Slowing down the action of a cylinder with flow control, mechanical or via software

[Randy Forgaard]

We are making some improvements to our robot in preparation for an off-season tournament. We have a pneumatics problem on our robot, and we're trying to find a solution that is allowable under the 2011 LogoMotion rules.

We have come up with a proposed solution for the problem, but we aren't sure if it is legal (under the rules) or practical, and are soliciting feedback.

NOTE: I have carefully read the thread, "Variable position of cylinder in compliance with <R74>," but our issue is a bit different. We don't need control over the variable position of a cylinder. That level of control is more than we require. We're just trying to reduce the speed of air flow from the exhaust port of a cylinder (see below).

Here is our issue: We have a long, pneumatically-actuated arm, with the pivot point at the bottom, that leans (rotates) down to pick up tubes. We use a double-acting cylinder to control the up-and-down movement of the arm. We are using one of the 5-port Festo valves that comes in the KOP for controlling a double-acting cylinder for this purpose.

To make the arm go down, we trigger the valve so that it sends pressurized air into the "down" port of the cylinder and exhausts from the other port of the cylinder. This sends the arm down fast (too fast, truthfully).

Our big problem comes when we reverse the valve to send the arm back up. The arm is rather long and heavy. When we send 60psi of air pressure into the "up" port of the cylinder (while exhausting from the other cylinder port), the pressure gradually builds in the cylinder for a couple of seconds until the cylinder is able to lift the arm. Then, when the cylinder lifts the arm, the arm starts going up slowly (fighting gravity), but as the arm rotates upward, there is less gravity to contend with (due to the angle of the arm), and the arm then slams very hard into the top stop bar on our robot. This makes us wince every time it happens. It really slams -- it's a wonder that the arm hasn't broken in the two tournaments we've been using the robot.

So, here's the thing. Even though we are using pneumatics to lower and raise the arm, we would like to be able to reduce the speed of the arm in both directions. We need to use the full 60psi of allowable pressure, because otherwise the cylinder cannot lift the arm from its lowest rotated down position. But using the full 60psi, the arm ends up slamming against the top stop when it is at the top and no longer fighting gravity, and we aren't happy about that. Also, in the down direction, the arm also goes too quickly.

For the down direction, the solution is fairly simple, but it doesn't appear to be allowable in the 2011 rules. We could simply use a COTS flow-control valve on the exhaust port of the Festo valve for the "down" direction of the arm, so that the air won't exhaust so quickly. A flow-control valve, for those who might not be familiar, is a simple, inexpensive, passive pneumatics part with a screw that restricts how fast the air can escape, so you can slow down the air flow. There are other, similar simple COTS pneumatics parts that achieve the same goal under different names: exhaust mufflers, speed controls, etc. They all work basically the same way -- they are passive little frobs that reduce the air flow so it doesn't move too fast. Any of these would allow the arm to go down nice and smoothly, and not overly fast. However, <R66> doesn't specifically list flow-control valves or similar devices as being legal for use this year, so it seems like we can't use a solution like that. I just want to confirm that we are reading the pneumatics rules correctly on this.

I can't quite understand why a passive flow-limiting device is not allowable. The main pressure vent plug valve will still release all stored air pressure in the system for our robot. There is no danger issue that I can see here of stored pressure, unless the flow control valve is completely closed (which would make the cylinder completely useless on the robot).

The "up" direction for the arm is also a problem, with the "slamming" at the end, and is more complicated to fix. A flow-control valve is not sufficient here. By the time the arm has rotated to the top (where it is about to "slam"), there is almost no gravity acting on the arm movement and the air exhaust is coming out of the cylinder very fast. If we used a flow-control valve for the exhaust in this direction, we would have to set the flow to be very constrained to prevent the slamming. But reducing the air flow that much would make the arm VERY slow to raise up when it is first coming up from the ground, fighting gravity -- too slow to use in competition.

A sort of obvious solution would be to use a solenoid-controlled valve on the exhaust port of the cylinder, to rapidly switch between allowing air to exhaust and blocking the exhaust. However, this would be in violation of <R74>, since we would be using multiple valves for a single commanded action of a cylinder, and this specific case is specifically clarified as illegal in Team Update #13. (This led to the <R74> discussion in this thread.)

So, here is our proposed solution for controlling the speed the air exhausts from the cylinder. If anyone sees a problem with this solution with respect to the rules, or can suggest an alternate solution, please let us know. Here's our idea:

When we are raising the arm, for the first few seconds, we will send all of the pressurized air into the "up" port on the cylinder port that raises the arm. After a few seconds, we will, under software control, slow down the arm by rapidly switching the Festo valve between the sending pressurized air into the "up" port of the cylinder and sending air into the "down" port of the cylinder. If we send air into the "up" direction 60% of the time, and send air into the "down" direction 40% of the time, the arm should still raise up, but more slowly, so it won't slam into the top stop bar of our robot.

How does that sound? It doesn't seem like it violates any of the rules, because we aren't adding any prohibited hardware, we're just using software to control the Festo valve very quickly. Also, the main pressure vent plug valve will still release all stored air pressure in the system for our robot. We haven't tried rapidly switching the Festo valve back and forth electronically before -- I wonder if that will burn out the Festo valve. Also, this solution is a kludge -- I wish we didn't have to do it this way, so if there is another way, that would be great.

Super sorry for this long message. In summary, we are looking for a way to effectively reduce the airflow out of the exhaust port on a pneumatic cylinder, in a variable manner (as the arm raises and needs to reduce the flow more), without violating the rules. And we are wondering if our software solution, above, makes any sense, or will just end up burning out our Festo valve.

I hope my problem explanation above is understandable. Any help appreciated. Thank you!

[Andrew Bates]

Have you considered just attaching some surgical tubing or springs so that when the arm is nearing it's uppermost position it has to stretch the tubing/springs. This should lessen the impact.

[Randy Forgaard]

Originally Posted by corpralchee

Have you considered just attaching some surgical tubing or springs so that when the arm is nearing it's uppermost position it has to stretch the tubing/springs. This should lessen the impact.

I really like that idea. Don't know why we didn't think of that. I'll bring it up with the team. Thank you!

[kwotremb]

Sounds like you have something similar to what we did this year on our robot. We had our entire mast and arm asembly rotate at a pivot point at the base that was controlled by a pnuematic cylinder. We used the flow control valves without any issues at all (basically the simple screw ones that replaced the straight or 90s that you screw onto the festo valvues). We had them pass inspection at 3 different events so I believe there should not be any issues with you using them. I thought it worked great for us.We only had the 'up' part of the piston hooked up to the air. Then for down all we did is have a flow control valve on the output that we controlled to make sure the fee fall of the arm was at a rate we needed.

Now for you, I would think for 'up' you guys should be able to use a flow control valve on the output just as you are thinking with the down. The flow control will make sure the rate at which the air can escape is limited, or more controlled. That way when the arm is moving slowly at first, the speed is not limited at all because the air can escape as fast as air is going in the other side of the cylinder. Then when you start to get to the top the air pressure will start to build up due to the fact that it will not be able to escape fast enough and slow down the rate of the arm. Hopefully that should be enough to prevent some of the slam you are experiencing on the top. Another simple way to melp out some of the slam is to put a bumper on the arm or frame where they hit, that way the slam is less violent. We were going to do this until we designed it so our cylinder maed out before we hit a hard stop. Hopefully this helps.

[Chris Fultz]

Flow control fittings are legal parts, and you can use them on both of the fittings of the pneumatic cylinder to control the speed in each direction.

You might want to also consider a gas shock of some type. McMaster Carr sells them in several lengths and forces.

We used one on our arm to provide some assist to the lift motors and to dampen the movement.

[Randy Forgaard]

Originally Posted by Chris Fultz

Flow control fittings are legal parts, and you can use them on both of the fittings of the pneumatic cylinder to control the speed in each direction.

You might want to also consider a gas shock of some type. McMaster Carr sells them in several lengths and forces.

We used one on our arm to provide some assist to the lift motors and to dampen the movement.

That's awesome that flow control fittings are legal parts. But I can't seem to find that in the rules. <R65> says that "no pneumatic parts other than those explicitly permitted by the Pneumatic System Rules may be used on the ROBOT." <R66> lists the pneumatic parts that are specifically allowed on the robot, and flow control fittings don't seem to be on the list. However, "kwotremb," who also responded on this thread, says that his team has used flow control fittings on their robot and it has passed inspection at 3 events, so it sounds like it's not a problem. But I'm still puzzled why flow control fittings don't seem to be explicitly allowed in the rules. I may be overthinking this.

[Randy Forgaard]

Originally Posted by kwotremb

Sounds like you have something similar to what we did this year on our robot. We had our entire mast and arm asembly rotate at a pivot point at the base that was controlled by a pnuematic cylinder. We used the flow control valves without any issues at all (basically the simple screw ones that replaced the straight or 90s that you screw onto the festo valvues). We had them pass inspection at 3 different events so I believe there should not be any issues with you using them.

That's interesting that your simple flow-control valves screwed into the exhaust on your Festo valve passed inspection at 3 events. Sounds like we may be more worried about the flow control valves than we should be. Clearly, it seems to me that flow control valves are in the spirit of the rules, since they don't store air and are entirely passive.

Originally Posted by kwotremb

We only had the 'up' part of the piston hooked up to the air. Then for down all we did is have a flow control valve on the output that we controlled to make sure the fee fall of the arm was at a rate we needed.

That's a good idea. We have been using pressurized air to send the arm down, because "at rest," the arm is straight up in the air so it doesn't naturally fall down. But there's no reason we couldn't change the stop bar so that the top of the arm is angled out slightly, so that gravity would drop the arm with no air pressure needed, and just use a flow control valve on the exhaust to reduce the speed of the drop.

Originally Posted by kwotremb

Now for you, I would think for 'up' you guys should be able to use a flow control valve on the output just as you are thinking with the down. The flow control will make sure the rate at which the air can escape is limited, or more controlled. That way when the arm is moving slowly at first, the speed is not limited at all because the air can escape as fast as air is going in the other side of the cylinder. Then when you start to get to the top the air pressure will start to build up due to the fact that it will not be able to escape fast enough and slow down the rate of the arm. Hopefully that should be enough to prevent some of the slam you are experiencing on the top.

Your description makes sense. We experimented with a flow control valve on the exhaust port for the "up" direction, but found that the arm still slammed. But I think we must not have done enough experimenting with the valve, because it makes total sense to me that we should be able to restrict the flow enough so that it won't slam, and it also won't have any impact on the speed that the arm raises since the air can escape as fast as the air is going into the other side of the cylinder, as you note above. If indeed we can legally use a flow control valve (and, from your experience at those 3 events, it appears we can), it seems like that should solve the problem nicely for us.

Originally Posted by kwotremb

Another simple way to melp out some of the slam is to put a bumper on the arm or frame where they hit, that way the slam is less violent. We were going to do this until we designed it so our cylinder maed out before we hit a hard stop.

I'm interested in your last sentence there. You mentioned you were going to install a bumper where the arm hits the frame, but you didn't need to do that because you designed the arm so the cylinder "maed out" before it hit a hard stop. I'm not sure what the means, but it sounds promising. Could you explain a little more?

And thanks very much for your detailed recommendations. It sounds like your robot uses a very similar arm mechanism to ours.

[Mr V]

Flow control valve between the solenoid and cylinder are legal. I know that a team who's lead mentor is also an inspector and has been before being associated with that team used them and I guarantee that he would not allow parts that aren't legal on his teams robot.

There are flow control valves that regulate the flow in only 1 direction. So put one that regulates the flow into the side that gets pressurized and/or one that regulates the flow out of the side that exhausts. You should be able to do what you want with one if that doesn't work add the other.

[Randy Forgaard]

Originally Posted by Mr V

Flow control valve between the solenoid and cylinder are legal. I know that a team who's lead mentor is also an inspector and has been before being associated with that team used them and I guarantee that he would not allow parts that aren't legal on his teams robot.

That's very good to know, thanks!

[kwotremb]

Originally Posted by Randy Forgaard

I'm interested in your last sentence there. You mentioned you were going to install a bumper where the arm hits the frame, but you didn't need to do that because you designed the arm so the cylinder "maed out" before it hit a hard stop. I'm not sure what the means, but it sounds promising. Could you explain a little more?

And thanks very much for your detailed recommendations. It sounds like your robot uses a very similar arm mechanism to ours.

Sorry, typo there.

Basically when our mast was all the way at the up we "maxed" (missed the x before) out on the stroke of the cylinder rather than hit a hard stop on the frame. We still had a little bit of hard 'slam' per say, but we were only at 35-40 psi and we felt that out mast was strong enough to take that hit, we were more worried about doing wheelies or ruining the cylinder. But with the battery and compressor in the front of the robot, it took a lot to do a wheelie. Also the cylinders should be designed to take more of an impact then we were putting it under. We were more worried about beding something if we kept hitting the mast onto the frame so we were just going to add some rubber or foam piece to add a shock absorber in that location.

What I would do with testing is start fully closed and slowly open it up as you test (that what we did for the free fall). Now I dont know if there will be enough air in the cylinder to compress to a psi high enough to counter the 60 psi you are lifting the arm up, but in theory I believe it should help.

Here is a decent pic of our robot in starting config, shows mostly what we were doing:
http://i271.photobucket.com/albums/j...1/IMG_4565.jpg

[Randy Forgaard]

Originally Posted by kwotremb

Basically when our mast was all the way at the up we "maxed" (missed the x before) out on the stroke of the cylinder rather than hit a hard stop on the frame.

That's a great idea as well. We could potentially replace the cylinder with one that has a somewhat shorter stroke, to prevent the slamming at the top.

Originally Posted by kwotremb

What I would do with testing is start fully closed and slowly open it up as you test (that what we did for the free fall). Now I dont know if there will be enough air in the cylinder to compress to a psi high enough to counter the 60 psi you are lifting the arm up, but in theory I believe it should help.

We'll do that. Thanks again!

[jspatz1]

Basically when our mast was all the way at the up we "maxed" (missed the x before) out on the stroke of the cylinder rather than hit a hard stop on the frame.

Originally Posted by Randy Forgaard

That's a great idea as well. We could potentially replace the cylinder with one that has a somewhat shorter stroke, to prevent the slamming at the top.

We'll do that. Thanks again!

A hard stop on the cylinder will probably not be any less problematic than a hard stop on the frame (it will still "slam"), and is very hard on the cylinder itself.

It is always easy to shorten the working length of a cylinder. Simply add a spacer between the clevis and the nose of the cylinder, so that it bottoms out externally on the spacer instead of internally on the piston. You can always use a cylinder that is a little longer than needed with this technique, but you can't get more stroke out of a cylinder that is too short.

[Tristan Lall]

There seem to be some errors in the information being given here. Let me try to clarify.

Flow control fittings are permitted (if they are "connecting fittings"). Flow control valves (and check valves, incidentally) are not permitted (because they're not any of the things permitted by <R66> and the rest of the pneumatics rules).

The fittings Chris was referring to are like an NPT-to-tubing elbow connector, with a needle-type adjustable flow control screw built in. Those are almost certainly legal, because they are usually understood to be a type of fitting. A flow control valve, that isn't also a fitting, is almost certainly illegal.

At an off-season event, do whatever the organizers require. If they're alright with teams taking reasonable liberties, go right ahead. (Usually there's no inspection.)

[Randy Forgaard]

Originally Posted by Tristan Lall

There seem to be some errors in the information being given here. Let me try to clarify.

Flow control fittings are permitted (if they are "connecting fittings"). Flow control valves (and check valves, incidentally) are not permitted (because they're not any of the things permitted by <R66> and the rest of the pneumatics rules).

The fittings Chris was referring to are like an NPT-to-tubing elbow connector, with a needle-type adjustable flow control screw built in. Those are almost certainly legal, because they are usually understood to be a type of fitting. A flow control valve, that isn't also a fitting, is almost certainly illegal.

At an off-season event, do whatever the organizers require. If they're alright with teams taking reasonable liberties, go right ahead. (Usually there's no inspection.)

Tristan: That makes a lot of sense to me. Thanks for pointing out that distinction between flow control fittings and flow control valves. This afternoon, we ordered some flow control fittings, with 1/8" NPT ports that screw right into the exhaust ports on the Festo valves. As fittings, those would seem to be within the spirit of the rules.

Interestingly, there was actually a question on the official Q&A about the legality of using flow controls. The questioner pointed out that in the Pneumatics Manual Rev B for 2011, the sample pneumatics diagram on Page 8, and the photos on Pages 9-10, show flow control fittings screwed into ALL of the cylinder ports. These are the NPT-to-tubing fittings with the needle-type flow control screws that you referred to above. But somewhat confusingly, <R66> seems to say that only the items it specifically lists are allowed in a pneumatics circuit on the robot, and flow controls are not specifically listed in <R66>. The questioner asked for a clarification on this apparent contradiction from the GDC. Unfortunately, the GDC's response was simply that "All pneumatic valves must comply with Rule <R66>," which still (to my mind) would seem to leave the answer ambiguous.

The wording of <R66> seems to add to the confusion, since it simply provides a list of "pneumatic system items" that are "specifically permitted," and <R67> lists pneumatic items that are prohibited. But what about the pneumatic items that are in the "no-man's land"; i.e., not shown on either the <R66> list or the <R67> list? Strictly speaking, it would seem that both flow control fittings and flow control valves are in this no-man's land.

However, <R66> Paragraph E does allow for unaltered COTS "connecting fittings" with a working pressure of at least 125psi. Does a flow control fitting that also provides connection to a tube count as a connecting fitting (which is legal), or is it a pneumatics part that provides extra functionality and thus it not permitted because it is not specifically listed as permitted in <R66>?

The point is mostly moot, because the official competitions are over for the year, and as you point out, most off-season competitions don't even do inspections. So, we'll feel free to use flow control fittings and not worry about it. But I kind of wish the GDC had made an explicit ruling to allow them. Maybe this ambiguity will be cleared up in next year's competition.

Thanks again for your thoughtful (and helpful) response, and for clarifying the difference between the two types of flow controls.

[jreuter]

Randy:

If you take a close look at the 2011 pnuematics manual, you will see flow control devices in both the schematic and photographs. The SMC NAS2201F-N01-07S flow control device was included in the 2008 KoP, and previous years, if I recall. We have some of these in our shop if your shipment doesn't arrive in time. Give me a call if you need them.

Also, you can indeed use separate Festos to switch between a high flow and low flow path, as long as you use a separate control signal for each one. You are correct that you are not allowed to control two valves from one control signal.

I have also read CD postings of people who rotate the valves with servos to provide software control of the flow rate.

With regard to rapid on/off pulses to the Festos, 1073 did this in 2010 to provide software-control preload pressures in our kicker. I was amazed that the Festos were so responsive, but we got very repeatable results, with no observed reliability problems.

Good luck at Beantown!
-Jeff