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Servo Speed — Parallax Forums

Servo Speed

Robert25Robert25 Posts: 53
edited 2006-03-17 17:57 in Robotics
Hi folks

Quick question about the speed of servos.· Is there any way to increase the rotational speed of an unmodified (not continuous rotation)·servo?· I can sacrifice torque for speed.· There might be a possible link to the EFX group as my application is animatronic in nature but thought it could post in robotics as well.

Bob

Post Edited By Moderator (Jon Williams (Parallax)) : 3/14/2006 6:49:42 PM GMT

Comments

  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-03-14 20:43
    Robert -

    There are only two ways I know of to increase the rotational speed of an R/C servo. One is a bit dangerous, and the other is not.

    You can increase the applied voltage, but you stand the risk of burning out the servo, or causing short life. The other method is to use external gearing. Here is a web site which has external gears for servos:
    http://www.servocity.com/html/gears___sprockets.html

    Regards,

    Bruce Bates

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  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-03-15 14:11
    The third way,use bigger wheels.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-03-15 14:27
    George -

    Bigger WHEELS in an animatronics project? Humm.

    Bruce

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  • Tom WalkerTom Walker Posts: 509
    edited 2006-03-15 18:29
    ...and, technically, bigger wheels might increase the speed of the movement, but it would not increase the speed of the servo output as Bruce's suggestions would. In fact, larger wheels would probably work to slow down the servo...at least, at startup and slowdown...rotational momentum an all...unless your bigger wheels were lighter than the small wheels...

    Physics is wonderful stuff [noparse]:)[/noparse]

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    Truly Understand the Fundamentals and the Path will be so much easier...
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-15 19:59
    I would somewhat disagree with the concept that a bigger wheel will not increase speed in this capacity.· If you have a small horn (even a round one, aka wheel)·on the servo moving a shaft, a bigger horn with the same shaft connected to the outside will, in fact move faster since the outer edge of the horn will travel at a faster rate as it is further from the center hub of the servo.· It really depends on how the servo is connected to what is moving, and how it is moving it.· See the attachment.· The servo in the left diagram moves at the same speed as the one on the right, but the shaft on the right moves faster, albeit with less torque than before.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
    servos.JPG
    413 x 443 - 15K
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-03-15 20:29
    Ah. The reciprocating shaft, or rod, going up and down, will move further and faster on the right instance than on the left one, that is true. The Servo Shaft (that the wheel is mounted on) will turn at the same speed in both instances.

    And yes, there's no such thing as a free lunch, so the left instance shaft will move with more FORCE than the right instance shaft. In many cases, that's not a problem.

    So, a servo has a certain torque it can put out. You can use that torque close to the servo shaft, and get more force over a shorter displacement, or use that torque further away, and get a larger displacement (and faster movement) with less force. In either case, the torque remains constant.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-15 21:09
    Allan,

    ·· That is correct.· I remember once having this discussion with some friends when we were younger.· They wanted to make their dirt-bike go faster by putting a larger sprocket on the back.· I told them a larger sprocket on the wheel side would make it go slower, but if they could increase the drive sprocket it would go faster.· Never could get my point across...

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-03-15 21:17
    Oh, it just occurred to me -- I recently was trying to develop a course in robot building for 6th graders, and we used Mindstorms. The nice thing about Mindstorms is how easy it is to put together different gear ratios, and explore the relationship of torque, speed, and step-up vs step-down gearing systems.

    And while we finished the semester, I'm still not entirely happy with the course-work. But it's on my to-do list.
  • Tom WalkerTom Walker Posts: 509
    edited 2006-03-16 15:07
    Chris,
    Just as an exercise in pickiness (I have not yet had my morning tea...and, besides, learned discussion/debate just tends to wander along "interesting" paths, or at least provides additional information), I was responding to Robert25's initial inquiry about increasing, specifically, the servo's rotational speed. Bruce's suggestion of a higher voltage would indeed effect this (effect used correctly <g>). Your and Bruce's suggestion of gearing would produce speedier output (with the necessary loss of torque), but not increase the servo's speed.

    </pickiness>

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    Truly Understand the Fundamentals and the Path will be so much easier...
  • Robert25Robert25 Posts: 53
    edited 2006-03-16 16:01
    Hi folks

    Interesting discussusion.· The larger servo horn might work in this instance.· The application, as I had indicated in the original post, was in animatronics.· Something that would work a movable jaw or eye cover etc.· Both of these don't require a lot of torque.· I have been surfing for various servos and looking at the speed specs.· Since I am relatively new to this hobby I might need some help in interpretation.· They use a·· sec/60 degree spec.· I assue it is the time to rotate 60 degrees.·· That being said I found the standard servos usually fall in the 0.2 range.··· I have also found servos that fall in the 0.07 to 0.1 range; presumably quicker.

    For the application, think of how fast your eye blinks.· Of course close or an approximation is ok.·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-16 16:19
    Tom,

    ·· In this application I think "speed" is a subjective term as it applies to what is being done.· While technically the shaft is not rotating any faster the fact that I can make a shaft move faster by simply increasing the size of the servo horn proves my point.· =)· And the servo doesn't have to suffer for it.· I was just offering one more possibly solution that may or may not be more attractive than pumping more voltage into the servo.· Besides, I have noted that on servos more voltage means little speed increase.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-03-17 13:27
    Chris Savage (Parallax) said...

    Besides, I have noted that on servos more voltage means little speed increase.

    very true, since the motors driving servos are so geared down, you have to substantial increase the speed in order to get an appreciable absolute change (relative change is constant but % compared to absolute is reduced because of the gears).

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  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-03-17 17:13
    Folks -

    The speed of an R/C servo is DIRECTLY proportional to the applied voltage, and depends also on the manufacturer. I find no surprise at all in those facts. If were we talking about PM DC motors only, and their respective characteristics, there probably wouldn't even be a discussion. Motors are wound by different manufacturers differently, as well as having a different number of poles·and DC motors used within R/C servos may also be geared differently.

    I spent about 2 weeks making inquires of various manufacturers, about 4 years ago, to make this very determination. Sadly, there is no standard measure. Some manufacturers offer two sets of data, one at 4.8 VDC, and the other at 6.0 VDC. We all know there are plenty of 7.2 VDC battery packs out there, so this is obviously a "nominal" voltage. Rarely did I see any manufacturer state a MAXIMUM PERMITTED voltage, although, when asked directly, they did have some particular, unpublished voltage in mind.

    The following table is quite incomplete, and DOES NOT represent similar servos, necessarily. All it is intended to show is the variety of data supplied by various manufacturers. Smaller, more common or "standard" servos were chosen for inclusion. Some of the servo model numbers are indicated, some are not. Some manufacturers also did not supply the number of degrees of travel, or didn't indicate the applied voltage. Those that did, have the data included.

    /code

    ·Mfgr········· Model·· Voltage 1······ Speed····· Voltage 2······ Speed
    ··············· Number···· D.C.····· Sec/Degrees···· D.C.····· Sec/Degrees

    Hitec········ HS-422····· 4.8··········· .21/60········6.0········· .16/60

    Futaba····· S-148······· 4.8············ .23/60········6.0·········· .19/60

    FMADirect················· 4.8············ .23/??········6.0·········· .20/??

    Airtronics···················································· 6.0 ········· .14/??

    Associated················ ?.?············ .19/60

    Hobbico···················· 4.8············.19/60······· 6.0·········· .15/60

    Multiplex··················· 4.8············.18/??······ · 6.0········· .15/??

    Tower······················ ?.?·············.22/60
    Hobbies

    Traxxas···················· ?.?············ .22/60

    code/

    As you can see, there is a good bit·of varience within some fairly tight ranges. Whether it matters, is·wholly dependant on the application.

    Regards,

    Bruce Bates

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-17 17:57
    Bruce Bates said...(trimmed)
    Folks -
    The speed of an R/C servo is DIRECTLY proportional to the applied voltage, and depends also on the manufacturer. I find no surprise at all in those facts. If were we talking about PM DC motors only, and their respective
    Most of the smaller motors have a maximum practical limit to speed that isn't going to change by applying more voltage, but WILL permanently damage the motor and/or servo control circuitry.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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