IROS2019国际学术会议论文集 0559

Development of an adaptive hexapod robot based on Follow the contact point gait control and Timekeeper control Yuki Murata1 Shinkichi Inagaki1and Tatsuya Suzuki1 Abstract In this paper a new control method for a hexapod robot walking on irregular terrain based on a human operator s foot placement navigation is proposed and evaluated The control method is based on the Follow the contact point FCP gait control that operates on the principle that each leg follows the contact point of its foreleg Hence planning the contact points for all the legs are summarized to one of the front legs To dedicate the FCP gait control to a hexapod robot three control architectures are added First new constraints in the transition from a stance phase to a swing phase are added to maintain static stability when a leg leaves the ground Second a real time posture control system for contacting legs is added The third is an adaptive control to adjust the time elapsed in each control mode by which specifi cations of deadlock free and static stability are satisfi ed The proposed control architecture is installed to a small hexapod robot and its performance is evaluated through experiments wherein the robot walks on uneven terrain I INTRODUCTION Mobile robots are often required to traverse irregular terrains during exploratory activities in disaster like sites and uncharted territories In particular hexapod robots are expected to have not only high stabilities but also diverse ranges of walking motions due to their redundant number of legs and the mechanisms that control them Such hexapod robots are designed to be capable of navigating many dif ferent environments and performing many tasks One major consideration of such robots is the management of all six legs to allow the robot to keep walking even on irregular environments One method called Free gait is based on the optimization of all the legs movements 1 5 The behaviors of the legs are planned by considering their contact points and the desired body posture simultaneously However since the computational burden of this method is high due to the many degrees of freedom involved real time control by this method is diffi cult for small onboard computers Another approach relies on the particular mechanism of the legs RHex 6 is a multi legged robot whose legs have high elasticities and only one joint This robot shows great traversability on complex irregular terrains while maintaining a low computational burden Unfortunately the robot is not able to maintain its body in one desired posture because of the low degrees of freedom Tasks such as carrying an object are therefore diffi cult for this robot 1Yuki Murata ShinkichiInagaki andTatsuyaSuzuki are withtheDepartmentofMechanicalSystemEngineering NagoyaUniversity Furo cho Chikusa ku Nagoya Aichi Japan y murata inagaki t suzuki nuem nagoya u ac jp Another approach to deal with the issue of high com putational burden is a decentralized calculation A model called the central pattern generator CPG which comprises a network of neurons in the central nervous system is used to control the locomotion of legged robots 7 10 CPG based robots are capable of adaptively walking on moderate irregular terrains using sensory feedback An event driven decentralized walking control called Follow the contact point FCP gait control proposed by Inagaki et al 11 was inspired by the earlier Follow the leader gait control 12 Both methods are based on the principle that each leg lands on the same contact point that the preceding foreleg has already contacted FCP gait control achieves this using a decentralized control architecture The advantage of this principle is that the overall planning of all the legs is centralized on just one of the front legs FCP gait control is easy to install on a small onboard computer thanks to its decentralized calculating and its concentrated foot placement planning However the original FCP gait control does not consider posture control because it has been proposed for a centipede like robot In this study a hexapod robot was used Because a hexapod robot has more legs than biped and quadruped robots it is easier for a hexapod robot to keep stable walk ing However it has also shown that even hexapod robots often fall down especially on complex irregular terrains Therefore a hexapod robot also requires adaptive control for tumble stability In addition this type of robot needs another type of adaptive control to suitably adjust the movement rhythms of legs that are disturbed by the fi rst adaptive control In this paper the former problem is solved with an improved FCP control The latter issue is solved using a Timekeeper control In order to solve the fi rst issue three points are added to the FCP gait control the fi rst one consists of the constraints which are added to the condition when a standing leg transitions into a swing phase The second is real time balance control which is added to the legs in the stance phase The third in order to guarantee the robot a statically stable walk is a new adaptive control architecture we ve called Timekeeper control This new method adjusts the timing to transition the control mode in the FCP gait control so as to guarantee a deadlock free statically stable walk Finally the effectiveness of the proposed control method is demonstrated by a small hexapod robot with a human in the loop control navigation interface The remaining sections of this paper are organized as follows the hexapod robot used in this paper is discussed in section II the improved FCP gait control for the hexapod 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 978 1 7281 4003 2 19 31 00 2019 IEEE3321 Fig 1 Hexapod robot Fig 2 Each mode and transition condition in control automaton robot is explained in section III the Timekeeper control is introduced in section IV the experimental specifi es of the hexapod robot and the human in the loop navigation interface are introduced in section V the experimental results are shown in section VI and this paper is fi nally concluded in section VII II MECHANISM OF THE HEXAPOD ROBOT The hexapod robot used in this study is shown in Fig 1 The robot consists of a body and six legs which are located radially Each leg has three links Link1 2 and 3 and three motors The two legs in the moving direction are defi ned as left and right front legs The right legs are labeled as Leg i i 1 2 3 from front to back and the left legs are similarly designated as Leg i i 4 5 6 The leg which is located laterally opposite to Leg i corresponds to Leg i 2 mod 6 1 A coordinate system B is fi xed on the body with its origin on the center of the body The y axis of B is defi ned as the front direction and the z axis as the upper one The reachable area of Leg i is defi ned as iand the boundary of i is defi ned as i Note that the legs are located so that the reachable areas of neighboring legs overlap In addition Link1 is tilted by 45 against the body so that each leg can exchange its contact points with its adjacent legs without the links colliding one another III FCPGAIT CONTROL MODIFIED FOR STATIC STABILITY A FCP gait control for a hexapod robot In this section the FCP gait control for a hexapod robot is introduced In this control each leg has a control automaton and is controlled according to the control modes in the automaton The control modes transition in the order of mode 1 2 3 4 1 Fig 2 The control modes and the transition conditions are detailed as follows 1 Control mode 1 The leg tip raises from the ground and approaches an intermediate point P1at a speed of v1 When the leg tip reaches P1 the control mode transitions to mode 2 2 Control mode 2 The speed of the leg tip is set to v2 0 and the leg tip stays at the intermediate point P1 For the leading front legs Leg i i 1 4 when a new contact point is identifi ed in the reachable areas 1and 4 respectively the control mode transitions to mode 3 The new contact points are given by the human operator in this study For trailing legs Leg i i 2 3 5 6 when the leg tips of the forelegs enter the reachable area i the control mode transitions to mode 3 3 Control mode 3 The leg tip moves to the next target point P3at a speed of v3 When a trailing leg exchanges its contact point with a foreleg P3is set to a point slightly apart from the contact point For the front legs Leg i i 1 4 P3is the new contact point Thus target point P3moves with the robot s progression from the point of view of the robot Therefore the new contact point is tracked using an image sensor The trailing legs Leg i i 2 3 5 6 always take over the former positions of the foreleg tips When each leg tip arrives at P3 the control mode transits to mode 4 4 Control mode 4 This control mode corresponds to the stance phase The legs in control mode 4 propel the body with a speed of v4 The derivation of the target point P4 of the leg tip is introduced in section III B The transition condition from control mode 4 to 1 is explained in section III C B Posture control of the body using a posture sensor In FCP gait control the supporting legs are supposed to contact the points which the front legs have previously contacted However in practice the leg tips slip and the robot occasionally falls down In this section posture control using a posture sensor equipped on the body is introduced and the target point P4of control mode 4 is derived based on posture control At fi rst the order in which the legs contact the ground is recorded The contacting order includes the next new contact point of the front legs At least three legs are ensured to contact the ground by the transition condition from control modes 4 1 as explained in the next section and by the Timekeeper control introduced in section IV From this it is possible to make a triangle whose vertices are the last three contacting points Fig 3 or two last contact points and one new contact point Fig 4 The coordinates of the vertices of 3322 the triangle are defi ned as Ca Cb and Ccin the coordinate system B A rotating matrix R g g from the global coordinates to the body coordinate system B is calculated in every control step gand gare the pitch and yaw angles in the global coordinate system respectively and are measured dynamically by a posture sensor equipped on the body Fig 1 and are the target values of yaw pitch and roll angles respectively and are given by the human operator Next the coordinates of the target positions of the body P in the coordinate system Bare calculated as follows P Ca Cb Cc 3 R 1 g g 0 0 h T 1 where h is the target height of the body also given by the operator The fi rst term on the right side is the center of the triangle in Figs 3 and 4 The second term is the vertical vector of length h which is converted into B A sub target P of the body is calculated as follows P P P v4 t P 0 0 0 Totherwise 2 where P is the norm of vector P t is the control step and is the threshold distance to stop moving the body Finally the coordinates of the target point P4 i of Leg i in control mode 4 are calculated as follows P4 i R 1 g g P4 i P 3 where signifi es updating the value in the next control step P4 i P is the current target position of Leg i tip in order to propel the center of body by P A rotational matrix R 1is then multiplied to it in order to make the current posture angles g gfollow C Transition condition from control mode 4 to 1 The original FCP gait control 11 was proposed for a centipede robot with over ten legs In this case there was no need to consider violating static stability due to the many supporting legs On the other hand in the case of a hexapod robot the combination of supporting legs must be considered in order to maintain the static walk In consideration of this the transition conditions from control mode 4 to 1 for Leg i are defi ned as follows Condition 1 For i 1 2 4 5 the rear leg Leg i 1 has already contacted the contact point of Leg i and the opposite side leg Leg i 2 mod 6 1 is in control mode 4 As for the rearmost legs Leg 3 and Leg 6 the condition is that the opposite side leg Leg 6 and Leg 3 both in control mode 4 Condition 2 All three legs Leg 2n 1 i mod 2 n 1 2 3 are in control mode 4 Condition 3 A point projected vertically downward onto the walking surface from the center point of the robot body walls within a triangle defi ned by the three Fig 3 The three last contact points and the target position of the body the target point of the leg tip of Leg i in control mode m is expressed as Pm i Fig 4 The two last contact points the next new contact point and the target position of the body Fig 5 The projected center of the body and the margined triangle made from three contact points points of Leg 2n 1 i mod 2 n 1 2 3 with a margin Fig 5 IV TIMEKEEPER CONTROL The improved FCP gait control proposed in the previous sections has no guarantee to avoid deadlock because added conditions may confl ict with one other Therefore an addi tional control architecture to adjust transition timing of each control mode is considered in this section First a timed automaton is used to express the behavior of legs controlled by the FCP gait control based on previous works 13 14 This automaton expresses not only the control modes of the FCP gait control but also the position of the leg The details to derive this automaton have been skipped in this paper for the sake of brevity The term ti m is defi ned as the time elapsed in each control mode 3323 Supporting leg d All legs are supporting c Only Leg i is not supporting a Odd or even numbered legs are supporting Leg i Leg i Another leg b Leg i 2 real time posture control 3 Timekeeper control to control time elapsed in each control mode Finally an experiment consisting of a small hexapod robot walking on uneven terrain with navigation dictated by a human operator was performed in order to validate the method Development of a new robot more suitable for the proposed FCP gait control is one future direction for this work which we are already undertaking ACKNOWLEDGEMENT This research is subsidized by Grant in Aid for Scientifi c Research 16K06181 REFERENCES 1 P eter Fankhauser Marko Bjelonic C Dario Bellicoso Takahiro Miki and Marco Hutter Robust Rough Terrain Locomotion with a Quadrupedal Robot IEEE International Conference on Robotics and Automation ICRA May 2018 2 Yuan Tian and Feng Gao Effi cient motion generation for a six legged robot walking on irregular terrain via integrated foothold selection and optimization based whole body planning Robotica 2018 vol 36 no 3 2018 pp 333 352 3 Ruiqin Li Hongwei Meng Shaoping Bai Yinyin Yao and Jianwei Zhang Stability and Gait Planning of 3 UPU Hexapod Walking Robot Robotics vol 7 no 3 2018 pp 48 4 Yue Zhao Xun Chai Feng Gao and Chenkun Qi Obstacle avoid ance and motion planning scheme for a hexapod robot Octopus III Robotics and Autonomous Systems vol 103 2018 pp 199 212 5 Baoling Han Xiao Luo 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