Effect of measurement noise in disturbance observer based control systems with two saturation constraints

Disturbance Observer (DOB) is a well known tool for control systems capable of compensating model un-certainty and rejecting exogenous disturbances. It has been reported previously that zero mean measurement noise may induce tracking error in DOB based control systems with a saturation nonlinearity due to saturating actuators. In this work, DOB based control systems with two saturation constraints are considered and the effect of measurement noise is investigated. Such system architecture arises from Robust Transient DOB (RTDOB) which intentionally bounds amplitude of disturbance compensation in order to avoid peaking phenomenon that can possibly worsen transient response of the closed loop system. Systems using RTDOB have two saturation constraints, one that naturally arises from saturating actuator, and the other artificially inserted to limit the amplitude of disturbance compensation. We show that zero mean measurement noise may induce tracking error in RTDOB based systems as well, analyze the amplitude of the error with systems parameters, and discuss its severity compared to that of a regular DOB based system. The accuracy of proposed analysis is demonstrated through simulations.