Abstract— Advances in technology have allowed unmanned

aerial vehicles (UAVs) to be utilized as tools in the power

industry. Power grids are routinely inspected to maintain a

reliable power supply. When maintaining transmission lines,

UAVs are primarily deployed for visual inspection, but there is

a need for ultraviolet (UV) inspections for the purpose of

detecting high voltage coronas. Coronas are potential indicators

of problems in high voltage conductors or insulating structures.

Ultraviolet cameras offer long range and high sensitivity to the

weak photon flux of high voltage coronas. The size, weight, and

cost of UV cameras limit their use in UAV applications. Compact

narrow band sensors do not have these limitations.

The range of the UV sensor is too short for flyby inspections

with a UAV, so methods of augmentation were researched. A

shallow parabolic reflector was developed to enhance the range

of a narrow band UV detector for the purpose of UAV based

autonomous high voltage line inspection. With safer standoff

distances, the UV sensor studied, in combination with a

parabolic reflector, exhibits potential for autonomous inspection

of high voltage structures for corona detection.

Index Terms—Corona, Parabolic Reflector, Solar-Blind UV

Sensors, Transmission Line Maintenance, Ultraviolet,

Unmanned Aerial Vehicle

I. INTRODUCTION

Advances in technology have allowed unmanned aerial

vehicles (UAVs) to be utilized as tools in the power industry

[1]. Power grids are routinely inspected to maintain a reliable

power supply. When maintaining transmission lines, UAVs

are primarily deployed for visual inspection [1]. Studies have

been conducted with image processing to locate insulators [2],

transmission lines [3], and towers [4] to aid in the inspection

process.

In addition, there is a need for ultraviolet (UV) inspections

for the purpose of detecting high voltage coronas. Coronas are

potential indicators of problems in high voltage conductors or

insulating structures [5]. Ground line crews conduct routine

inspections with UV cameras to detect the presence of

coronas, but ground-based inspections are labor-intensive and

slow [1][3]. Helicopters have been utilized in powerline

inspections for their superior speed and heavy payload

capacity. However, the operational cost and potential risk

levels of helicopter inspections are high [2][3]. Therefore,

recent attention has been focused on UAVs as tools for power

This work was performed with support from the NASA Safe Autonomous

Systems Operations and Unmanned Aerial System Traffic Management

programs and under space act agreements with Dominion Resources and Southern Company.

line inspection [2][4].

Research has been conducted on automating the

localization of coronas through the use of UV imagers [6][7].

However, while UV cameras have a long detection range,

they are fragile, heavy, and expensive [8]. These factors make

attaching a UV imager to a multirotor UAV problematic.

While compact UV sensors offer an alternative to UV

cameras, their range is limited. A flame detection sensor

[9][10] may be a low-cost alternative to the UV cameras. The

sensor is small, lightweight, and relatively inexpensive. These

three factors motivated this research effort. Detection of a

100KV corona source with the first generation (R9533)

sensor on calibrated machinery was proven at the range of

1.2m (4 feet) at Electric Power Research Institute (EPRI) [8].

Subsequent improvements in sensor technology have doubled

and tripled detection range (Figure 1) [9][10].

Figure 1. UV TRON sensor generation improvements and sensitivity to

100kV coronas.

The third generation (R13192) sensor has a range of 3.7m

(12 foot). However, this range is still shorter than desired for

flyby inspection with a UAV. A safety buffer of 6.1m (20

feet) from structures (e.g., conductors, towers, trees) is

required to accommodate pilot intervention time to avoid

collision [8][11]. Thus, improved methods of detection are

needed due to collision risks. Because GPS has a horizontal

uncertainty of 2m and vertical uncertainty 4m (95%

confidence interval, [12]), a longer range solution is required.

Affordable differential GPS solutions are potentially capable

of centimeter accuracy, but from previous experiments [11],

Inexpensive, Lightweight Method of Detecting

Coronas with UAVs Nicholas Rymer, Andrew J. Moore, and Matthew Schubert

2018 International Conference on Unmanned Aircraft Systems (ICUAS)Dallas, TX, USA, June 12-15, 2018

978-1-5386-1353-5/18/$31.00 ©2018 IEEE 452

positional integrity degrades at lower altitudes and near

objects. Sensor range improvement is needed because

positional data is often not reliable enough for close powerline

inspection [11].

Corona discharge is created by the ionization of the air

surrounding a conductor at high voltage. A corona will occur

when the strength of the electric field around a conductor is

high enough to form an electron avalanche region, but not

high enough to cause arcing to nearby objects [5][13].

The Hamamatsu UVtron (Figure 1) works like a Geiger-

Muller device for detecting UV in the 185nm to 260nm range

[9][10]. Because the ozone layer of the atmosphere filters

190nm to 290nm (UV-C band), the narrow band the UVtron

detects, no false detections are caused by solar radiation. The

sensor contains a pair of metal plates connected to electrodes

and is filled with an inert gas. When struck by a photon the

plates ionize and conduct momentarily [9][10]. The greater

the incidence of UV photons the higher frequency of

conduction. The companion circuit to the sensor uses a

capacitor to integrate multiple ionization events into a 10ms

square pulse. As the signal grows stronger, the pulse

frequency increases, saturating at 12Hz.

Previous studies have been conducted with focusing devices

to improve the detection range of these sensors. Engelhaupt,

et al. [14][15] used parabolic reflectors to increase the range

of fire detection from 8 meters to 80 meters with a high

curvature dish (the depth of the reflector is greater than the

focal point). Kim and Shong [16][17] used optical lenses and

varied the voltage levels applied to the sensor by the

companion circuit to improve its corona detection range. In

this paper, a comparison is made between the uses of lenses

and parabolic dishes to increase the range of these sensors.

In conjunction with NASA UAS Traffic Management

(UTM) program [18], research flights were conducted with

two major power companies near de-energized power lines

[8][11]. The R13192 UV sensors were attached to the side of

an octocopter, and a corona generator was suspended from

multiple power structures. When the UAV came within range

of the corona source a signal was reported and streamed to the

ground station. However, due to the short range of the sensor,

the corona source was intensified to maintain a safe distance

and still detect the source. Results determined that improved

sensor range was needed. During multiple flights the UAV

was tracked in national airspace via NASA UTM servers as

an added safety measure [8].

In addition to inspection flights, tests were conducted on

ICAROUS [19] trajectory management technology onboard

the UAV. For two flights, ICAROUS was tested. For the first

flight, ICAROUS monitored an autonomous waypoint flight

without perturbations. Then the flight was repeated, and

during this second test, the pilot took over control of the

autonomous flight and simulated a wind gust by deviating the

UAV perpendicular to the planned path. The ICAROUS

system detected the perturbation, assumed control, redirected

the UAV back to the flight course, then gave control back to

the autopilot to continue its mission [11].

II. DEVELOPMENT

A. Lens Development

Previous studies were conducted using lenses to focus UV

light on a narrow band UV sensors, and while increased range

was reported, the field of view (FOV) was not measured

[16][17]. The device itself has a wide FOV (>90°) but with

the lens attached, a reduction in angular sensitivity was

expected.

This experiment was conducted on a lens made of fused

silica, a material that allows photon transmittance in the UV

spectrum. The lens was 50mm in diameter with a focal point

of 100mm. A tube was drafted and manufactured by a 3D

printer to hold the lens with the focal point on the metal plate

of the R13192 sensor.

B. Dish Development

The UVtron was originally designed for fire detection but

high voltage coronas emit photons in the same narrow band

of UV that this sensor detects. Because parabolic dishes have

increased the range of UV sensors in the past [14][15],

reflective mirrors were developed. Engelhaupt et al. [14][15]

used parabolic reflectors of high curvature, however this

would cause the device to easily catch crosswinds. A shallow

dish (the depth of the reflector is less than the focal point) with

a short focal point has less wind resistance so a reflector with

a small focal point/diameter (F/D) ratio was developed.

A 10.2 cm (4 inch) diameter dish with a 0.35 F/D ratio was

designed in 3D drafting software and printed on a hobby grade

3D printer. Sanding and filing were required since the printed

parts were rough. Since aluminum has a high reflectivity in

the UV spectrum, a coating of aluminum foil was affixed to

the inside of the dish.

C. Comparative Testing of UV Lens vs UV Dish

A test comparing the R13192 UV sensor with lens and

parabolic dish enhancements was conducted. The sensor

remained stationary while the corona generator was placed at

0.3m (1 foot) from the sensor, and moved at 0.3m (1 foot)

increments out to 7.3m (24 feet). This was repeated at 15°

offsets from the center of the dish. The sensor output was

monitored by an oscilloscope and data was recorded at each

increment until no signal was registered (Figure 2).

The base sensor was tested as well (Figure 2, left), to

establish a ground truth because the corona generator flux

output was not calibrated. By comparing the slope of the lens

0° response curve and its intersection with the noise level (1.5

Hz) it was determined that the lens improved the detection

range of the sensor by approximately 39% as compared to the

unaugmented sensor. Inspection of the 0° response curve also

showed that at a higher UV intensity (8-12 Hz), the lens-

augmented sensor exhibited a 250% increase in range over the

unaugmented UV sensor. When the lens was tested at a 15°

offset, the device could detect a signal out to a max of 1.2m

(4 feet). The sensor’s FOV was markedly narrowed by the

lens augmentation. The 10.2 cm (4 inch) 0.35 F/D dish

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Figure 2. Profile of un-augmented R13192 UV sensor (left), profile of lens-augmented R13192 UV sensor (middle), and dish-augmented R13192 UV sensor

(right). The 10.2 cm (4 inch) diameter dish with a 0.35 F/D ratio was used in this comparison. As the frequency increases from 0-12Hz the color shifts from

white to black, respectively.

showed approximately 39% increased range over the base

sensor as well. However, the FOV of the dish-augmented

sensor (Figure 2, right) was notably greater than the FOV of

the lens-augmented sensor.

While the lens provided more range than the unaugmented

sensor, the narrow FOV produced by the lens rendered it

insufficient for flyby transmission line inspection. Since lens

augmentation is also heavier and more expensive than dish

augmentation, development and characterization of lens

augmentation ceased at this point. Development effort

continued on parabolic dish augmentation.

D. Further Dish Development

Multiple reflectors were designed with varying diameters

and curvatures. Dish diameters of 10.2 cm (4 inches), 12.7 cm

(5 inches), 16.24 cm (6 inches), and 25.4 cm (10 inch) were

designed. (The largest diameter designed was the maximum

size that would fit on the bottom of our test drone.)

Different curvatures were also designed. The larger

reflectors were designed to the 0.35 F/D ratio and the 10.2 cm

(4 inch) dishes were built with 0.525 and 0.7 F/D ratios (Table

1).

III. RESULTS

A. Dish Testing, Different Diameters and Curvatures

A new area was selected with more room to fully

characterize each dish and a new measurement method was

employed in this test. The corona source was stationary while

the sensor was placed 0.3 m (1 foot) from the corona generator

and moved away in 0.3 m (1 foot) increments. This was

repeated at 15° offsets from the center of the dish.

The corona source was not calibrated, requiring additional

measures to be taken to duplicate the UV intensity of a true

100kV corona. Even at the lowest setting the corona generator

produced more UV than the desired 100kV source. A hood

was placed over the generator with a window of wax paper

that attenuated the UV discharge.

A R9533 sensor was placed near the source as well. This

sensor was calibrated at EPRI to detect a 100kV corona at

1.2m (4 feet) [8]. Thus this reference sensor was placed a

fixed distance of 1.2m (4 feet) from the corona source and the

frequency was monitored by a microcontroller computing a

rolling average of the last 10 measurements to provide a

calibrated intensity. The R13192 UV test sensor was attached

to a microcontroller computing a rolling average and to an

oscilloscope for redundancy. Both microcontrollers were

attached to laptops displaying the rolling averages.

Periodically, the signal would drift due to heat buildup in

the generator. Testing would stop when the intensity deviated

and adjustments were made to return the source to a calibrated

intensity level.

As shown in Figure 3 and Table 1, the 0.35 F/D and 0.525

F/D parabolic dishes were similar in range, varying only by

0.3 m (1 foot) in range. However, the shallowest reflector (0.7

F/D) exhibited a lower range of 1.5 m (5 feet) and lower FOV.

Also, it was found that as the diameter increased, the range

increased but the FOV decreased (Figure 4, Table 1). The

largest increase in detection range was the jump from 10.2 cm

(4 inch) to 12.7 cm (5 inch) dish, with an increase in range of

2.4 m (8 feet). As compared to the 12.7 cm (5 inch) dish, the

16.24 cm (6 inch) dish only gains 0.6 m (2 feet) in range while

its FOV is cut in half. The 25.4 cm (10 inch) reflector had the

longest range of 11.6 m (38 feet) but a very narrow FOV. For

this set, the best balance of range and FOV was the 12.7 cm

(5 inch) 0.35 F/D ratio dish.

Figure 3. Comparison of measured range and FOV of dish reflectors with varied curvatures.

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Figure 4. Comparison of range and FOV of dish reflectors with varied

diameters

B. Adaptation to Flight Model and Testing

Modifications were made to the dish design for flight tests.

The bench test models lacked the structural integrity for

cross winds and vibrations caused by the UAV. The 25.4 cm

(10 inch) reflector was designed with a tri-arm support

design due to its large size and concerns about the structural

integrity of 3D printed Polylactic Acid (PLA). This tri-arm

support design was adapted to the 12.7 cm (5 inch) dish and

electrical components were enclosed in a 3D printed box to

protect it from the environment. Concerns with inaccurate

GPS data near towers (shadowing and multipath) and

dangers of wind gusts dictated that above-line inspections

would be safer [8]. Therefore the 12.7 cm (5 inch) dish was

mounted to the bottom of the UAV for flight testing (Figure

5). The finished product weighs 190g (110g is printed

material) and costs under 200 USD to build.

Figure 5. Research UAV with 12.7 cm (5 inch) dish installed.

C. Payload Architecture

The sensor ionizes and conducts in the presence of UV

photons. This is then formed into a 10ms square wave pulse

that travels to the microcontroller. The microcontroller

continuously counts the pulses and sends a pulse count via

serial communication to an onboard single board computer

twice a second. Onboard processing doubled the pulse count

to estimate the frequency from each half second sample.

Twice per second, the single board computer injects the

computed pulse frequency into the telemetry stream that is

transmitted to the ground station (Figure 6).

Figure 6. Payload architecture.

D. Flight Test

A corona generator was mounted on a tripod 1m above the

ground. The UAV, with a 12.7 cm (5 inch) dish attached, flew

an autonomous mission over the corona source in a grid

pattern over a 4 m x 4 m area, at 3, 5, and 7 meters above

ground. The flight was performed at 2 m/s velocity then

repeated at 4 m/s. The waypoints for the 2m/s flight were

plotted on the ground station and uploaded to the UAV. The

frequency reported by the sensor was injected into the

telemetry stream to the ground station. The purple lines are

the path of the UAV recorded on the ground station (bottom

left, Figure 7). The pulse count was fused with positional data

from the onboard single-ended GPS receiver to create the

maps seen at the bottom left and at the right of Figure 7.

IV. DISCUSSION

More accurate guidance systems are still required for close

powerline inspections. While GPS is suitable for higher

altitude autonomous navigation, it is not reliable enough for

close inspection of high voltage transmission lines [8][11].

GPS data deteriorates when close to obstacles such as steel

lattice towers due to multipath and shadowing [20][21].

A 12.7 cm (5 inch) parabolic dish increased the range of the

R13192 UV sensor enabling safer inspections of power lines

by allowing an adequate standoff distance. The half power

beam width of the dish used in these flights (Table 1) [22] was

8° at 6.7m (22 feet), providing an area of approximately 1.8m

(6 feet) in diameter.

455

Table 1. Range and beam width of the fabricated parabolic

mirrors

Diameter

(in)

Focal

ratio

Range

at 0° (ft)

Beamwidth

angle ϴ

Range

at ϴ (ft)

Unaugmented

sensor

17 41° 12.5

4 0.35 23 6° 16

5 0.35 31 8° 22

6 0.35 33 4° 22.5

10 0.35 38 2° 25

4 0.525 24 6° 17

4 0.7 18 3° 14

While the 12.7 cm (5 inch) reflector almost doubled the

range of the unaugmented UV sensor, there is room for

improvement. The dish was printed from PLA on a hobby

grade printer, warping and rough surfaces were present

distorting the mirror’s focal point. The coating was aluminum

foil, thus small wrinkles were present in the surface that could

cause scattering of the UV photons. Better coatings consisting

of multiple metals layers that reflect a higher percentage of

UV-C exist [14]. Kim and Shong [16][17] experimented with

voltage levels provided by the Hamamatsu UVtron driver

board and witnessed increased sensitivity when boosted to

700V over the board’s default voltage.

V. CONCLUSIONS

The process of evaluating sensors and the development of

multiple focusing devices to enhance payload range was

described. While UV cameras offer long range and high

sensitivity to the weak photon flux of coronas, cost and weight

limits their use in multirotor UAV applications. Because the

R13192 photoelectric sensor is compact, light, and

inexpensive, it is superior for UAV based inspections. With

the un-augmented UV sensor, only manual inspections of

transmission structures were possible but added range from a

UV dish allows for autonomous high voltage line inspections.

The parabolic mirror designed in this experiment almost

doubled the range of the standard R13192 UV sensor. Further

refinement should increase the range and widen the FOV.

This increased range gives a larger margin of safety for

positional uncertainty inherent in single ended GPS units.

With safer standoff distances, the R13192 UV sensor, in

combination with a parabolic reflector, exhibits great

potential for autonomous inspection of high voltage structures

for corona detection. The small size and light weight of the

Figure 7. Flight results. Top left: video frame from 2m/s autonomous flight. Bottom left: top-down rendering of flight telemetry. The star indicates the UV

source location. Right: side view. Measured UV intensity is indicated by color: 0Hz (grey), 1-12Hz (green-to-red color scale). Direction lines indicate the sensor attitude.

456

dish-augmented corona detectors allows for multiple sensors

to be flown simultaneously. Thus more efficient power line

maintenance programs can be implemented with the use of

dish-augmented corona detectors.

ACKNOWLEDGMENT

Dominion Virginia Power and Southern Company were key

partners in this research and flight campaign. ICAROUS

trajectory management technology was flown in collaboration

with the Safety Critical Avionics branch at the NASA

Langley Research Center. UAS Traffic Management

technology was provided by the UTM group of the NASA

Ames Research Center.

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