On the Design of Mobile Parallel Robots for Large Workspace Applications

In this paper, several considerations for designing industry oriented robots which combine the mobility of legged robots and advantages of parallel mechanisms are outlined. For designing such optimized robots in terms of simplicity and performance, a topology study is done based on the mobility analysis. Applying some design constraints, potential topologies of such robots are identified. One architecture is chosen for designing a tripod robot. Both inverse and forward kinematic problems of this robot are derived in order to simulate its gait motion. The analysis and simulations show that: integrating some clamping devices and some lockable passive joints, six actuators are enough to build a legged manipulator which can not only perform 6-axis machining but can also walk on a curved supporting media. INTRODUCTION Modern industry requires manufacturing systems to be more reconfigurable, flexible and agile to adapt to the increasing competitive climate with sophisticated customers demands [1]. In industrial fields such as automobile assembly lines or semiconductor manufacturing processes, robotic systems have been widely integrated. But when large workspaces are required, the traditional stationary-base robots can not be used. ∗Address all correspondence to this author. Therefore, the concept of mobile manipulator, classical serial robot arms mounted on a mobile base, has been considered for the automation of applications like welding, inspection, painting etc [2, 3]. However, most of these solutions will fail when a high precision and/or stiffness are required for applications like drilling or milling. Meanwhile, parallel mechanisms, with great potential to provide high rigidity and motion dynamics, suffer from their inherent limited operational workspace. Several approaches have been proposed recently in order to apply parallel mechanisms to the aeronautic industry, where a large operational workspace is required [4, 5]. The mobility of the base can be provided by linear guide ways, wheeled mobile robots, tracked mobile robots or legged mobile robots. On the one hand, long linear guide ways with high precision and stiffness represent high costs and tedious infrastructure adjustments of workshops. Beside the need of independent control for the mobile base, wheeled and tracked mobile robots, possessing lower degrees of freedom (DOF), have limited mobility, limited obstacle cross-over ability. On the other hand, legged robots, with mechanical structure inherently similar with parallel robots, rarely appear in the workshop because of the lack of efficiency and reliability [6]. An innovative solution which combines the mobility of legged robots and advantages of parallel mechanisms is studied in this paper (Fig. 1 shows a illustration of such robot). Un1 Copyright c © 2011 by ASME FIGURE 1: ARTIST’S VIEW OF WHAT COULD BE LEGGED DRILLING ROBOTS WORKING ON A WING BOX like classical legged robots, the extremity of every leg will be equipped with fixing devices which help to provide solid connections between the robot and the ground. Therefore the walking and machining capacities of such robots will not be seriously constrained by factors like center of gravity, friction between limbs and the ground etc. In the following sections, the design considerations and process are discussed. A tripod mobile robot which can ”walk” on surfaces with moderate curvature and perform as a 6-axis parallel manipulator once it is deployed on the working position is presented. Both inverse and forward kinematic problems of this robot are derived in order to simulate its gait motion. REDUCED DOF WALKING MANIPULATOR Legged robots have attracted attention because of their relatively good terrain pass-over capacity [7]. Most of these studies focus on improving the mobility and the reliability of mobile platforms in hazardous environments for exploration purposes. Many prototypes which imitate the limb structures of animals have been built and studied in universities and research centers. However, few of them have been used to solve industrial problems: both the human-like biped and animal-like quadruped or hexapod have legs with all their joints being actuated. Three actuators are needed for positioning the pinpoint-type foot to a point in the 3D space where no orientation capacity is required [8]. That is why a typical bio-mimetic quadruped has 12 actuators and a hexapod has 18 actuators. When the orientation of a foot needs to be controlled to fit well the terrain, more than five actuators are needed in each leg. It is difficult to consider using this kind of legged robots for manufacturing applications due to their high material cost and the complexity of their control. Designing a legged robot for (a) Picture of Roptalmu (b) Joint-and-loop graph of the crawler FIGURE 2: ROPTALMU, A DRILLING ROBOT, WITH MULTIPLE FUNCTIONAL X-AXIS manufacturing purpose is very different from designing a legged robot for explorations of hazardous unknown environments [9]. Principles to Reduce the Number of Actuators Several techniques are discussed below in order to build industry oriented legged robots which are capable to achieve tasks with high stiffness and accuracy. Sharing actuators for positioning each Limb The body of a legged robot can be moved to help positioning its limbs1. Generated by the supporting limbs, the DOFs of the body can be used to position the swinging limb. Instead of actuating each limb independently, sharing actuators for positioning each limb will help to reduce the total number of actuators [10, 11]. Using the same actuators for locomotion and for manipulation For conventional mobile robots, the locomotion actuation and the manipulation actuation are usually provided by two independent systems. In order to reduce the number of actuators, the 1The kinematic chains which connect the payload platform and the terrain are hereafter called limbs or branches 2 Copyright c © 2011 by ASME mobility of the locomotion system can also be used for manipulation purpose [12–17]. For example, Roptalmu, a 3-axis drilling robot designed for aeronautic industry applications, is composed by a wheeled mobile platform and a crawler robot. The wheeled mobile platform follows automatically the crawler, and its main goal is to compensate the gravity by exerting an upward vertical force on the crawler (Fig. 2 a). As it is outlined in the joint-andloop graph (Fig. 2 b), actuators X , Z1 are used for locomotion tasks. And actuators X , Y , Z2 are used for drilling tasks. Using the X axis actuator for both locomotion and machining tasks makes the mechanism of the crawler more efficient. Integrating lockers on the passive joints Legged robots, with closed kinematic chains (KC) formed between the body and the terrain, can be considered as parallel mechanisms. Noticing that the existence of passive joints in the branches of conventional parallel robots helps to build light-weight robot with relatively higher rigidity, passive joints will be introduced in the design of legged robots for this purpose. However, in order to keep the mechanism controllable during locomotion, lockers should be integrated on some of the passive joints. These lockers can eliminate temporarily the passive DOFs when it is necessary [18]. Docking system For robots which are supposed to provide high manipulation stiffness and accuracy, solid connections between the robot and the supporting media (tooling, workpiece itself, etc.) are required. The connection force can be provided by a magnetic device, a vacuum device or a mechanical clamping system [19]. MOBILITY AND TOPOLOGY ANALYSIS Mobility Requirements of A Walking Parallel Robot Following the presented techniques, the objective of the structure design is to build a mobile machining center which machines as a parallel machining center and walks as a legged robot. The various working modes of the desired robot can be roughly distinguished as: Machining Head (MH) mode and Branch Extremity (BE) mode. MH Mode: During the MH mode, all the branches of the robot are attached to the base. The robot which is supposed to perform 5-axis tasks can be considered as a classical parallel robot or a hybrid (serial-parallel) robot. When the payload platform (PP) possesses more than five DOFs, then the 5axis movement for the machining task can be provided by the PP. Otherwise an extra kinematic chain between the PP and machining head, which possesses the rest of the required mobility CS (CS in the Fig. 3 (a) denotes such supplementary connectivity) should be added on the PP. BE Mode: In the BE mode, one branch of the robot is detached from the base in order to reach another supporting point, (a) Machining head mode (b) Branch extremity mode FIGURE 3: GENERAL TOPOLOGY while the other branches remain attached to the base. Before detaching the branch from the supporting point, passive joints in the swing branch should be locked in order to control the extremity of this branch. Also, as there are less branches connected between the PP and the base, the DOF of the PP might be changed. In the case that the actuators in the branches attached to the base are not sufficient to control the PP, some of the passive joints in these branches also need to be locked in order to reduce the DOF of the PP.