Faults actuator

Voltage polarization depends upon the location of the relay and the location of the fault. It is possible that the residual voltage, at a particular location in the system, is not sufficient to actuate the voltage coil of the directional G/F relay. In such an event, current polarization is used to supplement voltage polarization. Current polarization is possible, provided that a star point is created on the system, even through a A/t> power transformer, if such a transformer is available in the same circuit. Figure 21.20. Else a grounding transformer may be provided as... 

A fault can be regarded as a not allowed deviation of at least one property or one characteristic parameter of the system compared to the normal operating conditions. The occurrence of a fault may have serious consequences in the process and may cause the facilities standstill, even to damage them. Several examples illustrating the gravity of the occurrence of sensor or actuators faults are reported in [25], [26], [62]. The fault detection refers to the determination of the presence of faults as well as their occurrence moment while the diagnosis is the determination of its amplitude and its behavior. 

Related studies to the diagnosis of bioprocess have been limited and have used mainly heuristic approaches. Moreover, they have been concerned with the detection of a disfunction of the bioprocess (detection of a desestabiliza-tion, state of the biomass, etc.) rather than the detection and the location of sensor and/or actuators faults. The interested reader will be able to refer to the following references [4], [5], [12], [14], [27], [28], [43], [45], [47], [52], [59], [60], [61], [63], [64], [71]. 

A fault is understood as an unpermitted deviation of at least one characteristic property or parameter of the system from the acceptable, usual or standard condition. A fault can stem from several origins as depicted by the Figure 1. It can be caused by an unexpected perturbation i.e., a major deviation from one input acting on the system) or by a disturbance i.e., the action of an unknown and uncontrolled input on the system). Another fault origin can be an error of any sensor or actuator, which is a deviation between the measured and the true or specified value. 

A traditional approach to fault diagnosis in the wider application context is based on hardware i.e. physical) redundancy methods which use multiple lines of sensors, actuators, computers and software to measure and/or control a particular variable. Typically, a voting scheme is applied to the hardware redundant system to decide if and when a fault has occurred and its likely location amongst redundant system components. The use of multiple redundancy in this way is common, for example with digital fly-by-wire flight control... 

Safety shutdown valves, which are normally wide open and operate infrequently, are expected to respond to a safety trip command reliably and without fault. To achieve the level of reliability required in this application, the safety valve must be periodically tested to ensure positive operation under safety trip conditions. To test the operation of the shutdown system without disturbing the process, the traditional method is to physically lock the valve stem in the wide-open position and then to electrically operate the pneumatic shutdown solenoid valve. Observing that the pneumatic solenoid valve has properly vented the actuator pressure to zero, the actuator is seen as capable of applying sufficient spring force to close the valve, and a positive safety valve test is indicated. The... 

Nc number of compounds involved in the reaction Ny number of considered actuator/process faults r scalar residual... 

Early approaches to fault diagnosis were often based on the so-called physical redundancy [11], i.e., the duplication of sensors, actuators, computers, and softwares to measure and/or control a variable. Typically, a voting scheme is applied to the redundant system to detect and isolate a fault. The physical redundant methods are very reliable, but they need extra equipment and extra maintenance costs. Thus, in the last years, researchers focused their attention on techniques not requiring extra equipment. These techniques can be classified into two general categories, model-free data-driven approaches and model-based approaches. 

The literature focused on model-based FD presents a few applications of observers to chemical plants. In [10] an unknown input observer is adopted for a CSTR, while in [7] and [21] an Extended Kalman Filter is used in [9] and [28] Extended Kalman Filters are used for a distillation column and a CSTR, respectively in [45] a generalized Luenberger observer is presented in [24] a geometric approach for a class of nonlinear systems is presented and applied to a polymerization process in [38] a robust observer is used for sensor faults detection and isolation in chemical batch reactors, while in [37] the robust approach is compared with an adaptive observer for actuator fault diagnosis. 

In this chapter, an FD framework for batch chemical processes is developed, where diagnosis of sensor, actuator, and process faults can be achieved via an integrated approach. The proposed approach is based on physical redundancy for detection of sensor faults [38], while an analytical redundancy method, based on a bank of diagnostic observers, is adopted to perform process/actuator fault detection, isolation, and identification [4],... 

If the dynamics of sensors and actuators is neglected, the fault-free condition is characterized by the following relations ... 

In a similar way, the actuator action, in the presence of an actuator fault, becomes... 

In this chapter, a bank of observers is adopted for isolation of process and actuator faults. Namely, it is assumed that only N different types of faults can occur. Then,... 

An actuator fault can be generated by a malfunction of the cooling system, such as electric-power failures, pomp failures, valves failures, and leaking pipes. Without loss of generality, actuator faults may be modeled as an unknown additive term affecting the state equation in (6.5), due to unexpected variations of the input u with respect to its nominal value, i.e., the value computed by the reactor control system. 

The above model includes the case in which a sensor and a process/actuator fault occur during the same batch operation. However, occurrence of multiple faults of the same nature (i.e., multiple process/actuator faults or multiple sensor faults), is not considered. 

Assumption 6.3 It is assumed that multiple process/actuator faults (i.e., two or more faults belonging to Ta occur) and multiple sensor faults (i.e., two or more sensors are subject to failures) cannot occur during the same batch operation. 

In the following, it is shown that multiple process/actuator faults (multiple sensor faults) can be detected but not correctly isolated and identified. 

On the contrary, occurrence of sensor and process/actuator faults during the same batch is allowed. 

Then, the healthy signal is used to feed a bank of /Vp + 1 nonlinear adaptive observers (where /Vp is the number of the possible process/actuator faults). The first observer is in charge of detecting the occurrence of process/actuator faults. The other /Vp observers, each corresponding to a particular type of process/actuator fault, achieve fault isolation and identification by adopting a suitable adaption mechanism. Figure 6.3 shows a block diagram representation of the overall architecture. 

The healthy measure, obtained via the diagnostic system described above, is used to feed a bank of observers providing process/actuator fault detection and isolation. One observer detects the occurrence of an actuator or process fault, while the other Np observers, each one corresponding to a fault type, are used for isolation and identification. 

Once a process/actuator fault has been detected, isolation and identification can be achieved via N-p nonlinear adaptive observers. Each observer is designed in such a way to be insensitive to a particular type of fault. In fact, the ith observer (hereafter i =, ..., Np) has the form... 

Hence, a sufficient condition for isolability for the Zth type of process/actuators faults is given by the two inequalities,... 

Decoupling Sensor Faults from Process and Actuator Faults... 

In order to make the observer (6.10) insensitive to process/actuator faults, the following modified dynamics can be adopted ... 

Three classes of actuator and process faults have been considered in the simulations. 

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