For the realization of compact and lightweight digital hydraulic cylinder drives for exoskeleton actuation the hydraulic binary counter concept was proposed. 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, the usage of a permanent magnet concept to realize the hysteresis function in an alternative way is studied. The valve spool is made of a ferromagnetic material and is attracted or repelled by a permanent magnet made of a Neodymium-Iron-Bor. The expected benefits are lower friction, lower demands on manufacturing tolerances, and an easier assembly of the valve. To find an advantageous embodiment of this functioning principle ring or disc shaped magnets of different sizes are analyzed. The magnetic forces exhibited by these different magnetic circuit designs are simulated with the Magnetic Finite Element code ‘FEMM’. The quasi-static magnetic forces at different spool positions are computed. Magnetic saturation and remanence are considered in this analysis. The aim is to achieve the required force on the piston and, thus, ensure the valve’s functionality. At the same time, however, the valve should be designed as compact and light as possible. The Finite element simulations are compared with an analytical model which provides a compact understanding of the influence of the design parameters on the functional and non functional performance criteria.

The hydraulic binary counter requires switching valves with a hysteretic response. In this paper an elastic snap through element is studied as means for that. The concept is based on a buckling beam which is elastically supported in axial direction in order to adjust its buckling properties with moderate manufacturing precision and to assure a well defined snap through behavior. The elastic support is provided by a cantilever beam. A rigorous optimization is performed heading for a most compact and fatigue durable design which exhibits the required lateral force displacement characteristics. A genetic algorithm is used to find the global design optimum. The stress/displacement properties of each design variant are computed by a compact model of the snap through system. It is derived by a Ritz method to obtain approximate solutions of the nonlinear buckling beam behavior. Its validity is checked by a Finite Element model. A compact design is possible if high strength spring steel is used for the elastic elements.

The use of a digital cylinder drive for exoskeletons was proposed in the recent year as a means to save weight, installation space, and energy. Also the hydraulic actuation of the switching valves of the digital drive was brought into discussion, in particular by a so called hydraulic digital counter concept. That concept was originally invented to realize a digital hydraulic amplifier. It uses a mechanical input and feedback, transfers this into binary switching states of the valves which switch the chambers of the digital cylinder either to tank or system pressure. It is now reconsidered with a hydraulic input, a special design of the valves appropriate for the sub-kilowatts power ranges of exoskeletons, and a special control strategy. The configuration of this system with a true binary counting property requires the input pressure thresholds to grow exponentially with the number of binary stages of the drive. That limits the number of feasible stages to approximately five or six, complicates the mechanical parts of the valves which realize the hysteretic response to the pilot pressure, and increases the hydraulic power for valve actuation. To overcome these problems the system is configured as a quasi binary counter which jumps over certain digits if the input flow is monotonous, thus, requires a reversal of input flow to realize those digits. A theory and a numerical model of the counter in this configuration and a proper counting control strategy of the hydraulic input are presented. With this much higher number of digits can be realized and valve design and configuration are eased.

Even though the majority of currently published exoskeletons prototypes employ electrical drives, great opportunities are seen for hydraulic drives. Their main advantages are the unrivalled force and power density, facilitating the low additional masses at the peripheral limb joints, and the simple realization of locking, damping, and recuperation functions. The latter function is feasible with some hydraulic control concepts, like primary or secondary motion control. In this paper a digital cylinder drive is studied for getting up from a crouch. This motion is a sound benchmark to test the ability of the drive for exoskeleton knee joint actuation. Digital cylinders can realize output torques only in steps and the transition between different steps can create jerky motions. In this study the motion quality and the losses are evaluated for a binary stepped digital cylinder with four different chambers.

Servo cylinders with hydrostatic bearings are employed when ultimate speed, low friction, or high precision are required. These advantages are opposed by a considerable leakage loss and high costs. The latter are caused by the high component precision required in order to avoid excessive leakage and to obtain high stiffness of the bearing. In this paper an alternative concept to realize such bearings with a considerable cost reduction potential is investigated. The sleeve is made of Polyetheretherketon (PEEK). A hydrostatic difference pressure or / and a hydrodynamic pressure deforms the PEEK sleeve such that a conical stabilizing sealing gap is created. A possible mechanical design is shown and the characteristics are analysed. To study the characteristics of the bearing system the finite element suite Abaqus is used. A Reynolds User Element is developed and included into Abaqus for the simulation of the fluid structure interaction. The Reynolds equation is discretised by finite elements and solved simultaneously with the mechanical model. With the developed user element in Abaqus, static and quasi-static analyses of mechanical models (linear, non-linear or inelastic material behaviour) containing lubrications gaps can be performed efficiently. The preliminary results showed the feasibility of the concept and, generally, the potentials of plastics as a flexible material to employ elastic deformation for the creation of lubrication effects.

Published theoretical work about fluid stiction between two separating plates was so far limited to a finite initial gap. It was shown that pressure and force evolution are well described by fluid film lubrication equations if cavitation is taken into account. The practically important case that plate separation starts from a mechanical contact condition was only studied by experiments. They showed that quite substantial negative pressures can occur in the gap for a very short time and that the peak forces are varying strongly even between consecutive experiments with equal test conditions. In this paper two models are presented which complement the Reynolds equations with dynamical bubble evolution equations. Initial gap height, bubble number density, and initial bubble radius are the three unknown parameters of these models. Initial gap height accounts for surface roughness, the two other parameters refer to the bubble nucleation of the fluid in the small roughness indentations of the gap. A first model employs the Rayleigh-Plesset bubble dynamic model. It requires that the bubbles stay small compared to the gap. Results show that its stiction force dynamics is two orders of magnitude faster than experimentally observed and that the bubble size condition is violated. The second model assumes that bubbles span over the whole gap height and that the flow of the liquid between the bubbles is guided by the Reynolds equation. This model can be brought into reasonable agreement with the experiments. Force variation from experiment to experiment can at least in part be reproduced by a random variation of the initial bubble sizes. The model exhibits a kind of boundary layer behavior close to the outer boundary. This layer represents the interaction zone between bubble growth dynamics, pressure distribution due to viscous flow, and the pressure boundary condition.

Energy efficiency improvements are forced by steadily increasing general performance and cost saving requirements, and for mobile machines mostly by stricter governmental emission laws. Such improvements can be realized by new system architectures, like hybrids or energy recovery systems, but also by optimizing existing systems. This publication discusses the reduction of systematic losses for an existing hydraulic Load Sensing System (LSS) used in mobile working machines, especially in compact excavators. Energy losses in a LSS are proportional to the pressure difference between pump and actuator in each section. These systematic losses are investigated and can be reduced by actuator adaptation or by splitting non-correlating sections. Energy losses along the hydraulic circuit, such as pump losses hydraulic line losses and actuator losses, which are affected by these adaptations indirectly, are neglected. The investigations are founded on measurements of a 5 ton compact excavator and their systematic evaluation. The actuator adaptation can be realised by changing the excavator’s geometry and/or hydraulic specifications (cylinder areas, displacement volume). The focus of this paper is limited to the hydraulic domain. Mathematical models and operation scenarios verified by measurements were taken as the basis to find optimum system parameter configurations by mathematical optimization, employing evolutionary algorithm. This included also different groupings of LSS circuits. Boom, stick, bucket and swing were taken into account and results are shown for a one, two and three pump LSS. Considering the introduced methods an effective way for reducing systematic losses up to 40% is shown in this exemplary case.

A series of detailed measurements of various mechanical and hydraulic system states of different excavators was performed. Main purpose of this study was to obtain a reliable information basis for assessing the potentials of hybrid drives, in particular the amount of recoverable energy. Differences concerned the size (tonnage) of the excavators and the hydraulic systems, open center versus load sensing. All machines were tested at the same set of operation scenarios, which are typical for practice, and with different operators. To this end, all test machines have been equipped with pressure, flow rate, temperature, angular and position sensors. These signals (about sixty) and several available from the machines CAN bus were recorded with a standard data acquisition system and electronically stored for later analysis. These raw data were processed to obtain the interesting data, like speeds, power flows, energies. In addition, videos of each test were recorded to facilitate the correct interpretation of the measurements and their correlation with the actual working processes. Power flows from the combustion engine, different pumps, and at each actuator and energetic losses at the different loss sources were plotted for the different operation scenarios. Total efficiencies of the machines for different scenarios and the energy in and outflow at each actuator were computed. From the latter so called relative and absolute recovery degrees for each actuator and for the total machine in the different operation scenarios were derived. The relative recovery degree is the ratio of the total outflow energy (second and fourth quadrant) and the total inflow energy (first and third quadrant). The absolute recovery degree is the ratio of the total outflow energy of an actuator and the total energy delivered by all pumps in an operation scenario. In most operation scenarios the total efficiency of consumed mechanical output energy at the hydraulic actuators relative to delivered hydraulic energy is in the range 15% to 25%. Reasonable recovery potentials do have the swing and the boom drive. For small machines, however, the boom drive dominates.

This paper presents simulation and experimental results of an energy saving hydraulic stepper drive prototype. Different concepts, advantages and the mechanical design of such kind of stepper drive were discussed in a previous publication. The excellent efficiency, the possibility of energy recuperation, and the control by switching and check valves only, may help to open new applications for hydraulic drives. Also the flow rate can be controlled rather directly by adjusting the switching frequency. This characteristic makes the sensorless position and speed control relatively easy. The drive is realized by a hydraulic cylinder piston unit which displaces a defined fluid quantum by the limited forward stroke of the piston controlled by a fast switching valve. This end to end motion of the piston in its cylinder generates a precise, incremental motion of an additional load cylinder; this enables a sensorless position control. Energy saving is achieved by storing the pressure surplus intermediately in the kinetic energy of the piston to displace a part of the fluid quantum without hydraulic energy from the supply line. A detailed simulation model of a stepper drive including transmission lines, flow channels, hydraulic accumulators and valve dynamics is applied to analyze the experimental results. This dynamic model in connection with the prototype allows to identify the potential for improvement. The different ways to improve the behavior are reviewed, in particular concerning energy losses: bearing friction, leakages in gaps, pressure losses and backflow through check valves. The measured dynamic characteristics and the energy efficiency are presented and compared to the simulation results. The preliminary results showed that the energy efficiency can be drastically increased by a better piston sealing and guidance system and faster check valves. Hence, the development of a fast plate type check valve for the hydraulic stepper drive is also proposed in this study.

The cushioning groove is a simple means to limit end stop speeds of small devices moving in some fluid, for instance, of spools in switching valves. A shallow groove limited by one or two edges is placed on one or both sides of the moving element. When the groove meets its opposing contact surface the fluid pressed out by the motion causes an increased pressure in the groove which provides the cushioning effect. It overcomes fluid stiction problems which are frequently encountered in squeeze gap type cushioning if the system is under high fluid pressure. The elementary cushioning groove concept assumes that the groove edges are exactly parallel to the contacting surface. In this paper, the performance of the cushioning groove in case of some slanting of the groove edges to the opposing surface is studied by means of a mathematical model. Slanting reduces the cushioning force and causes a resulting torque to the moving system due to an asymmetric pressure. Insufficient cushioning becomes more likely and, in turn, a repelling motion.

In digital hydraulic systems, switching valves have opening and closing times in the range of a few milliseconds. Due to this fast switching, high bandwidth pressure pulsation is excited, which is the stimulus for airborne noise up to some kilohertz. Since the human ear is very sensitive to audible noise in this frequency range, an analysis of the influence of the valve’s opening curve on the pressure surge in the pipe system is intended. The study is based on simulations employing dynamic pipe models for linear wave propagation and laminar fluid flow. In particular, a simple pipe system with a valve at one end and a pressure boundary at the other end of the pipe is investigated. It is shown, how the valve opening characteristics of spool and seat type switching valves influences the pipe responses. Also the role of parasitic inductances due to the valve block bores is discussed and it is shown how the switching characteristics influences the dynamical effects on the pressure pulsations in the pipe system.

In a series of experiments the peak current during switch on of a fast switching valve, which was found to be out of tolerances with respect to some armature dimensions, was varied to realize different switch on times. Despite the fact that the holding current was identical for all cases and the time between switch on and off was very long, the valve’s switch off time showed an unexpected dependency on the switch on peak current value. This paper presents an explanation of this phenomenon by coarse mathematical models, demonstrating that the manufacturing error in combination with friction, skewness, and fluid stiction are responsible for this behavior.

Stepper drives can realize quite precise, incremental motions without position sensors. Sensorless hydraulic motion control is strongly demanded by industry and, therefore, is an established idea in hydraulics for a while. Some concepts have been proposed in the past and a few of them have also been realized and applied in specific cases. But it is expected that digital hydraulics — due to its intrinsic discrete nature — can create new, more advantageous hydraulic versions of stepper drives. In this paper, a new stepper drive is presented and investigated. It creates the steps by fixed fluid quanta generated by a so called digital flow unit. That unit is realized by a hydraulic cylinder-piston unit which displaces a defined fluid quantum by each limited forward stroke of that piston. The unit is controlled by a fast switching valve which connects the piston areas alternately with the pressure-, tank- and output-line. The return motion is generated by a return spring. Energy saving is accomplished by storing the supply pressure surplus intermediately in the kinetic energy of the piston and converting that energy to displace part of the quantum to the consumer line without hydraulic energy from the supply line. Different detail concepts of this stepper drive are theoretically assessed. The transient behavior, the performance characteristics and the energy efficiency of a preferred concept are investigated by mathematical modeling and simulation. Furthermore, the main system parameters are identified and corresponding basic dimensioning rules are presented. In a second step, the influence of finite switching times of the valves, the hydraulic impedances of the various flow channels and of the dead volumes and the dynamical properties of the hydraulic cylinder attached to the device, are discussed.

Digital hydraulics is an opportunity to realize simple, robust, cheap and energy efficient hydraulic drives. In such systems digital on/off valves are used instead of proportional valves. Moreover, in hydraulic switching converters the valves are actuated within a few milliseconds, which create sharp pressure changes and, in turn, significant wave propagation effects in the pipe system. For a proper design of digital hydraulic systems a sound understanding of these effects is required to achieve the desired behavior of the switching drive system. In such converters, like the buck-, boost or boost-buck-converter, the inductance is one crucial component. It is realized by a simple pipe mainly for cost reasons. Furthermore, switching converters contain some components with nonlinear characteristics, like valves or accumulators, which prevent a comprehensive analysis in frequency domain. For a convenient analysis a qualified model of a hydraulic buck converter based on a mixed time frequency domain iteration is presented. Main parameters of this model are identified and wave propagation effects in the inductance pipe of the converter are investigated by simulation.

Oil stiction forces significantly influence the performance of fast switching valves. These forces stem from the significant lowering of the pressures between two oil filled plates relative to the surrounding pressure when the plates are quickly separated. If the pressure in the gap stays above the vapor pressure the stiction force can be derived from a solution of the Reynolds equation. However, for very fast motions — as occur in fast switching valves with a flat armature solenoid — cavitation is most likely to occur. The cavitation zone starts in central parts of the gap and extends as long as the gap volume increase cannot be fully compensated by the flow in the gap. Cavitation reduces the stiction force significantly. In many valves this stiction force reduction is decisive for a proper functioning of the valve. An important measure for stiction force control are flushing channels, in particular flushing bores. In this paper analytical models and Finite Volume method models are used to study the stiction force problems with and without cavitation and design measures for their mastering.

Fast check valves play a key role in digital hydraulics and high frequency oscillation pumps (HFOP); in these applications check valves with very short response times in the order of hundreds of microseconds are required. This paper presents a study on the dynamics of two different types of check valves, namely a ball valve available from the market and a plate valve designed specifically for a novel HFOP. A high frequency pumping cycle using such valves has been simulated mathematically in Matlab/Simulink for different working conditions. The same was reproduced physically by means of a test rig specifically designed for testing high frequency valve dynamics. The comparison between simulated and experimental results shows the influence of valve design (e.g. geometry of moving element) as well as fluid propagation effects on the dynamics of the process. The volumetric efficiency of the pumping cycle resulting from the collected data is a major input for the design of the valve for the specific HFOP application.