Radio Controlled Motor Problem?

Discussion in 'The Projects Forum' started by NM2008, Jul 5, 2008.

  1. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    Hi there,
    Does any body know of a radio controlled circuit, where once the transmitter stick is pushed left the motor will run clockwise and when pushed right the motor runs anti-clockwise. When the stick is at rest in centre the motor does not run.

    I have tried some circuits already, and have posted one below.
    In this circuit, due to its design I can only make the motor run one way, because it is only meant to switch a relay or glow plug on.
    Is it possible to build a similiar circuit to this, except have it triggering when the stick is pushed right.
    So that would mean two of these circuits one setup as in diagram and the other setup so it works when stick is pushed in the opposite direction.

    Any ideas greatly appreciated!
    Thanks NM
    rc-sw.gif
     
  2. jpanhalt

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    Jan 18, 2008
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    That is the way my analog servos work normally. All you need to do is modify a servo amp for continuous rotation (i.e., replace the pot with fixed resistors). Remember, the servo gets a certain pulse width from the receiver. If it is greater than center width (1.32 or 1.5 mS) it turns one direction and if less, it turns the other direction. They are brushed, DC motors.

    For more detail, you need to specify the type of motor you are using and its power requirements. Have you considered simply hacking a servo amp to do what you want?

    John
     
  3. Wendy

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    Mar 24, 2008
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    DOH! I knew this, look for combat robotic hacks. A lot of beginners will hack servo to use it for the wheels of a bot. It's slow (as bots go), but it is easy.

    After a not so brief search, here is one site I came up with....

    http://www.seattlerobotics.org/guide/servohack.html
     
    Last edited: Jul 6, 2008
  4. jpanhalt

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  5. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    Thanks for the links.

    But one problem, I've tried the servo hack, but it seems to deal with constant motion, by this I mean, with the resistor circuit instead of the pot the motor is constantly in motion when the transmitter stick is in neutral. So I decided to go back to the pot and adjust it manually, but it becomes very difficult/sensitive to maintain the stalled position. This seems to require very precise resistance, it appears to be in the region of .5 of an ohm or less out, and the motor starts to rotate again.

    Overall here is what I am trying to achieve.
    The past few months I have been building this machine similiar to those on robot wars or battle bots, controlled by a Futaba Challenger transmitter and a FP R107N receiver.
    It runs on a 5.5hp Honda engine and and will steer by its brakes.
    Basically it has three wheels two back on a diff driven by the engine and one front smaller wheel similiar to a heavy duty castor.

    So to steer and brake this thing, I have built on brake discs and cable callipers.
    When the calliper closes on left disc it will turn left and same for right turn.

    To pull the cable to close the callipers, I have used two 12v car electric sunroof motors, one for each brake. The cable is wound for one turn round a pulley on the motor output shaft.
    The motor will rotate on a pulse to reduce current when motor stalls, having pulled calliper to its maximum.

    Hopefully this make things a bit clearer as to what I am trying to do.

    The topic in question, where I am stuck is, finding a reliable circuit that once I push left on the transmitter stick the motor will rotate and pull the left calliper closed, stopping the left wheel, spinning the machine to the left. Once the stick is back in the neutral position the power to motor is shut off and calliper springs open and machine continues straight on. And the same to happen with right turn, when right stick is pushed.

    As with the circuit I posted earlier, I can only achieve this for one direction only. As I do not know if it is possible to build another similiar circuit, having it operate when the stick is pushed in the opposite direction.

    Even if somebody could help with the servo hack in order to get it to stall when the stick is in the neutral position, and have some way of making it less sensitive to resistance.

    Your help is greatly appreciated.
    Regards NM
     
    Last edited: Jul 6, 2008
  6. blocco a spirale

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    Jun 18, 2008
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    Have you considered using a servo to operate a couple of micro-switches?
     
  7. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    Ah? No actually, do you think this would be reliable in the long run?
    Regards NM
     
  8. blocco a spirale

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    Jun 18, 2008
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    It would be as reliable as the servo and micro-switches, which could be very good and certainly easy to diagnose if problems arise. I'm sure I've seen this method used on battle-bots.
     
  9. jpanhalt

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    @NM2008 What type of servo did you hack? What you describe sounds like a servo that twitches when on center. My analog servos don't. Some have a fairly broad dead band, which is bad for high-performance aircraft, but good for your purpose.

    In any event, you are going to have a certain pulse-width range that needs to be translated into zero movement. If your system is not stable, the problem is not in the hack per se. Do you have a o'scope that you could use to look at the decoded pulse width stability?

    Have you considered hacking something like a retract servo. They definitely don't move, until you give either an up or down signal. There are also designs for kill switches (not micro-switch based) on the hobby sites. They should give you a very positive full-on/full-off response. John
     
  10. bertus

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  11. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    jpanhalt,
    the servo is a Futaba S-148, that's exactly it, the servo twitches when on centre, with it dissembled and adjusting the pot manually it is very difficult to stop the motor rotating without it twitching. That's why I suspected a very precise resistance earlier, do you know of any method of creating a broader dead band, or stall position?

    bertus, thanks for the links, looks fairly complicated though.

    Regards NM
     
  12. jpanhalt

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    Off-hand I don't know how to increase the deadband. It is certainly worth a search. I don't have much experience with Futaba. Are the electronics accessible inside or are they potted or one of those epoxy blobs? If they are potted, your chance of being able to do something with them is slim.

    How close was your transmitter to the receiver? If you are too close, it can cause twitching too.

    John
     
  13. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    jpanhalt,
    The servo itself is an older model, there is no epoxy blob or surface mounted work and the electronics, appear to be somewhat workable.
    In a previous post it was suggested that I use the servo as is, and a couple of micro switches, what do you think of this idea and its reliability?
    Regards NM
     
  14. Wendy

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    Mar 24, 2008
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    It would work, I've seen servos press BIC lighters for one pound flame thrower bots. It would be on/off/on though, and there would be a weight penalty.
     
  15. jpanhalt

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    Servo-actuated microswitches have been used for years as ignition kill switches. Those have been replaced with more elegant solutions recently.

    That said, I assume you are building some sort of fighting robot and elegance may not be what you want. I would go with the simplest, workable solution. There are going to be lots of other problems to address. Then, once you have something working, refine it.

    That refinement could include a dedicated servo chip, maybe a microcontroller, and maybe proportional braking and turning. In other words, don't get stuck at some hurdle, when you have a workable solution at hand. Of course, that excludes putting Homer Simpson inside the 'bot.

    I hope we will hear from you again in 6 months, if not before, and try to solve this problem then. :)

    John
     
  16. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    Ok, I have now decided at this stage that it is best use the servo as is and the micro switches. This setup is way easier to troubleshoot(if at all needed) and has greater accuracy in comparison to adjusting a servo pot which does not tolerate more that .5 of an ohm of inaccuracy.

    After all the best ideas are the simplest.
    Thanks to everyone for their thoughts and help on this issue.
    Regards NM
     
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