Greenhouse gas emissions contribute to the dramatic climate change that we are facing, and that already started affecting our lives. Lowering emissions is one of the main global challenges at the moment, and how we produce electricity is one of the main sources of greenhouse gases. Part of the solution is finding new ways to exploit renewable, green energy sources, and this is exactly what Wave Hexapod does. Wave Hexapod is a company based in Dordrecht that aims at harvesting clean energy from ocean waves with the following concept (https://wavehexapod.com).
This assignment, which is carried out in the scope of a collaboration with Wave Hexapod, considers an advanced version of their original design. Their original design has a triangular platform consisting of three floaters, which is connected to a fixed base using six linear actuators in a hexapod configuration. The advanced version instead replaces these three linear actuator pairs with six DoF robot arms such that there will be one six DoF robot arm per floater.
The main research question is:
Which are the platform's dynamics model and simulator that consider all the proper interaction ports between the system elements?
The associated research output is a complete dynamics model in 20-sim that also includes a customizable model of the waves. Here are the steps of the assignment in more detail:
-Look for existing models of the ABB IRB 7600 series robot. If none is found, use a generic model of a 6 DoF robot arm -Model the system composed of the fixed base, three robot arms in 20-sim.
-Additional steps (they are a plus):
-Design the control of the robot arms so as to
-Harvest the maximum energy from the waves
-Avoid the arm singularities by exploiting the redundancy of the system as a whole
-Test the controller on a hardware setup
Modelling considerations
The Model will be developed using polymorphic port-based techniques, which are the best way to model the multi-physics and specifically the characterization of the energy harvesting. Polymorphic means that all submodels will have a "type" characterized by the kind of external ports to which such a submodel can be connected. It will then be easily
possible to change the implementation of such models by first starting with simple models and then extending their details as needed. Port-based means that a proper power structure of each port is preserved all the time, and in this way, the analysis of energy harvesting will be directly expressed. Furthermore, the package 20sim will be used, which will allow multi-body modelling as well as a consistent energy analysis.
Wave Generator: The wave generator will create a dynamic surface motion, which will be tunable for simulations.
Buoys: The Buoys will be modelled as wrenches applied to the connecting body.
Connecting Body: The Connecting body will be a rigid body with a certain inertial ellipsoid connecting the forces generated by the Buoys
Elastic Connections: The elastic connections will be dual port elements with wrenches and twists paired on both sides. One side will be connected to the Connecting Body, and the other one will be connected to the Robots.
Robots: The robots will have two mechanical ports and an electric one. The two mechanical ports will be connected to the elastic connection on one side and to the fixed base on the other.
Singularities
The relative motion of a rigid body is six-dimensional. If each of the legs of the Steward platform is implemented with 6 degrees of freedom (of) manipulators, this will result in at least 18 DoF, which gives redundancy in the null space of the mechanism of 12 DoF. Kinematic singularities should be identified and a joint-based repulsive potential created to prevent the system from reaching such singularities during operation.
Elastic Coupling
The elastic coupling will be fundamental to prevent damage to the robots and should be dimensioned based on the maximum forces and twists that the manipulator can support.
