[Isaac501]

I don't see double solenoids as required.. We use single solenoids with springs and it works like a champ. Even with no air pressure and medium load, the gearboxes still go into gear without issue on startup. This also allows for a "limp home" mode if your pneumatic system fails catastrophically (or a low noise, slow-speed demo)

To echo this again, do not use servos. They are weak and slow.