Post 657 -by Gautam Shah
Workshops till middle age were craftspeople oriented. A crafts person would occupy a place surrounded by tools and equipments and carry out the ‘Lot’ of work. After that another person took over that lot. For heavy duty jobs mechanical power drives were superior, but one had to shift around to use them. Mechanical devices like gears, leather belt transmission and offered greater productivity. Some degree of task assignment offered standardized products on a massive scale. The power shaft forced organization of workshops on linearity. The line production methods promoted productivity through time management and sequencing of task procedures. In many instances tasks began to be assimilated and handled simultaneously by many crafts-persons in a single time slot.
During the early part of the 20th C. machines became adjuncts to assembly-line production systems and multi tasking. For this machine became a device to handle a variety of tools, often simultaneously. The machines were operating at a faster speed requiring equally fast control system.
Mechanical power transmission systems were concurrent systems for the entire workshop, but this impediment was removed with the electric power. Electrical power offered local control through an electric motor through horse power rating, speed variation through voltage control and gears, and operational control like start-stop, etc. Independent electric powered machines with faster and multi tasking capabilities, however, were now difficult for human supervision. Control devices were actuators, for process regulation.
Control devices or systems are of two types: A Feed-forward system has inputs or predicts unusual happening, but would not oversee or govern the actualization of the action. A Feedback system improvises strategy for future actions in many instances oversees actions being taken.
Feed-forward systems: Jacquard weaving loom uses a feed-forward control as a programmed punched card to weave a pattern, but cannot stop the loom if there is a short feed of thread. Similarly a cutting machine cuts a large sized shape by moving the cutter tracing a small scale pattern through the arm of a pantograph.
Feedback systems: A wind mill keeps facing the wind with the help of a tail wane. A pressure cooker seals itself with heightened internal pressure of steam. Pressure valves are weight calibrated opening themselves at certain pressure levels only.
Control devices and systems have led to automation of processes. Automation began in the late 1940s with the development of the mechanical devices for moving and positioning objects on a production line, though observation and manual intervention were necessary. During 1960s digital computers began to offer control systems in three different manners: For supervisory or optimizing control, Direct digital control, and Hierarchy control. In the first instance, for the supervisory control a computer sets parametric levels for optimizing the operations. In the second instance, for the direct-digital control, several devices feed data to a single processor, which then decides a strategy of operation. The advantage here is very fast and objective evaluation of the data. The third system the hierarchy control applies to all the plant-control situations concurrently, often with the actuation of the control mechanisms.