Fully Actuated Multirotor UAVs: A Literature Review

TLDR: This article collects the different UAV designs having fully actuated aerodynamic wrench generation proposed in the literature to date and includes a systematic derivation of the control-allocation matrix for all of the concepts.

Abstract: In the last decade, the aerial robotics community has witnessed an increased interest in fully actuated multirotor unmanned aerial vehicles (UAVs) that have more capabilities than conventional underactuated multirotors. This article collects the different UAV designs having fully actuated aerodynamic wrench generation proposed in the literature to date. The work includes a systematic derivation of the control-allocation matrix for all of the concepts as well as a discussion of the different quantitative criteria used for optimizing UAV designs.

Figures

Table 2. The mapping-matrix variables for fixed-tilt concepts.

Figure 1. A schematic view of the reference frames used, illustrated on a fully actuated hexarotor.

Figure 3. The variable-tilt concepts. Each concept is accompanied by an illustration of the top view of the rotor configuration. Clockwiserotating rotors are yellow, while counterclockwise ones are green. Double arrows indicate the active tilt axis. The (a) QuadvC [36], (b) QuadvD [34], (c) QuadvCD [35], (d) QuadvCDc [37], (e) HexvC [38], and (f) HexvCc [40].

Table 1. The abbreviations used in the “Fixed-Tilt UAV Concepts” and “Variable-Tilt UAV Concepts” sections.

Table 4. The optimization criteria for designing fixed-tilt concepts.

Table 3. The mapping-matrix variables for variable-tilt concepts.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Fully actuated multirotor uavs: a literature review" ?

This article collects the different UAV designs having fully actuated aerodynamic wrench generation proposed in the literature to date. 

Q2. What is the common problem for fixed-tilt designs?

for variable-tilt designs, the control-allocation problem is more involved, as the rank of the M matrix might change during flight and the UAV might operate in an underactuated configuration. 

Q3. What is the common method of actuation of multirotor UAVs?

Full actuation has been realized mainly by using fixed propellers with dissimilar orientations, which the authors refer to as fixed-tilt concepts, and actively tilting the propellers using extra actuators, which the authors refer to as variable-tilt concepts. 

Q4. What is the problem with fully actuated multirotor UAVs?

The problem with fully actuated UAV concepts is that the optimal rotor configuration is application-dependent, contrary to underactuated concepts, which usually have the orientation of their rotors in a vertical in-plane symmetric configuration. 

Q5. What is the main advantage of the canting actuator?

It contains a canting actuator added to each rotor of a conventional hexarotor to achieve omnidirectional flight with unidirectional thrust generation. 

Q6. What was the top achievable pitch/roll angle?

Since the rotors the authors used were unidirectional and the tilting actuators had maximum limits, the UAV’s top achievable pitch/roll angle during hovering was limited to approximately 30°. 

Q7. What is the rotor position of the octarotor?

The rotor positions are fixed to the vertices of a cube, while the orientation of the rotors is optimized to maximize the vehicle’s agility, measured by the maximum attainable omnidirectional wrench. 

Q8. What is the main advantage of the cant and dihedral actuation method?

The main advantage of this actuation method is that it reduces the number of actuators required for the cant and dihedral actuation from eight to two. 

Q9. What is the common design principle used to optimize a fixed-tilt UAV?

In most of the works surveyed here, a two-stage control architecture is employed, where the control wrench from (7) is considered a virtual input to the rigid body model of the UAV and the mapping matrix is used to compute the desired propeller thrusts. 

Q10. What was the work of designing noncoplanar hexarotors?

The work of designing noncoplanar hexarotors was also studied in [24], in which the dynamic maneuverability and maximum lateral-translations measures were used to compute the optimal configurations of the three rotor pairs. 

Q11. What is the aerodynamic torque of a propeller?

It is well known from the aerial-robotics literature that, in quasi-static flights, the aerodynamic thrust and drag torque of a propeller are approximately proportional to the square of the propeller’s spinning velocity [5]. 

Q12. What is the advantage of a single actuator instead of six to cant the hex?

The advantage of a single actuator instead of six to cant the rotors is that the energy consumption and total mass of the system are greatly reduced compared to the parallel mechanism of the QuadvCDc concept. 

Q13. What design aspect was used to increase the thrust-generation capabilities of the UAV?

The UAV design in [39] had coaxial rotors to increase the thrust-generation capabilities of the UAV, with all other design aspects remaining the same. 

Q14. What is the thrust magnitude of the ith propeller in piW?

The thrust magnitude generated by the ith propeller in piW is denoted by ,im while the drag torque is expressed as ,,d i i ix cv m= where c is the propeller’s drag-to-thrust ratio, and { , }1 1i !v - specifies the propeller’s rotation direction (with 1iv = for clockwise rotation). 

Q15. What is the drawback of the HexC design?

As for the airflow interference between the top and bottom rotor, the drawbacks are less severe in this design (and for coaxial rotors in general) compared to the Quad4Hor design in [7] since both rotors have the same speed.