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The liquid cooling system (LCS) of the proton exchange membrane (PEM) fuel cell suffers from large time delay, coupling, uncertainties, and various disturbances, making it susceptible to temperature regulation overshoot and control oscillation. This article focuses on a composite control scheme for the LCS that has effectively addressed the aforementioned issues. We first propose a two-layer control approach for the pump to minimize coupling, and an average delay model (ADM) for radiator whose parameter uncertainties are removed by equilibrium optimizer (EO). To overcome disturbances, an adaptive sliding mode control (ASMC) based on extended state observer (ESO) is presented, and its finite-time convergence is demonstrated. Additionally, the indirect regulation for the stack temperature is designed to considerably reduce the time delay in the fan control loop. Co-simulation results indicate that the proposed ASMC significantly suppresses the chattering phenomenon compared with regular sliding mode control (SMC). In contrast to incremental fuzzy control, the proposed scheme possesses superiorities in response speed, tracking accuracy, anti-disturbance, as well as suppression of overshoot and oscillation. • A two-layer control approach is developed for the coolant pump. • An average delay model is presented for the radiator. • An adaptive sliding mode control is proposed for the cooling fan. • An indirect regulation method significantly reduces the time delay in the fan control loop. [ABSTRACT FROM AUTHOR]

In the work, the path tracking control methods are developed for autonomous agricultural tractors subject to wheel slip constraints by using second-order sliding mode (SOSM) and finite-time disturbance observer (FDOB) techniques. First of all, the path tracking error dynamics derived from the kinematic model is given and applied to controller design. In order to deal with the inevitable chattering problem existing in the conventional first-order sliding mode (FOSM) controller, a SOSM controller is constructed by regarding the controller derivative as the new control law, which means that the practical control law can be seen as an integration of the SOSM controller. Through a combination of the FDOB and the designed SOSM controller, the composite path tracking controller is further constructed to avoid high control gains in the designed SOSM controller. The strict Lyapunov stability analysis is carried out to ensure that the sliding variable can be finite-time stabilized to the origin under the proposed control algorithms. Finally, the comparative simulation results confirm that the developed guidance laws can achieve good tracking performance and strong robustness even in the presence of slipping effects. • The second-order sliding mode (SOSM) controller is designed to solve the path tracking control problem of agricultural tractors. • The composite control method is proposed to avoid high gains in the designed SOSM controller. • The slipping effects and unknown external disturbances are simultaneously considered in the controller design. [ABSTRACT FROM AUTHOR]

This work has developed an approach to a high-order sliding mode (HOSM) controller design for nonlinear systems subject to output constraints. To handle the output constraints, a barrier Lyapunov function (BLF) has been adopted. By inserting the BLF into the revamped adding a power integrator (API) technique, a HOSM controller, which can handle the closed-loop sliding mode dynamics synchronously with and without output constraints, is constructively framed. The strictly mathematical proof shows that the presented strategy can assure the finite-time stability of the closed-loop system and the achievement of a pre-set output constraint. The differences between the proposed HOSM controller and the traditional HOSM controller are discussed via a numerical example. Finally, the theoretical results obtained are successfully applied to a series elastic actuator system. The simulation results clearly confirm the merit of the presented HOSM algorithm. • By implanting BLF into API technique, a HOSM controller design scheme is developed. • The strategy can handle the HOSM dynamics with and without output constraints. • The HOSM controller ensures the system finite-time stability with output constraints. [ABSTRACT FROM AUTHOR]

• In this work, a new general control strategy for the asymptotic tracking problem of high-order uncertain nonlinear systems is proposed so that the convergence of tracking error is not affected by the initial value and the Lyapunov function. • The proposed approach ensures that all signals in the whole closed-loop system are bounded, the adaptive compensation is performed for each step, which avoids overestimating parameters and overcomes the inherent nonlinear obstacles of the system. • The flexibility of the control design is increased by introducing the ingenious coordinate transformations and algebraic transformations. The adaptive technology and the technique of adding a power integrator are successfully verified to update and extended to an universal adaptive asymptotic tracking control, which also provides guidance for constructing general adaptive controllers and verifying asymptotic convergence. This paper investigates the asymptotic tracking control of uncertain high-order nonlinear systems with time-varying desired signals.A new universal adaptive tracking control strategy is introduced, which combines the technique of adding a power integrator, adaptive technology and a series of algebraic transformations to achieve asymptotic tracking performance successfully. Compared to the traditional tracking control strategies, the proposed control scheme guarantees that the output tracking error of the high-order nonlinear system asymptotically converges to zero and is no longer limited to a time-varying compact domain. While ensuring that all signals in the closed-loop system are bounded, the controller also has sufficient ability to compensate for the unknown quantity, virtual control coefficient and parameter uncertainty of the system. Finally, the feasibility and effectiveness of the theoretical results are verified by a simulation example. [ABSTRACT FROM AUTHOR]

This communique proposes a novel second-order sliding mode (SOSM) control method to handle sliding mode dynamics with mismatched term, so as to reduce the terms in the control channel. Meanwhile, it is shown that the proposed control approach can be used to design SOSM controllers under disturbances bounded by positive functions rather than conventional constant upper bounds. The finite-time stability of the sliding variables has been shown by using finite-time Lyapunov theory. The validity of the proposed approach is verified by controlling a Buck converter. [ABSTRACT FROM AUTHOR]

This article aims at solving the straight-line tracking control problem of agricultural tractors in the presence of negative influences induced by unknown disturbances. Firstly, the path tracking dynamics model with unknown disturbance is established, and then it is further converted into a third-order sliding mode (TOSM) dynamics that facilitates control design. On this basis, a straight-line tracking control strategy for agricultural tractors is proposed by using TOSM. It is demonstrated by strict Lyapunov analysis that the lateral error and heading deviation are stabilized to the origin in a finite time. Finally, the comparative simulation results show that the proposed path-tracking algorithm can achieve the control objective of straight-line tracking of agricultural tractors. [Display omitted] • The third-order sliding mode controller is proposed to handle the problem of agricultural tractor navigation. • The proposed control scheme can ensure that the tracking errors converge to zero in a finite time. • The closed-loop sliding mode dynamics can be driven to the origin in finite time. [ABSTRACT FROM AUTHOR]

High-order sliding mode (HOSM) control is known to provide for finite-time-exact output regulation of uncertain systems with known relative degrees. Yet the corresponding universal HOSM controllers are typically constructed by special recursive procedures and have complicated form. We propose two new families of homogeneous HOSM controllers of a very simple form. Lyapunov functions are provided for a significant part of the first-family controllers. The second family consists of quasi-continuous controllers, which can be done arbitrarily smooth everywhere outside of the HOSM manifold. A regularization procedure ensures high-accuracy output regulation by means of control with required smoothness level. Output-feedback controllers are constructed. Controllers of the orders 3–5 are demonstrated. [ABSTRACT FROM AUTHOR]

• This is the first work to design a finite-time controller with a new adaptive mechanism by constructing a novel preset region. The dynamical change of the control gain is dependent on the case whether the state trajectories enter or leave a pre-designed domain or not. • The novel adaptive control strategy is presented to avoid the overestimation of the developed adaptive gain based on the adding a power integrator technique and the Lyapunov method. Meanwhile, the novel practical adaptive finite-time controller could improve the dynamic performance of systems and reduce the potential chattering influence. • In practical applications, the system states are usually finite-time stabilized to a neighborhood of the origin, rather than the origin. From this point of view, the investigation of practical finite-time stability (PFTS) for nonlinear systems is more meaningful. In this paper, the PFTS of second-order nonlinear systems has been verified under the developed adaptive finite-time controller. This paper considers the practical adaptive finite-time stabilization problem for a class of second-order systems. Based on the adding a power integrator technique and the adaptive method, a novel adaptive control strategy is presented to assure the trajectories of the closed-loop systems converge to a small neighborhood of the origin in a finite time. Under the adaptive control strategy, the dynamical change of control gain is dependent on the cases whether the state trajectories enter or leave a pre-designed domain or not. In addition, the practical finite-time stability (PFTS) of systems is testified via utilizing the Lyapunov method and adaptive mechanism. Last of all, the validity of the obtained practical adaptive finite-time algorithm is verified via simulation results. [ABSTRACT FROM AUTHOR]

Abstract: A global nonsmooth stabilization scheme is presented for a class of nonlinear cascaded systems with uncontrollable linearization. A stepwise constructive control methodology is proposed for the driving subsystem by using the adding a power integrator technique. Under suitable conditions and based on homogeneous properties, it is proved that the stabilization of the driving subsystem implies the stabilization of the overall cascaded system. Due to the versatility of the adding a power integrator technique and homogeneous properties, the proposed controller not only can be used to stabilize the cascaded system asymptotically, but also is able to lead to an interesting result of finite-time stabilization under appropriate conditions. [Copyright &y& Elsevier]

Abstract: In this paper, we consider the attitude stabilization problem for a rigid spacecraft with external disturbances. To obtain a better disturbance rejection property, we employ finite-time control techniques. In the absence of disturbances, by employing continuous finite-time control method, a continuous finite-time controller is designed such that the attitude of the rigid spacecraft will converge to the origin in finite time. In the presence of disturbances, by employing terminal sliding mode method, a discontinuous finite-time control law is proposed such that the states will eventually converge to a small region of the origin, which can be rendered as small as desired. Numerical simulation results show the effectiveness of the method. [Copyright &y& Elsevier]

• The paper projects a finite-time output feedback control framework subject to asymmetric output constraints. • The proposed control strategy can assure the finite-time stability of the whole system under output constraints. • Via an observer, only partial state knowledge is demanded to be measured in the proposed control method. This work has addressed the output-feedback finite-time stabilization issue for a type of asymmetric output-constrained planar systems. The innovation involved in the results of the work owes to the construction of a barrier Lyapunov function (BLF) to the handling of the asymmetric output constraint. By incorporated the BLF into the backstepping-like technique, a state feedback controller is first developed. An implementable observer is subsequently designed to estimate the unmeasurable system state. Lastly, an observer-based finite-time output feedback controller is explicitly proposed. Theoretically, the Lyapunov stability theory is adopted to show that the system states are finite-time stabilized under the presented control scheme. Meanwhile, the violation of a pre-established asymmetric output constraint is circumvented. To testify the derived theoretical results, the case studies of a permanent magnet linear motor system are offered. [ABSTRACT FROM AUTHOR]

This paper investigates the control design problem for the second-order sliding mode (SOSM) dynamics with mismatched uncertainty subject to the output constraint by means of the adding a power integrator technique. An unified method through the barrier Lyapunov function (BLF) technique and adding a power integrator technique has been proposed. The contributions of the paper can be summarized as follows. • This is the first time to investigate the stabilization problem for the SOSM dynamics with mismatched uncertainty subject to output constraint. • The homogeneity of the SOSM control method holds due to the modified BLF. • The proposed method can deal with the symmetric output constraint as well as the asymmetric output constraint. • The developed SOSM controller has the generic property since it is also valid in the absence of the output constraint or the mismatched uncertainty. This paper investigates a control design problem for the second-order sliding mode (SOSM) dynamics with mismatched uncertainty subject to the output constraint by means of the adding a power integrator technique. Firstly, a SOSM dynamics with mismatched uncertainty is constructed by retaining some information in the mismatched channel so as to reduce the uncertainty in the control channel, thereby relaxing the control requirement. Secondly, a barrier Lyapunov function (BLF) is designed in consideration of the output constraint to prevent the considered system from escaping from the output constraint. Moreover, the output constraint can be either symmetric or asymmetric. Thirdly, by embedding the above BLF into the adding a power integrator technique, a SOSM controller can be eventually constructed for the SOSM dynamics with the mismatched uncertainty to solve the output constraint problem. The finite-time stability analysis is proved by Lyapunov theory. The simulation results considering the symmetric constraint as well as the asymmetric constraint are provided to demonstrate the validity of the proposed control method. [ABSTRACT FROM AUTHOR]

The output constraints widely exist in the physical systems and severely affect the performance of the closed-loop system. This paper considers the second-order sliding mode controller design subject to an output constraint. By constructing a new barrier Lyapunov function and applying the technique of adding a power integrator, a novel second-order sliding mode control algorithm, which can be used to deal with the output constraint problem, has been developed. The proposed sliding mode algorithm enables the output variable not to violate the boundary of the constraint region. Meanwhile, it has been shown that under the output constraint the sliding variable can still be stabilized to zero in a finite time. Finally, an academic example is given to verify the feasibility of the proposed second-order sliding mode algorithm. [ABSTRACT FROM AUTHOR]

This paper considers the problem of finite-time stabilization in probability for a class of high-order stochastic nonlinear systems with output constraints in which the fractional powers are only required to be positive rather than not less than one. Based on a modified version of adding a power integrator technique and a novel tan-type barrier Lyapunov function, a systematic design approach is developed and a state-feedback controller is constructed. Rigorous mathematical proof shows that the origin of the system is finite-time stable in probability and the constraint requirement on the output is not violated. The effectiveness of the proposed finite-time control scheme is verified by a simulation example. [ABSTRACT FROM AUTHOR]

The mean-square exponential stabilization problem of stochastic Markovian jump system subject to time-varying delays and uncertainties has been investigated by using integral sliding mode control technique. Firstly, in order that the system trajectories can be kept on the integral sliding surface almost surely since the initial time, an integral sliding surface is constructed by properly choosing some matrices. On this basis, a novel sliding mode controller is designed to guarantee that the states will be kept on the sliding surface. Then, the sufficient condition in terms of linear matrix inequalities is presented to ensure that the mean-square exponential stability of the states can be guaranteed. Finally, a numerical example is provided to illustrate the effectiveness of the proposed design method. [ABSTRACT FROM AUTHOR]

• A modified design process of finite-time control (FTC) is achieved by utilizing prescribed performance control (PPC) and pre-assigned finitetime function (PFTF). • The states can converge to a predefined region within the pre-assigned finite-time. • The proposed method is more flexible and simpler since the convergence time is not related with the design parameters and initial values. • Nussubaum gain is employed to tackle the FTC problem for parameter systems with unknown signs. This paper mainly studies an adaptive finite-time control problem for a kind of uncertain strict feedback systems with unknown control gains. First, the original strict-feedback system is transformed into a pure-feedback system. And then, we propose a modified adaptive finite-time design idea on the basis of the property of pre-assigned finite-time function (PFTF) and Nussbaum gain. The designed controller can make sure that all states converge to an arbitrarily region in finite-time and signals in the closed-loop system are finite-time bounded. Moreover, the presented controller is smooth and the settling time does not rely on the initial values and design parameters. Finally, the simulation study is made to illustrate the innovativeness for the proposed scheme. [ABSTRACT FROM AUTHOR]

• This is the first work to investigate the design of SOSM controller with mismatched uncertainty and time-varying output constraint, and to further verify the stability of the uncertain system. • A new time-varying BLF has been constructed to handle the time-varying output constraint. By modifying the API technique and using the time-varying BLF, a SOSM controller for the SOSM dynamics under the time-varying output constraint is systematically projected such that the finite-time stability of closed-loop system can be guaranteed. Meanwhile, the time-varying output condition is never violated during operation. • In accordance with the Lyapunov approach, the finite-time stability of the closed-loop sliding mode dynamics is confirmed. Notably, the proposed method can handle nonlinear systems with the time-varying output constraint, static symmetrical output constraint, and can also be used to deal with nonlinear systems without output constraint. Therefore, the SOSM controller which is developed in this paper is more general. The current work develops a new approach to handle the second-order sliding mode (SOSM) controller design with mismatched term and time-varying output constraint. A novel time-varying Barrier Lyapunov Function (BLF) is firstly constructed to deal with the time-varying output constraint. A new SOSM dynamics subject to mismatched terms is established for the purpose of uncertainty reduction in control input channels. Meanwhile, by incorporating the time-varying BLF into the adding a power integrator (API) technique, a new finite-time SOSM controller, which can ensure that the system output variable will not violate the boundary of the constrained region, is explicitly constructed. Finally, a simulation example is given to illustrate the feasibility of the proposed SOSM algorithm. [ABSTRACT FROM AUTHOR]

In this technical communique, we propose a novel observer-based adaptive scheme to deal with the parameter estimation problem of biased sinusoidal signals. Different from the existing adaptive frequency estimation scheme, the proposed scheme can achieve fixed-time frequency estimation, whose convergence time is independent of the initial errors. Simulation example with different initial values shows the effectiveness of the theoretical result. [ABSTRACT FROM AUTHOR]

• A new concept, called fixed-time funnel boundary (FFB), is defined for the first time in this paper. • This work is first to introduce a new control mechanism for TCP/AWM system to ensures that the tracking error satisfies certain transient performances. • The proposed control scheme has the better performance and faster convergent rate than results without FFB or ACT. In this paper, TCP/AWM network congestion control problem is investigated. First, a new funnel boundary (FB) is proposed. And then, in the framework of backstepping, a new control technique is given by virtue of free-will arbitrary time stability and the novel FB, which guarantees that the tracking error converges to the origin within an arbitrary setting time. Besides, the transient performances of the error can also be achieved. Finally, simulation results verify the efficacy and superiority of the presented theoretical findings. [ABSTRACT FROM AUTHOR]

Aiming at identifying for tree branches obstacle in navigation automatically and picking process of agricultural harvesting robots, an iterative threshold segmentation of apple branch images based on contrast limited adaptive histogram equalization (CLAHE) is proposed. Firstly, the RGB color space of the branch images are converted to the XYZ and I 1 I 2 I 3 color space by transformation formula, and the X-Y and I 2 color factor of the apple branch images are extracted to analyze their gray-level difference. Then, The CLAHE is applied to the images whose gray-level difference is not intensity before iterative threshold. Finally, the apple branches are segmented from the original images by the iterative threshold. To verify the validity of the proposed method, 100 testing images in size of 1280 × 960 pixels under different illumination are utilized to compare the proposed method with other famous approaches, such as OTSU and histogram algorithm. Experimental results show that 94% of the apple branch images are correctly recognized and the segmentation quality of the proposed method is better than other approaches, which implies that the proposed method is effective for the tree branch segmentation. [ABSTRACT FROM AUTHOR]

This paper has developed a finite-time controller design for a category of multiple integrator nonlinear systems subject to input saturation. First of all, an unsaturated finite-time controller is constructed without considering input saturation. On this basis, by combining the saturation technique, a saturated controller is presented to finite-time stabilize the closed-loop system and to guarantee no transgression of pre-set input saturation. In simulation studies, an academic example is given to confirm the validity of the proposed approach. Moreover, the presented theoretical results are successfully applied to the saturated finite-time control of an agricultural vehicle straight-line tracking system (AVSTS). [ABSTRACT FROM AUTHOR]