RC Lawn Mower Control Circuit

Discussion in 'The Projects Forum' started by TelluriumCrystal, May 5, 2016.

  1. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    I have next to zero experience designing complex circuits and have taken the plunge by attempting to design a circuit I can use to control an RC lawn mower. I was hoping someone would be willing to look over my design and at least confirm that it won't blow itself up. To the best of my knowledge 24 volt systems drawing high amperage can be rather dangerous.

    There are a few unknowns in this system that I would like to point out:
    • Exact wheelchair motor model has not yet been determined, so I don't have a specific value on amp draw of M1 and M2
    • Exact model of motor controller has not yet been determined for the same reason as the above, but all the controller models function in the same manner
    • Batteries will most likely be lead acid but total battery capacity has not yet been determined
    Schematic and BOM are attached. Thanks for any help!
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Why not use off the shelf RC control systems found at any local or online hobby store?

    Basically plug and play going that route.
     
  3. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    There are a few reasons:
    1. I already have an Arduino Pro lying around doing nothing and I'd like to put it to use
    2. It is a good learning experience (I've already learned a lot about transistors)
    3. I would like more control over the motors than a typical RC controller offers (ex. limit max speed to 80% if it turns out 100% is hard to control)
    4. It makes the control system much easier to modify if I want to try and make it into a partially or full robotic system later
     
  4. wayneh

    Expert

    Sep 9, 2010
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    For safety's sake, be sure to program in a stop-by-default failsafe so that the mower stops moving at all if the control signal is lost or any error is detected. Or power is lost.
     
  5. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    That's actually why there is a relay in there, it allows the Arduino to govern when power is able to get to the motor controllers at all. If the Arduino stops receiving a transmitter signal for longer than a predetermined time it will cut power. Of course, if the Arduino loses power then the relay opens.

    Actually it looks like I forgot to add in the main on/off switch. That would go between the leftmost 12v battery and the wire junction between the batteries.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    What more control do you need than a common RC controller gives you? 0 - 100% or 0 - preset maximum in both directions. Is there a third or fourth function that a DC motor can do other than rotate one way or another to drive a wheel that can't be handled by one of the numerous other channels that the average low cost multi-channel RC controller and receiver set can handle? o_O

    Just asking.
     
  7. Brownout

    Well-Known Member

    Jan 10, 2012
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    According to your schematic, the Arduino is doing all the control. Or is that only a prototype?
     
  8. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    Good question. Nearly all RC systems out there are designed to control servos using PWM. I would guess that is because they are meant for RC aircraft and helicopters. In my case though, I need to control large 24 volt wheelchair motors that may be around 5A each during normal operation. That necessitates a motor controller or a set of large relays. Since I'd prefer to have variable control over the power output of the motors, that rules out the relay option and leaves me with a motor controller.

    The motor controllers I have the most experience with are the Sabertooths. They work really well for controlling a motor directly from the RC input and they usually support computer (or Arduino) input. If I wanted to drive the lawn mower using a tank-style control scheme then you are correct, the Sabertooth controller would suffice. However, these feature-packed motor controllers are anything but cheap (a Sabertooth 2x25 would cost around $130). Frankly, I only need something able to run the motors off a PWM signal, and that's exactly what the two Pololu motor drivers I plan to use do (and for a combined price of only $80).

    The nice thing about putting an Arduino between the RC receiver and the motor controllers is that it lets me do things like control the lawn mower with only one stick. I could even program in a "cruise mode" so I don't have to hold the stick forward on those long straight runs. It also gives me direct control over the timeout behavior of the motor controllers. Come to think of it I could use the throttle as a direct controller for the power relay, which would let me completely disable the mower's motors remotely if I needed to.

    I suppose in a way I am already using RC motor controllers, I'm just using an Arduino to control them.

    I'm referring to the Pololu motor drivers. There are different models that handle different amp loads.
     
    Last edited: May 5, 2016
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