Almost all modern robots are driven by brushless AC servo motors but many of the robots existing in industry use other drives.
Many of the simple pick and place arms are driven by pneumatics. This makes them cheap but has the disadvantage of being difficult to control. Pneumatics are still used with a number of modern robots to drive end effectors.
Hydraulic drives were used on a large number of the early robots as it was more rigid and controllable than pneumatics and it could provide more power than the electric drives then available. The problems with hydraulics are that it tends to be fairly slow in operation and that due to the high pressures involved leaks can be very messy.
There are three major types of electric drive that have been used for robots.
These are used mainly for simple pick and place mechanisms where cheapness is more important than power or controllability.
For the early electric robots the DC servo drive was used extensively. It gave good power output with a high degree of control of both speed and position.
In recent years the AC servo has taken over from the DC servo as the standard drive. These modern motors give higher power output and are almost silent in operation. As they have no brushes they are very reliable and require almost no maintenance in operation.