Nested control loops

Avoid nesting control loops. Control loops are nested if the operation of the external loop depends on the operation of the internal loop. Figure 8.11 illustrates a nested loop. A vapor sidestream is drawn off a column to hold the column base level, and a temperature higher up in the column is held by heat input to the reboilcr. The base liquid level is ouly affected by the liquid stream entering and the vapor boiled off, and therefore it is not directly influenced by the amount of vapor sidestream withdrawn. Thus the base level... 

Nested Control Loops More than one characteristic of a system s output to be controlled at the same time Robot controllers... 

NESHAPS (National Emission Standards for Hazardous Air Pollutants), 593 Nested control loops, 161... 

Note that the makeup of liquid A to the column is set by the level controller on the reflux drum of the column. This may seem like an unusual arrangement, and it certainly involves nested loops (the reflux drum level is not directly affected by makeup feed, but only through the bottoms composition loop changing heat input). If the makeup could be added directly into the reflux drum, there would be no nesting of control loops. 

Noise allocation followed by scalar quantization and Huffman coding. In this method, no explicit bit allocation is performed. Instead, an amount of allowed noise equal to the estimated masked threshold is calculated for each scalefactor band. The scalefactors are used to perform a coloration of the quantization noise (i.e. they modify the quantization step size for all values within a scalefactor band) and are not the result of a normalization procedure. The quantized values are coded using Huffman coding. The whole process is normally controlled by one or more nested iteration loops. The technique is known as analysis-by-synthesis quantization control. It was first introduced for OCF [Brandenburg, 1987], PXFM [Johnston, 1989b] and ASPEC [Brandenburg et al., 1991], In a practical application, the following computation steps are performed in an iterative fashion ... 

Changing distillate flow rate has no direct effect on temperatures in the column. Reflux and vapor boilup are the inputs that change temperatures (and compositions) inside the vessel. However, the distillate flow rate changes the level in the reflux drum, which results in a change in vapor rate when the level loop is on automatic. So the level loop is nested inside the temperature loop in this structure. The level loop must be on automatic for the temperature control loop to be effective. 

Since the reflux-drum level loop is nested inside the temperature loop, the level loop is tuned first. A relay-feedback test and Tyreus-Luyben tuning rules give the results shown in Table 8.5. Notice that the gain and integral times are very much different than a conventional level control loop. The integral time is about 8 min and the gain is only 0.3. 

Note that the TC4 temperature control loop is nested inside the reflux-drum level controller since a change in distillate flow rate has no direct effect on Stage 4 temperature. The tuning of the reflux-drum level controller (LC12) affects the tuning of the TC4 temperature controller, as Table 10.1 shows. Performance is improved by tightening up on the level controller. This is illustrated in Rgure 10.18 for a 20% increase in feed flow rate. 

The two feedback controllers are nested with the secondary control loop located inside the primary control loop. 

Cascade control is a common control technique that uses two controllers with one feedback loop nested inside the other [2- ]. The output of the primary controller acts as the set point for the secondary controller. The secondary controller controls the FCE. A typical cascade control loop is illustrated in Figure 6.1. 

Fig. 9.6 Hierarchical organisation of neuronal circuits governing the control of movement. Note the nesting of feedback loops. (After a figure in Shepherd, G.M. (1988). Neurobiology, Oxford University Press, Oxford.)...

If the temperature controller is on manual, the level loop cannot work. In this process it probably would be better to reverse the pairing of the loops control temperature with vapor sidestream and control base level with heat input. Notice that if the sidestream were removed as a liquid, the control system would not be nested. Sometimes, of course, nested loops cannot be avoided. Notice that the recommended Scheme B in Fig. 8.10 is just such a nested system. DistOlate has no direct effect on tray temperature. It is only... 

In order to achieve an accurate control of the internal reactor temperature, a cascade controller can be used. In this type of controller, temperature control is managed by two controllers arranged in cascade, that is, in two nested loops (Figure 9.14). The external loop, called the master, controls the temperature of the reaction mixture by delivering a set value to the slave, the inner loop, which controls the temperature of the heat carrier (Tc). 

From a Do or For loop, control is transferred to the statement following the Loop or Next statement, or, in the case of nested loops, to the loop that is one level above the loop containing the Exit statement. From a Function or Sub procedure, control is transferred to the statement following the one that called the procedure. 

To specify a parameter in a module as a design variable, you have to place a control block around the module and adjust the parameter such that design specifications are met. This arrangement creates a loop. If the values of many design variables are to be determined, you might end up with several nested loops of calculation. 

A regular computation-intensive signal flow—as occurs in algebraic analysis, Altering, or format conversion— is combined with nested branches and multiple data-dependent loops. This explicit irregularity complicates not only the controller synthesis but also other synthesis tasks, Uke scheduling and data-path allocation. These tasks need to deal with control-flow hierarchy explicitly, which is an important aspect of our approach (see section 3). 

The control flow, deflned by a procedural interpretation of the initial Silage description of figure 1, results in an inefficient solution. Indeed, the inner loop of the double nested loop construct is the fastest one executed. This type of control flow definition corresponds to a column-wise ordering of b [] [] and r [] [] instance computations. In terms of storage requirements, this means that all 562 input signal instances of s [] are read in before they are consumed. The same is true for all 50 instances of r [0] [], which are produced before one instance is consumed. Furthermore, P intermediate signals r [] [] have to be alive in parallel. The same is true for the column of return[] values. 

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