GB2187850A - Method and means for generating hydrodynamic pressure pulses - Google Patents

Abstract

To generate a hydrodynamic pressure pulse which is defined absolutely the closed internal volume of a measuring chamber (1) is acted on by a piston (4) which is actuated impulsively from outside. By determining the kinetic energy transmitted by the piston (4) and the relative development with time of the associated pressure pulse the respective system parameters, in particular the spring characteristic c and the damping characteristic d, of the liquid necessary for the exact absolute definition of the pressure pulse can be determined. Insofar as a pressure sensor (14) can be calibrated dynamically in an absolute manner in this way the relative development with time of the pressure pulse which arises can also be ascertained directly by taking into account the output signal of the pressure sensor to be calibrated. The piston 4 is acted on impulsively by a weight accelerated by a pre-loaded spring, magnetically or by a falling weight 5 dropped from adjustable heights. The velocity of the weight 5 immediately before impact with the piston 4 is measured optically 12 to determine the kinetic energy imparted to the piston. The time of contact between the weight 5 and piston 4 is also measured. <IMAGE>

Description

SPECIFICATION Method and means for generating a hydrodynamic pressure pulse The invention relates to a method of generating a hydrodynamic pressure pulse of duration and amplitude which are capable of being selected within limits, by dynamically compressing a liquid enclosed in a measuring chamber, and with means for carrying out this method.

Methods and apparatus for generating pressure pulses are known, both the hydrostatic and also the hydrodynamic case, but up to now it is only in the static case that a mathematical or absolutely definable pressure has been capable of being realised and generated.

In the cases of dynamic and highly dynamic pressure pulses such as would be required in particular for example for the further development of calibration of piezo-electric pressure transducers used in non-steady technical processes, it has not so far been possible to generate pulses which are mathematically defined or capable of being described for these reasons up to now also in connection with dynamic pressure pulses only relative estimates of the relevant factors have been carried out.

In connection with the dynamic pressure transducers mentioned above the so-called 'cold pressure shock' - that is a hydrodynamic pressure pulse without the high thermal shock load which arises at least simultaneously in the normal measuring operation of such transducers - has the appreciable significance as a testing method, since thereby it is possible to investigate the behaviour of the transducer in response to exclusively dynamic loads, that is to say without the influence of thermal loads. Accordingly such a method of testing is very significant because in comparison measurements in practical operation (with thermal and mechanical-dynamic loads) of individual pressure transducers which are to be calibrated under identical conditions, measurements are produce which differ to some extent.In this connection we may speak of a certain individual dynamic behaviour of a pressure transducer and by this we mean by those characteristics which cannot be explained either by the frequency response of the transducer nor can they by interpreted from the profile of the statically obtained response characteristic. The investigation which is sought into the individual dynamic behaviour of the transducer, and here in particular we speak of piezo electric transducers, therefore requires it to be possible to complete the pressure variations which arise in practical use for measurement as far as possible without simultaneous thermal loading; which however is not possible with the known method or apparatus for generating hydrodynamic pressure pulses, for the reasons mentioned above.

The aim of the present invention is to improve a method of the kind mentioned above and associated apparatus for carrying out this method so that the above-mentioned drawbacks do not arise and so that in particular it becomes possible to generate a mathematically definable and absolutely describable hydrodynamic pressure pulse, in principle for any desired purpose, but in particular for the dynamic calibration of pressure sensors.

According to the present invention this problem is solved in a method of the kind referred to in the introduction above in that the liquid present in the closed internal volume of the measuring chamber is acted on by means of a piston actuated dynamically from outside, producing kinetic energy transmitted by the piston, the associated relative change with time of the pressure pulse is observed and the system constants of the pressure system relevant for the absolute definition of the pressure pulse are derived from the kinetic energy transmitted from the piston and the observed change of pressure with time. Together with the other components of the pressure system the piston represents an oscillating system of which the relative system constants can be determined from the kinetic energy transmitted by the piston as well as the relative behaviour with time of the pressure pulse.The relative change of the pressure pulse with time can itself be ascertained either through a pressure sensor which is acted on by the compressed liquid or by an exact determination of the travel of the piston during the dynamic actuation of the piston, through an expansion measurement at the walls of the measuring chamber or by similar known methods; what is important here is only that the beginning, the position of the maximum and the end of the pressure pulse should be capable of being ascertained exactly.

According to one embodiment of the invention it is envisaged that the piston should be actuated by a falling weight of pre-determined mass; quite apart from this constructionally simple and cheap possibility, simple and safe to manufacture, the dynamic actuation of the piston could however also be achieved in another way, for example by means of a mass which is accelerated in any desired manner, or by means of a pre-loaded spring, or equally well magnetically.

According to a preferred embodiment of the invention, the spring characteristic c and the damping characteristic d are determined as the relevant system constants of the pressure system, the piston and the falling weight being regarded together as a single mass which oscillates under the influence of c and d. In this way therefore the pressure system in which according to the invention an absolutely definable hydrodynamic pressure pulse is to be generated - whethe it be as a dynamic pressure standard in itself or is to be used for various calibrating or measuring purposes - is regarded as a vibrating system with pre-deter mined springing and damping characteristics, the piston which transmits the exciting energy and the falling weight being regarded together as a so-called single-mass vibrator.For an exact description of the system one should in theory start from a two-mass vibrator because a spring effect and also a damping effect arises between the falling weight and the piston; this, however, increases very substantially the difficulty of evaluating the measurements and thereby defining the pressure pulse. However it has turned out that by taking a one mass vibrator as a model one can achieve a very high degree of accuracy in the mathematical description of the generated hydrodynamic pressure pulse, only insofar as the ratio of the mass of the falling weight to the mass of the piston can be set at figure of about 100 to 1 or more.

According to a further feature of the invention, with the mass of the falling weight remaining constant a series of pressure pulses can be generated in a sequence, each with different impact velocity of the falling weight, which pulses accordingly have different widths with time and amplitudes, and from the instantaneous spring constant c and damping constant d, determined point by point the spring characteristic c and damping characteristic d can be derived. As the spring force in a non-linear function of the displacement of the mass and likewise the damping force is not directly proportional to the velocity of the mass one must take as the starting point that a spring characteristic and a damping characteristic must be determined in order to be able to describe the pressure system in the whole of the pressure range than matters.According to the above-mentioned version the relevant constants of the system are determined point by point for different impact velocities of the falling weight and result in an overall picture of the corresponding characteristic over the pressure range of interest. Important for the determination of the relevant system constant and characteristics is therefore the production of pressure impacts or shocks of different maximum amplitude with otherwise constant system conditions; i.e.

contant mass of the falling weight and the piston and constant starting volume of the liquid present in the measuring chamber. Only under these condition is it true that for every pressure pulse the same inherent system spring behaviour and damping behaviour applies. Different amplitudes of the individual pressure pulses are then produced exciusively by variation of the starting energy, that is to say, the kinetic energy of the piston and the falling weight directly after they come into contact, dependent on the respective height from which the weight falls.

Some further details for the carrying out of the method of the invention are also contained in the description of the drawings and to avoid repetitions we will only draw attention to the following points here.

In a preferred embodiment of the invention it is provided that, for determining the kinetic energy, the velocity of the falling weight is determined immediately before it engages the piston. In the first place this allows very sim i ple and accurate determination of the kinetic energy imparted to the piston and which dif fers from the energy transmitted by the piston in practice only by the loss of pulse through the piston, and thereby an enhanced accuracy of the expression with regard to the absolute parameters of the dynamic pressure pulse.

In a further embodiment of the invention it can be provided that the duration of the con tact between the falling weight and the piston i is measured and is employed for monitoring or correcting the time-dependent determination of the pressure pulse. As a transfer of energy from the falling weight to the piston and from the piston to the liquid which is to be comt pressed can take place only during the period of contact between the falling weight and pis ton (this can be direct or with elements inter posed) the actual indicated duration in time of the pressure pulse is linked directly to the duration of contact of the above-mentioned parts, which results in making it simple to monitor and correct.

For the absolute dynamic calibration of a pressure sensor with the pressure pulse which has been generated one can, according to a particularly preferred embodiment of the inven tion, determine the relative change with time of the pressure pulse by observing of the out put signal of the pressure sensor to be cali brated. In this way no separate pressure sen sor is needed; the pressure sensor which it self is to be calibrated absolutely itself deliv ers the signal which is primarily of importance only in relation to the relative behaviour with time.

According to a further embodiment of the invention, in order to determined the dynamic compressibility of the liquid enclosed in the measuring chamber, the starting internal vol ume of the chamber can in addition be deter mined. This makes possible another advan tageous use of the invention for measuring or determining an initially unknown dynamic com pressibility of a liquid, the relevant formula be ing stated in the following shortly in the in terests of completeness: K=A2 c.V where: c is the spring stiffness A is the piston cross section V is the starting volume Apparatus for carrying out the method ac cording to the invention, with a largely rigidly defined measuring chamber, a liquid enclosed in it and a compressing device for dynamically compressing the liquid in the chamber is characterised, according to a preferred embodiment of the invention, in that the measuring chamber has a sealed opening in its wall in which is guided a piston of the compressing device which is actuated dynamically from outside, and a pressure sensor, and that the compressing device furthermore includes a falling weight of predetermined mass which is guided on guide means and co-operates with the piston.This results in a simple construction for the device and - as a consequence of the employment of the kinetic energy produced by the falling weight - this makes it possible to achieve a high accuracy of reproducibility.

A further embodiment of the apparatus according to the invention is distinguishable by the feature that the opening for the piston in the measuring chamber is provided with a low-friction pressure-compensated seal engaging the piston. In this connection the seal can furthermore have at the face which is towards the inside of the measuring chamber an annular projection concentric with the piston, of which the dimensions are chosen so that the clearance in relation to the piston stays substantially constant in the range of pressure which arises.In this way over a relatively wide pressure range (piezo-electric pressure pick-ups of the kind referred to above are for example tested with pressure pulses having a duration of between 1 and 5 milliseconds and an absolute pressure of about 1000 to 8000 bar) contancy of the movement relationships between the piston and the seal can be assumed and apart from this a tight and lowfriction guiding of the piston can be assured.

According to a further feature of the invention in the region of the end of the piston which projects out of the measuring chamber there is arranged a velocity measuring device for the falling weight, which operates in particular in a contact-free manner, e.g. by means of light beams. This feature provides for accurate determination of the actual kinetic energy acting on the piston and therefore on the liquid, which in principle, however, can also be achieved simply by calculation from the height from which the weight falls.

According to a further advantageous feature of the invention the guiding means for the falling weight can be in the form of seif-centering air bearings co-operating with guide rails to reduce the friction, which simplifies the reproducible attainment of pre-determined velocities of the falling weight.

In a further embodiment of the invention starting positions for the falling weight can be set on the guiding means for example by being defined by means of detents, simplifying the reproducible generation of predetermined kinetic energies.

According to a further feature of the invention the internal volume of the measuring chamber can be in communication with a filling device for the liquid, the filling device having a probe for engaging the piston, making it possible to maintain a reproducible starting volume. The connection of this filling device to the measuring chamber can either be disconnected entirely from it during the actual pressure impact or alternatively it can be cut off by a suitable non-return valve or the like.

For the liquid in the measuring chamber the requirements are substantially only the following: It must become solid at high pressures (up to 8 kilobars and above) and should have as far as possible linear compressibility characteristics, it should not lose its lubricating properties at high pressures and finally as a consequence of having a correspondingly adequate viscosity it should allow sealing of the chamber. With there requirements in mind particularly suitable liquids are glycerine or methyl alcohol; in principle, however various other known liquids could be used.

According to a further advantageous embodiment of the invention there is provided a degassing unit for the liquid, ensuring that it comes from filling device free of bubbles. In this way reproducible edge conditions or initial conditions are present in the measuring chamber, which likewise leads to an increase in the accuracy of the knowledge available about the pressure pulse. For the degassing unit one could envisage either means which orientate the measuring chamber on filling with fluid and the placing of different pressure sensors or other parts coming into contact with the liquid, in such a way that any air bubbles can escape upwards, or alternatively arrangements which, perhaps by chemical pre-treatment of the surfaces which come into contact with the liquid or by ultra-sound or the like, prevent adhesion of air bubbles to such surfaces.

The parts of the apparatus which carry the falling weight could, according to a further advantageous feature of the invention, be electrically insulated with respect to the piston, and in addition a time-measuring unit is provided, connected conductively with the falling weight and the piston, and serving to determine the duration of the period for which these parts come into contact. In this way, as already described above, it is possible to monitor the duration of the pressure pulse, e.g. accessible through a pressure pick-up output.

According to a further and advantageous embodiment finally the opening for the piston and those for the or each pressure sensor can be arranged substantially symmetrically with respect to the inner wall is the measuring chamber so that simple relationships are obtained with regard to the propagation of the pressure pulse in the liquid.

The invention is further explained in the fol lowing in conjunction with the drawings, in which: Figure 1 shows diagrammatically the important components of the pressure system of apparatus according to the invention, Figure 2 shows a spring and mass oscillating system, Figure 3 shows the variation in pressure p and displacement x with respect to time tin the idealised versions of the apparatus shown in Figs. 1 and 2, Figure 4 shows a diagram of the course with respect to time of the displacement x (t) in the oscillation respectively for the first halfperiod of the oscillation of an oscillating system with a single-mass oscillator with nonlinear spring and damping characteristics; such an arrangement is shown in Figure 5 with the important parameters.

Figure 6 shows a diagrammatic side view of apparatus according to the present invention, Figure 7 is a view of the device of Fig. 6 looking along the arrow VII in Fig. 6, Figure 8 shows a section through the measuring chamber of the apparatus of Figs. 6 and 7 to a larger scale, Figure 9 shows an illustration corresponding to Fig. 8 but of a measuring chamber of different construction, Figure 10 shows a section through the falling weight of another embodiment of the invention in the region of the guide rails and Figure 11 shows a partial elevation of a further from of apparatus according to the present invention during filling with the liquid.

As an example of high pressure indication particularly important in connection with measuring the development of pressure in the powder chamber of cartridge after firing of the shot - it can be shown how with a coid pressure shock the development of the pressure in the powder chamber of the cartridge can be simulated mechanically and under what assumptions this pressure shock can be recorded mathematically or absolutely.

For the development of pressure on firing very rapid compression; 0.5 to 1 millisecond to the peak pressure up to a few kilobars there results for the above-mentioned compression phase a curve which to a first approximation can be compared with the course of a harmonic function (since wave half cycle).

A harmonic movement is produced mechanically for example by a spring-mass oscillating system as illustrated in Fig. 2. By the use of a medium which is compressible within corresponding limits (in this case the liquid) as the spring and a piston instead of the vibrating mass m which compresses this spring in a closed chamber, the spring force proportional to the displacement x (t) can be converted over the piston cross sectional area A into a pressure - see Fig. 1. The development of the pressure p with time illustrate in Fig. 3 arises when the liquid (that is to say the pressure-transmitting medium) is compressed by the 'oscillating mass' at the initial velocity v,, where va==x (t=O).

With the 'pressure system' illustrated in Fig.

1, therefore, pressure shocks or hydrodynamic pressure pulses of varying amplitude and duration can be generated dependent upon the initial velocity v8, the compressibility K of the pressure-transmitting medium or liquid of starting volume V, piston cross-section A and piston mass m, these pulses being at least comparable with the rise in the gas pressure in the powder chamber of a cartridge and can be handles mathematically in the ideal case like the spring-mass oscillating system. In particular what applied in view of what has been said above is the equation p (t)=c.x(t) A where c here represents in practice the spring constant of the pressure system dependent upon the compressibility K and the volume V.

The single-mass oscillating system illustrated diagrammatically in Fig. 5 differs from that of Fig. 2 substantially by virtue of the damping which is now taken into account as well and which in the present case is dependent upon the piston velocity and originates substantially wholly from the friction in the liquid displaced by the piston. If we take different initial velocities (vai < v,2 < va3) - as shown in Fig. 4 - we obtain the maximum amplitudes of different heights and also a different behaviour of the pressure pulse in relation to time, as the velocity dependent damping has a retarding effect both in the build-up of pressure and also as it drops away (see Fig. 5).

The apparatus shown in Figs. 6 and 7 serves to generate a defined hydrodynamic pressure pulse of which the duration and amplitude can be selected within limits and comprises substantially a largely rigid-walled measuring chamber 1 which contains a liquid enclosed in the chamber and a compression device 2 for dynamically compressing the liquid in the chamber 1, and a pressure sensor which is not shown but which can follow the relative change in pressure with time arising in the liquid chamber 1. The chamber 1 has a sealed opening 3 in its wall, in which is guided a piston 4 of the compressing device 2, actuated dynamically from outside. The compressing device 2 also includes a falling weight 5 of pre-determined mass which is guided on guide means 6 and co-operates with the piston 4.

The guide means 6 of the compressing device 2 comprises two vertical columns or rails 7 which, like the measuring chamber itself, are mounted on a base plate 8 and at their upper ends they are held by a corresponding transverse head plate 9. In addition there is a pillar 7' which is likewise arranged between the base plate 8 and the head plate 9. The guiding means 6 for the falling weight 5, as shown in Fig. 10, can have self-centering air bearings 10 co-operating with the rails 7 to reduce the friction on relative movement between the weight 5 and the rails 7.As shown in Fig. 6 starting positions for the falling weight 5 or its holder 5' are determined on the guiding means 6 and the pillar 7' by detents 11 which also, in conjunction with the above-mentioned low friction bearings allow the maintenance of values which are reproducible within limits for the impact velocity of the weight 5 on the piston 4 and thereby for the kinetic energy transmitted to the piston 4.

An electromagnetically actuated holding and release device, not shown, for the weight 5 can be provided on the holder 5'.

For accurately determining the impact velocity and thereby the kinetic energy there is a velocity measuring device 12, shown in Fig. 7, in the region of that end of the piston 4 which projects from the chamber 1 and in the example illustrated it operates without contact, for example, by means of light beams and it ascertains the impact velocity of the weight 5.

Not shown in Figs. 6 and 7 are further additional devices such as for example a necessary electronic evaluating unit, connecting cables and so on as these parts are not important for an understanding of the invention.

Some embodiments of measuring chambers 1 are illustrated more closely in Figs. 8 and 9 and they differ from one another substantially only in that in Fig. 8 there are not pressure sensors in the inner wall 13 of the chamber 1 whereas the embodiment as shown in Fig. 9 there are two pressure sensors 14 projecting symmetrically into the chamber 1 or, rather, into its interior 15 which contains the liquid. In both embodiments of Figs. 8 and 9 there is a low-friction pressure-compensated seal 16 in the opening 3 for the piston 4, this seal having an annular projection 1 7 concentric with the piston 4 on that side of it which is towards the interior 15 of the chamber 1.The dimensions of this annular projection 17, that is to say its extension in an axial direction, its inside diameter and its outside diameter are chosen in relation to one another and in conjunction with the choice of material of the seal in such a manner that the clearance 18 with respect to the piston 4 remains at least substantially constant in the pressure range which arises and this makes it possible for the clearance 18 which increases at high pressures to be squeezed inwards by the radial forces acting from outside on the projection 17.

Finally from the Fig. 11 can be seen a filling device 19 which communicates with the interior of the chamber 1 in a manner not shown and which has a contact probe 20 for the piston 4, making it possible to maintain a reproducible starting volume in the chamber 1.

Within this filling device 19 or equally well separately from it there could be a degassing unit, not shown, which ensures that supply via the device 19 into the chamber 1 is free of bubbles as well as, circumstances permitting, ensuring bubble-free coating of the surface.

The falling weight 5 and the parts of the device which come into contact with it could furthermore be electrically insulated with respect to the piston 4 in a manner not shown and could be connected through leads to a time-measuring unit. In this way in a very simple manner the actual period of contact between the weight and the piston could be determined, corresponding in the idealised case to the duration of the respective pressure pulse generated and thereby it can be used for monitoring and also for correction.

Claims (21)

1. A method of generating a hydrodynamic pressure pulse of duration and amplitude which can be chosen within limits, in which a liquid enclosed in a measuring chamber is dynamically compressed, comprising the following steps - the liquid present in the closed interior of the measuring chamber is acted on by means of a piston which is actuated dynamically from outside, - the kinetic energy transmitted to the piston and by it to the liquid is determined, - the associated development of the pressure pulse in relation to time is determined and - the relevant spring and damping characteristics of the pressure system for the absolute definition of the pressure pulse are derived from the kinetic energy transmitted by the piston and the observed development of the pressure in relation to time

2.A method according to claim 1 in which the piston is actuated by a falling weight of predetermined mass.

3. A method according to claim 1 in which the spring characteristic c and the damping characteristic d are determined as the relevant system constants of the pressure system, the piston and the falling weight being regarded as one mass which oscillates under the influence of c and d.

4. A method according to claim 3 in which, with the mass of the falling weight kept constant, several pressue pulses in time sequence are generated, with different impact velocities of the falling weight, the pulses thereby having different widths and amplitudes, and the spring characteristic c and the damping characteristic d are determined from the spring constant c and the damping constant dj observed point by point in this way.

5. A method according to any one of claims 2 to 4 in which to determined the kinetic energy and the velocity of the falling weight is observed immediately before it en gages the piston.

6. A method according to any one of claims 2 to 5 in which the duration of contact between the falling weight and the piston is determined and is employed to monitor and correct the determination with time of the pressure pulse.

7. A method according to any one of claims 1 to 6 in which, for the absolute dynamic calibration of a pressure sensor using the pressure pulse generated, the course with time of the pulse is determined by making use of the output signal of the sensor to be calibrated.

8. A method according to any one claims 1 to 6 in which, for determining the dynamic compressibility of the liquid contained in the measuring chamber, the starting volume of the chamber is determined as well.

9. Apparatus for carrying out the method according to one of claims 1 to 8 comprising a largely rigid-walled measuring chamber, a liquid encloses in it and a compressing device for dynamically compressing the liquid in the chamber, in which the measuring chamber has at least one sealed opening in its wall, in which is guided a piston of the compressing device, actuated dynamically from outside, and a pressure sensor, and the compressing device furthermore includes a falling weight of predetermined mass which is guided on guide means and co-operates with the piston.

10. Apparatus according to claim 9 in which the opening in the measuring chamber for the piston is provided with a low-friction pressure-compensated seal with respect to the piston.

11. Apparatus according to claim 10 in which the seal has at that face of it which is towards the interior of the chamber an annular projection which is concentric with the piston and of which the dimensions are selected so that the clearance with respect to the piston remains largely constant in the range of pressures which arise.

12. Apparatus according to any one of claims 9 to 11 in which a velocity measuring device for the falling weight is arranged in the region of that end of the piston which projects from the chamber.

13. Apparatus according to claim 12 in which the velocity measuring device operates without contact, e.g. by means of light beams.

14. Apparatus according to any one of claims 9 to 13 in which the guide means for the falling weight has self-centering air bearings co-operating with guide rails to reduce the friction.

15. Apparatus according to any one of claims 9 to 14 in which defined starting positions for the falling weight are provided on the guide means, e.g. by means of detents.

16. Apparatus according to any one of claims 9 to 15 in which the interior of the chamber communicates with a filling device for the liquid, the filling device having a sensing probe for engaging the piston to allow the maintenance of a reproducible starting volume.

17. Apparatus according to claim 16 in which a degassing unit for the liquid is provided, ensuring bubble-free supply through the filling device.

18. Apparatus according to any one of claims 9 to 17 in which the parts of the apparatus which carry the falling weight are electrically insulated with respect to the piston and an electric time-measuring unit is provided, conductively connected to the weight and the piston and serving to determine the period of contact between these two components.

19. Apparatus according to any one of claims 9 to 18 in which the opening for the piston and for the or each pressure sensor is arranged at least substantially symmetrically with respect to the inner wall of the measuring chamber.

20. A method of generating a hydrodynamic pressure pulse of duration and amplitude which can be chosen within limits, substantially as described with reference to the accompanying drawings.

21. Apparatus for generating a hydrodynamic pressure pulse of duration and amplitude which can be chosen within limits, substantially as described with reference to the accompanying drawings.