EMI problems..?

Discussion in 'The Projects Forum' started by saiello, Aug 25, 2013.

  1. saiello

    Thread Starter Member

    Jul 1, 2007
    24
    0
    Hi,
    I am building an industrial machine that is to be used in manufacturing production. It is controlled by three separate systems, two of the systems each use Arduino style M/C boards, the other is a single phase AC motor controlled by user input. Both M/C boards are completely independent, one drives a coil that time controls a pneumatic cylinder, the other drives a stepper motor. Both boards are powered by the same 12V power supply. All systems function perfectly until I run the AC motor. When I do so, the software on the board that controls the stepper motor appears to get corrupted and the result is one of a few different states. Pressing the reset button or even pulling the plug and powering on again doesn't get the board back to normal function. I have to re-load the software and things are OK again.

    The board that controls the pneumatic cylinder is always completely unaffected.

    There is a main box in which is mounted the main power switch, all the power supplies, the stepper driver, and the "up/down" control system for the AC motor which utilises industrial contact breakers to change direction of the motor. Both boards and the button controls for the AC motor are remotely mounted away from the main box.

    From the problematic stepper driver M/C board I run about a 1.5 meter length of standard UTP network patch cable that carries two 5V control signals ( for direction and frequency ) and an earth down to the stepper driver that is mounted in the main box that the contact breakers are mounted in. Rather than using three single wires, I used three of the twisted pairs to each carry the signals and earth ( removing the rest ). I had no reason to use network patch cable other than one of convenience.

    As a last piece of information, I had the same problem when everything was separated and dispersed and hanging out of the machine prior to proper installation. I put it down to some possible electrical interference from the coiled pre-cut lengths of mains wire but I get the same problem when everything is installed as it should be.

    I don't know why running the AC motor should affect the stepper driver M/C let alone corrupt the software. Is it possible that I am getting EMI from the motor/contactors that is being picked up by the network cable and overloading the M/C's pins? This is the only real physical difference between the two M/C's and the only thing I can think of that is the cause of the problem. Am I making some silly schoolboy error in using the network cable in this way..?

    Any help much appreciated..! :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    I would suggest using optoisolators to drive the AC motor.
     
  3. saiello

    Thread Starter Member

    Jul 1, 2007
    24
    0
    Hi,
    Just to clarify, the AC motor is a completely independent system, it is manually operated and not electrically tied in to either of the M/C's. Are you suggesting that the contact breakers that control the motor are the main source of the problem and that using SSR's would be better? Is the use of the UTP cable on the problematic M/C actually the potential cause of the problem?

    Thanks.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Sorry, my suggestion assumed the mcu was driving the AC motor.

    Try replacing the mcu board and see if the problem still exists.
    You can try adding RF filters on the mcu power supplies.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    To start..You should use shielded wire for ALL wires and ensure you have everything fully grounded. Tie the shields to ground of course.

    This is a VERY common problem.
    some good info here (and a funny aluminum foil picture)
    http://www.cnccookbook.com/CCCNCNoise.html
     
  6. saiello

    Thread Starter Member

    Jul 1, 2007
    24
    0
    Thanks for info and link... new territory for me.. ;) I will try and isolate the emitter and receiver by trial and error. As an aside, if I run the offending cables through the steel box frame and ground the frame would this help with shielding..?

    Thanks again.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    In machine control systems there is two schools of thought, one is to galvanically isolate all systems OR make all system commons connected to earth ground but it has to be done in a certain manner.
    My personal preference has always been to common up supplies to a common star point together with the service ground and I have not had any problems.
    Even in the event of galvanic isolation, there can be inductive or capacitive coupling of EMI or other spurious signals.
    Here is a an excellent source that outlines something called Equi-potential bonding.
    http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
    Also see ch6 which outlines the current practice for shielding.
    Max.
     
Loading...