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Abstract: This paper deals with the design of an industrial robot controller t~aturing sensor-based control capabilities. The design tbllows the guidelines of a methodology aimed at structuring the design process, to assure traceability between the requirements and capabilities of the controller and a high degree of operational flexibility. The design consists of three steps, namely activity analysis, functional design and implementation design. 36297
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The methodology especially supports the tirst two steps with design principles and guidelines. The designed controller has the capabilities to execute machining operations, arc and laser welding, insertion of parts into others and other interesting operations from an industrial and commercial point of view. The new capabilities may be implemented optionally as an add-on to the base product. Key Words: Robot control; industrial robots; three control; open control systems; design methodology 1. INTRODUCTION The engineering design of a controller must satisfy a large number of requirements and constraints of different natures (e.g, performance, cost, safety and reliability), and it often involves a choice among several equally good solutions. Many factors, also subjective, may influence the choice. The definition of the right product features and the development of a design consistent with them are crucial to the realization of profitable products. The lack and need of a structured and systematic framework to assist this difficult task is well recognized (Astr6m and Wittenmark, 1990). This study concerns this subject. The design of a new industrial controller is tackled according to a codified methodology, Referring to a state-of-the-art commercial product, the Comau C3G 9000, the first objective of the study is to design a new controller ("next generation", since it should replace the current product) with innovations based on the forthcoming commercial needs and advances in sensor, actuators and information technologies. The design development follows the Control Development Methodology (CDM), a design methodology defined by the European Space Agency (ESA) for structuring the design process of automation and robotics control systems for space applications.
Actually, it is a second objective of the study to assess the effectiveness and possible drawbacks of the application of the CDM to industrial problems. According to the CDM, the design process consists of three major steps. The first step, referred to as activity analysis, defines precisely and formally the tasks the robot has to accomplish. The second one (requirements or functional analysis) establishes the control functions required to accomplish the tasks and the interaction with the human operators. This entails, for instance, the choice of control principles and algorithms, as well as of control and measured variables. The third step (architectural design) concerns the detailed design of the controller hardware and software. The CDM supports the first two steps in particular, supplying principles, methods and tools in order to proceed in a structured and formal way. The organization of the paper reflects the steps of the controller design process. In Section 2 the CDM is described. Section 3 deals with the activity analysis for next-generation industrial robots. Section 4 discusses, as an illustrative example, the functional analysis for hybrid position/force control, Section 5 outlines hardware and software implementation architectures. 2. THE CONTROL DEVELOPMENT METHODOLOGY 2.1 Objective and benefits The CDM (Putz and Man, 1992; Pntz and Elfving, 1992) provides principles and guidelines for the design of open and extendable robot control systems within a simple overall structure, using unified and unambiguous terminology, and above all maintaining traceability between solution- independent requirements and final realizations. 2.2 The Steps of the CDM The CDM defines a life-cycle model for the robot control system (which is seen as part of a larger "robot system"). Figure 1 illustrates the phases and steps within this life cycle. Guidelines and methods are offered in (Putz and Mau, 1992) for each of these phases, but special attention has been devoted to the initial steps 1 - 3. The first step is called activity analysis. Already, the initial user requirements need to be refined in a systematic (hierarchical) structure. The proposed output from this step is a so-called "activity script", a semi-formal and fairly detailed prescription of the process which has to be automated by the robot system (key question: "'FOR WHAT do I need automation and control ?"). The next step and phase is the refined analysis of the control (sub)system requirements. This has to take all the different kinds of requirements into account, most notably functional and performance requirements (stating WHAT control functions have to be performed HOW WELL) and (especially for space applications) operational requirements. The last step particularly emphasized in the CDM is the control system architectural design which, for the software part, coincides with the well-known S/W architectural design phase. Also, control hardware and human control operations Cbrainware") must be "designed" to the same level in this step. As opposed to the preceding requirements analysis phase, design starts to say HOW the required functions are going to be realized. This involves the specific processors, bus systems, real-time operating system services, algorithms, software communication schemes, operational procedures, etc.