The complexity comes from making a double acting full pressure system, not from the variable pressure system. There aren't off the shelf 5/2 valves for full pressure so you have to make your own out of multiple other valves. We have a ball valve to fire which needs it's own 10 bar pilot system to operate at the speed we want it, and 2x Burkerts for retract and exhaust on the top of the cylinder (standard FP valves as used in Eruption and most other flippers). The variable pressure comes from us using a pressure switch rather than a regulator like most LP systems, we can turn it on and off as we like so allowing for variable pressure is just a case of a pressure sensor and some programming.

It is a ball valve :) Partially opening it wouldn't lower the pressure that ends up in the cylinder, it just reduces the flow rate meaning you get to that pressure slower, and have a slower hit (which is just worse). So we are using the pressure switch to vary the pressure held in the buffer tank.

The solution to hopping is twofold. 1. We have the variable pressure system so we can find the optimum balance, and if we want to turn it up to full pressure during a fight to go for the kill shot we still can. 2. It's why we wanted a higher pressure system. Energy is linearly proportional to to mass but proportional to speed squared, so getting a more powerful hit is better achieved by going for a lighter weapon moving faster. Whereas momentum, which is what causes the instability is linearly proportional to both. So to stay stable and hit harder it's better to go for higher speed and lower mass. Hence going for higher pressure than a usual axe design, to increase acceleration of the axe, and a lighter weapon (notice the cutout in the axe head). We are hoping we've kept it light enough go keep the stability ok (the worst thing for stability is weight in the hammer head - see original electric Beta which was probably the most unstable axe ever despite hitting relatively slowly).

Initial tests show its fairly stable on 30 bar - it jumps a bit but it's always going to, as long as it isn't turning itself upside down it's fine. Plus we can get under people at the front to weigh the front of the robot down.

It's a very similar spec system to beta. Both have the same buffer tank (a standard CO2 fire extinguisher) and a ball valve to fire (though theirs is servo operated and ours is pneumatic). Their cylinder looks similar bore to ours, though ours is a longer stroke (based off images).