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-   -   Shifting with servos (http://www.chiefdelphi.com/forums/showthread.php?t=125171)

magnets 27-01-2014 17:21

Re: Shifting with servos
 
Quote:

Originally Posted by yash101 (Post 1332978)
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.

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!

Servo's aren't a very bad choice. They're a great solution if you're only looking to shift a few times a match while you're not moving.
Quote:

Servos have very little output torque, as compared to a piston.
This is false, as pneumatic cylinders have no torque. You can also gear up your servo or use an alternate motor.

Quote:

Also, designing the system with pneumatic pistons would be very easy!
Yes, but a servo is easier. The super shifter comes by default with a servo. You'd need to spend more money and assemble an extra cylinder mount. Also, you'd have to put tanks, valves, regulator... on the robot. I'd argue that servos are a lot easier.

Andrew Schreiber 27-01-2014 18:47

Re: Shifting with servos
 
Quote:

Originally Posted by MichaelBick (Post 1333287)
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.


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.

RobotDoktor 28-01-2014 21:40

Re: Shifting with servos
 
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?

Gregor 28-01-2014 21:50

Re: Shifting with servos
 
Quote:

Originally Posted by magnets (Post 1333296)
Servo's aren't a very bad choice. They're a great solution if you're only looking to shift a few times a match while you're not moving.

When would anyone ever want to do that?

pfreivald 29-01-2014 08:35

Re: Shifting with servos
 
Quote:

Originally Posted by Gregor (Post 1333947)
When would anyone ever want to do that?

Well, one would hope the catapult winch isn't driving while you shift into neutral.

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.

Andrew Schreiber 29-01-2014 10:39

Re: Shifting with servos
 
Quote:

Originally Posted by Gregor (Post 1333947)
When would anyone ever want to do that?

Heavily depends on your strategy. Shift on the fly is nice but if your ratios are spread too wide it can actually be jarring. Perhaps the game has multiple modes wherein one speed is beneficial and another wherein a vastly different speed is useful. OR in the case of a power takeoff.

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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