Process Control Loops

Various values for the control loop rate were used to evaluate the ability of the system to maintain real-time PID temperature control of the microreactor heaters. 

It was found that the control loop could successfully operate at a rate of 500 Hz with all 24 microreactor heaters being under closed-loop PID control. At faster rates, the processor had insufficient time to complete the TCP/IP communications with the HMI, which resulted in data loss. Somewhat conservative loop parameters were used because of the noise in the temperature measurement signal, but temperature control was quite stable imder a wide variety of both reactive and non-reactive conditions. The dynamics of the closed loop control algorithm were not explored since the data logging rate (4 Hz) was not fast enough to perform these types of evaluations. This would be possible with program modihcations, but this issue was not a project objective. It could be considered as a topic for future investigation. 

The few problems that were not solved really had minimal effect on the ability to use the system and did not detract at all from its safety. The most impressive finding was the ability of the PLC to perform closed-loop PID control on 24 microreactor heaters at a maximum rate of 500 Hz. Although this high loop rate is achievable in specialized control systems, this rate of control for hardware whose cost was less than 20k is a notable accomplishment. 

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Let s consider a typical basic process control loop (Figure 2.1). A pressure-indicating transmitter (PIT) sends the pressure signal to a proportional integral derivative controller (PID), which sends a signal to the control valve to... 

This simple approach allows the balance equation for the reactive gas partial pressure and deposition rate to be formulated. An example is given in Fig. 5.7 for the stability analysis of process control loops. 

Fig. 5.7. Stability analysis of a process control loop for the reactive magnetron sputtering of high-index metal oxides. The control of discharge power to stabilize the oxygen partial pressure set point is modeled within the framework of the Berg model. A cycle time of 100 ms and process uncertainties for discharge current and oxygen partial pressure measurements are assumed, (from [71])...

In analyzing individual components, what is required is the relationship between signal inputs (forcings) and outputs (responses). In the analysis of a whole process control loop what is required is the behavior... 

If the transfer function for all the components in the process control loop lumped together exclusive of the controller is G(s), and the transfer function of the controller (exclusive of the summer which takes the difference between set point and measured variable) is Gc(s), the transfer function of the closed loop is... 

Although automatic computers are used widely in economic control of process plants and organizations, the incorporation of computers in the process control loop to obtain fully automatic control (automation) has been explored only in preliminary fashion. 

The introduction of a digital computer in a process control loop changes the picture because a computer can handle information on a discrete-time basis only (i.e., at particular time instants). As we can see from Figure 26.5, in a computer control loop we have both continuous-and discrete-time signals present. The implication of this feature is twofold ... 

A sensor for the process control loop consists of these principal... 

Reduction of variability can be accomplished by the selection of the best control strategy for the distillation column to shed disturbances and by tuning process control loops for... 

This chapter provides an introduction to a feedback process control loop and a description of the action provided by Proportional (P), Integral (I), and Derivative (D). 

The tuning of process control loops can have a significant effect on the variability and robustness of the control of a process system. This chapter teaches methods of tuning controllers while they are running in automatic output mode, that is, closed loop. The pattern recognition methods are highly effective. 

The development of the Robbins closed-loop tuning methodology was the result of about 11 years of development with collaboration and feedback of comments from others. A number of different methods for tuning process control loops were tested along the way, and this new closed-loop method using pattern recognition was determined to be the most cost-effective. 

As shown in Figure F-1, the BPCS can be used to implement various functions (e.g., operator response to alarm, process control loops, and discrete process logic) provided the following ... 

Basic Process Control System (BPCSI. Failure of a process control loop is likely to be one of the main Initiating Causes. However, there may be another independent control loop which could prevent the Impact Event, and so reduce the frequency of that event. 

Final control elements are typically automated valves however, motors or other electrical devices can be used. The final control element is the last link in the modern control loop and is the device that actually makes the change in the process. Automatic valves open or close to regulate the process. Control loops usually have (1) a sensing device, (2) a transmitter, (3) a controller, (4) a transducer, and (5) an automatic valve. Automatic valves can be controlled from remote locations, making them invaluable in modern processing. 

Process analyzers are integral parts of the process control loop. Therefore, their parameters have to match the requirements of the control loop of the particular process. 

With the addition of the process control loops and the information flags, the PFD starts to become cluttered. Therefore, in order to preserve clarity, it is necessary to limit what data are presented with these information flags. Fortunately, flags on a PFD are easy to add, remove, and change, and even temporary flags may be provided from time to time. 

All process information that can be measured in the plant is shown on the P ID by circular flags. This includes the information to be recorded and used in process control loops. The circular flags on the diagram indicate where the information is obtained in the process and identify the measurements taken and how the information is dealt with. Table 1.10 summarizes the conventions used to identify information related to instrumentation and control. Exanple 1.9 illustrates the interpretation of instrumentation and control symbols. 

The final control element in nearly all chemical process control loops is a valve. 

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