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					Robotic Systems Laboratory

The Australian National University Research School of Information Sciences and Engineering

ADVANCING ACTIVE VISION BY IMPROVED DESIGN AND CONTROL
Orson Sutherland, Harley Truong, Sébastien Rougeaux & Alexander Zelinsky
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Introduction
• RSL has developed an extensive library of Vision Processing software for :
– Human/Machine interfaces – Human/Robot interaction – Navigation

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Introduction
• The intention is to mount these products into a powerful and robust sensor.

• For this we have chosen Active Vision.
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Exisiting Devices

The Agile Eye

ESCHeR

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Introduction
• This presentation will concentrate on:
– the mechanical design – the Hardware – Performance Specifications and Testing – and the Control

of a novel active Vision system named

CeDAR for Cable Drive Active Vision
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CeDAR

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Mechanical Design
• We wanted:
• a high performance • high precision rig • capable of moving a variety of payloads

• For this we required:
• a high precision transmission • and an optimised mechanical structure

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Mechanical Design Issues
• Speed & acceleration depend on:
• motor size • load inertia • transmission ratio

• Accuracy depends on:
• encoder resolution • transmission ratio and stiffness • joint stiffness
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Kinematics
• CeDAR is arranged in the more popular Helmholtz configuration. • An important property of the design is that the axes intersect at the optical center of each camera
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Helmholtz

Fick
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Transmission System
• Uses a cable drive transmission inspired by the Whole Arm Manipulator (Townsend, MIT) • Same as gear transmission except force is transmitted by tension in cables rather than contact between teeth
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Transmission System
• Advantages:
– – – – – – No Backlash No Slippage No lubrication High efficiency No speed limits Torque limited only by strength of cables

Bevel
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Transmission System
• Disadvantages:
• Limited range. • Miniaturisation is limited by the minimum bend radius of the cables.

HOWEVER • In our system, range is only 90°. • Pulleys are integrated into structure.
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Mechanical Architecture
• Parallel architecture allows:
– Motors to be fixed at base so they do not contribute to mass in motion

• But means axes are coupled
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Optimisation

The weight/rigidity trade-off was optimised so that maximum angular deflection was 0.01°.

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Hardware Overview
• Fully assembled CeDAR weighs 3.5kg.

• It has a moving mass of 1.7kg including 2x350g digital cameras.
• Maxon DC motors controlled by Motion Engineering, motion card. • Pentium III performs trajectory planning and vision processing
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Performance Specs
• The max range, payload and baseline were based on the use of motorisedzoom cameras (350g). • Performance needed to be at least comparable to the human eye:
– 5x90° saccades per second

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Performance Testing
• We achieved:
• Tilt: 600°.s-1 @ 18000°.s-2 with 0.01° and a saccade rate of 5Hz. • Vergence: 800°.s-1 @ 20000°.s-2 with 0.01° and a saccade rate of 6Hz.

• This means:
• Better performance than the human eye. • Better performance than ESCHeR. • Marginally inferior to the Agile Eye.
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Control
• Extends work by Murray et al. (1992) on Trapezoidal Profile Motion (TPM).
• In theory TPM can achieve optimal saccade and smooth pursuit with one compact algorithm.

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Control
Position profiles for saccade and smooth pursuit.

Velocity profiles for saccade and Smooth pursuit.

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Control
Two successive real-time saccades

Smooth pursuit of a 0.5Hz sinusoid, sampled at 4Hz.

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Future Work
• Applications:
• Tracking with ZDF and Optic Flows • Face/Feature tracking algorithms

• Hardware Improvements:
• A pan neck

• Future Prototypes:
• Use of plastics and other polymers • Fick configuration
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Conclusions
• CeDAR is a fast, accurate 3DOF stereo head. • CeDAR can carry relatively heavy payloads. • CeDAR’s controller is compact and simple and can implement both optimal saccade and smooth pursuit from the same algorithm.
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