CA2106148C - Device for resafing an armed mine - Google Patents

Abstract

A device for resafing an armed mine (3). The device comprises a resafing means (2) for performing a resafing movement in the mine, an electric motor (1) for providing said resafing movement, and a battery (5) for powering the electric motor. An artificial load (8) is connected across the battery (5) and is by means of a timer circuit (4) intermittently transformable from a normally non-conductive state to a conductive state. The artificial load is dimensioned to draw, in its conductive state, substantially as high electric current from the battery as the electric motor (1) draws during the resafing movement. A comparator (12) is provided to compare the battery voltage to a reference voltage (Vref), and to deliver a resafing signal to the electric motor when the battery voltage falls below the reference voltage in order to initiate the resafing movement.

Description

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TECHNICAL FIELD
The present invention relates to a device for resafing an armed mine. The invention relates particularly, but not exclusively, to a device for resafing an armed land mine.
BACKGROUND PRIOR ART
Land mines nearly always utilize a safe mechanism to pro-vide a delayed arming function. Arming is initiated by starting a timer mechanism or a battery-operated electric motor which drives a gearing device during a predeter-mined time interval whereupon arming will occur. During this time interval all personnel have to leave the danger zone of the mine.
The mine will then be armed during a certain preset time period, whereafter a resafing shall be performed. By means of a spring-biassed arming means (operated by a timer) which co-operates with a rotor element within a detonator house, a resafing may readily be performed by releasing a locking device.
When performing the resafing by electronic means, the mine often utilizes a so called reservoir capacitor which is able to, during a short time, deliver a higher electric current than can be delivered by the conventional mine battery. The energy of such a reservoir capacitor can, at an appropriate time, be fed to the powder fuze of a gas generator. A piston of the gas generator will then release the rotor element such that this will be returned to a fully safed condition, thereby accomplishing a resafing.
In those cases where the safety, arming and fuze functions are powered by an electric motor there will be required so large quantity of energy that such a reservoir capaci-tor will in practice not be able to store sufficient energy. Therefore, the battery itself has to deliver the current to the electric motor. When utilizing a fresh battery when burying the mine and a lay-time which is not too long, there will always be enough energy in the battery to provide a resafing.
The difficulties will of course increase if the battery has to be stored during a long time, and if great length of operation time is desired for the mine.
The mine fuze is often provided with a battery guard. The aim of the guard is to shut off the electronic means if the battery voltage has fallen so much that the normal function of the mine may be jeopardized. Preferably, the mine should also be resafed in order to avoid it from becoming a so called dud.
Since the electric motor draws 10 or 100 times more current than the rest of the electronic means, it will be understood that when the battery guard will sense a voltage drop such that it will switch on the motor, the load will be so heavy that the battery voltage will fall to a level where the motor is unable to function.
DISCLOSURE OF THE INVENTION
The object of the invention is to provide a device of the kind mentioned by way of introduction, which will guarantee automatic resafing of the mine before the battery voltage has fallen below a critical reference voltage level.
According to the invention there is provided a device for resafing an armed mine, including a resafing means for performing a resafing movement of a safety, arming and fuze unit in the mine, an electric motor operatively connected to the resafing means to cause said resafing movement, and a battery for powering the electric motor, 2a further comprising an artificial load connectable across the battery, the artificial load being dimensioned to be able to draw substantially as high electric current from the battery as the electric motor draws during said resafing movement, and by a comparator able to compare the battery voltage to a reference voltage when the artificial load is connected across the battery, and able to deliver a resafing signal to the electric motor if the battery voltage has dropped below the reference voltage, in order to initiate said resafing movement.

21os1~g BRIEF DESCRIPTION OF THE DRAWING
The sole figure of the drawing illustrates an electric circuit diagram of a device according to the invention.
PREFERRED EMBODIMENT
The figure illustrates electric motor 1 which is operati-vely connected to a movable resafing means in the form of slider 2 able to perform a resafing movement of a conven-tional, and therefore not shown Safety, Arming and Fuze Unit (so called SAFU) in only schematically shown land mine 3.
The mine is, in a conventional way, provided with timer circuit 4, which, in a known way, has output terminal 4a able to deliver an arming signal, after a predetermined time interval elapses, to first input terminal la of mo-tor 1 in order to start arming of the mine. Timer circuit 4 has also output terminal 4b, which after a certain lay-time, for instance 60 days, emits a resafing signal to second input terminal lb of motor 1 in order to start re-safing of the mine.
Electric motor 1 and timer circuit 4 are powered by elec-trio battery 5 which is able to be connected and discon-nected, respectively, by switch handle 6. The voltage of timer circuit 4 is stabilized by DC/DC converter 7. Other components of the mine, which are supplied with current from battery 5 are not shown, since they do not constitute any part of the invention.
According to the invention artificial load 8 is connected in parallel with battery 5. Artificial load 8 comprises, in the embodiment shown, a series arrangement of resis-tance 9 and normally non-conductive field effect transis-tor 10. Transistor 10 is connected with its gate l0a to third output terminal 4c of timer circuit 4. Timex circuit 4 is arranged to intermittently, for instance twice a day, emit at its output terminal 4c a control signal to gate 10a of the transistor in order to transform transistor 10 to a conductive condition during the duration of the con trol signal, which may be 2 to 4 sec., for instance.
Artificial load 8 is dimensioned to draw substantially as high electric current from battery 5 as electric motor 1 draws during its resafing movement. Each time artificial load 8 becomes conductive, the terminal voltage of battery 5 will, accordingly, drop. This terminal voltage is, via lead 11, supplied to first input terminal 12a of voltage comparator 12, second input terminal 12b of which is con-netted to reference voltage Vref.
Output terminal 12c of comparator 12, and output terminal 4b of the timer circuit are connected to respective input terminal 13a and 13b, respectively, of logical OR-circuit 13, output terminal 13c of which is connected to input terminal lb of the electric motor.
Accordingly, resafing of the mine will be automatically performed, when a connection of artificial load 8 will reduce the terminal voltage of battery 5 to a value below Vref, since input terminal 13a of the OR-circuit is high due to signal from comparator 12. However, i,f the terminal voltage of battery 5 never drops below Vref during the lay-time of the mine, the mine will be resafed only after the lay-time has elapsed, since second input terminal 13b of the OR-circuit then is high due to signal from timer circuit 4.
Battery 5 is suitably a lithium battery, since that kind of battery will function within a large temperature range (-35°C to 65°C). Battery 5 is typically a lithium thronyle chloride cell (LiSOCI2) in the range of R-14 having the following typical data ~10~148 -terminal voltage 3.65 volts -capacity 5 amperehours -output voltage at 1 milliampere, 10°C 3.6 volts 100 milliamperes, 20°C 3.4 volts 1 milliampere, -40°C 3.3 volts 100 milliamperes, -40°C 2.7 volts If the cell will be charged by 1 milliampere during a long time at room temperature, the voltage will gradually de-crease. If the reference voltage of comparator 12 is set at 2.5 volts, the battery cell will then in all probabili-ty be exhausted. However, by testing the cell intermit-tently according to the invention, automatic resafing will be guaranteed before the battery voltage, subjected to low charge, has dropped below 2.5 volts. Hence, correct opera-tion of the mine will always be obtained.

Claims (3)

1. A device for resafing an armed mine, including a resafing means for performing a resafing movement of a safety, arming and fuze unit in the mine, an electric motor operatively connected to the resafing means to cause said resafing movement, and a battery for powering the electric motor, further comprising an artificial load connectable across the battery, the artificial load being dimensioned to be able to draw substantially as high electric current from the battery as the electric motor draws during said resafing movement, and by a comparator able to compare the battery voltage to a reference voltage when the artificial load is connected across the battery, and able to deliver a resafing signal to the electric motor if the battery voltage has dropped below the reference voltage, in order to initiate said resafing movement.

2. A device in accordance with Claim 1, wherein the artificial load is normally non-conductive, and a timer circuit is provided to intermittently deliver to the artificial load a control signal, which during its duration transforms the artificial load to a conductive state.

3. A device in accordance with Claim 2, further comprising an electronic OR-circuit, one input terminal of which is connected to the timer circuit in order to receive an alternative resafing signal after a predetermined lay-time, and the other input terminal of which is connected to the output terminal of the comparator.