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  1. #81
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    Quote Originally Posted by quadtech View Post
    For anyone in India following this thread, Rhydolabz has
    the MH183 unipolar hall effect sensor for a decent price.

    It appears to be similar to the Honeywell SS441 and has the same pinout too.

    http://www.rhydolabz.com/index.php?m...roducts_id=391

    Datasheet (available on google quick view too)
    http://www.gwtusa.com/ic/ic/MH-183.pdf

    Nice find! Thanks for mentioning that sensor.

    I just read the datasheet and it looks similar to the Honeywell sensor, however there is no mention of temperature compensation. Also the graph on page 4 of the datasheet shows that it switches on at lower magnetic fields when hotter.

    I don't know how much effect this would have on actual sensing distance, or even how much temperature change would be expected in the sensor. Typically in a workshop the temperature might be from 15'C to 35'C which might be a couple of percent difference in the magnetic field strength (according to the graph). That might translate to a couple thou of position error, it's hard to say for sure without actual testing.

    Anyway, the more sensors we find, the better. It might be worth making a table of the possible sensors and their differences, prices etc. Hmm, it might be better to leave that until later to get more sensor types included in the table.



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    I came across another sensor which people seem to have used
    successfully - US5881LUA-ND and the SMT version costs $0.41 from Future!!

    The datasheet says it is a low magnetic sensitivity unipolar Hall-effect sensor. Perhaps it could be more suitable for use with a stronger
    Nd magnet?

    http://www.cnczone.com/forums/showthread.php?t=71155
    http://www.cnczone.com/forums/showth...t=31601&page=5
    http://www.turningaround.org/MechMate/Mechmate.htm

    Datasheet -
    http://www.melexis.com/Asset/US5881_...Link_4824.aspx

    Pinout-
    Pin 1 is Vcc
    Pin 2 is Gnd
    Pin 3 is the output



  3. #83
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    Nice find Quadtech.

    That US5881 sensor has similar pinout and pretty good specs.

    It is not temperature compensated, so again I think a temperature compensated sensor is better although testing would show whether that actually matters or not.

    The Digikey price for US5881 in singles was $1.70, so it's about average price i think.

    One good thing, the Melexis datasheet for the US5881 is excellent, and also on the Digikey page they had a link to this VERY nice PDF on general using hall switches! Definitely have a look at this one...


    Really well written 46 page PDF on using Hall Switches (with lots of pictures);
    http://www.melexis.com/prodfiles/0003715_hallapps.pdf

    (edit) I copied a couple of pictures out of that PDF so you can see the sort of info it contains;







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    How's everyone's home sensors going? Has anyone mounted on the machine yet?

    I'm curious what sensing distance people are getting with their particular magnets and/or info on the repeatability.

    ----------------------
    Mine are still going perfectly, nothing new to report there. I have noticed now the weather is colder at night in my workshop I get around 0.05 mm total heat expansion on that X axis from real cold to operating warm temps. I have a warm up routine coded in software that runs the axis back and forth for a few minutes and can be repeated if needed.

    For most jobs that amount of error will not matter but I can always warm the machine right up for critical small cutting jobs.



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    Those switches are an excellent choice for being used as homming switches. Now with that being said let me point out that they shouldn't be used as overtravel switches because of the fact that the are sinking or "normally open" switches and not sourcing or "normally closed" switches. It is good practice to use a normally closed switch in an e-stop type of situation. In the even that you have a wire break the machine will shut down when using a normally closed switch. Thank you for the open source design it is in fact a great idea to use for homing a machine and keeping the tolerences closer

    Is the voice in my head bothering you?


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    Ah, that gives me the opportunity for a small threadjack.

    There are home switches and limit switches. Obviously there are twice as many limit switches as home switches.

    But where are the home switches relative to the limit switches? Do you have some distance between 'home" and "limit" in the corner where home is?

    How far from the edge of travel do you put the limit switches? If the machine is barreling down the path and hits the limit switch, it doesn't stop dead, so do you give it any braking area?

    Clearly, the farther up the rail you put the limit switch, the less useful travel you get out of the machine.

    Home on the other hand, is a state of mind, right? It's not necessarily 0,0,0 on the DRO. It's just a fixed known reference point. Home could, theoretically, be in the middle of the table. Conventionally, it's in a corner though, right? Why?



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    brtech,
    Last one first, I put my home switch at a corner so the software will ALWAYS know which direction to go for home. If you put it in the middle of the travel, it might go the wrong way to find home!

    My limit switches are about 1/8 to 1/4" from a hard stop on my mill/drill and this has not caused any problems for me. Also, I use "home as limit" in my controller and therefore have reduced the number of switches. ON the X and Y axis, it is further reduced as the single physical switch is located near the center of travel and stops hit the single switch from either end of the axis.

    Art
    AKA Country Bubba (Older Than Dirt)


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    Having home in a corner also allows the best access for material loading onto the machine. That would be a pain with the cutting tool stopped in the centre all the time!

    I don't have limit switches. They are only ever needed in a fault situation and the computer should be competent to keep the machine from over-running anyway (unless it has missed steps). If you need limit switches it might be good to question *why*...

    That is a good point though about putting the home or limit switches inwards from the point of maximum travel. It can allow time to decelerate before crashing and also allows better homing correction as it can correct +/- a certain amount at power-up or after a crash.

    I placed my home switches about 10mm inward from the travel ends, so it can correct up to +/- 10mm homing error with no hassle. One of the good thing about magnetic sensors is they are perfect for "slide by" detecting, which is the way I set mine up (see post 83).

    This is convenient for me as I sometimes manually move the machine axes with the handles, for cleaning or setup etc and just have to move it back close to home and the machine can then do the +/- correction to re-home itself.



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    RomanLini,
    I use limit switches on my machine as my controller doesn't have soft limits and I am running servos.

    Art
    AKA Country Bubba (Older Than Dirt)


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    Default "normally clos

    has any one found a hall switch that is "normally closed" which is safer
    method for limit switches.... as pointed out...

    Last edited by eloid; 05-05-2010 at 10:30 PM.


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    Hi Eloid, sorry I haven't seen a hall switch "normally closed".

    It is a little meaningless with an electronic sensor. The safety benefits of a mechanical switch which is normally closed as that if the contacts fail (dirt etc) it fails as open, which triggers the limit. Same if the wires are cut.

    With an electronic switch there are no contacts to fail and no reason to expect the hall sensor to fail. If the magnet is brought close the output voltage drops to LOW, and there is very little to fail there (unless the magnet is lost).

    In the event that the wires are cut, the hall switch output (normally HIGH) drops to LOW and triggers a stop condition, so as far as the wiring goes it operates in a "fail safe" mode the same as a normally-closed mechanical switch.

    I'm interested if anyone thinks otherwise, but as far as I can see the hall switch should be as "safe" as any wiring option with a mechanical switch. Technically it may be safer as there should be a much reduced chance of failure, as there are no switch contacts or springs and no mechanical forces on the hall switch.



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    Default esd issues

    Quote Originally Posted by RomanLini View Post
    Hi Eloid, sorry I haven't seen a hall switch "normally closed".

    It is a little meaningless with an electronic sensor. The safety benefits of a mechanical switch which is normally closed as that if the contacts fail (dirt etc) it fails as open, which triggers the limit. Same if the wires are cut.

    With an electronic switch there are no contacts to fail and no reason to expect the hall sensor to fail. If the magnet is brought close the output voltage drops to LOW, and there is very little to fail there (unless the magnet is lost).

    In the event that the wires are cut, the hall switch output (normally HIGH) drops to LOW and triggers a stop condition, so as far as the wiring goes it operates in a "fail safe" mode the same as a normally-closed mechanical switch.
    have you..try different combo of wire get cut ie short to frame etc. ...
    +5v short with frame.... etc +5 v open other shorted out...
    remember safty if frist.....


    I'm interested if anyone thinks otherwise, but as far as I can see the hall switch should be as "safe" as any wiring option with a mechanical switch.

    with electronics you have worry about esd of the chip or transisitor logic fries what condiction will occure frist? with mechanical condition the switch is open.. all the time if a failure occurs...all the time... no esd issue. so i cant say hall switch is just as safe. other failure are if you power supply go low or
    you have large drop on them can get flaky.. etc if with low limit operation range.


    esd is not your friend here, you cant say where, how, what kind esd damage can occur, could be parimetic, Catastrophic, or intermit issues.

    Technically it may be safer as there should be a much reduced chance of failure, wrong!

    as there are no switch contacts or springs and no mechanical forces on the hall switch.
    read above over again


    what the esd threshold on these part 5000v or better?

    when you hear as esd zap that ~2000v when see the zapp it ~5000v
    also remember there a +/- threshold ... electronics with good immunity to esd are designed, have threshold +/- 5000V. cus static can have pos , or neg charge...at random....



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    That's an interesting point about the ESD. However static discharge of any significant level will blow the receiving electronics, regardless of whether it is connected to a mechanical switch or a hall switch.

    It's just good practice to have wiring connections covered to protect the switch and the receiving electronics anyway.

    I don't want to get into an argument about general reliability of sensor types, but one of the main reasons manufacturers use electronic sensors instead of mechanical ones in continually operated equipment is for reliability improvement. There's probably a number of research "white papers" available documenting the difference in life and reliability between hall sensors and mechanical switches. That would probably be the place to look if you want specifics about the sensor reliability.

    (edit) Re the actual ESD spec for the Honywell SS441 sensor, I just checked through the datasheet and that spec is not listed. I also looked through the "Using Hall Switches" PDF linked above in post #83, no mention of ESD there either. If i find any more info on hall switches and ESD issues I will post here in the thread.



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    Default Testing

    Good thread, RL. One thing you might want to try. Rig up a DI or DTI so that the end of the table or some other part will contact it just prior to "hitting" the home switch.

    The readings on a DI or DTI would be much more useful to compare than the position where your controller "thinks" it is.

    My theory isn't great on this, but might be interesting to try and use a bit of copper shielding aroung the magnet to make its field a bit more directional.

    As for reliability, Bosch switched to Hall Effect sensors to detect crankshaft rotational position over 25 years ago, after using points for perhaps 90 years. Suspect now that you'd be hard put to find a car or motorcycle still using a points and coil type ignition system anymore.

    One nice feature when using a Hall Effect to detect crankshaft rotational position, and hence to set ignition timing is that you don't have to go back a couple of times a year and reset the timing. Usually, just set it and forget it.

    Do they ever fail? Yeah, well, they do fail. Most common cause of failure is high temperature.

    The other common way to trigger a hall effect is to have the magnet permanently mounted across the way from the hall effect sensor, ie stationary. Then have a small blade stick down from the table which will interrupt the magnetic field.

    This method will give you a "normally on" condition, which will work nicely as an eStop.

    Tom



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    Default more esd

    That's an interesting point about the ESD.
    However static discharge of any significant level will blow the receiving electronics,

    Esd is not predictable event it picks the weakest link ...kinda like lighting picks the path of least resistance.... so there no way to know what it will blow usless the design if the chip has high speed esd diode protection buit within.


    regardless of whether it is connected to a mechanical switch or a hall switch.

    It's just good practice to have wiring connections covered to protect the switch and the receiving electronics anyway.

    I don't want to get into an argument about general reliability of sensor types, but one of the main reasons manufacturers use electronic sensors instead of mechanical ones in continually operated equipment is for reliability improvement. There's probably a number of research "white papers" available documenting the difference in life and reliability between hall sensors and mechanical switches. That would probably be the place to look if you want specifics about the sensor reliability.

    (edit) Re the actual ESD spec for the Honywell SS441 sensor, I just checked through the datasheet and that spec is not listed. I also looked through the "Using Hall Switches" PDF linked above in post #83, no mention of ESD there either. If i find any more info on hall switches and ESD issues I will post here in the thread. [/QUOTE]



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    Eloid- That's a good point about having a high speed ESD (Electro-Static Discharge) protect device built in.

    As far as I know there is no specific ESD protection in the hall switch, for the Honeywell brand or the other brands mentioned so far.

    BUT it would be a fairly simple matter to add a ESD protect device inside the mount, along with the pullup resistor and LED etc. This may be worthwhile if someone thinks their sensors might be subject to ESD and/or has exposed wiring.

    My personal thoughts is that extra ESD protection is not needed provided the wiring is not exposed, and can't be touched with fingers or metal objects etc. But there are also threads on the forum of people having issues with Static buildup on their machines so that has to be evaluated for each machine.

    TarHeelTom- Re using the Dial Indicator to check the accuracy of the home switch is a good idea. I did have a dial indicator handy when I calibrated my sensors but didn't think it would be worth the effort of making mountings to set it up to test each axis.

    With my setup (and most stepper setups) if it "jumps" one or more steps it is always to the next magnetic pole which is 1/50th of a motor rotation, so the home sensors basically just need to be able to detect that large error and can then correct. Any errors smaller than that come down to things like flex and thermal expansion so even with a dial gauge there will be some issues measuring the home switch accuracy.

    I would really like to see someone test these hall switches to a high accuracy, which probably needs a nice mill (or lab measuring) setup. With my machine it's flex/backlash/expansion etc means that I can only get readings to 0.02mm accuracy or so, and I can't really post any accuracy data better than the stuff already on page 1 of the thread.

    What you said about using a fixed magnet and moving metal "vane" is covered in that nice application note datasheet (I will provide the link again);
    Really well written 46 page PDF on using Hall Switches (with lots of pictures);
    http://www.melexis.com/prodfiles/0003715_hallapps.pdf

    Here is a picture from that app note;


    I'm not sure if it would give a sharper detect point and more accuracy. I think it might, but then there could be issues with the metal vane getting magnetised and that could ruin your accuracy...



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    Very interesting thread, RL. Just read about half of that brochure on Hall effect, extremely interesting.

    I may have to do some work on repeatability and accuracy once I get my "new" mill going. It's got mechanical limit switched, and I'd really like to have a set of home switches also.

    Tom



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    The moving vane would probably be made of a product called Mu Metal. It's commonly used as a magnetic shield for cathode ray tubes in older TVs, in oscilloscopes, and other applications.

    CarveOne

    CarveOne
    http://www.carveonecncwoodcraft.com


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    B]Eloid-[/B] That's a good point about having a high speed ESD (Electro-Static Discharge) protect device built in

    esd spec's need to request from the vendor... also esd protection diodes are not a off the self devices there built into the device... there very fast diodes...

    As far as I know there is no specific ESD protection in the hall switch, for the Honeywell brand or the other brands mentioned so far.

    BUT it would be a fairly simple matter to add a ESD protect device inside the mount, along with the pullup resistor and LED etc. This may be worthwhile if someone thinks their sensors might be subject to ESD and/or has exposed wiring.



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    TarHeelTom- Cool, when you add the home switches please feel free to post any results here.

    CarveOne- Yep good point, that metal would probably be a good choice for the interruptor vane. It is also very common as metal shielding inside cheap PC speakers, the ones that sit next to your monitor.

    But I don't think it is 100% free from magnetic retentivity as some TV's monitors etc still need de-magnetising after years of use. The best setup is probably still the simplest; with just a moving magnet and a fixed hall switch and no steel close by.

    Eloid- You can buy a range of different ESD devices; "transorbs" etc that are small and could be simply soldered to the hall switch inside the mount.



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