Abstract:
The present invention provides a device to monitor whether a work station operator has properly worn a grounding mechanism, and a novel cascading means to chain multiple devices into a chain for sharing electricity and ground. The present invention could achieve the following functions: (1) capable of monitoring both single-wire and two-wire grounding mechanisms; (2) providing backup grounding through the chain so that the static electricity discharging capability is maintained as long as a single device in the chain remains connected to the ground; (3) extending the ground through the chain so that grounding is not required to be previously prepared at every work station; (4) sustaining all devices on the chain by a single external power supply; and (5) providing networking connectivity through the chain&#39;s cascading cables so that the network cabling cost and effort is significantly reduced.

Description:
CROSS REFERENCE 
       [0001]    This is a continuation-in-part of U.S. patent application Ser. No. 11/964,151, filed on Dec. 26, 2007, which is incorporated herewith by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to monitoring electrical grounding, and more particularly to a device and system that automatically monitors an operator&#39;s grounding mechanisms only when the operator is actually present in front of a work station. 
         [0004]    2. The Related Art 
         [0005]    How to prevent electro-static discharge (ESD) from damaging valuable equipment or causing critical fabrication process to fail is still an important issue in high-tech industries. It is well known that proper grounding is the essential solution. However, despite the advancement of technology, ensuring such a proper grounding is not as easy as most people imagine. 
         [0006]    A typical manufacturing environment usually contains a number of assembly lines, and each assembly line usually contains a number of work stations, each for a specific assembly task or manufacturing operation. To prevent ESD from damaging the parts, devices, or the semi-products being assembled, the operator at the work station is usually required to wear an anti-static wrist strap, the floor is usually paved with an anti-static floor mat, and the table top of the work station is usually covered with an anti-static table mat. As illustrated in  FIG. 1   a,  the floor mat  10 , table mat  20 , and the wrist strap  30  are usually electrically connected to a common-point ground  40  of the work station by grounding cables, respectively (for simplicity, the drawing only shows a ground cable connecting the wrist strap  30  and the common-point ground  40 ). The common-point ground  40  is usually a metallic plate fixedly positioned at some place of the work station with a plastic cover for protection. The common-point grounds of an assembly line&#39;s work stations are series- or parallel-connected together, which are in turn connected to an equipment ground or an earth ground of the manufacturing facility (again, for simplicity, the equipment and earth grounds are not shown in the drawing). As such, the static electricity carried by or accumulated on an operator sitting or standing in front of the work station is discharged to the earth through the wrist strap, table mat, or the floor mat, via the common-point ground of the work station and then the equipment or earth ground of the manufacturing facility, thereby preventing potential hazards from ESD. 
         [0007]    The aforementioned grounding system is a proven solution and has been widely adopted for years. However, it suffers a number of disadvantages. First, this grounding system works only if the wrist strap, the floor mat, and the table mat are properly connected to the common-point ground. However, the grounding cables therebetween could be rusted or broken, or the grounding cables could be disconnected from the common-point ground due to the movement of the operator. In addition, when the operator has to take a break or to go for lunch, he or she may take down the wrist strap and leave it on the work station. Or, in most of the existing implementations, the grounding cable of the wrist strap has a plug at one end so as to plug into a socket of the common-point ground. Therefore, the operator unplugs the grounding cable (but still wears the wrist strap) before going for a break or lunch. When the operator returns, he or she then put the wrist strap back or plug the grounding cable again. As can be imagined, a lazy operator may avoid wearing the wrist strap; or an absent-minded operator may forget to put back or re-plug the wrist strap after returning to his or her post. The static electricity carried by or accumulated on the operator cannot be discharged to the ground, and may very possible damage the valuable equipment or parts or semi-product or completed product at the work station. 
         [0008]    As such, the present inventor has disclosed a wireless monitoring device for a work station operator to see if the operator has properly worn a wrist strap or similar grounding mechanism (U.S. patent application Ser. No. 11/964,151 filed on Dec. 26, 2007, hereinafter, the previous application). The representative drawing of the previous application is attached as  FIG. 1   b.  As illustrated, the monitoring device  100  mainly contains a microprocessor circuit  200  as its core. The device  100  is connected to the mains via a power cable or via an external power supply (e.g., a power adaptor such as those used by a notebook computer). The connection to the mains is very important in that, on one hand, the electricity extracted from the mains is processed by a power unit  500  of the device  100  to provide appropriate direct-current (DC) voltages to the microprocessor circuit  200 . On the other hand, the ground  60  of the mains is thereby electrically introduced into the device  100 . The device  100  is also connected to the manufacturing facility&#39;s equipment ground or earth ground  50  (hereinafter, jointly referred to as earth ground) via an interface  120 . This can be achieved by connecting a common-point ground  40  of the work station or, as illustrated, by directly connecting the earth ground  50 . Additionally, the device  100  is connected to two conducting wires  31 ,  32  of a wrist strap  30  via another interface  110 . As illustrated, an end of the wire  31  is electrically connected to the earth ground  50  inside the device  100  whereas an end of the wire  32  is electrically connected to the mains ground  60  via the microprocessor circuit  200 . The other ends of the wires  31 ,  32  are connected to two conducting plates embedded in an insulating casing of the wrist strap  30 , respectively. The conducting plates  33  are usually exposed from the inside of the insulating casing so as to contact an operator&#39;s wrist skin  70 . As such, when the operator has properly worn the wrist strap  30 , a discharge circuit, shown by the dashed lines of  FIG. 1   b,  is established from the mains ground  60 , through the earth, the earth ground  50 , the wire  31 , the skin  70 , the wire  32 , and then to the mains ground  60  via the microprocessor circuit  200 . A major function of the microprocessor circuit  200  is in determining if the discharge circuit has an appropriate resistance. The rest of the details could be found in the previous application and is omitted here. 
         [0009]    There are still some disadvantages for the monitoring device  100  taught by the previous application. First of all, the monitoring device  100  is applicable to grounding mechanism with two conducting wires, yet there are quite some commercially available single-wire grounding mechanisms. The monitoring device  100  is therefore not applicable to these single-wire grounding mechanisms. Secondly, the monitoring device  100  could only monitor the grounding mechanisms an operator is equipped with but cannot provide additional ESD protection to the operator. For example, if the connection between the common point ground  40  and the earth ground  50  is somehow disconnected (e.g., at a place marked by X in  FIG. 1   b ), the monitoring device  100  would signal an alarm and the operator has to stop his or her work until the disconnection is fixed. But, as can be seen in  FIG. 1   b , the mains ground  60  connected to the monitoring device  100  could provide the required grounding and the operator actually does not need to stop his or her work. In other words, the monitoring device  100  could actually provide the mains ground  60  as an auxiliary grounding for additional ESD protection. Additionally, according to the previous application, every monitoring device  100  would require a power cable or a power adaptor, and an outlet for plugging the power cable or the power adaptor, which would increase the product and installation costs. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, the present invention provides a device to monitor whether a work station operator has properly worn a wrist strap or similar grounding mechanism, and a system cascading these devices, so as to obviate the aforementioned shortcomings of the prior arts. 
         [0011]    A major characteristic of the device is that a wireless energy (e.g., infrared) transmission and detection mechanism is incorporated to sense if the operator is present in front of the work station. The device monitors the resistance of a loop composed of the operator&#39;s wrist strap. If the monitored resistance is not in a proper range, for example, when the wrist strap is not worn or the 1-MΩ resistor in the grounding cable is broken or shorted, the device will automatically issue alarms, only if the wireless energy transmission and detection mechanism has sensed that the operator is indeed present at the work station. 
         [0012]    Compared with the previous application, the present invention further provides a novel electrical connection within the device and a novel cascading means to chain multiple devices together for sharing electricity and ground. The present invention could further achieve the following functions: (1) capable of monitoring both single-wire and two-wire grounding mechanisms; (2) providing backup grounding through the chain so that the static electricity discharging capability is maintained as long as a single device in the chain remains connected to the ground; (3) extending the ground through the chain so that grounding is not required to be previously prepared at every work station; (4) sustaining all devices on the chain by a single external power supply; and (5) providing networking connectivity through the chain&#39;s cascading cables so that the network cabling cost and effort is significantly reduced. 
         [0013]    The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]      FIG. 1   a  is a schematic diagram showing a conventional grounding structure commonly found on a work station. 
           [0015]      FIG. 1   b  is a representative drawing of U.S. patent application Ser. No. 11/964,151. 
           [0016]      FIG. 2   a  is a schematic diagram showing two monitoring devices according to a first embodiment of the present invention are connected into a monitoring system. 
           [0017]      FIG. 2   b  is a schematic diagram showing a monitoring device according to a second embodiment of the present invention which monitors an additional series-connected grounding mechanism. 
           [0018]      FIG. 2   c  is a schematic diagram showing a monitoring device according to a third embodiment of the present invention which monitors an additional parallel-connected grounding mechanism. 
           [0019]      FIG. 3   a  is a functional block diagram showing a monitoring device&#39;s microprocessor circuit according to an embodiment of the present invention. 
           [0020]      FIG. 3   b  is a schematic diagram showing an active-typed personnel detection unit of a monitoring device of the present invention. 
           [0021]      FIG. 3   c  is a schematic diagram showing a passive-typed personnel detection unit of a monitoring device of the present invention. 
           [0022]      FIG. 3   d  is a functional block diagram showing a monitoring device&#39;s microprocessor circuit according to another embodiment of the present invention. 
           [0023]      FIG. 4   a  is a functional block diagram showing a monitoring device&#39;s microprocessor circuit according to yet another embodiment of the present invention. 
           [0024]      FIG. 4   b  is a schematic diagram showing multiple monitoring devices of  FIG. 4   a  networked together with a centralized console. 
           [0025]      FIG. 4   c  is a schematic diagram showing multiple monitoring devices cascaded to a centralized console. 
           [0026]      FIG. 4   d  is a schematic diagram showing multiple monitoring devices cascaded to a centralized console that also provides electricity. 
           [0027]      FIG. 5   a  is a schematic diagram showing the monitoring device of  FIG. 2   a  monitoring a single-wire wrist strap. 
           [0028]      FIG. 5   b  is a schematic diagram showing a monitoring device according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    The following descriptions are exemplary embodiments only, and are not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention as set forth in the appended claims. 
         [0030]      FIG. 2   a  is a schematic diagram showing two monitoring devices are connected into a monitoring system according to a first embodiment of the present invention. As illustrated, the two monitoring devices  100  are series-connected by a cable  700  containing at least two conducting wires  701  and  702  through their respective interfaces  150 . One of the monitoring device  100  (i.e., the upper one in  FIG. 2   a ) is further connected to an external power supply  600  by a cable  710  containing at least two conducting wires  711  and  712  through another interface  150 . Similar to the previous application, the monitoring device  100  is a stand-alone device and mainly contains a microprocessor circuit  200  as its core. However, by comparing  FIGS. 1   b  and  2   a,  it should be easy to see the difference between the monitoring devices of the present invention and the previous application. 
         [0031]    Please note that, even though not explicitly shown in  FIG. 1   b,  the monitoring device of the previous application also has an interface  150  connected to an external power supply  600 , just like the present invention. However, they are omitted in  FIG. 1   b  for simplicity and, in the present invention, they are explicitly illustrated. Compared to the monitoring device of the previous application, the monitoring device  100  of the present invention has an additional interface  150  for connecting another monitoring device  100  by the cable  700 . In other words, the monitoring device  100  of the present invention has two interfaces  150  and the two interfaces  150  are actually completely identical. There is no requirement which one of them is for connecting the other monitoring device  100  and which one is for connecting the external power supply  600 . The two interfaces  150  are interchangeable. Please note that, as shown in  FIG. 2   a,  each interface  150  has at least two terminals (not shown): a first terminal and a second terminal. The first terminals of the two interfaces  150  are connected together, and the second terminals of the two interfaces  150  are connected together, both inside the monitoring device  100 . However, the second terminals that are connected to either the wire  702  (in a cable  700 ) or the wire  712  (in a cable  710 ) are further connected to the power unit  500  of the monitoring device  100 . On the other hand, the first terminals that are connected to either the wire  701  (in a cable  700 ) or the wire  711  (in a cable  710 ) are further connected to the microprocessor circuit  200  and the interface  120  of the monitoring device  100 . 
         [0032]    The external power supply  600  on one hand is connected to the mains and, on the other hand, is connected to a monitoring device  100  by a cable  710 . As such, the external power supply  600  draws alternate-current (AC) electricity from the mains and supplies appropriate DC or AC voltage to the directly connected monitoring device  100  via the wire  712  of the cable  710 . The mains ground  60  is also provided to the directly-connected monitoring device  100  by the external power supply  600  via the wire  711  of the cable  710 . As shown in  FIG. 2   a,  the DC or AC voltage is fed to the power unit  500  which in turn provides appropriate DC voltage to drive the microprocessor circuit  200 . In the mean time, the mains ground  60  is also provided to the microprocessor circuit  200  and the interfaces  110  and  120 . Additionally, the voltage and mains ground  60  from the external power supply  600  is passed to a next monitoring device  100  via the wires  702  and  701  of a cable  700 , respectively. Even though only two monitoring devices  100  are illustrated in  FIG. 2   a,  more monitoring devices  100  could be cascaded in a chain by using cables  700  to connect the interfaces  150  of any two neighboring monitoring devices  100 . In this manner, the voltage and mains ground  60  from the external power supply  600  could be passed to every monitoring device  100  on the chain and a single external power supply  600  is shared by these monitoring devices  100 . The present invention therefore is more cost-effective compared to the previous application which requires an external power supply  600  for every monitoring device. Please note that the monitoring device  100  is not required to be used always with another monitoring device  100 . A monitoring device  100  could be used individually as long as it is connected to an external power supply  600  by a cable  710  by itself. 
         [0033]    As shown in  FIG. 1   b,  the previous application forms a closed loop (as denoted by the dashed lines) via the earth to determine if the wrist strap  30  is properly worn by an operator. In contrast, as shown in  FIG. 2   a,  the present application closes the loop (as denoted by the dashed lines) within the monitoring device  100  itself. As illustrated, the loop consists of the connection between the interfaces  120  and  110 , the wire  31 , the operator&#39;s skin  70 , the wire  32 , the microprocessor circuit  200 , and the connection between the microprocessor circuit  200  and the interface  200 . One of the major functions of the microprocessor circuit  200  is to determine whether the loop has an appropriate resistance. In other words, the microprocessor circuit  200  measures the resistance between the wires  31  and  32  to see whether the wrist strap  30  is properly installed by the operator. 
         [0034]    When a monitoring device  100  of the present invention is used individually (for example, considering only the upper monitoring device  100  of  FIG. 2   a ), the charges carried or accumulated on the operator could be discharged to (1) the earth ground  50  through to its interface  120 ; and (2) the mains ground  60  through the cable  710 . Therefore, when the monitoring device  100  and the earth ground  50  is for some reason disconnected, the charges carried or accumulated on the operator could still be discharged to the mains ground  60 . In other words, the present invention provides the mains ground  60  as an auxiliary grounding, for an additional level of protection. 
         [0035]    When a monitoring device  100  of the present invention is used in cascade (for example, using the lower monitoring device  100  of  FIG. 2   a  as example), the charges carried or accumulated on the operator could be discharged to (1) the earth ground  50  through to its interface  120 ; (2) the earth ground  50  connected to another monitoring device  100  series-connected by one or more cables  700 ; and (3) the mains ground  60  through the external power supply  600  connected to another monitoring device  100  series-connected by one or more cables  710 . Therefore, when the monitoring device  100  and the earth ground  50  is for some reason disconnected, the charges carried or accumulated on the operator could still be discharged. In other words, the present invention provides highly redundant grounding, for a significant level of protection. Assuming that there are N monitoring devices  100  cascaded together as described, each monitoring device  100  on the chain in effect has N+1 possible discharge paths. As long as one of the paths is functioning, the discharge capability of every monitoring device  100  on the chain is still preserved. Even when all monitoring devices  100  are disconnected from the earth ground  50 , they could still discharge to the mains ground  60 . This high level of redundancy is made possible by the cables  700  and the cable  710 . If one of the cables  700  or the cable  710  is disconnected, since they are also responsible for the delivery of electricity, this will lead to one or more monitoring devices  100  stop functioning due to the lost of power. This should immediately get the attention of the operators and remedies could be adopted right away. 
         [0036]    From the above description, it should be clear that there are various possible applications of the present invention. For example, only one monitoring device  100  on the chain is connected to the earth ground  50  and this is in effect a way to extend the earth ground  50  via the cascading monitoring devices  100 , instead of requiring the availability of the earth ground  50  at every work station. If each monitoring device  100  has an additional interface, this extended earth ground  50  could actually be shared to other devices through the additional interface. In other words, each monitoring device  100  could actually function as a common-point ground  40  at each work station (therefore, there is no need to install common-point ground  40  at every work station). Another example is not to use the earth ground  50  at all and to simply rely on the mains ground  60 . This would prove to be especially convenient when the earth ground  50  is not available. 
         [0037]    Identical to the previous application, the interface  110  between the grounding cable of the wrist strap  30  and the device  100  could be one that supports dynamically plugging and unplugging. For example, the wrist strap  30  has a plug at an end and the device  100  has a compatible socket. In alternative embodiments, the interface  110  could provide fixed connection only. Similarly, the interface  120  to the earth ground  50  could provide either fixed or dynamic connection. 
         [0038]    The present embodiment only monitors the wrist strap  30 .  FIG. 2   b  is a schematic diagram showing a monitoring device according to a second embodiment of the present invention which also monitors an additional grounding mechanism (such as a floor mat or a table mat). As illustrated, two wires  21 ,  22  of, for example, a table mat  20  are electrically connected to the earth ground  50  and the wrist strap  30 , respectively, via an interface  130 . Again, the interface  130  could provide fixed or dynamic connection. As illustrated, a loop, expressed by the dashed lines, is formed with the wrist strap  30  and the table mat  20  being series-connected, and the microprocessor circuit  200  is therefore able to monitor the table mat  20  and the wrist strap  30  simultaneously. In other words, in addition to the wrist strap  30 , the monitoring device  100  is able to incorporate the monitoring of at least one of any two-wire grounding mechanisms together. 
         [0039]    The microprocessor circuit  200  determines if the loop is normal by measuring the resistance of the loop. However, if there is indeed something wrong with the loop, the present embodiment is not able to tell which grounding mechanism is causing the problem.  FIG. 2   c  is a schematic diagram showing a grounding monitoring device according to a third embodiment of the present invention. As illustrated, separate loops (shown by the dashed lines) are formed through the wrist strap  30  and the table mat  20  respectively. As such, the microprocessor circuit  200  is able to monitor the wrist strap  30  and the table mat  20  individually and simultaneously. Again, a person with ordinary skill can easily extend the same idea to incorporate the monitoring of the floor mat  10  and therefore the detail is omitted here. Please note that the microprocessor circuit  200  of  FIG. 2   b  is not identical to the microprocessor circuit  200  of  FIG. 2   c,  as additional parts are required in the parallel configuration of  FIG. 2   c  to measure an additional loop. Again, a person of ordinary skill could easily extend the microprocessor circuit of series configuration to cover the microprocessor circuit of parallel configuration. Following the same line of thought, the present invention could be further extended to cover: (1) the monitoring of other grounding mechanisms similar to the wrist strap, the table mat, or the floor mat, as long as they also use two-wire grounding cables; (2) configurations where some grounding mechanisms are parallel-connected and some are series-connected; and (3) configurations with more than one wrist strap, table mat, or floor mat. For simplicity, the present specification will focus on the microprocessor circuit  200  of series configurations (e.g.,  FIG. 2   b ) and use only the wrist strap  30  as example to explain the details of the device  100 . 
         [0040]      FIG. 3   a  is a functional block diagram showing a grounding monitoring device&#39;s microprocessor circuit according to an embodiment of the present invention. In the drawing, Vin is a DC voltage produced by the power unit  500  after drawing electricity from the mains to drive the microprocessor circuit  200 . 
         [0041]    As illustrated, the microprocessor circuit  200  contains a comparison and amplification unit  210  which is mainly composed of at least an operation amplifier. The variable resistors R 1  and R 2  are actually an integral part of the comparison and amplification unit  210  but they are separately shown for easier explanation. The provision of the series-connected R 1  and R 2  allows the operation amplifier(s) in the comparison and amplification unit  210  to see if the resistance of the loop introduced by the wire  32  has a value between R 2  and R 1 . In other words, the function of the comparison and amplification unit  210  is to test if the resistance of the loop is bounded by a smaller first resistance (e.g., R 2 ) and a larger second resistance (e.g., R 1 ). If the operator does not put on the wrist strap  30 , or the ground cable of the wrist strap  30  is rusted or broken, the resistance of the loop would be greater than the second (i.e., larger) resistance. On the other hand, if the operator has properly worn the wrist strap  30  and the grounding cable and everything else is normal, the resistance shouldn&#39;t be less than the first (i.e., smaller) resistance either. Therefore, if the resistance of the discharge circuit is greater than the second resistance or less than the first resistance, the comparison and amplification unit  210  would trigger a microprocessor unit  220 . In alternative embodiments, it is possible to have only a single variable resistor R 1  (i.e., omitting the variable resistor R 2 ). These embodiments therefore will only detect if the resistance of the discharge circuit is greater than a specific value (i.e., the resistance of the variable resistor R 1 ). There are also embodiments where the first and second resistances are implemented by fixed resistors. The advantage of having variable resistors is that, depending on whether the loop covers only the wrist strap, or has additional grounding mechanism such as table mat series-connected, the first and second resistances can be dynamically adjusted to reflect these variations. The adjustment of the variable resistors R 1  and R 2  can be conducted by manually twisting knobs or by a control panel, both on the device  100 &#39;s casing. More details will be given later. 
         [0042]    The microprocessor unit  220  is the core of the device  100 . It could be a microcontroller unit (MCU), a single chip containing a processor, RAM, ROM, clock, and I/O control units. Millions of MCUs are in used in various devices ranging from automobiles to laser printers. The present specification therefore will not go into details. 
         [0043]    After being triggered by the comparison and amplification unit  210 , the microprocessor unit  220  activates an alarm unit  230  to issue alarms so as to remind the operator to wear the wrist strap or to get the attention of supervisors or managers. The alarm unit  230  contains one or more lamps, for example, made of light emitting diodes (LEDs). The alarm unit  230  turns on or flashes these lamps to provide visual alarms. The alarm unit could also contain one or more speakers or buzzers to provide audio alarms. These audio or visual alarms could be implemented individually or together. The alarm unit  230  could further contain electronic or mechanical relays to trigger additional devices. When the abnormality detected by the comparison and amplification unit  210  is resolved, the microprocessor unit  220  is notified to turn off the alarm unit  230 . In alternative embodiments, there are reset buttons on the casing or control panel of the device  100  to shutdown the audio or visual alarms. 
         [0044]    A personnel detection unit  240  is provided to see if there is indeed an operator present in front of the device  100  (i.e., in front of the work station). The personnel detection unit  240  may provide a presence signal when an operator appears or is present and an absence signal when the operator leaves or is absent. The presence and absence signals are delivered to the microprocessor unit  220  as well. As such, the microprocessor unit  220  is able to engage the detection of the loop&#39;s resistance and to trigger the alarm unit  230  if required, only when a operator is present in front of the device  100  (e.g., the microprocessor unit  220  has received. a presence signal but not an absence signal yet). When the operator has to leave the work station and take off the wrist strap  30  or disconnect the wrist strap  30  from the interface  110 , as shown in  FIGS. 2   a,    2   b,  and  2   c,  the microprocessor unit  220  will be triggered by the comparison and amplification unit  210  as the latter has seen an abnormal resistance from the loop (the loop is open-circuited). The microprocessor unit  220 , as it has already picked up an absence signal from the personnel detection unit  240 , will not initiate the alarm unit  230  to issue alarms. However, once the personnel detection unit  240  has sensed the presence of the operator, the microprocessor unit  220  automatically begins to activate the alarm unit  230  in accordance with the result of the comparison and amplification unit  210  so that the operator will be reminded to wear or re-plug the wrist strap  30 . In other words, the absence signal from the personnel detection unit  240  functions like an inhibitor to prevent the microprocessor unit  220  from activating the alarm unit  230  whereas the presence signal functions like an enabler to the microprocessor unit  220 . Please note that the personnel detection unit  240  only provides the detection result regarding whether the operator is present or absent. The decision about whether to activate the alarm unit  230  is still carried out by the microprocessor unit  220 . To prevent erroneous judgment and to allow the operator some time to settle, the microprocessor unit  220  will remain inhibited after receiving the presence signal for a period of time (e.g., 5 seconds) and, if there is no absence signal within this period of time, the microprocessor unit  220  will then activate the alarm unit  230  in accordance with the result of the comparison and amplification unit  210 . In contrast, if an absence signal is received at any point of time, the microprocessor unit will stop activating the alarm unit  230  immediately. 
         [0045]    The personnel detection unit  240  can employ either an active means or a passive means in detecting the presence of an operator.  FIG. 3   b  is a schematic diagram showing an active-typed personnel detection unit of a grounding monitoring device of the present invention. As illustrated, the active-typed personnel detection unit  240  has a wireless energy transmitter, such as the infrared LED  241  in the drawing or radar, which can radiate an electromagnetic or supersonic wave covering a limited range to a front side of the device  100  (i.e., towards the operator). The active-typed personnel detection unit  240  also requires a sensor to detect the energy reflected from the operator, such as the infrared receiver  242  in the drawing. This active-typed detection technique has been widely applied in various fields and there are many different transmitters, sensors, and related circuits disclosed and commercially available. To give a few examples, active-typed detection based on infrared is commonly found on auto-flush toilets, those based on supersonic waves are commonly found on automobile radar backup alarm systems. As illustrated, an output terminal of the microprocessor unit  220  controls an electronic switch  243  to turn on or off the infrared LED  241 . On the other hand, the output of the infrared receiver  242  is delivered to an input terminal of the microprocessor unit  242 . 
         [0046]    The active-typed detection is a rather effective solution to the present invention. However, there are usually chairs also positioned in front of the work stations. The personnel detection unit  240  couldn&#39;t distinguish whether it is the operator or the chair (after the operator has left) that is present in front of the work station. The passive-typed detection would provide a more accurate result in this respect. Currently the most common passive-typed detection is based of passive infrared (PIR) sensors, which are able to pick up the movement of a warm object within a specific range. PIR sensors are quite common in security-related applications. However, their adoption has declined in recent years as they cannot distinguish the movement made by a dog or a cat from the movement made by a human being, which are all warm bodies. Interestingly, PIR sensors are quite adequate for the present invention as they have no problem in differentiating the warm human body and the cold chair. As shown in  FIG. 3   c,  the passive-typed personnel detection unit  240  requires a single PIR sensor  244 , which is even simpler structurally. 
         [0047]    There is another passive-typed detection technique which uses a camera to capture images and performs image analysis to detect object movement. In security surveillance arena, such motion detection technique has already been proven to have a significant accuracy. However, to equip a camera in the personnel detection unit  240  and to make the microprocessor unit  220  powerful enough to carry out image processing would make the device  100  much more complicated and costly. 
         [0048]      FIG. 3   d  is a functional block diagram showing a grounding monitoring device&#39;s microprocessor circuit according to another embodiment of the present invention. In the present embodiment, the microprocessor circuit  200  contains an additional control interface unit  250 , which provide a human-machine interface to the device  100 . The control interface unit  250  signally connects one or more buttons (not shown) forming a control panel on the casing of the device  100 . The control interface unit  250  in turn connects a number of input terminals of the microprocessor unit  220  for configuring some operation parameters of the microprocessor unit  220 , such as the lead time after receiving a presence signal, turning on and off the detection function of the device  100 , turning on and off the alarms, etc. The control interface unit  250  can further connect a small-scale liquid crystal display (LCD) panel for showing the current status of the device  100 , for examining the parameter values, etc. The control interface unit  250  could also display alarm messages on the LCD panel. 
         [0049]    As a typical manufacturing environment contains multiple assembly lines and each assembly line contains multiple work stations, it could be rather time consuming and laborious to configure and monitor the device  100  at each work station. Therefore,  FIG. 4   a  shows another embodiment of the microprocessor circuit  200 , which contains an addition network interface unit  260 . The network interface unit  260  connects a network interface  140  of the device  100  and the input and output terminals of the microprocessor unit  220  for two-way data exchange. The network interface  140  provides the physical connection to an external network  300 , which could be a wired local area network conforming to the 802.11x specifications, or a control network conforming to the RS-485, Lonworks, etc. specifications, to name just a few. Depending on the requirement of the network  300 , the network interface  140  should have a compatible physical connection means (such as an RJ-45 socket for hooking onto a local area network). Then, as shown in  FIG. 4   b,  the devices  100  at different work stations can be remotely monitored by a centralized console  400  through the network  300 . Therefore, when the microprocessor unit  220  is triggered due to an abnormal resistance found on the loop, the microprocessor unit  220  not only activates the alarm unit  230  to issue visual or audio alarms, but also sends a message via the network interface unit  260  and the network  300  to the console  400 . In alternative embodiments, the console  400  could periodically poll and communicate with the microprocessor unit  220  of each device  100  to obtain the status (e.g., whether an abnormal condition in the discharge circuit is detected) thereof. The console  400  could also configure the parameters, turn on and off the detection function, etc. of all devices  100  simultaneously, or of a specific device  100  individually. 
         [0050]    Please note that what is shown in  FIG. 4   b  is a wired network in a bus architecture. It should be readily understandable that the present invention is applicable to networks of different architectures such those involve network switches and hubs. To extend even further, the network interface unit  260  could contain a wireless transceiver and the network  300  is a wireless local area network conforming to the 802.11a/b/g standard. In other words, the present invention does not pose specific requirement on whether the network  300  is wired, wireless, or adopting a specific protocol. 
         [0051]    In  FIG. 4   b,  the network interface  140  for connection to the network  300  and the interface  150  for sharing ground and electricity are implemented separately. As shown in  FIG. 4   c  which depicts another embodiment of the present invention, the network interface  140  could actually be integrated into the interface  150 , and the cable  700  not only carries the ground and electricity, but also provides two-way data exchange. For example, the interface  150  could be an RJ-45 socket having eight terminals and the cable  700  is an eight-wire twisted-pair cable. Among the eight terminals and wires, four could be used for data transmission and two of the remaining four are for ground and electricity. The details should be rather straightforward for a person skilled in network cabling and therefore are omitted here. Please note that, in the above example, since separate wires and terminals are employed, the cable  700  and the cable  710  for connection to the external power supply could actually be implemented using same kind of cable. In the embodiment shown in  FIG. 4   d,  the external power supply  60  is further omitted and the console  400  now is also responsible for the provision of electricity (i.e., the external power supply  60  and the console  400  are integrated together into a single device). Please note that, in  FIGS. 4   b  to  4   d,  each device  100  is still connected to the earth ground  50  but these connections are not drawn for simplicity. 
         [0052]    As mentioned earlier, each device  100 &#39;s two interfaces  150  could be used interchangeably; there is no requirement which one should be connected to a previous device  100  or a successive one along the chain. However, in the embodiment shown in  FIG. 4   c,  usually one interface  150  is specified for connection to the previous device  100  (e.g., one farther away from the console  400 ) and the other interface  150  is specified for connection to the successive device  100  (e.g., one closer to the console  400 ). The purpose of having such a sequential ordering is that, when the console  400  has learned that one device  100  is detecting abnormal condition, the console  400  is able to locate the specific device  100  through the foregoing sequential ordering of the devices  100 . This would be very convenient for maintaining and managing the system of  FIG. 4   c.  For those who are familiar with such daisy-chain configuration, the implementation details should be quite straightforward. 
         [0053]    Despite that wrist straps are the most common ground mechanism, and that the proper wearing of the wrist strap has been described so far as the main detection target of the grounding monitoring device, it has to be pointed out that the spirit of the present invention is not limited to the wrist strap only. The present invention could actually be applied to any grounding mechanism that employs two conducting wires to contact two separate spots of the human body to discharge the static electricity. Additionally, the present invention could further be applied to the monitoring of grounding mechanisms using a single conducting wire. This is another major characteristic differentiating the present invention from the previous application. 
         [0054]    As shown in  FIG. 5   a,  a single-wire wrist strap  30  is usually used along with a grounded and conductive floor  90 . The operator  80  wearing the wrist strap  30  usually also wears a shoe strap  33  to facilitate the static discharge. The device  100  of the present embodiment is basically identical to the monitoring device for two-wire grounding mechanisms. However, an appropriate plug or connector is required so as to connect the single conducting wire  32  of the wrist strap  30  to an appropriate terminal of the interface  110 .  FIG. 5   b  depicts another embodiment where an interface  160  specifically for connection to a single-wire grounding mechanism is adopted. By comparing  FIGS. 5   a  and  5   b,  the interface  160  also lacks the internal connection to the interface  120 . 
         [0055]    As denoted by the dashed lines of  FIGS. 5   a  and  5   b,  the loop monitored by the device  100  is constituted by the wire  32 , the wrist strap  30 , the body of the operator  80 , the conductive floor  90 , the earth, and the microprocessor circuit  200 . In other words, the device  100  monitors the resistance seen between the wire  32  and the interface  120 . Therefore, unlike the monitoring of a two-wire grounding mechanism where the loop is completed within the device, the monitoring of a single-wire grounding mechanism, the loop actually runs through the earth. Other than the foregoing differences, the devices  100  for two-wire grounding mechanisms and for single-wire grounding mechanisms could be completely identical. Please note that the backup grounding, the way the static electricity discharges, monitoring additional grounding mechanisms by series or parallel connection, networking multiple monitoring devices, wireless detection of the presence of the operator, etc., as detailed earlier could all be applied to the devices  100  of  FIGS. 5   a  and  5   b  as well. Furthermore, a single device  100  could connect and monitor both single-wire and two-wire grounding mechanisms simultaneously. These variations should be easy to extend from the aforementioned embodiments. 
         [0056]    Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.