Quick Question about Bigger Servos

[mkaa00x]

Hey guys!
okay So recently I've kinda been more active...

whatever anyways I was wondering, I wanna have a servo motor that can give me it's angle and I can move a wheel (out of 4 wheels) underneath a 120 pound robot.

Any suggestions on what to buy? I've got a program and everything for it and it has been tested on a smaller scale version, but if you were to recommend a servo that would move a wheel under a robot with ease, could you suggest which one to buy? (it's going to rotate the wheel on a XY-plane facing downwards flat to the robot)

[Gregor]

Make sure it doesn't exceed the specifications listed in R32 (2013 rules, may be different in 2014)

R32

The only motors and actuators permitted on 2013 FRC ROBOTS include the following:

COTS servos with a maximum power rating of 4W each at 6VDC

Per the Servo Industry,

Servo Max Power Rating = (Stall Torque) X (No Load Speed)

[EricH]

Is this for an FRC application? If it is, you're limited to 4W at 6VDC, calculated by stall torque*free speed, as of the 2013 rules. (That's not a whole lot, BTW... typically, a window motor would be used in this sort of application.)

Otherwise, there are a couple of questions that might be useful to know. For example, are we dealing with a Lunacy wheel or a Plaction wheel (or similar high/low traction wheels), as this may have implications on the torque needed to turn the wheel? What is the cost range you're looking for?

Now, that said, I think you might be looking in the range of what would be known as a Giant servo, something like what can be found at http://hitecrcd.com/products/servos/giant-servos. Some of the larger sport servos or boat servos may also be helpful, particularly high-torque variants (take a look around the Hitec site linked for some more servos that might be a better fit).

A small-scale test with a small servo won't necessarily produce the same results as a full-scale model, so proceed with caution.

One other thing. Maybe you can't find one servo that does the job... but two can. Don't be afraid to have 3-4 servos working together on the same pull if that's the best way you can think of to get the needed power.

[mkaa00x]

Okay.... thanks guys Ummm I'll discuss this with the other team members

[RyanCahoon]

Originally Posted by mkaa00x

it's going to rotate the wheel on a XY-plane facing downwards flat to the robot

Sounds like you're making a swerve drive (or similar).

The usual approach is to use a separate motor and rotation sensor (either a continuous rotation potentiometer or absolute encoder), then implement the control algorithm yourself on the cRIO. Check out CD media for some examples of the motors that other teams have used for steering.

[yash101]

A servo this big would be expensive if it would be allowed. However, like many people already stated above, it wouldn't be allowed in a competition. If you are trying to lift the robot, I would suggest a jack mechanism, to move the wheels up and down. It would be hard to design, but it is possible. Anyways, why would you like to switch drivetrains? I think you might be able to find a subsitute that has all the functions you need. Hopefully this was a little helpful.