1. Vítor M. F. Santos1 Filipe M. T. Silva2
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 Engineering
2 Department of Electronics and Telecommunications
University of Aveiro, PORTUGAL

2. Overview Introduction Initial considerations Mechanical conception
Actuators, power and batteries
Servomotor issues
Sensorial issues and force sensors
The control architecture
Some preliminary results
Conclusions and open issues

3. Project framework Motivation Why not a commercial platform?
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

4. 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

5. 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 considerations:
Robot dimensions
Mobility skills
Level of autonomy

6. The needed DOFs Total proposed: 22 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

7. 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

8. Mechanical conception
Final platfrom
3D model with 600+ components and 22 DOFs
Upper hip
Head base
Lower hip
Neck
Upper leg
Shoulder
Trunk
Lower leg
Arm
Trunk joints
Ankle
Forearm
Hip
Foot

9. 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)

10. 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

11. 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 373765 (mm3)

12. 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…

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

14. Envisaged sensorial capabilities
Vision unit (on the head)
Gyroscopes for angular velocity
GYROSTAR ENJ03JA from MURATA
Accelerometers for accelerations and inclinations
Potentiometer for position feedback
(HITEC Motor)
ADXL202E from
ANALOG DEVICE
Motor electric current
Serial power resistor
Sensitive feet
Strain gauges on a slightly compliant material

15. 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

16. Force sensors and motor connection
Servomotor
PIC local board +
Electric conditioning
Foot sensor
Servomotor reacts to differences on sensors located on the edges of the foot

17. Control system architecture
Main Control
RS232
Master
CAN BUS 1 2 3
Slaves
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 baud)

18. 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

19. 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

20. 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.)

21. Examples of piggy-back boards
Accelerometers
Strain gauges conditioning
Serial COM for master

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

23. Low cost... How Low? Servomotors Miscellaneous electronic components
Big size: ~50 € x 14 -> 700 €
Smaller size: ~30 € x 8 -> 240 €
Miscellaneous electronic components
Total -> ~300 €
Aluminium gears and belts
Batteries
~80 € x 4 -> ~320€
Sensors (except camera)
Negligible (<100€)
Raw materials (steel, aluminium)
Total ~ €2000
Excluding manufacturing and development costs (software, etc.)
Still missing:
Vision unit, central control unit (PC104+), lots of software...

24. 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
...

25. 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)

26. 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.

27. 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 …
Tel:
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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:
Fax: