Amount of tanks
 

if your operating 3 pistons how many tanks of air should you use?

Re: Amount of tanks
 

How big are the pistons you're using? The 3/4" bore 1" stroke cylinders used in the AM shifters take a whole lot less air than the 2" bore 24" stroke behemoth from Bimba.

Re: Amount of tanks
 

This is a difficult question to answer because we don't know what size cylinders you are going to use, and how much you are planning on using them. If you only plan on actuating the cylinder once, then you need to figure out how much volume of air is required to fill the cylinder at 60PSI and you can calculate the volume of the storage cylinder. Some small amount of calculations later you can find out how many storage cylinders you need. Now the rules state that you can only have 4 storage tanks (identical to the 2 supplied in the kit of parts) - refer to The Robot section, <R84>.

If you need more air, then it is time to start thinking about including the compressor on your robot. Then you might be able to reduce the amount of storage cylinders you have depending on how often you want to use your cylinders during a match.

Hope this helped! :)

Re: Amount of tanks
 

I agree with him, it is hard to know the amount of tanks that you would need. I would say to mock up a pneumatics set up and fire the cylinders that you have and look at the psi loss on the tanks as you fire the cylinders. One thought is to put multiple regulators on your system to save air as some applications do not need the full 60 pounds of pressure. Putting more regulators on could possibly save your from putting on that extra tank thus saving you a little room and a bit of weight.

- It's a thought

Re: Amount of tanks
 

i would use all 4 storage tanks unless you have a major weight problem
if you're extending/retracting your pistons a lot the air gets used up fast
and the air compressor really drains your battery, so you want to have lots of air stored

Re: Amount of tanks
 

just add all 4 of em :) just make sure you could take one out if you need to loose weight.

Re: Amount of tanks
 

You could do what 675 is toying with: A T-valve between the tanks and the pump. Add all 4 tanks. Charge in the pit, close T valve, disconnect pump (which ISN'T mounted to the robot), go to compete. As long as you have the regulation valve for the system at 60psi, you could charge the tanks at a higher pressure (we've been using 110psi for testing) which gives you more use out of the actuators.

Also, no mounted tank = Less weight and less power sap.

Re: Amount of tanks
 

Be sure to check the rules closely before doing this. In recent years, they've been making it harder to do this. I don't know what the rule is this year, but last year the rule was that everything had to be on board the robot, except for the compressor itself. That means all the plugs for powering the compressor, the sensor, the spike, etc. The software had to control turning the compressor on/off. Also, be sure that you have no leaks in your system. This is especially important if your compressor is off-board.

As long as you're not using too much air, I think it's a good idea to have the compressor off-board, to save weight. Two years ago(?) we used pneumatic shifters, which were the only pneumatics on the robot. It used two of about the smallest cylinders available, and two air tanks. This was plenty for us for a match.

Re: Amount of tanks
 

Depends on how much you'll use your pneumatics. If its only a few time a match you might be good with two. But if it is constant you might want one OR two AND a compressor aboard. Overall it comes to usage. If you know you won't even use two tanks, put three to be safe. Worst come to worst how many time a match will you use your pistons and prepare for the answer to that question. I suggest though a compressor and one or two accumulators though.

Pavan.

Re: Amount of tanks
 

Just to echo what everyone else is saying, to me (and I know how the "we need to shave off just .1 lbs. AHHHHHH) you should always put all four tanks on (you never, EVER, want to run out of air, its really not very fun, so you should play it safe) and if you're going to be using the pneumatics for, say something like scoring, you need to put the compressor on too; because that air will be gone really fast (note last comment about running out of air)

Number of tanks relates to how often you use them.
 

Last year, we had a compressor and 2 accumulators. Our robot used a piston to actuate the gripper (4 or 5 times a match) and 3 pistons to actuate each wing (once a match).
If your system won't be actuating many times a match, you should figure out how much air pressure you're using each time, and base how many accumulators you need on that. A compressor may be unnecessary depending on the amount of air you're spending a match.
Losing the compressor is a significant chunk of weight, so I suggest figuring out whether you're going to need to refill your accumulators.

Re: Amount of tanks
 

Give someone a fish, and they can eat for a day. Teach them to fish and they will eat for a lifetime. I’m going to answer the question I’m sure you meant to ask, which is “How do I determine how many tanks to use based on my selected actuators (pistons) and their usage?”

First some background you need to understand:

1. The volume of each tank is pi x (diameter/2)^2 x length. The volume of air used in each stroke of the actuator is pi x (diameter/2)^2 x stroke. It uses that volume each stroke, both extending and retracting.
2. Since all the volumes are linear functions of pi, you can take it out of the equation to simplify the calculations. (make your volume unit in^3/pi)
3. The pressure on the gage is the gage pressure (duh), which means the absolute pressure minus ambient pressure (about 14.7 psi for a standard atmosphere). This is the pressure used for force calculations (and the regulator settings).
4. From ideal gas laws, pressure x volume is a constant for some amount (mass) of air. In this case the pressure is the absolute pressure. So 100 in^3 of air at 60 psi absolute is the same amount of air as 200 in^3 of air at 30 psi.
5. You can fill the tanks to 120 psi. The maximum pressure allowed to actuate the actuators is 60 psi (the setting of the fixed regulator), which you will probably use for pushing or forcing a mechanism. To position something (like shifting gears in a gearbox or opening/closing against a stop that will take the load) 20-30 psi will probably be OK. You’ll need to use the extra regulator to set that pressure.
6. Each stroke of an actuator uses some amount of air from the tanks, which causes the remaining air in the tanks to expand and fill up the volume. This reduces the pressure in the tanks. You can continue to do this until the tank pressure drops to the operating pressure you need to operate your actuator.
7. The air in the plastic tubing is additional volume used for each stroke but it is very small compared to the actuator so we’ll ignore it.

So here (finally) is how you do the calculations:

For each tank, d=2.0, L=6.0, volume = 6 in^3/pi (at 120 psi). Suppose you want to run your actuators at 60 psi; the amount of air in each tank is equivalent to 6 x (120+14.7)/(60+14.7), or 10.82 in^3/pi at 60 psi. You will leave 6 in the tank – after that the pressure will drop below 60 – so you have 10.82-6=4.82 in^3/pi usable air at 60 psi.

Suppose you are using ¾ inch bore actuators. That’s 9/64 or .14 in^3/pi per inch of stroke. So each tank gives you 4.82/.14=34.27 stroke inches. For a 4 inch stroke actuator, that’s 8 strokes, or 4 cycles (extend and retract = 1 cycle)

Now suppose you are using 1-1/2 inch bore actuators. That’s 9/16 or .56 in^3/pi per inch of stroke. So each tank gives you 4.82/.56=8.56 stroke inches. For a 4 inch stroke actuator you will get 2 strokes (1 cycle). For the 2 inch bore actuator, it is 1 in^3/pi per inch of stroke. So each tank gives you 4.82 stroke inches, and you can only get 1 stroke out of a 4 inch stroke actuator.
Now try running the actuators at 30 psi. Using the calculations above at 30 psi, usable air in the tank is 12.08 in^3/pi at 30 psi. Scaling by 12.08/4.82, you get about 2.5 times as many cycles per tank.

You should be able to use the information for different combinations of pressures and actuators. I usually put it in an Excel spreadsheet so I can see the sensitivity to different changes. Good Luck.