Nt1310 Unit 8 Application Of Control Architecture

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This type of structure has two conditions – completely on or completely off. It is the most basic type of control strategy. It operates or switches on when the process variable (PV) drops below the desired set point and switches off when the set point is attained again. This type of control can have detrimental effects on electro-mechanical equipment such as output valves and relays as the constant switching of these will eventually wear the parts out. To prevent this constant switching, hysteresis is added to the control. Figure 4. On/Off Control with Hysteresis
2.2.2 Open-Loop Control
This type of control architecture is where there is no feedback signal from the process variable. Therefore, the controller output does not automatically
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The controller computes this and reduces the error signal until the desired set-point is acquired and maintained. The closed-loop structure is in wide use throughout industry. Figure 6. Closed-Loop System
2.2.4 Feed-Forward Control
Feed-forward control is considered when there is a need to reduce the effects of disturbances on a system. A detector will sense a disturbance or a load change within the system and the controller outputs the required signal to offset this. This will in theory allow the controller to counteract the disturbance before it influences the process. This type of control requires the engineer to have excellent knowledge of the system before implementing it. Figure 7. Feed-Forward System

2.3 System Identification and Modelling
System identification techniques are used by control engineers to gain an insight into how the process behaves and to design a model of the system as accurately as possible. The First-Order Plus Deadtime (FOPDT) model is a characterization of the dynamic response of the system to an input. It allows for calculation of the three-model parameters – process gain, process time-constant and process dead-time. [2] The standard method to develop FOPDT models is the process reaction curve (PRC) technique. To develop the PRC, the system must
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