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  1. #11
    Hi JW,

    Let me try to clarify what the homing on index function on a servo motor is.

    The incremental encoder on a servo motor generates pulses when the motor moves. The motor driver is capable of counting the pulses and store them on power off but if, for example, you move the motor with the power off or by a power failure where the axis moves from inertia, that axis position is lost. So the absolute position of a cnc machine cannot be determined reliable from the servo incremental encoders data and homing sensors are needed.

    Most incremental encoders have an additional "index" signal that provides a reference position of the rotor. It is output at each motor revolution. This signal can be used to increase the accuracy of the homing to an almost perfect and repeatable position. The homing on index routine moves the axis until the home sensor is triggered then rotates the motor to the index position. In this case the sensors accuracy need only to be grater than the axis movement of half a motor revolution. And the sensor be mounted so that when triggers the motor shaft is at about 180 degrees from the index position. Depending on the actual setup, the amount of axis movement per motor revolution, switches could have enough accuracy for homing on index.

    Why is homing important?

    First of all because if you have homing and set each axis maximum movement, the machine knows its limits! And will never hit a limit switch. If you load a g-code that exceeds the machine working area you get a warning. Otherwise you will only find out that you made a mistake when the axis hits the limit and ruins your work because...

    Secondly, if something bad happens and you have to cut power to the machine you loose the workpiece position. If you have accurate homing, the workpiece zero position is referenced to the machine zero (home). If those numbers are stored you can easily resume the work after a power failure or if you want to pause and continue the next morning... just home the machine and go to the stored workpiece coordinates. And continue cutting where you left off.

    Paul.

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  3. Paulus.v Explanation of Homing on an index is spot on, however, I think you'll find that most Closed-loop Hybrid servo encoders don't provide an index Pulse so can't use that feature.

    Regards Homing and Zeroing etc then you need to get into the habit of thinking in terms of Machine coordinates and Work coordinates. Home switches define the Absolute ZERO machine coordinate position. This positon NEVER changes and is used to reference WORK coordinate ZERO.
    So when you send the machine to HOME your sending it Machine ZERO. From this position, you can also define machine limits.

    WORK zero can be anywhere inside the work envelope of the machine. You'll often see reference to WORK offsets, these are just areas inside of the machine work area which can have separate WORK zero positions.
    This means you can assign each area to a Fixture G-code ie: G54 G55 G56 etc and use these codes in your cutting G-code file so it moves to the next fixture and starts cutting from new Zero point.
    This is useful for instance when have several different parts or materials setups on the bed. For instance could have G54 reference to point on the Bed while G55 references to corner of a Vice and so on.
    Each would have there own Zero WORK coordinate and all are referenced from the MACHINE Zero.(HOME). From MACHINE ZERO any location inside any Fixture Offset can be located accurately.

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  5. #13
    Picking up on question E) in post one - you could use a Hall effect sensor but they need a magnet 🧲 on the moving part and are likely to attract steel chips and filings and become unreliable. Proximity sensors detect steel or aluminium so are a better option. Your other options include cheap small micro switches or chunky industrial ones.
    Ideally have the detected object on the axis run past parallel to the face not up to it so a fault means the axis crashes into the hard end stop not the sensor.
    Oh and welcome back Jazz 🍻!
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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  7. #14
    JW1977's Avatar
    Lives in Vetlanda, Sweden. Last Activity: 3 Weeks Ago Has been a member for 5-6 years. Has a total post count of 82. Received thanks 2 times, giving thanks to others 20 times.
    Thanks!

    Proximity sensors seem like the option i will choose.
    Is there anything special i need to now about using them?
    As i understand i will use 6 of them and one of each axis will double as home switch right?

  8. #15
    If you are cutting steel do not use one requiring a magnet.

    The detector detects a passing object so it cannot be crushed if it fails.

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  10. #16
    In terms of proximity sensors :
    I would go for the 4mm distance sensing type

    Prox sensors are more sensitive on steel targets compared to aluminium

    They run on something like 12V -30V, typically 24V. When triggered they output this same level so they can’t go straight into a 5V BoB ( is my experience) without doing something to the signal. If your BoB can take this higher voltage then no problem.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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  12. #17
    Have a look at this thread

    http://www.mycncuk.com/threads/10262...oes-nt-it-work


    You can regard the LJ12A3-4-Z/BX as a normally open relay, where the output is switched to GND when activated. The fact that you supply 12-36V to the electronics of the induction sensor is a red herring. The unswitched output virtually floats and usually pull-up resistors in the BOB strap it to 5v. Meter your BOB input and you should find 5v. If you don't, then you have either buggered the BOB with mis-wiring or the BOB needs a 5v external pull-up resistor. I'll just clarify that last phrase - a resistor to pull the BOB input to 5v, I use a 10K if necessary.
    Last edited by cropwell; 02-09-2019 at 09:38 PM.
    “A DEAD (in a ditch) STATESMAN
    I could not dig: I dared not rob:
    Therefore I lied to please the mob.
    Now all my lies are proved untrue
    And I must face the men I slew.
    What tale shall serve me here among
    Mine angry and defrauded young?
    from EPITAPHS OF BREXIT 2016 - ????”

    Thanks to ― Rudyard Kipling 1865 - 1936

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  14. #18
    Quote Originally Posted by JW1977 View Post
    Thanks!

    Proximity sensors seem like the option i will choose.
    Is there anything special i need to now about using them?
    You have mentioned that you will be using the CSMIO IP-M controller. This controller is using the industry standard 24V inputs and you can use either PNP or NPN sensors, the 10-30V type.

    Quote Originally Posted by JW1977 View Post
    As i understand i will use 6 of them and one of each axis will double as home switch right?
    If you put the sensor on the moving part and have two metal triggers on each end of the axis you will actually need only 3 sensors, one for each axis. And yes, the sensors can be used as both home and limits.
    For security reasons it is advisable to also have limit switches that will cut power to the machine before reaching the end of axis travel.

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  16. #19
    JW1977's Avatar
    Lives in Vetlanda, Sweden. Last Activity: 3 Weeks Ago Has been a member for 5-6 years. Has a total post count of 82. Received thanks 2 times, giving thanks to others 20 times.
    Quote Originally Posted by paulus.v View Post
    You have mentioned that you will be using the CSMIO IP-M controller. This controller is using the industry standard 24V inputs and you can use either PNP or NPN sensors, the 10-30V type.



    If you put the sensor on the moving part and have two metal triggers on each end of the axis you will actually need only 3 sensors, one for each axis. And yes, the sensors can be used as both home and limits.
    For security reasons it is advisable to also have limit switches that will cut power to the machine before reaching the end of axis travel.
    Yes, i got the IP-M controller.
    Are you saying that i can use one sensor /axis that doubles as limit switch and homing switch or do i need one homing sensor + one limit switch /axis?
    Btw im using a driver by the name 2HSS86H.

  17. #20
    JW1977's Avatar
    Lives in Vetlanda, Sweden. Last Activity: 3 Weeks Ago Has been a member for 5-6 years. Has a total post count of 82. Received thanks 2 times, giving thanks to others 20 times.
    Quote Originally Posted by JAZZCNC View Post
    Paulus.v Explanation of Homing on an index is spot on, however, I think you'll find that most Closed-loop Hybrid servo encoders don't provide an index Pulse so can't use that feature.

    Regards Homing and Zeroing etc then you need to get into the habit of thinking in terms of Machine coordinates and Work coordinates. Home switches define the Absolute ZERO machine coordinate position. This positon NEVER changes and is used to reference WORK coordinate ZERO.
    So when you send the machine to HOME your sending it Machine ZERO. From this position, you can also define machine limits.


    WORK zero can be anywhere inside the work envelope of the machine. You'll often see reference to WORK offsets, these are just areas inside of the machine work area which can have separate WORK zero positions.
    This means you can assign each area to a Fixture G-code ie: G54 G55 G56 etc and use these codes in your cutting G-code file so it moves to the next fixture and starts cutting from new Zero point.
    This is useful for instance when have several different parts or materials setups on the bed. For instance could have G54 reference to point on the Bed while G55 references to corner of a Vice and so on.
    Each would have there own Zero WORK coordinate and all are referenced from the MACHINE Zero.(HOME). From MACHINE ZERO any location inside any Fixture Offset can be located accurately.
    Good explanation, thanks!
    Last edited by JW1977; 03-09-2019 at 08:43 AM.

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