Cascade Control Structure Research Articles

Three-Level Grid-Connected Inverter Based on Voltage-Oriented Control in Photovoltaic Generation Systems

The paper propose a new control of a three-phase three-level neutral-point-clamped (NPC) pulse width modulated (PWM) inverter in photovoltaic generation systems. The control scheme is mainly based on voltage-oriented control (VOC) with an improved maximum power point (MPP) tracking (MPPT). A cascaded control structure with an outer dc link voltage control loop and an inner current control loop is used. The currents are controlled in a synchronous dq reference frame using a decoupled feedback control. Furthermore, in order to achieve a unity power factor, the q-axis current reference is set to zero. The experimental results have proven an excellent performance and verified the validity of proposed system.

Human stick balancing: an intermittent control explanation

There are two issues in balancing a stick pivoting on a finger tip (or mechanically on a moving cart): maintaining the stick angle near to vertical and maintaining the horizontal position within the bounds of reach or cart track. The (linearised) dynamics of the angle are second order (although driven by pivot acceleration), and so, as in human standing, control of the angle is not, by itself very difficult. However, once the angle is under control, the position dynamics are, in general, fourth order. This makes control quite difficult for humans (and even an engineering control system requires careful design). Recently, three of the authors have experimentally demonstrated that humans control the stick angle in a special way: the closed-loop inverted pendulum behaves as a non-inverted pendulum with a virtual pivot somewhere between the stick centre and tip and with increased gravity. Moreover, they suggest that the virtual pivot lies at the radius of gyration (about the mass centre) above the mass centre. This paper gives a continuous-time control-theoretical interpretation of the virtual-pendulum approach. In particular, by using a novel cascade control structure, it is shown that the horizontal control of the virtual pivot becomes a second-order problem which is much easier to solve than the generic fourth-order problem. Hence, the use of the virtual pivot approach allows the control problem to be perceived by the subject as two separate second-order problems rather than a single fourth-order problem, and the control problem is therefore simplified. The theoretical predictions are verified using the data previously presented by three of the authors and analysed using a standard parameter estimation method. The experimental data indicate that although all subjects adopt the virtual pivot approach, the less expert subjects exhibit larger amplitude angular motion and poorly controlled translational motion. It is known that human control systems are delayed and intermittent, and therefore, the continuous-time strategy cannot be correct. However, the model of intermittent control used in this paper is based on the virtual pivot continuous-time control scheme, handles time delays and moreover masquerades as the underlying continuous-time controller. In addition, the event-driven properties of intermittent control can explain experimentally observed variability.

Theoretical Analysis of Close Loop Behaviour of Ideal Cascade Controller Structure for Positioning of XY Table Ballscrew Drive System

The implementation of Classical Cascade Controller for the purpose of controlling various type of engineering appliances has been vastly utilized by control engineer community. Cascade controller provides a simple structure of controller but yet functional and have the ability to satisfy the control design requirement. In general, the controller consists of two loops, namely velocity loop and position loop. This paper is focused on the fundamental aspect on how to analyzed the close loop behaviour for both velocity and position loop and to extract the mathematical formulation of the controller. The outcome from this paper which is in the form of mathematical formula is useful and very significant in order to validate the theoretical result with the simulation result. To be more precise, the result from this research work are in the form of transfer function for both velocity and position loop of the position output, position error, dynamic stiffness of the controller and the damping ratio of the system which is in this case the XY Table Ballscrew drive system.

A Cascade MPC-Feedback Linearizing Strategy for the Multivariable Control of Animal Cell Cultures

Abstract In this study, a multivariable control structure is developed to simultaneously control the concentrations of cells and of one of the nutrients in an animal cell cultivation system operated in perfusion. A cascade control structure is considered consisting of (i) an inner loop with a partially linearizing feedback controller, tuned so as to ensure robustness with respect to parameter uncertainties and non-canceled nonlinearities; and (ii) an outer loop involving two linear predictive controllers. The resulting control strategy shows robustness and performance properties similar to more computationally demanding strategies (such as a multivariable nonlinear MPC strategy), while requiring less measurements and involving an easier implementation.

Choosing a Proper Control Structure for a Mechatronic System with Variable Parameters

The paper presents design methods and control structure solutions dedicated to servo systems with continuously variable parameters. In order to obtain high control performance for the strip winding system – speed control – four cascade control structures (CCS) (with a inner proportional-integral (PI) current controller) with bump-less switching between several control algorithms are developed and tested. These solutions prove simplicity in adaptation on the representative operating points. The current controller is tuned on the basis of the Modulus Optimum method (MO-m). The basic speed controllers are tuned by the Extended Symmetrical Optimum method (ESO-m) and the developed continuous-time speed controllers are discretized using Tustin's method. The developed speed controllers are: (1) a classical PI controller, (2) a Takagi-Sugeno PI fuzzy controllers (TS-PI-FCs), (3) a two-degree-of-freedom PID (2-DOF PID) and (4) a sliding mode (SM) PI quasi relay control solution. The cost-effective feature is ensured by simple controller design and easy implementation, and it is illustrated by a case study which includes simulation results. A process with variable inertia, variable reference and load disturbance is discussed due to its applicability as a controlled plant in the field of mechatronic systems.

Advanced Stability Control for Electric Vehicles via a novel Hybrid Actuator System

The paper introduces a novel vehicle dynamics control concept based on a hybrid actuator system consisting of friction brake and traction machine. This configuration offers new degrees of freedom in the control of electric vehicles. The proposed approach is based on a cascaded control structure with an inner yaw rate loop and an outer sideslip angle feedback loop. The use of sliding mode algorithms accounts for plant uncertainties and reduces the effort for controller tuning to a minimum without neglecting the aspect of energy efficiency. With the help of selected driving maneuvers it is shown that the performance of a car equipped with the advanced stability control system is superior to a car with conventional electronic stability algorithms.

Simulation analysis of Series Cascade control Structure and antireset windup technique for a jacketed CSTR

Simulation analysis of Series Cascade control Structure and antireset windup technique for a jacketed CSTR

Control and experimental characterization of a methanol reformer for a 350 W high temperature polymer electrolyte membrane fuel cell system

This work presents a control strategy for controlling the methanol reformer temperature of a 350 W high temperature polymer electrolyte membrane fuel cell system, by using a cascade control structure for reliable system operation. The primary states affecting the methanol catalyst bed temperature is the water and methanol mixture fuel flow and the burner fuel/air ratio and combined flow. An experimental setup is presented capable of testing the methanol reformer used in the Serenergy H3 350 Mobile Battery Charger; a high temperature polymer electrolyte membrane (HTPEM) fuel cell system. The experimental system consists of a fuel evaporator utilizing the high temperature waste gas from the cathode air cooled 45 cell HTPEM fuel cell stack. The fuel cells used are BASF P1000 MEAs which use phosphoric acid doped polybenzimidazole membranes. The resulting reformate gas output of the reformer system is shown at different reformer temperatures and fuel flows, using the implemented reformer control strategy. The gas quality of the output reformate gas is of HTPEM grade quality, and sufficient for supporting efficient and reliable HTPEM fuel cell operation with CO concentrations of around 1% at the nominal reformer operating temperatures. As expected increasing temperatures also increase the dry gas CO content of the reformate gas and decreases the methanol slip. The hydrogen content of the gas was measured at around 73% with 25% CO2.

Enhanced cascade control for a class of integrating processes with time delay

Unlike self-regulating processes, cascade control strategies for control of integrating processes with time delay are limited. A novel series cascade control structure to enhance the closed loop performance is proposed for integrating and time delay processes. The proposed controller structure has only two controllers and a setpoint filter. The inner loop controller is designed based on IMC approach and the primary setpoint filter is based on optimal performance index. The primary load disturbance rejection controller, a PID controller in series with a lead/lag compensator, is designed on the basis of the desired closed-loop complementary sensitivity function. The robustness analysis is carried out using Kharitonov's theorem. Simulation results demonstrate the efficacy of the proposed method by showing satisfactory nominal and robust performances.

Tip tracking control of a micro-cantilever Timoshenko beam via piezoelectric actuator

In this paper, the tip tracking control problem of a Timoshenko micro-cantilever beam is investigated. The beam is actuated by a piezoelectric layer laminated on one side of the beam. Dynamic equations of the beam and piezoelectric layer are found using the Hamilton principle. By employing the Galerkin projection method, state space representation of the system is derived. Then, a cascade control loop is used for tracking control of the beam’s tip. The cascade control structure consists of an inner loop stabilizer and an outer loop proportional-integral-derivative controller. The stabilizer has a linear feedback form whose states are obtained through a linear observer which is based on the beam tip displacement measurement. Stability analysis of the inner loop stabilizer is performed to study the effects of higher un-controlled modes on performance of the controlled system. Simulation results show the effectiveness of the proposed control method.

Straightforward Current Control – One Step Controller based on Current Slope Detection

SummaryIn order to control the speed or the position of an electrical drive, the torque of the machine has to be set accordingly. The torque is a function of the machine current which is set commonly by means of a voltage source power converter. Because of the simplification of the set-up procedure, cascaded control structures are used. For a stable operation it is important that the speed of the inner control loop is much higher than the outer ones. Hence the speed of the current control limits the dynamics of the drive control. This means that the speed and the accuracy of the current control is decisive for the overall performance of the drive control.In this paper a new control method will be presented which is capable to reach the setpoint value in the shortest possible time of only one control period (comp. fig. 7) but without the knowledge of the machine parameters. This is the fastest possible response for digital controls which can be obtained theoretically by deadbeat controllers. However, high dynamics can hardly be achieved by deadbeat controls in practice because they are very sensitive against variations of the control parameters. Therefore the machine parameters like the inductance and the resistance as well as the course of the induced voltage have to be determined by a measurement for a proper operation. Furthermore the control has to be adjusted manually by the user. This leads to an high effort for the set-up procedure. In addition these parameters may vary during operation because of warming for instance, which can lead to a maladjustment of the control.The new control will adapt onto the load automatically which leads to a significant simplification of the set-up procedure. The identification of the control path is based on the detection of the current slopes of the load current ripple, caused by the alternating of the switching states of the power converter. Hence no additional test pulse are necessary for the identification of the system behaviour. This permanent adaptation leads to an optimal dynamic behaviour of the drive system. In contrast to many model predictive control methods, the new control has a low computation effort and no extensive model of the control path is necessary.The principle function of this new control approach will be explained and verified for an armature current control of a d.c.-drive system. However the principles of this control can also be applied on three-phase application which will be demonstrated by experimental results.

An improved parallel cascade control structure for processes with time delay

Parallel cascade control strategies, to improve the dynamic performance of a control system, have been proposed earlier mainly for control of stable processes. In this paper, further results are presented for a new parallel cascade control structure and controller design for controlling stable, unstable or integrating processes with time delay. The design of the disturbance rejection controllers and the setpoint filter are based on loop shaping and ISE performance measures, respectively. A modified Smith predictor scheme is used in the primary loop to enhance the closed-loop performance of the system. The stabilization, robustness and performances of time delay processes are analyzed. The disturbance rejection capability of the proposed scheme is superior as compared to some existing methods. Examples are given to illustrate the usefulness of the proposed method and its superiority over some parallel cascade control schemes.

Interactive Distributed Generation Interface for Flexible Micro-Grid Operation in Smart Distribution Systems

This paper presents an interactive distributed generation (DG) interface for flexible micro-grid operation in the smart distribution system environment. Under the smart grid environment, DG units should be included in the system operational control framework, where they can be used to enhance system reliability by providing backup generation in isolated mode, and to provide ancillary services (e.g. voltage support and reactive power control) in the grid-connected mode. To meet these requirements, the proposed flexible interface utilizes a fixed power-voltage-current cascaded control structure to minimize control function switching and is equipped with robust internal model control structure to maximize the disturbance rejection performance within the DG interface. The proposed control system facilitates flexible and robust DG operational characteristics such as 1) active/reactive power (PQ) or active power/voltage (PV) bus operation in the grid-connected mode, 2) regulated power control in autonomous micro-grid mode, 3) smooth transition between autonomous mode and PV or PQ grid connected modes and vice versa, 4) reduced voltage distortion under heavily nonlinear loading conditions, and 5) robust control performance under islanding detection delays. Evaluation results are presented to demonstrate the flexibility and effectiveness of the proposed controller.

Optimal Sliding Mode Cascade Control for Stabilization of Underactuated Nonlinear Systems

This paper presents an optimal sliding mode cascade control for stabilization of a class of underactuated nonlinear mechanical systems. A discrete-time, nonlinear model predictive control structure is used to optimally select and update the parameters of the sliding mode control surfaces at specified intervals in order to achieve a desired performance objective. The determination of these surface parameters is subject to constraints that arise from the stability conditions imposed by the sliding mode control law and the physical limits on the system such as input saturation. Nominal stability of the optimal cascade control structure is demonstrated and its robust performance is illustrated using an experimental rotary inverted pendulum system.

Sliding Back Pressure Control in Industrial CHP Plants

Operational characteristics and efficiency of the generation of electricity in combined heat and power (CHP) plants are discussed. CHP power plants produce both electric and heat power and the amount of the electric power can be adjusted by sliding the backpressure level in steam headers. The minimum back pressure level giving the maximum electric power output is detected by monitoring the positions of the load control valves in the steam consuming processes. Dynamic sliding of the backpressure level is carried out by a cascade control structure. The control structure is demonstrated in a simulator environment.

Cascade Controller Including Backstepping for Hydraulic-Mechanical Systems

Development of a cascade controller structure including adaptive backstepping for a nonlinear hydraulic-mechanical system is considered in this paper where a dynamic friction (LuGre) model is included to obtain the necessary accuracy. The paper compares the performance of two variants of an adaptive backstepping tracking controller with earlier results. The new control architecture is analysed and enhanced tracking performance is demonstrated when including the extended friction model. The complexity of the backstepping procedure is significantly reduced due to the cascade structure. Hence, the proposed control structure is better suited to real-time implementation.

Synthesis of PID tuning for a new parallel cascade control structure

Most of the published papers on parallel cascade control strategies present improvement of dynamic performance of stable processes. In this paper, a new parallel cascade control scheme is proposed for controlling stable and unstable processes with time delay. The two main features of the proposed scheme are: the primary process output completely tracks the primary setpoint and the servo response decouples the regulatory response in the nominal system. The proposed structure has only two controllers. The inner loop controller is designed based on IMC approach. The outer loop controller is a PID controller in series with lead/lag filter which is designed based on the desired complementary sensitivity function. Significant improvement in the load disturbance rejection performances are obtained when compared to some recent methods in the literature. Simulation results show the superiority and usefulness of the proposed control method over the existing ones.

Cascaded Multivariable Control of the Combustion in Diesel Engines

Feedback control of the engine-out pollutant emissions from diesel engines is a promising approach to relax issues regarding the requirement to fulfill strict limits on the cumulative tail-pipe emissions and regarding the complexity of calibration. This contribution proposes a multivariable controller for the center of main combustion and for the indicated mean effective pressure as inner loop of a cascaded control structure for the engine-out NOx and PM emissions. The combustion controller is synthesized as a discrete H∞ controller. The controller for the emissions is based on earlier work. The real-time estimation of the combustion characteristics is provided by an embedded device with limited computational capacity. The performance of the controller is demonstrated with engine experiments.

UAV Automatic Landing Control Law

Landing is the most important stage of the Flight of UAV, the study of automatic landing of UAVs has important engineering significance. In this paper, the UAV landing trajectory is divided into approach phase, steep glide phase and flare phase; a cascade control structure controller of height tracking was applied and the landing control law was designed. The digital simulation was done in the MATLAB / simulink environment. The results of simulation indicated that UAV can track the designed landing trajectory very well under the control law of automatic landing and safe landing can be achieved.

Development of a row guidance system for an autonomous robot for white asparagus harvesting

The special cultivation features of white asparagus pose new challenges to the research and development of the row guidance control. This paper presents a row tracking system of a two-layer structure for an autonomous robot developed to harvest white asparagus. At the low level, two independent speed control loops are suggested to ensure the actual revolutions of the drive motors to follow their demanded values. A cascade control structure, consisting of an inner orientation error controller and an outer lateral offset controller, is proposed for the high level to drive the robot to track the desired trajectory. The most important advantage of the cascade structure is that the inner loop directly regulates the most significant error (the orientation error), which minimizes the impact of the external disturbances upon the outer loop. Moreover, the cascade scheme allows the orientation angle to be limited within a given range. That ensures a collision-free tracking. The controllers are designed based on the traditional proportional-integral-derivative (PID) algorithms. The control parameters are selected by using root locus analysis, which guarantees the system stability. The effectiveness of the proposed control regime is evaluated by numerical simulations and validated by experiments. The experimental results demonstrate that with the suggested row guidance strategy a satisfactory tracking accuracy of ±0.5 cm is achieved.