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Servo Speed Disrepancy — Parallax Forums

Servo Speed Disrepancy

Tommy BotTommy Bot Posts: 60
edited 2006-10-27 18:48 in Robotics
hop.gifHEY!

I started working on my Boe Bot and got to the part where I test the servos. When I·pulsout 850 to one servo and 650 to the other (max speed different direction), then flip one servo upside down and hold it over the other, the speed of the two servos are slightly different.
freaked.gif·In no time one passes up the other.

Q-1. Is this common?

Q-2. If it is common, do I start playing with the numbers until I find the sweet spot?

Q-3. If it is not common, should I get another servo?

I did not have the time last night to test, but I suspect that one servo is running slow (as opposed to fast of course). I'll check this weekend to see if I get close to the 60 RPM on either, It didn't seem like the fast one went that fast.

Q. Could this be due to having a resistor/LED wired to the same pin (as part of the test circuitry)?

It happens·that the one that is rotating slowest came in with a broken servo horn. I thought nothing of it at first, knowing that I would need to remove them for contrustion of the Bot.

I sure don't want to have spend to much time fudging things to get them to work as they ought to. I don't know if the problem in linear or not, but I suspect that a custom speed table would have to be generated for straight runs and turns.

Advice?

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(Frequently heard from other's)

Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?

·

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2006-10-27 14:07
    1) Yes. Servos were originally developed for R/C model use where precise position was not an issue since visual feedback of the effect of movement was used. Continuous motion servos as is used for the BOE-BOT wheels are a further modification that removes the position sensor.
    2) Yes. Alternatively, there is a crude adjustment in the servo designed more to find the "stop" point and adjust that, but it may help bring the two servos closer together in speed. The Robotics manual talks about this adjustment.
    3) It's common and you should be able to reduce it by playing with the adjustments on the two servos
    4) No. The servo responds to a digital signal (anything above a certain voltage) and the resistor/LED shouldn't bring this below the level needed.

    A custom speed table might help somewhat, but another problem will be slippage of the wheels and mechanical load. I assume you're trying to determine the position of the BOE-BOT from its intended movement. The servos are just not that accurate. Parallax sells a kit to add wheel hole sensors so that you can compute the distance travelled based on the rotation of the wheels. That helps a lot, but there will still be some slippage of the wheels on the floor and you'll need some kind of recalibration periodically, possibly using the HM55B compass or something like that.
  • Tommy BotTommy Bot Posts: 60
    edited 2006-10-27 14:54
    So if I play with the dead band of the zero POT, it could bring them closer? Makes sense.

    If I were to use them as is, there would be no way to run a straight line without encoders.

    I already have the encoder kit, but did not intent to apply it until down the road. I just wanted to get the most accuracy out of it from the get-go, everything else added after will go much smoother. and will aid in the overall repeatability of the various aspects of the Bot.

    Thanks, it's a starting point.

    STAFF
    Ahhh to have the benefit of a box full of servos that could reletively be speed matched.... elbow elbow, wink wink, hint hint. (That would be a step in the right direction and make adjustments on the end user end less painful.)

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    (Frequently heard from other's)

    Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?

    ·
  • John R.John R. Posts: 1,376
    edited 2006-10-27 15:04
    Besides playing with the 'dead band' on the pot, you could also make adjustments to the numbers used for "neutral" on each servo.

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    John R.

    8 + 8 = 10
  • Tommy BotTommy Bot Posts: 60
    edited 2006-10-27 15:16
    John R. said...
    Besides playing with the 'dead band' on the pot, you could also make adjustments to the numbers used for "neutral" on each servo.

    How can this be applied toward syncronizing max speed on both wheels?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    (Frequently heard from other's)

    Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?

    ·
  • CCraigCCraig Posts: 163
    edited 2006-10-27 17:07
    I had the same problem to a lesser extent. You'll have to fine tune them. Look in the Boe-Bot manual Chapter 3, page 115.

    HTH, Chris
  • John R.John R. Posts: 1,376
    edited 2006-10-27 17:17
    I concept do the following:

    Define a variable or constant for each wheel that is the center point (neutral).

    Then in stead of a "hard coded" set point for forward and revers, use some math and set the speeds to something like LeftCenterPoint + 10 and RightCenterPoint - 10.

    If you find the servo response is not identical, then you could establish the following (numbers are ficticious examples):

    RightServoCenter = 755
    RightServoFullForward = 860
    RightServoFullReverse = 648

    LeftServoCenter = 748
    LeftServoFullForward = 639
    LeftServoFullReverse = 850

    Then when you want to move x% of full speed forward you'd set the speed like this:

    RightServoSpeed = ((RightServoFullForward - RightServoCenter) * x) + RightServoCenter
    LeftServoSpeed = LeftServoCenter - ((LeftServoCenter - LeftServoFullForward) * x)

    The numbers used for RightServoCenter, etc. would need to be determined by testing.

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    John R.

    8 + 8 = 10
  • Tommy BotTommy Bot Posts: 60
    edited 2006-10-27 18:48
    Thanks guys, all good advice and all may be employed. I'll need to see how close (or far off) it is after adjusting the centers again.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    (Frequently heard from other's)

    Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?

    ·
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