Architecture Model Dilemma..

Discussion in 'The Projects Forum' started by Connorp, Sep 19, 2013.

  1. Connorp

    Thread Starter New Member

    Sep 19, 2013
    3
    0
    Hi,

    I'm building an architectural model for one of my final year projects at uni, and i'm having trouble designing the operating circuit for it.

    The model is of what is commonly known as 'The Sliding House' by drMM architects (give it a google and you'll see what I mean about the motor) and it is in 1:40 scale, which puts it at about 80cm unextended.

    I'm a complete novice in electronics, although I'm having basic lessons from one of the pro Modelmakers at the company I'm doing work experience in, so I have a (very) basic grasp of it as of yet.

    I'll explain what I'm trying to do first and hopefully one of you would be kind enough to teach me how such a system would operate if you could?

    Ok, so I need to plug the model into the mains, just so you know where my power is coming from initially. (I know about fusing and transformers to convert so I'm ok up the the converted side of the transformer)

    Ideally, due to aesthetics and ease of demonstration, I'd like to use a 3 position rocker switch (on-off-on) to control the direction of the motor. Unless there are any better suggestions of low visual impact switches that would suit better? Also, i'd like the switch to only have to be depressed for a second (not held down) but the model to move until it reaches either end of the rack.

    I need to run a DC motor with low RPM to control a rack and pinion system, the rack being attached to the bottom surface of the outer part of the house, and the pinion being attached to the shaft of the motor to drive it.

    I need to reverse the polarity of the motor when the switch is presses from either left or right to move the model left or right (sorry if this is obvious, just trying to include as much info as possible.

    At both ends of the rack, I plan to install microswitches (have no idea what contact config) so that when the piece reaches the switch it stops the circuit and the model isn't working against itself, I believe this type of system is often found in model railways, although I want the piece to stop when it gets there.

    Hopefully this is clear as of yet, sorry if I'm getting anyone confused! I actually saw an image of a circuit similar with a blue train and microswitches which looks kind of what i'm after, without the constant movement. That guy got loads of help so hopefully I'll get the same!

    Ok, so here comes the hard part. The model has a 3rd position, the middle. It's important it stop at the right place as windows line up etc and it would ruin the effect if it wasn't precise. This is the part I'm really stuck on, I don't know if I need another micro switch in the middle (maybe a lever microswitch switch that gets depressed by a lump on the bottom of the moving piece that contacts it at the right position?), and even then I wouldn't have a clue how to wire it so it works from both directions.

    So, positions A, B and C. In terms of switch operation I'd love it if there was a way to have a 3 position switch, the middle position being no movement, and each momentary touch of a direction to send the model the appropriate direction. So imagine we are in position A, one touch of the right hand side of the switch would see the model move slowly right into position B, and another touch seeing it move into position C. A touch on the left side of the switch would see it move back into B, and so on.

    I know its long winded and fussy and everything, but I just have a vision of the movement in my head and I know with enough research and time (and grovelling) the solution is out there.

    Thanks in advance for the bundles of expert advice i'm about to receive, it's appreciated

    Cheers
     
  2. JoeFromOzarks

    Active Member

    Apr 14, 2010
    95
    27
    If this isn’t a job for an inexpensive microcontroller then I’ve never seen one! (grin) The alternative, mechanical switches and relays may prove to be somewhat of a wiring nightmare, though accomplishable.

    Where are you located, where can you source parts? (Put that info in your profile location.)

    “DC motor with low RPM” equals what for operating voltage and full load current draw? Since the motor will be bidirectional you’re looking at using an h-bridge whether electronic or electro-mechanical. “Low RPM” means either PWM or gearing, or both. Will the motor slow as it approaches its stop point, or suddenly jerk to a screeching halt, subject to gear lash and motor momentum?

    “At both ends of the rack, I plan to install microswitches” and “The model has a 3rd position, the middle” might want to consider magnetic “hall effect switches” so you may position the tiny magnets & sensors exactly where the motor should stop. I say “exactly” because “precisely” (and especially proportional speed control) may be a slight stretch using anything but an optical encoder to pinpoint position.


    :) joe
     
  3. Connorp

    Thread Starter New Member

    Sep 19, 2013
    3
    0
    First of all, thanks for the reply dude.

    Dont have a clue about micro controllers.. I was hoping to do it a more mechanical way as I'm still learning the basics, but if there are better alternatives then I'm all ears.

    Well in reality, the building takes about 6 minutes or so to move from one side of its rails to the other, so for a model of almost a meter, it should be somewhere between the 5-10 second mark per half (A to B etc)

    Like I said, I'm green to electronics so the theory behind the operating voltage etc will have to be idiot boarded I'm afraid.

    Whats an H-Bridge? I thought by wiring a motor a certain way you could just reserve its polarity? defined by the position of the switch?

    Magnet switches? never heard of them, but I'm guessing the attraction/repulsion of a set of magnets acts in the same way a push or lever switch would? so i could have 3 sets of magnets, one in A, B and C and the motor would do the same thing (stop) when it hits any of the 3?

    I'm from more of a mechanical based background before this so excuse my ignorance, I'm slowly getting there but its a bit more complex than i gave it credit for!

    Cheers

    Connor
     
  4. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Here. I erased most of the guts out of a chicken coop door controller. You can wire it like this nightmare? of wires or start learning microcontroller language.
     
  5. Connorp

    Thread Starter New Member

    Sep 19, 2013
    3
    0
    Thanks for the diagram. I'm not too fussed about complexity of physical wiring as long as I know where everything is going and that its going to work as intended really.

    I was speaking to one of the guys at work about it today, and he said to consider a servo with a simple program, is that the same kind of thing as a microcontroller? (unintended dumbest question of the day)

    The magnet idea seems quite solid though, but due to the nature of the building and how in all 3 positions the windows line up with openings in the sliding outer part of the house, it needs to be accurate to within 1-2mm. Would these magnet switched be able to do that or are those kind of devices less accurate due to waiting on magnetic power, not an on/off like a push button microswitch?
     
  6. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Yes, a programmable servo uses a microcontroller. The key words are, "programmable" = "microcontroller".

    Or, you can adjust the limit switches for the proper stopping point.
    If the unpowered glide isn't predictable, you can remove more of the chicken coop controller parts so that the motor is open circuited when the limit switch opens. That will make the motor seize up suddenly. However, you are moving this thing so slowly that I expect the motor to stop pretty quickly just because it lost power.

    and no, I do not think the wiring is a nightmare.
     
  7. JoeFromOzarks

    Active Member

    Apr 14, 2010
    95
    27
    Not a nightmare at all!!! :)

    :) joe
     
  8. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Wait. I can't remember whether stopping the motor with it shorted or open circuited causes the sudden stop. Anybody remember?
     
  9. JoeFromOzarks

    Active Member

    Apr 14, 2010
    95
    27
    ...though this might be. (A nightmare) :)

    :) joe
     
  10. JoeFromOzarks

    Active Member

    Apr 14, 2010
    95
    27
    Motor terminals shorted = brake. (Or both terminals charged at the same potential.)

    :) joe

     
    #12 likes this.
  11. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    In that case, the drawing I presented is already wired to make the motor stop suddenly.

    You can leave out the MOV's because there are no transistors in this design that will be whacked by the inductive kickback.
    Still, it would be easier on the relay contacts if you left them in. They are bi-directional, anything around 25 to 35 volts.
     
    JoeFromOzarks likes this.
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    A full short circuit to stop it can be a bit rough on a DC motor, it's better to put a resistor in circuit to reduce peak braking force and current. :)
     
    #12 likes this.
Loading...