Snake Aerial Manipulators: A Review <s> 4) TRANSFORMATION SEQUENCE <s> A backstepping approach is proposed in this paper to cope with the failure of a quadrotor propeller. The presented methodology supposes to turn off also the motor which is opposite to the broken one. In this way, a birotor configuration with fixed propellers is achieved. The birotor is controlled to follow a planned emergency landing trajectory. Theory shows that the birotor can reach any point in the Cartesian space losing the possibility to control the yaw angle. Simulation tests are employed to validate the proposed controller design. I. INTRODUCTION <s> BIB001 </s> Snake Aerial Manipulators: A Review <s> 4) TRANSFORMATION SEQUENCE <s> In this paper, we introduce a novel type of the multirotor aerial vehicle with two-dimensional multilinks which enables the stable aerial transformation for high mobility in three-dimensional environments. Our goal is to hold and carry object by using the whole-body manipulation in the air. The research involved three steps. First, we developed the modeling of the link modules that compose a multirotor with two-dimensional multilinks and conducted a quadrotor prototype. Second, we derived a stable flight control method for aerial transformation on the basis of linear-quadratic-integral optimal control. Third, we investigated the whole-body aerial manipulation based on the enveloping grasping method for the four-link type which takes the additional inertial parameters and joint torque into account. Successful aerial transformation and manipulation with the quadrotor prototype were demonstrated, confirming the feasibility of our proposed transformable multirotor for aerial maneuvering. <s> BIB002 </s> Snake Aerial Manipulators: A Review <s> 4) TRANSFORMATION SEQUENCE <s> This paper proposes a novel adaptive sliding-mode-based control allocation scheme for accommodating simultaneous actuator faults. The proposed control scheme includes two separate control modules with virtual control part and control allocation part, respectively. As a low-level control module, the control allocation/reallocation scheme is used to distribute/redistribute virtual control signals among the available actuators under fault-free or faulty conditions, respectively. In the case of simultaneous actuator faults, the control allocation and reallocation module may fail to meet the required virtual control signal, which will degrade the overall system stability. The proposed online adaptive scheme can seamlessly adjust the control gains for the high-level sliding mode control module and reconfigure the distribution of control signals to eliminate the effect of the virtual control error and maintain the stability of the closed-loop system. In addition, with the help of the boundary layer for constructing the adaptation law, the overestimation of control gains is avoided, and the adaptation ceases once the sliding variable is within the boundary layer. A significant feature of this study is that the stability of the closed-loop system is guaranteed theoretically in the presence of simultaneous actuator faults. The effectiveness of the proposed control scheme is demonstrated by experimental results based on a modified unmanned multirotor helicopter under both single and simultaneous actuator faults conditions with comparison to a conventional sliding mode controller and a linear quadratic regulator scheme. <s> BIB003
At this point, a method is required to transform the structure from an standard and coordinated flight mode (for example an helicopter or a tailsitter) to an aerial manipulator. The software implication is the right sequence of switching VOLUME 8, 2020 frequencies of the engines, which must be turned off and on in a precise order as the full system keeps hovering in the air (helibot patent pending MX/a/2016/014595) see also  , BIB002 Fig. 6,8 . Discussion: this section discussed about the main problems related to the tasks of free flight maneuvers to the workspace and the transformation sequence. The challenges related to cooperative control were highlighted, including the parameters and disturbances estimation either by centralized or decentralized modes. Finally the open problem of transition between the free flight mode and the aerial manipulation mode was stated. As indicated in the previous sections, it is convenient to investigate the boundary conditions between the free flight and the manipulation flight, because said transformation stage and in general the manipulation stage, entails adverse aerodynamic effects. In this manner, concerning to the topics referred into this part of the article, it is convenient to develop sensors or mathematical estimators (observers) to recalculate the position and orientation of a reference point affected by these perturbations, not considered nonlinear dynamics, and the effect of the trajectories (the center of mass, the center of gravity, the center of floating, etc., or, several of these reference points). On the other hand the coordination of the individual propulsion elements, implies the development of more efficient communication protocols in order to deal with data loss, and to incorporate selective or intelligent assignment of the event-based communication (for example by defining a leader according to whether or not there is a lot of wind concentrated at a specific part of the snake aerial manipulator), etc. On the side of the controllers, developing tolerant schemes to the failure of one or more propulsion aircraft are needed(there are already some schemes for multicopters in the case of failure of one or more propellers  BIB001 BIB003 , but in this case, the designer must remember that the thrusters could be full aircraft and not just individual propellers) Also is needed the development of sensors capable of measuring planar positions and altitude with an acceptable resolution. This sensors should be reduced in noise and drift effects. This is mentioned because these problems are already present and they are really important in a single vehicle, in this way, they are increased while using a cooperative system of aircraft. Additionally, many of the experiments currently carried out with the exception of LASDRA  , were done in the laboratory, under the comfort and reliability provided by external motion capture systems, but research must be done in the case of wanting to use snake aerial manipulators outdoors. It could be feasible to continue using external sensors, but by employing laser location systems, or specialized ultrasonic triangulation, or by developing passive or active ways of noise suppression so that the on-board measurements include less of the aforementioned inconveniences (here you could find specialized artificial vision algorithms, however, and since they require high processing, the reduction of weight, size and electrical consumption of these processing units, remains as a problem to be solved or improved)