Abstract:
A device detects the times of multiple explosions to within a microsecond thout the need for precise aiming. The device uses a linear array of photodetectors to detect light from the explosions. Each photodetector views a different area, so that explosions occurring in different areas are viewed by different photodetectors. To increase the sensitive area of each photodetector while keeping the device compact, the photodetectors are elongated in a direction orthogonal to the direction in which they are arrayed. A filter is used to transmit only the light produced at the beginning of each explosion.

Description:
FIELD OF THE INVENTION 
     The invention is directed to a system for detecting the timing of multiple explosions and is intended particularly for detecting the timing of explosions taking place in multiple warheads used to defeat reactive armor targets. 
     1. Description of Related Art 
     A multiple warhead includes a tip charge designed to defeat reactive (exploding) armor and a main charge designed to defeat the remaining armor on tanks and armored personnel carriers. In experiments conducted to assess the weapon and vehicle performance, the timing between explosions must be measured to within one microsecond. 
     Various attempts have been made to solve the problem of measuring this timing, including the use of a single optical detector with electronics connected to an oscilloscope. The known attempts have been successful only when conditions such as aiming have just happened to be sufficient for data collection. 
     2. Summary of the Invention 
     It is an object of the invention to provide a system for detecting the timing of multiple explosions without depending on fortuitous circumstances such as aiming. 
     To achieve this and other objects, the present invention is directed to a device for detecting a plurality of events that generate light, the device comprising: an array of detectors, each detecting a portion of the light and for generating an output; threshold detecting means, receiving the outputs from the array of detectors, for determining when any of the outputs exceeds a predetermined threshold value and outputting a trigger signal accordingly; and storage means, receiving the outputs and the trigger signal, for storing the outputs for a predetermined time period after the trigger signal. 
     It is desirable to design the detectors in the array so that the detectors have large sensitive areas, while the array covers only a small field of view. Otherwise, a particular detector might not detect a particular explosion because the image of that explosion on that detector might fall outside the most sensitive area of the detector. Therefore, it is preferable to design the detectors to be narrow in the direction of the array, which is aligned with the direction of detonation, but long in the orthogonal direction. 
     It is also desirable to include wavelength filters. According to experience in testing warheads, an explosion typically releases blue (˜450 nm) light for a very short duration, followed by infrared light. It is preferable to include narrow-band filters to pass the blue light, in combination with infrared blocking filters because the narrow-band filters do not by themselves block all infrared light. 
     Inclusion of a computer allows the user to communicate with the device both before and after testing. The computer can save multiple test results, thus removing the need to return to the target between tests. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment will now be set forth in detail with reference to the drawings, in which: 
     FIG. 1A shows a schematic diagram of an arrangement according to the invention used to detect explosions; 
     FIG. 1B shows a top view of the arrangement of FIG. 1A; 
     FIG. 2 shows a schematic block diagram of the device according to the invention; 
     FIG. 3 shows a detector array for use in the device of FIG. 2; and 
     FIG. 4 shows a camera body incorporating the array of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B schematically show respective side and top views of an arrangement according to the invention used to detect explosions. As shown, a detecting device 100 outfitted with zoom lens 102 is positioned to view explosions that occur as missile 104 having main charge 106 and tip charge 108 strikes reactive armor 110 on target 112, thus causing explosions that produce light 114. Light 114 includes light 114A from the explosion of tip charge 108, light 114B from the explosion of reactive armor 110, and light 114C from the explosion of main charge 106. 
     FIG. 2 shows a block diagram of detecting device 100. Light 114 from the explosion of missile 104 is collected by the zoom lens 102 and is filtered through blue narrow-pass filter 200A and infrared blocking filter 200B to produce filtered light 115. The filters 200A and 200B extract the blue (˜450 nm) wavelengths emitted at the time of each explosion and exclude wavelengths persisting after the explosion. The latter wavelengths could otherwise saturate the detectors. Zoom lens 102 and filters 200A and 200B could be reversed in order. 
     The device has a detector array 201 of N photodetectors 202-1, 202-2, . . . , 202-N for receiving the filtered light 115 from the explosion of missile 104. The output of each photodetector 202-1 . . . 202-N is amplified by a corresponding amplifier 204-1, 204-2, . . . , 204-N and is converted into an eight-bit word by a corresponding analog-to-digital converter (ADC) 205-1, 205-2, . . . , 205-N. Each photodetector provides a separate channel of data. 
     Threshold detector 206 is responsively coupled to the outputs of ADC&#39;s 205-1 . . . 205-N and has switch 216 for selecting a threshold. Threshold detector 206 scans the outputs to determine which output first exceeds the threshold. In response to such a determination, the threshold detector 206 produces a trigger signal to control storage of data. Alternatively, a separate threshold detector could be provided for each amplifier. 
     Storage device 208 coupled to threshold detector 206 stores the outputs of the detectors for a predetermined time after receipt of the trigger signal and is accessible by a computing device such as microprocessor 210. A single trigger signal starts data acquisition for all channels. The storage device 208 preferably has a non-volatile component such as non-volatile RAM or a hard drive. For each channel, 1,024 data points are stored, one for each interval of one microsecond, for a total time interval of 1.024 millisecond. 
     Timer 207 outputs clock pulses and counts the clock pulses until the appropriate number is reached to control the taking of the data points. Timer 207 produces one clock pulse every millisecond and counts from 0 to 1,023. 
     For comparison purposes, it can be useful to collect data from times before the trigger signal. In such cases, 128 data points in each channel can be taken when the device becomes ready to collect data. The device then waits for the trigger to collect the remaining 896 data points in each channel. 
     Battery 212 provides power for the device. The battery can be charged by any suitable charging system. 
     A user can communicate with the device by way of any suitable communication port, such as serial port 214, and thereby download the stored outputs to a PC or the like. The data can be downloaded with any suitable software, such as Procomm Plus for DOS or Windows. 
     Device 100 is armed (placed in the active state) via armed switch 218. The device can also be configured to wait for one minute after data have been taken and then rearm itself. 
     Event display 220 displays the number of events stored in storage 208. It can be a seven-segment LED display capable of showing any digit from zero through nine. The count shown by the event display can be reset by using reset switch 222. Data from the events remain in the non-volatile component of storage device 208 until overwritten, whether or not the count is reset. 
     FIG. 3 shows the detector array 201 for use in the device in greater detail. The detector array 201 may include, for example, six photodetectors 202-1, 202-2, 202-3, 202-4, 202-5, and 202-6, each of which measures 1 mm in its shorter direction and 8 mm in its longer direction. The shorter directions are aligned along axis A, which is in turn aligned to be parallel to the axis along which the explosions will be separated. The longer directions are aligned along axis B, which is orthogonal to axis A. When the detector array 201 is located at the focus of a 35 mm camera with a 70-210 mm zoom lens, the area viewed by each photodetector is about 17 inches by 136 inches. 
     In operation, each of the channels observes a different portion of the region in which the explosions take place, and data are taken every microsecond. Thus, explosions occurring in different portions of the region and at different times can be discerned independently, and the time between such explosions can be measured to within a microsecond. 
     The detecting device 100, or at least the photodetector array 201, can be incorporated into a camera body 400, as shown in FIG. 4. Light 114 from the explosion passes through zoom lens 102 and filters 200A, 200B to form filtered light 115, which enters camera body 400. Beam splitter 402 transmits one-half of the light 115 to photodetectors 202-1, . . . , 202-N and the other half of the light 115 via viewfinder 404 to human eye 406, thus allowing the user to aim the camera body. The viewfinder 404 can include a frame for aiming. 
     A working model of the preferred embodiment, known as the Warhead Detonation Timing Measurement System, has been built and tested. The specifications of the Warhead Detonation Timing Measurement System will now be given. Of course, these specifications are to be considered illustrative rather than limiting, as they refer to one specific working device. 
     Specifications 
     Package 
     Weight, 21 pounds, approximately 
     Size, 1 cubic foot, approx. 
     Material, Aluminum 
     Color, White 
     mounting, 1/4&#34; 20 threaded hole provided on bottom side for tripod 
     Handles, for carrying, located on sides 
     Battery 
     12V 4.6 AH Lead-acid battery (battery 212 of FIG. 2) supplies power. 
     Current drain, 330 mA standby, 440 mA operating, approximately 
     A rear panel green LED indicates when battery has more than approximately 5 hours charge life remaining; the LED is on when the battery charge is good, which is above 13.2 V. 
     Charge life is 5 hours minimum from fully charged battery. 
     Battery charge condition is available through the RS-232 port. 
     13.6 V for fully charged battery with charger on, 13.2 V for 5 hr (armed) life, 10.8 V for low battery. An external power source may be connected for longer battery life, use 16±V ac or 18±V dc, 400 mA, isolated from the ac line. 
     Charger 
     Wall transformer, 120 VAC. 60 Hz input. 1 A, 16 Vac output connects to front panel with a 3.5 mm P-1 connector 
     LED indicates charging when the charger is connected and on. 
     Data output 
     Serial output using Kermit protocol, 19.6 kbaud, 8 bit no parity. 1 stop bit 
     Connector, DB9 male on front panel (shown in FIG. 2 as serial port 214). 
     Terminal communications are in ASCII on the display. Trigger thresholds and signal levels are in hex format, with 1 bit equal to 20 mV. 
     Battery levels are in hex format with 1 bit equal to 80 mV. 
     Lens 
     70-210 mm Macro, Vivitar f4.5-5.6 1:4x type PK-AR (lens 102 of FIG. 2) 
     52 mm lens diameter, P/N 217186 with adapter for matching to Pentax camera body. Equivalent lens for other cameras used. 
     Camera Body 
     Modified SLR, Pentax K1000 
     Detector Array (array 201 of FIG. 2) 
     Detector, Silicon Photodiode UDT Pin 10379, 6 elements of 1×8 mm each 
     Element dark Current, 1 nA maximum at 5 V bias 
     Element responsivity, 0.35 A/W at 600 nm, peak responsivity at approximately 900 nm 
     Element Junction capacitance, 200 pf maximum 
     Element load resistance, 1 kΩ 
     Uniformity of Response, 5% overall 
     Electrical System for Signal Data 
     Digital Sampling Rate, 1 MHz 
     Frequency Response, 500 kHz 
     Data Resolution, 8 bits 
     Number of Data Acquisitions, 1024 
     Controls, Switches, Connectors and Indicators: These functions set, display and control the functions of the instrument and provide for information retrieval: 
     Events in Memory, 7-segment bar type LED (element display 220 of FIG. 2) shows how many exposures have occurred. The event counter is reset to one using the rear panel reset switch 222, or a power off, power on cycle. The events in memory remain until overwritten with new data whether or not the event counter has been reset. 
     Armed Switch 218 enables the electronics to trigger and perform a measurement sequence. The unit will automatically reset for the next event after approximately one minute. Red LED indicates when unit is armed. 
     Serial Interface, 9 pin, &#34;D&#34; connector 214 for data downloading, using &#34;Kermit&#34; protocol. 
     Thumbwheel Threshold Switch 216 sets the threshold Voltage, 4.99 Volts maximum in units, tenths, and hundredths of Volts, for a signal on any detector element to trigger a measurement event. The switch scale is voltage, which is the voltage from the amplifier section above which the trigger occurs. The noise floor of the threshold circuit is 25 mV. The threshold level should not be set for less than 30 mV, or spurious triggers may occur. 
     Triggered red LED shows if an event has been recorded for one minute after a trigger. The unit will rearm after this one minute period, and turn the trigger LED off. 
     Charging LED, green LED shows charging action from external charger. It is on when charger is connected to the AC line and the system. 
     &#34;Good Battery&#34; green LED indicator on rear panel indicates when battery is charged. 
     Reset Switch 222 resets event counter to 1 for beginning of up to 8 measurement events. The pre-recorded data are saved in non-volatile memory until overwritten with new data, except for the data in the indicated test number when the unit is armed. 
     Operating instructions 
     Before operation, the battery should be recharged, using the included charger, for 24 hours, or until the &#34;Good Battery&#34; LED is on. Alternatively, the battery voltage can be monitored for proper charge using the RS-232 serial port 214. 
     The unit should be placed on a holding fixture. The tripod mount is secured in the holding fixture to mount the unit. 
     Initial settings 
     At power on, the triggered light will come on for about one minute. The unit should not be used until the triggered light goes out. 
     The trigger level voltage should be set using the thumbwheel switches on the front panel. Previous data can be used for guidance. 
     If the lens requires adjustment, the access cover on the side of the system can be opened, to allow the user to perform the following: 
     Without neutral density filters placed on the macro lens filter adapter, look through viewfinder in camera body, adjust alignment of box for proper location of source on target. Adjust focus on macro lens for best image quality. 
     If required, place neutral density filters on the macro lens filter adapter. 
     Record the Events In Memory Number for this event, the trigger level, lens settings, focus distance and F#. 
     Operation 
     The Armed switch is pressed. The Armed LED should illuminate, whereupon the system is ready for a measurement sequence. The events counter should show &#34;1&#34;. The sequence will begin when a detector element in the camera sees enough light to reach the trigger threshold. 
     After the system is triggered, the data of 1024 data points (128 data points saved prior to trigger, 896 after trigger), for each of the 6 elements of the detector will be recorded. After each trigger, the event counter will increment by one. 
     Device 100 is configured to wait for a one-minute dead time between events; during this time the unit cannot be triggered. After the one-minute period the Armed LED will be on, and the unit will record the next event when triggered. 
     After eight events, the unit will be disabled from triggering, as the maximum number of events will have been recorded, and the Armed function will be automatically cleared. 
     The power can be turned off for data security. The data are saved in Non-volatile RAM. The data should be downloaded through the RS-232 port. 
     Pushing the reset switch will reset the event counter to &#34;1&#34;. Data in event &#34;1&#34; will be overwritten when the unit is armed. 
     When the device is armed, the previous test data on the indicated test number will be overwritten. This is because the pre-trigger data (128 data points) must be retained when the unit is next triggered. After a reset or power down, the unit will only start on test number &#34;1&#34;. All test data are retained and available for download. Data should be downloaded only when the device is in the unarmed state. 
     Theory of Operation 
     Analog Gain Stages 
     When light from the telephoto lens on the camera is imaged onto the photodiode array mounted at the camera focus, a current proportional to the optical power received is generated. Normal optical signals (explosions) will generate current in the micro-amp range. The first stage of electronics provides a low impedance load and voltage gain. 
     There are six identical channels, one for each photodiode, of first stage amplifiers. The six front end amplifiers drive six A/D converters. 
     Trigger Operation 
     When armed, the unit uses the optical input signals to decide when to take and store data. Analog comparators monitor the amplified signal and the remaining five channels. If any signal rises above the threshold set by the rear panel thumbwheel switches, the signal trigger is activated. A 4.00 Volt primary reference voltage is divided into 1 V, 0,100 V, and 0.010 V increments. These are selected by the thumbwheel switches, summed, and distributed by buffer amplifiers to the comparators. The maximum threshold voltage is 4.99 V. 
     Analog to Digital Converters 
     Each amplified photodiode output is converted to an 8-bit digital word by the A/D converter. The converters start the conversion on the negative edge of the 1 Mhz clock signal. When the conversion is complete, in less than 1 μs each, the converter generates a pulse that is used to control writing of the data into non-volatile RAM for later analysis. The converters have 5.120 Volts full scale range. This range results in 20 mV per bit resolution of the amplified signal. 
     Control Logic 
     The unit uses the occurrence of any one the six channels detecting an over-threshold condition to start the recording of date. If the unit is armed, the one-minute timer has expired, and the test number is 8 or less. Before the trigger occurs the 1 μs interval counter is free-running from count 0 to 1023 and recycling. A register contains the count where the previous 127 data words were stored prior to the unit being triggered. 
     During the data acquisition period, the interval counter continues counting for 896 additional steps, storing data from each of the six A/D converters every microsecond. When the last count is detected, further data recording is halted, and the test logic increments to the next test sequence number. After 8 events have been recorded, no further triggers will be accepted. 
     Pushing the Reset button and rearming the unit at any point during the test will erase all data from test 1. The data will be non-recoverable. 
     Microcontroller 
     Since the data are accumulated at 1 μs intervals, the microcontroller is too slow to manipulate the data in real time, although as faster microcontrollers become available, this should no longer be the case. Data are taken solely by the digital logic with the microcontroller used for status and interfacing to the external computer through the RS232 Serial Port. Communications protocol is &#34;Kermit.&#34; 
     While the unit is accumulating data, the microcontroller is prevented from accessing any data. The Armed signal controls which process is in control of the unit. If the unit is Armed, then data are accumulated; if the unit is not Armed, the microcontroller can access data and status through the RS232. The unit must not be armed when the microcontroller is trying to access the unit, as the data will be destroyed in one or more tests in memory. Power-up circuitry insures that the unit powers up not Armed. 
     The microcontroller is a standard 8031 with 2k×8k of nonvolatile RAM and 32k ×8k of ROM. Data memory and I/O control decodes are listed in the following table: 
     
         ______________________________________I/O Register  Hex Address                   Function______________________________________Program Data Memory         0000-07FF Read/Write 2k × 8Ram  Real Time Clock/Calendar     0800-087E       Read/Write Real Time Clock  Start Address, LSB           1000            Read LSB of Start Address                    Start Address, MSB           1800                   Read MSB of Star Address  Status Register              2000            Read Status  Battery Voltage              2800            Read Battery Volts 20.48V                                                                 FS                    Trigger Threshold Voltage    3000                   Read Threshold Voltage 5.12                                          VFS  Test Data Byte 0             4000            Read Byte 0 of Data                    Test Data Byte 1             4800                   Read Byte 1 of Data  Test Data Byte 2             5000            Read Byte 2 of Data                    Test Data Byte 3             5800                   Read Byte 3 of Data  Test Data Byte 4             6000            Read Byte 4 of Data                    Test Data Byte 5             6800                   Read Byte 5 of Data  Software Reset               7000            Reset all Hardware                   Registers  Memory Read Address          1800            Write Address Register                    (LSB)                   (LSB)  Memory Read Address      2000            Write Address Register                    (MSB)                    (MSB)______________________________________ 
    
     While an embodiment and the specifications of a working model have been set forth, those skilled in the art who have reviewed this specification will readily appreciate that modifications may be made within the scope of the invention. For example, various operations such as threshold detection can be performed in analog or digital mode as needed. Components disclosed as separate can be consolidated; for example, a single ADC can be used in place of ADC&#39;s 205-1, 205-2, . . . , 205-N. Likewise, an element having multiple functions can be replaced by multiple elements.