PhD defence by Viktor Hristov Donkov on Secondary Control of Multi-chamber Cylinders for Low-speed High-force Applications
15.01.2021 kl. 13.00 - 16.00
Viktor Hristov Donkov, Department of Energy Technology, will defend the thesis "Secondary Control of Multi-chamber Cylinders for Low-speed High-force Applications"
Secondary Control of Multi-chamber Cylinders for Low-speed High-force Applications
Viktor Hristov Donkov
Professor Torben Ole Andersen
Associate Professor Morten Kjeld Ebbesen
Associate Professor Michael Møller Bech
Associate Professor Mads Pagh Nielsen, Dept. of Energy Technology, Aalborg University (Chairman)
Professor Kalevi Huhtala, Tampere University, Finland
Professor Leonid Freidovich, Umeå University, Sweden
The ongoing global warming crisis and the increasing cost of energy demand improvements in many industries. Technologies and methods, which were once sufficient, must again and again be examined and improved. One of these technologies is hydraulic actuation. Hydraulic systems may have efficiencies as low as 30 %. Due to their many benefits such inefficiencies have been disregarded to some extent. In recent years both academia and industry have put a higher focus on efficiency, and different paradigm shifts are being investigated. Digital hydraulics is one such paradigm, which relies on the notion that a component, which is switched off, does not incur losses, while one that is used to its maximum potential can be designed to be efficient for that operation condition.
Multi-chamber cylinders are a part of the digital hydraulics field. They are hydraulic cylinders, which have more than two chambers. Each chamber has a different cross-sectional area. Different forces can be generated by connecting the chambers to different predefined pressures. This concept removes the need for proportional control valves and throttling. Removing throttling removes a major part of the losses in hydraulic systems. When a system is controlled in such a way as to only extract the necessary energy from a source, without changing the sources output, it is called secondary control.
One of the challenges in implementing secondary controlled multi-chamber cylinders is that the hydraulic cylinder in such a solution is more complex than a standard hydraulic cylinder. Very limited research has been conducted, documenting the performance of digitally controlled cylinders on low-speed, high-force applications. Concerns about the smoothness of motion, the possibility of pressure spikes, and the computational complexity of the control structure have been raised. This thesis addresses these difficulties. First it investigates the performance of a multi-chamber cylinder in a low-speed, high-force application such as a knuckle boom crane. It is shown that while simpler control structures can get the job done, a more complicated structure such as Model Predictive Control (MPC) can give better performance. The optimization problem inherent in MPC, the controllers sensitivity, and the controllers robustness are also studied. In trying to study the smoothness of motion of a cylinder, the project faced a lack of tools with which to measure it and the use of total harmonic distortion, a concept from electrical engineering is proposed.
The project concludes that multi-chamber cylinders can have smooth, energy efficient motion if MPC with integral action is used and certain conditions are met - mostly known or selected system parameters. When changes are introduced the energy efficiency suffers first followed by smoothness of motion. The project also concludes that when MPC is used for position control, a short time horizon can be sufficient. In this case a much simpler optimization method can still give good results. In this case MPC works more like model based control. Overall, multi-chamber cylinders are an intriguing solution, but more research into making their performance more robust is needed before they can find wider popularity.
THE DEFENCE IN ENGLISH - all are welcome.
Department of Energy Technology