System Parameter Study for a Light-Weight Series Hydraulic Hybrid Vehicle

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.

A Real Time Controlled Test Rig for High Bandwidth Force Control

This paper describes an electronically controlled active force control system developed to test the tail rotor actuator of a new medium size helicopter. As for all hydraulic force control systems, the critical control issue is to mitigate the disturbance generated by the actuator velocity. For this particular case, the problem was accrued by the high bandwidth of the tail rotor actuator. To define the optimum control algorithm a model based approach was followed, estimating, when unable to measure directly, mechanical and hydraulic model parameters with a dedicated experimental campaign. A controller was eventually developed able to cope with the severe dynamic disturbances by introducing velocity and acceleration compensation laws. The controller was then implemented in a high recursion rate real time machine interfacing with a servovalve controlling the flow to a hydraulic actuator provided with hydrostatic bearings to minimize the friction force. The actuator force was sensed by a load cell providing the feedback signal for the force servoloop. A critical feature of the control was the need to develop a dedicated complex filter for the velocity signal able to cancel out the signal noise while allowing to retain the correct real time information of the actuator velocity and maintain adequate stability margins.

Design Method for Fast Switching Seat Valves for Digital Displacement® Machines

Digital Displacement® (DD) machines are upcoming technology where the displacement of each pressure chamber is controlled electronically by use of two fast switching seat valves. The effective displacement and operation type (pumping/motoring) may be controlled by manipulating the seat valves corresponding to the piston movement, which has been shown to facilitate superior part load efficiency combined with high bandwidth compared to traditional displacement machines. However, DD machines need fast switching on-off valves with low pressure loss for efficient operation, especially in fast rotating operation, where switching times must be performed within a few milliseconds. These valve requirements make a simulation based design approach essential, where mechanical strength, thermal dissipation, fluid dynamics and electro-magnetic dynamics must be taken into account. In this paper a complete design method for DD seat valves are presented, taking into account the significant aspects related to obtaining efficient DD valves with basis in a given DD machine specifications. The seat area is minimized and the stroke length is minimized to obtain fast switching times while considering the pressure loss of the valves. A coupled optimization is finally conducted to optimize the electro-magnetic actuator, leading to a valve design based on the chosen valve topology. The design method is applied to an example DD machine and the resulting valve design fulfilling the requirements is presented.

HIL Simulation of Elastomer Supported Machine Bed Dynamics

This paper presents a Hardware-in-the-Loop (HIL) test setup used for studying the dynamics of an elastomer supported machine bed. The setup uses real elastomer dampers and modeled machine dynamics (process model) connected together via real-time interface. The HIL approach was chosen since the elastomers are a critical part of the system, however, determining their properties for engineering needs can be a challenging task. Accurate elastomer models include many parameters that can only be determined by experimentally, and even then their implementation for real-life applications is not always practical. Using real elastomers supports in the simulation removes uncertainties associated with classic elastomer models, while simulated process makes it possible to test different scenarios fast and with good repeatability. The process model includes a description of the machine body, a rotating unbalanced drive mechanism creating cyclic loading and external excitation forces acting on the machine. The method enables testing of machine bed supports in a realistic operating environment. A test rig was built for housing the elastomers incorporating a hydraulic actuator for producing the process movement. The hydraulic circuit was designed for good dynamic performance with predictive control to minimize delays in the real-time interface. It was found that the HIL-setup can provide fast and accurate information about the plant model behavior in different operating scenarios using the elastomer supports.

Electro Hydraulic Gas Exchange Valve Actuation of 4-Stroke Large Bore Diesel Engine

After several years of study, a large bore diesel engine Electro-Hydraulic Valve Actuation (EHVA) system has reached a development point where the system has been successfully used for years in laboratory test engine environment too. During the evolution of the EHVA, insight of the hydraulic and control systems features has been cleared. This paper concludes main findings and results of EHVA research in Tampere University of Technology / IHA. The hydraulic circuit effect to the power consumption of EHVA is clear. 3-way controlled actuator has advantages compared to 4-way controlled system. Direction control valve defines controllability of the system, and has source of the largest single component power loss. Hydraulic actuator design has also a fair effect to the overall power consumption when pressure force vs. required flow changes heavily, due to load forces. Mechanically, return spring design of the gas exchange valve has great effect to power consumption too. Controller design is dependent on what kind of performance is required. The controller compensates lack of control valve bandwidth, and reacts to changing environmental variables. In this state, the Iterative Learning Controller (ILC) has proved the best choice. If gas exchange valve exact lift motion during the lift event is not important but variation between the strokes is kept in narrow range, a Model-Based Controller (MBC) is a strong option.

Quadratic Programming to Optimize Energy Efficiency of Speed- and Displacement-Variable Pumps

Within the last years, speed-variable pump drives were investigated in numerous applications. In combination with a variable displacement pump, the volume flow and the drive speed can be decoupled. In this paper the resulting degree of freedom will be used to minimize the energy consumption of hydraulic processes by means of a novel model predictive control concept. A dynamic loss model of all drive components will be transformed to a mathematical quadratic optimization problem. The optimum use of the two control variables can achieve energy savings of up to 25% in comparison to known control strategies of speed-variable variable-displacement pumps. Especially in highly dynamic process cycles the proposed optimization guarantees optimum energy efficiency while known approaches become inefficient.

The Impact of the Surface Shape of the Piston on Power Losses

Axial piston machines are widely used in industry thus new cost-effective and highly efficient designs are needed. One way to increase efficiency and decrease cost is by altering the geometry along with the configuration of the piston/cylinder interface influencing the fluid film generation and in turn the energy dissipation and load carrying capacity while still having a design that is cost effective and easy to manufacture. This paper presents a study on a reduction of energy dissipation between the piston and cylinder over a wide range of operating conditions at both full and partial displacements based on the surface shape of the piston along with the minimum clearance. First, it is necessary to measure a base design and then compare those results to simulations in order to verify the simulation results. Once a baseline is established, various piston surface shapes and minimum clearances are then also simulated and compared back to the simulated baseline. Not only is energy dissipation important to compare, but also the minimum gap height over one revolution. The minimum gap height is in direct correlation to friction loss and wear. Therefore, this paper also includes an understanding of how the gap height affects the total losses thus leading to the importance of finding a relative clearance that satisfies a median between torque losses and leakage along with the importance of reducing the occurrence of critical gap heights to reduce the need for wear in in the machine.

Dynamics of Transmission Line Junctions: Comparison of CFD Results Against Measurements

Computational Fluid Dynamics (CFD) has become a valuable tool in the development of fluid power components due to its ability to model flows in complex geometries where simple analytical models are difficult to apply. The downside of CFD is the high computational cost of the method which prevents the application to whole fluid power systems. However, large parts of such systems can usually be modelled by more simple approaches like transmission line modelling. A coupled approach for a simple transmission line system is shown in this paper: Cylindrical parts of the flow domain are modelled with a spatially one-dimensional transmission line model augmented with frequency-dependent friction. The geometrically more complex intersections of cylindrical geometries like elbow joints and T-junctions are treated with a CFD solver in the Open-FOAM system. The simulation results from this coupled model are compared against a set of measurements from a test rig for elbow joints where cylindrical transmission line sections are connected at an angle of 90 degrees.

Advanced Hydraulically Actuated Patient Transfer Assist Device

A new, advanced patient transfer device is being developed for moving mobility limited patients, for example, from a wheelchair to a bed or a floor into a chair. Current market patient lift devices are antiquated and insufficient for customer needs, with only one actuated degree of freedom. The high power to size ratio of hydraulic actuation makes it suitable for moving larger, heavier patients. We have developed a prototype pump-controlled hydraulically actuated patient transfer device that is more maneuverable and agile, using multiple actuated degrees of freedom. We are also working toward developing a more intuitive and safe caretaker interface and control strategy. We have performed an extensive needs assessment; these are a few associated key design requirements relevant to this presented work. A compact package is needed for ease of maneuvering the patient around obstacles in an uncertain environment. The device is capable of producing large forces, so it is desirable for the controller to minimize any unintentional large external contact forces, or provide force feedback. In this system, the caretaker and device work together to maneuver a complex payload, a human body; the operator’s mental workload must be minimized. With humans in the device workspace, safety and stability are necessary, including environment interactions. This new application presents several challenges related to the hydraulic control. First, we are using a separate bidirectional fixed displacement pump with a reversible brushed DC motor for each degree of freedom. The low level control involves obtaining desirable response from each motor-pump-actuator system, while compensating for significant static friction. At a higher level, we are testing several approaches to attain the desired intuitive control and desired dynamics, and minimize the operator workload. First, we are developing a coordinated control using a force input, such that the operator simply pushes on the device in the desired direction of motion. We are testing several different controllers to attain the desired dynamics. This paper presents the design of the machine, the proposed control structures as applied to this application, operator interface options, some preliminary results, and future work.

Novel Open Circuit Displacement Control Architecture in Heavy Machinery

Motivated by the ever-stricter demands by lawmakers to lower emissions of mobile machinery and increasing fuel prices, mobile machinery has gone through a paradigm shift. Fuel efficiency has become a major selling point of machine producers. Even the heavy machinery branch, which is mainly dominated by reliability, productivity and serviceability, has started to feel this change. Hydraulic systems of large scale, as can be found in mining excavators, have typically been based on simplicity and durability. Typical architectures are open-center hydraulic systems, which were designed with robustness and productivity in mind; however they lack competiveness with other hydraulic systems in terms of energy efficiency. Displacement control has shown promising potential especially in multi-actuator machines such as excavators. The technology has so far been demonstrated in closed circuit applications on small-scale machines (below 30 t). Large scale excavators however should in general be more suitable for displacement control due to their relatively small hydraulic component cost compared to the machine and operating cost, larger energy recovery potential due to larger mass movement, more flexibility in space management and greater hydraulic power installed. Large machines feature already several smaller pumps instead of a single large pump, which is important with respect to the fact that displacement control is based on one pump per actuator. A challenge for displacement control on large-scale machinery is handling their high volumetric flow-demands on the system. Today many large excavators feature a float valve, which short-circuits the cylinder chambers and ensures rapid lowering of the attachment under aiding load. Float valves ensure fast cycle times and are essential for high productivity, however incorporating this feature in displacement control is a challenge, especially in closed circuit systems. Open circuit displacement control systems have greater flexibility than closed circuit solutions in working with float-valves and dealing with the high volumetric flows. Additionally the open circuit architecture is ideal for pump-flow-sharing, the strategy to connect two or more pumps with one actuator, which can be practiced when not all actuators move at the same time. This paper compares displacement control in open circuit form with valve-controlled actuation in a mining excavator and shows several fuel saving potentials. The Open Center system was simulated and results were validated with measurements. The proposed open circuit displacement control solutions are implemented virtually and replace the valve-controlled system. Components and system-architecture were carefully chosen in order to ensure reliability, minimal component changes and redundancy that compare to the robustness of today’s system.