Note for Industrial Automation And Control - IAC by Er.Ramji Tripathi
- Industrial Automation And Control - IAC
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INDUSTRIAL AUTOMATION An Industrial Automation System consists of numerous elements that perform a variety of functions related to Instrumentation, Control, Supervision and Operations Management related to the industrial process. These elements may also communicate with one another to exchange information necessary for overall coordination and optimized operation of the plant/factory/process. Below, we classify the major functional elements typically found in IA systems and also describe the nature of technologies that are employed to realize the functions. INDUSTRIAL CONTROL SYSTEMS By industrial control systems, we denote the sensors systems, actuator systems as a controller. Controllers are essentially (predominantly electronic, at times pneumatic/hydraulic) elements that accept command signals from human operators or Supervisory Systems, as well as feedback from the process sensors and produce or compute signals that are fed to the actuators. Control Systems can be classified into two kinds. Continuous Control This is also often termed as Automatic Control, Process Control, Feedback Control etc. Here the controller objective is to provide such inputs to the plant such that the output y(t) follows the input r(t) as closely as possible, in value and over time. The structure of the common control loop with its constituent elements, namely the Controller, the Actuator, the Sensor and the Process itself is shown. In addition the signals that exist at various points of the 1 Er Ramji Tripathi, Assistant Professor, Mechanical Engineering Department Rajkiya Engineering College, Banda
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Disturbances di + r (t) e uA Controller GC(s) Actuator GA(s) do uP Command/ reference/ Setpoint Plant GP(s) y(t) Output ds + + Sensor GS(s) Fig. 2.3 Typical control loop system are also marked. These include the command (alternatively termed the set point or the reference signal), the exogenous inputs (disturbances, noise). The difficulties in achieving the performance objective is mainly due to the unavoidable disturbances due to load variation and other external factors, as well as sensor noise, the complexity, possible instability, uncertainty and variability in the plant dynamics, as well as limitations in actuator capabilities. Most industrial control loop command signals are piecewise constant signals that indicate desirable levels of process variables, such as temperature, pressure, flow, level etc., which ensure the quality of the product in Continuous Processes. In some cases, such as in case of motion control for machining, the command signal may be continuously varying according to the dimensions of the product. Therefore, here deviation of the output from the command signal results in degradation of product quality. It is for this reason that the choice of the feedback signals, that of the controller algorithm (such as, P, PI pr PID), the choice of the control loop structure (normal feedback loop, cascade loop or feedforward) as well as choice of the controller gains is extremely important for industrial machines and processes. Typically the control configurations are well known for a given class of process, however, the choice of controller gains have to be made from time to time, since the plant operating characteristics changes with time. This is generally called controller tuning. A single physical device may act as the controller for one or more control loops (single- loop/multi-loop controller). Today, many loop controllers supplement typical control laws such as PID control by offering adaptive control and fuzzy logic algorithms to enhance controller response and operation. PID and startup self-tuning are among the most important features. Among other desired and commonly found characteristics are, ability to communicate upward with supervisory systems, as well as on peer-to-peer networks (such as Fieldbus or DeviceNet), support for manual control in the event of a failure in the automation. Software is an important factor in loop controllers. Set-up, monitoring and auto-tuning and alarm software for loop controllers is now a common feature. The controllers also accept direct interfacing of process sensors and signals. Choice of inputs includes various 2 Er Ramji Tripathi, Assistant Professor, Mechanical Engineering Department Rajkiya Engineering College, Banda
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types of thermocouples, RTDs, voltage to 10 V dc, or current to 20 mA. While most sophisticated controllers today are electronic, 3 Er Ramji Tripathi, Assistant Professor, Mechanical Engineering Department Rajkiya Engineering College, Banda
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pneumatic controllers are still being used. Pneumatic controllers are easy to use, easy to maintain, and virtually indestructible. SEQUENCE / LOGIC CONTROL Many control applications do not involve analog process variables, that is, the ones which can assume a continuous range of values, but instead variables that are set valued, that is they only assume values belonging to a finite set. The simplest examples of such variables are binary variables, that can have either of two possible values, (such as 1 or 0, on or off, open or closed etc.). These control systems operate by turning on and off switches, motors, valves, and other devices in response to operating conditions and as a function of time. Such systems are referred to as sequence/logic control systems. For example, in the operation of transfer lines and automated assembly machines, sequence control is used to coordinate the various actions of the production system (e.g., transfer of parts, changing of the tool, feeding of the metal cutting tool, etc.). There are many industrial actuators which have set of command inputs. The control inputs to these devices only belong to a specific discrete set. For example in the control of a conveyor system, analog motor control is not applied. Simple on-off control is adequate. Therefore for this application, the motor-starter actuation system may be considered as discrete having three modes, namely, start, stop and run. Other examples of such actuators are solenoid valves, discussed in a subsequent lesson. Similarly, there are many industrial sensors (such as, Limit Switch / Pressure Switch/ Photo Switch etc.) which provide discrete outputs which may be interpreted as the presence/absence of an object in close proximity, passing of parts on a conveyor, or a given pressure value being higher or lower than a set value. These sensors thus indicate, not the value of a process variable, but the particular range of values to which the process variable belongs. A modern controller device used extensively for sequence control today in transfer lines, robotics, process control, and many other automated systems is the Programmable Logic Controller (PLC). In essence, a PLC is a special purpose industrial microprocessor based real- time computing system, which performs the following functions in the context of industrial operations 4 Er Ramji Tripathi, Assistant Professor, Mechanical Engineering Department Rajkiya Engineering College, Banda
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