GB2278694A - Servo hydraulic circuit - Google Patents

Description

2278694 Servo hydraulic circuit The invention concerns a control for a

hydraulic consumer in accordance with the preamble of claim 1, and a safety circuit for a servo hydraulic control system for hydraulic control and actuation of a consumer,, in particular of a working unit, e.g. of a lifting unit on a utility vehicle or on an agricultural machine, according to the preamble of claim 1, which may be combined therewith.

is Servo hydraulic control systems are primarily needed in automotive hydraulics in order to provide the users of a working unit with maximum operating safety or safety against accident. In particular for working units on utility vehicles and agricultural machines, e.g. the lifting unit of a tractor.. such servo hydraulic control systems are required to serve as a positioning device for the working unit to maintain a predetermined position relative to the vehicle. for example to adjust a predetermined penetrating depth of the plough Into the solli and furthermore as a possibility to compensate lift and pitch oscillations of the vehicle which may occur during practical work with the vehicle due to surface roughness, e.g. while driving on agriculturally used areas. and propagate to the working unit. Often having a large mass, the working unit may exert a considerable influence on the pitch oscillations of the vehicle, whereby operational precision as well as the driving safety of the utility vehicle might be impaired.

There is a tendency for the demands to the safety of machines and devices of the kind described above to become stricter. In the area of servo hydraulic control systems, Page - 1 - e.g. servo hydraulic control systems for lifting units on tractors, hereinafter referred to as SHR systems, the safety functions are lagging behind those of electro hydraulic systems in certain specialized areas. Up to now, the lack of certain safety functions has consequently been a feature distinguishing servo hydraulic systems from electro hydraulic systems.

Servo hydraulic controls for lifting units., such as those f or example known from DE-OS 29 40 403 or DE-OS 31 06 086, operate e.g. by using a main valve in the form of a directional control valve which is pressurecontrolled in two directions, or a proportional valve controlled by a closed loop control circuit comprising a hydraulic measuring bridge having fixed and variable throttles. One of the variable throttles serves as a setting means and the other one serves as a controlled output sensor which is mechanically coupled to the actuated part of the lifting unit. When a new signal value (desired value) for the lifting unit is set via a variable throttle, i.e. via a manual lever, the directional control valve or proportional valve shifts from the neutral position and will only return there after the hydraulic measuring bridge has been balanced, i.e. when the control pressures applied to the proportional valve spool are balanced, i.e. when the deviation has become Zero.

in former servo hydraulic control systems for lifting units, a safety hazard mightr for example, occur due to an imbalance of the measuring bridge manifesting itself in a sudden control reaction, unexpected by the operator, which brings about an undesirable and thus dangerous movement of the lifting unit. Namely, the measuring bridge might have become imbalanced during engine standstill if one or several measuring orifices are imbalanced, i.e. for example due to a shifted lever, a change in the force at the force transducer, or a positional change of the lifting unit.

Page - 2 - It is therefore the object of the invention to provide a control for a hydraulic consumer where the above described safety hasards will not occur, and which preferably may be incorporated in a safety circuit f or a servo hydraulic control system according to the preamble of claim 2, for example according to DE-OS 29 40 403 or DE-OS 31 06 086, and wherein the possibility of documenting serial testing of the safety function on the device proper should be ensured.

This object is attained by the features of claim 1 as concerns the control, and by the features of claim 2 as concerns the safety circuit.

is What is particular about the control for a hydraulic consumer according to the invention is that the control liquid for the load retaining valve controls pressure build-up of the working fluid,, or the connection f rom the consumer to the tank. via a pair of hydraulically controllable directional valves, namely by utilizing an additional flow path of the directional valve. This creates a prerequisite for the fact that a movement of the consumer may only be effected If the directional valve has been actuated in a certain way. Hereby it becomes possible that. as a rule,. a movement of the consumer may only take place if the operator actively actuates the directional valve in solne manner, preferably past: the Zero position. Serial testing of the safety function on the device proper is possible due to the f act that a pressure build-up of the working fluidr or the connection from the consumer to the tank, is Initiated by means of the control liquid having a flow path via the directional valve.

According to the invention, the safety circuit ensures that whenever the deviation at the proportional valve spool is not Zero, i.e. when the proportional valve spool. i.e.

Page - 3 - the SHR spool, is not in the neutral position, a consumer movement, for example a lifting movement or a lowering movement of the lifting unit, is precluded until the two safety valves have been safety-released by the operator manipulating the adjusting lever whereby the signal value is adjusted, for which purpose the setting lever must be shifted past the neutral position. This positively ensures that whenever the SHR spool is not in the neutral position when the servo hydraulic control system is switched on, the working unit does not move and Is always set in motion from the neutral position. This results in the following advantages: the safety valves can be equipped with clearly defined shifting functions, consequently they may have a simple construction and may be designed safely. Violent reactions will not be occasioned by moving the working unit out of "neutral" during switching on. The safety function may be incorporated in the control device, inasmuch as the safety valves are governed in correspondence with signal bridges which are clearly determined by the position of the proportional valve. Thus serial testing of the -safety function may also be carried out an the device proper. In other words, the safety function can already be tested and documented at the component level of mvalves".

Advantageous embodiments of the safety circuit form the subject matters of the subclaims 3 through 12.

When a control pressure and a signal pressure determined by the position of the proportional valve are applied to a respective one of the safety valves, the result Is a very simple structure of the safety circuit, inasmuch as additional control lines leading to an external setting means are not required.

The circuitry according to claims 5 and 6 requires a minimum amount of additional isignal lines, resulting in the Page-4- z additional advantage that the constructional design of the safety valves may be unified.

If a safety throttle is incorporated into the line for the further signal pressure leading to the safety valve for the first direction of movement, for example for the lifting movementy simple measures bring about an additional insurance against pressure peaks in this signal pressure line during switching on, when the proportional valve pool is In the position initiating the second direction of working unit movement during switching on. Possibly occurring signal pressure peaks are attenuated by the safety throttle, with the possibility of further improving insurance against the pressure peaks by means of the further development of claim 7. Exactly this development,. howeveri reveals the particular safety margin of the concept according to the application. For even in the event of signal pressure peaks, which might bring about a premature safety-release of the safety valve provided for the first direction of movement, e.g. for the lifting movement, an undesirable reaction of the lifting unit cannot occur as in this case, the proportional valve is still in the position InitiatIng the other direction of movement, i.e. the lowering movement of the working unit,, and the associated safety valve is still in its safety position.!.a. it is still locked.

The safety valves are preferably pretensioned in their respective safety positions by strong reset springst such that these valves reliably assume the secured positions when the servo, hydraulic control system is switched off.

In the following. several embodiments of the Invention are explained in more detail by making reference to schematic drawings, wherein..

Page - 5 - Fig. 1 shows a circuit diagram of a servo hydraulic control system comprising the safety circuit; rig. 2 shows a cl=cuit diagram for a possible way of generating the signal pressures at the proportional valve; and rig. 3 shows a block diagram illustrating a further embodiment of a lifting unit control system comprising a positioning branch and a force branch.

In Fig. it reference numeral 10 designates a proportional valve serving as a main valve of a servo hydraulic control system for hydraulic control and actuation of a consumer Z. The consumer Z has, for example, the f orm. of a working unit on a utility vehicle or on an agricultural machine, such as e.g. the working cylinder of the lifting unit on a farming tractor.

The proportional valve 10 has three main positions, namely the positions N ("neutral"), H ("lift") andS ("lower"). By means of two springs 12, 14, the valve spool of the proportional valve 10 is centered in the position N.

The control pressures X1 and X2 corresponding to the output signals of a measuring bridge for the position of the lifting unit or of the consumer, which is represented in Fig. 2,r are furthermore applied to the valve spool in opposite directions. Fig. 2 shows that the signal pressures X1 and X2 are applied to the left and right sides of the proportional valve 110, respectively. Both signal pressures X1 and X2 are derived from a pilot pressure Pl, i.e. a pressure in a further closed loop control circuit.

The signal pressure Xl is determined by the throttles 116, 118 which preferably are adjustable, such that a reference pressure prevails on the left-hand side of rig.

2. The signal pressure X2 on the other aide of the Page - 6 - proportional valve 110 is determined by the throttles 120r 122. with the throttle 120 being coupled to a transducer and the throttle 122 being coupled to a sensor. Fig. 2 thus shows a measuring bridge for the position of the lifting unit. When the measuring bridge is balanced, the pressure signals Xl and X2 are just about large enough for their force effects to cancel each other out at the main valve spool of the proportional valve 110. On the other handr i.e. in the case where the measuring bridge has developed an imbalance, possibly due to the throttle 120 (transducer) having been repositioned or due to a change in the position of the lifting unit and therefore a readjustment of the throttle 122. the valve spool of the proportional valve 110 is shifted until the signal value has been reached at the consumer and the measuring bridge has become balanced again. in other words, the valve spool of the proportional valve 10 is adjusted until the deviation has been madeZero.

Fig. 3 shows another variant f or generating the signal pressures Xl and X2 at the proportional valve 210. In this casef the signal pressure Xl being a reference pressure constantly,. it remains at a constant value. for example at 1/2 p,. This is the purpose of the fixed throttles 216 and 219. The signal pressure X2 on the other side of the proportional valve 210 is generated in a different manner.,A positioning branch shown under 224 and a force branch shown under 226 are provided,, wherein two adjustable throttles 220a and 220b, or 222a and 222b. are respectively provided. The throttles 220a and 220b are connected to a transducer, for example in the form of a manual lever, whereas the throttles 222a and 222b are connected to force sensors or position sensors. The pressure between the throttles 220 and 222 is detected and 228 supplied to the signal pressure line 230 leading to the proportional valve 210 via a shuttle valve.

Page - 7 - - In order to prevent the occurrence of a sudden control reaction of the lifting unit, unexpected to the operator and probably originating from the measuring bridge having been imbalanced unintentionally during engine standstill or during the switching-on phaser when the servo hydraulic control system is switched on, e.g. when the engine of the farming tractor is started, a safety circuit is provided which shall be described below in more detail:

A power circult and a _control or pilot pressure circuit are conducted via the proportional valve 10. This is indicated by -the symbols P and P1. In other words, a working line 32 f ed by the main pump and an auxiliary or pilot pressure line 34 are routed via the proportional valve 10. The line sections downstream of the proportional valve 10 are shown under 36 and 381 respectively.

A line 40 comprising an orifice 42 branches of f from the line section 36, with a pressure feedback being effected to the output side 44 of -a flow control valve 46 indicated by dash-dotted lines when the proportional valve 10 is not in the neutral position N. The flow control valve 46 comprises a connection 48 with the tank T and an inlet 50 connected to the working line 32. A flow control measuring orifice 52 is arranged in parallel with a pressure compensator 54, with the pressure compensator ensuring that an excessive amount of fluid displaced by the pump is supplied directly to the tank via a separate control land, such that the pump only needs to work against the respective load pressure plus the pressure differential at the flow control valve. i.e. at the pressure compensator.

The pressure in the line 40 represents a signal pressure X4 used for the safety circuit.

Page - 8 - The second signal pressure X3 utilized by the safety circuit is detected from the pilot pressure P1, in particular from the line 38 downstream of the proportional valve 10. Reference numeral 56 designates a branch point where the line 38 separates into a branch line 38a and a branch line 38b. The branches 38a and 38b lead up to the safety valves 60 and 62, respectively, having the form of spool valves, the functions of which shall be explained in the following:

The safety valves 63, 62 may present a largely Identical structure, in particular the same nominal diameter of their valve spools. such that e.g. they may be received in an identical valve block bore. Each valve has the form of a is multiway/2-position valve, with each valve body being pretensioned into the safety position S shown in Fig. 1 by strong springs 64 and 66. respectively.

The safety valve 60 is associated with the lowering operation of the consumer Z and the safety valve 62 is associated with the lifting operation of the consumer Z. For this purpose, the circuitry is designed as follows:

By means of the safety valve 60. an unblockable check valve 68. i.e. a releasable non-return valve. may be controlled to open, in order to be able to lower the consumer such as the cylinder of the lif ting unit on a farming tractor. In order to control the blocking unit 68 to open, a control line 70 is provided which in the released position E of the safety valve 60 in connected to the branch line 38B conducting the signal pressure X3. The control pressure for the blocking unit 68 Is detected between two measuring orifices 72. 74, with the measuring orifice 72 being located in a branch line leading to the tank T.

Page - 9 - -- In the safety position S. the control line 70 is relieved as the safety valve 60 closes the corresponding control land for transmitting the signal pressure X3. When controlled to an open position, however, the safety valve 60 in this position S constitutes a connection between the signal pressure connector SX4 and a control line 76 which is conducted on the one side of the valve spools of the safety valves 60 facing away from the spring 64. The signal pressure connector SX4 conducts the pressure in the signal pressure line 40 applied to a control land EX4 of the second safety valve 62 via a branch line 78, with the line section 78 leading to the tank T being in an open position when the second safety valve 62 is in the safety position. SX3 designates a signal pressure connector at the second safety valve 62,, i.e. the one provided for the lifting operation of the consumer Z. which is connected to a control pressure line 80 in the safety position S shown in Fig. 1; this control pressure line - similar to the structure of the safety valve 60 - is routed toward the one side of the valve spool of the safety valve 62 which faces away from the spring 66.

Besides the respective control lands EX3, SX4 and SX3, EX4,, each of the two safety valves 60, 62 furthermore comprises a pressure-retaining'control land RS connected to the pilot pressure P1 via a llne branch 82; In the respective safety position S of the safety valves 60, 62, this pressure-retaining control land is blocked, and In the safety-released position E it is connected to a related control pressure line 76 or 20 leading to the positioning element in order to keep the valve sleeve of the respective safety valve 60 or 62 in a position of stable transmission after unlocking or safety-release has taken place.

in the signal pressure line 38A leading to the signal pressure connector SX3, a safety throttle shown under 84 for smoothing or compensating pressure peaks 12 Page - 10 - incorporated, which shall be described in the following. Finally It should be noted that the safety valves 60, 62 preferably are sliding valves having relatively large amounts of stroke.

The structure of the proportional valve 10 in accordance with the embodiment shown In rig. 1 is such that in the neutral position N it blocks the working line 36 and the pilot pressure. line 38 while relieving the signal pressure line 40, which conducts the signal pressure X4. toward the tank T. In the lowered position Sr. the power pressure P is controlled shut, the pilot pressure applied via the line 34 is transmitted to the line 38, and the working line 36 is connected to the tank just like the signal pressure line 40. In the raised position H. the control land shuts off the connection between the lines 34 and 38 and the connection to the tank and connects the signal pressure line 40 just as the working line 36 to the power pressure P.

A dotted line finally indicates a control line 96 loading to the limit switch (not represented) of the position sensor and receiving the signal pressure X4 in the safety-released position E of the safety valve 60.

The circuit according to Fig. 1 basically operates along the following lines:

In order to enable the generation of a pump pressure corresponding to the consumer pressure inside the pressure line 32 when the control valve 10 is shifted to the %lifting" position, It is necessary for the connection 50 from the pressure line 32 to the tank line 49 to be sufficiently throttled by means of the control spool of pressure compensator 54 for a correspondingly high pressure to occur. This throttling action of the control spool is only possible If the control spool, having the pumping Page-11- pressure applied to it in opening direction via a control line, has a control pressure applied to it in the opposite direction, i.e. in the closing direction and in the effective direction of the spring forcef with the amount of this control pressure being about equal to the amount of the opposed pump pressure minus the force of the closing spring. In order to enable build-up of this control pressure X4 on the spring side of the pressure compensator, the valve 62 must initially be shifted into the control position E whereby a connection between the control line 40 conveying the control pressure X4 and the tank T is Interrupted via the control valve 62. The only remaining connection between the control line 40 and the tank is then provided by the control valve 10 in its illustrated central position X.

In this central position wherein the connection from the pump to the consumer is interrupted. the pump conveys to the tank via the pressure compensator while counteracted by the small shutting force of the spring which is applied to the control spool of the pressure compensator. As soon as the control valve 10 is shifted into its position E, the connection between the control line 40 and the tank is interrupted, while a connection with the consumer is simultaneously established to allow a control pressure building up inside the control line 40. This control pressure takes effect on the control spool of the pressure compensator in the shutting direction and throttles the connection with the tank to such an extent that a pump pressure suited for the consumer Is generated Inside the pressure line 32. This pressure build-up would not be possible if the control line 40 were still connected to the tank via the control valve 62. The position of the control valve consequently determines whether or not a control pressure for an effective control position of the pressure compensator is possible; or whether or not a pressure build-up of the working fluid is possible. The control 1 Page - 12 - position of the other control valve 60 determines whether or not the working fluid is to be applied to the releasable check valve, such that working fluid may or may not emanate from the consumer to the tank via a control land TX of the control valve 10. This control valve consequently creates the possibility of a connection from the consumer to the tank.

The structure described above results in the following manner of functioning:

Fig. 1 shows the safety circuit in the state where the single components are in their respective positions when the servo hydraulic control system (SHR) is switched off.

The relatively strong springs 94, 66 moved the valve spools into their safety positions S when the pilot pressure P1 was switched off. Various constellations are imaginable to have caused the valve spool of the proportional valve 10, during the phase of switching off the serva hydraulic control system, from leaving its balanced position (position "neutral" N). This may, for example, have been occasioned by a variation in the force at the force transducer due to a variation in the position of the lifting unit during engine standstill or, however, by some other readjustment of a lever.

It may initially be assuraed that the spool of the proportional valve 10 is in the position S ("lower,,) during or after starting up. This does establish a connection between the lines 34 and 38 via the proportional valve 10l such that the signal pressure X3 is transmitted via the signal pressure connector SX3 for the control side of the safety valve 62, whereby the latter is moved into the unlocked or safety-released position E. The safety valve 60, however, continues to remain in the secured locked position S which cannot be abandoned because the signal pressure X4 applied to the signal pressure connector SX4 in Page - 13 - the saf ety position S of the oaf ety valves 62 is relieved to the tank T via the control land EX4. in the control position E of the safety valve 62, the signal pressure X4 continues to be relieved to the tank via the control land TX.

In order to move the lifting unit. the operator must at first shift the setting lever 120, or 220A, or 220B in the direction "lift",, whereby the control spool passes through the neutral position N. When leaving the neutral position N in the direction of "lift', the lifting unit, or the consumer Z, is then drivent with the same reaction occurring at the consumer as during a regular shift from "neutral', N to 1111ft,' H. While the pressure compensator 54 is closed, i.e. while the flow control valve 46 is beginning to take effect, a sufficiently high signal pressure X4 gradually builds up in the signal pressure line 40 and is transmitted via the safety valve 60 to the control pressure line 76 and in the lifting mode switches this safety valve to the safety-released position E. where it remains due to the transmitted pilot pressure P1. The lifting unit Z may then be moved as usual. i.e. it may also be lowered.

The following is a description of the second case where the spool of the proportional valve 10 is in "lift", i.e.

in the position H during or after starting up. In this case, both safety valves 60 and 62 remain in the safety or locked position S, and the lifting unit comes to a standstill as the signal pressure X4 in the signal pressure line 40, due to being relieved to the tank at the safety valve 62, practically equals Zero. A sufficient pressure to push open the valve ball of the non-return valve 68 correspondingly cannot build up in the working line 36 downstream of the proportional valve 10. In order to trigger a consumer movement, the setting lover must first be moved in the direction "lower". Only during the Page - 14 - 62 be unlocked transition from "neutral,, to "lower" can the safety valve r i.e. moved into the safety-released position E. if, however, the setting lever is at first moved in the direction "lift",, a reaction of the lifting 5 unit or a variation of the spool positions does not ensue.

Due to the movement of the setting lever in the direction "lower", the safety valve 62 now associated with the lifting operation could be unlocked. It the setting lever is now moved in the direction the signal pressure X4 can become larger than Zero as the safety valve 62 is unlocked. whereby the safety valve 60 is also unlocked. The lifting unit is then simultaneously driven during the transition from 1'neutra1H to "li-fto. When the setting lever is subsequently moved in the direction nlowero, the lifting unit is also driven during the trans ition from "neutral,, to "lower", which is in accordance with a common operating characteristic of the lifting unit.

Finally a third case is imaginablef according to which the control spool is in the position "neutral during 9tarting up. If the setting lover is moved toward "lift,, in this case,. the safety valve 60 as before remains locked because the signal pressure X4 remains at Zero. Only by shifting the setting lever to glower". a lifting motion of the working cylinder Z may be prepared. A reaction does, howeverr not ensue upon the first movement toward Olowerm, for it merely serves to unlock the safety valve 62. The setting lever is subsequently moved to "lift". The safety valve 60 is thereby unlocked and the lifting unit follows to "lift". When the setting lever is subsequently moved to olowerllt the lifting unit also follows. for now both safety valves 60 and 62 are in their unlocked positions.

It is equally possible to first effect an unlocking of the safety valve 62 from the neutral position by first Page - 15 - moving the setting lever to "lower". A reaction does, however. not ensue yet as the safety valve 60 has not been unlocked yet. The safety valve 60 can only be unlocked by moving the setting lever to "li:ft". Here the lifting unit follows upon transition from "neutral,, to nlift" just as in the case described above.

Due to the construction of the safety circuit according to the invention, the operator will therefore get used to always at first moving the setting lever in the direction oelolimrtf in order to unlock the lifting movement. Subsequently the lifting unit follows In the second direction. This manner of functioning applies to the normal case as well as to the case of a safety hasard.

The manner of functioning described above also applies to the case where the lifting unit is in its extreme top or bottom position during switching on. When the lifting unit is in its lowest position, the servo hydraulicclosed loop control circuit is in the "lower" mode, such that the scenario of the above described first case applies.

When the lifting unit is in its top position, the servo hydraulic closed loop control circuit is in the "lift" mode, such that the scenario for the second case described applies.

Due to the above described manner of functioning of the safety circuit, there results the advantage of the operator always passing through the "neutral" position when carrying out the releasing movement with the setting lever, irrespective of whether the actual value utilised by the control mode refers to force or position. An undesirable violent reaction at the consumer is consequently precluded in any case. Also, when the servo hydraulic control system is started up in the "neutral" position, there only results a single reaction because the servo hydraulic control Page - 16 - system remains inactive upon unlocking the first safety valve 60. Furthermore there results the advantage that additional operating elements are not required.

In summary. the concept on which the safety circuit is based may thus be characterised in that unlocking the safety valve associated with the lowering operation is effected by moving the setting lever to "lift". and that unlocking the safety valve associated with the lifting operation in effected by moving the setting lover to Olower'. Thust in the case of a safety hasard. it Is a rule that the olifto inode, must always first be unlocked by moving the setting lever to "lower". Only then may pressure build up in the signal pressure line X4, and only then may is the pump in the system circuit build up pressure while the pressure compensator is closed. and directly push open the non-return valve 68 to the cylinder.

The core element of the described safety circuit therefore is a control for a hydraulic consumer, in particular for a lifting unit,, which consists of the hydraulically controllable directional valve 10 for feeding and draining the working fluid, and of an hydraulically unlockable load retaining valve 68 arranged inside the working line 36 between the directional valve 10 and the consumer. The control fluid for the load retaining valve 68 is routed via an additional flow path 10a of the directional valve 10 and additionally controls the pressure build-up of the working iluidi or the connection from the consumer to the tankg via another hydraulically controllable directional valve 60 an well as another directional valve 62 which Is also hydraulically controllable. Hydraulic control of the additional directional valves 60 and 62 is determined by the respective control position SE, N. or H of the directional valve 10 controlling the working fluid.

Page - 17 - As a matter of f act, deviations f rora the embodiments described above are possible without departing from the principle of the invention. Namely, the saf ety circuit is also applicable to double action consumers. The invention is furthermore equally applicable to control devices where the control signals X1 and X2 at the control spool are formed by other signals, for example by electrical signals.

The invention thus provides a control f or a hydraulic consumer which may advantageously be combined with a safety circuit for a servo hydraulic control system, in particular for hydraulic control and actuation of the lifting unit on a utility vehicle. Controlling the lifting unit is effected, in correspondence with the position of a setting lever, via a proportional valve to one side of which a reference pressure is applied and to the other side of which a pressure in proportion with the deviation is applied, such that in the case of a deviation of Zero, the lifting unit will assume a neutral position wherein the working line leading to the consumer or from the consumer will be blocked. In order to preclude undesirable spontaneous consumer movements during switching on of the control system, even in a case where there is a deviation during switching on., two safety valves are provided the pretension-ing of which blocks the lifting and lowering operations, respectively, in the switched off state of the control system. The safety valves are switched such that when the control system is switched on, at least the one safety valve whereby the procedure controlling the consumer as predetermined by the currently set position of the proportional valve may be blocked., is kept in the locked position until a setting lever has been moved sufficiently far in one direction for the spool of the proportional valve 10 to be shifted past the neutral position N.

page - 18 -

Claims (15)

1. Control for a hydraulic consumer, in particular a lifting unit, consisting of a hydraulically controllable directional valve (10) for feeding and draining the working fluid, and a hydraulically unlockable load retaining valve (68) arranged in the working line (36) between the directional valve (10) and the consumer, characterized in that the control liquid for said load retaining valve (68) is routed via an additional flow path (10a) of said directional valve (10) and additionally controls pressure build-up is of said working fluld, or the connection from the consumer to the tank, respectively, via another hydraulically controllable directional valve (60) and another directional valve (62) which is also hydraulically controllablet with hydraulic control of said additional directional valves (60, 62) being determined by the respective control position (SE, Ny H) of said directional valve (10) controlling said working fluid.

2. Safety circuit, in particular utilizing a control according to claim 1, for a servo hydraulic control system for hydraulic control and actuation of a consumer, in particular of a working unit e.g. of a lifting unit on a utility vehicle or on an agricultural machinet wherein controlling the consumer is effected in correspondence with the position of a setting lever via a proportional valve to the one side of which a reference pressure is applied and to the other side of which a pressure in proportion with the deviation is applied,, such that in the case of a deviation of Zero (Xl - X2), it assumes a neutral position wherein the working line leading to the consumer or from the consumer is blocked, page - 19- - is characterised in that two safety valves (60, 62) are provided, one of which (60) is pretensioned Into a position (S) locking the lifting operation (H) and the other one (62) of which is pretensioned into a position (S) locking the lowering operation (E) when the control system (SHR) is in the OFF-state, said valves being switched such that. when the control system (SER) is switched on, at least the one safety valve (62 andlor 60) whereby the procedure controlling the consumer (Z) as predetermined by the currently set position (SE,, N, or H) of the proportional valve (10) may be blocked, is kept in the locked position (S) until the proportional valve (10) has been shifted past the neutral position (N) by moving a setting lover (120; 220A/ 220B).

3. Safety circuit according to claim 2,. characterized in that a respective control signal pressure (X3, X4) determined by the position of said Proportional valve (10) is applied to each of said safety valves (60. . 62).

4. Safety circuit according to claim 2 or 3, characterised In that one of said safety valves (62) determines or provides the control pressure (X4) required for safety-releasing the other safety valve (60) through Its set position (S or E).

5. Safety circuit according to any one of claims 2 to 4g characterised In that in the safety position (S)i the safety valve (62) associated with one direction of movement (H) of the consumer (Z) relieves a signal pressure (X4) derived from the power circuit (32. 36) downstream of said proportional valve to the tankt with a line branch conducting said signal pressure (X4) is routed via said other other safety valve (60) Page -1p- ? is to the control side of said safety valve (line 76) in its safety position (S).

6. Safety circuit according to claim 5, characterized in that in the safety-rel eased position (E).. the safety valve (60) associated with the other direction of movement (gE) of the consumer (Z) conveys another signal pressure (X3) derived from a pilot pressure circuit (Ply 34, 38) downstream of the proportional valve (10) to a releasable non-return valve (68) located inside a working line (36) leading to the consumer (Z)t -thile the safety valve (62) associated with the first direction of movement (H) conducts this signal pressure (X3) via a control land (5X3) of said safety valve (62) to its control side (control pressure line 80).

7.

Safety circuit according to claim 6, characterized in that a safety throttle (84) is incorporated in the line (38A) for the further signal pressure (X3) which leads to the safety valve (62) for the first direction of movement (H).

8. Safety circuit according to claim 7y characterized In that the safety valve (62) associated with the first direction of movement (H) has a sufficiently long stroke which preferably is harmonised with the safety throttle (84) in order to compensate pressure peaks of the signal pressure (X3).

9. Safety circuit according to any one of claims 5 to By characterised in that the signal pressure (X4) derived from the power circuit (36y P) downstream of the proportional valve (10) represents the pressure prevailing inside a branch line (40) branching off from the working line (36) behind a measuring orifice (42), raid branch line (40) being connected to the Page -11 - -- output (44) of a f low control valve (46) the inlet (50) of which communicates with the power circuit (32, P) upstream of the proportional valve (10).

10. Safety circuit according to any one of claims 5 to 9 for controlling a lifting unit, characterised in that the first direction of movement represents the lifting direction (H).

is

11. Safety circuit according to any one of claims 2 to 10, characterised in that said safety valves (60, 62) comprise respective strong reset springs (64, 66).

12. Safety circuit according to any one of claims 2 to 11, characterised in that said proportional valve (10) is controlled by a pair of output signals (Xl, X2) of a measuring bridge (Xl=X2) such as to assume the neutral position (N) when said measuring bridge is balanced.

13.

A servo hydraulic circuit for a hydraulic device, said circuit comprising a directional valve for selectively feeding and draining hydraulic fluid to and f rom such a device via a control valve which is hydraulically controllable via valve means whose actuation is dependent upon the position of said directional valve.

Page-21-

14. A servo hydraulic circuit comprising a directional valve for selectively controlling flow of hydraulic fluid to and from a hydraulic device and having a neutral position for blocking f low of hydraulic f luid both to and f rom said device, and further valve means f or precluding flow of hydraulic means to and from said device via said directional valve on actuation of said circuit until the directional valve is moved by actuating means thereof in a predetermined way.

15. A servo hydraulic circuit substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

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