Scandinavian International Conference on Fluid Power

The Hydraulic Infinite Linear Actuator for Efficient and Flexible Timber and Agricultural Logistics

Magnus Landberg — 2021-06-24

In forestry and agriculture industry, robust and power-dense hydraulics have long played an important role for a rational and cost-effective logistics. In these fields there is a trend towards longer, larger and heavier vehicles and machines. Within the forestry business there is a need to develop transport vehicles with lower energy consumption. This can be done by improving the vehicles' aerodynamics. The air resistance of unloaded timber trucks and timber trains will be significantly reduced if stakes and banks are put together. Furthermore, there is a need to place banks and stakes at individual distances individually to accomplish different load combinations. In agriculture when sowing, it is an advantage to be able to flexibly position row units on a seed drill in optimal distances from each other to secure productive and sustainable farming. However, most present seed drills have fixed row distances and thus have a low adaptability for different crops or soil conditions. There is also a need for a faster transport of the seed drill between different fields.

Future actuation systems for forestry and agricultural vehicles and machines can be improved by utilizing a new sort of hydraulic linear actuator technology, the Hydraulic Infinite Linear Actuator (HILA). HILA technology also has an impact on heavy and dangerous manual steps when changing banks and stakes on timber vehicles. The adjustment can be controlled from the cabin, thus eliminating manual steps. Heavy bank elements on a timber vehicle can be positioned individually with high locking force. In the agricultural context, it is possible to quickly change between different inter-row spacing on a seed drill when using HILA technology, enabling a multi-purpose seed drill and inter-row cultivator. HILA long stroke capability also facilitates a smooth folding into a compact transport position. With the bills are gathered, a low center of gravity will be accomplished. This enables a more stable vehicle dynamics and enables a higher speed.

HILA is based on a well-known hydraulic clamping element technology, where the piston and the piston rod can be coupled and uncoupled by means of the clamping element. The HILA invention, in its simplest usage, provides new features to hydraulic cylinders, such as providing very long strokes and small chamber volumes, which means high stiffness and low capacitance. However, the invention also enables lower weight and volume of the actuator when compared to conventional hydraulic cylinders. This is a new way to generate and distribute mechanical linear movement and force by using hydraulic actuators in a cost effective way. The technology also represents a new sort of digital hydraulics. The technology is best suited for relatively slow dynamics and where the movement pattern is well-known.

Benchmarking the performance of hydrostatic pumps

Robin Mommers — 2021-06-24

In search for sustainable and clean solutions, the hydraulic industry is forced to develop more efficient alternatives to traditional systems. For mobile applications, battery driven machines are becoming an essential solution. But, electric driven hydraulic systems set completely different demands than classical systems. Since batteries are expensive and bulky, it is no longer acceptable that the majority of the battery stored energy is lost in the hydraulic system.

One of the promising solutions for efficiency increase is the application of electrohydraulic actuators (EHAs). Aside from all the inherent control advantages, EHAs deliver energy to each load on demand. This makes them much more efficient than current valve controlled systems, at least in principle. In practice, EHAs require both low and high-speed operation of pumps. Almost all hydrostatic pumps have high friction losses, strong wear and often also high volumetric losses at speeds below 500 rpm. Additionally, it is obvious that the pumps must have the highest efficiency possible.

Given these constraints and demands it is understandable that information is needed about the performance of pumps and motors. In the past years, Innas has measured and tested several positive displacement machines and published a comprehensive report about these measurements. This paper will analyse the outcome of the test results, with a special focus on the application in EHAs.

Extremal Optimisation Approach to Component Placement in Blood Analysis Equipment

Magnus Sethson — 2021-06-24

This reports present an initial study on generative mechatronic design of equipment for blood analysis where the samples and chemicals are forwarded in thin single millimeter vessels. The system of vessels in the equipment transfers the fluids to different stations where chemical reactions and studies are performed. One of the stations is an optical inspection that requires controllable lighting conditions using an array of LEDs of different types.

The focus is on the generative design of the placement and configuration of the LEDs. The placement of the LEDs has been taken as a studying case for the method of Extremal Optimisation (EO) approach to mechatronic design. This method forms an opposing strategy to methods like genetic algorithms and simulated annealing. This is because it discriminate the individual parts or components of the configuration that underperform in a particular aspect instead of the more classical strategy of favouring good configurations from global measures. The presented study also relates to the class of many-objective optimisation methods (MaOP) and originates from the concept of self-organised criticality (SOC). The characteristics of avalanche barrier crossings in the parameter search space is inherited from such systems.

The test case used for the evaluation places occupying circles onto a quarter ring domain representing LEDs and circuit board. The fluid vessels are represented by lit up small domains that are also approximated by a circular disc. Some conclusion upon the methods capability to form a valid solution are made. A framework for describing a set of local flaw-improvement rules, called D2FI is introduced.

Control Strategies for a Dual AC Motor Pump System in Aircraft Hydraulic Power Packages

Nils Trochelmann — 2021-06-24

Hydraulic power packages (HPP) integrate electric motor-driven pumps (EMP) and hydraulic equipment to supply (on demand) hydraulic power to specific functions. To achieve a high operational availability two redundant EMPs are installed per HPP. To produce maximum output power, the EMPs need to operate in parallel. In the first part of this paper a baseline pressure control strategy is developed that enables the parallel operating mode. It has to cover the performance requirements and it is crucial to avoid stability issues of today’s aircraft multi-pump hydraulic systems, induced by slightly differing pump characteristics. A central pressure controller, which calculates the total (cumulated) control effort and allocates it evenly to both EMPs, is selected as the basic controller structure. A loopshaping approach, where the requirements are mapped on target loop shapes, is applied. The control design is verified by nonlinear simulation and by experiments using representative aircraft prototype EMPs. Their slightly differing characteristics are utilized for an implicit proof of robustness. The second part of this paper makes use of the low utilization of the EMPs during most parts of the flight to achieve secondary objectives (efficiency, dynamic performance) and to improve the handling of operational constraints (e.g. electric input power limit). A model predictive control allocation (MPCA) algorithm flexibly allocates the total control effort to the EMP units. The allocation is derived from the solution of an optimization problem with the operational limits as constraints. Non-linear simulations of two exemplary scenarios show that the MPCA algorithm minimizes power losses by increasing the utilization of the more efficient unit. In addition, the MPCA algorithm improves the dynamic performance in case of an assymetric performance degradation by prioritizing the unit with better performance capability. The potential for improvement increases with the difference between the units and therefore offers advantages in dynamic allocation in particular for fault-tolerant operation.

Hydraulic Trainer for Hands-on and Virtual Labs for Fluid Power Curriculum

Hassan Assaf — 2021-06-24

Hands-on experiences constitute a high value, perhaps unreplaceable, element of applied engineering disciplines such as fluid power. Hydraulic and pneumatic trainers have been developed over the years to expose students to applications of fluid power technology. However, the traditional approach for educating students through hands-on lab is recently under high pressure due to the following aspects: a) the outdated design of the traditional trainers that seldom integrate modern electro-hydraulic components, data acquisition systems, and visual aids; b) the increased need for online education. These factors have been endangering the number of students – already low compared to the industry needs – enrolled in fluid power programs.

This paper describes the effort made at Purdue University to develop a modern hydraulic trainer along with its digital twin that tackles the above challenges. A novel physical trainer was formulated to allow 29 lab experiences that span from basic concepts of single actuator control to more sophisticated layouts for controlling multiple actuators. The trainer largely uses electro-hydraulic components, sensors as well as a DAQ system connected with a touch base screen, aimed at maximizing the student’s feeling of experiencing modern technology. A virtual trainer that replicates the physical trainer is developed and implemented with the commercial software Unity 3D. The virtual trainer uses the CAD drawings of the physical components of the actual trainer, and it allows reproducing all the main aspects of the real lab experience, including typical students’ mistakes and realistic operating noise. This trainer simulator was successfully used for the first time at Purdue in Fall 2020, and it will represent a valid option for virtual hands-on experiences for distance learning students for years to come.

Comparison of Sliding Mode Controller for Classical and Direct Driven Electrohydraulic System

Juraj Benić — 2021-06-24

In this paper, a novel energy-efficient Direct Driven Hydraulic (DDH) drive is made in parallel with a classical valve-controlled proportional electrohydraulic system. In the proposed concept asymmetrical hydraulic cylinder is controlled with two reversible pumps directly connected to a servo motor. Due to direct control of the oil flow, such system provides higher energy efficiency in comparison to a valve-controlled hydraulic system. The experimental setup is designed with the possibility of easy switching between both systems, which enables an exact comparison of the experimental results. Sliding Mode Controller (SMC) is designed for the DDH system and also for the classical proportional electro-hydraulic system. A comparison study of two systems is done based on the experimental results obtained with the SMC while reference results are obtained with a widely used PID controller. Parameters for the PID controller are obtained with the Ziegler-Nichols method. The same parameters are used for both controllers in all test cases. The performance and energy efficiency of the proposed system is evaluated through a step and sine wave reference signal with different payloads varying from 0 to 200 kg with 20 kg increments. Additionally, system performance is evaluated based on six parameters calculated from the system response while energy efficiency is calculated based on input and output powers of the systems.

Experimental investigation of the influence of fluid viscosity on the efficiency of a crawler excavator

Deuster Sebastian — 2021-06-24

In view of decreasing energy resources and the rising problems associated with CO2 emissions and global warming, there is a strong interest in reducing the fuel consumption of machines in all sectors. Manufacturers of mobile machinery, such as hydraulic excavators, are also striving to develop increasingly efficient machines. Triggered by this development trend, the power density of hydraulic systems and their components continues to increase. This results in higher pressures, temperatures and lower oil volumes in the system. As a result, the hydraulic fluid used is subject to greater thermal stress and the systems thermal properties are becoming increasingly important. Further developments of tribological systems, for example in hydraulic displacement units, also create new demands on the hydraulic components and fluids. The fluid properties in particular are increasingly coming to the fore. For this reason, the content of this paper is to consider the influence of the hydraulic oil’s viscosity on the efficiency of a crawler excavator and to present the viscosity dependent losses of valves and pipe/hose lines. To this purpose, experimental tests are discussed by regarding viscosity related energy losses of the hydraulic system of a crawler excavator. Therefore the results of experimental tests of a gravel cycle at different temperatures will be discussed. The results are divided into different types of energy losses. Finally, a hypothesis can be made about the dependence of the viscosity of the hydraulic fluid on the efficiency of the system.

Improving the performance of self-contained cylinder drives with load-holding capabilities

Viktor Donkov — 2021-06-24

A self-contained pump-driven cylinder drive system based on two electric motor and pump (TEMPO) units used for actuation of a two-link, medium-sized knuckle boom crane, is investigated. The TEMPO drive system is equipped with load holding valves and is a closed hydraulic system with a gasless self-pressurizing reservoir. Besides accommodating prescribed safety requirements, the load holding valves allow to reduce energy consumption during standstill. The transition between load holding and motion modes should be unnoticeable for the operator, hence the transition between these modes must be fast, a feature that has not been accomplished in previous attempts. An additional challenge with the system is a constant position error appearing during the locking process. By refining the hydraulic circuit and the control structure, these shortcomings are addressed. A simulation study with a realistic loading cycle is conducted in order to investigate the motion performance of the system. The simulation results demonstrate that the mentioned shortcomings are mitigated, suggesting a potential for future load carrying applications based on the proposed TEMPO drive.

A Study of Hydraulically Actuated Oscillating Pitch to Increase Energy Production in Wind Turbines

Daniel Escobar — 2021-06-24

The focus of this paper is to oscillate the pitch of the wind turbine blades using hydraulic actuation to increase the power captured by a hydrostatic wind turbine. This is based on the fact that oscillating an airfoil under specific conditions increases the instantaneous lift coefficient by up to 97%. Different conditions were investigated by varying airfoil shape, waveform shape, waveform amplitude, waveform frequency and average angle of attack. The following conditions were found to give the best performance: a low camber airfoil, a lower average angle of attack than used for a non-oscillating airfoil, and an optimized waveform shape called a “tilted sinusoid.” A multi-level factorial experiment determined the most impactful variables were frequency, amplitude, and average angle of the tilted sinusoid. The best frequencies and amplitudes were studied through numerical simulations and hill climbing optimization. Results showed that oscillating the pitch of the wind turbine blades would, under all conditions tested, never outperform steady-state turbine operation, the main reason being the higher drag coefficients that are also present when oscillating the pitch of the blade.

Improving the Energy Efficiency of Single Actuators with High Energy Consumption

David Fassbender — 2021-06-24

A load sensing (LS) supply in combination with control valves is one of the most common solutions for the actuation of implements on heavy-duty mobile machines (HDMMs). A major drawback of this approach is its relatively low energy efficiency due to metering losses—especially for multi-actuator operation and load braking. Several novel, more efficient concepts have already been proposed but involve high component costs for each actuator, which is not acceptable for HDMMs with many actuators that have a medium to low energy turnover. Therefore, this work proposes a novel system design which is based on a conventional LS system—for cheap operation of a high number of low-energy-consuming actuators—but allows to avoid metering losses for single high-energy-consuming actuators by replacing their metering valves with electric-generator-hydraulic-motor (EGHM) units that work similar to pump-controlled concepts. The benefits of the novel concept are explained in detail by looking at the three main throttling functions of an actuator in a typical valve-controlled LS systems, which the novel concept avoids by applying pressure in the actuator return lines and recuperating energy electrically instead of dissipating it by throttling. Furthermore, advantages and challenges for the novel concept are analyzed, and ways to address the latter are presented. Before the novel concept is simulated, the required control algorithms are presented. The simulation study in Amesim and Simulink focuses on a telehandler that utilizes the novel concept for the boom, extension and tilt actuators. Simulation results show that the novel system can decrease the required input energy for typical duty cycles by up to 34% compared to a conventional LS system. At the same time, simulations show that, from an economic perspective, it seems most reasonable to utilize the novel EGHM units only for the boom and extension actuators of the studied telehandler.

Load weight estimation on an excavator in static and dynamic motions

Mehmet Ferlibas — 2021-06-24

Excavators are one of the commonly used types of hydraulic machines in earth moving operations. The material handled is often transferred by dump trucks having a payload capacity that should not be exceeded. Payload monitoring systems are needed in order to prevent the possible problems during the delivery, increase the work efficiency, reduce the cost, and obtain the product information automatically without the requirement of truck scales. In this study, we propose a novel approach to estimate the load weight in the bucket of the excavator when the machine links are in motion. We consider the excavator as a three-revolute joint manipulator in vertical plane with the boom, the stick, and the bucket links. We rewrite the dynamic torque equations in a decoupled form as the linear combination of dynamic parameters and functions of joint angles, velocities, and accelerations. We perform least squares estimation to identify these parameters allowing us to predict the no load joint torques for any configuration of the links. We show that the most accurate torque prediction is the difference between the boom torque and the stick torque. We then derive the relation between the joint torques with and without the load, which are functions of the dynamic parameters. Using these equations, we can estimate the load weight. The relation becomes simpler when the links are stationary, since only the gravitational parameters remain present in the torque equations. The relation in dynamic case requires the parameters of the polar coordinates for the center of gravity of the bucket and we show that these parameters can be estimated with the knowledge of the empty bucket mass. We summarize our findings on load weight estimation for different cases including stationary poses and dynamic trajectories on free space and discuss the results. Although the friction is neglected throughout the modeling, the results obtained indicate that the effect of the static friction plays an important role in the accuracy of the estimated payload mass. We show that our dynamic model based solution is very promising, and exhibit only 2% error for high enough velocities.

Experimental Investigation of New Design Concepts for the Tribological Contact between the Valve Plate and the Cylinder Block in Axial Piston Machines

Stefan Geffroy — 2021-06-24

Axial piston machines are widely used in both stationary and mobile hydraulic systems due to their efficiency and robustness in high pressure applications. Important tribological contacts in axial piston machines are between pistons and bushings, sliding shoe and swash plate, as well as between cylinder block and valve plate. The analysis of these contacts is imperative to improve the overall performance of the machine, since they influence its efficiency to a high extend. This paper focuses on the contact between the valve plate and the cylinder block. High pressure forces acting on the cylinder block result in a tilted position, defining the shape of the interface with the valve plate. The tilted position is overlayed by the cylinder block’s rotation, resulting in unfavorable lubrication conditions and high contact pressure.

Measures to actively influencing the cylinder block’s position during its rotation is currently researched at ifas. Using pressure pockets in the kidney grooves of the cylinder block is one of these measures and presented in this paper. The investigation is done simulative and experimental, using a 140 cm³ pump on a special test rig, measuring the cylinder block movement. The results of this are presented in this paper.

A New Degree of Freedom for Variable Axial Piston Pumps with Valve Plate Rotation

Thomas Heeger — 2021-06-24

Conventionally, variable axial piston machines vary displacement by adjusting the length of the piston stroke, which can be done by adjusting the angle of the swash plate in swash plate machines or the angle of the cylinder block in bent axis machines. Another possibility to achieve variable displacement is to rotate the valve plate and thus adjust the effective use of the piston stroke. An advantage of this method is that it only requires small forces and is easier to control in comparison to conventional displacement adjustment.

This idea is not new, and the concept was studied decades ago, but unfortunately, cavitation and high pressure peaks in the bridge between the kidneys hindered a successful implementation of valve plate rotation.

Using a double pump with opposing pistons offers the potential to overcome these obstacles, as the displacement can be adjusted by joint rotation of both valve plates, and the effective bridge angles can be adjusted by relative rotation of the valve plates. This paper presents a methodology to optimise valve plate kidney angles for a double pump with rotating valve plates. Optimisation results for exemplary sets of operating points are presented. At high setting ratios, power losses and flow pulsations can be reduced. The risk of cavitation and high pressure peaks can be eliminated, but at the expense of cross-porting and increased losses at low setting ratios and high speeds.

Strategies to Minimize Data Sample Size for Regression-Based Pump/Motor Models

Jack L. Johnson — 2021-06-24

This work presents an analysis for tracking the evolution of regression coefficients and the Root-Mean-Square of their residuals on a test dataset for a hydraulic pump. The method starts by iteratively regressing data points that are undergoing sequencing by adding one new data sample at a time, then regressing with each iteration. This process was named Progressively Sequenced Regression Analysis, shortened to “PSR analysis” in this paper. The motivating and guiding postulate of PSR analysis is based on the belief that a plateau of the regression coefficients and statistical figures of merit had to exist if sampling theory is accepted to be real. It was anticipated at the outset that both the regression coefficients and the Residual RMS would converge on respective plateau values; however, it was discovered that the coefficients were very volatile, with some, more volatile than others. Tracking the Residual RMS was found to produce the more reliable measure of information saturation because the convergence is more obvious, provided that the sample sequencing was done with the experience learned from performing PSR analysis. This document is focused on explaining how orthogonally sequenced data can be mined for the limits or hyperspace vertexes of the sampled data, and the source data optimally sequenced (rearranged) to produce results that are as efficacious as Latin Hypercube (LHC) sampling for achieving information saturation at a predictable number of samples. PSR analysis has led to an objective method for verifying that the proper arrangement, i.e., optimized sequencing, of the source data set can predict the condition of information saturation and minimum useful sample size. It ends with a postulate of how this can be achieved using a combination of LHC sampling and vertex pre-test planning, or vertex mining of legacy data. The content of this paper has concentrated solely on the output flow model of hydraulic, positive displacement pumps.

Pump-Controlled Actuators with Dump Valves

Samuel Kärnell — 2021-06-24

The electrification trend means an increased focus on the energy efficiency of mobile hydraulic systems. In turn, this means an increased focus on so-called pump-controlled systems, where actuators are controlled by pump flow rather than valve throttling. However, one of the main problems with pump-controlled systems is matching the flow into the pump with its outlet flow. There are many ways of solving this problem, but the solutions tend to be rather bulky and not ideal from a service perspective, since they often rely on accumulators. This paper presents simple but novel pump-controlled concepts, without accumulators. The energy consumption of the presented concepts is analysed and compared to other concepts. The analysis is based on measurements taken on a backhoe loader and the results show that implementation of the presented concepts on the backhoe’s boom actuator can reduce the total power consumption by 50-60 % compared to the original load sensing system. This can be compared with 55-65 %, which is the yield for an ideal, accumulator-based, pump-controlled system on the boom.

Control Optimisation of a Pump-Controlled Hydraulic System using Digital Displacement Pumps

L. Viktor Larsson — 2021-06-24

When dealing with electrification of working machines, energy-efficient operation is key to maximise the usage of the limited capacity of on-board batteries. Previous research indicate that plenty is gained by reducing component and system losses by means of design. In contrast, this paper focuses on how to maximise energy efficiency by means of control optimisation. Dynamic programming with backward-facing simulation is used to find the optimal electric motor speed trajectory for a scooptram machine equipped with pump control, using digital displacement pumps with dynamic flow sharing as enabling technology. The results show that low shaft speed is preferred to minimise drag losses from parasitic components, partly facilitated by the relatively high and operation point-independent efficiencies of the pumps and electric motor. The results indicate energy reduction of 5 - 9 %, where higher figures could be expected for other, more hydraulic-intense applications, such as excavators.

Adaptive Identification and Application of Flow Mapping for Electrohydraulic Valves

Jianbin Liu — 2021-06-24

Good estimates of flow mapping for electrohydraulic valves are important in automation of fluid power system. The purpose of this paper is to propose adaptive identification methods based on a recursive least squares method (RLSM), a recursive maximum likelihood method (RMLM) and radial basis function neural network (RBFNN) to estimate the uncertain parameters in flow mapping for electrohydraulic valves. In order to reduce the complexity and improve the identification performance, model structures derived from prior knowledge are introduced. The methods are applied to map the pressure-flow characteristic of an electrohydraulic valve. With the help of simulation results, the accuracy and efficiency of these algorithms are demonstrated. Some issues like invertibility of flow mapping are discussed and suggestions to apply these methods are made.

The Determination of Hydraulic Motor Displacement

Paul Michael — 2021-06-24

Because the geometrical displacement of a pump or motor is very difficult to measure directly, the derived capacity of motors is used to assess the efficiency of positive displacement machines. The current internationally accepted method for deriving the displacement of hydraulic pumps and motors is ISO 8426:2008. Difficulties in accurately assessing derived displacement via ISO 8426:2008 have been reported by several authors. These inaccuracies can lead to efficiency results that exceed 100% in ISO 4409:2019 performance tests. In the presented work, fixed axial, variable axial, and radial piston motors were evaluated at 50°C and 80°C in dynamometer tests. Linear, orthogonal, and semi-randomized data sets were collected. The Wilson, Toet, and an analytical form of the Toet were compared with ISO 8426:2008. In general, the differences between the various methods for deriving displacement were not statistically significant, except in the instance of the axial piston motor. In the axial piston motor, the ISO 8426:2008 derived displacement was approximately 1% lower than the other methods. Use of this lower ISO 8426:2008 displacement in efficiency calculations produced values exceeding 100%. The error in the ISO 8426:2008 derived displacement determination was attributed to difficulties in detecting speeddependent factors that affect displacement when testing is conducted at a single speed. The ISO 8426:2008 method does not provide instructions for calculating derived displacement when data is collected at more than one speed. It is proposed that the One-Step Toet method be incorporated into ISO 8426 as a method for calculating the derived displacement when users opt to measure performance at multiple speeds. This revision will reduce the potential for speed-dependent errors in the determination of derived displacement.

Modelling, Simulation and Validation of the Pneumatic End-Position Cylinder Cushioning

Fedor Nazarov — 2021-06-24

In this paper a model of the pneumatic cylinder with an integrated pneumatic end cushioning is presented. This model is needed to simulate and analyze the thermodynamical possesses in the pneumatic end cushioning and to elaborate a novel design strategy for damping systems with a higher capability on kinetic energy absorption and robust performance even with fluctuating operational conditions, such as supply pressure, inertial load or travel speed. To validate this model, the results of the experimentally based parametrization of the friction force in the cylinder sealings are compared for a Stribeck friction model and its modifications. A new approach suitable for an accurate approximation of the measured friction data within a wide range of pressure (2…8 barrel) and piston speed (0…0.8 m/s) is proposed. In the next step a flow rate characteristic of the integrated end cushioning throttle is experimentally obtained and analyzed. These data are used to parametrize the lumped parameters model of the cylinder with an end cushioning. Pressure and temperature in the cushioning volume and piston displacement are measured for different openings of the cushioning throttle to prove the validity of the model. The model will be used further for sensitivity analysis and robust optimization of the cushioning system design.

Improving the efficiency of valve-controlled systems by using multi-chamber actuators

Henrique Raduenz — 2021-06-24

This paper outlines how multi-chamber actuators can improve the efficiency of valve-controlled systems. Resistive control is a major source of energy losses in valve-controlled systems that share the same pump to drive multiple loads. In the proposed concept, by selecting different chambers, the load on the multi-chamber actuator can be transformed into different pressure and flow rate levels, allowing the adaptation of its load to the loads on other actuators. This can lead to a reduction of resistive control energy losses that occur between pump and actuators when driven simultaneously. Such systems are seen as an intermediate solution between resistive conventional hydraulics and throttle-less digital hydraulics. As a case study to highlight the possible efficiency improvement, a concept of a load sensing system with a conventional and a multi-chamber actuator is analysed. To determine its efficiency, the equations that describe its static behaviour are presented. Evaluating them for a set of load forces and speeds demonstrates how the load transformation occurs and how it can improve efficiency.

Loss Analysis and Concept Comparison for Electrically Driven Hydraulic Loader Crane

Amy Rankka — 2021-06-24

The load-sensing system has for a long time been the most energy efficient hydraulic system widely used for mobile machines. When replacing the combustion engine drive with an electric drive with a battery as energy source an incentive for using more energy efficient systems arise. Some promising examples of more energy efficient systems are independent metering systems, pump controlled systems and open flow control systems. Before getting to deeply involved in a specific design, an investigation of the intended application is of importance. The objective of this study is to present a large number of energy efficient designs for the hydraulic system of an electrified loader crane by the means of a loss analysis and a high level concept comparison. To be able to cover a large design space all systems are modeled based on static pressure-flow relations for their components.

Based on measurements, the losses from simultaneous operation, backpressure losses and load holding valve losses are found to be the largest loss contributors in the hydraulic system. If an electrical supply system is added to the reference load sensing system, the overall efficiency is found to be 23 %. The hydraulic system it is found to account for 62 % of the losses and the drive system for 38 %.

The concept comparison shows that a two or four pump system with recuperation possibilities can decrease the energy consumption of the complete system on the studied working pattern by about 50 %.

Design of Pressure Control for Optimal Damping in Individual Metering Systems

Gerhard Rath — 2021-06-24

Modern oil-hydraulic systems for moving heavy payloads are designed for optimised motion, but also for minimal energy loss. Individual metering technique, using separate control of the two actuator chambers, offers some advantages. A common strategy when moving the load is to control the incoming oil flow to obtain a desired speed, and the pressure at the downstream side for good efficiency. In this work analysis and design of PI (proportional-integral) pressure control is done. The adjustment of the control parameters of this loop is usually uncritical. In the worst case, the damping of the mechanical system is the only contribution. It is shown in this work, that pressure control can increase the damping of load oscillations. The influence of the P and I parameters to the system properties is investigated using the poles of the transfer function of the system. It is shown, that there is a point, where the damping factor of the system has its maximum value, and a design method for this optimisation is given. The problem ends up in a system of two equations of fourth order. A method is shown how to reduce the problem to solving one third-order equation, which is done numerically. Finally, the results are verified using simulation.

Concept, Simulation Studies and Design for a Novel Concept of the Hydraulic Binary Counter

Rudolf Scheidl — 2021-06-24

For the realization of compact and lightweight digital hydraulic cylinder drives for exoskeleton actuation the hydraulic binary counter concept was proposed and studied previously. This counter principle is based on hydraulically piloted switching valves which feature a hysteretic response with respect to the pilot pressure. In first prototypes of that counter bistable mechanical buckling beams realized the hysteretic response. Their performance suffered from high friction in the hinges and high local stresses. Furthermore, they require tight manufacturing tolerances not only of themselves but also of their bearing structure. In this paper, hydraulic feedback from the multi-chamber cylinders is proposed and investigated to realize the reset of lower order valves when a higher order valve switches. To make the hydraulic feedback independent from the system pressure feedback to the valve is done via spring compression. This principle makes bi-stable elements obsolete. The functioning of this principle for a small drive for exoskeleton use is proven by a simple mathematical model and its numerical solution by a MATLAB program. An exemplary embodiment design of the valves with the proposed feedback mechanism shows the feasibility of its practical realization. The application of this concept is not limited to small drives but can be applied for larger drives where multi-chamber cylinders are advantageous, such as for excavators, to save cost and installation space of the many solenoid valves required otherwise.

Simulating a high frequency piezo pump with disc reed valves

Nathan Sell — 2021-06-24

Piezo pumps provide an attractive alternative for driving small actuators (e.g. less than 100W) compared to traditional valve controlled cylinders powered by a central hydraulic supply. This provides the ability to distribute power electrically rather than hydraulically, which can bring both weight and efficiency savings. Currently the use of piezo pumps is severely limited by the maximum power and flows that can be provided. This paper documents the simulation of a new pump which makes use of disc type reed valves to rectify the flow generate by a single piezostack-driven piston. The proposed valves have the potential to overcome frequency limitations of more conventional poppet or ball type check valves. This enables the pump to operate at higher frequencies and thereby produce larger flows. Simulation results suggest that a pump capable of producing a no load flow in excess of 1L/min would be possible using an off-the-shelf piezo stack.

Multidimensional Trajectory Tracking for Numerically Stiff Independent Metering System

Goran Stojanoski — 2021-06-24

This paper presents a new approach for solving an optimal control problem in a hydraulic system, using a variational calculus method. It uses a path tracking method of two different states with different units and of different magnitude. To ensure the uniqueness of the solution, two regularization terms were introduced, whose influence is regulated by regularization parameters. The system of differential equations, obtained from the Euler-Lagrange equations of the variational problem, was solved by a mass matrix method and discretized with linear differential operators at the interstitial points for numerical stability. This enabled the calculation of the control variables, despite the stiffness of the numerical problem. The results obtained show an energy-efficient performance and no oscillations. Finally, a Simulink model of the hydraulic system was created in which the calculated control variables were inserted as feed-forward inputs, to verify the results.

Novel hydraulically relieved electromechanical direct actuation system for large scale switching valves

Tobias Vonderbank — 2021-06-24

Extensive actuation forces and strokes are required for the actuation of large sized valves normally implemented in high power hydraulic systems. A hydraulic piloted operation is, as for now, the most suitable solution and the state of the art. However, recent research has shown that the application of new electromechanical valve actuation systems is possible in various cases. In this contribution a novel electromechanical valve actuation system for large sized 4/3-way directional control valves in a displacement controlled system is presented. The new actuation system is characterized by a hydraulic relief of the centering springs. Therefore, the springs are only active in safety-critical conditions, as in a power outage. Since the actuator is not working against the spring force during every displacement, the necessary actuation force is reduced drastically, so that common electromechanical actuators can be used. In case of a power outage, the spring relief is deactivated causing the stored energy to center the spool in neutral position. The performance of the novel actuation system is examined based on measurements, which are conducted on a manufactured demonstrator for valves of nominal size 25 with a flow rate of up to 600 l/min.