Juhoon Back

Brief paper: Consensus of high-order linear systems using dynamic output feedback compensator: Low gain approach

In this paper, we study the consensus (and synchronization) problem for multi-agent linear dynamic systems. All the agents have identical MIMO linear dynamics which can be of any order, and only the output information of each agents is delivered throughout the communication network. It is shown that consensus is reached if there exists a stable compensator which simultaneously stabilizes N-1 systems in a special form, where N is the number of agents. We show that there exists such a compensator under a very general condition. Finally, the consensus value is characterized as a function of initial conditions with stable compensators in place.

Adding robustness to nominal output-feedback controllers for uncertain nonlinear systems: A nonlinear version of disturbance observer

The linear disturbance observer (DOB) approach has been successfully employed as a tool for robust control and disturbance rejection in practice. In this paper, we present a nonlinear DOB which inherits all the benefits of the classical linear DOB approach. In particular, we prove that the proposed nonlinear DOB recovers not only the steady-state performance but also the transient performance of the nominal closed-loop system under plant uncertainties and input disturbances. One novel feature of the proposed controller is that it is an add-on type inner-loop output-feedback controller; thus any type of outer-loop controller can be combined to enhance the closed-loop system performance. Our result is a semi-global one and the stability proof is given when the nonlinear system has a well-defined relative degree with a stable zero dynamics.

An Inner-Loop Controller Guaranteeing Robust Transient Performance for Uncertain MIMO Nonlinear Systems

An output-feedback controller has been recently proposed that has the following features: 1) it is an inner-loop controller so that it can be added on the existing closed-loop system working in harmony with a pre-designed (possibly non-robust) outer-loop controller; 2) it robustifies the closed-loop system in a way that the uncertain plant under external disturbance behaves like a disturbance-free nominal plant; and 3) it recovers the trajectory of the nominal closed-loop system in time domain. However, it is restricted to the single-input-single-output nonlinear systems. In this technical note, we extend this result for a class of multi-input-multi-output (MIMO) nonlinear systems having the same number of inputs and outputs. The tools used in this synthesis are the singular perturbation theory and the multi-variable circle criterion. An example shows the effectiveness of the proposed method.

Consensus of output-coupled linear multi-agent systems under fast switching network: Averaging approach

This study addresses the problem of consensus of multi-agent systems, consisting of a set of identical MIMO LTI systems, under a time-varying network that has a well-defined average (with uniform convergence to the average). The information delivered through the communication network is the output of each system. First, it is shown that consensus is reached asymptotically by using a group of compensators if the network switches sufficiently fast and the compensators are designed such that the multi-agent system asymptotically achieves consensus for the average of the network. Further, we find a relation between the two agreements, one obtained from considering the switching network and the other obtained from replacing the network with its average. Then, for a class of minimum phase systems, we remove the fast switching condition by redesigning the compensators. Finally, the formation stabilization of unicycle-type mobile robots is dealt with as an application of the problem, and it is demonstrated via a computer simulation.

Sliding mode controller for torque and pitch control of wind power system based on PMSG

Recently, the size of wind turbines is increased. As a result, the permanent magnet synchronous generator (PMSG) plays an important role because of its simple structure and high efficiency. This paper proposes a torque and pitch control scheme for variable speed wind turbines. A torque controller is designed to maximize the output power below the rated wind speed and a pitch controller is designed to regulate above the rated wind speed. The controllers exploit sliding mode control scheme considering the rapid variation of wind speed. Since the aerodynamic torque and rotor acceleration are difficult to measure in practice, an observer is designed which estimate them. In order to verify the proposed control strategy, we provide stability analysis and present simulation results.

Yet another tutorial of disturbance observer: robust stabilization and recovery of nominal performance

This paper presents a tutorial-style review on the recent results about the disturbance observer (DOB) in view of robust stabilization and recovery of the nominal performance. The analysis is based on the case when the bandwidth of Q-filter is large, and it is explained in a pedagogical manner that, even in the presence of plant uncertainties and disturbances, the behavior of real uncertain plant can be made almost similar to that of disturbance-free nominal system both in the transient and in the steady-state. The conventional DOB is interpreted in a new perspective, and its restrictions and extensions are discussed.

Embedding Internal Model in Disturbance Observer With Robust Stability

The disturbance observer has been widely employed in applications due to its powerful ability for disturbance rejection and robustness under plant uncertainties. However, it rejects the disturbance approximately rather than exactly since it is usually designed without considering structural properties of disturbance. In order to improve the disturbance rejection performance, we propose a design method to embed the internal model of disturbance into the disturbance observer structure. Furthermore, a systematic design procedure is proposed so that one can always design the disturbance observer to guarantee robust stability of the closed-loop system even though uncertain parameters of the plant belong to an arbitrarily large (but bounded) set.

Reduced-Order Dynamic Observer Error Linearization

Abstract For an unforced nonlinear single output system, we propose a new approach to observer error linearization called reduced-order dynamic observer error linearization (RDOEL), which is a modified version of dynamic observer error linearization (DOEL). While RDOEL also has an auxiliary dynamics and a generalized output injection like DOEL, it offers a lower dimensional observer than DOEL, and allows a complete constructive algorithm in contrast to DOEL when the auxiliary dynamics is a chain of integrators. For RDOEL whose auxiliary dynamics is a chain of integrators (RDOELI), we provide a complete constructive algorithm which presents not only a coordinate transformation between the original system and a nonlinear observer canonical form, but also the minimum number of integrators needed to perform RDOELI. Moreover, we show that if the original system is of dimension n , then the minimum number of integrators is less than or equal to n –- 2. In order to describe our algorithm well, we present an example that the auxiliary dynamics is a chain of two integrators.

Rejection of polynomial-in-time disturbances via disturbance observer with guaranteed robust stability

The type-k disturbance observer, known as ‘high order disturbance observer’, is a variant of disturbance observer which contains an internal model for polynomial-in-time disturbances so that it can reject those disturbances exactly as well as unmodelled low frequency disturbances approximately. This paper presents a systematic design procedure for the type-k disturbance observer so that the closed-loop system is robustly internally stable and at the same time polynomial-in-time disturbances are rejected exactly. To develop the procedure, we employ the Kharitonov theorem and a root-locus based design tool to deal with the uncertain plant parameters explicitly, and as a result it is guaranteed that one can always find a solution step by step even if the uncertain parameters belong to an arbitrarily large (but bounded) set.

Consensus of Multi-Agent Systems Under Periodic Time-Varying Network *

Abstract In this paper, we address the consensus problem for a class of feedback linearizable nonlinear multi-agent systems under a dynamically changing and periodic network communication. The information delivered throughout the communication network is the output of each identical MIMO autonomous agent which can be of any order. First of all, with a local-level control strategy, we show that the consensus is reached by only using a linear time-invariant compensator if the network changes sufficiently fast in time and the compensator is designed so that the multi-agent system achieves the consensus for the average of the communication network. In this case, it is also shown that the state trajectories are close to those of average system if one of the agents is. Finally, in the case of integrator chains, we relax the fast switching condition of the network and give an example that verifies the result.