Docking joint for autonomous self-assembly

DOCKING JOINT FOR AUTONOMOUS SELF-ASSEMBLY Mehdi Delrobaei, Kenneth A. McIsaac Electrical and Computer Engineering Department The University of Western Ontario London, ON N6A 5B9, Canada Email: mdelroba@uwo.ca, kmcisaac@uwo.ca Abstract Automatic docking between separate parts is a fundamental challenge that arises in engineering systems which autonomously change their structures. This ability enables free bodies in the same environment to join together in order to complete a task that would otherwise not be achievable with the independent modules. Docking capability is highly desirable in applications such as self-reconfigurable robots, autonomous undersea vehicles, and automated recharging of security robots. The major problem for this task is to overcome alignment errors and ensure a firm connection. We have developed a mechanism to overcome this challenging problem. This paper focuses on the design details of our proposed docking joint and presents the results of finite element analysis (FEA) on the joint as well as experimental results. Our final goal is to develop an autonomous multi-rove robot. In this concept, a number of independent mobile robots can self-assemble into a single modular robot. Index Terms— Self-assembly, Docking joint, Connection mechanism design. 1. INTRODUCTION Self-assembly is a concept that offers a new approach in robotics, in which robot modules are able to assemble and form a connected structure, or a rigid body can disassemble into a group of unconnected units. Following on from Whitesides and Grzybowski [1], autonomous docking can be defined as a reversible process in which separate parts securely connect together to form a single structure without human intervention. This process, which is also called selfassembly, involves components at scales from the molecular (crystals) to the planetary (weather systems). In robotics, autonomous robot docking is generally divided into two classes [2]: intra-robot docking, which deals with the problem of docking among modules in the same connected group (e.g., ATRON, a lattice-based selfreconfigurable robot [3]), and inter-robot docking; docking between two independent and unconnected groups of modules. A situation where a self-reconfigurable robot disassembles into a set of independent and autonomous mobile units to spread out in an area, and later reassembly them back into a single robot can be an example of interrobot docking [4]. To build such robots, however, some technical challenges must be conquered. Of primary concern is to implement a reconnectable joint which autonomously and easily connects and disconnects the modules. For this purpose, the design must overcome the following challenges: (1) build the connector as lightweight and compact as possible; (2) form a secure and reliable connection; (3) overcome unavoidable alignment errors; (4) design the locking/unlocking mechanism to be considerably power efficient. This paper reports the design and implementation of our proposed automatic docking joint, and it is organized as follows. Section 2 surveys related work. Section 3 contains a description of the joint design. In Sections 4 and 5, we present the results of Finite Element analysis and experimental results where we examine to what extent the mechanism is capable of tolerating larger forces. In Section VI, we discuss the results. Finally, Sections 6, 7 give a brief summary of our future work, and conclude the paper. 2. RELATED WORK Among all applications of automatic docking joints, perhaps the one that requires autonomous docking the most is the self-reconfigurable robot [2]. Fukuda et al. studied a docking system for a cell-structured robot; a hook-type coupling mechanism using a DC motor and a worm gear to engage the hooks [5]. PolyBot is a modular reconfigurable robot system composed of 1-DOF modules each equipped with 2 connection ports [6]. A shape memory alloy actuator integrated in each connection plate can rotate a latch to CCECE/CCGEI May 5-7 2008 Niagara Falls. Canada 978-1-4244-1643-1/08/$25.00 ” 2008 IEEE 001025 catch lateral grooves in the pins from the mating connection plate. The CONRO self-reconfigurable robots [7] are made of a set of self-sufficient, autonomously connectable modules. Each CONRO module has two degrees of freedom and is equipped with four docking connectors for connecting with other modules. Khoshnevis et al. [8] presented a design of CONRO connectors and considered a number of parameters such as alignment, size, power consumption, and weight of the docking connectors. Gross et al. [9] presented Swarm-Bots, which is a new distributed robotic concept. The basic component of the system, called s-bot, is equipped with a gripper and a surrounding ring matching the shape of the gripper. The design of the connection mechanism allows for some misalignment. A further fine-grained alignment occurs during the grasping, favored by the shape of the two teeth at the end of the gripper’s jaws, as well as the relatively high force by which the gripper is closed (15 N). Bererton and Khosla [4] have used a docking connector which has forklift pins for docking between separate mobile robots with repair capabilities. The docking connector allows approximately 30-degree alignment errors. Docking is also essential for security robots to get automatically recharged. A docking strategy has been proposed by Luo et al. for security robots recharging; including connection pins and recharging adapters located at the inside of cone-shaped holes [10]. A critical capability for a network of autonomous undersea unmanned vehicles is the ability to dock for the purpose of sleeping, recharging batteries and transferring data [11]. Singh et al. [12] proposed a docking mechanism for autonomous underwater vehicles (AUVs) which is mounted on the bow of the vehicle and consists of a Vshaped, bilaterally symmetrical, titanium latch body with two fixed tines and a pivoting titanium capture bar. Recently, Allen et al. [13] developed and demonstrated a novel underwater docking connection system which is basically a linear actuator and two guide pins on the dock frame engaging into the docking connector on the vehicle. Fuel loading robots are another application which needs a docking system. Dun et al. [14] demonstrated an autodocking system for fuel loading robots which consists of a 3-DOF parallel mechanism (parallel robot) and revolute joints. Satellite docking is a relatively new area of study and practice in aerospace studies. Docking an in-orbit satellite by another satellite is a very challenging task; Kasai et al. [15] have reported the results of a docking space mission. Some of the mentioned joints do not lock (security robots). Some of them are not power efficient. For instance, Swarm-bots have rigid and flexible grippers actuated by DC motors. The motors are continuously activated while the sbots are connected. In some others, the joint does not necessarily make a rigid connection. In the case of undersea robots, the robot (sometimes even the docking station) is plunged in water. Our design most closely resembles the joints used for self-reconfigurable robots (CONRO and PolyBot). The difference is that both PolyBot and CONRO modules need to be exactly aligned to get connected whereas our design permits considerable misalignment. 3. JOINT DESIGN The purpose of the work presented in this paper is to develop a mechanical joint which allows separate modules to automatically couple to form a connected structure. Our final goal is to develop an autonomous multi-rove robot. In this concept, a number of self-sustained mobile robots can self-assemble into a single serial chain modular robot. This multi-rover robot will demonstrate new features, such as snake-like [16] and eel-like locomotion [17], to respond to different terrain conditions. There would be a universal joint in the middle of each rover enabling the robot to move its body up and down (pitch), and from side to side (yaw). The size and weight of each module would be around 0.1×0.2×0.1 m³ and 0.7 kg, respectively. Therefore, the proposed docking joint should provide a rigid link capable of supporting at least two modules in all directions (X-Y-Z). It should be mentioned that snake-like locomotion requires at least three rigid links joined by rotary actuators. We describe the design of the joint in details. 3.1. The Connectors According to the following figures, the mechanism includes a male part (Fig. 1.(a)) and a female opponent (Fig. 1.(b)). The conical head of the male part provides a smooth docking, and the slot allows a key to lock the joint. Fig. 1.(c) shows the way these two parts fit together. 3.2. The Latching Mechanism For physically docking and undocking, the female part also houses a latch (Fig. 1.(d)). The latch consists of two linear actuators which are located on the sides of the pipe (the female part), and a key attached to the ends of the actuators. The key is placed on the slot of the pipe. When the pin (the male part) enters the pipe and reaches the end of it, a contact switch is closed which activates the linear actuators. The attached key moves down into the groove of the pin and locks the joint (Fig. 2). To unlock the joint, the linear actuators are activated to move back, so that the key is disengaged and the pin is released. This mechanism is advantageous in that it is compact and easy to use, which makes it reliable in any environment. Moreover, the mechanism is quite power efficient, because activation of the actuators is needed only during the opening and closing phase. The latch consists of two linear actuators; each driven by a small DC motor linked to a gear-train, and finally a lead-screw. This configuration is normally locked 001026 unless the actuators are powered. So, the latch mechanism is activated only during the docking/undocking operation which takes a few seconds. of safety (FOS) is the ratio between the breaking load on a part and a safe permissible load on it. Key Female Part Male Part Linear Actuator (a) Linear Actuator (b) Contact Switch (c) (d) Fig. 1. The main idea of the connection mechanism; (a) The male Part; (b) The female part; (c) The merger of the parts; (d) The locking mechanism. 4. FINITE ELEMENT ANALYSIS This section presents the results of a finite element analysis (FEA) study on the joint to evaluate the performance of the proposed design. The finite element analysis package COSMOSWorks from SolidWorks corp. has been used in the analysis. As the results of analysis directly depend on the specified loads and constraints, the boundary conditions should be carefully indicated. In our analysis, the edges of the plate holes are fixed (all translational and rotational degrees of freedom are set to zero). The applied load to the joint is a combination of three normal forces (with uniform distributions), and a torque. Three equal 100 N forces are applied to the joint in X-Y-Z directions (Front, side, and top of the joint; Fig. 3), and the acting torque is 1 Nm. The torque and forces values indicate the worst-case of our application. A force of 100 N is equal to the weight of nearly 14 robot modules connected to one another. Fig. 3 and Fig. 4 show the stress test analysis results based on Von-Mises criteria. As can be seen in Fig. 3, the maximum stress is 4.019e+7 N/m2 while the yield strength of the used material (aluminum 2018) is 3.171e+8 N/m2. Therefore, the minimum safety factor of our design (yield strength divided by the maximum stress) is 7.89. The factor Fig. 2. The mechanism includes a male part (pin) and a female opponent which houses a latch. If a torque is applied to the joint, it affects the latch as well. Fig. 4 shows the stress analysis result for the key. The bottoms of the actuators are considered to be fixed while a 1 Nm torque is applied to the joint. In fact, when a torque is applied to the pin, the edge of groove applies an upward normal force to the key. Table 1 shows the minimum FOS of the joint and the key for different materials. The use of these data helps to select a proper material. For instance, if the part was made of rubber or Aluminum 1350 Alloy, the joint would certainly fail under this load. But, using Acrylic or Aluminum 2018 Alloy is quite safe. As the weight of the joint can be critical in our application, the density of materials is also listed for comparison. It should be noted that the results are based on linear static analysis, and the material is assumed isotropic. 5. JOINT CONSTRUCTION AND VERIFICATION We finally decided to build two joints from cast Acrylic and Aluminum 6061-T6 alloy which is more common than the 2018 alloy and stronger (Fig. 5). For the actuators, we used two miniature high force linear actuators (PQ-12f) from Firgelli Tech. Inc. Each linear actuator is driven by a tiny DC motor linked to a brass gear-train, and finally a leadscrew. 001027 According to the manufacturer specifications [18], the actuator statically holds about twice of its maximum rated dynamic force (2×24 N). Therefore, with two linear actuators, the key is capable of applying nearly 96 N forces to the pin. Above 200 N permanent damage to the actuators may occur. The actuator housing is made from Delrin (Acetyl) plastic, and the total mass is 19 g. The actuators position along their strokes can be accurately controlled based on feedback from built in 2 k precision linear potentiometers. Fig. 6 shows the physical implementation of the docking mechanism. The total height is 8 cm. As can be seen, a contact switch is used to sense the pin once it reaches the end of the pipe. This configuration is quite compact and power efficient. Fig. 5. The actual joints from Aluminum 6061-T6 alloy, and acrylic Our experiments show that the mechanism works well, and we manually verified that this configuration allows ±10 degrees misalignment (Fig. 7). 6. DISCUSSION While the proposed joint works well, we believe the performance of this docking mechanism is highly dependent on the joint head design. The current configuration allows ±10 degrees misalignment which could be improved by using a different shape for the pin head. It is clear that compared to docking, undocking is relatively a simple process, since no re-alignment is needed. This docking mechanism is reliable and easy to construct. It is made mainly from aluminum 6061 which makes it lightweight yet sturdy. The locking mechanism is power efficient in that it is in a passive mode most of the time. Fig. 3. Stress test analysis result; the male joint. 7. FUTURE WORK Fig. 4. Stress test analysis result; the key. Table 1. Material Comparison Material Nylon 6/10 Acrylic Aluminum 1350 Alloy Aluminum 2024 Alloy Aluminum 2018 Alloy Brass Copper Steel AISI 1020 Alloy Steel Min. FOS (Joint) 3.4 5.2 0.69 1.9 7.9 6.0 6.6 8.6 15 Min. FOS (Key) 26 39 4.0 14 61 46 49 67 120 Density (Kg/m3) 1700 1400 2700 2770 2800 8550 8900 7850 7600 As the firmness of the docking mechanism directly depends on the size of the joint, in our future work we intended to identify the relationship between the joint size and the joint strength. We also have planned to improve the joint design to overcome larger alignment errors. Now that the joining of modules is possible, it is interesting to study a docking guidance system to coordinate and align the modules. We believe a set of infrared emitter/receivers can be used for directing the modules once a rough alignment is achieved. Once the modules are connected together, it would be quite useful if they could exchange data. Since there will be point-to-point connections between the docking plates of the modules, infrared transceivers (aimed in the same direction) can be embedded on the docking plates allowing data to be shared between the modules without any physical connection. 8. CONCLUSION This paper presented the design and implementation of a reconnectable joint which autonomously connects and disconnects two robot modules. The proposed mechanism is lightweight, compact, and powerful enough to secure a 001028 reliable connection. It overcomes alignment errors, and it is considerably power efficient. Our current goal is to integrate this mechanism with an exact yet wide-ranging relative pose system for detection and localization of the robots. The overall objective is to develop an autonomous multi-rover robot in which a team of autonomous mobile robots can assemble into a single serial chain modular robot. Fig. 6. The physical implementation of the docking mechanism Fig. 7. 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