Pneumatic devices are principally very similar to hydraulic systems. A source of pressurized
air is used to power and drive linear or rotary cylinders, controlled by manual or electrically
controlled solenoid valves. Since the source of pressurized air is separate from the moving
actuators, these systems have lower inertial loads. However, since pneumatic devices
operate at a much lower air pressure, usually up to 100—120 psi max, their power to weight
ratio is much lower than hydraulic systems.
The major problem with pneumatic devices is that air is compressible and, as a result,
its volume changes under load. Consequently, pneumatic actuators are usually only used
for insertion purposes, where the actuator is all the way forward or all the way backward,
or they are used with 1/ 2—-DOF joints that are hilly on or fully off. Otherwise, controlling
the exact position of pneumatic cylinders is very difficult.
One way to control the displacement of the pneumatic cylinders is called diflerential
dithering. In this system, the exact location of the piston is sensed by a feedback sensor
such as a linear encoder or potentiometer. This information is used in a controller that
controls the air pressure on the two sides of the cylinder through a servovalve to control the exact position.
Pneumatic actuators are simple, rugged, and safe. Even if they leak, the air is not a
contaminant. Most components are off the shelf, and therefore, easy to use and
inexpensive. They are mostly used either as on/off devices or as accessories in a robotic
cell in conjunction with robots for material handling and similar purposes.