Electric field sensor

An electric field sensor for measuring the electric field outside a body having an electrically conductive surface, particularly a hovering aircraft, consists of a sensor element including a plate of electrically conductive material, a slab of dielectric material having a pair of opposite surfaces, a first of the pair of surfaces being fixed to the plate, and an electrometer means positioned within the body adjacent to a second of the pair of surfaces and arranged for measuring the electric field within the slab of dielectric material, the electrometer being calibrated to indicate the electric field outside the body on the basis of the measured electric field within the slab of dielectric material.

BACKGROUND OF THE INVENTION 
This invention relates to devices for measuring electrostatic field 
strength and particularly to devices for detecting and indicating the 
strength of an electrostatic field in the vicinity of a hovering aircraft 
such as a helicopter. 
It is well known that the operation of an aircraft often tends to cause an 
accumulation of electrostatic charges through various effects including 
atmospheric conditions and the motion of the aircraft or parts thereof 
through the atmosphere. The accumulation of the electrostatic charges 
tends to build up potentials than can become dangerous and can cause radio 
frequency interference, electrical shock to persons connected with the 
aircraft, and unwanted ignition of fuel and armament. It therefore becomes 
necessary to provide an apparatus for measuring the strength of the 
electrostatic field between the aircraft and the surrounding environment 
in order to determine whether or not the electrostatic potential of the 
aircraft is at a dangerous level. 
While several methods of measuring electric fields have been developed 
which have enjoyed varying degrees of success, it has been difficult to 
reliably determine the potential differences between a charged body and 
surrounding structures when the intervening atmosphere includes any 
substantial number of charged particles or ions. For example, when 
triboelectrically charged particles from a rotating helicopter rotor mixed 
with the ions resulting from corona discharge from the helicopter rotor 
are forced downward by the flow of air sustaining the helicopter in 
flight, the helicopter is immersed in a sea of charged particles and ions 
which all but prevent any meaningful measurement of electrical fields in 
the vicinity of the helicopter. Likewise, ions carried by a flowing fluid 
such as fuel through high velocity fuel pumps can create similar problems 
if potential difference measurements are attempted in an effort to reduce 
fire and explosion hazards. Often in such situations, ions and charged 
particles can become bound to nonconductive portions of the structure in 
the vicinity of the electric field sensor, resulting in apparent field 
modifications having little or no relation to the actual potential 
differences between the body as a whole and surrounding stuctures. 
Examples of electrostatic field measuring devices particularly adapted for 
use in environments of this type are illustrated in U.S. Pat. Nos. 
3,260,893; 3,600,632; 3,812,419; 3,857,066; and 3,874,616. Attempts have 
been made to use the sensed field to signal an appropriate device to 
discharge the charged or charging body. The electrical discharge of 
aircraft is illustrative of such attempts and has been reasonably 
successful when the aircraft has enjoyed a sufficent velocity vector with 
respect to any triboelectrically charged particles and ion products, but 
has generally been unsuccessful in the case of hovering aircraft since the 
fluid borne electric charges adversely affect the electric field sensor's 
ability to accurately detect either the polarity or magnitude of the 
charge on the aircraft. 
The electrostatic fields have typically been measured with the use of 
conventional electrometers of the so called field mill type. Generally, a 
field mill is a device for measuring electrostatic field strength on the 
basis of charges induced on a sensor electrode within the field mill which 
is either electrically or mechanically alternately covered and exposed to 
the electrostatic field sought to be measured. The induced charges on the 
sensor electrode of the field mill are proportional to the external field 
impinging on the sensor electrode. With appropriate phase convergence, 
both the strength and polarity of the electrostatic field sought to be 
measured can be determined. 
It is an object of this invention to combine a conventional field mill with 
other appropriate apparatus so as to more accurately measure the 
electrostatic field surrounding a body. It is a further object of this 
invention to combine a conventional field mill with a sensing means 
postitioned so as to intercept or "short out" at least a portion of the 
equal potential lines surrounding the body and thus more accurately 
determine the electrical field strength. Yet a further object of this 
invention is to provide means by which the sensor means may be maintained 
free from any ion or charged particle buildup due to the presence of ions 
and charged particles in the surrounding fluid. A further object of this 
invention is to utilize an electric field sensor as herein disclosed to 
indicate the electric charge state of a body in such a manner that the 
indication may be used to operate an active discharge system to discharge 
the charged bodies. 
SUMMARY OF THE INVENTION 
An electrostatic field measuring device, according to this invention for 
measuring the electric field outside of a body, comprises a sensor element 
at least a portion of which is positioned externally to the body, and 
comprises at least in part a plate of electrically conductive material 
which is electrically insulated from the body. The apparatus further 
comprises a segment of dielectric material having a pair of opposite 
surfaces, a first of the pair of surfaces being fixed to the plate. The 
sensor element and the segment of dielectric material are joined to a 
conventional electrometer positioned within the body. The electrometer is 
arranged adjacent to the second of the pair of surfaces of the segment of 
dielectric material for measuring the electric field within the dielectric 
material. The electrometer is calibrated so as to indicate the electric 
field outside of a body on the basis of the measured electric field within 
the segment of dielectric material. The electrometer can also be arranged 
to detect the rate of change of the electric field with respect to time 
(dE/dt). 
In the most basic embodiment the sensor element consists of a single 
electrically conductive plate arranged at a preselected distance outside 
of the body. In this embodiment the segment of dielectric material 
preferably comprises a dielectric extending from the plate continuously 
through an aperture in the body to a surface within the body against which 
the electrometer is positioned. This configuration can be viewed as an 
electromechanical analog to a Gaussian pillbox designed to sample the 
electrostatic field outside of a body by intercepting or "shorting" a 
portion of the equal potential field lines which exist about the charged 
body and from the information thus gained determine the charge on the 
body. This embodiment is most useful when free ions and charged particles 
will not impinge on the conductive sensor plate. The thickness and height 
of the dielectric material selected is dependent upon conventional design 
considerations such as potential differences anticipated, dielectric 
strength of the material selected, and lowest threshold of electric field 
strength necessary to obtain useful signals from the electrometer. The 
dimensions of the sensor plate may vary widely, being dependent only upon 
the objective of developing an electric field within the dielectric 
material suitable for detection by the electrometer used. In most 
applications the electrometer signal will be maximized by making the 
sensor plate slightly larger than the area of the electrometer's sensing 
aperture. 
Where the sensor plate is to be exposed to fluidborne ions and charged 
particles, it becomes necessary to periodically drain away accumulated 
charge from the sensor plate. This is achieved by incorporating in the 
electrostatic field measuring device of this invention a normally open 
switch means attached to the sensor element and to the body about which 
the field is to be measured for periodically connecting the sensor element 
and body to eleminate any electric charge inbalance. It is usually 
desirable to disable the electrometer simultaneously with the closing of 
the switch means so as to avoid erroneous indication from the 
electrometer. In a preferred embodiment, the electrostatic field measuring 
device further comprises a signal sampling and storage means attached to 
the electrometer and to the switch means for periodically sampling and 
storing the indicated or measured electric field, the signals indicating 
the field to be sampled at least once between each closing of the normally 
open switch means thereby achieving a substantially continuous indication 
and record of the electrostatic field sought to be measured. The switching 
means can be either a mechanical device such as a relay or an electronic 
apparatus such as a triac and in each case will include appropriate means 
for triggering the switch means from its normally open or high impedance 
condition to its closed or high conduction condition. The disabling means 
can be attached to the swith means so as to activate or trigger the switch 
means in response to a given preselected indication on the electrometer or 
can be attached to the switch means so as to disable the electrometer in 
response to a self-initiated closing of the switch means or both. The 
signal storage and sampling means can be any of a number of devices to be 
found in the prior art for periodically sampling and storing electrical or 
electronic, digital or analog information indicative of any variable 
sought to be measured. Preferably the signal storage and sampling means is 
attached to the switch means such that the indication of field strength 
given by the electrometer is sampled as least once between each 
functioning of the switch means. 
In a preferred embodiment, and particularly where an electrostatic field 
measuring device according to this invention is mounted within a 
helicopeter, the plate of the sensor element is mounted within a body of 
dielectric wholly contained within the helicopter. The sensor element 
further comprises an elongated linear member of electrically conductive 
material connected to the sensor plate and extending outward from the body 
a predetermined length so as to intercept a large number of equal 
potential force lines surrounding the body. The elongated linear member or 
drop line is itself insulated from the body by suitable dielectric 
materials. The drop line may be from rigid to non-rigid and is maintained 
in place, despite any fluid turbulence from any operating downwash, by 
means of an appropriate weight located near the end of the dropline. Ions 
and charged particles impinging on the drop line can be periodically 
discharged by means as previously described and/or by the use of a low 
threshold passive discharger mounted on the extreme lower end of the drop 
line. The drop line preferably contains a high resistance element mounted 
very close to the body so as to decrease the possibility of large current 
density discharges through the line which might be harmful to apparatus or 
personnel coming in contact with the line. 
Additional features and advantages of an electrostatic field measuring 
device according to this invention will be appreciated by those skilled in 
the art upon further consideration of the following description of a 
preferred embodiment together with the attached figures.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In FIG. 1, an electrostatic field measuring apparatus 10 according to this 
invention is mounted in an aperture in the wall 12 of a body so as to 
measure the electric field in the region R outside the body. The electric 
field may be schematically represented by a series of equal potential 
surfaces; E1, E2, E3, etc. existing in the region R surrounding the body. 
The apparatus 10 for measuring the electric field E outside of the body 
comprises a sensor element 14 illustrated in FIG. 1 to comprise a plate of 
electrically conductive material electrically insulated from the body and 
positioned externally to the body. The apparatus 10 further comprises a 
segment of dielectric material 16 having a pair of opposite surfaces 18 & 
20. A first of the surfaces 18 is fixed to the flat plate 14. The 
apparatus 10 further comprises an electrometer means 22 positioned within 
the body adjacent to the second of the pair of surfaces 20 and arranged 
for measuring the electric field within the slab of dielectric material 
16. The electrometer means 22 is calibrated to indicate the electric field 
outside the body on the basis of the measured electric field within the 
slab of dielectric material 16. The electrometer means 22 preferably 
comprises a field mill which includes an electric field sensitive vane 24, 
the field mill being situated such that the electric field sensitive vane 
24 and flat plate 14 are essentially parallel. The field mill may be of 
the conventional type as illustrated in U.S. Pat. No. 2,815,483 or 
alternatively of the newer electronic type as illustrated in U.S. Pat. No. 
3,812,419. The sensor plate 14 extending outward from the wall 12 of the 
body acts to intercept or "short out" one or more of the equal potential 
surfaces surrounding the body and with the parallel electric field 
sensitive vane 24 forms a capacitor having a potential difference between 
the plates which can be measured by the electrometer means 22. The 
dielectric 16 may be of any suitable material ranging from natural 
dielectric materials to synthetic plastics and including air or vacuum 
properly encased. 
As illustrated in FIG. 2, the electrostatic field measuring device 
according to this invention comprises sensor element 14, the segment of 
dielectric material 16 and the electrometer means 22 as previously 
described and additionally certain other elements intended to permit the 
device to operate even in the presence of fluid borne ions and charged 
particles. Attached to the sensor element 14 and to the wall of the body 
12 is a normally open switch means 26 for periodically connecting the 
sensor element 14 and body 12 to eliminate any undesired electric charge 
imbalance between the two. The switch means 26 thus periodically drains 
any charge from the sensor element 14 resulting from the impingement of 
ions and charged particles onto the sensor element 14. These switch means 
26 can be either electromechanical or electronic yet still perform the 
intended function. Preferably the switch means 26 is further connected to 
a disabling means 28 attached to the electrometer means 22 for temporarily 
disabling the electrometer means 22 each time the switch means 26 
functions to eliminate the accumulated charges on sensor element 14. The 
disabling means 28 can again be either an electromechanical or electronic 
means capable of performing the intended function. In this embodiment the 
electrostatic field measuring apparatus also preferably includes a signal 
storage means 30 attached to the switch means 26 and to the electrometer 
means 22 for periodically sampling, storing and comparing the indicated 
electric field measured by the electrometer means 22. The signal storage 
means 30 is preferably maintained so as to sample the indicated field at 
least once between each functioning of the switch means 26 thereby 
generating a continuous record of the measured electric field. 
In FIG. 3, there is illustrated an embodiment of the present invention 
particularly suited to determining the electric field about a hovering 
aircraft 32. The electrostatic field measuring device in this embodiment 
comprises a sensor element including plate 14 and an elongated linear 
member 34 of electrically conductive material electrically connected to 
the plate 14. The elongated linear member 34 is electrically insulated 
from the aircraft 32 and extends generally downward from the aircraft 32 
despite any downwash or air turbulence by means of a weight 36 placed near 
the end of the elongated linear member 34. The member 34 is preferably a 
multistranded cable of conductive material but includes, either 
continuously throughout or as an independent discrete element, a 
resistance means 38 to prevent large currents in the elongated linear 
member 34. At the extreme lower end of the linear member 34 is a passive 
discharge means 40 typically used to control corona discharge aboard 
aircraft. The electrometer means 22 switch means 26 disabling means 28 and 
signal storage means 30 are essentially the same and perform the same 
intended function as that discussed with the apparatus indicated in FIG. 
2. It will be noted that the sensor plate 14 is now found wholly within 
the aircraft's body 32 and is further encased in a second layer of 
dielectric material 42 to prevent electrical contact with the aircraft 32 
and therefore more accurately reflect the electrical field existent below 
the aircraft in the region of the elongated member 34. It will be 
appreciated that the elongated member 34 can extend outward from the 
aircraft 32 a much greater distance than the dielectric body 16 
illustrated in FIG. 1 and thus intercept a greater number of equal 
potential surfaces in the region or outside the body and as a consequence 
more accurately indicate the surrounding electric field. 
As illustrated the signal storage means 30 can be utilized to provide 
information to a conventional active electrostatic discharge system 44 for 
dispensing ions of the proper charge so as to maintain the electrostatic 
potential of the aircraft as low as possible with respect to the 
surrounding environment. In experimental test flights of an electrostatic 
field measuring device according to this invention as illustrated in FIG. 
3, it was noted that a good approximation of the potential difference 
between the hovering aircraft and the earth prior to contact of the drop 
line and the earth was experienced. It is believed that the enhanced 
sensitivity of the measurement is due at least in part to the existance of 
the several foot length of drop line that extends below the aircraft 
thereby intercepting a considerable portion of the equal potential force 
lines which surround the aircraft. 
It will be appreciated that when the drop line 34 contacts the earth, an 
extremely accurate measurement of potential difference between the 
aircraft and the earth can be made. In such a case the electrostatic field 
measuring device according to this invention performs the same function as 
an infinite impedance volt meter. The potential between the aircraft and 
the earth can thus be calibrated by utilizing a drop line 34 of variable 
length and maintaining the hovering aircraft at certain preselected 
altitudes to establish a repeatable pattern of electric potential. The 
drop line 34 can then be shortened to a more generally utilized length of 
only a few feet and the field again measured by repeating the previous 
flight patterns and the indicated fields being correlated with the 
previously known potentials so as to calibrate the field measuring 
equipment. 
In a series of experimental flights it has been acceptably demonstrated 
that when the electrostatic field measuring device illustrated in FIG. 3 
is coupled with an appropriate active electrostatic discharge system 44, 
the hovering aircraft can be discharged to residual energy levels in the 
micro-joule region. 
One embodiment of the circuitry of the invention is shown in FIG. 4. This 
circuitry can be used in the environment illustrated in FIG. 3. In FIG. 4 
the electrometer means 22 is shown to include the field sensitive vane 24 
which is periodically either mechanically or electrically shielded from 
the sensor element 14 by means M. The field sensitive vane 24 is connected 
to amplifier means 46 for amplifying the signals sent by the field 
sensitive vane 24. An indicator means 48 may be included to indicate both 
the polarity and magnitude of the field sensed. 
The disabling means 28 includes means 50 for sensing signal levels from the 
output of the amplifier 46 which are above a preset minimum value in 
magnitude. The means 50 are so constructed that a signal of the 
established minimum magnitude and of either polarity will trigger the 
disabling means 28 to function. The disabling means 28 is connected by way 
of an appropriate current limiting device 52 to the switch means 26 which 
is illustrated in FIG. 4 to be a triac connecting the linear member 34 to 
the aircraft chassis 12. The signal storage means 30 is shown to consist 
simply of a capacitor of an appropriate value which continuously reflects 
the output potential of the amplifier 46. The output 54 can be attached to 
a conventional electrostatic discharge system 44 as illustrated in FIG. 3. 
While the forms of apparatus herein described constitute preferred 
embodiments of the invention, it is to be understood that the invention is 
not limited to these precise forms of apparatus and that changes may be 
made without departing from the scope of the invention which is defined in 
the appended claims.