Depending on the requirements in terms of accuracy, servo cylinders (Figure S 53) are used with two construction designs, which differ in terms of the bearing of the piston and piston rod:
1st cylinder with hydrostatic bearing and 2nd cylinder with contact seals, which must have a particularly low level of friction. These special seals are normally made of Teflon.
Servo cylinders, together with the load, form a system capable of oscillation with a relatively low natural frequency (between 10 and 100 Hz) and very low damping, between 0.05 and 0.2 (Figure S 54). As their stiffness also depends on the oil volume in the cylinder, they are not easy to handle in terms of control technology.
Figure S 53: Servo cylinder
Figure S 54: Bode diagram of a control chain servo valve –– cylinder