WO1999029143A1 - Control of static electricity in micro-electronic fabrication - Google Patents

Abstract

An electrostatic grounding strap (12) with at least two elements (26, 28) for making electrical contact with a wearer's skin is disclosed. The sufficiency of contact is measured via the skin resistance between the contacts. When sufficiently depressed, the electrical circuitry within the strap (30) is powered. The circuitry determines the status of a plurality of ground conditions and signals them via an RF transmitter mounted on the strap to a central monitoring unit (16).

Description

  
 



   CONTROL OF STATIC ELECTRICITY IN
 MICRO-ELECTRONIC FABRICATION
 Backqround
Technical
 This invention relates to a grounding strap for use in controlling static electricity during micro-electronic fabrication and, more particularly, to such grounding strap in combination with a personal monitoring system.



  Background
 Static electricity creates many problems in the electronics industry, particularly with the advent of integrated circuits and other microelectronic components.



  Components such as integrated circuits, for instance, may be disabled or destroyed by excessively high voltages resulting from static electricity. Junctions in such circuits can be destroyed by as little as a 50 volt potential, which radically changes the doping structure in their lattices.
 yet a person walking on carpet on a dry day can accumulate as much as 30,000 volts of static electricity potential and can generate thousands of volts by simply changing position in a chair or handling a styrofoam cup.



  The danger is that such static potential can be discharged into a circuit or component simply by touching. the resulting damage can be severe.



  Alternatively, the static potential can induce a charge in the circuit. this induced charge can cause overvoltage or excessive power density when the circuit is subsequently grounded.



   More and more frequently, therefore, those in industries in which integrated circuits and other microelectronic components are handled or assembled are taking measures to keep them, as well as their environment, at zero electrical potential in order to limit the failure rate of those components. Such  measures include providing workers and work stations with anti-static carpet, conductive or dissipative grounded work surfaces, hot air ion generators which emit ions to neutralize static charges, and static electricity controlling grounding straps to keep workers at zero potential.



   The situations in which grounding straps are used heighten the importance of reliably maintaining continuous electrical contact with the skin. The person working on microelectric components or integrated circuits may be completely unaware of accumulated static electrical charges, and may therefore unknowingly be in a position to disable circuits being worked on or handled.



   If, therefore, the strap does not effectively ground the person by, for example, not making appropriate contact by being loose or removed, the person may be unaware that electrical discharges are being transmitted and disabling the circuits. No one may discover this damage until hours, days, or weeks later when the circuits have been placed in devices which fail when used. Removal and repair or replacement of these circuits is far costlier than avoiding potential failure while the wearer is handling the circuits. To achieve a lower failure rate of such electronic circuits and components, maintaining continuous electrical contact between ground, the grounding strap and the wearer's skin is very important.



   Still situations occur where the appropriate contact is not achieved. These include the wearer removing the strap to leave the workplace and then returning and forgetting to reconnect the strap or not connecting the grounding strap to ground. Also, contact between the strap and the wearer's skin, even when the strap is in place, is not always assured.



   An attempt to address this problem can be found in
U.S. Patent 4,745,519 (Breidegam) which provides for a  grounding strap which has at least two sections of conductive material for contacting a wearer's skin. One or more conductors connect each conductive section to ground. The system includes circuitry, remote from the grounding strap, capable of actuating warning indicators if any of the conductive sections are not contacting the wearer's skin or if the connection between any conductive section and ground is broken.



   Unfortunately, the Breidegam system has a number of disadvantages. First, it is a hard wired system without the capability of providing "intelligent" information regarding the strap or its wearer. It can therefore, make provision for complex personnel monitoring beyond the simple monitoring of the grounding condition of a wearer and strap located at a seat. Second, the
Breidegam system relies on a power supply remote from the wearer. As such, the wearer must be physically connected to ground and the remote power supply. The Breidegam system can, accordingly, not provide monitoring of wearers that constantly move around.



   Thus, the need has arisen for a versatile grounding strap which provides both effective grounding and can be used with a sophisticated personnel monitoring system.



   Summary of the Invention
 Briefly, therefore, this invention provides an electrostatic grounding strap with two members for making electrical contact with a wearer's skin. The sufficiency of contact between the contact members and the wearer's skin is determined by measuring skin resistance between the contact members. The strap is activated by a pressure contact switch which, when sufficiently depressed, causes electrical circuitry to be powered.



  The circuitry determines the status of a plurality ground condition indicators and, in response to these determinations, is capable of causing the generation of
RF signals indicating the strap's grounding condition.  



  Typically the ground conditions determined include: a low resistance between the wearer skin contact members; sufficiency of strap grounding; and monitoring of the resistance across a discharge resistor on a strap ground connector. In operation, an RF transmitter mounted on the strap transmits a periodic status signal to a remote receiver which is coupled to a memory and a visual status indicator. The memory provides analysis of fault conditions which are, in turn, displayed on the visual status indicator. The memory can also function as a personnel monitoring system.



   Preferably, the electronic circuitry determines whether sufficient contact has been made with the wearer's skin by measuring the conductivity of the wearer's skin between the two conductive contact members.



   Typically, one of the contact members is the pressure sensitive switch. The switch is depressed into a closed position when sufficient contact is made between the associated contact member and the wearer's skin.



  When the switch is in the closed position, it allows power from a DC battery, to power the circuitry and the
RF transmitter. When the pressure switch is in the open (off) position the battery is effectively isolated, thus saving battery power when the grounding strap is not in use.



   The strap also includes a sound generator which receives signals from the circuitry. This enables the device to generate sound in response to various signals received from the circuitry.



   In use, the grounding strap is connected to ground by means of two grounding leads. One grounding lead includes a 1.0   MQ    electrostatic discharge resistor. When the strap and, as a result, the circuitry is grounded along the grounding leads, the contact switch is sufficiently depressed to be in closed position and the conductive members are appropriately contacting the skin of the wearer, a complete "closed" circuit is formed.  



   In operation, the RF transmitter transmits a periodic status signal to the remote receiver.



  Typically, this signal is in the form of an "all's well" signal indicating that the contact switch is in the closed (on) position, a conductive path is established along the wearer's skin to contact members and that the circuitry is grounded.



   Should any of these conditions not be met, an appropriate signal can be transmitted to the remote receiver. In the event that the contact switch is not in the closed (on) position, no power is supplied to the circuitry. Accordingly, the transmitter on the grounding strap will not be able to transmit. this situation will be determined by the receiving monitor when it does not receive the normally transmitted periodic status signal. this, in turn, will trigger an error condition and an appropriate warning can be sounded.



   This invention also provides for a personnel monitoring system for use together with the grounding strap and monitoring unit.



   Typically, the monitoring system includes status indicators in the form of visual and audio status and warning indicators. Light emitting diodes (LED's) and speakers are the preferred visual and audio indicators respectively.



   Further, The monitoring system is configured to interface with a computer which includes memory and processing means. Thus, the computer can provide sophisticated monitoring and status control functions.



   Further, details of the present invention will become apparent to one skilled in the art from the following detailed description when taken in conjunction with the accompanying drawings.  



   Description of the Drawings
 In the accompanying drawings:
 Figure 1 is a schematic overview of the system of the invention;
 Figure 2 is a schematic representation of a grounding strap suitable for use with this invention;
 Figure 3(a) is a partially sectional top view of a housing associated with the strap of Figure 2;
 Figure 3(b) is a bottom view of the housing shown in
Figure 3(a);
 Figure 3(c) is a sectional view taken along line
C - C in figure 3(a);
 Figure 3(d) is a sectional view taken along line
D - D in Figure 3(a);
 Figure 4 is a schematic illustration of the monitoring control unit of the invention; and
 Figure 5 is a schematic illustration of another embodiment of the invention.



   Description of Embodiments
Overview
 The personnel monitoring system of the invention, generally indicated as 10, is shown in overview in Figure 1.



   As shown, the system 10 is constituted by a plurality of personnel static electricity control grounding straps 12 in radio frequency communication, as shown by arrows 14, with a control monitoring unit 16.



  Each grounding strap 12 is connected to ground 18 by means of first and second grounding leads 20, 22. The grounding strap 12 is designed to encircle and make electrical contact with a wearer's limb, such as the wrist shown in part by broken line 24. Electrical contact between the strap 12 and the wrist 24 is primarily through a pressure contact switch 26 and a contact plate 28, both of which bear onto the skin of the wearer.  



   Sufficient electrical contact between the strap 12 and the wearer is insured by a combination of the contact switch 26 and the contact place 28. When the grounding strap 12 encircles the limb 24, pressure is brought to bear on the switch 26. As will be described below, this connects a power source to circuitry (not shown) in
Figure 4 mounted, in a housing 30, on the strap, thereby activating it. Also, the pressure switch 26 and the plate 28 are physically separate from one another. As the human skin is conductive with an associated amount of resistance, the resistance of the skin between the switch 26 and the contact plate 28 can be measured by the circuitry. Should very high or infinite resistance be measured between the switch 26 and the plate 28, a fault condition exists and an appropriate warning signal generated.



   The circuitry, which is mounted on the strap 12, is adapted to measure contact with ground 18 along a path completed by conductors 20 and 22. Once again, if a determination is made that a connections to ground does not exist, a warning signal is generated.



   As indicated, sensing of these "conditions" is achieved by means of the electronic circuitry housed in the housing 30 connected to the strap 12. The housing 30 also contains a radio transmitter (illustrated in Figure 3) capable of transmitting signals for receipt by the control monitoring unit 16. The transmitter is in communication with the circuitry and receives signals associated with any of these "conditions," from it.



   The control monitoring unit 16 typically incorporates a visual status indicator such as an array of light emitting diodes (LEDs) 40: an audible alarm generator including a device such as a speaker 42; and an internal memory (not shown). The unit 16 can also be connected to an external visual status indicator 44; interfaced with devices such as a computer 46 and be connected to a supplemental antennae 48.  



   In operation, a user attaches the grounding strap 12 to the wrist 24. As will be described in detail below, the circuitry determines whether the strap 12 is appropriately connected to an in electrical contact with the wearer's wrist 24 and whether the strap is grounded.



  Should all contact be appropriate and the grounding strap grounded, an audible tone is generated at the housing 30 on the strap 12. At the same time an appropriate signal 14 is transmitted to the unit 16. this unit 16 logs various information, such as the status, the tags identification, etc., in its internal memory. Should a fault be detected, however, warning signal is sounded at the strap, through the control unit's audio alarm generator 42 and/or given visually on the visual status indicators 40 or 44.



   The apparatus of this invention provides a relatively reliable and flexible system for insuring that a worker is appropriately grounded. In addition, the transmitter associated with the grounding strap 12 can transmit identifying information about the tag, thereby facilitating personnel monitoring. This will become more apparent from the more detailed description of the individual components and the operation of the system given below:
Grounding Strap
 The grounding strap 12, as shown in greater detail in Figure 2, is constituted by an electrostatic discharge wrist or ankle band 50. Many different forms of band 50 can be used. Typically, a "stretch fabric" band is appropriate. Whatever the form, the band is preferably an ISO 9000 certified wrist band.



   The band 50 is secured to the ends of a generally rectangular molded plastic housing 30 which supports a micro-transmitter. The transmitter (shown in Figure 3(a)) typically has a maximum transmission power of 1 milliwatt and consumes an average power of less than 1 microwatt. It operates at 2.4 GHZ with a transmission  range of approximately 1000 feet (300 meters). The transmitter (and any other circuitry within the housing 30) is powered by two 3 Volt batteries sufficient to power the transmitter and circuitry for at least 18 months. The transmitter can be any device suitable of transmitting signals over a distance of about 1000 feet.



  The batteries are typical lithium/manganese dioxide batteries supplied by Duracell or part number DL 2025.



   The housing 30 also supports a pressure contact switch 26 and an aluminum contact plate 28. These are mounted to protrude slightly from the underside 52 of the housing 30 so that, when the wrist strap 12 is in position encircling the wrist 24, both the pressure switch 26 and the contact plate 28 bear against the wrist. The pressure switch 26 and the grounding plate 28 are respectively grounded along first and second grounding cords 20, 22. One of the grounding cords 20 includes a conventional 1.0   MQ    electrostatic discharge resistor 90 positioned between the housing 30 and ground 18. Although this resistor 90 is shown on the grounding cord 20, it could very well be on the second grounding cord 22.

   However, as a preferred alternative to installing the resistor 90 either one of the outline grounding cords 20, 22 the resistor 90 can be built into the housing 30.



   Further details of the construction of the housing and its operation are illustrated in Figures 3(a)-(d).



  These figures should be read in conjunction with Figure 2.



   The housing 30 comprises rectangular, injection molded outer shell 60. Pin connectors 62a, 62b are provided, one at each of the "short" sides of the shell 60. In use, the band 50 of the strap 12 is connected to these pin connectors 62a, 62b.



   The shell 60 includes a cavity 64 into which two clip-on or clip in electrical connector jacks, of which one 66 is shown, are mounted. These connector jacks  removably receive standard plug-in ends of the grounding cords 20, 22. The shell 60 also includes a recessed space 68 for receiving and housing electrical circuitry, the transmitter and an antenna, schematically represented by blocks 70, 72 and 74 respectively.



   Two 3 Volt, LiMnO2 batteries 76 provide power to the circuitry 70, transmitter 72 and antenna 74. Power from these batteries is connected to the circuitry 70, etc. under action of the pressure contact switch 26 supported by the underside of the shell 60. When the switch is in its open position (as shown) no power is supplied to the circuitry 70, etc., however, when the strap 12 properly encircles the wrist of a user, the contact switch is depressed to a position 26 shown in broken lines in
Figure 3(c). In this closed position, power is supplied to the circuitry 70, etc. The switch 26 is spring biased into its off (open) position, so that, when the strap 12 is removed or too loose, the switch 26 opens to interrupt power.



   These figures also show the contact plate 28 as a generally rectangular plate mounted on the underside 78 of the shell 60. The plate 28 is in electrical contact with the circuit 70 by means of two pins 80a, 80b extending perpendicular to the plate 28 and away from the underside 78 of the shell 60.



   In addition, the shell 60 includes an upper cover plate 82 which has a piezo ceramic "bender" 84 epoxied into its underside. The bender may be one such as Model
No. KBI-1541 supplied by a company called Projects
Unlimited. Preferably, it has a resonant frequency of 4.lkHz, a maximum resonant resistance of 10000   Q    and electrostatic capacitance of 8nF. It is typically 15mm in diameter.



   When the cover plate 82 is in place, the bender 84 fits into a cavity 86 formed in the shell 60. The bender 84 is connected to the circuitry 70 by means of wires or lead lines 88. In operation, the circuitry is capable of  generating a signal which, when communicated to the bender 84, causes it to resonate to generate an audio signal.



   Typically, the housing 30 is about 0.5 inches thick, 1.30 inches wide and 1.70 inches long. As such, it is relatively small and does not impose significantly on the wearer.



   When the wrist strap 12 is in place around the wrist, and appropriate connection between the limb 24, the pressure switch 26 and pressure plate 28 is achieved, a circuit is completed. This occurs because the pressure switch 26 is depressed into the closed position and the skin of the wearer located between the pressure switch 26 and the contact plate 28 acts as a conductor.



  Furthermore, both the switch 26 and the connector plate 28 are conductively connected to ground via the circuitry 54 and the first and second grounding leads 20, 22.



   Circuitry in the housing 30 regularly checks to ensure a user is properly grounded. This is a three part check. In the first part of the check, current from the batteries in the housing 30 is driven through the skin between the pressure switch and the contact plate 28.



  The resulting resistance of the skin is measured by the circuitry. Should this resistance fall below a certain predetermined value, it can be safely assumed that sufficient connection between the user's skin and the grounding strap 12 has been achieved. Without such sufficient connection, the wearer cannot be assumed grounded. Typically, the circuitry checks to ensure that the skin resistance is below about   10MQ.   



   The second part of the check is achieved by sensing the resistance across the   1MQ    discharge resistor 90 in the grounding lead 20 or 22. If the resistance is typically between the limits of   700or    and   1.3MQ    it is assumed that this portion of the circuitry is in order.



   As the third part of the check of the circuitry makes the simple determination of whether the circuit is  grounded via the grounding leads 20, 22. Should all these conditions be met, it is certain that the wearer is appropriately grounded.



   In actual operation, the circuitry continuously monitors the state of the ground connection i.e., the third part of the check. The first and second parts of the check, i.e., the checks for skin resistance and across the   1MQ    discharge resister 90 are, however, performed when ground is first connected and approximately every five minutes thereafter. These two checks are also performed on "power-up" (when switch 28 is first depressed into the "on" state) if ground is already connected.



   When powered and in use, the transmitter transmits a periodic status signal 14, every 5 minutes (although any period can be selected) with a random interval.



  Signal 14 is also transmitted when a fault condition is detected; when ground is connected or disconnected; and on "power-up." Typically, the signal 14 is comprised of a Direct Sequence Spread Spectrum (DSSS) data signal transmitted at 33.33 kpbs and includes at least a tag ID and the status of the various "sensors", i.e., each status determined in each part of the three part check described above. The signal 14 is typically repeated 5 times with the same message, with an approximate 3 second random interval between each signal bursts. The carrier frequency of the signal is 2414 MHz +/- 1 MHz.



   After the first of these periodic signals is transmitted and received by the monitoring unit 16, the unit 16 "expects" to receive the next signal at the end of a predefined period. If it does not, a fault condition is defined and an appropriate warning or indication is given at the monitoring unit 16.



   In addition to the RF status signal 14, the strap can produce audio alert tones of various kinds. These are generated by the piezo-ceramic tone generator mounted  at the top of the housing 30 and can be any of the following:
 1. That the system checks came out OK immediately
 following ground connect. (Happy Tones)
 2. That the system checks came out OK immediately
 upon power-up if the ground is present.



   (Happy Tones)
 3. That ground is lost. (Sad Tones 1)
 4. That the resistance across the skin resistance
 is high. (Sad Tones 2)
 5. That the resistance across the discharge
 resistor is high. (Sad Tones 3)
 6. That the discharge resistor is low. (Sad
 Tones 4)
 The user will normally only hear a Happy tone when first hooking up, and Sad Tone 1 when disconnecting. The only time other tones are heard is when there is something wrong.



   In this regard, there are several unique cases that need to be addressed:
Ground is lost then re-established before Sad Tones 1 is complete.



   This might occur when the ground strap is
 intermitted. The aural tones consist of both Sad
 Tones 1 and Happy Tones. Thus, the suer can
 identify the ground was lost and then re-established
 without unplugging. This indicates a problem.



  Ground is established but there is high wrist resistance or another problem.



   In this case the aural indication is a Happy Tone
 followed by an appropriate Sad Tone (1-3). that way
 the user gets the feedback that the ground is
 established, but also is alerted that something else
 is wrong.  



  Central Monitoring Unit
 As illustrated in greater detail in Figure 4, the control monitor unit 16 includes a number of discreet individual components and features.



   The unit 16 includes a built-in LED display 40; a audio generator in the form of a speaker 42; and optional supplemental LED visual display 44; and RS 232 port 100 for attaching the unit 16 to a computer such as a PC 46; an internal antenna 102; an optional external antenna 48; and a modem port 104 for connecting the unit 16 to a modem. The unit 16 also includes a key pad 108 through which the unite 16 can be programmed. Associated with the key pad 108 is a LCD display 110.



   In operation, the control monitor unit 16 receives a signal 14 from a wrist strap 12. Should the wrist strap be within about 250 feed (approximately 80 meters) the signal is received via internal antenna 102. Should the unite 16 be designed to operate at greater distances - up to about 1000 feet (300 meters) an optional external antenna 48 must be used.



   On receipt of the signal 14 a visual status indication is given in the LED display 40 or the external display 44. Both displays operated in approximately the same manner and the following description of the built-in
LED display 40 will suffice to illustrate of the use of the external LED display 44 as well.



   As shown, the built-in LED display includes ten sets of LED lights 40.1, 40.2, . . . The top LED in each set 40.1, 40.2 is green, the central LED red and the lower
LED yellow. Each set of LED 40.1, 40.2 provide visual status indications of the state of a single wrist strap 12. The green LED's indicates a wrist strap which is both in use and sufficiently grounded; the red LED indicates a wrist strap which is in use but in which a fault condition exists; and the yellow LED indicates a wrist strap which is not in use.  



   Although the illustrated visual status indicator 40 includes only ten sets 40.1, 40.2, . . . of LEDs, (each set representing a signal grounding strap 12), the control monitor unit 16 is, adapted for use with more than ten grounding straps 12. This is accomplished by providing a selector in the form of a rotatable knob 112 which can, in this example, be rotated through 5 different position. In the first position, the selector knob 112 will cause the control unit 16 to display the status of the first ten grounding straps 12; in the second position, the status of the second 10 grounding straps 12, and so on. Thus, the control unit 16 can be used to monitor a large number, fifty in this example, of individual grounding straps 12.



   The unite 16 also includes a 128 K memory module (not shown) capable of maintaining control and status data for up to fifty individual grounding straps 12 for 24 hours a day for up to 5 to 10 days. The alarm conditions represented by this data can be reviewed using the LCD display 110. This is done by entering specific instructions into the unit 16 using key pad 108.



   Typically, the unite 16 is powered by a 9 volt DC from an external 110 volt power supply. The modem 106 is preferably a 9600 b.p.s. (Baud) or higher speed capability modem. the unite also includes RS 232 port 100 for porting the historical data to a central processing unit such as may form part of computer 46.



  This historical data can also be downloaded via a traditional RJ 11 telephone modem port 104 to an external and remote processing unit such as a different computer (not shown).



   The optional computer 46, is used to provide sophisticated monitoring in conjunction with the control monitoring unit 16. Typically, the 46 uses a Windows   95"    or 3.0 or DOS compatible software. The computer and compatible software is preferably adapted to support traditional MRP and MRP II production management tools.  



  Typically, the monitor 120 of the computer 46 can display information such as an employee name/number (which can be determined from a correlation between current wrist strap status, confidence signal intervals and historic even status information for a 24 hour period. In addition, the system will typically include an automatic archive system that saves data for each 24 hour period as a separate file. this file can then be accessed for analysis through an archive record retrieval system by use of a standard "windows type" pull down menu. furthermore, the historical data saved on the computer 46 can be downloaded via ASCII format to a traditional database or spreadsheet software packages for further manipulation and analysis.



   The use of the computer 46 provides the advantage of a substantial increase computing power over that of the monitoring unit 16. As such, it can be used to display in a number of different statuses associated with a grounding strap 12. For example, the computer can indicate the status of the 1   MQ    grounding resistor 90 whether the employee associated with the wrist strap is grounded; whether the contact switch is in an open or closed position; or where in a manufacturing facility a specific grounding strap 12 (and hence, the wearer associated with that grounding strap 12) is located.



  This last facility is particularly important as it can be used with a number of employee productivity and safety monitoring functions. Examples of these are automated time cards status for productivity, and employee identification and positioning in the case of disasters such as fires, earthquakes or bombings.



  Alternative Embodiment
 An alternative embodiment of the invention is illustrated in Figure 5.

 

   This figure shows a slightly different version of the control monitoring unit, here indicated generally as 216. The unit 216 includes a built-in LED display 40; a  audio generator in the form of a speaker 42 that could be built in; an RS 232 port 100 for attaching the unit 16 to a computer such as a PC 46; an internal antenna 102; an optional external antenna 48; and a modem port 104 for connecting the unit 16 to a modem. In these respects, the unit is very similar to that illustrated before.



   The unit 216 also ten tag registration switches 218, five tag range selection switches 220, a tag range selection switch 222, a lamp test switch 224 and a volume control knob 226 for speaker 42.



   In operation, the control monitor unit 216 receives a signal 14 from a wrist strap 212 in very much the same way a monitor unit 216 can, however, be used with more than 10 grounding straps 212. This is accomplished by a combination between the five tag range LED's 220 and the tag range selection switch 222.



   Successively activating the selection switch 222, causes successive groups of ten straps 212 to be monitored and a corresponding one of the range LED's 220.1, 220.2 . . . to illuminate. Accordingly, activating the switch 222 once will select monitoring of the first ten straps 212 and the first tag range LED 220.1 will illuminate. Thus the red, yellow and green
LED's 40.1, 40.2 . . . will indicate the status of this first set of straps. Activating the switch 222 again will illuminate the second range LED 220.2 and cause the red, green and yellow LED's to reflect the status of a second st of ten straps 212. The unit 216 can, as before, include an additional display module (not shown).



   The unit 216 also includes a memory module (not shown) capable of maintaining data of a ten grounding strap system for 24 hours a day for up to 5 days (1200 hours). This internal memory can, however, be increased.



   As before, the optional computer (PC 46), is used to provide sophisticated monitoring in conjunction with the control monitoring unit 16. Typically, the PC 46 uses a
Windows   95"    or DOS compatible software.



   The PC 46 can, for example, be used to provide indications of the 1 mega ohm grounding resistor status; employee grounding status; personnel location; both historical and real time data; the confidence status of the strap; employee identification; automated timecard information; audible tone fault indications; a "comments column" for event status information; other indications of specific event status; and an ASCII import/export capability.



   In addition, the strap 212 shown in this figure is slightly different from that shown in previous figures.  



   First, the strap as shown has the once mega ohm grounding resistor housed within the housing 230. thus the resistor is not   "in    line" on either of the grounding cords 20, 22. Second, the strap does not include a pressure based activation switch between the housing 230 and the wearer's wrist 24. Instead, the housing has three skin contact electrodes 226, 227, 228. Having three contact electrodes reduces provides a greater degree of grounding certainty.



   The housing 230 includes circuitry capable of monitoring at least four different conditions:
 i) that the resistance across the 1.0   MQ    resistor
 is within a 10% range;
 ii) The resistance between the wearer and the
 strap is within the acceptable range of 200   kQ   
 and 10   MQ;   
 iii) that the employee is properly wearing the
 strap by polling the three contacting
 electrodes 226, 227, 228; and
 iv) that proper grounding connection is made to
 the work station ares.



   As before, when the strap is first worn (a condition that is determined by determining electrical conductivity or non-infinite resistance between the contact electrodes) a confirming audible tone is generated. When a fault condition is detected, the circuitry within the housing immediately transmits a signal to the monitoring unit 216 and causes the strap 212 to generate a distinctly different audible tone.



   Thus, the system of the different embodiments of the invention provides a flexible and efficient monitoring system which can be used to continually monitor the grounding status of each wrist strap and produce suitable warning indicators should the appropriate status not be maintained.



   In addition, the system provides for employee monitoring which can be beneficial both from productivity  and from safety points of view. The added computing power provided by the computer allows for improved analysis and monitoring functions. For example, the use of the computer can indicate where and when a certain fault (non-grounding) situation arises which will allow a component manufacturer to identify a small subset of components for testing prior to shipping or installing these components into devices. This type of selective fault identification and component testing is far more accurate and reliable in reducing the number of damaged components resulting from static electricity related failures.



  Other Applications
 This invention has been described in particularly with regard to monitoring personnel wearing static electricity control grounding straps. But, its principles could be applied to a number of other situations. For example, it could be applied to monitoring infants in hospital nurseries or childcare facilities.



   Similarly, the system could be used for other types of personnel monitoring applications such as, for example, monitoring prison parolees or other people in hazardous situations such as mines.



   All publications and patent application mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.



   Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, ti would be obvious that certain changes and modifications may be practiced within the scope of independent claims.
  

Claims

WHAT IS CLAIMED IS: 1. A personnel monitoring system comprising: a) a receiver; b) a personnel identification device associated with a person to be monitored remote from the receiver, and including: i) a sensor for determining the status of at least one predetermined grounding condition associated with the person; ii) a signal generator for generating a status signal associated with the determination of the condition; and iii) a transmitter for transmitting an electromagnetic signal, in response to the status signal, to the receiver; c) a power supply; and d) at least one pressure activated switch which, when sufficient pressure is brought to bear on it, activates the power supply to supply electrical power to the personnel identification device.

2. The personnel monitoring system of Claim 1 further comprising: a) at least one contact, associated with the sensor, for making electrical contact with the person; and b) at least one ground connector along which the contact can be connected to ground, wherein the transmitter is capable of generating the electromagnetic signal to indicate at least one predetermined condition associated with the grounding condition of the person.

3. The personnel monitoring system of Claim 2 wherein the device further includes: a) two spaced apart contacts for making electrical contact with the skin of person to be grounded; and b) resistance measuring circuitry for determining whether the electrical resistance of the skin between the spaced apart contacts is below a predetermined range and for generating a signal associated with such determination, wherein the transmitter is adapted to receive such signal and generate an electromagnetic signal indicating the determination.

4. The personnel monitoring system of Claim 3, wherein the pressure activated switch defines one of the contacts.

5. The personnel monitoring system of Claim 4, wherein the device further includes: a) at least two ground connectors along which a connecting to ground can be established; and b) at least one electrostatic discharge resistor associated with at least one such ground connector.

6. The personnel monitoring system of Claim 5, wherein the resistance measuring circuitry is capable of determining any one or more of the following resistance conditions: whether the electrical resistance of the skin between the spaced apart contacts is below a predetermined range; whether at least one grounding strap is connected to ground; whether a resistance across the discharge resistor is above a predefined resistance; and whether the resistance across the discharge resistor is below a predefined resistance; and capable of generating a signal associated with such determination, and wherein the transmitter is capable of receiving such signal and generating an electromagnetic signal indicating such determination.

7. The personnel monitoring system of Claim 6 wherein the device further includes an indicator for generating an indication signal based on the determination by the circuitry to the person associated with the strap.

8. The personnel monitoring system of Claim 7 wherein the indicator is an audio tone generator.

9. A grounding strap for facilitating the electrical grounding of a person, comprising: a) at least one contact for making electrical contact with the person to be grounded; b) at least one ground connector along which the contact can be connected to ground; c) a transmitter for generating an electromagnetic signal indicating at least one predetermined condition associated with the grounding condition of the person; d) two spaced apart contacts for making electrical contact with the skin of person to be grounded;

and e) resistance measuring circuitry for determining whether the electrical resistance of the skin between the spaced apart contacts is below a predetermined range and for generating a signal associated with such determination, wherein the transmitter is adapted to receive such signal and generate an electromagnetic signal indicating the determination.

10. The grounding strap of Claim 9, further comprising: a) a power supply; and b) at least one pressure activated switch which, when sufficient pressure is brought to bear on it, activates the power supply to supply electrical power to the circuitry.

11. The grounding strap of Claim 10, wherein the pressure activated switch defines one of the contacts.

12. The grounding strap of Claim 11, further comprising: a) at least two ground connectors along which a connecting to ground can be established; and b) at least one electrostatic discharge resistor associated with at least one such ground connector.

13. The grounding strap of Claim 12, wherein the resistance measuring circuitry is capable of determining any one or more of the following resistance conditions: whether the electrical resistance of the skin between the spaced apart contacts is below a predetermined range; whether at least one grounding strap is connected to ground; whether a resistance across the discharge resistor is above a predefined resistance; and whether the resistance across the discharge resistor is below a predefined resistance; and capable of generating a signal associated with such determination, and wherein the transmitter is capable of receiving such signal and generating an electromagnetic signal indicating such determination.

14. The grounding strap of Claim 13, further comprising an indicator for generating an indication signal based on the determination by the circuitry to the person associated with the strap.

15. The grounding strap of Claim 14, wherein the indicator is an audio tone generator.