Tetrix Servos 2013

[immukage]

Hi everyone.

So my team is looking to lift about 1Kg of weight using a 4 or 6 bar linkage system through a Tetrix servo (the ones that come in the kit). However, we just can't seem to figure out how strong these servos are in relation to other servos. Does anyone know if we can lift 1Kg with these servos? Using DC Motors is not an option, however an alternative is NXT servos.

All help much appreciated!

- Team 7767

[CENTURION]

Hello, welcome to Chief Delphi! Good to see more FTC people on here!

According to Pitsco, the HiTechnic standard-scale servos have up to 84 oz/in. of torque.

And the HiTechnic quarter-scale servos have up to 183 oz/in.

You can do the math yourself if you'd like (I'm not very knowledgeable with physics honestly), but my instinct would be to say that the standard-scale servo couldn't do it, especially with the weight out at the end of a four-bar (which would act as a lever against it).

I don't even know that the quarter-scale could do it. It might be able to pull it off if you run it through at worm gear, although it would move more slowly.

Hobby servos aren't generally made for moving loads like that, they're better for gripping claws, latches, and small, low-force applications of that nature.

If you'd like to test it, make a quick prototype. Throw a bar of aluminum about the length of the proposed 4-bar on it, and stick the weight on the end of it. (keep in mind that the servo has to lift the four-bar mechanism along with the weight).

Is this the robot's primary mechanism? Why aren't you able to use the Tetrix DC motors?

[Bryce Paputa]

Yeah, one of these servos is not going to cut it. Lets assume that the 4 bar linkage is 6 inches long, and that 2kg~5lb, that would require 30lb-in, or 480oz-in, of torque to hold, plus the weight of the mechanism, plus the torque to accelerate it, plus inefficiencies like friction. This is well above the stall torque, you'd need some where between 16:1 and 4:1 of reduction depending on the servo used, which would probably mean that you would need a continuous rotation servo which has even less torque (not to mention that it would be insanely slow). A NXT motor might be a tiny bit better, but not much. Why can't a DC motor be used?

[immukage]

Thanks for the replies guys. What if we used 2 of these servos? Still not gonna cut it?

As for DC Motors, all motor controllers are out of stock atm and won't be in stock until after our competition so we're trying to make do with what we have.

[Bryce Paputa]

Maybe, you'd still need a reduction, but it would be less. There are other teams near you that you could possibly borrow one from, right? If not, the Michigan FTC season just "ended" and you might be able to get one from here.

[magnets]

If you put a heavy weight on the end of an arm on a servo, and you set an angle, the servo will overshoot past the setpoint, and then oscillate and never settle. For powering heavy stuff, try to avoid servos. Instead, you could try to make it easier for the servo with some surgical tubing to help pull the heavy arm up, so the servo has an equal load lifting and putting down the box. Again, you will need to do the math to see if it's possible.