You've kinda hit upon the reason why there are few pneumatic robots.
There are no viable valves. Proportional servo valves are big,
expensive, and power hungry. Dynalloy (www.dynalloy.com) uses nitinol to
make a proportional valve. It's small, quiet, and less expensive than
comparable solenoid or servo valves ($8 when ordered in bulk), but it's
still difficult to drive and a huge power hog. Plus Dynalloy only offers
it as a demonstration product, so you have to buy tons of them for the
price per unit to become reasonable.
Maybe in a decade we'll see some improvements with polymers that'll
solve the problems inherent with SMAs. But for now robotic pneumatics is
a bleak and cumbersome field.
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