On board compressor vs Stored air

[WesleyK]

Our team has been debating whether to have a air compressor on the robot vs just having 4 or so air tanks on board the robot. Our robot is rocken a 3 cim shifting gearboxes for our drive train so it leaves use kind of stuck between a rock and a hard place.

On one hand With the compressor on-board we can shift unlimited times and have other pneumatic pistons on board. The pneumatic system would be small too since we do not need as many air tanks allowing us to store more balls on the robot. The negative side is additional weight and needing to Manually run the compressor through software so we don't trip the 120 amp breaker during a pushing match.

What experience have you had with pneumatic systems? have you had more success with a compressor on board or not on board?

[GeeTwo]

If you know just what you're going to use the air for, and have a good cap on how many times you'll need air in the match, it's probably worth going 20-50% over and accept the risk of needing it a few extra times.

However, if you do make this compromise, seriously consider your selection, wiring, and plumbing of solenoid valves. I heartily recommend that you use single solenoid valves (with spring return), figure out which state is preferred in the unlikely event that you do run out of air, and plumb/wire the system so if the air runs out, you have that default state. (For robots with shifters designed for speed, I'd rather be stuck in low than stuck in high; if designed for pushing wars, possibly in high.)

[Jon Stratis]

You shouldn't need to worry about the compressor tripping the main breaker during a pushing match. Check out the second graph on the datasheet and you'll see why. The breaker is a temperature driven device, and you can actually put 240A through it for 10 seconds before it trips, according to the graph. The snap action breakers respond much quicker, and they are actually what prevent you from tripping the main breaker in a pushing match, as each CIM is capable of pulling 133A when stalled, all by itself. a 4 CIM drive train has the potential to stall and pull over 500A!

[Kevin Sevcik]

As GeeTwo said, you should be able to calculate your air usage. If you're only using it for shifting, you can probably get by with one or two 36 cu in tanks. A single tank would get you 160 actuations of a 3/4" x 1/2" cylinder at 60 psi. So 40 low-high-low shifts for a double acting cylinder. 80 for the Andymark spring return cylinder. Shifting from low to high and back every 3 seconds seems reasonable, unless you have really aggressive auto shifting. If you can drop the working pressure to 40 psi without shifting too slowly, you can double the amount of actuations. 50 psi = 40% more shifts.

An onboard compressor isn't really going to save you much space, because it's big and you'd still need some sort of storage tank. A 36 cu in tank is 12 x 2.7, 88 cu in if you call it a box. The compressor is 2.1 x 4.5 x 6, 57 cu in as a box. So it's pretty close on volume consumption. You'd probably be better off running 2 tanks and no compressor than 1 tank and 1 compressor. You'd definitely be better off running just 1 tank.

[MrForbes]

be sure to figure in the little bit of extra effort required to make sure the tank is full before each match. And the fun of having a small leak that drains the storage tank during a match, but if you had a compressor on board, it would not be a problem. We had offboard compressor our rookie year (mainly because no one was keeping track of weight and the robot was way over weight), but on board ever since. I'm a fan of KISS and that means making it easy for the drive team, by not making them do any extra work.

[RichQuinn]

For Stronghold last year, we ran a small pneumatic kicker for the shooter and were able to run an entire match on a single tank. We chose to keep the compressor off-board to avoid the weight penalty.

Carrying an off-board compressor means always having to keep an external system on hand and nearby before every practice match, competition match, sponsor demo, etc. It is an inspected device at competition and requires duplicating the relief valve, gauges, pressure switch, battery, etc. Since humans will continue to be humans, there will be cases where the system fails to get charged before an event and you will ask yourself if those 2.4 pounds were worth it. For us, they were not.

Put me in the On Board camp from now on.

[GeeTwo]

Originally Posted by Kevin Sevcik

An onboard compressor isn't really going to save you much space, because it's big and you'd still need some sort of storage tank. A 36 cu in tank is 12 x 2.7, 88 cu in if you call it a box. The compressor is 2.1 x 4.5 x 6, 57 cu in as a box. So it's pretty close on volume consumption. You'd probably be better off running 2 tanks and no compressor than 1 tank and 1 compressor. You'd definitely be better off running just 1 tank.

If you're more limited by volume than weight and doing such small shifts, you can do a compressor and a much smaller (metal) accumulator; I'd recommend something at least 3x as large as the combined volume of your shifter cylinders. You can get these from Bimba or other sources. As a temporary solution (but as I read the rules not at competition) you can use the rod (retract) side of a medium-sized cylinder as an accumulator. There are probably also smaller plastic accumulators out there, but I haven't searched.

[theun4gven]

Originally Posted by MrForbes

be sure to figure in the little bit of extra effort required to make sure the tank is full before each match. And the fun of having a small leak that drains the storage tank during a match, but if you had a compressor on board, it would not be a problem. We had offboard compressor our rookie year (mainly because no one was keeping track of weight and the robot was way over weight), but on board ever since. I'm a fan of KISS and that means making it easy for the drive team, by not making them do any extra work.

I can't reiterate this enough. If you are running an offboard compressor you'd best be sure you've removed your leaks. First few matches of the day and they want your robot on the field or queued up before opening cermeonies? Someone having field connection issues or the field sits with your robot for 30+ minutes? Made it to Einstein and your robot is put on the field ready to play and you only have to wait for a small speech by Dean before you get going? That small leak adds up.

The Viair compressors are small and weigh around 2.4 lbs. At that size I'll take the peace of mind and removal of another step from the drive team's checklist over the weight savings.

[rich2202]

IMHO, weight won't be a problem this year. Space will be at a premium. I would go with a compressor and a small storage cylinder.

In your code, you can deactivate the compressor when power to the CIM's is more than 75% (or whatever number you want).