Device and method of monitoring grounding of personnel and equipment in ESD-sensitive areas

• US 6,930,612 B1
• Filed: 10/04/2002
• Issued: 08/16/2005
• Est. Priority Date: 10/04/2002
• Status: Active Grant

• Current Assignee: Desco Industries Incorporated
• Original Assignee: Credence Technologies, Inc. (3M Company)
• Inventors: Kraz, Vladimir, Martin, Kirk Alan
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): PREVIL, DANIEL
• Application Number:
  US10/264,381
• Time in Patent Office: 1,047 Days
• Field of Search: 340/649, 340/650, 340/651, 340/652, 340/653, 340/660, 340/664, 340/561, 340/661, 340/635, 340/657, 361/212
• US Class Current: 340/649
• CPC Class Codes: G08B 21/185 Electrical failure alarms

Abstract

A method and apparatus for monitoring grounding of personnel and equipment in electrostatic discharge (ESD) sensitive areas. The device generates a low voltage alternating current control signal (60) which is applied to personnel (50) and equipment (350) being monitored. The device further includes signal conditioning (66) and detection means (70) to distinguish static charges from the control signal and transmit an alarm (72) upon detection of such static charge. The method encompasses the use of an alternating current control signal to detect static charge.

Claims

1. A device for monitoring grounding of equipment comprising:

an oscillator circuit that generates an alternating current control signal;

a connector that is connectable between the oscillator circuit and a piece of equipment to supply the alternating current control signal to the piece of equipment;

a signal level detector connected to the connector that detects the alternating current control signal wherein a magnitude of the alternating current control signal detected by the signal level detector is reduced if the piece of equipment is connected to ground; and

a signal conditioning circuit connected between the connector and the signal level detector that rejects signals not related to the alternating current control signal.

2. The device of claim 1, wherein the signal conditioning circuit further comprises a filter.

3. The device of claim 2, wherein the filter further comprises a digital filter.

4. The device of claim 2, wherein the alternating current control signal has a frequency different from a frequency and harmonics of a power line.

5. The device of claim 4, wherein the alternating current control signal frequency is less than 50 Hz.

6. The device of claim 4, wherein the filter has a peak in a filter frequency response that corresponds to the alternating current control signal frequency and has one or more nulls in the filter frequency response that corresponds to the power line frequency and harmonics.

7. The device of claim 1, wherein the signal level detector further comprises a circuit that generates a signal as a function of the alternating current control signal that indicates an actual impedance of a connection between the piece of equipment and an external ground and wherein the magnitude of the generated signal indicates the magnitude of the ground impedance.

8. The device of claim 1 further comprising:

a high-pass filter with an input connected to the connector; and

a second signal level detector circuit having an input connected to the output of the high-pass filter and an output connected to an alarm circuit, the alarm circuit indicating a high-frequency signal on a ground line connected to the piece of equipment.

9. The device of claim 8 further comprising a second alarm circuit that indicates that the ground line is properly connected to the piece of equipment.

10. The device of claim 8, wherein the signal level detector generates a signal, as a function of the strength of the high-frequency signal, that indicates an actual magnitude of the high-frequency signal.

11. A device for monitoring the grounding of a piece of equipment, comprising:

an alarm circuit, connectable to a piece of equipment, that generates an alarm signal when a piece of equipment is not properly connected to ground; and

a switch connected to the alarm circuit that one of automatically enables and automatically disables the alarm circuit based on a need to monitor the piece of equipment so that a false alarm, when the piece of equipment is not connected to the monitoring device, is eliminated.

12. The device of claim 11, wherein the alarm circuit further comprises an indicator of a good ground connection of the piece of equipment, a second indicator of a bad ground connection of the piece of equipment and a third indicator of a disabled alarm circuit when the switch has disabled the alarm circuit.

13. The device of claim 11, wherein the alarm circuit further comprises an indicator of a good ground connection of the piece of equipment and a second indicator of a bad ground connection of the piece of equipment wherein there is no indication of a disabled alarm circuit.

14. A device for monitoring the grounding of a plurality of pieces of equipment, comprising:

an output circuit, connectable to a plurality of pieces of equipment, that generates an output signal on a single wire to a remote system having multiple signal levels indicating the ground connections of the plurality of pieces of equipment, wherein the level of the output signal is a function of the number of ground connections of the pieces of equipment.

15. The device of claim 14, wherein the level of the output signal is a function of a ground impedance for each piece of equipment.

16. A device for monitoring the grounding of an operator in an electrostatic discharge sensitive area, the device comprising:

a connector, connected to a dual wrist strap worn by an operator that is in electrical contact with the operator'"'"'s body in plurality of separate places, the connector having a separate terminal for each connection to the body of the operator;

an oscillator circuit, connected to the connector, that generates an alternating current control signal that is capable of being supplied to the operator'"'"'s body through the wrist strap when the wrist strap is connected to the connector; and

a signal level detector circuit connected to the connector that measures a change in the alternating current control signal, based on an electrical resistance of the body of the operator, indicating that the operator is wearing a wrist strap connected to the connector.

17. The device of claim 16 further comprising a resistor, connected between the connector and a ground connection, that dissipates static charges from the operator.

18. The device of claim 16 further comprising a signal conditioning circuit, connected between the connector and the signal level detector circuit, that reject signals not related to the alternating current control signal.

19. The device of claim 18, wherein the signal conditioning circuit further comprises a filter.

20. The device of claim 19, wherein the filter further comprises a digital filter.

21. The device of claim 19, wherein the alternating current control signal has a frequency different from a frequency and harmonics of a power line.

22. The device of claim 21, wherein the alternating current control signal frequency is less than 50 Hz.

23. The device of claim 21, wherein the filter has a peak in a filter frequency response that corresponds to the alternating current control signal frequency and has one or more nulls in the filter frequency response that corresponds to the power line frequency and harmonics.

24. The device of claim 16, wherein the signal level detector circuit generates a signal that indicates an actual impedance of the body of the operator.

25. The device of claim 16 further comprising a direct current voltage level detector having an input connected to the connector and an output connected to an alarm circuit, wherein the direct current voltage detector generates a signal to the alarm circuit when a voltage on the operator exceeds a preset alarm level.

26. The device of claim 25 further comprising a second alarm circuit that indicates the connection of the operator to a wrist strap connected to the connector.

27. The device of claim 16 further comprising an output circuit that generates an output signal having multiple signal levels wherein the level of the output signal is a function of an impedance of the operator connected to the wrist strap.

28. A device for monitoring the grounding of an operator in an electrostatic discharge sensitive areas, the device comprising:

a connector, connected to a dual wrist strap worn by an operator that is in electrical contact with the operator'"'"'s body in plurality of separate places, the connector having a first contact and a second contact capable of being connected to different portions of the wrist strap that contact different places of the body of the operator;

an oscillator circuit, connected to the first contact of the connector, that generates an alternating current control signal that is capable of being supplied to the operator'"'"'s body through the first contact of the connector; and

a signal level detector having an input connected to the second contact of the connector so that the alternating current control signal is passed through the body of the operator from the first contact to the second contact, wherein the signal level detector generates an output signal that indicates when the wrist strap is connected to the operator.

29. The device of claim 28 further comprising a resistor, connected between the connector and a ground path, that dissipates static charges from the operator.

30. The device of claim 28 further comprising a signal conditioning circuit, connected between the connector and the signal level detector circuit, that reject signals not related to the alternating current control signal.

31. The device of claim 30, wherein the signal conditioning circuit further comprises a filter.

32. The device of claim 31, wherein the filter further comprises a digital filter.

33. The device of claim 31, wherein the alternating current control signal has a frequency different from a frequency and harmonics of a power line.

34. The device of claim 33, wherein the alternating current control signal frequency is less than 50 Hz.

35. The device of claim 33, wherein the filter has a peak in a filter frequency response that corresponds to the alternating current control signal frequency and has one or more nulls in the filter frequency response that corresponds to the power line frequency and harmonics.

36. The device of claim 28, wherein the signal level detector circuit generates a signal that indicates an actual impedance of the body of the operator.

37. The device of claim 28 further comprising a direct current voltage level detector having an input connected to the connector and an output connected to an alarm circuit, wherein the direct current voltage detector generates a signal to the alarm circuit when a voltage on the operator exceeds a preset alarm level.

38. The device of claim 37 further comprising a second alarm circuit that indicates the connection of the operator to the wrist strap connected to the connector.

39. The device of claim 28 further comprising an output circuit that generates an output signal having multiple signal levels wherein the level of the output signal is a function of an impedance of the operator connected to the wrist strap.

40. A device for monitoring the grounding of an operator in an electrostatic discharge sensitive area, the device comprising:

a connector, connected to a dual wrist strap worn by an operator that is in electrical contact with the operator'"'"'s body in plurality of separate places, the connector having a first contact and a second contact capable of being connected to different portions of the wrist strap that are connected to different places on the body of the operator;

an oscillator circuit having a first and second output, the first output connected to the first contact of the connector and the second output connected to the second contact of the connector, the oscillator circuit generating an alternating current control signal that is supplied to the operator'"'"'s body through the first and second contacts of the connector; and

a signal level detector having a first and second input, the first input connected to the first contact of the connector and the second input connected to the second contact of the connector, the signal level detector indicates a difference between the first and second inputs, wherein the signal level detector generates an output signal that indicates when the wrist strap connected to the connector is connected to the operator.

41. The device of claim 40 further comprising a resistor, connected between the connector and a ground path, that dissipates static charges from the operator.

42. The device of claim 40 further comprising a signal conditioning circuit, connected between the connector and the signal level detector, that reject signals not related to the alternating current control signal.

43. The device of claim 42, wherein the signal conditioning circuit further comprises a filter.

44. The device of claim 43, wherein the filter further comprises a digital filter.

45. The device of claim 43, wherein the alternating current control signal has a frequency different from a frequency and harmonics of a power line.

46. The device of claim 45, wherein the alternating current control signal frequency is less than 50 Hz.

47. The device of claim 43, wherein the filter has a peak in a filter frequency response that corresponds to the alternating current control signal frequency and has one or more nulls in the filter frequency response that corresponds to the power line frequency and harmonics.

48. The device of claim 40, wherein the signal level detector generates a signal that indicates an actual impedance of the body of the operator.

49. The device of claim 40 further comprising a direct current voltage level detector having an input connected to the connector and an output connected to an alarm circuit, wherein the direct current voltage detector generates a signal to the alarm circuit when a voltage on the operator exceeds a preset alarm level.

50. The device of claim 49 further comprising a second alarm circuit that indicates the operator being connected to the wrist strap connected to the connector.

51. The device of claim 40 further comprising an output circuit that generates an output signal having multiple signal levels wherein the level of the output signal is a function of an impedance of the operator connected to the wrist strap.

52. A device for monitoring the grounding of an operator in an electrostatic discharge sensitive area, the device comprising:

at least two connectors wherein a first connector is connected to a wrist strap of an operator and the grounding of the operator is monitored and a second connector is connected to a wrist strap of a visitor and the grounding of the visitor is not monitored; and

a circuit that detects the presence of the first and second connectors wherein a connection to the second connector without any connection to the first connector generates an alarm and wherein the connection to the second connector after the connection to the first connector does not generate an alarm.

53. The device of claim 52 further comprising:

a signal source circuit connected to the first connector to provide a control signal to the operator through the first connector;

a signal level detecting circuit, connected to the first connector, that measures a magnitude of the control signal that has passed through the operator;

an alarm circuit connected to an output of the signal level detector circuit;

a first switching jack having first and second contacts and a second switching jack having first and second contacts wherein the first contact of first switching jack is connected to a first contact of the first connector, the second contact of first switching jack is connected to a first contact of the second connector and to the first contact of the second switching jack and the second contact of the second jack is connected to the second contact of first switching jack;

wherein, with no wrist strap is connected to either connector, the switching jacks provide a low-impedance circuit across the first connector thus providing a low control voltage to the signal level detecting circuit indicating an absence of a wrist strap plugged into either connector and, with a wrist strap connected to the second connector, the switching jacks providing an open circuit across the first connector thus providing a high level of control voltage; and

wherein the alarm circuit indicates a failed connection to an operator when said control voltage across said first connector exceeds a predetermined value.

54. A method for monitoring the grounding of an object in an electrostatic discharge sensitive area, the method comprising:

generating an alternating current control signal;

applying the alternating current control signal to an object to be monitored;

monitoring a level of the alternating current control signal after being applied to the monitored object; and

activating an alarm if the level of the alternating current control signal exceeds a preset alarm level.

55. The method of claim 54 further comprising filtering the alternating current control signal to remove signals not associated with the alternating current control signal prior to monitoring the level of the alternating current control signal.

56. The method of claim 55 where the filtering of the signal further comprises using a digital filter.

57. The method of claim 54, wherein generating the alternating current control signal further comprises generating an alternating current signal with a frequency that is different from a frequency and harmonics of a power line.

58. The method of claim 57, wherein the frequency of the alternating current is less than 50 Hz.

59. The method of claim 57, wherein the signal filtering further comprises using a filter having a peak in filter frequency response that corresponds to the alternating current control signal frequency and having one or more nulls in filter frequency response that corresponds to the frequency and harmonics of the power line.

60. A method for monitoring the grounding of an operator in an electrostatic discharge sensitive area, the monitoring device having a first connector for an operator whose grounding is monitored and a second connector for a visitor whose grounding is not monitored, the method comprising:

detecting a connection to a connector that is not monitored for grounding; and

generating an alarm when no connection is detected for a second connector that is being monitored for grounding.

61. A method for monitoring the grounding of a user, comprising:

generating an alternating current voltage that is applied to the user;

monitoring the grounding of the user based on a decrease in the alternating current voltage caused by the resistance of the user; and

monitoring a magnitude of the alternating current voltage applied to the user; and

generating an alarm if the magnitude of the alternating current voltage exceeds a preset level.

62. A device for monitoring the grounding of a user based on an alternating current voltage, comprising:

a circuit that measures a decrease in the alternating current voltage caused by the resistance of a user to determine if the user is properly grounded;

a circuit that monitors a magnitude of the alternating current voltage applied to the user and generates an alarm if the magnitude of the alternating current voltage exceeds a preset level.