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Shifting with servos
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? |
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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. |
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Thanks for the input guys!
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This video of Anymark's sonic shifter shows pneumatic and servo switching.
http://www.youtube.com/watch?v=cK3PqMj1TeM |
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to move an arm that will pull & push the dog gear mechanism. Jim |
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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. |
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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.
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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. |
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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! |
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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. |
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Or because they haven't got anyone with experience using them. 67 has not used pneumatics since 2004 from what my brother was saying. Bear in mind that when you are disengaging the dog gear you are fighting friction. If you don't have something else taking the load from the winch you will be fighting a LOT of friction. You can overcome it but chances are it won't be easy. A simpler solution might be to use a ratchet style system that takes the load. You'd need to figure out the math behind your particular system to tell if a servo or a motor with a cam could achieve the desired results. |
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I hope you are geared pretty low, considering the tension levels needed to launch the ball well. A ratcheting mechanism of some type will also be needed to avoid stalling the motor. Is using electrical solenoids an option for shifting?
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As to the original question, I have two points to consider: 1. Using a ratchet to take the load off of the gearbox entirely is a good idea. We built a custom dog system for a winch, and once we get it into place the motor drives back a little and the ratchet/pawl takes the entirety of the load, so we can use a tiny little pneumatic cylinder to disengage the winch. (We have a fatter-but-not-insanely-so cylinder then disengaging the pawl, which takes about 12 lbs of force.) Given that you probably need a ratchet of some kind to keep the system from back driving anyway, this is a good idea to consider. 2. If you're not going to have any other pneumatics on your robot and a servo just won't do, consider using a screw drive. They're as strong/fast as you need them to be (based on motor power, of course--in 2010 we lifted our robot using a CIM-driven screw), accurate, easy to control, and for your application perhaps the best thing is that they're motor driven. What they're not is efficient, but you can use this to your advantage to prevent back drive in some applications where that's desired. |
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I've also run bots that servo shifted dewalts and when a shift was requested we backed off the power to the drive motors momentarily while the shift happened. Not the best, but it worked. Course, I'm loving the simplicity of a 6CIM single speed right now. |
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