GB2318872A

GB2318872A - Blast wave detection

Info

Publication number: GB2318872A
Application number: GB9622614A
Authority: GB
Inventors: Kevin John Sharpe; Stuart James Cameron Dyne
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1996-10-30
Filing date: 1996-10-30
Publication date: 1998-05-06
Also published as: GB9622614D0

Abstract

A method for determining the direction of propagation of a blast wave involving arranging at least four sensors (S 1 , S 2 , S 3 , S 4 ) in a non co-planar array in the expected path of the blast wave, the sensors (S 1 , S 2 , S 3 , S 4 ) each being capable of detecting the passage of the blast wave and being disposed so that the positions of the sensors relative to one another are known. Upon passage of the blast wave the time delay of passage of the blast wave past different sensors is measured and from these time differences and the positions of the sensors relative to one another the direction of propagation of the blast wave is determined relative to the sensor array. The velocity of the blast wave may also be determined and the magnitude of the blast wave calculated.

Description

Blast Wave Detection This invention relates to a method of detecting the direction of propagation of a blast wave and in particular to a method of taking measurements on blast waves in enclosed spaces and to an apparatus for use therein.

Tests on explosives often require measurements to be made on the blast waves, for instance to determine explosive yield. Tests such as these are usually carried out in large spaces or outdoors both for safety reasons and because the direction of propagation of the blast wave is easy to predict. The measurements are made by positioning sensors in the path of the blast wave and recording the information following detonation.

Known sensors are blast gauges having a relatively large flat surface provided with at least one piezoelectric transducer which measures the pressure of the blast wave as it passes. Blast gauges such as these are quite expensive however and the measurements taken are dependant upon the angle of incidence of the sensors to the blast wave. The sensors need to be positioned therefore such that the angle of incidence is known, with measurements usually being taken at grazing incidence. Also the sensors require continuing calibration throughout their lifetime to be effective.

It is sometimes necessary to do internal explosion tests, for example to determine the effects of an explosion in an enclosed space, which still require measurements to be made on the blast waves produced. In internal explosion tests however the direction of propagation of a blast wave at a measurement point is difficult to predict due to reflection of the blast waves from various surfaces. Therefore correct alignment of the sensors for grazing incidence is almost impossible. As noted above, the output from a sensor can vary significantly between grazing incidence and normal incidence and so knowledge of the angle of incidence is required in order to interpret the output of the sensors.

It is therefore an object of the present invention to provide a means for making measurements on blast waves in situations where the direction of propagation is difficult to predict.

Thus, according to the invention there is provided a method of determining the direction of propagation of a blast wave comprising the steps of, arranging at least four sensors in a non co-planar arrangement wherein the location of each sensor relative to the others is known, each sensor being capable of detecting the passage of a blast wave, determining the times at which a blast wave reaches each sensor, and calculating from said times the direction of propagation of the blast wave relative to the sensors.

The time difference between arrival of the blast wave at each sensor is determined by the separation of the sensors, the velocity of the blast wave and the angle of incidence of the blast wave. The blast wave has a substantially planar wavefront at any distance from the point of explosion and travels with a velocity, v, perpendicular to the wavefront. This wavefront will pass the sensors in a set order dependant upon the orientation of the sensors relative to the source of the wave. As the arrangement of the sensors relative to one another is known the time difference between the blast wave passing the first sensor it reaches and another sensor will yield information about the components of the velocity of the blast wave. The components can be combined to give the direction of propagation of the blast wave. At least four sensors are required so that a non coplanar configuration can be achieved to ensure that the direction of propagation is resolved in three dimensions.

Locating the direction of propagation of the blast wave not only allows for the blast magnitude to be inferred but also enables the effects of confined spaces or interposed objects on the propagation of the blast wave to be assessed without having to deduce those effects from the results of the blast. Also, the method can give an indication of the source of a blast when the source is not known. Preferably the velocity of the blast wave is also calculated.

In a second aspect of the invention there is provided a method for determining the magnitude of a blast wave comprising the steps of; arranging at least four sensors in a non co-planar arrangement wherein the arrangement of each sensor relative to the others is known, each sensor being capable of detecting the passage of a blast wave, determining the times at which a blast wave reaches each sensor, calculating from said times the velocity of the blast wave and the direction of propagation of the blast wave relative to the sensors, and calculating the magnitude of the blast wave from the velocity and direction of propagation of the blast wave.

The velocity of a blast wave is proportional to the pressure of the blast wave and so determining the velocity of propagation of the wave provides the blast wave magnitude. This method therefore allows for measurements to be taken on blast waves without requiring expensive blast gauges and without requiring the direction of the blast waves to be known at the outset. Also, the method could be used to provide information on blasts which could aid in identifying the characteristics and source of a blast of unknown origin.

Conveniently the sensors are pressure sensors capable of measuring the passage of a blast wave. As the magnitude of the blast wave is not measured directly complex and expensive sensors which measure the magnitude of the pressure discontinuity are not required. Simple and inexpensive transducers can be used which only record passage of a blast wave reducing drastically the cost of measuring a blast wave. Useful pressure sensors may be piezoelectric transducers. Also, as the sensors can be set for a threshold pressure, complex filtering of the output is not required. Further, continual calibration throughout the life of the sensors is no longer required.

A large separation between the sensors will allow for a greater time difference between detection of the blast wave at different sensors which would increase the accuracy of the time measurements. However, too large a separation would allow for the possibility of the velocity or planarity of the wavefront altering between sensors. Also, the greater the separation the greater the chance that more than one wave may pass the sensors during measurement and that a reflected wavefront may confuse the measurement. The separation of the sensors is therefore chosen to be great enough to allow for accurate time measurements but small enough to ensure that the sensors all detect passage of the same blast wave and that the wave is substantially unmodified. A useful range of separations is 75 - 250mm The sensors may either be affixed directly to a structure in the blast wave path, such as an internal wall, or alternatively may be located on a support frame. The sensors could then be located in a known arrangement with respect to an axis through the frame. As the location of the sensors relative to each other and the frame is already known the set up for taking a measurement would simply involve locating the frame at the position at which measurement is required and orienting the frame in a known direction. A cubic skeleton frame gives a simple and stable shape which allows for a variety of sensor positions.

In a another aspect of the invention there is provided an apparatus for measuring blast waves according to the above methods comprising a support means, at least four sensors located in a non coplanar arrangement by said support means, each of the said sensors being capable of detecting the passage of a blast wave, a means for measuring the time differences between the detection of the blast wave by each of the sensors and a logic unit adapted to determine the direction of propagation of the blast wave from the time differences measured and the location of the sensors relative to each other.

Preferably the logic unit also determines the velocity of the blast wave.

A further aspect of the invention is an apparatus for determining the magnitude of a blast wave comprising a support means, at least four sensors located in a non co-planar arrangement by said support means, each of the said sensors being capable of detecting the passage of a blast wave, a means for measuring the time differences between the detection of the blast wave by each of the sensors and a logic unit adapted to determine the direction of propagation of the blast wave and the velocity of the blast wave from the time differences measured and the location of the sensors relative to one another and calculate the magnitude of the blast wave from the velocity and direction of propagation of the blast wave. Such an apparatus could be permanently incorporated into a structure to record the magnitude of a blast wave from an explosion and give an indication of the source of the blast wave and could be fitted, for example, to aircraft holds to provide information for the flight data recorder or for military targets to monitor the effects of explosive attack.

The sensors may be pressure sensors. Convenient pressure sensors are piezoelectric transducers. The sensors are preferably far enough apart that the errors in time measurement are not significant but are close enough together that the same wave passes all sensors substantially unmodified. A convenient range of sensor separations may be 75 - 250mm but other separations could be used in different circumstances.

The support means may comprises a frame and means for attaching the sensors thereto. Usefully, the frame may be a cuboid skeleton frame.

The invention will now be described by way of example only with reference to the following drawing(s) in which; Figure 1 shows a sensor arrangement suitable for use in the method, and Figure 2 is an illustration of the principle of operation of the invention.

Referring now to figure 1 a generally cuboid frame, indicated generally by 2, is constructed from aluminium rods 4. Four sensors Si, S2, S3, S4, are located at various points on the frame 2 in such a manner that the sensors are not co-planar. The sensors Si, S2, S3 and Sq may be for example piezoresistive transducers but it will be apparent to one skilled in the art that other pressure sensitive sensors could be employed. In the lower corner of the frame 2 is located a shielded unit 6 containing a logic unit, timer and power unit (not shown). In use detection of the blast wave by any one of the sensors starts the timer and the logic unit records the time at which each of the other sensors detects the blast wave. The logic unit may then process the results to determine the direction of propagation of the blast wave or, to simplify the apparatus in the shielded unit 6, the logic unit may simply store the information and may be provided with a simple interface means to download the information to a suitably programmed computer. Alternatively the timing means, logic unit and power unit may also be located remotely from the sensors.

The sensors are deployed on the frame 2 such that the separations of any two sensors are of an order of magnitude. An average separation of about 150mum allows for reasonable time intervals between detection of the wave by different sensors but is not so large that the blast wave will be substantially modified between sensors or that there will be appreciable interference from a reflected wavefront. In this example the sensors are disposed toward one side of the cuboid frame 2 in order to minimise the possibility of reflection from any frame support interfering with measurement. Were the frame to be placed on a flat surface and the blast wave expected to come from a generally sideward or topward direction then the frame would be oriented so that the sensors were at the top to avoid reflection from the flat surface. The sensors may be conveniently screwed to the frame or alternatively could be held in specialised mountings.

The principle of operation of the device shown in figure 1 is illustrated by reference to figure 2 which shows a plan view of four sensors designated Si, 52, S3 and Sq which could either be attached to a frame as shown in figure 1 or affixed directly to an internal structure. The blast wave is shown as a plane wave front labelled B moving in a direction perpendicular to the wavefront at a velocity v, as shown by the arrow. The wavefront passes the first sensor S, which activates the timer. The wavefront then passes the sensors S2, S3 and S4 in order at times t2 ,t3 and t4 respectively. The time taken for the wave to travel between sensor S1 and any other sensor is given by the distance travelled divided by the blast wave speed. The distance travelled is perpendicular to the wave front and for sensors S1 and S2 is represented by d12. In vector form d12 is a projection of the vector S12 onto a unit vector in the direction of v.

Therefore, the distance = S12.v and hence the elapsed time, t12 = v2 v v 512 can be expressed as (x2 - xl)i +(y2 - yi)j + (z2 - zl)k where x, y, and z represent Cartesian co-ordinates in the direction of unit vectors i, j and k respectively.

Therefore,

where v2 = vx2 + vy2 + vz2 and vx, vy and vz are the components of V in the i, j and k direction respectively.

As similar equations can be constructed for the other two sensors a matrix relation may be formed;

which can be represented by T = S.u where

The matrix equation can be solved for u giving u = S-1 T. The vector u is proportional v As V.V 4 V then v 1 U to V withu v v As u.u vector v can be calculated from the vector u which is obtained from the locations of the transducers and the arrival times.

Thus by knowing the locations of the sensors in an arbitrary co-ordinate frame the direction of the blast wave and its velocity can be determined. Referring back to figure 1 a convenient co-ordinate scheme would correspond with the axes of the cuboid frame. It is then a simple matter to orientate the frame in a desired manner.

It can be seen from the above that the maximum time delay of passage of a blast wave past two sensors occurs when the blast wave is perpendicular to the line joining the sensors. As the blast wave will generally have a velocity greater than 300my~' the maximum time delay for a separation of 150mm will be 0.5my. The actual time difference measured however will depend on the orientation of the sensors to the blast wave. The accuracy of the system is determined by the bandwidths of the sensors and the recording system. A bandwidth of 50KHz is sufficient for most applications.

It can also be seen from the above that only a detection of the passage of the blast wave is required to determine the direction of propagation of the wave or the velocity and hence the magnitude, however, it will be apparent to one skilled in the art that sensors could be used to also record characteristics of the blast wave in order to provide further information.

Claims

1. A method for determining the direction of propagation of a blast wave comprising the steps of; arranging at least four sensors in a non co-planar arrangement wherein the arrangement of each sensor relative to the others is known, each sensor being capable of detecting the passage of a blast wave, determining the times at which a blast wave reaches each sensor, and calculating from said times the direction of propagation of the blast wave relative to the sensors.

2. A method for determining the direction of propagation of a blast wave according to claim 1 wherein the method also includes the step of calculating the velocity of the blast wave from the times of arrival of the blast wave at each sensor.

3. A method for determining the magnitude of a blast wave comprising the steps of; arranging at least four sensors in a non co-planar arrangement wherein the arrangement of each sensor relative to the others is known, each sensor being capable of detecting the passage of a blast wave, determining the times at which a blast wave reaches each sensor, and calculating from said times the direction of propagation of the blast wave relative to the sensors and the velocity of the blast wave, and calculating the magnitude of the blast wave from the velocity and direction of propagation of the blast wave.

4. A method according to any preceding claim wherein the sensors are pressure sensors.

5. A method according to claim 4 wherein the pressure sensors are piezoelectric transducers.

6. A method according to any preceding claim wherein the sensors are located such that the separation of the sensors is 75 - 250mm.

7. A method according to any preceding claim wherein the sensors are located on a support frame in a known arrangement with respect to an axis through the frame.

8. A method according to claim 8 wherein the support frame is a cuboid skeleton frame.

9. A method for taking measurements of a blast wave substantially as hereinbefore described with reference to the accompanying drawings.

1O.An apparatus for determining the direction of propagation of a blast wave comprising a support means, at least four sensors located in a non co-planar arrangement by said support means, each of the said sensors being capable of detecting the passage of a blast wave, a means for measuring the time differences between the detection of the blast wave by each of the sensors and a logic unit adapted to determine the direction of propagation of the blast wave from the time differences measured and the location of the sensors relative to one another.

1 1.An apparatus for determining the magnitude of a blast wave comprising a support means, at least four sensors located in a non co-planar arrangement by said support means, each of the said sensors being capable of detecting the passage of a blast wave, a means for measuring the time differences between the detection of the blast wave by each of the sensors and a logic unit adapted to determine the direction of propagation of the blast wave and the velocity of the blast wave from the time differences measured and the location of the sensors relative to one another and calculate the magnitude of the blast wave from the velocity and direction of propagation of the blast wave.

12.An apparatus according to claim 10 or claim 11 wherein the sensors are pressure sensors.

13.An apparatus according to claim 12 wherein the pressure sensors are piezoelectric transducers.

14.An apparatus according to any of claims 10 - 13 wherein the sensors are located such that the separation of the sensors is in the range 75 - 250mm.

15.An apparatus according to any of claims 10 - 14 wherein the support means comprises a frame and a means for attaching the sensors thereon.

16.An apparatus according to claim 15 wherein the frame is a cuboid skeleton frame.

17.An apparatus for taking measurements on a blast wave substantially as hereinbefore described with reference to the accompanying drawings.