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I need a very fast servo — Parallax Forums

I need a very fast servo

RL600RL600 Posts: 40
edited 2011-08-17 13:30 in Robotics
Hello,

I have a boe-bot. But the normal continious rotation servo is for me a little to slow. Do someone
got an idea where I can buy an faster servo. I know how to transform a standard servo into an continious rotation. But I think it's better to buy an continious servo. Has somebody an idea.

Many thanks for looking,
RL600

Comments

  • ercoerco Posts: 20,256
    edited 2011-08-15 14:25
    You can compare relative servo speeds by looking at their timing specs to see how fast they rotate 60 degrees: http://www.ehow.com/list_7633254_futaba-servo-specs.html

    My Vigor VS-11 says 0.19 sec/60 degrees at 6V, but after conversion to CR, its peak RPM is only about 60, running on 7.2V. I don't think CR servos go much faster than that. DC motors will.

    All servos obviously operate somewhat faster at higher voltages; 6V is faster than 4.8V. 7.2V is faster, but pushing it.
  • Rick BrooksRick Brooks Posts: 67
    edited 2011-08-15 15:46
    A pair of Futaba S9254 will get a BOEBot over 25 inches per second.
    Rick
  • RL600RL600 Posts: 40
    edited 2011-08-15 22:49
    Many thanks for your reactions,
    The next problem is I don't know where I must buy these things. I can put
    It on Google but I don't know which website is good or not. And the website
    must also be sell them servos outside Amerika.

    RL600
  • ercoerco Posts: 20,256
    edited 2011-08-15 22:57
    Rick Brooks wrote: »
    A pair of Futaba S9254 will get a BOEBot over 25 inches per second.
    Rick

    Rick: That's a $120 digital servo at http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXFZP1&P=7

    Have you modified one for CR and tested? Love to see that!
  • Rick BrooksRick Brooks Posts: 67
    edited 2011-08-16 14:55
    RL600...see erco's link to Tower Hobbies. That is where I purchased them.

    erco....Our group has four high school students running them now. Our major focus is on the National Robotics Challenge in Marion, Ohio. This year Marie took first in the tactile maze and Weilin took first in the non-tactile maze. See videos below:
    http://brooksbots.com/Movies/Ohio%20Tech%202011/Maze/Marie.wmv
    http://brooksbots.com/Movies/Ohio%20Tech%202011/Maze/Weilin.wmv
    I do expect to see more teams with them next year.
    They are easily modified for continuous rotation.
    The kids complain that there is not a pot that allows them to center the servo so that the off position matches "750". They don't get much sympathy from me on this one.
    I should note that the servo dead band is very narrow, so you may not be able to get them to stop with a BS2. Too bad Futaba doesn't provide user programming for their servos.
    Also, it is not recommended that the servos be run over 4.8 volts, so we run them through the voltage regulator on the BOE. Since Weilins second run was fast, she tried the third run on battery voltage to the servos. That is where the BOEBot tried to climb the wall.
    Make sure that the students can set up the robot for a maze with standard servos before switching to fast servos. The faster BOEBots are an order of magnitude harder to set up. We have maze contests before Christmas and the winners get the fast servos.
    My BOEBots all have servos running on external H Bridges and much higher voltages and they are not as fast as the S9254 equipped BOEBots.
    We are trying a Summer project this year. The kids are making a RoboMagellan robot out of a MadeUSA kit with a Propeller brain. I was hoping that it would be running in either autonomous or R/C mode by UPENE, but it won't happen unless one of them gets busy this weekend and does a lot of soldering and programming.
    Their Heavy Sumo is devastating.
    Rick
  • RL600RL600 Posts: 40
    edited 2011-08-16 23:36
    many thanks erico and rick,

    Now I can finally buy a fast servo!!!

    RL600
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-08-17 06:35
    @Rick, It's very interesting to hear about these fast servos being converted to CR. I had thought digital servos don't make very good CR servos.

    @RL600, These heli tail servos are an expensive way to gain speed. You might want to look into using a different type of motor with a H-bridge.

    Duane
  • RL600RL600 Posts: 40
    edited 2011-08-17 12:22
    What do you mean with H-bridge

    RL600
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-08-17 13:15
    A H-bridge is used to control motors. There are lots of options.

    I like to use these for relatively small motors.

    There are options for larger motors too.

    There are lots of tutorials on the web about using H-bridges.

    Servos are often used because they are so easy to control. If you want speed and/or power you'll spend a lot less money by using a H-bridge with a motor than trying to find fast/powerful servos.

    Duane
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-08-17 13:30
    I failed to include some Parallax options.

    L293D chips can be used with small motors. (I like the SN754410 chips better(see second link in my previous post).)

    Parallax's HB-25 can be controlled with the same pulses used to control a servo. They are probably overkill unless you have a very powerful motor.

    I believe these are the motors used on the Stingray. These look like a really nice set of motor/gearbox/wheels. You might be able to control them with two SN754410 chips stacked on top of each other.

    H-bridges can also be made from transistors. I think PJ Allen has a thread in the Projects forum about designing a H-bridge.

    If you want to learn about robots, learn how to use H-bridges.

    Duane
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