Six Legged Mobile Robot based on Tripod Gait

Six Legged Mobile Robot based on Tripod Gait Victor Adîr, Tempea Iosif, George Adîr Theory of Mechanisms and Robots Dept., University Politehnica of Bucharest Spl. Independenţei no. 313, 77206, sector 6 Tel:410 39 85; 410 04 00 / 632 Abstract. The paper presents a six legged mobile robot with a structure inspired from the living life, namely insects. There are introduced some constructive solutions, the main technical characteristics of this robot, the constitutive systems, the phases of walking, the command and control systems, some experiments. 1. INTRODUCTION The Earth is permanently traversed by a great variety of insects and animals that have fascinated man, who wanted to copy their skill for movement. Thus, by copying the gait of insects and animals, have been achieved many walking vehicles, that have supported man to move in hostile environments, where the vehicles with wheels or caterpillar couldn t work, namely: marsh, ice, desert, outer space, under water. The achievement of a walking robot, based on biological principles, similar to an insect, is not an easy activity, because of the complexity to provide the stability and the safe movement of the walking robot. Usually, there is a strategy of movement for these robots, based on the design of some walking types in a defined environment, with the possibility of changing the moving speed from the gentle, wavy gait to one faster or even gallop. The use of this kind of robots has many advantages, as regards the ecological point of vue: it s no need to have roads for their movement; the pollution is quite diminished; it is not destroyed the soil, as it s happened when there are used vehicles with wheels or caterpillar. Fig. 2 2. CONSTRUCTIVE SOLUTIONS OF WALKING ROBOTS There are some classification criteria for walking vehicles, namely: a) The constructive principle,that shares walking vehicles in two categories: - machines based on biological principles (Fig. 1, Fig. 2); - machines based on a personal conception (Fig. 3, Fig. 4) Fig. 3 Fig. 1

Fig.7 c)the type of mechanisms for the supporting elements (legs): a. plane (Fig. 8); b. spatial (Fig. 4, Fig. 5) d)the number of legs: 2, 3, 4, 6, 7, 8 or more (Fig. 1 6 and Fig. 8 13) Fig. 4 b) The operating system, that can be: - electric (Fig. 1 4); - hydraulic (Fig. 5); - pneumatic (Fig. 6, Fig. 7); - a combination of these ones Fig. 5 Fig, 8 Fig. 6

Fig. 11 Fig. 9 Fig. 12 Fig. 13 e)the goal of their achievement 3. EXPERIMENTAL SIX LEGGED WALKING ROBOT 3.1 The structure of the walking robot Fig. 10 The walking robot depicted in Fig. 14 has the following main technical characteristics: -length: 590 mm; width: 450 mm; height: 230 mm; mass: 8.9 kg; operating: 8 stepping electrical motors

In the position c (Fig. 16 c) the legs 2, 3, 6 (already lifted in b position) are moved to forward, followed by a coming down motion to the ground. In the same time, the legs 1, 4, 5 are lifted. Now, the supporting phase is achieved by the legs 2, 3, 6. By operating the stepping motors M1, M2, the walking robot is moving forward.. In the position d (Fig. 16 d) the legs 1, 4, 5 are moved to forward, followed by a coming down motion to the ground. In the same time, the legs 2, 3, 6 are lifted, so, the supporting phase is achieved now by using the legs 1, 4, 5. By operating the stepping motors M1, M2, the walking robot is moving forward. For moving backward it has to be inverted the moving sense of the axes of motors. 3.3 Command & control system of the six legged walking robot Fig. 14 Also, it has the following systems: mechanical system, operating system, command and control system. So, the mechanical system is built of modular structure and has 6 identically legs. The operating system has 8 stepping electrical motors, that command the movement of legs, by using steel wires. (Fig. 15) The command and control electronic system has, as main parts, a controller (MINICONTROL RA) and a computer (PC) having a card with A / D conversion functions. The controller is a leading system to act the 8 axes of the stepping electrical motors (M1,, M8) according to the computer programme. In addition, the command system has all the necessary interfaces to operate the walking robot, to connect to an external process and, also, to connect to data acquisition system. The computer has a PCL 812 (card with A / D conversion functions) with the possibility to read 16 analog input of 12 bits resolution. The data transfer from computer to controller is accomplished on a parallel port. The general draft of connections is shown in Fig. 17, where: RP2000 M1 M2 M3 M4 M5 M6 M7 M8 SSw LSw HP ADC ITF DRIVE UNIT DRIVE UNIT CONTROLLER H/F E PC PPI SAFETY UNIT DIGITAL IN Fig. 15 3.2 The locomotion study of the walking robot The phases and subphases for the forward movement are presented in Fig. 16. PROGRAM & CONTROL UNIT M1,,M8 SSw LSw H / F E HP PPI ADC ITF PC MINICONTROL RA2 Fig. 17 - stepping electrical motors - synchronization switch - limit switch - half step / full step - enable - display home position - programmable port interface -analog digital conversion interface - personal computer - monitor DIGITAL OUT EXTERNAL PROCESS Fig. 16 Thus, it is possible to identify 2 phases: a supporting phase and a moving phase. In the initial position (Fig. 16 a) the walking robot is supported by the 6 legs, driven by steel wires from the M3,,M8 stepping motors. This is the synchronization position of the walking robot. In the next position (Fig. 16 b) the legs 1, 4, 5 are raised and moved to forward, with an angle, followed by a coming down motion to the ground. Then, supported on the legs 1, 4, 5, the legs 2, 3, 6 are lifted. By operating the stepping motors M1, M2, the walking robot is moving forward. When command the stepping motors, their dynamics may be controlled by the characteristic diagram between torque and the rotational frequency. Taking into consideration that the torque reserve of motors is limited by the rotational frequency and the walking robot is moving on irregular surface, is very necessary to have adaptable command. In this case, the adaptable command demands a continuous measurement of the torque in the motor shaft. This thing is easy to do if is measured the electric current into the reeling stator. The electric current is measured as a medium value in the rotational period of the electromagnetic field in stator. The instantaneous maximum value of the current is limited by the protection circuits of the operating level to an adjustment value for each stepping motor. In order to have such command model, there have been achieved some experiments, by measuring the medium current (after every 8 steps) correlated to the open - loop programmed rotational frequency, when the walking robot is moving, with different speeds, on various surfaces of a variable inclination. In the diagram from Fig. 18 there are presented the open loop programmed speed diagram for the two first motors of the walking robot and the torque boundary curve diagram for this kind of motors, according to the rotational frequency of the engines.

In Fig. 19 there are presented the torque diagrams for the two first motors, comparative to the boundary torque diagram obtained in the testing programme. 250 200 150 100 50 T[N*mm ] RF[steps/s] 0 1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 Steps T[s/10] Rotational frequency Fig. 18 on the Theory of Machines and Mechanisms, Oulu, Finland, June 20 24, 1999 6. Popescu I., Biomecanismele deplasării insectelor şi mecanisme bionice echivalente, Proceedings of the Sixth IFToMM Int l Symposium, SYROM 93, vol.2, pp.219 226 7. Popescu I., Iordăchiţă I.,Dumitru N., Rinderiu P., Mecanisme biologice, Ed. SITEH, Craiova, 1997 8. Tempea I., Adîr G., Adîr V., Roboţi păşitori soluţii constructive şi structurale, Ed. Casa Şcoalelor, Bucureşti, 1999 9. Tempea I., Adîr G., Moise V., Adîr V., Roboţi păşitori sinteza mecanismelor pentru acţionarea roboţilor păşitori, Ed. Casa Şcoalelor, Bucureşti 1999 10.Tempea I., Adîr V., The Mechanism of walking legged machines, Proceedings of the Seventh International Congress on the Theory of Machines and Mechanisms, September 3 5, 1996, Liberec, Czech Republic, pp. 611-616 T[N*mm] 205 210 200 195 200 190 190 185 180 180 175 170 165 170 160 150 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 Steps number 1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 Steps number/8 Torque motors Torque M1 Torque M2 Fig. 19 It can be noticed that the torque value doesn t pass beyond the admissible maximum value for all the electric motors, when the walking robot is travelling. This is possible because the testing conditions (surface, speed) have a necessary reserve, specifically to the open-loop command system. The closed-loop shifting to the limit of admissible parameters motor presume a higher shifting speed on irregular surfaces over the admissible limit of the mechanical compliance in the mechanisms of the walking robot. In this case, the real diagram will be situated nearby the limit of the reference diagram T (rf) without pass beyond it and the reference torque diagram float depending on frequency rotation variation of the engine (Fig. 20). Fig. 20 These diagrams present only two motors of the walking robot. The characteristics of the other 6 stepping motors are similar (the difference consists in the torque reference diagram and the programable speed of the engines). 4.CONCLUSIONS The design of mobile robots, similar to animals or insects, is based on a very strong analysis of a large documentation in different fields. It is important to underline that is very difficult or, in some cases, impossible, with the actual technique, to achieve, command and control different mechanical systems, similar to those that asssure the movement of animals and which contain about 400 operating devices. But, in spite of these difficulties man has to try to copy the nature for improving his life. REFERENCES 1. Adîr V., Ph. D. Thesis, University Politehnica, Bucharest, 2000 2. Brooks R.A., A Robot that walks; Emergent Behavi - ors from a Carefully Evolved Network, IEEE Int l Conference on Robotics and Automation,1989, pp 629 696 3. Dumitru N., Contribuţii la analiza şi sinteza unor mecanisme pe baza studiului mişcării insectelor, Teză de doctorat, 1996, Universitatea din Craiova, Facultatea de Mecanică 4. Hirose S., Three Basic Types of Locomotion in Mobile Robots, IEEE 1991, pp 12 17 5. Morecki A., Walking Machines Modelling, Design, Applications,The 10th World Congress