Work station monitor

• US 5,057,965 A
• Filed: 07/06/1989
• Issued: 10/15/1991
• Est. Priority Date: 07/06/1989
• Status: Expired due to Term

• Current Assignee: 3M Company
• Original Assignee: 3M Company
• Inventors: Wilson, Robert W.
• Primary Examiner(s): Pellinen, A. D.
• Assistant Examiner(s): Osborn, David
• Application Number:
  US07/376,113
• Time in Patent Office: 831 Days
• Field of Search: 361/42, 361/212, 361/220, 361/573, 340/573, 340/649, 340/650, 340/652, 340/653, 324/64, 324/692, 324/722
• US Class Current: 361/212
• CPC Class Codes: G01R 31/50 Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections testing of sparking plugs H01T13/58
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults
• CPC Class Codes: G01R 31/54 Testing for continuity

Abstract

A device for monitoring the grounding characteristics of both an electrically dissipative work surface and a conductive wrist strap having a grounding tether. The device includes a mat monitor and a strap monitor. The mat monitor checks the resistance of the mat ground path and alerts the user if this resistance exceeds a predetermined resistance. The strap monitor checks the resistance of the wrist strap and tether ground path, and employs a timer to send electrical pulses to the wrist strap rather than having a constant voltage output to the strap. The strap monitor establishes a resistance window for the strap ground path, and alerts the user if the strap ground path resistance falls outside of the resistance window.

Claims

1. A device for indicating the grounding ability of both an electrically dissipative surface and a conductive wrist strap having a grounding tether, comprising:

housing means;

first jack means located in said housing means for receiving the grounding tether;

second jack means located in said housing means for receiving a wire attached to the electrically dissipative surface;

first means for monitoring the ground path resistance of the conductive wrist strap and grounding tether, said first means performing said monitoring periodically on a continual basis in response to timing means which pulses an input to said first means, said input also being connected to the tether and wrist strap;

second means for continually monitoring the ground path resistance of the dissipative surface; and

means for electrically connecting said first and second monitor means to an external power supply and an external ground.

2. The device of claim 1 further comprising first indicator means responsive to said first and second monitoring means for indicating acceptable ground path resistances of both the wrist strap and the dissipative surface.

3. The device of claim 2 wherein said first monitoring means includes means for disabling said timing means when an unacceptably high ground path resistance through the wrist strap and tether exists, said disabling means thereby causing said first monitoring means to latch to a constant, non-interrupted and non-pulsed output until the ground path resistance through the wrist strap and tether returns to an acceptable value.

4. The device of claim 3 further comprising:

second indicator means responsive to said first monitoring means or indicating an unacceptably high ground path resistance through the wrist strap and tether;

third indicator means responsive to said first monitoring means for indicating an unacceptably low ground path resistance through the wrist strap and tether; and

fourth indicator means responsive to said second monitoring means for indicating an unacceptably high ground path resistance through the dissipative surface.

5. The device of claim 4 wherein:

said first monitoring means further includes first and second voltage comparison means for establishing maximum and minimum strap resistance thresholds, respectively, for the grounding path through the wrist strap and tether; and

said second monitoring means further includes third voltage comparison means for establishing a maximum surface resistance threshold for the grounding path through the dissipative surface.

6. The device of claim 5 wherein:

said first voltage comparison means outputs a low voltage level when the resistance through the wrist strap and tether is below said maximum strap resistance threshold, and outputs a high voltage level when the resistance through the wrist strap and tether is above said maximum strap resistance threshold;

said third voltage comparison means outputs a low voltage level when the resistance through the dissipative surface is below said maximum surface resistance threshold, and outputs a high voltage level when the resistance through the dissipative surface is above said maximum surface resistance threshold; and

said outputs of said first and third voltage comparison means are connected as inputs to a NOR gate, the output of said NOR gate being connected to said first indicator means.

7. The device of claim 6 wherein:

said first indicator means comprises a green LED;

said second indicator means comprises a first red LED and an audible alarm;

said third indicator means comprises a yellow LED; and

said fourth indicator means comprises a second red LED.

8. The device of claim 3 wherein:

said first monitoring means further includes first and second voltage comparison means for establishing maximum and minimum strap resistance thresholds, respectively, for the grounding path through the wrist strap and tether; and

said second monitoring means further includes third voltage comparison means for establishing a maximum surface resistance threshold for the grounding path through the dissipative surface.

9. The device of claim 8 wherein:

said first voltage comparison mans outputs a low voltage level when the resistance through the wrist strap and tether is below said maximum strap resistance threshold, and outputs a high voltage level when the resistance through the wrist strap and tether is above said maximum strap resistance threshold;

said third voltage comparison means outputs a low voltage level when the resistance through the dissipative surface is below said maximum surface resistance threshold, and outputs a high voltage level when the resistance through the dissipative surface is above said maximum surface resistance threshold; and

said outputs of said first and third voltage comparison means are connected as inputs to a NOR gate, the output of said NOR gate being connected to said first indicator means.

10. The device of claim 2 further comprising:

second indicator means responsive to said first monitoring means for indicating a high ground path resistance through the wrist strap and tether;

third indicator means responsive to said first monitoring means for indicating an unacceptably low ground path resistance through the wrist strap and tether; and

fourth indicator means responsive to said second monitoring means for indicating an unacceptably high ground path resistance through the dissipative surface.

11. A dual workstation monitor which alerts its user to the existence of unacceptable resistances in the ground paths of both an electrically dissipative surface and a conductive wrist strap having a grounding tether, comprising:

a housing;

a first jack located in said housing for receiving the grounding tether;

a first wire for attachment to the electrically dissipative surface;

a second jack located in said housing for receiving said first wire;

a second wire for attachment to ground;

a third jack located in said housing for receiving said second wire;

a fourth jack located in said housing for connection to an external DC power supply;

strap monitor means for monitoring the resistance of the ground path of the conductive wrist strap and grounding tether, said strap monitor means being connected to said first, third and fourth jacks, and performing said monitoring periodically on a continual basis in response to timing means which pulses an input to said strap monitor means, said input also being connected to said first jack; and

surface monitor means for continually monitoring the resistance of the ground path of the dissipative surface, said surface monitor means being connected to said second, third and fourth jacks.

12. The dual workstation monitor of claim 11 further comprising:

first indicator means responsive to said strap and surface monitor means for indicating contemporaneous acceptable ground path resistances of both the wrist strap and the dissipative surface;

second indicator means responsive to said strap monitor means for indicating an unacceptably high ground path resistance through the wrist strap and tether;

third indicator means responsive to said strap monitor means for indicating an unacceptably low ground path resistance through the wrist strap and tether; and

fourth indicator means responsive to said surface monitor means for indicating an unacceptably high ground path resistance through the dissipative surface.

13. The dual workstation monitor of claim 12 wherein:

said strap monitor means further includes first and second voltage comparison means for establishing maximum and minimum strap resistance thresholds, respectively, for the grounding path through the wrist strap and tether; and

said surface monitor means further includes third voltage comparison means for establishing a maximum surface resistance threshold for the grounding path through the dissipative surface.

14. The dual workstation monitor of claim 13 further wherein:

said first voltage comparison means includes a first operational amplifier which outputs a low voltage level when the resistance through the wrist strap and tether is below said maximum strap resistance threshold, and outputs a high voltage level when the resistance through the wrist strap and tether is above said maximum strap resistance threshold;

said third voltage comparison means includes a second operational amplifier which outputs a low voltage level when the resistance through the dissipative surface is below said maximum surface resistance threshold, and outputs a high voltage level when the resistance through the dissipative surface is above said maximum surface resistance threshold; and

said outputs of said first and second operational amplifiers are connected as inputs to a NOR gate, the output of said NOR gate being connected to said first indicator means.

15. An electrostatic discharge monitoring system comprising:

an electrically dissipative mat having first and second wires connected thereto at different locations, said first wire being for attachment to an external ground;

a wrist strap having two conductive portions;

a two wire tether connected to said wrist strap;

a DC power supply;

a housing;

a first jack located in said housing for receiving said tether;

a second jack located in said housing for receiving said second wire;

a third jack located in said housing for connection to the external ground;

a fourth jack located in said housing for connection to said power supply;

a strap monitor connected to said first, third and fourth jacks, and including;

timing means for pulsing the output of said strap monitor to said first jack,

first voltage comparison means for establishing a maximum strap resistance threshold for the grounding path through said wrist strap and said tether, and

second voltage comparison means for establishing a minimum strap resistance threshold for the grounding path through said wrist strap and said tether;

means for establishing said timing means when the ground path resistance through said wrist strap and said tether exceeds said maximum strap resistance, said disabling means thereby causing said strap monitor to latch to a constant non-pulsed output until the ground path resistance through said wrist strap and said tether falls below said maximum strap resistance;

a mat monitor connected to said second, third and fourth jacks and including third voltage comparison means for establishing a maximum mat resistance threshold for the grounding path through said dissipative mat;

first indicator means responsive to said first and third voltage comparison means for indicating contemporaneous acceptable ground path resistances of both said wrist strap and said dissipative mat;

second indicator means responsive to said first voltage comparison means for indicating an unacceptably high ground path resistance through said wrist strap and said tether;

third indicator means responsive to said second voltage comparison means for indicating an unacceptably low ground path resistance through said wrist strap and said tether;

fourth indicator means responsive to said third voltage comparison means for indicating an unacceptably high ground path resistance through said dissipative mat.

16. The electrostatic discharge monitoring system of claim 15 wherein:

said first voltage comparison means includes a first operational amplifier which outputs a low voltage level when the resistance through said wrist strap and said tether is below said maximum strap resistance threshold, and outputs a high voltage level when the resistance through said wrist strap and said tether is above said maximum strap resistance threshold;

said third voltage comparison means includes a second operational amplifier which outputs a low voltage level when the resistance through said dissipative mat is below said maximum mat resistance threshold, and outputs a high voltage level when the resistance through said dissipative mat is above said maximum mat resistance threshold; and

said outputs of said first and second operational amplifiers are connected as inputs to a NOR gate, the output of said NOR gate being connected to said first indicator means.

17. The electrostatic discharge monitoring system of claim 15 further comprising:

means for coupling the output of said strap and mat monitors to a central monitoring location; and

means for remotely connecting said tether to said first jack.

18. A device for indicating the grounding ability of a conductive strap having a ground tether, comprising:

housing means;

jack means located in said housing means for receiving the grounding tether;

means for monitoring the ground path resistance of the conductive strap and grounding tether, said means performing said monitoring periodically on a continual basis in response to timing means which pulses an input to said means, said input also being connected to the tether and strap; and

means for electrically connecting said monitoring means to an external power supply and an external ground.