GB2243188A

GB2243188A - Apparatus for open-loop or closed-loop hydraulic flow control

Info

Publication number: GB2243188A
Application number: GB9106302A
Authority: GB
Inventors: Peter Lindeckert; Gunter Worner; Arno Rohringer; Heiko Bublitz
Original assignee: Daimler Benz AG; Mercedes Benz AG
Current assignee: Daimler Benz AG
Priority date: 1990-03-26
Filing date: 1991-03-25
Publication date: 1991-10-23
Anticipated expiration: 2011-03-25
Also published as: DE4009641C2; FR2661263A1; ITRM910195A0; JP3081869B2; GB9106302D0; DE4009641A1; FR2661263B1; JPH04228368A; GB2243188B; ITRM910195A1; IT1244949B

Abstract

The apparatus is characterised by a spring or springs 23, (30, Fig 3) of a shape memory alloy. Only at relatively high temperature do these springs 23, 30 develop notable actuating forces, by which an only relatively slightly throttled connecting path 8 between delivery side 15 and intake side 14 of a pump 2 is opened and a pressure relief valve 10 may be opened. <IMAGE>

Description

:2 2.4::,: 1 'd - a, ' 1 Apparatus for open-loop or closed-loop hydraulic

flow control The invention relates to an apparatus for open-loop or closed-loop hydraulic f low control in a hydraulic system with a pump which is driven or runs at variable speed and is connected on the intake side to a reservoir and on the delivery side via a throttle to a consumer, for example a power-assisted steering system of a motor vehicle, with a flow control piston controlling a connecting path between intake side and delivery side of the pump, which piston is actuated by the pressure of the delivery side into a position releasing the connecting path and by a restoring spring and the pressure downstream of the throttle into a position shutting off the connecting path, as well as with a pressure relief valve, arranged on the delivery side of the pump, which valve releases with a throttling effect a connecting line leading to the intake side if a pressure threshold value is exceeded.

In the case of hydraulic systems in motor vehicles, the associatcd hydraulic pumps are usually driven directly by the vehicle engine, so that the delivery member of the pump is driven with greatly varying speed, corresponding to the greatly varying speeds of the vehicle engine during driving operation. In such cases, the pump is generally dimensioned in such a way that the volume flow necessary for operation of the respective hydraulic system is already generated at the idling speed of the vehicle engine. In other words, at relatively high speeds of the vehicle engine there may be a delivery capacity available which far exceeds what is required. The volume flow passed to the consumer is then limited to the respective requirement by the flow control piston.

A corresponding apparatus for open-loop or closed-loop hydraulic flow control is described in Swiss Patent Specification 362,613 in connection with a power-assisted steering system of a motor vehicle. In normal operating states of the power-assisted steering system, hydraul ic medium constantly flows from the delivery side of the pump via the servo valve to a hydraulic reservoir. Consequently, a pressure drop occurs at the throttle connecting the servo valve to the

2 delivery side of the pump, the said throttle being arranged and designed in the construction represented in Swiss Patent Specification 362,613 as part of the flow control piston, the drop in pressure having the effect that a force in the opening direction is exerted on the flow control piston. Consequently, in normal operating states of the servo valve, the flow control piston can adapt the supply of hydraulic medium to the servo valve according to requirements; depending on the requirements of the servo valve and the delivery capacity of the servo pump, a greater or lesser proportion of the hydraulic medium delivered by the pump is introduced into the connecting path to the intake side of the pump.

In extreme operating states of the servo valve - for example if a great resistance in the vehicle steering has to be overcome - no hydraulic medium can flow off via the servo valve to the hydraulic reservoir. Consequently, the pressure on the delivery side of the pump increases, until the pressure relief valve opens, which in the arrangement represented in Swiss Patent Specification 362,613 controls a connecting line, branching off from the delivery line of the pump upstream of the connecting path controlled by the flow control piston, to the intake side of the pump.

In particular when the vehicle engine is running at high speed and the pump is correspondingly generating a very strong delivery flow, great quantities of the hydraulic medium f low through the pressure relief valve. For as long as the servo valve assumes the said extreme operating state, in which no hydraulic medium can flow via the servo valve to the hydraulic reservoir, the connecting path, controlled by the f low control piston, to the intake side of the pump also remains closed, because no pressure drop can occur at the throttle leading to the servo valve and, correspondingly, no hydraulic forces can be generated which would urge the flow control piston into a position opening the said connecting path.

Due to the great hydraulic flow passing through the pressure relief valve, there may be extreme heating of the pressure medium, it being possible for the pump to be destroyed 3 already after a relatively short time.

It is admittedly possible in principle to design the servo valve or other consumer connected to the pump in such a way that, in all possible operating states, hydraulic medium can flow back from the delivery line via the servo valve or the consumer to the reservoir, in order to ensure that an appropriate quantity of fresh and cooled hydraulic medium must always be supplied to the pump from the reservoir via the intake line. As a result, an adequate cooling of the pump could be ensured. However, with such a design of the hydraulic system, a pump with increased capacity must be arranged, in order to be able to achieve a predetermined maximum pressure differential between intake side and delivery side of the pump. In other words, an increased drive power is required for the pump. This may cause problems in the case of motor vehicles, because the vehicle engine can only deliver limited power in idling operation, at least if the idling speed is to be kept as low as possible.

-ion, in principle it would be possible to let In addit the connecting line controlled by the pressure relief valve open out into the reservoir, so that the hot hydraulic medium coming from the pressure relief valve could mix with the distinctly cooler hydraulic medium in the reservoir and correspondingly cooler hydraulic medium would be fed to the intake side of the pump. In this case, however, the connecting line would have to be relatively long if the reservoir is arranged some distance away. In addition, an additional connection would have to be provided on the reservoir. If, finally, the pressure relief valve and the flow controller are to be accommodated together with the pump in a common block or housing to achieve as compact a design as possible, an additional connection for the connecting line would also have to be provided on this block as well.

The problems pointed out above have not been solved satisfactorily so far.

German Patent Specification 3,303,492 merely relates to a special way of designing a flow controller in order to prevent the occurrence of flutter of the flow control piston.

4 German Offenlegungsschrift 3,822,970 merely discloses shock absorbers on which throttle valves controlling the damping resistance are provided with springs of shape memory alloys, the elastic stress of which increases greatly at relatively high temperature and consequently increases the throttling resistance of the aforementioned valves. In this way, the damping force remains approximately constant even if the hydraulic medium in the shock absorber is heated considerably, and consequently becomes increasingly fluid.

German Offenlegungsschrift 3,313,666 relates to a pressure control valve which is urged in the closing direction by means of a first spring. An additionally provided bimetal spring opposes the aforementioned spring. This counteracting force can be controlled by means of heating acting on the bimetal spring. In this way, the closing pressure of the pressure control valve can be controlled externally.

Finally, in German Auslegesschrift 1,132,400 a thermostatically controlled valve is represented, the closing body of which is urged in the closing direction by a plurality of different bimetal springs, which are arranged in series. In this way, the closing pressure can be increased with increasing temperature.

The present invention seeks to provide an apparatus for closed-loop or open-loop hydraulic flow control which, with a simple design, prevents an excessive temperature increase of the hydraulic medium in the pump even when the volume flow taken from the consumer drops extremely or ceases.

According to the present invention there is provided apparatus for openloop or closed-loop hydraulic flow control in a hydraulic system with a pump which is driven or runs at variable speed and is connected on the intake side to a reservoir and on the delivery side via a throttle to a consumer, with a flow control piston, controlling a connecting path between intake side and delivery side of the pump, which piston is actuated by the pressure of the delivery side into a position releasing the connecting path and by a restoring spring and the pressure downstream of the throttle into a position shutting off the connecting path, as well as with a pressure relief valve arranged on the delivery side of the pump, which valve releases with throttling effect a connecting line leading to the intake side if a pressure threshold value is exceeded, wherein the connecting line branches off downstream of the throttle and the flow control piston and/or the shut-off member of the pressure relief valve are actuated by a spring consisting of a shape memory alloy and having a pronouncedly temperature-dependent actuating force, in such a way that the connecting path controlled by the flow control piston and the connecting line controlled by the pressure relief valve are released if the hydraulic medium exceeds a temperature threshold value.

The invention is thus based on the general idea of controlling the actuating forces acting on the flow control piston and the pressure relief valve temperature-dependently by means of springs of a shape memory alloy, in order to shift the flow control piston completely into its open position if the temperature threshold value is reached. Consequently, the pump is relieved of virtually all the throttling resistances opposing the pump work, so that a further temperature increase of' the hydraulic medium is reliably prevented. Moreover, the invention makes use of the finding that, due to the arrangement of the branching of the connecting line downstream of the throttle, the heat of the hydraulic medium coming from the pump, which may be heated considerably, can heat a spring of a shape memory alloy acting on the flow control piston and a spring of a shape memory alloy actuating the shut-off member of the pressure relief valve and consequently change the spring force significantly. In both cases, the connecting path controlled by the flow control piston is then released, greater quantities of the hydraulic medium without excessive heating being able to flow through the said path.

A particular benefit of the invention is that the design of conventional apparatuses for open-loop or closed-loop hydraulic flow control only has to-be changed slightly.

Essentially, only at least one spring of a shape memory 6 alloy need be additionally arranged in heat-conducting contact with the hydraulic medium.

According to a preferred embodiment of the invention, it is envisaged in this case that the spring consisting of a shape memory alloy has a greatly dropping or ceasing actuating force below the temperature threshold value and a greatly increasing actuating force above the temperature threshold value, and actuates the flow control piston or the pressure relief valve respectively in the opening direction. In this way, with increasing temperature of the hydraulic medium, an active opening force is exerted on the flow control piston or the pressure relief valve.

It is desirable if a redundant arrangement is also possible with the invention, in that both the flow control piston and the pressure relief valve each interact with a spring of a shape memory alloy. As a result, a particularly high reliability against an excessive heating of the hydraulic medium in the pump is achieved.

For the remainder, as far as preferred features of the invention are concerned, reference is made to the claims and the following explanation of particularly advantageous embodiments, which are represented in the drawing, in which:

Fig. 1 shows a schematicised, circuit diagram-like representation of a power-assisted steering system of a motor vehicle, Fig. 2 shows an axial section of a servo pump with hydraulic flow control according to the invention and shows an axial section of a flow control piston with pressure relief valve arranged therein, which valve interacts with a spring of a shape memory alloy.

According to Fig. 1, a pump 2, connected on the intake side to a reservoir 1, is connected on the delivery side via a throttle 3 to two throttling sections leading to the reservoir 1, which sections are formed by the controllable throttles 41, 411, 51 and 511. The throttles 41, 411, 51 and 511 are controlled by the steering wheel (not shown), to be precise in such a way that the throttling resistance of the throttles 41 and 51 is changed (for example increased) in one direction, while the 7 throttling resistance of the two other throttles 411 and 511 at the same time experiences a change in the other direction (for example a reduction). In the extreme case, a pair of throttles, for example the throttles 41 and 51, are closed, while the other pair of throttles, i.e. the throttles 411 and 511, are completely open.

In normal straight driving of the vehicle, all the throttles are set to approximately the same throttling resistances.

Connected between the throttles 41 and 511 on the one hand and the throttles 411 and 51 on the other hand are opposite sides of a servo motor 6, which in the example of Fig. 1 is designed as a double-acting piston-cylinder unit. Depending on the setting of the pairs of throttles 41 and 51 and 411 and 511, respectively, a greater or lesser hydraulic pressure differential acts on the piston of the servo motor 6, so that the piston is urged with greater or lesser force in one direction or the other. A maximum actuating force is achieved when one pair of throttles or the other (for example 41, 5") is closed and what is respectively the other pair of throttles (for example 411, 511) is completely open.

The pump 2 is driven by the vehicle engine (not shown) which runs at a varyingly high speed, depending on driving state, in the extreme case either only at idling speed or at maximum speed. correspondingly, the pump 2 is thus driven with greatly variable speed, i.e. the delivery capacity of the pump 2 is also subject to correspondingly great variations.

In order to adapt the flow of the hydraulic medium delivered to the throttles 41 to 511, and to the servo motor 6, the pump 2 is provided with a closed-loop control arrangement or open-loop control arrangement 7. This has on the one hand a connecting path 8, leading directly from the delivery side of the pump 2 to its intake side, the degree of opening of which 11 path depends on the pressure drop at the throttle 3. A f low control piston, explained further below, serves for controlling the degree of opening. Arranged in parallel with the connecting path 8 there is also a connecting line 9, which is controlled by a pressure relief valve 10.

8 In this arrangement, the pressure relief valve 10 is designed in such a way that it is not opened until there is a relatively high pressure, which as a rule can only be reached if one of the pairs of throttles 41, 51 or 411, 511 is closed and the piston of the servo motor 6 remains stationary, in particular because it has reached its respective end position. Without the pressure relief valve 10, the connecting path 8 could not be opened in this operating state because no hydraulic medium could flow off via the delivery line of the pump 2 due to the closed one pair of throttles 41, 51 or 411, 511 and the stationary, piston of the servo motor 6, and consequently there could also be no pressure drop at the throttle 3, necessary for opening the connecting path 8.

Thus, it is ensured under all circumstances by the pressure relief valve 10, opening when appropriate, that hydraulic medium constantly flows through the throttle 3 and that correspondingly there is at this throttle 3 a pressure drop, which for its part effects an opening of the connecting path a.

However, in this operating state great throttling resistances occur, which result in the hydraulic medium being heated very considerably, at least when pump 2 is running fast. This can cause extremely high temperatures, because the pump 2 only works in circulation operation, i.e. no fresh, cooled hydraulic medium is fed from the reservoir 1 to the pump 2.

By the invention, however, an overheating of the hydraulic medium in the pump 2 can be avoided. The invention namely provides, in a way described further below, that the connecting path 8 and the connecting line 9 are also controlled temperature-dependently in such a way that the throttling resistance in particular of the connecting path 8 is greatly reduced at relatively high temperature. The pump 2 then correspondingly only works against a comparatively low resistance, as a result of which a further temperature increase is prevented.

The pump 2 and the flow control arrangement 7 of Fig. 1, i.e. all elements between the interfaces A and B, can be accommodated in a common housing 11, according to Fig. 2.

1 9 In this arrangement, the pump 2 may be designed as a vane pump, the vanes 12 of which are driven by means of the drive shaft 13 in such a way that hydraulic medium is passed from an intake chamber 14 into a delivery chamber 15 of the pump 2. An intake line (not visible in Fig. 2), which connects the intake chamber 14 to the reservoir 1 (cf. Fig. 1), opens out into the intake chamber 14. The delivery chamber 15 is arranged essentially concentrically to the axis of the drive shaft 13 and connected via housing bores (not visible in Fig. 2) to a delivery line 16, in which the throttle 3 is arranged.

Arranged coaxially with the drive shaft 13 in the housing 11 is a cylindrical bore 17, which is open towards the delivery chamber 15 and communicates via radial bores 18 with the intake chamber 14. At its end remote from the delivery chamber 15, the bore 17 is connected via the connecting line 9, designed as a housing bore, to a section of the delivery line 16 downstream of the throttle 3 (seen in the direction of flow of the hydraulic medium), the mouth of the connecting line 9 into the delivery line 16 being designed as throttle 19.

Arranged displaceably inside the bore 17 is a flow control piston 20, which, in its closed position represented in Fig. 2, bears with a necklike continuation against a side plate 21 of the pump 2 and, with its left end face. closes off the delivery chamber 15, or the end of the bore 17 on the delivery chamber side, with respect to the radial bores 18 and the intake chamber 14. If the flow control 20 is shifted sufficiently far to the right in Fig. 2, the connecting path 8 between the delivery chamber 15 and the intake chamber 14 is opened, because the left end of the flow control piston 20 no longer protrudes far enough into the section of the bore 17 between the delivery chamber 15 and the radial bores 18.

A compression spring 22, which attempts to urge the flow control piston 20 constantly into the closed position represented is clamped between the right end face of the flow control piston 20 and a bottom of the bore 17. This compression spring 22 works virtually temperature-independently.

A further compression spring 23, which consists of a shape memory alloy and generates pronouncedly temperature dependent actuating f orces, is clamped between the other end face of the flow control piston 20 and the side plate 21 of the pump 2: at low temperature, the compressive stress of the compression spring 23 can be largely ignored; at relatively high temperature, on the other hand, the compression spring 23 attempts to push the flow control piston 20 to the right with great force.

Arranged inside the flow control piston 20 is the pressure relief valve 10, which in the opened state connects the end of the bore 17 communicating with the connecting line 9 to an annular space formed by a circumferential groove 24 of the flow control piston 20, which space for its part overlaps the radial bores 18 and correspondingly communicates with the intake chamber 14. The annular space formed by the circumferential groove 24 is connected via a radial bore 25 to an axial blind bore 26 of the f low control piston 20, which receives a valve seat part 27 of the pressure relief valve 10 at its open end. Arranged axially movably inside the blind bore 26 is the valve body 28 of the pressure relief valve 10, which is stressed by a valve spring 29 against the valve seat part 27 in such a way that a conical nipple formed on the valve body 28 can shut off the valve channel passing axially through the valve seat part 27. The valve spring 29 works largely temperature-independently in the usual way.

In Fig. 2, the valve body 28 of the pressure relief valve 10 has a rodlike continuation, protruding into the valve spring 29, which continuation limits the opening stroke of the valve body 28 in interaction with the bottom of the blind bore 26.

In the case of the embodiment according to Fig. 3, the valve body 28 embraces the valve spring 29 with a sleeve-shaped region, the left end of which in Fig. 3 likewise limits the opening stroke of the valve body 28 by hitting the bottom of the blind bore 28.

Arranged on the. outside of this sleeve-shaped section is a further compression spring 30 of a shape memory alloy, which is clamped under compression between the valve seat part 27 and an annular flange on the valve body 28. The compression 11 spring 30 has a pronouncedly temperature-dependent behaviour, such that the effect of the compression spring 30 can be ignored at low temperature. only at relatively high temperature does the compression spring 30 generate notable spring forces, which attempt to move the valve body 28 to the left in Fig. 3, in the sense of an opening of the pressure relief valve 10.

The arrangement of Figs. 2 and 3 functions as follows: As long as hydraulic medium can flow off to the consumer via the delivery line 16, a pressure drop occurs at the throttle 3 of the delivery line 16, i.e. the hydraulic pressure downstream of the throttle 3 in the direction of flow is distinctly less than upstream of the throttle 3. In this case, the pressure upstream of the throttle 3 corresponds to the greatest extent to the pressure in the delivery chamber 15. Due to the pressure drop at the throttle 3, the mutually remote end faces of the flow control piston 20 are subjected to different hydraulic pressures, i.e. the left end face in Fig. 2 is loaded by the high pressure of the delivery chamber 15, while the other end face is subjected to the lower pressure, as prevails downstream of the throttle 3.

As soon as the pressure drop at the throttle 3 is great enough, i.e. as soon as the difference between the hydraulic pressures to which the end faces of the flow control piston 20 are subjected exceeds a threshold value, the flow control piston 20 is shifted to the right in Fig. 2 against the force of the compression spring 22, so that the connecting path 8 is opened with a larger or less large cross-section and a part of the hydraulic medium can flow over directly from the delivery chamber 15 to the intake chamber 14. It is thus ensured that the volume flow of the hydraulic medium discharged via the delivery line 16 to the consumer is limited to a desired amount independently of the respective delivery capacity of the pump, which under certain circumstances, corresponding to the driving engine of the vehicle, can run very fast.

As long as hydraulic medium flows off to the consumer via the delivery line 16, the hydraulic medium can only be heated insignificantly, if at all, inside the housing 11, because the hydraulic medium flowing after from the reservoir 12 1 (cf. Fig. 1) always ensures adequate cooling.

As soon as the consumer connected to the delivery line 16 reaches an operating state in which no f urther hydraulic medium f lows off any longer via the delivery line 16, virtually no pressure drop can occur at the throttle 3, so that a very high pressure also prevails downstream of the throttle 3 in the direction of flow. This pressure is transmitted via the connecting line 9 into the region of the bore 17 on the righthand side of the flow control piston 20. As soon as a threshold value is reached, this pressure opens the pressure relief valve 10 so that hydraulic medium can flow off from the right end region of the bore 17 to the intake chamber 14, hydraulic medium constantly flowing after via the connecting line 9. This f low ef f ects a pressure drop at the throttle 3, or at the throttle 19, so that the mutually remote end faces of the flow control piston 20 are in turn subjected to unequal pressures and the flow control piston 20 is shifted in the opening direction; thus, in this case as well, the connecting path 8 is again opened to a greater or lesser extent.

In this operating state, in which no hydraulic medium can flow of f to the consumer via the delivery line 16, and consequently also no fresh, cooled hydraulic medium can flow via the intake line to the pump 2, the hydraulic medium is exclusively passed in circulation by the pump 2, it being possible for very considerable heating to occur due to the throttles in the flow paths of the hydraulic medium.

The associated temperature increase of the hydraulic medium effects a correspondingly considerable heating of the compression springs 23 and 30, respectively, consisting of shape memory alloys. This has the consequence that these compression springs 23 and 30 become effective if a temperature threshold value is reached. The compression spring 23 thereby urges the flow control piston 20 directly into its opening position, and the compression spring 30 acts in the sense of an opening of the pressure relief valve 10, as a result of which the right end face in Fig. 2 of the flow control piston 20 is constantly relieved of the pressure and the comparatively high hydraulic pressure in the delivery chamber 15 can shift the i i 13 flow control piston 20 into its opening position.

Thus, in every case the connecting path 8 is opened relatively wide, so that the hydraulic medium can flow over via a short path and without excessive throttling resistance from the delivery chamber 15 to the intake chamber 14.

Consequently, the circulation operation takes place against low throttling resistances, with the consequence that a further undesired temperature increase of the hydraulic medium, which could destroy the pump 2, is avoided.

One of the compression springs 23 and 30 of a shape memory alloy is sufficient for reliable functioning. If both springs 23 and 30 are arranged, a redundant arrangement with particularly high reliability is obtained.

A further benefit of the invention is that only slight modifications with respect to previous designs are necessary. The arrangements represented in Figs. 2 and 3 differ essentially from conventional arrangements merely by the springs 23 and 30 of shape memory alloys.

14 claims 1. Apparatus for open-loop or closed-loop hydraulic flow control in a hydraulic system with - a pump which is driven or runs at variable speed and is connected on the intake side to a reservoir and on the delivery side via a throttle to a consumer, with a flow control piston, controlling a connecting path between intake side and delivery side of the pump, which piston is actuated by the pressure of the delivery side into a position releasing the connecting path and by a restoring spring and the pressure downstream of the throttle into a position shutting off the connecting path, as well as with a pressure relief valve arranged on the delivery side of the pump, which valve releases with throttling effect a connecting line leading to the intake side if a pressure threshold value is exceeded, wherein the connecting line branches off downstream of the throttle and the flow control piston and/or the shut-off member of the pressure relief valve are actuated by a spring consisting of a shape memory alloy and having a pronouncedly temperature-dependent actuating force, in such a way that the connecting path controlled by the flowcontrol piston and the connecting line controlled by the pressure relief valve are released if the hydraulic medium exceeds a temperature threshold value.

Claims

2. Apparatus according to Claim 1, wherein the spring consisting of a

shape memory alloy has a greatly dropping or ceasing actuating force below the temperature threshold value and a greatly increasing actuating force above the temperature threshold value, and actuates the flow control piston or the pressure relief valve respectively in the opening direction.

3. Apparatus according to Claim 1 or 2, wherein the spring 21 C consisting of a shape memory alloy is clamped in the delivery chamber of the pump between a stationary abutment part or housing part and the end of the f low control piston on the delivery chamber side.

4. Apparatus according to Claim 1 or 2, wherein the spring consisting of a shape memory alloy is arranged in the hydraulic flow path of the opened pressure relief valve.

5. Apparatus according to any one of Claims 1 to 4, wherein the pressure relief valve is accommodated inside the f low control piston between an inlet opening on the end f ace subjected to the pressure downstream of the throttle and a radial opening of the f low control piston, leading to the intake side.

6. Apparatus for open-loop or closed-loop hydraulic flow control in a hydraulic system substantially as described herein, with reference to, and as illustrated in, the accompanying drawings.

Published 1991 at The Patent Office. Concept House. Cardiff Road. Nvwport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cu-nifelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Marv Cray. Kent