Let’s start with a 7 gal (1617 cu in) charged to 117.6 psig (that’s +8atm, selected to simplify the math). As we regulate this down to 58.8 psig (+4 atm), we are making 1617 * 4 = 6,468 scfm, or 1,293 cu in at +4 atm (58.8 psig). Multiplying this out, I show a potentially usable energy of over 76,000 lb-in. A tote weighs 7.8 lb, so that’s about 9,750 tote-inches, or 5 totes times 1950 inches. If each lift is 25", this is still a total of 78 lifts. That’s 78 lifts of 5 totes 25 inches. There were only 30 totes behind the walls, so even without doing any optimization beyond not pressurizing the down stroke, there is more than twice the required energy in one 7 gal tank to stack all of the totes behind the wall. (Though I haven’t done any flow calculations to determine if it can be done in 2:15!)
With a 10 pound air tank, this obviously won’t get down to 15 pounds, but I expect that it can be done for well under the available 35 pounds, including tank, cylinders, frame, tether, and electronics.
R18 is the list of motors, so that assemblies including motors do not fall under the CUSTOM CIRCUIT rules. Motor controllers are required to be connected directly to the PDP. I am 99+% certain that the GDC would consider a switching power supply which steps up from ~9V to ~12V would constitute “directly altering the power pathway”. (OBTW, a transformer cannot be used to step up DC voltage; that’s why Tesla’s AC power distribution system eventually won out over Edison’s DC system.) Consider the exceptions named in the second sentence of R44:
Stepping up the voltage is definitely consequential.
Thanks for clarifying the meaning of Power pathway. I thought that it meant that must be a direct pass-through without being interrupted by a sensor or relay etc. I was taking it in a rather literal sense.
We went the pneumatics route this year, so I have some first hand results as to how our testing went. Not powering the downstroke sounds like a good idea until you learn that most solenoids need 20 psi minimum on each side to open and close properly. We eventually decided on 15 psi, and it was still very slow going down. Also, don’t forget that once your tank drops below 60 psi, you will see decreased performance. For this reason, we went with slightly bigger cylinders at 30 psi so we wouldn’t see that effect until later in the match. No matter the size of your air tanks, at 60 psi you will see that effect halfway through your lifts.
Also, if you did what many teams did this year and let the second tote fall into the first one without lifting the first one, you will only need to lift 4 totes per 6 stack, not 5. You also won’t need to lift them as high, only from 1 tote high to above the chute instead of from the ground. IIRC that’s less than 25". Or you could do what we did and build a ramp attached to our stacker for the totes to slide to (almost) ground level, then you only need to lift the height of one tote.
I agree that powering the down stroke with 15psi is probably worse than not powering it at all, but relying on a spring return or (if there’s enough weight) gravity. Alternately, the two sides of the lift cylinders could be controlled through a 3-state solenoid or controlled separately so that you don’t need to fill the upper chamber of the cylinders to 60psi, even though the supply is that high.
Yes, if you read more closely you can see that I was calculating based on the air in the tank above 60 psi.
That would make it even better on several counts - fewer than half as many lifts, as well as a lighter peak lift. The lift would have to be more than 25", however. Each six-stack would involve lifting 6 totes about 30", which is much less than lifting 15 totes 25". With a bit of optimization and some sensors, I estimate that five six-stacks would take about 3 gallons, or 3 six-stacks would need about 2 gallons of tank.
I think you’re misunderstanding what I’m suggesting. The bottom of the piston’s travel would be at the lip of the second tote on the stack. The piston would then travel up so the bottom of the second tote would just clear the top of the chute. Granted this is more than 25" of travel, but it’s 12" less than it would be otherwise, you have to lift one less tote per 6 stack, and you only ever have to lift 4 totes at a time instead of 5. Therefore, since air consumption is lift force * lift height, you can decrease both lift force and height for an overall decrease in air consumption.
No, I understood, that you never need to lift more than four totes at a time. Perhaps I took your strategy even farther than you meant. Here’s what I’m thinking as “making a stack”:
Drop two totes through the chute. The first lands on the floor, the second on top of the first.
Lift those two totes high enough for the next step (about 30-36") (2 totes lifted)
Drop two more totes through the chute.
Lower the raised stack onto the stack on the floor, and continue to the low end of the stroke.
Lift the four totes high enough for the next step (4 totes lifted).
Drop two more totes through the chute.
Lower the raised stack onto the stack on the floor, and continue down at least far enough to disengage from any totes.
Open the release gate for the main robot to score it.
Six totes per stack when doing pairs is 2 for the first lift and 4 for the second lift; 2 + 4 = 6. Fifteen totes on single-stacking is 1 + 2 + 3 + 4 + 5. It doesn’t matter where the lift engages the tote; the length of stroke must be as high as the bottom of the tote moves up plus the “engagement distance”, that is the amount of motion between the bottom of the stroke and the tote being lifted off the floor. I’m estimating that to clear a 2-stack with a few inches for free entry of the second tote would be about 30-36".
If you were thinking of leaving a tote on the floor the whole time, that would be lifting 10 totes a bit over half as far as the six above. It’s probably a wash in terms of air, but two lifts should take less time than four.
That would be “leaving the downstroke unpowered”, which has been considered and largely discarded. Unless you have a separate mechanism to engage and disengage from the tote, your downstroke will have to work against some sort of spring action to get around the top edge of the tote. For reliable action, either the down stroke needs to be powered, or a spring return is needed, or the carriage must be heavy enough to force through the ratcheting action.
I was thinking of leaving one tote on the ground the whole time and stacking from there. Your way sounds like it could also work very well. Mine would use less air per cycle (shorter throw), but yours would use less cycles (2 vs. 4). I don’t have the numbers in front of me to do the exact calculations. For anyone that does, it would be a wash if the height to lift a ground-level tote is double the height to lift a tote stacked 2 high to above the chute. If it’s more, your idea uses less air; if less, my idea uses less.
Either way, your idea is almost definitely faster. You have one lift of two and one lift of four. I have that and also a lift of one and three. So either way, yours would have to be faster.
I just wish I thought of all of this during the season instead of now.
Also for the OP, if you do decide to go with pneumatics as GeeTwo suggested, you can move the PCM onto the tethered bot. Then you can decrease the tether to two wires (power and CAN). The power wire has a pretty low amperage when not running a compressor, so if you make it a big-ish wire you shouldn’t see too bad of a voltage drop.
EDIT: One more thing for the OP. When totes fall from the chute to ground level, they tend to not land correctly because the front of the tote tips down as it comes out of the chute. Some HPs tried to negate this by pushing on the tote with the chute door as it slid down the chute. I see this as a big source of human error that should be avoided if possible. You may want to experiment with what base height will allow the bottom tote to land properly while still giving clearance for the second tote to fall on top of the first one. This will keep your human player’s job down to a minimum, and it will also decrease the height you have to lift, which will decrease air consumption and lift time.
Depending on the design, this can be pretty trivial. Hinge-style tote lifters have nearly no resisting force in the opposite direction and if the weight of the carriage alone doesn’t do it, a tiny amount of surgical tubing would.
Powering the lift in one direction and only lifting the top 4 totes reduces the number of powered strokes required to stack to 4, with half the travel of lifting the bottom tote as well. It is also probably much faster to do this as it takes a long time for a tote to settle on the bottom level. I would have to run the calculations for air consumption but this could make 3 or 4 stacks with a reasonably small amount of air; several Clippard tanks would do it.
That’s not hard. The weight of the shaft alone will probably be enough to overcome the friction of the cylinder. Anyway Ari423’s reasoning for not powering the downstroke was not due to friction but due to the solenoids switching restrictions (what I’m questioning).
Well the reason I gave was solenoid switching restrictions, but it was actually more than that. We did devise a method of dealing with the solenoid. When we tried using in without any down pressure, it worked but it was painfully slow when only lifting one tote. Gravity just wasn’t strong enough to overcome the friction (granted we also had a pulley system to make sure both sides were the same height that could have added some friction). We settled on about 15 psi of down force, which was below the rated min pressure but it worked out ok. Perhaps with a strategy where you lift 2 totes minimum you can get away with a lower pressure or even no pressure.
You definitely either have too much friction, or you have the wrong flow control valve installed. This is a great application for one-way flow control valves, which restrict air going in but not out. My team had a stacking mechanism that would quickly and easily lower itself even without the weight of any totes on it; unless you have an extremely inefficient system this should work fine.
I can’t quite speak for the mechanical aspect of the system, as I had no hand in that. I only worked on the control aspect of the system. And since I’m the only member of my team on CD, I don’t expect we’ll ever get a good answer as to why our system had so much friction.
I was under the impression that one-way valves were illegal. Is that not true?
In 2015, Rule R66 F allows (among other things) flow control valves as pneumatic components. Festo’s page of one-way flow control valves describes the same thing Chris is describing - allowing free air flow one direction, throttled flow the other. There does not seem to be a rule allowing check valves, which allow air to travel in one direction but completely prevent airflow in the other direction. Apart from the interface between an external compressor and the robot, it is difficult to come up with an application for a check valve that complies with 2015’s rule R78, which has been a game rule for some time:
I wanted to share my reasoning for putting the PCM on the robot side. Either way has benefits and costs relative to the other.
The way I set things up, all of the wires carried simple DC signals that switched on the order of a second. CAN carries rather faster switching, so I wanted to leave it just on the main hull. I’ve since checked CAN lengths, and you should be OK running from one hull to the other, and even back. However, to simplify things and reduce losses, I would use an external terminator on the stacker end rather than bring the line back to the robot for termination at the PDB as most teams usually do.
I didn’t want to run CAN down the tether, because it’s inherently more vulnerable. I was worried that if something happened to the tether, the whole robot might freeze up. If you are using PWM for motor control, this may not be an issue for you.
I didn’t want to run compressor power down the tether. There are workarounds, including a spike relay on the main hull side, and using a “tether bypass” when filling the tanks*. Still, much less current is required to power a compressor than some CIMs.
Note that even with the PCM on the stacker hull, any limit switches or other sensors (except the pressure switch) still have to go back to the 'RIO. Limit switches and thoughtful design can significantly reduce the amount of air needed.
I haven’t found any rules in 2015 that would have prohibited making internal electrical changes (e.g. disconnecting the tether and wiring directly across it using molex or Anderson power poles) during pressurization process. Just make sure you put things back! The more steps in a checklist, the more likely something will get missed.