JRobo: simulation and control with java for quadruped robot.

Vatau, Steliana ; Cioi, Daniel

1. INTRODUCTION

In the past few decades, the development of walking or running machine has drawn significant attention in the field of robotics. Many previous studies of legged robots have been performed. About walking on terrain, monopod biped and quadruped robots have been studied. Most of these earlier studies employed precise models of a robot and an environment and also involved planning foot trajectories as well as controlling joint motions on the basis of an analysis of the models (Hong et al., 1999).

With advances in science and technology, the interest to study the animals walking has developed the demand for building the legged robots. The development of legged robot involves research in heterogeneous areas. Design of legged robot involves equal amount of mechanical and electronics considerations. There are many factors which are to be considered are cost, actuator, size, weight and controlling of actuators. All these factors have been considered and designed.

2. ROBOT MODEL

The JRobo robot has 12 degrees of freedom, with three degrees of freedom per leg. Each leg has hip, knee and ankle. The hip joint is actuated in vertical plane (Pitch) and horizontal plane (Roll), knee joint is actuated in vertical plane (Pitch) and ankle is not actuated. Figure 1 shows the quadruped robot model. The mechanical design is divided into four phases: determining the mechanical constraints, conceptual design, building the prototype model, specification and fabrication of the model. There are various design considerations when designing a quadruped robot. Among them, the major factors that have to be considered are robot's size selection, degrees of freedom (D.O.F) selection, link design, stability and foot pad design. Robot size plays a major role. Based on this the cost of the project, materials required for fabrication and the number of actuators required can be determined. In this project miniature size of the robot is preferred so a height of 500mm is decided which includes mounting of the control circuits, but the actual size of the robot is 400mm without controlling circuits. The leg has got six degrees of freedom (Hip-3 D.O.F, Knee-1 D.O.F, Ankle-2 D.O.F), but implementing all the six D.O.F is difficult due to increase in cost of the project and controlling of the actuators which become complex, so in this project reduced degrees of freedom is aimed so 3 D.O.F per leg has been finalized.

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In this project U shaped bracket like arrangement is used for joints formation. The bracket consists of two parts namely servomotor bracket A and B (figure 2). Servomotor are fixed in the bracket A and the bracket B is used to transmit the output of the servomotor. Bracket B and servomotor are coupled using servomotor horn. By using the brackets there is a greater flexibility and individual joint can be actuated without disturbing the other joints. The servomotor brackets are designed in accordance with the motor size.

With quadruped mechanism, three points will be in contact with the ground surface. The stability of the robot is determined by the foot pad. In a statically stable gait, the vertical projection of the center of gravity (G) onto a horizontal plane is kept within the support area at all times, as shown in figure 3. In the absence of any inertial or external forces and if the ground is sufficiently rigid, the robot can remain stable as long as the G is within the support area (Fukuoka et al., 2003). For robots with point feet, a necessary condition for static stability is that the robot has at least three legs on the ground at all times. This is necessary in order to form an area of support that can contain the projection of G within its borders.

In figure 4, in the left part of the figure, three legs provide support and the projection of center of gravity is located inside the support area such that the robot is statically stable. The foot placement in the right part projects the center of gravity outside the support area, which leads to instability due to a tipping moment caused by gravity.

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Generally there is a concept that over sized and heavy foot pad will have more stability due to more contact area. But there is a disadvantage in using the oversized and heavy foot pad, because the torque requirement of the motor is more and lifting the leg against the gravity becomes difficult. By considering this disadvantage an optimal sized foot pad was used.

Stable walking pattern can be obtained only if the centre of mass and centre of pressure are within the supporting (Buehler et al., 1998). Generally walking cycle consists of two steps namely "initialization" and "walking". In the "initialization" step the robot will be in balanced condition and in this step the servomotors are made to return to home position. This will certainly help the robot to advance into the next step.

The support diagrams of four typical quadruped robotics' gait (trot, gallop, pace, walk) (Schmiedeler & Waldron, 1999) are shown in figure 5. A solid circle indicates that the leg is in contact with the ground providing support, and an open circle indicates that the leg is in the air returning to the appropriate position for its next period of support.

3. ROBOTIC CONTROL SYSTEM

The quadruped robot is controlled using a GUI (Graphical User Interface), which implementing different diagram gait. In figure 6 is show a screenshot. This GUI is made in Java and Java 3DAPI (Sun, 2008). In the buttom part of interface it is displayed the result of positional values combinations, which are possible to implementing a certain style for walk. In this mode, the users can choice the optimal positional value for locomotion. In right part of interface, in Keys control panel, there is four control buttons (Forward, Left, Right, and Back).

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With these buttons the robot can be controlled manual. In Results panel there are two buttons. With "Save" button can be saved the optimal positional value for all motors, which are corresponding to a style gait.

When the user click on the "Run it" button, the robot walk in concordance with the style gait choiced from the bottom part of interface. The buttons from File panel perform the following operation: permitte to user to open an existing file, save the result in an xml file, or import a compatible file.

The robot has four sensors for collision detection. Figure 7, shows the robot crossing an obstacole. On the real robot there are four micro-switches for collision detection with obstacles (whitch are implementing in foot leg's robot).

The algorithm for crossing the obstacoles is simple. The robot up his leg, and beat the obstacol. If is obstacol, the robot increase up leg and verified again is obstacol in face. When is no more obstacol in face, then robot move in concordance with a strategy to displacement over obstacle.

4. CONCLUSION

In this paper, the authors present a simulator for study different walking gait for a quadruped robot. Using this simulator, the user obtains the positional values for implementing in a microcontroller.

Complex movements can be achieved by increasing the D.O.F. Proximity sensors (which are able to detect the presence of nearby objects without any physical contact) and remote control through wireless ethernet mode will be considered.

5. REFERENCES

Buehler, M.; Battaglia, R.; Cocosco, A.; Hawker, G.; Sarkis, J. & Yamazaki, K. (1998). Scout: A simple quadruped that walks, climbs and runs. Proceedings of ICRA98, 1707-1712, (1998).

Fukuoka, Y.; Kimura, H. & Cohen, A. H. (2003). Adaptive Dynamic Walking of a Quadruped Robot on Irregular Terrain Based on Biological Concepts. In The International Journal of Robotics Research, Vol. 22, No. 2, February 2003, pp. 187-202.

Hong, Y.S.; Lee, H.K.; Yi, S.Y. & Lee C.W. (1999). The design and control of a jointed-leg type of a quadrupedal robot for locomotion on irregular ground. In Robotica vol.17, 1999, pp. 383-389.

Schmiedeler, J. & Waldron, K. J. (1999). The Mechanics of Quadrupedal Galloping and the Future of Legged Vehicles. In The International Journal of Robotics Research, Vol. 18, No. 12, December 1999, pp. 1224-1234

Sun Microsystem Inc., Java tutorial, (2008). Available from: http://java.sun.com/docs/books/tutorial/ Accessed: 2008-0412