Electricity: measuring and testing – Electrostatic field – Using modulation-type electrometer
Reexamination Certificate
2001-11-13
2003-08-19
Le, N. (Department: 2858)
Electricity: measuring and testing
Electrostatic field
Using modulation-type electrometer
C324S457000
Reexamination Certificate
active
06608483
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates generally to Electrostatic Field measuring devices and, more particularly, to sensor configurations and signal processing methods that provide enhancements in the bandwidth and suppression of unwanted signals or noise.
There is a long history of people studying atmospheric electricity. Storm prediction and analysis of electric fields and discharges (lightning) associated with storm clouds have necessitated the creation of several types of geophysical sensors and instruments. The field mill is a geophysical sensor that is designed to measure the electric fields below and near storm clouds.
Chalmers, J. Alan,
Atmospheric Electricity
, 1957, New York Pergamon Press, pp. 212-217 provides a description of the distribution of charges in clouds and the resulting field observed at the ground level. The most common storm cloud has a positive charge distribution in the top of the cloud and a negative charge distribution in the bottom of the cloud. This is called a positive cloud and is most common in the regions studied, although other charge distributions do exist in clouds. Monitoring the electric field at ground level as a positive storm cloud approaches would show a positive field if the cloud is at some distance and a negative field as the cloud passes overhead. As the cloud recedes, a positive field would again be observed. This can be predicted by considering the geometry of the cloud charge distribution in relation to the observation point. In clear weather, the typical field at ground level has been measured to be about 100 to 300 volts/meter, whereas the field associated with storm clouds is typically in the thousands of volts/meter. These field measurements are considered to be quasi-static since they are either constant or vary at a low rate of change. In order to measure these quasi-static fields, the field mill was developed. Field levels associated with lightning exhibit a rate of change that is higher than can be tracked by the typical field mill.
Chalmers, supra, also cited a study in England by Wormell involving the use of an inverted test plate connected to an electrometer. The test plate was shielded from above to protect it from rain. As the test plate was covered and uncovered, the electrometer registered the field due to the cloud charge in the region. This prior art study could be considered a manual version of the field mill developed later. Wormell stated that he could only measure the quasi-static or stable fields before the lightning flashes, not the rapidly varying values that occurred after the flashes. This limitation was due to the limited bandwidth of the instrument.
Kessler,
Instruments and Techniques for Thunderstorm Observation and Analysis
, vol. 3, 1983, Normand: University of Oklahoma Press, pp. 89-102, in the chapter authored by Edward T. Pierce, it is stated that although many designs for field mills exist, all operate on the same basic principles. The principles Pierce describes come from basic electrostatic theory and can be summarized as follows. If a conducting plate having a surface area A is exposed to an electrostatic field of magnitude E in a medium such as a vacuum or air (free space) that has a permittivity e0, a surface charge of e0*E*A will be generated on the surface of the plate. If the conducting plate is shielded, this surface charge will be effectively zero. Thus, if the conducting plate is repetitively exposed to and shielded from the electrostatic field, the resulting charge waveform will be an alternating one. This charge waveform can be further processed to extract the magnitude of the field E.
The typical field mill cited in the Kessler reference consists of a set of conducting plates, insulated and alternately shielded from and exposed to the electrostatic field by a rotor having open areas and solid areas. This structure produces an exposed plate area that varies as a triangular function of time and in this example has a frequency of 100 Hz. After this triangular signal is demodulated by a signal that is synchronized with the rotor, the triangular signal is integrated over a time constant that often exceeds 0.1 second. This integration (low pass filtering), along with the variation in exposed plate area over time rendered this prior art field mill unsuitable for studying electrostatic fields that change rapidly, such as those related to lightning discharges. The need for additional instrumentation in such measurement applications is recognized by the prior artisan Pierce.
According to the Kessler reference, cited above, it is apparent that prior art investigator Pierce was aware of the inherent bandwidth limitations of the typical field mill. His statement that the time constant exceeds 0.1 second is equivalent to saying that the upper bandwidth limit of a field mill was less than 1.6 Hz. Since a field mill can measure quasi-static fields, this is equivalent to saying that the lower field mill bandwidth limit extends down to DC. Combining these facts gives a specification of the field mill bandwidth of DC to 1.6 Hz.
Studies of atmospheric electricity and other field measurement activities require the capability of measuring the changes in field level that occur at a rate faster than a typical field mill can measure. This requires an instrument with an upper bandwidth limit greater than that of the field mill.
In the Kessler reference, Pierce describes some of the additional instrumentation that could be used to measure rapid changes in the magnitude of electrostatic fields that extend beyond the capability of the prior art field mill. This additional instrumentation takes the form of a slow antenna and a fast antenna. One form of the slow and fast antennae, preferred due to its ease of calibration, is a flat conducting plate of area A, insulated from, but flush with the earth's surface. In accordance with the basic electrostatic theory of field mill operation summarized above, the resulting charge on the flat conducting plate due to the varying field E(t) above the plate is e0*E(t)*k The inherent structure of these antennae and the subsequent amplification can be modeled as a source with a shunting resistance R and capacitance C such that the output voltage will be proportional to e0*E(t)*A/C, with a decay time constant of R*C. In the interest of measurement accuracy, it is prudent that this decay time constant be at least ten times the duration of the change in the electrostatic field to be measured. For a slow antenna, with a typical 10-second decay time constant, the duration is limited to 1 second. For a fast antenna, with a typical 100-microsecond decay time constant, the duration is limited to 10 microseconds.
The time constants that Pierce described can be also stated as the lower bandwidth limit of these types of antennas. A slow antenna with a time constant of 10 s has a lower bandwidth limit of 0.016 Hz, and a fast antenna with a time constant of 100 us has a lower bandwidth limit of 15.9 KHz. Neither of these types of antennas can measure the quasi-static field such as the field mill does, since their lower bandwidth limit does not extend down to DC. This is not possible due to the inherent resistance in the insulator holding the antenna, as well as that of the amplifier circuit. The upper bandwidths of these antennas are limited by the amplifier circuit and the need to discharge the previous measurement before a second measurement is performed. This is why a fast antenna with a low time constant is used for measuring the multiple rapid field changes associated with a return lightning stroke rather than using a slow antenna for both measurements. If this were not done, the amplifier would saturate and thereby be rendered useless for subsequent measurements until returning to equilibrium.
It may be seen that it is necessary to utilize three prior art instruments to measure over the range of bandwidths required to study the field changes associated with storm clouds and the field changes caused by lightning within those storm clouds.
It
Hein William E.
Le N.
Teresinski John
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