FIGURE 1.1 The objective is to regulate the level of liquid in the tank, h, to the value H. Curtis...

FIGURE 1.1 The objective is to regulate the level of liquid in the tank, h, to the value H.

Curtis JohnsonProcess Control Instrumentation Technology, 8e]

FIGURE 1.2 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.

FIGURE 1.3 An automatic level-control system replaces the human with a controller and uses a sensor to measure the level.

FIGURE 1.4 Servomechanism-type control systems are used to move a robot arm from point A to point B in a controlled fashion.

FIGURE 1.5 This block diagram of a control loop defines all the basic elements and signals involved.

FIGURE 1.6 The physical diagram of a control loop and its corresponding block diagram look similar. Note the use of current- and pressure-transmission signals.

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.

FIGURE 1.7 A control system can actually cause a system to become unstable.

FIGURE 1.8 One of the measures of control system performance is how the system responds to changes of setpoint or a transient disturbance.

FIGURE 1.9 In cyclic or underdamped response, the variable will exhibit oscillations about the reference value.

FIGURE 1.10 Two criteria for judging the quality of control-system response are the minimum area and quarter amplitude.

FIGURE 1.11 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.

FIGURE 1.12 An ADC converts analog data, such as voltage, into a digital representation, in this case 4 bits.

FIGURE 1.13 This ON/OFF control system can either heat or cool or do neither. No variation of the degree of heating or cooling is possible.

FIGURE 1.14 An analog control system such as this allows continuous variation of some parameter, such as heat input, as a function of error.

FIGURE 1.15 In supervisory control, the computer monitors measurements and updates setpoints, but the loops are still analog in nature.

FIGURE 1.16 This direct digital control system lets the computer perform the error detection and controller functions.

FIGURE 1.17 Local area networks (LANs) play an important role in modern process-control plants.

FIGURE 1.18 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.

FIGURE 1.19 Electric current and pneumatic pressures are the most common means of information transmission in the industrial environment.

FIGURE 1.20 One of the advantages of current as a transmission signal is that it is nearly independent of line resistance.

FIGURE 1.21 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.

FIGURE 1.22 Uncertainties in block transfer functions build up as more blocks are involved in the transformation.

FIGURE 1.23 Hysteresis is a predictable error resulting from differences in the transfer function as the input variable increases or decreases.

FIGURE 1.24 Comparison of an actual curve and its best-fit straight line, where the maximum deviation is 5% FS.

FIGURE 1.25 A P&ID uses special symbols and lines to show the devices and interconnections in a process-control system.

FIGURE 1.26 Computers and programmable logic controllers are included in the P&ID.

FIGURE 1.27 The dynamic transfer function specifies how a sensor output varies when the input changes instantaneously in time (i.e., a step change).

FIGURE 1.28 Characteristic first-order exponential time response of a sensor to a step change of input.

FIGURE 1.29 Characteristic second-order oscillatory time response of a sensor.

FIGURE 1.30 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.

FIGURE 1.31 Figure for Problem 1.4.

FIGURE 1.32 Figure for Problem 1.5.

FIGURE 1.33 Figure for Problem S1.1.

FIGURE 1.1 The objective is to regulate the level of liquid in the tank, h, to the value H. Curtis...

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