Identification of the Pneumatic Servo System Using the Self-excited Oscillations

The paper describes the identification of the variable critical gain and natural frequency of the pneumatic cylinder using the self-excited oscillations method. The pneumatic drives are characterized by the low stiffness of the pneumatic cylinder which depends on the piston position and determines the natural frequency of the cylinder. The variety of the stiffness in dependence on the piston position can be obtained using the curve describing the course of the critical gain and natural frequency in dependence on the piston position. This feature is a disadvantage by the closed loop control of the piston position. The low stiffness should be taken into the account by the design of the pneumatic servo system operated in the closed loop, because the natural frequency of the cylinder depends on the stiffness and limits the closed loop gain and quality of the step response of the controlled system. The identified curve can be used for the design of the adaptive controller with variable on the piston position dependent gain. The described approach is presented by the measurements and results realized on the test rig.