WO1994012795A1 - Systeme hydraulique - Google Patents

Systeme hydraulique Download PDF

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Publication number
WO1994012795A1
WO1994012795A1 PCT/DK1993/000370 DK9300370W WO9412795A1 WO 1994012795 A1 WO1994012795 A1 WO 1994012795A1 DK 9300370 W DK9300370 W DK 9300370W WO 9412795 A1 WO9412795 A1 WO 9412795A1
Authority
WO
WIPO (PCT)
Prior art keywords
valve
tank
pilot
load
connection
Prior art date
Application number
PCT/DK1993/000370
Other languages
English (en)
Inventor
Thorkild Christensen
Siegfried Zenker
Original Assignee
Danfoss A/S
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Danfoss A/S filed Critical Danfoss A/S
Priority to EP94900773A priority Critical patent/EP0670967A1/fr
Publication of WO1994012795A1 publication Critical patent/WO1994012795A1/fr

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B11/00—Servomotor systems without provision for follow-up action; Circuits therefor
    • F15B11/16—Servomotor systems without provision for follow-up action; Circuits therefor with two or more servomotors
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/30—Directional control
    • F15B2211/305—Directional control characterised by the type of valves
    • F15B2211/30525—Directional control valves, e.g. 4/3-directional control valve
    • F15B2211/3053—In combination with a pressure compensating valve
    • F15B2211/30535—In combination with a pressure compensating valve the pressure compensating valve is arranged between pressure source and directional control valve
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/50—Pressure control
    • F15B2211/505—Pressure control characterised by the type of pressure control means
    • F15B2211/50509—Pressure control characterised by the type of pressure control means the pressure control means controlling a pressure upstream of the pressure control means
    • F15B2211/50536—Pressure control characterised by the type of pressure control means the pressure control means controlling a pressure upstream of the pressure control means using unloading valves controlling the supply pressure by diverting fluid to the return line
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/50—Pressure control
    • F15B2211/505—Pressure control characterised by the type of pressure control means
    • F15B2211/50563—Pressure control characterised by the type of pressure control means the pressure control means controlling a differential pressure
    • F15B2211/50581—Pressure control characterised by the type of pressure control means the pressure control means controlling a differential pressure using counterbalance valves
    • F15B2211/5059—Pressure control characterised by the type of pressure control means the pressure control means controlling a differential pressure using counterbalance valves using double counterbalance valves
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/50—Pressure control
    • F15B2211/515—Pressure control characterised by the connections of the pressure control means in the circuit
    • F15B2211/5159—Pressure control characterised by the connections of the pressure control means in the circuit being connected to an output member and a return line
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/50—Pressure control
    • F15B2211/52—Pressure control characterised by the type of actuation
    • F15B2211/528—Pressure control characterised by the type of actuation actuated by fluid pressure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/60—Circuit components or control therefor
    • F15B2211/605—Load sensing circuits
    • F15B2211/6051—Load sensing circuits having valve means between output member and the load sensing circuit
    • F15B2211/6054—Load sensing circuits having valve means between output member and the load sensing circuit using shuttle valves
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/60—Circuit components or control therefor
    • F15B2211/605—Load sensing circuits
    • F15B2211/6051—Load sensing circuits having valve means between output member and the load sensing circuit
    • F15B2211/6055—Load sensing circuits having valve means between output member and the load sensing circuit using pressure relief valves
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/70—Output members, e.g. hydraulic motors or cylinders or control therefor
    • F15B2211/71—Multiple output members, e.g. multiple hydraulic motors or cylinders
    • F15B2211/7142—Multiple output members, e.g. multiple hydraulic motors or cylinders the output members being arranged in multiple groups
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
    • F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
    • F15B2211/00—Circuits for servomotor systems
    • F15B2211/70—Output members, e.g. hydraulic motors or cylinders or control therefor
    • F15B2211/78—Control of multiple output members
    • F15B2211/781—Control of multiple output members one or more output members having priority

Definitions

  • the invention relates to a hydraulic system having a pump with fixed positive displacement, a tank and a valve group connected to the tank by way of a tank line, the valve group having at least one load valve and a control section containing a pilot overflow valve for control of the hydraulic fluid supplied on demand to the load valve.
  • a system of that kind is known from the Danfoss range of proportional valves, for example from the company publication "Directional Valve, Proportional Valve, Type PVG", edition 2/88, page 4.
  • several load valves each of which controls a respective load, are combined to form a block.
  • Hydraulic fluid not required by the loads is returned to the tank again. So that this fluid also can be used, it is known from US 4 262 580 to transfer hydraulic fluid not required by the loads by way of an auxiliary valve arrangement to a downstream hydraulic load, which has the advantage that the loads controlled by way of the valve group are given preference, that is to say, priority between the loads and further loads is guaranteed.
  • the pressure in the tank line may change, namely, as a function of the pressure requirement of the further loads.
  • this is of no great importance since the flow of liquid supplied to the load valves is not controlled according to demand.
  • the simple connection of an auxiliary valve arrangement to the tank line (the so-called "power-beyond") is impossible, because the effect on the control of the hydraulic fluid supplied to the load valves would be intolerable.
  • parts connected to the tank line would have to be of sufficiently robust dimensions to cope with the high pressures. For that purpose a relatively robust or strong material would be required, which more often than not has to be ruled out for reasons of cost.
  • the discharge side of the pilot overflow valve it is especially preferred for the discharge side of the pilot overflow valve to be connected directly to the tank.
  • the tank in itself is without pressure, that is to say, the pressure prevailing in it is the atmospheric or ambient pressure.
  • a direct connection of the discharge side of the pilot overflow valve therefore ensures that the pressure on the discharge side of the pilot overflow valve is, as before, atmospheric pressure.
  • the pressure is constant. This simplifies subsequent control of the hydraulic fluid through precision control of the load valves.
  • the load valve prefferably in the form of a proportional valve. In this way the connected loads can be controlled relatively precisely with minimum expense.
  • the load valve is hydraulically operable, in particular electro- hydraulically operable, and has a first supply connection which is connected to the pump by way of a pilot reducing valve; the pilot reducing valve is connected by way of a leakage oil outlet directly to the tank.
  • Hydraulically operable load valves are known per se. They can advantageously be remote- controlled. For that purpose, using a remote-control device which can be operated, for example, electrically and/or mechanically, hydraulic pressure is applied to an end face of a slide in the load valve in order to displace the slide, if desired against the force of a spring. The hydraulic pressure is supplied via the first supply connection. To ensure that this hydraulic pressure remains at a predetermined value, the pilot reducing valve is provided.
  • the pilot reducing valve can have, for example, a slide, as explained in detail in DE 38 40 865 C2, which on one side is biased by pressure behind the pilot reducing valve, that is, by pressure at the first supply connection, and on the opposite side is biased by a spring and the pressure in the control outlet, which in the known case is formed by the tank outlet. If the pressure at the first supply connection now increases beyond a predetermined extent, the pilot reducing valve opens against the force of the spring and against the pressure at the control output and allows hydraulic fluid to flow away. If the control output were then to be subjected to changing pressures, the pressure at the first supply connection would also change correspondingly. By connecting the control output directly to the tank, fluctuations in pressure that could be caused by the further hydraulic loads can be avoided.
  • the load valve has a second supply connection acting oppositely to the first supply connection and connected to the tank.
  • a somewhat more complicated control of the movement of the slider of the load valve could also compensate for the changing pressures at the first supply connection. But it is still impossible to ensure that the load valve will not be subjected to pressures that are too high. Some materials, such as diecast aluminium, for example, would be unable to withstand such a loading. Connecting the second supply connection directly to the tank eliminates this problem. The pressures then at the load valve are the same as those that previously existed in the valve group.
  • the load valve to have a tank connection which is directly connected to the tank.
  • the auxiliary valve group is still supplied with the hydraulic fluid that is not taken up by the load valves, but because the hydraulic fluid flowing back through the load valve or valves generally has only a slight pressure or no pressure at all, the use of this fluid can be omitted without appreciable loss.
  • the valve group is preferably constructed in a modular manner with elements flange-mounted on one another; a first end element (input module) comprises the pilot overflow valve and the tank line connected to the auxiliary valve arrangement and a second end element (final module) comprises the pilot reducing valve and a pilot tank connection connected directly to the tank.
  • the pilot overflow valve and the pilot reducing valve are arranged separately from one another in different end elements. This simplifies the construction of the valve group and enables the connections, which can be connected to the auxiliary valve arrangement, to be separated more easily from the connections that have to be taken directly to the tank. Even unskilled operatives are able to assemble such a valve group relatively easily.
  • the tank connection of the load valve advantageously terminates in one end element and is led through the other.
  • the tank connection of the loads is therefore connected either still to the tank line connected to the auxiliary valve arrangement, or is led directly to the tank.
  • connection to the tank line and to the pilot tank connection to be provided in each end element, and to be interruptible.
  • interruption it is sufficient, for example, for a plug or other barrier to be inserted in the corresponding connection.
  • the valve group can therefore be switched over relatively easily from one function to another.
  • Fig. 1 shows a state-of-the-art hydraulic system
  • Fig. 2 shows a first embodiment of a hydraulic system with an auxiliary valve arrangement
  • Fig. 3 shows a second embodiment.
  • FIG. 1 A state-of-the-art hydraulic system is illustrated in Fig. 1.
  • a pump 1 of fixed positive displacement that is to say, a constant volumetric displacement, is connected via a pump line 2 to a control section 3.
  • the control section 3 forms an end element of a valve group 4.
  • the valve group 4 comprises a series of proportional valves 5, 6, 7 which, in that order, control a rotary motor 8, a bidirectional hydraulic piston-cylinder unit 9 and a unidirectional hydraulic piston cylinder unit 10.
  • Each proportional valve 5 to 7 has a pump connection 11 connected to the pump line 2 and a tank connection 12. Furthermore, a load-sensing line 13 is provided on which, by way of change-over valves 14, the largest pressure required from all loads is relayed to the control section 3.
  • the load-sensing line 13 is connected to the tank connection 12 by an end element 15 flange-mounted on the valve group 4 at the opposite end to the control section 3.
  • the control section 3 comprises a pilot overflow valve 16 which is connected at its inlet side to the load-sensing line 13 by way of a throttle 17 and on its output side is connected to a tank line 18.
  • the pilot overflow valve 16 has a slide which is biased on one side by a spring and on the other side by the pressure upstream of the pilot overflow valve 16.
  • the pilot overflow valve guarantees a constant pressure on its input side corresponding to the force of the spring 19. This pressure acts together with a spring 20 on a slide of a pressure balance valve 21, the other side of which is biased by the pressure of the pump 1.
  • the pressure balance valve 21 connects the pump line 2 with the tank line 18.
  • the tank line 18 is also connected to the tank connection 12.
  • the pressure balance valve 21 can be controlled so that the proportion of hydraulic fluid not required by the loads 8 to 10 can be returned directly to the tank again. Control of the hydraulic fluid supplied to the loads 8 to 10, or rather to the proportional valves 5 to 7, is therefore effected in accordance with demand, the demand being communicated to the control section 3 by way of the load-sensing line 13.
  • the proportional valves 5 to 7 each have a slide 23 which can be displaced by remote control.
  • a remote control connection 24 is provided to which remote control signals can be applied.
  • a pressure that is present at a first supply connection 25 can be passed to the end face of the slide 23 in order to displace this correspondingly against the force of a restoring spring 26.
  • Displaced hydraulic fluid is then able to flow to the tank connection 12 via a line which forms a second supply connection 27.
  • hydraulic fluid can be suctioned subsequently via this second supply connection 27.
  • the first supply connection 25 is connected via a pilot reducing valve 28 to the pump 1.
  • the pilot reducing valve 28 ensures, in a manner known per se. that there is always a constant pressure at the first supply connection 25.
  • the pilot reducing valve 28 has a slide which is biased from one side with the pressure behind the pilot reducing valve 28, that is to say, the pressure at the first supply connection 25, and is biased on the other side by a spring 29 and by the pressure at a control output 30. If the pressure at the first supply connection 25 increases beyond an extent that is determined by the force of the spring 29 and the pressure at the control output 30, then the pilot reducing valve 28 opens and allows the hydraulic fluid supplied from the pump 2 to flow away to the tank 22 again.
  • a first modification compared with the construction shown in Fig. 1 relates to the control section 103.
  • a pilot tank line 32 has been added.
  • the pilot tank line 32 is connected to the output of the pilot overflow valve 16.
  • the pilot tank line 32 is led through the proportional valves 105, 106, 107 and opens directly into the tank 22.
  • two tanks 22 are shown here. It is clear, however, that these can be embodied physically by a single container.
  • a second modification in the control section 103 consists in that the pilot reducing valve 28 has been removed. It is now arranged in the end element 115. As before, it is arranged between the pump connection 11 and the first supply connection 25. Only the control output 30 opens into the pilot tank line 32.
  • a third modification concerns the proportional valves 105, 106, 107.
  • the second supply connection 127 is no longer connected to the tank connection 12, but to the pilot tank line 32.
  • the valve 23 can be controlled only in one direction is illustrated. In that instance, the second supply connection 127 is connected directly to the tank. If it is desirable to operate the valve 23 in the other direction as well, the connections would have to be interchanged, if desired using suitable auxiliary valves.
  • the effect of the first modification that is, connecting the output of the pilot overflow valve 16 directly to the tank 22, is that the proportional valves 105, 106, 107 can continue to be supplied with hydraulic fluid on demand, without having to take pressure fluctuations in the tank line 118 into account
  • the two other modifications essentially serve to protect the proportional valves 105, 106, 107 against pressures that are too great, so that, as before, they can be manufactured from materials that withstand only a limited pressure stress.
  • isolating the proportional valves 105, 106, 107 from the pressures by way of the slide 23 also simplifies remote control.
  • Fig. 3 shows a further embodiment in which parts that correspond to parts in Fig. 2 are provided with the same reference numerals and corresponding parts are provided with reference numerals increased by 100.
  • the single modification to the embodiment shown in Fig. 3 compared with the embodiment of Fig. 2 is that the tank connection 12 is no longer, as in Fig. 2, connected to the tank line 118 by way of the control section 103. On the contrary, this connection is interrupted in the control section 203.
  • the tank connection 12 is instead connected in the end element 215 to the pilot tank line 32.
  • the hydraulic fluid flowing back by way of the proportional valves 105, 106, 107 from the loads 8, 9, 10 no longer flows through the auxiliary valve arrangement 31, but directly to the tank 22.
  • the loads 8, 9, 10 are also isolated from pressure changes because of the auxiliary valve arrangement 31.
  • the tank connection 12 is therefore connected to the tank 22 either only by way of the control section 103, which forms an end element of the valve group 104, or only by way of the other end element 215 of the valve group 204.
  • the connection between the tank connection 12 and the tank line 118 and between the tank connection 12 and the pilot tank line 32 can be arranged respectively both in the control section 103 and in the control section 203 and both in the end element 115 and in the end element 215.
  • This connection can be interrupted however, if this should be necessary for the construction of the hydraulic system.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Fluid Mechanics (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Fluid-Pressure Circuits (AREA)

Abstract

Système hydraulique possédant une pompe (1) à caractéristique volumétrique constante, un réservoir (42) et un groupe de soupapes reliées au réservoir (22) par l'intermédiaire d'un conduit (118) du réservoir. Le système possède au moins une soupape de pression (105 à 107) et une section de commande (103) comportant une soupape pilote de trop-plein (16) servant à réguler le fluide hydraulique alimentant la soupape de pression en fonction de la commande. Dans un système de ce type, il est souhaitable de pouvoir doter le conduit d'un ensemble à soupapes auxiliaires sans nuire au fonctionnement du système. A cette fin, on a prévu, outre l'ensemble à soupapes auxiliaires (31) dont est doté ledit conduit (118), de relier le côté de refoulement de la soupape pilote de trop-plein (16) à un point à pression constante afin de réguler une pression hydraulique supplémentaire.
PCT/DK1993/000370 1992-11-20 1993-11-11 Systeme hydraulique WO1994012795A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
EP94900773A EP0670967A1 (fr) 1992-11-20 1993-11-11 Systeme hydraulique

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DEP4239109.1 1992-11-20
DE19924239109 DE4239109C1 (de) 1992-11-20 1992-11-20 Hydraulisches System

Publications (1)

Publication Number Publication Date
WO1994012795A1 true WO1994012795A1 (fr) 1994-06-09

Family

ID=6473310

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/DK1993/000370 WO1994012795A1 (fr) 1992-11-20 1993-11-11 Systeme hydraulique

Country Status (4)

Country Link
EP (1) EP0670967A1 (fr)
CA (1) CA2147477A1 (fr)
DE (1) DE4239109C1 (fr)
WO (1) WO1994012795A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2754571A1 (fr) * 1996-10-16 1998-04-17 Rexroth Sigma Dispositif de distribution hydraulique multiple

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19536663A1 (de) * 1995-09-30 1997-04-03 Wabco Gmbh Einrichtung zur Steuerung des Druckes in Druckmittelleitungen
DE19714141A1 (de) * 1997-04-05 1998-10-08 Mannesmann Rexroth Ag Hydraulische Steueranordnung
DE102004048684A1 (de) * 2004-10-06 2006-04-13 Bosch Rexroth Ag Hydraulische Steueranordnung
WO2015094142A1 (fr) * 2013-12-17 2015-06-25 Hema Endüstri̇ Anoni̇m Şi̇rketi̇ Système de soupape qui supporte une charge et qui égalise la pression dans des installations hydrauliques

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4262580A (en) * 1972-12-15 1981-04-21 Trw Inc. Power steering system with auxiliary power capability
DE3840865C2 (fr) * 1988-12-03 1990-09-13 Danfoss A/S, Nordborg, Dk

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4262580A (en) * 1972-12-15 1981-04-21 Trw Inc. Power steering system with auxiliary power capability
DE3840865C2 (fr) * 1988-12-03 1990-09-13 Danfoss A/S, Nordborg, Dk

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2754571A1 (fr) * 1996-10-16 1998-04-17 Rexroth Sigma Dispositif de distribution hydraulique multiple

Also Published As

Publication number Publication date
DE4239109C1 (de) 1994-06-30
EP0670967A1 (fr) 1995-09-13
CA2147477A1 (fr) 1994-06-09

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