Abstract
A piezoelectric sensor arrangement for use in a projectile comprises a piezoelectric sensor enveloped by a damping layer adapted to attenuate signals of frequencies above a predetermined cutoff frequency whereby the voltage output signal of the piezoelectric sensor upon impact on a desired hard target can be discriminated from voltage output signals originating from impact on undesired soft objects. A method of discriminating voltage outlet signals by means of the piezoelectric sensor arrangement and the use of a piezoelectric sensor arrangement in a projectile to safeguard unintentional detonation does not occur is described herein.
Claims
1. Piezoelectric sensor arrangement for use in a projectile comprising a piezoelectric sensor fully or partially enveloped by a damping layer adapted to attenuate signals of frequencies above a predetermined cutoff frequency whereby the voltage output signal of the piezoelectric sensor upon impact on a desired hard target can be discriminated from voltage output signals originating from impact on undesired soft objects.
2. Piezoelectric sensor arrangement according to claim 1, wherein the damping layer allows signals below the predetermined cutoff frequency to pass.
3. Piezoelectric sensor arrangement according to claim 1, wherein the damping layer is glued or soldered to the piezoelectric sensor.
4. Piezoelectric sensor arrangement according to claim 1, wherein the damping layer has a thickness ranging from 0.01 to 1 mm.
5. Piezoelectric sensor arrangement according to claim 1, wherein the damping layer is composed of silicone or a silicone-based material.
6. Piezoelectric sensor arrangement according to claim 1, wherein the material of the damping layer does not substantially change its mechanical properties within a temperature ranging from 60 to 80 C.
7. Piezoelectric sensor arrangement according to claim 1, wherein the piezoelectric sensor is a piezoelectric crystal.
8. Method of discriminating voltage outlet signals by means of the piezoelectric sensor arrangement according to claim 1 comprising i) integrating the voltage output signals obtained from the piezoelectric sensor upon impact over time to calculate the electrical energy ii) setting a threshold limit based on knowledge of the calculated electrical energies obtained in step i) thus safeguarding discrimination of low-energy content originating from undesired soft objects below said threshold limit from high-energy content originating from desired hard targets.
9. Use of a piezoelectric sensor arrangement in a projectile to safeguard unintentional detonation does not occur.
10. Projectile arranged with a piezoelectric sensor according to claim 1.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 shows electric output voltage signals of a piezoelectric crystal without a damping layer.
[0041] FIG. 2 shows electric output voltage signals of a piezoelectric crystal embedded in a damping layer of silicone.
[0042] FIG. 3 shows electric output voltage signals having been integrated over time with respect to an arrangement in which the piezoelectric crystal is embedded in a damping layer of silicone.
[0043] FIG. 4 shows a projectile comprising a piezoelectric crystal positioned in a fuze structure.
DETAILED DESCRIPTION OF THE INVENTION
[0044] FIG. 1 shows simulated raw data of piezoelectric voltages for various target types. The X-axis shows time (in seconds) and the Y-axis shows voltage (in Volts). It may be noted that voltages start to arise after a period of time of about 0.05 ms. At that time, a shock wave is sensed by the piezoelectric crystal (without damping layer) via the mechanical structure of the shell impacting on a target. From a mechanical movement in the piezoelectric sensor, an electric current is output from the piezoelectric sensor. The electrical current charges a capacitor and other electrical components arranged to be charged by an electrical current. The capacitor is electrically connected to the logical device and/or processor device of the electronics of the fuze. The signals from the piezoelectric sensor are thus transmitted to the processor of the fuze that analyzes the signals. The signal is analysed with respect to time and amplitude including rise time, amplitude and integration of the voltage over time. Integration of the voltage over time corresponds to the generated electrical energy increasing over time. As the projectile impacts on brushwood, the electrical energy is declining after an initial increase in generated electrical energy. As can be noted in FIG. 1, impact on brushwood 8 creates a short but high peak in the piezoelectric crystal whereas impact on a steel target 9 at an angle of 5 results in a signal of lower amplitude of the peak but also a signal reflecting the retardation of the projectile over time. Numeral 10 indicates the deceleration upon impact on the steel target 8.
[0045] FIG. 2 shows simulated data of piezoelectric voltage signals impacting on brushwood and a steel target at an angle of 5. The piezoelectric crystal is embedded in a damping layer of silicone with a thickness of 0.8 mm. The Y-axis corresponds to the digital reference value the analogue-digital converter (ADC) yields. 0 corresponds to 0 V and 255 corresponds to the reference value of the ADC (in this particular case, 2.3 V). The different numerals represent the following impacts:
[0046] Brushwood 11 (the signals of which have been considerably reduced relative to impact on the steel target 14). The signals of the silicone-embedded piezoelectric crystal is to be compared to the non-embedded piezoelectric signals in FIG. 1 where the signals of the brushwood have higher peaks than the signals following impact on the steel target. Signals 12 represent freely flying shell which not yet has reached the steel target. The signals 13 represent the impact of the fins of the shell on the steel target (impact angle 5). The peak 14 represents impact on the steel target by the shell body. The signal 15 represents the shell having rebounded and flying freely toward a sand heap (represented by signal 16) where the shell stops. FIG. 2 clearly shows the filtering effect of the silicone layer resulting in considerably reduced signals, in particular following passage through the brushwood.
[0047] FIG. 3 shows simulated data of integrated piezoelectric voltages. The same piezoelectric crystal as in FIG. 1 is used but in FIG. 3, it is provided with a silicone damping layer attached around the surface of the piezoelectric crystal. The piezoelectric signals illustrated in FIG. 3 have been filtered, i.e. frequencies above a certain level have been cut-off by means of the silicone layer and integrated. Also, the graphs of FIG. 3 illustrating the retardation following the impact on the steel target at an impact angle of 5 have been integrated over time. The input signals have been transmitted to the fuze electronics having a predetermined threshold triggering whether the projectile shall detonate or not. In FIG. 3, as the signals have been integrated, the signal of the desired target can be discriminated from the brushwood, i.e. the undesired object. This method of integrating signals over time thus enables safe setting of a threshold voltage signal when a detonation shall occur without running the risk of premature detonation, for example as the shell passes brushwood. The threshold set will determine at which minimum level the detonation shall be initiated. The threshold voltage for one and the same damped piezoelectric crystal will vary depending on the mechanical structure of the device, e.g. a shell.
[0048] FIG. 4 shows a projectile 2 in flight in the direction indicated by arrow 1. The projectile 2 comprises a booster charge 3 and a fuze structure 4. In the fuze structure 4, a damping structure 5 (not to scale) composed of a silicone layer is joined to the piezoelectric crystal 6 (only the damping layer 5 below the crystal 6 is shown in FIG. 4the damping layer 5 may in an alternative embodiment also entirely envelope the crystal 6). The piezoelectric crystal 6 is in contact with the projectile body via boundary surface 7 (and via the enveloping damping layer 5) from which projectile body the shock wave upon impact is transmitted to the piezoelectric crystal 6.
