Method and device for the evalution of a hydraulic system

0001The invention relates to a method for determining of a hydraulic system, comprising the combination of a hydraulic excitation pump with an external pump leak outlet and a hydraulic operating circuit connected thereto at least one of the following quantities: pump loss and leakage flow in the excited hydraulic circuit, as well as to a device for effecting such a method.

0002To obtain information about the state of a hydraulic system one determines in practice on the one hand the pump losses of the hydraulic excitation pump as used therein and on the other hand the loss in the system, which is excited by said pump. To evaluate the state of the pump it is necessary to remove the pump and to test it on a testing stand and to evaluate the system it is necessary to connect between the the pump outlet and the inlet ofo the system a volume flow meter for measuring the leak losses in the excited system. When one does not want to change meters this volume flow meter must not only be able to show the, important, volume flow which occurs during the operation of the system but this meter must also be able to show accurately the small system leak flow which is present at rest state, and which is between 2 and 10 of the maximum output flow. These two requirements are difficult to combine. The invention aims to provide a method and a device for executing the method as defined hereinabove by means of which the quantities which are of importance for judging the state of a hydraulic system can be determined without the necessity to remove the pump or to use a volume flow meter which must satisfy two, mutually contrary, requirements.

0003This aim is obtained by the invention in that one: <ul id="ul0001" list-style="none" compact="compact"><li>records temperature and pressure of the hydraulic medium at respectively pump inlet, pump outlet and pump leak outlet under each of the following conditions: <ul id="ul0002" list-style="none" compact="compact"><li>1) pump leak outlet connected to the system return conduit or the environment, combined with a determination of the leak flow under these circumstances, and</li><li>2) pump leak outlet directly connected to the pump inlet upwardly of the tempreature recording position thereof, or pump leak outlet blocked, and:</li></ul></li><li>asumming that in both cases (1) and (2) respectively the pump efficiency will be equal:</li><li>determining the pump efficiency and/or the leak flow of the operating circuit respectively from on the one hand the measured leak flow and on the other hand the thermodynamic relations between these quantities and the enthalpy and entropy values of the hydraulic medium as used on the other hand, as derived from the recorded values of temperature and pressure.</li></ul>

0004The method according to the invention is determined in more detail in claim 2.

0005The invention furthermore relates to a device for effecting the method as described hereinabove, such as defined in the claims 3-5.

0006The invention is thus based on the insight that the hydraulic fluid (of which the properties must be known) is used as information bearer using the thermodynamic processes such as occuring at the pump. By means of the known thermodynamic interrelations pump loss and system leakage are determined. This interrelations are described in the publications of the inventor (Dr. Ing. K. Witt) in "Ölhydraulik und Pneumatik", 1976-1977, combined in the publication "Thermodynamisches Messe" in der Ölhydraulik" (published by the "Instituut voor Aandrijftechniek", TU Eindhoven), particularly 21(1977) no. 3, page 162.

0007The method and device according to the invention have the important advantage that the implementation thereof requires only three temperature sensors (at the pump inlet, at the pump outlet and at the pump leakage outlet) and, starting from the presumption that both the pressure at the inlet respectively the pressure at the leakage outlet are equal to the pressure of the surrounding air, only one extra pressure sensor. The necessary volume flow meter connected to the pump leakage outlet must only be able to show only a small flow. Finaly only a two-way valve with a connection to the pump inlet, or a simple shut-off valve connected to the pump leakage outlet are required.

0008Registration of temperature and pressure and measuring the pump leakage flow can be done before and after the starting up and shutting down respectively of a hydraulic installation. More particular this means that, when method and device are used in a system such as a hydraulic aircraft system, the state thereof can be monitored permanently and the result of this monotoring action can be stored. This results into a "case history" and at each moment one has an insight in the state of the installation as it is, without the necessity of any removal of any component.

0009The invention is elucidated on the hand of the drawing. Herein shows: <ul id="ul0003" list-style="none" compact="compact"><li>fig. 1a a first hydraulic diagram,</li><li>fig. 1b a second hydraulic diagram,</li><li>fig. 1c the entropy-enthalpy curves of the hydraulic medium,</li><li>fig. 2a a second hydraulic diagram,</li><li>fig. 2b the enthalpy-entropy curves thereof.</li></ul>

0010Fig. 1a shows a hydraulic system with a storage tank 2, filled with a hydraulic fluid of which the physical properties are exactly known. Via the conduit 4 this tank is connected to the inlet 6 of a hydraulic pump 8, of which the shaft 10 is driven by a driving motor, not shown. The outlet 12 of the pump 6 is via the conduit 14 connected to a hydraulic system which is schematically indicated by the broken lines 16, and which is for instance an aircraft system as known as in itself with, for instance, various actuators driving the flight controls. The pump 8 has, as known in itself, a pump leak outlet 18 which is connected via a volume flow meter 20 - which needs only be able to show a small flow - to the inlet 22 of the two-way valve 24. This two-way valve 24 has two outlets: the first, 26, is connected via the conduit 28 to the pump inlet 6 (and more particularly the position where the temperature (T1) of the inlet medium is measured) and the second, 30, leads via the conduit 32 to the system return conduit 33 which ends in the tank 2.

0011In the conduit 28, between the outlet 26 of the two-way valve and the pump inlet 6, a non-return valve 29 can be inserted which opens in the direction of the pump inlet. In that case the part 28a of the conduit is omitted.

0012Fig. 1b shows a diagram which closely resembles the one of fig. 1a, but here the volume flow meter 20 is not connected to a two-way valve, but to a simple shut-off valve 25. When the shut-off valve 25 is closed the pump 8 operates as a pump without external leak outlet - there are hydraulic pumps, particularly the ones used in hydraulic aircraft installations, which can be operated with a closed leak outlet for a short time.

0013However it is also possible that the pump leak outlet is connected to the pump inlet 6 via conduit 31 and the non-return valve 28', thus behind the volume flow meter 20.

0014Fig. 2a shows the situation in which the pump leak outlet 18 is connected, via the two-way valve 24, with the system return conduit 32.

0015Temperature and pressure of the hydraulic medium are recorded or measured respectively at three points: <ul id="ul0004" list-style="dash" compact="compact"><li>directly before the inlet 6 of pump 8 temperature (T1) and pressure (P1) of the medium passing this point are recorded. When the system is an "open system" the pressure P1 is known: it is the ambient pressure. The temperature is measured with the temperature sensor 9.</li><li>Directly at the outlet 12 of said pump temperature (T2) and pressure (P2) of the medium which leaves the pump and enters the hydraulic system 16 are recorded. The temperature is measured with the temperature sensor 11; the pressure is measured by means of the pressure sensor 34.</li><li>At the pump leak outlet 18 temperature (T3) of the medium which leaves this outlet is measured with the sensor 19. As the pump leak outlet 19 is connected, via the volume flow meter 20, with either the pump inlet 6 or the return conduit 32 the pressure P3 will be , in fact, equal to the ambient pressure.</li></ul>

0016The situations according to fig. 1a and 1b respectively thus, in fact, correspond with same of a pump <u>without</u> an external leeak outlet, thus with only an inlet (6) and an outlet (12). It is known that for such a pump the pump efficiency η_T is given by the following relation:<maths id="math0001" num="(1)"><math display="block"><mrow><msub><mrow><mtext>η</mtext></mrow><mrow><mtext>T</mtext></mrow></msub><mtext> = </mtext><mfrac><mrow><msub><mrow><mtext>h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>*-h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mtext>'*</mtext></mrow><mrow><msub><mrow><mtext>h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>*-h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mtext>*</mtext></mrow></mfrac></mrow></math><img file="EP1069312A1_D0001.tif" /></maths>

0017Herein is: <ul id="ul0005" list-style="none" compact="compact"><li>h1* = enthalpy of the hydraulic medium at the pump inlet with temperature T1 and pressure P1,</li><li>h2* = enthalpy of the hydraulic medium at the pump outlet with temperature T2* and pressure P2*,</li><li>h2'*: enthalpy of the hydraulic medium as obtained from the entropy-enthalpy diagram in the way as shown in fig. 1c:</li><li>the entropy-enthalpy diagram of a hydraulic medium is known from its physical properties. Fig. 1c and fig. 2b respectively give an example of the entropy-enthalpy diagram of a hydraulic medium used in the system according to the figs. 1a and 1b. Such an entropy-enthalpy diagram comprises lines of constant pressure - some of which are shown in fig. 1c and 2b respectively, indicated with Pa..Pd.-, as well as lines of constant temperature, some of which are shown in fig. 1c and 2b respectively and are indicated with Ta..Ti. For each combination of pressure and temperature of the hydraulic medium results from the entropy-enthalpy diagram a value of both the enthalpy and the entropy.</li></ul>

0018From the values of temperature (T1) and pressure (P1) as measured at the inlet 6 results in the entropy-enthalpy diagram the point 1* and therefrom the value of h1* which is to be used in equation (1); in a similar way results from the value of T2* and P2* results the value of h2*. h2'* is determined by drawing, starting from the point 2* in the diagram of fig. 1a (which relates to T2* and P2*) a line perpendicular to the horizontal (entropy) axis, thus a line representing points of equal entropy, determining the intersection of this line with the line of constante pressure (Pa), corresponding with poin 1. This intersection, indicated with 2'*, is then the required value of the enthalpy h2'*.

0019In the preceding discussion the quantities and values respectively, indicated with an asterisk, are those which are measured when het pump leak outlet is either returned to the pump inlet 6 (fig. 1a) or is blocked (fig. 1b). This situation is, in fact, the situation of a pump without external leak outlet and in both these cases the pump efficiency η_T is given by the preceding equation (1).

0020Thus η_T is known.

0021A second measurement is effected with the pump outlet 8 connected to the return conduit of the system; this is the situation during the normal use of a hydraulic system, be it with the addition that now in the connection between the pump leak outlet 18 and the return conduit 33 a volume flow meter is connected.

0022Fig. 2a and the entropy-enthalpy diagram according to fig. 2b relate to this situation: the leak outlet 18 of the pump 8 is connected directly via conduit 32 to the return conduit 33. It is known that in such a case the pump efficiency η_T is given by the following equation:<maths id="math0002" num=""><img file="EP1069312A1_D0002.tif" /></maths>

0023The values h1 and h2 in this equation (2) are again determined from T1, P1 and T2, P2 respectively; (in the way as described hereinabove) h2' follows from the entropy-enthalpy diagram while h3 represents the enthalpy of the hydraulic medium at the pump leak outlet, such as determined by T3 and P3. Equation (2) further comprises the following quantities: <ul id="ul0006" list-style="none" compact="compact"><li>M_L = the mass hydraulic medium per time unit, discharged via the pump leak outlet, and</li><li>M_s = the mass hydraulic medium per time unit, fed into the system.</li></ul>

0024The two equations (1) and (2) respectively are, in fact, two equations with two unknown factors: η_T on the one hand and M_s on the other hand.

0025Solved to M_s results into:<maths id="math0003" num="(3)"><math display="block"><mrow><msub><mrow><mtext>M</mtext></mrow><mrow><mtext>S</mtext></mrow></msub><mtext> = </mtext><mfrac><mrow><msub><mrow><mtext>M</mtext></mrow><mrow><mtext>L</mtext></mrow></msub><msub><mrow><mtext>η</mtext></mrow><mrow><mtext>T</mtext></mrow></msub><msub><mrow><mtext>(h</mtext></mrow><mrow><mtext>3</mtext></mrow></msub><msub><mrow><mtext>-h</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext>)</mtext></mrow><mrow><msub><mrow><mtext>(h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>-h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>')η</mtext></mrow><mrow><mtext>T</mtext></mrow></msub><msub><mrow><mtext>(h</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>-h</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext>)</mtext></mrow></mfrac></mrow></math><img file="EP1069312A1_D0003.tif" /></maths>

0026η_T is known from (1) and can thus be introduced in equation (3). The volume flow meter 20 measures Q_L and from Q_L follows directly M_L, because: M_L = Q_L . p, in which p represents the specific mass of the hydraulic medium. So M_L, too, can be introduced into (3). In this way the system leak, M_s, is determined.

0027This value of the system leak yields information about the state of the hydraulic system when excited by the pump.

0028The power loss P_v of the power pump results from the difference between the supplied power and the discharged power and is given by:<maths id="math0004" num="(4)"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>v</mtext></mrow></msub><msub><mrow><mtext> = (P</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> - P</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext>) Q</mtext></mrow><mrow><mtext>s</mtext></mrow></msub><mtext> (</mtext><mfrac><mrow><mtext>1</mtext></mrow><mrow><msub><mrow><mtext>η</mtext></mrow><mrow><mtext>T</mtext></mrow></msub></mrow></mfrac><mtext> -1)</mtext></mrow></math><img file="EP1069312A1_D0004.tif" /></maths>