Ruqi Ding, Min Cheng - Independent Metering Electro-Hydraulic Control System-Springer (2024)

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Independent Metering Electro-Hydraulic

Control System

Ruqi Ding

Min Cheng

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Independent Metering Electro-Hydraulic Control

System

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Independent Metering

Electro-Hydraulic Control

System

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Ruqi Ding

East China Jiaotong University Nanchang, Jiangxi, China

Min Cheng

State Key Laboratory of Mechanical Transmission

Chongqing University Chongqing, China

ISBN 978-981-99-6371-3 ISBN 978-981-99-6372-0 (eBook) https://doi.org/10.1007/978-981-99-6372-0

Jointly published with Shanghai Jiao Tong University Press.

The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Shanghai Jiao Tong University Press.

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The first author of this book is Prof. Ding Ruqi from East China Jiaotong University, and the second author of this book is Prof. Min Cheng from Chongqing university.

They both have been rooted in teaching and researching advanced electro-hydraulic control systems for a long time. Compiling textbooks not only requires the author to have rich scientific research practice but also needs the correct grasp and under- standing of the frontier of scientific and technological development in related fields.

It can be said that the book is the conclusion of his long-term scientific research work.

The subject of this book is the study of advanced technology used in hydraulic systems. The technology in question is termed Independent Metering (IM). Because of its many advantages, Independent Metering Control Valve or System (IMCV or IMCS) has become the research hotspot and is used in a range of very important applications such as construction machinery. The IMCS was proposed in the 1980s by Professor Backe. W of RWTH Aachen University in Germany. In China, Zhejiang University also carried out research on the IMCS for decades, in which Professor Qingfeng Wang and I have cultivated a large number of talents in this field. Each generation follows in the footsteps of their predecessors, and the younger generation gradually takes on the responsibility of society. The new era of the fluid power transmission and control needs more and more youngers like the author of this book.

This book is rich in content and rigorous in structure. The publication of this book will further promote in-depth research in the field of the IMCS. This book consists of seven chapters and shows the independent metering electro-hydraulic system involving its flexible hardware layouts, complex software control, representative products, and interesting applications. Starting from the traditional electro- hydraulic system, the background and motivation of IMCS are deeply analyzed. In addition, various hardware layouts are summarized involving the utilized hydraulic components and circuits, together with their advantages and disadvantages. Followed by the flexible configuration, a variety of working modes can be realized. Then, multi- variable control strategies including three levels: load, valve, and pump, as well as the fault-tolerant control under the fault condition, are demonstrated in detail. Finally,

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vi Foreword products of IMCV and their applications in some typical construction machinery are reviewed in the state of the arts.

Therefore, this book has great significance and reference value for the application of the IMCS in engineering. Research on the IMCS will learn a lot from this academic monograph.

I hope that more and better excellent monographs will be published in the future.

This book is interesting and useful to a wide readership in the various fields of fluid power transmission and control.

September 2023 Bing Xu

Executive Deputy Director of the State Key Laboratory of Fluid Power and Mechatronic Systems Zhejiang University Hangzhou, China

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Due to several advantages, such as a high power-weight ratio and high load capability, fluid power transmission technology has been used in all types of construction machines, such as excavators, rotary drilling rigs, concrete pump trucks, and other construction machinery. The electro-hydraulic control system is the core driving and controlling device of construction machinery, which directly determines its energy efficiency, control performance, and safety reliability. Traditional electro- hydraulic control systems, such as load-sensing systems and positive/negative flow control systems, are most commonly utilized in construction machinery, such that a trade-off between energy efficiency and steering quality can be captured. However, three typical drawbacks are inevitable in traditional systems: low energy efficiency, insufficient compatibility, and poor controllability. Therefore, due to the complexity and inflexibility of conventional mobile hydraulic systems, it is difficult to meet the growing demand, which motivates the development of better components and creative circuits to overcome the three aforementioned weaknesses.

By decoupling the inlet and outlet, an Independent Metering Control System (IMCS) improves the freedom of control and provides the possibility for a more intelligent control strategy. At the same time, it can improve energy efficiency while meeting the control requirements. The content of this book is derived from the scientific research practice of the authors. This book’s publication will help readers understand different control theories of electro-hydraulic systems and promote the development of electro-hydraulic control systems.

In Chap. 1, the concept and characteristics of an IMCS are introduced in detail.

From the aspects of energy efficiency, compatibility, and controllability, the characteristic of an IMCS is presented. In Chap. 2, the valve assembly used in the IMCS and the possible layout of the main and pilot stages are described in detail. In Chap. 3, multiple operating modes are established according to the achievable flow paths by the independent metering control, and the energy-saving functions, including regeneration and recuperation, are realized. In addition, energy-saving characteristics and force-velocity capability are analyzed to design the mode-switching logic, such that the most efficient operating mode without losing controllability for precise motion tracking can be automatically selected. In Chap. 4, the different independent

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viii Preface metering valve control architectures are described. To minimize the strong vibration generated by low damping, a hybrid control method combining dynamic pressure- feedback control with independent metering control is proposed for active damping compensation. In Chap. 5, the pump-valve coordinate control system is established by incorporating the electronically controlled pump (ECP) into the IMCS, and a novel method for three-level multi-mode transfer, considering the cylinder, valve, and pump, is developed. In Chap. 6, both active valve and sensor fault-tolerant control systems parallel to the normal controller are proposed to adapt to valve or sensor fault conditions. In Chap. 7, well-known products of Independent Metering Control Valve or Systems are introduced and their representative features are discussed.

Due to the limited level of the author and the short time, there may be omissions and inadequacies in the book. I hope the majority of readers and members of the academic and industrial affiliations can make criticisms and corrections.

Nanchang, China November 2023

Ruqi Ding

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This work was supported by the National Key Research and Development Program of China(Grant No.2020YFB2009703), the Natural Science Foundation of China(Grant No.52175050), and Major Scientific and Technological Research and Development Project of Jiangxi Province(No. 20233AAE02001).

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Introduction

1.1 Background

Due to several advantages, such as a high power-weight ratio and high load capability, fluid power transmission technology has been used in all types of construction machinery [1]. These applications are often referred to as “mobile hydraulic systems.” High power is often required to precisely control the simultaneous motion of all of the actuators, which distinguishes mobile hydraulic systems from industrial hydraulic systems (Fig. 1.1a) [2]. The required flow and pressure vary continuously, and the load frequently alternates between resisting and overrunning to cover the four quadrants (Fig. 1.1b) [3].

Conventional electro-hydraulic control systems, such as HLS (Hydraulic Load Sensing) systems, are commonly used mobile hydraulic systems that make trade- offs between energy efficiency and control characteristics. In Fig. 1.2, an adaptable flow and pressure control strategy, where the pump is pre-set to maintain a certain pressure margin beyond the highest load pressure is shown [4]. Hydro-mechanical pressure compensators (PC) makes the motion of actuator load-independent [5].

However, conventional systems have three typical disadvantages: (1) mechanical coupling between the inlet and outlet, (2) pressure feedback network via long pipelines, and (3) pressure compensation between different loads. The weaknesses arising from the three above constraints include the following:

(1) Low energy efficiency

Figure 1.3 displays the distribution of energy losses in an excavator. The heaviest energy losses, which consist of the three following aspects, reside in the valves [6]:

Inlet throttling loss: the pump pressure margin pm is set to overcome losses across the hoses, directional valves, and pressure compensation valves. To satisfy the requirements of all operating points, the hydro-mechanical pump always considers the worst working conditions to preset the pressure margin, which causes unnecessary pressure losses.

R. Ding and M. Cheng, Independent Metering Electro-Hydraulic Control System, https://doi.org/10.1007/978-981-99-6372-0_1

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2 1 Introduction

(a) (b)

Fig. 1.1 Distinct features of mobile machinery

Load 1 Load 2

Ps

PL1 PL2

Pb Pa

Pr

Fig. 1.2 Schematic diagram of the HLS system

Outlet throttling loss: due to the mechanical coupling of the inlet and outlet, the meter-out valve cannot open as large as possible under resisting loads, leading to a noticeable pressure loss. Under overrunning loads, the energy under lowering loads cannot be recuperated and is wasted by outlet throttling losses.

Load difference: because multiple actuators are supplied by one pump, waste occurs in the pressure compensator owing to pressure differences between lower loads and higher load.

(2) Insufficient compatibility

Due to the complexity of hydro-mechanical systems, commissioning methods are often simplified to a combination of experience and trial-and-error, and the commissioning itself is inconveniently conducted by changing hardware, such as using meter- in and meter-out orifices in valves, as well as adjusting springs, spool design, and

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Fig. 1.3 Energy consumption distribution of an excavator

hose volumes in the pump [7]. Such tedious commissioning with slow cycle time produces unnecessary waste, including time, manpower, and resources. Furthermore, the compatibility for various applications is limited due to the structure coupling, as depicted in Fig. 1.4.

(3) Poor controllability

By introducing the LS-pressure feedback, the system is often poorly damped with complex dynamics that oscillate. Additionally, pressure feedback via long pipelines causes a delay between the handle input and displacement regulator mechanism of the pump, decreasing the dynamic response [8].

Fig. 1.4 Commissioning process of the traditional system

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4 1 Introduction

Presently, construction machinery is increasingly confronted with the following challenges:

(1) Rising energy costs and stringent emission regulations and standards [9], (2) Constant demands for higher productivity and enhanced flexibility, and (3) Tough requirements of operator comfort and safety [3].

Due to the complexity and inflexibility of conventional mobile hydraulic systems, they have difficulty meeting these requirements, which motivates the development of better components and creative circuits to overcome the three aforementioned weaknesses.

1.2 Independent Metering Control System (IMCS)

The energy efficiency of mobile hydraulic systems has become an important topic in recent years. In mobile applications, controllability can be a secondary design objective. A good example of energy inefficient machine is the hydraulic excavator, whose total energy efficiency can be as low as 10%, contributing to approximately two hundred million tons of CO₂emissions in all construction machinery [10, 11].

Today, strict administrative regulations demand energy consumption and CO₂emissions reductions for the industry. In December 2017, 29 countries signed the Carbon Neutral Alliance Statement at the One Earth Summit, making a commitment to zero carbon emissions by the mid-twenty-first century [12]. As of 12 June 2020, 125 countries have committed to achieving carbon neutrality by the mid-twenty-first century [13].

To address the problem of low energy efficiency, the first approach is pump control, which can make the pump outlet pressure and flow automatically adjust with the load pressure and velocity without excess as possible. Benefiting from the absence of throttling losses, the direct pump control is highly energy efficient. There are two solutions to achieve pump control: variable displacement and variable speed [14]. The former achieves the purpose of pump control by changing pump displacement. Exten- sive research on this subject is conducted by Professor Ivantysynova et al. [15]. The latter acheives the purpose of pump control by changing motor speed, where the actuator and motor have their own pump, as shown in Fig. 1.5. Research on the characteristics and performance of speed variable pump circuits was carried out [16]. The main characteristics of these systems are lower energy consumption ratios, hence better fuel economy and fewer greenhouse gases. However, owing to the large inertia mass, leakages, and high nonlinearities, it is much more complicated to achieve accurate tracking performance and fast dynamic response of the actuator compared to the valve control systems. Moreover, compared to valve control, it needs a significant increase in the component effort: each cylinder needs to be coupled with a high dynamic variable pump unit.

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Fig. 1.5 Individualization levels of pump control

When using valve control, the IMCS is an inevitable choice to increase energy efficiency. It is an abbreviation for systems where the meter-in orifice and the meter- out orifice are independently controlled. The IMCS breaks the structural coupling of the spool, which changes from a single splool to twin spools, as shown in Fig. 1.6.

After IMCS was proposed by Jansson and Palmberg [17], intensive investigations have been performed. Most research has focused on energy efficiency. Theoretically, a specific piston force f p can now be obtained with an infinite number of chamber pressure combinations. This enables a possibility for hydraulic actuators’ energy consumption reductions if such high-precision chamber pressure trackings can be designed for pa and pb, such that the pressures can be set as low as practically possible [18]. Moreover, independent control of inlet and outlet orifices increases energy efficiency by allowing individual control paths or modes. For example, the system can change flow paths during operation, i.e., recuperation and regeneration can be utilized to reduce energy consumption.

Fig. 1.6 Comparison of the structure of traditional multi-way valve and Independent Metering Valve (IMV)

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6 1 Introduction

In addition to energy efficiency, high-precision motion, and force-tracking controls are vital functionalities for mobile hydraulic systems. Introducing mechanically decoupled orifices at the cylinder chambers increases the system’s degree of freedom (DOF) from 1 to 2 [6]. The additional DOF changes the system from Single Input Single Output (SISO) to Multiple Input Multiple Output (MIMO), making it possible to control both speed and pressure. Meanwhile, it changes the control system from the hydro-mechanical concept into an intelligent control system that relies on software. The improvement of energy efficiency and control controllability depends on the software programming. More advanced and complex control methods can now be applied to the hydraulic systems to adapt to various working conditions of different machines, such as an adaptive robust control (ARC) strategy by Yao et al. [19] and Virtual Decomposition Control (VDC) approach by Zhu etc. [20].

In addition, pressure feedback via sensitive sensors and electric circuits increases the dynamic stability. Famous companies such as Eaton and Husco have developed emedded integrated controllers with high-performance computing power and information transmission rate (the fastest 3 ms) [6]. To solve the uncertainty and nonlinear problems in IMCS, Li Chen et al. proposed a nonlinear valve flow model, which can capture the nonlinearity of valve flow well and achieve high control accuracy [21].

Moreover, the IMCS depends on software to determine performance, which improves hardware versatility and modularization simultaneously. It is convenient for manufacturer to efficiently programme software and tune parameters, instead of trial-and-error. This also greatly decreases the commissioning cycle and costs in practical application.

1.3 Conclusion

In a summary, there are several advantages including higer energy efficiency, sufficient compatibility and better controllability in IMCS compared with conventional hydraulic control system. The core of IMCS is the improvement of energy efficiency and control controllability depending on the software programming, instead of hardware. Therefore, more advanced control strategies can be introduced to adapt to various working conditions of different machines. The book will introduce IMCS in detail from the following chapters:

In this chapter, in terms of the conventional electro-hydraulic system, the background and motivation of IMCS are deeply analyzed. From the aspects of energy efficiency, compatibility, and controllability, the main characteristic of IMCS is presented.

In Chap. 2, this chapter presents the representative hardware configurations of independent metering valves, including both the pilot and main stage, and analyzes their pros/cons.

In Chap. 3, the system degree of freedom (DOF) increases from one to two in IMCS. It first benefits the flexible configuration of individual flow paths, including regeneration and recuperation. Multiple operating modes and the mode-switching

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logic are established according to the achievable flow paths by the independent metering control, such that the most efficient operating mode without losing controllability can be automatically selected.

In Chap. 4, the additional DOF restructures the system from SISO to MIMO.

Therefore, the multi-variable valve control architecture is described, and a detailed decentral control algorithm is proposed. Meanwhile, to minimize the strong vibration generated by low damping, a hybrid control method combining dynamic pressure- feedback control with independent metering control is proposed for active damping compensation.

In Chap. 5, the pump-valve coordinate control system is established by incorporating an electronically controlled pump (ECP) into the IMCS, such that higher energy efficiency and accurate motion control can be accomplished by different mode configurations and associated multi-variable control strategies.

In Chap. 6, active VFTC (Valve fault-tolerant control) and SFTC (Sensor fault- tolerant control) systems parallel to the normal controller are proposed to adapt to different valve or sensor faults.

In Chap. 7, the application of the IMCS in construction machinery is discussed in detail.

References

1. H. Shi, H.Y. Yang, G.F. Gong, H.Y. Liu, D.Q. Hou, Energy saving of cutterhead hydraulic drive system of shield tunneling machine. Autom. Constr. 37, 11–21 (2014). https://doi.org/10.1016/

j.autcon.2013.09.002

2. H. Murrenhoff, S.S. Milos, An overview of energy saving architectures for mobile applications, in 9th International Fluid Power Conference, ed. by H. Murrenhoff (RWTH Aachen University, Aachen, Germany, 2014), 978-3-9816480-0-3, pp. 24–26

3. B. Xu, M. Cheng, Motion control of multi-actuator hydraulic systems for mobile machineries- recent advancements and future trends. Front. Mech. Eng. 13(2), 151–166 (2018). https://doi.

org/10.1007/s11465-018-0470-5

4. J. Weber, B. Beck, E. Fischer, R. Ivantysyn, G. Kolks, M. Kunkis, et al., Novel system architectures by individual drives, in 10th InternationalFluid Power Conference, ed. by J. Weber, vol. 2 (Dresden University of Technology, Dresden, Germany, 2016), pp. 29–62. http://nbn-res olving.de/urn:nbn:de:bsz:14-qucosa-199972. Accessed 18 July 2018

5. B. Eriksson, J. Palmberg, Individual metering fluid power systems: challenges and opportuni- ties, Proc. Inst. Mech. Eng. I J. Syst. Control Eng. (225), 196–211 (2011). https://doi.org/10.

1243/09596518JSCE1111

6. R. Ding, J. Zhang, B. Xu, et al., Programmable hydraulic control technique in construction machinery: status, challenges, and countermeasures. Autom. Constr. 95(NOV.), 172–192 (2018)

7. K. Abuowda, I. Okhotnikov, S. Noroozi, P. Godfrey, M. Dupac, A review of electrohydraulic independent metering technology. ISA Trans. 98, 364–381 (2020). https://doi.org/10.1016/j.

isatra.2019.08.057. https://www.ncbi.nlm.nih.gov/pubmed/31522820

8. R. Ding, B. Xu, J. Zhang, et al., Self-tuning pressure-feedback control by pole placement for vibration reduction of excavator with independent metering fluid power system. Mech. Syst.

Signal Process. (2017)

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8 1 Introduction

9. T.A. Minav, L.I.E. Laurila, J.J. Pyrhonen, Analysis of electro-hydraulic lifting system’s energy efficiency with direct electric drive pump control. Autom. Constr. 30, 144–150 (2013). https://

doi.org/10.1016/j.autcon.2012.11.009

10. M. Vukovic, R. Leifeld, H. Murrenhoff, Reducing fuel consumption in hydraulic excavators—a comprehensive analysis. Energies 10(5), 687 (2017)

11. J. Chen, Q. Shi, W. Zhang, Structural path and sensitivity analysis of the CO2 emissions in the construction industry. Environ. Impact Assess. Rev. 92, 106679 (2022)

12. Carbon Neutrality Coalition. Plan of action: carbon neutrality coalition (2017) [2020-08-20].

https://www.carbon-neutrality.global/plan-of-action/

13. Energy & Climate Intelligence Unit. Net zero emissions race (2020) [2020-08-20]. https://eciu.

net/netzerotracker/map

14. A. Helbig, Energieeffizientes elektrisch-hydrostatisches Antriebssystem am Beispiel der Kunststoff-Spritzgießmaschine. Dissertation, TU Dresden (2007)

15. M. Ivantysynova, Quo Vadis fluid power? in ASME/BATH 2015 Symposium on Fluid Power and Motion Control, Chicago (USA) (2015)

16. E. Siemer, Variable-speed pump drive system for a 5000 kN ring expander, in 8th International Fluid Power Conference, Dresden, vol. 3, pp. 45–56 (2012)

17. A. Jansson, J.-O. Palmberg, Separate controls of meter-in and meter-out orifices in mobile hydraulic systems. SAE Trans. 377–383 (1990)

18. J. Koivumäki, J. Mattila, Energy-efficient and high-precision control of hydraulic robots.

Control Eng. Pract. 85(2019), 176–193 (n.d.)

19. B. Yao, L. Xu, et al., Adaptive robust control of linear motors for precision manufacturing, in IFAC Proceedings Volumes (1999)

20. W.H. Zhu, Virtual decomposition control. Springer Tracts Adv. Robot. 60 (2012)

21. C. Li, L. Lyu, B. Helian et al., Precision motion control of an independent metering hydraulic system with nonlinear flow modeling and compensation. IEEE Trans. Ind. Electron. 69(7), 7088–7098 (2021)

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Hardware Layout of Independent Metering Control

2.1 Introduction

The Independent Metering Valve (IMV) is a general term referring to a valve architecture class that allows for individual control of the inlet and outlet flow of hydraulic actuator working ports, as well as the flexible flow path for multiple operating modes [1]. For mechanical decoupling of the inlet and outlet, there are various valve arrangements according to the utilized components and different layouts. There is no common consensus about which arrangement of IMV is the preferred one because each arrangement presents distinguished features and performance. This chapter presents the representative hardware configurations of IMV, including both the pilot and main stage, and analyzes their pros/cons. The two-stage structure is considered because it is always utilized to address the increased flow force under the large flow rate condition.

2.2 Valve Component

Proportional spool valve

Spool valves have been widely used as proportional directional valves in both the mobile and industrial hydraulic fields [2, 3]. The flow of spool valve can be regu- lated accurately because the opening area is a linear function of the spool displacement. However, complicated spool structures with small valve displacements increase manufacturing costs. Other disadvantages include large leakage and poor resistance to contamination.

Proportional poppet valve

Compared with the spool valve, the flow forces and fluid inertance effects of the poppet valve are more significant, and the relationship between the open area and

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10 2 Hardware Layout of Independent Metering Control

spool displacement is highly nonlinear [4, 5]. Traditionally, spool valves are suited to proportional control, and poppet valves are suited to on/off control. However, proportional poppet valves are still available, although the flow controlled by the poppet valve is less precise than a spool valve. In contrast, there are also many benefits with poppet valves compared to spool valves. Due to the seat structure, poppet valves have advantages including leakage-free and higher resistance to contaminants. Further- more, Poppet valves are produced with lower manufacturing costs. For example, they have less demanding manufacturing tolerances [6].

Digital valve

Both the proportional spool and poppet valves are controlled by an analogue method, which means that their spool displacements are continuous [7]. The digitally controlled valve distinguishes itself from them because the valve only has two states, i.e., on and off. The digital control manner includes Pulse Width Modulation (PWM), Pulse Number Modulation (PNM), pulse code modulation (PCM), etc. Numerous parallel connected on/off valves are used to form a DFCU (digital flow control unit), and a large number of unique flow rates to the chamber are achieved by opening a combination of valves. Therefore, the flow controlled by a DFCU is discrete [8, 9].

2.3 Main-Stage Layout

The mechanical decoupling principle is based on changing the main-stage valve types from the traditional 4/3-valve into the 2-way valve, which leads to different configurations of IMV [10]. The functional decoupling relies on the switching and proportional valves, where the functionality depends on the switching valve. According to the valve component, the main-stage arrangements can be mainly classified into three types: two 3-way spool valves, four 2-way poppet valves, or numerous digital valves [11]. Some classifications also contain several varieties, especially for two 3-way spool types. Each type of IMV has its distinct characteristics. Their pros and cons are compared in Table 2.1 referring to the commercial products in the state of the arts.

(1) Two 3-way spool valves

Combinations of two industrial directional valves or a developed twin 3-way valves both belong to such an arrangement. It is a natural approach deriving from the conventional 4/3-valve of spool type, which is similar to the conventional technique [12, 13]. The individually controlled metering edges are introduced by adding a control input of the spool, such that an additional state variable can be controlled. Since they are 3-way valves, they must be spool valves. This layout can not be constructed with 2-way poppet valves unless increasing the number of valve components. One of the representative commercial products is the CMA from Eaton [14, 15]. Two spools both connect the supply and the drain lines, and the only work port of the two spools connects two cylinder ports respectively, which is a standard layout using two

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Table 2.1 Main valve configuration of independent control type hydraulic valve Main-stage layout

Two 3-way spool valvesFour 2-way poppet valvesNumerous digital valves ProductsWessel PASDanfoss PVXEaton CMAHusco/Caterpillar Mechanical decoupling Recuperation /regeneration Float Integrated pressure compensator Low leakage Control accuracy Features anti-leakage sensitive to contamination

flexibility redundancy

flow accuracy manufacture cost

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3-way spools. This layout enables each cylinder port to connect to the pump or tank, but not to both simultaneously. Recuperation, regeneration, and float modes are all possible. Pros of the layout include that only two components are required at each cylinder to achieve separate metering control and free multi-operating mode. The spool valve technique is suitable for continuous and proportional metering control.

However, leakage is inevitable in the spool valve, and certain safety features can be hard to achieve without the check valve functionality. Another con is a difficult task to integrate a pressure compensator, because of the bidirectional flow across the spool under regeneration and float modes.

There are some variants in the two 3-way spool layout. Some traditional proportional multi-way valve also integrates the recuperation or regeneration flow path in the spool. These flow paths can be activated in a special spool position, which means the spool should be fabricated to a longer displacement. The representative commercial product utilized in his layout is the PVX multi-way valve from Danfoss [16]. The spool design in this layout usually complies with the machinery’s operating condition, losing certain flexibility compared to the standard two 3-way spool layout.

Another alternative of the two 3-way spool layout is utilized in PAS multi-way valve from Wessel [17]. Instead of connecting one of the cylinder ports to either pump or tank, one of the valves connects the pump/tank to one or the other cylinder port, then the system cannot connect the cylinder chambers. Consequently, its cons contain not only all the cons of two 3-way spool layouts but also the absence of free operating modes. A pro is introduced by this layout—a possibility to partly distribute the system, e.g., have the meter-out valve distributed and the meter-in valve not distributed.

(2) Four 2-way poppet valves

This layout employs poppet valves because 2/2-valves are used, such that the advantages of the poppet valve can be inherited, including excellent sealing capabilities, higher resistance to contaminants, and lower manufacturing costs. With four 2-way poppet valves, it is possible to connect one cylinder port to both the pump and tank simultaneously. Since the objective is often to reduce energy consumption, such a connection is not preferable. Owing to accuracy limits and/or dynamic limits, there may be situations where the undesirable flow will occur from the pump directly to the tank. Nevertheless, the possibility to connect each cylinder port to both pump and tank simultaneously makes this layout more flexible than the two 3-way spool layout, since the dynamics of the system can be changed with the use of this connection.

Therefore, the layout is a more flexible and redundant solution. What’s more, the integration of the pressure compensator can be achieved by a specially designed valve component, such as EHPV developed by Husco [18–20], and a Valvistor valve developed by Linkoping University [21]. However, more components are needed than two 3-way spool layout, and poppet valves can be difficult to control proportionally. The representative products with this layout can be seen in the INCOVA hydraulic control from Eaton [22] and Adaptive Control System (ACS) from Caterpillar [23].

Another four 2-way valve layout is a combination of the standard layout and an additional 2/2-valve that connects the cylinder chambers directly [24]. It has the same

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properties as the system described in the previous paragraph, with the extension of the additional properties of the system due to the extra valve which connects the load ports. It is an even more flexible solution than the standard four 2-way valve layout.

The extra valve can be used for example in regenerative and floating operations to reduce pressure losses. It is also conceivable to use the extra valve for dynamic improvements.

Although more controlled components in the two layouts introduce higher flexibility and redundancy of the system, the risk of failure also increases accompanied by the number of components (an extra bypass valve makes this even worse). The safety issue is an important consideration in mobile applications.

(3) Numerous digital valves

This layout is similar to the aforementioned one, but numerous digital valves are utilized as a poppet proportional valve. For example, Linjama from Tampereen University proposed a “Digital Flow Control Unit (DFCU)”, which consists of several parallel-connected on/off valves [25]. Opening different combinations of parallel valves is employed to control the flow charged into or discharged from the cylinder.

The on–off valves may be of the same flow capacity (pulse number modulation of bits with equal significance) or not (pulse code modulation of bits whose significance is set according to, e.g., binary series). Then, the flow rates can be achieved according to the number of on–off valves. A DFCU consisting of five valves is capable of producing unique flow rates (and zero flow). An increase in the number of bits results in greater resolution: in the case of ideal binary series, each bit doubles the resolution. Since the proportional control of the poppet valve is difficult, such a digital control method results in relatively good controllability for many applications. An IMV arranged by four or five DFCUs involves three main positive features:

deterministic operation, fast response, and fault tolerance.

(4) Other layouts using a combination of different valve types

The aforementioned three layouts always utilized a single valve type, such as proportional spool valves, proportional poppet valves, or on–off switching valves. One of the biggest obstacles to applying independent metering control techniques in mobile machinery is the increased costs. The costs consist of not only the hardware, including more components and advanced sensors but also the sophisticated software control.

For example, four metering edges are arranged in the system, and energy-saving regeneration/float modes mean that both two directional flows are required. Because the hydro-mechanical pressure compensator is difficult to integrate, an electrohydraulic pressure compensation must be designed based on the calibration of orifice flow characteristics, which requires sophisticated software control. There are several measures to overcome this challenge by a combination of different valve types.

a. Reducing the number of high-tech components

Generally, four metering edges are mounted because each actuator port may be charged or discharged by the supply and drain line, respectively. However, only two metering edges are responsible for a certain operation mode, and others are

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Fig. 2.1 Examples of incorporating directional on/

off valves into IMV to reduce the number of proportional valves

prepared for the next modes when the flow paths are changed. Due to the high costs of proportionally controlled units, As depicted in Fig. 2.1, some researchers proposed novel valve arrangements consisting of only two 2-way proportional valves combined with directional on/off valves [26, 27].

Another feasible solution is to replace all the proportional controlled valves in the IMV with digital control valves, as mentioned in Sect. 2.2. The costs of switching valves are much lower than proportional valves.

Plockinger et al. adopt four fast-switching valves with the PWM control method [28]. Linjama and his co-worker developed a digitally controlled valve, which was a group of parallel-connected switching valves controlled by the PCM [29, 30].

b. Eliminating requirements for complex control strategy

As depicted in Fig. 2.2, the utilization of individual PCs can reduce the complexity of the control strategy considerably by controlling a constant flow with a hydro- mechanical method. This approach requires a specifically designed bidirectional valve integrated with a PC or specific IMV structure. The decrease in electronic sensors also accompanies the simplification of the control strategy. Sitte and Weber proposed a layout in which two proportional valves are arranged with four on–off two- way directional valves and an individual pressure compensator is always configured in the inlet flow path, such that the reversing flow through the PC is avoided [27].

A novel control strategy using only one pressure sensor in the common supply pipe and spool stroke sensors at the individual PC [31]. The strategy uses the PC as a sensor for detecting the load situation and does not need any other pressure sensors, as shown in Fig. 2.2. Both the control software and sensor hardware are simplified.

c. Simplifying the hardware layout

Although the uses of proportional valves can be decreased, in the aforementioned studies, the circuit still includes both meter-in and meter-out edges. An interesting question is whether or not both these edges are necessary. Inspired by the findings of meter-out control, Vukovic and Murrenhoff further simplified the IMV system by utilizing a single-edge meter-out control circuit, in which the meter-in valve is no longer included [32], as shown in Fig. 2.3. Compared with the classic IMV arrangement with four proportional valves, at present, only one proportional valve

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y1,ref

y u

y₂,_ref

Fig. 2.2 Control schematic diagram of IMV using the IPC as a sensor

Fig. 2.3 Single-edge meter-out control valve

and three switching valves are needed. Another advantage is the ability for simple integration of the pressure compensator.

2.4 Pilot Stage Configuration

In the above section, only main-stage layouts of IMV have been discussed. The pilot stage associated with the valve arrangement of a specific topology is another issue.

Generally, the main-stage valve is directly actuated by an electro-mechanical actuator or indirectly actuated by a pilot pressure. Preferably a directly actuated way

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can improve dynamical properties. However, current state-of-the-art electromechan- ical actuators do not provide enough force for handling the flow forces in spool valves, unless increasing electrical power and the cost of power electronics. This issue becomes significant for the large flow condition. The flow force compensation can be used with a specially designed sleeve, which is too expensive and less compact for the mobile application. Therefore, a main stage actuated via a pilot stage is necessary, especially for large flow rate valves.

The primary requirement of the pilot stage to actuate the independent metering valve is a fast response. In the conventional proportional multi-way valve, the necessary pressure compensator is operated hydro-mechanically, making them very responsive with natural frequencies typically above 50 Hz [32]. The main stage should be actuated fast enough about the varied load pressure to reach similar response times using electro-hydraulic flow control. Second, the fail-safe state that all orifices of the main stage are closed must be obtainable if electrical power fails.

This gives one restriction on the pilot actuation circuit that it must balance the pressures in pilot chambers facing the ends of the main stage if power fails. Four pilot actuation schemes whereby this is obtained are sketched. The 2-way valves of the three upper circuits are on–off valves but continuously controllable ones are also possible. As shown in other combinations of fixed and variable orifices are possible, but not all of these fulfill the fail-safe condition. Besides, the control performance, such as flexibility, accuracy, etc., is also to be taken into account for the precise adjustment of the main valve opening.

A proportional directional spool has been employed in Eaton CMA [14, 15], in which the voice coil is used as the electro-mechanical actuator to improve the dynamic properties. The control of the two pilot pressure is coupled by a four-way spool, which provides precise displacement control and a sufficient fail-safe state of the main stage. However, a large dead zone and mass decrease the response time. Based on this pilot layout, a decoupled pilot layout is proposed by Zhejiang

Table 2.2 The main configuration of the pilot drive of the high-flow hydraulic valve

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University, in which a 4-way 3-position proportional spool is substituted by two independent 3-way 2-position spools [18]. This coupling contributes to a smaller dead zone and mass, such that a 20–40% increase in the frequency response can be obtained. The two proportional spools also can be replaced by a high-speed digital switching valve, which utilizes a PWM control method. Furthermore, the two 3-way 2-position spool valves can be decoupled to four 2-way poppet valves, which is the pilot layout of the Danfoss PVG multi-way valve. The comparison of different pilot layouts is depicted in the following Table 2.2 [16].

2.5 Conclusions

This chapter started by presenting the different valve components utilized in the IMV.

Proportional spool and poppet valves are mainly utilized in the state of the arts, while the digital on–off switching valve become more and more popular due to its distinct advantages. According to the three valve types, the standard hardware layouts of the main stage, as well as their variants, are all discussed in detail. The combination of the three valve types and other switching valves also attracts more attention to address the challenges of each standard layout, such as the difficult integration of pressure compensator, expensive cost, bidirectional flow, etc. In addition to the main stage, the possible layouts of the pilot stage are also analyzed.

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Chapter 3

Multi-Mode Load Control

3.1 Introduction

Using the IMCS, the increased control DOFs in the hardware introduce many advantages including energy saving, functionality extension, etc. To make full of these advantages, more complex and intelligent control software attracts more attention than the hardware design. The improvements of the control software reside in three aspects, as shown in Fig. 3.1 [1]:

(1) the control architecture is extended from the conventional one level to two or three levels by the separated meter-in and meter-out valves, as well as the electronically controlled pump;

(2) the operating modes for each level are exploited from unicity to multiplicity regarding different load characteristics;

(3) at least two control variables can be steered to achieve the first objective of actuator motion and simultaneously another secondary objective [2, 3].

In the electro-hydraulic control system, the upper level is always named the load control to regulate the flow paths (hydraulic circuits) [3, 4]. In the traditional electro- hydraulic system, the flow paths of the actuator chambers are not individual due to the mechanical decoupling between the inlet and outlet. It means that the flow can only be charged into one chamber and discharged out from another chamber according to the direction of the velocity, which is referred to as “Normal mode” (Nor. mode). With the independent metering control, the flow paths of the actuator chambers become individual to perform more operating modes, such as regeneration and recuperation [5]. The upper level controls a group of flexible switching of flow paths according to the load condition. Therefore, it is also referred to as mode switching.

Functions of the upper load control level first include the selection of high energy- efficiency operating, and then control of the transfer from one mode to another.

The former depends on the mode-switching logic, while the latter depends on the mode transition control. The operating mode decides the distribution of independent

21

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Fig. 3.1 Architecture of independent metering control system

metering valves to construct an energy-saving g hydraulic circuit and enable the controllability for precise motion tracking at the same time. Therefore, the upper level plays the most important role in the IMCS.

3.2 Multiple Operating Modes for the Actuator

The load conditions of the hydraulic actuator (cylinder or motor) are described by the load quadrants. As shown in Fig. 3.2, four quadrants are distinguished by the combinations of load force directions and actuator velocity. The same directions of the force and velocity vectors define an overrunning load quadrant; otherwise is a resistive load quadrant [6, 7].

For the aforementioned load quadrants, the operation modes of the hydraulic actuator can be divided into four types in terms of flow paths, as shown in Fig. 3.3:

Normal mode (Nor.): The flow is supplied by the pump, the inlet of the actuator is connected to the pump, and the meter-out chamber is connected to the drain line. This mode is the only one that can be realized by the traditional single-spool control valve.

It is suitable for both resistive and overrunning loads among four load quadrants, with the high-pressure or low-pressure oils supplied by the pump respectively.

Regeneration mode (Reg.): Two chambers are connected to the pump simultaneously, and the flows are regenerated fro