Process control, automated feedback

We discuss in this chapter analysers that arc highly automated, such as flow injection and discrete analyzers. In addition, laboratory robotic systems that are becoming more and more commonplace for sample handling and preparation arc also described. The latest advances in automation involve the development of microlluidic systems, which are sometimes called lab-on-a-chip or micro total analysis systems. These recent developnienis are also described here. It is important to note that the same principles of automatic analysis discussed here also apply to process control systems, which analyze the current state of a process and then use feedback to alter experimental variables in such a way that the current state and the desired state are nearly identical,... 

Decision aids need to be designed carefully. With the goal of providing assistance to the human controller, automated systems may provide feedforward (as well as feedback) information. Predictor displays show the operator one or more future states of the process parameters, as well as their present state or value, through a fast-time simulation, a mathematical model, or other analytic method that projects forward the effects of a particular control action or the progression of a disturbance if nothing is done about it. 

Another important field of application for chemical sensors is process control. Here the sen- sor is expected to deliver some crucial signal to the actuators (valves, pumps, etc.) that control the actual process. Fully automated process control is feasible with certain types of feedback circuits. Since key chemical parameters in many chemical or biochemical processes are not subject to sufficiently accurate direct determination by the human senses, or even via the detour of a physical parameter, sensor technology becomes the key to automatic process and quality control. Often the performance of an entire industrial process depends on the quality and reliability of the sensors employed. Successful adaptation of a batch process to flow-through reactor production technology... 

The relay feedback experiment was made popular in the field of process control by Astrom and Hagglund (1984). This experiment was suggested as a means to automate the Ziegler-Nichols scheme for determining ultimate gain and frequency information about a process. Their approach followed directly from a describing function approximation (DFA) to the nonlinear relay element. The objective was to use the obtained process information for automatic tuning of PID controllers. 

The distinction between controlled and automated processes was first defined and studied by Schneider and Shiffrin (1977). In a series of laboratory studies they demonstrated that the process by which we learn to deal with complex situations involves the automation of various sequences of behavior. Prior to automation each component in that behavior is controlled through monitoring and feedback. This process is relatively slow, requires much attention, and prevents us from doing other tasks simultaneously. As we repeatedly perform some of these sequences, the process becomes automated, in the sense that once it is initiated, the sequence of actions is hardly monitored, requires minimal attention, and is performed more or less unconsciously. Changing manual gears has often been used as an example of a controlled process that through repeated experience becomes automated. The concepts of controlled and automated processes are discussed in more details in Chapter 5 on Information Processing. 

Thirdly, there will be a need for some sort of sizing analysis procedure that can form part of a fully automated production unit This will involve automatic saunpling, size analysis and feedback of the information to the process control ... 

The human body is a remarkably complex biochemical process, and it shares many attributes with more traditional process control problems that have been discussed in earlier chapters. In the event that a body fails to achieve the robust level of self-regulation that occurs naturally (cf. Chapter 24), there are opportunities for medical intervention, often involving the administration of a therapeutic agent (or drug) in a prescribed manner. The therapy can be optimized using open-loop methods, but it is often advantageous to automate the process, thus removing the human from the feedback loop (much as a chemical plant removes the operator from the loop in the transition from manual control to... 

It is often desirable to automate a process, peirticularly when a process is repeated for a number of cycles. This can be achieved with either microprocessor-based controllers or via computer programs to operate valves [26]. The process control system can also be used to monitor, record and change the separation conditions such as pressure, pH, conductivity, flow rate and temperature. Use of sensors allows feedback control of the process. These can be used to control flow rate and pressure and to detect the presence of air in the system. The emergence of protein from the column can be monitored by measuring the absorbance of the eluate and this can be used to initiate isolation of the product [26]. 

While the decrease in extraction time is favourable for laboratories in general, it can be critical when laboratory analyses are used in feedback control of production cycles and quality control of manufacturing processes. The volume of solvents used in PFE can be some 10 times less than traditional extraction methods (cf. Table 3.36). PFE cuts solvent consumption by up to 95 %. Because so little solvent is used, final clean-up and concentration are fast direct injection in analytical devices is often possible. Automated PFE systems can extract up to 24 sample cells. 

Ranky, P. G. (2003), A real-time manufacturing/assembly system performance evaluation and control model with integrated sensory feedback processing and visualization, Assembly Automation. 

---