Per Johansen

Optimum Design of a Moving Coil Actuator for Fast-Switching Valves in Digital Hydraulic Pumps and Motors

Fast-switching seat valves suitable for digital hydraulic pumps and motors utilize direct electromagnetic actuators, which must exhibit superior transient performance to allow efficient operation of the fluid power pump/motor. A moving coil actuator resulting in a minimum valve switching time is designed for such valves using transient finite-element analysis of the electromagnetic circuit. The valve dynamics are coupled to the fluid restrictive forces, which significantly influence the effective actuator force. Fluid forces are modeled based on transient computational fluid dynamics models. The electromagnetic finite-element model is verified against experimental measurement, and used to design an optimum moving coil actuator for the application considering different voltage-current ratios of the power supply. Results show that the optimum design depends on the supply voltage-current ratio, however, the minimum switching time obtained is nearly independent on this voltage-current ratio. Selecting a suitable power supply based on thermal considerations yields a switching time just above one millisecond for a travel length of 3.5 mm while submerged in oil. The proposed valve has a pressure drop below 0.5 bar at 600 L/min flow rate, enabling efficient operation of digital hydraulic pumps and motors.

Optimum design of seat region in valves suitable for digital displacement machines

Digital displacement fluid power is an upcoming technology setting new standards for the achievable efficiency in variable displacement fluid power pumps and motors. In the present work, an annular seat valve suitable for use in digital displacement units is considered, and the valve geometry is optimised considering both the mechanical strength during pressure loading and fluid flow restriction in the open valve state. Material stresses are modelled using finite element (FE) analysis including non-linear material behaviour, contact elements and fluid pressure penetrating load, closely reflecting the actual load of the seat valve connected to a fluid pressure chamber. Valve pressure losses are modelled using computational fluid dynamics (CFD). On basis of an overall physical size requirement and material specification, optimum valve geometry and stroke length are given as function of a defined normalised flow coefficient directly related to the machine efficiency.

Optimization of geometry of annular seat valves suitable for Digital Displacement fluid power pumps/motors

Digital Displacement Fluid Power is an upcoming technology setting new standards for the achievable efficiency of fluid power pumps and motors. The core element of the Digital Displacement technology is high performance electronically controlled seat valves, which must exhibit very low flow pressure loss and switching times within a few milliseconds to enable high efficiency operation. These valves are mechatronic components and special attention to both the mechanical, electromagnetic, fluid dynamical and control system design must be paid to ensure the needed performance. In the present work an annular seat valve suitable for use in Digital Displacement units is considered, and the ring geometry is optimized using finite element analysis including non-linear material behaviour, contact elements and fluid pressure penetrating load, closely reflecting the actual load of the seat valve connected to a fluid pressure chamber. The search for optimal design points is conducted using a brute force strategy with subsequent selection of the dominating design points.

Modeling of movement-induced and flow-induced fluid forces in fast switching valves

Fast switching fluid power valves set strict requirements on performance, size and energy efficiency and simulation models are therefore needed to obtain good designs of such components. The valve moving member is subject to fluid forces depending on the valve flow rate and movement of the valve member itself. These fluid forces may be accurately simulated using Computational Fluid Dynamics (CFD) analysis, but such models suffer from being computationally expensive and is not suited for optimization routines. In this paper, a computationally inexpensive method for modeling the fluid forces is proposed, which includes both the flow-induced fluid forces and the movement-induced fluid forces resulting from movement of the valve moving member. The movement-induced fluid force model is based on a known solution to the linearized Navier-Stokes equations. A method for accurately simulating the flow-pressure relationship of a switching valve based on CFD results is presented along with the fluid force model, to constitute a complete valve fluid model. The parameters needed for the proposed model are determined based on CFD analyses, and the process of finding these parameters are described based on a reference valve design. Simulated results of the total fluid force are presented showing the movement-induced fluid force to be significant for a reference application. The model form established is useful for valve designers during development and for accurate operation simulation.

Design Method for Fast Switching Seat Valves for Digital Displacement Machines

Digital Displacement® (DD) machines are upcoming technology where the displacement of each pressure chamber is controlled electronically by use of two fast switching seat valves. The effective displacement and operation type (pumping/motoring) may be controlled by manipulating the seat valves corresponding to the piston movement, which has been shown to facilitate superior part load efficiency combined with high bandwidth compared to traditional displacement machines. However, DD machines need fast switching on-off valves with low pressure loss for efficient operation, especially in fast rotating operation, where switching times must be performed within a few milliseconds. These valve requirements make a simulation based design approach essential, where mechanical strength, thermal dissipation, fluid dynamics and electro-magnetic dynamics must be taken into account. In this paper a complete design method for DD seat valves are presented, taking into account the significant aspects related to obtaining efficient DD valves with basis in a given DD machine specifications. The seat area is minimized and the stroke length is minimized to obtain fast switching times while considering the pressure loss of the valves. A coupled optimization is finally conducted to optimize the electro-magnetic actuator, leading to a valve design based on the chosen valve topology. The design method is applied to an example DD machine and the resulting valve design fulfilling the requirements is presented.Copyright

On the application of reynolds theory to thermo-piezo-viscous lubrication in oil hydraulics

The efficiency of fluid power motors and pumps is a subject to research, which has generated numerous publications during the last three decades. The main incentives for this research are optimization of reliability and efficiency through the study of loss and wear mechanisms, which are very difficult to study experimentally, whereby modeling and simulation are necessary. A common approach to theoretical investigation of the pressure generated in the lubricated joints is the use of Reynolds equation, in which the oil viscosity is modelled with dependency of both pressure and temperature. In this paper the derivation of Reynolds equation from the continuum assumption is reviewed and it is shown that the validity of Reynolds theory based pressure field solutions in oil hydraulic thermo-piezo-viscous lubrication models are subject to maximum bounds on the pressure and temperature field gradients. These bound must be evaluated a posteriori to validate that model results is complying with the principle of conservation of mass and momentum.

Modeling of Dynamic Fluid Forces in Fast Switching Valves

Switching valves experience opposing fluid forces due to movement of the moving member itself, as the surrounding fluid volume must move to accommodate the movement. This movement-induced fluid force may be divided into three main components; the added mass term, the viscous term and the so-called history term. For general valve geometries there are no simple solution to either of these terms. During development and design of such switching valves, it is therefore, common practice to use simple models to describe the opposing fluid forces, neglecting all but the viscous term which is determined based on shearing areas and venting channels. For fast acting valves the opposing fluid force may retard the valve performance significantly, if appropriate measures are not taken during the valve design. Unsteady Computational Fluid Dynamics (CFD) simulations are available to simulate the total fluid force, but these models are computationally expensive and are not suitable for evaluating large numbers of different operation conditions or even design optimization. In the present paper, an effort is done to describe these fluid forces and their origin. An example of the total opposing fluid force is given using an analytically solvable example, showing the explicit form of the force terms and highlighting the significance of the added mass and history term in certain fast switching valve applications. A general approximate model for arbitrary valve geometries is then proposed with offset in the analytic model terms. The coefficients in this general model are determined based on CFD analyses, which are evaluated throughout the movement range of the moving member on an example valve geometry. The proposed model is compared to complete unsteady CFD simulations and found to generally predict the opposing fluid force well and gives accurate predictions under certain conditions. The proposed model is suitable for valve designers who need a computationally inexpensive fluid force model suitable for optimization routines or efficient dynamic models.Copyright

Oil Stiction in Fast Switching Annular Seat Valves for Digital Displacement Fluid Power Machines

Digital Displacement (DD) fluid power machines utilizes electronically controlled seat valves connected to pressure chambers to obtain variable displacement with high operational efficiency and high bandwidth. To achieve high efficiency, fast valve switching is essential and all aspects related to the dynamic behaviour of the seat valves must be considered to optimize the machine efficiency. A significant effect influencing the valves switching performance is the presence of oil stiction when separating the contact surfaces in valve opening movement. This oil stiction force is limited by cavitation for low pressure levels, e.g. valves connected to the low pressure manifold, however for valves operated at higher pressure levels, the oil stiction force is dominating when the separating surfaces are close to contact.This paper presents an analytic solution to the oil stiction force for annular seat valves suitable for DD applications based on the Reynolds equation and considers contact surface curvature and attack angle. A dynamic cavitation zone is included in the stiction model, and cavitation is found to be present even for seat valves surrounded by high pressure levels.Copyright

Topology selection and analysis of actuator for seat valves suitable for use in Digital Displacement pumps/motors

Digital Displacement (DD) Fluid Power is an upcoming technology setting new standards for the achievable efficiency of fluid power pumps and motors. To enable high efficiency operation fast switching electronically controlled seat valves are needed, preferably exhibiting very low flow pressure loss and switching times within a few milliseconds. These valves are mechatronic components and attention to both the electromagnetic, fluid dynamical, mechanical and control system design must be paid to ensure the needed performance. In the present work, a quick response linear electro-magnetic actuator for DD seat valves is developed, and the resulting dynamic response of the seat valve is presented. Requirements for the valve actuator is established based on the DD application, and three feasible actuator topologies are analyzed by means of transient electro-magnetic FEA simulation. From this analysis a moving coil actuator topology is selected for the DD valve. A lumped parameter model including magnetic diffusion effects of this chosen actuator is developed, which is included in the dynamic valve simulation. Result show a switching time below 2 ms and a pressure loss below 0.5 bar at 500 litres/minute for the proposed DD seat valve.

Simulation of dynamic behaviour of a digital displacement motor using transient 3d computational fluid dynamics analysis

A fast rotating 1500 rpm radial piston digital displacement motor connected to a 350 bar high pressure manifold is simulated by means of transient 3D CFD analysis of a single pressure chamber. The analysis includes dynamic piston and valve movement, influencing the boundaries of the fluid domain. Movement of the low and high pressure valves is coupled to fluid forces and valve actuation is included to control the valve movement according to the pressure cycle of the digital displacement motor. The fluid domain is meshed using a structured/unstructured non-conformal mesh, which is updated throughout the simulation using layering zones as required by the moving fluid boundaries. The effect of cavitation at low pressures is included by implementing a pressure dependent density, based on an effective bulk modulus model. In addition, pressure dependent oil viscosity is included in the analysis. As a result of the CFD analysis, the dynamic response of the pressure chamber and valve movement of a digital displacement motor is presented, along with the total efficiency of a multi-chamber motor at full and partial displacement. Simulation predicts that pressure over- and undershoot may be reduced to a few percent of the high pressure manifold level by precise timing and fast switching of the seat valves.Copyright