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Servo Movement At Power Up Before Initialization

colporteur

I am experiencing a problem with the servos operations on this project that members helped me develop.

When the Mega is powered on the servo track to a position before going to the initialized position. The power on position for the servo is outside the range of motion for the animation. The movement has over extended some mechanical connections and caused breakage.

I went in search of a solution and found this post. It doesn't solve the problem but the discussion is an example of what I am experiencing. One of the suggestions is to relay control power. Only apply power to the servo after full initialization thereby avoiding the issue.

I'm curious if anyone has experienced a servo movement issue on power up and how they may have solved it.

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  • 0

    Hi Sean,

    The power on position is really the position with no PWM applied. I can think of four potential solutions.

    (a) Ideally there would be a mechanical solution (i.e. as Frank mentions, to try to have that 'no PWM' position to be a valid position within the desired range of motion). If that's not currently feasible (e.g. the mechanical work is complete and a software or electronic solution is needed) then the following two options could be worth considering:

    (b)initialize the PWM as early as possible within the Arduino code, to any value within the valid range. By doing it as soon as possible, there's less time for the servo to jump to the invalid position. However, there's a good chance it might be impossible to make this option (b) work, because of the time that the Arduino may take resetting and then running the user code.

    (c) have power control over the servos electronically, so that you can switch off the servo power, apply the PWM, and then switch on the servo power. An easy way to do that is to have a single relay, controlled by the Arduino, and have it wired to power all the servos when the relay is energized. The relay would be programmed in the Arduino to be energized after the PWM is initialized and within valid ranges. The relay could be replaced with a MOSFET if desired.

    A slightly luxury variation of (c) is to have a separate MOSFET per servo, i.e. ability to power off/on each servo separately. There are use-cases for that, but you may not need that level of servo power control granularity for the situation that you mention, and besides, it is a lot of circuitry to now retrofit into the project. But for future designs, that could be worth considering.

    (d) A manual version of (c) could be to simply have a switch to the servo power, so that you can apply it after the Arduino is running. But if the user forgets to toggle it off afterwards, then things will break when everything is next powered up. Perhaps it is solvable using warning messages on the power switches, with the power-on procedure labelled. Better yet, you could even use a rotary switch with three positions. The first position being all off. The next position powers on the Arduino, and the third position also powers on the servos. That way, the user has no choice but to power it up or down in the correct sequence. That would solve things with no code modifications, and no electronic modifications beyond obtaining a three-position switch with at least two gangs. (You'd still need a label on the mains plug to inform the user to ensure the switch is in the fully off position before plugging in).

    image

  • 0 in reply to shabaz

    I am working on a combination of your suggestions.

    My first attempt is (a). I wish I had known of the issue before assembly, it would have been easier to implement.

    I'm looking at (c) all in with servos. I have a server power bus that is easily switched. 

    It is two days away from a train show to discover the issue. I  didn't recognzie the issue during prototyping. The layout is finished and I am just getting to install everything in its final resting place to discover AH CRAP!

    I am at the site this afternoon to start working the problem.

    Really appreciate the insight. It is my problem but reaching out makes me feel like I am not alone to solve.

  • 0 in reply to colporteur

    Could you use a weak spring link between the servo and the animals so that the spring gives outside of limits.

    Alternatively modify the linkage in such a way that the home position moves to within the allowable limits of movement.  

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  • 0 in reply to beacon_dave

    I successfully adjusted the physical connection on one servo today after determining "horn initialization position" I was fortunate since the horn works from 90 to 0. 90 being the start and the initialization position worked out great. I have started another post within this post looking at some discovery. I don't want to confuse the issue. 

    I do like the spring idea. Going to investigate that.

  • 0 in reply to colporteur

    The likes of a cam, bell crank, or four bar linkage can often be used to limit the physical travel extent independently from that of the driving motor.

    However, if you reduce the length of the servo horn (radius), you may be able to reduce the length of travel at the animal to within safe operational limits.

    Do you have a photo / sketch that shows the actual servo to animal set up ?