Research Papers

On Fluid Compressibility in Switch-Mode Hydraulic Circuits—Part I: Modeling and Analysis

[+] Author and Article Information
James D. Van de Ven

Department of Mechanical Engineering,
Worcester Polytechnic Institute,
100 Institute Road,
Worcester, MA 01609

Four-quadrant control is defined as sinking and sourcing power with reversing flow direction, allowing a hydraulic unit to act as a pump and a motor in both rotational directions.

1Present address: University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455, e-mail: vandeven@umn.edu.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received June 7, 2011; final manuscript received October 9, 2012; published online December 20, 2012. Assoc. Editor: Robert Landers.

J. Dyn. Sys., Meas., Control 135(2), 021013 (Dec 20, 2012) (13 pages) Paper No: DS-11-1177; doi: 10.1115/1.4023062 History: Received June 07, 2011; Revised October 09, 2012

Fluid compressibility has a major influence on the efficiency of switch-mode hydraulic circuits due to the release of energy stored in fluid compression during each switching cycle and the increased flow rate through the high-speed valve during transition events. Multiple models existing in the literature for fluid bulk modulus, the inverse of the compressibility, are reviewed and compared with regards to their applicability to a switch-mode circuit. In this work, a computational model is constructed of the primary energy losses in a generic switch-mode hydraulic circuit with emphasis on losses created by fluid compressibility. The model is used in a computational experiment where the system pressure, switched volume, and fraction of air entrained in the hydraulic fluid are varied through multiple levels. The computational experiments resulted in switch-mode circuit volumetric efficiencies that ranged from 51% to 95%. The dominant energy loss is due to throttling through the ports of the high-speed valve during valve transition events. The throttling losses increase with the fraction of entrained air and the volume of fluid experiencing pressure fluctuations, with a smaller overall influence seen as a result of the system pressure. The results of the computational experiment indicate that to achieve high efficiency in switch-mode hydraulic circuits, it is critical to minimize both the entrained air in the hydraulic fluid and the fluid volume between the high-speed valve and the pump, motor, or actuator. These computational results are compared with experimental results in Part II of this two part paper series.

Copyright © 2013 by ASME
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Fig. 1

Some configurations of switch-mode hydraulic circuits. Circuit (a) is a unidirectional virtually variable displacement pump. Circuit (b) is a unidirectional virtually variable displacement motor. Circuit (c) is a bidirectional virtually variable displacement pump/motor using a 3-way valve and a direction reverse valve that is only used to change the torque direction. The bold fluid paths are defined as the switched volume.

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Fig. 2

Bulk modulus versus pressure with 2% entrained air by volume at atmospheric pressure using the various models found in the literature. Note the Yu et al. model is plotted twice with different values for the air bubble variation coefficient, c1, to demonstrate the impact.

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Fig. 3

Geometry of the port areas created by circular orifices that are partially obstructed by the moving spool, represented as a moving plane. Note, the positive direction of lT and lP is toward the open area of the moving plane.

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Fig. 4

On–off valve command, spool displacement, and orifice areas as a function of time during two switching periods with a duty cycle of 60%

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Fig. 5

Pressure in the switched volume and flow rate and power loss through the on–off valve and the check valve as a function of time during two switching periods at a duty cycle of 60%, a system pressure of 20.7 MPa, a switched volume of 10 cm3, and air entrainment of 1%

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Fig. 6

Influence of the entrained air, system pressure, and switched volume on the three forms of throttling energy loss. To scale the results for comparison purposes, the y-axes are the throttling energy divided by the energy reaching the hydraulic motor.

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Fig. 7

Volumetric efficiency as a function of entrained air, switched volume, and system pressure

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Fig. 8

Pressure in the switched volume, flow rates through the three ports into the switched volume, and corresponding throttling energy losses for a duty cycle of 60%, system pressure of 20.7 MPa, a switched volume of 40 cm3, and air entrainment of 10%

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Fig. 9

Volumetric efficiency as a function of entrained air utilizing different bulk modulus models with a switched volume of 10 cm3, an accumulator pressure of 20.7 MPa, and a duty cycle of 60%

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Fig. 10

Volumetric efficiency of the switch-mode valve circuit as a function of duty cycle and bulk modulus model with a switched volume of 10 cm3, an accumulator pressure of 20.7 MPa, and air entrainment of 1%




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