SEMESTER PROJECT Design and implementation of a force/torque sensor - - PowerPoint PPT Presentation

SEMESTER PROJECT

Design and implementation of a force/torque sensor for a quadruped robot

Nicolas Sommer, master MT 20/06/2011 Supervisers : Alexander Spröwitz Rico Möckel

• Prof. :

Auke Jan Ijspeert

PRESENTATION OUTLINE

Introduction Cheetah/Oncilla and CPGs : sensory feedback Electronics Sensor design Simulation of sensor’s deformation Results Conclusion/questions

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INTRODUCTION

Two-parts semester project :

• Analyze and possibly correct previous project on the

Roombots

• Develop a multi-axis sensor for the Cheetah-robot

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Roombots Old Cheetah prototype

CHEETAH AND CPGS: INTEGRATION OF SENSORY FEEDBACK

• Cheetah + CPG : Gait up to 1m/s [1]
• It is possible to use sensory information in a CPG so that the
• scillator is better coupled with the mechanical system [2]
• 6 axis sensor gives the GRF  correlate with phase of the gait
• Enables to detect collision

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[1] A. Tuleu, A. Sproewitz, M. Ajallooeian, P. Loepelmann, and A. J. Ijspeert. Exploiting Compliance with a Cat-sized Quadruped Robot for Trot Gait Locomotion. *Biomechatronics, TU-Ilmenau, Germany. Biorobotics Laboratory, EPFL, Lausanne, Switzerland. [2] L. Righetti and A. J. Ijspeert. Pattern generators with sensory feedback for the control of quadruped locomotion. Proceedings of the 2008 IEEE International Conference on Robotics and Automation (ICRA 2008), Pasadena, May 19-23, 2008.

DATA ACQUISITION ELECTRONICS

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3 software interfaces :

• PC :
• Matlab
• dsPic :
• UARTPC
• SPIADC

Debugging the SPI communication Clock Chip select Data out (SDO) Data in (SDI)

Digital 24-bits values

COMPUTER (Matlab)

dsPIC (microcontroller)

Analog-Digital Converter

COM PORT

SPI USB/serial (UART)

Wheatstone bridges

about |DC|<200mV Analog voltage Formatted Values, home- made protocol

SENSOR DESIGN FOR QUADRUPED ROBOT

Hypothesis : 1. Three forces between foot and floor (GRF) 2. No moments transmission. 3. Position of contact (below: A) varies and position of the foot unknown

𝒰

𝑔𝑚𝑝𝑝𝑠→𝑔𝑝𝑝𝑢 =

𝐺

𝑦

𝐺

𝑧

𝐺

𝑨

0 𝑩 = 𝐺

𝑦

𝑁𝑦,𝐶 = 𝐺

𝑧 ∗ 𝑨𝐵𝐶 − 𝐺 𝑨 ∗ 𝑧𝐵𝐶

𝐺

𝑧

𝑁𝑧,𝐶 = −𝐺

𝑦 ∗ 𝑨𝐵𝐶 + 𝐺 𝑨 ∗ 𝑦𝐵𝐶

𝐺

𝑨

𝑁𝑨,𝐶 = 𝐺

𝑦 ∗ 𝑧𝐵𝐶 − 𝐺 𝑧 ∗ 𝑦𝐵𝐶 𝑪

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A B

yAB zAB . z y x

Useful to define dimensions and to compute forces in A from B Third segment and foot schematic A : contact between foot and floor B : position of the sensor (third leg segment)

Foot

SENSOR DESIGN FOR QUADRUPED ROBOT (2)

• Choice of a 6-axis sensor
• No restriction on the location of the contact
• Simplicity of the design
• Design inspired from robot’s finger 6-axis sensor [1]
• Small, already tested and very precise
• Same range of forces
• One Wheatstone bridge sensitive to only one force/moment

component by design

[1] G-S Kim 2004 Development of a small 6-axis force/moment sensor for robot’s fingers.

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SENSOR DESIGN FOR QUADRUPED ROBOT (3)

• Sensitivity and resistance considerations to determine the

dimensions

• Specifications on minimum sensitivity for each component
• Maximum forces admissible

 Reduce stress concentrations

• Third part for a better access to glue the gauges.
• Rounded edges to avoid stress concentrations
• Attachement points at the extremities

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Prototype parts

DEFORMATIONS FROM FORCES

Each picture shows the deformation along the axis of the drawn gauges

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Gauges

DEFORMATIONS FROM MOMENTS

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Values sampling done in the middle of each gauge but tested on larger surfaces and little difference.

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SIMULATION RESULTS

1,93 mV

• 15
• 10
• 5

5 10 15 20 B1 B2 B3 B4 B5 B6 Output Voltage (mV)

Bridges-to-force voltage

Fx=1N Fy=1N Fz=1N

• 600
• 400
• 200

200 400 600 B1 B2 B3 B4 B5 B6 Output Voltage (mV)

Bridges-to-torque voltage

Mx = 1N.m My = 1N.m Mz = 1N.m

• Each bridge (from B1 to B6) measures one force/moment component only
• Good output ratios
• Low sensitivity on Fz
• Still a Fz resolution of about

𝟐 𝟓 𝑶𝒇𝒙𝒖𝒑𝒐 (25g) with 0,5mV reading precision

Full bridge configuration - gauge factor K = 150 - Bridge Vin=5V

𝑊

𝑝𝑣𝑢 = 𝑊 𝑗𝑜 ∗ 𝐿 ∗

∆L 4

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SIMULATION RESULTS

1,93 mV

• 15
• 10
• 5

5 10 15 20 B1 B2 B3 B4 B5 B6 Output Voltage (mV)

Bridges-to-force voltage

Fx=1N Fy=1N Fz=1N

• 600
• 400
• 200

200 400 600 B1 B2 B3 B4 B5 B6 Output Voltage (mV)

Bridges-to-torque voltage

Mx = 1N.m My = 1N.m Mz = 1N.m

• Each bridge (from B1 to B6) measures one force/moment component only
• Good output ratios
• Low sensitivity on Fz
• Still a Fz resolution of about

𝟐 𝟓 𝑶𝒇𝒙𝒖𝒑𝒐 (25g) with 0,5mV reading precision

Full bridge configuration - gauge factor K = 150 - Bridge Vin=5V

𝑊

𝑝𝑣𝑢 = 𝑊 𝑗𝑜 ∗ 𝐿 ∗

∆L 4

EXPERIMENTAL SETUP

• Half-bridges  Time constraint - Only drawback = sensitivity
• Only Fx and Mz
• Acquisition with Labview

EXPERIMENTAL TESTS

44 mV 41 mV 5 10 15 20 25 30 35 40 45 50

Fx=10N

Output voltage (mV)

Bridge n°1 Output voltage (mV)

Simulation Experiment

180 mV 172 mV 20 40 60 80 100 120 140 160 180 200

Mz=1N.m

Output voltage (mV)

Bridge n°6 Output voltage (mV)

Simulation Experiment

• Good results
• Fx error < 7%
• Mz error < 5%

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Linearity not tested yet, possible causes :

• Gauges non-linearity, specified values :
• Better than ±0.25% to 600 µm/m (

∆𝑀 𝑀 < 6E-4 )

• Better than ±1.5% to 1500 µm/m (

∆𝑀 𝑀 < 1.5E-3 )

• Elastic limit reached  much higher deformation

~180mV

CONCLUSION

• Design of a 6-axis force/moment sensor that fits in the

robot’s leg’s third segment and is lightweight (~15g)

• Separation of the forces/moments components on

each bridge and good output ratios

• First experimental results match simulations well

(5 and 7%)

• Improvements : Complete electronics, run resistance

tests, increase FEA part precision, improve test setup

• Next work : Make use of the data

QUESTIONS

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