EP1045992B1 - Control arrangement for a hydraulic motor - Google Patents

Abstract

A control arrangement for a hydraulic motor (1) is, at least in one working direction, loadable by an external load (2). It has a control valve (3) with an inflow control throttle (5) and a return control throttle (7). The control valve (3) is part of a pressure control, in which the motor inflow pressure (PA) is substantially determined by the control valve (3). Further, there is a load retaining valve (14) in the return path (8), which opens in dependence of the difference between the motor inflow pressure (PA) and a reference pressure (PR), which can be picked off between the load retaining valve (14) and the return control throttle (7). A compensation valve (13) maintains a constant pressure drop at an adjustable throttle arranged in the return path (8). This gives a control arrangement, which is to a high degree load independent during lowering and stable towards oscillations.

Description

• The invention concerns a control arrangement for a hydraulicmotor, which is, at least in one working direction,loadable by an external force, with a control valve havingfor this working direction an inflow control throttle inan inflow path extending between the pump connection andthe inflow motor connection and a return control throttlein a return path extending between the return motor connectionand the tank connection, with a load retainingvalve having a load retaining throttle in the return path,and with a compensation valve maintaining a constantpressure drop at a throttle arranged in the return path.
• When hoisting or lowering a load, for example in connectionwith crane booms, lifting platforms, fork lifts,excavators etc., it is important out of regard for thecontrollability that the movements are performed withoutoscillations. It is also desirable for the movements to beperformed at a speed which is determined by the operatornot by the load size. With hoisting movements there arenormally no problems in meeting both wishes. With loweringmovements, however, it is very difficult to meet theserequirements.
• A control arrangement as mentioned in the introduction isknown from DE 38 00 188 C2. A normal manually operatedthree-position valve constructed as a proportional valveserves as control valve. In the inflow path a switchingvalve has a first throttle assuming such a position thatthe pressure drop at the throttle is constant. Fixedlyconnected with the first throttle is a second throttlearranged in the return path, the second throttle servingas load retaining valve and assuming a position determinedby the first throttle. A compensation valve is connected in series with the load retaining valve in the returnpath, which compensation valve keeps the pressure drop atthe load retaining valve constant. Due to the connectionbetween the inflow-side first throttle and the return-sidesecond throttle and the balancing system with suction pipebetween the return path and the inflow path required forthis reason it is difficult to obtain an accurate, loadindependent control, which is stable towards oscillations.
• From DE 38 02 672 it is known to arrange an auxiliaryvalve in series with a control valve on the inflow side ofa hydraulic motor. When this auxiliary valve is operatedso that the pressure drop at the inflow control throttleis kept constant, a constant inflow quantity occurs independentlyof the load pressure. If, however, a pilot pipeis provided between inflow path and return path, whichpilot pipe has a series connection of a fixed pilot throttleand a pilot throttle adjustable by means of the controlvalve, the auxiliary valve can be activated in dependenceof the pressure drop at the fixed pilot throttle,which causes that a pressure which is substantially dependenton the position of the control valve prevails atthe inflow motor connection, which pressure is independentof the load.
• The invention is based on the task of providing a controlarrangement of the kind described in the introduction,which is to a large extent load independent and stabletowards oscillations.
• According to the invention this task is solved in that thecontrol valve is part of a pressure control in which themotor inflow pressure is substantially determined by theposition of the control valve, and that the load retainingvalve opens in dependence of the difference between themotor inflow pressure and a reference pressure, which can be picked off between the load retaining valve and thereturn control throttle.
• This construction gives an inflow side pressure controland a return side flow control. Seen from a purely controltechnical point of view the pressure control causes aninternal pressure feedback, which is extremely stabilisingand enables an approximately oscillation-free lowering.Due to its control, the load retaining valve ensures thata lowering is only possible when a positive motor inflowpressure is available, that is cavitation cannot occur,and together with the compensation valve the load retainingvalve provides that during lowering the returningquantity is limited and cannot exceed a maximum quantityspecified by the full opening of the control valve. Thisrelation with the motor inflow pressure causes a loadindependent lowering. In this connection the return quantitymay certainly have a different value than the inflowquantity, as is the case with a hydraulic cylinder withdifferent piston areas.
• It is recommended that the pressure control has a pilotpipe extending between the inflow path and the returnpath, which pilot pipe comprises the series connection ofa fixed pilot throttle and a pilot throttle adjustable bymeans of the control valve, as well as an auxiliary valveopening in dependence of the pressure drop at the fixedpilot throttle and arranged in series with the inflowcontrol throttle. A pressure control of this kind has avery simple construction and can be accommodated in thehousing of the control valve.
• Further, it is advantageous that the load retaining valvehas a slide, which is loaded in one direction by thereference pressure and a spring and in the other direction by the motor inflow pressure. This gives a very simpleconstruction of the load retaining valve.
• In this connection it is advantageous that the slide isadditionally loaded by the inlet pressure of the loadretaining valve, the slide having a larger pressure surfacefor the motor inflow pressure and a smaller pressuresurface for the inlet pressure. Thus, the load retainingvalve can also work as pressure-relief valve. As theinfluence of the inlet pressure proportional to the pressuresurface only amounts to a fraction of that of themotor inflow pressure, the load retaining valve does notopen until the excess pressure has reached a value whichis a multiple of the normal working pressure.
• Further, it has turned out to be advantageous that theload retaining valve has a spring chamber, which can bepressure-relieved independently of the reference pressure.
• The intended effect particularly occurs when the compensationvalve maintains a constant pressure drop at the loadretaining valve. However, also the frequently preferredopportunity exists that the compensation valve maintains aconstant pressure drop at the return control throttle. Inthis case the inflow side pressure control and the returnside flow control occur in a particularly pronounced wayat the control valve.
• Expediently, the compensation valve is arranged in thereturn path. It may be arranged between the return motorconnection and the load retaining valve, between the loadretaining valve and the return control throttle or betweenthe return control throttle and the tank connection.
• Another preferred opportunity is that the compensationvalve is arranged in a pilot pipe system and additionally influences the pilot pressure for an adjustable throttle.In this connection only a small compensation valve isrequired, which is interesting for weight and space reasons.
• In a preferred embodiment it is provided that a pilot pipeextending between the inflow motor connection and the tankconnection comprises a series connection of a fixed throttleand the compensation valve and therebetween thebranching to a pressure surface of the load retainingvalve. The fact that the additional control with thecompensator overrides the control of the load retainingvalves causes that a constant pressure drop can be maintainedat the return control throttle.
• Another, also preferred embodiment provides that thecompensation valve bypasses the pilot throttle adjustableby means of the control valve. Thus, the overridden controlof the auxiliary valve available in the inflow sideby means of the compensation valve causes the auxiliaryvalve to maintain a constant pressure drop at the returncontrol throttle.
• An additional advantage in these latter cases is obtainedin that the compensation valve is built into the controlvalve, which is usually possible because of the smalldimensions of the compensation valve.
• An additional advantageous embodiment involves that theload retaining valve is constructed as a pressure reliefvalve and the return control throttle is open in theneutral position of the control valve. This gives anadditional utilisation of the load retaining valve.
• In the following the invention is described on the basisof preferred embodiments on the basis of the drawings,showing:

Fig. 1

the connection diagram of a control arrangementaccording to the invention

Fig. 2

an associated working diagram

Fig. 3

a second embodiment of a control arrangementaccording to the invention

Fig. 4

a third embodiment

Fig. 5

an associated working diagram

Fig. 6

a fourth embodiment

Fig. 7

a fifth embodiment

• The control arrangement in Fig. 1 operates a motor 1, hereshown as a so-called piston-cylinder unit, which can beloaded in the lowering direction by anexternal force 2.The diagram only shows the details required of the loweringmovement. For the hoisting a usual flow control can beused.
• Acontrol valve 3, which is adjustable manually by aregulating arrangement 4 or via a remote control, has aninflow control throttle 5 in aninflow path 6 extendingbetween a pump connection P and a motor connection A, anda return control throttle 7 in areturn path 8 extendingbetween a return motor connection Bc and a tank connectionT. Apilot pipe 9 connects theinflow path 6 at the inletof theinflow control throttle 5 with thereturn path 8 atthe outlet of the return control throttle 7. In this pilot pipe 9 afixed pilot throttle 10 and a pilot throttleadjustable together with the inflow andreturn throttles 5and 7 are arranged. Theinflow control throttle 5 isconnected in series with anauxiliary valve 12, whichmaintains a constant pressure drop at thefixed pilotthrottle 10. This results in a pressure control accordingto which the motor inflow pressure P_A has a value, whichis mainly determined by the position of thecontrol valve3.
• In the return path 8 acompensation valve 13 and aloadretaining valve 14 are arranged in series. Theload retainingvalve 14 receives the motor inflow pressure P_A viaapilot pipe 15 and a reference pressure P_R via anadditionalpilot pipe 16, which reference pressure rules atthe outlet B of theload retaining valve 14. Thus the loadretaining valve adjusts under the influence of aspring 17so, that it does not open until the pressure difference P_A- P_R has overcome the spring force. Thecompensation valve13 is controlled so that it maintains a constant pressuredrop at theload retaining valve 14. The series connectionofcompensation valve 13 and load retainingvalve 14 isbypassed by an antiparallel connection of a pressurerelief valve 18 and anon-return valve 19 opening duringthe lifting movement.
• In the working diagram of Fig. 2 the motor inflow pressureP_A is applied on the right half via the adjusting path Xof the control valve, that is in particular the slide of aproportional valve. In the left half it is shown how thecontrol path Z of theload retaining valve 14 changes independence of the motor inflow pressure P_A. The area from0 to X0 corresponds to the dead band of a proportionalvalve. Here the pressure is built up from a standby pressureto PO. This pressure is required to overcome thepreload of theload retaining valve 14. From X0 to X1 the motor inflow pressure changes from P0 to P_stop, which isthe pressure required to move the slide of theload retainingvalve 14 towards the end stop. The area from X0 toX1 is the real control area when lowering the load. Whenthe movable part of the motor meets a resistance, forexample, is pressed into the earth or has to be packed up,a pressure higher than P_stop may be required. In such casesthe slide of the control valve must move beyond X1. Thisgives a higher motor inflow pressure P_A, while the movementspeed of the motor is still limited by the opening oftheload retaining valve 14 in the position Zmax and fromthe throttling through the compensation valve.
• The vertical line starting at S is idealised. It onlyappears when the control throttles of thecontrol valve 3work without losses, which is not possible in practice.The dash-and-dot line L shows that a certain pressure P_Aon the motor inflow connection A belongs to a certainposition X of the slide of thecontrol valve 3, and thatwith this motor inflow pressure a certain position Z ofthe load retaining valve is specified, which position Z inconnection with thecompensation valve 13 results in apredetermined return quantity. Thus, on the inlet side itis ensured that in the inflow path 6 a cavitation preventingpressure is maintained and that in thereturn path 8the quantity drained off is limited.
• In the embodiment according to Fig. 3 the slide of theload retaining valve 114 is loaded in the closing directionby thespring 17 and the reference pressure P_R and inthe opposite direction by the motor inflow pressure P_Aand, via anadditional pilot pipe 20, by the inlet pressureP_E of theload retaining valve 114. However, themotor inflow pressure P_A is still prevailing, which appearsfrom the fact that the pressure chamber connectedwith thepilot pipe 15 has a larger pressure surface 21 than thepressure surface 22 of the pressure chamberconnected with thepilot pipe 20. The pressure surfaceexposed to the reference pressure through P_R is preferablyequal to the sum of the pressure surfaces 21 and 22.
• The embodiment of Fig. 4 differs from the one in Fig. 3 inthat thecompensation valve 13 is connected to the outputside of theload retaining valve 114 in thereturn path 8,and that the compensation valve maintains a constantpressure drop at the return control throttle 7. Here againthe outlet pressure of theload retaining valve 114 ischosen as reference pressure P_R. Further, the function ofthe pressure relief valve 18 is incorporated in theloadretaining valve 114, meaning that thespring 17 gets acorresponding preload and the pressure surface relation ischosen accordingly. The chamber for thespring 17 can berelieved separately from the chamber for the referencepressure P_R, for example towards the atmosphere or thetank.
• For the embodiment in Fig. 4 the working diagram of Fig. 5applies, which corresponds to the right half of the diagramof Fig. 2. Here again the area from 0 to X0 forms thedead band of thecontrol valve 3. Here the motor inflowpressure P_A is built up from a small stand-by pressure toa pressure P0, which must be high enough to serve as pilotpressure, to open the load retaining valve withoutavailableforce 2. This is explained by the following example:Theload retaining valve 114 has a pilot relation 4:1. Aspressure relief valve it is set at 300 bar. Without theforce 2 a load pressure of 20 bar is measured due to theown weight. The required pilot pressure PPmax therefore is(300-20)/4 = 70 bar. As shown in Fig. 5, the start pressureP0 at the end of the dead band is slightly largerthan this maximum pilot pressure. With aforce 2 the required pilot pressure gets lower. With maximum force,for example 240 bar, the pilot pressure Ppmin = 15 bar.
• Thus, it is provided that on each operation the motorinflow pressure P_A is so high that theload retainingvalve 114 is opened. When the pressure drop over thereturn control throttle 7 is then kept constant by thecompensation valve 13, for example at 10 bar, the quantitydrained off is so limited that it can do no damage.
• In the embodiment according to Fig. 6 thecompensationvalve 113 is made as a small pilot valve. It is arrangedin apilot pipe 115 in series with a fixed pilot throttle23. The pick-off 24 arranged between them leads to thepressure chamber with thelarger pressure surface 121 oftheload retaining valve 114. Thecompensation valve 113maintains a constant pressure drop at the return controlthrottle 7, as deviations are balanced by a superposedadjustment of the slide of theload retaining valve 114.
• In the embodiment according to Fig 7 thecompensationvalve 113 is again made as a pilot valve. It maintains aconstant pressure drop at the return control throttle 7 inthat theadjustable pilot throttle 11 is bypassed throughapilot branch 25 meaning that the inflow sideauxiliaryvalve 12 is additionally acted upon. Thecompensationvalve 113 can be arranged in the housing or in the slideof thecontrol valve 3. Further, the advantage is obtainedthat only the required pressure is built up on the outletside.
• Deviations from the embodiments shown are possible within the scope of the appended claims.Forexample, the compensation valve can also be connected tothe output side of the return control throttle.

Claims (15)

1. Control arrangement for a hydraulic motor, which is,at least in one working direction, loadable by anexternal force, with a control valve having for thisworking direction an inflow control throttle in aninflow path extending between the pump connectionand the inflow motor connection and a return controlthrottle in a return path extending between the returnmotor connection and the tank connection, witha load retaining valve having a load retainingthrottle in the return path, and with a compensationvalve maintaining a constant pressure drop at athrottle arranged in the return path,characterisedin that the control valve (3) is part of a pressurecontrol in which the motor inflow pressure (P_A) issubstantially determined by the position of the controlvalve (3), and that the load retaining valve(14; 114) opens in dependence of the difference betweenthe motor inflow pressure (P_A) and a referencepressure (P_R), which rules between the load retainingvalve (14) and the return control throttle (7).
2. Control arrangement according to claim 1,characterisedin that the pressure control has a pilot pipe (9)extending between the inflow path (6) and the returnpath (8), which pilot pipe (9) comprises the seriesconnection of a fixed pilot throttle (10) and a pilotthrottle (11) adjustable by means of the control valve(3), as well as an auxiliary valve (12) opening in dependenceof the pressure drop at the fixed pilotthrottle (10) and arranged in series with the inflowcontrol throttle (5).
3. Control arrangement according to claim 1 or 2,characterisedin that the load retaining valve (14; 114) hasa slide, which is loaded in one direction by the referencepressure (P_R) and a spring (17) and in theother direction by the motor inflow pressure (P_A).
4. Control arrangement according to claim 3,characterisedin that the slide is additionally loaded by theinlet pressure (P_E) of the load retaining valve (114),the slide having a larger pressure surface (21) forthe motor inflow pressure (P_A) and a smaller pressuresurface (22) for the inlet pressure (P_E).
5. Control arrangement according to one of the claims 1to 4,characterised in that the load retaining valve(14; 114) has a spring chamber, which can be pressure-relievedindependently of the reference pressure (P_R).
6. Control arrangement according to one of the claims 1to 5,characterised in that the compensation valve(13) maintains a constant pressure drop at the loadretaining valve (14; 114).
7. Control arrangement according to one of the claims 1to 5,characterised in that the compensation valve(13; 113) maintains a constant pressure drop at thereturn control throttle (7).
8. Control arrangement according to one of the claims 1to 7,characterised in that the compensation valve(13) is arranged in the return path (8).
9. Control arrangement according to claim 8,characterisedin that the compensation valve (13 is arrangedbetween the return motor connection (B_C) and the load retaining valve (14).
10. Control arrangement according to claim 8,characterisedin that the compensation valve (13) is arrangedbetween the load retaining valve (114) and the returncontrol throttle (7).
11. Control arrangement according to one of the claims 1to 7,characterised in that the compensation valve(113) is arranged in a pilot pipe system and additionallyinfluences the pilot pressure for an adjustablethrottle.
12. Control arrangement according to claim 11,characterisedin that a pilot pipe (115) extending between theinflow motor connection (A) and the tank connection(T) comprises a series connection of a fixed throttle(23) and the compensation valve (113) and therebetweenthe branching (24) to a pressure surface (121) of theload retaining valve (114).
13. Control arrangement according to claims 2 and 11,characterised in that the compensation valve (113) bypassesthe pilot throttle (11) adjustable by means ofthe control valve (3).
14. Control arrangement according to claim 13,characterisedin that the compensation valve (114) is builtinto the control valve (3).
15. Control arrangement according to one of the claims 1to 14,characterised in that the load retaining valve(114) is constructed as a pressure relief valve andthe return control throttle (7) is open in the neutralposition of the control valve (3).

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