Algorithms for Manipulation of Control Variables

We know that the temperature control of the shower in a bathroom is not so easy. To have a comfortable shower, a hot water valve needs to be manipulated carefully (control action) however, we usually rely on a trial and error action until the proper temperature of a shower is achieved. How can we achieve the most comfortable shower temperature more quickly This is a common problem in the control action of many processes that are controlled by a feedback loop. The difficulty comes from a delay in the response, which naturally exists in any process - in other words, the dynamic characteristics of a process. Therefore, the control action should be determined based on the dynamics of the process. In particular, some bioprocesses are known to have serious delays in response. 

There are two frequently used algorithms to determine the control action in a feedback control system for bioprocess control, an on-off (two-positioned) control, and a PID (proportional-integral-derivative) control. 

On-Off (Two-Positioned) Control An on- off (two-positioned) controller is an algorithm that has two manipulating actions completely ON or completely OFF. Here, the controller gives the ON signal when the deviation exceeds a threshold value and the OFF signal when the deviation falls below another threshold value. Automatic on-off valves or switches are usually employed in actual processing plants in order to perform on-off (two-positioned) control. 

The on-off control is a simple and cheap algorithm for manipulating a control variable, and, therefore, it has been apphed to bioprocess controls such as temperature control and pH control. 

Proportional-lntegral-Derivative Control The most common algorithm for control action in the feedback loop of processing industries is the PID control, which is a combination of proportional action (P), integral action (1), and differential action (D). 

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