process control see also control theory automatic process control in continuous production processes is a combination of control engineering and chemical engineering disciplines that uses industrial control systems to achieve a production level of consistency economy and safety which could not be achieved purely by human manual control it is implemented widely in industries such as oil refining pulp and paper manufacturing chemical processing and power generating plants there is a wide range of size type and complexity but it enables a small number of operators to manage complex processes to a high degree of consistency the development of large automatic process control systems was instrumental in enabling the design of large high volume and complex processes which could not be otherwise economically or safely operated the applications can range from controlling the temperature and level of a single process vessel to a complete chemical processing plant with several thousand control loops history early process control breakthroughs came most frequently in the form of water control devices ktesibios of alexandria is credited for inventing float valves to regulate water level of water clocks in the 3rd century bc in the 1st century ad heron of alexandria invented a water valve similar to the fill valve used in modern toilets later process controls inventions involved basic physics principles in 1620 cornlis drebbel invented a bimetallic thermostat for controlling the temperature in a furnace in 1681 denis papin discovered the pressure inside a vessel could be regulated by placing weights on top of the vessel lid in 1745 edmund lee created the fantail to improve windmill efficiency a fantail was a smaller windmill placed 90 of the larger fans to keep the face of the windmill pointed directly into the oncoming wind with the dawn of the industrial revolution in the 1760s process controls inventions were aimed to replace human operators with mechanized processes in 1784 oliver evans created a waterpowered flourmill which operated using buckets and screw conveyors henry ford applied the same theory in 1910 when the assembly line was created to decrease human intervention in the automobile production process for continuously variable process control it was not until 1922 that a formal control law for what we now call pid control or threeterm control was first developed using theoretical analysis by russian american engineer nicolas minorsky his goal was stability not general control which simplified the problem significantly while proportional control provided stability against small disturbances it was insufficient for dealing with a steady disturbance notably a stiff gale due to steadystate error which required adding the integral term finally the derivative term was added to improve stability and control development of modern process control operations a modern control room where plant information and controls are displayed on computer graphics screens the operators are seated as they can view and control any part of the process from their screens whilst retaining a plant overview process control of large industrial plants has evolved through many stages initially control would be from panels local to the process plant however this required a large manpower resource to attend to these dispersed panels and there was no overall view of the process the next logical development was the transmission of all plant measurements to a permanentlymanned central control room effectively this was the centralisation of all the localised panels with the advantages of lower manning levels and easier overview of the process often the controllers were behind the control room panels and all automatic and manual control outputs were transmitted back to plant however whilst providing a central control focus this arrangement was inflexible as each control loop had its own controller hardware and continual operator movement within the control room was required to view different parts of the process with the coming of electronic processors and graphic displays it became possible to replace these discrete controllers with computerbased algorithms hosted on a network of inputoutput racks with their own control processors these could be distributed around plant and communicate with the graphic display in the control room or rooms the distributed control system was born the introduction of dcss allowed easy interconnection and reconfiguration of plant controls such as cascaded loops and interlocks and easy interfacing with other production computer systems it enabled sophisticated alarm handling introduced automatic event logging removed the need for physical records such as chart recorders allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs and provided high level overviews of plant status and production levels hierarchy functional levels of a manufacturing control operation the accompanying diagram is a general model which shows functional manufacturing levels in a large process using processor and computerbased control referring to the diagram level 0 contains the field devices such as flow and temperature sensors process value readings pv and final control elements fce such as control valves level 1 contains the industrialised inputoutput io modules and their associated distributed electronic processors level 2 contains the supervisory computers which collate information from processor nodes on the system and provide the operator control screens level 3 is the production control level which does not directly control the process but is concerned with monitoring production and monitoring targets level 4 is the production scheduling level control model to determine the fundamental model for any process the inputs and outputs of the system are defined differently than for other chemical processes the balance equations are defined by the control inputs and outputs rather than the material inputs the control model is a set of equations used to predict the behavior of a system and can help determine what the response to change will be state variable x this is a measurable variable that is a good indicator of the state of the system such as temperature energy balance volume mass balance or concentration component balance input variable u this is a specified variable that commonly include flow rates its important to note that the entering and exiting flows are both considered control inputs the control input can be classified as a manipulated disturbance or unmonitored variable parameters p the parameters are usually a physical limitation and something that is fixed for the system such as the vessel volume or the viscosity of the material output y the output is the metric used to determine the behavior of the system the control output can be classified as measured unmeasured or unmonitored types processes can be characterized as one or more of the following forms batch some applications require that specific quantities of raw materials be combined in specific ways for particular duration to produce an intermediate or end result one example is the production of adhesives and glues which normally require the mixing of raw materials in a heated vessel for a period of time to form a quantity of end product other important examples are the production of food beverages and medicine batch processes are generally used to produce a relatively low to intermediate quantity of product per year a few pounds to millions of pounds continuous often a physical system is represented through variables that are smooth and uninterrupted in time the control of the water temperature in a heating jacket for example is an example of continuous process control some important continuous processes are the production of fuels chemicals and plastics continuous processes in manufacturing are used to produce very large quantities of product per year millions to billions of pounds such controls use feedback such as in the pid controller a pid controller includes proportional integrating and derivative controller functions hybrid applications having elements of batch and continuous process control are often called hybrid applications control loops example of a continuous flow control loop signalling is by industry standard 420 ma current loops and a smart valve positioner ensures the control valve operates correctly the fundamental building block of any industrial control system is the control loop which controls just one process variable an example is shown in the accompanying diagram where the flow rate in a pipe is controlled by a pid controller assisted by what is effectively a cascaded loop in the form of a valve servocontroller to ensure correct valve positioning some large systems may have several hundreds or thousands of control loops in complex processes the loops are interactive so that the operation of one loop may affect the operation of another the system diagram for representing control loops is a piping and instrumentation diagram commonly used controllers are programmable logic controller plc distributed control system dcs or scada example of level control system of a continuous stirredtank reactor the flow control into the tank would be cascaded off the level control a further example is shown if a control valve were used to hold level in a tank the level controller would compare the equivalent reading of a level sensor to the level setpoint and determine whether more or less valve opening was necessary to keep the level constant a cascaded flow controller could then calculate the change in the valve position economic advantages the economic nature of many products manufactured in batch and continuous processes require highly efficient operation due to thin margins the competing factor in process control is that products must meet certain specifications in order to be satisfactory these specifications can come in two forms a minimum and maximum for a property of the material or product or a range within which the property must be all loops are susceptible to disturbances and therefore a buffer must be used on process set points to ensure disturbances do not cause the material or product to go out of specifications this buffer comes at an economic cost ie additional processing maintaining elevated or depressed process conditions etc process efficiency can be enhanced by reducing the margins necessary to ensure product specifications are met effective process control strategies increase the competitive advantage of manufacturers who employ them list of techniques and mechanisms used actuator automation automatic control check weigher closedloop controller control engineering control loop control panel control system control theory controllability controller control theory cruise control current loop digital control distributed control system feedback feedforward fieldbus flow control valve fuzzy control system gain scheduling intelligent control laplace transform linear parametervarying control measurement instruments model predictive control negative feedback nonlinear control openloop controller operational historian proportional control pid controller piping and instrumentation diagram positive feedback process capability programmable logic controller regulator automatic control scada servomechanism setpoint signalflow graph simatic s5 plc sliding mode control temperature control transducer valve watt governor process control monitoring walker mark john 20120908 the programmable logic controller its prehistory emergence and application pdf phd thesis department of communication and systems faculty of mathematics computing and technology the open university archived pdf from the original on 20180620 retrieved 20180620 the michigan chemical engineering process dynamics and controls open textbook pid control theory and best practices process control equipment video tutorials pid control virtual laboratory free video tutorials online simulators advanced process control schemes process control publication 