Method and apparatus for generating a precisely defined dynamic pressure pulse

Method and apparatus for generating a precisely defined dynamic pressure pulse. A rigid pressure chamber contains a pressure transmission agent. The pressure chamber has a through opening in which is disposed a piston. A drop weight outside the pressure chamber coacts with the piston to generate a pressure pulse. The mass of the drop weight and the cross section of the piston, along with the chronological curve of acceleration of the drop weight during coaction with the piston, are used to generate a curve representing the pressure pulse. The curve satisfies the following equation: ##EQU1## wherein: m=mass of the drop weight; PA0 a(t)=said acceleration curve; and PA0 A=said cross sectional area of said piston.

TECHNICAL FIELD 
The present invention is directed to a method and apparatus for generating 
a precisely defined dynamic pressure pulse having a duration and amplitude 
selectable within predetermined limits. 
BACKGROUND OF THE INVENTION 
Methods and apparatus are known which generate static (hydrostatic) 
pressure pulses, as well as dynamic (hydrodynamic) pressure pulses. 
However, such devices and methods are capable of describing only static 
pressure pulses with absolute mathematical precision. Difficulties have 
arisen in precisely mathematically describing dynamic or hydrodynamic 
pressure pulses (as particularly required, for example for the further 
development or calibration of piezoelectric pressure sensors that are 
employed in non-stationarily sequencing technical processes). 
For example, German OS 37 07 565 discloses method and apparatus that at 
least theoretically generate a precisely defined dynamic pressure pulse. 
This method and apparatus employs a piston and drop weight to charge a 
pressure transmission medium in order to generate a pressure pulse, the 
chronological curve of which is ascertained. The relevant spring and 
damping characteristics of the pressure system are identified from the 
kinetic energy transmitted by the piston and from the above mentioned 
chronological curve. These characteristics are used to draw conclusions 
about the course of the absolute pressure pulse. However, in order to 
ascertain the chronological curve of the pressure pulse, the apparatus 
employs a pressure sensor of the very type that the apparatus is designed 
to calibrate. Since the pressure sensor influences the dynamic 
characteristics of the chronological curve of the pressure pulse, it 
inevitably introduces an undesirable level of uncertainty into this known 
method. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a method and 
apparatus to generate a mathematically definable, precisely describable 
dynamic pressure pulse. 
It is another object of the invention to provide a method and apparatus for 
the dynamic calibration of pressure sensors that is independent of the at 
least initially unknown dynamic characteristics of such pressure sensors, 
i.e. a method and apparatus whose dynamic characteristics are 
ascertainable without the use of pressure sensors. 
These and other objects are achieved by providing a largely rigid pressure 
chamber that surrounds a pressure transmission agent. The pressure chamber 
includes a sealed through opening for a piston that is dynamically 
actuatable from outside the pressure chamber via a drop weight. 
Conclusions regarding the chronological curve of the absolute pressure 
pulse are made from individual measurements at the pressure system using a 
connected evaluation mechanism. 
The chronological curve of the acceleration drop weight is identified 
during its coaction with the piston. Using this chronological curve in 
conjunction with measurements of the mass of the drop weight and the cross 
section of the piston, the curve of the absolute pressure pulse may be 
determined with mathematical precision. 
The dynamic pressure curve of a gaseous or fluid medium which is charged 
with pressure in a closed measuring chamber and acted upon by a piston and 
drop mass, can be portrayed as the product of the compressibility of the 
medium and the penetration of the piston, dependent on time, divided by 
the cross sectional area of the piston. The compressibility of the medium 
(analogous to a type of spring constant) is at least in part dependent 
upon temperature and thus is, as a practical matter, extremely difficult 
to quantify. There are corresponding tabular values only for individual, 
pure agents. It is practically impossible to obtain compressibility values 
for other media, particularly for fluid mixtures typically used in 
generating high pressure pulses. 
The product of the compressibility of the agent and the penetration path of 
the piston dependent on time during its coaction with the drop weight, 
corresponds to a force. This force can in turn be expressed as product of 
the mass of the drop weight and the acceleration or deceleration of the 
drop weight during its contact with the piston. Hence, given a 
predetermined piston cross section, by calculating the drop mass and the 
chronological curve of the acceleration of the drop weight during its 
coaction with the piston, one can determine the dynamic pressure curve 
with mathematical precision without the assistance of a pressure sensor. 
It should be noted that the above method is actually based on a two mass 
resonator model, since certain spring and damping affects occur between 
the drop weight and the piston. Strict adherence to such a model would 
make the evaluation of the measurements and thus, the precise definition 
of the pressure pulse, extremely complicated. However, in the known method 
that was initially described, as well as in the method of the present 
invention, it has been shown that a very high degree of precision in 
mathematical description of the generated dynamic pressure pulses can be 
achieved with a single mass resonator model, as long as the ratio of the 
mass of the drop weight to the mass of the piston can be set approximately 
in the range of 100:1 and above. In a preferred embodiment of the present 
invention, this ratio is set at about 1000:1. 
In a further embodiment of the method of the present invention, the 
absolute pressure pulse p(t) is calculated according to the following 
relationship: 
##EQU2## 
wherein: m=mass of the drop weight (g) 
a(t)=curve of the acceleration of the drop weight (g) in interaction with 
the piston (k) 
A=cross-sectional area of the piston (k). 
The apparatus of the present invention includes a measuring arrangement 
having an acceleration measuring unit that generates measured signals to 
an evaluation system. These measured signals are dependent on the 
acceleration curve of the drop weight during its coaction with the piston. 
Acceleration measuring units that are capable of precisely measuring the 
chronological curve of acceleration are well known. The present invention 
also provides measuring equipment to determine the mass of the drop weight 
and the relevant cross section of the piston. Once measured, these 
quantities may be considered constants, and therefore have to be 
identified only once. 
In another embodiment of the invention, the acceleration measuring unit 
includes an acceleration sensor, preferably a piezosensor, attached to the 
drop weight. Since such sensors, in contrast to pressure sensors, can also 
be dynamically calibrated on what are referred to as vibrating tables or 
similar arrangements, the chronological curve of the acceleration of the 
drop weight can thereby be simply and precisely calculated. 
In contrast to direct measurement by an acceleration sensor, the 
acceleration may also be calculated by differentiation of a velocity timed 
curve of the drop weight. Any known method can be employed for the 
determination of the velocity timed curve itself. However, in a further 
preferred embodiment of the present invention, the velocity timed curve is 
calculated in "non-contact" fashion, preferably with laser velocimetry 
using the Doppler effect. 
Commercially available laser velocimeters form a Doppler signal from an 
emitted beam. The beam is reflected at the test subject, and provides an 
extremely precise determination of the velocity of the test subject using 
the Doppler frequency of the emitted and reflected beams. 
Other objects and advantages of the present invention will become apparent 
upon reference to the accompanying description when taken in conjunction 
with the following drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus of FIGS. 1 and 2 generate a precisely defined dynamic 
pressure pulse having a selectively variable duration and amplitude. The 
apparatus includes a generally rigid pressure chamber 1 that contains a 
pressure transmission agent (a gas, a gas mixture, or a fluid) enclosed by 
the pressure chamber. The apparatus also includes a compression mechanism 
2 for dynamically compressing the pressure transmitting agent. A piston k 
is sealingly guided in a through opening 3 of the pressure chamber 1. A 
drop weight g having a predetermined mass m is mounted for vertical 
movement on a guide assembly 4 to dynamically actuate the piston k. The 
guide assembly 4 of the compression mechanism 2 includes two vertical 
columns or guide rails 7 that are mounted, with the pressure chamber 1, on 
a base plate 5 and are held at their upper ends by a cross head 6. A stand 
7' is arranged between the base plate 5 and the cross head 6. The guide 
assembly 4 for the drop weight g can include low friction bearings (not 
shown) interacting with the guide rails 7 or with the stand 7' Such 
bearings, for example self-centering air bearings, are used to reduce the 
friction generated by the relative motion between the drop weight g and 
the guide assembly 4. As may be seen in FIG. 1, predetermined initial 
positions for the drop weight g or for a drop weight holder 9, are defined 
at the rails 7 or at the stand 7'. In this embodiment, these positions 
occur at latches 8 that facilitate the observation of reproducible initial 
values. A holding and trigger mechanism (not shown) for the drop weight g 
can also be provided with the guide assembly 4. 
FIGS. 1 and 2 show an acceleration measuring unit 10 arranged at the cross 
head 6. The acceleration measuring unit 10 includes a laser velocimeter 11 
directed onto the drop weight g. A beam path 12 generated by the laser 
velocimeter 11 is shown in broken line. The acceleration measuring unit 10 
is connected to an evaluation system (not shown) via a line 13. The 
evaluation system uses the curve of the velocity generated by the 
acceleration measuring unit 10, in conjunction with values representing 
the cross sectional dimension of the piston k and the mass of the drop 
weight g, to generate a mathematically precise description of the pressure 
pulse within the chamber 1 as described with reference to the following 
equation: 
##EQU3## 
wherein: m=mass of the drop weight (g) 
a(t)=curve of the acceleration of the drop weight (g) in interaction with 
the piston (k) 
A=cross-sectional area of the piston (k) 
As an alternative to the laser velocimeter 11, FIGS. 1 and 2 also shown an 
acceleration sensor 10' (in broken line) that is directly attached to the 
drop weight g. The acceleration sensor 10' can, for example, be formed by 
a piezosensor and directly enables a registration of the acceleration or 
deceleration of the drop weight g upon its coaction with the piston k. The 
acceleration measuring unit 10' may be connected to an evaluation system 
(not shown) via a line 13'. 
FIG. 3 shows a detailed sectional view of the pressure chamber 1 having a 
low friction, pressure-compensated seal 14 between the piston k and the 
through opening 3. The seal 14 is concentric with the piston k, and 
includes an annular projection extending into the interior 15 of the 
pressure chamber 1 from the surface of an insert 16. The dimensions of the 
projection (i.e., its axial extent and inner and outer diameters) can be 
selected, along with the material of the insert 16, to provide a generally 
constant seal between the piston k and the opening 3. This enhanced seal 
occurs due to the fact that increasing pressure in the interior 15 of the 
pressure chamber 1 increases the radial forces biasing the projection 14 
inwardly towards contact with the piston k. 
As shown in FIG. 3, two pressure sensors 17 (for example piezoelectric 
sensors) are inserted at a lower side of the pressure chamber 1, and are 
connected via lines 18 to corresponding measuring equipment (not shown). 
The pressure sensor 17 can be dynamically calibrated with the precisely 
defined , dynamic pressure pulse generated by the apparatus of FIGS. 1 
through 3. Note, however, that the dynamic pressure pulse generated with 
the illustrated equipment in the interior 15 of the pressure chamber 1 is 
not limited to the calibration of the pressure sensors. The pressure pulse 
can also be used for other purposes for which the exact knowledge of all 
relevant parameters of the pressure pulse is required. 
The cure a(t) shown in FIG. 4 was generated with an acceleration sensor 10' 
attached to the drop weight g as shown in FIGS. 1 and 2. After reaching 
maximum deceleration (e.g., at the bottom of its travel), the elastic 
properties of the pressure transmission agent act through the piston k to 
accelerate the drop weight g in an upward direction. This acceleration and 
deceleration is expressed in the reversal of the a(t) curve. The highly 
zig-zag appearance of the a(t) curve in FIG. 4 is due to vibrations caused 
by the impact of the drop weight g on the piston k, of from slight flaws 
in the selection and attachment of the acceleration sensor 10' itself. 
Such minor fluctuations, however, can be easily filtered out or 
mathematically smoothed in the evaluation system. 
The curve p(t) shown in FIG. 4 has been calculated from the curve a(t) 
according to the equation set forth hereinabove, taking into consideration 
the mass m of the drop weight g as well as the cross sectional area a of 
the piston k. Although it is obvious that the operational signs and the 
base lines of the two curves do not agree, it is equally clear that the 
maximums of both curves are coincident. 
The velocity curve v(t) shown in FIG. 5 represents the output of the laser 
velocimeter 11 of FIGS. 1 and 2. By differentiating the function v(t), an 
acceleration curve a(t) is obtained that is similar to that shown in FIG. 
4. This acceleration curve can then be used in the equation as described 
hereinabove, the result of which is the curve p(t) as shown in FIG. 5. 
It is therefore clear that the method and apparatus of the present 
invention generate a precisely defined dynamic pressure pulse using 
relatively easily obtained information, while eliminating the use of a 
pressure sensor requiring initial calibration. 
Although the present invention has been described with reference to 
embodiment, those of skill in the art will recognize that may be made 
thereto without departing from the scope and the invention as set forth in 
the appended claims.