Hydraulic and Pneumatic Actuation
Fluid power pushes a piston with pressure. Hydraulics use oil, which barely compresses, giving enormous, stiff force for excavators and big legged robots. Pneumatics use air, which compresses like a spring, giving cheap, fast, and naturally soft motion that is hard to stop precisely.
Force from pressure
A piston of area \(A\) under pressure \(p\) pushes with force
Hydraulic systems run at 100 to 300 bar, so a 4 cm piston pushes over 10,000 N. Air systems run at about 6 bar, so the same piston gives only a few hundred newtons. Air also compresses, so a pneumatic cylinder behaves like a spring with stiffness about \(k \approx p A^2 / V\) for air volume \(V\).
What to notice
Add load. The pneumatic cylinder overshoots and bounces, because the trapped air acts as a spring, and with a heavy load it may not be able to lift at all. The hydraulic cylinder barely notices the load and stops crisply, since oil is nearly incompressible. The electric screw is precise and stiff but slower, and its force is limited by the motor.
Robots use all three: hydraulics for big legged robots and construction machines, pneumatics for simple grippers and soft robots, and electric motors for nearly everything else, because they are clean, quiet, and easy to control.