[FTC]: dual synchronized servos or geared up winch servo?

[BSV]

The kids have ball dumper that is controlled by a quarter-scale HS-785HB servo. Apparently the dumper is a little heavy, as the spline on the servo stripped (it may have not been properly seated, which may have contributed).

With a competition coming up this weekend and the girls also on an FRC team, there isn't time for a full redesign of something that was working pretty well. There seems to be two options we've discussed that might work:

(1) buy two HS-755HB servos, put them on opposite sides of the ball dumper, and synchronize them.

(2) buy a HS-785HB winch servo (3.5 rotations) and gear it up.


On #1, we are afraid that if they don't sync perfectly there will be problems (they may fight each other).

On #2, we are afraid of gear slippage because it need some amount of precision.

Anyone have experience with either option? Or other thoughts? Thanks!

[ehochstein]

I would suggest option #1 - Provided they are programmed correctly, you shouldn't have any issue syncing them up.

[wgardner]

I vote for #1 also.

Another thing that could work depending on if your design has room is to add a counter-weight on the opposite side of the ball dumper. That would decrease the overall torque that the servo would have to provide.

[BSV]

Thanks for the quick replies. We went with #1, and the servos should arrive by Tuesday evening. The dumper/loader is already pretty well balanced (it tilts at the middle, and has standard servos at both ends to load and unload, but those contribute to the weight).

The backstory on this is that the dumper was working okay with a lower-torque standard servo, but it was pretty strained and seemed like it would break at any time, so we got one with 4 times the torque figuring that it would have no problem. However, they mounted it with a top-mount bracket and there were screws on the back of the servo horn that were rubbing against the mount each time it turned. Additionally, I think it was only pressed halfway down on the spline, so it was easier to strip half the spline than the whole spline. I guess those top-mount Tetrix servo brackets aren't meant for use with actual servos because the clearance isn't large enough to attach anything to them! Ours now has a nice, larger hole to prevent rubbing.

(We might be able to get away with one, given that fix, but two seems so much safer for a critical component of the robot.)

[Al Skierkiewicz]

BSV,
I would not be surprised if the servos do not link up. You will need some kind of mechanical device to link them together and remember that you can't simply turn one around and switch the power lead to get it to go in the other direction as you could with a motor. Some kind of spring linkage might work if they do not seem to follow each other close enough for your application.

[BSV]

Originally Posted by Al Skierkiewicz

BSV,
I would not be surprised if the servos do not link up. You will need some kind of mechanical device to link them together and remember that you can't simply turn one around and switch the power lead to get it to go in the other direction as you could with a motor. Some kind of spring linkage might work if they do not seem to follow each other close enough for your application.

Thanks for the info. We did order three of them to improve our chances that we could get two to act as mirror images of each other. Our previous experience is with Lego motors, and those frequently have different startup time lag and we try to find two of them that share the same lag. Of course, those are plastic gears so I expected the Karbonite ones to be better.

We are thinking that "servo2 = 255 - servo1" given that both of them start at the same (mirrored) position, and that we'd have to always make sure they are initialized properly or we will run into issues where they are fighting each other.

[Al Skierkiewicz]

I would power up the robot with the two servos not connected to each other. Then I would command them to move and observe the reaction of each compared to the other. Observe jitter when the servo reaches commanded position. A flex coupling might be all that is needed.

[BSV]

Originally Posted by Al Skierkiewicz

I would power up the robot with the two servos not connected to each other. Then I would command them to move and observe the reaction of each compared to the other. Observe jitter when the servo reaches commanded position. A flex coupling might be all that is needed.

Thank you. Do you have any recommendations on a flex coupling that will work with Tetrix (or can be modified to work)?

[Al Skierkiewicz]

I would have to search around. I would check with hobby outlets and see what they might have.

[emmell]

Maybe how about using two LEGO motors (ports B and C) as they try to naturally sync when you apply the same power to them (PID control kept by the NXT). If you're using ROBOTC, I think they keep it that way as well in the firmware as does the LabVIEW version does.

[BSV]

So maybe a block of rubber with holes punched into it would work?

Originally Posted by emmell

Maybe how about using two LEGO motors (ports B and C) as they try to naturally sync when you apply the same power to them (PID control kept by the NXT). If you're using ROBOTC, I think they keep it that way as well in the firmware as does the LabVIEW version does.

We thought about this but the cables to the motors would be required to be longer than the maximum allowed cable length, plus probably not enough torque. The loader is out at the end of an arm that is attached to a linear lift.

We'll run some tests on the servos when they come in. May have to improvise a little.

Thanks!

[BSV]

Got the servos today, and they aren't an exact match, but a "sleep(10)" delay between start times syncs them perfectly.

The loading tray that both of these support also has some flexibility, and we did find some flexible couplings in the Tetrix supply, so I think we are good to go.


Thanks to everyone for your feedback.

[ftelliott]

We had a similar problem this year. The two opposing servos will always be fighting each other to some degree. Being careful with setting their synhcronized positions using the method described previously and then having a rigid cross linkage with some a flexible coupling is sound practice. The 7** series have twice the torque of the 4** series so depending on your situation that may have been sufficient.

We use the ServoCity Servo Blocks anywhere we can in place of the TETRIX servo mounts. Support for the servo shaft and careful alignment are critical in all servo uses. I've found my kids do not have the degree of close observation and fine motor skills needed to install servos unless I coach them through this very carefully and then double check their work. They think, "Hey, its on there!" is good enough. That usually works for motors but rarely for servos.