vítor m. f. santos 1 filipe m. t. silva 2

Centre for Mechanical Technology and Automation

Institute of Electronics Engineering and Telematics

TEMA

IEETA http://www.mec.ua.pt/robotics

Engineering Solutions to Build an Inexpensive Humanoid Robot Based on a Distributed Control Architecture

Vítor M. F. Santos1

Filipe M. T. Silva2

1 Department of Mechanical Engineering2 Department of Electronics and Telecommunications

University of Aveiro, PORTUGALUniversity of Aveiro, PORTUGAL

UNIVERSITY OF AVEIRO, PORTUGALCentre for Mechanical Technology and AutomationInstitute of Electronics Engineering and Telematics

Overview Introduction Initial considerationsMechanical conceptionActuators, power and batteriesServomotor issuesSensorial issues and force sensorsThe control architectureSome preliminary resultsConclusions and open issues


Project framework Motivation

Develop a humanoid platform for research on control, navigation and perception.

Offer opportunities for under & pos-graduate students to apply engineering methods and techniques

The utopia of Man to develop an artificial being with some of its own capabilities…

Why not a commercial platform? Versatile platforms imply prohibitive costs! Reduces the involvement at lowest levels of machine

design Current status

So far, it is only a development engineering approach The platform prototype already performs some motion Early studies on control just began


To build or to buy? Several commercial platforms already exist:

Only a few offer great versatility, DOFs, possibilities of control, …

Good platforms (e.g., Fujitsu) have high costs (tens of thousands of Euros); others are not even for sale

Commercial platforms favour mainly high level software development

Developing a platform from scratch allows using hardware more oriented to the desired approach: Distributed control, special sensors, alternative central units …

Developing a platform from scratch takes longer, but hopefully can be done at lower costs…

Sony QRIO Fujitsu Sumo Bot

ZMP Nuvo

Kawada HRP1S

Honda Asimo


Initial Considerations Main Objectives

Build a low-cost humanoid robot using off-the-shelf technologies, but still aiming at a fully autonomous platform

Have a working prototype capable of participating in the RoboCup humanoid league (Germany2006)

Design Concerns Consider a distributed control architecture due to

the expected complexity of the final system Assume modularity at several levels to ease

development and scalability Provide rich sensorial capabilities

Initial design Initial design

considerationsconsiderations::− Robot dimensionsRobot dimensions− Mobility skillsMobility skills− Level of autonomyLevel of autonomy

Initial design Initial design

considerationsconsiderations::− Robot dimensionsRobot dimensions− Mobility skillsMobility skills− Level of autonomyLevel of autonomy


The needed DOFs 6 DOFs per leg

Universal joint at the foot (2 DOFs) Simple joint on the knee (1 DOF) Spherical joint on the hip (3 DOFs)

• Total DOFs for the Legs = 2 x 6 = 12

Trunk with 2 DOFs To envisage better balance control

3 DOFs per arm without hand and wrists Universal joint on the shoulder (2 DOFs) Simple joint on the elbow (1 DOF)

• Total DOFs for the arms = 2 x 3 =6

Neck/head accounts for 2 DOFs To support a camera for vision perception

Total proposed: 22 DOFs


Kinematics simulations Four Denavit-Hartenberg open

kinematics chains: One leg on the floor up to the

opposite foot not on the ground Starting on the hip, a 2nd chain

goes up to the neck. A 3rd and 4th chain for left and

right arms. Allows the analysis of:

Static torques Path of CoM and its projection on

the ground Opens the way to simulate:

Dynamics in higher speeds ZMP paths

Inputs for the model in Matlab: Links’ DH parameters Links’ masses Links’ centers of mass Path planning at joint level


Mechanical conception

Foot

Ankle

Lower leg

Upper leg

Lower hip

Upper hip

Hip

Trunk joints

Trunk

Neck

Head base

Shoulder

Arm

Forearm

Final platfrom

3D model with 600+ components

and 22 DOFs


Summary of mechanical properties Complete humanoid model

22 degrees of freedom Weight - 5 kg Height - 60 cm Max width - 25 cm Foot print - 20 8 (cm2)

Materials used for the body and accessory parts Aluminium (2.7 g/cm3) Bronze (8.9 g/cm3) Steel (7.8 g/cm3) Nylon (1.4 g/cm3)


Actuators

Static (and some simplified dynamic) simulations were carried out to estimate motor torques in a simulated step

Best low cost actuators in the market are Futaba RF servos or similar (HITEC,…).

Available models best suited for our application are:

Application Model Mass (g) Torque (Nm)

Arms & small torque joints HS85BB ~20 0.35

Legs & high torque joints HS805BB 119 2.26

Additional mechanical issues for motors Use gear ratios up to 1:2.5 to rise torques Use tooth belt systems for easier tuning Use ball bearings and copper sleeves to

reduce friction


Power requirements and batteries Motors

Max current: 1.2 – 1.5 A per motor (big size model)

Electronics and control Estimated to less than 200 mA per

board with a total of ca. 1.5 A. Voltage Levels

5 V for logic; 6.5 V for motors Two ion-lithium batteries were

installed (from Maxx Prod.) 7.2 V/9600 mAh per pack Maximal sustained current of 19A Each pack weights circa 176g Confined to a box of 373765 (mm3)


Servomotor velocity “control”

Servomotors have an internal controller based on position User cannot directly control

velocity! Either replace motor own

control electronics or do some software tricks

Example: Two similar motors with different velocities Dynamic PWM generation Stepped target points Without load, open-loop and

feedback based actuation give similar results…


First results with one leg in motion Simple open loop actuation of the leg joints PWMs generated by dedicated boards (shown further on)


Envisaged sensorial capabilities

Accelerometers for Accelerometers for accelerations and inclinationsaccelerations and inclinations

Gyroscopes for angular velocity Gyroscopes for angular velocity

Potentiometer for Potentiometer for position feedbackposition feedback

(HITEC Motor)(HITEC Motor)

GYROSTAR ENJ03JA GYROSTAR ENJ03JA from MURATA from MURATA

ADXL202E fromADXL202E fromANALOG DEVICE ANALOG DEVICE

Vision unit (on Vision unit (on the head)the head)

Motor electric currentMotor electric current

Serial power Serial power resistorresistor

Sensitive feet Sensitive feet

Strain gauges on a Strain gauges on a slightly compliant slightly compliant materialmaterial


The sensitive foot A device was custom-made using strain gauges

properly calibrated and electrically conditioned Four strain gauges arranged near the four corners of the foot

Strain Gauge

Flexible beam

Foot base

Adjustable screw


Force sensors and motor connection

Foot sensor

PIC local board +Electric conditioning

Servomotor

Servomotor reacts to differences on sensors located on the edges of the foot


Control system architecture

Distributed control system A network of controllers connected

by a CAN bus A master/multi-slave arrangement Each slave controller is made of a

PIC device with I/O interfacing.

Asynchronous communications Between master and slaves: CAN

bus at 1 Mbit/s Between master and high level

controller (currently serial RS232 at 38400 baud)

Main ControlMain Control

RS23RS2322MasteMaste

rr

CAN CAN BUSBUS

1

23 1

2

3

1

2

31

2 1

2

3

1

2

3

1

2

3

1

2

SlaveSlavess


Functions of the control level units Main control unit

Global motion directives; high level planning. Vision processing Interface with possible remote hosts

Master CAN controller Receives orders to dispatch to the slaves Queries continuously the slaves and keeps the

sensorial status of the robot• Currently does it at ca. 10 kHz

Slave CAN controllers Generate PWM for up to 3 motors Interface local sensors Can have local control algorithms


The set of local controllers 7 slaves controllers for joints and sensors 1 master controller

Interfaces slave controllers by CAN Interfaces upstream system by RS232


Local control boards All master and slave

boards have a common base upon which a piggy-back unit can add I/O (sensors, additional communications, etc.)

Power resistor (0.47)

16:1 multiplexer

CAN connector

Piggy-back socket

PIC Cristal oscillator

CAN driver

PIC

Unit CAN Address

PWM plugs

Servo fuse

Fuse status LED

Piggy-back board 2

Piggy-back board 1

Connector to sensor

CAN bus Power plug

Power regulator Reset button

RS232 plug

Connector to sensor


Examples of piggy-back boards Accelerometers

Dual accelerometer

Dual amplifier

25 mm Strain gauges

conditioning Serial COM for master


First humanoid motion The robot is able to stand, lean on sides, for/backward Primitive locomotion motions have been achieved


Low cost... How Low? Servomotors

Big size: ~50 € x 14 -> 700 € Smaller size: ~30 € x 8 -> 240 €

Miscellaneous electronic components Total -> ~300 €

Aluminium gears and belts Total -> ~300 €

Batteries ~80 € x 4 -> ~320€

Sensors (except camera) Negligible (<100€)

Raw materials (steel, aluminium) Negligible (<100€)

Total ~ €2000 Excluding manufacturing and

development costs (software, etc.) Still missing:

Vision unit, central control unit (PC104+), lots of software...


On-going and open issues Next concerns for the platform

Joint position feedback from dedicated sensor (not servo’s own!)

Safety issues to automatic cut of power on controller failure

Better adjustable tensors for belts Selection and installation of central

control unit (Embedded Linux) Selection and installation of the vision

unit (FireWire..?) Research concerns

Localized/distributed control algorithms Elementary Gait definition ...


Hints for local control The difficult relation between planning and

stability opens the way to localised control Minimal dependence on planned variables Better adaptation to changing conditions (e.g., load, ground)

Force-based perception is the key issue:• Reaction forces• Joint torques / motor currents

What local control can do Accept a global directive and act locally based on an

associated rule. Example:

• Top order: keep standing immune to perturbations• Local rules: try to actuate the joints you control in order to

keep force balance (e.g., try to have a distribution of forces as uniform as possible on the foot area)


Conclusions A highly versatile platform is possible to be built with

constrained costs and off-the-shelf components. The distributed control architecture has shown several

benefits: Easier development Easier debugging Provides modular approaches

• The generic local controller using piggy-back modules is a confirmation of the modularity

Local controller capabilities include the possibility of localised control based simply on local perception and global directives.

A prototype system has been built and the selected technological solutions ensure a platform for research

A huge field of research issues can be addressed, mainly on control, perception and other autonomous navigation matters.


Author’s Short Biography

Vítor Santos is Associate Professor at the Department of Mechanical Engineering of the University of Aveiro He received his PhD from the University of Aveiro in 1994 …

His research interests include …

Filipe Silva is Assistant Professor at the Dept. of Electronics and Telecommunications of the University of Aveiro He received his PhD in Electrical Engineering from the University of Porto

in 2002; modelling and control of biped locomotion systems

His main research interests are centred in the areas of Humanoid Robotics and Healthcare Robotics

Tel: +351 234 370 531 Email: [email protected]: +351 234 370 545 http://www.ieeta.pt

Tel: +351 234 370 828 Email: [email protected]: +351 234 370 953 http://www.mec.ua.pt

vítor m. f. santos 1 filipe m. t. silva 2

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