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Discussion in 'Electronic Basics' started by Cableguy, Jun 20, 2007.

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  1. Cableguy

    Cableguy Guest

    Hy guys,

    Hope all of you are doing ok....

    I'm a newbie in mciro-electronics, alrhough I have an electronics course,
    i've never worked with PICs before...

    I'm trying to design an "actuator" alike circuit, wich would run a dc motor
    foward or backward, depending on it's position....Kinda like the CD-Rom draw
    does, or a garage door opener....

    I'm guessing that to figure the current position out I would have to use
    some micro-switch or optic barrier along with one or two relays to invert
    the direction....

    Anyone ever deone or came across something similar?....

    Thanks From Portugal

    Paulo Gomes
     
  2. Lord Garth

    Lord Garth Guest


    Aside from the optical beam across a garage door, they might sense the
    motor current to make the reverse direction decision. Will your design
    stop at each extreme as a garage door or CD tray does or will it
    continuously
    reverse at the limit of travel?
     
  3. Rich Grise

    Rich Grise Guest

    It's done routinely. In fact, with a microprocessor, you shouldn't even
    need a reversing relay - just do it in software. Of course, you'll need
    drivers for "forward" and "reverse", but that can be done with relays,
    power transistors or FETs, H-bridge drivers, or whatever you want.

    You might want to add "fail-safe" limit switches, such that if the uP
    doesn't respond to the "at the end" signal, and the motor continues to
    run, you have a switch that cuts power to the motor completely.

    (i.e., two switches at each end, one to the micro, and one to cut power
    if the micro fails.)

    Have Fun!
    Rich
     
  4. default

    default Guest

    "Like" a garage door opener or "a garage door opener - "

    Do you need relative position, absolute position, variable speed,
    proportional speed control, how much over/undershoot is tolerable,
    what size horsepower, 360 rotation, etc. etc.

    Describing the application goes a long way to helping make good
    decisions on how best to implement what you want to do.

    This is a common problem and been solved lots of times, so yes,
    someone has done it.
     
  5. Cableguy

    Cableguy Guest

    Thanks for all of your inputs...

    My project is to motorize a compartiment door activated by a single button
    like "open/close".
    Since DC motors are alot more inexpensive and I have afew of them around
    from slot-cars, Speed is not an issue, of course the more smooth the
    moviment the better,
    I've tryed to buil a "relay logic" circuit but got stuck in the midle and
    eventually dropped that concept...and turned to uP....I was planing to use
    PicBasic since I do some Basic related programing in Windows and WIndows
    Mobile enviroments....

    The movements should be like this:

    If on point A go to point B and stop
    If on point B go to point A and stop
    If in between go to point A

    Position would be determine by either micro-switch or integrated IR
    barriers....I dont know waht sp+pecs to search in order to choose a uP so I
    was counting on Your input and experience...I figure I only need 5 I/O pins,
    and the code should be that big either....

    Thanks for your time...
     
  6. default

    default Guest

    A reversible motor can turn in either direction so you may want a
    double pole double throw switch and relays to implement the simplest
    solution. (I didn't automatically assume that there were A and B and
    the motor had to reverse to get there - an endless loop chain, or
    platter with 360 rotation could also stop at two points with - state
    your problem better and you have a better shot of getting a good
    answer)

    For instance a ball valve without stops can be told to go to "open" or
    "closed" with 360 rotation and a simple shaded pole gear motor.
    Likewise other devices where the motor rotates in only one direction -
    I replaced a "voice valve" in an acoustic organ with a single gear
    motor that operated a slide valve - taking advantage of the mechanical
    advantage (low motor loading) at full open or full closed.

    Well that would be a perfect relay logic application so that's what I
    would use.

    What you want is simple (keep it simple). In the old days they called
    it "zero seeking."

    For instance you turn on a motor and it revolves a platter (opens a
    valve, moves a chain, moves something) and it rotates until it reaches
    a switch that turns off the motor.

    That is "zero seeking." in its basic form. Windshield wiper motors
    use it today so they stop at the bottom of the stroke when power is
    removed.

    You have two positions you want to access and a default position to
    goto when it starts up in between stations. (presumably it would be
    at A or B most of the time unless something outside intervened).

    You are probably familiar with those single pole double throw
    switches? (from electronics suppliers or hardware stores to control a
    light from two locations?)

    Well, in its simplest iteration you would transfer power to one or the
    other limit switch and have it move the motor until that switch was
    "satisfied" (actuated - normally closed then opened by the motor or
    gearing) whereupon it would switch off power.

    move switch to A motor goes to A then stops. Ditto B.

    NO microprocessors works with AC or DC and no electronics.

    Now you also want it to goto A when in between - the in between a
    power failure or something physically moved it despite the motors
    wanting to return it to the desired location???


    The power failure scenario can be done with two relays. One comes on
    with applied power (assuming that's how it got between stations) and
    runs to A until it energizes a second relay that stays energized
    dropping the first relay out of the circuit and latching in that
    state.

    If, for some reason you want it to do all that and you have
    over-riding clutches or can physically overpower the drive mechanism
    resulting in an in-between state you could use a PICAXE
    micro controller to do that for about $3 plus $10 in cabling to
    program
    it - then you could add delays, and have it make decisions based on
    time or other inputs - motor loading, hazard items, etc..

    So I come back to my earlier statement; you should just tell us what
    you want to do, and that will determine the best way.

    Don't jump into a programmable device as the first solution you try -
    they offer a lot of versatility and lots of options for later change -

    But if you only want to do one thing - the simplest way is often the
    most reliable way and that could be the lowly relay. (no special
    power supplies, no power up reset hassles, less/no interfacing
    circuitry, better noise immunity, current sensing, etc.)

    Relay logic rules! (until you want to get fancy)
     
  7. Cableguy

    Cableguy Guest

    Default, Thank You very much for you input, with it I learned the "Zero
    Seeking" term...

    What I want to acomplish is a movable faceplate for a car audio system that
    would Act when the power was feed to the circuit and then return when
    powered off.

    Like this.

    car key in On position go from A to B and Stop
    car key in OFF position go from B to A and stop

    I havent yet startred the face plate movement design because I havent yet
    decided how far the movement should go, althought taht is not important to
    the circuit design...

    Once again Thank you for your input...


    Paulo Gomes
     
  8. There must be a million ways to skin this cat, but you can do it with a
    DPDT relay, a permanent-magnet motor, two limit switches and two diodes.

    The motor and both limit switches are in series. Each limit switch has a
    diode across it, allowing the actuator to move away from that limit. The
    relay, driven by the ignition-switch "accessory" contact switches the
    polarity applied to the motor, driving the actuator toward one limit or
    the other until that limit switch opens. The motor, of course, has to be
    powered from some point that's always on. If nothing else, you can use
    the "battery" contact of the ignition switch and an inline fuse holder.

    The simplest microcontrollers are so cheap that they can profitably
    replace relay logic in many cases, but in this case you need the relay
    anyway, to carry the motor load, and all the logic is done by the two
    diodes. I suppose you _could_ use a microcontroller and an H-bridge to
    drive the motor, but a small relay is probably cheaper, and comes
    pre-programmed.
     
  9. Jasen

    Jasen Guest

    you can do it without the diodes. just put the switches in the hot
    wires to the two halves of the DPDT.

    Bye.
    Jasen
     
  10. Cableguy

    Cableguy Guest

    Thank You Guys....!!!

    I never expected this kind of foward help from a news group...
    You guys rock!!!

    I think that I willgo with the relay option, as it is cheaper for me, since
    I don't even have a uP programer, and also attending to the caution notes
    posted....

    As far as the "mechanical design, I'm thinking on using a "blind" like
    faceplate that rolls up and down...My only doubt is ...
    Waht will happen if the motion is stoped by exterior interaction, like a
    slider getting stuck'
    What will burn first? The motor, the relay, or the fuse?

    Anyway all that rest me to do is experimenting and give you guys a big THANK
    YOU for all your input..
     
  11. Rich Grise

    Rich Grise Guest

    Get a schematic/wiring diagram of a car with retractable headlights, and
    see how they do it. They'd have these things at the local auto parts store
    or possibly the public library.

    Good Luck!
    Rich
     
  12. Peter

    Peter Guest

    Sure a microswitch would do it.

    You could use a magnet and a reed switch. When the magnet comes need
    the reed switch if goes open or closed circuit.

    Another option is to monitor the current to the motor. When the current
    goes high switch the motor off. Have you ever seen a garage door
    that closes so far stops because it hits something and then reverses.
    They can monitor the current to the motor to do that.

    Or use an infra red transmitter and receiver.

    Probably depends on how easy mechanically it is to do it.

    Paul
    Bear Technologies
    www.beartech.com.au
     
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