Continuous cycling method

Table 4.3 Ziegler-Nichols PID parameters using the Continuous Cycling Method ...

The settings obtained by this method are good initial estimates but are not optimal and some retuning may be necessary as for the Continuous Cycling method. 

Process model is unknown If the process model is unknown, the continuous cycling method may be used [7]. The mles of Ziegler and Nichols [24] have proven... 

Tab. 12.2. Zeigler-Nichols controller tuning parameters based on the continuous cycling method [7, 22, 24].

The continuous cycling method, or a modified version of it, is frequently recommended by control system vendors. Even so, the continuous cycling method has several major disadvantages ... 

Astrom and Hagglund (1984) have developed an attractive alternative to the continuous cycling method. In their relay auto-tuning method, a simple experimental test is used to determine Kcu and Pu> For this test, the feedback controller is temporarily replaced by an on-off controller (or relay). After the control loop is closed, the controlled variable exhibits a sustained oscillation that is characteristic of on-off control (cf. Section 8.4). The operation of the relay auto-tuner includes a dead zone as shown in Fig. 12.14. The dead band is used to avoid frequent switching caused by measurement noise. 

Compare the closed-loop set-point changes that result from the Tyj-RIT -S pairing and the Ziegler-Nichols continuous cycling method in Chapter 12. Consider two cases ... 

Process and Operating Conditions The major parameters that must be fixed or identified are the solvent to be used, the temperature, the terminal stream compositions and quantities, leaching cycle (batch or continuous), contact method, and specific extractor choice. 

The process can take one of two forms. In one, the sample and liquid are shaken (or otherwise agitated) together in the same container, the resultant mixture filtered, and the filtrate, which then contains the analyte, collected. In the other, fresh liquid is continuously cycled over a period of hours through the solid sample via a continuous evaporation-condensation process (that usually does not require an extra filtration step), and the liquid is collected. This latter method is known as a Soxhlet extraction. Soxhlet extraction will be discussed in more detail in Chapter 11. 

A solid-liquid extraction is the selective dissolution of a component of a solid material through contact with a liquid solvent. Two methods of accomplishing this are 1) shaking the solid with the solvent in the same container for a specified period of time, and 2) using a Soxhlet extractor, in which fresh solvent is continually cycled through the solid for a specified period of time. 

The forced vibration methods away from resonance can again be subdivided into those which apply deformation cycles and those which apply force cycles, the more usual being deformation cycles. An alternative form of test uses transient loading instead to continuous cycling. 

One approach using deformation in tension is worthy of note. When the deformation at low temperatures is applied repeatedly the apparent modulus becomes lower until an equilibrium level is reached. Eagles and Fletcher20 described a dynamic low temperature test in which the test piece is continuously cycled in tension and the force monitored so that both initial and equilibrium moduli can be calculated. Furthermore, tests could be made at different applied strains. This method could undoubtedly provide more comprehensive precise data but, despite claims of better reproducibility, it was not adopted as a standard method, principally because it involved rather... 

Figure 8 Standing oscillations in Continuous Cycling (Ziegler-Nichols) method...

Process Reaction Curve Method (Cohen-Coon Tuning). For some processes, it may be difficult or hazardous to operate with continuous cycling, even for short periods. The process reaction curve method obtains settings based on the open loop response and thereby avoids the potential problem of closed loop instability. The procedure is as follows ... 

J. 2.2.2 Moving Belt System. Initially developed by McFadden et al. [13], the moving belt system was based on the physical method of evaporation of the mobile phase through heat and vacuum that leave analytes as a thin coating on a continuously cycling polyimide belt. The analytes were transported from atmospheric pressure region to the vacuum of the ion source through differentially pumped vacuum locks. Ionization methods used... 

Describe the Ziegler-Nichols tuning methodology. This procedure is often called the continuous cycling tuning method. Why ... 

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