a system design concept based on omni-directional mobility, safety and modularity for an autonomous...

Corresponding author: Yasunori TakemuraE-mail: [email protected]

Journal of Bionic Engineering Suppl. (2008) 121–129

A System Design Concept Based on Omni-Directional Mobility, Safety and Modularity for an Autonomous Mobile Soccer Robot

Yasunori Takemura1, Yu Ogawa1, Amir A. F. Nassiraei1, Atsushi Sanada1,

Yuichi Kitazumi1, Ivan Godler2, Kazuo Ishii1, Hiroyuki Miyamoto1

1. Kyushu Institute of Technology, Kitakyushu, Fukuoka 808–0196, Japan 2. The University of Kitakyushu, Kitakyushu, Fukuoka 808–0135, Japan

Abstract In this paper, we describe the concept, design and implementation of a series of autonomous mobile soccer robots, named

Musashi robots, which are designed referring ISO safety standards and have mechatronics modular architecture. The robots are designed to participate in the RoboCup Middle Size League. Using a modular design philosophy, we show that the selection of a proper moving mechanism, a suitable vision system and a mechatronics modular architecture design can lead to the realization of a reliable, simple, and low cost robot when compared with most car-like robots that include many kinds of sensors and have a complex design structure. Keywords: entertainment robotics, mechanism design, wheeled robot

Copyright © 2008, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved.

1 Introduction

RoboCup is an international joint project to pro-mote Artificial Intelligence (AI), robotics, and related fields. It attempts to foster AI and robotics research by providing standard problems in which a wide variety of technologies can be integrated and examined. In Ro-boCup, a soccer game is used as a main topic of research, and the aim is for innovations to be applied to socially significant problems and industries in the future[1].

In the RoboCup Middle Size League (MSL), two teams of mid-sized robots (maximum 50 cm × 50 cm × 80 cm) with all sensors on-board play soccer on a field (12 m × 18 m). Relevant objects (ball, field and line) are distinguished by colors (orange, green and white). Ro-bots communicate with each other by wireless LAN. No external control by humans is allowed, except to bring or remove robots in/from the field. Duration of a game is 15 minutes half, so total 30 minutes. Therefore, the robots should be autonomous and have tough mechanism.

Hibikino-Musashi is a soccer team in the RoboCup MSL[2,3]. Members of the team are from three different

research and educational organizations located in the Kitakyushu Science and Research Park, Kitakyushu, Japan.

In this paper, a design methodology to realize a safe, simple, robust, and mobile platform for Musashi robots is presented. Musashi robots are a series of autonomous mobile soccer robots that have modular architecture in their hardware. The robot includes an omni-directional movement mechanism, omni-vision and a novel ball-kicking device, and is developed as a reliable and robust soccer robot with a high degree of simplicity, mobility, and maneuverability.

Therefore, the design concepts of the Musashi ro-bot have two high priority concepts: safety and modu-larity. Safety concept consists of safety design, safety guarding and safety informing. Modularity concept consists of easy assembling, maintenance, trouble-shooting, reliability and low cost. Modularity concept also helps safety mechanism.

This paper presents details of the current state of hardware architectures. The 2nd section introduces our old robot problems and points of the modification. In the

Journal of Bionic Engineering (2008) Suppl. 122 3rd section, we describe how to design the new Musashi robot by using the omni-directional motion, safety and modularity concepts.

2 Concept of design

Fig. 1 shows the first version of our robot, which was developed in the Fraunhofer AIS as we organized an international joint team GMD Musashi[4]. This robot has a car-like locomotion mechanism that includes two ac-tive wheels and two caster wheels at the back and front, respectively. A digital camera with a 70 degree wide-angle lens is mounted on top of the robot. The camera is controlled by a DC motor and an absolute encoder to have 360 degree rotation in the horizontal plane. In order to move in a dynamic environment avoiding obstacles, the robot is also equipped with dif-ferent kinds of sensors such as two infrared (IR) sensors, two distance sensors, and touch sensors. A pneumatic kicker whose air is supplied by two small air pressure tanks is installed on the robot.

Fig. 1 The first Musashi robot (The first Musashi robot equipped with 11 sensors).

The existing problems of the mechatronics design

outlined above are as follows. (a) Poor mobility functions for performing re-

quested motions such as rotation around the ball and lateral movements.

(b) Complex data processing owing to the wide variety of sensors

(c) Complexity in the development of the hardware and software involved in camera motion control for tracking a ball.

(d) A design that is not compatible or safe for in-

teraction with humans. (e) Low reliability and insufficient robustness for

dynamic environments such as that of RoboCup. (f) Complex mechatronics from the viewpoints of

assembly, maintenance, extendibility, troubleshooting, and transportation.

(g) Low speed in kicking of the ball, in comparison with other teams’ robots, which can shoot the ball at high speeds up to 6.0 m·s�1[5,6].

To solve the above problems and achieve the re-quired characteristics for a RoboCup scenario, a new mobile robot named “Musashi” is designed and con-structed for the Hibikino-Musashi team. In this design approach, we show that selection of a proper moving mechanism, a suitable vision system and mechatronics modular architecture design can lead to the realization of a more reliable, simple, and low cost robot.

3 Musashi architecture

The design of the Musashi robot is based on three significant and fundamental concepts: that the robot must be 1) omni-directional, 2) safe, and 3) modular[7–9].

3.1 Concept of omni-directional mobility The Musashi robot is an omni-directional mobile

platform with omni-vision (Fig. 2). The dynamic and kinematic characteristics of the omni-directional design

Fig. 2 Three-dimensional computer-aided design of Musashi (Musashi includes an omni-directional platform, omni-vision, and a strong novel ball-kicking device, designed by Autodesk Inven-tor 3D-CAD.).

Takemura et al.: A System Design Concept Based on Omni-Directional Mobility, Safety and Modularity for an Autonomous Mobile Soccer Robot 123

allow for high maneuverability in the field. The me-chanical structure is designed with the three-dimensional CAD (computer-aided design) software Autodesk In-ventor. Each robot is equipped with three omni-wheels, each of which is driven by a 70 W DC motor with re-duction ratios of 12:1. The rotation velocity of a wheel is fed back to the motor driver using 540 ppr digital in-cremental encoders. The velocities of the three wheels are controlled by three Faulhaber motor drivers (MCBL 2805) with each having an RS232 communication port. The robot can move with a maximum speed of 3.4 m·s�1. Problem (a) mentioned in the previous section can be solved using this design.

Problems (b) and (c) are solved by changing from mono-directional vision (the first version of our robot) to an omni-directional vision system consisting of a digital camera (IEEE 1394) and a hyperbolic mirror. The main parameters, the height of the mirror H and distance be-tween the mirror and the camera h illustrated in Fig. 2, are determined so that the area invisible to the omni-vision camera almost completely corresponds to the area occupied by the robot[10]. The omni-vision can be used for not only object detection and localization, but also collision avoidance, whereas our first robot required different types of sensors to avoid contact with other robots by low computational power processors. The image processing requires high CPU power, so that various sensors are needed.

Table 1 shows the specifications of Musashi. The sensors used for Musashi are an omni-directional camera and three DC motor encoders, whereas the first version

Table 1 Musashi specificationsItem Specifications Size 500 mm × 500 mm × 800 mm

Total weight 18.0 kg

Actuator

DC-motor × 3 (Maxon, 24.0 V, 70 W)

Motor driver × 3 (Faulhaber, MCDC 2805)

Power supply Li-polymer battery (3.7 V × 7 cells, 2000 mAh)

Duration 0.5 hours

Kicking device DC motor × 1

(Faulhaber, 24.0 V) Torsion spring × 3

Sensors Omni-directional camera DC motor encoder × 3

of our robot was equipped with 11 sensors (two IR sensors, two distance sensors, a camera, two DC motor encoders, touch sensors, two limit switches for robot fingers, and an absolute encoder for camera motion). Fig. 3 is a flowchart of the Musashi power system, which includes a main Li-polymer battery (25.9 V) and an extra Li-polymer battery (7.2 V) for bursts of high accelera-tion and speed while catching and carrying a ball. The necessary voltages for the camera and the micro com-puter power supply are converted from 25.9 V to 12.0 V and 5.0 V, respectively. The power consumption of the robot is approximately 40 W and the estimated operation duration of the robot is 0.5 h.

Fig. 3 Flowchart of the Musashi power system (All power is supplied by a Li-polymer battery (25.9 V). Lines describe the voltage of the wiring connection. 5.0 V lines are dash line, 12.0 V lines are dash and dot line, 25.9 V lines are dot line and 7.2 V lines are dash and two dot line.).

3.2 Concept of safety

This section describes the safety strategy. Many robots such as industrial and service robots have recently been developed. Industrial robots are mostly manipula-tors having a necessary number of degrees of freedom. When the controller fails, there is a possibility that the robot could harm the human operator. ISO provides principles for machine design in general, under the ISO 12100 standard[11]. In addition, the ISO 10218 stan-dard[12], which is based on ISO 12100, sets out industrial robot design principles for keeping the operator safe. Engineers should reduce risk based on the principles of safe design so that the robot can satisfy the demands of

Journal of Bionic Engineering (2008) Suppl. 124 society. In RoboCup soccer, there is a direct interaction between robots and humans and thus there is the possi-bility of severe injury for the robot operators. Therefore, we should consider human safety when designing our robot for RoboCup.

One of the final goals in RoboCup is to win against the human World Cup champion team. However, at present there are no rules on safety in the RoboCup Soccer MSL. It is important to keep humans safe in the event of a collision with a robot. We conducted a risk assessment referring to ISO 1412[13,14], which details risk assessment relating to making robots safe. Table 2 presents the high risks for humans related to Musashi.

A risk assessment method is needed to determine compliance with ISO 12100. ISO 12100 explains how to decrease risk using a three step method. Here, we im-plement the method to reduce the risks we consider as high for Musashi.

Step 1 involves intrinsically safe design. In this way, the construction of hazardous exterior features can be avoided in the design stage. However, many of the ro-bots that are used in RoboCup have already been built before modification by the contestants. Therefore, it is difficult to reduce the dangers by design. We need to increase safety by the next step.

In the Musashi robots, safe design is considered for critical elements. As an example, for autonomous robot, batteries are one of the most important parts for loco-motion. In the Musashi robot case, a battery pack con-sists of 7 cells Li-Polymer battery. Each cell has nominal 3.6 V, full charge 4.2 V (more than 4.2 V is called over

charging), and over discharging critical voltage 3.1 V (less than 3.1 V is called dead cell). Therefore, nominal, full charge and discharge voltages of battery pack are 25.2 V, 29.4 V and 21.7 V, respectively. In the battery safety design concept, the minimum requirement is that protection elements and circuits should be designed to avoid over discharging, over current and over charging. To realize these three concepts, a breaker for avoiding over current is installed in the robot, and a special de-signed circuit is developed for over charging and dis-charging. This special protection circuit that also is mounted in the robot has two main functions.

Over discharging protection: this means the battery never discharge lower than 22.5 V (21.7 V (dead voltage) + safety rate) under any condition. For example, if a user forgets to turn off the robot, the protection circuit will shut down the robot power in adjusted voltage (22.5 V) automatically.

Over charging protection: that is to say, the robot cannot be turn on, if the battery voltage inserted to the robot is higher than 30 V (29.4 (full charge voltage) + safety rate). In this approach we realize safety concept by considering that using over charging battery is dan-gerous for the robot and human.

Moreover, the developed battery charger has a battery protection circuit to avoid over charging in process of charging the battery.

Step 2 involves safeguarding and adding protective elements. It is possible to add an emergency switch for each hazard. If something in the robot breaks or mal-functions while around people in a game scenario, the

Table 2 Risk assessmentHazards Crush Impact Cut

Hazards Contents of hazards Wheel mechanism run-

ning over human Collision with human Sharp parts contact human

Severe of an injury High High High Frequency High High High Evasion Possible Possible Possible

Risk analysis

Scale of risk High High High

Safety strategy Set emergency switch, use sensors

Use a soft cover, install emergency switch

Remove sharp parts, install emergency switch,

ensure hazards are covered, use sensors

Remaining risk Breakdown of emer-gency switch and sensors

Breakdown of emergency switch

Breakdown of emergency switch and sensors


robot should be stopped immediately. Therefore, emer-gency switches are effective. However, there remains the risk of an emergency switch malfunctioning, and this risk is passed on to the referee and participants.

As shown in Table 2, some dangerous places of the robot are remaining. For example, bottom of the parts have sharp edges, therefore, for considering when the robot crush the humans, the robots should be install emergency switch and rubber guard. It is not only our problem, but also RoboCup MSL problem.

Step 3 involves providing information to the users. Risks that cannot be mitigated in the first and second steps must be explained in the form of information or instructions provided to team members. For example, the referee and participants may need to wear shoes upon entering the playing area.

The referee is the person at greatest risk of being injured by robots during a game of robot soccer. There-fore, the overall risk of an accident occurring can be dramatically reduced by keeping the referee safe. In the case of industrial robots, the safety of workers and op-erators can be guaranteed by fencing around the robot work space and signs prohibiting entry when the robot is working. However, in the case of the RoboCup MSL, it is impossible to fence the robots and keep the referee away from the robots. In fact, it is necessary for the referee to have direct interaction with the robots during the game as in real soccer (for example, to place the ball and make judgments). Therefore, the game organizer needs to explain the specifics of each robot to the referee and participants, including the robot’s expected erro-neous behavior and the results of its risk assessment. In this way, the designer of the robot can relinquish re-sponsibility. However, there are at present no RoboCup MSL rules regarding responsibility. Therefore, when an accident happens, no one can be held to account. It is important for participants to be clear about critical haz-ards and implement safety measures beforehand.

We propose that global safety standards will be used in the RoboCup MSL in the future. It will then be necessary to account for safety in the process of robot design. Our team attempts to reduce the risk and make safe robots while considering concepts of simple design

and modularity.

3.3 Concept of modularity Step 1: Description of the robot system architecture To describe and emphasize the concept of modu-

larity, it is necessary to present a short overview of the design of the robot system architecture. Musashi is equipped with a laptop on which the image processing, control, communication and data exchange are per-formed. The behavior commands such as start, stop, and corner kick are received from a referee box PC located outside the field via a wireless LAN. To achieve a safe, simple, and robust system, the samplings of sensor data and actuator control are executed using conventional interfaces: IEEE 1394, USB, and RS232. The commu-nication between an omni-directional camera and a mounted laptop PC is performed using the IEEE 1394 interface. The laptop PC sends the motor control com-mands (target velocities) to the motor drivers via a USB interface and USB/serial converters because the motor driver has only a RS232 serial port (Fig. 4). Another USB/RS232 converter is used for communication be-tween the laptop PC and the circuit of the kicking device.

Step 2: Definition of basic module Based on the flowchart of the robot architecture

(Fig. 4), the basic modules should be determined by considering similar hardware structures or similar

Fig. 4 Flowchart of the Musashi architecture (Each dash square area describes basic modules. Area 1 indicates the USB module, area 2 the motor driver (MD) module, area 3 the kicking device (KD) module, area 4 the motor and wheel (MW) module, area 5 the kicker circuit (KC) module, area 6 the main, power and cam-era switches (SW) module and area 7 battery module.).

Journal of Bionic Engineering (2008) Suppl. 126 mechanical connections. Fig. 4 shows the seven basic modules for Musashi hardware illustrated with dash square area and number. For example, a USB hub and four USB/RS232 converters can be a module (USB module) because they have a common interface. Another consideration is the mechanical similarity such as an omni-wheel and a motor (MW modules). It is important to note that one of the considerations in the design of a basic module is that a mechanical interface should be able to attach the basic modules to the back plane “di-rectly”. The mechanical interface including the con-nector and the fixture should be designed to accept ex-ternal force by the fixture, not to occur wrong installa-tion to the back plane, and to be easy to exchange the modules.

Another consideration for definition of the basic module, we consider the safety. For example, battery module is defined by considering safety. Our team’s battery is Li-polymer battery, therefore, we should con-sider explosion. The battery module consists of top and bottom parts. Both parts is covered a battery and free wires. Therefore, the dangerous area of the battery is hiden almost 60 % for the module parts. Also, the battery module has the battery box, so that means the dangerous area of the battery is reduced to less than 20 % by the use of battery module.

As mentioned above, the modularity concept and safety concept has a good relationship for making the reliable robot.

Step 3: Definition of the merged module The concept of modularity can be extended by

merging the basic modules in an effort to decrease the number of wires. A merged module can be theorized by considering the flow chart connections of the single modules and realized by design and implementation of the “back plane” concept. A back plane can be regarded as a basic module’s communication port in a merged module. For example, considering the connections among the USB, kicker circuit (KC) and motor driver (MD) modules, a back plane can be designed to merge the five modules (a USB, a KC, and three MD modules) and solve the problem of complex wiring connections. The new merged module is referred to as a central con-trol module (Fig. 5).

Fig. 5 Modules of Musashi hardware (Battery and SW modules can be recognized as basic modules, and Central Control module as an merged module.).

The Musashi modular architecture is summarized as follows. We designed the robot as comprising two main modules, a bottom module and an upper module (Fig. 6), considering the ease of assembling, mainte-nance, troubleshooting, and transportation. Conse-quently, the bottom module consists of six single mod-ules (a switch, a battery, a kicking device, and three motor-wheel modules) and one merged module (the central control module). Using this approach, we could solve problems (e) and (f) described in the second sec-tion.

Fig. 6 Main merged modules of Musashi: bottom module and top module. 3.4 Special function of Musashi robot

In 1997, RoboCup MSL robots had no kicking de-vice. One year later at the second RoboCup tournament,


the Freiburg team introduced the first kicking device to be used, and was champion for that year[15]. After 1998, the development of kicking devices became a key con-sideration for all participating teams.

A new kicking device for Musashi robots was de-signed using torsion springs and is based on two main functions: (a) shooting the ball at high speed (with a target velocity of up to 5.0 m·s�1 – 6.0 m·s�1 and (b) lifting the ball (with a target height in excess of 1.0 m).

In general, three mechanisms are necessary to use the energy of a spring: (a) a mechanism for charging the spring (storing energy), (b) a mechanism to lock the spring (maintaining energy), and (c) a mechanism to release the spring (releasing energy). In the developed novel kicking device presented in this section, instead of

these three mechanisms, we designed a cam mechanism that saves, keeps and releases the energy of a series of torsion springs using only a motor and a limit switch for controlling. The basic concept of this mechanism is also introduced to a jumping robot “JumpingJoe”[16–17].

This mechanism, during the charging time, the perpendicular distance between the direction of the contact force F and the center of the cam decreases when the spring torque �S and the contact force F increase.

At the completion of charging, the direction of force F is towards the center of the cam. The mechanism has an inherent characteristic to lock without any motor torque as shown in Fig. 7. It is clear that when the motor continues to rotate after the locking process, the spring will be released rapidly.

Fig. 7 Sequence of charging of the spring at the beginning, middle, and end of the charging process. At the completion of charging, because of the direction of force F passing the center of the cam, the mechanism has the inherent characteristic to lock.

4 Conclusion

In this paper, we described the concept, design and implementation of a series of autonomous mobile soccer robots, named “Musashi” robots. The designs of Musashi robots are based on three significant and fun-damental concepts: the robots must be 1) omni-direc-tional, 2) safe and 3) modular.

With regard to the first concept, an omni-direc-tional mobile platform was designed for the Musashi robot to extend its maneuverability and performance beyond those of traditional car-like robots, whose plat-forms are limited by poor kinematic and dynamic mo-

tions. In addition, we selected an omni-directional vision system consisting of a digital camera (IEEE 1394) and hyperbolic mirror, which are mounted on top of the robot. In this approach, the omni-camera can be used not only for object detection and localization but also for colli-sion avoidance, whereas previous robots needed differ-ent types of sensors and hardware to avoid contact with other robots and objects. As a result of the omni-directional concept, Musashi can be operated us-ing a sensor system by means of an omni-directional camera and three DC motor encoders with the minimum setting, whereas our previous car-like robot was equipped with 11 sensors.

Journal of Bionic Engineering (2008) Suppl. 128 We proposed a mechatronics concept of safety, re-

ferring to ISO standards. In addition, we conducted a risk assessment referring to ISO14121, which details the conducting of risk assessments for making robots safe. We suggest that global safety standards should be used for the RoboCup MSL in the future. It will then be im-portant and necessary to account for safety in robot de-sign. Our team attempts to reduce risks and make safe robots while considering the design concepts of sim-plicity and modularity.

We proposed mechatronics modularity, which consists of three steps, starting with a description of the robot system architecture, defining single modules and finally merging modules. Regarding the Musashi modular architecture, we designed and divided the robot into two main modules, a bottom module and an upper module. The bottom module consists of six single modules and one merged module. Using this approach, we could realize a simple, reliable, and robust robot, for which troubleshooting would be relatively easy and manufacturing could be done at low cost.

In addition, we explained the design and develop-ment of a strong kicking device having the capability of shooting (up to 5.0 m·s�1) and lifting (up to 120 cm) a ball. The ball-kicking device is achieved by designing a unique spring charging mechanism referred to as a cam charger. The key idea is to charge a series of strong tor-sion springs using a special cam. One of the features of the cam charger mechanism is that charging, maintain-ing and releasing the spring energy is realized by only employing a simple DC motor gearhead and controlling using a limit switch. Employing the developed Musashi robots, the Hibikino-Musashi team was ranked among the eight best teams at the RoboCup 2006 world cham-pionships in Bremen, was placed fourth at the RoboCup 2007 world champion ship in Atlanta and was champion and awarded the Most Valuable Player award at the RoboCup Japan Open in 2008.

Acknowledgements

Many people and graduate students have helped us in our research. Their guidance, good humor, advice, and inspiration sustained us through the years of work. In addition, this work was supported by the 21st Kyushu

Institute Technology of Century COE program, Ooba Sangyou Co., Ltd., the Nippon Telephone Center Cor-poration, and Neural Image Co., Ltd.

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