Dead-time Compensation Research Articles

An enhanced feedback-feedforward control scheme for process industries

Abstract In this article, a unified control scheme is proposed for dead-time compensation and disturbance rejection via feedback and feedforward controller. The objectives of this work are suggested in two folds, first tuning of fractional order feedback controller via delayed Bode’s ideal transfer function instead of conventional Bode’s ideal transfer function with the benefits of dead time compensator and second feedforward controller for disturbance rejection. An existing method is utilized for comparison with the proposed scheme. To examine the efficacy of the proposed method robustness test is also carried out via sensitivity analysis. For quantifiable evaluation of the proposed scheme Integral Absolute Error (IAE) and Integral Square Error (ISE) are utilized. For the usefulness of the proposed scheme, two practical problems are demonstrated in this paper. The limpidity and instinctive appeal of the proposed scheme make it beautiful for industrial applications.

Dead-Time Compensation for the First-Order Dead-Time Processes: Towards a Broader Overview

The article reviews the results of a number of recent papers dealing with the revision of the simplest approaches to the control of first-order time-delayed systems. The concise introductory review is extended by an analysis of two discrete-time approaches to dead-time compensation control of stable, integrating, and unstable first-order dead-time processes including simple diagnostics of the model used and focusing on the possibility of simplified but reliable plant modelling. The first approach, based on the first historically known dead-time compensator (DTC) with possible dead-beat performance, is based on the reconstruction of the actual process variables and the compensation of input disturbances by an extended state observer (ESO). Such solutions play an important role both in a disturbance observer (DOB) based control and in an active disturbance rejection control (ADRC). The second approach considered comes from the Smith predictor with two degrees of freedom, which combines feedforward control with output disturbance reconstruction and compensation by the parallel plant model. It is shown that these two approaches offer advantageous properties in the case of actuator limitations, in contrast to the commonly used PID controllers. However, when applied to integrating and unstable first-order systems, the unconstrained and possibly unobservable output disturbance signal of the second solution must be eliminated from the control loop, due to the hidden structural instability of the Smith predictor-like solutions. The modified solutions, usually referred to as filtered Smith predictor (FSP), then no longer provide a disturbance signal and thus no longer fully fit into the concept of Industry 4.0, which is focused on further optimization, predictive maintenance in dynamic systems, diagnosis, fault detection and fault identification of dynamic processes and forms the basis for the digitalization of smart production. Nevertheless, the detailed analysis of the elimination of the unstable disturbance response mode is also worth mentioning in terms of other possible solutions. The application of both approaches to the control of a thermal process shows almost equivalent quality, but with different dependencies on the tuning parameters used. It is confirmed that a more detailed identification of the controlled process and the resulting higher complexity of the control algorithms does not necessarily lead to an increase in the resulting quality of the transients, which underlines the importance of the simplified plant modelling for practice.

On-line dead time compensator for PMSM drive based on current observer

An on-line compensation for the dead time strategy without the knowledge of the current direction is proposed for PMSM drive based on a current observer. The modifications of the motor reference voltages are achieved by an uncomplicated current observer working in conjunction with the feedforward dead time voltage estimator. Therefore, the distortions on phase currents and torque due to dead time are greatly reduced. The proposed method does not require look-up table and coordinate transformations. It also does not need phase current polarity information or calculation of dead time period. Since the proposed scheme requires no extra hardware and time consuming signal processing algorithms, it is appropriate to use in real-time implementation of AC drives. The proposed scheme has been validated in simulation and implemented using TMS320F28335 DSP. Simulation and practical results validate the effectiveness and the feasibility of the proposed scheme.

Dead-Time Compensation Based on a Modified Multiple Complex Coefficient Filter for Permanent Magnet Synchronous Machine Drives

In field-oriented vector control of permanent magnet synchronous machine (PMSM) drives, the dead time will cause the 5th and 7th current harmonics in the stationary reference frame. They are mapped to the negative and positive 6th harmonic current in the synchronous reference frame (SynRF), resulting in torque ripple and deterioration of control performance. To solve this problem, a new dead-time compensation method is proposed in this article. First, a multiple complex coefficient filter is modified with a low-pass filter being paralleled to extract positive and negative sequence components of 6th harmonic from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -axes currents. Then, based on the extracted harmonics, a voltage feedback path with variable gains is designed in SynRF to eliminate the dead-time effect. Finally, the proposed method is implemented in a prototyped PMSM drive to verify its effectiveness. Both the simulations and experiments indicate that the proposed method can significantly weaken the dead-time effect with no additional hardware. Besides, it is also proved robust to the variable machine parameters.

Model predictive control with dead-time compensation applied to a gas compression system

The natural gas produced in primary separation is passed through a compression system to be pressurized and conditioned before being sent to its final destination. The operation of that system needs to be safe and efficient to avoid equipment damage and reduce energy consumption. The stable and secure operation of the equipment in a compression system is provided, many times, by a classical regulatory control layer. In this work, we present a Model Predictive Control (MPC) strategy to provide setpoints for the regulatory control layer of a gas compression system, aiming to avoid excessive energy consumption, decrease the plant variability, and guarantee a stable and safe operation against load disturbances. The proposed method is tested in a digital twin of a typical industrial unit using a Dead-Time Compensator Generalized Predictive Controller (DTC-GPC). Some disturbances in the feed flow rate of gas were considered as case studies. The controller responded satisfactorily to these disturbances keeping the plant operation stable and returning the controlled variables in the desired operating range after a short time.

Distributed predictor-based stabilization of interconnected systems with network induced delays

This paper presents sufficient stability conditions for distributed prediction-based control of interconnected systems subject to network-induced time-varying delay. Due to the flexibility of the proposed criteria, continuous-time, sampled-data control and output-feedback problems can be handled by the same framework. A detuning procedure is combined with a distributed LQR design in order to ensure the closed-loop stability for a given network-induced delay bound. If compared with related works, the proposed framework can be applied to obtain the time-varying delay bound derived from distinct combinations of distributed dead-time compensation strategies with stabilizing control laws. Two simulation case studies based on a benchmark problem and the level control of a irrigation canal are presented to illustrate the usefulness of the proposed framework.

A Novel Predictive Direct Torque Control Using an Optimized PWM Approach

This article presents a novel predictive direct torque control (DTC) scheme using an optimized pulsewidth modulation (PWM) and a dead-time compensation. In the proposed method, two effective voltage vectors are selected according to the predicted torque and flux errors based on zero-voltage vector. The back electromotive force and flux-torque coupling are also taken into consideration. The torque and flux variations per sampling period are predicted by the zero-voltage vector and the two effective voltage vectors. The switching period of the three voltage vectors (zero, first effective, and second effective) are determined by the torque and flux errors along with the predicted torque and flux variations that have been calculated in the previous step. To improve the prediction result, a dead-time compensation algorithm and a modified PWM switching scheme are adopted to reduce dead-time effect. The detailed implementation of the proposed control scheme is presented. The effectiveness of the proposed predictive DTC-PWM is verified by both simulation and experiments. The results are also compared with the conventional method.

Model Predictive Current Control with Fixed Switching Frequency and Dead-Time Compensation for Single-Phase PWM Rectifier

The research object of this paper is single-phase PWM rectifier, the purpose is to reduce the total harmonic distortion (THD) of the grid-side current. A model predictive current control (MPCC) with fixed switching frequency and dead-time compensation is proposed. First, a combination of an effective vector and two zero vectors is used to fix the switching frequency, and a current prediction equation based on the effective vector’s optimal action time is derived. The optimal action time is resolved from the cost function. Furthermore, in order to perfect the established prediction model and suppress the current waveform distortion as a consequence of the dead-time effect, the dead-time’s influence on the switching vector’s action time is analyzed, and the current prediction equation is revised. According to the experimental results, the conclusion is that, firstly, compared with finite-control-set model predictive control, proportional-integral-based instantaneous current control (PI-ICC) scheme and model predictive direct power control (MP-DPC), the proposed MPCC has the lowest current THD. In addition, the proposed MPCC has a shorter execution time than MP-DPC and has fewer adjusted parameters than PI-ICC. In addition, the dead-time compensation scheme successfully suppresses the zero-current clamping effects, and reduce the current THD.

A New Dead-time Compensation Method Based on LMS Algorithm for PMSM

This paper presents a new dead-time compensation method for permanent-magnet synchronous motor (PMSM) at low speed. Based on least-mean-square (LMS) algorithm and combined with the traditional average voltage compensation method, the proposed method can determine the current polarity through extracting the fundamental current. Compared with traditional compensation methods, the proposed method can detect current polarity more accurately at low speed due to the narrower notch bandwidth. Moreover, this method is robust to motor parameters and easy to implement. The proposed method is simulated in a voltage-source inverter PMSM drive controlled by I/F and the results show that it meets the requirement of dead-time compensation and eliminates the influence of dead-time effect.

Analysis and experimental application of a dead‐time compensator for input saturated processes with output time‐varying delays

Dead-time compensators (DTCs) are a family of classical controllers derived from the Smith Predictor (SP). Their main characteristic is that they explicitly employ the model of the open-loop process to feedback a predicted value of the non-delayed system, thus obtaining \textit{compensation} of the delay. Such a perfect compensation is not achievable in the case of time-varying delays. In this paper, we address stability analysis of a DTC structure in this situation, in addition to considering saturating actuators and disturbances of limited energy. Specific challenges related to the DTC closed loop are taken into account in the developed theoretical conditions, which are expressed in terms of linear matrix inequalities (LMIs) by using an adequate Lyapunov-Krasovskii functional (LKF) and generalized sector conditions. Furthermore, a new approach for the definition of the set of initial conditions in an augmented space in conjunction with the LKF is presented. Besides theoretical innovations, practical discussion about the relation between the tuning of DTC controllers and robustness for this class of systems is presented through numerical examples. An experimental application on a neonatal incubator prototype is carried out to emphasize the effectiveness of the results.

Design of a Sliding Mode Controller for a Conical Tank Process

This paper demonstrates the implementation of a Sliding Mode Controller (SMC) for a First Order System with Time Delay (FOPDT). The transit time/ dead time compensation has been provided to minimize its effect on the related process. The SMC algorithm is implemented on the mathematical model of pilot conical tank system and closed loop system is observed for both servo as well as the regulatory response. The results are investigated for its efficacy and observations have been provided.

A Review on Control System Applications in Industrial Processes

Present paper attempts to review the literature related to design of P-I-D control for time delayed complex industrial process for single as well as for multivariable process with interaction considerations, their decoupler design and time delay compensators. General instrumentation of the industrial feedback control systems along with control system analysis has been covered. Also it covers, control features of some paper mill sub-processes like headbox operation, basis weight and retention. The importance of eliminating the effects of interactions, among the process control loops inside a multi input multi output industrial control system, has been discussed with the help of literature study. The importance of the process dynamics knowledge for designing a control system has also been investigated. This paper also investigates the significance and effectiveness of PID controllers through various literature studies. The problems of the classical PID controllers such as constraints, presence of disturbances etc.) can be removed by designing in combination with soft computing techniques. Moreover, possibility of further enhancements in the PID controller with the utilization of various schemes available, has been presented. The present status of control systems in industrial processes in terms of various control parameters such as stability, dead time compensation etc. has been presented and the future improvements have been stated.

Anti-windup strategy for processes with multiple delays: A predictor-based approach

This paper proposes an anti-windup implementation framework with dead-time compensation for processes with delays. The new approach is based on a causal modified implementation of the primary controller which unifies continuous-time and discrete-time approaches. Multivariable processes with multiple input-output delays can be handled by applying the proposed framework. No additional design parameter is required to control square multivariable processes with the proposed anti-windup strategy. Moreover, Linear Matrix Inequality conditions are presented in order to perform stability analysis and to define the domain of attraction of the constrained control system. Three simulation case studies are presented to illustrate the usefulness of the anti-windup framework for processes with delay. An experimental temperature control based on a prototype of a neonatal intensive care unit (NICU) is also shown.

Switching Signals Based Condition Monitoring for Submodule Capacitors in Modular Multilevel Converters

The capacitors are one of the most important com- ponents in modular multilevel converter (MMC). Due to the large numbers of capacitors applied in MMC, the primary technical challenge about the capacitors is reliability. To identify the abnormal capacitors and improve the performance of MMCs, the capacitor condition monitoring strategy is proposed in this brief. In the monitoring algorithm, the relationship of the capacitances in an arm is deduced. The actual and nominal sums of switching signals for submodules (SM) are used to estimate the capacitances. Furthermore, a compensation for dead time is added to enhance the monitoring accuracy. The proposed condition monitoring algorithm not only simplifies the calculation of capacitances with high accuracy, but also has no adverse influence on the normal operation of MMCs. The feasibility of the proposed strategy is verified by the simulation results in MATLAB/Simulink.

Tuning rules for unstable dead-time processes

Despite many advances in the field of dead-time compensators (DTCs) for unstable first-order plus dead-time (UFOPDT) processes, the tuning, in general, is manually carried out. Therefore, this paper proposes simple tuning rules for a DTC intended to UFOPDT processes. The rules are based on the relative dead time, desired closed-loop time constant, and achievable robustness. Besides, as the practical implementation is always in the discrete-time domain, a method to choose the sampling period for UFOPDT processes is presented. Four simulation examples from the literature are used to show the advantages of the proposed method. In addition, an experiment with a propeller pendulum is performed to confirm such advantages in the control of a real unstable process with dead time.

A Newly Designed VSC-Based Current Regulator for Sensorless Control of PMSM Considering VSI Nonlinearity

A novel d-axis variable-structure control (VSC) current regulator is proposed for realizing fast and accurate sensorless control in the surface-mounted permanent magnet synchronous motor (SPMSM) in this article. In this proposed regulator, the sigmoid function is employed as a switching control law to reduce the chattering effect and increase the degrees of freedom in tuning the regulator. The sensorless control performance under the d-axis VSC current regulator is testified by using the model reference adaptive system (MRAS) and sliding-mode control (SMC) method. In the MRAS observer, the adaptive law of speed and resistance is designed so that resistance, speed, and position can be estimated simultaneously, which can eliminate the influence of the resistance variations theoretically. In the sliding-mode observer, the back electromotive force observer is employed to estimate the speed and position of PMSM so that the estimation can cover a relative low-speed range because pure integration is not employed. Furthermore, the dead-time compensation voltages (DTCV) strategy is employed to online estimate disturbance voltage in a feed-forward manner, which can improve the sensorless control performance of PMSM. Experimental results obtained from an SPMSM drive system validate the superiority of the proposed d-axis VSC current regulator.

Optimal Fractional Order IMC-Based Series Cascade Control Strategy with Dead-Time Compensator for Unstable Processes

In industrial unstable processes, disturbance rejection is more challenging task than setpoint tracking. So, cascade control structure is widely used in many chemical processes to reject disturbances. In this work, an advanced dead-time compensator-based series cascade control structure (SCCS) is suggested for unstable processes. The suggested SCCS has three controllers (named as primary, secondary and stabilizing controllers). Both primary and secondary controllers are designed using fractional order-based internal model control (IMC) approach. The stabilizing proportional–derivative controller is designed using maximum sensitivity considerations and Routh–Hurwitz stability criteria. Optimal values of the closed-loop time constants and fractional orders of IMC filters are obtained using constrained artificial bee colony (ABC) algorithm. This ABC algorithm uses a multi-objective function involving minimization of integral of absolute error, integral of time weighted absolute error and integral of squared error. Simulation studies are conducted using some benchmark plant models used in literature for illustrating the advantages of the proposed strategy compared to the state of the art. Moreover, robust stability of the proposed design is analysed and quantitative performance measures are also computed.

Simplified filtered Smith predictor for high-order dead-time processes

This paper proposes a control structure suitable for high-order non-minimum phase (NMP) processes. In general, dead-time compensators (DTCs) firstly predict the process output with zero error at a steady-state and then a primary controller with an integrator is designed based on the delay-free model. The main advantage of the proposed structure is that the primary controller is only a state feedback gain with no integrators. This leads to fewer parameters to tune and lower order filters, while a robustness filter is used to reject disturbances and guarantee zero error at a steady state. Simulation results show better or equivalent performance compared to other recently published works, even kept the controller design simplicity.

An Impedance Source Multi-Level Three Phase Inverter with Common Mode Voltage Elimination and Dead Time Compensation

Currently, most electro-mechanical drive systems that require speed control use pulse-width modulated (PWM) variable frequency drives known as adjustable speed drives (ASD). The high switching speeds of the electronics switches are essential for proper operation of the ASD. Common mode voltage (CMV) has its origin in the PWM switching. The CMV increases the stress on the coils and windings, reduces the life of the bearing and, therefore, has a significant impact on motor life cycle. In this paper, a variant of a PWM-based space vector modulation (SVPWM) switching algorithm is proposed to control both the shoot-through intervals and the dead time of the power switches that could be compensated. The proposed algorithm is implemented on a platform consisting of an impedance source network in the DC side of the topology with the purpose of mitigating the CMV and capability of voltage boosting. Since similar methods have achieved a CMV reduction of 1/6 of the DC link voltage so far, in this paper, while surpassing the disturbing current harmonics, the high efficiency is fully accessible. The presented experimental results verify the effectiveness of the proposed approach by slightly increasing the total harmonic distortion (THD) and reducing the converter losses.

Closed-loop dynamic real-time optimization (CL-DRTO) of a bioethanol distillation process using an advanced multilayer control architecture

The ethanol production has become an important part of the worldwide economy driven by its use as renewable energy and environmentally clean fuel. To obtain a high-level quality product, a large quantity of energy is used in the distillation stage. This work proposes to optimize the bioethanol production process applying a closed-loop dynamic real-time optimization (CL-DRTO) framework associated with advanced control strategies in the ethanol distillation facilities to improve production and minimize energy losses. A high-fidelity computational platform is developed using a software-in-the-loop (SIL) technique to demonstrate the feasible application in real cases. OLE (Object Linking and Embedding) Automation integrates Matlab and Aspen Hysys software to simulate practical scenarios currently applied in the industry. The optimization and control algorithms are developed in Matlab and the ethanol distillation process is modeled in the Aspen Hysys. Different advanced control strategies, such as IHMPC (Infinite Horizon Model Predictive Control), MIMO FSP (Filtered Smith-Predictor) and DTCGPC (Dead-Time Compensator Generalized Predictive Controller), are used to overcome the process dynamic issues, for instance, strong nonlinearity, dead-times, long time constants and coupled loops, on ethanol distillation process. A CL-DRTO layer is developed in the proposed control structure to consider economic and production aspects, guiding the process to optimal operating points. Performance indices and computational effort are used to evaluate the control behavior for disturbance rejection and reference tracking scenarios. Furthermore, economic performance is analyzed to ensure the advantages of the proposal. Results show that the proposed computational platform is able to reproduce industrial scenarios with high-fidelity. In addition, preliminary results demonstrate that advanced control structure can improve the production and profitability of the bioethanol distillery and present a powerful alternative to replace classical controllers currently used in this type of industry.