Søren Ketelsen, AAU Energy, will defend the thesis "Efficient Actuation of Load Carrying Applications by Electro-Hydraulic Compact Drives"

Title

Efficient Actuation of Load Carrying Applications by Electro-Hydraulic Compact Drives

PhD defendant

Søren Ketelsen

Supervisor

Professor Torben Ole Andersen

Co-supervisor

Associate Professor Lasse Schmidt
Associate Professor Morten Kjeld Ebbesen, University of Agder, Norway

Opponents

Associate Professor Henrik Sørensen, Aalborg University, Denmark (Chairman)
Professor Katharina Schmitz, RWTH Aachen University, Germany
Professor Petter Krus, Linköbing University, Sweden

Abstract

Hydraulic actuation technology is used extensively in many industries due to well-known advantages such as the ability to generate large actuation forces and its robustness, to mention a few. Especially for applications characterized by low speed and high force operation hydraulic actuation is the preferred solution. Such applications may include mobile machinery such as excavators, wheel loaders, cranes etc.
Today hydraulic cylinders are predominantly controlled by metering the flow, resulting in high power losses. This leads to low energy efficiencies being the main drawback of conventional hydraulic systems. Previous studies have found the average energy efficiency of hydraulically actuated systems to be between 21 % and 50 % on the average, across industries (L. Love, E. Lanke, and P. Alles, “Estimating the Impact (Energy, Emissions and Economics) of the US Fluid Power Industry”,  Oak Ridge National Laboratory, United States, 2012). The same study estimates that at least 1.5 % of the total energy demand in the US are consumed by hydraulic systems, suggesting that inefficient hydraulic systems is not a minor issue. This dissertation is motivated by the poor efficiency of conventional hydraulic systems, and focuses on energy-efficient hydraulic actuation using a differential cylinder for load carrying applications working in four quadrants. A medium sized knuckle boom crane is used as a theoretical case study. 
As the cost of energy is of increasing importance, and due to an increased political and societal awareness on energy consumption and carbon emissions, energy-efficient system designs eliminating the need for throttling control are highly needed. Several of such concepts have been developed by industry and academia recently. 

The technology investigated in this study is known as pump-controlled cylinders, electro-hydraulic actuators or electro-hydraulic compact drives (ECD). Throttling losses are eliminated by removing the metering valves, such that each cylinder is  equipped with a decentralised hydraulic supply. This also eliminates the need for extensive hydraulic piping to the cylinder and permits installing a fully decentralised actuator only requiring electrical power. 

Replacing the valve-based hydraulic actuation system with a decentralised compact actuator possesses challenges which are addressed in this dissertation.   
These include the reduced payload capacity of the considered crane and the need of a gasless and compact oil reservoir permitting the system to operate in tilted orientations. Further challenges include the need of a load holding functionality not preventing the ECD from recovering potential energy, as the crane lowers its load, as well as  thermal considerations. 

Based on a systematic review of existing system layouts, a novel system architecture is proposed to address these challenges. This system consists of a self-pressurising reservoir charged by internal system pressures and a so-called indirectly controlled hydraulic lock. The lock ensures that the cylinder is hydraulically locked in case of power loss or hose burst, but may be unlocked by properly controlling the internal pressure states. Two electrical motor-pump units are included to permit sufficient control possibilities and to increase the scalability properties. To assess the extent to which cooling of the oil is necessary, a thermo-hydraulic simulation model has been developed including a description of the passive heat transfer to the ambience. Based on experimental results obtained from a smaller version of an ECD, the modelling method has been experimentally verified to anticipate the thermal behaviour within sufficient accuracy for thermal design of ECDs.

The dissertation finds that a large potential for energy savings is present if replacing the conventional valve-technology with the developed ECD system. Theoretical studies find an efficiency of the ECD of up to 83 %. By simulating a realistic motion cycle of the crane, a reduced energy consumption of 58 % to 75 % compared to a valve-controlled system is found, while good motion tracking performance has also been established. At ambient temperatures of 20 C, the thermo-hydraulic model finds that passive heat transfer is sufficient, thus active cooling of the oil may be avoided. This is advantageous in terms of investments cost, system compactness and to reduce the mass of the actuator. The latter is important, as the decentralised location of the actuator reduces the static payload capacity of the crane. The developed system is estimated to be 56 % heavier than the cylinder used for valve-controlled actuation, reducing the static payload capacity of the crane by around 3 %.

The dissertation contributes with knowledge to the research field by suggesting new technical solutions to the load holding and oil reservoir design challenges. Also, insights into how the thermal behaviour of ECDs may be simulated are provided.  Combined, these findings may lead to an expansion of the application range to also include safety critical load carrying applications. This includes a wider acceptance of ECDs as efficient, viable alternatives to conventional valve-controlled hydraulic systems. Further studies include experimental validation of the proposed system architecture and associated control methodology, using a downscaled version of the ECD, which has been dimensioned and manufactured within the project. 

The defence will be in english - all are welcome.