EP0552841A2

EP0552841A2 - Method and device for detecting undissolved gas in a hydraulic control system

Info

Publication number: EP0552841A2
Application number: EP93200129A
Authority: EP
Inventors: Klaus Leonhard Witt; Willi Schneider
Original assignee: Sun Electric Systems BV
Current assignee: Sun Test Systems BV
Priority date: 1992-01-23
Filing date: 1993-01-19
Publication date: 1993-07-28
Anticipated expiration: 2013-01-19
Also published as: DE69325190T2; EP0552841B1; EP0552841A3; ES2134239T3; DE69325190D1; ATE181142T1

Abstract

Method and device for detecting undissolved gas in a hydraulic control system of the type containing a hydraulic-medium-pressure pump and a feed reservoir, based upon checking the compressibility of the medium and deducting the amount of undissolved gas from the thus obtained information by taking into account that an increase of the amount of undissolved gas in the medium results in a decrease of the compressibility of this medium.

Description

The invention relates to a method of detecting undissolved gas in a hydraulic control system of the type containing a hydraulic-medium pressure pump and a feed reservoir, and also to a device for carrying out said method.
The presence of undissolved gas in a hydraulic system can result in cavitation damage, impairs the operational reliability and may even cause an unexpected, sudden and complete failure of the hydraulic system. The presence of undissolved gas is therefore a latent danger, which is furthermore increased by the fact that the occurrence of gas bubbles and the displacement thereof through the system is unpredictable. It is clear that the presence of free gas in the positive-displacement pump can have catastrophic consequences for the operation of the system if the pressure in the system completely or partly disappears.
Up till now detecting the presence of undissolved gas in the hydraulic medium was done by taking samples which were analyzed in an external measurement set-up; it was hoped to counteract the occurrence of free gases by frequently venting the system; it was however, impossible to determine the need for this measure.
The object of the invention is to provide a method and a device for carrying it out which make it possible to detect the presence of free gas and to determine the amount thereof in a simple way without removing any component and without the need for a complicated external measurement set-up.
The invention is based on the insight that the presence of undissolved gas in the hydraulic medium of a hydraulic control system has a direct influence upon the compressibility of thin medium. The method of the invention is therefore carried out by checking the compressibility of the medium and deducting the amount of undissolved gas from the thus obtained information by taking into account that an increase of the amount of undissolved gas in the medium results in a decrease of the compressibility of this medium.
Practice has shown that the method according to the invention is very reliable; it can be carried out easily and yields immediately positive and valuable information about the actual presence of undissolved gas.
A first way of carrying out the method according to the invention is described in the claims 2 and 3; this method has the advantage of being universally usable without the need of access to the reservoir of the hydraulic system.
This method thus implies that the entire control system, both the high-pressure side and the low-pressure side, is brought to a first, predetermined pressure in a first phase and then is brought to a second, predetermined pressure in a second phase which means identical compression conditions for any gas bubbles present for both the high-pressure side and the low-pressure side, with the result that the amount of undissolved gas can be unambiguously determined from the observation of the volume fed to the system to be tested in the changeover from the first phase to the second phase.
This hydraulic medium can be drawn from a reservoir provided with a filling-level indicator and the change in the filling level of said reservoir can be observed, but it is also possible to supply the hydraulic medium via a volumetric flow meter.
A second way of carrying out the method according to the invention is described in claims 4-6. This way of carrying out the inventive method is based on the insight that a reduced compressibility, caused by and indicating the presence of undissolved gas, is associated with aanother value of the difference between the filling level with the system at rest and the filling level with the system under pressure than the value of this difference which is obtained when tere is no undissolved gas in the system, as determined during a reference measurement. It is therefore possible to only determine the respective filling levels at regular moments in order to be able to detect the presence of undissolved gas reliably. If necessary, these determinations can be carried out in a controlled way by a suitable regulating system.
Devices for carrying out the method, according to the invention are subject of the claims 7 and 8 on the one hand and claims 9-13 on the other hand.
It is observed that a hydraulic system of the kind as described in the preamble of claim 12 is known in itself from DE-U-2735554.
The invention is explained by reference to the drawing, wherein:

Figure 1 is the (simplified) diagram of a first embodiment of a device for carrying out the method according to the invention,
Figure 2 is the (simplified) diagram of a second embodiment of a device for carrying out the method according to the invention.
Figure 3 shows the diagram of a closed hydraulic system having two reservoirs each containing a piston and suitable for carrying out the method of the invention;
Figure 4 shows the diagram of a hydraulic system containing a reservoir and also suitable for carrying out the inventive method.

In figures 1 and 2, in which corresponding elements are indicated with idential reference numerals, the reference numeral 2 indicates a hydraulic pressurised-medium reservoir having the normal fittings associated therewith and not explained in greater detail. Said reservoir is connected via the conduit 4 to a feed pump 6 which is driven by the motor 8. The high-pressure side 10 thereof is connected to a pressure-regulating device 16 with which a desired pressure can be accurately set at the output 18 thereof and which is of a type known per se. The connection 20 of the pressure-regulating device 16 is connected to the return conduit 22, shown by broken lines, which leads to the reservoir 2. The high-pressure conduits are shown by continuous lines in the figure; the low-pressure conduits by broken lines.
An overload protection 24, also of the type known per se, is incorporated between the point 10 and the return conduit 22.
The installation contains a hydraulic shuttle valve 26 having the connections 26a, 26b, 26c and 26d; in the position of the shuttle valve shown, indicated by the symbol
, the connection 26a is connected to the connection 26d and the connection 26c is connected to the connection 26b; in the second position indicated by the connection 26a is connected to the connection 26c and the connection 26b is connected to the connection 26d.
The connection 26a is connected to the output 18 via the conduit 19, and the connection 26 is connected via the non-return valve 28 and via the conduit 30 to the high-pressure connection point 32 of the installation, which connection point 32 is designed to be connected via the aeroplane connection 34 to the high-pressure side of the control system to be tested. The pressure at the connection 32 can be measured with the manometer 48 if the valve 52 is opened.
The connection point 26b is connected via the conduit 38 and the conduit 40, in which a shut-off valve 42 is incorporated, to the connection point 44. The installation is connected via the aeroplane connection 46 to the low-pressure side of the control system to be tested. The pressure at the point 44 is measured with the manometer 48.
The conduit 40 is connected via the conduit 50 to the return conduit 22.
Finally, the conduit 30 can be connected via the shut-off valve 52 to the connection point 44, with the result that a direct connection can be made via said valve between the high-pressure side and the low-pressure side of the control system to be tested and both are then at the same pressure, shut-off valve 42 being closed.
The above applies to both the embodiment shown in Figure 1 and the embodiment shown in Figure 2.
The embodiment shown in Figure 1 has, in addition, a measuring cylinder 52 in which a piston 44 can move freely along with the piston rod 56 which has a sealed bushing. The piston 54 interacts with a graduated scale 58 with which the instantaneous position of the piston 56 can be determined. The chamber 60 underneath the piston is connected via the conduit 62 to the conduit 30 and, therefore, to the high-pressure side 34 of the system to be tested, the annular chamber 64 above the piston being connected via the conduit 66 to the connection 26c of the shuttle valve 26.
The embodiment shown in Figure 1 is operated as follows:
The pump 6 is put into operation and a first pressure is set with the aid of the regulating device 16 at the output 18 of the latter. The valve 52 is opened, the valve 42 is closed and the shuttle valve 26 is in the position shown, with the result that the connection 26a is connected to the connection 26d and the connection 26c is connected to the connection 26b.
The pressure medium flows from connection 18 via 26a-26d, the non-return valve 28 and conduit 30 to the high-pressure connection 32 and to the low-pressure connection 44 of the installation to be tested. Via the open valve 52, the same pressure prevails at the low-pressure connection 44. All this has the consequence that the reservoir of the hydraulic control system, which is not in operation at that instant, is completely filled with pressure medium.
The pressure prevails also in the chamber 60 underneath the piston 54. The chamber 64 above the piston 54 is connected to the return conduit 22. The lower surface of the piston 54 is larger than the upper surface, with the result that the piston 54 will move fully upwards until it comes to rest against the upper edge of the cylinder 60.
The shuttle valve 26 is now set to the position ⇄ , with the result that 26a is connected to 26c and 26b is connected to 26d. Then the pressure at the connection 18 is increased until the pressure at the measurement point 48 assumes a first, predetermined value which is higher than before. For this purpose, a somewhat higher pressure must prevail at the connection 18 because the upper surface, which is under pressure, of the piston 54 is smaller than the lower surface of said piston. The piston 54 will then have moved downwards to a first position indicated by S1 and indicated by broken lines.
This position is indicated by means of the graduated scale 58 and noted.
Then the pressure at the connections 32 and 34 is increased to a second, predetermined higher value. This is achieved by increasing the pressure at the connection 18, as a result of which the piston 54 moves from the position S1 to the position S2 and an amount of pressure medium is therefore displaced from the chamber 60 underneath the piston and enters the hydraulic control system to be tested, via the conduit 62. The amount of pressure medium displaced is directly dependent on the compressibility of the pressure medium contained in the control system and this compressibility is dependent in turn on the amount of undissolved gas in said pressure medium. The difference between the positions S1 and S2 therefore represents directly the amount of undissolved gas in the system to be tested.
The procedure described above can be repeated, if necessary, one or more times.
The embodiment shown in Figure 2 differs from that shown in Figure 1 in that the displacement cylinder 60, together with piston 54, is replaced by a volumetric flow meter 60 in series with a non-return valve 62 and a flow limiter 64, all this being incorporated between the conduit 30 and the connection 26c of the shuttle valve 26. The test procedure is essentially as described above, but with the difference that, after setting the pressure at the connections 32 and 34 to the first predetermined higher value with the aid of the volumetric flow meter 60, the amount of hydraulic medium which flows through the meter 60 is measured while the pressure is being increased to the second predetermined higher measurement value, for example 6 bar. This amount is directly dependent on the compressibility of the medium contained in the system and, therefore, on the amount of undissolved gas contained therein.
The hydraulic system depicted in Figure 3 comprises the conventional positive-displacement pump 102 (which, when it is at rest, may be replaced by an additional external pump indicated by 102') whose high-pressure side 102a is connected via a nonreturn valve 104, on the one hand, via the conduit 106 to a load 108 to be driven by the hydraulic system and, on the other hand, via the conduit 110 to the cylinder volume 112 of a positive-displacement cylinder 114 which contains the piston 116. Said piston 116 is connected via the common piston rod 118 to the second piston 120 present in a second positive-displacement cylinder 122 in which the chamber 124 behind the piston is connected via the conduit 126 to the low-pressure side 102b of the positive-displacement pump 102 and via the conduit 128 to the load 108. Such systems containing closed feed reservoirs on the high-pressure side and the low-pressure side respectively are used, in particular, in aircraft. They are therefore known per se and are described, for example, in DE-U-2735554.
As already described above, the presence of undissolved, that is to say free, gas in such a system is very undesirable and the invention provides the possibility of detecting the presence thereof by checking the compressibility of the hydraulic medium.
In the method according to the invention, the system is first well vented via the existing venting connections on the high-pressure side and the low-pressure side, indicated in Figure 3 by 130 and 132, respectively, and the reservoirs 114 and 122 are filled with the correct amount of hydraulic medium. For this purpose, the system can be vented, for example, by a venting system to be connected separately, in which case air is extracted from the system with the aid of a vacuum system, or in some other suitable way. Then, with the system in the rest state, that is to say with the positive-displacement pump 102 at rest, the filling level of one of the reservoirs is measured. In the example shown in Figure 3, this is the filling level of the cylinder 114, which is determined by observing the position of the piston 116. In Figure 3, this position is indicated by a solid triangle 134 opposite the scale 136. This position is noted, that is to say recorded.
The system is then put into operation, which can be done by driving the pump 102 or by connecting the external driven pump 102' via the connections 130', 132' and the conduits 120', 126', which has the consequence that the position of the piston 116 will change: the piston 116 will move to the right, for example, to the position indicated by the triangle 134a drawn with broken lines. This position is also noted.
The data thus obtained are reference data which apply to a system in which there is no undissolved gas.
If there is, in fact, undissolved gas in the system, the difference between the two measurements 134, on the one hand, and 134a, on the other hand, will be greater. This is a consequence of the fact that undissolved, free gas can be compressed much more easily than liquid.
In order to determine, after a time has elapsed and at desired instants, whether undissolved gas is present in the system, it is not necessary to do anything other than repeat the determinations described above: the position of the piston 116 is thus first determined with the system at rest and then the position of the piston 16 with the system in operation. If the difference between the two positions is greater than the difference determined in the reference measurement, this indicates the presence of undissolved gas.
The invention has been explained above by reference to a simple visual determination in which use can be made, for example, of a scale division provided on a transparent wall section of the cylinder 114 and which can also be carried out, for example, with a ruler. Of course, the method can be refined with the aid of electronic means available to the person skilled in the art. Thus, it is conceivable, for example, as shown in Figure 3, to provide, next to the cylindrical reservoir 114, for example, an inductive or optical position-measuring device 140 or another suitable sensing element whose output signal represents the position of the piston 116. This signal is fed to a memory 144 present in an electronic processing unit 142 and stored therein. After two determinations, the memory 144 thus contains two measure values, the difference between which can be determined and can be retrieved via the addressing device 146; this difference can then be shown on a display 148 as an indication of the presence of undissolved gas. In determining the difference to be shown on the display 148, the electronic signal processing device can, if necessary, allow for varying operational conditions, for example for the temperature of the medium present in the reservoirs 114 or 122 and for the actual filling level of the reservoir, and for the actual pressures prevailing during the measurements.
Figure 4 illustrates the application of the method and device according to the invention to a hydraulic system having a single reservoir. In Figure 4, parts and components corresponding to those which have already been discussed by reference to Figure 3 have been indicated with the same reference numerals as those used in Figure 4.
Just as is the case in the embodiment shown in Figure 3, there is the conventional positive-displacement pump 102 whose high-pressure side 102a is connected to the load 108 via the nonreturn valve 104 and the conduit 106; the low-pressure side 102b is connected via the conduit 126 to the open reservoir 127 which is filled with hydraulic medium 129. The low-pressure side of the load 108 is also connected via the conduit 128 to said reservoir.
Figure 4 also shows the connections 130', 132', via which the system can be vented or connected to the external hydraulic pump 102'.
Here, again, the system is first well vented and then the reservoir 129 is filled with the correct amount of hydraulic medium. With the system in the rest state, the filling level of the reservoir 127 is measured by observing the position of the surface 131 of the medium 129. This position is indicated by the solid triangle 134'. Said position is noted, that is to say recorded.
After the system has been put into operation, the liquid level 131 will drop, for example to the level indicated by the outline triangle 134a'. This position is also noted.
As already described by reference to Figure 3, these data are reference data which are applicable to a system which contains no undissolved gas.
If a system does in fact contain undissolved gas, the difference between the two measurements 134', on the one hand, and 134a', on the other hand, will be greater.
In the embodiment shown in Figure 4, use can again be made of a suitable position-measuring device 140, in this case, for example, an optical one, which interacts with the electronic processing unit 142 having the memory 144 present therein, the addressing device 146 and the display 148, all these features being as described by reference to Figure 3.
It is pointed out that the reservoir 127 shown as open in Figure 4 may also be sealed by means of a membrane or piston present between the hydraulic medium and the outside air, while the method according to the invention when applied to a system as shown in Figure 4 can also be carried out in conjunction with the pressurisation of the reservoir 127, with the result that air present in the low-pressure side is also reliably detected.

Claims

Method of detecting undissolved gas in a hydraulic control system of the type containing a hydraulic-medium pressure pump and a feed reservoir, characterised by the steps of checking the compressibility of the medium and deducting the amount of undissolved gas from the thus obtained information by taking into account that an increase of the amount of undissolved gas in the medium results in a decrease of the compressibility of this medium.
Method according to claim 1, characterised in that both the high-pressure side and the low-pressure side of the system is brought to a predetermined, first pressure by means of a separate source of said medium, then the pressure on both the high pressure side, and the low pressure side is brought to a second, predetermined higher pressure using said source and the amount of hydraulic medium fed to the control system from said source during this operation is observed.
Method according to claim 1-2, characterised in that the hydraulic medium is drawn from a reservoir provided with a filling-level indicator and the change in the filling level of said reservoir is observed.
Method according to claim 1, characterised in that
- 1. the system is vented while it is at rest and the filling level of the reservoir is determined and stored;

- 2. the system is put under pressure, for example in the operational state, and the filling level of the same reservoir then arising is determined and stored;

- 3. the difference in the filling levels is determined and stored;

- 4. the determinations described under 1) and 2) are repeated at a later instant in time without venting and a conclusion is drawn as to whether the difference in the filling level(s) deviates from the difference determined earlier to such a significant extent that undissolved gas must be present in the system.
Method according to Claim 4, characterised in that, in concluding whether free gas is present in the system, account is taken of variations in filling level due to variations in temperature, and of the actual filling level or pressures.
Method according to Claims 4-5 as applied to a hydraulic control system of the type containing a hydraulic pressure pump whose high-pressure side is connected to a first reservoir in which a first displaceable sealing element having a relatively small surface area is provided and whose low-pressure side is connected to a second reservoir in which a second sealing element coupled to the first sealing element and having a relatively large surface area is provided, all these features being such that, during normal operation, a constant pressure ratio is maintained, characterised in that the respective filling levels are derived from the respective piston positions.
Device for carrying out the method according to claims 1-3, characterised by a pressure-medium source which is able to deliver said medium at a settable pressure to a pressure output connection (18) and to accept it in return from a return conduit (22), and by a shuttle valve (26) having a first input connection (26a) connected to the pressure output connection (18) and having a second input connection (26b) connected to the return conduit (22), and having a first output connection (26c) which, in the first position of the shuttle valve, is connected to the second input connection (26b) and which, in the second position of the shuttle valve, is connected to the first input connection (26a), and having a second output connection (26d) which, in the first position of the shuttle valve, is connected to the first input connection (26a) and which, in the second position of the shuttle valve, is connected to the second input connection (26b), the return conduit (20, 22, 50) being connected via a controllable valve (42) to a device output connection (44) designed for a possible connection to the low-pressure side of the control system to be tested, the second output connection (26d) of the shuttle valve (26) being connected via a non-return valve (28) and a conduit (30) being connected to a second device output connection (32) designed to be connected to the high-pressure connection of the system to be tested, a controllable valve (52) being incorporated between the two output connections (44, 32), and also characterised by a cylindrical reservoir having a piston, freely movable therein, whose piston rod is passed in a sealed manner through one of the cylinder end walls and which piston co-operates with a piston position indicating device, it being possible to determine the instantaneous piston position, the annular chamber above the piston of said reservoir being connected via a conduit (66) to the first output connection (26c) of shuttle valve (26), while the cylindrical chamber (60) underneath the piston (54) is connected via a conduit (62, 30) to the high-pressure output connection (32) of the device and can be connected via the valve (52) to the low-pressure side (44) thereof.
Device for carrying out the method according to claims 1 and 3, characterised by a pressure-medium source which can deliver said medium at a settable pressure to a pressure output connection (18) and can accept it in return from a return conduit (22), and by a shuttle valve (26) having a first input connection (26a) connected to the pressure output connection (18) and having a second input connection (26b) connected to the return conduit (22), and having a first output connection (26c) which, in the first position of the shuttle valve, is connected to the second input connection (26b) and, in the second position of the shuttle valve, is connected to the first input connection (26a), and having a second output connection (26d) which, in the first position of the shuttle valve, is connected to the first input connection (26a) and, in the second position of the shuttle valve, is connected to the second input connection (26b), the return conduit (20, 22, 50) being connected via a controllable valve (42) to an output connection (44) of the device designed to be connected to the low-pressure side of the control system to be tested, the second output connection (26d) of the shuttle valve (26) being connected via a non-return valve (28) and a conduit (30) to a second device output connection (32) designed to be connected to the high-pressure connection of the system to be tested, a controllable valve (52) being incorporated between the two output connections (32 and 44, respectively), and also characterised by a volumetric flow meter connected, on the one hand, to a first output connection (26c) of the shuttle valve (26) and, on the other hand, via a non-return valve (62) to the device high-pressure output connection (32).
Device for carrying out the method according to Claim 1 and claims 4-6, characterised in that a reservoir (114, 127) is provided with means (136) for determining the filling level.
Device according to Claim 9, characterised in that said means (136) supply a measure value which can be stored in a memory (144) and can be read-out at a later moment for comparison with a new measure value.
Device according to Claim 10, characterised in that said means yield a visually observable filling-level value.
Device according to Claims 9-11 in a hydraulic system of the type containing a hydraulic pressure pump (102) whose high-pressure side (102a) is connected to a first reservoir (114) in which a first displaceable sealing element (116) having a relatively small surface area is provided and whose low-pressure side is connected to a second reservoir (122) in which a second sealing element (120) coupled to the first sealing element (116) and having a relatively large surface area is provided, all these features being such that, during normal operation, a constant pressure ratio is maintained, characterised in that the means for determining the filling level interact with a piston (116) present in one of the reservoirs.
Device according to Claim 12, characterised in that said means comprise a piston position indicator (140) which delivers an electrical signal representing the piston position.