Mulit-postioning with pnuematics this year?

[FadyS.]

Originally Posted by pras870

I've actually been looking for a way to do this for the past 2 years. I have a pretty firm grasp on pnuematics from the past 2 years, but I still fail to see how this could be done

I've also found, that amazingly, pneumatics actually weigh less then a motor driven assembly. Our team has always seemed to have a weight problem, and I'm always looking for a way to incorporate pneumatics instead of motor driven for the fact that they weigh less and are actually more reliable, but I've always been under the assumption, with the materials first gives us, you can only open and close a cylinder in 2 positions (full and closed).

Our team has been successfully able to get a multi-positioning system working for our arm using the pneumatics hook up talked about here.

We use the following bit of code for the positioning system:

	// Position Control System (PCS) of arm
	if( p3_sw_aux1 == 1 )
	{
		// Open arm
		relay5_fwd = 1;
		relay5_rev = 0;
		relay6_fwd = 1;
	}
	else if( p3_sw_aux2 == 1 )
	{
		// Close arm
		relay5_fwd = 0;
		relay5_rev = 1;
		relay6_fwd = 1;
	}
	else
	{
		relay6_fwd = 0;
	}

EDIT: Relay5 = double solenoid valve
Relay 6 = Single solenoid exhaust valve.

We use this setup along with the SMC speed controllers to slow down the opening and closing of the arm and stop it when we close the exhaust valve.

[Jeffrafa]

If having >60 psi is a concern (~ 66 psi), then an easy fix would be to use a rodless cylinder, this would make both sides have the same amount of pressure when it is stopped mid-stroke.

I dont think going a little over 60 is an issue because the rule is there to make shure it is safe, and all the cylinder's have a max pressure of 150, so 66 certinly is still safe, as well as you aren't (and couldnt) use it as working force, its just a result of moving the cylinder. You still only have 60psi running in your pneumatics system.

[Astronouth7303]

There are 3 simple ways of doing this: easy, easier, and easiest. But I can't say which is which.

Style 1: Use 2 solenoids: Direction and Brake. Direction is a 'double throw' it has 2 ins; connect normally. Brake is a single: use on exhaust, only use A or B.

Style 2: Use 2 solenoids: Up and Down. Both are singles. Tubing could be a problem. (Exhaust through closed solenoid). the basic Idea is that when you turn on Up, it goes up. Turn on Down, it goes down.

Style 3: Use 1 solenoid: a 'double pull, double throw'. Basiclly: when off, A and B are off. When on A side, A gets pressure, B is exhaust. On B side it's opposite: B gets pressure, A is exhaust.

That should be easy enough.

[Squirrelrock]

Currently, I am in the process of trying this idea out on our 1st year robot, (umm, 2000 I think) and I will try to use the desgin in the aattached thumbnail. Besides what's already been said, has anyone had any problems doing it this way?

Thanx
Squirrel

[greencactus3]

Originally Posted by Liz*599

the way our team achieved three positions from each joint, as opposed to two, was by coupling two cylinders together and then only fireing them independently, which worked very well for us.

if you used two separate stroke cylinders, you would be able to get 4 positions. 0-0, 0-1, 1-0, and 1-1 with the same stroke cylinders, 1-0 amd 0-1 are the same so only3.

and by the way.. if you are only using a single acting cylinder (ex. spring loaded or bungee loaded or just plain gravity drivin back, mulit positioning is possible with two single solenoids. i have used this method and the "normal" double and single solenoids multipositioning both with no problems ever. and for people who still dont quite get how multipositioning works, think of when you presss against the force of the air in a cylinder (only try it with a small diameter cylinder running on low pressure or you may hurt yourself). by pushing in a rod that is pushed out by the air. with the right amount of force to the rod from your pushing, the rod will stay "inbetween" the full out and full in stroke. that is multipositioning. only you dont use your hands to push in the rod but something else, gravity, elastic, or air (pressure).


*edit* oh and remember, some solenoids wont work with zero pressure buildup in the input side. (sorry, can someone word that better?)

[Squirrelrock]

Originally Posted by greencactus3

oh and remember, some solenoids wont work with zero pressure buildup in the input side. (sorry, can someone word that better?)

Maybe... Some solenoids need input pressure to work?

Anyways, this morning I was able to get the cylinder to creep to midpoint by telling it to stop a little bit above the midpoint and it equalizes slowly. Also, it I tell it to stop too late, then it creeps all the way down. Could this have something to do with the lever action of gravity on the extended arm? Could this be fixed by using a spring to act against the arm?

Thanx

[Jeffrafa]

Actually, all solenoids need imput pressure in order to function. Solenoid valves themselves work just like cylinders, they use air pressure in order to shift the flow of the air, so every solenoid valve needs at least 25 - 30 psi in order to work. Some, however, have an additional port in which to supply pressure for the solenoid to function separate from working off the imput pressure - these are common in systems that carry negative pressure (vacuum).

The easiest method for multi-positioning is simply using one dual solenoid, three position valve in which the center position is all ports blocked, with this you can make the cylinder move whichever direction you want by activating each solenoid, or you can make it stop anywhere by not activating either solenoid. We had this on our hook cylinder last year and it worked flawlessly.

I can draw up a schematic of it later today if you want me to.

[ngreen]

Jeffraffa scheme seems to be the best under 2004 rules (Since three position solenoids are available and legal). I see no reason why they would limit this availability this year so the three position solenoids are the right to for the right job.

squirrelrocks schematic is a modifyed version of what was allowed in 2003 rules. The pressure regulator is the additional part to regulate the exhaust gas pressure. To explain it's need in this design read earlier post.

As far as how it is reacting there are a lot of variable to consider. The first two are important for all pneumatic systems. 1. Keep all your tubes short. Especially exhaust side. This will cause the device to react quicker. 2. Check for leaks.

The weight of the arm will affect the how quickly it reacts. The stopping force must compensate for both the pressure force (PressurexArea) and the weight (Massxgravity). When you are stopped the sum of the forces=0. I would imagine the if you were lifting the arm it might go slower up and stop quicker as opposed to lowering when it would go down quicker and take more time to stop.

There will be some play in the cyclinder when you stop it but if you make sure the pneumatics are correct and use the right cylinder at a good speed it should work.

[Rickertsen2]

Not ALL valves are air piloted. There are quite a few companies that make valves with entirely magnetically actuated spools.

BTW i would highly caution against the above setup for anything with a dynamically varying load. I will also note that changing direction wastes alot of air and is generally somewhat unstable. If the cylinder will only be required to resist in one direction, there is a much more stable and air efficient version of this.
Use a spring return cylinder. If the assembly will return by itself (IE by the weight of gravity etc, the spring is counterproductive and not necessary).
Use a 5/3 double actuated vavle with a closed center as mentioned above. Plug port 2/A/output1 (different manufacturers call it different things. Plumb port 1/input/whatever to supply. Plumb port 4/B/output2/whatever to the port on the cylinder that corresponds to the direction that you want it to apply force in. To regulate extension speed put a flow regulator in series with the supply to the valve. To regulate retraction speed, screw a flow regulator into port 5/BE/exhaust2. you could also use unidirectional flow regualtors in series with the line from the valve to the cylinder. This setup will still drift when the load changes, but it uses MUCH less air and is MUCH more stable when changing directions.

Food for thought: The REAL way to accomplish multipositioning is with a proportional valve, a position sensor, and a feedback controller of some sort.
The position sensor, porportional valve and feedback controller are normally
electronic. This wouldn't be a problem except that FIRST tells us that we can only drive solenoids from spikes. Most porportional valves take either an analog input or have some sort of digital interface. I will give people a hint and tell you that there are a number of ways that are FIRST legal an to create a entirely analog entirely pneumatic feedback position control loop. WOOT for pneumatic amplifiers differential regulators etc! In fact, the first PID controllers were entirely pneumatic. First person to come up with a workable scheme wins a prize.. well not really but i would like to see what people come up with.

[Squirrelrock]

to fix the possible leaks, we (jdiwnab and I) retubed the whole system and replaced the old valves with Festos from last year. I also modified the design so that each Festo controlled one side of the cylinder, basically A goes to the cylinder, B is a blocked loop, P is pressure, EA is blocked, and EB is open to atmosphere. This is true for both valves. Why would the piston get stuck at midway and not move up or down from there? Have I inadvertently made it so that the air cannot exhaust itself from either side?

Thanx

Squirrel

[Squirrelrock]

Solved!
Whatever problems we had we fixed. It turns out that I had blocked the wrong port on the second valve (see thumbnail on my post, about eight back from this one) . Thanx for the help!

Squirrel

[bombadier337]

After months of reading, I finally made my own account. Anyway, I'm from team 1541, we are a rookie team, but we are doing pretty good so far. We have some video of our testing of multi-position pneumatics at http://www.sartanyac1541.com . We used the SMC to plug into the actuator, and then the exhaust is routed into the festo valve which opens whenever the extend or retract button is being held down. It took a little fooling around to get it, but now we have everything running off one spike. :-)