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![Figure 1.5.: The wave converter from [39]](profile/Helmut-Kogler/publication/279186617/figure/fig5/AS:669397890920453@1536608302768/The-wave-converter-from-39_Q320.jpg)
This thesis concerns the Hydraulic Buck Converter (HBC), which is a concept transferred from electric drive technology to hydraulics. In contrast to resistance control the HBC enables an energy efficient operation of hydraulic drives. This type of power control belongs to the class of fast switching hydraulic systems. Conventional resistance contro...
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... Considering, that the hydraulic power supply is designed to the maximum occurring load, which is only rarely demanded at a large number of applications, a huge amount of energy is wasted. However, considering again the configuration of Fig. 1.1 now in case of lowering the load, when the orifice R S is shut, the efficiency ...
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... a basin at a higher level, like depicted in Fig. 1.2. For this purpose the potential energy of water from a reservoir or a small river is converted into kinetic energy in the slopy drive line by opening the waste valve V W . When the water flow is high enough the pressure at the end of the drive line exceeds the closing force of the waste valve, which will be shut rapidly. The ...
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... is remarkable, that nowadays the principle of the hydraulic ram is still in use in some high valleys in the mountains to ensure the water supply for the cattle on the alp. In Fig. 1.3 the realisation of the hydraulic ram by Easton & Amos is illustrated, which was introduced at the world exhibition in Crystal Palace in the year 1851. ...
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... the middle of the 1990's Gall and Senn presented in [8] an energy efficient linear hydraulic drive, illustrated in Fig. 1.4. The main intention of this concept is to raise the efficiency by exploiting the inertia of the load in combination with so called free-wheeling valves for saving energy. For lifting the load the supply sided valve V S is opened for a certain amount of time and when the valve is closed again, the kinetic energy of the load initiates ...
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... exploiting the inertia of a dead load nearly arbitrary high inductances can be realised, which is important to reduce the ripples in the flow rate due to the switching process and, thus, to reduce the losses of the drive. The equivalent inductance of the configuration depicted in Fig. 1.4 compared to the fluid inertia in a simple pipe can be assessed ...
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... of about A ≈ 3 cm 2 , which leads according to Eq. (1.3) to an equivalent inductance of L h = m A 2 ≈ 3 · 10 9 kg m 4 . A simple pipe line, as used in an HBC, with the length of 1.5 m and an hydraulic diameter of 8 mm delivers only an inductance L ≈ 3 · 10 7 kg m 4 and, thus, the fluid inertia is about 1 100 of the system illustrated in Fig. 1.4. The main deficiency of the Gall and Senn concept is the low force capacity of the drive due to the small cross-section of the cylinder, which restricts this approach to a lower number of applications. Furthermore, for an optimal operation also very large and very fast valves are required, which were not available at that time. How- ...
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... side-branch has half the length of the first pipe, a further pipe attached at its mid-point has no pressure pulsa- tions of orders 2, 6, 10, . . .. Further side-branches following that design strategy can filter even higher pressure modes. This is the basic principle of the so called wave converter, which was presented in [40] and is depicted in Fig. 1.5. [39] As mentioned, the switching valve operates in pulse width mode and connects the pipe alternately to supply pressure and tank pressure, respectively. The average pressure value is set by the on-time of the valve, which is called pulse width t rel . Since the whole system is resonating at its eigen frequencies its energy ...
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... well established converter concept in electrical engineering is the resonance converter. The basic circuit is depicted in Fig. 1.6. A resonance converter of this type operates at switching frequencies just above the resonance point of the oscillating system. In this frequency range the decay of the output voltage strongly depends on the frequency. This property allows to control the output voltage of the drive. In [38,9,31,32] the successful Figure 1.6.: ...
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... property allows to control the output voltage of the drive. In [38,9,31,32] the successful Figure 1.6.: Electrical resonance converter transfer of this electrical drive concept to hydraulics is presented. The hydraulic resonance converter (HRC) is shown in Fig. 1.7, where the oscillating part is realised by a spring mass oscillator. ...
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... frequency range the decay of the output voltage strongly depends on the frequency. This property allows to control the output voltage of the drive. In [38,9,31,32] the successful Figure 1.6.: Electrical resonance converter transfer of this electrical drive concept to hydraulics is presented. The hydraulic resonance converter (HRC) is shown in Fig. 1.7, where the oscillating part is realised by a spring mass oscillator. The displacement of this oscillator corresponds to the flow rate at the output, hence it is a dual working principle to the wave converter. Also this converter uses the sensitive dependence of the output flow rate on frequency above the resonant point of the spring ...
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... Also this converter uses the sensitive dependence of the output flow rate on frequency above the resonant point of the spring mass oscillator. The simple version of the hydraulic resonance converter uses 3 valves for operation which is called single chamber converter (SCC). The control of the 3 valves can be examined in the upper right diagram in Fig. 1.7. The symmetric double chamber converter (DSC) uses 6 valves and has some performance advantages compared to the single chamber converter, but both types are rather costly. The design of the hydraulic resonance converter depends again strongly on the achievable switching frequency. To obtain handy proportions of the converter a ...
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... a 4 bit hydraulic D/A-converter can be realised. Figure 1.8 shows the basic principle and the corresponding drawing symbol. ...
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... the digital approach the valves have to be arranged in parallel for best flow conditions. For this reason the different on/off valves have to be located on a compact valve block, for instance, as depicted in Fig. 1.9. The corresponding characteristic properties are listed in Tab. 1.1. In this case the controllability of the flow rate differs from conventional proportional control. In theory a common servo valve can deliver any flow rate within its working range, but in practice non-linearities as, e.g., hysteresis, valve overlap or even flow ...
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... on the quality and in turn on the price of the valve. In the opposite, the flow rate through a certain digital valve is strictly quantised, but this flow characteristics is realised without any uncertainty. This is another benefit of the digital approach. The degree of controllability depends on the number of the used on/off valves as shown in Fig. 1.10. Another major advantage of the digital hydraulic valve concept is the robustness in terms of oil contamination and malfunction of a valve. If one valve is breaking down, the application is still controllable, even though at reduced controllability and higher energy consumption. To provide this benefit with conventional hydraulics a ...
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... most simple converter which can be transferred from electronics to hydraulics is the so called buck converter. The electrical circuitry of this traditional step down converter is depicted in Fig. ...
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... hydraulic pendant of the electric buck converter is depicted in Fig. 1.12. Its corre- sponding elements are a switching valve, a check valve, the inductance -realised simply by a pipe -and an accumulator to smoothen the pressure ripples at the load, which is rep- resented by an ordinary orifice in this simple case. The hydraulic buck converter (HBC) Figure 1.12.: Simple hydraulic buck converter also uses ...
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... corre- sponding elements are a switching valve, a check valve, the inductance -realised simply by a pipe -and an accumulator to smoothen the pressure ripples at the load, which is rep- resented by an ordinary orifice in this simple case. The hydraulic buck converter (HBC) Figure 1.12.: Simple hydraulic buck converter also uses the spill-over of kinetic energy stored in the inductance to raise the efficiency of the system. After closing the active switching valve, the inertia of the fluid enforces a flow through the tank sided check valve. ...
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... are in the kHz-range, switching in hydraulics will take place in the range of fifty up to a few hundred Hertz depending on the application. This limit does not only depend on the dynamic performance of the available valve, but also on hydraulic parasitic capacity effects of the fluid, which make switching at much higher frequencies unfeasible. In Fig. 1.12 a simple HBC for only one flow direction is depicted. A more relevant design is the both-way HBC which can be seen in Fig. 1.13. By spending Figure 1.13.: Both-way hydraulic buck converter another valve stage the flow through the converter can be controlled in both directions. With this type of the HBC it is possible to control, ...
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... This limit does not only depend on the dynamic performance of the available valve, but also on hydraulic parasitic capacity effects of the fluid, which make switching at much higher frequencies unfeasible. In Fig. 1.12 a simple HBC for only one flow direction is depicted. A more relevant design is the both-way HBC which can be seen in Fig. 1.13. By spending Figure 1.13.: Both-way hydraulic buck converter another valve stage the flow through the converter can be controlled in both directions. With this type of the HBC it is possible to control, e.g., a differential cylinder. To this end the cylinder has to be operated in plunger mode, i.e., the annulus chamber has to be ...
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... more relevant design is the both-way HBC which can be seen in Fig. 1.13. By spending Figure 1.13.: Both-way hydraulic buck converter another valve stage the flow through the converter can be controlled in both directions. With this type of the HBC it is possible to control, e.g., a differential cylinder. ...
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... simple switching concept constitutes the hydraulic boost converter, shown in Fig. 1.14. As the name promises, this is a step up converter which allows to boost the Figure 1.14.: Hydraulic boost converter pressure at the load to higher levels than the supply pressure. The converter also operates in PWM-mode and its working cycle starts with accelerating the oil in the inductance by opening the valve V T . After closing ...
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... simple switching concept constitutes the hydraulic boost converter, shown in Fig. 1.14. As the name promises, this is a step up converter which allows to boost the Figure 1.14.: Hydraulic boost converter pressure at the load to higher levels than the supply pressure. The converter also operates in PWM-mode and its working cycle starts with accelerating the oil in the inductance by opening the valve V T . ...
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... comparison to the simple hydraulic buck converter from Fig. 1.12 the hydraulic boost converter actually consists of the same basic components, which are arranged in a way to achieve a totally different behavior of the system. This structural flexibility shows the high potential of switching ...
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... advanced switching principle is the hydraulic boost buck converter, which is depicted in Fig. 1.15. The converter unifies the benefits of the boost and the buck converter, which consequently consists of two converter stages, the boost and the buck stage, respectively. Hence, the considered configuration is able to operate in one flow direction either as a step down or as a step up converter, depending on the power need at the ...
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... converter unifies the benefits of the boost and the buck converter, which consequently consists of two converter stages, the boost and the buck stage, respectively. Hence, the considered configuration is able to operate in one flow direction either as a step down or as a step up converter, depending on the power need at the load, which Figure 1.15.: The hydraulic boost-buck converter shows a broad variability. In buck mode simply the switching valve V D has to be pulsed at a certain switching frequency in PWM -mode. ...
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... switching valve V D has to be pulsed at a certain switching frequency in PWM -mode. On the other hand, the boost mode is characterised by pulsing the valve V B , while the valve V D has to be kept open constantly. Thus, the load pressure p A equals the boost pressure p B , at least assuming that the resistance of the valve can be neglected. In Fig. 1.15 one can also discover the preloaded tank system to avoid cavitation due to the limited size and dynamics of the check valves. In the mentioned figure, the tank system is not determined in detail. But, if the tank pressure is realised simply by a pressure relief valve, then both valves V B and V D have to be pulsed simultaneously in ...
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... electric buck converter depicted in Fig. 1.11 operates at two different modes, called the discontinuous and the continuous mode, as explained for instance in [45]. In the discontinuous mode, the current through the inductance vanishes during the off-time of the switch before the next switching cycle starts. Thus, in discontinuous mode the duty ratio κ corresponds to a certain ...
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... mode the duty ratio κ corresponds to a certain average current, depending on the load voltage. The continuous mode is characterised by the proper balance between the rise and decay of the current during the on-and off-time of the switch, which means, that the duty ratio corresponds to a mean voltage at the node Y of the converter from Fig. 1.11. The transition from one to the other mode of operation is determined by the duty ratio κ and the load resistance or load pressure, respectively. Because the HBC is analog to the electric buck converter, there exist also two different modes of operation in hydraulics, which are named flow control mode and pressure control mode, ...
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... the load resistance or load pressure, respectively. Because the HBC is analog to the electric buck converter, there exist also two different modes of operation in hydraulics, which are named flow control mode and pressure control mode, respectively. In this section the considerations are focused on the simple hydraulic buck converter as shown in Fig. 1.12. As already mentioned before, the inductance of the converter is simply realised by a pipe. At first the modes of operation will be derived for a lumped parameter pipe model, i.e., an inductance L and a static resistance R. Of course, the flow through a pipe is a more complicated process, because of wave propagation in the fluid. ...
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... the HBC is designed to operate in both flow directions, as depicted in Fig. 1.13, it is also possible to recuperate energy by switching of the tank sided valve. The overspill of kinetic energy of the oil in the inductance initiates an energy feed back into the supply line. The very basic principle of the recuperation mode is again illustrated in Fig. 2.6 under the assumptions, that had been declared at the ...
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... For this reason it is also important to define the load requirements precisely. A differential cylinder is the most frequently used actuator for most linear hydraulic drives. Hence, in the following a design procedure of a fast switching hydraulic drive consisting of a differential cylinder with a dead load controlled by an HBC as illustrated in Fig. 1.13 is proposed. In the following sub-sections simple design rules for the HBC and for its components, respectively, are presented. These rules are based on the simple models of Sections 2.1 and ...
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... first step in configuring a hydraulic drive is to get information about the hydraulic actuator as depicted, for instance, in Fig. 1.13. In this circuitry, the annulus chamber of the cylinder is connected to supply pressure, hence the corresponding cross-sections of the cylinder has to be defined to achieve the desired forces at the load. For an efficient operation of the system, the ratio of both cross-sections should be determined such, that the mean pressure of ...
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... equipment to enable energy saving and recuperation, respectively. First, the tank system has to be pre-pressurised to avoid cavitation during the suction phase. Therefore, a certain tank pressure above the ambient pressure has to be provided. One possibility of realisation would be the use of a simple pressure relief valve, like depicted in Fig. 1.13. The dynamics of this relief valve has to be much slower than the switching frequency to avoid oscillating effects due to the switching process. Its size depends on the waste flow rate, i.e., the difference between the released and the flow drawn in the suction phase. The realisation of the tank pressure by a pressure relief valve ...
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... a convenient control of a hydraulic drive as depicted in Fig. 1.13 the converter characteristics have to be compensated. Therefore, the calculations must be carried out in both flow directions. In Fig. 4.16 the simulation results of both flow rate directions are illustrated, where p A is a function of the duty ratio κ and the load flow rate q L . For a reasonable compensation, the characteristics ...
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... observer gain L(t), and the mea- sured quantity η, is intended. Under the assumption that the flatness based control keeps the system state sufficiently close to the desired trajectoriesûtrajectoriesˆtrajectoriesû = ˆ q A represents the input of the closed loop system. The intended flatness based control with a nonlinear observer is depicted in Fig. 5.14. ...
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... block diagram of the flatness based control (FBC) employing a complete observer is illustrated in Fig. 5.15, where the advantage of this observer type becomes clear. The nonlinear observer estimates the whole system ...
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... Controller . No external load force is applied to the piston, thus, F P = 0 is assumed. The corresponding results are illustrated in Fig. 5.16, where the flatness based control is compared to a conventional HPD according to Subsection 5.2.2. In contrast to the performance of the linear controllers according to Fig. 5.11 a remarkable improvement can be achieved with the FBC. Beside some fluctuations in the piston velocity, the trajectory of the HBC is even closer to the ...
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... load scenarios play In the simulation results of the flatness based control presented so far no external loads were applied. In most cases a hydraulic drive is designed to handle big loads and forces. The simulated performances of the HBC exploiting a flatness based control and an HPD under a nearly stepwise process force are presented in Fig. 5.18. The results show the big advantage of the conventional proportional hydraulics. Due to the stiffness of the HPD no relevant deviation in the piston position is noticeable despite of the applied process force. In contrast to that, the proposed controller for the HBC drive is not qualified for an adequate compensation of the deflection ...
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... performance of the load observer is also studied by simulation experiments, which were carried out in accordance with Fig. 5.19. Beside the state of the control system (5.1), the observer also estimates the uncertain process force F P . The estimated valuê F P is used in the nonlinear controller to compensate its influence on the closed loop ...
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... of the mentioned configurations at a cyclic sinusoidal position reference at a frequency of 2 Hz is illustrated. In fact, the deviations in the position compared to the ramp simulations increase due to the process force. But the performance of the HBC with the load observer is much better than with the complete observer. The second diagram in Fig. 5.21 shows the energy consumption. In contrast to the HPD the HBC needs less than the half power for almost the same output performance. As a consequence, the power dimension of the hydraulic supply unit can be reduced dramatically, if an HBC is applied. Thus, not only the running energy costs would be lowered by the application of an HBC, ...
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Citations
... These were theoretically and experimentally confirmed in the hydraulic switching converters with a compact long pipe. [15][16][17] In a hydraulic converter using flow-dependent fluid inertance, 18 when the fast switching valve switches from a high-pressure line to a tank line, the fluid kinetic energy stored in the inertance pipe should have continuously drawn the flowing fluid from the tank line to the load due to the existence of fluid inertance. 19 However, as an inherent behavior of SIHCs, 20 the performance of the fluid inertance is heavily affected by nonlinear characteristics including parasitic inductive effect 21 and wave propagation effect 22 within the inertance pipe at each switching period. ...
To acquire a linear load pressure with the potential of energy-saving compared with conventional throttling hydraulic systems, a switched hydraulic circuit (referred to as a flow-dependent inertia hydraulic converter) featuring a flow-dependent inertial effect was proposed. In this device, the flowing fluid modulated by an equivalent fast switching valve allows the fluid inertance at the connection pipe and rotation inertia incorporated in the flywheel to be combined. The theoretical and simulation models of the flow-dependent inertia hydraulic converter were established, and then the flow-dependent inertia was discussed and quantified. The predictions of the load pressure, flywheel speed and system efficiency were confirmed by experimental measurements. Results indicated that the theoretical and simulation predictions match well with the experiments for this configuration. The pressure wave propagation effect within the connection pipe is responsible for the variation in the transient load pressure and flywheel speed that is initially modulated by the rotation inertia theoretically, which seriously questions the predecessor's hypothesis of using a compressible volume to model the upstream connection pipe. The parabolic mean suction flow induced by the flow-dependent inertia draw our attention to the available supply flowrate and duty cycle. Finally, the throttling loss and system efficiency that varies with the desired characteristics is superior to that of conventional throttling hydraulic systems theoretically, and an acceptable system efficiency was obtained under the premise of ignoring friction experimentally.
... In the accumulator the elasticity of the gas spring of the accumulator is much softer than the oil stiffness. Thus, for simplicity and with regard to [32], the compressibility of the fluid is completely neglected and the dynamic model of the drive configuration ...
Digital hydraulics offers a cost-effective and robust opportunity to replace expensive and sensitive hydraulic servo valves for cylinder drives. If the digital directional valves are operated in pulse-width mode at a constant switching frequency, the mean flow rate through the valves into the actuator is controlled by the duty ratio of the pulse-width-mode signal. However, for a proper implementation of digital hydraulic linear drives, some adaptions must be made compared to common cylinder drives. In this article, a gas-loaded accumulator is used in order to tune the dynamics of the system for a smooth behavior of the drive, which is generally nonlinear. An energy-based controller according to the passivity concept in combination with a load observer is presented. Simulations show the achievable performance.
... Most upcoming alternative solutions can be summed up with the term Digital Fluid Power [4]. Actively researched concepts are for example Secondary Controlled Multi-Chamber Hydraulic Cylinders [5], Hydraulic Buck Converter [6], and Hydraulic Stepper Drive [7]. One other concept, the topic of this contribution, is called Linear Incremental Hydraulic Actuators, LIHA, or Hydraulic Infinite Linear Actuator, see [1] [8] [9] [10] [11]. ...
Linear Incremental Hydraulic Actuators are a class of actuators consisting of one or more double-acting cylinders with are detachable from a common piston rod. The actuator works in a kind of rope-climbing motion. The cylinders engage to the rod, drive the load, disengage and retract. The various patterns, how these base actions of the realized cylinders are concatenated form the gaits of Linear Incremental Hydraulic Actuators. Depending on the gait the base actions are realized digital or continuous. For example, in the gait Smooth Motion engaging / disengaging is realized digital and the driving / retraction continuous. Other gaits may be realized fully digital. This contribution presents the concept of the Linear Incremental Hydraulic Actuator, an overview over various gaits, and how they can be combined to form advantages in applications. Advantages of Linear Incremental Hydraulic Actuators are or may be lower resource utilization and environmental impact, applicable for long or even infinite stroke, and simple integration.
... Conventional Hydraulic Linear Actuators, i.e. valve controlled cylinders, are common and mature components of hydraulic systems. Ongoing research focuses on secondary control with multi-chamber cylinders [6], various concepts of digital hydraulics [7], individual metering [8], and advanced control concepts [9], [10]. A further topic is sensor-less positioning with stepper drives [11]. ...
The concept of the Hydraulic Infinite Linear Actuator, HILA, consists of two double-acting cylinders with hydraulically detachable pistons driving a common rod. Alternatingly one cylinder engages and drives the load while the other retracts; the actuator thus works in a kind of rope-climbing motion. The actuator has three degrees of freedom, DoFs, which are temporarily coupled compared with one in a conventional cylinder. Hence, the actuator is a hybrid system with both, continuous and discrete states and control inputs. This contribution presents a tracking control problem and a solution for high-performance motion, which is interpreted as utilizing the hardware limits in the sense of velocity, accuracy, smoothness and usability. The concept is demonstrated by simulation for the case of a rest-to-rest motion, but the intended use-cases also include arbitrary trajectories.
... In switching hydraulics (Linz style digital hydraulics), e.g. [Kog12], fast on/off valves are used to control the flow of fluid exploiting the inductance of the fluid or other system components. The control input is implemented as pulsewidth modulation (PWM) or pulse-frequency modulation (PFM). ...
... The control input is implemented as pulsewidth modulation (PWM) or pulse-frequency modulation (PFM). For example, one switching hydraulic system is the hydraulic buck converter [Kog12]. A linear hydraulic actuator exploiting the inertia of the load in combination with so-called freewheeling valves has been presented in [GS94]. ...
... Characteristic is that a short strong thrust is followed by a free motion period. The ballistic gait brings switching technology, see for example the Hydraulic Buck Converter [Kog12], to the hydraulic-mechanical field. The control inputs are for example how often the thrusts are executed, how long the thrusts are, or how many cylinders engage for a thrust. ...
Linear Incremental Hydraulic Actuators combine one or more short-stroke cylinders, and two or more engaging/disengaging mechanisms into one actuator with long, medium, or even unlimited stroke length. The motion of each single short-stroke actuator concatenated by the engaging/disengaging mechanisms forms the motion of the linear incremental hydraulic actuator.
The patterns of how these motions are concatenated form the gaits of a specific linear incremental hydraulic actuator. Linear incremental hydraulic actuators may have more than one gait. In an application, the gaits may be combined to achieve optimal performance at various operating points.
The distinguishing characteristic of linear incremental hydraulic actuators is the incremental motion. The term incremental actuator is seen as analogous to the incremental versus absolute position sensor. Incremental actuators realize naturally relative positioning. Incremental motion means also that the behavior does not depend on an absolute position but only on the relative position within a cycle or step.
Incremental actuators may realize discrete incremental or continuous incremental motion. Discrete incremental actuators can only approach discrete positions, whereby stepper drives are one prominent example. In contrast, continuous incremental actuators may approach any position. Linear electric motors are one example of continuous incremental actuators. The actuator has no inherent limitation in stroke length, as every step or cycle adds only to the state at the beginning of the step or cycle and does not depend on the absolute position. This led to the alternative working title Hydraulic Infinite Linear Actuator.
Linear incremental hydraulic actuator provides long stroke, high force, and linear motion and has the potential to
* decrease the necessary resource usage,
* minimize environmental impact, e.g. from potential oil spillage,
* extend the range of feasible products: longer, stiffer, better, etc.
This thesis presents an analysis of the characteristics and properties of linear incremental hydraulic actuators as well as the gaits and possible realizations of some gaits. The gait for continuous, smooth motion with two cylinders is comprehensively studied and a control concept for the tracking problem is proposed. The control concept encapsulates the complexity of the linear incremental hydraulic actuator so that an application does not have to deal with it. One other gait, the ballistic gait, which realizes fast, energy-efficient motion, enabling energy recuperation is studied.
... Conventional hydraulic linear actuators, i.e. cylinders, are common and mature components of hydraulic systems. Ongoing research focuses on secondary control with multichamber cylinders [6], various concepts of digital hydraulics [7], individual metering [8], advanced control concepts [9,10], and sensor-less positioning with stepper drives [11]. ...
... Gall and Senn presented a linear hydraulic actuator exploiting the inertia of the load in combination with so-called freewheeling valves for saving energy in [12], see also [7]. In the ballistic gait, this basic idea is applied on HILA. ...
... A second concept, fully digital, later referred to as system D, is shown in Figure 7. In cases where other gaits are required too, this concept can be combined with for example hydraulic switching technology such as the Hydraulic Buck Converter [7], or equipped with parallel proportional valves to resemble the system in Figure 6. ...
The Hydraulic Infinite Linear Actuator, HILA, has been presented in previous publications.
The novel actuator consists of one, two or more double-acting cylinders with a common piston rod and hydraulically detachable pistons. In the basic gait, one cylinder alternatingly engages and drives the load while the other retracts. The HILA thus works in a kind of rope-climbing motion. The concept also allows other gaits, i.e. patterns of motion.
This contribution focuses on the ballistic gait, a pattern of motion where one cylinder engages to give the load a push. The load then carries on with its motion by inertia, cylinders disengaged. The actuator thus realizes hydro-mechanical pulse-frequency modulation (PFM).
This gait is energy efficient and able to recuperate energy.
... Many researchers in the field of fast switching hydraulic systems, particularly in high frequency oscillation pumps and hydraulic converters, classified the valves as bottleneck of these systems (Leati [23] and Kogler [24]). Various problems like cavitation, overpressure peaks and cross-flow can occur if the valve is not fast enough in relation to the movement of the piston. ...
This paper presents the development of an energy efficient low power stepper converter.
A prototype with a hydraulic output power of ≈ 600 W was designed, manufactured, investigated and
improved. The converter consists of a hydraulic cylinder piston unit controlled by a fast switching
valve to displace a defined fluid quantum by the limited forward stroke of the piston in its cylinder.
The displaced fluid generates a precise, incremental motion of a load cylinder which should be
controlled. 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. Energy recuperation can be done in a similar way. Simulations and experiments showed two
main efficiency improvement measures of the first converter prototype. The weak points were the
commercially available check valves and the used guidance system for the pistons. The second part
of the paper reports about the development of a fast check valve and of a combined hydrostatic
hydrodynamic bearing system based on the elastic deformation of plastics. The theoretical and
experimental results show a significant improvement of the energy efficiency, the potential of this
drive technology and further improvement potential. Expressed in terms of numbers an energy
efficiency increase compared to a resistance control up to 30% and a maximum recuperation energy
efficiency over 60% were measured.
... Hydraulic switched inertance converters can be arranged in two distinct configurations, a step-down configuration, and a step-up configuration [1]. A stepdown (buck) converter, shown in Figure 1, works by accelerating flow through the system inertance, , and upon closing the switching valve, the fluid inertia creates suction pressure at the node . ...
... The gas spring accumulator located after the inertance tube has a dynamic equation that was derived by Kogler [1]. The rate of change of load pressure is given by the following equation. ...
... The switching valve uses the turbulent orifice equation which is well established in the literature, however the rate limited square waveform for the orifice area should be checked against the literature to see if the wave shape is acceptable. A typical digital hydraulic valve has a spool movement curve such as the one determined by Kogler [1] and is shown below in Figure 5. It is known than the relationship between spool position and valve orifice area is linear, therefore an area waveform can be directly employed. ...
Hydraulic switched inertance converters have gained recent research interests due to their highly efficient method of converting system pressures over typical restrictive type methods. Step-down pressure converters are the topic of this paper, and one composed of a fast switching valve, check valve, inertance tube, accumulator, and a flow control valve is modeled. The dynamic equations of the converter are introduced, and then validated. Several models for the inertance tube are available in the literature, and out of interest to the author a Transmission Line Model (TLM) was used to model the system's inertance tube. Valve switching ratio (duty cycle) was found to be the main parameter that controls load pressure. System response was analyzed in detail for three different switching ratios. It was found that a nearly linear relationship between switching ratio and load pressure exists. This made the implementation of a PI controller to accurately control load pressure possible.
... [3], various concepts of digital hydraulics, e.g. [4], and hydraulic stepper drives as in [5]. The commercially available system most similar to the presented one is strand jacks, e.g. from ENERPAC [6]. ...
This contribution presents the novel Hydraulic Infinite Linear Actuator (HILA). It focuses on the control of motion and is based on simulation and analysis.
The novel actuator consists of two symmetric double acting cylinders with a common piston rod and hydraulically detachable pistons. Alternatingly one cylinder engages and drives the load while the other retracts, the HILA thus works in a kind of rope climbing motion.
The purpose of this contribution is to study the motion control of the HILA and to mimic the behaviour of a conventional cylinder. The HILA has three degrees of freedom which are temporarily coupled compared with one in a conventional cylinder. Further, the HILA with the chosen hydraulic system has two continuous and two digital control inputs. The challenge to be tackled is to combine the short stroke back and forth motions of the cylinders into a continuous smooth motion of the whole actuator.
Results from simulations and analyses show that the investigated concepts can keep the jerk within acceptable limits for many applications.
... Today, in advanced conventional casters, the mold oscillation is realized by a hydraulic proportional servo drive (HPD) as depicted in Fig. 6 on the right hand side. As an energy efficient alternative, a hydraulic buck converter (HBC, see for instance [16,17]) shown on the left hand side in Fig. 6 is studied. The HBC represents a hydraulic switching converter, which operates at a constant switching frequency in pulse-width-mode. ...
Digital fluid systems are controlled solely by on-off valves, in contrast to analogue fluid systems which use continuously adjustable valves. Many digital fluid systems have been in practical use for some decades. The steel industry relies on several fluid technologies. Apart from the handling of the liquid steel, several other fluid processes exist to fulfill indispensable functions; examples include cooling, motion control, torch cutting, descaling, and lubricating. Many of these processes need better control concerning precision, dynamics, resource demand, reliability, and environmental impact or must offer additional func-tionalities for use in factories of the future. In several cases digital fluid systems are the better solutions. In this paper this is demonstrated by four examples, ranging from proven simple solutions and concepts going currently into industrial application to promising concepts for the future.
