Design and control of a small size humanoid robot

Chapter 1

Introduction

It has always been the dreams of many for man to co-exist with humanoid robots,
to live and work in the same environment. Japan, as a leading country in robots and
their applications, has incorporated robotics in their manufacturing industries for
years. However, most of the robots involved are limited to robot arms that are
fixed to the ground and tasks allocated to them are straightforward and repetitive.
The desire to build robots resembling ourselves is reflected in the works of many
researchers in recent years, where a significant focus is placed on building
humanoid robots.
Robotics competitions around the world have also included humanoid category in
recent years, and it is perceived as one of the most challenging groups that would
draw crowds of spectators. Team RO-PE, formed in 2002, by the Mechanical
Engineering Department of National University of Singapore, has also been
playing a part in striving to advance the technology in humanoid robots and it had
participated in the humanoid category of both RoboCup and FIRA, two famous


CHAPTER 1: INTRODUCTION

2

international robotics competitions. RO-PE-V, the fifth humanoid robots built by
Team RO-PE, had represented the team to participate in RoboCup 2006 and 2007,
and had achieved encouraging results. And RO-PE-V is employed as the subject of
this thesis.
This project involved the design and building of a humanoid robot, RO-PE-V.
With RO-PE-V setup as a platform, walking control had been implemented.
Experiments on localization and slope walking were also performed. And these are

presented in this thesis in 8 chapters.
Chapter 1

Introduction – Some background information on the topic

are provided in this chapter, the scope of the thesis is also laid down.
Chapter 2

Literature Review – In this chapter, related works from other

researchers are discussed, reviewing the current state of technology and general
approach in this field.
Chapter 3

Sensors, Actuators and Computer Systems – The important

hardware mounted on the robot are explained in this chapter.
Chapter 4

Mechanical Design – The design philosophy and approach

are presented in this chapter.
Chapter 5

Walking Control – In this chapter, the approach used to

control the walking of the robot is presented.


CHAPTER 1: INTRODUCTION

Chapter 6

3

Slope Walking – Some experiments are done on a simple

approach to slope walking, the logic of this approach will be presented in the
chapter.
Chapter 7

Localization – Localization in a colour-coded environment

(RoboCup Competition) is experimented with the robot as the platform and will be
discussed in this chapter.
Chapter 8

Conclusions and Recommendations – In this chapter,

conclusions to this project and this thesis are given, some recommendations for
further investigation in this topic are also provided.


Chapter 2

Literature Review
The study of humanoid is an interesting field of research which is highly complex
and multi-disciplinary. It had for a long time been only the dream and fantasy of
man, and exists only in science fictions, novels or movies. The earliest engineering
records of humanoid would probably be the design of a humanoid automaton by

Leonardo da Vinci in around year 1495 [30], and it is still unknown whether it was
physically built or just a paper design. This line of research remained largely
unexplored for many years until the last three to four decades. This is primarily due
to the fact that technology at that time, especially in terms of hardware, was still
unable to handle the stringent requirements of humanoid robot, making the topic
extremely difficult to handle.
There is a significant advancement in humanoid research in the last three to four
decades. Waseda University from Japan began their humanoid research in about
1966 and built the world first full-scale humanoid, WABOT-1, in 1973 [1][30].
The interest in humanoid research did not stop at research institutes and
universities and commercial companies also took up the challenge in research. The
pioneers in this field is the Japanese car manufacturing giant, Honda, which began
their research in about 1986. Many versions of humanoid robots had evolved from


CHAPTER 2: LITERATURE REVIEW

5

Honda through the years, with ASIMO being its latest version [5][28]. HRP-2 is
another famous humanoid produced by Kawada Industries Inc. [2][3], and it is able
to cooperate with human to carry some load. Furthermore, it had demonstrated the
ability to get up from a face-down position, which is very challenging given its
height of 158cm.
The three robots mentioned above are relatively larger robots that have heights of
more than 1m. They are expensive and more difficult and dangerous to handle.
Many researchers then turn to scaled-down humanoids, of height of about 50cm,
where in terms hardware, are much more affordable. Qrio from Sony [4] and
HOAP from Fujitsu [40] are two commercial small size humanoids that are
produced a few years back. Though the robots could be for sale, the price is

extremely steep for them to dominate the small size humanoid market.
Realizing the growing interest in humanoid robots, motors manufacturers are
coming up with their own humanoid for sale. URIA from Robotis [32],
ROBONOVA from Hitec [33] and KHR-1HV from Kondo [34] are some of the
relatively low priced humanoid available in the market. RoboSapien [35] is another
budget humanoid built for the toy industry. Because it is meant to be a simple toy,
it does not carry a powerful processor that would make it more ‘intelligent’. In fact,
some researchers use RoboSapien as a walking platform, replacing the processor
with a more powerful one like a PDA for more intensive computation like image
processing and collaboration between robots [24].
International robotics competitions like RoboCup [36] and FIRA [37] had also
called upon researchers from around the world to construct their own humanoid


CHAPTER 2: LITERATURE REVIEW

6

robots. Among them are VisiON NEXTA from Vstone Corporation [38] and
NimbRo from University of Freiburg [39], which had both shown exceptional
performance in competitions. Fig. 2.1 shows the different humanoids seen around
the world today.

(a) WABOT-1 [30]

(d) QRIO [42]

(b) ASIMO [28]

(e) HOAP2 [40]

(c) HRP-2 [41]

(f) URIA [32]


CHAPTER 2: LITERATURE REVIEW

(g) ROBONOVA [33]

(i) RoboSapien [35]

7

(h) KHR-1HV [34]

(j) VisiON NEXTA [38]

(k) Nimbro [39]

Fig. 2.1 Humanoid robots constructed for different purposes.
Though there are already many humanoids built and walking, there are still many
areas in this topics that are not fully covered, and it would still take a lot of effort
and time before these robots could be made to work safely (for both human and the
robots) in an unstructured area. The current research approach in humanoid could
broadly be classified into three areas, (1) mechanical design and hardware, (2)
walking control and (3) artificial intelligence.


CHAPTER 2: LITERATURE REVIEW

2.1

8

Mechanical Design and Hardware

In the area of mechanical design, one of the important areas is to decide the
number and locations of degrees of freedom for the biped. Fred R. Sias, Jr and
Yuan F. Zheng [9] had done an indepth research on this and came to a conclusion
that eight degrees of freedom are required on each leg to have a good
approximation of human gaits by a biped robot. However, they also remarked that
the degree of freedom with a vertical axis at the ankle is unnecessary for most gaits
used for locomotion, while the degree of freedom at the foot is significant only for
rapid walking. Therefore, in most situations, a leg with six degrees of freedom
(three at the hip, one at the knee and two at the ankle) is employed so as not to
complicate the design of the robot. P2 from Honda is example of humanoid with
six degrees of freedom on each leg [5].
Valuable design experience and lessons learnt are shared among the research
community through publication. Research work by Honda [5] shows that impact
absorption at the foot is of paramount importance. Not only that it would help to
protect the hardware on the robot from potential damage caused by the impact
force, damping by rubber-like protection could also help to prevent vibration by
acting as a mechanical lowpass filter. It was pointed by the designers of SDR-4X
from Sony [4] that the yaw axis of the leg should be offset towards the back. By
doing so, a wider turning angle could be achieved by this yaw motion before
having the two feet hitting each other. Fig. 2.2 explains the logic of the shift in a
pictorial form.

CHAPTER 2: LITERATURE REVIEW

9

Fig. 2.2 Offsetting the yaw axis to achieve wider turn angle [4].
Motors and power transmission are necessary components of humanoids and some
robot researchers like Honda and Sony are using their own customized motors for
actuation. While harmonic gears are getting popular in the large humanoid robots
community, normal gearbox remains the common selection by small size
humanoid robots as they are usually integrated with motors as a compact package
by the manufacturer and are much cheaper. However, backlash would be a
potential problem for using gearbox, compromising precision in motor control.
Timing belt is an alternative to overcome the problem of backlash in gearbox
system, HOAP2 from Fujitsu uses timing belt for power transmission.
As for the main frame of the robot, the general idea would be to have the building
material to be as light and as strong as possible. However, one would expect a
strong material to be heavy and a light material to be weak. Therefore, compromise
on this is required to identify an optimum material. Common choice for this
application would be aluminum alloy, well known for its low density of about
2700Kg/m3, and also machinability. Recently, there is a trend for small size
humanoid robots to use composite materials like carbon fibre sheets or tubes,


CHAPTER 2: LITERATURE REVIEW

10

which has a typical density of about 1750Kg/m3, as the structural material.
NimbRo from the University of Freiburg is an example of humanoid robot built
with carbon fibre.

2.2

Walking Control

Given the complexity of a humanoid robot, walking stably is a challenging task.
Even human beings need months to learn how to walk. Bipedal walking control
has been the focus for many researchers. And many approaches to achieve stable
walking had been considered, and they could generally be classified into five main
categories [12], (1) model-based, (2) ZMP (zero moment point)-based, (3)
biologically inspired, (4) learning and (5) divide-and-conquer.
2.2.1 Model-based approach
In model-based approach, mathematical models derived from laws of physics are
used to generate control algorithm. Approximations are made to vary the
complexity of the mathematical model. Using this approach, Kajita et al. [14] had
come out with the linear inverted pendulum model by approximating that the mass
of robot legs to be negligible compared to the body mass. The system would then
be similar to an inverted pendulum pivoted at the ankle joint. By constraining the
mass to move in a linear path, a closed-form solution could be found for the linear
differential equation.


CHAPTER 2: LITERATURE REVIEW

2.2.2

11

ZMP-based approach

ZMP or zero moment point is a widely adopted concept in humanoid robotics. The
term was first coined by Vukobratovic [27], which refers to the point on the ground
where the resultant of the reaction forces from the ground acts on the robot. It is
believed that ZMP is an indication to the stability of the walking biped. Therefore,
by planning the desired ZMP positions, the required positions of the centre of mass
could be obtained, and through inverse kinematics, obtaining the joint trajectories.
Wasaeda University was the first to implement this control approach on a real
robot [30].
2.2.3

Biologically inspired

Passive dynamic walking is a form of walking behaviour that was discovered by
Tad McGeer [6]. It was shown that a passive walker could walk down a slope
based on just gravity and no actuation was needed. This was also inspired by the
fact that human being does not need to exert a lot in order to walk.
2.2.4

Learning

Learning is a natural concept for walking control for the fact that human beings
need to learn in order to walk properly. The general idea in learning is to allow the
robot to try to walk and gain experience through the process, repeating and
improving the task until the final goal is achieved.
2.2.5

Divide-and-conquer

As the name suggest, this is a very common approach to a complex problem,
where this complex problem is handled by breaking down to a few simpler sub-

CHAPTER 2: LITERATURE REVIEW

12

problems and be tackled individually. In the case of bipedal walking, it could be
broken down into the frontal and sagittal plane for better analysis.

2.3

Artificial Intelligence

In the field of humanoid research, works done on artificial intelligence are rather
limited. A typical interpretation on artificial intelligence for humanoid would be
for humanoid robots to perceive the environment and make appropriate decisions,
it is also suppose to learn and become more intelligent in the process of learning.
This would then depend on the task allocated to the robot, and currently, the tasks
given to humanoid robots are rather simple and they usually operate in a structured
area. ASIMO from Honda [28] had demonstrated an encouraging level of
intelligence by recognizing voice of people and moving around with people. But
still, there are much works to be done before humanoids could really be intelligent.


Chapter 3

Sensors, Actuators and Computer
Systems

A fully autonomous humanoid robot could complete a task by itself, without

assistance from outside system. Thus, all the components that are required to
complete the task has to be mounted on the robot. The three groups of basic
components that are needed are (1) Sensors, which receive signals from the
surrounding environment and feedback to the brain of the robot for necessary
reaction, (2) Actuators, which are the components that perform the physical
movements upon receiving signals from the controller, and it is the combination of
motions from many actuators that allows the humanoid robot to walk and (3)
Computer Systems, which acts as the brain of the robot, take in signals from
sensors, compute, come out with the necessary reaction plan and produce the
signal to instruct the robot or, the actuators, the actions to be carried out.


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

3.1

14

Sensors

For a robot to be fully autonomous, it is necessary that it carries some form of
sensors that it could use to collect information on the surrounding environment and
also on its own status. And with these information, the robot could come out with
the necessary reaction plan for execution later.
For human, we are equipped with numerous sensors. Our eyes, ears, nose, tongue
and skins are the most basic sensors everyone is familiar with. So it would be
intuitive that the primary sensor of RO-PE-V to be its vision system.
3.1.1

Vision System

An omni-directional vision system was selected to be the primary sensor of ROPE-V, instead of the conventional pan-tilt vision system. The concept of omnidirectional vision system was first proposed in 1970. The idea is to have a camera
looking up at the curved mirror that is reflecting the image of the surrounding. Fig.
3.1 illustrates the schematics of this vision system.

Light rays

Fig. 3.1 Schematics of omni-directional vision system.


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

15

The main advantage of using the omni-directional vision system is that it allows
the robot to see 360o around itself, identifying several landmarks simultaneously
and this feature is especially useful for localization which will be discussed in
Chapter 7 of this thesis. Another reason for using this new type of vision system is
weight reduction. For the conventional pan-tilt system, actuators are required to
execute the pan and tilt motions for the camera to see a larger region. But since the
omni-directional vision system is already able cover 360o, there is no need for the
pan and tilt motions, thus, shaving the weight of two actuators that would about
50g each, whereas the additional mirror in the omni-directional vision system only
weighs about 30g.
However, these advantages are accompanied by some short-comings of the system.
The main disadvantage of an omni-directional vision system is that there will be
distortion in the image captured by the camera due to the fact that the camera is
seeing the surrounding through a curved mirror. With this, the distance of an object
could not be obtained straight-forwardly, the distorted image also affects the
visibility of objects that are relatively far away. Fig.3.2 shows an image obtained

through the omni-directional vision system. In addition, there are also blind spots
for this vision system, the view of regions just around the robot are blocked by the
shoulder and the body of the robot, though this could be overcome by some actions
of the robot to clear the obstructions.


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

16

Fig. 3.2 An image captured by the vision system of RO-PE-V.
3.1.2

Magnetic Tilt Switch

Two magnetic tilt switches from Assemtech are mounted on RO-PE-V to detect
the orientation of the robot with respect to the ground. They are important sensors
because they provide the feedback on the status of the robot, i.e. whether the robot
has fallen down, and the appropriate reactions could be carried out, for example,
the ‘getting up’ routine. Fig. 3.3 shows the picture of the tilt switch employed.

Fig. 3.3 Magnetic tilt switch (MTA 240) from Assemtech.
The magnetic tilt switch is really an on/off switch governed by the position of a
movable ball bearing, which rolls along a guided path depending on the orientation


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

17

of the tilt switch. These tilt switches, thus, provide digital signals to the computer
system of RO-PE-V for decision making. Fig. 3.4 shows the schematics of the
working principles of the magnetic tilt switch.

When tilted,
magnet falls to
the right

After the magnet falls to
the right, the metal
connectors will be attracted
towards, the magnet,
closing the circuit in the
process.

Fig. 3.4 Schematics of the working principle of the magnetic tilt switch.
3.1.3

FlexiForce

Two force sensors are mounted on each of the foot of RO-PE-V to detect the
ground contact of every step. The use of force sensors are more for slope walking
which will be discussed with more details in Chapter 6. Fig. 3.5 shows the picture
of the FlexiForce employed on RO-PE-V.

Fig. 3.5 FlexiForce sensor employed on RO-PE-V.


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

18

FlexiForce from Tekscan was selected for it is small and light weight, such that
they could be installed on the robot with minimum disturbance to the motion of the
robot. FlexiForce is a resistive force sensor that changes resistance depending on
the amount of force applied to the sensing area. When there is no load, the sensor
has a high resistance of about 20M , while the resistance of the sensor would drop
to range of K

when it is loaded. To measure the contact force, through measuring

the change in resistance, the sensor is connected in a potential divider as shown in
Fig. 3.6 to output an analogue voltage signal for measurement.
5V
1k
Output to
analogue to
digital converter
FlexiForce
Ground
Fig. 3.6 Circuit for measuring the change in resistance in Flexiforce.

3.2

Actuators

Actuators could be considered the most important component of a robot. They are
the actual moving mechanisms that would allow a robot to perform an action, just
like muscles on human. For a long time, servo motors from Japanese companies
like Hitec, JR and Futaba have been dominating the market of actuators for small

size robots, primarily because of their light weight and compactness in size.


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

19

However, as technology in robotics advances and with this research topic getting
popular worldwide, competitors from other countries appear. Robotis from Korea
is among the leading competitor, and Dynamixels DX-117 are employed as the
only type of actuator on RO-PE-V. Fig. 3.7 gives a picture of DX-117 while Table
3.1 shows a comparison between HSR-5995 from Hitec and DX-117 from Robotis.

Fig. 3.7 Dynamixel DX-117 from Robotis.
Table 3.1 Comparisons between HSR-5995 from Hitec and DX-117 from Robotis.
HSR-5995 (Hitec)

DX-117 (Robotis)

Max. Torque (Kg-cm)

30

37

Weight (g)

62

66

Speed (sec/60o)

0.12

0.129

Operating Angle (degree)

180

300

Supply Voltage (V)

4-6

12-16


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

Link

Through PWM generator

RS485

Feedback

No

Yes

Daisy Chain

No

Yes

20

The increase in torque and operating angle, the existence of feedback and daisy
chain capability are the primary pull factors for the switch from Hitec motors to
Robotis motors. The increase in torque and operating angle would allow RO-PE-V
to have a higher payload and to execute more demanding actions. The daisy chain
connections would minimize wires within the robot, cutting down weight and
chances of wires being snipped by the mechanical structure in motion. And the
availability of feedback in position and torque gives the possibility of
implementing more sophisticated action algorithms, while the feedback in
temperature and voltage could be used to protect the motors from overloading.
DX-117 uses RS485 for communication with the controller of the robot, which is a
standard protocol in the field of data acquisition, and it is this employed protocol
that allows DX-117 to be daisy chained and a high transmission rate of up to
1Mbps. Each motor is given a unique ID in the setting phase, and because all of
them are connected in the same lines, they will receive all instructions given by the
controller. However, each instruction packet is led by the ID(s) of the desired
receiving motor(s). Thus, the motors will only respond to instructions meant for
them and ignoring the rest.

CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

3.3

21

Computer Systems

Computer systems serve as the brain of the robot. It makes decisions according to
the environment information from the sensors’ feedback and a set of rules predetermined in the program. It disseminates its decisions in the form of instructions
to the motors for execution. This sequence could be simple and does not require a
very powerful processing unit. However, the processor on RO-PE-V would be
tasked to perform image processing as well. This would be a demanding routine
and the overall workload would require a powerful but compact processor.
CRR3 CoolRoadRunnerIII from the PC104 family is selected for this application.
It has a processing speed of 650MHz and is relatively compact in size. It is
effectively a Pentium 3 computer in a small form factor. Real-time Windows is
installed as the operating system for RO-PE-V with Microsoft Visual Studio as the
programming environment. Table 3.2 lists some important specifications of CRR3.
Table 3.2 Specifications of CRR3 from LIPPERT.
Processor Speed

650MHz

RAM

512MB

Serial Communication

2 x RS232

USB

2 x USB1.1 compliant

Supply Voltage

5V


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

Power Consumption

<15.5W

Cooling

Passive heat sink

Board Format

PC104+

22

Some other parts have to be included for the interfacing the components that are
mentioned earlier. For example, to send the image information from the omnidirectional vision system to the computer, a frame grabber of the PC104+ format is

used, for controlling the DX117 that requires RS485 communication link, a RS232
to RS485 converter is employed. Finally, a Power Management and Data
Acquisition Board developed by the team is used to interface the batteries and the
sensors to the power supply and RS232 port of the computer. Table 3.3 lists the
components used on RO-PE-V while Fig. 3.8 shows connections between these
components.


CHAPTER 3: SENSORS, ACTUATORS, AND COMPUTER SYSTEMS

Table 3.3 Components of RO-PE-V.

Fig. 3.8 Flow chart showing how components are connected to each other.

23


Chapter 4

Mechanical Design

The mechanical design of the robot includes the consideration on the number of degrees of
freedom, the positions of joint, and the design of the linkages. In this chapter, the
requirement and limitation shall first be discussed, followed by a detailed presentation on
the actual robot design.
Humanoids are modeled after human, that has numerous degree of freedom and are very
flexible, but it would not be realistic to expect humanoid to have as many degree of
freedom. Studies have shown that the minimum numbers of degree of freedom required
for humanoid to achieve most of human’s lower limbs actions are six on each leg, which
includes three at the hip, one at the knee and two at the ankle. This is used as the primary

guide in the design of RO-PE-V. As for the upper limbs, their main purpose is to assist in
getting up when the robot has fallen and only two degrees of freedom are allocated to each
arm in order not to incur extra weight on an overly complicated arm design. Recovery
from fallen positions would be a requirement for RO-PE-V because the sole purpose for
RO-PE-V is to participate in RoboCup, and falls are expected during interaction with other
robots. Thus, the ability to get up by itself could avoid the penalty given teams that need to
bring the robot upright manually.


CHAPTER 4: MECHANICAL DESIGN

25

Light weight, simplicity and ease for maintenance are the keys to the design of RO-PE-V.
To minimize the weight of RO-PE-V, aluminum alloy, which has a low density of about
2700Kg/m3, are used for the main skeleton of the robot. Linkage designs are simple and
connectors are positioned such that minimum dismantling is required in order to access
and tighten any of them during use of the robot.
RoboCup has a set of robot specifications which participants has to adhere to when
designing their robot. Therefore, this poses as one of the main limitations that constraint
the design. Some of the important competition specification regarding to the mechanical
design are listed below, the detailed competition rule is in Appendix A.

4.1

1.

H (height of robot) = min( 2.2 x centre of gravity of robot, physical height)

2.

30cm

3.

Foot size

4.

The robot must fit into a cylinder of diameter 0.55H

5.

0.35H

H

60cm
H2/24

Length of legs

0.7H

Robot Design

The design of RO-PE-V was done using SolidWorks, a 3D Computer Aided Design (CAD)
software. Analyses were done on certain critical linkages using a SolidWorks extension,
COSMOXpress to ensure that design is sufficiently safe from failing by excessive flexure
due to body weight or impact at a fall.