PID Control Loop

The process variable to be controlled, PV that is, any stream- or operation-related variable in the flowsheet (e.g., pressure, temperature, liquid level, species mass or mole fraction, mass or molar flow rate). In addition, the minimum and maximum values of the PV are used to express the PV as a percentage of its full range  

The controller output, OP, to be manipulated by the controller, as a percentage of its full range. This variable is usually either a stream flow rate or the rate of heat transfer of an energy stream. Grenerally, its minimum value is specified as zero and its maximum is taken as twice its nominal value. Note that occasionally the nominal value is not positioned midway between the minimum and maximum values (e.g., when the nominal flow rate of a bypass stream lies near its maximum or minimum flow rate). 

The tuning parameters. For a PID (Proportional-Integral-Derivative) controller, the output, OP t), is a function of the tracking error, E[t]  

As mentioned above, SP, PV, and OP are expressed as percentages of their full ranges. Consequently, the controller gain, Kc is dimensionless, and represents the percentage change in OP for a one-percent change in PV. In the absence of other information, c-tory settings are used Kc = 1, Tj = 10 min, and = 0. These are tuned for improved performance, as discussed in the next section. 

The PID controller is the most commonly used feedback controller in industry, with three tunable parameters as stated previously. The integral component ensures that the tracking error, E t), is asymptotically reduced to zero, whereas the derivative component imparts a predictive capability, potentially enhancing the performance. Despite its apparent simplicity, the subject of PID controller tuning has been discussed in several textbooks and thousands of research papers since the landmark work of Ziegler and Nichols (1942). In practice, despite these developments, most PID controllers are tuned as PI controllers for several reasons. 

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Therefore, the control loop shown in Fig. 5.28 was developed to solve the problem of symmetry control [121]. Two additional PID control loops are used to control the homogeneity of the reactive gas partial pressure because of appropriate regulation of the threefold gas inlet (top/center/bottom). The... 

Are there likely to be timing issues with the interface (e.g., for PID control loops) Is the interface designed to handle bulk FO ... 

In the multiloop controller strategy each manipulated variable controls one variable in a feedback proportional integral derivative (PID) control loop. Taking a single-feed, two-product distillation column with a total condenser and a reboiler as an example, a basic list of possible controlled variables includes the distillate and bottoms compositions, the liquid levels in the reflux accumulator and the column bottom, and the column pressure. The main manipulated variables are the reflux, distillate, and bottoms flow rates and the condenser and reboiler heat duties. 

The following procednre is recommended for tnning PID control loops ... 

Although the size of the flow manifold and reactor components have been substantially reduced in the AIMS, no such reduction has occurred in the control system since both the MARS and AIMS rely on components that have already been miniaturized. Figure 12.17 shows both the Siemens S7-300 controller for the MARS Version VI and the National Instruments controller for the AIMS. Their footprints are essentially the same, with both having a similar number of inputs and outputs. The Siemens controller used for the MARS Version VI could have been used for operating the AIMS, but it was not capable of PID control loop rates faster than 20 Hz. 

Many PID controller loops remain on their factory settings long after plant startup. When the controller action is in the right direction and the PV range is defined wisely, these settings often give adequate performance. 

The outlet flow rate is larger than the inlet flow rate therefore, the pressure starts to increase. This is again counteracted, in this case by opening a mass flow controller, controlled using a PID control loop. Again, the temperature at the outlet of the bed is used to determine the point to switch. 

Chapters 2, 3, and 4 deal with the distillation variables, and Chapter 5 covers distillation process control strategies. Chapter 6 describes some of the constraints on distillation variables and separation capabilities. Chapter 7 introduces the concepts that are critical to product quality and the measurements that evaluate performance criteria such as frequency of failure. Chapter 8 describes the concepts and nomenclature that are fundamental to PID control loops. Chapter 9 covers the concepts of tuning process controllers when they are operating in automatic output mode. Chapter 10 is about measuring the response of process variables when the controller is in manual output mode, that is, with no feedback from the process variable. 

Shardt YA, Huang B (2014) Minimal required excitation for closed-loop identification implications for PID control loops. In ADCONIP conference proceedings, Hiroshima, Japan, pp 296-301. doi http //www.nt.ntnu.no/users/skoge/prost/proceedings/adconip-2014/pdf/... 

Some controls in the process are more complex than just PID control loops. One example is the wet end control which ensures uniform process conditions in the approach flow of the paper machine and in the forming section by maintaining constant consistencies, gas content, charge and other parameters. It involves e. g. special software tools (soft sensors) using wet end data instead of direct measurements to predict basis weight during start up of the machine, when the paper has not yet reached the reel or the quality measurements. Such, wet end controls not only reduce the start up time of the process, but also minimize the consumption of chemicals, i. e. only as much retention aid is used as is really needed to reach a required retention level in the forming section. 

The final step. Step 7 of Fig. 20.9, is to implement the calculated control actions, usually as set points to regulatory PID control loops at the DCS level. 

In order to monitor the performance of a single standard PI or PID control loop, the basic information in Table 21.5 should be available. 

A sample of the air/gas mixture is fed into the analyzer cell at a fixed rate. The cell and sensor are maintained at approximately 1500°F (812°C). the sample ignites, burners and the hot products of combustions (POC s) flow past the ceramic/platinum sensor and produce an electrical voltage proportional to the amount of excess O2 or excess hydrocarbon. The signal is amplified, conditioned and lineraized. A discrete step function occurs when excess hydrocarbon (unburnt fuel) is present in the POC s. This signal is in used an input to the PLC which has a PID control loop output to operate a gas flow control valve to maintain the specified Plasma Valve. 

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