Load Distribution and Body Angle Measurement in Static Walking of a Quadruped Walking Robot

Copyright @ IF AC Bio-Robotics, Information Technology and Intelligent Control for Bio-Production Systems, Sakai, Osaka, Japan, 2000

LOAD DISTRIBUTION AND BODY ANGLE MEASUREMENT IN STATIC WALKING OF A QUADRUPED WALKING ROBOT Hiroshi N akashima' Masaru Tokuda" Hiroaki Yamamoto ••• Masafumi Funakoashi •••• ,1

• Associate Professor, Division of Environmental Science and Technology, Graduate School of Agriculture, Kyoto University, 606-8502 JAPAN. •• Instructor, Department of Agricultural and Environmental Engineering, Faculty of Agriculture, Kobe University, 657-8501 JAPAN. ••• Professor, ditto . •••• Master Program Student, Graduate School of Science and Technology, K obe University, 657-8501 J A PA N.

Abstract: A prototype quadruped walking robot has been developed to examine the possibility of application in agricultural works. The purpose of current study is to obtain some information on load distribution and inclination angle of vehicle body in three typical motion of forward , lateral and turning walk. The result showed that the current configuration of robot body required the severe translation of CG point which results in increasing of pitching angle of vehicle body. Copyright © 2000 IFAC Keywords: robotics , quadruped robot , static walking, load distribution , pitching and rolling angle

1. INTRODUCTION

less destroyed (Song and Waldron , 1989 ) and thus ideal for the growth of plant within the field . In mountainous areas, enough space for farm vehicle introduction to small fields cannot be prepared and this may be regarded as one of the reasons for abandoning of agriculture in the mid-mountainous areas in Japan . The walking agricultural robot may be applied in such areas to assist agriculture as a power source, which is comparable to an animal assistance in the early stage of agricultural mechanization. This is why our study on an introduction of walking mechanism to agriculture has come into action.

The conventional wheeled and tracked vehicles , such as farm tractors and combine harvesters , travel with continuous contact mechanism of running device. Thus, it is difficult to maneuver the non-fiat pathway, such as ridge in the field , without causing any destruction in field terrain. It is obvious that the discontinuous contact of running device which is only realized by walking mechanism is good at keeping the surface I

Corresponding author. E-mail:994a·[email protected]

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Fig. 2. Load cell for load distribution measurement

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1. Prototype quadruped walking robot MODEL-II Table 1. Specification of MODEL-I! Items

Data

Length(m) 0.86 Width(m) 0.66 Height(m) 0.92 Number of legs 4 N umber of DO F per leg 3 Number of DC Motors 12 51 Wd N ) 314.9 Wv(N) Note: WL=Leg Weight; Wv=Vehicle Weight

Fig. 3. Sensor for body inclination measurement 2. MEASUREMENT SYSTEM The MODEL-II robot was used in the measurement. Two types of sensors; (i)ring type load cell for load distribution measurement and (ii)inclination sensor for pitch and roll angle measurement, were added on the robot(Yamamoto , 2000). The power supply was also modified to increase the revolution velocity of DC motors.

Walking can be divided into two types, one is static walking which always keeps the projected center-of-gravity(CC) point in the support polygon domain and the other is dynamic walking where legs are activated fast without falling into statically unstable condition. In our research, our main purpose is to discuss the possibility of application of walking mechanism and operation of a walking robot in dynamic conditions is not necessary. The effect of total number of legs has already been surveyed and a prototype quadruped walking robot , MODEL-Il, as shown in Fig.1(Nakashima, 1999)(Ninomiya, 1999)(Funakoshi, 1999) was constructed whose specifications were listed in Table 1. With MODEL-U robot, the static walking with the average forward velocity of 0.26cm / s could be realized by using the predetermined gait pattern which was obtained by trial-and-error method.

As for load cell, rectangular ring-type load cell was manufactured and installed at the tip of each leg as shown in Fig.2. It should be noted that the measurement of load distribution is done in the direction of leg and not in the direction of normal axis to the horizontal plane. This is why the total summation of measured load from four legs does not show the same value in the next section. The commercial inclination sensor, SL-OIJS by Ohmic Co., was used for the inclination measurement of the vehicle. Two inclination sensors were installed onto the cent er point, or reference point, of the robot body (Fig.3), to detect pitching angle rj) and rolling angle wof the robot.

The purpose of current study is to collect some information on load distribution and robot body inclination while in forward walk. lateral walk and turning walk. After obtaining these data, one can further develop a flexible walking pattern where the location of CC will automatically be controlled by the computer while walking.

As for the pitching angle , the downward inclination with respect to reference point is assumed to be positive in sensor output. The rolling angle have a positive sign when the clock-wise rotation with respect to vehicle's forward direction is detected .

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Four IC-type strain amplifiers are installed and mounted on the interface board which is set on the robot body. Moreover, A/D conversion board is also added on the computer unit so that one can obtain the measured result while controlling the vehicle's walk.

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Fig. 5. Result of body inclination for forward walk

In the following walking pattern result figures, same leg symbols are used; RR: rear right leg, FR: front right leg, RL: rear left leg and FL: front left leg. As for loading results, we schematically classified the state of loading at the tip of leg as .A.: 0 < P ::; 49(N), .: 49 < P ::; 98(N), .: P > 98(N) and 0: P = O(N) which means noncontact.

to its shoulder. This is how the vehicle can walk. Moreover, in steps 5, 9 and 13, the center of gravity adjustment action is applied to translate the CG point into or onto the support triangle which is defined by the supporting three legs. When a leg was detached and moved, the opposite leg at the end of diagonal line of the moving leg should be activated upward to transfer the load for maintaining the static stability of the vehicle. Thus it was confirmed that the load transfer which results in the increase of pitching angle was necessary to keep the stability of the vehicle in case of static walking of a quadruped vehicle.

3. MEASUREMENT RESULT AND DISCUSSION 3.1 Forward walk

In this walk, the motor IW3 which activated the adduction/abduction of a leg was not used and the each leg was activated in 2DOF. The pattern was quite similar to so-called wave gait. Schematic walking pattern is shown in Fig.4. The load distribution of each leg is shown in Fig.5. and the corresponding inclination angle result is shown in Fig.5.

The leg FR becomes non-contact in steps 5 and 7, but the leg RR supports nearly half of the vehicle weight as shown in Fig.5. As is already stated, the summation of the output of four load cell does not show the unique value , but the average load was found to be 334.1 N. '\loreoyer. in Fig.5. the behavior of pitching angle curve is significant and the largest value reaches about 12 deg. This result explains the fact that the vehicle showed the severe movement of backward bow in steps 5 to 9. The rolling angle shows rather smooth behavior and the maximum value is found to be 3 deg. These phenomena show that the pitching angle affects the vehicle balance more than the rolling angle.

The number of 0-18 in Fig.4 implies the leg movement step as follows; steps 0-1: the vehicle body moves forward with respect to its shoulder. steps 2-4: the leg RR is activated to occupy the new position. The leg FR, RL and FL is moved like RR sequentially and finally the vehicle body is once again moves forward with respect

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Fig. 9. Result of body inclination for lateral walk Three contacting legs do not support as large load as in the case of forward walk in every step. The vehicle could keep the static stability of the body not only by applying the load transfer in pitching direction but also by applying the load transfer in rolling direction. Average value of the summation of four load cell result was 331.1 N.

Fig. 7. Schematic walking pattern for lateral walk ~ ,-----------------------------.

The consumed time of the lateral walk is 82 seconds, and the walked distance is 12.5 cm. The one cycle distance was found to be shorter than in the case of forward walk because of the available stroke of slide mechanism for M3 motor.

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In this walk, M 1 , M2 and M3 motors were activated and each leg was become actuated with 3 DOF. This walk pattern is shown in Fig.lO. The load distribution on each leg is shown in Fig.11, and the result on inclination angle of the body is shown in Fig.12.

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Fig. 8. Result of load distribution for lateral walk The consumed time of the one cycle of forward walk was 83 seconds, and the walk distance was 32.0 cm, thus the average velocity was 0.39 cm/so

In Fig.1O, the body turned its direction in steps 0-1. The order of the leg movement to be able to maintain the statical stability of the vehicle was decided by trial-and-error method. The leg RL was activated in steps 3-5, and other legs were moved similarly in the order of FL,FR and RR. Moreover, the adjustment of the CG point was done in steps 2, 6, 10, 14, 15 and 19. The load distribution result of Fig.II indicates that one or two leg supports a large amount of vehicular weight when a non-contacting leg is activated. Fig.I2 shows that the change in rolling angle is not so large as that in pitching angle. In this case, the average total load was found to be 323.3 1\. The difference in the average total load was thought to be the result of complicated movement of each leg in 3 DOF.

3.2 Lateral walk In this experiment, the motor Ml which rotates the leg unit at its shoulder was not used, but each leg was actuated with two degrees of freedom using M2 and lv13 motors. This walk pattern is shown in Fig.7. The load distribution on each leg is shown in Fig.8 , and the result of the inclination angle of the body is shown in Fig.9. Firstly, the legs FL and RL which becomes the rear leg of the forward direction in lateral walk were activated in steps 0-9, and the body was moved to the reverse direction with the activation of all M2 motors in four legs in step 10. Then, the legs FR and RR were activated to the reverse direction in steps 11-19. Thereby, the one cycle of lateral walk was completed.

The consumed time of the one cycle of turning walk is 75 seconds, and the turning angle was 28 degrees.

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The obtained results by current study are summarized as below .

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(1) By using our prototype quadruped walking robot, some information on load distribution and body inclination for three typical motion of forward, lateral and turning walk could be obtained. The body inclination angle in case of forward walk was found to be 12 deg. (2) When a leg was detached and moved, the opposite leg at the end of diagonal line of the moving leg should be activated upward to transfer the load for maintaining the static stability of the vehicle in case of our quadruped vehicle. (3) Obtained information will further be utilized to develop a flexible walking pattern where the location of CC will automatically be controlled by the computer while walking. [Acknowledgements] This study was partly supported by

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the Grant-in-aid for Scientific Research from MESC(No.094601 The assistance given by Mr. F. Tanimori, formerly senior student , in the experiments was fully acknowledged.

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5. REFERENCES

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Funakoshi, A. (1999). On an automatic control system for a prototype quadruped walking vehicle for agriculture. Unpublished thesis. Faculty of Agriculture, Kobe University. Nakashima, H. et al. (1999). Development of a quadruped walking robot for agricultural environment. Proceedings of the 13th International Conference of IS T VS, Munich pp. 711718. Ninomiya, A. (1999). On manufacturing of a prototype quadruped walking vehicle for agriculture. Unpublished thesis. Faculty of Agriculture , Kobe University. Song, S. M. and K. J. Waldron (1989). Machines that walk. The MIT Press. Boston. Yamamoto, H. et al. (2000) . Studies on an agricultural walking vehicle-part ii. on sensor system for applied walking. K ansai Branch Report of JSAM 87, 73-76 .

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