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Thanks to the incorporation of robotic systems, the development of industrial processes has generated a great increase in productivity, yield and product quality. Nevertheless, as far as technological advancement permits a greater automation level, system complexity also increases, with greater number of components, therefore rising the probability of failures or anomalous operation. This can result in operator's hazard, difficulties for users, economic losses, etc. Robotic automatic systems, even if helped in minimizing human operation in control and manual intervention tasks, haven't freed them from failure occurrences. Although such failures can t be eliminated, they can be properly managed through an adequate control system, allowing to reduce degraded performance in industrial processes.


Introduction
Thanks to the incorporation of robotic systems, the development of industrial processes has generated a great increase in productivity, yield and product quality. Nevertheless, as far as technological advancement permits a greater automation level, system complexity also increases, with greater number of components, therefore rising the probability of failures or anomalous operation. This can result in operator's hazard, difficulties for users, economic losses, etc. Robotic automatic systems, even if helped in minimizing human operation in control and manual intervention tasks, haven't freed them from failure occurrences. Although such failures can´t be eliminated, they can be properly managed through an adequate control system, allowing to reduce degraded performance in industrial processes. In figure 1 we see a scheme showing the different performance regions a given system can adopt when a failure occurs. If the system deviates to a degraded performance region in, presence of a failure, it can recover itself moving into an optimum performance region, or www.intechopen.com near to it. These systems are called fault tolerant systems and have become increasingly important for robot manipulators, especially those performing tasks in remote or hazardous environments, like outer space, underwater or nuclear environments. In this chapter we will address the concept of fault tolerance applied to a robotic manipulator. We will consider the first three degrees of freedom of a redundant SCARAtype robot, which is intended to follow a Cartesian test trajectory composed by a combination of linear segments. We developed three fault-tolerant controllers by using classic control laws: hyperbolic sine-cosine, calculated torque and adaptive inertia. The essays for such controllers will be run in a simulation environment developed through MatLab/Simulink software. As a performance requirement for those controllers, we considered the application of a failure consisting in blocking one of the manipulator's actuators during trajectory execution. Finally, we present a performance evaluation for each one of the above mentioned fault-tolerant controllers, through joint and Cartesian errors, by means of graphics and rms rates.

Fault tolerant control
The concept of fault tolerant control (Zhang & Jiang, 2003) comes first from airplane fault tolerant control; although at scientific level it appears later, as a basic aim in the first congress of IFAC SAFEPROCESS 1991, with an especially stronger development since the beginning of 21 th century. Fault tolerant control can be considered both under an active or passive approach, as seen in figure 2a. Passive tolerant control is based on the ability of feedback systems to compensate perturbations, changes in system dynamics and even system failures (Puig, Quevedo, Escobet, Morcego, & C., 2004). Passive tolerant control considers a robust design of the feedback control system in order to immunize it from some specific failures (Patton, 1997). Active tolerant control is centered in on-line failure, that is, the ability to identify the failing component, determine the kind of damage, its magnitude and moment of appearance and, from this information, to activate some mechanism for www.intechopen.com rearrangement or control reconfiguration, even stopping the whole system, depending on the severity of the problem (Puig, Quevedo, Escobet, Morcego, & C., 2004). Fault tolerant control systems (being of hybrid nature) consider the application of a series of techniques like: component and structure analysis; detection, isolation and quantification of failures; physical or virtual redundancy of sensors and/or actuators; integrated real-time supervision of all tasks performed by the fault tolerant control, as we can see in figure 2b (Blanke, Kinnaert, Lunze, & Staroswiecki, 2000). In the evaluation of fault tolerant controllers it is assumed that a robotic manipulator where a failure has arisen in one or more actuators, can be considered as an underactuated system, that is, a system with less actuators than the number of joints (El-Salam, El-Haweet, & and Pertew, 2005). Those underactuated systems present a greater degree of complexity compared with the simplicity of conventional robot control, being not so profoundly studied yet (Rubí, 2002). The advantages of underactuated systems have been recognized mainly because they are lighter and cheaper, with less energy consumption. Therefore, a great deal of concern is being focused on those underactuated robots (Xiujuan & Zhen, 2007). In figure  3 it is shown a diagram displaying the first three degrees of freedom of a SCARA type redundant manipulator, upon which essays will be conducted considering a failure in the second actuator, making the robot become an underactuated system.

SCARA-type redundant manipulator
For the evaluation of fault-tolerant controllers, we consider the first three degrees of freedom of a redundant SCARA-type robotic manipulator, with a failure occurring in one of its actuators; such a system can be considered as an underactuated system, i.e., with less actuators than the number of joints (Xiujuan & Zhen, 2007). Those underactuated systems have a greater complexity compared with the simplicity of conventional robots control, and they haven't been so deeply studied yet (Rubí, 2002). The advantages of underactuated systems have been remarked mainly because they are lighter and less expensive; also having less energy consumption, consequently an increasing level of attention is being paid to underactuated robots (Xiujuan & Zhen, 2007). In figure 3 it is shown the scheme of a redundant SCARA-type robotic manipulator, and in figure 4 we can see a diagram showing the first three degrees of freedom of such manipulator, on which the essays will be carried on. The considered failure is the blocking of the second actuator, what makes this robot an underactuated system. Having in mind the exposed manipulator, it is necessary to obtain its model; therefore we will consider that the dynamic model of a manipulator with n joints can be expressed through equation (1): where: τ : Vector of generalized forces (n×1 dimension). M : Inertia matrix (n×n dimension). C : Centrifugal and Coriolis forces vector (n×1dimension). q : Components of joint position vector. : Components of joint speed vector. G : Gravity force vector (n×1dimension). : Joint acceleration vector (n×1dimension). F : Friction forces vector (n×1dimension). Under failure conditions in actuator number 2, that is, it's blocking, the component 2 of equation (1)

Considered controllers
Considering the hybrid nature of fault tolerant control, it is proposed an active fault tolerant control having a different control law according to the status of the robotic manipulator, i.e. normal or failing, with on-line sensing of possible failures and, in correspondence with this, reconfiguring the controller by selecting the most adequate control law (changing inputs and outputs). Next, we will present a summary of the controllers considered for performance evaluation when a failure occurs in the second actuator of the previously described manipulator.

Fault tolerant controller: hyperbolic sine and cosine
This controller is based on the classic controller hyperbolic sine-cosine presented in (Barahona, Espinosa, & L., 2002), composed by a proportional part based on sine and hyperbolic cosine functions, a derivative part based on hyperbolic sine and gravity compensation, as shown in equation (2). The proposed fault tolerant control law includes two classic hyperbolic sinecosine controllers that are "switched" to reconfigure the fault tolerant controller.
According to equations (2) and (3) In (Barahona, Espinosa, & L., 2002) it is established that robotic manipulator's joint position error will tend asymptotically to zero as long as time approaches to infinite: This behavior is proved analyzing equation (5) and pointing that the only equilibrium point for the system is the origin (0,0).

Fault tolerant controller: Computed torque
Another active fault tolerant controller analyzed here uses a control law by computed torque, consisting in the application of a torque in order to compensate the centrifugal, Coriolis, gravity and friction effects, as shown in equation (6).
K v : Diagonal definite positive matrix (n×n dimension).

Fault tolerant controller: Adaptive inertia
The fault tolerant control under examination is based on an adaptive control law, namely: adaptive inertia (Lewis, Dawson, & Abdallah, 2004), (Siciliano & Khatib, 2008), for what it is necessary to consider the manipulator dynamic model in the form expressed in equation (11). The term corresponding to centrifugal and Coriolis forces is expressed through a matrix V m .
K v : Diagonal definite positive matrix (n×n dimension). The adaptive control updating rule can be expressed by: where: Γ: Diagonal definite positive matrix (n×n dimension).

Fault tolerant control simulator
The three above mentioned control laws, along with the dynamic model of the redundant SCARA-type manipulator considering the first three degrees of freedom (Addendum A), are run under the simulation structure shown in figure 5, where we can see the hybrid nature of this kind of controller.
In Addendum B we show the set of parameter values employed in the manipulator dynamic model, and the gains considered for each kind of fault tolerant controller.

Results
After establishing the control laws being utilized, we determine the trajectory to be entered in the control system to carry out the corresponding performance tests of fault tolerant control algorithms. This trajectory is displayed in figure 6.       Where e i represents articular trajectory as well as Cartesian errors.

Conclusions
In this work we presented a performance evaluation of three fault tolerant controllers based on classic control techniques: hyperbolic sine-cosine, calculated torque and adaptive inertia. Those fault tolerant controllers were applied on the first three degrees of freedom of a redundant SCARA-type robotic manipulator. The different system stages were implemented in a simulator developed using MatLab/Simulink software, allowing to represent the robotic manipulator behavior following a desired trajectory, when blocking of one of its actuators occurs. In this way we obtained the corresponding simulation curves. From the obtained results, we observed that the adaptive inertia fault tolerant controller have errors with less severe maximums than the other controllers, resulting in more homogeneous manipulator movements. We noticed that greater errors were produced with the calculated torque fault tolerant controller, both for maximums and rms. Consequently, the best performance is obtained when using the adaptive inertia controller, as shown in figures 14 and 15. It is also remarkable that the hyperbolic sine-cosine fault tolerant controller have a lesser implementation complexity, since it does not require the second derivative of joint position. This can be a decisive factor in the case of not having high performance processors.

Further developments
Thanks to the development of this work, from the implemented simulation tools and the obtained results, fault tolerant control systems essays are being currently carried out, in order to apply them to actual robotic systems, with and without link redundancy, like the SCARA-type robots shown in figure 14 and figure 15, respectively.

Addendum A: Manipulator's dynamic model
The manipulator's dynamic model is given by equations a1 to a14.  www.intechopen.com m 3 : Third link mass. l 1 : First link length. l 2 : Second link length. l 3 : Third link length. l c1 : Length from 1 st link origin to its centroid. l c2 : Length from 2 nd link origin to its centroid. l c3 : Length from 3 rd link origin to its centroid. I 1zz : 1 st link inertial momentum with respect to the first z axis of its joint. I 2zz : 2 nd link inertial momentum with respect to the first z axis of its joint. I 3zz : 3 rd link inertial momentum with respect to the first z axis of its joint.

Addendum B: Considered parameter values
Parameter values considered for the manipulator as well as controller gains values are shown in tables B1 and B2, respectively.