need help using limit switches

Discussion in 'The Projects Forum' started by carsonc, Jan 25, 2008.

  1. carsonc

    Thread Starter New Member

    Jan 25, 2008
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    1st post so hello to all
    I am working on a machine called a draw bench. It is 50 ft long and has a clamping device mounted on wheels (it is called a donkey) which rolls back and forth from the center of the bench to one end. It is powered by a 15 hp vector drive motor that is controlled by a Baldor motor driver.
    I need to have limit switches at the end of travel and at the starting point, the problem is that the motor controller only has one input for a controlled stop (de-acceleration curves). So I have to put the 2 limit switches in series, but then when the donkey trips the switch it stops and is holding the switch open. The motor controller will not allow movement in the other direction while the stop circuit is open. I have only been able to get around this by using a bunch of timed relays, there must be a better way.
    Does anyone have a sugestion??
    Carson
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Does the donkey need to stop and wait for another switch input before returning to the other limit switch point?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    When working with industrial machines of this power level, safety must be the primary concern.

    That being said, I suggest that you have "absolute end of travel" limit switches on either end that will shut off the motor in case the circuit controlling the "normal travel limits" fails. Additionally, you should have a number of mushroom-style "panic stop" buttons along the bench, in case there is an emergency need to stop the motor.

    What is the voltage and current in this control circuit?
     
  4. carsonc

    Thread Starter New Member

    Jan 25, 2008
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    beenthere Yes it sits there until the operator presses the return button, at which time it will travel to the start position and close the other limit switch.

    SgtWookie The motor runs on 220 3 phase I dont recall the exact current it pulls but the supply line has 60 amp fuses. The motor controller circuits are 24 v dc opto isolated inputs
    The controller has 3 ways to stop operation, the one I am using on the limit switches has programable de-acceleration curve which we need. There is a regular stop circuit but it just cuts power and the motor coasts to a stop. The other stop circuit is in the failure - trip circuit. I will have to check if it is a coast to a stop also. I will incorperate some emergency off switches and the idea of a back up set of limit switches sounds like a good idea, They will be much more simple to set up than the ones I am working on now.

    Any ideas on how to bypass the limit switch while the donkey is triggering it.
    Thanks
    Carson
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK, so to recap:
    1) The control circuit is 24VDC.
    2) The limit switches when tripped, engage the de-acceleration curve.
    3) To get the donkey back, the return button needs to be pressed and the limit switch closed.

    How long does it take from the moment a limit switch is tripped, until the motor is completely stopped?

    How long does it take from the moment the return button is pressed until the motor moves away from the limit switch, thus closing it?

    Is the return button a NO or NC switch? (I'm assuming it's on the 24VDC control circuit as well?)
     
  6. carsonc

    Thread Starter New Member

    Jan 25, 2008
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    correct it is a 24 v dc control circuit, the de-accel takes about 2 1/2 sec. (I may shorten the de-accel time to 1 1/2 sec. after I get it working) The foward and return button are both N.O.and momentary The de-cel is a NC.The return circuit will not work until the the limit switch is closed, also closing the limit switch (inserting a NO push button for testing) it appears that the limit switch must be closed before you press the return button, not pressed at the same time. The donkey will clear the switch in approx 1 sec, when I do it manually.
    One of my ideas was to have the limit switch trip a timer relay (NC) that would open and send the sig to the de-cel circuit for 1 sec and then close. However with the other control circuits for the JOG and switching for pre_programed return speed and pot adjustable foward speed It would look like an advertisment for relays.
    I keep thinking about the circuit that one would use to turn on the same light on and off from 2 different locations
    but so far nothing has come of it.
    Thanks SgtWookie for your help
    Carson
     
  7. carsonc

    Thread Starter New Member

    Jan 25, 2008
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    0
    Some one must have an idea ?
    Carson
     
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