Mechatronic design of NAO humanoid
David Gouaillier, Vincent Hugel, Pierre Blazevic
Chris Kilner, J´
ome Monceaux, Pascal Lafourcade, Brice Marnier, Julien Serre, Bruno Maisonnier
Abstract— This article presents the mechatronic design of the
autonomous humanoid robot called NAO that is built by the
French company Aldebaran-Robotics. With its height of 0.57 m
and its weight about 4.5kg, this innovative robot is lightweight
and compact. It distinguishes itself from existing humanoids
thanks to its pelvis kinematics design, its proprietary actuation
system based on brush DC motors, its electronic, computer and
distributed software architectures. This robot has been designed
to be affordable without sacriﬁcing quality and performance.
It is an open and easy-to-handle platform. The comprehensive
and functional design is one of the reasons that helped select
NAO to replace the AIBO quadrupeds in the 2008 RoboCup
Why build another humanoid robot? This introduction
will answer this question by describing the guidelines the
French company Aldebaran-Robotics 1 followed in the
mechatronic design and making of the NAO humanoid robot
(ﬁgure 1). These guidelines are affordability, performance,
Affordability means that the robot should be made avail-
able to the maximum of people who would like to work
or play with a performant biped robot. Current functional
humanoids are somewhat expensive (see table I). Even the
HOAP small-sized humanoid robot from Fujitsu costs about
50KUS $. The NAO robot has been devised with the
concern of cost reduction without sacriﬁcing quality and
performance. It is a completely custom designed robot as
the whole process of design and manufacturing is mastered
(mechanics, electronics, software). The target price of NAO
will be about 10K euros for academics. Thanks to mass
production and reduction of functionalities a version will be
publicly available for approximately 4K euros.
A biped robot must show good motion performance rela-
tive to its height to weight ratio or body mass index (BMI).
Table I gives the BMI for different functional humanoid
robots. NAO has a BMI of about 13.5[kg/m2], which means
that it is very light compared to other existing robots of
the same height. Compared to a heavy robot, a lightweight
robot means smaller and less powerful motors, less thermal
dissipation, larger acceleration range, and better dynamic
capabilities. Due to their reduced weight, lightweight robots
are less dangerous and less subject to breakdown.
D. Gouaillier, V. Hugel and P. Blazevic are with the Engineering System
Laboratory, University of Versailles, 10/12 avenue de l’Europe, 78140
All other authors are with the Aldebaran-Robotics, 168 rue Raymond
Losserand, 75014 Paris, France
1Aldebaran-Robotics is a French company founded in 2005 by chief
executive Bruno Maisonnier.
Fig. 1. Aldebaran-Robotics NAO humanoid.
Fig. 2. Detailed kinematics of NAO. Wrist joint not represented.
Biped robots are built by robotics research teams to
focus on a particular research subject. One recurrent subject
is the development of efﬁcient walking gaits, like in the
case of the ETL-humanoid , the BIP biped , the 2D
Rabbit biped , or the Johnnie and LOLA humanoid
robots , , . Other subjects concern the design
of artiﬁcial limbs for disabled people, like the Robian biped
, or the introduction of natural dynamics through the
use of ﬂexible actuators, like the series of robots designed
at MIT . All these robots were not devised to be fully
autonomous in terms of energy, neither operational. Other
2009 IEEE International Conference on Robotics and Automation
Kobe International Conference Center
Kobe, Japan, May 12-17, 2009
978-1-4244-2789-5/09/$25.00 ©2009 IEEE 769
CHARACTERISTICS OF FUNCTIONAL HUMANOIDS.
Height (h) Weight (w) BMI Price
(m) (kg) (kg/m2)
KHR-2HV 0.34 1.3 10.9 1K US $
HOAP 0.50 7.0 28.0 50K US $
NAO 0.57 4.5 13.5 10K euros
QRIO 0.58 6.5 19.0 NA
ASIMO 1.30 54.0 32.0 NA
REEM-A 1.40 40.0 20.4 NA
HRP-2 1.54 58.0 24.5 400K US $
(5 year lease)
Human 1.5-2 50-100 18-25 NA
BMI: body mass index = w/h2, NA: not available
robots were designed in collaboration between researchers,
engineers and manufacturing companies to reach these ob-
jectives. Among performant robots the Asimo humanoid
built by Honda may be the most impressive , . It is
capable of walking fast, up to 3[km/h]forward, change
direction and walk up/down stairs smoothly. The HRP-2 and
HRP-3 robot manufactured by Kawada Industries are also
advanced technological achievements , , , . HRP-
2 can walk up to 2.5[km/h], and can lie down and get up
again by itself. These Japanese robots can be considered
energetically autonomous and capable of achieving a wide
range of movements. NAO was designed to perform smooth
walking gaits, even when changing speed and direction. The
walking speed must be similar to the walking speed of 2 year
old children of the same size, that is about 0.6[km/h]. The
performance targets for NAO also include the capability of
performing a rich panel of movements with smoothness and
precision, and a certain degree of interactive autonomy.
Modularity is the third guideline followed by the French
designers of NAO. Firstly, modularity refers to actuator
modules that could be used for different joints. Secondly,
the modular design of the robot’s limbs is also very useful
to promote further evolution. The head of NAO can be easily
unplugged and replaced by a more specialised one. Hands
and forearms can also be changed. Thirdly the problem of
maintenance is not negligible. Since NAO will be for sale
on a large scale, its maintenance must be optimized so that
spare parts can be changed quickly.
This paper describes the kinematics, the dimensioning of
leg actuators, the modular design of the actuator units and
the computer-electronic architectures.
Table III gives the main characteristics of the NAO hu-
manoid. NAO has a total of 25 degrees of freedom, 11
degrees of freedom (DOF) for the lower part that includes
legs and pelvis, and 14 DOF for the upper part that includes
trunk, arms and head.
Each leg has 2 DOF at the ankle, 1 DOF at the knee and
2 DOF at the hip. A special mechanism composed of two
coupled joints at each hip equips the pelvis. The rotation axis
SEN SOR S TH AT EQU IP NAO.
30 FPS CMOS videocamera 1
Magnetic rotary encoder (MRE) 34
Infrared sensor (emitter/receiver) 2
Ultrasonic sensor 2
MAIN CHARACTERISTICS OF THE NAO HUMANOID.
Height (m) 0.57
Weight (kg) 4.5
Capacity 55 Wh
Degrees of freedom (DOF): 25
Head 2 DOF
Arms 5 DOF X 2
Pelvis 1 DOF
Leg 5 DOF X 2
Hands 1 DOF X 2
Upper Arm 163
Lower Arm 87
of these two joints are inclined at 45◦towards the body. This
mechanism replaces the classical set of three active rotary
joints encountered in most humanoid robots (see ﬁgure 3):
the horizontal axis rotary joint at the waist and the rotary
joints of vertical axis for each leg hip. The coupling of
the pelvis joints prevents the trunk from rotating along the
vertical axis (yaw rotation) when both legs are in support, but
this is not a problem for walking and other motion behaviors.
The proposed mechanism presents several advantages. Only
one motor is required to drive the pelvis instead of three in
the classical design, this allows to reduce building cost and
to save space in the lower part of the trunk. In addition, this
structure helps to better distribute the power between the hip
roll joint and the pelvis joint, and confers a speciﬁc motion
style to the NAO humanoid. Unlike prototypes such as H7
, Wabian-2R  or LOLA , the foot sole of the
present version of NAO does not feature any passive or active
joint that would enhance higher speed gait performances
In addition, each arm features 2 DOF at the shoulder, 2
DOF at the elbow, 1 DOF at the wrist and 1 additional DOF
for the hand’s grasping. The head can rotate about yaw and
pitch axes. Figure 2 gives the kinematics details, and table
IV lists the joints with their range.
Fig. 3. Left-hand side: classical set of three rotary joints, one horizontal
axis rotary joint at the waist and two vertical axis rotary joints for the legs.
Right-hand side: coupled inclined axis rotary joints for the NAO pelvis.
JOINTS TYPE,RANGE,AN D ACT UATOR TY PE .
Part Motion Range (◦) Actuator type
hip twist (45◦)-68 to 44 M1R11
hip roll -25 to 45 M1R11
hip pitch -100 to 25 M1R12
knee pitch 0 to 130 M1R12
ankle pitch -75 to 45 M1R12
ankle roll -45 to 45 M1R11
shoulder roll 0 to 95 M2R22
shoulder pitch -120 to 120 M2R21
elbow roll -120 to 120 M2R22
elbow yaw 0 to 90 M2R21
Head yaw -90 to 120 M2R21
pitch -37 to 31 M2R22
III. DIMENSIONING OF LEG ACTUATORS
Some humanoid designers developed an iterative process
for the mechanical design and ran dynamic simulations to get
parameters of joint torques and velocities that were used for
motor and gear selection . The design process of NAO is
not iterative and relies on a dimensioning methodology. The
robot’s model is simpliﬁed and simulated dynamically in the
sagittal plane and in the frontal plane. The software used
for simulation was WorkingModelTM linked with Matlab-
SimulinkTM for the control part. A set of basic movements in
the sagittal plane for the one part, and in the frontal plane for
the other part, were deﬁned and simulated. Movements in the
sagittal plane helped dimension the knee actuators and the
pitch joint actuators of hips and ankles. Movements in the
frontal plane helped complete the dimensioning of hip and
ankle roll joint actuators. The duration of each movement
was set for the robot to achieve lively motion. Motor and
gear selection must ensure that the robot will be capable of
achieving these movements.
The robot’s model in the sagittal plane is composed of a
rectangular trunk and a single leg with rectangular femur,
tibia and foot. The model in the frontal plane features
both legs. Hip, knee and ankle joints are rotary joints. In
the frontal plane knee joints are blocked. The set of basic
movements in the sagittal plane are the following:
1) knee ﬂexion on the spot, 1[sec]
2) standing up from ﬂexed knee position, 1[sec],
3) leg transfer during walking step, body motionless,
4) body translation in the direction of motion during leg
5) body sinusoidal movement in the direction of motion
during leg stance, 1/3[sec](ﬁg. 4),
6) simultaneous knee ﬂexion and body forward bending
for pick up, 1.2[sec](ﬁg. 4).
For basic movements 3 and 4, the target walking velocity
can be taken into account by changing the duration of the
Fig. 4. Sagittal plane simulation experiments of body sine move and pick
Fig. 5. Frontal plane simulation experiments of sideways move and leg
lift-off in lopsided position.
The set of basic movements in the frontal plane are the
1) lopsided move on both legs, 0.5[sec](ﬁg. 5),
2) leg lift off in lopsided position , 0.5[sec](ﬁg. 5),
All motors are controlled using a PID law. Velocity and
output torque are recorded for all the movements. Actuator
power is also recorded. For each joint the data relative to
all the simulated movements are grouped to get the velocity
variations as a function of torque over all the movements.
Then the convex envelop is calculated. Figure 6 presents
the curves of speed vs torque relative to the knee joint for
experiments 1 to 6 in the sagittal plane. The convex envelope
is also represented.
The rpm versus torque speciﬁcations of off-the-shelf mo-
tors were compared with the desired variations to select
the best suited motors thanks to a custom designed and
interactive graphical interface. Figure 7 shows the speed vs
torque speciﬁcations of two motors, one from MaxonTM and
the other from MabuchiTM. The convex envelopes are relative
to the knee, hip, and ankle pitch joint data drawn from the
family of experiments simulated in the sagittal plane. This
study is very helpful to select the best motor that can be used
for several joints.
Fig. 6. Speed versus torque of knee actuator relative to experiments 1 to
6 in the sagittal plane.
Fig. 7. Software used to choose motor and reduction ratio for the hip,
knee and ankle pitch actuators.
In the process of dimensioning it is interesting to check
the load to motor inertia ratio k.
where Nis the reduction ration, JMis the motor inertia,
and JLis the load inertia.
A too high value of load to motor inertia can cause instabil-
ities and oscillations due to resonance. The more compliant
the system, the more subject to oscillation it will be. A low
value of the load to motor inertia leads to easier control and
better dynamic response (fast acceleration/deceleration), but
reduces the bandwidth of the system. Therefore the load to
motor inertia results from a trade-off. Whatever the value of
k, the most important parameter for actuators is the stiffness
of the gear mechanism. If the actuators are stiff enough,
high values for the load to motor ratio are acceptable, as the
control module can deal with close loop corrections. Taking
into account that the drive mechanism is made of ﬂat plane
gears, the load to motor inertia ratio should be comprised
between 1 and 10. Since the load varies when the robot
walks, the ratio will vary inside this range.
Let us check the load to motor inertia ratio at the knee
joint in the case of simple support and in the case of double
support. The mass seen above the knee is the mass of the
robot’s upper part and the masses of the thighs, that is
approximately M= 3.2[kg], see table III. For the robot’s
lower part, the actuators are based on motor M1 whose
inertia is Jm= 4.17.10−7[kg.m2], and reduction ratio N
is 130.85. The distance dof the knee joint to the center
of mass is around 0.15[m]in the standard upright position.
In the case of double support, we consider that each knee
supports half of the mass M,
In the simple support phase, the load inertia must take into
account the mass of the robot’s upper part, the thigh of the
supporting leg and the mass of the leg in the air. We assume
that the position of COG does not vary.
The values of load to motor ratio are within the acceptable
IV. MODULAR ACTUATOR UNIT DESIGN
Modularity was also the concern of designers of the LOLA
bipeds  or the QRIO humanoid .
Off-the-shelf RC servomotor modules were used for the
design of most humanoid robots of the kid-size league of
RoboCup. Despite of good performance for some of them,
RC servos were not selected for NAO because the packaging
is generally not suited and bulky, and the gear reduction
mechanism and the joint control system are ﬁxed and cannot
Taking into account that actuators represent the major cost
it is necessary to conduct a careful study of how to choose
and assemble motor, driving mechanism and sensor in the
same module. Fukushima et al.  listed the properties
of a good actuator in their design of the ISA-4 –Intelligent
Servo Actuator – for the SONY SDR-4X robot. The actuator
must be compact, lightweight, highly back-drivable, efﬁcient,
precise and reliable. Backdrivability was studied by , it
deﬁnes the facility of movement transmission from output to
input axis. The performance criteria of an actuator are power
over weight ratio, temporary high torque generation, band-
width, and response time. In the case of NAO the response
time must be less than 6[ms]. This value was obtained from
speciﬁcations of joint maximal angular velocity and maximal
angular deviation at the ankle for motions that involve one
leg in support.
Off-the-shelf motors for legged robots do not exist. Hu-
manoid designers often use existing brush DC motor ,
Fig. 8. CAO design of ankle module. There is a double supporting structure
for the pitch joint. The half of this structure is represented in transparency in
the right hand-side ﬁgure but is not represented in the left-hand side ﬁgure.
, , brushless DC motor , or proprietary motors
. Brushless DC motors present a better power density,
higher torque and speed bandwidths compared to brush
DC motors. However the electronics is more complex and
therefore more expensive. The motors used for the NAO
actuators are MaxonTM coreless brush DC motors, that are
known for precision and reliability.
Even though harmonic drives are widely used for human-
size humanoids , they were not selected because they
remain expensive, and there were not many providers. In
addition, off-the-shelf harmonic drives do not present enough
backdrivability  and can therefore be more sensitive to
shocks. In the case of the HRP-2, harmonic drives were
used in conjunction with timing belt and pulley. Taking this
into account the designers of NAO decided to use spur and
planetary gears in order to have a fairly good backdrivability.
This strategy was also adopted for the development of the
ISA actuators . These kinds of gear offer very small
backlash. In addition special plastics loaded with PTFE
(Polytetraﬂuoroethylene) and carbon ﬁber were used to meet
torque and longevity requirements.
Thanks to the dimensioning study the number of actuators
was reduced to 4 for all the joints, with two kinds of motor
(M1, M2), and two types of reduction gear (R11, R12)
(see table IV). The innovation brought to the design of
the NAO actuators consisted of grouping two rotary joints
together to make a Universal joint module that includes
packaging. This allowed costs to be reduced and to take
into account the mechanical constraints imposed by the outer
shell. Thus the same Universal joint module composed of
actuators M2R21 and M2R22 is used for shoulders, elbows
and head. Regarding the lower part of the robot, the same
motor M1 is used for the pelvis joint and all leg joints, the
gear mechanism R11 is the same for pitch joints and the
pelvis joint, and R12 is the same for roll joints. Tables V
give the technical data for these actuators. Figure 8 gives a
CAO view of the ankle joints module.
V. COMPUTER ARCHITECTURE
NAO’s head is equipped with an x86 AMD GEODE
500 MHz CPU motherboard with 256 Mb SDRAM. An
additional 1Gb Flash memory is available. Communication
with the robot is possible through the WiFi 802.11g protocol
ACT UATOR 1S PEC IFI CATI ONS ,MOT OR (M) A ND RE DU CT IO N (R)
Characteristics M1 R11 R12
ratio = 201.3 ratio = 130.85
No load speed 8000 RPM 238.45◦/s 366.83◦/s
Stall torque 59.5 mNm 11.97∗Nm 7.78∗Nm
Nominal speed 6330 RPM 188.67◦/s 290.25◦/s
Nominal torque 12.3 mNm 2.47∗Nm 1.61∗Nm
*: without ratio efﬁciency
Shoulder: 2 motors + MRE
Inertial sensor, 3 acc. + 2 gyros
LEDS, 16 RGB
RS 232 port
Head: 2 motors + MRE
Elbow: 2 motors + MRE
Wrist: 1 motor + MRE
Hip: 1 motor+ MRE
Pelvis: 1 motor + MRE
Knee: 1 motor + MRE
Ankle: 2 motors + MRE
x2 4 FSR RGB LED
Hand: 1 motor + MRE
Hip: 1 motor+ MRE
Hip: 1 motor+ MRE
Fig. 9. Electronic architecture of NAO. MRE stands for magnetic rotary
and through Ethernet port. The CPU manages audio, video,
and WiFi and other advanced modules. One ARM7-60MHz
microcontroller located in the torso distributes information to
all the actuator module microcontrollers (Microchip 16 bit
dsPICS) through a RS485 bus (throughput of 460[Kbits/s]).
There are two RS485 buses, one that connects the ARM7
microcontroller to the dsPICS modules of the upper part
of the body, and the other that connects the ARM7 to the
dsPICS modules of the lower part of the body. This bus
partition permits to increase the data throughput.
The ARM-7 microcontroller communicates with the CPU
board through a USB-2 bus with a theoretical throughput of
11[Mbits/s]. It can be used to control the robot’s stability
using the inertial unit. The operating system is based on
Linux, but the whole system can be modiﬁed.
Custom designed integrated circuits based on Microchip
16 bit dsPICs microcontrollers were designed to control the
actuators. These circuits are responsible for servo-control,
bus control, sensor management, and power converters. Each
circuit can drive up to two actuators. Each actuator is
equipped with magnetic rotary encoders (MRE) that yield
absolute outputs. Figure 9 shows the overall electronic ar-
chitecture of the system. Table II gives the list of sensors
that equip NAO. One dsPIC based circuit, connected to the
ARM7 board through I2CTM bus, is devoted to the signal
acquisition from two gyrometers and three accelerometers.
Signals issued from accelerometers and gyrometers can be
combined to get an acceptable feedback of the robot’s trunk
orientation , . Another dsPIC based circuit manages
an infrared transmitter/receiver and a series of LEDS.
The ﬁrst operational prototype of NAO was capable of
reaching a forward walking velocity of 0.36[km/h]with an
open-loop walking algorithm that did not take into account
the inertial system feedback (see video). This velocity is half
the velocity that was speciﬁed for the design –0.6[km/h], but
the robot is expected to reach such a target velocity thanks
to the design of closed loop control. The robot is capable
of making turns showing a speciﬁc style due to the pelvis
kinematics (see video). The algorithm was based on a ZMP
cascading procedure inspired from .
This paper presented the design of the small-size and
lightweight humanoid named NAO developed by the French
Aldebaran-Robotics company in collaboration with research
laboratories with objectives of affordability and performance.
NAO presents the following interesting and innovative fea-
•the pelvis is made of two coupled hinge joints inclined
at 45◦driven by a single motor.
•the motorization uses actuator modules that include
custom universal joint, custom gear mechanism, MRE
sensors, and custom servoboard.
The main advantages of NAO are its light weight and its
affordability compared to other existing robots of similar
performance. The maintenance required for this robot is
easy and not excessive. The architecture of control and the
software are customizable. The robot also comes with a rich
environment of development.
A speciﬁc version of NAO was delivered to 16 teams of
the standard RoboCup league in 2008 to play soccer. This
brought very positive feedback to improve the robustness and
the reliability of the robot.
The partners of the NAO project would like to thank the
students of the French design school Creapole for designing
the outer shell of NAO.
The authors would also like to thank the French National
Agency of Research and Technology for allotting a CIFRE
grant to fund a PhD in the frame of this project.
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