instrumentation for process measurement and control, third editon

[...]... characteristics Both static (steady) and dynamic (changing with time) responses must be considered in the operation and understanding of a process control system Types ot Processes The simplest process contains a single capacity and a single resistance Figure 1-2 illustrates a single-capacity, single-resistance process in (a) electrical, (b) hydraulic, (c) pneumatic, and (d) thermal forms To show how these behave... temperature, and then to the transmitter before the change can be seen Dead time is a function of the physical dimensions of a process and such things as belt speeds and mixing rates Second, the capacity of a process is the material or energy that must enter or leave the process to change the measurements -for example, the gallons necessary to change level, the Btu's necessary to change temperature, or the standard... of the proportional band Two basic properties of proportional control can be observed from this graph: 1 For every value of proportional band, whenever the measurement equals the set paint, the normal output is 50 percent 2 Each value of the proportional band defines a unique relationship between measurement and output For every measurement value there is a specific output value For example, using the... cycling of the measurement This proportional band value, known as the ultimate proportional band, is a límit on the adjustment of the controller in that loop On the other hand, if too wide a proportional band is used, the controller response to any change in measurement is too small and the measurement is Dot controlled as tightly as possible The determination of the proper proportional band for any application... out Process Characteristics and Controllability The automatic controller uses changes in the position of the final actuator to control the measurement signal, moving the actuator to oppose any change it sees in the measurement signal The controllability of any process depends on the efficiency of the measurement signal response to these changes in the controller output For proper control, the measurement. .. controlloop I Measurementmust be made to indicate the current value of the variable controlled by the loop Commonmeasurements used in industry include ftow rate, pressure,level, temperature,analytical measurements suchas pH, ORPand conductivity; and manyotherg particular to specificindustries 2 For every processthere mustbe afina! actuator that regulatesthe supply of energyor materialto the processand changes... filling the tub with cold water, for example, and then adding enough bot water to reach the desired temperature; or by mixing the bot and cold to get the same resulto Controlling the Process In performing the control function, the automatic controller uses the difference between the set-point and the measurement signats to develop the output signat to the valve The accuracy and responsiveness of these signats... a process will contain many capacitance and resistance elements Figure 1-4 illustrates a process containing two resistance elements and two capacitance elements Figure 1-5 shows the resulting process reaction curve Note that the additional capacitance and resistance essentially affect the initial curve shape, adding a delay to the processo Dead Time Dead time is a delay between two related actions For. .. a balance between supply and demand over a period of time As noted previously, supply and demand are defined in terms of energy or material into (the manipuINSTRUMENT FOR SINGLE MEASURED INSTRUMENT INSTRUMENT MOUNTED ON BOARD LOCALLY MOUNTED VARIABLE INSTRUMENT MOUNTED BEHIND BOARD NSTRUMENT FOR TWO MEASURED Fig 1-7 Instrumentfor measured variables VARIABLES INTRODUCTIONTO PROCESS CONTROL II Table... vats, the large capacity causes MEASUREMENT 100 % SIGNAL TO VALVE Fig 1-12 On-off control for reverse-acting controllerand air-to-closevalve 20 FEEDBACK PROCESS CONTROL Fig 1-13.Automatic controller with artificial signal a long time constant Therefore, the measurement can change only slowly As a result, the cycle occurs within a very narrow band around the set point, and this control mar be quite acceptable, . alt="" THIRD EDITION for NORMAN A. ANDERSON CHILTON COMPANY RAONaR, PENNSYLVANIA CONTENTS Preface SECTION I FEEDBACK PROCESS CONTROL 1 1 Introduction to Process Control 1 Types of Processes 5 Processes. hydraulic, (c) pneumatic, and (d) thermal. Each has a single capacity and a single resistance and all bave identical response characteristics. regulate a process and certain aspects of process behavior. static (steady) and dynamic (changing with time) re- sponses must be considered in the operation and understanding of a process control system. Types ot Processes The simplest process contains