Feed-forward mechanisms

Figure 10-2. Regulation of purine synthesis by the nucleotides and the intermediate, 5 -phosphoribosyl-1 -pyrophosphate (PRPP). Both feedback and feed-forward mechanisms are utilized in this intricate scheme. IMP, inosine monophosphate.

In addition to this feed-forward mechanism, Fru-2,6-P2 functions in feedback control of sucrose synthesis. When sucrose accumulates in the leaf, the hexose phosphate concentration increases [51] (the reason for this is still unclear), leading to an activation of Fru-6-P,2K. The resultant increase of Fru-2,6-P2 then restricts the activity of cytosolic FBPase so that less sucrose, and more starch, is synthesized. In this way, when photosynthesis exceeds the rate at which sucrose can be exported, or stored in the leaf, an increasing proportion of the photosynthate is diverted into starch, which provides a store of carbohydrate that is especially important at night (see above). 

Fig, 17.12 Schematic dia am showing feedback and feed-forward mechanisms involved in the Pasteur effect. (Modified from reference [46].)... 

The behaviour of clock-gene products in the SCN provides strong support to the current model of the circadian clockwork as an autonomous intracellular mechanism based upon interlocked feedback and feed-forward loops. Intercellular signalling, more specifically convergent mechanisms for up- and down-regulation of Per expression, is critical to circadian entrainment, which can now be explained in mechanistic terms rather than described by formalisms. 

The first involves various feedback and feed forward control mechanisms associated with metabolic pathways. Here a chemical present in the cell, usually an end product of a metabolic sequence, will influence the activity of an enzyme at the beginning of the pathway. This is usually achieved by the presence of an allosteric enzyme whose properties are significantly changed by effector molecules (Fig. 5.31). 

The relevant block diagram is composed of two loops (Fig. 7.97)—a normal feedback loop and a loop which contains the parameter adjustment mechanism (comparable to feed-forward compensation—Section 7.14.1). 

As a second example, latex, used to produce latex gloves, is a natural ingredient produced from tree sap, which varies in consistency with weather conditions and geographic location. Each batch of latex must be analyzed to determine its properties and then adjusted to get a more consistent performance. This is an example of a feed-forward control mechanism. One cannot set an operating window on the amount of solids added because it depends on the batch of latex. One cannot set an operating window on the percent solids in the latex initially because nature cannot be controlled. However, a control mechanism can still be established that ensures that the product will consistently meet requirements. Again adjustment mechanisms are required. 

The control plans considered include feed-forward and feedback mechanisms. Designed experiments can provide the detailed understanding required to establish more complex control mechanisms where they are needed. 

A control plan is the sum of the procedures and equipment used to ensure that the internal requirements are met. It includes control charts, sampling plans, 100% inspection, feed-forward/feedback mechanisms, and mistake-proofing tech-niques/devices. Some items may be performed by equipment like automatic controllers, and some are performed by operators and checkers. There may not be a single document called a control plan. Instead, these controls might be spread across a number of documents including a statistical process control plan, an inspection plan, an operator manual, and various other standard operating procedure and specifications. 

ASM), contrast (CON), entropy (ENT), homogeneity (HOM), and correlation (COR) a mnltiple-layer feed-forward nenral network model was established to predict the three mechanical parameters. These investigators obtained a correlation coefficient above 0.84 in comparison to a conventional method. Also, heated-oil qnalities (acid value, total polar component, and viscosity) were qnantitatively predicted nsing the VIS-NIR hyperspectral analysis and partial least sqnares calibration models (Kazemi et al., 2005). The values for all the quality parameters were above 0.92, indicating a good prediction. 

---