Question about PLC's and Motor switching circuit

Discussion in 'The Projects Forum' started by creyta, Apr 5, 2014.

  1. creyta

    Thread Starter New Member

    Apr 5, 2014
    2
    0
    First I want to say hello, I am excited to learn more about this forum and am excited to be here. The reason that I am posting is because I am working on building a autonomous, constantly roaming, robot. This is the way it will be set up, It is using a Micrologix 1000 Allen Bradly PLC as the brains. The PLC will be mounted on an RC car chassis and will control the motors. The robot is constantly roaming so I'm using micro limit switches to keep it away from obstacles, walls, and stairs. Its a very simple design, but I am having trouble integrating two things.
    I hacked the RC car about and just left the motors and wheels. The RC car has two motors, one is for propulsion and the other for steering. I am able to get the car to go forward no problem with a relay switching circuit (basically its in two states, if its not in reverse the RC car is going forward. I want to do something similar to the steering motor, but I need a neutral state, otherwise it will always be turning one direction if it isn't turning the other direction. This is where I am having my first problem. The picture attached shows the working relay circuit for propulsion using a DPDT relay, but I need to figure out how to modify the circuit to have a neutral position so that the car can go straight.
    Secondly I am having trouble with the power supply. My original plan was to have a voltage divider and drop 18V across 3 resistors. I had a 10K Ohm and two 2900 Ohm resistors to give me ~12V, ~3V, and ~3V. These values are more than enough to trip the relay and give juice to the motors. I tested it on a bench power supply and got exactly what I needed, but then when I connected two 9V batteries, I had just enough to trip the relay and had nothing across the last two resistors. Is there a way to fix this? Should I use a different type of battery for my power supply?
    Thank you for sticking this out and reading my long winded question. Any help on one or both or my problem would be greatly appreciated. Once again I am looking forward to participating more on this forum. If you have questions or need me to clarify something please let me know

    -Creyta
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,503
    2,367
    First, using resistor to drop voltage for supplies is very wasteful.
    Usually RC motor and ESC's are one voltage.
    Also remember you are using a PLC for logic, so keep the majority of the logic internal, as little external logic as possible, this is the general idea behind a PLC.
    e.g. any relay interlocking can be done in the PLC.
    RC servo's usually receive a command and wait until the feedback component says it is in position, pot etc, unless bang-bang, two position servo's?
    To keep the PLC powered if portable you will need a decent supply, if a 24v PLC, a couple of Gell-Cells could be used.
    Max.
     
  3. creyta

    Thread Starter New Member

    Apr 5, 2014
    2
    0
    I forgot to mention that the PLC will have its own separate power supply. The issue is with switching polarity right now. How would I program the PLC to internally switch the polarity of the motor for left/right and have a neutral?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,503
    2,367
    Two DPST relays, one FWD one REV both OFF for neutral.
    Weren't there controller for the motors on the original car?
    It would have been worth looking into using these, if so.
    Are you using proportional control? if so you would need some feedback of some kind.
    Max.
     
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