SkySweeper is a high wire robot designed to maneuver on power lines. It has two identical links connected by a rotary actuator hub, and each link has a 3-position clamp. It can translate using an "inchworm" gait by alternating clamp positions. It can also perform swing-up and backflip maneuvers to circumvent obstacles by controlling clamp positions and actuator torque. Dynamics and control are modeled using Lagrangian mechanics. Hardware uses 3D printed parts, off-the-shelf electronics, and series elastic actuators. Simulation matches experimental results of maneuvers like inchworm locomotion.

1. SkySweeper: A High Wire Robot
Nick Morozovsky, PhD
Robotics Consultant
@DrNickMo
April 20, 2015
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2. Nick Morozovsky Apr 20, 2015
SkySweeper
𝜃
𝜶
x
y
JL ,mL
JL ,mL
JJ
2L
2L
1
2
Outline
• Introduction
• Architecture & Maneuvers
• Dynamics & Control
• Hardware Design
• Results
• Conclusions
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3. Nick Morozovsky Apr 20, 2015
SkySweeper
Challenges in Robotics
3
MobilityPerception
Manipulation“Get me
a beer”
Stairs
Opening
a door
Sand
Eggs
Unstructured
terrain
How to
grasp object
Localization
Mapping
High Wire

4. Nick Morozovsky Apr 20, 2015
SkySweeper
Introduction
• Several existing power line robots
• Many degrees of freedom–complex and expensive
• Slow, quasistatic maneuvers
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ve player in the development of such technology
has several collaborating teams working in par-
l on a number of projects.
Among the most advanced of these projects is
ual-arm robot designed for live-line inspection
extra-high-voltage power transmission lines (see
ure 2). As presented by Wang, Fang, Wang, and
ao (2006), this platform is designed to hang from
OGW on its two wheeled arms, in order to have
optimal view of the conductors below. Inspections
a video camera pointed downward at the lines.
length of the arms adjusts to keep the robot hori-
tal and help keep the distance between the camera
its target constant.
Each of the two wheels is equipped with a grip-
that can securely grasp the conductor. The hous-
containing electrical and electronic components
be shifted forward or backward to center the
ss on either of the two arms, as explained in Zhu,
ng, Fang, Zhao, and Zhou (2006a). An earlier ver-
n of the prototype and the governing kinematic
ations were presented by Wang, Wang, Fang, and
of the obstacle, and then repeat the process for the
front arm. These sequences and the associated control
methods are presented in Zhu et al. (2006b, 2006c).
These methods enable the robot to cross coun-
terweights and crimp connection pipes and, using
the second method, single overhead anchor clamps.
Because the two arms are 240 mm apart, pairs of
anchor clamps separated by a sufficient gap for one
of the wheels can also be crossed in sequence. The
prototype weighs 40 kg and travels at up to 2 m/s.
To ensure safe operation, a motor current watchdog
was implemented to detect any abnormalities and
immobilize the unit until the situation could be
analyzed by an operator. A prototype has been tested
in the field but the research team plans to further test
its reliability under windy conditions.
Its control scheme is based on an expert system
that uses information from various onboard sensors
and a static database to attempt to navigate along
transmission lines autonomously. When the robot
encounters an obstacle on its path, it matches it to
a six-bit code to which a motion sequence can be
Figure 2. CAS prototype presented in Zhu et al. (2006, 2006b; c⃝2006 IEEE).
reembarking onto it. The first, described in Zhu et al.
(2006c), uses two laser sensors on each gripper. The
second, described in Wang, Wang, and Fang (2007),
uses the video signal from a single microcamera
on each gripper. For this method, stereovision was
deemed unnecessary because the distance to the
conductor is a function of the ratio of the apparent
to actual (known) wire diameter. The control system
has not been fully tested, but Sun, Wang, Zhao, and
Ling (2007) have proposed precision enhancement
methods.
2.2.2. Hydro-Qu ´ebec LineScout
The LineScout Technology developed at Hydro-
Qu´ebec’s research institute (IREQ), shown in
Figure 3, was first presented by Montambault
et al. (2005) and then by Montambault and Pouliot
(2006). The latter paper was selected to appear in
Montambault and Pouliot (2007a).
The two-wheel LineScout platform can cross ob-
stacles by deploying a two-gripper auxiliary frame
under the cable and securing a grasp on both sides of
the obstacle. The traction wheels can then be released
from the conductor, flipped down, and moved to the
other side of the obstacle. The geometrical analysis
underlying the optimization of the platform’s struc-
ture was detailed in Pouliot and Montambault (2008).
The mobile robot is designed to travel along
single energized conductors, including one of the
ences (EMI/RFI) from lines of up to 735 kV. The
LineScout’s obstacle-crossing sequence takes less
than 2 min and is versatile enough to clear obstacles
up to 0.76 m in diameter and most series of adjacent
obstacles. Such obstacles include warning spheres,
spacer-dampers, and single- and double-suspension
clamps. Crossing dead-end structures and jumper
cables was not included in the design specifications.
LineScout’s top speed is 1 m/s, and its weight is
98 kg. To the authors’ best knowledge, this is the first
and only robot of its kind that has been successfully
used in the field to date. The thorough validation
to which LineScout was subjected is described in
Montambault and Pouliot (2007b), and the methods
associated with its field deployment can be found in
Montambault and Pouliot (2008).
The decision was made to control the robot in a
teleoperation mode, whereas systems were designed
to later shift to an autonomous mode. LineScout
relies on a variety of sensors for control and safety:
three programmable pan-and-tilt cameras (PPTC),
inclinometers, and motor encoders keep track of
attitude. The core of the control system, at the op-
erator’s ground station to which information from
these sensors is sent, is a LabVIEW program and
interface. To simplify and ensure safe control of its
11 motors (excluding the PPTC motors), it makes use
of a mode operation strategy (MOS), which limits
the number of actuators being controlled, depending
on the specific task mode. Also, software interlocks
Figure 3. LineScout on a live 315-kV line.
Journal of Field Robotics DOI 10.1002/rob
CAS Robot, Tang et al., c. 2005 LineScout, Pouliot et al., c. 2008 Expliner, Debenest et al., c. 2008

5. Nick Morozovsky Apr 20, 2015
SkySweeper
Introduction
5
Expliner, Debenest et al., c. 2008

6. Nick Morozovsky Apr 20, 2015
SkySweeper
Architecture
• 2 identical links pivotally
connected by rotary series
elastic actuator (SEA) hub
• 3 position actuated clamp
(a) Open
(b) Rolling - allows axial
translation
(c) Pivoting
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7. Nick Morozovsky Apr 20, 2015
SkySweeper
Maneuvers: Inchworm
• One pivoting clamp and one rolling clamp
• SEA actuates to increase the angle between the links
• Switch clamp configuration, decrease the angle between the links, repeat
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8. Nick Morozovsky Apr 20, 2015
SkySweeper
Maneuvers: Swing-Up
• One pivoting clamp and
one open clamp
• Sine sweep control input
to the SEA
• Second clamp closes
once it reaches cable
• Useful for installation on
the cable
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9. Nick Morozovsky Apr 20, 2015
SkySweeper
Maneuvers: Backflip
• One pivoting clamp and one open clamp
• Preload SEA, release one clamp, swing to grab other end, repeat
• Circumvent obstacle on the cable
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10. Nick Morozovsky Apr 20, 2015
SkySweeper
Dynamics
• Define generalized coordinates
for Lagrangian dynamics
• Assume rigid, horizontal wire
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𝜃
𝜶
x
y
JL ,mL
JL ,mL
JJ
2L
2L
1
2
q = x y ✓ ↵
T
L = T1 + TJ + T2 V
d
dt
✓
L
˙qi
◆
L
qi
= Qi, i = 1, 2, ..., n
M(q)¨q + F(q, ˙q) = B⌧ + A(q)T
A(q) ˙q = 0, S(q) = null(A(q)), ˙q = S(q)⌫
S(q)T
M(q)S(q) ˙⌫ + S(q)T
F(q, ˙q) = S(q)T
B⌧

11. Nick Morozovsky Apr 20, 2015
SkySweeper
𝜃
𝜶
x
y
JL ,mL
JL ,mL
JJ
2L
2L
1
2
Dynamics
• Dynamic constraints depend on
the configuration of the clamps
• 0, 1, or 2 constraints per clamp
• Holonomic vertical constraint
when clamp is rolling or pivoting
• Additional non-holonomic
horizontal constraint when clamp
is pivoting
• Stack applicable constraint
matrices and find orthonormal
basis for null space
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y = 0
Ay1(q) = (0 1 0 0 0)
˙x = 0
A˙x1(q) = (1 0 0 0 0)
y 2L(cos ✓ + cos ↵) = 0
Ay2(q) = (0 1 2L sin ✓ 0 2L sin ↵)
˙x + 2L( ˙✓ cos ✓ + ˙↵ cos ↵) = 0
A˙x2(q) = (1 0 2L cos ✓ 0 2L cos ↵)
q = x y ✓ ↵
T

12. Nick Morozovsky Apr 20, 2015
SkySweeper
Control: Finite State Machine
• Actions: clamp positions, SEA
• Transitions defined by sensor
readings: spring deflection,
separation angle, cable
detection
• Implemented in code as a
switch structure
• Simulation performed with
switched system of equations
of motion with different
constraint matrices
𝜃+π-𝜶 > 1.9
𝜃+π-𝜶 < 1.0
State 0: Open
Clamp 1: Pivoting
Clamp 2: Rolling
u = -0.65
State 1: Close
Clamp 1: Rolling
Clamp 2: Pivoting
u = 0.40
State 8: Swing 1
Clamp 1: Pivoting
Clamp 2: Open
u = -0.20
State 9: Charge 2
Clamp 1: Pivoting
Clamp 2: Pivoting
u = 1
Cable in grasp of clamp 2
State 7: Charge 1
Clamp 1: Pivoting
Clamp 2: Pivoting
u = -1
𝜶-γ > 1
State 10: Swing 2
Clamp 1: Open
Clamp 2: Pivoting
u = 0.20
𝜶-γ < -1
Cable in grasp of clamp 1
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State 5: Swing
Clamp 1: Pivoting
Clamp 2: Open
u = 0.7t*sin(π t)
State 6: Hold
Clamp 1: Pivoting
Clamp 2: Pivoting
u = 0
Cable in grasp of clamp 2
Inchworm
Swing-Up
Backflip

13. Nick Morozovsky Apr 20, 2015
SkySweeper
Hardware Design
• 3D printed parts with off the shelf electronics
• 3 position actuated clamp
• Servo drives symmetrically coupled clamp arms
• Magnets align clamps, teeth prevent rotation in pivoting position
• IR emitter and phototransistor pair detect cable
• Series elastic actuator (SEA) hub
• DC motor and two unidirectional torsion springs
• Energy storage for dynamic maneuvers
• Potentiometers measure spring deflection and angle between links
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14. Nick Morozovsky Apr 20, 2015
SkySweeper
Results: Inchworm
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15. Nick Morozovsky Apr 20, 2015
SkySweeper
Results: Inchworm
• Simulation matches
experimental results,
although greater spring
deflection is predicted
• 50ms delay in switching
clamp positions
• Unmodeled effects of
slippage and rope vibration
contribute to the
discrepancy between plots
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0 0.5 1 1.5 2 2.5 3 3.5
0.5
1
1.5
2
2.5
t(s)
LinkSeparation
θ+π−α(rad)
0 0.5 1 1.5 2 2.5 3 3.5
−0.5
0
0.5
1
1.5
t(s)
SpringDeflection
α−γ(rad)
Simulation
Experimental

16. Nick Morozovsky Apr 20, 2015
SkySweeper
Results: Swing-Up
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17. Nick Morozovsky Apr 20, 2015
SkySweeper
Results: Backflip
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18. Nick Morozovsky Apr 20, 2015
SkySweeper
Conclusions
• Novel, simple robot design for multimodal locomotion on high wires
• Prototype successfully executes dynamic maneuvers
• 3D printed mechanism and COTS electronics keep cost low
• Parameters can be optimized using dynamic simulation
• Acknowledgements
• Thanks to Victor Ruiz, Amber Frauhiger, Alexandros Kasfikis, and Antonios
Kountouris for their assistance in developing the prototype
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19. Nick Morozovsky Apr 20, 2015
SkySweeper
Nick’s Rules of Robotics
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1. Never disassemble a working robot.
1. Always have a demo ready.
2. Video or it didn’t happen.
2. If it works the first time, you’re testing it wrong.
1. How good is good enough? Have defined metrics.
2. If you can’t measure it, you can’t control it.
3. When in doubt, lubricate.

20. Nick Morozovsky Apr 20, 2015
SkySweeper
Nick’s Rules of Robotics
4. Never underestimate the estimation problem.
1. “but it works in simulation”
5. If specs for a part are listed differently in two places, they’re both wrong.
1. How can you validate it yourself? Or deal with uncertainty?
6. Glue, tape, and zip-ties are not engineering solutions (though they might work in
a pinch).
1. You should be able to open your robot.
2. The component that’s hardest to access will be the first to fail.
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21. Nick Morozovsky Apr 20, 2015
SkySweeper
Nick’s Rules of Robotics
7. Do not leave lithium polymer batteries charging unattended.
1. It’s not worth the risk.
2. Use a charging sack.
8. Always have a complete CAD model, including screws and fasteners, before
constructing your robot.
1. Plan out order of operations for assembly.
2. Have extra parts on hand.
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22. Nick Morozovsky Apr 20, 2015
SkySweeper
Nick’s Rules of Robotics
9. Avoid using slip rings if at all possible.
1. Intermittent contact, high/variable resistance
10.Clamping collars are always better than set screws.
1. If you have to use set screws (e.g. for cost reasons), use a driving flat
and an appropriate thread-locking agent.
11.Always check polarity before plugging a component into a power source.
1. Label battery connectors and components.
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ServoCity.com clamping hub