This technical note studies the output consensus problem for a class of heterogeneous uncertain linear multi-agent systems. All the agents can be of any order (which might widely differ among the ...agents) and possess parametric uncertainties that range over an arbitrarily large compact set. The controller uses only the output information of the plant; moreover, the delivered information throughout the communication network is also restricted to the output of each agent. Based on the output regulation theory, it is shown that the output consensus is reached if the (state) consensus is achieved within the internal models among the agent's controllers (even though the plant's outputs, rather than the internal model's outputs, are communicated). The internal models can be designed and embedded into the controller, which provides considerable flexibility to designers in terms of the type of signals that are agreed on among the agents.
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.
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.
This paper addresses the problem of preserving the nominal performance of a conventional cascade controller for electric motor drives in the presence of parameter uncertainties and external ...disturbances. Practical reduced-order proportional-integral (PI) observers are incorporated with predesigned PI controllers in order to enhance the robust performance of current and speed controllers. A unified analysis based on the singular perturbation theory is presented to confirm the desired approximation of the augmented system with the PI observers to the nominal closed-loop system. The validity of the proposed dual observer-based control scheme is tested via computer simulations and experiments using a dc motor system. Both results verify that the proposed controller can be used effectively to recover the nominal performance of the conventional cascade scheme for electric motor drives.
In most research works on the disturbance observer (DOB)-based controller, stability conditions are only sufficient and not effective for handling large uncertainties. To deal with large ...uncertainties, a necessary and sufficient condition for robust stability has been recently provided by utilizing a novel design technique for the Q -filter. Although this design method can deal with large uncertainties, it is not capable of suppressing the effect of sensor noise. To achieve noise suppression as well as disturbance attenuation, this paper proposes a robust controller by utilizing a modified DOB structure. In contrast to the conventional DOB structure, noise suppression ability of the proposed controller can be adjusted by an appropriate choice of the Q-filter. Moreover, a necessary and sufficient condition for the modified DOB system to be robustly stable is newly presented. To show the effectiveness of the proposed controller, the performance is compared with other disturbance attenuation methods through simulation studies. Finally, in order to verify the validity of the proposed method, laboratory experiments are also conducted with brushless dc motor drives, which confirms that the proposed controller is quite useful for attenuating both input disturbance and sensor noise, while DOB controller is not capable of suppressing sensor noise.
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.
This paper investigates a class of sampled-data control systems in which at least one of the discrete-time zeros remains on the unit circle regardless of the sampling period. This type of zero is ...referred to as an "uniformly marginally stable" (UMS) zero. Utilizing this novel concept, we present necessary conditions as well as sufficient conditions for the continuous-time plant whose sampled-data model has an UMS zero. The theoretical finding of this paper newly highlights the significance of the symmetry regarding the zeros and poles of the continuous-time plant, as it plays a vital role in enforcing a discrete-time zero of the sampled-data system to be UMS, for both discretization zeros and intrinsic zeros. We develop the theory for sampled-data systems employing a conventional sampler and a zero-order hold together with a single-input single-output continuous-time plant, and then observe that certain results can be extended to the generalized holds. As an application, we construct a discrete-time stable inversion-based control in the sampled-data setting and establish a condition for continuous-time plants that allow for this type of control, in which absence of such an UMS zero is crucial.
This note points out certain limitations of our results from the paper mentioned in the title, and provides a modified approach to overcome these limitations. Specifically, the observer design ...addressed in the aforementioned paper is, in general, only applicable to switched linear systems with invertible state reset maps and this note presents a modified algorithm for state estimation that can also handle non-invertible state reset maps. In the process, we also identify some equalities from that paper which may not hold in general for arbitrary state reset maps.
In this paper, we consider the problem of designing a robust controller for a two-inertia system which contains arbitrarily large (but bounded) model uncertainties and disturbances. The research is ...motivated by the fact that a two-inertia system represents most of an industrial robot arm system that has a flexible joint, for which vibration suppression and robust control against model uncertainties and external disturbances are very important. The proposed controller consists of two parts: the outer-loop controller designed by the backstepping approach and the inner-loop controller by the new partial disturbance observer (DOB). The DOB is responsible for compensating the input-matched uncertainties and disturbances, while the backstepping controller is designed for dealing with the rest. The proposed controller makes tracking error and vibration of the system suppressed within an arbitrarily small bound during operation time when full states are measured. The results are verified by simulations and experiments with a Luenberger observer.
This technical note studies robustness of synchronization against heterogeneity of multi-agent systems. Here, heterogeneity also includes uncertainties and external disturbances in each agent. In ...order to effectively deal with the heterogeneous agents, we introduce the concept of the averaged dynamics which is the average of all agents' dynamics, and then claim that two sources enhance the robustness of synchronized behavior against heterogeneity of agents' dynamics. First, in spite of the heterogeneity of agents' dynamics, we show that strong coupling among agents makes the trajectories of all agents arbitrarily close to the averaged dynamics. Second, in the averaged dynamics, the effect of variations in each agent gets smaller as the number of agents increases. Simulation results confirm the claim that a large number of agents with strong couplings have robust synchronization.
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