Switchmode Hydraulic Power Supply Theory

Switchmode hydraulic power supply is a new kind of energy-saving pressure converting system, which is originally proposed by the authors. It is mainly applied in multiple-actuator hydraulic systems, and installed between hydraulic pump and actuators (one switchmode hydraulic power supply for one actuator). It can provide pressure or flow rate that is adapted to the consumption of each actuator in the system by boosting or bucking the pressure, with low power loss, and conveniently, through high-speed switch valves, just like a hydraulic pressure transformer. There are two basic types of switchmode hydraulic power supply: pressure boost and pressure buck. Their structures and working principles are introduced. The dynamic characteristics of two typical types of switchmode hydraulic power supply, the pressure boost type and the pressure buck type, were analyzed through simulations and experiments. The performances were evaluated, and improvements on the efficiency of switchmode hydraulic power supply were proposed.

Electric Motors Coupled to Hydraulic Motors as Actuators for Hydraulic Hardware-in-the-Loop Simulation

Hardware-in-the-Loop (HIL) Simulation enables testing of an actual physical component of a system under a variety of conditions without the expense of full scale testing. In hydraulic systems, flows or pressures that interface with the component in question are controlled by a computer running a simulation designed to emulate a complete system under real operating conditions. Typically, servo valves are used as actuators to control the flows or pressures. This paper investigates the use of electric servo-motors coupled to hydraulic gear motors as alternative actuators, and discusses some of the advantages and disadvantages that motors have in comparison to valves. A demonstration HIL simulation involving a mobile proportional flow control valve attached to an emulated backhoe is described, and results are compared to data from a real backhoe.

Experimental and Numerical Investigation of Centrifugal Pump Performance When Handling Viscous Fluids

On the test bed of centrifugal pump, the centrifugal pump performance has been investigated using water and viscous oil as Newtonian fluids, whose kinematic viscosities are 1 × 10−6, 43 × 10−6 and 62 × 10−6 m2/s, respectively. Also, the finite volume method is used to model the three dimensional viscous fluids for different operating conditions. For these numerical simulations the SIMPLEC algorithm is used for solving governing equations of incompressible viscous/turbulent flows through the pump. The κ-ε turbulence model is adopted to describe the turbulent flow process. These simulations have been made with a steady calculation and using the multiple reference frame (MRF) technique to take into account the impeller-volute interaction. Numerical results are compared with the experimental characteristic curve for each viscous fluid. The data obtained allow the analysis of the main phenomena existent in this pump, such as: head, efficiency, power and pressure field changes for different operating conditions. Also, the correction factors for oils are obtained from the experimental for part loading (PL), best efficiency point (BEP) and over loading (OL) and the results are compared with proposed factors by American Hydraulic Institute (HIS) and Soviet Union (USSR). The comparisons between the numerical and experimental results show a good agreement.

Comparison of Hydraulic Pump Faults Diagnosis Methods: Wavelet vs. Spectral Analyses

This paper introduces two faults diagnosis methods, a conventional spectral analysis method and a wavelet transform method, for hydraulic pump applications. The fundamental technologies of both methods, as well as their performance in detecting a few common hydraulic pump defects, are described in this paper. The performance of both diagnoses methods were evaluated based on experimental results. In order to eliminate the effects of border distortion arising from applying wavelet transform to finite-length signals, the pump outlet pressure in this case, a preprocess on the obtained signals is carried to clean up the errors prior to faults diagnosis analysis. Validation results obtained from both methods in analyzing the same data sets indicated that the wavelet transform based method showed a more sensitive and robust detecting capability than that obtained from a spectrum analyses approach.

Selection of Operating Modes of a Multi-Functional Hydraulic Device

The field of earth moving equipment is experiencing a transformation due to the introduction of more electronic control capability and advanced control concepts. Conventional hydraulic control systems are controlled by proportional directional spool valve. The construction of the spool valve is such that a given position of the spool determines the flow in and the flow out restriction sizes. Thus, metering in and metering out are dependent or coupled. A certain restriction size on the inlet corresponds to a certain restriction size on the outlet. Therefore, we have one degree of freedom. It can provide for good motion control but it cannot achieve energy saving potential at the same time. In this paper, the concept of ‘independent meter in / meter out’ will be emphasized. Decoupling of meter in from meter out provides for more controllability and potential for energy saving in overrunning load cases when compared with a conventional spool valve controlled hydraulic system. A four-valve configuration controlling a hydraulic cylinder will be stressed. The four-valve configuration can operate in several modes because each of the four valves is controlled separately from the others. Five of these metering modes will be pointed out; two conventional modes and three regenerative modes. An industry system supplier has introduced a novel distributed independent metering in/meter out hydraulic system with actuator force feedback. This control system is going to presented in brief. The distributed, actuator mounted implementation of this system maximizes the efficiency of the regeneration flow by minimizing the line losses associated with the recirculating fluid This control system can be used to control a multi-functional hydraulic device, e.g. excavators, tractor loader backhoes, etc... An experiment was done where a typical dig cycle was performed by a tractor loader backhoe equipped with this new technology. The data recorded from this experiment is used to perform an off-line optimization of metering modes selection for each function during the cycle. this off-line optimization can be used to maximize machine productivity or machine efficiency or a weighted combination of both. A general objective function will be presented and results will be given for maximizing machine productivity. Finally conclusions and future work suggestions are presented.

Experimental Operation and Characterization of a Free Piston Compressor

The ongoing design evolution of a free piston compressor (FPC) is presented in this paper. The FPC is a proposed device that utilizes combustion of a hydrocarbon fuel to compress air into a high-pressure supply tank. This device is designed to extract chemically stored energy from the fuel and convert it to potential energy of compressed air, while achieving high conversion efficiency relative to other small-scale portable power supply systems. The chemically stored energy of the hydrocarbon fuel is first converted into kinetic energy of the free piston by the end of the combustion phase. Subsequently, the moving piston acts as a pump and air compressor during a compression phase. The proposed technology is intended to provide a compact and efficient pneumatic power supply source appropriate for human-scale robots. The design and implementation of this version of the FPC is shown, and experimental results relating all phases (combustion, expansion and pumping) are discussed. The total efficiency of the system is experimentally measured and compared to its theoretical prediction.

Sliding Mode Controller and Filter Applied to a Model oft an Electrohydraulic Actuator System

A new robust state and parameter estimation strategy called the Variable Structure Filter (VSF) has recently been proposed and used for state and parameter estimation. A very common problem of linear stochastic systems is to design a combined robust control and estimation strategy, given system and noise uncertainties. Variable Structure Control (VSC) and its special form of Sliding Mode Control (SMC) show superb robustness. This paper proposes a new strategy involving the Sliding Mode Control and the Variable Structure Filter. Both the estimator and controller are based on the concepts of Variable Structure Systems (VSS). In the presence of bounded parametric uncertainties and noise, a robust stability is guaranteed. Further more, the combined strategy can be used to achieve high regulation rates or short settling times. The object of this paper is to introduce this combined VSF and SMC strategy and to demonstrate its application to a third order model of a high precision hydrostatic system, referred to as the Electrohydraulic Actuator System (EHA).

Experimental Simulation of Piston Leakage in an Axial Piston Pump

A method used to introduce “artificial leakage” into an axial piston pump to simulate leakage from a worn piston is described in this paper. A pressure control servo valve with a very high frequency response was employed to divert flow from the pump outlet in a prescribed waveform directly to tank. The purpose was to simulate piston leakage from the high pressure discharge chamber to the pump case drain chamber as the “simulated worn piston” made contact with the high pressure chamber. The system and associated control algorithms mimiced the action of a single worn piston at various degrees of wear. The experimental results indicated that the experimental system could successfully introduce artificial leakage into the pump which was consistent with a unit with a “real” worn piston. Comparisons of the pressure ripples from an actual faulty pump (with one worn piston) and the artificial faulty pump (with one simulated worn piston) are presented.

Torque Ripple Attenuation of an Axial Piston Pump by Continuous Swash Plate Adjustment

One of the major drawbacks of a hydraulic system is the disturbing noise generated by the hydraulic pump. Based on the accepted theory about noise generation in an axial piston pump, various studies suggesting modification of the port-plate relief groove geometry and addition of hydraulic attenuators have shown a limited success in reducing the noise. The noise level is still high and may not be acceptable for future applications. A recent industrial study shows that the noise apparently has relation with the torque acting on the input shaft of the axial piston pump. The primary objective of this paper is to describe a new method to reduce the noise level by varying the swash plate of the pump continuously to eliminate the torque ripple. The paper begins by deriving the equation of torque acting on the input shaft as a function of the average torque and the total number of pistons that are used within the rotating group. A control law is derived according to which if the swash plate is dithered, should give a constant shaft torque. By attenuating the torque ripple characteristics of the pump, other vibrational aspects of the pump are also expected to be reduced. The secondary objective of this paper is to present a Simulink® model of a nine piston pump to describe a functional pump. The swash plate control law earlier derived will be applied to this model to see the behavior of the output parameters like the shaft torque, output flow and pressure. The results will be investigated to see if any relationship exist between the pump torque ripple and pressure ripple, and the theory of pump noise attenuation by reduction in torque ripple will be corroborated.

Effect of Parameter Uncertainty on Reliability of Hydraulic Transmission System Simulation

Simulation of fluid power systems has become a tool widely used for designing and testing purposes. The usefulness of this approach is however highly dependent on the results that have uncertainty to some extent. In the theoretical approach uncertainty depends mainly on the simplifications made in the modeling, algorithms used and parameters. In the experimental approach uncertainty depends on the transducers and the construction of the measurement system and data transfer. This paper discusses the effect of parameter uncertainty on the reliability of virtual testing of hydrostatic transmission systems in general. The definition of the total uncertainty by using a sum of variances is presented and applied to basic equations of fluid power. As an application a simplified hydrostatic transmission circuit is investigated. Importance of grade of models in reducing the computational inaccuracy is discussed.