Category: Control Systems
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Ratio Control
In some processes, it is a common requirement for two flows to be kept in precise ratio to each other for example, a gas or oil and air in combustion control, and reagents being fed to a chemical reactor hence the need for ratio control. In simple ratio control, one flow is declared to be…
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Feedforward Control
Feedforward control is based on principle of pre-emptive load counter-action i.e. if all major loads (i.e. disturbances) on a process variable are monitored, and their effects on that process variable are well-understood, a control system programmed to take the appropriate action based on load changes will shield the process variable (PV) from any adverse effect.…
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Benefits of Cascade Control
We know very well that closed loop control provides increased performance over open loop control. So it would be rational to expect benefits from adding inner control loop around plant items that have degrading overall performance. The Figure (a) below illustrates a typical example of a cascade control: In Figure (a) above, the outer loop…
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Proportional-Integral-Derivative (PID) Control Systems
Proportional-Integral-Derivative For processes that are able to operate with continuous cycling, the relatively inexpensive two position controller is sufficient. For processes that cannot tolerate continuous cycling, a proportional controller is used. For processes that tolerate neither cycling nor offset error, a proportional plus integral/reset controller can be used. For processes that require improved stability and…
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Proportional plus Derivative (PD) Control System
Proportional plus derivative (rate) control is a control mode in which a derivative section is added to the proportional controller. This derivative section responds to the rate of change of the error signal, not the amplitude as with the integral control; this derivative action responds to the rate of change the instant it starts. This…
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Proportional Plus Integral (PI) Control System
This type of control is combination of two control modes, proportional and integral (reset). The Combining of the two modes results in gaining the advantages and compensating for the disadvantages of the two individual control modes. The main advantage of the proportional control is that an immediate proportional output is produced as soon as an…
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Integral (Reset) Control Systems
Integral (Reset) control Reset/Integral control describes a controller in which the output rate of change is dependent on the magnitude of the input. Specifically, a smaller amplitude input causes a slower rate of change of the output. This controller is termed to as integral controller because it approximates the mathematical function of integration. This integral…
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Proportional Control Systems
The control mode In the proportional control mode, the final control element is throttled to various positions that are dependent on the process systems conditions. The controller operates within a band that is between the 0 % output point and the 100 % output point and where the output of the controller is proportional to…
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Modes of Automatic Control
The mode of control can be defined as the way or manner in which a control system makes corrections relative to an error that exists between the desired value (setpoint) of a controlled variable and its actual value. Any particular mode of control used for a specific application depends on the characteristic of the process…
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Two Position Controller
A Two position controller is a device that has two operating conditions: Completely ON or completely OFF. From the figure above, we can see the characteristic waveform for a two position controller that switches from its ‘’OFF’’ state to ‘’ON’’ state when the measured variable increases above the setpoint. Conversely, it switches from its ‘’ON’’…