Hydraulic Component Design LibraryRev 9 – November 2009Copyright © LMS IMAGINE S.A. 1995-2009AMESim® is the registered trademark of LMS IMAGINE S.A.AMESet® is the registered trademark of LMS IMAGINE S.A.AMERun® is the registered trademark of LMS IMAGINE S.A.AMECustom® is the registered trademark of LMS IMAGINE S.A.LMS b is a registered trademark of LMS International N.V.LMS b Motion is a registered trademark of LMS International N.V.ADAMS® is a registered United States trademark of MSC.Software Corporation.MATLAB and SIMULINK are registered trademarks of the Math Works, Inc.Modelica is a registered trademark of the Modelica Association.UNIX is a registered trademark in the United States and other countries exclusively licensed by X / Open Company Ltd.Python is a registered trademark ofthe Python Software Foundation.Windows is the registered trademark of the Microsoft Corporation.All other product names are trademarks or registered trademarks of their respective companies.TABLE OF CONTENTS1. Introduction (1)2. Tutorial examples (3)2.1. Constructing hydraulic check valves using HCD (3)2.2. Constructing a hydraulic jack using HCD (10)2.3. Constructing spool valves (15)2.4. 3-position 3-port hydraulic directional control valve (18)2.5. Hydraulic jack with moving body (23)3. A Few General Rules (25)3.1. Introduction (25)3.2. Causality (25)3.3. Use of special facilities for setting parameters (26)3.4. Use of the mass dynamics blocks (26)3.5. Setting chamber length at zero displacement (26)3.6. Major reconstructions (27)Using theHCD Library1. IntroductionHCD stands for Hydraulic Component Design (previously named Hydraulic AMEBel -which stands for AME Sim asic e lement ibrary-). You can use the HCD library to construct a submodel of a component from a collection of very basic blocks. HCD greatly increases the power of AMESim but it is a good idea to be thoroughly familiar with the standard AMESim submodels before you start using HCD.Why was it necessary to create this library? This question will be answered in this section. After this, five examples of the use of HCD are presented. In the last section a few general rules are established to enable you to use HCD effectively.The first four examples are concerned with absolute motion. The majority of applications of HCD that you will use will likely fall into this category. The fifth example is concerned with relative motion. It is recommended that you reproduce the first FOUR examples using AMESim.When you use AMESim, you build a model of an engineering system from a collection of components. For these components AMESim originally used graphical symbols or icons based on standard representations (such as ISO symbols for hydraulic components). For an engineer working in a particular field, this makes the final system sketch look very standard and very easy to understand. However, there are two problems associated with this approach:diversity of components,diversity of skills.The diversity of components problem can be stated quite simply: ‘no matter how many components you have, there are never enough’. As an example think of a hydraulic jack. Here are some possibilitiesthe jack can have one or two hydraulic chambers,it can have one or two rods,it can incorporate one, two or zero springs.This gives 12 combinations and each would need a separate icon. Behind each icon must be at least one submodel. For many AMESim icons, one submodel is enough. In this case, we would have 12 submodels. If we consider telescopic jacks, the number of possibilities doubles. Sometimes it is useful to allow different causalities on the ports. With all the possible combinations of causality on ports there could be well over a hundred submodels of jacks.It is not possible to provide huge numbers of icons and submodels within the standard AMESim libraries. Hence only the more common component icons and submodels are provided. The expert AMESim users can of course create extensions by using AMESet to add both new icons and new submodels but at this point we encounter the second diversity problem.What skills are necessary to produce good submodels of components in AMESim or any other software? Here is a list:an understanding of the construction and operation of the component;an understanding of the physics governing the operation of the component;an ability to translate the physics into a mathematical algorithm to determine the outputsof the submodel from its inputs;an ability to translate this algorithm into a working piece of code.Implicit within these skills are also the ability to test, debug and correct the submodel. This means that the submodel developer requires ability in engineering, physics, mathematics and computer science. This is the problem of diversity of skills. People having all these skills are relatively rare - constructing good submodels is a specialist activity.HCD was developed to overcome these diversity problems. Remember the traditional AMESim library uses symbols, where possible, based on standard ISO symbols. These symbols impose a subdivision of the model into submodels. Clearly this subdivision is not the only one possible; neither is it necessarily the best. We could use subdivision based on larger units or smaller units. HCD uses a subdivision that enables us to build the greatest number of engineering system models from the smallest number of icons and submodels. Returning to the case of the hydraulic jack, we can easily see that all possible jacks can be built up from various combinations of the following elements:hydraulic fluid under pressureannular variable volume chambermechanical springpiston generating force due to differential pressures and areasThis suggests that this is a good subdivision to use. Compared with subdivision based on standard ISO symbols it is clear that the basic blocks are much smaller. We could describe them as technological units since each element is a tangible object for an engineer. With most HCD icons, you could almost go to the engineering store, collect the corresponding physical objects and use them to make a component.Shopping List1 piston2 annular variable volumes2 mechanical springs2 tins of hydraulic oilWe will return to this example in the next Chapter 2 were we introduce a series of examples which progressively introduce features of HCD.2. Tutorial examples2.1. Constructing hydraulic check valves using HCDIn this section you will create the hydraulic check valves shown inFigure 1. These components were chosen because their method ofworking is clear, even to the non-specialist.Figure 1The standard AMESim library already supplies submodels for these components and they are useful for general simulation of hydraulic systems. They do not include any dynamics since it is assumed they react sufficiently fast, compared with the rest of the system.Figure 2Figure 3Figure 2 shows the category icon for HCD. The components in this category are shown in Figure 4. The first 19 components are used as for absolute motion and the 18 following are used for relative motion. Figure 3 shows two special purely hydraulic components. With the relative motion icons there is one body inside another and both are capable of motion. With the absolute motion icons, if there is an outer body, it is considered fixed. We will concentrate first on the absolute motion icons.Figure 4 : Components of the Hydraulic Components Design libraryFor most of the absolute motion icons, there are two linear shaft ports and at leastone hydraulic port which supplies a pressure. Of very great importance is theactive area on which pressure acts. The icon indicates this by the use of thickerlines or curves on the concerned part and, to make it even clearer, arrows alsoindicate the active area. These icons are normally joined together by their linearshaft ports to form an object which might be a spool valve, a hydraulic actuator or, as in the present case, a check valve. However, many other objects can be constructed in the same way such as hydraulic brake components, parts within an automatic gearbox or fuel injection systems.The most commonly used hydraulic icon is the hydraulic volume with compressibilitywhich is associated with a submodel which computes hydraulic pressure. This icon hasprovision for 4 hydraulic flow ports which receive a flow rate and a volume. From this the total volume and the total inflow can be calculated. If this total inflow is positive the pressure rises, if it is negative the pressure falls.The simplest possible check valve consists of a ball which is free to move over a limited displacement. In one extreme position it is fully closed and completely blocks the flow, and in the other extreme position it is fully open. In equilibrium, the position depends on the pressures at the two hydraulic ports.Figure 5HCD contains two icons of a ball in a hydraulic flow path. One has theball on a plain circular seat and the other on a conical seat. Asubmodel associated with the plain seat icon is shown in Figure 5.Note that:- there are two hydraulic flow ports and at each a pressure is received as an input;- if the ball is in the extreme right position, the flow path will be blocked whereas if it is in the extreme left position the flow path is at its maximum opening;- the rods attached to the ball are optional and have a default diameter of zero in the submodels.The ball will be subject to forces due to the pressure and, if they do notbalance, the ball will move. This means we must take into account theinertia of the ball. Since the movement of the ball in the check valve islimited, we need the right hand inertia icon shown. Details of its externalvariables are shown in Figure 6.Figure 6Figure 7 shows two possible versions of the system we are building. Each contains the check valve and two pressure sources to perform a simple test on it. Why two versions? The reason is simple. In order to make HCD as easy to use as possible, many HCD icons are associated with two submodels. Looking again at Figure 5, you will see the external variables of submodel BAP21. The external variables of BAP22 are a mirror image of these. You will get essentially the same results from either of these systems but, to make the example easier to follow, build the system shown in Figure 7(a). Note that there are zero force sources (F000) plugged into the free mechanical ports.Figure 7In Submodels mode it is easier to set submodels by selecting Premier Submodel. However, if you set the submodel for the inertia manually, you will find there are two possible submodels which differ only in the way they treat the limitations in the displacement. These are often referred to as end stops. The two methods of modeling differ in the way they deal with the contact at an end stop:- a perfect inelastic collision with the velocity coming instantaneously to rest or- a mechanical spring and damper.Both methods are valuable but the problem with the second method is knowing how to set the spring and damper rates. MAS005 uses the first method.In Parameters mode for the submodel MAS005 set the mass to 10 g(0.01 kg), the lower displacement limit to 0 mm, the upper displacement limit to 4 mm (0.004 m). The submodel takes the weight into account and hence an angle can be set. In our case the weight force is probably insignificant compared with the pressure force so the value set for the angle is not critical. It is probably not appropriate to set Coulomb friction and stiction. A non-zero viscous friction would make the unit more stable but in practice it is normally fully open or fully shut. Set the viscous friction to 0. The other parameters refer to Stribeck friction. This was introduced because it gives a smoother transition from stiction to Coulomb friction. Normally the Stribeck friction parameters can be left at their default value. Since we set Coulomb friction and stiction to zero they will not be effective in any case.In the submodel BAP22 both rod diameters must be set to zero. The maximum flow rate coefficient is never far from the default value of 0.7. The critical flow number controls how fast this flow coefficient is reached and normally can be left at its default value.The total force on the ball is calculated from the pressures acting on the ball and from the external forces. The pressure force is calculated on the assumption that, referring to Figure 7(a), the right hand port pressure acts on an area adjacent to the orifice and the left hand port pressure acts on the rest of the ball. This assumption is satisfactory under most circumstances but there is provision for a correction term which is known as a jet force. This force tends to close the ball valve. A coefficient, the jet force coefficient, is used to disable or enable this term. It is defaulted to 0 to disable the term and when set to 1 will enable it. It can be set to other values if experimental data is available and fine-tuning of the submodel is desired.Set the left-hand pressure source to a constant value of 50bar. Set the right hand pressure source to ramp from 0 bar to 100 in 1second and back to 0bar in a further 1 second. Perform a simulation over 2seconds with a communication interval equal to 0.01second. Figure 8 shows a typical plot of the flow rate through the check valve plotted against the differential pressure. Remember this is a dynamic submodel and the flow rate can be non-zero even when the differential pressure is negative. Even though the steady state characteristic for a particular pressure drop is for the valve to be closed, the inertia causes a ball position to lag behind the steady state position resulting in a reverse flow. Note that for similar reasons the opening and closing curves are not the same.Figure 8To get the steady-state characteristic, ramp the pressure much more slowly and increase the simulation time accordingly.Note that the ball submodels compute some volumes which are external variables at the two flow ports. The explanation of these will be deferred to the next section on hydraulic jacks where they are of great importance.Figure 9Next you will add a spring (SPR000) to convert the check valve to a spring-loaded unit. The modified system is shown in Figure 9. Attach a zero velocity source (V001) to the other port of the spring.Note that:- The spring is always in compression.- There are two ways of constructing the valve shown as (a) and (b) in the Figure 9. It does not matter which side of the ball the inertia effect is positioned. However, the spring must be to the left or else it will be tending to open the valve instead of closing it.- The spring supplies a force at both ports and so the left spring port must be closed witha zero velocity source rather than a zero force source.We must adjust the spring rate and preload to give a desired characteristic. By choosing appropriate values we can set a cracking pressure and a flow rate pressure characteristic.Figure 10The basic displacement and the corresponding velocity are calculated within the mass submodel MAS005. As shown in Figure 5 and Figure 6, these values are passed through the submodel BAP21. Figure 10 shows the external variables of the spring submodel. SPR000 will accept the velocity from BAP21 and another velocity (which is always zero) from V001.Figure 11When setting the parameters for the spring, we will try to give a small pre-load to the check valve which will determine its cracking pressure. The parameters shown in Figure 11 will give a pre-load of 10N.Figure 12Rerun the simulation with the same pressure sources as in the previous example. Figure 12 shows the flow rate pressure characteristic of the check valve indicating a cracking pressure of about 5bar. The change in the slope of the curve at about 22bar is caused by the ball reaching the limit of its travel. Figure 13 shows the velocity of the ball. Note that there are signs of instability in the unit when it is partially open. (It is better to reduce the communication interval to 0.001 seconds to see this more clearly.) This can be cured by including a damping orifice. We will follow this idea in the third tutorial example.Figure 13Figure 14As an optional exercise you can alter your check valve to make it into the valve shown in Figure 14. This valve senses the pressure in two supply systems and connects the demand system to whichever supply system has the higher pressure. The two ports in the center are in reality probably only one port. Make sure the line submodel DIRECT is set for both of the lines joining the ball submodel to the node, and a HL000 for the line joining the flow rate source.The valve is set up for testing and the two pressure sources represent the two supply systems and the flow source is the demand.Set a demand ramping from 0 to 10 L/min (in the flow source this will be -10L/min) in 10 seconds, the left pressure source ramping from 0 to 100bar and the right source from 100 to 0 bar also in 10seconds. To allow movement of the double ball you must set the lift corresponding to zero displacement in the left ball submodel. Set the lower limit in mass end stops to 0 and the upper limit to 0.005m (= 5mm). For the right ball, set the ball opening corresponding to zero displacement to 0 and same parameter to 5mm for the left ball submodel. Choose a length of 0.01 m for the line. Run the simulation for 10seconds then plot the flow through each ball valve and also the output pressure.2.2. Constructing a hydraulic jack using HCD(b)(a)Figure 15In this section we will return to hydraulic jacks discussed in the introduction. We will consider the simple jack shown in Figure 15(a). Note this has a mass included and is one of the standard AMESim jacks. The simplest HCD construction for this is shown in Figure 15(b).Begin by constructing the double system shown in Figure 16 so that the results using HCD can be compared with those using the standard AMESim library equivalent. Note that inertia icon has been placed so that it gives a sign convention for displacement in agreement with the one used in the standard jack submodel HJ000. Use the Premier Submodel to select as many submodels as possible automatically. Select the mass submodel with ideal end stops. In Parameters mode set the parameters so as to make the two systems as nearly the same as possible. This does require some care so here is a little advice.Figure 16The submodels BAP11 and BAP12 represent the piston and the two volumes either side of the piston. There are not two pistons but one. Each submodel deals with the pressure force on one side of the piston. The arrows and the thick line indicate the area to which the pressure is applied. Note that the mass submodel could be placed on the left or even between the two halves of the piston. The rod diameter must be set to 0 in the left submodel. In both submodels the piston diameter must be set to 25mm to agree with the standard jack submodel HJ000. In the right submodel the rod diameter must be set to 12 mm. At this stage do not worry about the parameters labeled chamber length at zero displacement. We will return to this parameter later. Note that it can be very useful to use the following features when you set the parameters in HCD submodels :Global parametersCopy parametersCommon parametersThus for the diameters of pistons we could introduce a Global parameter named pdiam set to 25 mm. This could then be manually set once and copied to the other submodel. Alternatively it could be set with the Common parameters facility.In HJ000, the default stroke is 0.3 m and the default mass is 1000kg. Hence in the mass with ideal end stops we set the mass to 1000 kg, the lower limit to 0 m and the upper limit to 0.3 m. The arrow and + sign indicate that, when the displacement is zero, the mass is in the extreme left position. The initial displacement in HJ000 is 0 which means that the piston is on the left side. So we keep the initial displacement also to 0 m in MAS005.Set the supply pressure to 100 bar, adjust the input signal frequency to 1 Hz and run a simulation. Figure 17 shows some typical results for the displacement.Figure 17 Why are the results slightly different? The answer is quite simple. In the bottom system (Figure 16(b)) there are DIRECT (direct connections) submodels between the valve and the jack. This means that there is no dynamics in the lines at all. This is equivalent to saying the valve is attached directly to the jack. The pressure dynamics is due solely to the hydraulic volumes within the jack and these volumes vary with the piston position. In contrast, in the upper system (Figure 16(a)), there are no hydraulic volumes within the jack but there are HL000 hydraulic pipe submodels between the valve and the jack. These have pressure dynamics but are based on a fixed volume.Figure 19It is easy to add pressure dynamics corresponding to the hydraulic volumes within the jack.Figure 19 shows the modified system.The crucial icon here is the hydraulic chamber which is connected to the hydraulic flow ports of the two halves of the piston. The corresponding submodel is BHC11 and this is used to model the pressure dynamics. There are 4 ports and each requires as input a flow rate in L/min and also a volume in cm 3 .The submodel sums the 4 volumes and adds a dead volume. It also sums the 4 flow rates. From these values the derivative of the pressure can be calculated.Figure 18The submodel can be used in complex situations involving several separate hydraulic volumes and can also accommodate leakage flows. In the current situation only 2 ports are really required and so 2 ports are plugged with the zero flow rate and volume source shown in Figure 18.Make the changes to produce the system shown in Figure 19. Set the dead volumes in BHC11 to 50 cm³ to agree with the values in HJ000.When the displacement of the mass is zero, the piston is at the extreme left position. This means that the length of the right hydraulic chamber is 0.3 m or 300 mm and that of the left chamber is 0. Hence set the parameter labeled chamber length at zero displacement in BAP11 to 300 mm and the corresponding parameter in BAP12 to 0mm.Why the mixture of length units in mm and m? As well as jacks, HCD submodels are also used to construct numerous types of valves. Often units of m are far too big and mm are much more convenient. The mass models, however, use m because they are likely to be connected to standard AMESim submodels.Figure 20Figure 20 shows a comparison of the displacements produced using our HCD jack and HJ000. Now the two models return the same results.You can also examine the volumes in the two hydraulic chambers BHC11 as shown in Figure 21.Figure 21One option that we have not yet included in our HCD library jack is leakage past the piston. This is easily remedied by inserting the leakage icon between the two halves of the piston as shown in Figure 22. The corresponding submodel, BAF11 (and its mirror image BAF12), computes leakage flow rates which are outputs on ports 1 and 2 and in addition supplies a volume which is always zero. This means that these ports can be connected to the hydraulic chamber submodel BHC11.The leakage flow rate is calculated based on the piston diameter, clearance, length of piston and viscosity. A viscous friction term is also calculated.Figure 22We now consider the jack shown in Figure 23. This jack is not included in the standard AMESim library. By now it should be clear that this can be constructed using the system shown in Figure 24.Figure 24Note that with HCD submodels, it is very easy to see the assumptions on which the model is based. It is clear from Figure 24 that the pressure dynamics is taken into account, that there is leakage and that the end stops are modeled. With Figure 23, the assumptions are not clear at all. Figure 232.3. Constructing spool valvesFigure 25 Figure 26We will construct the specific pressure regulation valve shown in Figure 25. The supply pressure is at the port labeled P and the load is supplied from port A. The port A pressure is also supplied as a pilot pressure to the valve. The idea behind the valve is to try to maintain a predetermined pressure at port A. The spring tries to maintain the valve open and the pilot pressure opposes this. If the load pressure is low, the spring opens the valve allowing more flow. If it is high, the pilot pressure partially or fully closes the valve. There is a drain to tank.Figure 27Figure 26 shows a sketch of a typical unit and Figure 27 a simple HCD representation. Note that: - the hydraulic chamber dynamics is modeled using the pipe submodel HL000;- no leakage is incorporated;- there is a stabilizing or damping orifice which is part of port A without which the component can perform very badly;- there are 3 circular or annular faces of the piston on which pressure acts to oppose or assist the spring.Figure 28 presents the same model with some extensions. The line submodels HL000 are replaced by compressibility elements BHC11. We must stress that the variable volume of the pilot chamber is passed to the BHC11 submodel. In contrast in Figure 27 a conventional orifice isused together with a HL000 line between the orifice and the pilot port. This means a fixed volume is used to represent the variable volume pilot pressure chamber. There will be some differences but, provided the parameters for HL000 are set to correspond to the mean volume of the pilot pressure chamber, results should be very similar.Figure 28The submodel corresponding to the damping orifice is BHO11. This differs from OR000 in that it provides a zero volume output on each port in addition to the flow rate output.Many different variations are possible. We can of course put the mass dynamics in alternative positions but this will not change the results. Other variations are due to different assumptions and these may produce significantly different results. In Figure 27, two models of line with compressibility are used (HL000). The volume of the pilot chamber is not included in the compressibility effect.Also the leakage between the left (damping) and the supply port is accommodated in Figure 28. This is equivalent to having an additional orifice in parallel to the damping orifice.Which set of assumptions is better? If the volume of the pilot chamber is very small compared with the pipe to which it is attached and there is no significant restriction between them, the model in Figure 27 is probably enough. But if the damping orifice is directly connected to the pilot chamber or if the volume of this chamber varies with the displacement of the valve, the model in Figure 28 will be better. However, remember that with HCD it is possible to test different combinations of assumptions and compare results.Figure 29Figure 29 presents another way to model the spring chamber. Figure 27 and Figure 28 are closer to the physical situation with a drain to tank. However, closer inspection shows that there is no provision for leakage. We could easily put in a leakage but is likely to be extremely small. It follows that the only difference between the treatment of spring chambers is that in Figure 29, the pressure force is always zero whereas in Figure 27 and Figure 28 it will be zero only if the tank pressure is zero.Construct the system shown in Figure 28 and use Premier Submodel to set the submodels.In Parameter mode set the characteristic of both orifices by specifying a diameter. For the variable orifice set the maximum diameter to 8mm. Do not forget to set the integer parameter to specify that the characteristics are defined by the orifice diameter. Set the damping orifice diameter to 0.5mm. For the signal source attached to the variable orifice ramp the signal from 0。
