Posted by Elliot Johnson at 1/9/2001 2:19 PM EST
Student on team #218, HP Robotics Team, from Highland Park High School and Double-E Inc..
In Reply to: Re: Gears? Chain?
Posted by Jim Meyer on 1/8/2001 3:04 PM EST:
: On the pneumatics, has anyone figured out how to controll the position of the piston other than mechanically limiting its range? It's supposed to be so much better for robot arms but it seems you could only have two positions of your arm.
There are at least two different ways I know of to slow the cylinders down to a controllable speed.
The first (easiest & most reliable) is to put one of the little pressure valves inline with the cylinder's feed. I'm not sure the exact name for them, they look like little 90 degree elbows and have a screw coming out of them, like a small water faucet. You adjust the screw until the cylinder moves at a comfortable speed. To move it to a certain position, just turn on the pressure switch for the cylinder until it is where you want it and turn off the switch. The air pressure will remain in the cylinder so it will maintain it's position. You can also use two valves, one for each entry into the cylinder, to have different speeds in different directions.
The other way I've read about it to use a model like PWM for controlling motors. The solenoid valve is quickly pulsed on and off rapidly. The pulses begin at a constant frequency (the solenoid is turned on every x milliseconds) but the length of the pulse depends on how fast you want the cylinder to move. The length of the pulse can be between 0 (not moving at all) and the full period (full power). Anywhere in between should give a proportional speed. This setup is more complicated, but the speed isn't fixed like with the pressure regulators. For a better description of PWM, check out this article in the SRS newsletter:
http://www.seattlerobotics.org//enco...68hc12pwm.html
Hope that helps!
Elliot Johnson
Team 218
Double-E Inc & Highland Park High School Robotics