Pistons not extending fast enough?

[TheHolyHades1]

Originally Posted by TikiTech

All the mentioned changes will help for sure.
Set the cylinders at a better angle.
Since your cylinders are a larger boar and stroke it will need lots of air quickly.
A separate solenoid per cylinder is a must with, if possible, higher Cv flow.
We got these really quickly using one of the vouchers from FIRST.
http://www.automationdirect.com/adc/...)/AVS-3211-24D
They are 24v and the price was reasonable.. About 20 dollars. Not sure if you are using the older 12v solenoid from previous kits. Some FIRST KOP came with 12v others 24v. You will have to make some adjustments if you are running 12v.
You will have to have the solenoid breakout on the cRio run it.
Ideally, like mentioned, mounted on the cylinder. If not as close as possible!
Air lines after the solenoid have to be filled before any air reaches the cylinder.
Storage of high pressure as well as having working pressure tanks for each cylinder. Try having the low pressure tank as close as you can to the solenoid.

As for venting. Take off the unused hose fittings!!! You are reducing the flow of escaping air and slowing it down.

We are running a system similar to 842, ( well done by the way 842!)
(2) 10" 1-1/2"bore cylinders and it will consistently hit the high goal at 20 feet away with a release point of 3.5 feet off the floor..
You can look at our Youtube stuff and freeze frame for ideas..
http://www.youtube.com/watch?v=LfKh6...po3XAn15-6TXDg

Send me a message if you have additional questions. I will try to get some photos from the students.

Good luck this season,

Aloha!

Thank you. How are you able to use the 24v solenoids? I thought the breakout board and spikes only apply battery voltage (12v)?

[tim-tim]

I didn't get a chance to respond to this thread last night, but I would like to re-iterate and maybe add a few things to what others have already said. We are using a 2 piston "catapult" and have had much luck; albeit after a lot of math and testing.

For starters,
What pistons are you using (bore and stroke)? We use 1.5" bore, 8" stroke (2ea)
The idea of gravity return is your friend. Remove fittings and hosing from the return port - this will help the air escape the piston faster as you "shoot".
You may want to check some of your angles. We start our "catapult" roughly 35(deg) below horizontal and have an angle of about 50(deg) above horizontal at full extension.
What we found was that the regulator was also a very constricted part of the system. We added accumulators after the 60psi (working pressure) regulator to store air that we would be dumping into the piston.
We are using the 12V Solenoids, and have one for each piston.
We have very short tubing runs from the accumulators to the solenoids and pistons.
You may want to look at force vectors. Our system keeps the pistons relatively vertical throughout the motion of travel. This is especially important when the arm is horizontal.
As others have said, test at the pressure you will be using on the field. We found that dropping the pressure to 55psi had a huge effect on the system's performance; we operate at 60psi.
I would caution that you match the weight as closely as possible to the real arm. From the hinge on, our arm weighs 6lbs. Switching the weight of the arm had a drastic effect on the characteristics of the shot. We experimented with a 4lb arm (more a proof of concept) and then a 5lb arm. We are still attempting to tweak the 5lb arm, but right now the 6lb arm is the most consistent and controlled. Weight matters
We use a ratchet strap as a "hard" stop and angle adjustment on the system. We stop the pistons anywhere from 37 to 41 (deg) above horizontal. This also has a great effect on the ball's trajectory - we don't switch often.

Here is a screenshot from on of the early CAD models of our system. There isn't a huge difference between the the screenshot and the actual robot when it comes to the setup of the "catapult".

Let me know if you have any questions - or just want to bounce ideas off of me. Hopefully I didn't contradict too many people, but this is what is working for us.

[gpetilli]

I do want to clarify few things:

1) You absolutely need a storage tank on the high pressure side. 120PSI is twice the working 60PSI so you can store twice the air.

2) several posts mention a storage tank per cylinder after the solenoid. This only works if you use the latch and are pre-charging the air spring.

3) The automation direct solenoids have 3.3x the Cv of the previously legal valves, however they are much bigger and heavier. I think there is a voucher on TIMS. http://www.automationdirect.com/adc/...)/AVS-5211-24D

4) The input side of the cylinder is only half of the problem. You also need to exhaust the air on the other side of the piston. Instead of connecting the exhaust side to the other side of the solenoid, we do not put a fitting on the exhaust side of the cylinder and use gravity (and a small diameter cylinder if necessary) to return the catapult to the latch position.

[jwfoss]

Originally Posted by gpetilli

2) several posts mention a storage tank per cylinder after the solenoid. This only works if you use the latch and are pre-charging the air spring.

There may be some confusion. With a high flow solenoid, the regulator becomes a restriction in flow, having a storage tank (@60psi) with as short of a hose as possible right before the solenoid (directly mounted to the cylinder) will absolutely benefit you. I do agree that after the solenoid will not make a large difference.

[ttldomination]

Man, this thread really blew up. There's tons of awesome nuggets of information.

Originally Posted by TheHolyHades1

Thank you. How are you able to use the 24v solenoids? I thought the breakout board and spikes only apply battery voltage (12v)?

To answer this question, the solenoid breakout can power 12V solenoids or 24V solenoids. It all depends on where you plug into on the power distribution board. Here's a link to the wiring diagram, if you can see, the cRio is hooked up into a 24V slot, and there's room to also attach the solenoid breakout board.

- Sunny G.

[gpetilli]

Originally Posted by jwfoss

There may be some confusion. With a high flow solenoid, the regulator becomes a restriction in flow, having a storage tank (@60psi) with as short of a hose as possible right before the solenoid (directly mounted to the cylinder) will absolutely benefit you. I do agree that after the solenoid will not make a large difference.

Okay, that makes sense that the regulator could be the bottleneck - just slightly different than the problem our team had. Since we are building an air spring, putting the storage tank after the solenoid also takes out the flow restriction of the solenoid. Before the solenoids would improve flow rate, regardless of latched cylinder or not. If you are not latching and pre-charging the cylinders, the tank(s) should be before the solenoids (after would effectively increase the air required and actually make things worse).

[Matt C]

Originally Posted by TheHolyHades1

The gate latch - when your pistons retract, I'm assuming that it locks back into place until the next time you're shooting? Also, do you have the pistons retracting with gravity or some other mechanism, or are you having them retract by sending air in the other side?

Gravity retract, the gate latch remains open until the 3/8" bolt that goes into the latch hits the microswitch to signal that the catapult is back in the down position. The solenoid for the small 1/2" stroke piston is then actuated and the latch closes.

We actually only have one solenoid going to both pistons. It's a slightly longer pressurization (about 2 seconds) for a full shot, but allows us a better lob/pass "shot" with no piston gate latch retention with slower motion and less air consumption (the solenoid is switched only for "x" time before it's switched back, allowing the ball to be shoved out, but not allowing the tanks and pistons to reach the full 60 psi, thus saving a bit on air.)

[Oblarg]

Originally Posted by Joey1939

A single solenoid will not give you enough airflow to extend with enough force to launch the ball.

This is fundamentally not true. You might not be able to score from 18 feet out with a single solenoid, but 449 has a shooter which scores consistently in the high goal using two 1.5''-bore, 8''-stroke cylinders off of one solenoid with no downstream tanks, and no latch. It's all about the geometry if you're going to go that route, though; it's subtle.

[chrisfl]

Team 1699 has a magnificent pneumatic catapult. we are using 1, 2in bore piston with a 10in throw. If you take the connection that retracts the piston off, you will have the speed of a small piston with a small bore, but the strength of the 2in bore and longer throw. We can shoot the ball very accurately and we have a good 10ft throw. In testing we were shooting more than twice this distance away. However, adjustments were made to accommodate our frame parameters.

[hrench]

Originally Posted by Oblarg

This is fundamentally not true. You might not be able to score from 18 feet out with a single solenoid, but 449 has a shooter which scores consistently in the high goal using two 1.5''-bore, 8''-stroke cylinders off of one solenoid with no downstream tanks, and no latch. It's all about the geometry if you're going to go that route, though; it's subtle.

Geometry is right, but there is also another trick you can use. If you use a longer-stroke cylinder (we use 11") and don't retract it all of the way, you'll have a volume chamber right in the cylinder you can fill to 60 psi before you release. Also, yes, the other side of the cylinder should be open to atm.


https://www.facebook.com/bob.hrenchi...type=1&theater

[TikiTech]

Originally Posted by TheHolyHades1

Thank you. How are you able to use the 24v solenoids? I thought the breakout board and spikes only apply battery voltage (12v)?

The breakout boards are supplied power, usually 12v, from the PB Board. One for the analog breakout the other for the solenoid breakout. The cRio is supplied power form the 24v Wago connector on the PD board.
Remove the wiring from the solenoid breakout at the PD board end and put them in the 24v Wago connecter on the PD Board ( The one supplying 24v to the cRio ) The 24v Sauro connector is labeled as V,C,NC,C
Usually you put the cRio positive connection on (V) and put the negative connection on the outer (C).
Instead wire it with the cRio positive on (V), like before, put the cRio negative on the (C) next to it. Then put the positive to the solenoid breakout on (NC) and the negative solenoid breakout wire in the outer (C).

You can still use 12v solenoids but will have to actuate them with Spikes.

We do this on our robot, using the 24v higher Cv flow solenoid from the solenoid breakout for our catapult and the older 12v solenoid controlled by a spike off the sidecar for out shifter.

Works very well.

Aloha!

[TikiTech]

Originally Posted by hrench

Geometry is right, but there is also another trick you can use. If you use a longer-stroke cylinder (we use 11") and don't retract it all of the way, you'll have a volume chamber right in the cylinder you can fill to 60 psi before you release. Also, yes, the other side of the cylinder should be open to atm.


https://www.facebook.com/bob.hrenchi...type=1&theater

This will require a latch and releasing mechanism to keep the slightly extended cylinder in place as it becomes pressurized.

As well you should install a safety mechanism.
You will most likely be pressuring your system in queue and swapping batteries prior to taking the field.
We are planning on charging our system in queue. Our system has four 120psi and two 60psi tanks. Since we are not putting our cylinders in a pre charged state we simply extend the catapult and put a channeled PVC pipe on the extended rod keeping it at maximum throw. If our system accidently fires the catapult is already on its stop point and will not put strain on the safety system.
I am a little concerned for some of the straps over the tensioned and charged catapults. Unless the straps a sufficiently strong and TIGHT there could be trouble

Aloha!