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MSR1 Power problems — Parallax Forums

MSR1 Power problems

JonathanRainesJonathanRaines Posts: 3
edited 2013-04-27 07:16 in Propeller 1
Hi,

I'm attempting to build a hexapod robot using the MSR1 (retired) board to run the servo motors. There are 3 servo motors in each leg so 18 total. I have used the jumpers to set all the pins to "V IN" setting, and am supplying 6V from a DC power supply. However, every time I turn on the propeller, it resets itself with in seconds and continues to do so. Is this the current limiter? If so what can I do to prevent the current to the chip spiking? I would like to run the chip on a separate 3.3V supply but I can't stop it from running of the 6V.

Any suggestions of what might be causing this/ how to solve it?

Many thanks,

Jon

Comments

  • whickerwhicker Posts: 749
    edited 2013-04-24 23:18
    Well, first of all, you said it was a 6V power supply, but how many watts or amps?

    Also 6V is the lowest that the board appears to run with. There's even mention of 6.5V in the first couple of paragraphs of the documentation.

    Does the board stay stable when you don't try turning on any servos?
    I guess what I'm asking is, is it resetting because the motors are trying to move, or would it still be resetting without the motors plugged in?
  • JonathanRainesJonathanRaines Posts: 3
    edited 2013-04-26 12:56
    It's fine without the servos, the amperage depends on the load of the servos which is related to how much force they are exerting to maintain their position. The servos can only take 6V
  • cavelambcavelamb Posts: 720
    edited 2013-04-26 13:09
    JonathanRaines wrote: »
    It's fine without the servos, the amperage depends on the load of the servos which is related to how much force they are exerting to maintain their position. The servos can only take 6V

    Well, it looks like you have two problems...

    If the servo voltage limit is 6 volts (and 5 preferred?) and the Prop regulator want to see 6.5?
    That put's you 1/2 volt below a guaranteed limit.

    The other is current, not voltage.
    When the current load increases, the voltage is being pulled down below the above limit.

    Figure 1 amp for each servo.
    They don't pull that all the time, as you noted.
    But if they EVER all run at the same time, they will all pull their own 1 amp - each.

    The best solution?
    Raise the Vin to 7.5 volts and add a 5 volt regulator (LM7805?) for each servo.

    But you HAVE to supply enough current to run all the do-dads.
    And that could be as high as 18 AMPS.....

    You will not get that from any wall wart I've ever met.
  • pedwardpedward Posts: 1,642
    edited 2013-04-26 15:16
    The MSR1 uses switching power supplies to make 3A a 3.3v and 3A at 5v and is rated for 6-15 volts.

    I don't see any big electrolytic capacitors on the board, so if your power supply isn't giving you 15A at 6v, you're probably sagging it down too much. Servos run all the time, they are closed loop analog devices and have an idle current of 15ma, and a no-load operating current of 140ma +/- 50ma.

    So, at least 270ma at 6V idle, 2520ma no-load moving, and probably around 9A peak load current.

    I'm guessing your power supply is too small and 18 servos is creating a lot of sag/noise on the power bus and causing the Propeller to brown out. The other option is that you are encountering the un-powered USB bug where the Propeller powers the USB chip via parasitics because the P30/P31 are being driven by software and the USB isn't plugged in.
  • JonathanRainesJonathanRaines Posts: 3
    edited 2013-04-26 15:36
    Thanks cavelamb, that was the problem, just measured the voltage across the propeller directly and it drops below 3.3V when the servos move. Adding a regulator to each servo will be a bit messy, I was using the board for the convenience of the pins having the same connectors as the servos. I may just go back to the drawing board.
  • pedwardpedward Posts: 1,642
    edited 2013-04-26 21:15
    Adding a 7805 is like using a roofing hammer to do body work on a car.

    The board already has 2 switchmode power supplies, your main power supply you are feeding it with is underpowered. Adding regulators won't fix that problem.

    You need a battery (what it was designed for) or a beefy 6v power supply.

    Like I said above, you need around a 10A surge capacity at 6V for the board and servos to work correctly. You should be using AT MINIMUM a 6v/3A power supply, and that won't be enough if you move a lot of servos at once.
  • cavelambcavelamb Posts: 720
    edited 2013-04-27 07:16
    JonathanRaines wrote: »
    Thanks cavelamb, that was the problem, just measured the voltage across the propeller directly and it drops below 3.3V when the servos move. Adding a regulator to each servo will be a bit messy, I was using the board for the convenience of the pins having the same connectors as the servos. I may just go back to the drawing board.

    Granted. That may be a bit of over-kill.
    Another possible way to go would be run the servos directly from a 6 volt source (but make it a much bigger source)
    and run the Propeller from a DIFFERENT source.
    If this is to be battery operated, use separate batteries? :)
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