Robustness of PID-controlled Manipulators vis-à-vis Actuator Dynamics and External Disturbances

Robustness in control systems is without doubt a property as important as stability; it is the property by which a system preserves an acceptable behavior under uncertainty, external disturbances, measurement noise, etc. Generally speaking, uncertainty and disturbances may take different forms and meanings, especially for the theoretician and for the practitioner. Uncertainty may stem from imprecision on numerical values of some parameters, inadequacy of the dynamic model, neglected dynamics, approximation of unavailable measurements, etc. Disturbances may take the form of noise in the measurements or external forces from physical interaction with the environment such as friction and, more generally in robotics, all forces that depend on time, position and velocity which affect the motion.