Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2001-06-12
2004-07-13
Sircus, Brian (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S056000
Reexamination Certificate
active
06762917
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to ESC monitoring devices and ESC protective devices, and more particularly is a method of monitoring tribolelectric current generated in an operator's body, and thereby controlling ESC levels on the operator's body.
BACKGROUND OF THE INVENTION
The present invention is based on the underlying mechanisms which cause electrostatic potentials to build up on human or other bodies. These mechanisms are used to provide improved and simplified methods for elimination of electrostatic potentials. The present invention also addresses the voltage potentials which are capacitively coupled into human or other bodies by power distribution wiring or by other naturally occurring or man-made electric fields.
The build up of electrostatic potentials on the human body when a person walks over a carpeted surface and when that person subsequently feels the unpleasant discharge when touching a door-knob has been experienced by countless numbers of people. Likewise, existence of capacitively coupled low frequency potentials, mostly from power distribution wiring, can be relatively easily demonstrated by touching an input of an audio amplifier with a finger, which results in a loud hum emanating from the speakers.
The capacitively coupled voltage potentials from power distribution wiring have been traditionally largely ignored because their peak amplitudes are relatively small in comparison with the ones from electrostatic origin. In practice, when measuring these potentials, one finds peak amplitudes well below one kilovolt, whereas peak amplitudes from triboelectric charging are easily an order of magnitude higher. The neglect is further compounded by the fact that a human body discharge below 3 kilovolts is usually not physically felt. However, state of the art electronic devices and micro-structures have become so electrostatic discharge sensitive that one can no longer neglect the effects of low voltage potential discharges such as the ones from capacitively coupled low power line electric fields. All further references to human body voltage potentials in the application therefore imply potentials of triboelectric origin as well as potentials resulting from capactively coupled electric fields.
In order to reduce and eliminate the detrimental effects of voltage potential discharges from human bodies, a number of devices have been developed. The most common of these devices is the “grounded wrist strap”. The grounded wrist strap is basically a wrist-worn bracelet made of a conductive material. The bracelet makes electrical contact with a person's skin, and is connected to protective earth or other electrical common reference plane (subsequently referred to as the “ground”) via a lead wire. Usually a resistor with a value of at least one megaohm is inserted in this path-to-ground. The choice of the resistance value is a compromise between the need to adequately bleed-off the voltage potential charges, and, the need to avoid electric shock and/or electrocution when a person wearing such a grounded bracelet accidentally touches a live power distribution wire.
However, as shall be demonstrated below, the 1 Mohm or higher resistor does not reduce the voltage potentials sufficiently close to zero, and does thus not provide adequate protection during handling of state of the art electronic structures and devices. Under certain circumstances, instantaneous peak voltages of tens to hundreds of volts can be measured in spite of such grounding. State of the art Magneto Resistive (GMR, TMR, etc. . . . ) magnetic disk drive heads and sub-micron semiconductor structures can be destroyed with a human body voltage discharge of as low as 5 volts.
In addition to the wrist straps, it is also common industry practice to use “Workstation Monitors” in conjunction with the wrist straps. The purpose of the workstation monitors is at least twofold: (1) to establish a controlled path-to-ground, and, (2) to verify whether or not a wrist strap is being properly worn. Verification of whether or not a wrist strap is being properly worn is usually accomplished by dividing the wrist strap into two sections isolated from each other. In a typical detection approach, a small measuring current is sent through the wearer's skin between the isolated sections. By measuring the voltage drop across the isolated sections, a decision can be made whether the user's wrist is present or not. If a wrist is indeed present, it is further assumed that a proper connection to ground most likely exists.
The disadvantages of existing wrist straps, whether the split or the non-split version, in conjunction with their existing workstation monitors, are as follows:
1. The 1 Mohm or higher resistor in the path-to-ground allows for body voltage potential excursions well beyond the safe limits of state of the art electronic structures and devices.
2. The value of the resistor in the path-to-ground cannot be lowered due to safety considerations.
3. None of the existing wrist strap/workstation monitor combinations determine with scientific certainty whether a true path from wrist to ground actually exists. All known approaches rely on the assumption that the hardware components used in this path seldom fail, and, therefore, the path is assumed to be of high integrity.
4. The interface between commonly used wrist strap materials and the human skin is not optimum. Well understood bioelectric effects cause variability in the contact resistance. Bioelectric potentials, because of the ionic nature of the skin/wrist strap interface, do interfere with the small measurement currents used in the split wrist strap approaches. As will be demonstrated later, it is unlikely that existing wrist straps will ever allow reliable and repeatable paths-to-ground of sufficiently low resistance, so as to meet the necessary 5 volt and lower sensitivities.
5. Certain split wrist strap measurement circuits in existing workstation monitors apply voltages above 5 volts to the wrist-under-test, thus charging the body capacitance to these levels, hence possibly contributing to device destruction rather then to their protection.
The present invention intends to improve on or eliminate all these constraints and limitations. The present invention establishes a method of monitoring and controlling electrostatic charge on a human body. The method utilizes the discovery that the first order phenomenon in the charging of a human body to a voltage potential is in fact an electrical current, and that this current is driven from a near perfect current source of atomic nature. Proof of existence of this triboelectric current can be easily reproduced and demonstrated by connecting a human-under-test to a Current Amplifier, such as the Keithly 428 or equivalent, or, to a simple homemade current-to-voltage operational amplifier circuit with a 1 microamp per volt conversion gain. The output of the amplifier can be observed with a suitable recorder or oscilloscope while the human-under-test walks over a typical electrifying floor surface. In order to eliminate interference from often overwhelming capacitively coupled stray electric fields, the test is best done in a shielded room. Peak triboelectric currents generated by typical shoe sole/floor surface interactions were found to be as high as tens of microamperes. However, the vast majority, estimated at better then 95%, do not exceed 10 microamperes.
The voltage potential which develops on a human body, and which is measurable with any suitable electrometer or electrostatic voltmeter, is thus a direct result of a triboelectric current charging the body capacitance to an instantaneous voltage according to the general equation dv=(i*dt)/C. The typical body capacitance is 100 to 150 picofarads, and depends on the size, shape and posture of the body. In fact, the body voltage potential will continuously vary because of the continuous body shape or posture variations, all of which will change the body capacitance. This variation of body voltage potential is a s
Heymann Steven B.
Nelsen Lyle
Verbiest Noel
Nguyen Danny
Novx Corporation
Sircus Brian
The Kline Law Firm
LandOfFree
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