Pistons not extending fast enough?
 

Hello. Today we conducted a test for a pneumatic system which we are looking to add to the robot before the next competition, but we were met with a rather unsuccessful attempt at shooting a ball. I'm posting here to hopefully get some suggestions to improve the design and/or point out any fundamental flaws with the system.

The tanks were charged at around 90 PSI (the working end pressure guage was displaying about 70 PSI, this needs to be reduced). The setup was constructed sort of arbitrarily, in that I didn't make any exact measurements to emulate the robot in any particular way. This was set up more as a proof of concept, and unfortunately, the proof of concept seems to have failed.

Differences from the design that'll go on the robot (see pictures):

1. When fully retracted the pistons are attached so that the ramp is upright; this is opposite of what the actual design is at the moment, whereby the pistons will be attached so that the ramp will extend from about its current 30 degrees below horizontal to approximately 45 degrees above horizontal (or as close as possible, that being the idea angle for kinematics. This could certainly change if/when I get around to reading some of the more complex analysis that has been done on the physics of the ball). To account for this, we angled the setup (as demonstrated in the videos), though I'm not sure how closely we replicated the movement.

2. The ramp we're using is made of wood and lacks the metal side rails that are on the ramp currently. This would have made it easier to launch the ball as it is significantly lighter.

3. The hinges are strap hinges instead of piano hinges as are currently supporting the ramp on the robot, though I doubt that matters much.

4. We were shooting at a PSI that was slightly higher than the allowed 60 PSI (70); at least, that's what our guage read.

That being said, I think that this test should have at least proved to some degree whether the concept would work. However, after doing it, I found that the ramp rotates much too slowly to be able to do anything productive; notice that even the light basketball from two years ago (rebound rumble) hardly launched off the board.

Would attaching the pistons at the very bottom of the board make that much more effective that it can launch a ball across the room? It also seems as though the pistons are extending much too slowly; is there anything we can do to fix this?

Piston setup:



Robot Ramp (in bag):



Walkthrough of testing setup:

https://www.dropbox.com/sh/eob698yuo...kthrough.mp 4

Test 1:
https://www.dropbox.com/sh/eob698yuo...320_175538.mp4
Test 2:
https://www.dropbox.com/sh/eob698yuo...320_175720.mp4
Test 3:
https://www.dropbox.com/sh/eob698yuo...320_175801.mp4

Re: Pistons not extending fast enough?
 

Sorry, a better question is what's the PSI on the regulator?

Re: Pistons not extending fast enough?
 

A single solenoid will not give you enough airflow to extend with enough force to launch the ball. What you need is a way to help the piston along. I have seen two different ways to do this.

1. I haven't done this, but I have read about it. You can add a spring to the arm of the piston so that it extends outward more quickly.

2. The second way is how my team did it. You pull the piston in and have it lock in somehow(we use a gate latch). You then push the piston outward. The piston tries to extend but can't because it is locked in. Then when the lock is released, the piston releases with more energy because you have pre-pressurized the piston. (The one problem is that our catapult system requires two huge pistons. These two pistons take a lot of air and therefore require several tanks.)

I hope this helps but it might be hard to implement on your robot because it has already been mostly built.

Re: Pistons not extending fast enough?
 

You will get much better results using smaller diameter cylinders. Those large ones in the picture require too large a volume of air to fill, thus the extension speed is not fast. Use much smaller cylinders (same length, smaller diameter) for a fast launch.

You may also wish to look at the flow rate on your solenoids. We are using a new solenoid we bought that has a much higher flow rate than the solenoids that were FRC legal last year and before. No flow rate restriction this year - only limit is port size.

Re: Pistons not extending fast enough?
 

So, I have a basic understanding of pneumatics, so I'll attempt to advise you the best I can. Warning, this is going to be a long read, so take a deep breath, and try to understand as much as you can.

The name of the game is flow rate. How do you move air as quickly as possible into the cylinders? If you move the air in quicker, the cylinder will shoot out faster.

Before I get into this, I'm going to set up an analogy for us to follow. Think of the cylinders as buckets, and the solenoids as hoses that carry water. The goal is to fill up the buckets of water as fast as you can.

Now, based on the videos, you have one solenoid hooked up to two, fairly large, cylinders. You're essentially trying to fill two big buckets with one hose. So, how do you make the system faster? How can you change up your variables (buckets and hoses) to make the system fill up faster?

Let's go through a few scenarios:
Use more hoses - Of course, if you use, say, one more hose (read: solenoid), that'll help with the flow rate, and it'll help you fire quicker. Now, I believe the rules limit you to maximum one solenoid per cylinder, so keep that in mind.

Use smaller buckets - While this may seem counter intuitive, this is also correct. Using smaller cylinders will help you fire faster. There's a tipping point between cylinder size that gets you the best speed vs. power. I'd recommend reaching out to a team to find out what they're using.

Use a larger hose - Now, this is interesting. If you use a solenoid that's rate for a faster flow rate, you can move air quicker, which will ultimately shoot the pistons out faster. FIRST doesn't impose a solenoid flow rate limit, but given a set of constraints (price, power, etc.) you may be limited on what's actually available to you.

Pool your water into a smaller bucket ahead of time - Now, this is a little bit more difficult to understand, but bare with me. Let's say you pop the solenoid and let the cylinders build pressure, but you lock down the shooter. Then, at a later point in time, you manually release the shooter (via a latch or something). Here, the cylinders are essentially acting like compression springs, but you are able to get over the flow rate problem.

That's the tip of the iceberg, and I recommend you do some more soul searching (read: researching) into your issues, your system, and which solutions would best fit your setup.

Here are some other threads that may help:
Team 842's Amazing Machine
Creating Pneumatic Launchers
This talks more about letting the solenoids fire before you release the catapult.

Let me know if this makes sense, if there are any errors, or if you need any further assistance/clarifications.
- Sunny G.

Re: Pistons not extending fast enough?
 

As mentioned, you should really be using a solenoid for each piston, we found that attaching them directly to the shooter worked better. Additionally, you may want to move your pistons farther towards the pivot point and start the catapult at a lower position. It sounds like a weird thing to do but when the catapult requires a greater amount of force to move initially it causes the air to build up in the piston first, basically doing the latch thing without an additional mechanism. Here's a video from our first prototype, we currently have two pistons that I believe are the same size that you guys are using. You may also want to check out 842's design, they have a great pneumatic shooter.

Re: Pistons not extending fast enough?
 

This might give you some ideas, and it's the clearest picture I have of our shooter which appears to be similar in geometry to what you are attempting to accomplish. The pistons are 7" stroke, both have a small air tank on the working pressure side attached to each piston. The pistons push just short of over center (they almost line up with the hinge point on the front of the "catapult"). This puts only a slight force on the modified gate latch(not seen on the left side of the picture inside the robot, actuated by a .5" stroke piston which lets go to release the pre-pressurized shooter. The pivot is a pair of door hinges and there is a strap to prevent the pistons from smacking into the end of the cylinders whey they extend.




https://www.youtube.com/watch?v=rbYT...m-upload_owner

Re: Pistons not extending fast enough?
 

I'm not quite sure I understand what you mean, but I can tell you that for each of the tests the pressure gauge on the high pressure side (before regulator) read about 90 PSI and the pressure gauge after the regulator was at a solid 70ish PSI.

Re: Pistons not extending fast enough?
 

Okay so I was reading that correctly. If you are using a single noid for both of the pistons, switch to 2 of them for each, like others have already said. Better flow rate means faster extension.

Re: Pistons not extending fast enough?
 

There isn't actually a solenoid in the testing setup, but I do see your point with the airflow problem. Would I be able to improve this by adding two small tanks on the working side and pressurizing them to 60 PSI, plumbing each tank to its own cylinder?

As for the latch system, it seems that many teams are doing that this year. We may have to end up doing something similar, perhaps with a small stroke piston (or a rotational cylinder we have lying around?), though I would like to avoid this option as it may be at the edge of our capability technically.

Re: Pistons not extending fast enough?
 

In addition to what everyone else has said, be sure that you test with 60 psi working pressure as mandated in the rules. Also, I suggest that you include a hard stop before the piston reaches the end-cap of the cylinder to prevent damage to the cylinder.

P.S.
Piston is the thing that moves, while cylinder is the entire actuator.

Re: Pistons not extending fast enough?
 

Could you perhaps provide a link or a google search directing me to solenoids similar to those you're using? Currently we've got some solenoids left over from the early 2000s, and I'm not sure what teams are using now.

As for the cylinders; if the diameter becomes too small, wouldn't they not have enough force to push the ball + platform? I will do some calculations and figure out how much force would be needed from each piston to move the ball however, and will certainly see what the smallest piston we can use is.

Re: Pistons not extending fast enough?
 

FYI, R90 prohibits plumbing to solenoids two one cylinder.

Re: Pistons not extending fast enough?
 

I believe what is occurring is that one solenoid is currently connected to both cylinders, and he was recommending to use one solenoid per cylinder for a total of two in the entire assembly.

Re: Pistons not extending fast enough?
 

It's good to be sure... Some would overlook that and see that a higher flow rate copied be a how'd by running two together. Better at this point to keep everyone well informed to save hassles at competition.

Re: Pistons not extending fast enough?
 

Agreed, I would not want to spend precious time between competitions tuning a system only to arrive at an event and find its illegal.

Re: Pistons not extending fast enough?
 

Thanks so much for the great explanation, it really helps understand the whole concept of airflow. Apart from changing from one solenoid to one per cylinder and adding a higher airflow solenoid (if possible), would there be anything else I could do to increase my airflow?

Re: Pistons not extending fast enough?
 

The pressure regulator also restricts flow rate somewhat. To compensate, you could add additional accumulator tanks on the low pressure side of the regulator.

Re: Pistons not extending fast enough?
 

What do you mean by directly attaching them to the shooter?

I will be working on modifying the catapult so that it starts in a lower position and the pistons are closer to the hinge point. I'm also going look into 842's design a bit more, thank you for pointing that out. It seems they're using the same size pistons as us; if they are having no issues shooting, maybe we'll be able to do the same.

Re: Pistons not extending fast enough?
 

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?

Re: Pistons not extending fast enough?
 

Thanks for the clarification. I will go ahead and calibrate the system tomorrow so that we are working at the regulated 60 PSI.

By a hard stop, do you mean something to stop the ramp before the pistons make it all the way to the end of the cylinder?

Re: Pistons not extending fast enough?
 

We are using this trick as well, to eliminate unneeded tubing and fittings between the solenoid and the piston to increase flow rate. See this image of our setup; it shows the solenoid valves directly attached to the pistons and how our plumbing is laid out.

Re: Pistons not extending fast enough?
 

Yes. A strong strap (like a tie-down strap) would be a good option. You just want to prevent the piston from striking the end-cap with high speed/force.

Re: Pistons not extending fast enough?
 

This is a recommendation I've seen before, and it seems reasonable to me.... however, we have something on the order of 1000 cycles on our practice robot catapult with no noticeable wear or potential failure of the cylinders, and we are using the cylinders with no hard stop. Mind that we avoid dry-firing like the plague, and under normal launching conditions the cylinders aren't hitting their end of travel with such an absurd velocity.

Re: Pistons not extending fast enough?
 

This is immensely helpful. Thank you.

Apart from having the solenoids connected directly to the cylinders, adding tanks before the pistons, and having them vented to the atmosphere is there anything else you've done to increase you efficiency? While I certainly believe that will help I find it hard to believe that this will bring our dysfunctional setup to working capacity, though I'm certainly going to try.

EDIT: Also, how do the pistons fall back in the cylinder? Does gravity do the work?

Re: Pistons not extending fast enough?
 

The only other big trick is to use a high-flow solenoid valve; we are using McMaster part number 6124K511 to feed each of our pistons. The valves you get from AndyMark or Vexpro flow about 3x less air (these were the maximum flow rate that we could use in the past... this year's rule set has allowed us to use a much larger variety of solenoids).

The geometry of the arm also plays a big role. Based on the videos you posted, I would extend the length of the platform that is shooting the ball, and move the piston locations on the base mounting board back about 0.5"-1" from the pivot location to allow it to sit "flatter" before firing. These pivot locations will all probably need a bit of adjustment as you continue testing.

Oh, and if you can get rid of the miles and miles of tubing between your pneumatics testing board and the catapult, you will probably see an immediate improvement. :)


EDIT: as far as gravity returning the catapult arm to the starting position, we have never had any trouble with this. It falls right back into place with ease.

Re: Pistons not extending fast enough?
 

Yep.

Re: Pistons not extending fast enough?
 

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!

Re: Pistons not extending fast enough?
 

All glory to the gate latch!!!

Every since I built a 16ft trebuchet for my 9th grade woodshop project I have LOVED using gate latches as a quick, easy, reliable, and cheap release mechanism. Picture Here Additional Info Here

Re: Pistons not extending fast enough?
 

We have found similar results with McMaster 6425k11

There are a large number of teams successfully launching with various length 1.5" Bores, with a single solenoid on each of the cylinders "extend" side and no valve on the "retract" side.

You may want to take a look at the geometry of 842, 624, or 2587.

Re: Pistons not extending fast enough?
 

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

Re: Pistons not extending fast enough?
 

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.

Re: Pistons not extending fast enough?
 

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.

Re: Pistons not extending fast enough?
 

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.

Re: Pistons not extending fast enough?
 

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

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.

Re: Pistons not extending fast enough?
 

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).

Re: Pistons not extending fast enough?
 

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.)

Re: Pistons not extending fast enough?
 

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.

Re: Pistons not extending fast enough?
 

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.

Re: Pistons not extending fast enough?
 

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

Re: Pistons not extending fast enough?
 

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!

Re: Pistons not extending fast enough?
 

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 ::safety::

Aloha!