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Servo Refresh Rate-does it really help torque? — Parallax Forums

Servo Refresh Rate-does it really help torque?

bulkheadbulkhead Posts: 405
edited 2005-12-15 22:28 in Robotics
I guess I could just make some sort of a test, but I wanted to get some expert advice on this. How significant is the refresh rate of a servo to its torque output? For example, if a servo is rated for 50oz/in on standard RC equipment (which I believe, has a standard refresh rate), then by how much could that refresh rate be increased using a stamp and how much could the torque be increased by?

On that note, I usually use a refresh rate of "2304" with the Javelin's PWM class. What is the largest value that it could have, and what (if anything) is wrong with having it refresh more often?

Thanks

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2005-12-13 14:05
    The repitition of the positioning pulse merely tells the Servo comparator hardware where to move the output shaft. If you don't refresh often enough (50 mSec), then the Servo comparator has to 'relax' and ignore the position of the output shaft. Since it's not being driven at that point, the shaft can be turned by external loads.

    However, if you DO refresh as specified (20 to 50 mSec) then the Servo drives the shaft, with the full Servo torque, to the position you've specified. Shortening the refresh period below 50 mSec does NOT increase this torque.

    So, when a Servo is rated at 50 oz/in, that's the maximum torque that Servo can put out. Changing the control signal won't increase this value.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-12-14 15:45
    Hello,

    ·· I don't think it's·a matter of improving your torque...I think the real thing is that servos require refresh every 20 - 50 mS...Somewhere in there is the sweet-spot for your servos.· The further you deviate from that the worse your torque is going to get.

    ·· The same analogy works when centering/calibrating servos on a BOE-Bot or SumoBot.· When the Boy veers to one side you don't speed up a servo to compensate, you slow the other down.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • bulkheadbulkhead Posts: 405
    edited 2005-12-15 05:27
    Ok, well, the servos that I'm having trouble with are the parallax continuous rotation servos. What refresh rate will work best with them? Currently, I'm running two of them with 3"wheels on a 6lb carpet robot, and they can barely move it. I know this may be a large load, but is there any way to increase their power? Currently I'm running the servos on 6V with a 20ms refresh rate. If it could work out, I would prefer using these to using a larger geared motor, with an H-bridge, etc.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-12-15 22:28
    Not in that case...You're definately overloading the servos at 6lbs.· You might consider heavy duty servos with a higher torque rating.


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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