Research and Development of a Multi-copter
for Autonomous Inspectionof Headrace Tunnels of Hydraulic Power Stations
Sustainable System Research Lab.
Central Research Institute of Electric Power Industry

Koichi Yonezawa

Japan-Korea Joint Workshop on Rotorcraft
February 10th , 2023
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Background: Maintenance of Hydro Power Stations
Large-scale earthquakes

e. g. 2016 Kumamoto earthquakes： (Magnitude 6.2 -> 7.3 -> 7.1->… 400 times, in a week)

Flood Disasters

Typhoons, Torrential rains -> landslides, floods, etc.
Typical hydraulic power
station (run-of-river type)
PenstocksRiverHeadrace Tunnel
Hydro-turbine

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River （Downstream）

It is necessary to ascertain
damages of facilities.
Inspection by workers is dangerous.
Unmanned inspection systems should bedeveloped.
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Autonomous Drone for Disaster Response
Why drones in headrace tunnels ?
1) Water or sediment remains on the floor.
2) There are large step structures or steep slopes.
3) Distances can range to several kilometers.
Technical issues
1) Many tunnels have smaller cross-sections than traffic tunnels.
2) GPS is not available and radio control from the outside is difficult.
3) No lighting facilities.
4) Hard environmental conditions, including water leakage from ceilings.

Today’s topics
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 Development of a prototype of TUNNEL DRONE system
 Investigation of aerodynamic characteristics
3DEVELOPMENT OF A PROTOTYPE OF TUNNELDRONE SYSTEM
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Specifications of the Aircraft
Variable pitch rotor
1-D Lidar(Upper)

2-D Lidar 360 Camera 1-D Lidar（Altitude）

Visual-SLAM
Tracking Camera

Light panel

360 Camera

Fully autonomous flight system

Rotor

4 Units/15 inched/2 blades：5400r.p.m. +Variable pitch control

Battery

Li-Po 6S 20000mAh （Flight duration 12 minutes）

 High maneuverability Robust sensing and navigation system

Dimensions

1074(L) x 806(W) x 320(H) mm

Waterproof / dustproof

IP55

Veridical positioning

Upper/lower wall distance control or Altitude control

Horizontal positioning

Left/right wall distance control

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Rotors

Thrust control by variable collective pitch：
Quicker response than rotational speed changeHigh maneuverability contributes to stabilize
the attitude and motion of the aircraft in
turbulence in tunnel.
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Collective pitch control
vs rotational speed control
6Sensors and Navigation
Tracking camera ID=1
（Upside view）
Tracking camera ID=2
（Up-left view）

Radius=30m

Speed detection using 2 tracking cameras
 Switching/restarting are possible in case of error→Robustness is enhanced by redundant system View in two directions (Upside and up-left directions):
Autonomous flight is possible even when water remains on
the floor
 Seamless flight through tunnels and open conduit is possible

2-D LiDAR detects obstacles and walls in front and on the side.
Smooth distance control are realized even when the wall surface has
discontinuous2023
curve.

7Flight Demonstration (1)MISSION:
A) Flight in low temperature（-5℃）
B) Flight in fog• Low temperature→ Condensation occurs on optical sensors• Fog→ Cause of Visual-SLAM error = Significantly exceeded target point
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Flight Demonstration (2)MISSION:
1. Flight in tunnels with small cross sections
2. Flight in a water conduit with a series of tunnels and open channel
 Small cross sections -> Highly turbulent around the aircraft Dirty water droplets from the ceiling in tunnel / Flying leaves in open channel
Variable-pitch-rotors contributed to the flight in turbulence.
Robust optical-system maintained SLAM in various disturbances.
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INVESTIGATION OF AERODYNAMIC
CHARACTERISTICS
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Geometry of the TUNNEL DRONE

Symmetric configuration

D: Rotor diameter (381 mm)
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Asymmetric config.
a/d: Upper, b/c: Lower

There is overlap on the left and rightpropeller rotation planes.
Narrow width is advantage for flightin small tunnel
11Aircraft width vs Torque coef. (Hovering)
Aerodynamics are evaluated using CFD (rFLow3D ©JAXA )Overlap 0.2D
Overlap 0.04D Asymmetric configuration results in ahigher efficiency (lower torque).
 Negative effect is small when overlap issmall on rotating planes.
 Inter-rotor interference can be more
harmful when propellers are placed on
the same plane.
rFLow3D
Accuracy in descritization:
4th order in convective terms
2nd order in viscous terms
Unsteady simulation by LU-SGS implicit scheme
Overlap mesh2023
system was used

Torque coef. vs Aircraft width at
thrust coef. CT＝0.0108
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Observation of Flows around Symmetric Config.
CT＝0.0108
CQ=0.126
Blade angle＝9.26°

Upwash

Tip vortex interaction

Diametrical cross section

Cross section through adjacent
propeller axes

Normalized velocity magnitude for middle W/D
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Normalized axial velocity
for various W/D

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Observation of Flows around Asymmetric Config.
A
C

B

A

Lower rotational plane

C

B

A

Upper rotational plane

Normalized axial velocity inrotational plane （W/D＝1.96）

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BC

Normalized axial velocity incross-section A-C （W/D＝1.96）
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Observation of Flows around Asymmetric Config.
CTCQ

2.24

W/DALLθ0.0108

0.126

9.26

Symmetric config.
CT

θ

1.96

W/D
Upper

0.0108

0.127

9.27

Lower

0.0108

0.123

9.04

Ave.

0.0108

0.125

9.16

Asymmetric config.

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CQ

Upper rotor：
Aerodynamic loss increases
・Interaction between front/rear rotor is small
・Suction flow due to lower propeller induces loss on theupper rotor.
Lower rotor
Aerodynamic loss decreases
・ Interaction between front/rear rotor is small
・Downwash from the upper rotor suppress the blade tip
vortex and loss.
Propeller interaction effect between the left and right rotors
is dominant. Propeller interaction effect between the front
and rear rotors is small due to longer distance.
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Conclusion
 A drone for unmanned inspection of headrace tunnels of hydraulic power
station was developed.
 High maneuverability and robustness of sensing and navigation systems areconfirmed by carrying out fully autonomous flight demonstrations in highly
disturbed environments.
 An asymmetric aircraft configuration for flying through narrow tunnels wasdesigned and its aerodynamic characteristics were investigated.
 The rotor efficiency increases by overlapping the rotating planes less than 25%.
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