Design and control of a small size humanoid robot

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... 2.1 Mechanica

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

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 6 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

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

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] (b) ASIMO [28] (c) HRP-2 [41]

(d) QRIO [42] (e) HOAP2 [40] (f) URIA [32]

(g) ROBONOVA [33] (h) KHR-1HV [34]

(i) RoboSapien [35] (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

2.1 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

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,

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

2.2.2 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-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

3.1 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

RO-PE-V, instead of the conventional pan-tilt vision system The concept of

omni-directional 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

Fig 3.1 Schematics of omni-directional vision system

Light rays

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

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

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

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

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

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

5V

Ground

Output to analogue to digital converter 1k

FlexiForce

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

Link Through PWM generator RS485

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

3.3 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

pre-determined 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

Power Consumption <15.5W

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

omni-directional 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

Table 3.3 Components of RO-PE-V

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

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

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

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

RO-PE-V consists of seventeen degrees of freedom, six on each leg, two on each arm and

one at the body to give the extra flexibility to help in getting up from fallen Fig 4.1 shows

the locations of the degree of freedom on RO-PE-V

Fig 4.1 Locations of degrees of freedom on RO-PE-V

4.1.1 Head Design

The design of the head is rather straight forward, primarily because of the use of the

omni-directional vision system, which requires no actuation compare to a pan-tilt system which

needs two actuators to traverse the camera The main idea is to adapt the off-the-shelf

vision system to the shoulder blade of the robot Fig 4.2 shows the omni-directional vision

system mounted on the shoulder blade of RO-PE-V

Fig 4.2 Omni-directional vision system as the head of RO-PE-V

4.1.2 Upper Limbs Design

For the arm design, there are actually two versions, the first version targets at weight

saving while the second version aims at rigidity and reliability For the first version, to

minimize the weight of the arm, Perspex, of density of about 1190Kg/m3, is used as the

material for the arm linkages This cuts down the weight of the upper body, which could

be a potential problem because of the weight concentration at the upper body due to the

weight of the high performance processor Fig 4.3 shows the first version of the arm

design

Fig 4.3 Arm design using Perspex as linkages

The use of Perspex indeed cuts down on weight, however, the tradeoff in using Perspex is

a reduction in impact strength Due to the nature of the application, impact on the Perspex

portion of the robot is unavoidable because of falling, and after one year of usage, a couple

of the Perspex rods started to break And therefore, the second version of arm design,

which uses aluminum alloy, is used to replace the earlier design Fig 4.4 shows the new

arm design

Fig 4.4 New arm design using aluminum alloy as linkages

4.1.3 Body Design

The main purposes of the body are to connect the limbs and to house the computer systems

and the batteries A simple rectangular casing was designed for that, with some holes

drilled on the plates to cut down on weight Another function of this body casing is to

dissipate heat The supply voltage from the batteries is about 16V, which is meant for

supplying the actuators, but at the same time, the computer system and the camera are

taking power from this source as well However, the two items only take in 5V and 6V

respectively, and this would require voltage regulators to step down the voltage level

Energy are lost through the regulators, take the one for the computer system for example,

Aluminum alloy

the amount of energy lost in the form of heat, is equal to (16V – 5V) x 2.5A = 27.5W This

would mean a large amount of heat and a passive heat sink would be required To save

weight, instead of putting an additional heat sink on the robot, the shoulder blade is used to

help dissipate the heat, and its temperature would rise up to about 40oC at region closer to

the heat source, the regulators To cut down on the heat generated, the analog voltage

regulators are replaced with switching regulators recently, which could regulate the

voltage level without the undesirable heat dissipation The implementation of switching

regulators is not part of this thesis and shall not be discussed here in detail

To assist in the getting up motions, an additional degree of freedom is implemented at the

torso of RO-PE-V This would provide RO-PE-V with the extra degree of flexibility in

achieving these taxing motions Fig 4.5 shows the design of the upper body of RO-PE-V

Fig 4.5 Design of the upper body of RO-PE-V

4.1.4 Lower Limbs Design

The lower limbs are the most important portion of humanoid robots It is because the two

for humanoid research and also for RoboCup The design of the lower limbs could be

broken down into four parts, (1) hip joint, (2) thigh, knee joint and shank, (3) ankle joint

and (4) foot, and they shall be explained in details below

Hip Joint

As mentioned above, studies have shown that the minimum degree of freedom required at

the hip for humanoid is three, and the three degrees of freedom are the hip pitch, hip roll

and hip yaw The arrangement of the three actuators is as shown in Fig 4.6

Fig 4.6 The arrangement of the three actuators at the hip of RO-PE-V

Axial needle roller bearings are also employed at the hip joint It is used as a thrust bearing

to absorb the vertical force resulting from the impact between the landing foot and the

ground, so as to prevent this vertical force from reaching and thus damaging the yaw

motor In addition, this axial needle roller bearing also serves to fill up the gap between the

hip plate and the hip roll motor, limiting the only relative motion between them to be the

yaw motion Fig 4.7 shows the actual CAD drawing of the hip joint while Fig 4.8 shows

a picture of the axial needle roller bearing employed

Yaw

Roll

Pitch

Fig 4.7 CAD drawing of hip joint of RO-PE-V

Fig 4.8 Axial needle roller bearing used on hip joint of RO-PE-V

Thigh, Knee Joint and Shank

There is only one degree of freedom at the knee joint, that is, the knee pitch Connecting to

knee pitch are the two most critical linkages in the humanoid robot, the thigh and the

shank, of which their structural rigidity are of utmost importance It is because flexing or

twisting of these two linkages could happen if they are not sufficiently rigid, especially in

the single support phase where the supporting leg is taking the weight of almost the entire

robot And flexing and twisting of the linkages would result in uncertainty and make the

One of the primary considerations for the design of the thigh link is the range of motion

that the link could achieve The requirement for RO-PE-V to recover from fallen positions

would require the robot to be able to tuck in its legs as far as possible, and the design of

the thigh would allow the robot to swing forward to about 100o While the design of the

shank has a similar requirement, that is, to allow the legs to fold up as far as possible

Therefore, the designs of the two linkages attempt to achieve the maximum range of

motion without compromising the structural rigidity Fig 4.9 shows the designs of the

thigh and shank while Fig 4.10 shows the COSMOXpress analyses on the two linkages,

which indicate a safety factor above 1.5 for both linkages

Fig 4.9 Designs of the thigh and shank links of RO-PE-V

Fig 4.10 COSMOXpress analysis results on the thigh and shank links

Ankle Joint

There are two degrees of freedom required at the ankle joint, ankle pitch and ankle roll,

and the design of this joint takes a similar form as the hip joint That is, the pitch and roll

motors are connected in the same way This would mean the design of the connecting

piece at the ankle joint would be the same as the one used at the hip joint, reducing the part

count so as to cut down on the manufacturing cost Fig 4.11 shows the design of the ankle

joint of RO-PE-V

Fig 4.11 Design of the ankle joint of RO-PE-V

Foot

Foot design is important for the stability of the robot In general, a larger foot would give

better stability because there is a larger area for the centre of gravity of the robot to fall

within, thereby maintaining stability However, too large a foot would render the research

uninteresting due to the lack of realism, and for the same reason, RoboCup has a foot size

specification that the participating teams are supposed to adhere to

Another foot design consideration is the need to accommodate force sensors that would be

Pitch

Roll

should not be overly complicated and heavy, which would otherwise hinder the smooth

execution of the robot’s actions

The final design could accommodate up to four force sensors Fig 4.12 shows the concept

of the foot design with the locations of the force sensors and Fig 4.13 shows the foot of

RO-PE-V equipped with force sensors

Fig 4.12 Concept of foot design that could accommodate force sensors

Fig 4.13 Foot of RO-PE-V equipped with force sensors