In the design of any complex system, all the relevant design details must be considered 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 actuator 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 understanding 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
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 operation 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 production 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 significant 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 lifecycle.
• 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, operating 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 programmable 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 communication 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 development of industrial robots. This was followed during the 1980s by the integration of robots, machine tools, and material handling systems into computer-controlled factory environments. The logical conclusion of this trend is that individual productnquality 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. 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 sequencing 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 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.