Process measuring device control

The analytic control problem deals with the appraisal of existing control systems. What is the overall dynamic behavior of the system, and what individual contributions are made to the overall behavior by the various components in the system How may processes be characterized as to their dynamic behavior in terms useful for control system analysis What are the control characteristics of typical instruments such as the measuring devices, controllers, and regulating units ... 

Consider the generalized closed-loop system shown in Figure 13.1b. For each of its four components (process, measuring device, controller mechanism, and final control element) we can write the corresponding transfer function relating its output to its inputs. In particular, if we neglect the dynamics of the transmission lines, we have ... 

To use the Bode criterion, we need the Bode plots for the open-loop transfer function of the controlled system. These can be constructed in two ways (a) numerically, if the transfer functions of the process, measuring device, controller, and final control element are known and (b) experimentally, if all or some of the transfer functions are unknown. In the second case the system is disturbed with a sinusoidal input at various frequencies, and the amplitude and phase lag of the open-loop response are recorded. From these data we can construct the Bode plots. 

When we use continuous analog controllers, all signals in a loop are continuous in time. Then the dynamic behavior of each component in the loop (process, measuring device, controller, final control element), as well as the response of the overall control system, can be effectively analyzed by continuous models (differential equations in the time domain or transfer functions in the Laplace domain). 

In Chapter 14 we examined the dynamic characteristics of the response of closed-loop systems, and developed the closed-loop transfer functions that determine the dynamics of such systems. It is important to emphasize again that the presence of measuring devices, controllers, and final control elements changes the dynamic characteristics of an uncontrolled process. Thus nonoscillatory first-order processes may acquire oscillatory behavior with PI control. Oscillatory second-order processes may become unstable with a PI controller and an unfortunate selection of Kc and t,. 

Until now, most of modules for the microreaction system are made of metals. These robust modules have high resistance to various harsh chemical conditions as well as temperature and pressure. Each module works as a sensor, reactor, measuring device, control unit, analytic device, pump, thermostat, and so on. These modules can be combined together to correspond to the needs of researchers for the regarded process. This easily modularized aspect can enhance a wide application range of the DCF system. 

Figure 5.50 shows the typical components found in a control loop. It can be seen that there are four main components involved process, measuring device, controller, and the final control element. The controller mechanism gets the value of the set point and directs the final control element to carry out the actions. Disturbances enter the system through the process, which affect the process variables. 

What types of sensors should be used in monitoring the process This is a specific question on the design of measurement devices for process control. A general presentation of different methods for the collection of data on accident risks is given in Chapters 12-14. 

Kp= first-order process static gain = first-order valve constant K = first-order measurement constant ItjS proportional gain for the three-mode controller f= Laplace transform of the output temperature deviation f = Laplace transform of the input load temperature deviation = first-order lime constants for the process, measurement device, and process valve, respectively. 

Process industries frequently need to weigh and control the flow rate of bulk material for optimum performance of such devices as grinders or pulverizers, or for controlling additives, eg, to water suppHes. A scale can be installed in a belt conveyor, or a short belt feeder can be mounted on a platform scale. Either can be equipped with controls to maintain the feed rate within limits by controlling the operation of the device feeding the material to the conveyor. Direct mass measurement with a nuclear scale can also be used to measure and control such a continuous stream of material. 

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

Fig. 7. Instmment components of a control loop, where A = process measurement devices, in this case, pressure measurement B = transducer ...

The process and instrumentation (P I) diagram provides a graphical representation of the control configuration for the process. The P I diagrams illustrate the measurement devices that provide inputs to the control strategy, the actuators that will implement the results of the control calculations, and the function blocks that provide the control logic. 

For regulatory control, repeatability is of major interest. The basic-objective of regulatory control is to maintain uniform process operation. Suppose that on two different occasions, it is desired that the temperature in a vessel be 80°C. The regulatoiy control system takes appropriate actions to bring the measured variable to 80°C. The difference between the process conditions at these two times is determined by the repeatability of the measurement device. 

Dynamics of Process Measurements Especially where the measurement device is incorporated into a closed loop control configuration, dynamics are important. The dynamic characteristics depend on the nature of the measurement device, and also on the nature of components associated with the measurement device (for example, thermowells and sample conditioning equipment). The term mea-.sui ement system designates the measurement device and its associated components. 

Measurement of the hotness or coldness of a body or fluid is commonplace in the process industries. Temperature-measuring devices utilize systems with properties that vaiy with temperature in a simple, reproducible manner and thus can be cahbrated against known references (sometimes called secondaiy thermometers). The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers and are applicable over different temperature regimes. 

With the advent of the microprocessor, digital technology began to be used for data collection, feedback control, and aU other information processing requirements in production facUities. Such systems must acquire data from a variety of measurement devices, and control systems must drive final actuators. 

Measurement Devices and Actuators Often referred to as level 0, this layer couples the control and information systems to the process. The measurement devices provide information on the cur-... 

Regulators, though not controllers or final control elements, perform the combined function of these two devices (controller and final control element) along with the measurement function commonly associated with the process variable transmitter. The uniqueness, control performance, and widespread usage of the regulator make it deseivang of a functional grouping of its own. 

Also, the electronic control-valve device s level of immunity to, and emission of, electromagnetic interference (EMI) can be an issue in the chemical-valve environment. EMI requirements for the control-valve devices are presently mandatory in the European Community but voluntary in the United States, Japan, and the rest of the world. International Electrotechnical Commission (lEC) SOI, Parts I through 4, Electromagnetic Compatibihty for Industrial Process Measurement and Control Equipment, defines tests and requirements for control-device immunity. Immunity and emission standards are addressed in CENELEC (European Committee for Electrotechnical Standardization) EN 50 081-1 1992, EN 50 081-2 1993, EN 50 082-1 1992, and prEN 50 082-2 1994. 

An interlock is a protec tive response initiated on the detection of a process hazard. The interlock system consists of the measurement devices, logic solvers, and final control elements that recognize the hazard and initiate an appropriate response. Most interlocks consist of one or more logic conditions that detect out-of-hmit process conditions and respond by driving the final control elements to the safe states. For example, one must specify that a valve fails open or fails closed. 

These tests must encompass the complete interlock system, from the measurement devices through the final control elements. Merely simulating inputs and checking the outputs is not sufficient. The tests must duplicate the process conditions and operating environments as closely as possible. The measurement devices and final control elements are exposed to process and ambient conditions and thus are usually the most hkely to fail. Valves that remain in the same position for extended periods of time may stick in that position and not operate when needed. The easiest component to test is the logic however, this is the least hkely to fail. 

Instrumentation normally is denoted by a circle in which the variable being measured or controlled is denoted by an appropriate letter symbol inside the circle. When the control device is to be located remotely, the circle is divided in half with a horizontal line. Table 1.3 gives various instrumentation symbols and corresponding letter codes. The specific operating details and selection criteria for various process instrumentation are not discussed in this book. The reader is referred to earlier works by Cheremisinoff [1,2] for discussions on essential control and measurement instrumentation. 

Automation and instrumentation are critical to tlie safe control of processes. Suitable measurement devices and control of system variables should be provided for nonnal operating conditions as well as for emergencies. Tliere are six major components to a control system ... 

Continuous in-line measurements and control of the mass material balance in the process, with automatic feedback to the reactants dosing devices (performed either by computerized system or by traditional flow control loops). 

Hardness does not produce a complete characterization of the strengths of materials, but it does sort them in a general way, so it is very useful for quality control for the development of new materials and for developing prototypes of devices and processes. Furthermore, mechanical hardness is closely related to chemical hardness, which is a measure of chemical bond stability (reactivity). In the case of metals the connection is somewhat indirect, but nevertheless exists. 

In addition, suitable equipment and suitable monitoring and measuring devices should be available so that the laboratory processes operate effectively. The procedures for the release of the laboratory product, i.e. the acceptance of the laboratoiy resnlts as well as that for the delivery of snch results to the customers, have to be specified and controlled. 

Crystallization from solution is a widely utilized separation and purification technique in chemical industry. It is characterized by the formation of a spectrum of differently sized crystals. This spectrum, called the Crystal Size Distribution or CSD, is highly important for the performance of the crystallizer, the crystal handling equipment like centrifuges and dryers, and the marketability of the produced crystals. However, in many industrial crystallizers, the observed CSD s show large transients due to disturbances or are unstable because of the internal feedback mechanisms of the crystallization process ). The main limitation for effective CSD control was the lack of a good on-line CSD measurement device, but recent developments show that this hurdle is taken (2). 

Fig. 6.9 shows the control algorithm and a process w/ith a phase shift of the first order and a dead time. The dynamics of the measuring device and the control elements (in our case the evaporator and the power supply) are... 

During the entire coating process the coating thickness is continuously monitored with an optical measuring system or by means of electrical resistance measurement devices. The measured values are compared with the coating thickness setpoints in the system and the evaporator power is thus automatically controlled. 

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