Increased demands on fuel-efficient propulsion motivate the use of complex hybrid hydromechanical transmissions in heavy construction machines. These transmissions offer attractive fuel savings but come with an increased level of complexity and dependency on computer-based control. This trend has increased the use of computer-based simulations as a cost-effective alternative to hardware prototyping when developing and testing control strategies. Hardware-In-the-Loop (HWIL) simulations that combine physical and virtual model representations of a system may be considered an attractive compromise that combine the benefits of these two concepts. This paper explores how HWIL simulations may be used to evaluate powertrain control strategies for hybrid hydromechanical transmissions. Factors such as hardware/software partitioning and causality are discussed and applied to a test rig used for HWIL simulations of an example transmission. The results show the benefit of using HWIL simulations in favour of pure offline simulations and prototyping and stress the importance of accurate control with high bandwidth in the HWIL interface.

Hopsan is an open-source simulation package developed as a collaboration project between industry and academia. The simulation methodology is based on transmission line modelling, which provides several benefits such as linear model scalability, numerical robustness and parallel simulation. All sub-models are pre-compiled, so that no compilation is required prior to starting a simulation. Default component libraries are available for hydraulic, mechanic, pneumatic, electric and signal domains. Custom components can be written in C++ or generated from Modelica and Mathematica. Support for simulation-based optimization is provided using population-based, evolutionary or direct-search algorithms. Recent research has largely focused on co-simulation with other simulation tools. This is achieved either by using the Functional Mock-up Interface standard, or by tool-to-tool communications. This paper provides a description of the program and its features, the current status of the project, and an overview of recent and ongoing use cases from industry and academia.

This thesis deals with control aspects of complex hybrid hydromechanical transmissions for heavy mobile working machines. Control problems are identified and solved to facilitate the implementation and use of these systems.

Fuel prices and environmental concerns have increased the interest in hybrid hydromechanical transmissions for heavy mobile working machines. Hybridisation, the introduction of a secondary energy source in the transmission, offers attractive improvements in terms of both fuel efficiency and performance. These improvements are, in turn, enabled by software control. A complex transmission architecture has several components that need to interact in a stable manner. In addition, optimal utilisation of the added energy source is required to maximise fuel savings. Meanwhile, there is a strong trend towards automation, where many of the operator’s difficult control tasks need to be managed by computers. Sophisticated control strategies are therefore needed, along with a deeper understanding of dynamic properties.

Previous research on the control of hybrid hydromechanical transmissions has primarily focussed on on-road applications. The working conditions of heavy working machines place different requirements on the transmission, which is also reflected in the control strategy.

This thesis highlights the importance of fast response of the variable displacement pump/motors used in hybrid hydromechanical transmissions. Their central position in the interface between hydraulics and mechanics makes their dynamic performance a limiting factor both for the stability of the transmission control loops and in the realisation of smooth mode shifts in multiplemode transmissions. Dynamic models and control strategies for displacement actuators are derived and validated in simulation and experiments. A linear model for dynamic analysis of a general hybrid hydromechanical transmission for heavy working machines is derived and a powertrain control strategy based on decoupled control is proposed. The strategy is verified in simulations and experiments in hardware-in-the-loop simulations, and may be used in a working machine with or without a human operator.

Fuel efficiency has become an increasingly important property of heavy mobile working machines. As a result, Hybrid Hydromechanical Transmissions (HMTs) are often considered for the propulsion of these vehicles. The introduction of hybrid HMTs does, however, come with a number of control-related challenges. To date, a great focus in the literature has been on high-level control aspects, concerning optimal utilization of the energy storage medium. In contrast, the main topic of this article is low-level control, with the focus on dynamic response and the ability to realize requested power flows accurately. A static decoupled Multiple-Input-Multiple-Output (MIMO) control strategy, based on a linear model of a general hybrid HMT, is proposed. The strategy is compared to a baseline approach in Hardware-In-the-Loop (HWIL) simulations of a reference wheel loader for two drive cycles. It was found that an important benefit of the decoupled control approach is that the static error caused by the system’s cross-couplings is minimized without introducing integrating elements. This feature, combined with the strategy’s general nature, motivates its use for multiple-mode transmissions in which the transmission configuration changes between the modes. 

This paper focuses on the low-level control of heavy complex hydraulic hybrids, taking stability and the dynamic properties of the included components into account. A linear model which can describe a high number of hybrid configurations in a straightforward manner is derived and used for the development of a general multiple input multiple output (MIMO) decoupling control strategy. This strategy is tested in non-linear simulations of an example vehicle and stability requirements for the low-level actuators are derived. The results show that static decoupling may be used to simplify the control problem to three individual loops controlling pressure, output speed and engine speed. In particular, the pressure and output speed loops rely on fast displacement controllers for stability. In addition, it was found that the decoupling is facilitated if the hydrostatic units have equal response. The low-level control of heavy complex hydraulic hybrids may thus imply other demands on actuators than what is traditionally assumed.

Fuel efficiency and environmental concerns are factors that drive the development of complex solutions for propulsion in heavy working machines. Although these solutions, such as power-split hydromechanical transmissions and hydraulic hybrids, indeed are promising in terms of energy efficiency, they also tend to increase the dependency on accurate, stable control. The realization of this aspect, in turn, relies on continuous testing throughout the development process, usually carried out on expensive, time-consuming prototypes. To lower the development costs and time, hardware-in-the-loop (HWIL) simulations may be introduced as a middle-way between pure prototyping and computer-based simulations. In this concept, some parts of the transmission are represented as hardware while others are included as mathematical models running in real-time in a data acquisition system. This mix of hardware and software allows for high versatility while maintaining a high level of reliability of the results. This paper reports on parts of a study on HWIL simulations of heavy complex hybrid hydromechanical transmissions. Control algorithms for the hardware/model interface in a test rig are derived and their performance are evaluated in HWIL simulations of a mid-sized wheel loader. The results show the importance of fast rig controllers to capture the fast dynamics of the software simulations. It was also found that an important aspect of HWIL simulations is that they are well aligned with their purpose. If so, the simulation yields more reliable knowledge, which is of higher use in the design process of these complex systems. To summarize, HWIL simulations may, if implemented properly, be an important asset in the development of heavy complex hybrid hydromechanical transmissions.

The need for efficient propulsion in heavy vehicles has led to an increased interest in hybrid solutions. Hydraulic hybrids rely on variable hydraulic pumps/motors to continuously convert between hydraulic and mechanical power. This process is carried out via the implementation of secondary control which, in turn, is dependent on a fast displacement controller response. This paper reports on a study of a prototype axial piston pump of the in-line type, in which the displacement is measured with a sensor and controlled using a software-based controller. A pole placement control approach is used, in which a simple model of the pump is used to parametrise the controller using desired resonance and damping of the closed loop controller as input. The controller’s performance is tested in simulations and hardware tests on the prototype unit. The results show that the pole placement approach combined with a lead-compensator controller architecture is flexible, easy to implement and is able to deliver a fast response with high damping. The results will in the future be used in further research on full-vehicle control of heavy hydraulic hybrids.

Demands for low cost sustainable solutions have increased the use of and interest in complex hydromechanical transmissions for heavy off-road vehicles. In transmissions with multiplemodes, an important condition is to maintain the tractive force during the mode shifting event. For hybrid hydromechanical transmissions, with a direct connection to a hydraulic accumulator, the impressed system pressure caused by the hydraulic accumulator has not yet been observed to interfere with this condition. In this paper, a black box model approach is used to modify the hydraulic system after obtaining knowledge regarding how it is affected by a mode shift. A comparative study is carried out where a full vehicle model of a mobile working machine is simulated with two different hydraulic systems. The results show that different system solutions imply different demands on the included components, and that the mode shifting event is not a negligible factor in heavy hydraulic hybrid vehicles.

Strängare krav på lägre bränsleförbrukning och miljöpåverkan har lett till att tillverkare av tunga arbetsmaskiner i allt högre utsträckning ser sig om efter mer energieffektiva transmissioner. Som en konsekvens har komplexa hydraulmekaniska power-splittransmissioner blivit allt mer intressanta som alternativ till de mer traditionella koncepten. Genom att kombinera den klassiska hydrostaten med en mekanisk länk höjs verkningsgraden samtidigt som en kontinuerligt varierbar utväxling kan realiseras. Införandet av olika växelsteg (moder) gör transmissionen än mer mångsidig, samtidigt som designrymden utvidgas. Detta medför att modellering och simulering har blivit ett viktigt verktyg under utvecklingsprocessen, eftersom transmissionernas egenskaper kan studeras i detalj utan att behöva tillverka dyra prototyper. På så sätt kan viktiga beslut fattas, och slutsatser dras, i ett tidigt skede i utvecklingen. I denna artikel beskrivs hur både linjäriserade och olinjära modeller av en power-splittransmission med två moder för en traktorgrävare tagits fram. De olinjära modellerna har implementerats och simulerats i det Flumes-utvecklade simuleringsprogrammet Hopsan. Förslag ges på en enkel reglerstrategi, och ett antal viktiga slutsatser om konceptet kan dras och stå till grund för hur konceptet skall tas vidare i utvecklingsprocessen.

This paper demonstrates the use of high-speed simulation in transmission conceptual design and presents a transmission test bed for hardware-in-the-loop simulations of hydromechanical transmission concepts. Complex transmissions, such as multiple-mode hydromechanical transmissions and hydraulic hybrid transmissions, present new difficulties and costs in the development process. There is today a greater demand for more efficient product development and more work has shifted towards simulation. The Hopsan simulation package allows robust, high-speed simulations suitable for both offline and hardware-in-the-loop simulation. New simulation models for hydromechanical transmissions are developed and used to simulate a known two-mode transmission concept. The same concept is also tested in hardware-in-the-loop simulations in the proposed transmission test bed. Results show good agreement with the hardware tests and highlight the proficiency of the simulation tools.