A Self-Contained Cylinder Drive with Indirectly Controlled Hydraulic Lock

Abstract

This paper presents a self-contained pump-controlled hydraulic linear drive including an innovative load holding sub-circuit. For safety critical applications such as crane manipulators, locking valves or load holding valves are enforced by legislation, but the load holding functionality may also be used actively to decrease the energy consumption for applications where the load is kept stationary for longer periods of time. The system proposed in this paper is based on a simple hydraulic architecture using two variablespeed electric motors each connected to a fixed-displacement pump. This architecture is well-known in academic literature, but in this paper a novel load holding sub-circuit has been included. To control this load holding functionality, the low chamber pressure needs to be controlled accurately, while still being able to control the motion of the cylinder piston as well. Due to strong cross-couplings between cylinder piston motion and chamber pressures this task is non-trivial. The control for opening the locking valves is indirect in the sense that it is controlled via the chamber pressures, which are actively controlled. The fundamental control strategy presented in this paper is based on transforming the highly coupled physical states to virtual states, significantly reducing cross-couplings. As a case study, a two link medium sized knuckle boom crane is selected as an application example. Simulation results confirm the applicability of the proposed system. Appropriate position tracking performance has been achieved for the considered motion trajectory, while the low chamber pressure is controlled in a satisfying manner. A smooth transition from motion operating mode to load holding mode is achieved, with the system not requiring any input energy to keep the load stationary when the hydraulic cylinder lock is engaged.

Novel Concept for Electro-Hydrostatic Actuators for Motion Control of Hydraulic Manipulators

“…The motor is rated thermal class F which allows a temperature rise of 105 • C above ambient. The calculations are shown in Equations ( 18)- (20). Figure 20 shows the allowable continuous current to stay below 105 • C as a function of the motor speed ω.…”

“…A study of a differential cylinder connected to two pumps is conducted in [18], studying the effect of pump displacement ratio and cylinder area ratio on energy recovery. Other topics of pump-controlled cylinders and EHAs include: load-holding topolgies [19,20], analysis of EHAs [21][22][23], energy regeneration in EHAs [24], optimization of EHAs [25], and EHAs for more-electric aircraft [26].…”

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

“…If used in pump-driven cylinder drives the CBV can prevent the recovery of energy [3]. A different system for load holding with TEMPO drives was explored in [4]. The feature allows load holding valves to lock the movement of the cylinder in case of a loss in pressure.…”

“…The feature allows load holding valves to lock the movement of the cylinder in case of a loss in pressure. The authors of [4] focused on the safety of the system and not on making the locking and unlocking process fast. The load holding valves can also be engaged in order to reduce energy losses during standstill, but in order for this to be done it is desired that the locking process happens as fast as possible.…”

“…If the locking and unlocking happens sufficiently fast, then it can happen automatically without impeding the work flow of the operator which normally drives such a crane. In order to lock the load holding valves, the pressure in one chamber needs to be reduced rapidly which can cause a deviation in the cylinder's position [4]. A method to address this was proposed in paper [5], but it was only tested for open loop control.…”