Fewer connections means fewer chances of leaks in your pneumatic system. This is especially important on any portions of your pressure system connected to your storage. Plumbing your system in series allows for any leaks on the storage side to drain all your remaining reserve air. Using a manifold mitigates the risk of that happening.
There are also weight, size/space, and aesthetic considerations.
Cleaner installation. They are easy to install and mount near the pressure regulating valve.
Flexibility with design. We usually decide how many solenoids we will need, and then add one more solenoid than that. As the design develops, we have that extra solenoid readily available to connect to if we add another actuator.
Simplified tube routing. A single input line at the manifold.
Failures. If a solenoid fails, it is easy to connect the needed lines to the spare solenoid and quickly get back in play.
The negatives are that they are a little heavier, and can be expensive. However, they last several years and can be re-used.
Automation Direct sales manifolds for their valves that is cost competitive with the additional plumbing that singles require. A lot easier to mount one manifold than multiple individual valves.
One reason not to use a manifold is if you trying to minimize pressure losses to a high flow device like a shooter cylinder. Then you want to mount the vavle close to the device and minimize your tubing length. (You could use bigger tubing, but you wouldn’t pass robot inspection.)
Chris covered all of my team’s reasons. I just wanted to +1 his last comment. Although we did not end up using pneumatics in 2016, they were a key part of our development strategy in several previous years. We started using the manifold pictured here on our 2014 competition robot, and have re-used it several times – on our 2015 competition robot and on a few off-season projects.
I’ll add one thing I’ve seen in inspection… fill all slots on the manifold appropriately! Solenoids attach to them at the top with a rubber gasket to ensure a good seal. If you aren’t using one of those slots, either put a solenoid in there anyways (It’s always good to have a backup, if you can afford the weight) or put in an appropriate cap that’s designed and sold for that use. I’ve seen teams block unused ports with epoxy or hot glue before, which just means that manifold is coming off the robot when the inspector sees it!
Taken to the extreme you mount the valve directly to the port on the side of the cylinder that needs the most speed. Run supply tubing directly back to the working pressure regulator locating the component for the shortest runs and bends possible. This will not take you from worst to first, but might help when you are battling your arch rival on Einstein for the fraction of advantage. (Of which we have not have had that pleasure yet) :]
Based on some number crunching I did during build this year, it appeared that the fastest cylinder performance would be to have a large tank (at least twice and preferably three times as large as the cylinder capacity) on the working pressure side of the regulator, then feeding directly from the tank to the solenoid valve to the cylinder. We did not get to test this, as it turned out the valves we were using had much too low a cv for this to make a difference.
Always good to know that the math gave the right answer!
Essentially, the air on the working side of the regulator is already there, ready to work. The air on the high side has to filter its way through the pressure regulator, which is not as fast. The tank needs to be significantly larger than the working piston so that there is not too much pressure drop in the supply during the stroke.
Also note that if you are going for a fast stroke, your cylinder should be large enough to move the load (slowly) at a rather low pressure, somewhere around 35psi/(desired acceleration + gravity), with acceleration measured in gees. There will be around 20 psi drop going through the solenoid valve, and pressure reduction as the cylinder extends (or retracts); the remainder is F=ma. (I am assuming here that your fast stroke is for the purpose of working against gravity to throw a ball or other object.)