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Small dc motor control circuit help regd please.

  1. Oct 26, 2009 #1
    Hi,
    I am a 24/7 carer for my wife who has Huntingtons disease. This has caused her to loose the ability to hold a cup.
    I want to devise a swinging arm with a cup attatched to one end. It will be operated electricaly with a small LEGO motor that has been geared to give plenty of torque.

    Imagine a gantry crane in miniture, the base of the crane will be a bearing block that is fixed under a small table, the crane tower is a 10" piece of 12mm silver steel rod that goes through a cut out in the table and is fixed in the bearing block with grub screws. Fixed at the top of the "tower" is the crane jib, it is a piece of metal tubing that is very strong and appx 2ft in length. At the end of the "jib" a cup holder will hold a cup .

    The idea is when my wife wants a drink the jib will swing from a position away from her to a position where the cup will stop just in front of her mouth so she can drink through a straw.

    Fixed to the end of the "tower" under the bearing block I have mounted a small lego motor that runs on a 9v battery pack. It has plenty of torque and turns the "crane" without a problem.

    I used a combination of micro switches and relays with a push to make switch to control the motor.
    The circuit is very similar to that used in an electricly operated curtain "drapes". system.

    When the switch is pushed the motor is activated and swings the arm to the drinking position, it stops at this position by hitting a limit switch. The limit switch is a bog standard (com, no,nc) There is another limit switch that operates a latching relay to reverse polarity to the motor. When the switch is pushed again the arm will swing away until it activates another limit switch in the "park position".

    This works fine, BUT!!,

    My wife cannot hold the switch down for very long so the arm moves in jerks.

    Could some one come up with a circuit diagram that will require her to push the button switch only momentarily to start the motor, and not to keep it pushed as is the case at present.

    In effect she will touch the push button, this will start the arm swinging to the drink position and stop. When she has finished drinking she touches the push button again and the arm will swing away to park position and stop ready to be used again, and so on.

    I am not an electical engineer, but I am ok with basic circuits, relays, micro switches and the like. I am sure to someone who know what they are doing this will be a simple circuit, unfortunately my little grey cell is worn out so please bear this in mind :smile:

    If any one could help I will be very grateful. I will send my email through private message if wanted.

    josalsam001
     
  2. jcsd
  3. Oct 26, 2009 #2

    dlgoff

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    You could make a start/stop circuit like what is used for motor starters. Once the start switch is pressed, it energies a relay that has a set of contacts in parallel with it that "seals" it until a stop switch (probably a micro switch in this case) that is in series with the stop switch is opened. Check out the Motor Starter Animation diagram at the end of this page. All you are concerned with is the start-stop portion of this operation.
    http://www.exman.com/ims.html"
     
    Last edited by a moderator: Apr 24, 2017
  4. Oct 26, 2009 #3
    Hi Hi josalsam001
    I am very sorry to hear about your wife. Here in thumbnail is a simple latching relay circuit. The manual switch is normally open, and will supply power to the relay when it is closed. The limit switch is normally closed, and will continue to supply power to the relay until it opens. I hope this helps.
    Bob S
     

    Attached Files:

  5. Oct 26, 2009 #4

    dlgoff

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    Just parallel the relay contacts with your momentary Manual Switch and you have the motor starter circuit I was suggesting.
     
  6. Oct 26, 2009 #5
    For my post #3 with the latching relay circuit, note

    1) Pressing the momentary manual switch in my tumbnail sketch will override the limit switch and permit non-latching contol beyond the limit. If you want to prevent manual override, then [STRIKE]another pole of the same limit switch should be between the manual switch and the relay coil[/STRIKE] move the limit switch downstream to the far right side of the circuit diagram, between the latching curcuit and the motor.

    2) If you want a bi-diectional system, then you will need two separate latching relay circuits, one for each direction (there may be other ways), each using limit switches on separate ends. In this case, the arm can be moved in either direction between the limits, using separate manual switches.

    3) For a bi-directional system, a separate pole of one of the latching relays will need to operate a downstream DPDT polarity reversing relay to change voltage polarity on the motor.

    Bob S
     
    Last edited: Oct 27, 2009
  7. Oct 26, 2009 #6

    Ouabache

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    Welcome josalsam to PF !! I admire your creativity at devising an electro-mechanical solution to help your wife to hold a cup.
    As an additional thought, I wonder if you've considered a http://www.monkeyhelpers.org/"? I remember seeing monkeys on public television, that had been trained to assist people who are unable to use their own hands or legs. If you follow the link, this site has a couple of video clips which indicate the possibilities.
     
    Last edited by a moderator: Apr 24, 2017
  8. Oct 27, 2009 #7
    Hi all,

    Many many thanks for all your kind reply's. I will wait till "Madam" is tucked up tonight and have a look in detail at the suggestions and diagrams.

    I liked the idea of a monkey helper, my wife would love it, unfortunately it would not be practicable. Looking after someone 24/7 is very tiring, having to care for a monkey as well would not be possible. Not only that, I doubt the laws in England would allow it as a monkey is classed as a wild animal. Nice thought though.
     
  9. Oct 28, 2009 #8
    I just realized that if you have two manual switches with separate latching relays for opposing directions, the two latching relays have to be cross-connected to the switch labeled "limit switch" on the other relay in my thumbnail to release the latch when direction is reversed.
    Bob S
     
  10. Oct 29, 2009 #9
    Hi Bob & Don



    Bob, I had already worked that out. :smile:

    I contacted Lego for motor information as it has four contacts (terminals), I was not sure what each contact/terminal did.

    What I have found is the battery pack for the motor, (LEGO XL power function) has a reverse polarity switch incorporated. see diagram

    http://philohome.com/pf/pf.htm

    I have managed to get a couple of double latching relays DPDT being sent me as samples, when they arrive I will have a try at your suggestion.

    Looking at the circuitry in the diagram might make things clearer for you to understand what I am trying to do.
    There is a common +9v , c1 and c2 reverse the motor, and OV Ground.

    Having this information might make things easier for you.. I have not got the remote reciever, just the battery box with three switch positions, (1) 9v>C1 Forward Out1
    .(2) 9v>C2 Reverse Out2 and (3) 0V Ground.

    I will obviously have to cannibalise the lego wire loom to suit.

    To summerise, wife will momentarily push start button which will swing the arm to drinking position and stop, momentarily push start button again to swing arm back to parking position and stop.

    Don, I had a look at the animation, this has given me an idea of what is required. It is the reverse bit that has me stumped. This is where the DPDT latching relays will have to be used. Linked with Bobs diagrams I am sure my little grey cell will eventualy grasp how it should be done.

    John
     
  11. Oct 29, 2009 #10
    It looks like the new Lego Power Functions (PF) system may be a PWM (pulse width modulation) form of the old standard PM (permanent magnet) motor system, where the motor had only two terminals, which would use one polarity for forward, and the other polarity for reverse. The PF system will give better RPM control, with better low RPM torque, but requires the complexity of two added connections for +9 volts and ground. Can the new PF motors operate on only two wires? I think so, because if necessary a 4-diode bridge rectifier at the motor can recover the required(?) +9 volts and ground from the C1 and C2 wires. try it.
    Bob S
     
  12. Oct 29, 2009 #11

    dlgoff

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    This is how I would do it. I don't see that you would need latching relays. Note that K1A, K1B,and K1C are relay contacts for relay R1 and similarly for R2 relay. The limit switches are for both ends of the arms travel.

    Edit: As wired, you would not want to press the return until the cup has reached its limit switch and has opened the circuit. Might require some safety feature to prevent accidental switching before travel is complete.

    Regards
     

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    Last edited: Oct 29, 2009
  13. Oct 29, 2009 #12
    Why don't you want latching relays. Read the OP:
    "My wife cannot hold the switch down for very long so the arm moves in jerks.

    Could some one come up with a circuit diagram that will require her to push the button switch only momentarily to start the motor, and not to keep it pushed as is the case at present.

    In effect she will touch the push button, this will start the arm swinging to the drink position and stop. When she has finished drinking she touches the push button again and the arm will swing away to park position and stop ready to be used again, and so on."

    Bob S
     
  14. Oct 29, 2009 #13

    dlgoff

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    The relay contacts across the momentary switch seals the switch so no latching relays are required. i.e. the K1A or K2A contacts.
     
  15. Oct 29, 2009 #14
    Nice. Your circuit eliminates the need for the DPDT motor-reversing relay.
    [Edit] But what happens if the operator decides to reverse the direction of the motion before the limit switch is reached?
    Bob S
     
    Last edited: Oct 29, 2009
  16. Oct 29, 2009 #15

    dlgoff

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    Here is the circuit current path with red showing the "moving of the cup forward".
     

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  17. Oct 29, 2009 #16
    I agree, but it looks like if the direction of motion were changed, both K1A and K2A would be closed and both relay coils R1 and R2 would be energized at the same time.
    Bob S
     
  18. Oct 29, 2009 #17

    dlgoff

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    That's why I put in
    [STRIKE]One would need to put in another set of contacts to remove the momentary switch common terminal from power after the sealing relay has "made up". Maybe another relay?[/STRIKE] Here's how to take care of it. The relays will need 3 normally open and 1 normally close set of contacts. The normally closed ones are used to disable the momentary switch until the limit switches are opened.

    Edit: Changed drawing. The limit switches need to be before the relay coil and the normally closed contacts should have been K1D and K2D.
     

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    Last edited: Oct 30, 2009
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