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FIGURE 1.1 The objective is to regulate the level of liquid in the tank, h, to the value H.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE A human can regulate the level using a sight tube, S, to compare the level, h, to the objective, H, and adjust a valve to change the level. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE An automatic level-control system replaces the human with a controller and uses a sensor to measure the level. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Servomechanism-type control systems are used to move a robot arm from point A to point B in a controlled fashion. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.5 This block diagram of a control loop defines all the basic elements and signals involved.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE The physical diagram of a control loop and its corresponding block diagram look similar. Note the use of current- and pressure-transmission signals. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.6 (continued) The physical diagram of a control loop and its corresponding block diagram look similar. Note the use of current- and pressure-transmission signals. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.7 A control system can actually cause a system to become unstable.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE One of the measures of control system performance is how the system responds to changes of setpoint or a transient disturbance. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE In cyclic or underdamped response, the variable will exhibit oscillations about the reference value. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Two criteria for judging the quality of control-system response are the minimum area and quarter amplitude. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Graph (a) shows how output variable b changes as an analog of variable c. Graph (b) shows how a digital output variable, n, would change with variable c. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE An ADC converts analog data, such as voltage, into a digital representation, in this case 4 bits. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE This ON/OFF control system can either heat or cool or do neither. No variation of the degree of heating or cooling is possible. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE An analog control system such as this allows continuous variation of some parameter, such as heat input, as a function of error. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE In supervisory control, the computer monitors measurements and updates setpoints, but the loops are still analog in nature. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE This direct digital control system lets the computer perform the error detection and controller functions. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.17 Local area networks (LANs) play an important role in modern process-control plants.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE A programmable logic controller (PLC) is an outgrowth of ON/OFF-type control environments. In this case the heater and cooler are either ON or OFF. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Electric current and pneumatic pressures are the most common means of information transmission in the industrial environment. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE One of the advantages of current as a transmission signal is that it is nearly independent of line resistance. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE A transfer function shows how a system-block output variable varies in response to an input variable, as a function of both static input value and time. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Uncertainties in block transfer functions build up as more blocks are involved in the transformation. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Hysteresis is a predictable error resulting from differences in the transfer function as the input variable increases or decreases. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Comparison of an actual curve and its best-fit straight line, where the maximum deviation is 5% FS. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE A P&ID uses special symbols and lines to show the devices and interconnections in a process-control system. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.26 Computers and programmable logic controllers are included in the P&ID.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE The dynamic transfer function specifies how a sensor output varies when the input changes instantaneously in time (i.e., a step change). Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Characteristic first-order exponential time response of a sensor to a step change of input. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.29 Characteristic second-order oscillatory time response of a sensor.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE Multiple readings are taken of some variable with an actual value, V. The distributions show that sensor A has a smaller standard deviation than sensor B. Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.31 Figure for Problem 1.4.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.32 Figure for Problem 1.5.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.33 Figure for Problem S1.1.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.34 Figure for Problem S1.5.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.35 Figure for Problem S1.7.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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FIGURE 1.36 Figure for Problem S1.8.
Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.
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