Single-loop control

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

In addition to single-loop process controllers, products that have benefited from the implementation of fuzzy logic are ... 

The ISA symbology provides different symbols for different types of actuators. Furthermore, variations for the controller symbol distinguish control algorithms implemented in DCS technology from panel-mounted single-loop controllers. 

Remote control units. These units are used to control unit processes. Basic control functions such as the PID algorithm are implemented here. Depending on other hardware components used, data acquisition capability may be required to perform digital control. They may be configured to supply process set points to single-loop controllers. Radio telemetiy may be installed to communicate with MUX units located at great distances. 

With the exception of pneumatic controllers for special applications, commercial single-loop controllers are almost entirely microprocessor-based. The most basic products provide only the PID control algo-... 

Single-loop controllers provide both the process control functions and the operator interface function. This makes them ideally suited to very small applications, where only two or three loops are required. However, it is possible to couple single-loop controllers to a personal computer (PC) to provide the operator interface function. Su(m installations are extremely cost effec tive, and with the keen competition in PC-based produc ts, the capabilities are comparable and sometimes even better than that provided by a DCS. However, this approach makes sense only up to about 25 loops. 

Initially, the microprocessor-based single-loop controllers made the power of digital control affordable to those with small processes. To compete with these products in small applications, the DCS suppliers have introduced micro-DCS versions of their products. As a PC-based operator interface is usually a component of the micro-DCS, there is sometimes little distinction between a micro-DCS and a system consisting of single-loop controllers coupled to a PC-based operator interface. 

More microprocessor-based process equipment, such as smart instruments and single-loop controllers, with digital communications capability are now becoming available and are used extensively in process plants. A fieldbus, which is a low-cost protocol, is necessary to perform efficient communication between the DCS and these devices. So-called mini-MAP architec ture was developed to satisfy process control and instrumentation requirements while incorporating existing ISA standards. It is intended to improve access time while... 

Single-Loop Controller The single-loop controller (SLC) is a process controller that produces a sin e output. SLCs can be pneumatic, analog electronic, or microprocessor-based. Pneumatic SLCs are discussed in the pneumatic controller section, and analog electronic SLC is not discussed because it has been virtually replaced by the microprocessor-based design. 

Presently, fieldbus controllers are single-loop controllers with 8- and 16-bit microprocessors and are options to digital field-control devices. These controllers support the basic PID control algorithm... 

Pneumatic Controllers The pneumatic controller is an automatic controller that uses pneumatic pressure as a power source and generates a single pneumatic output pressure. The pneumatic controller is used in single-loop control applications and is often installed on the control valve or on an adjacent pipestand or wall in close proximity to the control valve and/or measurement transmitter. Pneumatic controllers are used in areas where it would be hazardous to use electronic equipment, in locations without power, in situations where maintenance personnel are more familiar with pneumatic controllers, or in applications where replacement with modern electronic controls has not been justified. 

Taxonomj No. 2.2.1,1 Equipment Description CONTROLLERS - ELECTRONIC PANELBOARD (SINGLE LOOP) ... 

Regulatory / Control / Seconds/Minutes Single Loop/... 

Shinskey (1984) has shown that there are 120 ways of connecting the five main parts of measured and controlled variables, in single loops. A variety of control schemes has been devised for distillation column control. Some typical schemes are shown in Figures 5.22a, b, c, d, e (see pp. 234, 235) ancillary control loops and instruments are not shown. 

Design and Tuning of Single-Loop Control Systems... 

In a simple single-loop system, we measure the outlet temperature, and the temperature controller (TC) sends its signal to the regulating valve. If there is fluctuation in the fuel gas flow rate, this simple system will not counter the disturbance until the controller senses that the temperature of the furnace has deviated from the set point (Ts). 

No interaction. Controller design is like single-loop systems. Strong interaction if 8 is close to 1 weak interaction if 8 1. One-way interaction... 

After proper pairing of manipulated and controlled variables, we still have to design and tune the controllers. The simplest approach is to tune each loop individually and conservatively while the other loop is in manual mode. At a more sophisticated level, we may try to decouple the loops mathematically into two non-interacting SISO systems with which we can apply single loop tuning procedures. Several examples applicable to a 2 x 2 system are offered here. 

The purpose of retrieving precursors is to analyse them with the help of the control model derived in the previous Chapter. This control model establishes why reoccurrence has taken place first by checking the single loop control elements and second by checking the double loop control elements. 

The double loop analysis questions the steering process if any single loop control element seems to be ineffective. So the analysis discriminates between an ineffective single loop control element and an ineffective corresponding (double loop) steering element, where the steering element has the benefit of the doubt. 

In the next steps the elements of the single control loop, i.e. the control elements observation , judgement and intervention , are checked by questioning their presence and effectiveness, according to the left flow scheme of Figure 35 (single loop). 

The additional double loop analysis gives more detailed information regarding the 44 ineffective control elements, distinguishing true ineffective elements of the single loop from the false ineffective elements that in fact concern the double loop learning , i.e. an ineffective steering element. The results of this additional analysis are presented in Figure 37. 

From this figure, it can be seen that in more than 60 % of the single loop control elements initially indicated as ineffective (total of 44 elements), the true ineffective element originates from the double loop learning cycle, i.e. the ineffectiveness originates from the steering element. In about only 40 % of the ineffective elements the initial single loop indication was correct. 

Single-line processing units, 21 847 Single-loop feedback controller, 20 695, 696 Single-mode cavity microwave applicators, 16 521-522... 

Importance. The control room is the major interface with the plant. Automation is increasingly common in all degrees of sophistication, Irom single-loop systems to computer-control systems. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

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