TRBR: Flight body posture compensation for transverse ricochetal brachiation robot
Introduction
Brachiation robots mimic the movements of primates that use their limbs to swing from branch to branch [1], [2], [3]. The locomotion of these robots can be categorized as continuous brachiation [4], [5] or ricochetal brachiation [6]. Continuous brachiation is generally used in cases where the gaped distance is within hand-holding range, whereas ricochetal brachiation is generally used in cases where the gaped distance exceeds hand-holding range. Most robots that use continuous brachiation employ a two-link mechanism [7] and swing control strategies aimed at providing precise tracking control of end-effectors (grippers) [4], [8], [9], [10], [11], [12], [13], efficient energy consumption [7], [14], [15], [16], [29], and transition schemes to reduce the impact of forward grabbing [15]. A variety of motion planning and control strategies have been proposed to investigate brachiation performance under various hand-holding conditions, including regular branches [15], [16], [17], [18], [19], irregular branches [20], moving branches [16], and flexible bars [21], [22].
Research into ricochetal brachiation robots (RBRs) has focused mainly on the development of locomotion models based on high-DOF dynamic systems [6], [24] and simulations to facilitate motion planning and control for two-link robots [23], [25], [26]. Little research has gone into the mechatronic design and control methods for ricochetal brachiation. Results obtained from previous experimental studies [27], [28] give some indication of the difficulties this entails.
- (1)
It is difficult to predict timing for the grabbing of target hand holds due to the effects of modeling errors and air resistance during this short flight time. A gripper must be designed to compensate for uncertainties in the grabbing motion.
- (2)
All existing motion control methods to adjust the midair posture of the robot were developed using a simplified 2D model [23], [25], [26], [31], [32], despite the fact that an unbalanced body mass and external disturbances can introduce undesired body rotations during the actual 3D flight phase [30].
- (3)
Providing the desired locomotion as well as the ability to switch from swing phase to flight phase for leaping makes the mechatronic design and implementation a non-trivial task.
The issues mentioned above were encountered in the development of transverse ricochetal brachiation robots (TRBRs) [27], [28]. The term “transverse” highlights the difference between the movement of this type of ricocehtal brachiation robot and conventional RBRs. The design of TRBRs is based on the locomotive style of athletes swinging their bodies transversely with their hands on a ledge in order to propel themselves into a leap to a target ledge [27]. The principles of TRBR locomotion lie in a smooth transition from the swing phase into the flight phase, and then into the landing phase. TRBRs can be applied to climbing tasks requiring movement between ledges on a wall, such as exterior window sills or eaves. Existing RBRs were designed to climb horizontal ladders with large gaps between the bars [23].
First-generation TRBRs (TRBR-I) [27] comprised two arms, a body, and a tail. A parallelogram mechanism formed by the arms and body can help to maintain body posture during the swing phase and reduce the accumulation of excessive angular momentum, which could cause body rotation during the flight phase. We derived a simplified 2-DOF dynamic model for the swing phase to empirically determine the resonant frequency with which to formulate a swing strategy for ricochetal brachiation. Our preliminary results on swing excitation and one-hand grabbing motion demonstrated the feasibility of the prototype based on a pair of pneumatic grippers. Robots that rely on the simultaneous actuation of arm and tail motors tend to be inefficient in accumulating kinetic energy, due to the effects of friction associated with geared mechanisms at the shoulder joints. This issue was addressed in the design of TRBR-II [28] by integrating electromagnetic clutches between arm linkages and actuating motors. We implemented a switchable shoulder stiffness scheme to eliminate all stiffness during the swing phase and thereby promote the efficient accumulation of kinetic energy (using only tail swing) and then impose high stiffness during the flight phase to facilitate arm posture control. We integrated an electrically-driven robotic gripper with a trigger mechanism as an alternative to pneumatic grippers, thereby allowing tight handhold capability during the swing phase and rapid hand release capability at the moment of transition from the swing phase to the flight phase. We developed a model-based optimization method to derive the parameters that would ensure the desired tail reference trajectory in order to improve swing excitation efficiency. We also developed a dynamic model of the flight phase by which to analyze the midair body posture and assess the feasibility of using full tail rotation for posture compensation. Unfortunately, the simple open-loop compensation method developed in this study is susceptible to inefficient control energy consumption and poor robustness against unknown disturbances.
In this paper, we describe the development of a novel TRBR with improved mechanical design (TRBR-III) and a variety of strategies to actively control body posture during the flight phase of transverse ricochetal brachiation. Robot design parameters were obtained by formulating model-based optimization problems under specific system constraints to minimize the size of the robot and expand the leaping distance. We employed a controlled arm swing strategy during the flight phase to extend the transverse distance travelled by the grippers and improve the likelihood of holding the target ledge, without having to increase the leap distance of the center of mass (COM) of the robot. Experiments on ricochetal brachiation were conducted to evaluate the efficacy of the proposed robot design and analyze the performance of the proposed posture compensation methods. The contributions of this paper are summarized as follows:
- (1)
The design parameters of the robot and gripper were optimized to enhance the efficiency of resonant excitation to provide a more reliable hand-release between complex gait transitions.
- (2)
We developed a motion control scheme based on successive loop closure (SLC) [33] to deal with the issue of body posture compensation in an underactuated robot during the flight phase.
- (3)
We developed a strategy incorporating arm swing with tail swing to extend leaping distance, as a demonstration of the potential of using arm-body-tail brachiation robots for novel forms of locomotion styles.
The remainder of this paper is organized as follows. Section 2 presents our problem statement explicitly identifying the design goal of the proposed robot. Section 3 identifies crucial design parameters and details pertaining to the mechatronic implementation of the proposed robot. Section 4 reviews a dynamic model of the robot during the flight phase. Section 5 introduces the methods proposed for body posture compensation during flight and corresponding simulation results. Section 6 presents experiments involving ricochetal brachiation. Concluding remarks are presented at the end of the paper.
Section snippets
Problem statement
As shown in Fig. 1, the process of transverse ricochetal brachiation can be broken down into a swing phase, a flight phase, and a landing phase. Accumulating the energy required for leaping requires swinging the tail repeatedly at the resonant frequency of the robot. After releasing the hands, the robot must maintain appropriate posture in preparation for a two-handed landing. The above process is deemed successful only if the robot comes to a stable stop on the target ledge without falling to
Development of transverse ricochetal brachiation robot
In this study, we adopted the arm-body-tail robot design proposed in the previous generations of TRBRs [27], [28]; however, we made a number of fundamental changes to improve locomotion performance. In this section we analyze the design parameters, the configuration of the mechatronic systems, the design of the gripper within the context of the design requirements, and sensory and control configurations.
Flight dynamics of TRBR
As mentioned in Section 2, we simplified the complex flight dynamics of the ricochetal brachiating robot as transverse (2D) flight motion to facilitate the design of a posture control system. Even in this simplified configuration, the flight dynamics of the robot included two parts: translational dynamics and rotational dynamics. Translational dynamics was analyzed using the equation of projectile motion (2), and we present in the following a brief review of rotational dynamics [28].
The
Active compensation for robot posture in midair
The yaw rotations introduced by the angular momentum during the flight phase can have a profound effect on the posture of the robot, which can lead to a decrease in the overall flight distance and/or skew the position of the grippers, thereby preventing a successful landing. In a previous study [28], it was shown that the tail of the robot could be rotated to alleviate yaw rotation. In this study, we implemented an active tail-assisted compensation strategy to adjust the body posture with the
Experiments and discussion
In this study, extruded aluminum bars were used in constructing the experiment platform for experiments on transverse ricochet locomotion [27], [28]. Based on the active tail control methods presented in Section 5, we implemented a number of flight posture compensation strategies to evaluate the effectiveness of the proposed scheme.
Conclusions
This paper presents a robot based on the arm-body-tail configuration for transverse ricochetal brachiation. We also present in-depth analysis of posture compensation methods and their effects on flight performance. We formulated an optimization problem aimed at maximizing the flight distance under given constraints with the aim of identifying the robot design parameters that are critical to flight performance. We employed shoulder joints with switchable stiffness as well as custom-made grippers
Declaration of Competing Interest
None.
Acknowledgment
This work was supported by the Ministry of Science and Technology, Taiwan, under grant number MOST-105-2628-E-011-004 and partially supported by the “Center for Cyber-physical System Innovation” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
Chi-Ying Lin received the B.S. and M.S. degrees from National Taiwan University, Taiwan in 1999 and 2001, respectively, and the Ph.D. degree from University of California, Los Angeles in 2008, all in mechanical Engineering. He is currently an Associate Professor in the Department of Mechanical Engineering at National Taiwan University of Science and Technology, Taiwan. His recent research interests include (1) design, modeling, and locomotion control of bio-inspired brachiating robots; (2)
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Chi-Ying Lin received the B.S. and M.S. degrees from National Taiwan University, Taiwan in 1999 and 2001, respectively, and the Ph.D. degree from University of California, Los Angeles in 2008, all in mechanical Engineering. He is currently an Associate Professor in the Department of Mechanical Engineering at National Taiwan University of Science and Technology, Taiwan. His recent research interests include (1) design, modeling, and locomotion control of bio-inspired brachiating robots; (2) active vibration control of smart flexible structures using piezoelectric electrode configuration techniques; and (3) development of novel robotic systems integrated with the techniques of visual servoing and force feedback.
Zong-Han Yang received the B.S. and M.S. degrees from National Taiwan University of Science and Technology, Taiwan in 2016 and 2018, all in mechanical engineering. He is currently an Engineer in Yulon Motor CO., LTD. His research interests include mechatronic design of bio-inspired robots and intelligent autonomous vehicles.