SkySweeper: A High Wire Robot

SkySweeper: A High Wire Robot

Nick Morozovsky, PhDRobotics Consultant@DrNickMo

April 20, 2015

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Nick Morozovsky Apr 20, 2015

SkySweeper

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Outline

• Introduction

• Architecture & Maneuvers

• Dynamics & Control

• Hardware Design

• Results

• Conclusions

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SkySweeper

Challenges in Robotics

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MobilityPerception

Manipulation“Get mea beer”

StairsOpeninga door

SandEggs

Unstructuredterrain

How tograsp object

LocalizationMapping

High Wire


SkySweeper

Introduction

• Several existing power line robots

• Many degrees of freedom–complex and expensive

• Slow, quasistatic maneuvers

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480 • Journal of Field Robotics—2009

2.2. Complete SystemsAmong the various projects that exist in this field ofrobotics, only few resulted in validated prototypesthat are close to implementation or highly developed.This section presents two exceptions in order to intro-duce key aspects of the technology, further discussedlater in the paper.

2.2.1. Chinese Academy of ScienceThe Chinese Academy of Science (CAS), in collabo-ration with other academic institutions, has been anactive player in the development of such technologyand has several collaborating teams working in par-allel on a number of projects.

Among the most advanced of these projects isa dual-arm robot designed for live-line inspectionof extra-high-voltage power transmission lines (seeFigure 2). As presented by Wang, Fang, Wang, andZhao (2006), this platform is designed to hang fromthe OGW on its two wheeled arms, in order to havean optimal view of the conductors below. Inspectionsuse a video camera pointed downward at the lines.The length of the arms adjusts to keep the robot hori-zontal and help keep the distance between the cameraand its target constant.

Each of the two wheels is equipped with a grip-per that can securely grasp the conductor. The hous-ing containing electrical and electronic componentscan be shifted forward or backward to center themass on either of the two arms, as explained in Zhu,Wang, Fang, Zhao, and Zhou (2006a). An earlier ver-sion of the prototype and the governing kinematicequations were presented by Wang, Wang, Fang, and

Zhao (2005) and Zhang, Zhang, and Jian (2007). Initialsimulations and design optimization are described bySun, Wang, Zhao, and Liu (2006).

This technology has two different methods ofcrossing obstacles. The first, referred to as the“cankerworm method,” consists of centering themass on the rear arm, lifting the front wheel, movingforward until the front has cleared the obstacle, andthen setting the front wheel back onto the conductor.The same process is repeated for the rear. Using thesecond method, once the robot is near the obstacle, itcan grip the wire with the front gripper for stability,lift and rotate the rear laterally to the opposite sideof the obstacle, and then repeat the process for thefront arm. These sequences and the associated controlmethods are presented in Zhu et al. (2006b, 2006c).

These methods enable the robot to cross coun-terweights and crimp connection pipes and, usingthe second method, single overhead anchor clamps.Because the two arms are 240 mm apart, pairs ofanchor clamps separated by a sufficient gap for oneof the wheels can also be crossed in sequence. Theprototype weighs 40 kg and travels at up to 2 m/s.To ensure safe operation, a motor current watchdogwas implemented to detect any abnormalities andimmobilize the unit until the situation could beanalyzed by an operator. A prototype has been testedin the field but the research team plans to further testits reliability under windy conditions.

Its control scheme is based on an expert systemthat uses information from various onboard sensorsand a static database to attempt to navigate alongtransmission lines autonomously. When the robotencounters an obstacle on its path, it matches it toa six-bit code to which a motion sequence can be

Figure 2. CAS prototype presented in Zhu et al. (2006, 2006b; c⃝2006 IEEE).

Journal of Field Robotics DOI 10.1002/rob

Toussaint et al.: A Survey on Transmission Line Robots • 481

associated. Two distinct methods were developed toenable the gripper to locate the ground wire whenreembarking onto it. The first, described in Zhu et al.(2006c), uses two laser sensors on each gripper. Thesecond, described in Wang, Wang, and Fang (2007),uses the video signal from a single microcameraon each gripper. For this method, stereovision wasdeemed unnecessary because the distance to theconductor is a function of the ratio of the apparentto actual (known) wire diameter. The control systemhas not been fully tested, but Sun, Wang, Zhao, andLing (2007) have proposed precision enhancementmethods.

2.2.2. Hydro-Quebec LineScoutThe LineScout Technology developed at Hydro-Quebec’s research institute (IREQ), shown inFigure 3, was first presented by Montambaultet al. (2005) and then by Montambault and Pouliot(2006). The latter paper was selected to appear inMontambault and Pouliot (2007a).

The two-wheel LineScout platform can cross ob-stacles by deploying a two-gripper auxiliary frameunder the cable and securing a grasp on both sides ofthe obstacle. The traction wheels can then be releasedfrom the conductor, flipped down, and moved to theother side of the obstacle. The geometrical analysisunderlying the optimization of the platform’s struc-ture was detailed in Pouliot and Montambault (2008).

The mobile robot is designed to travel alongsingle energized conductors, including one of the

conductors of a conductor bundle, and is immunizedto electromagnetic and radio-frequency interfer-ences (EMI/RFI) from lines of up to 735 kV. TheLineScout’s obstacle-crossing sequence takes lessthan 2 min and is versatile enough to clear obstaclesup to 0.76 m in diameter and most series of adjacentobstacles. Such obstacles include warning spheres,spacer-dampers, and single- and double-suspensionclamps. Crossing dead-end structures and jumpercables was not included in the design specifications.LineScout’s top speed is 1 m/s, and its weight is98 kg. To the authors’ best knowledge, this is the firstand only robot of its kind that has been successfullyused in the field to date. The thorough validationto which LineScout was subjected is described inMontambault and Pouliot (2007b), and the methodsassociated with its field deployment can be found inMontambault and Pouliot (2008).

The decision was made to control the robot in ateleoperation mode, whereas systems were designedto later shift to an autonomous mode. LineScoutrelies on a variety of sensors for control and safety:three programmable pan-and-tilt cameras (PPTC),inclinometers, and motor encoders keep track ofattitude. The core of the control system, at the op-erator’s ground station to which information fromthese sensors is sent, is a LabVIEW program andinterface. To simplify and ensure safe control of its11 motors (excluding the PPTC motors), it makes useof a mode operation strategy (MOS), which limitsthe number of actuators being controlled, dependingon 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


SkySweeper

Introduction

5Expliner, Debenest et al., c. 2008


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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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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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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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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SkySweeper

Dynamics

• Define generalized coordinates for Lagrangian dynamics

• Assume rigid, horizontal wire

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M(q)q + F(q, q) = B⌧ +A(q)T�

A(q)q = 0, S(q) = null(A(q)), q = S(q)⌫S(q)TM(q)S(q)⌫ + S(q)TF (q, q) = S(q)TB⌧


SkySweeper

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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

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x1(q) = (1 0 0 0 0)

y � 2L(cos ✓ + cos↵) = 0

Ay2(q) = (0 1 2L sin ✓ 0 2L sin↵)

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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

!+π-# > 1.9

!+π-# < 1.0

State 0: OpenClamp 1: PivotingClamp 2: Rollingu = -0.65

State 1: CloseClamp 1: RollingClamp 2: Pivotingu = 0.40

State 8: Swing 1Clamp 1: PivotingClamp 2: Openu = -0.20

State 9: Charge 2Clamp 1: PivotingClamp 2: Pivotingu = 1

Cable in grasp of clamp 2

State 7: Charge 1Clamp 1: PivotingClamp 2: Pivotingu = -1

!-γ > 1

State 10: Swing 2Clamp 1: OpenClamp 2: Pivotingu = 0.20

!-γ < -1


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State 5: SwingClamp 1: PivotingClamp 2: Openu = 0.7t*sin(π t)

State 6: HoldClamp 1: PivotingClamp 2: Pivotingu = 0


Inchworm

Swing-Up

Backflip


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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SkySweeper

Results: Inchworm

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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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SimulationExperimental


SkySweeper

Results: Swing-Up

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Results: Backflip

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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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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.


SkySweeper


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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SkySweeper


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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SkySweeper


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

http://ServoCity.com