Equivalence of LADRC and INDI controllers for improvement of LADRC in practical applications

This paper explores the equivalence between the linear active disturbance rejection control (LADRC) and incremental nonlinear dynamic inversion (INDI) controllers. The equivalence is verified using an n-order, single-input-single-output, perturbed, pure integration plant which represents a class of feedback linearizable systems. And the linear extended state observer (LESO) inside the LADRC is interpreted from a filter perspective, which shows that the core of the LESO is a low-pass filter. A better parameter tuning method is proposed for the LESO. Then, based on the equivalence, an actuator model is integrated into the LADRC to improve its performance on the plant with non-negligible actuator dynamics that are ignored by the original LADRC. Through the modification, compared with the original LADRC, the bandwidth of the LESO and the whole controller is extended, so that improved performance can be achieved on the plants with low-bandwidth actuators. The equivalence between two controllers and the effectiveness of the improved LADRC are both demonstrated by experiments conducted on a quadrotor.