Mobile Mechanism Suitable for Rough Terrain Vehicle∗

646

Haruo HOSHINO∗∗, Atsuo ISHIKAWA∗∗, Toshino FUKUDA∗∗∗ and Yasuhisa HASEGAWA∗∗∗∗

We propose an interconnected suspension mechanism that is applicable to rough terrain mobile robots for construction sites. All wheels can follow on uneven ground by moving mutually with an interconnected suspension mechanism. And, it is possible to control the posture of the robot simply using the mechanism. In this paper, the structural and mechanical characteristics of the mechanism are discussed and the ground contact loads of wheels are analyzed. It was conﬁrmed the mechanism has enough ability for moving on an uneven ﬂoor and posture control through experiments with an experimental rough terrain moving robot.

Key Words: Mechatronics and Robotics, Hydraulic Actuator, Construction Machinery, Vehicle, Rough Terrain, Suspension, Posture Control

ground surface. 1. Introduction Since postures of vehicles such as machines and The aging of workers and lack of skilled workers robots change when these vehicles move in construction ff are more serious problems in the construction industries sites having many height di erences and unevenness and than in other industries, it is predicted that the produc- such a posture change leads to overturning, it is impor- tion capacities in the former will be decreased in the aged tant to control vehicular postures when moving on rough (5) society in the near future. Although mechanization and terrain . Previous studies have reported moving mech- robotization seem to be necessary countermeasures for anisms or posture controlling methods for rough-terrain this problem, difficulties such as unevenness of floor sur- running robots, including examples of a rough-terrain run- (6) faces and height differences are existent in construction ning active suspension robot by Tani et al. and a TAQT (7) sites and the today’s moving mechanisms have lower capa- carrier by Hirose et al. However, for rough-terrain run- bilities for dealing with these difficulties, thereby requir- ning and posture control, it has been required to control ing to develop a mechanism capable of efficiently mov- the posture of a vehicular body while simultaneously and ing on a rough terrain having unevenness or height differ- independently controlling four wheels to ensure ground ences. Meantime, although development of running parts contact of the wheels. It has thus been required to con- has been progressed in view of increased needs for rough- duct extremely complicated control for rough-terrain run- (5) – (10) ffi terrain running in the fields of engineering works and agri- ning , thereby problematically making it di cult to culture and forestry, while other development efforts have rapidly ensure the ground contact of wheels and the stabi- been conducted for planet rovers(2), (3) and mobile robots(4) lized posture of vehicular body. (11) – (14) such as adapted to off-road or sea-bottom, the main pur- We have proposed an interconnected suspen- pose of development is to transmit a driving force to a sion mechanism having all wheels move in an interconnected way to allow a vehicle to naturally follow uneven- ∗ Received 10th November, 2003 (No. 02-0299). Japanese ness of a floor surface and to easily control posture, and Original: Trans. Jpn. Soc. Mech. Eng., Vol.69, No.677, C experimentally produced a rough terrain vehicle with this (2003), pp.126–131 (Received 18th March, 2002) mechanism to confirm its effectiveness. This paper con- ∗∗ Research & Development Institute, Takenaka Corpora- siders the scheme of our interconnected suspension mech- tion, 1–5–1 Ohtsuka, Inzai-shi, Chiba 270–1395, Japan. anism, while giving the results of an experiment with the E-mail: [email protected] experimental rough terrain vehicle. ∗∗∗ Dept. of Micro System Engineering, Nagoya Univer- sity, Furo-cho, Chigusa-ku, Nagoya-city, Aichi 464–8603, 2. Interconnected Suspension Mechanism Japan ∗∗∗∗ Dept. of Mechanical Systems Engineering, Faculty of En- 2. 1 Scheme of four-point ground contact gineering, Gifu University, 1–1 Yanagido, Gifu 501–1193, Vehicles with fixed four wheels and without suspen- Japan sion have the center of gravity generally located on an in-

Series C, Vol. 47, No. 2, 2004 JSME International Journal 647 tersection of diagonal lines. Thus, when one wheel has in Fig. 3, four hydraulic cylinders supporting respective passed over a bump, the whole load of the vehicle is borne wheels are serially connected in an alternating manner in a by two wheels comprising this wheel and another wheel closed loop piping. When the four cylinders have the same on the same diagonal line, while the other two wheels configurations, two paired cylinders on one diagonal line float, thereby causing an unstable posture. Nonetheless, are moved in the direction opposite to and differentially by providing a differential mechanism as schematically from the other two cylinders on the other diagonal line in shown in Fig. 1 such that two pairs of diagonal wheels are this mechanism, thereby enabling the four-point ground mutually opposed and move vertically relative to the ve- contact identically to Fig. 2. hicular body, the load is necessarily distributed among all 2. 2 Action of the interconnected suspension mech- diagonal wheels so that no wheels partly float. Namely, as anism shown in Fig. 2, inclinations θ1, θ2 are caused for the di- Figure 4 shows a state where rod ends of cylinders agonal lines in addition to a height difference d f between C1, C2, C3, C4 of the interconnected suspension mecha- the diagonal lines caused by the differential mechanism, nism of this proposal shown in Fig. 3 contact with four thereby enabling ground contact at four points of differ- points h1, h2, h3, h4 having different heights from a ref- ent heights. One example of the mechanism exhibiting erence plane. Since the rod lengths of two cylinders on such an action includes a rocker bogie suspension used in one diagonal line are always the same, middle points M1, (3) Rocky IV (JPL) . This mechanism is constituted such M2, M3, M4 between neighboring two ground contacting that two diagonal lines are differentially moved by rocker points equal distances from the upper surface of the main arms. body and are located within the same plane. When a coor- In the basic constitution of the interconnected sus- dination system {F} having an origin at its center is settled pension mechanism proposed in this research as shown on this plane, the coordination system {F} has an X f −Y f plane representing the posture of the upper surface of the vehicle main body as shown in Fig. 5. The vehicular posture is represented by gradients θx, θy relative to the reference plane of X f , Y f , and the rela- tionships of height differences hx, hy and the heights of cylinder rod ends relative to Lx, Ly are represented by the following equations: h +h −h −h h = −L sinθ = 1 2 3 4 (1) x x x 2 Fig. 1 Principle of interconnected suspension

Fig. 2 Adaptation to uneven ground

Fig. 4 Mechanism of interconnected suspension

Fig. 3 Outline of interconnected suspension Fig. 5 Posture of vehicle

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h2 +h3 −h1 −h4 hy = Ly sinθy = (2) 2 In this mechanism, the strokes of two cylinders on one diagonal line are the same, and the cylinders on the diagonal lines are moved in the mutually opposite directions, respectively, so that the operating configuration of the suspension is uniquely determined by obtaining the stroke difference between one pair of diagonal cylinders and the other. This stroke difference between the cylinders is represented by d f in Fig. 4. The height difference da on the reference plane of the d f is represented by the heights of cylinder rod ends as follows: Fig. 6 Posture control method h +h −h −h d = 1 3 2 4 (3) a 2 The relationship between da and d f is represented in the following equation by utilizing Z-Y-X Euler angles (α,β,γ): d d = a (4) f cosβcosγ

β = θx (5) sinθy sinγ = (6) cosβ (a) State of inclined vehicle from Eqs. (1) and (2), L2 −h2 cosβ = x x (7) L x 2 2 1 Lxhy cosγ = L2 − (8) y 2 − 2 Lx Lx hx

The stroke difference d f between cylinders is represented by the following equation: 2 2 (b) After control LxLy d = d (9) f a 2 2 − 2 2 − 2 2 Fig. 7 Schematic of posture control LxLy Lyhx Lxhy From the above consideration, the vehicular posture can be determined by the ground contacting heights of represented by a broken line, thereby enabling control of the four points of the suspension cylinders, and the stroke the inclination in the pitching direction. difference between the suspension cylinders can be calcu- To explain the posture controlling method, Fig. 7 (a) lated. Further considering that a vehicle is always placed shows the mechanism of Fig. 4 viewed in the X f direction on a ground surface, it can be understood that the stroke and horizontally relative to the reference plane. As can difference between the cylinders has an unequivocal value be understood, although the rods have only two lengths ds in this mechanism, which enables ground contact of four and dl, the rod ends contact the ground surface at different wheels in a stabilized posture. heights due to inclination θy of the X f axis and inclination 2. 3 Posture control θx of the X f axis. Here, by transferring the operating oil As shown in Fig. 6, this mechanism enables inclina- communicated in the X f axis direction from the right side tion control by adopting pumps, each having the same to the left side by the pump as shown in Fig. 6, the cylin- input and output amounts, so as to transfer operating oil ders at the left side are expanded by the same stroke as the from two suspension cylinders on one side into two cylin- contracted cylinders at the right side. This enables control ders on the opposite side. When the operating oil at the of the inclination of the vehicle in the Y f axis direction and pushing side is transferred as shown by the arrow in Fig. 6, control of the vehicle horizontally as shown in Fig. 7 (b), the operating oil is allowed to be passively transferred by transferring the operating oil by the stroke obtained by between these cylinders also communicated on the other multiplying 1/2 of the right-left height difference by the sides, thereby allowing control of the inclination in the inclined amount of X f axis. Since the same control can rolling direction. This is also true for the pulling side as be performed in the fore-and-aft direction, posture control

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Fig. 10 Possible area for weight distribution

The load distribution in the Y-axis direction is similarly represented by the following equation, since the Fig. 8 Distribution of weight to wheels pulling sides are communicated with each other in the Y- axis direction:

WGyG = P1SLy − P3SLy (16) Assuming that the pressurization is zero upon initially sealing the hydraulic pressure into this system, P2 = P4 = 0 when the position of center of gravity is at the center. When the center of gravity is moved to the position of Fig. 8, the action for stopping rotation due to the moment around the Y-axis is performed by P2 and then P4 = 0. Fig. 9 Balance of moment Thus, the loads at the rod ends are obtained as follows, = from the Eqs. (10) – (16) and from P4 0: = 1 − 2 y + 2 can be performed by the above-mentioned method based W1 WG 1 G xG (17) 4 Ly Lx on outputs of sensors, which are installed on the vehicle 1 2 2 and are capable of measuring gradients of the vehicle. W2 = WG 1− yG − xG (18) 4 Ly L 2. 4 Load distribution x There will be a load distribution to wheels depend- 1 2 2 W3 = WG 1+ yG − xG (19) 4 Ly L ing on the position of center of gravity in this mechanism. x There is a situation where the interconnected suspension 1 2 2 W4 = WG 1+ yG + xG (20) mechanism is horizontally placed as shown in Fig. 8, and 4 Ly Lx W1 to W4 are assumed to represent the distribution of the Broken lines in Fig. 10 showing a plan view of this total weight WG to rod ends when the total weight WG is at system represent the insides of brackets of Eqs. (17) – (20), the center of gravity deviated from the center of the mech- and it is understood that W1 to W4 are positive when the anism. Assuming the pressure-receiving surface areas of position of center of gravity is inside the broken lines the hydraulic cylinder are S at the pushing side and N of Fig. 10 thereby achieving a stabilized state where the at the pulling side while the respective pressures are P1 whole load is duly distributed to four wheels, and that through P4 asshowninFig.8,therodendloadsW1 to W4 the partial load to the diagonally arranged wheel becomes and the total weight WG are represented as follows:. negative when the position of center of gravity is out-

W1 = P1S − P2N (10) side the broken lines thereby resulting in an unstable state switched into a 3-point ground contact. W2 = P3S − P2N (11) 2. 5 Features of this interconnected suspension = − W3 P3S P4N (12) mechanism W4 = P1S − P4N (13) The features of this interconnected suspension mechanism are described below. WG = 2P1S +2P3S −2P2N −2P4N (14) a. All the wheels are allowed to contact an uneven Since the pushing sides communicate with each other surface without requiring any power, by virtue of the X in the -axis direction as shown in Fig. 9, the load distribu- mechanism for interconnecting all wheels. X tion in the -axis direction is represented by the following b. The inclination control in the rolling direction and W P P equation based on the moment balance by G and 2, 4 pitching direction can be individually performed, by trans- Y around the -axis: ferring the operating oil between the cylinders at the op- WG xG = P2NLx − P4NLx (15) posite sides.

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c. More power is never required, because inclination control is performed in a balanced state and the vertical movement of the position of center of gravity is small. Previous research includes systems for ensuring ground contact by all wheels, such as a rhombic 4-wheel rover(2) and JPL Rocky IV(3). The research diﬀerentially moves the diagonal lines similarly to the suspension of this research, but fail to consider posture control. Further, posture control plays an important role in rough-terrain running, and there is an extensive research including a rough- terrain running active suspension robot(6), a TAQT car- (7) (9) rier and TOYOTA MOGULS . Many actively control Fig. 11 Experimental model the four wheels, and thus require measurements at many points such as for suspension strokes, wheel loads, and ac- celerations, as well as complicated calculations. Although research has been performed to ensure the ground contact of wheels by interconnecting two wheels such as in a swinging crawler type of rough terrain vehicle(5),because limitations are considered to be existent in the control of active suspension such as due to simultaneous es- tablishment of ground contact performance and stability of wheels, the changeover for such an interconnection and the posture control are complicated. The interconnected suspension mechanism proposed in this research ensures the ground contact of four wheels by virtue of the scheme Fig. 12 Outline of system of the mechanism, thereby enabling posture control. 3. Experimental Production of Rough Terrain Vehi- cle 3. 1 Outline of experimental apparatus Figure 11 shows part of the suspension of the experimental rough terrain vehicle and Fig. 12 shows its system constitution. At each of the right and left, three wheels are coupled via double rocker arms into a bogie structure, and the fulcra of the rocker arms are supported by an interconnected suspension mechanism based on hydraulic cylinders (inner diameter 20 mm, stroke 150 mm). Since the fulcra are coupled to each other, mounting portions Fig. 13 Slope for experiments of front two cylinders are made into rotatable trunnion structures. Running was performed by coupling the ex- 3. 2 Running experiment perimental model to a battery-type electromotive carriage The experiment was performed by measuring a pos- with a looped tensioned wire. As shown in Fig. 12, the ture change depending on the presence/absence of pos- posture controlling system uses dual-rod hydraulic cylin- ture control and on running speed when the experimen- ders (inner diameter 32 mm, stroke 200 mm) acting as tal model was driven to run such that its wheels at only pumps which are driven by AC servomotors (rated output one side passed over a mountain-shaped slope having a 120 W, manufactured by Sinano Electric Co., Ltd.) and height of 100 mm and a gradient of about 14.5◦ shown in pinion/rack gears. Posture control was performed by de- Fig. 13. Although the experimental model had six wheels, tecting gradients in the fore-and-aft direction and right- its analysis is the same as that in the previous chapter when and-left direction by a biaxial inclination sensor (detecting considering four points at the suspension ends, such that ◦ ◦ angle: ±60 ; precision: 1%; manufactured by A-PLUS), the posture of vehicular body can be obtained as θx = 5.8 ◦ and by feedback controlling the AC servomotors based and θy = 10.7 by the Eqs. (1) and (2) without posture con- on the gradients and angular velocities with the computer trol in Fig. 13. Although each rotatable cylinder caused (NEC-9801RA). The total weight of the experimental ve- agradientδ, four points necessarily contacted the ground hicle was about 35 kg. surface in the mechanism, thereby causing no operational

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Fig. 15 Experimental result without posture control

(a) Movement without control

(b) Movement with control Fig. 14 Movement on slope inconvenience. Further, the θ = 3◦ or less at the maximum in the experiment, so that the eﬀect on the cylinder stroke was extremely small on the order of 0.3%. Figure 14 shows the experiment. Without performing Fig. 16 Experimental result with posture control posture control, although ground contacts of wheels were ensured by the interconnected suspension mechanism as shown in Fig. 14 (a), the gradient was caused depending tional resistance of each cylinder while the response was on the slope as shown in Fig. 15. The above-mentioned low in a stable range. posture of vehicular body according to the analysis result 4. Conclusion of Fig. 13 corresponds to the position at nearly 7 seconds in Fig. 15, showing a substantially matching result. We have proposed, as a mobile mechanism adapted Upon performing posture control, running was real- for a rough terrain vehicle, an interconnected suspension ized at a lower speed (6 m/min) while substantially main- mechanism allowing all wheels to move interconnectedly taining the horizontal posture, and the absolute value of to ensure ground contact of the wheels and to facilitate the maximum gradient was decreased to 40% or less as posture control, and have considered eﬀectiveness such as shown in Fig. 16 than the situation without the posture concerning operation, posture control, load distribution of control. Although swinging was generated at increased this mechanism. As a result of the running experiment on speeds, this is considered to be caused by the fact that an a slope with the subsequently and experimentally produc- increased feedback gain resulted in instability due to fric- ing a rough terrain vehicle with the interconnected suspen-

JSME International Journal Series C, Vol. 47, No. 2, 2004 652 sion mechanism, it was proven that running was possible ( 7 ) Hirose, S., Sensu, T. and Aoki, S., The TAQT Carrier: at a lower speed while maintaining a substantially horizon- A Practical Terrain Adaptive Quadruped-Track Carrier tal posture. Since the ground contact of wheels is ensured Robot, IEEE/ RSJ Int. Conf. on Intelligent Robotics by virtue of the scheme of the interconnected suspension and Systems, (1992), pp.2068–2073. ( 8 ) Horiuchi, E., Usui, S., Tani, K. and Shirai, N., Control mechanism, it has been confirmed that horizontal control of the Active Suspension for Wheeled Vehicle, Trans- is enabled simultaneously in two directions with only one actions of the Japan Society of Mechanical Engineers, inclination sensor for posture control of a rough terrain ve- (in Japanese), Vol.55, No.515, C (1989), pp.1697– hicle, and this is considered to provide a sufficiently high 1702. practicability. ( 9 ) Fujii, K., Development of TOYOTA MOGULS’s Ac- tive Posture Controlling System, Toyota Technical Re- References view, (in Japanese), No.33 (1996), pp.2–8. ( 1 ) Ito, N., Perspective and Prediction Concerning Run- (10) Terai, A., Stepped Axes Type Vehicle Movable on ning Parts of Rough Terrain Vehicle, Journal Construc- Slope, Journal of the Forestry Mechanization Society, tion Mechanization, (in Japanese), Vol.451 (1987), (in Japanese), No.440 (1990), pp.37–42. pp.22–26. (11) Hoshino, H. and Ishikawa, A., Development of ( 2 ) Hirose, S. and Ootdukasa, N., Design and Develop- Rough Terrain Vehicle, 1st Report, Posture Controlling ment of Quadra-Rhomb Rover for Mars Exploration, Method Using Interconnected Suspension Mechanism, JSME Conference on Robotics and Mechatronics, (in 1998 JSME Conference on Robotics and Mechatron- Japanese), (1993), pp.356–359. ics, (in Japanese), CD-ROM, (1998), 1BV1-4. ( 3 ) Reynolds, K., Mars Rover Test. JPL Rocky IV, Road & (12) Hoshino, H. and Ishikawa, A., Development of Rough Track, Vol.44, No.8 (1993), pp.92–97. Terrain Vehicle, 2nd Report, Experiments of Posture ( 4 ) Waldron, K.J., Terrain Adaptive Vehicles, Transactions Control with Experimental Model, 1999 JSME Con- of the ASME, Vol.117 (1995), pp.107–112. ference on Robotics and Mechatronics, (in Japanese), ( 5 ) Yamamoto, H., Development of Off-Road Vehicle with CD-ROM, (1999), 1P1-02-011. Rotating Crawlers (First Report), Mechanism of the (13) Ishikawa, A. and Hoshino, H., Development of Rough Vehicle and Characteristics of Its Running Device, Terrain Vehicle, Proc. of the 7th Symposium on Con- Journal of Japanese Society of Agricultural Machinery, structional Robotics, (in Japanese), (1998). (in Japanese), Vol.56, No.1 (1994), pp.3–12. (14) Hoshino, H., Ito, M., Eguchi, J. and Furuya, M., Devel- ( 6 ) Tani, K., Matsumoto, O., Usui, N. and Horiuchi, opment of Rough Terrain Mobile Elevating Work Plat- H., Rough-Terrain Running Active Suspension Robot, form, Proc. of the 8th Symposium on Constructional Journal of Mechanical Engineering Laboratory, (in Robotics, (in Japanese), (2000). Japanese), Vol.46, No.2 (1992), pp.211–217.

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