Resumen de Proprioceptive force estimation and control for teleoperation in radioactive environments
Enrique Constantino del Sol Acero
As the energy on the particle accelerators or heavy ion accelerators such as CERN or GSI, fusion reactors such as JET or ITER, or other scientific experiments is increased, it is becoming increasingly necessary to use remote handling techniques in order to interact with the remote and radioactive environment. So far, the dose rate at CERN could present values near several mSv for cooling times in the range of hours, which allowed human intervention for maintenance tasks. At JET, values close to 200 μSv have been measured after a cooling time of 4 months. Since then, the use of remote handling techniques became usual. There is a clear tendency to increase the radiation levels in the future. A good example is ITER, where values of 450 Sv/h are expected in the centre of the torus after 11 days of cooling. Also, the new energetic levels of CERN are expected to lead towards a more advanced remote handling means in this facility.
The framework of this thesis is based on these environments. It develops a bilateral control system based on force-position, trying to avoid the use of force/torque sensors, whose electronic content makes them very sensitive under radiation. Thus, the research it is focused on teleoperating industrial robots, which due to its well-known reliability, easiness to be adapted to harsh environments, cost-effectiveness and high availability, are considered as an interesting alternative to expensive custom-made solutions for remote handling tasks.
Firstly, the kinematic problem of teloperating master and slave with dissimilar kinematics is analysed and a new general approach for solving this issue is presented. The solution includes the use of assistive forces in order to guide the human operator.
Coming up next, I explain with detail the experiments accomplished with an ABB robot, showing the difficulties encountered and the proposed solutions. This section is concluded with a method to match the master’s and slave’s workspaces when they present dissimilar kinematics. Afterwards, the research enters into the robot dynamics, with special focus on robot modelling. This has the purpose of obtaining a method that allows to estimate external forces acting on these devices. During the characterisation of the robot’s parameters, a set of tests are performed in order to get to a compromise between computational complexity and estimation error. Key points for modelling and characterising robots with a parallelogram structure are also given, and the desired control architecture is introduced. Once a complete model of the slave is obtained, different alternatives for external force estimation are reviewed. They allow to predict forces in real time, minimizing the position differentiation to reduce the noise of the estimation. The research starts by implementing classic state observers and then it evolves towards the use of Luenberger-Sliding observers whose implementation is relatively easy and the results are convincing.
I also analyse the use of proposed observer during a simulated bilateral control. During this experiment, the force feedback obtained with the classic techniques based on the position error is compared with a control architecture based on force-position, where the force is estimated instead of measured. It is proved how the proposed solution gives results comparable with the classical techniques, however, it introduces an alternative method for teleoperating industrial robots whose teleoperation in radioactive environments would have been impossible in a different way.
Finally, the problems originated by the practical application of teleoperation in the before mentioned scenarios are analysed. Due to the prohibitive conditions for every electronic equipment, the control systems should be placed far from the manipulators.
This provokes that the power cables that fed the slaves devices can present lengths of hundreds of meters. In these circumstances, reflections are developed when implementing drives based on PWM technique. The occurrence of overvoltage is very dangerous for the system composed by drive, wiring and actuator, and has to be eliminated.
During this work, a solution based on commercial LC filters is proposed and it is extensively proved that its use does not introduce adverse effects into the actuator’s control.
