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  1. #21
    Join Date
    Aug 2006
    Posts
    2758
    Quote Originally Posted by pminmo View Post
    Depending on noise source, open outputs of the opto isoltors will not be of any help.
    Switches are normally closed as specified on the schematics, when used as limit switches one of them will open when activated , in that case noise won't be an issue. Used as Home switches and standing in a home position in a very electromagnetically "noisy" system it could be of some concern, in that case, a 1Kohm resistor from pins 1 and/or 4 of the opto-coupler to Auxiliary ground will help a little more.

    Remember that home switches should be one for each Parallel port pin (unless your software moves only one axis at a time when going to the home position), you could add another opto-coupler IC and use other input pins for the other two axes. The diagram is only one example. You are not limited by the amount of series connected limit switches except for the fact that the more switches you use more cable is used between switches and more noise will be present in the circuit.

    Other important consideration when chosing limit switches is switch hysteresis, mecanical switches have hysteresis, it shows as different home or limit positions depending on the direction of movement, (nothing to do with backlash), Opto-switches have the least amount of hysteresis and show no bouncing..

  2. #22
    Join Date
    Jun 2003
    Posts
    3312
    Kreutz, I just looked at the schematic, didn't see the note on the normally closed. (In fact I've never seen normally closed switches drawn normally open) I've never had any issues running with closed switches, but have normally open. One spindle motor I have is just a pure noise generator! It's easy to see the effect on open switches by just turning on the machine and disconnecting the switches. Virtually all the noise is switching emi from the choppers, sometimes spindle motor commutator noise. Physical layout of wiring can cause issues, avoid parallel wires.

    KISS principle, don't make it more complex than it needs to be.
    Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
    Vist my websites - http://pminmo.com & http://millpcbs.com

  3. #23
    Join Date
    Aug 2006
    Posts
    2758
    Quote Originally Posted by pminmo View Post
    KreutZ, I just looked at the schematic, didn't see the note on the normally closed. I've never had any issues running with closed switches. Have you? But it's easy to see the effect on open switches by just turning on the machine and disconnecting the switches. Virtually all the noise is switching emi from the choppers, sometimes spindle motor commutator noise. Physical layout of wiring can cause issues, avoid parallel wires.

    KISS principle, don't make it more complex than it needs to be.
    No problem, your comment served to add more detail to the explanation. I did not find any switch diagram normally closed on my schematics' library, will have to make it in order to avoid misunderstandings. Most people, including myself, don't read the written notes on the schematics.

    The only problem I have found using N.C. mechanical micro-switches as limit switches is when somebody replaces the original switch with a generic micro-switch, or when there is excessive vibration on the machine. Note that I don't have CNC experience, but my designs and work experience is in the medical equipment field and we design/use a lot of electro-mechanical assemblies similar to a CNC machine. We use twisted cable to the micro switches connected in a star fashion in order to avoid wire loops acting like R.F. antennae. In a very electro-magnetically noisy environment (like in an operating room using old technology electro-surgical units) we also use shielded twisted pairs.

  4. #24
    Join Date
    Apr 2006
    Posts
    512
    Thanks Guys. I should have drawn the switches in my diagram NC.

    Had some luck last night. I isolated all the connectors from the metal driver box. This solved the interference with the slaved A and Z stepper motors. Unfortunately I still have a high debounce(22000!!) when I connect the Y homing and limits. I suspect it is the ACME rod causing this as they pass within an inch of it. Unfortunately I can't seem to find a route to avoid the ACME.

    I have yet to buy the capacitors . Will I also need new micrswitches? Mine are rated 0.1A 125VAC.

    Parrallel wires? Should I also twist the wire pairs leading to the microswitches?

  5. #25
    Join Date
    Aug 2006
    Posts
    2758
    Quote Originally Posted by bigz1 View Post
    Thanks Guys. I should have drawn the switches in my diagram NC.

    Had some luck last night. I isolated all the connectors from the metal driver box. This solved the interference with the slaved A and Z stepper motors. Unfortunately I still have a high debounce(22000!!) when I connect the Y homing and limits. I suspect it is the ACME rod causing this as they pass within an inch of it. Unfortunately I can't seem to find a route to avoid the ACME.

    I have yet to buy the capacitors . Will I also need new micrswitches? Mine are rated 0.1A 125VAC.

    Parrallel wires? Should I also twist the wire pairs leading to the microswitches?

    If you need a high debounce value on the software, replace the micro-switches, there is nothing external that influences bouncing on the switches, it is a switch feature. Also, the applicable switch rate is the DC rate, sometimes they write it on the switch body, sometimes you need to look at the datasheet. Look for a low DC current rated micro-switch. If the contact resistance measured with a digital ohmeter, exceeds 1 ohm, replace the switch.

    If you can twist the wires, that is going to add extra inmunity to R.F. induced noise, 2 twists per inch is OK.

  6. #26
    Join Date
    Apr 2006
    Posts
    512
    Thanks for your help and patience Kreutz.

    Could you please confirm that these will be suitable?

    Cheers Liam
    Attached Thumbnails Attached Thumbnails 1a.jpg   1b.jpg  

  7. #27
    Join Date
    Aug 2006
    Posts
    2758
    Quote Originally Posted by bigz1 View Post
    Thanks for your help and patience Kreutz.

    Could you please confirm that these will be suitable?

    Cheers Liam

    The Capacitor is OK, the switch seems to be OK too, look at the yellow highlights on the data-sheet in order to clarify relevant information (the highlights are mine). Make sure the actuator or cam can deliver the required plunger travel, do not over travel the plunger!, make an adjustable mounting
    for the switch body.
    Attached Files Attached Files

  8. #28
    Join Date
    Apr 2006
    Posts
    512
    Cheers kreutz for going to the trouble of highlighting the data sheet. I will definately make sure the soft limits are in place to avoid crashing into the switch.

    Ordering the parts now. I will keep you all informed.

    Thanks Liam

  9. #29
    Join Date
    Apr 2006
    Posts
    512
    Good news guys the limits/homing switches now work. Debounce signal is now 400 as opposed to 24000. The 2 main problems appear to be a dirty earth for the shielded cable on the limit switches(Shielding is no longer earthed but appear s to work fine). The other problem was the metal connetors earthing the shielded wire to the metal driver case.

    Many thanks for all your help it has been appreciated.

    Liam

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