ISO 4409 is the most important international standard for measuring the efficiency of hydraulic pumps and motors, the latest edition being 4409:2007. The standard describes methods for determining the steady-state performance in terms of overall efficiency. It also defines equations for calculating the volumetric efficiency of pumps and motors. The hydromechanical efficiency is only defined for motors, not for pumps. This paper analyses the efficiency and losses of pumps and motors in an alternative way. The preference is on loss analysis instead of efficiencies. Especially the effects of the bulk modulus are considered in a different and more inclusive manner. The new methodology results in a higher total loss for motor and a lower total loss for pumps than the current ISO 4409 standard. Furthermore, it results in significant changes of the hydro-mechanical and volumetric losses. The differences between the new methodology and ISO 4409 become larger for high load pressures. The new methodology demands knowledge about the minimum volume of the displacement chamber. The ratio between this volume and the full displacement of a single displacement chamber strongly influences the hydro mechanical and volumetric losses of the pump or motor. The new methodology is valid for all positive displacement hydrostatic pumps and motors. The volumetric efficiency, as defined in ISO 4409, can still be used as a flow rate factor, but should not be regarded as an energy conversion efficiency. The importance of adopting the proposed methodology is further demonstrated by analyzing and comparing the measurement data about a fixed displacement pump and motor, showing the differences in the loss analysis by means of ISO 4409 and the new equations. The methodology, observations and validation results presented in this paper are significant and can pave the road for improving the current ISO 4409:2007 standard, which would ultimately benefit the industry.

This thesis showcases the challenges, downsides and advantages to building a MultiDisciplinary Optimization (MDO) framework to automate the generation of an efficientpropeller design built for lightly loaded operation, more specifically for humanpowered aircrafts. Two years ago, a human powered aircraft project was initiatedat Linköping University. With the help of several courses, various students performedconceptional design, calculated and finally manufactured a propeller bymeans of various materials and manufacturing techniques. The performance ofthe current propeller is utilized for benchmarking and comparing results obtainedby the MDO process.The developed MDO framework is constructed as a modeFRONITER project wereseveral Computer Aided Engineering softwares (CAE) such as MATLAB, CATIAand XFOIL are connected to perform multiple consequent optimization subprocesses.The user is presented with several design constraints such as blade quantity,required input power, segment-wise airfoil thickness, desired lift coefficientetc. Also, 6 global search optimization algorithms are investigated to determinethe one which generate most efficient result according to several set standards.The optimization process is thereafter initialized by identifying the most efficientchord distribution with a help of an initial blade cross-section which has been previouslyused in other human powered propellers, the findings are thereafter usedto determine the flow conditions at different propeller stations. Two different aerodynamicoptimized shapes are generated with the help of consecutively performedsubprocesses. The optimized propeller requires 7.5 W less input power to generatenearly equivalent thrust as the original propeller with a total efficiency exceedingthe 90 % mark (90.25 %). Moreover, the MDO framework include an automationprocess to generate a CAD design of the optimized propeller. The generatedCAD file illustrates a individual surface blade decrease of 12.5 % compared tothe original design, the lightweight design and lower input power yield an overallpropulsion system which is less tedious to operate.

This thesis explores and evaluates the design of control systems for a variable Power Take-Off (PTO) pump used in agricultural machinery, focusing on powering fans in air seeding drills and planters. The motivation stems from the increasing size of agricultural equipment and the consequent demand for increased hydraulic power, which current systems fail to provide efficiently under varying operating conditions. The primary objective is to evaluate different systems’ ability to compensate for disturbances while maintaining stability and efficiency. The research addresses responsiveness, stability, and energy consumption of different control system designs compared to existing solutions. A simulation model was created to benchmark various designs under different load cases. The control systems were implemented and evaluated using a purpose-built physical test rig. The physical test rig was employed to validate the benchmark performances and evaluate oil temperature dependence. The findings indicate significant performance improvements with the proposed variable PTO pump designs compared to an existing PTO unit. The authors’ recommendation is to move forward with a pressure-controlled pump utilising feedforward for major disturbances and feedback to compensate for varying operational conditions.

The hydraulic system in a fighter aircraft is not fully utilised during large parts of the flight mission were more electrical power is needed. To better utilise the hydraulic power the current Auxiliary Hydraulic Pump (AHP) and the Emergency Hydraulic Pump (EHP) can be exchanged to an Electro Hydraulic Energy Converter (EHEC). The EHEC has the possibility to provide hydraulic power to the system, but also convert hydraulic power into electric power. The control for such a unit can be implemented in different ways.

A literature study was performed to decide a suitable architecture for use in a fighter aircraft application. A simulation model representing the resulting architecture was created. The simulation model was successful in describing the basic behaviour of the hydraulic system, but lacks a realistic representation of hydraulic consumers. 

Different control strategies were created and tested on the simulation model with several test scenarios based on real flight data from tests performed on the aircraft. The control strategies were compared and suitable candidates were presented based on their relative performance and compared with the current hydraulic system behaviour.

An architecture consisting of a variable displacement pump with over-center capabilities combined with a permanent magnet synchronous machine (PMSM) was decided to be used. A PI-controller with a feedforward on consumer flow was the best performing controller for use in emergency operation of the EHEC. For the case when regenerating electrical power a PI-controller with load pressure feedback is desired initially during start up. When reaching steady state a PI-controller without load pressure feedback is then advantageous.

At Linköping University work has been done to automate the design process of Micro or Mini Air Vehicles. A design optimization framework that links together a CAD system for airframe design and panel code for aerodynamic design has been developed. This paper describes the experience made so far, and demonstrates a case study on fully automated design where a 3D printer is used for manufacturing.

A methodology for an automated design and fabrication of micro-air vehicles (MAVs) is presented. A design optimization framework has been developed that interfaces several software systems to generate MAVs to optimally fulfil specific mission requirements. By means of amulti-objective genetic algorithm, families of MAVs are tailored with respect to objectives such as weight and endurance. The framework takes into consideration the airframe and aerodynamic design as well as the selection and positioning of internal components. The selection of propulsion system components is made from a database of off-the-shelf components. In combination with a three-dimensional printer, physical prototypes can be quickly manufactured. A validation of the framework results from flight tests of a real MAV is also presented.

Aircraft design is an inherently multidisciplinary activity that requires integrating different models and tools to reach a well-balanced and optimized product. At Linköping University a design framework is being developed to support the initial design space exploration and the conceptual design phase. Main characteristics of the framework are its flexible database in XML format, together with close integration of automated CAD and other tools, which allows the developed geometry to be directly used in the subsequent preliminary design phase. In particular, the aim of the proposed work is to test the framework by designing, optimizing and studying a transport aircraft wing with respect to aerodynamic, geometry, structural and accessability constraints. The project will provide an initial assessment of the capability of the framework, both in terms of processing speed and accuracy of the results.

This paper explores novel methodologies for enabling Multidisciplinary Design Optimization (MDO) of complex engineering products. To realize MDO, Knowledge Based Engineering (KBE) is adopted with the aim of achieving design reuse and automation. The aim of the on-going research at Linköping University is to shift from manual modelling of disposable geometries to Computer Aided Design (CAD) automation by introducing generic high-level geometry templates. Instead of repeatedly modelling similar instances of objects, engineers should be able to create more general models that can represent entire classes of objects. The proposed methodology enables utilization of commercial design tools, hence taking industrial feasibility into consideration. High Level CAD templates (HLCt) will be proposed and discussed as the building blocks of flexible and robust CAD models, which in turn enables high-fidelity geometry in the MDO loop. Quantification of the terms flexibility and robustness is also presented, providing a means to measure the quality of the geometry models. Finally, application examples are presented in which the outlined framework is evaluated. The applications have been chosen from three ongoing research projects aimed at automating the design of transport aircraft, industrial robots, and micro air vehicles.

This paper addresses a co-simulation method for fluid power driven machinery equipment, i.e. oil hydraulic machinery. In these types of machinery, the fluid-structure interaction affects the end-product performance to a large extent, hence an efficient co-simulation method is of high importance. The proposed method is based on a 1D system model representing the fluid components of the hydraulic machinery, within which structural 3D Finite Element (FE) models can be incorporated for detailed simulation of specific sub-models or complete structural assemblies. This means that the fluid system simulation will get a more accurate structural response, and that the structural simulation will get more correct fluid loads at every time step, compared to decoupled analysis. Global system parameters such as fluid flow, performance and efficiency can be evaluated from the 1D system model simulation results. From the 3D FE-models, it is possible to evaluate displacements, stresses and strains to be used in stress analysis, fatigue evaluation, acoustic analysis, etc. The method has been implemented using two well-known simulation tools for fluid power system simulations and FE-simulations, respectively, where the interface between the tools is realised by use of the Functional Mock-up Interface standard. A simple but relevant model is used to validate the method.

Öhlins CES Technologies in Jönköping have in the last 30 years been developing control valves for semi active suspension systems used in the car industry. The system, marketed by Öhlins under the brand name CES (Continuously controlled Electronic Suspension), enables a wide working range and ability to adapt to the current road conditions. By controlling  the valve in different ways there are also possibilities to decide on a specfic damper characteristic such as sport or comfort.

The CES valve is working as a pilot controlled pressure regulator and is continuously controlled with help of an electro magnet. The CES valve is mounted in a uniflow damper which in turn guarantees the flow through the valve to go in only one direction independently ofdamper stroke direction.

The rst part of the thesis investigates the damping characteristics in the latest model of the CES valve (i.e the CES8700). A simulation model is made to approximate the damping in the solenoid plunger. Questions that are answered are: How is damping dened, what creates damping in the valve, how large is the damping, what parameters aect the damping. The second part of the thesis investigates new and already prototyped damping concepts with help of simulation. This has been done in order to optimize the valve damping and in turn the damper performance. The simulation results show that the valve dynamics can be improved but often at the expense of a slower valve.

This master's thesis has evaluated dierent system designs for the hydraulic system on an articulated hauler at Volvo Construction Equipment (CE). The current system suers from great losses when running on low pump displacement settings. This is due to large installed displacements as a result of regulations and market demands. New system concepts have been generated and simulations in Matlab and Amesim show that some of the concepts can be implemented in order to increase energy eciency up to 65%. However, increasing eciency does in most cases also increase cost, making some of the concepts unrealistic to implement. The suggested solution for Volvo CE is to remove one pump and allow for the fan pumps to supply oil to the steering and dumping, as described in the Displacement reduction concept. They should also examine the possibility to implement clutches further.

Aircraft system identification has been widely used to this day in applications like control law design, building simulators or extending flight envelopes. It can also be utilized for determining flight-mechanical characteristics in the preliminary design phase of a flight vehicle. In this thesis, three common time-domain methods were implemented in MATLAB for determining the aerodynamic derivatives of a subscale aircraft. For parameter estimation, the equation-error method is quick, robust and can provide good parameter estimates on its own. The output-error method is computationally intensive but keeps account of the aircraft's evolution in time, being more suitable for fine-tuning predictive models. A new model structure is identified using multivariate orthogonal functions with a predicted squared error stopping criteria. This method is based on linear regression (equation-error).

The code written is flexible and can also be used for other aircraft and with other aerodynamic models. Simulations are compared with experimental data from a previous flight test campaign for validation. In the future, this tool may help taking decisions in conceptual design after a prototype is tested.

I mobila maskiner finns en mängd olika systemtyper för arbetshydrauliken. Det vanligaste är fortfarande det kostnadseffektivaoch robusta öppet-centrum systemet med fast pump. Det ger operatören en mjuk styrning med hög dämpning. Ennackdel är dock den låga energieffektiviteten. Under de senaste decennierna har det lastkännande systemet blivit mer ochmer populärt. Energieffektiviteten i detta system är högre även om tryck och flöde varierar stort över tid och mellan olikafunktioner. En nackdel är dock att systemets dämpning är låg. Det finns ett antal olika systemtyper som kombinerar en mjukstyrning med relativt hög energieffektivitet. Nyckeln är att begränsa flödet genom det öppna centrumet i riktningsventilen.Detta kan göras med en variabel tryckstyrd pump som regleras så att öppet-centrum flödet hålls på en konstant nivå. Alternativetär en flödesstyrd pump som styrs direkt från operatörens joystickkommandon eller via flödet i det öppna centrumet. Iden här artikeln görs en jämförande analys av olika systemtyper för arbetshydrauliken i mobila maskiner. Fokus ligger påenergieffektivitet, dynamiska egenskaper och pumpreglering. En genomgång av mer avancerade systemtyper såsom individuellastrypställen och pumpstyrda aktuatorer har också gjorts.

This thesis studies an innovative working hydraulic system design for mobile applications. The purpose is to improve the energy efficiency and the dynamic characteristics compared to load sensing systems without increasing the complexity or adding additional components.

The system analysed in this thesis is referred to as flow control. The fundamental difference compared to load sensing systems is that the pump is controlled based on the operator’s command signals rather than feedback signals from the loads. This control approach enables higher energy efficiency since the pressure difference between pump and load is given by the system resistance rather than a prescribed pump pressure margin. High power savings are possible especially at medium flow rates.

Furthermore, load sensing systems suffer from poor dynamic characteristics since the pump is operated in a closed loop control mode. This might result in an oscillatory behaviour. Flow control systems have no stability issues attached to the load pressure feedback since there is none.

Pressure compensators are key components in flow control systems. This thesis addresses the flow matching problem which occurs when using conventional compensators in combination with a flow controlled pump. Flow sharing pressure compensators solve this problem since the pump flow will be distributed between all active functions. A novel control approach where the directional valve is controlled without affecting the cylinder velocity with the objective of optimizing the damping is proposed.

In this research, both theoretical studies and practical implementations demonstrate the capability of flow control systems. Experiments show a reduced pump pressure margin and energy saving possibilities in a short loading cycle for a wheel loader application.

This thesis deals with innovative working hydraulic systems for mobile machines. Flow control systems are studied as an alternative to load sensing. The fundamental difference is that the pump is controlled based on the operator’s command signals rather than feedback signals from the loads. This control approach enables higher energy efficiency and there is no load pressure feedback causing stability issues. Experimental results show a reduced pump pressure margin and energy saving potential for a wheel loader application.

The damping contribution from the inlet and outlet orifice in directional valves is studied. Design rules are developed and verified by experiments.

A novel system architecture is proposed where flow control, load sensing and open-centre are merged into a generalized system description. The proposed system is configurable and the operator can realize the characteristics of any of the standard systems without compromising energy efficiency. This can be done non-discretely on-the-fly. Experiments show that it is possible to avoid unnecessary energy losses while improving system response and increasing stability margins compared to load sensing. Static and dynamic differences between different control modes are also demonstrated experimentally.

Svängningar är ett vanligt förekommande problem i hydrauliska system. Den här artikeln analyserar dämpningen i ett system bestående av en tryckkompenserad ventil, en cylinder samt en last. Genom att designa utloppsstrypningen i ventilen på ett bra sätt kan en hög dämpning erhållas. Resultaten som presenteras i den här artikeln kan ligga till grund vid systemdimensionering för att erhålla önskad dämpning i tryckkompenserade hydraulsystem. Volymerna på respektive sida av cylinderkolven har stor inverkan på dämpningen. Om inloppsvolymen är liten blir dämpningen låg. Det absolut viktigaste är dock att designa utloppsstrypningen på ett bra sätt. En optimal öppningsarea existerar; både mindre och större area ger lägre dämpning. I den här artikeln föreslås en design av utloppsstrypningen som optimerar dämpningen i det mest ogynnsamma fallet samtidigt som förlusterna över strypningen hålls relativt små. Experimentella resultat som bekräftar de teoretiska förväntningarna presenteras också.

This paper proposes a novel working hydraulic system architecture for mobile machines. Load sensing, flow control and open-centre are merged into a generalized system description. The proposed system is configurable and the operator can realize the characteristics of any of the standard systems without compromising energy efficiency. This can be done non-discretely on-the-fly. One electrically controlled variable displacement pump supplies the system and conventional closed-centre spool valves are used. The pump control strategies are explained in detail. Experimental results demonstrate one solution to the flow matching problem and the static and dynamic differences between different control modes.

This paper presents a hybrid pump controller approach for mobile hydraulic systems, influenced by both pressure and flow. The controller is tuneable to be able to set the order of importance of the pressure and flow controller, respectively. It is thus possible to realize a load sensing system, a flow control system or a mix of the two. Using a low load pressure feedback gain and a high flow control gain, a system emerges with high energy efficiency, fast system response, high stability margins and no flow matching issues. In this paper, both theoretical studies and practical implementations demonstrate the capability of a hybrid pump control approach.

This paper presents a flexible and energy efficient system solution which mimics the behaviour of an open-centre system. An electro-hydraulic variable displacement pump and closed-centre valves are used. Instead of having a flow in the open-centre gallery, that flow is calculated using a pressure sensor and a valve model. The variable pump is then controlled in order to only deliver the flow that would go to the actual loads. It is also possible for the operator to decide how much load dependency there should be. The extreme case is not having any load dependency at all, resulting in a system where the pump displacement setting is controlled according to the sum of all requested load flows. It is thus possible to realize a system design with open-centre characteristics, a flow control system or something in between. Each operator can thereby get their optimal control characteristic while having high energy efficiency.

This work studies an innovative working hydraulic system design for mobile applications, referred to as flow control. The fundamental difference compared to load-sensing systems is that the pump is controlled based on the operator’s command signals rather than feedback signals from the loads. This control approach enables higher energy efficiency since the pressure difference between pump and load is given by the system resistance rather than a prescribed pump pressure margin. Furthermore, load-sensing systems suffer from poor dynamic characteristics since the pump is operated in a closed-loop control mode. This might result in an oscillatory behaviour. Flow control systems have no stability issues attached to the load pressure feedback since there is none. This allows the pump to be designed to meet the response requirements without considering system stability. Pressure compensators are key components in flow control systems. This study addresses the flow matching problem which occurs when using traditional compensators in combination with a flow-controlled pump. Flow sharing pressure compensators solve this problem since the pump flow will be distributed between all active functions. Simulation results and measurements on a wheel loader application demonstrate the energy-saving potentials and the dynamic improvements for the flow control system.

Interest has increased in flow controlled systems in the field of mobile fluid power. The capital distinction between traditional load-sensing (LS) systems and flow controlled systems is that the pump is controlled based on the operator’s total flow demand rather than maintaining a certain pressure margin over the maximum load pressure. One of the main advantages of flow controlled systems is the absence of the feedback of the highest load pressure to the pump controller. In this paper, a dynamic analysis is performed where flow controlled and LS systems are compared. It is shown how instability can occur in LS systems due to the pump controller and proven that no such instability properties are present in flow controlled systems. A drawback with one type of flow controlled system is that the highest load dynamically will disturb the lighter loads. This paper shows a novel way to optimize the damping in such systems by controlling the opening position of the directional valve independently of the flow. The mentioned disturbance between the highest load to the others can thereby be reduced.

This paper analyses the damping in pressure compensated closed centre mobile working hydraulic systems. Both rotational and linear loads are covered and the analysis applies to any type of pump controller. Only the outlet orifice in the directional valve will provide damping to a pressure compensated system. Design rules are proposed for how the system should be dimensioned in order to obtain a high damping. The volumes on each side of the load have a high impact on the damping. In case of a small volume on the inlet side, the damping becomes low. However, the most important thing is to design the outlet orifice area properly. There exists an optimal orifice dimension for maximized damping; both smaller and larger orifice areas give lower damping independently of the volumes. This paper presents a method to dimension the outlet orifice area and the load volumes in order to obtain a desired system damping. Experimental results, which confirm the theoretical expectations, are also presented. The conclusions are that it is possible to obtain a high damping contribution from the outlet orifice if the system is dimensioned correctly. However, the energy efficiency needs to be considered while improving the damping

This paper analyses the damping of a flow controlled cylinder with a mass load and an outlet orifice. By using linear models, a mathematical expression for the damping is derived. It is shown that the volumes on each side of the piston have a high impact on the damping. In case of a small volume on the inlet side, the damping becomes low. However, the most important thing is to design the outlet orifice area properly. There exists an optimal orifice dimension; both smaller and larger orifice areas give low damping independently of the size of the volumes. In this paper a design is proposed of the outlet orifice area that optimizes the damping of the system. Experimental results which confirm the theoretical expectations are also presented. The conclusions are that without an outlet orifice, the hydraulic system will not contribute with any damping at all. Furthermore, large dead volumes in the cylinder will increase the damping, but at the expense of the system’s efficiency.

Safety, performance and driving comforts are given high importance while developing modern day cars. All-Wheel Drive vehicles are exactly designed to fulfill such requirements. In modern times, human concern towards depleting fossil fuels and cognizance of ecological issues have led to new innovations in the field of Automotive engineering. One such outcome of the above process is the birth of electrical hybrid vehicles. The product under investigation is a combination of all wheel drive and hybrid system. A superior fuel economy can be achieved using hybrid system and optimized vehicle dynamic forces are accomplished by torque vectoring action which in turn provides All-Wheel Drive capabilities.

Heat generation is inevitable whenever there is a conversion of energy from one form into another. In this master thesis investigation, a thermal simulation model for the product is built using 1D simulation tool AMESim and validation is done against the vehicle driving test data. AMESim tool was chosen for its proven track record related to vehicle thermal management. The vehicle CAN data are handled in MATLAB. In a nutshell, Simulation model accounts for heat generation sources, oil flow paths, power loss modeling and heat transfer phenomena.

The final simulation model should be able to predict the transient temperature evolution in the rear drive when the speed and torque of motor is supplied as input. This simulation model can efficiently predict temperature patterns at various locations such as casing, motor inner parts as well as coolant at different places. Various driving cases were tried as input including harsh (high torque, low speed) ones. Simulation models like this helps Engineers in trying out new cooling strategies. Flow path optimization, flow rate, convection area, coolant pump controlling etc are the few variables worth mentioning in this regard.

Hybrid transmissions are characterized by their utilization of more than one form of energy storage. They have the potential to help reduce overall fuel consumption and vehicle emissions by providing the possibility of brake energy recuperation and prime mover operation management. Electric hybrids and electric vehicle drives are nowadays ubiquitous, and mechanical energy storage in flywheel has been investigated in the past. The use of fluid power technology with a combustion engine has also been investigated since the late 1970s, and is frequently revisited.

Hydraulic hybridization is especially attractive for heavy vehicles with frequent braking and acceleration which benefit most from fluid power components’ high power density, typically busses, delivery or refuse vehicles, and vehicles with existing hydraulic circuits and transmissions, such as forest and construction machinery, but have been considered for smaller vehicles as well.

Due to the characteristic discharge profile of hydraulic energy storage, special attention needs to be paid to control aspects in the design process to guarantee drivability of the vehicle. In this respect, simulation models can be of use in early design process stages for cheaper and faster evaluation of concepts and designs than physical experiments and prototyping, and to generate better understanding of the system studied. Engineering optimization aids in the systematic exploration of a given design space, to determine limits and potentials, evaluate trade-offs and potentially find unexpected solutions. In the optimization of a hydraulic hybrid transmission, the integration of component and controller design is of importance, and different strategies (sequential, iterative, bi-level and simultaneous approaches) are conceivable, with varying consequences for the implementation.

This thesis establishes a simulation-based optimization framework for a hydraulic hybrid transmission with series architecture. Component and control parameter optimization are addressed simultaneously, using a rule-based supervisory control strategy. The forward-facing dynamic simulation model at the centre of the framework is built in Hopsan, a multi-disciplinary open-source tool developed at Linköping University. The optimization is set up and conducted for an example application of an on-road light-duty truck over standard drive cycles. Both results from these experiments as well as the framework itself are studied and evaluated. Relevant design aspects, such as explicit design relations to be considered and performance requirements for more robust design, are identified and addressed, and the optimization problem is analysed with regard to algorithm performance and problem formulation. The final result is an optimization framework that can be adjusted for further in-depth studies, for example through the inclusion of additional components or optimization objectives, and extendable for comparative analysis of different topologies, applications and problem formulations.  

Hybridization of a vehicle’s drivetrain can in principle help to improve its energy efficiency by allowing for recuperation of kinetic energy and modulating the engine’s load. How well this can be realized depends on appropriate sizing and control of the additional components. The system is typically designed sequentially, with the hardware setup preceding the development and tuning of advanced controller architectures. Taking an alternative approach, component sizing and controller tuning can be addressed simultaneously through simulation-based optimization.

The results of such optimizations, especially with standard algorithms with continuous design variable ranges, can however be difficult to realize, considering for example limitations in available components. Furthermore, drive-cycle based optimizations are prone to cycle-beating. This paper examines the results of such simulation-based optimization for a series hydraulic hybrid vehicle in terms of sensitivity to variations in design parameters, system parameters and drive cycle variations. Additional relevant aspects concerning the definition of the optimization problem are pointed out.

With the focus on energy efficiency for many different kinds of vehicle applications,hybridization is considered a possible solution to reduce fuel consumption. While hybrid electric concepts are already available for passenger vehicles, and also considered for heavier applications, hybrid hydraulic alternatives have been mainly limited to the latter, which benefit most from the higher power density available. To study the different hybrid architectures and applications, a modeling framework for the system design is developed using the simulation tool Hopsan from Linköping University.  Previously,  the  model  of  a  series  hydraulic  hybrid  vehicle  was introduced, a light-duty vehicle simulated over two standard urban drive cycles, and its potential for further work established. In this paper, the model is extended by including a simple combustion engine power management to provide for more realistic propulsion of the hydraulic drivetrain, showing the potential to operate a series hydraulic hybrid vehicle’s engine in more efficient regions. Additionally, the design is studied concerning the effects of a variation of key component sizes on the accuracy and energy efficiency objectives. Instead of subjecting the system to (multi- objective) optimization, at this stage the individual component’s influence is studied, and the objectives are dealt with separately from each other to eliminate the need for compromise   between   them,   both   to   gain a better understanding of the interdependencies.

Hydraulic hybrid drives are one potential way of improving the fuel efficiency of vehicles, including the possi-bility of recuperating braking energy in a hydraulic accumulator. The high power density of fluid power is mainly advantageous for heavy vehicles, or duty cycles characterized with frequent braking and acceleration. For smaller vehicles, hydraulic hybrid drives are thus most interesting under urban and suburban driving conditions. Amongst the existing architectures, the series hydraulic hybrid offers the advantage of operating the internal combustion engine independently of the vehicle speed, at the cost of a less efficient transmission path than a purely mechanical one. Previously, a series hydraulics hybrid light-duty vehicle was modelled in the transmission-line modelling (TLM)-based simulation software Hopsan from the division of Fluid and Mechatronic Systems (Flumes) at Linköping University. This paper studies through simulation-based optimi-zation how the fuel-optimal vehicle design is affected by various mixes of urban and suburban driving requirements. Both the system’s hardware and the parameters of a basic control strategy are considered. The results show quite similar designs for most performance requirements combinations, and can be the base for further studies addressing additional requirements, conditions and objectives.

Hybridisation of hydraulic drivetrains offers the potential of efficiency improvement for on – and off-road applications. To realise the advantages, a carefully designed system and corresponding control strategy are required, which are commonly obtained through a sequential design process. Addressing component selection and control parameterisation simultaneously through simulation-based optimisation allows for exploration of a large design space as well as design relations and trade-offs, and their evaluation in dynamic conditions which exist in real driving scenarios. In this paper, the optimisation framework for a hydraulic hybrid vehicle is introduced, including the simulation model for a series hybrid architecture and component scaling considerations impacting the system’s performance. A number of optimisation experiments for an on-road light-duty vehicle, focused on standard-drive-cycle-performance, illustrate the impact of the problem formulation on the final design and thus the complexity of the design problem. The designs found demonstrate both the potential of energy storage in series hybrids, via an energy balance diagram, as well as some challenges. The framework presented here provides a base for systematic evaluation of design alternatives and problem formulation aspects.

Hydraulic hybrids continue to receive attention as a possible solution in the search for improved fuel economy for different vehicle types. This paper presents a model for a series hydraulic hybrid light-duty vehicle, developed in the Hopsan simulation tool of Linköping University’s Division of Fluid and Mechatronic Systems (Flumes). Focusing on the hydraulic transmission which employs a pump control based on the hydraulic accumulator’s state-of-charge (SoC), several simplifications were made, especially concerning mechanical components. Simulation over two urban standard cycles shows promising results concerning dynamic performance and energy recuperation potential, provided sufficient component sizing to compensate for the mechanical limitations. This paper lays the foundation for both a further refined model - suitable for optimizing the full drivetrain, including component sizing and controller parameterization - and the development of comparable models for parallel and power-split hybrid architectures.

Simulation-based optimization is a useful tool in the design of complex engineering products. Simulation models are used to capture numerous aspects of the design problem for the objective function. Optimization results obtained can be assessed from various perspectives. In this study, component and control optimization of a series hydraulic hybrid vehicle is used as an application, and different robustness and performance aspects are evaluated. Owing to relatively high computational loads, efficient optimization algorithms are important to provide sufficient quality of results at reasonable computational costs. To estimate problem complexity and evaluate optimization algorithm performance, the definitions for information entropy and the related performance index are extended. The insights gained from various simulation-based optimization experiments and their subsequent analysis help characterize the efficiency of the optimization problem formulation and parameterization, as well as optimization algorithm selection with respect to parallel computation capabilities for further development of the model and optimization framework.

Amongst the hybrid vehicle propulsion solutions aiming to improve fuel efficiency, hybrid electric solutions currently receive most attention, especially on the market. However, hydraulic hybrids are an interesting alternative, especially for heavier vehicles due to higher power density which is beneficial if higher masses are moved. As a step towards a comprehensive design framework to compare several possible hydraulic hybrid architectures for a specified application and usage profile, the model of a series hydraulic hybrid vehicle was previously introduced and initially studied concerning component sizing for an exemplary light-duty vehicle in urban traffic. The vehicle is modeled in the Hopsan simulation tool. A comparably straight-forward engine management is used for the vehicle control; both pump and engine controls are based on the hydraulic accumulator’s state-of-charge. The model is developed further with respect to the accumulator component model. Based on that, the influence of several system and component parameters, such as maximum system pressure and engine characteristics, as well as controller parameters on the vehicle’s performance is analyzed. The goal is to allow for more understanding of the system’s characteristics to facilitate future optimization of the system.

One of the largest energy losses in an excavator is the compensation loss. In a hydraulic load sensing system where one pump supplies multiple actuators, these compensation losses are inevitable. To minimize the compensation losses the use of a multi chamber cylinder can be used, which can control the load pressure by activate its chambers in different combinations and in turn minimize the compensation losses. 

For this proposed architecture, the control of the multi chamber cylinder systems is not trivial. The possible states of the system, due to the number of combinations, makes conventional control, like a rule based strategy, unfeasible. Therefore, is the reinforcement learning a promising approach to find an optimal control. 

A hydraulic system was modeled and validated against a physical one, as a base for the reinforcement learning to learn in simulation environment. A satisfactory model was achieved, accurately modeled the static behavior of the system but lacks some dynamics. 

A Deep Q-Network agent was used which successfully managed to select optimal combinations for given loads when implemented in the physical test rig, even though the simulation model was not perfect.

The pace of development in aviation technology is increasing, and there is a constantneed for new concepts to keep up. An innovative concept is desired to reach the netzero emission and sustainability target visualized in Flight path 2050. Introducing digital models and virtualization into aviation fields reduces time consumption onmanual modelling and increases design accuracy. Digital mock-up models also helpin minimizing costs due to errors in the later stage of development or manufacturing. The Institute of Systems Architecture in Aeronautics at German AerospaceCenter (DLR) works in digitizing cabin design environments with extensive implementation of the Knowledge-Based Engineering (KBE) approach. The virtual cabindesign system tool also known as Fuselage Geometry Assembler (FUGA) providesa digital model of the cabin of both single and twin aisle configurations of commercial aircraft. The information of aircraft characteristics is provided to FUGA using Common Parametric Aircraft Configuration Schema (CPACS). CPACS coupled with FUGA provides the user with a consistent model of aircraft and cabindesign, when viewed through a virtual platform provides an immersive experienceto be inside an aircraft cabin before physical production. The multidisciplinary capability of FUGA provides experts from different disciplines to perform analysis such as vibration analysis on the cabin environment. For ease of usage and better visualization of information from FUGA, a web-based application through Flask is hosted for FUGA. This enables the user to access the FUGA tool without the needof installing the tool on their devices. With the world now moving towards a greener approach, an alternative propulsion system may require a different fuel tank configuration. Retro-fit of liquid hydrogen fuel tank into an existing aircraft’s fuselage is done using FUGA tool and aircraft performance analysis is conducted and the outcomes are studied. The enhanced and advanced model of twin-aisle configuration, now on par with single-aisle configuration is used for hydrogen tank sensitivity analysis. The comparative study of different aisle configurations retro-fitted with liquidhydrogen fuel tank is further conducted for arriving at an optimal design point fora balance in range and passenger capacity.

This thesis investigated if decoupled design of the air intake and exhaust systems for four-stroke internal combustion engines is possible. Using the information found design guidelines were set up for the formula student team ELiTH Racing. The literature study revealed that the systems are not uncoupled, and the inuence of exhaust geometry on air intake behavior needed more thorough investigation. Experiments were designed, using a single cylinder engine with simple intake and exhaust geometries. The tests were attempted, but had to be abandoned due to time constraints. Successful tests would have yielded results in the form of pressure measurements, froma Prandtl-tube, in the air intake, and footage of smoke tests. As a secondary task the potential of computer simulations during the design process was investigated, which yielded a suggestion on how to set up a complete reasonable computational model of the systems. This also resulted in that the design guidelines included how to use computer simulations for the design process. Finally a few ways to expand this work are presented.

The scope of this report is the development of a mathematical model and a control system for a three degrees of freedom (3DOF) helicopter rig. This 3DOF-system offers a good simplification of a real world tandem helicopter for evaluating performanceof different automatic control principles.

A mathematical model of the system is developed using free-body diagrams. This mathematical model is then linearized and a controller is developed by decoupling the system. Due to model imperfections external disturbances and similar effects integral action is added as well as feed-forward compensation to reduce nonlinear effects.

After the controller has been decoupled the two different controllers are tuned. The Linear-Quadratic Regulator, described in section 3.6, is used for selectingstate-feedback gains. Due to the highly nonlinear nature of the system an Extended Kalman Filter is developed to estimate unmeasurable states.

The model and controller is then implemented on the actual rig and evaluated. The results displayed that the elevation controller had good performance. The travel controller also showed good performance but not as good as the elevation controller.

The main goal of this thesis was to develop a controller for the 3DOF helicopter system. The results clearly show that an LQR-controller is able to successfully control a system like this with decent performance characteristics despite the highly nonlinear system.

The aim of this thesis was to determine the stability criteria of an inline axialpiston pump with an eccentric control system. The hydrostatic/hydrodynamic bearings of the swash plate was inspected with model building and simulation as method. The general approach has been to regard the swash plate as stiff, leading to the hydrostatic friction between the swash plate and the pump house to be considered more or less constant. Recent research treat the swash plate as bendable which could lead to the swash plate bearings’ friction being replaced from mixed friction to a full contact interaction between the swash plate and the pump house, with an increased friction as result. This increase in friction is theorized to increase the stability ofthe pump by limiting the movement of the swash plate in comparison to the fully lubricated bearing. 

Distributed system simulation can increase performance, re-usability and modularity in model-based product development. This thesis investigates four aspects of distributed simulation: multi-threaded simulations, simulation tool coupling, distributed equation solvers and parallel optimization algorithms.

Multi-threaded simulation makes it possible to split up the workload over several processing units. This reduces simulation time, which can save both time and money during the product development cycle. The transmission line element method (TLM) is used to decouple models to independent sub-models.

Different simulation tools are suitable for different problems. Tool coupling makes it possible to use the best suited tool for simulating each part of the whole product. Models from different tools can then be coupled into one aggregated simulation model. An emerging standard for tool coupling is the Functional Mock-up Interface (FMI). It is investigated how this can be used in conjunction with TLM.

Equation-based object-oriented languages (EOOs) are becoming increasing popular. A logical approach is to let the equation solvers maintain the same structure that was used in the modelling process. Methods for achieving this using TLM and FMI are implemented and evaluated.

In addition to parallel simulations, it is also possible to use parallel optimization algorithms. This introduces parallelism on several levels. For this reason, an algorithm for profile-based multi-level scheduling is proposed.

As the speed increase of single-core processors keeps declining, it is important to adapt simulation software to take advantage of multi-core technology. There is a great need for simulating large-scale systems with good performance. This makes it possible to investigate how different parts of a system work together, without the need for expensive physical prototypes. For this to be useful, however, the simulations cannot take too long, because this would delay the design process. Some uses of simulation also put very high demands on simulation performance, such as real-time simulations, design optimization or Monte Carlo-based sensitivity analysis. Being able to quickly simulate large-scale models can save much time and money.

The power required to cool a processor is proportional to the processor speed squared. It is therefore no longer profitable to keep increasing the speed. This is commonly referred to as the "power wall". Manufacturers of processors have instead begun to focus on building multi-core processors consisting of several cores working in parallel. Adapting program code to multi-core architectures constitutes a major challenge for software developers.

Traditional simulation software uses centralized equation-system solvers, which by nature are hard to make parallel. By instead using distributed solvers, equations from different parts of the model can be solved simultaneously. For this to be effective, it is important to minimize overheadcosts and to make sure that the workload is evenly distributed over the processor cores.

Dividing an equation system into several parts and solving them separately means that time delays will be introduced between the parts. If these occur in the right locations, this can be physically correct, since it also takes some time for information to propagate in physical systems. The transmission line  element method (TLM) constitutes an effective method for separating system models by introducing impedances between components, causing physically motivated time delays.

Contributions in this thesis include parts of the development of the new generation of the Hopsan simulation tool, with support for TLM and distributed solvers. An automatic algorithm for partitioning models has been developed. A multi-threaded simulation algorithm using barrier synchronization has also been implemented.

Measurements of simulation time confirm that the simulation time is decreased almost proportionally to the number of processor cores for large models. The decrease, however, is reduced if the cores are divided on different processors. This was expected, due to the communication delay for processors communicating over shared memory. Experiments on real-time systems with four cores show that a four times as large model can be simulated without losing real-time performance.

The division into distributed solvers constitutes a sort of natural cosimulation. A future project could be to use this as a platform for linking different simulation tools together and simulating them with high performance. This would make it possible to model each part of the system in the most suitable tool, and then connect all parts into one large model.

Simulation of fluid power systems typically requires models from multiple disciplines.Achieving accurate load dynamics for a system with complex geometry, for example, mayrequire both a 1D model of the hydraulic circuit and a 3D multi-body model. However, mostsimulation tools are limited to a single discipline. A solution to these kinds of problems isco-simulation, where different tools are coupled and simulated together. Co-simulation canprovide increased accuracy, improved modularity and facilitated collaboration between dif-ferent organizations. Unfortunately, tool coupling typically requires tedious and error-pronemanual work. It may also introduce numerical problems. For these reasons, co-simulation isoften avoided as long as possible. These problems have been addressed by the developmentof an open-source framework for asynchronous co-simulation. Simulation tools can be inter-connected through a stand-alone master simulation tool. An extensive range of tools is alsosupported via the Functional Mockup Interface standard. A graphical user interface has beenimplemented in the OpenModelica Connection Editor. System models can be created andedited from both a schematic view and a 3D view. Numerical robustness is enforced by theuse of transmission line modelling. A minimalistic programming interface consisting of onlytwo functions is used. An example model consisting of a hydraulic crane with two arms, twoactuators and a hanging load is used to verify the framework. The composite model consistsof nine multi-body models, one hydraulic system model and a controller. It is shown thatmodels from various simulation tools can be replaced with a minimal amount of user input.

A major concern in the forest industry is impact on the soil caused by forest machines during harvesting. A six-wheel pendulum arm forwarder is being developed. The new forwarder aims at reducing soil damage by an even pressure distribution and smooth torque control and thereby also improving the working environment. The suspension contains pendulum arms on each wheel controlled by a hydraulic load sensing system in combination with accumulator.

A natural approach is to model each part of a system in the bestsuited software. In this case, the hydraulic system is modelled in the Hopsan simulation tool, while the vehicle mechanics is modelled in Adams. To understand the whole system it is necessary to simulate all subsystems together. An open standard for this is the Functional Mock-up Interface. This makes it possible to investigate the interaction between the hydraulic system and the multi-body mechanic model.

This paper describes how different simulation tools can be combined to support the development process. The technique is applied to the forwarder’s pendulum suspension. Controllers for height and soil force are optimized to minimize soil damage and maximize comfort for the operator.

Modeling and simulation are important tools for efficient product development. There is a growing need for collaboration, interdisciplinary simulation, and re-usability of simulation models. This usually requires simulation tools to be coupled together for co-simulation. However, the usefulness of co-simulation is often limited by poor performance and numerical instability. Achieving stability is especially hard for stiff mechanical couplings. A suitable method is to use transmission line modeling (TLM), which separates submodels using physically motivated time delays. The most established standard for tool coupling today is the Functional Mock-up Interface (FMI). Two example models in one dimension and three dimensions are used to demonstrate how the next version of FMI for co-simulation can be used in conjunction with TLM. The stability properties of TLM are also proven by numerical analysis. Results show that numerical stability can be ensured without compromising on performance. With the current FMI standard, this requires tailor-made models and custom solutions for the interpolation of input variables. Without using custom solutions, variables must be exchanged using sampled communication and extrapolation. In this case, stability properties can be improved by reducing communication step size. However, it is shown that stability cannot be achieved even when using unacceptably small communication steps. This motivates the need for the next version of FMI to include an intermediate update mode, where variables can be interchanged in between communication points. It is suggested that the FMI standard should be extended with optional callback functions for providing intermediate output variables and requesting intermediate input variables.

Numerical stability is a key aspect in co-simulation of physical systems. Decoupling a system into independent sub-models will introduce time delays on interface variables. By utilizing physical time delays for decoupling, affecting the numerical stability can be avoided. This requires interpolation, to allow solvers to request input variables for the time slot where they are needed. The FMI for co-simulation standard does not support fine-grained interpolation using interpolation tables. Here, various modifications to the FMI standard are suggested for improved handling of interpolation. Mechanical and thermodynamic models are used to demonstrate the need for interpolation, as well as to provide an industrial context. It is shown that the suggested improvements are able to stabilize the otherwise unstable connections.

Combining performance and numerical stability is a key issue in co-simulation. The Transmission Line Modeling method uses physically motivated communication delays to ensure numerical stability for stiff connections. However, using a fixed communication delay may limit performance for some models. This paper proposes Steady-State Identifcation for enabling variable communication delays. Three algorithms for online Steady-State Identification are evaluated in three different co-simulation models. All algorithms are able to identify steady-state and can thereby determine when communication delays can be allowed to increase without compromising accuracy and stability. The results show a reduction in number of the solver derivative evaluations by roughly 40-60% depending on the model. The proposed method additionally supports connections with asymmetric communication delays, which allows each sub-model to independently control the delay of its input variables. Models supporting delay-size control can thereby be connected to those that do not so that the step length of each individual sub-model is maximized. Controlling the delay-size in sub-models also makes the method independent of the master co-simulation algorithm. 

The Modelica language offers an intuitive way to create object-oriented models. This makes it natural also to use an object-oriented solver, where each sub-model solves its own equations. Doing so is possible only if sub-models can be made independent from the rest of the model. One way to achieve this is to use distributed solvers by separating sub-models with transmission line elements. This offers robust and predictable simulations, simplified model debugging and natural parallelism. It also makes it possible to use different time steps and solver algorithms in different parts of the model to achieve an optimal trade-off between performance and accuracy. The suggested method has been implemented in the Hopsan simulation environment. Different modelling techniques for taking advantage of the distributed solver approach are explained. Finally, three example models are used to demonstrate the method.

By introducing physically motivated time delays, simulation models can be partitioned into decoupled independent sub-models. This enables parallel simulations on multi-core processors. An automatic algorithm is used for partitioning and running distributed system simulations. Methods for sorting and distributing components for good load balancing have been developed. Mathematical correctness during simulation is maintained by a busy-waiting thread synchronization algorithm. Independence between sub-models is achieved by using the transmission line element method. In contrast to the more commonly used centralized solvers, this method uses distributed solvers with physically motivated time delays, making simulations inherently parallel. Results show that simulation speed increases almost proportionally to the number of processor cores in the case of large models. However, overhead time costs mean that models need to be over a certain size to benefit from parallelization.

The demand for large-scale real-time simulations of fluid power systems is in-creasing, due to growing demands for added functionality. Real-time simulationscan be used in for example hardware-in-the-loop experiments and embeddedcontrol systems. In order to achieve real-time performance, it is often necessaryto use small or simplified models, reducing the usefulness and accuracy of theresults. This article proposes the use of transmission line modelling (TLM) forexploiting multi-core hardware in real-time and embedded systems. The charac-teristics of the TLM method are analysed to identify difficulties and possibilities.A method for how to parallelise TLM models is then presented. Subsequently, aprogramming interface for implementing the parallel models in the target systemsis introduced. Practical experiments show that the approach works and that themethod is applicable. So far, however, it has required great effort on the part ofthe engineer, both when it comes to programming, compiling and importing themodel into the target environments, although some attempts to automate the pro-cedure have been successful, reducing the level of complexity.

Even though direct-search optimization methods are more difficult to parallelize than population-based methods, there are many unexploited opportunities. Five methods for parallelizing the Complex-RF methods have been implemented and evaluated. Three methods are based on the unchanged original algorithm, while two require modifications. The methods have been tested on two test function and one real simulation model. An analysis of the algorithm has been performed. This provides a basis for parametrization of the parallel methods. Without changing the original algorithm, speed-up of 2.5-3 is achieved. With allowing modifications, a speed-up of up to 5 is obtained without significantly reducing the probability of finding the global minimum. Speed-up does not scale linear to the number of threads. When more threads are added, parallelization efficiency decreases. However, a comparison with a particle swarm method shows that Complex-RF performs better regardless of the number of threads, due to its fast convergence rate.

This paper describes how models from different simulation tools can be connected and simulated on different processors by using the Functional Mockup Interface (FMI) and the transmission line element method (TLM). Interconnectivity between programs makes it possible to model each part of a complex system with the best suited tool, which will shorten the modelling time and increase the accuracy of the results. Because the system will be naturally partitioned, it is possible to identify weak links and replace them with transmission line elements, thereby introducing a controlled time delay. This makes the different parts of the system naturally independent, making it possible to simulate large aggregated system models with good performance on multi-core processors. The proposed method is demonstrated on an example model. A suggestion of an XML extension to the FMI standard for describing TLM ports is also presented.

Distributed solvers provide several benefits, such as linear scalability and good numerical robustness. By separating components with transmission line elements, simulations can be run in parallel on multi-core processors. At the same time, equation-based modelling offers an intuitive way of writing models. This paper presents an algorithm for generating distributed models from Modelica code using bilinear transform. This also enables hard limitations on variables and their derivatives. The generated Jacobian is linearised and solved using LU-decomposition. The algorithm is implemented in the Hopsan simulation tool. Equations are transformed and differentiated by using the SymPy package for symbolic mathematics. An example model is created andverified against a reference model. Simulation results are similar, but the equation-based model is four to five times slower. Further optimisation of the algorithm is thus required. The future aim is to develop a distributed simulation environment with integrated support for equation-based modelling.