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Application of innovative analysis methods in the development of a Load Sensing System

Authors: Dipl. Ing Daniel Brcky Dipl. Ing Dennis Berghaeger Dr. Elys Botell

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I IntroductionThe problem HAWE would like to know if AMESim was able to handle two situations in a system that is used normally in fork-lifters and similar applications as seen in Figure 1.

Figure 1 - Fork lifter

The system example that was sent to us can be seen in Figure 2 and represents a load sensing system. This kind of system has the particularity of allowing load variations (in the fork lifter for example) without a speed velocity change in the movement of the load. This is obtained with a pressure balance valve (Druckwaage) which maintains the pressure drop over the orifice constant and allows a flow non dependant of the load. In this kind of system, engineers faced vibrations found in the lowering movement of the mass. The vibrations were happening not always in the same situations but when it happened it produced a vibration that was around 2 to 3 Hz (visual observation). This vibration problem was solved along the years at HAWE and the company is now able to avoid this. The proposal of HAWE to IMAGINE was to give us a chance of analyzing these vibrations using our software AMESim.

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The questions of HAWE were the following ones: Is it possible with AMESim to calculate the ideal orifice diameters in the system in order to reduce the vibrations of the Load? When the load is vibrating the whole system is affected by this oscillation. This means that all the valves are oscillating as well, implying a geometrical change. Is it possible to simulate this geometry variation during the simulation?

The answer to both questions is yes. AMESim is able of doing this two operations in a very rapid and efficient way.

Figure 2 Scheme

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II AMESim ModelII.1 The systemIn Figure 3 we have access to the geometry of the relevant details for a simulation. From this detail and some other additional details supplied by HAWE we were able to built a model that can reproduce the normal behavior as well as the unwanted behavior of the valve.

Figure 3 - Load sensing detail With the information gathered from HAWE we managed to build a preliminary model and with it we managed to achieve some results. It was our concern to avoid to build a to complex model as what we were looking for was a typical behavior of the system. We resumed the model to the most important parts of the system as shown in Figure 4 and even in this reduction we concentrated mainly on the modeling of the pressure balance valve (1). The pressure balance valve (PB valve) was modeled with relative accuracy due to the importance of it in the working of the system. From HAWE came the information that the system was sometimes unstable (load oscillations) and the way this was avoided was by reducing the diameter of the orifice (2). Since the diameters involved were very small an alternative solution was encountered. This restriction is obtained by putting several blades together

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with small orifices that are opposed in 90 degrees which should mean (according to HAWE) an equivalent diameter of about 0,2mm.

Figure 4 - AMESim Model (see Load_Sensing_ame41_Lin_01.ame) In AMESim we do not have this kind of limitations. For the modeling of this particular geometry we choose to introduce only an orifice (2) with an approximated diameter (we started with 0,2mm). Concerning the flexible line, an equivalent bulk modulus corresponding to the hose plus the fluid have been fixed to 500bar. For this type of application the range of the equivalent bulk is inbetween 300 and 800 bar. The 500 bar match with the frequency oscillation that were observed by HAWE hydraulic on the real system. We will notice that this bulk modulus parameter is the only one that has been tuned. The other parameters come from the real dimensions, mass, overlaps, etc...

II.2 Verification of the modelWith the information we had from HAWE we started to try to reproduce behaviors of the real system in order to see if the system was working accurately. In a first stage we concentrated in the PB valve and tried to be sure that it was correctly modeled. Only after introducing all the relevant geometrical details did our PB valve worked as expected and presented as result a constant speed value independent of the load as seen in Figure 5.

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Figure 5 Load velocity for different loads After having a working PB valve we connected it to the remaining system and it was then that we had some interesting results. As it is possible to see in Figure 5 the system was oscillating and with it the load.

II.3 Linear analysisII.3.1 Overview of linear analysis theoryWe think that it is not necessary to work to much in the time domain when we have an unstable system. Actually, the time domain leads to a lot of runs and a lot of parameter changes. In order to better understand the dynamic behavior of this valve, we are going to use the linear analysis facilities of AMESim. This part of the work is presented in paragraph II.3.

Figure 6 - Damped oscillator Let us take as example a damped oscillator to take a brief overview of linear analysis theory. Often, the characteristic equation for this kind of system can be expressed as 2nd order equation like the one presented by eq.1.2 s2 + 2 z n s + n = 0

eq.1

In this 2nd order equation the undamped natural frequency is given by n and the damping ratio given by z . The solutions for this characteristic equation are :

s1 = z n + j n 1 z 2 ,

s2 = z n j n 1 z 2 eq.2

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for the behavior of the system of relevance will be the value of z . Z1 Z=1 Z