Scaling Flying Modular Structures Based on Quadrotors

Modular robotics implies in general flexibility and versatility to robots. In theory, you could design a modular robot basically on the way you would want it to be, by simply adding or removing modules from the already existing robot. Changing the robot configuration by adding more individuals, generally increases the system redundancy, meaning that now, there are probably many different ways to achieve a specific goal. From a naive standard point of view, more modules could imply in practice more robustness due to this redundancy. In fact, it does get more robust by the cost of becoming more complex, and probably harder to control. Added to that, other issues may arise when you take into account that your modular robot is flying, and how physical properties and actuation scales as the number of modules grow.

In the GRASP Laboratory at the University of Pennsylvania, one of our focuses is to allow robots to achieve a specific task. In this work, we present ModQuad-DoF, which is a modular flying platform that enlarges the configuration space of modular flying structures based on quadrotors (crazyflies), by applying a new yaw actuation method that relies on the desired roll angles of each flying vehicle. This research project is coordinated by Professor Mark Yim , and led by Bruno Gabrich (PhD candidate).

Scaling Modular Robots

Scaling modular robots is a very challenging problem that usually limits the benefits of modularity. The sum of the performance metrics (speed, torque, precision etc.) from each module usually does not scale at the same rate as the conglomerate physical properties. In particular, for ModQuad, saturation from individual motors would increase as the structures became larger leading to failure and instability. When conglomerate systems scale up in the number of modules, the moment of inertia of the conglomerate often grows faster than the increase in thrust capability for each module. For example, the increase in the moment of inertia for a fifth module added to four modules in a line can be approximated by the mass of the module times half the distance to the center squared. This quadratic increase gives us the intuition that the required yaw actuation grows faster than the actuation authority.

Yaw Actuation

An inherit characteristic of quadrotors is to have their yaw controlled by the drag moments from each propeller. For ModQuad as more modules are docked together, a decreased controllability in yaw is noticed as the structure becomes larger. In a line configuration the structure’s inertia grows quadratically with the distance of each module to the structure center of mass. On the other hand the drag moments produced scales linearly with the number of modules.

The new yaw actuation method relies on the fact that each quadrotor is capable to generate an individual roll enabled by our new cage design. By working in coordinated manner, each crazyflie can then generate structure moments from moment arms provided by the propellers given its roll and its distance from the structure’s center of mass.

Cage Design

The Crazyflie 2.0 is the chosen platform to enable thrust and attitude to the individual modules. The flying vehicle measures 92×92×29 mm and weights 27 g while its battery lasts around 4 minutes for the novel design proposed. In this work the cage performs as pendulum relative to the flying vehicle. The quadrotor is joined to the cage through a one DOF joint. The cages are made of light-weight materials: ABS for the 3-D printed connectors and joints, and carbon fiber for the rods.

Although the flying vehicle does not necessarily share same orientation as the cage, the multiple connected cages do preserve same orientation relative to each other. With the purpose of allowing such behavior, we used Neodymium Iron Boron (NdFeB) magnets as passive actuators to enable rigid cage connections. Docking is only allowed at the back and front face of the modules, and each one of these faces contains four magnets. Those passive actuators have dimensions of 6.35 × 6.35 × 0.79 mm with a bonding force of 1 kg.

Structure Flying Performance

Conclusions

ModQuad-DoF is a flying modular robotic structure whose yaw actuation scales with increased numbers of modules. ModQuad-DoF has a one DOF jointed cage design and a novel control method for the flying structure. Our new yaw actuation method was validated conducting experiments for hovering conditions. We were able to perform two, four and, six modules cooperatively flying in a line with yaw controllability and reduced loss in thrust. In future work we aim to explore the structure controllability with more robots in a line configuration, and exploring different solutions for the desired roll angles. Possibly, with more modules in the structure, only a few would be required to roll in order to maintain a desired structure yaw. Given that, we could explore the control allocation for each  module in a specific structure configuration, and dependent on its desired behavior. Further, structures that are not constrained to a line will also be tested using the basis of the controller proposed in this work.

Detailed Video Explanation (ICRA 2020)

This work was developed by:

Bruno Gabrich, Guanrui li, and Mark Yim

Additional resources at:

https://www.modlabupenn.org/
https://www.grasp.upenn.edu/

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