Nested controllers

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stream entering and the vapor boiled off, and therefore it is not directly influenced by the amount of vapor sidestream withdrawn.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... 

What makes the scheme impractical is that each level controller relies on the other to take corrective action. If one of the controllers were switched to manual then the output of the other would eventually saturate. While in some instances it is unavoidable, it is not generally advisable to design such nested controllers - particularly mutually nested controllers. All of the schemes described as impractical in Table 12.5 involve unnecessary nesting. 

Iterative statements control how many times a simple statement or a compound statement is repeated, and also may be nested with conditional and/or other iterative statements. They may be... 

Experience in the baking industry since World War II has shown that proper use of residual spray materials such as DDT and chlordan will most effectively control these casual invaders. When housekeeping in a bakery is maintained as near perfect as possible, the application of a residual layer of toxic insecticide on areas upon which casual invaders are most likely to travel in seeking harborage will effectively kill the invader individual before it has an opportunity to nest. Experience has borne out this theory in a general way, and specific data are now being collected which will describe it in more scientific terms. 

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.)...

Two Experimental versus control subjects One-at-a-time designs Factorial designs Fractional factorial designs Nested designs Special designs... 

Also, due to the nature of the physical factors involved, the values of some of the factors may not be under the experimenter s control. Thus, some factors may be nested, while others may not be. 

Bradbury et al. (2004), however, recently reanalyzed the relation between tamoxifen and cataracts and described it as a null association, They used a nested, matched, case-control study design and data collected in the General Practice Research Database. They identified all women 30-79 years old who were diagnosed with breast cancer and treated with tamoxifen within 6 months, or with bladder cancer, colorectal cancer, or nonmelanoma skin cancer between January 1991 and December i999. From this population they identified all newly diagnosed cases of cataract and matched four female controls to each case on age, index date, and study entry data. They assessed the risk of cataracts for current, past, and sometime users of tamoxifen... 

Hippisley-Cox J, Coupland C (2005) Effect of combinations of drugs on all cause mortality in patients with ischaemic heart disease nested case control analysis. Br Med J... 

Nested case-control (pottery workers) M (43, 6353) F (15,1972) M + F (58, 8325) No association with duration, cumulative exposure , or mean silica concentration Turner39... 

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