Pump-controlled hydraulic cylinder drives may offer improved energy efficiency, compactness, and plug-and-play installation compared to conventional valve-controlled hydraulic systems and thus have the potential of replacing conventional hydraulic systems as well as electro-mechanical alternatives. Since the late 1980s, research into how to configure the hydraulic circuit of pump-controlled cylinder drives has been ongoing, especially in terms of compensating the uneven flow requirements required by a differential cylinder. Recently, research has also focused on other aspects such as replacing a vented oil tank with a small-volume pressurized accumulator including the consequences of this in terms of thermal behavior. Numerous references describe the advantages and shortcomings of pump-controlled cylinder drives compared to conventional hydraulic systems or electro-mechanical drives. This paper presents a throughout literature review starting from the earliest concepts based on variable-displacement hydraulic pumps and vented reservoirs to newer concepts based on variable-speed electric drives and sealed reservoirs. By classifying these drives into several proposed classes it is found that the architectures considered in the literature reduce to a few basic layouts. Finally, the paper compares the advantages and shortcomings of each drive class and seek to predict future research tasks related to pump-controlled cylinder drives.

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Classification and Review of Pump-Controlled Differential Cylinder Drives

With the rapid development of the global economy, more and more attention has been paid to the energy conservation of construction machinery. The hydraulic system is the key component of construction machinery, and improving its energy utilization rate has become an important means to achieve energy conservation. In conventional valve-controlled or pump-controlled hydraulic systems of construction machinery, controllability and energy-saving performance typically cannot be considered at the same time. The pump-valve coordinated system combines the energy-saving characteristics of the pump-controlled system and the high-precision and high-frequency response of the valve-controlled system, which has the potential to become a primary research direction of electro-hydraulic systems. This review summarizes the recent research progress in energy-saving technologies based on pump-valve coordinated systems. Particularly, we discuss the structures of hydraulic systems in different categories of construction machinery, various control methods of the electro-hydraulic system, novel hydraulic hybrid energy regeneration systems, and key components. In addition, future directions and challenges of the pump-valve coordinated systems are described, such as independent metering system (IMS), common pressure rail (CPR), and hybrid power source (HPS).

Energy-saving technologies for construction machinery: a review of electro-hydraulic pump-valve coordinated system

This paper is considering the application of a novel pump controlled cylinder drive, the so-called Speedvariable Switched Differential Pump (SvSDP), for knuckle boom crane actuation. Especially the control system for the SvSDP drive is considered, and aiming on improving energy efficiency a refinement of the existing control structure is proposed. An energy efficient sizing algorithm for the SvSDP drive is developed, and fundamental differences between the achievable operating range for the SvSDP drive compared to a conventional valve-cylinder drive are discussed. A case study is conducted with knuckle boom crane actuation, and compared to a conventional valve actuation. Simulation results show that the motion tracking performance is on a similar level compared to the valve actuation approach, while the energy consumption is drastically decreased. For the given test trajectory the valve actuation system consumes 0.79 kWh of electrical energy, while the SvSDP drive consume 0.06 kWh, if ideal energy recovery and storage is assumed.

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Energy Saving Potential in Knuckle Boom Cranes using a Novel Pump Controlled Cylinder Drive

This article presents a force control solution for a throttle-free multi-pressure hydraulic cylinder actuator. A model-based force controller was developed and the position and velocity tracking co...

Model-based force and position tracking control of a multi-pressure hydraulic cylinder:

For Discrete Displacement Cylinder (DDC) drives the control task lies in choosing force level. Hence, which force level to apply and thereby which pressure level each cylinder chambers shall be connected to. The DDC system is inherently a force system why often a force reference is generated by a tracking controller and translated into a discrete force level in a Force Shifting Algorithm (FSA). In the current paper the tracking controller and the FSA are combined in a Model Predictive Control algorithm solving the tracking problem while minimizing the energy use. Two MPC algorithms are investigated and compared to a PID like tracking controller combined with a FSA. The results indicate that the energy efficiency of position tracking DDC systems may be improved significantly by using the MPC algorithm.

Energy Optimal Tracking Control with Discrete Fluid Power Systems using Model Predictive Control

The decentralisation of hydraulic systems is a recent trend in industrial hydraulics. Speed variable drive is one concept where an actuator is driven by an integrated pump, thus removing the need for control valves or complex centralised variable displacement hydraulic units and long pipelines. The motivation for the development is the need to improve the energy efficiency and flexibility of drives. A similar solution to mobile hydraulics is not currently available. This paper studies a digital hydraulic approach, which includes a local hydraulic energy storage located together with the actuator, the means to convert efficiently energy from the storage to mechanical work and a small start-/stop-type pump unit sized according to mean power. The simulation results and first experimental results show that the approach has remarkable energy saving potential compared to traditional valve controlled systems, but further research is needed to improve the controllability.

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Digital hydraulic multi-pressure actuator – the concept, simulation study and first experimental results

A digital hydraulic multi-pressure actuator is a new actuator concept, which aims at lowering energy losses and decreasing dynamic requirements of a prime mover in mobile hydraulic applications. The actuator consists of an integrated hydraulic accumulator, which serves as an energy storage and a number of asymmetric cylinders acting as discrete pressure transformers. Leak-free on/off-valves are used to direct flow from the discrete pressure transformers to the actuator. Input power is supplied by charging the local accumulator with a small fixed displacement pump. Thus, the actuator requires only mean input power, while the output power peaks can be multifold. This paper concentrates on studying the controllability of the actuator concept and analyses the power losses and their sources through experimental study. The energy losses of the concept are measured in a mobile hydraulic boom mock-up and compared to earlier measured losses of a load sensing proportional valve based system. The measurements show that up to 77 % of the losses can be avoided by using the new concept. Three controller types are studied numerically and experimentally and their effect on control resolution and energy efficiency is evaluated.

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Arttu Aalto

Papers

Digital hydraulic multi-pressure actuator – the concept, simulation study and first experimental results

Study of Energy Losses in Digital Hydraulic Multi-Pressure Actuator