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ICRA '04. 2004 IEEE International Con ference on Robotics and Automation, volume 5. Chapter 1 Electromechanical systems In the design of any complex system, all the relevant design details must be con sidered to ensure the development of a successful product. In the development of motion systems, problems in the design process are most likely to occur in the ac tuator or motor-drive system. When designing any actuation system, mechanical designers work with electrical and electronic systems engineers, and if care is not taken, confusion will result. The objective of this book is to discuss some of the electric motor-drive systems in common use, and to identify the issues that arise in the selection of the correct components and systems for specific applications. A key step in the selection of any element of a drive system is a clear under standing of the process being undertaken. Section 1.1 provides an overview to the principles of industrial automation, and sections 1.2 and 1.3 consider machine tools and industrial robotics, respectively. Section 1.4 considers a number of other applications domains. 1.1 Principles of automation Within manufacturing, automation is defined as the technology which is concerned with the application of mechanical, electrical, and computer systems in the opera tion and control of manufacturing processes. In general, an automated production process can be classified into one of three groups: fixed, programmable, or flexible. • Fixed automation is typically employed for products with a very high pro duction rate; the high initial cost of fixed-automation plant can therefore be spread over a very large number of units. Fixed-automation systems are used to manufacture products as diverse as cigarettes and steel nails. The signif icant feature of fixed automation is that the sequence of the manufacturing operations is fixed by the design of the production machinery, and therefore the sequence cannot easily be modified at a later stage of a product's life cycle. 1 2 1.1. PRINCIPLES OF AUTOMATION • Programmable automation can be considered to exist where the production equipment is designed to allow a range of similar products to be produced. The production sequence is controlled by a stored program, but to achieve a product change-over, considerable reprogramming and tooling changes will be required. In any case, the process machine is a stand-alone item, oper ating independently of any other machine in the factory; this principle of automation can be found in most manufacturing processes and it leads to the concept of islands of automation. The concept of programmable automation has its roots in the Jacquard looms of the nineteenth century, where weaving patterns were stored on a punched-card system. • Flexible automation can be considered to be an enhancement of pro grammable automation in which a computer-based manufacturing system has the capabiUty to change the manufacturing program and the physical configuration of the machine tool or cell with a minimal loss in production time. In many systems the machining programs are prepared at a location remote from the machine, and they are then transmitted as required over a computer-based local-area communication network. The basic design of machine tools and other systems used in manufacturing processes changed Uttle from the eighteenth century to the late 1940s. There was a gradual improvement during this period as the metal cutting changed from an art to a science; in particular, there was an increased understanding of the materials used in cutting tools. However, the most significant change to machine-tool technology was the introduction of numerical-control (NC) and computer-numerical-control (CNC) systems. To an operator, the differences between these two technologies are small: both operate from a stored program, which was originally on punched tape, but more recently computer media such as magnetic tapes and discs are used. The stored program in a NC machine is directly read and used to control the machine; the logic within the controller is dedicated to that particular task. A CNC machine tool incorporates a dedicated computer to execute the program. The use of the computer gives a considerable number of other features, including data collection and com munication with other machine tools or computers over a computer network. In addition to the possibility of changing the operating program of a CNC system, the executive software of the computer can be changed, which allows the performance of the system to be modified at minimum cost. The application of NC and CNC technology permitted a complete revolution of the machine tool industry and the manufacturing industries it supported. The introduction of electronic systems into conventional machine tools was initially undertaken in the late 1940s by the United States Air Force to increase the quality and productivity of machined aircraft parts. The rapid advances of electronics and computing systems during the 1960s and 1970s permitted the complete automation of machine tools and the parallel devel opment of industrial robots. This was followed during the 1980s by the integration CHAPTER 1. ELECTROMECHANICAL SYSTEMS External Computer Network User Interface Interface T J 1 I £ ^ System Computer I Process ma 0 Control T Process Individual axis controllers L Figure 1.1. The outline of the control structure for CNC machine tool, robot or similar multi-axis system. The number of individual motion axes, and the interface to the process are determined by the system's functionality. of robots, machine tools, and material handling systems into computer-controlled factory environments. The logical conclusion of this trend is that individual prod uct quality is no longer controlled by direct intervention of an operator. Since the machining parameters are stored either within the machine or at a remote location for direct downloading via a network (see Section 10.4) a capability exists for the complete repeatability of a product, both by mass production and in limited batches (which can be as small as single components). This flexibility has permitted the introduction of management techniques, such as just-in-time production, which would not have been possible otherwise. A typical CNC machine tool, robot or multi-axis system, whatever its function, consists of a number of common elements (see Figure 1.1). The axis position, or the speed controllers, and the machining-process controller are configured to form a hierarchical control structure centred on the main system computer. The overall control of the system is vested in the system computer, which, apart from sequenc ing the operation of the overall system, handles the communication between the operator and the factory's local-area network. It should be noted that industrial robots, which are considered to be an important element of an automated factory, can be considered to be just another form of machine tool. In a machine tool or 4 1.2. MACHINE TOOLS industrial robot or related manufacturing systems, controlled motion (position and speed) of the axes is necessary; this requires the provision of actuators, either Hnear or rotary, associated power controllers to produce motion, and appropriate sensors to measure the variables. 1.2 Machine tools Despite advances in technology, the basic stages in manufacturing have not changed over the centuries: material has to be moved, machined, and processed. When considering current advanced manufacturing facilities it should be remem bered that they are but the latest step in a continuing process that started during the Industrial Revolution in the second half of the eighteenth century. The machine- tool industry developed during the Industrial Revolution in response to the de mands of the manufacturers of steam engines for industrial, marine, and railway applications. During this period, the basic principles of accurate manufacturing and quality were developed by, amongst others, James Nasmyth and Joseph Whit- worth. These engineers developed machine tools to make good the deficiencies of the rural workers and others drawn into the manufacturing towns of Victorian England, and to solve production problems which could not be solved by the exist ing techniques. Increased accuracy led to advantages from the interchangeability of parts in complex assemblies. This led, in turn, to mass production, which was first realised in North America with products (such as sewing machines and type writers) whose commercial viability could not be realised except by high-volume manufacturing (Rolt, 1986). The demands of the market place for cost reductions and the requirement for increased product quality has led to dramatic changes in all aspects of manufacturing industry, on an international scale, since 1970. These changes, together with the introduction of new management techniques in manu facturing, have necessitated a considerable improvement in performance and costs at all stages of the manufacturing process. The response has been a considerable investment in automated systems by manufacturing and process industries. Machining is the manufacturing process in which the geometry of a component is modified by the removal of material. Machining is considered to be the most versatile of production processes since it can produce a wide variety of shapes and surface finishes. To fully understand the requirements in controlling a machine tool, the machining process must be considered in some detail. Machining can be classified as either conventional machining, where material is removed by direct physical contact between the tool and the workpiece, or non-conventional machin ing, where there is no physical contact between the tool and the workpiece. 1.2.1 Conventional machining processes In a conventional machining operation, material is removed by the relative motion between the tool and the workpiece in one of five basic processes: turning, milling. CHAPTER 1. ELECTROMECHANICAL SYSTEMS Figure 1.2. The turning process, where a workpiece of initial diameter D is being reduced to d; Ft is the tangential cutting force, A^ is the spindle speed, and / the feed rate. In the diagram the depth of the cut is exaggerated. drilHng, shaping, or grinding. In all machining operations, a number of process parameters must be controlled, particularly those determining the rate of material removal; and the more accurately these parameters are controlled the higher is the quality of the finished product (Waters, 1996). In sizing the drives of the axes in any machine tool, the torques and speed drives that are required in the machin ing process must be considered in detail. Figure 1.2 illustrates a turning operation where the tool is moved relative to the workplace. The power required by the turn ing operation is of most concern during the roughing cut (that is, when the cutting depth is at its maximum), when it is essential to ensure that the drive system will produce sufficient power for the operation. The main parameters are the tangential cutting force. Ft, and the cutting speed, Vc. The cutting speed is defined as the relative velocity between the tool and the surface of the workpiece (m min~^). The allowable range depends on the material being cut and the tool: typical values are given in Table 1.1. In a turning operation, the cutting speed is directly related to the spindle speed, N (rev min~^), by Vc = DTTN (1.1) The tangential force experienced by the cutter can be determined from knowledge of the process. The specific cutting force, K, is determined by the manufacturer of the cutting tool, and is a function of the materials involved, and of a number of other parameters, for example, the cutting angles. The tangential cutting force is given by 1.2. MACHINE TOOLS Table 1.1. Machining data Material Cutting Speed, Specific Cutting Material Removal Vc Force, K Rate, Rp Low carbon steel 90-150 2200 25 Cast iron 60-90 1300 35 Aluminium 230-730 900 80 Kf{D - d) Ft^ (1.2) Knowledge of the tangential forces allows the power requirement of the spindle drive to be estimated as Power — (1.3) 60 In modem CNC lathes, the feed rate and the depth of the cut will be individually controlled using separate motion-control systems. While the forces will be consid erably smaller than those experienced by the spindle, they still have to be quantified during any design process. The locations of the radial and axial the forces are also shown in Figure 1.2; their magnitudes are, in practice, a function of the approach and cutting angles of the tool. Their determination of these magnitudes is outside the scope of this book, but it can be found in texts or manufacturers' data sheets relating to machining processes. In a face-milling operation, the workpiece is moved relative to the cutting tool, as shown in Figure 1.3. The power required by the cutter, for a cut of depth, Wc, can be estimated to be Power = (1.4) Rp where Rp is the quantity of material removed in m^ min~^ kW~^ and the other variables are defined in Figure 1.3. A number of typical values for Rp are given in Table 1.1. The determination of the cutting forces is outside the scope of this book, because the resolution of the forces along the primary axes is a function of the angle of entry and of the path of the cutter relative to the material being milled. A value for the sum of all the tangential forces can, however, be estimated from the cutting power; if Vc is the cutting speed, as determined by equation (1.1), then E« =6 0000 X Power (1.5) Vr. The forces and powers required in the drilling, planing, and grinding processes can be determined in a similar manner. The sizes of the drives for the controlled axes in all types of conventional machine tools must be carefully determined to en sure that the required accuracy is maintained under all load conditions. In addition. CHAPTER L ELECTROMECHANICAL SYSTEMS Figure 1.3. The face-milling process where the workpiece is being reduced by d: f is the feed rate of the cutter across the workpiece, Wc is the depth of the cut and N is the rotary speed of the cutting head. a lack of spindle or axis drive power will cause a reduction in the surface quahty, or, in extreme cases, damage to the machine tool or to the workpiece. 1.2.2 Non-conventional machining Non-conventional processes are widely used to produce products whose materials cannot be machined by conventional processes, for example, because of the work- piece's extreme hardness or the required operation cannot be achieved by normal machine processes (for example, if there are exceptionally small holes or complex profiles). A range of non-conventional processes are now available, including • laser cutting and electron beam machining, • electrochemical machining (ECM), • electrodischarge machining (EDM), • water jet machining. In laser cutting (see Figure 1.4(a)), a focused high-energy laser beam is moved over the material to be cut. With suitable optical and laser systems, a spot size with a diameter of 250 /xm and a power level of 10^ W mm"^ can be achieved. As in conventional machining the feed speed has to be accurately controlled to achieve