A Gasless Reservoir Solution for Electro-Hydraulic Compact Drives With Two Prime Movers

Abstract

Due to an increased focus on improving the energy efficiency and compactness of hydraulic linear actuators, the electro-hydraulic compact drive (ECD) has received increased attention lately. In this study the ECD consists of variable-speed electric motors and fixed-displacement pumps, which are directly connected to the cylinder, thus controlling the linear motion in a throttleless manner. Furthermore, ECDs are self-contained systems, i.e. based on a fully enclosed oil circuit, in order to avoid external contamination and air to enter the system and to increase system compactness. Conventionally a low-pressure gas-loaded accumulator is used as an oil reservoir to compensate for the flow imbalance occurring whenever utilizing single-rod cylinders in closed systems. The accumulator pressure is to be kept relatively low to stay within the required limits governed by the permitted pump housing’s pressure. Generally, this pressure is not allowed to exceed 1–3 bar. To avoid violating this limitation, the gas volume must be significantly larger than the actual oil volume, which needs to be stored in the accumulator. This requirement decreases the obtainable compactness of the ECD, especially for systems with a large cylinder stroke. Furthermore, the accumulator represents a potential of gas leakage, which ultimately could result in the ECD being non-functional. This paper presents a gasless reservoir solution, improving the system compactness and avoiding the risk of gas leakage. The proposed solution is based on a bootstrap reservoir which is charged by the lowest cylinder chamber pressure. This strategy is feasible for the class of ECDs that is capable of controlling the lowest cylinder chamber pressure alongside the cylinder motion. An ECD consisting of two electric prime movers is considered as a case study. It is shown how the gasless reservoir may be integrated into the system, and an analysis of how this affects the operating range and the dynamic couplings of the system is presented. This leads to the derivation of a control strategy for the Multi-Input-Multi-Output (MIMO) system based on state decoupling, by defining virtual inputs to control virtual outputs. A numerical study suggests that the reservoir volume may be reduced by approximately 50% for the given system dimensions. The proposed control strategy shows good position tracking performance while also being able to control the reservoir pressure within the pre-defined limits of 1 to 3 bar.

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

“…In [2] it was found that the system as presented in the previous section is highly coupled. In order to decouple the motion of the cylinder from the back pressure a number of virtual inputs and outputs need to be designed.…”

“…This parameter is assumed to be known. A study showing how changes in the parameter will affect stability can be seen in [2] and…”

“…This was done in order to improve the stiffness of the system. In [2] the aim of P B is twofold. On one hand the chamber pressure is kept constant improving system stiffness, but also the reservoir pressure P R is controlled in order to prevent cavitation or damage to the pumps' seals.…”

“…Pump-driven cylinder drives have been proposed for knuckle boom cranes in [1]. Furthermore, self-contained pump-driven cylinder drives were proposed in [2]. The system offers energy efficiency improvements over the standard hydraulic system and also offers other benefits such as plug-and-play capabilities i.e.…”

“…A method to address this was proposed in paper [5], but it was only tested for open loop control. In this paper the controller methods from [4] and [6] will be combined to facility faster locking and unlocking of the TEMPO drive proposed in [2]. The error during locking is addressed before the valves fully lock through manipulation of the references in the control structure.…”