The invention relates to a hydraulic system comprising a pressure source and a pressure sink, and also several hydraulic loads which are hydraulically connected by way of a common line branch to the supply unit and by way of an electrical bus are connected to a central processing unit, and where each load is assigned a control device which is arranged between the load and the line branch.

Hydraulic systems of that kind are needed in many different areas of application, for example on ships, in processing plants of the chemical industry, on oil- drilling platforms and similar installations. The hydraulic loads can here be of many different constructions. For example, the hydraulic loads can be in the form of hydraulic motors, cylinder-piston units, rotary actuators and similar devices. In such systems, one difficulty is that the individual hydraulic loads are often located at a considerable distance from the supply unit and also from each other. A substantial number of line branches are therefore necessary in order for the supply unit to be able to target specifically each individual load.

Only a single line branch with a pump line and a tank line as the supply and discharge line is therefore required for all hydraulic loads to be supplied with the necessary hydraulic fluid. The hydraulic fluid acts here only as an energy carrier which serves to operate the hydraulic loads, that is, for example, to drive motors or other actuators. The control information is made available by way of the electrical bus with which the individual loads are connected. The central processing unit is in this case able to

address each load specifically by way of the bus, so that on the basis of the electrical signals sent on the bus, the load is able to draw hydraulic fluid from the line branch or not. Moreover, communication opportunities between the load and the central processing unit, which together with the supply unit often forms one component, are expanded. The loads can be operated, if desired several loads simultaneously, and monitored simultaneously. Thus, for example, using a pressure sensor it is possible to monitor the hydraulic pressure at the load by way of the bus and at the same time, likewise by way of the bus, the electrical voltage.

A system as described above is known from "Hydraulics & Pneumatics", May 1979, p. 81-106, in an article titled "Microprocessor controls clinder sequencer". The described control devices are in the article shown as magnetic valves controlled by means of a signal from a micro computer. When these known magnetic valves are activated, you will have a displacement of the slide against a spring, causing that the magnetic valves require electric energy during the full activation period.

The object of the invention is to reduce the consumption of energy in a hydraulic system as described in the present invention in order to avoid dangerous situations.

This problem is solved in a system of the kind mentioned in the introduction in that the control device comprises a hydraulically operable valve arrangement and a self-latching control valve, which when operated, produces a pressure difference which is present across the valve arrangement, the control valve remaining in the switch state assumed after operation.

The hydraulic loads are often additionally affected by the problem that they are located in regions at risk from fire or explosions. This danger

is quite considerable in particular on tanker ships or oil-drilling installations. In addition to structural measures to safeguard against explosions, precautions must also be taken to ensure that as few dangerous situations as possible are able to develop, in which, for example, sparks or undesirable heat can occur. With the control device described, this is achieved in that the actual control of the valve arrangement is effected hydraulically. The valve arrangement is hydraulically operated, that is to say, opened or closed. No electrical energy is therefore required for this purpose. Only to operate the control valve is auxiliary energy briefly required and this can be produced, if desired, by means of electrically operating means. Since this auxiliary energy is required only briefly, however, namely, merely to operate the control valve, that is to say, to move a valve element from one position into another, the risk of this causing damaging heat, an undesirable rise in temperature or sparks to occur is relatively small. Because the control valve is self-latching, electrical energy is not required for holding purposes either.

The system can also be used in conjunction with very long electrical and hydraulic lines (100 m to 500 m and more) , in which the transferable power is frequently limited for protection against explosion. The control power arriving at the other end used frequently to be insufficient for operation of the loads. In the present invention it does suffice, however, because it is required here only as auxiliary energy.

It is espacially advantageous for the line branch to be in the form of a ring circuit. In this way the dependency of the pressure of the hydraulic fluid available to the hydraulic loads on the distance from the supply unit is decreased.

The control valve is preferably hydraulically operable, and an auxiliary valve arrangement which generates a hydraulic pressure difference across the control valve is provided to operate it. The control valve itself is therefore also operated not electrically but hydraulically. Only the auxiliary valve arrangement is operated electrically here. Since the auxiliary valve arrangement merely has to switch a control pressure, however, it can be of relatively small construction, which in turn reduces the electrical power consumption. An arrangement of that kind is advantageous in particular in high pressure systems, (100 bars and above) where the valves controlling the actual pressure have a considerable power requirement.

The control valve preferably comprises a slider which is arranged between two pressurizable work chambers, the slider moving towards one work chamber following a pressure drop in this work chamber and thereby creating a connection between this work chamber and the tank. Here and in the following text the term "tank" means generally the pressure sink; in a closed system, however, the pressure sink can also be formed by the intake side of a hydraulic pump. Correspondingly, "pump" denotes the pressure source. Since the two work chambers are normally pressurized with a pressure that is higher than the tank pressure, a pressure drop in one of the work chambers automatically effects a movement of the slider into this chamber. The movement of the slider creates a connection between this work chamber and the tank, so that this chamber is now permanently at the lower pressure so that the slider remains in that position until pressure equilibrium between the two work chambers is restored by other measures. It is therefore merely necessary to effect an initial movement of the slider in the control valve. It is

only for this that auxiliary forces are required. All other movements can then be controlled hydraulically.

For that purpose, the control valve has for each work chamber a slider-operable tank valve. This tank valve is operated when the slider has moved a predetermined distance towards the particular work chamber. There are various options for the design of the tank valves. The slider may operate, for example, a valve member, for instance when it has ramp-like sections. Alternatively, the tank valve can be constituted merely by apertures in the wall enclosing the slider which are caused to open or close by the slider as this moves.

Advantageously, the auxiliary valve arrangement switches a connection of the work chamber to the tank. The auxiliary valve arrangement need only effect this connection briefly, in order to reduce the pressure in the work chamber sufficiently far for the slider to move towards this work chamber. As soon as a further connection of the work chamber to the tank has been effected by means of the slider, for example, by means of the tank valve, the auxiliary valve arrangement can be disabled again. For its operation, no further energy, and in particular no electrical energy, is then required. In this manner, despite the low power of the electrical auxiliary energy, several loads can be operated simultaneously. Only the start of the operation is then slightly staggered as regards time. As soon as the loads are working, however, they can do so simultaneously. This is especially important in regions at risk from explosion where it is desirable to keep electrical power to a minimum.

The control valve is preferably also manually operable. On failure of the auxiliary energy, the function of the hydraulic system or the individual loads is not endangered by this.

The valve arrangement is preferably arranged parallel to the control valve between two control lines, the control valve reducing the pressure in one of the two control lines on being operated. This control line is then advantageously connected to the work chamber of which the pressure is being lowered for movement of the slider of the control valve. Because two control lines are now provided, in which the pressures can be set independently of one another, a very accurate control of the valve arrangement can be achieved. This can even be direction-dependent, depending on the control line in which the pressure is lowered with respect to the other.

A replenishing valve is preferably provided, the inlet connection of which is connected to a pump line of the line branch, and which is arranged parallel to the control valve and supplies both control lines with a pressure that is greater than the tank pressure when the control valve is inoperative. The line branch which serves to supply the hydraulic loads contains not only a tank line for transporting the used hydraulic fluid back to the tank or the pressure sink again, but also a pump line, which supplies pressurized hydraulic fluid to the load. This hydraulic fluid under pump pressure now also reaches the replenishing valve, whereupon the pump pressure can likewise be lowered somewhat so that the replenishing valve and the control device are not overloaded. In the inoperative state, that is, during "idling", the replenishing valve ensures that both work chambers of the control valve are at least approximately at the same pressure, which is high enough to be able to displace the slider, even against the force of a restoring spring, towards a chamber when pressure in this chamber drops. The replenishing valve here has a kind of monitoring function. The pump pressure is not introduced into the two work chambers in an uncontrolled manner.

For that purpose, the replenishing valve preferably has a slider arranged between two work chambers which is movable by a pressure difference between the work chambers, the slider displaced from its position of rest producing a smaller flow resistance from the inlet connection to the work chamber having the higher pressure than from the inlet connection to the work chamber having the lower pressure. This ensures, firstly, that the work chamber at the higher pressure necessary for displacing the slider of the control valve is continuously supplied with hydraulic fluid to prevent a standing pressure drop caused by the increase in volume of the work chamber. Secondly, this ensures that the hydraulic fluid supplied by the pressure source does not flow away to the tank without effect. The replenishing valve increases the flow resistance towards the tank so that only a little hydraulic fluid, and in an extreme case none at all, is able to enter the lower pressure work chamber of the control valve.

Preferably, the slider of the replenishing valve is arranged in a recess of a housing and unblocks or blocks control channels in the inner wall of the recess or has control slots on its outside. Such control channels can be made with the required accuracy. The flow cross-section or flow resistance of the inlet connection to the individual work chambers can therefore also be made dependent on position, which ensures that in a predetermined position of the slider the desired pressure also reaches the individual work chambers. The control channels or slots can be provided both in the inner wall of the recess and in the outer wall of the slider.

Parallel to the control valve there is preferably provided an equalizing valve arrangement with which the pressure difference across the control valve can be lowered to a predetermined minimum value. This

equalizing valve arrangement can be used in particular in two cases. Firstly, it can ensure that before the start of operation of the valve there is a pressure equilibrium between the two work chambers of the control valve so that too low a pressure is not inadvertently present in the chamber which is intended to apply the pressure required for displacing the slider. Secondly, it can be used to return the control valve to its neutral position again, that is, to terminate operation of the hydraulic load.

It is especially preferable for the equalizing valve arrangement to have a pressure-limiting function. The pressure difference between the two work chambers cannot therefore exceed a predetermined value so that the control device cannot be overloaded.

In a preferred construction, the equalizing valve arrangement can also have a manually operable hand valve. The hand valve provides a guarantee that the control arrangement can be operated even when the auxiliary energy has failed.

It is also preferred for the equalizing valve arrangement to have at least one valve operated by auxiliary force. The work of the control device can thereby be automated or remote-controlled.

Advantageously, the valve arrangement is in the form of a bridge in which diagonally opposite valves are opened simultaneously by the pressure difference, the line branch being connected to two diagonally opposite bridge points and the load being connected between the two remaining bridge points. Such a valve arrangement enables the load to be supplied with hydraulic fluid in dependence on direction. The directional control is then effected simply by reversing the pressure being applied by way of the valve arrangement.

The auxiliary valve arrangement preferably has two valves, each of which is connected to a respective work

chamber of the control valve. Operation of one of the two auxiliary valves therefore determines the direction in which the hydraulic load is supplied. If one of the two auxiliary valves is operated, the pressure on one of the two control lines is lowered and is kept lowered even after operation has ended. On operation of the other auxiliary valve, the pressure on the other control line is correspondingly lowered and kept lowered.

A hand pump is preferably arranged parallel to the load between the valve arrangement and the load. The load can therefore still be operated even in the event of failure of the hydraulic supply.

Adjustable throttles are advantageously arranged in the pump and/or tank line, the work chambers of the control valve being connectable to a point between throttle and tank. The throttles can be used specifically for each individual load for adjustment of the pressure and the quantity which is supplied to the load in operation. The tank pressure used for the control remains unaffected by the throttles.

The control device is preferably arranged to be remote-controlled. For example, the electrical signals arriving by way of the bus or bus line can be used for that purpose. Altogether, this creates a more compact construction of the load with associated control device, although the complete system manages with relatively few lines, both hydraulic and electric.

The control device preferably has a control unit connected to the bus line which operates the auxiliary valve arrangement and/or the equalizing valve arrangement on the basis of signals present on the bus line. The control unit is therefore the interface between the electrical signals and the hydraulic changes in state in the control device resulting therefrom.

The control unit preferably operates the equalizing valve arrangement before each operation of the auxiliary valve arrangement. This can be effected automatically without separate commands that have to be fed through the bus line being required for that purpose. As mentioned above, the equalization of pressure before operation of the auxiliary valve arrangement produces improved operating behaviour.

The control unit is preferably connected to a state detector of the hydraulic load and operates the equalizing valve arrangement when an actual state detected by the state detector corresponds with a predetermined desired state. The control unit therefore terminates operation of the load, for example, when the load is in the form of an actuator and has reached a desired position. Here, the desired state, that is, for example, the desired position or the desired deflection, can be fixed in advance in the associated control unit for each load individually. Alternatively, the desired state can be transferred by electrical signals by way of the bus line. In each case, movement of the hydraulic load can be specifically controlled.

In an especially advantageous construction, provision is made for the replenishing valve and the control valve to assume a neutral position in the event of failure of the pump pressure and to ensure that the valve arrangement closes and the load stays in its assumed position. On failure of the pump or control pressure, operation comes to a standstill, that is, no dangerous actions occur.

The invention also relates to a method of controlling a load in a hydraulic system by means of electrical signals, in which method the load starts its operation on receiving an electrical signal and continues its operation once the electrical signal has

ended until it is mechanically blocked or receives an electrical END signal.

By means of such a method it is possible to restrict the non-hydraulic auxiliary energy required for operation of the hydraulic load to the absolute minimum amount. In principle, a short auxiliary energy pulse which can be constituted, for example, by a short electrical signal, is sufficient. With such a control method "dangerous" times, in which heat or sparks may occur because of transfer of electrical energy, are kept relatively short. In addition, several loads can be operated simultaneously. The start of the operation is merely slightly staggered in time. Electrical energy consumption remains low.

Here, when several loads are present, in the normal state all loads are preferably kept in an electrical state of rest, in which they are merely ready to receive electrical signals and no opportunity for action exists, and on the appearance of an electrical signal on the bus line all loads are changed to a standby state, in which they are capable of evaluating subsequent electrical signals for their relevance and initiating the actions ordered by these signals. The time in which heat or sparks can be generated is thereby reduced to the absolute minimum. Normally, all loads are electrically inactive, that is to say, they must not even be able to perform any computing or control operations, excepting functions that serve for safety, for example monitoring functions. Only when an electrical signal appears on the bus line are all loads changed to a standby state in which they can be specifically addressed by the signal or by subsequent signals. In the standby state, for example, each load can evaluate and monitor addresses from the signal, whether it itself is being addressed or not. Since all loads are actuated simultaneously, potential errors, for example through

failure of an addressed load to respond, are kept relatively low. In addition, in the time in which all loads are actuated, the loads can be monitored by way of the bus system.

Each load preferably returns to its state of rest a predetermined time after the last electrical signal. No separate termination signal therefore has to be sent to effect this change to the state of rest. On the contrary, this change is effected automatically after a predetermined time, which can be selected to be relatively short. It only has to be long enough for the load to be able to distinguish it reliably from other signal pauses.

The invention is explained hereinafter with reference to a preferred embodiment in conjunction with the drawing, in which Fig. 1 shows a first embodiment of a hydraulic system, Fig. 2 shows a second embodiment of a hydraulic system, Fig. 3 is a diagrammatic illustration of a load and Fig. 4 shows an example of the construction of a load.

A hydraulic system 1 comprises a supply unit 2 and a central processing unit 3. The supply unit 2 comprises a pump, not specifically illustrated and a tank, also not specifically illustrated, for hydraulic fluid. The pump removes hydraulic fluid from the tank and conveys it at a high pressure (100 bars and above) , which corresponds at least to the operating pressure of hydraulic load stations 4, in a line branch 5, which is in the form of a ring circuit. The line branch 5 has a pump line, with which hydraulic fluid under pressure is conveyed from the supply unit 2 to the load stations 4, and a tank line, in which pressureless hydraulic

i fluid is returned from the load stations 4 back to the tank of the supply unit 2 again.

Virtually any hydraulically-operable elements can be used as loads in the load stations 4, such as motors, actuators, valve-operating elements, piston- cylinder units, rotary actuators or similar devices.

The central processing unit 3 is connected electrically by way of a bus line 6 to the load stations 4. By way of the bus line 6, the central processing unit 3 is able to inform each load station 4 whether and how it is to be set in operation. The use of a bus line for controlling remotely located addresses is known per se. (see, for example, US 4 530 045 or US 4 701 938) .

When the central processing unit sends the corresponding signals by way of the bus line 6 (generally also referred to as the "bus"), it is able to operate and monitor each individual load station 4 specifically. The load station 4 can draw the energy necessary for its drive from the line branch 5. The required line lengths and the number of lines is accordingly kept to the absolute minimum, both in respect of hydraulic lines and in respect of electrical lines. Nevertheless, reliable operation of the hydraulic system 1 is possible. The system can therefore also be used even when there are large distances between the load and the supply unit.

Fig. 2 shows a modified embodiment of the hydraulic system 1', in which only the bus line 6' is in the form of a ring circuit. The other components remain unchanged. Here too, the line branch 5 for the hydraulic fluid is in the form of a ring circuit. The load stations 4 are controlled remotely by way of the central processing unit 3.

Fig. 3 shows a single load station 4 with a hydraulic load 7, which is in the form of a linear operating element, for example, and moves a plunger 8

to and fro in the direction of the double-ended arrow 9. The movement, that is, the position, of the plunger 8 is detected by a position detector 10.

The load 7 is connected by way of a valve arrangement 11 to the line branch 5 which has a pump line P and a tank line T. The valve arrangement 11 is connected by way of two control lines I and II to a control device 12, which in its turn is hydraulically connected likewise to the line branch 5 and is also, by way of a control unit 13, electrically connected to the bus line 6. The control unit 13 is also connected to the detector 10 which is able to report the position of the plunger 8 to the control unit 13.

The more detailed construction of the valve arrangement, control device and control unit is apparent from Fig. 4.

Identical parts are here provided with the same reference numbers.

The control device 12 comprises the valve device 11 consisting of four valves VI, V2, V3, V4. The valve device 11 is arranged between the load 7 and the line branch 5. The four valves VI to V4 of the valve arrangement 11 are arranged in the form of a bridge. In this case, the pump line P of the line branch is connected by way of a stop valve 15 and an adjustable throttle 16 to a point of the bridge. The tank line T of the line branch is connected by way of a stop valve 17 and an adjustable throttle 18 to the diagonally opposite point of the bridge. The load 7 is connected to the two remaining bridge points. The valves VI, V4 of the valve arrangement 11 are hydraulically controllable. They are therefore connected to the first control line I and to the second control line II respectively, so that two diagonally opposite valves VI and V4, and V2 and V3 are always jointly moved into an open position. Depending on which valve pair is

opened, there is a flow of hydraulic fluid in one or other direction through the hydraulic load 7.

Between the valve arrangement 11 and the load 7 there is provided a hand pump 19 parallel to the load 7 and, in an emergency, if the hydraulic pressure on the line branch 5 fails, the hand pump can be used to control the hydraulic load, for example to bring a hydraulic actuating element into the desired position.

To actuate the valve arrangement 11, a control valve 14 is provided in the control circuit 12. This control valve 14 has a slider 20 which is arranged between two work chambers 21, 22, and is capable of being displaced against the force of springs 23, 24 into one or other work chamber 21, 22. The slider 20 has a constriction 25, for instance in its axial middle area, so that with its housing 26 it there forms a tank chamber 27 which is connected by way of a line 28 to the tank line T. The line 28 emerges between the stop valve 17 and the adjustable throttle 18 into the line connected to the tank line T between the line branch 5 and valve arrangement 11. The constriction 25 simultaneously forms camming faces for operation of the tank valves 29, 30.

The adjustable throttles 16 and 18 serve to limit the pressure, or the volume, through the hydraulic load 7.

The tank chamber 27 is connected by way of a tank valve 29 to the first control line I and by way of a tank valve 30 to the second control line II. The two tank valves 29, 30 are operated by displacing the slider 20. In the embodiment, they are illustrated as separate valves. Alternatively, they can be formed by slots in the inner wall of the housing 26 which are caused to open or close by movement of the slider 20. The slider 20 is also movable by means of a handle 31 in the direction of the double-ended arrow 32.

Parallel to the contro] valve 14 there is provided a replenishing valve 33, which likewise comprises a valve slider 34 which is arranged between two work chambers 35, 36. The slider is movable against the force of springs 37, 38 in the replenishing valve. The springs 37, 38 are, like the springs 23, 24 of the control valve 14, designed so that the sliders, 34 and 20 respectively, remain in a neutral or central position without the presence of external forces.

The replenishing valve 33 has an inlet connection 39 which is connected to the pump line P by way of a pressure-compensating valve 40, which ensures a constant low control pressure. The inlet connection is connected by way of control slots 41, 42, shown only diagrammatically, to regions inside the replenishing valve 33 which in the neutral position of the slider 34 are covered by the slider. If, however, the slider 34 is moved, it produces a connection between inlet connection 39 and the work chambers 35, 36. Furthermore, in a manner not illustrated, a flow path that is open in the neutral position for a small quantity of hydraulic fluid can be provided in the slider or even in the housing of the replenishing valve 33, and ensures that at least in the neutral position of the slider 34 the two work chambers 35, 36 are filled with hydraulic fluid under pressure.

Since the replenishing valve 33 is arranged parallel to the control valve 14, the one work chamber 35 is connected to the work chamber 21 of the control valve 14 connected to the first control line I, whereas the other work chamber 36 of the replenishing valve 33 is connected to the work chamber 22 of the control valve 14 which is also connected to the second control line II.

Furthermore, parallel to the control valve 14 there is arranged a manually operable equalizing valve 43, which can be opened by way of a handle 44.

Parallel to the manually operable equalizing valve, an electromagnetically-operable equalizing valve 45 is connected between the two control lines I and II. This equalizing valve has at the same time a pressure- limiting function, that is to say, the differential pressure between the two control lines I and II cannot exceed a predetermined value.

The control line I is connected by way of a first electromagnetic valve MV1 to the line 28, which leads to the tank. The second control line II is connected by way of a second electromagnetic valve MV2 to the same line 28. Both valves MV1 and MV2 also have a pressure-limiting function. The two valves MV1 and MV2 together form an auxiliary valve arrangement 46.

The pressure in the first control line I causes the valve VI to close, the valve V2 to open, the valve V3 to open and the valve V4 to close. The pressure in the second control line II causes the valve VI to open, the valve V2 to close, the valve V3 to close and the valve V4 to open. All valves VI to V4 of the valve arrangement 11 are held in the closed position by springs. They can be opened only when the pressure difference in the control lines I and II exerts a force on the valves that is greater than the force exerted by the springs.

The electromagnetically-operable valves 45, 46 are actuated by the control unit 13. This control unit derives the information on whether a valve, and if so which valve, is to be actuated from the bus line 6.

The control unit 13 is normally inactive. It is then in a state of rest in which it is merely able to recognize whether electrical signals are arriving on the bus line 6 or not. In this state, however, it has no opportunity to operate, that is to say, cannot produce electrical signals to actuate the electromagnetic valves 45, 46. Only when any signals appear on the bus line is the control device 13, or

more accurately, is evaluating part 47, activated and then evaluates the signal arriving on the bus line 6 for its relevance, for example as to whether an address contained in the signal applies to its own station or not. If it does apply, the following commands are given.

The development of such a command trace proceeds as follows: first of all the electromagnetic valve 45 is briefly actuated to create a pressure equilibrium between the two control lines I and II and thus also between the work chambers 21 and 22 and work chambers 35, 36, respectively. For that purpose, the electromagnetic valve 45 needs to be opened only very briefly. The current pulse which causes the opening is correspondingly short. No appreciable increase in temperature can take place. The risk of spark formation is kept low.

The equalization of pressure between the two control lines I and II also creates a defined state of the valve arrangement 11. All valves VI to V4 are then in a closed state. The sliders 20, 34 of the control valve 14 and replenishing valve 33 respectively are in their middle position.

Depending on the direction in which the hydraulic fluid is to flow through the load 7, one of the two electromagnetic valves MV1, MV2 of the auxiliary valve arrangement 46 is then actuated. The following account is given by way of example with reference to the electromagnetic valve MV1 and the control line I. A corresponding account applies analogously to the electromagnetic valve MV2 and the control line II.

When the electromagnetic valve MV1 is opened, a connection is established between the control line I and the tank line T. The pressure in the control line I therefore drops to the tank pressure, which is lower than the pressure at the inlet connection 39 of the replenishing valve 33. Up until this time, this

pressure at the inlet connection 39 of the replenishing valve 33 has been the pressure prevailing in the work chambers 21, 22 and the work chambers 35, 36. Opening of the electromagnetic valve MV2 causes the pressure in the work chamber 21 of the control valve 14 and the pressure in the work chamber 35 of the replenishing valve 33 likewise to drop to tank pressure, so that the pressure on the respective other side of the slider pushes the slider downwards into the work chamber 21 and into the work chamber 35 respectively. In the case of the control valve 14, the tank valve 29 is now actuated, which creates a connection between the line. 28 and the control line I so that the control valve 14 is self-latching even after closure of the electromagnetic valve MV1, that is to say, the slider 20 stays in this position.

At the same time, the slider 34 of the replenishing valve 33 is also pushed into the work chamber 35 so that hydraulic fluid under pump pressure is able to pass by way of the control slot 42 into the work chamber 36. This pressure in the work chamber 36 is then, of course, also the pressure in the control line II. It also assists the self-latching of the control valve 14 by keeping the slider 20 in the desired position. Since the control line I is now virtually at tank pressure, but the control line II is at control pressure, for example pump pressure, the valves VI and V4 of the valve arrangement 11 are opened and the load 7 is correspondingly actuated. The actuation of the load 7, that is, for example, its deflection or movement, is continued until the load 7 meets a mechanical stop and is unable to move further. In that case, the pressure at the load 7 may be pump pressure, but with suitable design of the load 7 this is tolerable.

Provision may also be made for the position of the load 7 to be detected by the detector 10 and reported

back to the evaluating part 47 of the control unit 13. As soon as the position of the load has reached a predetermined desired value, the evaluating part 47 can now supply a new current pulse to the electromagnetic valve 45 which opens, and thus creates a pressure equilibrium between the two control lines I and II. This pressure equilibrium firstly causes all valves V1-V4 of the valve arrangement 11 to close, and secondly, causes the two sliders 20, 34 of the control valve 14 and the replenishing valve 33 respectively to return to their neutral position. Operation of the hydraulic load 7 therefore ends. The load 7 remains in the position it has assumed until it is returned again at a new command. The desired value of the state, that is, for example, the position of the hydraulic load 7, can be reported by way of the bus line 6 to the evaluating part 47. Alternatively, it can be stored in the evaluating part 47 in advance for each load station.

Should the electrical control option fail, the manually operable equalizing valve 43 is provided. This also enables pressure between the two control lines I and II to be equalized. Similarly, if the electromagnetic valves MV1 and MV2 fail, the valve arrangement 11 can also be operated by way of the handle 31, by means of which the slider 20 of the control valve 14 can be moved in the direction of the double-ended arrow 32.

The control unit 13 need not be separately returned to its passive state or state of rest after operation of the electromagnetic valves 45, 46. This change-over can be effected automatically a predetermined short time after the end of the last signal. This ensures that currents are able to flow at all only at very short, clearly defined time intervals. Since the electromagnetic valves 45, 46 each have to open only for a very short time to effect

a control pressure pulse which initiates the further procedure, here too there is little danger that excess heat will be generated or sparks will be formed to an appreciable extent. For safety reasons, however, in many cases a part of the control unit 13 which contains the electromagnetic valves 45, 46, will additionally be housed in an explosion-proof area 48.