Classical Cascade Control Research Articles

State Feedback Speed Control with Periodic Disturbances Attenuation for PMSM Drive

This paper proposes an auto-tuned constrained state-feedback controller (SFC) to attenuate periodic disturbances present in permanent magnet synchronous (PMSM) motor drives. An online auto-tuning process of SFC has been made using a powerful nature-inspired optimization algorithm—artificial bee colony (ABC)—to achieve high-performance operation of the drive. A novel performance index is proposed to minimize the impact of pulsating torque and obtain smooth-velocity of the drive. The proposed approach is a practical application of classic control theory with novel engineering-tools for improving the operational quality of a PMSM drive system. The obtained results are compared with a classical cascade control structure (CCS) based on proportional-integral (PI) regulators and disturbance observer-based control (DOBC). A detailed time- and frequency-domain analysis has been conducted in respect to periodic disturbances present in a PMSM drive system. Moreover, the robustness of SFC against parameter variations of inductance and resistance has been tested.

Incremental Passivity Control in Multilevel Cascaded H-Bridge Converters

A low-capacitance static compensator (LC-StatCom) based on a cascaded H-bridge (CHB) multilevel converter can be an efficient and low-cost solution for reactive power compensation in power grids. However, the reduced capacitance, and, therefore, faster capacitor voltage dynamics, makes it challenging to control it. Classical cascade control schemes, in which the bandwidths associated with the inner current loop and the outer capacitor voltage loop need to be separated enough, are the most reported approaches. Nevertheless, for the LC-StatCom case, the voltage loop can be almost as fast as that one of the current loop. For this reason, this article proposes a control approach that does not separate voltage and current dynamics. Concretely, we propose a multi-input linear time-variant control law, based on incremental passivity theory for CHB multilevel converters. The proposed control guarantees stability despite saturation in the control signal. In addition, it provides capacitor voltage balance without the need to use a specific balancing stage. This article, first, reviews the average model of the converter and, then, analyses the steady-state dynamic behavior to determine the desired and coherent reference signals. Then, it applies an incremental passivity control approach to drive the LC-StatCom state variables to the desired references. After introducing the proposed control law, experimental results on a seven-level 1-kVA CHB LC-StatCom are shown to demonstrate its excellent performance in steady state and transients.

Cascaded Nonlinear MPC for Realtime Quadrotor Position Tracking

In this paper we present a cascaded control approach using nonlinear model-predictive controllers for both stages. Using a quadrotor platform as an exemplary target plant with fast nonlinear dynamics, a realtime capable design is proposed that does not require the plant dynamics to exhibit clearly separable time constants as in classical cascaded control. In contrast to similar work we employ NMPC for the inner control loop instead of classical control approaches such as PID control, allowing to keep predictive properties as well as explicit constraint handling. We demonstrate via Monte-Carlo simulations that our design is able to achieve a significantly better position tracking performance of the quadrotor while being equally computationally expensive compared to monolithic position tracking NMPC.

Output voltage tracking controller embedding auto‐tuning algorithm for DC/DC boost converters

This study suggests a non-linear output voltage tracking controller for DC/DC boost converters in the form of a classical cascade control structure. Non-linearities in the converter dynamics as well as parameter uncertainties and load variations are considered. The first contribution of this study is the design of an auto-tuner, which automatically adjusts the control gain according to the output voltage error to enhance transient performance. The second contribution involves proving that the closed-loop system ensures the performance recovery without any steady-state errors in the presence of parameter and load variations. The effectiveness of the proposed algorithm was verified through experimental investigations using a 5 kW prototype DC/DC boost converter.

MFC/IMC Control Algorithm for Reduction of Load Torque Disturbance in PMSM Servo Drive Systems

The following article presents the hybrid control scheme (MFC/IMC) that involves model following control (MFC) and internal model control (IMC) for velocity control of permanent magnet synchronous motor (PMSM) with the aim of reducing the influence of torque load disturbance. A brief description of the MFC/IMC algorithm is given, its structure is an expansion of a classical cascade control loop; the PMSM model was linearized in order for it to be applicable to the proposed solution. Numerical analysis of sensitivity for both systems (i.e., the MFC/IMC cascade and the classical one) is given with regards to their robustness to torque load disturbance. The behavior of these two control schemes is then analyzed on a laboratory test stand with various load disturbances introduced to the system; the methodology and the test stand itself are also briefly presented in the article. The results presented in the form of performance indices and graphs prove that the developed MFC/IMC method is an improvement over the classical cascade for velocities near the operating point.

Speed Finite Control Set Model Predictive Control of a PMSM Fed by Matrix Converter

This paper presents a new speed finite control set model predictive control algorithm, which has been applied to a permanent-magnet synchronous motor driven by a matrix converter (MC). This method replaces the classical cascaded control scheme with a single control law that controls the motor currents and speed. Additionally, unlike classical MC modulation methods, the method allows direct control of the MC input currents. The performance of the proposed work has been verified by simulation studies and experimental results.

Control of Adjustable Compliant Actuators

Adjustable compliance or variable stiffness actuators comprise an additional element to elastically decouple the actuator from the load and are increasingly applied to human-centered robotic systems. The advantages of such actuators are of paramount importance in rehabilitation robotics, where requirements demand safe interaction between the therapy system and the patient. Compliant actuator systems enable the minimization of large contact forces arising, for example, from muscular spasticity and have the ability to periodically store and release energy in cyclic movements. In order to overcome the loss of bandwidth introduced by the elastic element and to guarantee a higher range in force/torque generation, new actuator designs consider variable or nonlinear stiffness elements, respectively. These components cannot only be adapted to the walking speed or the patient condition, but also entail additional challenges for feedback control. This paper introduces a novel design method for an impedance-based controller that fulfills the control objectives and compares the performance and robustness to a classical cascaded control approach. The new procedure is developed using a non-standard positive-real Η2 controller design and is applied to a loop-shaping approach. Robust norm optimal controllers are designed with regard to the passivity of the actuator load-impedance transfer function and the servo control problem. Classical cascaded and positive-real Η2 controller designs are validated and compared in simulations and in a test bench using a passive elastic element of varying stiffness.

Modeling and control of temperature of heat-calibration wind tunnel

This paper investigates the temperature control of the heat air-flow wind tunnel for sensor temperature-calibration and heat strength experiment. Firstly, a mathematical model was established to describe the dynamic characteristics of the fuel supplying system based on a variable frequency driving pump. Then, based on the classical cascade control, an improved control law with the Smith predictive estimate and the fuzzy proportional-integral-derivative was proposed. The simulation result shows that the control effect of the proposed control strategy is better than the ordinary proportional-integral-derivative cascade control strategy.

Cascade Control Scheme for Tubular Reactors with Multiple Temperature Measurements

In this work, we introduce a new structure for cascade control scheme based on a weighted average temperature of three-temperature measurement distributed along the axial position of the tubular reactor. The control configuration exploits the information provided by two additional temperature sensors located close to the feed and the output positions of the tubular reactor. The configuration with the averaged temperature improves the behavior of the classical cascade control scheme by enhancing the disturbance rejection. Numerical simulations of two axial dispersion models of tubular reactors are used to illustrate and compare the control performance.

Dynamic stiffness enhancement of direct-driven machine tools using sliding mode control with disturbance recovery

This paper presents a disturbance adaptive discrete sliding mode controller for feed drives equipped with linear motors. The control law is expressed as a function of friction and cutting force disturbances which are estimated from the motor force and control states. The accurate prediction of disturbance forces is used to actively compensate low frequency machine tool structural modes which are within the bandwidth of the controller. The proposed control system is experimentally demonstrated on a high performance linear drive, which exhibited high bandwidth and significant increase in dynamic stiffness compared to classical cascaded control methods.

Optimal State Space Control of DC Motor

In comparison to classical cascade control architecture of DC motors, the state feedback control offers advantages in terms of design complexity, hardware realization and adaptivity. This paper presents a methodic approach to state space control of a DC motor. The state space model identified from experimental data provides the basis for a linear quadratic regulator (LQR) design. The state feedback linear control is augmented with a feedforward control for compensation of Coulomb friction. The controller is successfully applied and the closed loop behavior is evaluated on the experimental testbed under various reference signals.

A Neural Network Assisted Cascade Control System for Air Handling Unit

In the centralized heating, ventilating and air-conditioning (HVAC) system, air handling units (AHUs) are traditionally controlled by single-loop proportional-integral-derivative (PID) controllers. The control structure is simple, but the performance is usually not satisfactory. In this paper, we propose a cascade control strategy for temperature control of AHU. Instead of a fixed PID controller in the classical cascade control scheme, a neural network (NN) controller is used in the outer control loop. This approach not only overcomes the tedious tuning procedure for the inner and outer loop PID parameters of a classical cascade control system, but also makes the whole control system be adaptive and robust. The multilayer NN is trained online by a special training algorithm-simultaneous perturbation stochastic approximation (SPSA)-based training algorithm. With the SPSA-based training algorithm, the weight convergence of the NN and stability of the control system is guaranteed. The novel cascade control system has been implemented on an experimental HVAC system. Testing results demonstrate the effectiveness of the proposed algorithm over the classical cascade control system

A model predictive formulation for control of open-loop unstable cascade systems

Cascade control is commonly used in the operation of chemical processes to reject disturbances that have a rapid effect on a secondary measured state, before the primary measured variable is affected. In this paper, we develop a state estimation-based model predictive control approach that has the same general philosophy of cascade control (taking advantage of secondary measurements to aid disturbance rejection), with the additional advantage of the constraint handling capability of model predictive control (MPC). State estimation is achieved by using a Kalman filter and appending modeled disturbances as augmented states to the original system model. The example application is an open-loop unstable jacketed exothermic chemical reactor, where the jacket temperature is used as a secondary measurement in order to infer disturbances in jacket feed temperature and/or reactor feed flow rate. The MPC-based cascade strategy yields significantly better performance than classical cascade control when operating close to constraints on the jacket flow rate.

Fuzzy Logic Scheduling in PI Control of the Servo System Under High Stiction Torque

This paper is concerned with a controller design for a servo system with a high stiction torque. A complete friction model is adopted that caters for a dynamic stiction torque. A classical cascaded control structure that is composed of the inner velocity and outer position control loops is employed. The PI controllers are used in this loops. Due to a high stiction torque a high accuracy of servo tracking in the steady-state and a high dynamic performance cannot be simultaneously achieved by fixed parameter controller under uncertainty in the friction model parameters. High gains of the controller needed to reject friction in order to obtain required steady-state error would lead to a loss of stability. Gain scheduling is proposed based on the servo speed measurements. The gain scheduling functions cannot be derived using classical techniques due to the uncertainty and nonlinearities. A fuzzy logic approach is then applied that easily utilises available qualitative representation of the functions. Performance of the controller is illustrated by simulation results.

Cascaded control of feedback interconnected nonlinear systems: Application to robots with AC drives

In this paper we are interested in the design of global tracking controllers for feedback interconnected nonlinear systems with partial state measurements. We assume the forward subsystem is an underactuated Euler-Lagrange system and that, in the absence of the latter, a stabilizing controller is known for the feedback subsystem. This scenario, which appears in many practical applications with the forward subsystem representing the actuator dynamics, leads naturally to a classical cascaded (nested-loop) control scheme. Our main contribution is the establishment of conditions under which we can design an inner-loop controller for the Euler-Lagrange system such that global tracking is achieved. These are expressed in terms of actuator-sensor couplings, the “strength” of the subsystems interconnection, and the requirement of linear dependence on the unmeasurable variables. Interestingly enough, this analysis does not invoke the standard time-scale separation assumptions prevalent in cascaded schemes, but uses instead some “growth” conditions on the interconnections. The design is carried out using passivity arguments, and relies on energy shaping plus damping injection ideas. To insure asymptotic tracking of the inner loop the derivative of its reference, which is the output of the outer loop controller, is typically needed. To overcome this problem we add a nonlinear observer. The procedure is used to design an output feedback global tracking position controller for robot manipulators actuated by AC drives. To illustrate the generality of the method we consider a fairly large class of AC drives, which includes as particular cases induction, synchronous and stepper motors. Instrumental for the observer design is the utilisation of a new robot controller which is linear in the link velocities.

VS-Cascade Control

A variable structure cascade control is presented in order to avoid the increasing loss in the bandwidth of the outer loops, typical drawback of classical cascade control. In the choice of the coefficient of the VS-controller the compatibility of the command signals with the physics of the controlled plants is also guaranteed. As a test case a DC-motor speed and position control is considered.

Multivariable Cascade Control of Purex Process

A differential-difference equation model is introduced which can describe the dynamics of any type of extractor, whether, mixer settler, pulsed column or rapid contactor. The differential or difference equation models used by several investigators prove to be particular cases of this model. From the results calculated by this model, it is shown that there exist an optimal stage and an optimal component for monitoring the performance in the control of extractors. A computer control system for the solvent extraction step of the Purex process is devised with use made of a generalized version of the classical cascade control law, which the authors have named the Multivariate Cascade Control Law. This law can easily be extended for application to the control of the Purex process as a whole.

Multivariable Cascade Control of Purex Process

A differential-difference equation model is introduced which can describe the dynamics of any type of extractor, whether, mixer settler, pulsed column or rapid contactor. The differential or difference equation models used by several investigators prove to be particular cases of this model. From the results calculated by this model, it is shown that there exist an optimal stage and an optimal component for monitoring the performance in the control of extractors. A computer control system for the solvent extraction step of the Purex process is devised with use made of a generalized version of the classical cascade control law, which the authors have named the Multivariate Cascade Control Law. This law can easily be extended for application to the control of the Purex process as a whole.