The present invention refers to the use of a displacement machine known per se as a pressure control valve.
The control of the flow of a liquid usually is gained by restricting the flow in a restriction in some kind of valve, usually a seat slide valve. In such valves it is re- quired, however, a very small area change for providing a very great change of the pressure drop, i.e. the pressure drop varies exponentially with the change of the position of the valve body. When opening seat valves very little the flow velocity becomes very high and the liquid pressure locally becomes so low that the liquid tends to be vapori¬ sed, giving rise to cavitation. In order to avoid this it would be desirable to provide a control device operating, if possible, entirely proportional.
For the control of flows the use of rotary-dynamic machines is previously known in various connections. In US-A-3 709 245 thus is disclosed a control valve in which a rotor is brought to rotate by means of a whirling rotation of a fluid flow. Energy then can be tapped from the rotor in various ways, e.g. by the generation of electric current or the like. Due to the inherent characteristics ' of the rotary-dynamic machines they are, however, not suited for a more exact control of a fluid flow and particularly the pressure thereof.
Displacement machines also have been used as flow meters. Such flow meters then also have been used such that the out¬ put controls a valve or the like for providing a flow con¬ trol, cfr. DE-A-2 440 078. Furthermore, it is also known to use a displacement machine having a rotary member mounted in a liquid conduit for controlling the flow and/or the pressure of the liquid by braking said rotary member. In
this respect it might be referred to e.g. DE-B-2 840 134, DE-A-3 210 098, FR-A-2 380 585 and US-4 815 278.
Quite surprisingly it has now been found that a particular use of a displacement machine as a pressure control valve in connection with motor vehicle shock absorbers allows the achievement of unexpected advantages hitherto not obtain¬ able with known shock absorber valve structures.
The main object of the present invention thus is to suggest the use of a displacement machine known per se as a pres¬ sure control valve for the liquid in a vehicle shock absor¬ ber in order to provide a direct proportional pressure con¬ trol and hence a desired damping force of the latter, said displacement machine being mounted within the piston of the cylinder together with associated actuation means operable from the outside of the cylinder in response to a pressure sensor inside said cylinder.
Owing to the invention a very desirable and entirely pro¬ portional control action thus can be obtained by applying a braking action on the rotary member of the displacement machine mounted within the piston. This manner of control¬ ling the liquid pressure is of particular advantage in con- nection with motor vehicle shock absorbers.
In a preferred embodiment of the present invention the dis¬ placement machine is a screw pump, particularly of so-cal¬ led IMO-type. By the use of such a screw pump it can be ob- tained a pressure control which is proportional to the bra¬ king momentum exerted on the rotary member of the machine. This implies that the control action becomes independent of the viscosity of the fluid or liquid and the great equiva¬ lent flow area eliminates the risks of cavitation. However, the present invention also might be advantageously used in connection with other kinds of hydraulic cylinders.
It is also to be mentioned that in known displacement machi¬ nes of screw-type which are used as flow or volume meters the rotary members have no output shaft but the rotary speed is registered by e.g. an inductive sensor. When using the displacement machine for the control of the pressure of a liquid according to the present invention the machine now also includes braking means and a pressure sensor for setting the pressure to a desired value. The invention also might be applied in hydraulic systems in which a common servo valve or a direct-controlled valve can be substitu¬ ted with a control by means of a displacement machine accor¬ ding to the present invention.
By way of example the invention will be further described below with reference to the accompanying drawing in which Fig 1 is a longitudinal section of a displacement machine of screw-type for the use according to the invention, as seen in a plane containing the axis of rotation of the screws, Figs 2a, 2b and 2c diagrammatically illustrate va- rious application examples of the present invention in a shock absorber with variable restriction, a hydraulic cylin¬ der having internal variable damping and a single-acting hydraulic cylinder having internal damping, respectively, and Figs 3 and 4 diagrammatically illustrate further embo- diments of the invention.
The displacement machine for pressure control illustrated in Fig 1 is mounted within a piston 1 of a hydraulic cylin¬ der such as a vehicle shock absorber of telescope type, not further illustrated. In this case the screw pump is of well- known type having three screws, namely a central screw 2 and two idle screws 3 laterally thereof. The idle screws of the pump are journalled at each end in axial bearings 4 as known.
At its end remote from the piston rod 9 the piston accor¬ ding to the invention is provided with suitable braking means, in the present case of electromagnetic type and com-
prising a circumferential electric coil 5 inserted into the piston 1 and an annular metal pressure disc 6 located axially beyond said coil and therebetween a brake disc 7 connected with an entraining member 8 secured to the cen- tral screw of the displacement machine. Through conductors not furhter illustrated the coil 5 can be excited, due to which the pressure disc 6 urges the brakedisc 7 to a desi¬ red extent towards the end surface of piston 1 and hence applies a braking momentum onto the drive screw 2 such that the pressure drop through the machine increases (or the liquid flow decreases) . The liquid pressure drop over the screw then is proportional to the braking momentum which in turn is proportional to the current in the coil 5.
Suitably the central screw 2 is provided with a central longitudinal bore 10 for carrying the liquid pressure beyond the end of the piston to a pressure sensor 11 moun¬ ted in the piston rod 9. The output of said pressure sen¬ sor then passes through the piston rod 9 and outwardly out- side the shock absorber and to a preferably electronic con¬ trol device. Preferably the axial bearing at the end of the screw facing the piston rod 9 is made double-acting and furthermore such that it seals the space at the pres¬ sure sensor 11 from the adjacent screw end. By suitable dimensioning of the areas it can thereby be provided an axial pressure equilibrium over the central screw.
In shock absorbers the present invention can provide an active motion damping in wheel suspensions which for isola- ting vibrations require a rapid-acting and easily control¬ lable valve. The same also applies to various machine sup¬ ports.
Above it has been described a kind of- electromagnetic fric- tion brake for braking of the displacement machine and thus achieving the desired control action. To the artisan it is obvious, however, that for the same purpose and with only
minor design changes an eddy current brake known per se might be used but also an electric generator circuit.
In Fig 2 have been illustrated diagrammatical views of fur- ther applications of the present invention. In Fig 2a thus is disclosed a shock absorber having variable restriction while in Fig 2b has been diagrammatically illustrated a hydraulic cylinder with variable internal restriction, in which the displacement machine can allow the hydraulic liquid to be carried between the spaces at either side of the piston and thus eliminating the necessity of having an external hydraulic source. In Fig 2c, finally, has been illustrated a single-acting hydraulic cylinder having in¬ ternal damping.
Finally, in Figs 3 and 4 have been illustrated diagrammati¬ cally various further application possibilities of the pre¬ sent invention. Fig 3 shows the application in pressure control in common (or flow control) . Pressure or rotary speed are measured and processed by the control device 20 which in turn acts upon e.g. the current to the brake mem¬ ber such as the coil 5. Fig 4 discloses pressure control with feed-back of real pressure. It is also an inherent characteristic of the displacement machine that in the pre- sent connection the same also might be used for measuring or metered discharge of the liquid quantity, in which only the number of revolutions of the rotary machine part (the central screw) needs to be determined.
Within the scope of the present invention also other types of displacement machines might be used, such as gear pumps but due to the greater initial starting inertia thereof than in machines of screw-type they are less suitable for satisfying the objects of the present invention.
In general the brake-equipped displacement machine accord¬ ing to the invention used as valve might be incorporated in pistons in most kinds of hydraulic mechanisms.