Servo questions...

Need some quick assistance with servos please (we’re behind schedule)…
The docs say that we can use as many servos as we need, each with a maximum spec of 55oz-in and 100rpm (0.1 sec/60 deg) at 6V. These are the specs on the servos listed as possibilities…
HS-322HD = 51oz-in, 0.15 sec/60°
HS-325HB = 51oz-in, 0.15 sec/60°
HS-475HB = 76 oz-in, 0.18 sec/60°

Speed is a secondary consideration in my application, so I’m looking for more torque…

  • Since the FTC servo is higher than 55oz-in, I assume it’s okay since it’s slower. So can I legally use a 110oz-in servo if it’s 0.2sec/60deg (half the speed)?

  • Anyone have a better (legal) option. I’ve not found a better candidate (yet) at TowerHobbies, etc.

  • Since we can use as many as we need, can I double-up servos (mechanically parallel them) to get more torque? Or triple up? Yeah, I would have to work out the specifics, but for now I’m cuious in case it gets to that.

  • Anyone know of a gear I can bolt onto a servo, without machining a custom part? Alternate option is to screw a plastic gear to a servo horn, but that does not seem like it would be strong.

Thanks!

Nope. The FTC servos are explicitly allowed separately from the rest of the servos. Any servo that isn’t FTC would have to meet the specs. ( != )

  • Anyone have a better (legal) option. I’ve not found a better candidate (yet) at TowerHobbies, etc.
  • Since we can use as many as we need, can I double-up servos (mechanically parallel them) to get more torque? Or triple up? Yeah, I would have to work out the specifics, but for now I’m cuious in case it gets to that.
    I see no rules against that. If there had been, then at least one team would have been jumped on about a week ago when they posted a picture of three that were obviously designed to work together.

Sorry, I don’t know of any other options right now.

  • Anyone know of a gear I can bolt onto a servo, without machining a custom part? Alternate option is to screw a plastic gear to a servo horn, but that does not seem like it would be strong.

Thanks!
I don’t think there is such a thing… Though you could ask your local hobby store.

Here is a post on the FIRST Q&A forum that has some information on it concerning use of multiple servos, and some potential problems connecting them mechanically. Hope this helps a bit. :slight_smile:

This was posted on FIRST Q&A, but referred by FIRST Q&A to Control System Forum. It is therefore not official, and should be used for informational purposes only.

http://forums.usfirst.org/showthread.php?p=26616#post26616

Before anyone else catches the error, I just noticed that the current inspection checklist does not technically permit FTC servos since they’re rated to supply torque > 55 oz-in. I need to change the checklist to accommodate FTC servos as an exception to the torque spec.

I was trying to keep the checklist simple for the inspectors but I apparently went a bit overboard. It will be changed to comply with the Section 8 rule that permits an unlimited number of FTC servos in addition to any servo < 55 oz-in and 100 rpm at 6Vdc (which includes the two in the KoP).

Russ

Thanks for the quick reply – gotta place an overnight order real soon here.

That’s clarifies that. Thx.

RE: doubling-up servos:

I see no rules against that. If there had been, then at least one team would have been jumped on about a week ago when they posted a picture of three that were obviously designed to work together.

Then that will probably be my solution… I can think of about 3 ways to do this (1) back-to-back with the signal reversed to one of them (2) dual control links from each servo’s horn to the same fulcrum point on the controlled arm, or (3) gear on the controlled arm with servos pushing that gear at different points around the circumference of the controlled gear.

I can’t find that post with the other team’s implementation of this, so if you know any keywords to find that post, it would be much appreciated.

Much thanks!
-Neil.

http://www.chiefdelphi.com/forums/showthread.php?t=73780

Looks like they used a custom unit, though. By the looks, those gears were made with a 3D printer of some form.

you can also gang them together with plastic sprockets & #25 chain. or toothed belts.