head gasket (there are two heads) at about 140,000 miles last year. Replacement was not cheap or easy: The problem was caused by a badly designed head gasket. The problem was perpetuated by Subaru from 1999 through 2011. The problem was well known, but ignored by Subaru. Same with Mercedes Benz and their crankshaft position sensor. Now, why would Subaru continue to build engines with known defective head gaskets for 12 years, and Mercedes Benz continue to ship engines which they knew would kill the crankshaft position sensor every 75,000 miles?
Easy. The engines are tested for and blessed by various government agencies to comply with smog and safety regulations. Changing the design of the head gasket and possibly the crankshaft position sensor would require retesting the engine and all the vehicles that used the engine. The cost would be prohibitive. The manufacturer would rather deal with a few lawsuits by irate customers, than to issue a service bulletin or recall and replace all the head gaskets and sensors.
It's the same with changing sensor types and designs. The cost of testing, retrofit, warranties, and recalls is far too much to justify the expense of the changes. Making improvements to a component is also de facto admission that there was something wrong with the original component. That often results in a class action lawsuit, allegedly on behalf of the purchasers of the vehicle, who might be entitled to a free safety upgrade at the expense of the manufacturer.
146 complaints, zero recalls on my Subaru. 7 of the 146 complaints were for blown head gaskets.
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No, it wasn't. The proffered solution (to avoid a class action and/or recall) was a progression of better gaskets, but the gasket was not the original problem.
The problem is that the open head design (which is difficult to gasket) is susceptible to localised overheating due to air that has not (can not) be adequately bled from the cross-over pipe between the left and right cylinder banks.
The problem can be prevented in the first place by fitting a tiny bleed tube to the topmost point of this pipe, returning some coolant to the radiator. That makes the coolant simple to bleed, you get no hot air pockets around the cylinder head, and no gasket failure.
This problem and solution was diagnosed by an engineer who specialised in safety analysis, and who had used one of these engines in his home-built light aeroplane, which was heavily instrumented and would log huge volumes of data to the laptop computer. He detected the head distortion at a very early stage from noticing short pulses of high crank-case pressure on overruns, and started investigating. The details are findable on the Internet, I believe.
I doubt that the bleed tube change would have affected certification, but IANAE.
I can't explain why Subaru never fixed it. It cost us $4500 in an engine rebuild and a huge amount of hassle, getting my young family back from a skiing trip, no family car for weeks, and back up to the repair centre later... and then fighting with them a year later because they had installed the clutch wrongly, resulting in another $2000 repair.
Reed Relays can be rate for 10e9 contact cycles at 10mA. Some are rated less but up to 1A if used on resistive loads. Max current and Reactive loads reduces MTBF drastically.
All Relays 2A are not gold plated and if oxidized need 10% rated current for removing contact oxide with the "wetting" current to work almost like mercury switches. I used 10uF tant and a pullup R on DC to burn the oxide on sense contacts to achieve the same reliability with TTL.
A good design needs better specs. - 1,2,3 values are not engineering specs but sounds like the old Memorex method of decision-making with analog weighting values 1~10.
Reviewing State of the Art is the 1st step in making "better specs" before the design begins.
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A created a virtual design group in my last job before I retired and the the company sold out to the biggest contract manufacturer around who then sold out to the biggest Mfg world wide.
I solicited design skillsets from each city/country of sister plants and stressed the most important assets are ability to create detailed specs without forcing choices on components (unless this was a design iteration/revolution) and creative thinking-out-of-the-box solutions for reliable form-fit and function and allow team players to compete on schedule to see who wins.
This was how I succeeded in getting Lucent/AVAYA's first outsourced design and delivered early, on-budget in 7 wks 1st prototype.
If closed for a long time, the contact can cold-weld and stay stuck, especially because reeds get hot when left on for a long time. The opening forces are small.
Tiny amounts of contamination can deposit insulating films on the contacts. There is basically no wiping motion in a reed.
PCB stresses can be translated up into the capsule and mess things up.
They are big, expensive, power-hogging SPST contacts. The rare SPDT or multi-pole versions are even worse. They need to be spaced apart for thermal and for magnetic isolation.
They also twang, make complex millivolt-range ringing-bell waveforms as the closing contacts bounce and then vibrate in the magnetic field. They have bad thermoelectric offsets too.
Some people fall in love with reeds. They should get over it.
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John Larkin Highland Technology, Inc
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Not that hot. The magnetic coils are wound with enamelled copper wire.
True, but they are adequate.
Dry-reed relays are hermiticly sealed into vacuum-tight glass capsules. How is this mythical contamination going to get at the contacts?
The glass capsules area great deal stiffer than printed circuit board.
Compared with transistors they are big. Compared with conventional open frame relays, less so.
The reeds are essentially an iron-based alloy. Solder them onto copper tracks and you do have a thermocouple, but at microvolt levels. It's hard to enough of a thermal gradient to generate anything more.
Mercury-wetted reed relays don't twang. The mercury damps the bounce.
Reed relays have their virtues. They aren't any kind of universal solution, any more than semiconductors switches are.
That's so weird! Reluctance of a coil does change when you insert a diama gnet or ferromagnet into its field, but the reluctance is its AC impedance. The c urrent from a changing B flux is dynamo effect, not reluctance!
I'm offended by this misuse of language! It's just as bad as the motor-st art item called 'relay' which is actually a PTC resistor.
There will be lots of moving parts and plumbing in a ventilator and a Reed Relay is the least of your worries if you must choose one and do not know how to make that extremely reliable. It seems many have had bad experiences and suppliers have made many improvements over the decades.
Many products use IR interrupters now for position end-stop detection. A spec of 1e9 cycles at 10mA should exceed lifetime of many patients.
**Without detailed specs, no design choice is perfect and with, any design that meets all specs is perfect. The devil is in the details.**
One of the end bag valve mask specs for critical features are:
Oxygen connection and reservoir Pop-off valve for safety (location not important) One-way valve that guides air to the patient Exhalation valve (this stays closed while there is any pressure on the bag) PEEP valve that is installed post the exhalation valve and maintains backpressure Sensing port for manometer connection ( for air pressure sensor connection)
Back when I repaired ATMs for a living, reed switches and passive magnets were used to detect the type of currency in a particular cartrdge. A rather crucial job; you don't want the machine thinking it's dispensing 10s when the cart is 20s.
OTOH, a big part of our maintenance routine (and no small percentage of our repair calls) was dust/dirt clogging the IR interrupters used in the bill transports to the customer.
Needless to say, I'm not a fan of IR for long-term reliability situations. Not because the elements themselves conk out ... you just can't keep them clean.
I would have dismissed the purported issues with dirt clogging IR interrupters, but there are too many here citing issues. So I will be pointing that out to the group when we discuss it. Sounds like it will either be Hall effect or shaft encoder.
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Yes even on laser mice they can become erratic if not cleaned with Q tips.
It's a matter of gain margin vs loss accumulation over time to perform error free.
I remember the look on the guys face when he saw a bunch of early '70 vintage photocopied bills in the change machine on campus. He wore a gun holstered after that.
I'd omit the battery during storage. Design to take common batteries. No battery will survive long enough, and nobody will be tenderly charging them every two months for years to decades.
Field hospitals look a lot like a tent.
Sounds like lots of dirt traps that will defeat cleaning and sterilization.
Sealed switches can be pretty large, and the seals hinder motion. And they break a lot.
Indoors but in random buildings, it can be very humid, and in some part of the day there will be condensation. Virginia can be like that.
The bigger problem is surviving decades of indifferent storage.
Many are optical in nature. Not sure I'd expect them to last decades.
Hmm. Are you sensing rotary motion, or linear motion? Be careful that you are sensing motion of the critical mechanical thing, and not something in the drive chain moving that critical thing.
id deployment... or something. There is presently a decision being made ab out various methods of detecting end of travel for the back stroke of the m otor. One of the selection criteria is "simplicity", rated 1 to 3. I don' t see that as an objective value, rather a subjective rating of what they t hink of each approach. Cost is rated from 1 to 3 without any real consider ation of a dollar (or pound since this is a UK project) figure. The three methods that use a detector each are rated 1 on cost (because there is some ) and two methods are rated 3 on cost because they use timing (guessing) or motor current on reaching a mechanical stop.
h 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 suppose e ven if they are used in a hospital like environment.
They have to be plugged in to be used. Even in a field hospital, I think t hey don't have dirt floors. Separate storage from use.
nterference? 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 impact it, like the motor current (brushed DC)? The sensor will pr obably be 9 inches away. Is a magnet built into Hall effect sensors or is that required to be added?
optical interference! That seems rather unlikely in a closed box especial ly. There are openings, but not large. For some reason this one is listed as poorest in simplicity. Seems simple to me and reliable.
o 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 days 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 NA SA ground station, forty years ago. Likely they would not accept that now?
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 circui t board and after development is done leave out the one you don't use. The y seem to think they can make all the decisions without knowing for sure ho w this will need to be used really.
ere
the front with clearance in the metal case. The bag protrudes both sides again, with clearance in the metal case. There is a smaller hole for the p ower jack, again with clearance.
This is not an OR. The outside can be sterilized. Virtually every machine has openings for cooling. This is like that.
ed, but that will need to be specified.
or.
cs.
gh humidity condensation can form, but even then, interrupting a beam is a pretty high contrast ratio. Dust is another matter.
I've never seen condensation on anything that wasn't cool.
If storage is long term the device would be prepped according to the manual . Sealing in a plastic bag will prevent dust. Storage in a hospital for i mmediate use on need would either be plugged in or would be rotated and cha rged once a month or two. Are you telling me they don't keep equipment plu gged in when not in use? It all has batteries and if you pull it into serv ice with a flat battery it has no backup in a power failure. Hospitals hav e been to this rodeo before. They know how to maintain equipment. We just need to identify the issues in the manual.
ly more if other needs arise.
Don't care that the machine lasts for decades. That's not a requirement. The shaft encoder is sealed and won't dust up, is non-mechanical, so won't wear any faster than the motor.
Yes, it's measuring the motor, but that is not important. Really, trust me . The shaft encoder is not the part that will detect a critical failure.
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Rick C.
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Yes it's a kind of dynamo, but with a soft iron rotor the thing it responds to is changing reluctance.
It senses the changing reluctance of the magnetic path through the coil tip it does this by using a DC bias which is provided not by a DC current, but by a permanent magnet.
Reluctance is measured in turns per henry and bringing iron close to an open inductor (like a bar inductor or a slotted ring) will increase the inductance and thus reduce the reluctance.
Latching reed relays offer a way of reducing the heat dissipated in the vicinity of the coil. You only have energise the coil long enough to change the magnetic field - the ad says 1.5msec - and the remnant field keeps the contact open or closed.
You might want to reset the field from time to time in case somebody had reset the field by waving a magnet nearby.
I came up with this as potential solution for the thermocouple voltage problem back in 1979 - nobody was worried enough about thermocouple voltages to bother to try it - so it's a fairly obvious solution.
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