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  1. #21
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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by Atomic_Sheep View Post
    Looks like I opened a can of worms
    I think you can see it clearly that there are at least two camps and several ways to look at the simple problem of limit switches. Never the less, regardless which side you are on, the main thing is that you have limit switches because regardless how careful you are, one day you will need them. If you have a table with fixed home position and ATC you can use soft limits and that will stop your machine in a controlled manner, but I set zero on the work piece, and not consistently on the same spot every time, I also change tools manually so soft limits are not for me. I also rarely jog out of space but sure, everyone can make mistakes, so as he says, entering the wrong value in the MDI or missing a decimal point or a +/- sign can cause hitting of the limit switch, but so what? I lose a few seconds, or at tops 2-3 minutes because I have to reset my machine.



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    Default Re: Limit Switches and Acceleration

    Gocha.



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by Atomic_Sheep View Post
    Gocha.
    Ask if people are using mechanical or electronic proximity switches and which is best...



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by A_Camera View Post
    Ask if people are using mechanical or electronic proximity switches and which is best...
    I'm planning on using hall effect switches. Seem to not have any drawbacks compared to mechanical or electronic... unless... electronic is hall effect.



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    Default Re: Limit Switches and Acceleration

    Yes, Hall Effect is an electronic thing, verging on quantum mechanics too.

    Cheers
    Roger



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by Atomic_Sheep View Post
    I'm planning on using hall effect switches. Seem to not have any drawbacks compared to mechanical or electronic... unless... electronic is hall effect.
    ...and I prefer good old traditional mechanical limit switches. ALL the other type have drawbacks and may cause unwanted/random triggering, or no triggering when needed. Basically, mechanical switches are all 100% reliable and fail safe, don't need power to function and noise is not an issue, no need to think about what is near and which materials you work with, if they get dirty you just brush or blow them clean, and if they break then the machine stops. Of course, it assumes correct wiring... but here is another advantage... they are easy to wire... so only benefits compared to any other type, unless I am missing something.

    I know hall effect is the trend and mechanical switches are so out of date, but who cares?



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    Default Re: Limit Switches and Acceleration

    Unfortunately, mechanical switches fail too, especially in an environment of dust, chips and oil. But yes, they have one important advantage of being fail safe - if you push a normally closed switch, it WILL open. Unless, of course, the lever is broken off...



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by CitizenOfDreams View Post
    Unfortunately, mechanical switches fail too, especially in an environment of dust, chips and oil. But yes, they have one important advantage of being fail safe - if you push a normally closed switch, it WILL open. Unless, of course, the lever is broken off...
    Yes, everything can fail, but as you say, unless the arm is off, if it fails - and is wired right = normally closed it will open or break the circuit, so dust in my opinion is not an issue. The problem with limit switches if they fail and DON'T break the circuit, which practically never can happen with a mechanical switch unless the arm is broken and you should see that pretty easily. Speaking of not likely to happen... one of my switches failed once, and actually the arm broke. but I still believe that mechanical switches are best for this purpose.



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    Default Re: Limit Switches and Acceleration

    Basically, mechanical switches are all 100% reliable and fail safe,
    Unfortunately, mechanical switches fail too, especially in an environment of dust, chips and oil. But yes, they have one important advantage of being fail safe - if you push a normally closed switch, it WILL open. Unless, of course, the lever is broken off...
    I guess I will have to disagree here too. At least as it concerns "reliability". I think the term reliability here is inappropriately applied. Reliability means it works the way it is supposed to every time, meaning when the switch is supposed to be closed it is closed and when it is supposed to be open it is open. Fail safe I would agree with in terms of the "correct" switch itself, but the way most hobby CNC machines are wired the switch signals the software controller to stop the machine. This is far from "fail safe" as the mechanical switch is telling the controller what to do through electronic switches in the BOB, parallel port, smoothstepper, whatever. So I don't understand how this can be termed "fail safe" as it relies on all that stuff plus the software to do the right thing. If you want better fail safe operation, the limit switches should be wired to a latching contactor and if the switches trip the power is taken off of everything. No software, and only one contactor in addition to the switches.

    In regard to switches themselves, any switch, mechanical or electronic can fail, but where is your supporting evidence that electronic switches are less reliable? Your argument is not based on any factual evidence but simply on your opinion. Also, saying that the switch will activate unless the arm is broken is dependent on how the mechanical switch is made. Certainly some mechanical switches may be made such that his is true, but you cannot possibly say that all mechanical switches will break the circuit

    I will give a simple example from my direct experience. Points. I have a homebuilt airplane that uses a VW Beetle engine, and a distributor that originally came with points. A purely mechanical switch. It originally flew with the points and they were replaced after about 100 hours with a hall effect sensor. Note: they were replaced every 25 hours during that first 100 hours as precautionary maintenance. Why did I replace the points with a hall effect sensor? Reliability. The points required periodic maintenance and periodic replacement. I'm old enough to remember having a spare set of point, condenser, and tools in the glove box to replace a set of dead points on the side of the road. Since putting in the hall effect sensor in my airplane I have put on about 300 hours and the cruise RPM of this engine is 3400 RPM, but lets say 3000 for an average. Switching 2 times per rev, the math works out to 300 hr x 60min/hr x 3000 rev/min x 2 switches/rev = 108 million switch events.

    Another example is the Piper PA-30 that I used to own and the mechanical switches in the retractable landing gear system. In the time I owned the airplane I had to replace the switches that show the gear fully down and locked at least twice in the 10 years I owned the airplane. These switches are exposed to the elements and is directly related to why they failed. They were sealed, supposedly, but the contacts would still somehow corrode and prevent proper operation requiring replacement. These switches only had to operate once per flight.

    So I know someone is going to say that a limit switch doesn't have to operate under this kind of severity or operating parameters, and it is true. However, a limit switch may have to deal with coolant, cutting oil, chips on the plunger, etc. So, a limit switch has its own bucket of severity to deal with. In any case, the reliability of any switch used as a limit switch is a red herring in my opinion. Mechanical is fine, but so is electronic.

    Personally, I would and did use inductive proximity switches for a few reasons. First, it is physically sealed by potting in electrical potting compound so it is impervious to coolant, oil, etc. Second, it has amazing accuracy. I measured the homing repeatability of mine and they repeat to within 0.0002" every single time. Third, they are cheap. Comparing the price of a properly sealed mechanical switch for coolant/oil resistance, you can get the prox switches for about $2 each compared to about $10 each for the mechanical switches.



  10. #30
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    Default Re: Limit Switches and Acceleration

    The subject is not aviation, so let's leave that example.
    I have also not spoken about home switches, so let's leave that as well.
    Accuracy and repeatability is irrelevant in limit switches, so that can be left out as well.
    I also believe that limit switches are always connected to BOB or some other type of electronics, even in professional machines, so we can stop discussing that also.
    How reliable a BOB or whatever inputs you are using was also not the subject, so we can leave that out as well.

    Evidence...? This is not a court, and is not something I need to win, just a discussion forum to discuss ideas and opinions, so I don't need to provide evidence other than saying "it is common sense". Electronic switches are more complicated, so of course, they have larger percentage of failure than mechanical switches. The simplicity of mechanical switches should be evidence enough that it is easier for an amateur to install, maintain and fault find errors in installations with mechanical switches than electronic ones.

    Reliability means you can rely on the switch for the function expected, which in this case is to break the circuit if necessary, or if a wire is cut. What is after is a totally different subject, but reliability ends where the limit circuit ends, which in my case is the screw terminals of my BOB. How reliable the BOB is was not something I brought up, and I will not discuss that at all.

    So, what is it you disagree with? Put a screw driver or a wise near a magnetic sensor and tell me how reliable that sensor is. Put the same wise near a mechanical switch and tell me that it is not reliable.
    Yes, I knew somebody will mention the fluids, but come on, is that really such a big issue? They have invented water sealing a long time ago, that is hardly a reason to use magnetic switches, but yes, in that situation it is SIMPLER to make water tight limit switches, but still, a mechanical is just as reliable if properly isolated, and don't live in hope that electronic switches don't need to be sealed, because they do. Why assume that everyone uses poor quality switches...?

    Again, you confuse Emergency stop with Limit switch caused stop. I don't think there are any machines breaking everything just because a limit switch is hit. Normally people want to jog out of that situation, not totally breaking the power. Emergency switch is something different, that's the only situation I would want to completely shut the machine down. It is "fail safe" to it's expected function (limit switch function), not compared to an emergency switch function.

    BTW, I also have a pilot license... or more like had... got bored with flying.



  11. #31
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    Default Re: Limit Switches and Acceleration

    so I don't need to provide evidence other than saying "it is common sense". Electronic switches are more complicated, so of course, they have larger percentage of failure than mechanical switches.
    Unfortunately, I have to point out that 'common sense' does not apply here. There is nothing 'common' about fail-safe switches: you have only to read the Standards which cover them. And relying on 'common sense' in novel situations is NOT 'common sense. Some careful engineering analysis is needed instead.

    The counter argument is that mechanical switches have moving, abrading and arcing parts, while electronic switches have none of those. If you can eliminate all the 'common sense' failure mechanisms, it would be 'common sense' to expect greater reliability. I have had mechanical switches fail.

    For the record: I have ordinary micro-switches, industrial fail-safe switches, magnetic sensors and optical sensors on the shelf - quite a few of each, and I have used them all. They all worked when used according to the instructions. Each of them have an area where they are superior to the others - where the other cannot work.

    Cheers
    Roger



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by RCaffin View Post
    so I don't need to provide evidence other than saying "it is common sense". Electronic switches are more complicated, so of course, they have larger percentage of failure than mechanical switches.
    Unfortunately, I have to point out that 'common sense' does not apply here. There is nothing 'common' about fail-safe switches: you have only to read the Standards which cover them. And relying on 'common sense' in novel situations is NOT 'common sense. Some careful engineering analysis is needed instead.

    The counter argument is that mechanical switches have moving, abrading and arcing parts, while electronic switches have none of those. If you can eliminate all the 'common sense' failure mechanisms, it would be 'common sense' to expect greater reliability. I have had mechanical switches fail.

    For the record: I have ordinary micro-switches, industrial fail-safe switches, magnetic sensors and optical sensors on the shelf - quite a few of each, and I have used them all. They all worked when used according to the instructions. Each of them have an area where they are superior to the others - where the other cannot work.

    Cheers
    Roger
    Yes, everything can fail, that is true. But "common sense" was sort of sarcastic joke. I did not demand any evidence about that mechanical switches are less reliable than electronic switches. There is simply no factual evidence of such thing, just as there is no factual evidence that electronic switches are more reliable than mechanical ones. It depends on many things, not just the quality of switches, but also how they are installed and by whom. Of course electronic switches can be just as reliable, but there is no question about that it is is simpler to install mechanical switches and if there is an error, it is also much simpler for an amateur to find the faults. Just look around on this forum and ask people who have random reset/limit switch/home switch/E-stop problems... most, if not all of them use electronic switches and have VERY limited electrical knowledge, so yes, in a way, this should be "evidence" enough, but there is no way I can present any factual evidence to support one or the other side. Personally I chose mechanical switches, but that's just my choice, not necessarily shared by the majority, because mechanical switches are not sexy enough, and are definitely not the trendiest way. Like I said, this is not a court and there is no case to win. Yes, in some situation one is better than the other, but that's it.

    My opinion is mine. Use whatever suits your needs or fit your opinion.



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    Default Re: Limit Switches and Acceleration

    The subject is not aviation, so let's leave that example.
    The example was provided as an example of electronic switch "reliability". You stated that mechanical switches were more reliable, which is an opinion. Whether a switch is used in aviation, a car, a boat, a printer or whatever is not immaterial whether you want it to be or not. The examples showed in one case that the electronic switch had superior reliability and the other example showed that a sealed switch exposed to only ambient environmental conditions can and do fail.

    I have also not spoken about home switches, so let's leave that as well.
    And yet on many hobby machines the switches in question serve double duty as both home and limit switches. So lets not leave that one.

    Accuracy and repeatability is irrelevant in limit switches, so that can be left out as well.
    Not if used as combination home/limit switches. Keep this one as well.

    I also believe that limit switches are always connected to BOB or some other type of electronics, even in professional machines, so we can stop discussing that also.
    Wrong. All of the industrial machines I ever had the privilege to run had the limits wired to a contactor that would completely power down the system when a limit is tripped. The limit trip required taking a wrench to the drive screw to actually move off of the limit switch. Jogging off of the limits was not an option because the limit could not be overridden. They do this for a reason. If the controller is operating correctly, and the machine parameters haven't been mucked up then an industrial machine should never be able to even get to the actual switch due to the software limits, let alone trip it. If the actual switch trips on an industrial machine then something is bad wrong, like the control software is haywire, or an operator has set the soft limits outside of the physical switch. In any case a limit trip on an industrial machine is a big deal.

    but reliability ends where the limit circuit ends
    Really? Well the switch reliability of that circuit ends, but proper limit activation doesn't if wired through anything else. If this is really your opinion, then my opinion is you are flat out wrong.

    Again, you are confusing reliability with fail safe operation. A reliable system is one the both trips and un-trips. A fail safe system is one that is only required to trip.

    Put a screw driver or a wise near a magnetic sensor and tell me how reliable that sensor is.
    I can't because my switches are buried inside my machine and I can't get a screwdriver to them. In any case the switch is still reliable in that it activates when a suitable target is within in sensing range, so this does not at all speak to switch reliability.

    Again, you confuse Emergency stop with Limit switch caused stop. I don't think there are any machines breaking everything just because a limit switch is hit.
    No I am not confusing limit and e-stop. You were the one who muddied up those waters earlier in the thread. Where did I even mention e-stop? Saying there aren't many machines breaking due to a limit trip is a huge assumption. Many small stepper driven machines can likely be driven right to a hard stop at full speed without damage, but how about a machine with servo motor capable of short bursts of 3 hp? This is a size servo motor used on many hobby machines. You cannot assume that just because your hobby machine will not be damaged that someone else's hobby machine won't be as well. Different machines have different requirements yet you make blanket statements such as this that are blatantly not true.

    Why assume that everyone uses poor quality switches...?
    Did I say anything about this? No I did not, so you are the one making assumptions. For example, you assume everyone that installs mechanical limit switches will install ones that have a design that is fail safe. Just because a switch is mechanical does not mean it is fail safe, as I have stated. It depends on the switch design and the installation. Something you want to totally ignore.

    Since we are talking about home-brew hobby machines mostly, we have the choice on how to implement safety systems. We can ignore them completely, ignore parts, or implement them as if they were an industrial machine. We are not bound to any standard. Each machine is different, and all I am trying to do is to provide information that others can use to make their own decision. You seem to want people to make the same decision you did apparently because no other opinion matters. In the end, you are entitled to your opinion, but I am also entitled to mine.



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    Default Re: Limit Switches and Acceleration

    ask people who have random reset/limit switch/home switch/E-stop problems...
    I did, and it was a MECHANICAL switch which was giving the problem.

    But, to each his own.
    Cheers
    Roger



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by RCaffin View Post
    ask people who have random reset/limit switch/home switch/E-stop problems...
    I did, and it was a MECHANICAL switch which was giving the problem.

    But, to each his own.
    Cheers
    Roger
    OK, fine, but if a person can't connect and wire a limit switch circuit with mechanical switches he should look for other hobbies or should hire someone to wire it up. Never the less, of course even that can be connected or wired wrongly. but a person who can't manage mechanical switches can probably manage even less the electronic ones. The most common mistakes people make with mechanical switches is that they wire in NO mode and leave inputs floating, this mistake can be made even with electronic switches. Now, in which way is that a problem connected to the fact that the used switch is mechanical? To me, that's a user error. As I said already in post #26...

    Of course, it assumes correct wiring...
    Everything else is irrelevant discussion because it has NOTHING to do with mechanical vs. electronic limit switches. Everything needs correct wiring, so you can't take improperly wired machines as examples of why one is better than the other. Why would a person who is not capable of wiring a mechanical switches based limit switch circuit be capable of wiring an electronic switches based limit switch circuit?

    Other than that, yes, to each his own.



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    Default Re: Limit Switches and Acceleration

    if a person can't connect and wire a limit switch circuit with mechanical switches he should look for other hobbies or should hire someone to wire it up.
    Actually, I agree - but plenty try. Where do we start charging? :-)

    Cheers
    Roger



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    Default Re: Limit Switches and Acceleration

    Quote Originally Posted by RCaffin View Post
    if a person can't connect and wire a limit switch circuit with mechanical switches he should look for other hobbies or should hire someone to wire it up.
    Actually, I agree - but plenty try. Where do we start charging? :-)

    Cheers
    Roger

    We are hired slaves, free labor.
    The price they must pay is that they have to decide which one of us to listen too and who's advise they follow.



  18. #38
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    Default Re: Limit Switches and Acceleration

    Ooh - hard choice!
    Cheers
    Roger



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