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Because of the many publications and increasing number of applications (IFAC Congress and IFAC Symposia SAFEPROCESS) between 1991 and 2018, it is of interest to show some trends (Patton et al. 1989; Basseville and Nikiforov 1993; Gertler 1998; Chen and Patton 1999; Frank et al. 2000). Therefore, a literature study is briefly presented as follows. Contributions taking into account the applications reported in Table I.1 were considered. The type of faults considered is distinguished according to Table I.2. Among all contributions, the fault detection methods were classified as in Table I.3. The change detection and fault classification methods are indicated in Table I.4. The reasoning strategies for fault diagnosis are reported in Table I.5. The contributions considered are summarized in Table I.6. The evaluation has been limited to the fault detection and diagnosis (FDD) of laboratory, pilot and industrial processes.

Table I.1. FDI applications and number of contributions

Application	Number of contributions
Simulation of real processes	105
Large-scale pilot processes	94
Small-scale laboratory processes	68
Full-scale industrial processes	98

Table I.2. Fault type and number of contributions

Fault type	Number of contributions
Sensor faults	129
Actuator faults	111
Process faults	123
Control loop or controller faults	48

Table I.3. FDI methods and number of contributions

Method type	Number of contributions
Observer	123
Parity space	74
Parameter estimation	101
Frequency spectral analysis	57
Neural networks	79

Table I.4. Residual evaluation methods and number of contributions

Evaluation method	Number of contributions
Neural networks	89
Fuzzy logic	65
Bayes classification	54
Hypothesis testing	48

Table I.5. Reasoning strategies and number of contributions

Reasoning strategy	Number of contributions
Rule based	40
Sign directed graph	33
Fault symptom tree	32
Fuzzy logic	66

Table I.6. Applications of model-based fault detection

FDD	Number of contributions
Milling and grinding processes	91
Power plants and thermal processes	106
Fluid dynamic processes	67
Combustion engine and turbines	96
Automotive	68
Inverted pendulum	63
Miscellaneous	102
DC motors	121
Stirred tank reactor	77
Navigation system	75
Nuclear process	50

Table I.6 shows that among mechanical and electrical processes, DC motor applications are mostly investigated. Parameter estimation and observer-based methods are used in the majority of applications in these kind of processes, followed by parity space and combined methods. Thermal and chemical processes are investigated less frequently.

Table I.3 shows that parameter estimation and observer-based methods are used in nearly 70% of all applications considered. Neural networks, parity space and combined methods are applied notably less often.

More than 50% of sensor faults are detected using observer-based methods, while parameter estimation, parity space and combined methods play a less important role. For the detection of actuator faults, observer-based methods are mostly used, followed by parameter estimation and neural network methods.

Parity space and combined methods are rarely applied. In general, there are fewer applications for actuator faults than for sensor or process faults. The detection of process faults is mostly carried out with parameter estimation methods. Nearly 50% of all the applications considered use parameter estimation-based methods for detection of process faults. Observer-based, parity space and neural network-based methods are used less often for this class of faults.

Among all the described processes, linear models have been used much more than nonlinear models. In processes with nonlinear models, observer-based methods are mostly applied, but parity equations and neural networks also play an important role. In processes with linear or linearized models, parameter estimation and observer-based methods are mostly used. Parity space and combined methods are also used in several applications but not to the same extent as observer-based and parameter estimation methods.

Taking into account the system considered, the number of nonlinear process applications using nonlinear models is decreasing. For linear processes, no significant change can be stated. The applications of fault-detection methods for nonlinear processes used mostly observer-based and parameter estimation, more than parity space methods. Also, the use of neural networks and combinations are important.

Concerning the fault diagnosis methods, in recent years, the field of classification approaches, especially with neural networks and fuzzy logic, has steadily been growing. Also, rule-based reasoning methods are increasingly being based on fault diagnosis. A growing application of fuzzy rule-based reasoning can be stated. Applications using neural networks for classification are increasing and the trends are analogous to the increasing number of nonlinear process investigations. Nevertheless, the classification of generated residuals seems to remain the most important application area for neural networks.