# Review of Fault-Tolerant Control Systems Used in Robotic Manipulators
## 1 Introduction  
The basic concepts and techniques of fault tolerance in computer systems were described in [1], which presented the different error classifications and the application of redundancy techniques to ensure the reliable operation of computers. The modeling and prediction of faults were also described, along with examples of fault-tolerant computers. In the early 1990s, [PERSON] [2], [PERSON] [3], and [PERSON] [4] published the results of different applications of FTC systems. In [5], state-of-the-art methods for fault-tolerant control applications up until 1997 were presented. In a fault-tolerant control system, it is necessary to distinguish concepts such as fault definition and fault tolerance. A fault is defined as the deviation of a parameter from an acceptable value. Fault tolerance is the ability of the system to continue operation regardless of faults. The origins of FTC systems are related to the need to use special control in critical safety devices and equipment. Examples of such devices are aircraft or nuclear power plants. In [6], which was published in 1990, one of the first applications of fault-tolerant control in robot manipulators was described and the measure of joint failure influences on the remaining dexterity of a kinematically redundant manipulator was quantified. This measure was used as a criterion and technique for calculating optimal configurations of fault-tolerant redundant manipulators. An example of a three-dimensional robot was shown that used a fourth actuator, which guaranteed half of the original dexterity, after the joint failure. In the same period, i.e., in the 1990s, extensive research was undertaken to develop FTC systems for applications in manipulators intended for space research [7; 8; 9]. The foundations for the design of fault-tolerant manipulators were presented in [10]. In this article, the modeling tools and reliable evaluation methods of robots were described. The design aspects, together with a reliability evaluation, were presented. Based on the performed analyses, a methodology for designing fault-tolerant manipulators was proposed. Particularattention was paid to the development of recommendations for the design of the arm with 10 degrees of freedom. It was proposed to use two actuators on each joint. Techniques for a robust residual generation have been proposed by several authors. In order to ensure the successful operation of autonomous robots in remote or hazardous environments, fault tolerance is essential. In [11], different fault tolerance algorithms were proposed, where the possible failures of the robot components and the interdependence of these failures must be determined first. To achieve this, fault tree analysis can be a very useful tool. The tree structures developed using this type of analysis make it possible to both determine a block diagram of possible occurrences of robot faults and define the cause-and-effect relationships between failures. In [12], a control method was proposed in which a model reference algorithm and a proportional integral derivative controller were applied in the fault-tolerant operation of robotic manipulators. This method was tested in a simulation that showed the effectiveness of the recovery algorithm, which enabled it to continue the movement on the assumed trajectory and reach the given end effect. In 1987, [PERSON] and [PERSON] [13] proposed the use of artificial intelligence, i.e., a real-time expert system, in a fault control solution, which was used in an algorithm to supervise fault detection and localization. In the case of sensor damage, the system automatically replaced the faulty measurement with an updated predictor model output signal. The investigated system was successfully tested in a pilot process using artificial intelligence to overcome physical faults. The authors of [14] proposed the use of intelligent fault-tolerant control in a flexible assembly cell, where the fault-tolerant controller would immediately recognize errors and react in real time to such situations.