Christoph Gradl

An Oil Stiction Model for Flat Armature Solenoid Switching Valves

Oil stiction forces significantly influence the performance of fast switching valves. These forces stem from the significant lowering of the pressures between two oil filled plates relative to the surrounding pressure when the plates are quickly separated. If the pressure in the gap stays above the vapor pressure the stiction force can be derived from a solution of the Reynolds equation.However, for very fast motions — as occur in fast switching valves with a flat armature solenoid — cavitation is most likely to occur. The cavitation zone starts in central parts of the gap and extends as long as the gap volume increase cannot be fully compensated by the flow in the gap. Cavitation reduces the stiction force significantly. In many valves this stiction force reduction is decisive for a proper functioning of the valve.An important measure for stiction force control are flushing channels, in particular flushing bores. In this paper analytical models and Finite Volume method models are used to study the stiction force problems with and without cavitation and design measures for their mastering.Copyright

A Basic Study on the Response Dynamics of Pulse-Frequency Controlled Digital Hydraulic Drives

Various control strategies in digital hydraulics have been proposed and studied so far. In hydraulic switching control Pulse Width Modulation (PWM) of one or two switching valves was mostly considered. This paper deals with Pulse Frequency Control (PFC) which — opposite to PWM — uses the pulse repeating frequency and not the pulse width as control input. PFC may be to be preferred if the hydraulic switching device can realize a very particular pulse in a quite favorable way.This paper studies the influences of the flow rate pulse shapes and of the pulse frequency on the overall system dynamics. Based on a dimensionless mathematical model of a simple linear hydraulic drive and on elementary performance requirements (e.g. overshooting and pressure pulsations) dimensioning rules are derived. In addition to a repeated pulsing single or just a few pulses are investigated. It turns out that particular single or twin pulses can realize stepping motions of the drive without subsequent pulsations. In this way a hydraulic stepping drive can be realized. In case of repeated pulsing, high pulsing frequencies, in particular frequencies well above the natural frequency of the drive system, reduce oscillations considerably. Such frequencies may be realized either by one high frequency pulse device or by several pulse devices which are arranged in parallel and are operated in a phase shifted mode.© 2013 ASME

An Approximate Computational Method for the Fluid Stiction Problem of Two Separating Parallel Plates With Cavitation

Stiction forces exerted by a fluid in a thin, quickly widening gap to its boundaries can become a strongly limiting factor of the performance of technical devices, like compressor valves or hydraulic on–off valves. In design optimization, such forces need to be properly and efficiently modeled. Cavitation during parts of a stiction process plays a strong role and needs to be taken into account to achieve a meaningful model. The paper presents an approximate calculation method which uses qualitative solution properties of the non cavitating stiction problem, in particular of its level curves and gradient lines. In this method, the formation of the cavitation boundaries is approximated by an elliptic domain. The pressure distribution along its principle axis is described by a directly integrable differential equation, the evolutions of its boundaries is guided just by pressure boundary conditions when the cavitation zone expands and by a nonlinear differential equation when it shrinks. The results of this approximate model agree quite well with the solutions of a finite volume (FV) model for the fluid stiction problem with cavitation.

Modeling and Design of a Cushioning Groove Considering a Slanting Impact

The cushioning groove is a simple means to limit end stop speeds of small devices moving in some fluid, for instance, of spools in switching valves. A shallow groove limited by one or two edges is placed on one or both sides of the moving element. When the groove meets its opposing contact surface the fluid pressed out by the motion causes an increased pressure in the groove which provides the cushioning effect. It overcomes fluid stiction problems which are frequently encountered in squeeze gap type cushioning if the system is under high fluid pressure. The elementary cushioning groove concept assumes that the groove edges are exactly parallel to the contacting surface. In this paper, the performance of the cushioning groove in case of some slanting of the groove edges to the opposing surface is studied by means of a mathematical model. Slanting reduces the cushioning force and causes a resulting torque to the moving system due to an asymmetric pressure. Insufficient cushioning becomes more likely and, in turn, a repelling motion.Copyright

Investigation of a Switch-Off Time Variation Problem of a Fast Switching Valve

In a series of experiments the peak current during switch on of a fast switching valve, which was found to be out of tolerances with respect to some armature dimensions, was varied to realize different switch on times. Despite the fact that the holding current was identical for all cases and the time between switch on and off was very long, the valve’s switch off time showed an unexpected dependency on the switch on peak current value. This paper presents an explanation of this phenomenon by coarse mathematical models, demonstrating that the manufacturing error in combination with friction, skewness, and fluid stiction are responsible for this behavior.Copyright

Development of an Energy Saving Hydraulic Stepper Drive

Stepper drives can realize quite precise, incremental motions without position sensors. Sensorless hydraulic motion control is strongly demanded by industry and, therefore, is an established idea in hydraulics for a while. Some concepts have been proposed in the past and a few of them have also been realized and applied in specific cases. But it is expected that digital hydraulics — due to its intrinsic discrete nature — can create new, more advantageous hydraulic versions of stepper drives.In this paper, a new stepper drive is presented and investigated. It creates the steps by fixed fluid quanta generated by a so called digital flow unit. That unit is realized by a hydraulic cylinder-piston unit which displaces a defined fluid quantum by each limited forward stroke of that piston. The unit is controlled by a fast switching valve which connects the piston areas alternately with the pressure-, tank- and output-line. The return motion is generated by a return spring. Energy saving is accomplished by storing the supply pressure surplus intermediately in the kinetic energy of the piston and converting that energy to displace part of the quantum to the consumer line without hydraulic energy from the supply line. Different detail concepts of this stepper drive are theoretically assessed.The transient behavior, the performance characteristics and the energy efficiency of a preferred concept are investigated by mathematical modeling and simulation. Furthermore, the main system parameters are identified and corresponding basic dimensioning rules are presented. In a second step, the influence of finite switching times of the valves, the hydraulic impedances of the various flow channels and of the dead volumes and the dynamical properties of the hydraulic cylinder attached to the device, are discussed.Copyright

Simulation and Experimental Investigations of a Hydraulic Stepper Drive

This paper presents simulation and experimental results of an energy saving hydraulic stepper drive prototype. Different concepts, advantages and the mechanical design of such kind of stepper drive were discussed in a previous publication. The excellent efficiency, the possibility of energy recuperation, and the control by switching and check valves only, may help to open new applications for hydraulic drives. Also the flow rate can be controlled rather directly by adjusting the switching frequency. This characteristic makes the sensorless position and speed control relatively easy.The drive is realized by a hydraulic cylinder piston unit which displaces a defined fluid quantum by the limited forward stroke of the piston controlled by a fast switching valve. This end to end motion of the piston in its cylinder generates a precise, incremental motion of an additional load cylinder; this enables a sensorless position control. Energy saving is achieved by storing the pressure surplus intermediately in the kinetic energy of the piston to displace a part of the fluid quantum without hydraulic energy from the supply line.A detailed simulation model of a stepper drive including transmission lines, flow channels, hydraulic accumulators and valve dynamics is applied to analyze the experimental results. This dynamic model in connection with the prototype allows to identify the potential for improvement. The different ways to improve the behavior are reviewed, in particular concerning energy losses: bearing friction, leakages in gaps, pressure losses and backflow through check valves. The measured dynamic characteristics and the energy efficiency are presented and compared to the simulation results.The preliminary results showed that the energy efficiency can be drastically increased by a better piston sealing and guidance system and faster check valves. Hence, the development of a fast plate type check valve for the hydraulic stepper drive is also proposed in this study.Copyright