Abstract:
A device ( 10 ) for detecting the presence of an airborne, electrically conductive particle, the device including spaced conductors ( 24,26 ) and a circuit ( 12 ) for detecting when the electrically conductive particle forms a conducting path between the spaced conductors ( 24,26 ). The conductors ( 24,26 ) are provided in a grid ( 14 ) of alternate elongate conductors, and the circuit ( 12 ) applies a voltage to one set ( 24 ) of conductors sufficient to detect and destroy the particle when it creates a conductive path to the alternate set ( 26 ) of conductors.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a device for detecting an electrically conductive particle.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrical equipment may not perform correctly if undesirable electrically conducting paths form between circuit elements at different electrical potentials. Installations with low voltage circuitry such as telephone exchanges and computer rooms are particularly vulnerable. Electrically conducting paths may form when airborne, electrically conductive particles settle upon circuit elements. For example, zinc particles or “whiskers”! are known to grow on zinc electroplated metalwork used in such installations. If a whisker breaks away from the metalwork, the result is an extremely light, virtually invisible, needle-like and highly electrically conductive, airborne whisker. This whisker may fall upon sensitive electronic circuitry, resulting in equipment failure. This mode of failure is known to occur in many installations, yet can be particularly difficult to diagnose. Failure due to metallic whiskers is an ongoing problem which is extremely costly for providers of data processing and switching equipment. It is desired, therefore, to provide a device for detecting an electrically conductive particle, or at least a useful alternative to existing detection devices.  
         SUMMARY OF THE INVENTION  
         [0003]    In accordance with the present invention, there is provided a device for detecting the presence of an airborne, electrically conductive particle, said device including spaced conductors and a circuit for detecting when said electrically conductive particle forms a conducting path between said spaced conductors.  
           [0004]    The present invention also provides a device for detecting an electrically conductive particle present in the air, including:  
           [0005]    a detection grid of spaced conductors; and  
           [0006]    a detection circuit for detecting when said particle electrically connects said conductors.  
           [0007]    The present invention also provides a device for detecting an electrically conductive  5  whisker, including:  
           [0008]    a sensor of spaced conductors; and  
           [0009]    a detection circuit for detecting when said whisker electrically connects said conductors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 is a schematic diagram of a preferred embodiment of a particle detector;  
         [0012]    [0012]FIG. 2 is a diagram of an electrode structure of the detector;  
         [0013]    [0013]FIG. 3 is a block diagram of the electrical components of the detector;  
         [0014]    [0014]FIG. 4 is a circuit diagram of the electrical components of the detector; and  
         [0015]    [0015]FIG. 5 is a flow diagram showing a preferred embodiment of a particle detection process executed by the detector. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    A detector  10  of electrically conductive particles, as shown in FIGS.  1  to  4 , includes a sensor  14  and a processing circuit  12 . The detector  10  detects electrically conductive particles larger than a certain size that contact the sensor  14  and stores the number of detection events in non-volatile memory  40 . The detector  10  includes status indicators  38  for providing visual information to a user of the detector  10 , such as when a particle is detected, and the number of particles detected by the detector  10 . The detector  10  may be used to qualitatively or quantitatively indicate the presence of airborne metallic whiskers in electrical installations prone to failures caused by whiskers from equipment metalwork. Because the detector  10  is small enough to be held in the hand, it may be easily used by maintenance personnel to evaluate whisker counts in several locations within a single installation, for example. The early detection of whiskers allows subsequent equipment failure to be avoided by removing whiskers from an installation, and provides an indication of the likelihood of whiskers as the cause of equipment failure.  
         [0017]    The sensor  14  comprises a number of detection grids  16 ,  18 . The detector  10  is built on a standard fiberglass PC board (PCB)  22 , and most of the surface of the PCB  22  is occupied by two detection grids  16 , one on either side of the PCB  22 . The detector also includes two additional detection grids  18 , mounted perpendicular to the plane of the PCB  22  to increase the detection probability. The four detection grids  16 ,  18  are connected in parallel. The PCB has a small rubber foot in each of its four corners for supporting the detector  10 .  
         [0018]    Each detection grid  16 ,  18  comprises a pair of electrically conductive, interdigitated tracks or fingers  24 ,  26 , as shown in FIG. 2 for the two mounted detection grids  18 . The fingers  24 ,  26  are supported on fiberglass PC boards, and the outer surfaces of the fingers  24 ,  26  are coated with gold to ensure good electrical contact with impinging particles. The use of gold plating is significant as it provides good conductivity and does not form an insulating surface oxide layer on the fingers. The PCB material is electrically insulating, so that there is essentially no electrical conductivity between the two fingers  24 ,  26  unless an electrically conductive particle contacts the two fingers simultaneously, such as when an airborne metallic whisker impinges upon the two fingers  24 ,  26 .  
         [0019]    The processing circuit  12 , as shown in FIGS. 3 and 4, includes a power supply  28  powered by a 9V lithium battery  20 , a DC step-up circuit  30 , a detection circuit  32 , a microcontroller  34 , non-volatile, EEPROM memory  40 , status indicators  38 , and control switches  36 . The power supply  28  in this implementation is a National Semiconductor LM2936-5.0 5V regulator. The DC step-up circuit  30  generates a high (51V) voltage that is applied across the fingers  24 ,  26  of the sensor  14  to break down oxide layers of metallic whiskers that impinge upon the sensor  14 .  
         [0020]    An EPROM-based microcontroller  34 , such as the PICmicro PIC16C73B-20, executes a particle detection process, as shown in FIG. 5. This process is implemented as a software program stored in the microcontroller&#39;s internal program memory. When the detector  10  is first powered, by inserting the battery  20 , or when the microcontroller  34  is reset by pressing a reset switch of the control  36 , the microcontroller  34  performs an initialisation step  502 , and then switches the grid voltage at step  504 . Step  504  is achieved by enabling the output  31  of a DC-DC controller chip, such as the Maxim MAX773, of the DC step-up circuit  30  through an output port  6  of the microcontroller  34 .  
         [0021]    The microcontroller  34  detects the appearance of a conductive path between opposing fingers  24 ,  26  of the sensor  14  by sensing a change in the voltage on a single-bit input port  45  (RB7) connected to the sensor  14  via the detection circuit  32 , which includes a voltage divider having two resistors  44 ,  46  in series. Particular resistance and voltage values are described below for one implementation of the detector  10 , but it will be understood by those skilled in the art that different values may be selected, particularly if a different microcontroller is used. For example, a microcontroller with an internal voltage comparator may be employed, or a microcontroller that is coupled to an external voltage comparator.  
         [0022]    A first resistor  46 , of value 62 kΩ, is connected between the input port  45  and ground. A second resistor  44 , of value 2 MΩ, is connected between the port  45  and one set of fingers  24  of the detection region  14 . These fingers  24  are also connected to the 51V supply  31  from the DC step-up circuit  30  through a current-limiting resistor  42  of value 560 Ω. The other set of fingers  26  is connected to ground. When there is no conductive particle between opposing fingers of the sensor  14 , current flows from the 51 V supply to ground through the three resistors. Because the value of the current-limiting resistor  42  is negligible in comparison with the first and second resistors  44 ,  46 , the 51 V supply potential is essentially divided across the first and second resistors, so that the potential at the input port  45  of the microcontroller  34  is at a level other than low, approximated by 51 V (62 kΩ/2MΩ)≈1.6 V. When a conductive particle such as a metallic whisker forms a conductive path between opposing fingers  26  and  26  of the sensor  14 , most of the current from the 51 V supply passes through the sensor  14  to ground, provided that the resistance of the conducting path through the particle is substantially less than 2 MΩ. The potential at the input port  45  is therefore a low level, ≈0 V.  
         [0023]    After turning the grid voltage on at step  504 , a check is performed at step  506  to see if a particle is stuck between opposing fingers  24  and  26  of the sensor  14 . If this occurs, the input port voltage will remain at 0 V. Normally, the current flowing through the conducting particle will be sufficient to melt the particle and destroy the conducting path.  
         [0024]    However, if this does not occur, the detector cannot detect any more particles, and the battery power will simply drain away. Consequently, the grid voltage is shut off to save power and the process stops at step  508 . Otherwise, the input port potential returns to 1.6 V, and the process proceeds to step  510  with a check to see if an erase EEPROM function has been selected by the user controls  36 . If so, then the EEPROM memory  40  is erased at step  512 . After this step, or if the function was not selected, a detection event count is read from the EEPROM memory  40  and the count value is displayed on the status indicator  38  and output to a serial port of the microcontroller  34  for transfer to an external device such as a notebook computer. The status indicator  38  is a light-emitting diode (LED) that flashes a number of times equal to the count value.  
         [0025]    Subsequently, the microcontroller  34  enters a sleep mode at step  516  and waits for a particle to impinge upon the sensor  14 . Sleep mode is a low power consumption mode of the microcontroller  34  which conserves battery power. If a particle forms a conducting path between opposing fingers  24  and  26  of the sensor  14 , the potential at the input port RB7 of the microcontroller  34  changes from ≈1.6 V to ≈0 V. The input port circuitry of the microcontroller  34  monitors the potential on this port and generates an interrupt when its value differs from a previously latched value. The interrupt wakes the microcontroller  34  from sleep mode at step  520 . The detection event is written to the EEPROM memory  40  at step  522  by simply reading the currently stored detection count value, incrementing it by one, and storing the incremented value. The process then delays for a predetermined period of time, for instance 200 ns, at step  524 , and then checks to see if the particle has been removed, as described above, at step  526 . If the particle has been removed, the process loops back to step  516  and enters sleep mode. If the particle has not been removed, the grid voltage is turned off at step  528 , a flag is written to EEPROM memory at step  530 , and the process stops at step  532 .  
         [0026]    Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as herein described with reference to the accompanying drawings. For example, a more sophisticated display such as a liquid crystal display may be used instead of the status LEDs. More sophisticated communications methods may also be employed; for example, the detector  10  may include a Bluetooth module for wireless communication of particle detection events, event counts, and status information to a remote processing module. The storage of particle detection events may include storing a timestamp with each detection event.