An Earth Auger as Excavator for Planetary Underground Explorer Robot Using Peristaltic Crawling H. Omori *, T. Murakami, H. Nagai, T. Nakamura **, and T. Kubota *** * Department of Precision Mechanics, Chuo University, Japan h_omori@bio.mech.chuo-u.ac.jp ** Department of Precision Mechanics, Chuo University, Japan nakamura@mech.chuo-u.ac.jp *** Japan Aerospace Exploration Agency (JAXA) kubota@isas.jaxa.jp Abstract Exploration of planets has significant chances of discoveries e.g. discovery of new substances, as well as finding keys to the origin of our planet. However, some terrains are poorly explored, especially the underground ones. Thus, we have developed underground explorers that use peristaltic crawling of an earthworm for a propulsion part and an earth auger (EA) for an excavation. In this paper, we introduce our concept of an underground explorer. Next, we explain the two different diameter EAs that can be used for the propulsion part. Then we perform excavation experiments, pushing force experiments and rotation speed experiments. We confirmed that both EAs could excavate and the Auger 5 showed good excavation performance. 1 Introduction We have performed space exploration for many years. It has great potential for discoveries, e.g. the discovery of new substances as well as finding keys to the origins of our planet. However, some terrains are poorly explored, especially underground ones. Shield and boring constructions are used for digging the earth. This equipment tends to be large-sized and consume large amount of energy. However, small and unmanned explorers are needed to address issues of safety and development concerns for planetary exploration. Both mole [1] [] and screw type [3] equipment have been developed as small, unmanned explorers. These explorers can excavate tens of centimetres in regolith simulant. It will be necessary, however, for underground explorers to excavate deeper on surface materials. Thus, we focused on the peristaltic crawling of an earthworm. This locomotion mechanism maintains a large area of the body in contact with the surrounding regolith, which supports the body position against rotating excavation and transport mechanism. The units contraction and extension velocities are controllable to produce peristaltic crawling locomotion, which can also control the velocities of an excavator along a vertical axis against the front regolith. In addition, the robot has space for transport mechanism that excavates the surface regolith and shifts it to the rear. Therefore, this mechanism is suitable as the propulsion mechanism for an underground explorer robot. A variety of earthworm robots have been developed using different actuators []-[]. Taking aerospace tasks into consideration, we developed an earthworm robot created with servomotors, which can move downward and upward in perforated soil [7]. However, the robot was initially not equipped with an excavation tool for drilling holes in the regolith. Here, we develop an excavation tool and combine it with the peristaltic crawling robot. In this paper, we explain our concept of an underground explorer robot using peristaltic crawling for propulsion and an earth auger (EA) for excavation. In the process of excavation, the EA head excavates soil, and its spiral carries the excavated soil to the rear. Different head shapes and spirals were examined for this excavation process []. The fish tail single spiral was found to exhibit higher excavation efficiency than any of the other types. This explorer robot is equipped with both a propulsion part and an excavation part. The excavation mechanism must excavate a hole of the same diameter as the propulsion unit to allow movement. The excavated soil is carried through the propulsion unit by the screw of the EA. Therefore, the EA requires different diameters for the excavation mechanism and the transport mechanism. We examined two different diameter EAs with different skirt angles and found that both could excavate. Then, we performed excavation experiments, evaluating excavation velocities and motor torques against motor rotation speeds and pushing force. From these experiments, we determined specifications of the propulsion part that is required to combine to the excavation part. i-sairas 1 August 9-September 1, 1, Sapporo, Japan 7
Peristaltic crawling of an earthworm An earthworm moves by peristaltic crawling [9] [1]. Fig. 1 shows the locomotion pattern of an earthworm during peristaltic crawling. First, the earthworm contracts its anterior segments. This increases the friction between the segments and the surrounding surface, as the thicker segments are in contact with the surface during locomotion. This friction generates a reaction force that extends the contracted front segments in the desired direction. The contraction propagates continuously towards the rear. This movement pulls the rear segments in the direction of movement. After the contraction is complete, the anterior segments of the earthworm are extended in the axial direction. This type of locomotion mechanism has the following advantages: 1. It requires less space than other mechanisms, e.g., bipedal, wheel-based and meandering locomotion.. It should be more stable underground because contact is maintained with a large area of surrounding soil. This mechanism is, therefore, suitable for an underground explorer robot. Contraction Friction area EA with an excavation, transport and discharge mechanism. The processes of excavation and transportation are performed by a simple mechanism. The EA can dig the hole with the top part excavating the regolith and the screw part removing and transporting the excavated regolith to the rear of the EA. The robot excavates underground making complete use of these mechanisms. Fig. 3 shows the motions of excavation. 1) Contracted units maintain contact with the wall of the hole and hold the body position and orientation against the rotation of the excavating EA. At the same time, the EA excavates the regolith in front of the robot. -) Contractions propagate towards the rear and the spiral of the EA (transport mechanism) carries excavated regolith to the rear. The regolith is then discharged from the rear. -1) The second and third units from the front contract and hold their position, and the front unit extends. At this time, the robot can move downward. The excavation robot performs phases 1- and excavates underground. Discharge Mechanism Direction of movement Transport Mechanism Propulsion Mechanism Propulsion Mechanism Excavation Mechanism Figure 1. Pattern of earthworm locomotion with peristaltic crawling 3 Concept of an underground explorer robot In this section, we explain the concept of an underground explorer robot, as illustrated in Fig.. The robot consists of two parts: a propulsion part and an excavation part. We adopted the peristaltic crawling of an earthworm as the propulsion mechanism, with an EA as the excavation tool. The propulsion part of the robot consists of four units, corresponding to the segments of an earthworm. Each unit can expand in a radial direction when it contracts in an axial direction and generates large amount of friction between the surrounding regolith and its body. In contrast, the excavation part of the robot includes an Figure. Concept of the underground explorer 1) ) 3) ) 1) Figure 3. Motions of excavation 75
Different diameters of EA excavation mechanism through the transport mechanism because of the different diameters of the EA and skirt part. Here, the pitch of the transport mechanism Pr is longer than the top pitch Pf, which minimizes the compression as less as possible. Fig. 7 shows the developed excavation part. A covered (ABS) DC motor (Maxon) rotates the EA. A pipe is fastened to the skirt part. Aluminium shafts connect the skirt part and the motor cover, to maintain the cover from rotating and to support the axial position of the EA. Its total weight is 1.9 [kg]. In the process of excavation, the EA head excavates regolith and the spiral carries excavated regolith to the rear. We examined different head shapes and spirals for this excavation process. Two types of auger heads, the fish tail [11] and the tip cone, have been developed. Both auger heads have two spiral types: single and double spirals. In excavation experiments, we confirmed that the fish tail single spiral could excavate more effectively; however, the excavation mechanism of this robot excavates a hole the same diameter as the propulsion part to allow movement. The excavated soil is carried through the propulsion part by the screw; therefore, the diameter of the EA must differ between the excavation and the transport mechanisms. In this paper, we develop two types of different diameter EAs for combination with the propulsion part, and conduct some excavation experiments. Pipe Pr Skirt angle T Pf.1 Development of a different diameter EA Fig. shows the concept of an EA with different diameters of spiral. It has a fish tail single spiral with a larger diameter at the top than in the transport mechanism. It has a skirt part that assists in smoothly carrying the excavated regolith to the transport mechanism. The skirt angles T were set at 3 and 5 [deg]; the former is called Auger 3 and the latter is called Auger 5. We conducted experiments to determine if those EAs could excavate and then examined the effect of the skirt angle on motor torque (excavation reaction) and excavation speed. Skirt d D Auger Figure 5. Different diameter EA Table 1. Specifications of Auger 3 and 5 Auger 3 Auger 5 Diameter D [mm] 13 Diameter d [mm] Pf [mm] Pr [mm] Skirt angle [deg] Auger Weight㩷 [g] Skirt weight [g] Propulsion part Pipe 5 55 3 97.5. 55 5 9.5.7 Skirt pat T d D 5[deg] 3[deg] Figure. Concept of the improved F-S EA Fig. 5 shows the EA parameters and Table 1 shows the dimensions corresponding to each parameter. As seen in Fig., the reach of Auger 3 from the top of EA to the excavation part is shorter than that of Auger 5 and excavated regolith tends to compress from the top of the Auger 3 Auger 5 Figure. Two types of Augers and skirt parts 7
Table. Comparison of soil characteristics Motor part Regolith Regolith simulant Reddish soil EA Pipe (PVC) Bulk density [g/cm3] Internal friction angle I [deg] 1.5~1.79 1.~. 37. 1.1 3. 5 Excavation experiments Aluminum shafts In the experiments, we added loads on the excavation part and measured the excavation velocity and reaction force, which is the motor torque. Next, we controlled the motor rotation speed. Fig. 9 shows the experimental results for Auger 5. We confirmed that it could excavate. Skirt Part Figure 7. Developed excavation part a b c d. Experimental setup Fig. shows the excavation experimental setup. The rear end of the motor part is fixed on linear guides with a plate. Rotation of the excavation part is suppressed and the excavator can excavate downward smoothly due to the linear guides. The wire sensor, which measures the excavation depth, is set at the top of the frame. A counterweight is fixed to the rear of the motor through a wire and the pushing force for excavating soil is controllable by changing its weight. EAs excavate reddish soil. Table shows a comparison of the internal friction angle among regolith and regolith simulant [1]. The internal friction angle of reddish soil is different from that of regolith, while the characteristics of regolith and regolith simulant are similar. Figure 9. Excavation of Auger 5 5.1 Load experiments The pushing force and velocity of the excavation part onto regolith is controllable by the propulsion part (Fig. 3). In this experiment, we added loads on the excavator and measured the depth of excavation and motor torque. The added loads were 7., 1, 17,, 7 and 3 [N] at the beginning of the experiments. The motor rotation speed was 1 [rpm]. Figs. 1 and 11 show the depth of excavation of Augers 3 and 5 versus time. The excavations were nearly linear, except at the start of the excavation. We think the reason the excavation speeds were faster for a couple of tens of seconds from the beginning of experiments is that the earth pressure of surface soil was smaller than in deeper soil. Fig. 1 compares excavation velocities. With Auger 3, the excavation velocity reached a maximum with a pushing force of [N] and it was 17 [N] for Auger 5. We assume that it was because this the excavator has a skirt part. The maximum amount of excavated soil is defined by the space between the skirt part and the top of EA. Figs. 13 and 1 show the results for motor torque versus excavation depth. Both results indicate that motor Wire sensor Linear guides Counter weight Excavation part Reddish soil Figure. Experimental setup 77
.. Excavation velocity [mm/s] 15 1 5 1 15 1 5. 1.5 1..5. 3 [N] 7 [N] [N] 17 [N] 1 [N] 7. [N] Time [s] Figure 1. Depth of excavation (Auger 3) 1 1 3 [N] 7 [N] [N] 17 [N] 1 [N] 7. [N] Time [s] Figure 11. Depth of excavation (Auger 5) Auger 3 Auger 5 7. 1 17 7 3 Pushing force [N] Figure 1. Velocity comparison (1[rpm]) 3 [N] 7 [N] [N] 17 [N] 1 [N] 7. [N] 5 1 15 Figure 13. Excavation reaction force (Auger 3) torque increased from the beginning, and then reached a steady value. The steady values were reached when EA screws were filled with excavated soil. We found that Auger 3 needed more motor torque than Auger 5. We assume this is because the stroke from the top of the excavation mechanism to the transport mechanism of the skirt part for Auger 5 is longer than that of Auger 3, and that the excavated soil in Auger 3 was compressed more and carried to the transport mechanism. 1 3 [N] 7 [N] [N] 17 [N] 1 [N] 7. [N] 5 1 15 Figure 1. Excavation reaction force (Auger 5) 5. Changing rotation speed Next, we experimented with changing rotation speeds. With a pushing force of 17 [N] and rotation speeds of 3, 5, 1 and 1 [rpm], we measured motor torque and excavation velocities. 1 [rpm] 1 [rpm] 5 [rpm] 3 [rpm] 5 1 15 Figure 15. Rotation speed experiment (Auger 3, 17[N]) 1 [rpm] 1 [rpm] 5 [rpm] 3 [rpm] 5 1 15 Figure 1. Rotation speed experiment (Auger 5, 17[N]) 7
Excavation velocity [mm/s]. Figs. 15 and 1 show motor torques versus the excavation depth. Increasing the rotation speeds did not show great differences; however, increasing the rotation speeds also increased the deflection angles of the excavation mechanism, making the excavation process unstable. Fig. 17 compares the excavation velocities at the rotation speeds. The excavation velocities were faster when the rotation speeds were increased. We conclude that the amounts of soil taken into EA increased per second..5. 1.5 1..5. Figure 17. Velocity comparison of rotation speed experiment (17[N]) Conclusions Auger 3 Auger 5 3 5 1 1 Rotaion speed of earth auger [rpm] A novel underground explorer was suggested using the peristaltic crawling of an earthworm as the propulsion mechanism and an EA for excavation. We then developed two EAs, Auger 3 and Auger 5, which had different diameters of excavation and transport mechanisms. We confirmed that this improved excavator can effectively excavate soil. Several excavation experiments were conducted, with various pushing forces and changing the rotation speed, and concluded that Auger 5 had higher efficiency than Auger 3. The largest velocity of Auger 5 was. [mm/s] with a pushing force of 17 [N] at 1 [rpm] and the largest motor torque was 9 [Nm] with a pushing force of 3 [N] at 1 [rpm]. Therefore, the propulsion unit should withstand at least 9 [Nm] of rotation reaction of the EA. In the future, an underground explorer robot will be developed having both an excavator and a propulsion part, and it will perform several excavation experiments in regolith simulant. Symposium on Artificial Intelligence, Robotics and Automation in Space, 3 [3] K. Nagaoka, T. Kubota, and M. Otsuki, Experimental Study on Autonomous Burrowing Screw Robot for Subsurface Exploration on the Moon, Proc. of IEEE International Conference on Intelligent Robots and Systems,, pp. 1-19 [] N. Saga, and T. Nakamura, Development of peristaltic crawling robot using magnetic fluid on the basis of locomotion mechanism of earthworm, Smart Material and Structures, Vol.13, No.3, pp. 5 59, [5] N. Saga, and T. Nakamura, A prototype of peristaltic robot using pneumatic artificial muscle, Intelligent automation system, no., pp. 5 95, [] J. Zuo, G. Yan, and Z. Gao, A micro creeping robot for colonoscopy based on the earthworm, Journal of Medical Engineering & Technology, vol. 9, no. 1, pp. 1 7, 5 [7] H. Omori, T. Nakamura, and T. Yada, An underground explorer robot based on peristaltic crawling of earthworms, Industrial Robot, An International Journal of Industrial and Service robotics, Vol. 3 No., pp. 35-3, 9 [] H. Omori, T. Nakamura, T. Yada, T. Murakami, H. Nagai, and T. Kubota, Excavation Mechanism for a Planetary Underground Explorer Robot, Pro. of International Symposium on Robotics, 1, to be presented [9] H. Sugi, Evolution of Muscle Motion, the University of Tokyo Press, pp. 7, 1977 (in Japanese) [1] R. M. Alexander, Exploring Biomechanics, Animals in Motion, W. H. freeman and Company, 199 [11] A. Kato, Excavating Machines and particular kind of Excavation, Japan Society of Civil Engineers, pp. 73 7, 1979 (in Japanese) [1] S. Wakabayashi, K. Matsumoto, Development of Slope Mobility Testbed using Simulated Lunar Soil, JAXA Research and Development Memorandum, References [1] H. Kudo, and K. Yoshida, Basic examine by mole-type moon excavation robot, The Society of Instrument and Control Engineers Tohoku Chapter. 13rd, no. 13-, July, 1999 (In Japanese) [] K. Watanabe, S. Shimoda, T. Kubota, and I. Nakatani, A Mole-Type Drilling Robot for Lunar Subsurface Exploration. Proc. of the 7th International 79