Technical Field of the Invention

This invention relates to the field of explosion detection, in particular to apparatus and methods for detecting explosively generated overpressure.

Background to the Invention

Explosives are used in a variety of applications including in demolition, ammunition, propulsion and pyrotechnic displays. During manufacture the explosive composition, shape, and arrangement, amongst other attributes, can be tailored to their intended use. Variations in these attributes can affect the ability of an explosive to perform its intended function.

There exists an ongoing requirement for testing explosives during and after manufacture. In particular, understanding the propagation of the shockwave generated from an explosive detonation (the explosively generated overpressure) is critical in specifying the size, shape, and other attributes of an explosive, as well as in predicting how buildings or structures in proximity to an explosion will be affected. Computer modelling of explosive blasts can provide a useful insight, but empirical data from experiments is still desirable and is ultimately required to validate computational models.

The very nature of an explosion renders empirical measurements practically difficult to achieve. Measurements can be obtained sufficiently remotely from an explosion such that measurement equipment is not damaged (for instance using pressure sensors), however measurements in the near-field (within reach of the explosive fireball itself) have remained difficult to achieve. This is because any measurement equipment must overcome issues such as bombardment by detonation products, high temperatures, and radiation generated during the explosion, which ultimately can lead to inaccurate measurements and destruction of expensive measurement apparatus. Therefore it is an aim of the present invention to provide an alternative apparatus and method for detecting an explosion that mitigates these issues.

Summary of the Invention

According to a first aspect of the invention there is provided use of means for detecting ionisation to detect an explosion.

According to a second aspect of the invention there is provided a method of detecting an explosion, the method comprising the steps of locating means for detecting ionisation remotely to an explosive source; and detecting at least a first explosively generated ionisation event using the means for detecting ionisation following detonation of the explosive source.

The explosion may occur in an ionisable environment for instance a gaseous environment comprising atoms, molecules or substances, within which an explosion can occur. When an explosion occurs in an ionisable environment (for instance in air), energy is generated (often visually represented by a fireball propagating from the source of explosion). This energy can be imparted to the ionisable environment and cause ionisation of atoms or molecules. This ionising effect propagates away from the source of explosion. Alternatively as an explosive is detonated it may generate ionised explosive products that travel away from the source of explosion with the explosively generated overpressure. The inventor has shown that the rate of this propagation of ionisation is similar to the rate of propagation of the shock wave or overpressure generated from the explosion in the near-field (within reach of the fireball generated by an explosion). Providing means for detecting ionisation located remote to the explosive source (not in contact with the explosive source) enables an explosion to be detected by detecting the propagating ionisation effect. Means for detecting ionisation can be manufactured relatively cheaply in comparison to direct pressure measurement devices. Therefore the method of the invention provides a more cost effective approach to blast detection. Furthermore the means for detecting ionisation is using the phenomenon that occurs close to an explosion to its advantage that would otherwise render alternative pressure measurement device unreliable. For instance the temperature and heat effects that add noise to or damage other sensor types.

In preferred embodiments of the invention the means for detecting ionisation is arranged in the near-field of the explosive source. The near-field includes any location within reach of the explosive fireball or the ionisation products generated by the explosion. Near-field measurements are practically difficult to achieve with current approaches to explosion detection, without destroying costly measurement equipment. Prior art approaches include use of pressure sensors positioned remotely to an explosive source. Such sensors directly measure the overpressure generated from an explosion. Whilst these sensors are suitable for detecting and measuring the pressure of an explosion in the far-field (away from the fire ball), they are not suitable for near-field measurements. Pressure sensors can be highly sensitive to heat and mechanical effects associated with near-field measurement, and are often relatively expensive and therefore not considered disposable. The inventor has shown that the propagating ionisation effect of an explosion can be used to detect and measure characteristics of an explosion, enabling relatively low cost disposable measurement instruments such as ionisation pins, to be used.

In particular, preferred embodiments of the invention use one or more ionisation pins as part of the means for detecting ionisation. Wire probes have been used to measure detonation of velocity within an explosive material, by detecting the detonation wave as it propagates through the charge. Critical to these measurements is the wire probe being embedded within, or in contact with, the explosive charge, such that the detonation wave can generate an electrical path between the probe and a further electrical conductor, within the explosive charge. The inventor has shown, against the general teaching of the art, that an ionisation pin can be used as a relatively cheap, accurate and disposable means for detecting explosively generated ionisation effects as they propagate away from the source of an explosion. An ionisation pin may comprise an inner electrically conductive core surrounded by an outer electrically conductive sheath. The inner core and outer sheath are separated by a radial gap. In use a voltage can be applied across the radial gap such that upon the intermediate dielectric (for instance air) becoming ionised (as the fireball passes the ionisation pin for instance), a spark across the gap is generated, thereby generating a brief electrical signal which can be measured.

In preferred embodiments the step of locating the means for detecting ionisation comprises the step of arranging a plurality of ionisation pins in a linear array. The linear array may be any linear direction leading away from the explosive source. This allows an ionisation effect to be detected at a plurality of distances away from the source of an explosion. Other arrays may be used in addition to a linear array - for instance two or three dimensional arrays of ionisation pins will provide two or three dimensional information about the propagation of an explosive blast.

Some embodiments of the invention further comprise the step of measuring a time of arrival for each of the ionisation events. The time of arrival is the time after an explosion is initiated that an ionisation event is detected by a corresponding means for detecting ionisation. For instance the time of arrival may be the time after detonation that the ionisation effect propagates past an ionisation pin. This enables not only for an ionisation event, and therefore an explosion, to be detected, but also provides information suitable for establishing characteristics of the explosion itself. Even more preferred embodiments further comprise the step of calculating a velocity function from the times of arrival. The means for detecting ionisation may be positioned at a known distance/s from an explosive source. As such by measuring a time of arrival of one or more ionisation events, a speed of propagation can be calculated. If multiple ionisation pins are used each at respective known distances from an explosive source, a distance versus time relationship can be established, from which a velocity function can be calculated (for instance by differentiation). A velocity function can then be used to predict when an explosion will affect buildings or structures in proximity to a blast. Even more preferred embodiments further comprise the step of calculating a pressure function from the velocity function. This enables a peak overpressure at the means for detecting ionisation to be determined.

According to a third aspect of the invention there is provided apparatus for remotely detecting an explosion, the apparatus comprising means for detecting ionisation. The apparatus is intended to be used to detect or measure an explosive effect without requiring physical contact with the explosive source (for instance an explosive material). In preferred embodiments the means for detecting ionisation is connected to a means for processing data, wherein the means for processing data is configured to: receive location information of the means for detecting ionisation; and to receive from the means for detecting ionisation an indication of at least a first explosively generated ionisation event, and to determine therefrom respective times of arrival. The apparatus of the third aspect of the invention thereby can be used to detect the propagating ionisation effect from an explosion by receiving signal indications of ionisation events, and therefrom reliably detecting whether an explosion has occurred, particularly in the near-field.

In some embodiments of the third aspect of the invention the means for processing data is further configured to calculate a velocity function from the location information and times of arrival. Calculating a velocity function enables the propagation of the ionisation effect, and therefore the overpressure generated from the explosion, to be determined. In even more preferred embodiments the means for processing data is further configured to calculate a pressure function from the velocity function, thereby enabling pressure information to be determined in the near-field of an explosion.

The means for processing data may be a computer system. The means for processing data is connected to the means for detecting ionisation by cable or wireless means, such that the means for processing data can be located away from the means for detecting ionisation, and therefore away from the damaging effects of an explosive blast.

The location information describes the position of the means for detecting ionisation relative to an explosive source from which an explosion will be generated. The location information may be received by the means for processing data by a user inputting the location information (for instance inputting via a keyboard to a computer). The data processor may calculate a time of arrival of an ionisation event as the time an electrical signal or impulse is received via a cable connected to the means for detecting ionisation, or alternatively as a wireless signal from a transmitter attached to the means for detecting ionisation. The means for processing data is intended to be programmable to optionally calculate a velocity function and required overpressures from the time of arrivals and location information (for instance using computer code/software held within internal memory).

Some embodiments of the third aspect of the invention comprise one or more ionisation pins as the means for detecting ionisation. The ionisation pins may each comprise an electrically conductive inner core surrounded by an electrically conductive outer sheath, the inner core and outer sheath being separated by a radial gap. The radial gap may be variable according to the strength of the ionising effect being measured, or indeed the environment within which the measurements are being performed. A voltage applied across the radial gap can be applied such that upon the dielectric within the gap (which may be air) being ionised, a spark is generated, causing a current flow and a signal that can be detected and measured to indicate an ionisation event. Different voltages may be applied depending on ambient conditions. Even more preferred embodiments further comprise one or more means for measuring pressure, such as piezoelectric pins. The inventor has shown that a combination of ionisation pins (for measuring ionisation propagation in the near-field and therefrom overpressure) and piezoelectric pins (for measuring pressure change in the far- field and therefore overpressure) provides apparatus that can provide reliable empirical blast data across a range of distances from the source of an explosion.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1A shows an illustration of an embodiment of apparatus for remotely detecting an explosion;

Figure IB shows an illustration of the apparatus of Figure 1A remotely detecting a first ionization event;

Figure 1C shows an illustration of the apparatus of Figure 1A detecting a second ionization event;

Figure ID shows an illustration of the apparatus of Figure 1A detecting a third ionization event; Figure IE shows an illustration of the apparatus of Figure 1A detecting a fourth ionization event;

Figure 2 shows an illustration of a distance-time relation based on location information and time of arrival information from ionisation pins;

Figure 3 shows an illustration of a velocity function;

Figure 4 shows an illustration of a pressure function; and

Figure 5 shows an illustration of an embodiment of an ionisation pin.

Detailed Description

Figure 1A shows an illustration of an embodiment of apparatus for remotely detecting an explosion 10, arranged remotely to an explosive 11. The apparatus 10 comprises means for detecting ionisation in the form of ionisation pins (12, 13, 14, 15) arranged at respective distances XI, X2, X3 and X4 from explosive 11. The ionisation pins (12, 13, 14, 15) are within the range of the fireball generated when explosive 11 detonates (the near-field). Each ionisation pin comprises an electrically conductive inner core surrounded by an electrically conductive outer sheath. The core and sheath of each pin (12, 13, 14, 15) are separated by a radial gap. A 300V voltage is applied to the ionisation pins (12, 13, 14, 15) across the radial gap. The ionisation pins (12, 13, 14, 15) are also cabled to a means for processing data 16. The means for processing data 16 comprises buffering electronics, a pin mixer and a computer. The computer of the means for processing data 16 comprises an input interface for connecting to the pin mixer output. The computer system of means for processing data 16 also comprises internal memory onto which computer code has been loaded. The computer code is programmed to receive and store location information (XI, X2, X3, X4) for the ionisation pins (12, 13, 14, 15), ambient pressure and ambient temperature as user inputs (for instance through use of a keyboard). The computer code is also programmed to receive and store signals received via the input interface indicating ionisation events. The computer code also stores respective times of arrival of signals received through the input interface. To mitigate received signal noise, the computer code thresholds any received signals and applies a peak detection algorithm to detect the true time of arrival of an explosive ionisation effect at the respective ionisation pin. Subsequent processing is then applied to calculate velocity and pressure functions. Figure IB illustrates the apparatus 10 detecting a first ionisation event tl. Explosive charge 11 has detonated and generated energy resulting in ionisation effect 17 propagating away from the source of explosion 11. The ionisation effect 17 has reached ionisation pin 12 resulting in a spark across the radial gap of the pin 12. A first ionisation event tl is generated and a pulsed signal is communicated to means for processing data 16. The means for processing data 16 buffers the signal and then records the pulsed signal tl and associated time of arrival. Figure 1C shows the ionisation effect 17 at a later time arriving at ionisation pin 13; Figure ID shows the ionisation effect 17 at an even later time arriving at ionisation pin 14; and Figure IE shows the ionisation effect 17 at a later time arriving at ionisation pin 15. As the ionisation event 17 passes successive ionisation pins (13, 14, 15) the ionisation events t2, t3, t4, are detected and signals communicated to the means for processing data 16.

Figure 2 shows an illustration of a distance-time relationship as may be generated by a computer system using location information (XI, X2, X3, X4) and time of arrival information for ionisation events (tl, t2, t3, t4) as detected by the ionisation pins (12, 13, 14, 15).

Figure 3 shows an illustration of a velocity function as may be generated by computer system 16 calculating the derivative of the distance-time relationship shown in Figure 2, and applying curve fitting to determine velocity as a function of location information (XI, X2, X3, X4).

Figure 4 shows an illustration of a pressure function as may be calculated from the velocity function of Figure 3. A suitable function is provided by McNesby et al ("Optical measurement of peak air shock pressures", Propellants Explosives and Pyrotechnics, 2014) that calculates peak overpressure as a function of shockwave velocity. The inventors have shown that in the near-field, shock wave velocity can be calculated from the time of arrival of detected ionisation events, thereby enabling pressure to be determined using the McNesby formula. Other pressure functions also exist, such as the Rankine Huigoniot equations. Figure 5 shows an illustration of an embodiment of an ionisation pin 50 having an electrically conductive inner core 51 and electrically conductive outer sheath 52. Both inner core 51 and outer sheath 52 are formed from metal. A radial gap A exists between inner core 51 and outer sheath 52. Between inner core 51 and outer sheath 52 is a dielectric 53. A voltage is applied between inner core 51 and outer sheath 52 to generate a potential difference across radial gap A. The presence of sufficient ionisation effect across gap A when pin 50 experiences an explosive blast generates a spark temporarily bridging the circuit between inner core 51 and outer sheath 52. A signal is consequently generated that can be used to indicate an ionisation event.

Whilst the embodiments described specify a means for detecting ionisation being used in isolation of other sensors, this is not intended to be limiting. The apparatus may be used in conjunction with other sensors, particularly pressure sensors (piezoelectric pins for instance) to provide both near-field and far-field measurements. The apparatus may be used with any explosive substance that generates a propagating ionisation effect - for instance solid, powder, gaseous or liquid explosives. A cabled connection between the ionisation pins and computer system is described in these embodiments, but a wireless means of communication could be achieved through use of a suitable transmitter and receiver. An oscilloscope or other data capture means may be preferred to a computer system, for the processing of data. A single linear array of ionisation pins are described herein, however multiple linear arrays, two dimensional arrays or three dimensional arrays, of ionisation pins or other suitable means for detecting ionisation, could be used to establish empirical measurements to support the analysis of an explosive blast. The means for detecting ionisation may be mounted on stands that are robust to the overpressure experienced during a blast, but that also allow for minor adjustment to position the means for detecting ionization precisely. Anchoring may be used to fix stands in position. Where ionisation pins are used as part of the means for detecting ionisation, the voltages applied across the pins, and the dielectric used within the pins, may vary depending upon ambient conditions. Air may be used as a suitable dielectric.