H. Noura

Fault-tolerant control in dynamic systems: application to a winding machine

The general fault-tolerant control method described in the article addresses actuator and sensor faults, which often affect highly automated systems. These faults correspond to a loss of actuator effectiveness or fault sensor measurements. After describing these faults, a fault estimation and compensation method was proposed. In addition to providing information to operators concerning the system operating conditions, the fault diagnosis module is especially important in fault-tolerant control systems where one needs to know exactly which element is faulty to react safely. The methods abilities to compensate for such faults are illustrated by applying it to a winding machine, which represents a subsystem of many industrial systems. The results show that once the fault is detected and isolated, it is easy to reduce its effect on the system, and process control is resumed with degraded performances close to nominal ones. Thus, stopping the system immediately can be avoided. However, the limits of this method are reached when there is the complete loss of an actuator. In this case, only a hardware redundancy is effective and could ensure performance reliability. The method proposed here assumes the availability of the state variables for measurement.

Fault diagnosis and accommodation of a three-tank system based on analytical redundancy

This paper investigates the application of a fault diagnosis and accommodation method to a real system composed of three tanks. The performance of a closed-loop system can be altered by the occurrence of faults which can, in some circumstances, cause serious damage on the system. The research goal is to prevent the system deterioration by developing a controller that has some capabilities to compensate for faults, that is, the fault accommodation or fault-tolerant control. In this paper, a two-step scheme composed of a fault detection, isolation and estimation module, and a control compensation module is presented. The main contribution is to develop a unique structured residual generator able to isolate and estimate both sensor and actuator faults. This estimation is of paramount importance to compensate for these faults and to preserve the system performances. The application of this method to the three-tank system gives encouraging results which are presented and commented on in case of various kinds of faults.

Multivariable identification of a winding process by subspace methods for tension control

Abstract The challenging problem of multivariable tension control in winding systems is addressed in this paper. Two control configurations, the SS model and the ST model configurations (SS: speed-speed and ST: speed-torque), are considered and applied to a pilot winding plant. The relevance of using linear models in such multivariable systems is examined. Tension simulations based on a proposed linear ST model yield interesting results in both open and closed-loops. Emphasis is also laid on the applicability of subspace methods to winding tension state-space model identification. Such methods simplify the problem of multivariable identification by reducing the associated polynomial model structural identification to the easier problem of order estimation. It is shown that the design of a Linear Quadratic Gaussian (LQG) controller capable of ensuring efficient performance over the entire winding zone can be based on an ‘average’ state-space model identified around a nominal operating point.

Fault tolerant control in dynamic systems using convex optimization

In this paper a method for fault diagnosis and control reconfiguration in dynamic systems is presented. The proposed approach is composed of two steps. In the event of a fault, the first step is the detection of the failed component, and the estimation of the fault parameters. The second step consists of the design of a new control structure which implicitly reconfigures the control law using the online estimates of the altered system dynamics. The application to a winding machine is presented to illustrate the proposed approach.

Sensor fault diagnosis based on energy balance evaluation: Application to a metal processing

This paper deals with the design of a residual generator for fault detection and isolation in the dynamic closed-loop systems based on the balance of energy which enters and leaves plants. The main contribution of this paper consists in developing a suitable fault detection and isolation technique to detect faults in single-input single-output closed-loop system based on major signals without the requirement of an accurate static or dynamic model. Indeed, in the absence of conventional input-output models, the proposed method involves the on-line energy balance evaluation to detect a sensor fault. The application to the monitoring of a galvanizing line in steel industry shows the effectiveness of the suggested approach when a sensor fault occurs.

Sensor Fault Diagnosis and Accommodation Based on Analytical Redundancy: Application to a Three-Tank System

Abstract In this paper, a sensor fault accommodation method is developed. This method makes possible the compensation of additive or multiplicative sensor faults and failures. Its principle is based on the on-line detection, localization and estimation of the faulty element when a fault occurs on the system. Then, a new control law is computed in order to compensate the fault effect on the system. The performances of the method are tested in simulation on a pilot plant.

Sensor fault tolerant control design applied to a winding machine

This paper describes an Active Fault Tolerant Control System based on a Linear Time Varying (LTV) model for a winding machine. It shows that, according to an experimental identification approach, a LTV model is ideally suited to improve the control of web tension. Moreover, based on this model, a sensor fault detection, isolation and estimation module is developed. This estimation is of paramount importance to compensate for this fault using sensor masking principles and to preserve the system performances. The effectiveness and performances of the strategy are illustrated via real tests.

Limit Checking Using the Algorithmic Redundancy Approach

Abstract A new type of redundancy to be used by fault diagnostic methods is defined in this paper. The algorthmic redundancy concept is presented in the context offered by the temporal, material, and analytical redundancies. An original solution in the field of diagnostic assistant systems is brought today by the DIALOGS software, the result of more than three years of collaboration between five industrial and three academic European partners. The main concepts of the DIALOGS project are briefly presented. and a statistical limit checking method is used to illustrate the implementation of the algorithmic redundancy approach.

Non-Linear Identification of a Steel Cooling in a Galvanising Plant in View of Fault Diagnosis

Abstract The supply on the steel market is always increasing and the quality of the steel coating becomes important for car industries. This quality depends on a thermal cycle which brings the steel to a specific temperature before entering the zinc bath. This temperature is measured by means of two pyrometers and one of them is used in the control-loop. It is required to know if a sensor is faulty and a triple redundancy is then necessary to locate the faulty sensor. Due to the cost of a new sensor and the non-existent place to implement it, the solution is to model the strip temperature in order to obtain this triple redundancy. The desired global model is obtained by combining a static model using a regression method with a dynamic one.

Reconfiguration Against Major Actuator Failures

Abstract This paper deals with a reconfiguration method against major actuator failures. These failures, as a blocking or a complete loss of an actuator, may reduce drastically the desired performances of a closed-loop system. The aim of the reconfiguration method is to set the faulty system to its optimal operating order with respect to desirable performances. The nominal control law is initially designed using an input-output linearization. The sufficient conditions to solve the reconfiguration problem are given. The effectiveness of the proposed strategy is illustrated by a numerical example. Finally, an optimisation problem making possible the generation of optimal reference trajectory is addressed.