On-board systems in fighter aircraft are expected to deliver high performance under extremeand hostile operational conditions. As technology advances, system architectures are shiftingfrom traditional federated configurations toward integrated and electrified designs characteristicof more electric aircraft. A range of architectural configurations, combining different powersources and consumers, can each fulfil the required system functionalities, offering distinctadvantages and drawbacks. To tackle this problem, energy and power management offers asolution-independent, agnostic framework for assessing on-board system architectural decisionswith respect to their impact on top-level aircraft requirements. Nonetheless, a clear understandingof the state-of-the-art and design sensitivities of these systems are needed in early stagesof design. This study describes on-board system architectures and their associated trade-offs toquantify and compare architectural options available to system designers. It reviews on-boardsystems from both federated and more electric aircraft architectures, linking them to the aircraftlevelfunctions they fulfill and outlining key design trade-offs. The systems reviewed includeflight control systems, hydraulic systems, fuel systems, electrical systems, pneumatic systems,environmental control systems, auxiliary power systems, emergency power systems, and landinggear systems. The review highlights that the interdependence and diversity of options of onboardsystems require robust integration frameworks that assess them collectively, rather than inisolation, to achieve a balanced architecture at the aircraft level.

This paper presents a multi-chamber hydraulic actuator controlled by digital pumps and on/off valves, in order to improve the efficiency of hydraulic systems applied in aircraft for flight control. Hydraulic positioning systems are used in many different applications, such as mobile machinery, industry and aerospace. In aircraft, the hydraulic actuators are used at flight control surfaces, cargo doors, steering, landing gear and so one. However, the mas-sive use of resistive control techniques, which throttles the passages of the hydraulic fluid, associated with internal leakage of the hydraulic components, make these systems low energy efficient. In order to improve their energy efficiency, digital hydraulics emerges as a promising solution mainly for mobile applications. In this paper a hydraulic positioning system for aircraft control surfaces using a multi-chamber actuator controlled by on/off valves and a digital pump is proposed. The use of a digital pump with three fixed displacement pumps can provide eight different volumetric displacement out-puts. The multi-chamber actuator with four areas can operate in two different modes, normal or regenerative, resulting in six different equivalent areas. The regenerative mode allows the actuator to achieve higher actuation velocity values with smaller pumps. These equivalent areas combined with the dif-ferent supplied flow rates can deliver 43 different discrete output velocity values for the actuator, in steady-state. For the system dynamic analyses, three mathematical simulation models were developed using MATLAB/Simulink and Hopsan, one for the digital system, and two for the conventional solutions applied in aircraft (Servo Hydraulic Actuators (SHA) and Electro Hydrostatic Actuator (EHA)). The simulation results demonstrate that the digital actuator can achieve, for position control, a maximum position error, in a steady-state, of 0.7 mm. From the energy consumption point of view, the digital circuit consumes 31 times less energy when compared with the SHA and 1.7 when compared to the EHA, resulting in an energy efficiency of 54%.

Today, electrification is a widely discussed topic within all fields of engineering and especially in the field ofaeronautics. Whether electrification of aircraft systems is the only viable option to overcome environmentalissues is a highly complex question which involves many aspects. In this paper, fundamental differences between electrified and hydraulic aircraft actuators are mapped, compared and discussed to understand how and if these systems can be compared without misleading results.In this paper, different actuator architectures were collected from literature, classified and analyzed by usingan ontological approach. The classification was created by utilizing defined classes where the class definition corresponded to the functional requirements which had to be fulfilled by the actuator architecture.This work has shown that ontologies can sufficiently be used for classification of actuator architectures. Thebuilt in reasoner can be used to sort a large set of actuators into comparable classes. Meanwhile, the ontologydefines a framework where important information of the actuators and their components can be stored and structured.

Imagine a system with a pump driven by a speed-controlled electric motor. What and how much can be gained by using a pump with discretely variable displacement instead of a conventional fixed pump in such a system? This question is the focus in this paper, in which a simulation study based on a drive cycle for a loader crane is presented. The results indicate that the system efficiency from inverter input to pump output can increase by a few percentages. This might be considered small in relation to the increasing complexity that comes with discrete displacement. However, the results also show that a system with discrete displacement substantially reduces torque and cooling requirements on the electric motor. The required maximum torque can be reduced by 30 to 50 % and the motor can generate up to 40 % less heat since it can work in more efficient conditions. These potential benefits will be obtained with only a few discrete displacement settings available.

Digital hydraulic piston pumps that use electrically controlled on/off valves to individually control the flow from each piston is a promising technique as these pumps are highly efficient at part displacement and respond quickly. However, digital pumps can still use check valves on the inlet, a fact that makes analysing wobble plate pumps (WPP) and their check valves interesting. Here, we measured the cylinder pressures of a WPP and compared these results with results from a simulation model we developed. In addition, we used linear analysis to investigate how different design parameters affect the valves behaviour. From the measurements we found that the cylinder pressure is clearly affected the flow from other pistons and also that the system is not as stiff as expected. From the linear analysis, a criterion of how to design the valve to avoid instability was derived.

This paper investigates the static and dynamic behaviour of a pressure control valve with nonlinear negative characteristics. The pressure control valve has both reducing and relieving capability and is actuated by a solenoid. The static characteristics have been measured over the entire working range, covering the dynamic response of the solenoid, as well as the complete valve. A model is proposed that considers the flow as a mix of laminar and turbulent flow and flow forces with a flow angle that varies with the stroke of the spool. The model shows good agreement with measurements. The investigations show that the flow forces decrease with higher flow rates as a result of a flow angle that tends to go towards a vertical angle. This results in an increase in pressure with flow during pressure reducing mode. A linear analysis is also presented, explaining this as a negative spring constant in the low frequency range. Stability is, however, maintained.

This thesis deals with the Electrohydraulic Power Steering system for road vehicles, using electronic pressure control valves. With an ever increasing demand for safer vehicles and fewer traffic accidents, steering-related active safety functions are becoming more common in modern vehicles. Future road vehicles will also evolve towards autonomous vehicles, with several safety, environmental and financial benefits. A key component in realising such solutions is active steering.

The power steering system was initially developed to ease the driver's workload by assisting in turning the wheels. This is traditionally done through a passive open-centre hydraulic system and heavy trucks must still rely on fluid power, due to the heavy work forces. Since the purpose of the original system is to control the assistive pressure, one way would be to use proportional pressure control valves. Since these are electronically controlled, active steering is possible and with closed-centre, energy efficiency can be significantly improved on.

In this work, such a system is analysed in detail with the purpose of investigating the possible use of the system for Boost curve control and position control for autonomous driving. Commercially available valves are investigated since they provide an attractive solution. A model-based approach is adopted, where simulation of the system is an important tool. Another important tool is hardware-in-the-loop simulation. A test rig of an electrohydraulic power steering system, is developed.

This work has shown how proportional pressure control valves can be used for Boost curve control and position control and what implications this has on a system level. As it turns out, the valves add a great deal of time lag and with the high gain from the Boost curve, this creates a control challenge. The problem can be handled by tuning the Boost gain, pressure response and damping and has been effectively shown through simulation and experiments. For position control, there is greater freedom to design the controller to fit the system. The pressure response can be made fast enough for this case and the time lag is much less critical.

In steering-related active safety systems, active steering is a key component. Active steering refers to the possibility to control the road wheel angle or the required torque to turn the wheels by means of an electronic signal. Due to the high axle loads in heavy vehicles, hydraulic power is needed to assist the driver in turning the wheels. One solution to realize active steering is, then, to use electronically controlled valves that are of closed-center type. This means that the assistance pressure, or force, can  be set to any feasible value and still benefit from the high power density of fluid power systems. A closed-center solution also implies that a significant reduction in fuel consumption is possible. This paper investigates such an electrohydraulic power steering system, and a comparison with the original system is also made. The findings have shown that while a high response of the pressure control loop is desired for a good steering feel, instability might occur at higher steering wheel torque levels. This has effectively been shown and explained by simulation and hardware-in-the-loop simulation, together with linear analysis. For any desired boost curve, the response of the pressure control loop must be designed to preserve stability over the entire working range.

One of the major concerns in the forest industry is the impact on the soil caused by the forest machines duringharvesting, where damages can have a negative impact on e.g. further growth. One of the main reasons is wheel slip.Another concern is the working environment of the operator due to the harsh ground in the forest. Both these issueshave a negative impact on productivity. An attempt to overcome these challenges is made within a collaborative researchproject, which among others also includes Linköping University, where a new six-wheel pendulum arm forwarder isbeing developed. The new forwarder aims at reducing the soil damage by an even pressure distribution and smooth torquecontrol, as well as increased damping of the complete chassis, and thereby improving the working environment. This ispossible since each wheel, driven by its own hydraulic motor, is attached to a pendulum arm allowing to control the heightof each wheel independently of each other. The forwarder has a total maximum weight of 31 tonnes, including 14 tonnesmaximum load. It consists of two steerable joints and is driven by a 360 bhp diesel engine. The transmission consists oftwo hydraulic pumps and six hydraulic motors.This paper deals with the development of the driveline and presents the first experimental tests of the implementedcontrol strategies, where a secondary control approach is chosen for its ability to individually control the torque on eachwheel. The control strategies, presented in the paper, include pressure control, velocity control of the vehicle and ananti-slip controller. To support the development of the control strategies, models of the vehicle and hydraulic subsystemsare derived. The aim with this paper is to verify the concepts on the actual vehicle. The initial results are promising,indicating that the suggested concept is feasible.

One of the major concerns in the forest industry is the impact on the soil caused by the forest machines during harvesting, where damage can have a negative impact on growth at replanting for example. Another concern is the working environment of the operator. Both these issues have a negative impact on productivity. A new six-wheel pendulum arm forwarder is being developed within a collaborative research project. The new forwarder aims to reduce soil damage by means of an even pressure distribution and smooth torque control. This paper presents the first step in the development of the driveline, where a secondary control approach is chosen for its ability to control the motion of each wheel individually. Simulation models of both vehicle and driveline have been constructed developed, partly for the development of the control strategy, and partly for evaluation. A speed control concept and a torque control concept have both been evaluated for different scenarios with regard to their ability to reduce wheel slip. Results have shown that a velocity control approach is more sensitive to kinematic model accuracy while wheel slip is handled automatically. A torque control approach is more robust towards model accuracy while the reduction of slip is dependent on an accurate model.