Patent Publication Number: US-6984971-B1

Title: Low power, low maintenance, electric-field meter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119(e) of provisional U.S. Ser. No. 60/275,763, filed on Mar. 14, 2001, entitled “ELECTRIC FIELD METER WITH OSCILLATING SHUTTER,” the contents of which are hereby expressly incorporated in their entirety by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Portions of the invention were funded under NSF Grant ATM-9724594. 

   BACKGROUND OF THE INVENTION 
   Lightning is a well known natural hazard. Every year in the United States, lightning kills approximately one hundred people and injures on the order of a thousand. Approximately 50 million cloud-to-ground lightning discharges occur each year in the U.S. Damage to equipment and disruption of commercial and industrial operations is measured in billions of dollars. For these reasons, and for many safety-related and equipment protection purposes, it is desirable to provide objective information about impending electrical storms, active thunderstorms and expiring thunderstorms. 
   Instruments known as electric-field meters are currently used to measure atmospheric electric fields at the surface of the earth. Almost all such electric-field meters are based on a design originally developed by C. T. R. Wilson in the early part of the 20th Century. One of the prior art electric-field meters developed by Wilson used a flat circular metal plate mounted flush with, but well insulated from the ground. The flat plate was connected to a gold-leaf electrometer. To make a measurement of electric field with this instrument, a grounded cover was placed over the sensor plate (thus shielding the sensor plate from the ambient electric field) and a zero for the electrometer was determined. Then the cover was removed, allowing charge to be induced on the plate and causing a deflection of the electrometer leaves. By means of a calibrated variable capacitor and a power supply, Wilson was able to null out the induced charge and thereby determine the electric field at the sensor plate when it was uncovered. All mechanized electric-field meters that followed have been, essentially and simply, variations with varying degrees of automation of the basic concepts employed by Wilson. 
   Mechanized electrical field meters have been employed for atmospheric research and thunderstorm warning for about seventy years. Mechanized field meters have been used as stand-alone instruments and in networks in which multiple individual sensors are installed some distance apart on the surface of the Earth to give measurements of electric fields over a wide area. 
   Multiple field meters in a network have been employed at the NASA Kennedy Space Center for more than 20 years as one component of a decision support system to inform official judgement as to propriety of fueling operations, launch, etc. Single field meters are employed at high-risk installations such as armament caches, etc. The cost of commercial field meters currently available is high, they have great electrical power requirements, and they usually need frequent preventive and periodic maintenance. These disadvantages preclude widespread application of commercially available field meters. 
   More specifically, the prior art electric-field meters suffer from at least three problems which make their wide spread use generally too costly. These three factors are relatively high power consumption, difficult calibration procedure, and stringent requirements for frequent maintenance. For example, on all high input impedance electric-field meters it is necessary to clean the insulators and/or the electrodes of the insulated sensing electrode assembly periodically. Cleaning is necessary because when the insulators become covered with films of dust, moisture and salt spray, conductive paths can form, defeating the purpose of the insulators. Over time, the sensing electrode assembly also becomes covered with a film of dust and salt spray. In the prior art, the cleaning operation is difficult because the prior art electric-field meters require extensive and complex disassembly of the instrument to remove electrodes and thereby clean insulators and electrodes. The disassembly of the prior art electric-field meters for electrode and insulator cleaning thus requires a highly skilled technician adding considerably to the on-going expenses associated with the electric-field meter. 
   Commercial field meters typically consume tens to hundreds of watts of electrical power. Such high power consumption precludes or discourages application of commercial field meters on most of the existing remote, solar-powered weather stations where electrical power is severely limited. 
   Commercial field meters, when mounted for practical use in elevated configurations, e.g., above ground, on top of buildings, on weather station masts or poles, for which the electric field enhancement factor is unknown must be properly adjusted to compensate for the mechanically increased gain due to the mounting. Typically this correction is performed by changing the value of a resistor or by adjusting a variable resistor inside the instrument to effect a reduction of the electrical gain of the instrument by the same factor that the gain is enhanced by the mounting arrangement. This gain adjustment process typically involves disassembly of the instrument to gain access to electronic components. This process also typically involves a skilled technician and involves risks of opening and improperly closing sealed enclosures in the field. 
   Prior-art field meters suffer from two types of uncorrected errors that change with time, temperature, humidity and atmospheric pollutants. Typical instruments that predate the present invention have a zero-signal output (defined as the output value of the field meter with an imposed electric field of zero) that is typically set during manufacture but which subsequently changes in an unknown way with use and time. Because valuable information about atmospheric electrical conditions can be obtained around zero and at the zero-crossing, i.e., when the electric field reverses polarity, there is a significant advantage in having a zero-signal reading that is known with confidence throughout the operating life of the instrument. 
   Prior-art field meters also suffer from variations in leakage current at the charge-amplifier input due to conduction across insulators associated with the sense electrode and the circuitry used for charge measurement. For prior-art field meters at the place and time of manufacture, the average leakage current at the charge-amplifier input is typically negligible but it invariably increases over time and with changes in atmospheric conditions. The average leakage current in prior-art field meters is an unknown variable that can degrade an instrument to a state of improper operation without warning. Uncorrected increases in average leakage currents tend to reduce the magnitude of the measured electric field, possibly leading to improper assessment of atmospheric electrical threats. 
   Field meters that suffer from unknown and uncorrected zero offsets and average leakage currents do not always provide information of high quality over long periods of use and such field meters typically require labor intensive testing, adjusting and cleaning at times that have to be determined empirically. Here we teach methods for making field meters that measure and correct zero-signal offset errors and errors due to leakage current at the charge-amplifier input as part of each measurement cycle so that every measurement made and reported is of high quality. 
   Thus, a need exists in the art for an electric-field meter with low operating power requirements, ease of installation and field calibration, minimal on-going maintenance expenses, and continuous and automatic error detection and correction. It is to such an improved electric-field meter that the present invention is directed. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a side elevational view of an electric-field meter constructed in accordance with the present invention, and installed in typical fashion. 
       FIG. 2  is a block diagram of the electric-field meter when the electric-field meter is characterized as a field mill. 
       FIG. 2A  is a block diagram of the electric-field meter when the electric-field meter is characterized as an induction voltmeter. 
       FIG. 2B  is a block diagram of the electric-field meter when the electric-field meter is characterized as an electrostatic fluxmeter. 
       FIG. 2C  is a block diagram of the electric-field meter when the electric-field meter is characterized as an agrimeter. 
       FIG. 3  is a perspective view showing a lower portion of one embodiment of the electric-field meter. 
       FIG. 4  is a bottom plan view of the electric-field meter depicted in  FIG. 3 . 
       FIGS. 5 and 6  are diagrammatic views of other embodiments of the electric-field meter wherein the electric-field meter is provided with an adjustable rod or plate for mechanically adjusting the gain of the electric-field meter. 
       FIGS. 7 and 8  are diagrammatic views of other embodiments of the electric-field meter wherein the electric-field meter is adjustable relative to a mounting device for mechanically adjusting the gain of the electric-field meter. 
       FIG. 9  is a perspective view of another embodiment of a mechanical gain adjustment assembly constructed in accordance with the present invention. 
       FIG. 10  is an exploded, side elevational view of an electrode assembly constructed in accordance with the present invention. 
       FIG. 11  is a perspective view of one embodiment of the electric-field meter having a housing removed. 
       FIG. 12  is a schematic diagram of the electric-field meter depicted in  FIG. 1  wherein a position detection assembly is shown in more detail. 
       FIG. 13  is a top plan view of an electrical contact assembly constructed in accordance with the present invention. 
       FIG. 14  is a side elevational view of the electrical contact assembly depicted in  FIG. 13 . 
       FIG. 15  is a fragmental view of a second version of an electrical contact assembly constructed in accordance with the present invention. 
       FIG. 16  is a side elevational view of a third version of an electrical contact assembly constructed in accordance with the present invention. 
       FIG. 17  is a top plan view of a fourth version of an electrical contact assembly constructed in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to improvements in electric-field meters to allow more cost effective and reliable monitoring of atmospheric electrification at single or multiple remote stations. The electric-field meter of the present invention exploits the physical principle that charge is induced on a conductor placed in an electric field. By alternately covering and exposing a conducting and properly insulated electrode assembly, induced charge flows back and forth to the successively exposed, then unexposed, electrode assembly through charge-sensing electronic circuits. The electronic signal produced is indicative of (e.g., proportional to) the applied electric field at the electrode assembly when exposed. Thus, the electric-field meters can be used for measuring a magnitude and/or a polarity of a steady or changing electric field. 
   The electric-field meters of the present invention can be used either individually or as a part of a network for research or for widespread monitoring and warning of impending electrical storms at the following exemplary locations and activities: a golf course, an airport, a marina, an agricultural operation, an offshore oil rig, a hiking trail, an outdoor stadium (football, soccer, baseball, track and field), a theme park, a swimming pool, explosive and munition handling, or any other outdoor operation, such as loading of freight on an aircraft, fueling or the like. When more than one of the electric-field meters of the present invention are utilized, data from separate electric-field meters arranged in a network or grid can be taken to build contours of electric field at ground level as a function of time for operational purposes (hazard warnings, all-clear notice, or the like) and for meteorological research. The electric-field meter can be used for measurement of atmospheric electric fields for a variety of purposes, such as thunderstorm research, early warning of impending lightning, air pollution monitoring, industrial and process control, industrial safety, high-voltage laboratories, physics experiments and educational demonstrations, and intrusion detection and alarm systems. 
   Some of the improvements to the electric-field meters in accordance with the present invention can be characterized according to the following classes: mechanical gain adjustment assemblies, insulated electrode assemblies, rotating electrical contact assemblies, and in some embodiments, elimination of the need for such assemblies, and position detection assemblies. Each of these improvements can be used individually or in combination with other improvements discussed herein to achieve improved performance in comparison to prior art electric-field meters. 
   The mechanical gain adjustment assemblies and the insulated electrode assemblies 1) permit less expensive and more reliable electric-field meters to be constructed, and 2) simplify installation procedures for electric-field meters. 
   The electrical contact assemblies of the present invention, or the elimination of the need for them in some embodiments of the present invention, improve the reliability of electric-field meters and also improve the grounding of or signal conduction from rotating or moving parts thereby improving the signal-to noise ratio of the output of the electric-field meter. The position detection assemblies greatly reduce electrical power consumption as well as complexity of setup and maintenance of electric-field meters constructed in accordance with the present invention relative to the prior art electric-field meters. 
   Various devices are described herein for implementing each of the improvements discussed above. However, it should be understood that each of the improvements can be implemented in various manners so long as the functions described herein are achieved. 
   Referring now to  FIG. 1 , shown therein and designated by a reference numeral  10  is an electric-field meter constructed in accordance with the present invention. In one preferred embodiment, the electric-field meter  10  requires power of only six hundred (600) milliwatts or less. The electric-field meter  10  can be provided with a complete internal battery-powered sub-system that enables the electric-field meter  10  to operate for extended periods of time while remaining independent of external power sources. Alternatively, the electric-field meter  10  may be powered by a small, inexpensive solar panel that can provide adequate recharge of sealed, internal lead-acid (or other types) batteries, for example, for unattended long term operation. The prior art electric-field meters discussed in the background of the present invention typically require tens to hundreds of watts. Thus, the electric-field meter  10  is a substantial improvement over the prior art electric-field meters. 
   As shown in  FIG. 1 , the electric-field meter  10  can be mounted and supported by a mounting device  12 . The electric-field meter  10  is preferably mounted on the mounting device  12  in an “inverted” position wherein an electrode assembly of the electric-field meter faces the ground so as to provide some protection for the electrode assembly from the elements, such as rain, snow, hail and ice. However, the electric-field meter  10  can be mounted to face upwards. 
   Mounting methods, such as the mounting device  12  for supporting the electric-field meter  10  are well known in the art. Thus, no more comments are deemed necessary to teach one of ordinary skill in the art how to construct and/or use a mounting device, such as the mounting device  12 . The present invention is not limited to any particular type of mounting device. 
   Referring now to  FIG. 2 , shown therein, in block diagram form, is one preferred embodiment of the electric-field meter  10 . The electric-field meter  10  described hereinafter is also known as a “field mill”. However, as will be discussed in more detail below with reference to  FIGS. 2A   2 B and  2 C it should be understood that the present invention is equally adapted to other electromechanical electric-field sensing instruments, such as electrostatic fluxmeters ( FIG. 2B ) induction voltmeters ( FIG. 2A ) and agrimeters ( FIG. 2C ), which share the common problems of calibration, making proper reliable contact with moving or rotating conductors, achieving and maintaining proper insulation of signal carrying conductors, correcting for signal modification due to non-ideal mounting configuration and detecting the position of moving conductors for the purpose of synchronous rectification. 
   In general, the electric-field meter  10  shown in  FIG. 2  is provided with one or more electrode assembly  26 , a shield assembly  28 , a movement assembly  30 , one or more position detection assembly  32 , a charge measurement circuit  34 , and a microcontroller  36  with an analog-to-digital converter. 
   In the field-mill implementation, the electrode assembly  26  is maintained in a stationary position and is selectively exposed to an electric field  38 . Charge is induced on the electrode assembly  26  when the electrode assembly  26  is exposed to the electric field  38 . Charge passes to ground from the electrode assembly  26  when the electrode assembly  26  is shielded from the electric field  38 . The electric-field meter  10  can be provided with one or a plurality of electrode assemblies  26  preferably mounted in a symmetrical or evenly spaced relationship. For example, as shown in  FIG. 4 , the electric-field meter  10  has four electrode assemblies  26  with each of the electrode assemblies  26  mounted 90 degrees apart and in which opposing pairs are electrically common. 
   When the electric-field meter is a “field mill”, the shield assembly  28  is maintained at a ground reference potential  40  and is movable with respect to the electrode assembly  26  through a predetermined path for alternately covering and exposing the electrode assembly  26  to the electric field  38 . In this example, the shield assembly  28  is provided with one or more vanes  29  ( FIG. 4 ), but the shield assembly  28  can take whatever form is needed for covering and exposing the electrode assembly  26 . 
   The movement assembly  30  has a linkage  42  operably connected to the shield assembly  28  for moving the shield assembly  28  with respect to the electrode assembly  26 . The linkage  42  can be a mechanical linkage, such as the shaft of a motor. The movement of the shield assembly  28  by the movement assembly  30  alternately covers and exposes the electrode assembly  26  to the electric field  38 . 
   The movement assembly  30  can move the shield assembly  28  in a rotary manner, a reciprocating manner, or an oscillatory manner. The rotary manner of movement of the shield assembly  28  refers to movement of the shield assembly  28  in only one direction through a closed path encompassing at least 360 degrees. The movement assembly  30  providing the rotary manner of movement of the shield assembly  28  can be a stepper motor, a DC brushed motor, a DC brushless motor, a DC servo motor, a solenoid, a rotary actuator, or a clockwork-spring-like mechanism, or any of a variety of single or multiphase AC motors. 
   The reciprocating manner of movement refers to linear or rotary motion involving back and forth movement, up and down movement or the like. In the case of rotary reciprocating motion, the shield assembly  28  may move through any angle desired up to the limit imposed by an electrical contact assembly discussed herein below with reference to  FIGS. 13–17 . The reciprocating manner of movement can be implemented with a stepper motor, a DC brushed motor, a DC brushless motor, a solenoid(s), a rotary actuator, a clockwork-spring-like mechanism, a linear actuator, a DC motor with servo-control, a mechanical spring with an assist from an electromechanical device, an air motor, or a fluid drive. The oscillatory manner of movement refers to the shield assembly  28  and movement assembly  30  being a driven mechanical oscillator, i.e., a mechanical system having a natural frequency of oscillation. 
   The stepper motor provides many advantages over other ways of moving the shield assembly  28 . That is, the stepper motor allows for (1) low-noise brushless DC operation and allows for rapid, repeatable motion of the shield assembly  28 ; (2) electromagnetic braking of the shield assembly  28  and precise repeatable repositioning of the shield assembly  28 ; (3) very low average power consumption because the stepping motor coils can be de-energized after a move is complete; (4) complete symmetry of covering and uncovering “moves” that can be used to advantage for improved signal processing; and (5) control with only two I/O lines from the microcontroller  36  thereby providing economy in hardware and software. The stepper motor can give a reduction in number of parts and thereby a reduction in cost to manufacture and cost to maintain and an increase in reliability. 
   In the reciprocating and oscillating manners of movement, there optionally may be a pause (hereinafter referred to herein as “dead time”) between movement of the shield assembly such that 1) the shield assembly  28  is not moving and 2) the shield assembly  28  is not covering the electrode assembly  26 . dE/dt signals can optionally be received by the charge measurement circuit  34  and monitored during dead time when the electrode assembly  26  is exposed to electric-field change signals. This in effect allows the electric-field meter  10  to function as an electric-field-change meter, which monitors the environment for changes of electric-field during dead time. Using the electric-field meter  10  during dead time as the electric-field-change meter makes the electric-field meter  10  responsive to both electric field changes caused by lightning, and the relatively slow variations in electric-field changes associated with growth and decay of as well as advection charge accumulations in clouds. Both of these functions are useful if the electric-field meter  10  is being used for monitoring thunderstorms. For example, the electric-field meter  10  can be used for counting lightning-like field changes, watching for lightning and other large field changes, and other conditions which are indicative of a thunderstorm threat. 
   The position detection assembly  32  monitors the position of at least a portion of the shield assembly  28  and outputs a shield position detection signal. The shield position detection signal indicates the portion of the shield assembly  28  for typically determining the covering or uncovering of the electrode assembly  26  by the shield assembly  28 . The charge measurement circuit  34  is connected to the electrode assembly  26  and produces a charge measurement signal indicative of the charge accumulated, or, released by the electrode assembly  26 . The charge measurement circuit  34  can be implemented with a charge amplifier, or with a voltage amplifier that measures the voltage drop across a resistor connected between the electrode assembly  26  and the ground reference potential  40 . 
   The microcontroller  36  receives the shield position detection signal and the charge measurement signal and determines the sign and magnitude of the electric field  38  based on the shield position detection signal and the charge measurement signal. 
   The microcontroller  36  outputs a signal indicative of the electric field  38  to a meter, a computer system, or a dedicated display located locally or at a central monitoring station. The signal output by the microcontroller  36  can be either an analog or a digital signal, or both. Optionally, the electric-field meter  10  can be provided with a fiber-optic communication link for communicating the signal output from the microcontroller  36  to the central monitoring station. The type of signal output by the microcontroller  36  can vary widely depending on the application. In one preferred embodiment, the output from the microcontroller  36  is provided as a digital representation of an analog signal with full-scale equal to plus or minus 2.5 volts which corresponds to plus or minus 10 kilovolts per meter of electric field. 
   When the movement assembly  30  is the stepper motor, the microcontroller  36  is typically used to control the stepper motor, and to sample the charge measurement circuit  34  (in conjunction with an AND converter) for performing a sampling of the peak output signal from the charge measurement circuit  34  on both covering and uncovering operations. This scheme of using an A/D converter to digitize the peak voltage out of the charge measurement circuit  34  provides a sample and hold function with infinite hold time. This feature greatly relaxes the requirements for perfect insulators and long amplifier decay times for the charge measurement circuit  34 , thus reducing maintenance requirements even further, beyond the improvements afforded by the mechanical designs of the present invention. 
   This simple combination of features (moving the shield assembly  28  with a microprocessor-controlled stepper motor detecting position and sampling the peak voltage resulting) replaces the entire synchronous rectifier portion of a conventional field meter. The electric-field meter  10  is thus automatically synchronous. Since the microcontroller  36  causes the covering and uncovering motions, the microcontroller  36  knows at all times the position of the shield assembly  28  so that the output signal of the charge measurement circuit  34  can always be sampled at the proper time and thus the proper sign can be assigned to the electric field measurement. 
   As shown in  FIGS. 2 ,  2 A,  2 B, and  2 C, the electric-field meter  10  can be characterized as a field mill ( FIG. 2 ), an induction voltmeter  10   a  ( FIG. 2A ), an electrostatic fluxmeter  10   b  ( FIG. 2B ), or an agrimeter  10   c  ( FIG. 2C ). The characterization of the electric-field meter  10  as the field mill  10 , the electrostatic fluxmeter  10   b , the induction voltmeter  10   a  or the agrimeter  10   c  depends on the relative movement and grounding of the shield assembly  28  and the electrode assembly  26  (as will be discussed below). The making and using of field mills, electrostatic fluxmeters, induction voltmeters and agrimeters is well known in the art. For purposes of brevity, the induction voltmeter  10   a , electrostatic fluxmeter  10   b  and agrimeter  10   c  will only be shown schematically. However, it should be understood that one skilled in the art will be readily able to make and use the induction voltmeter  10   a , electrostatic fluxmeter  10   b  and agrimeter  10   c  based on the description set forth in the present patent application. For purposes of brevity, similar components between the electric-field meter  10 , the induction voltmeter  10   a , electrostatic fluxmeter  10   b  and agrimeter  10   c  are provided with similar numerical prefixes, and different alphabetic suffixes. 
   The field mill  10  has the movable shield assembly  28  that is held at the ground reference potential  40 . The movable shield assembly  28  alternately exposes and covers the electrode assembly  26  which is connected to the ground reference potential  40  through the charge measurement circuit  34 . In early implementations, field mills measured the alternating potential difference developed across a resistor as charge on the electrode assembly  26  passed to and from the ground reference potential  40 . More modern implementations use an operational amplifier in the charge measurement circuit  34  to hold the electrode assembly  26  near the ground reference potential  40  and to perform charge-to-voltage conversion. 
   As shown in  FIG. 2A , the induction voltmeter  10   a  is similar to the field mill except that the electrode assembly  26   a  is alternately connected to the ground reference potential  40   a  when exposed and to the charge measurement circuit  34   a  when shielded. 
   As shown in  FIG. 2B , the electrostatic fluxmeter  10   b  is provided with an electrode assembly  26   b  which is movable through a predetermined path, and the shield assembly  28   b  is maintained at the ground reference potential  40   b . The shield assembly  28   b  is maintained in a stationary position adjacent the predetermined path of the electrode assembly  26   b  to alternately cover and expose the electrode assembly  26   b  to the electric field when the electrode assembly  26   b  moves relative to the shield assembly  28   b . The electrode assembly  26   b  is connected to the ground reference potential  40   b  through the charge measurement circuit  34   b.    
   As shown in  FIG. 2C , the agrimeter  10   c  is similar to the electrostatic fluxmeter  10   b  except that an electrode assembly  26   c  is alternately connected to the ground reference potential  40   c  when exposed and to the charge measurement circuit  34   c  when shielded. 
   Mechanical Gain Adjustment 
   As shown in  FIG. 3 , In accordance with the present invention, the electric-field meter  10  can be provided with a mechanical gain adjustment assembly  50  for adjusting the effective area of the electrode assembly  26 . Typically, the mechanical gain adjustment assembly  50  is utilized for correcting the signal augmentation that results from elevating and inverting the electric-field meter  10  above the surface of the Earth. 
   For example, it is usually desirable to know the actual electric field that exists at ground level. However, to reduce the effects of precipitation splash, mud, dust, insects, plant growth etc., the electric-field meter  10  is typically mounted on the mounting device  12  in the elevated and inverted position, resulting in an increase or enhancement of the actual electric field. 
   A variety of different embodiments of the mechanical gain adjustment assembly  50  will be discussed below. In each of the embodiments, the mechanical gain adjustment assemblies  50  vary the effective “aperture” of the electric-field meter  10  to mechanically vary, i.e., augment or reduce, the exposure of the electrode assembly  26  to the electric field. The effective aperture can be adjusted manually by an operator who has knowledge of how much the gain of the electric-field meter  10  needs to be changed. 
   Shown in  FIG. 3  is a perspective view of a lower portion of one embodiment of the electric-field meter  10 . The electric-field meter  10  includes a housing  52  at least partially constructed of a conductive material. As shown in  FIG. 4 , the housing  2  has a lower end  54 . The electrode assembly  26 , and the shield assembly  28  are positioned on the lower end  54 . 
   In this embodiment, the mechanical gain adjustment assembly  50  includes a shroud  60  having an open end  62 . The open end  62  of the shroud  60  defines an aperture  63  that permits exposure of the insulated electrode assemblies  26  to the electric field  38 . The shroud  60  surrounds the housing  52  and is affixed to provide lengthwise adjustment between the shroud  60  and the housing  52 . The lengthwise adjustment permits the open end  62  of the shroud  60  to extend past the lower end  54  of the housing  52  such that the distance between the open end  62  and the electrode assembly  26  can be adjusted. This adjustment mechanically increases or reduces the electric-field to which the electrode assembly  26  is exposed. The mechanical gain adjustment assembly  50  is also provided with a securing mechanism  64  for selectively permitting and preventing movement of the shroud  60  on the housing  52  after a desired position has been set. The securing mechanism  64  can be any device capable of selectively permitting and preventing movement of the shroud  60  relative to the housing  52 . For example, the securing mechanism  64  can be a band clamp, screws, bolts, cams or combinations thereof. 
   Although the mechanical gain adjustment assembly  50  has been shown and described herein as the shroud  60  and the securing mechanism  64 , it should be understood that other manners of constructing the mechanical gain adjustment assembly  50  are contemplated. For example, the mechanical gain adjustment assembly  50  can be constructed of the following components either singularly or in combination: one or more selectively movable and securable conductive rods or conductive plates  66  positioned near the electrode assembly  26  as shown in  FIGS. 5 and 6 ; an iris or shutter assembly having a plurality of movable vanes for changing the size of an aperture (not shown) providing access to the insulated electrode assemblies  26  and the shield assembly  28 ; a pivotal connection  68  between the electric-field meter  10  and the mounting device  12  for moving the electrode assembly  26  closer to a grounded, conducting object, such as a mounting mast or stanchion as shown in  FIG. 7 ; or a sliding or telescoping mechanism  70  permitting adjustment of the height of the electric-field meter  10  relative to the ground as shown in  FIG. 8 . 
   Referring now to  FIG. 9 , shown therein and designated by a reference numeral  80  is a second embodiment of a mechanical gain adjustment assembly constructed in accordance with the present invention. In general, the mechanical gain adjustment assembly  80  includes a fixed shield  82  which is spaced a distance away from the electrode assembly  26 , and a movable shield  84 . The fixed shield  82  is at least partially constructed of a conductive material and is connected to ground. The fixed shield  82  has one or more apertures  86  formed therethrough. The movable shield  84  has one or more apertures  88  formed therethrough. The movable shield  84  is movable through a predetermined path configured to overlap the aperture  88  in the movable shield  84  with the aperture  86  in fixed shield  82 . By varying the amount of overlap between the apertures  88  and  86 , the covering and uncovering of the electrode assembly  26  is effected. 
   In one preferred embodiment, the electrode assembly  26  is provided with an opening  90  formed therethrough. The linkage  42  extends through the opening  90  and supports the movable shield  84 . The movement assembly  30  in this embodiment is preferably the stepper motor. In this instance, the movement assembly  30  is controlled by the microcontroller  36  to move the movable shield  84  either to the fully overlapped position, or to some position that only partially overlaps the apertures  86  and  88  in the fixed and movable shields  82  and  84 . For example, if a stepper motor having small steps is used, e.g., 1.8 or 0.9 degrees per step, and if the stepper motor is controlled to half-step the stepper motor, very fine position changes can be made in the overlapping of the apertures  88  and  86 . In other words, the stepper motor alternately moves the movable shield  84  to a closed or non-overlapped position, and then moves the movable shield  84  to the partially overlapped position so that the electric-field at the electrode assembly  26  is reduced by some predetermined fraction that is controlled automatically with every measurement cycle. When the movable shield  84  is moved to overlap the apertures  84  and  86  only partially, a reduction in effective aperture is achieved. 
   As shown in  FIG. 9 , the mechanical gain adjustment assembly  80  can optionally be provided with an adjustable shield  92 . The adjustable shield  92  is constructed of a conductive material and is connected to the ground reference potential  42 . The adjustable shield  92  has one or more apertures  94  formed therethrough. The adjustable shield  92  can be adjusted (manually or automated) such that the aperture  94  in the adjustable shield  92  overlaps the aperture  86  in the fixed shield  82 , effecting the same purpose as the mechanical gain adjustment assembly  50 . By varying the amount of overlap between the apertures  94  and  86 , the effective aperture is also varied. In other words, the adjustable shield  92  can be moved and fixed to constrict or open the size of the aperture  86  in the fixed shield  82 . In one preferred embodiment, the adjustable shield  92  and the fixed shield  82  can be formed of conductive material, such as cast metal, or sheet metal that move relative to one another. 
   The size and/or the shape of the apertures  86 , 88  and  94  can be varied. The shape of the apertures  86 ,  88  and  94  can be any of a variety of geometric, non-geometric or fanciful shapes. The relative location of the movable shield  84 , fixed shield  82  and adjustable shield  92  can be varied. 
   Although the mechanical gain adjustment assembly  80  has been described herein as having the movable shield  84 , fixed shield  82  and the adjustable shield  92  with the apertures  86 ,  88  and  94 , it should be understood that the mechanical gain adjustment assembly  80  should not be limited to the apertures  86 ,  88  and  94  unless such apertures  86 ,  88  and  94  are specifically recited in the appended claims. The function of the apertures  86 ,  88  and  94  is to vary the size of the effective aperture. For example, the mechanical gain adjustment assembly  80  can be implemented with the aperture  86  formed in the fixed shield  82  and the movable shield  84  formed of a vane or a blade movable to completely overlap or partially overlap the aperture  86 . 
   Insulated Electrode Assemblies 
   Referring now to  FIGS. 10 and 11 , shown therein in greater detail is one version of the electrode assembly  26 . The electrode assembly  26  is designed to permit easy cleaning and/or replacement of the insulators with minimal disassembly of the electric-field meter  10 . As mentioned in the background section, it is necessary to clean the insulators periodically when the insulators become covered with films of dust, moisture and salt spray that after a time cause degradation of insulation. 
   The electrode assembly  26  is provided with a sensing electrode  100 , a standoff  102 , a fixed insulator  104 , a replaceable insulator  106 , and an electrode mount  108 . The sensing electrode  100  is constructed of a conductive and preferably non-corrosive material, such as stainless steel or gold, or plated metal. The fixed insulator  104  is constructed of an insulating material, such as Teflon brand insulator, KEL F brand insulator, glass or wax. The standoff  102  is constructed of a conductive material, such as stainless steel or gold-plated metal. The fixed insulator  104  is connected to the standoff  102  such that the fixed insulator  104  extends beyond the periphery of the standoff  102 . In one embodiment, the fixed insulator  104  is connected, e.g., press-fit, threaded, or epoxied, to the standoff  102 . 
   The electrode mount  108  removably connects the sensing electrode  100  to the standoff  102 . The sensing electrode  100  is electrically connected to the standoff  102 . The electrode mount  108  can be a screw, a snap fastener, a friction mount or the like. Although  FIG. 10  shows the electrode mount  108  and the sensing electrode  100  being two separate components which are connected together, it should be understood that the electrode mount  108  and the sensing electrode  100  can be of unitary construction. When the electrode mount  108  and the sensing electrode  100  are formed of separate components, the electrode mount  108  is desirably affixed to the sensing electrode  100  so as to provide a secure mechanical and electrical connection therebetween. The replaceable insulator  106  is positioned between the sensing electrode  100  and the fixed insulator  104 . The replaceable insulator  106  can be Teflon brand insulator, KEL F brand insulator, glass or wax, for example. 
   To connect the electrode assembly  26  to the electric-field meter  10 , the housing  52  of the electric-field meter  10  is provided with a mounting surface  114  having an electrode mounting opening  116 . The standoff  102  and the fixed insulator  104  are disposed in the electrode mounting opening  116 . Desirably, the fixed insulator  104  is received within the electrode mounting opening  116  so as to space the standoff  102  away from the mounting surface  114  thereby insulating the standoff  102  from the mounting surface  114  of the housing  52 . 
   The electrode assembly  26  can be mounted to the mounting surface  114  with any suitable mechanical scheme or linkage. For example, a terminal  117  can be mounted to the standoff  102  via a screw  118 . In this instance, the terminal  117  and the fixed insulator  104  are on opposing sides of the mounting surface  114 . The terminal  117  also functions to provide an electrical connection between the electrode assembly  26  and the charge measurement circuit  34 . Alternatively, the electrode assembly  26  can be mounted to the mounting surface  114  by threading the fixed insulator  104  and the electrode mounting opening  116 , or using tabs, adhesive bonding, cohesive bonding, press fitting or the like to secure the fixed insulator  104  to the mounting surface  114 . 
   Once the electrode assembly  26  is mounted to the mounting surface  114 , the housing  52  defines a cavity  120  that is sealed. A desiccant pack (not shown) is loaded into the housing  52  before sealing of the cavity  120 . 
   To clean or replace the sensing electrode  100 , replaceable insulator  106 , and the fixed insulator  104  of the electrode assembly  26 , the electrode mount  108  is manipulated from outside of the housing  52  so as to remove the sensing electrode  100  and the replaceable insulator  106  from the standoff  102 . For example, when the electrode mount is a screw, the screw can be removed with a screwdriver. Once the sensing electrode  100  and the replaceable insulator  106  are removed from the standoff  102 , the electrode  100  and the replaceable insulator  106  can be cleaned, or replaced and the fixed insulator  104  can be cleaned. Thus, it will be understood by those skilled in the art that the sensing electrode  100  and the replaceable insulator  106  can be removed and/or cleaned with minimal disassembly of the electric-field meter  10 . This permits replacement or cleaning of the sensing electrode  100  and the replaceable insulator  106  in the field by a relatively unskilled technician. This is a vast improvement over the prior art electric-field meters which required an extensive disassembly of the electric-field meter to clean and/or replace the electrodes and/or insulators. 
   Position Detection Assemblies 
   Referring now to  FIG. 12 , shown therein in schematic or block diagram form is the position detection assembly  32 . In every type of electro-static field measuring machine, the position of the moving conductor, e.g., the shield assembly  28  of a field mill, must be known in order to determine the polarity of the field being measured and to know the time at which the signal is at peak value. In the electric-field meter  10 , when the movement assembly  30  is a stepper motor the position of the shield assembly  28  is known automatically and intrinsically as a result of the microcontroller  36  controlling the stepper motor as discussed above. However, even with the movement assembly  30  being the stepper motor, position errors such as missteps can occur. For this reason, the position of the shield assembly  28  is preferably detected directly by the position detection assembly  32 , rather than by using additional components that must be referenced to the shield assembly  28  by subsequent mounting and adjustment. 
   In general, the position detection assembly  32  is provided with a first element  130  mounted onto one or more vane  29  of the shield assembly  28 , a second element  132   FIGS. 11 and 12 ) mounted in a known relationship to the predetermined path of the shield assembly  28 , and a detect circuit  134  receiving an output from the first element  130  or the second element  132 . The first element  130  is epoxied, soldered, riveted, bolted, spot welded, threaded, or otherwise mechanically attached to the shield assembly  28 . The second element  132  is desirably mounted adjacent to the mounting surface  114  of the housing  52  such that the shield assembly  28  and the second element  132  are mounted on opposing sides of the mounting surface  114 . However, it should be understood that the first and second elements  130  and  132  can be mounted on the same side of the mounting surface  114 . The second element  132  detects the movement of the first element  130  when the first element  130  passes near the second element  132 , and in response thereto outputs one or more signals in synchrony with the position of the shield assembly  28  over the electrode assembly  26 . The detect circuit  134  receives the signals from the first element  130  or the second element  132  and in response thereto conditions such signals to be in a form recognizable by the microcontroller  36 . 
   In a preferred embodiment shown in  FIGS. 11 and 12 , the first element  130  is a magnet, the second element  132  is a coil and the detect circuit  134  is a comparator. This embodiment requires only a low-power comparator to perform pulse detection and minimal low-power combinatorial logic as active components, therefore enabling synchronous shield assembly  28  detection at microwatt power levels with no adjustment or calibration required. The coil is desirably located in a co-axial relationship with the sensing electrode  100 . Alternatively, the first or second elements  130  and  132  can include, for example, modulation of the self-inductance of a coil that is used as an element of an oscillator by sweeping a ferrous object near the coil, optical components and a light source, printing or etching to provide variations in reflected light to an appropriately mounted sensor, solid state magnetic field sensors such as Hall Effect devices, SQUIDS, etc. For example, a light source and light detector (first element  130 ) can be attached to the vane  29  of the shield assembly  28 , and a retroreflector (second element  132 ) can be attached to the mounting surface  114 . Alternatively, the retroreflector (first element  132 ) can be attached to the vane  29  of the shield assembly  28 , and the light source and light detector (second element) can be attached to the mounting surface  114 . 
   Electrical Contact Assembly 
   In accordance with the present invention, the electric-field meter  10  is provided with an electrical contact assembly  140  ( FIGS. 13 and 14 ) for grounding or receiving a signal from a moving electrode assembly  26 , or the shield assembly  28 . In one preferred embodiment, the electrical contact assembly  140  engages a moving conductor  142 , which is typically the linkage  42  of the movement assembly  30 . For example, when the electric-field meter  10  is configured as a field mill, the electrical contact assembly  140  grounds the shield assembly  28 , whereas when the electric-field meter  10  is configured as an agrimeter, the electrical contact assembly  140  receives a signal from the electrode assembly  26   c.    
   Shown in  FIG. 13  is an embodiment of the electrical contact assembly  140  for use in applications where the movement assembly  30  moves the shield assembly  28  a in rotating, reciprocating or oscillating fashion. The electrical contact assembly  140  engages the linkage  42  of the movement assembly  30  and thereby provides electrical contact therebetween. The electrical contact assembly  140  is provided with a pair of conducting members  144   a  and  144   b . The conducting members  144   a  and  144   b  are spaced a distance apart. The conducting members  144   a  and  144   b  are electrically connected to and supported by a standoff  146 . The standoff  146  is connected to the mounting surface  114  of the electric-field meter  10 , or frame of the electric-field meter  10  to provide support for the conducting members  144   a  and  144   b . Each of the conducting members  144   a  and  144   b  supports a pad  148   a  and  148   b , respectively, engaging the sides or an end of the linkage  42 . The pads  148   a  and  148   b  can be constructed of a conductive cloth material, such as carbonized cloth, graphite cloth, metal cloth or the like. 
   To provide low friction, low noise, electrical contact between the conducting members  144   a  and  144   b  and the linkage  42 , each of the pads  148   a  and  148   b  is loaded with a conductive lubricant, such as Nyogel  753 G obtainable from Nye, other conductive particles, or oils or greases loaded with conductive particles. The conducting members  144   a  and  144   b  can be connected to the ground reference potential  40  to ground the linkage  42 . Alternatively, the conducting members  144   a  and  144   b  can be electrically connected to the charge measurement circuit  34  so that the charge measurement circuit  34  receives a signal from a moving electrode. 
   Shown in  FIG. 15  and designated by a reference numeral is another example of an electrical contact assembly  150  constructed in accordance with the present invention for use in applications where the movement assembly  30  moves the shield assembly  28  in a rotating, reciprocating or oscillating fashion. The electrical contact assembly  150  is provided with a sealed antifriction bearing  151  having an inner surface  152 , an outer surface  154 , one or more rolling element(s)  156 , and a cage  158  for retaining the rolling element(s)  156  in position. The surfaces  152  and  154  are typically designed to reduce (as far as possible) all friction. However, the surfaces  152  and  154  could be formed on a bushing. The cage  158  retains the rolling element(s)  156  in a spaced apart position between the inner surface  152  and the outer surface  154 . The rolling elements  156   a  can be spherical balls, or rollers. The rollers can be provided with any suitable shape, such as cylindrical, tapered, barrel-shaped, needle-shaped or the like. 
   The outer surface  154  and the inner surface  152  are constructed of conductive material(s), such as stainless steel, aluminum or brass. The outer surface  154  is typically supported by the mounting surface  114 , frame or other bodies of metal in the electric-field meter  10 . When it is desired to ground the shield assembly  28 , the mounting surface  114 , frame or other bodies of metal in the electric-field meter  10  can be connected to ground, e.g., in a field mill. 
   When it is desired to receive a signal from a moving electrode (e.g., an agrimeter), a connector can be connected to the mounting surface  114 , frame or other bodies of metal in the electric-field meter  10  for receiving the signal. The inner surface  152  engages the linkage  42 . 
   The rolling elements  156  are spaced a distance apart to form at least one void therebetween. To lubricate the rolling elements  156   a  as well as increase the conductivity between the outer surface  154  and the inner surface  152 , the anti-friction bearing includes conductive lubricant positioned about the rolling elements  156   a  and within the void. The conductive lubricant can be Nyogel  753 G obtainable from Nye or any of a variety of grease-like or lubricating substances that are loaded with conductive particles. The conductive lubricant is sealed within the void of the anti-friction bearing  151  by any suitable sealing device, such as grease grooves in the bore of the housing, felt washers, leather or synthetic-rubber seals, labyrinth washers or the like. 
   The construction and use of bearings is well known in the art. Thus, no further comments are deemed necessary to teach one skilled in the art to make and use the electrical contact assembly  150  of the present invention. 
   Shown in  FIGS. 16 and 17  are two other examples of an electrical contact assembly  160  constructed in accordance with the present invention for use in applications where the movement assembly  30  moves the shield assembly  28  or the electrode assembly  26  in the reciprocating or oscillating manner. The electrical contact assembly  160  includes a flexible conductor  161  bonded to the linkage  42  and bonded either to the ground reference potential  40  or the charge measurement circuit  34 . This completely obviates the need for any sort of wiping or probing contact and therefore eliminates the limitations imposed by such means. The flexible conductor  161  is arranged to have sufficient freedom of movement so that it can perform a large number, such as billions of cycles without breaking or losing contact. The flexible conductor  161  can be bonded to an end or side  162  of the linkage  42  in various manners. For example, the flexible conductor  161  can be clamped, crimped, screwed, soldered, or welded to the linkage  42 . The flexible conductor  161  can be implemented as a solid wire (as shown in  FIG. 16 ), a stranded wire, a coiled metal spring (as shown in  FIG. 17 ), a flexible sheet metal strip or the like. 
   One of the electrical contact assemblies  140 ,  150  or  160  can be used for grounding the shield assembly  28  or receiving a signal from a moving electrode, or more than one of the rotor contact assemblies  140 ,  150  or  160  can be used to provide redundancy and thereby increase the reliability of the electric-field meters  10 , 10   a ,  10   b  and  10   c . Alternatively, combinations of the electrical contact assemblies  140 , 150  and  160  can be used to provide redundancy. 
   Error Detection and Correction 
   As discussed above in the Background section, prior-art field meters suffer from two types of uncorrected errors that change with time, temperature, humidity and atmospheric pollutants. Typical instruments that predate the present invention have a zero-signal output (defined as the output value of the field meter with an imposed electric field of zero) that is typically set during manufacture but which subsequently changes in an unknown way with use and time. Because valuable information about atmospheric electrical conditions can be obtained around zero and at the zero-crossing, i.e., when the electric field reverses polarity, there is a significant advantage in having a zero-signal reading that is known with confidence throughout the operating life of the instrument. 
   Prior-art field meters also suffer from variations in leakage current at the charge-amplifier input due to conduction across insulators associated with the sense electrode and the circuitry used for charge measurement. For prior-art field meters at the place and time of manufacture, the average leakage current at the charge-amplifier input is typically negligible but it invariably increases over time and with changes in atmospheric conditions. The average leakage current in prior-art field meters is an unknown variable that can degrade an instrument to a state of improper operation without warning. Uncorrected increases in average leakage currents tend to reduce the magnitude of the measured electric field which can lead to improper assessment of atmospheric electrical threats. 
   Field meters that suffer from unknown and uncorrected zero-signal offset errors and average leakage currents at the charge amplifier input do not always provide information of high quality over long periods of use and such field meters typically require labor intensive testing, adjusting and cleaning at times that have to be determined empirically. Here we teach methods for making field meters that measure and correct zero-signal offset errors and errors due to leakage current at the charge-amplifier input of the charge measurement circuit  34  automatically and continuously so as to enhance the quality of the measurements made. The zero-signal offset errors can be determined during each measurement cycle, or as frequently as desired. For example, the zero-signal offset errors can be determined during every measurement cycle, every third measurement cycle, or every tenth measurement cycle. 
   Field meters of the present invention can also be preset, at the factory or by the user, to warn in some way of increases in the average leakage current at the charge-amplifier input of the charge measurement circuit  34  allowing the instrument to inform users when maintenance, e.g., insulator cleaning or replacement, is required, rather than relying on scheduled maintenance or waiting for malfunction. These error detecting, correcting and monitoring features can be used automatically or manually to prevent use of degraded measurements. 
   Specific features of the present invention permit these novel error detection and correction capabilities when combined with measurement steps taken in sequence by the microcontroller  36 . Zero-signal output is tested once per measurement cycle when the shield assembly  28  is in the position that completely covers the sense electrode of the electrode assembly  26 . As shown in  FIG. 12 , the charge measurment circuit  34  includes a charge-amplifier, i.e., an operational amplifier  170  configured as a charge-to-voltage transducer, with a capacitor C as the gain determining element and a selectable resistance R across the capacitor C that sets the decay rate of charge on the capacitor, allows optimum setting of circuit parameters for error detection purposes and normal measurements. 
   The selectable resistance R with at least two known resistance values may be implemented with a mechanical switch or relay having two known states (such as closed circuit and open circuit). Alternatively, the selectable resistance R may be implemented with switchable, fixed resistors, a motor-driven variable resistor, or a solid-state voltage-controlled resistor such as a field-effect transistor. These various methods of implementing a selectable resistance are well known in the art. The resistance value of the selectable resistance R is selected by the micro-controller  26  via a signal path  172 . 
   The error detecting, correcting and monitoring features of the present invention may be implemented with field meters  10 ,  10   a ,  10   b  and  10   c  of all types including field meters  10 ,  10   a ,  10   b  and  10   c  that use rotary, reciprocating or oscillating motion of one conducting element relative to another conducting element, providing that proper considerations for motion control, moving conductor (i.e., shield assembly  28  or electrode assembly  26 ) position detection, etc., are taken into account. 
   The steps taken by the microcontroller  36  to measure and correct for zero-signal offset error are:
         a. Position the movable shield assembly  26  (or the electrode assembly  26 ) so that the sense electrode  100  is completely shielded from the external electric field.   b. Select a resistance across the capacitor C in the charge-amplifier that gives a short decay-time typically less than 5 milliseconds of the RC circuit in the charge-amplifier.   c. Select a resistance across the capacitor C in the charge-amplifier that gives a long decay-time typically at least 1 second of the RC circuit in the charge-amplifier.   d. Allow for settling time and digitize the zero-signal offset error (hereafter referred to as Verror) and store the measurement.   e. Position the movable shield assembly  26  to selectively expose the sense electrode to the external electric field.   f. Digitize the charge-amplifier output (hereafter referred to as Vsig).   g. Compare the previously stored zero-signal offset error and the normal measurement using mathematical operations in such a way that the zero-signal offset error contribution to the overall measurement is removed. For example, subtraction can be used (Vsig−Verror) to remove the zero-signal error.   h. Store and/or communicate the corrected measurement of electric field to the outside world.       

   The steps above for determining the zero-signal offset error and correcting for the zero-signal offset error can be taken at any desired rate including but not limited to one or more times per measurement cycle or at any desired rate. Moreover, the correction for the zero-signal offset error can be accomplished locally or at a remote site. 
   The steps taken to measure and correct for the average leakage current at the charge-amplifier input of the charge measurement circuit  34  are:
         a. Set the selectable resistance R to give a long time-rate-of-decay of charge on the capacitor C in the charge-amplifier with the sensing electrode  100  covered by the shield assembly  28 . Digitize and store the zero signal value.   b. Position the movable shield assembly  28  or electrode assembly  26  to selectively expose the sensing electrode  100  to the electric field.   c. Digitize the charge amplifier output Vsig after the shield assembly  28  (or the electrode assembly  26 ) fully exposes the sensing electrode  100 . Store the time T1 between the readings of the zero signal value1 and Vsig.   d. Position the shield assembly  28  or the electrode assembly  26  to fully cover the sensing electrode  100  and digitize the charge amplifier output (zero signal value2). Store the time T2 between the readings of Vsig and zero signal value2.   e. Compute and store, using the stored signal values and the total time (T1+T2) between the signal samples, the average time-rate-of-decay of the peak signal value Vsig. Use this average time-rate-of-decay of the charge-amplifier output and the total stored time (T1+T2) to complete the move to correct the measurement for loss of signal amplitude due to average leakage currents at the charge-amplifier input.       

   The steps above can be taken at any desired rate including but not limited to one or more times per measurement cycle. Moreover, the correction for the average leakage current can be accomplished locally by the microcontroller  36 , for example, or at a remote site. In the instance where the correction for the average leakage current and/or the zero-signal offset error is accomplished at the remote site, all measured and stored information would be output to the remote site by the field meter  10 . 
   The selectable resistor R can be replaced with a fixed resistor having a high value to provide the RC circuit with a long-time constant, such as greater than 1 second. In this instance, one must wait until the capacitor is discharged before determining the zero-signal offset error. 
   Changes may be made in the construction and operation of the various components, elements and assemblies described herein and changes may be made in the steps or the sequence of steps of the methods described herein without departing from the spirit and the scope of the invention as defined in the following claims.