Shifting with servos

[Micah Chetrit]

Hello,
our team has never used a shifting gearbox on our robots before, and we are considering using an andymark 2774 to have a shifting system similar to how RI3D released their shooting mechanism. We don't plan on having pnumatics on our robot because the only place we would use them would be on the shifter, so we thought about using a servo to actuate it.
Can a servo handle it well? I doubt that the force required is as simple as the force required to move a lightweight dog gear since there are other forces acting on the gear, so I don't know how much resistance to expect. Are there any issues you foresee with using a servo to release a launching mechanism?

[RonnieS]

Originally Posted by Micah Chetrit

Hello,
our team has never used a shifting gearbox on our robots before, and we are considering using an andymark 2774 to have a shifting system similar to how RI3D released their shooting mechanism. We don't plan on having pnumatics on our robot because the only place we would use them would be on the shifter, so we thought about using a servo to actuate it.
Can a servo handle it well? I doubt that the force required is as simple as the force required to move a lightweight dog gear since there are other forces acting on the gear, so I don't know how much resistance to expect. Are there any issues you foresee with using a servo to release a launching mechanism?

I don't have any math on hand to back this up but I will venture to say no. Remember, you might shoot a max of 10 times a match maybe more...might be able to just have storage with off board compressor..of course go by the rules but it shouldn't be ruled out (pneumatics that is)

[Isaac501]

In my experience, servos are poor choices for shifting mechanisms on FRC robots.

I've observed them to have a difficult time shifting under load, and noted delays between left/right gearboxes when shifting.

[Micah Chetrit]

Thanks for the input guys!

[KamalRC]

This video of Anymark's sonic shifter shows pneumatic and servo switching.

http://www.youtube.com/watch?v=cK3PqMj1TeM

[Willyspu]

Originally Posted by Micah Chetrit

Hello,
our team has never used a shifting gearbox on our robots before, and we are considering using an andymark 2774 to have a shifting system similar to how RI3D released their shooting mechanism. We don't plan on having pnumatics on our robot because the only place we would use them would be on the shifter, so we thought about using a servo to actuate it.
Can a servo handle it well? I doubt that the force required is as simple as the force required to move a lightweight dog gear since there are other forces acting on the gear, so I don't know how much resistance to expect. Are there any issues you foresee with using a servo to release a launching mechanism?

We haven't tested it yet, but we are looking at using one of these https://www.servocity.com/html/hs-805mg_servo.html
to move an arm that will pull & push the dog gear mechanism.


Jim

[EricH]

Originally Posted by Willyspu

We haven't tested it yet, but we are looking at using one of these https://www.servocity.com/html/hs-805mg_servo.html
to move an arm that will pull & push the dog gear mechanism.


Jim

You sure that's within the servo spec for FRC? 4W, free speed*stall torque is the max.


With respect to the video, that's a non-loaded drivetrain, which will behave differently than a competition drivetrain, and there was at least once when the servo stopped in the middle of a shift. And the servo in general was slower than the cylinder. Both of those can easily be deal-breakers for a drivetrain servo shifter.

[aciarniello]

We are using an AM supershifter for our prototype winch mechanism. With a full load on our catapault, we could not get the dog to disengage even at 60 psi. I would be VERY doubtful that a servo could get it done.

[Willyspu]

Originally Posted by aciarniello

We are using an AM supershifter for our prototype winch mechanism. With a full load on our catapault, we could not get the dog to disengage even at 60 psi. I would be VERY doubtful that a servo could get it done.

that begs the question . . . did you find a solution?

[Willyspu]

Originally Posted by EricH

You sure that's within the servo spec for FRC? 4W, free speed*stall torque is the max.

Hmm, I guess 830mAx6v=4.98watts won't fly. Thanks!

[Micah Chetrit]

Originally Posted by aciarniello

We are using an AM supershifter for our prototype winch mechanism. With a full load on our catapault, we could not get the dog to disengage even at 60 psi. I would be VERY doubtful that a servo could get it done.

Interesting. thanks for the heads up. do you have a work around yet?

[Micah Chetrit]

Would this help? http://www.andymark.com/product-p/am-2584.htm
It has about twice the torque of a normal servo but only has 2 positions which apparently gives it more torque. Power drain isn't any more than other servos.

[Tom Line]

Originally Posted by Willyspu

Hmm, I guess 830mAx6v=4.98watts won't fly. Thanks!

Power = Voltage * Amps is one way to calculate power, but it is NOT the way the servo industry calculates it, and it is not the way FIRST calculates it. Please look at Eric's post that explains what the industry standard formula is. That servo is much much higher than 4.98 watts.

[yash101]

Actually, FIRST goes by the output power, not the input power.

Servos are a very bad choice because you will be shifting at top speeds. Servos have very little output torque, as compared to a piston. Also, designing the system with pneumatic pistons would be very easy!

You just need to have a pneumatic tank, valve, solenoid, all the basic components. You may be able to run off an external compressor, and precharge the cylinders!

[MichaelBick]

Originally Posted by yash101

Actually, FIRST goes by the output power, not the input power.

Servos are a very bad choice because you will be shifting at top speeds. Servos have very little output torque, as compared to a piston. Also, designing the system with pneumatic pistons would be very easy!

You just need to have a pneumatic tank, valve, solenoid, all the basic components. You may be able to run off an external compressor, and precharge the cylinders!

Some teams don't like using pneumatics because of the weight. If you are just running shifters than it may not be worth it.

67 has been one of these teams in the past. They have used the window motors instead of servos to successfully shift. You cannot shift on the fly, but it can also be lighter than a pneumatics system.