Parallel Robotic Machine Tools

The demands for higher performance of general-purpose industrial robots are increasing continuously. In particular, the need for truly adaptive automation in many applications has led to higher requirements for operational accuracy, load capacity, task flexibility, reliability, and cycle time with robots. Examples of such needs are higher precision assembly, faster product handling, better measurements, surface finishing, and milling capabilities. Furthermore, there is a high demand for off-line programming to eliminate touch-up of programmed positions; in other words, robots must perform their task with better load capacity and accuracy in operations. A general trend of meeting these demands and requirements is to make use of parallel robots, which have excellent potential capabilities, including high rigidity, high accuracy, and high loading capacities.

Parallel robots generally comprise two platforms, which are connected by at least two kinematic chains, and to provide relative motion between a moveable platform and a base platform. In fact, parallel robots have become an indispensable part of general robots both in industry and in academia. Besides, with the rapid development of parallel robots a few decades ago, the research on mechanism theory, mobility analysis, dimensional synthesis, kinematics and dynamics modeling, and design optimization have been increasing in a large scale.

Robot kinematics is the study of the motion (kinematics) of robotic mechanisms. In a kinematic analysis, the position, velocity, and acceleration of all the links are calculated with respect to a fixed reference coordinate system, without considering the forces or moments. The relationship between motion and the associated forces and torques is studied in robot dynamics. Forward kinematics and inverse kinematics are the main components in robot kinematics.

Forward kinematics (also known as direct kinematics) is the computation of the position and orientation of a robot’s end effector as a function of its joint angles. Inverse kinematics is defined as: given the position and orientation of a robot’s end-effector, calculate all possible sets of joint motion that could be used to attain this given position and orientation.

One of the objectives of this book is to find the most promising kinematic structures that can be used for machine tool design. Hence, some well-known principles are applied to investigate all the possibilities of structure in detail. A mechanism is defined as a kinematic chain with one of its components (link or joint) connected to the frame. A kinematic chain consists of a set of links, coupled by joints (cylindrical, planar, screw, prismatic, revolute, spherical, and Hooke) between adjacent links. In this chapter, a topological study of different combinations of kinematic chain structures are performed using a graph representation approach. The number of links and joints for the desired system and their interconnections, neglecting geometric details (link length and link shape), are described. The possible architectures that provide 5 degrees of freedom between the tool and the workpiece are generated. In Sect. 3.2, basic kinematic elements of mechanisms are introduced, and the classification of mechanisms is given based on the motion relation. In Sect. 3.3, the basic concept of the graph representation of a kinematic structure is addressed. Then, the Chebychev–Grübler–Kutzbach criterion is introduced in Sect. 3.4. A topological study of the kinematic structures is described in Sect. 3.5. Requirements for possible kinematic structures are set up. Furthermore, the structural representation of kinematic chains and architectures with consideration of parallel and hybrid cases is illustrated. In Sect. 3.6, a remark on the role of redundancy is given. A summary with discussion of related work is presented in Sect. 3.7.

Parallel kinematic machines with their unique characteristics of high stiffness (their actuators bear no moment loads but act in a simple tension or compression) and high speeds and feeds (high stiffness allows higher machining speeds and feeds while providing the desired precision, surface finish, and tool life), combined with versatile contouring capabilities have made parallel mechanisms the best candidates for the machine tool industry to advance machining performance. It is noted that the stiffness is the most important factor in machine tool design since it affects the precision of machining. Therefore, to build and study a general stiffness model is a very important task for machine tool design. In this chapter, we will build a general stiffness model through the approach of kinematic and static equations. The objective of this model is to provide an understanding of how the stiffness of the mechanism changes as a function of its position and as a function of the characteristics of its components. This can be accomplished using stiffness mapping.

There are two methods to build mechanism stiffness models [170]. Among them, the method which relies on the calculation of the parallel mechanism’s Jacobian matrix is adopted in this book.

In this chapter, we first introduce a fully six degrees of freedom fully-parallel robotic machine with prismatic actuators. Then several new types of parallel mechanisms with prismatic actuators whose degree of freedom is dependent on a constraining passive leg connecting the base and the platform is analyzed. The mechanisms are a series of n-dof parallel mechanisms which consist of n identical actuated legs with six degrees of freedom and one passive leg with n degrees of freedom connecting the platform and the base. This series of mechanisms has the characteristics of reproduction since they have identical actuated legs, thus, the entire mechanism essentially consists of repeated parts, offering price benefits for manufacturing, assembling, and maintenance.

A simple method for the stiffness analysis of spatial parallel mechanisms is presented using a lumped parameter model. Although it is essentially general, the method is specifically applied to spatial parallel mechanisms. A general kinematic model is established for the analysis of the structural rigidity and accuracy of this family of mechanisms. One can improve the rigidity of this type of mechanism through optimization of the link rigidities and geometric dimensions to reach the maximized global stiffness and precision. In what follows, the geometric model of this class of mechanisms is first introduced. The virtual joint concepts are employed to account for the compliance of the links. A general kinematic model of the family of parallel mechanisms is then established and analyzed using the lumped-parameter model. Equations allowing the computation of the equivalent joint stiffnesses are developed. Additionally, the inverse kinematics and velocity equations are given for both rigid-link and flexible-link mechanisms. Finally, examples for 3-dof, 4-dof, 5-dof, and 6-dof are given in detail to illustrate the results.

In this chapter, first, a six degrees of freedom fully parallel robotic machine with revolute actuators is presented and analyzed. Then, a serial of parallel manipulators with 3-dof, 4-dof, and 5-dof whose degree of freedom is dependent on an additional passive leg, this passive leg is connecting the center between the base and the moving platform. Together with the inverse kinematics and velocity equations for both rigid-link and flexible-link mechanisms, a general kinetostatic model is established for the analysis of the structural rigidity and accuracy of this family of mechanisms, case studies for 3-dof, 4-dof, and 5-dof mechanisms are given in detail to illustrate the results.

The evolution of manufacturing systems is triggered by the dynamic customer environment of its time. The main characteristics of today’s customers’ environment are mass customization and responsiveness to market demand, and thus the reconfigurable manufacturing system has been suggested for such environment. A reconfigurable manufacturing system (RMS) is one designed at the outset for rapid change in its structure, as well as its hardware and software components, in order to quickly adjust its production capacity and functionality within a part family in response to sudden market changes or intrinsic system change [87]. Ideal reconfigurable manufacturing systems possess six main characteristics: Modularity, Integrability, Customized flexibility, Scalability, Convertibility, and Diagnosability (US patent, No. 6,349,237). These characteristics provide a RMS with exactly the functionality and production capacity needed, and also the system can be economically adjusted exactly when needed [105].

The components of RMS are: CNC machines [86], Reconfigurable Machine Tools [90], Reconfigurable Inspection Machines (US patent No. 6,567,162), and material transport systems (such as gantries and conveyors) that connect the machines to form the system. As the main component of reconfigurable manufacturing systems, the reconfigurable machine tools are machine tools that are built from machine modules [46]. Therefore, research and development in reconfigurable robots can generally be divided into two categories. One studies the most suitable modular architecture for robots. This includes the development of independent joint modules with various specifications and link modules as well as rapid interfaces between joints and links. The other is aimed at providing a CAD system for rapid formulation of a suitable configuration through a combination of those modular joints and links – a modular robot in its best conformity to a given task. In this chapter, first, we give some general idea about design procedures of reconfigurable parallel robotic machine tools, and then focus on the design of reconfigurable machine tools.

Global stiffness and optimal calibration are the two crucial issues for parallel robotic machines for their performance, since global stiffness is directly related to the rigidity and accuracy of a parallel robotic machine, while optimal calibration can effectively improve the performance of the parallel robotic machine. In this chapter, both issues will be introduced and discussed. An example of a novel 3DOF parallel robotic machine will be illustrated in the chapter to show the detail of how to implement the global stiffness evaluation and optimal calibration. The method introduced in this chapter is very generic and can be applied in all types of robotic systems.

Optimization plays an important role in engineering design problems; it deals with problems of minimizing or maximizing a function with several variables. The purpose of optimization design is aiming at enhancing the performance indices by adjusting the structure parameters such as link length, radii of fixed platform and moving platform, and its distance between the center points of the two platforms. The approach can been called dimensional-synthesis-based performance optimization of parallel manipulator. In the optimum design process, several performance criteria could be involved for a design purpose, such as stiffness, dexterity, accuracy, workspace, etc.

Because of the recent trend toward high-speed machining HSM, there is a demand to develop parallel kinematic machine with high dynamic performance, improved stiffness, and reduced moving mass [2, 11, 93, 148]. However, as researchers at Giddings and Lewis have indicated, full integration of standard automation components, CAD, and a user interface are required before making its parallel kinematic machine readily available for the general market. A virtual environment that can be used for PKM design, analysis, and simulation is urgently demanded. Several efforts have been done on this topic. Pritschow [122] proposed a systematic methodology for the design of different PKM topologies. Merlet [106] developed the software for the optimal design of a specific PKM class – Stewart platform-based mechanisms. Jin and Yang [79, 80] proposed a method for topology synthesis and analysis of parallel manipulators. Huang et al. [75] made some efforts on conceptual design of 3dof translational parallel mechanisms. Nevertheless, there is no complete virtual system existing for PKM design and analyze from the literature.

With the objective of developing a practical methodology and related virtual environment for PKM analysis and design, several activities have been conducted at Integrated Manufacturing Technologies Institute of National Research Council of Canada. PKM is a key component of reconfigurable manufacturing systems in different industrial sectors. It is very important for PKM designers to design and analysis the potential PKM with an integrated virtual environment before fabrication. The virtual environment is used for modeling, simulation, planning, and control of the proposed PKM.