Category: Control Systems
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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’’…
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Process Time Lags and Stability of Automatic Control Systems
Processes have the characteristics of delaying and retarding changes in the values of the process variables. Process time lags generally describe these process delays and retardations. Process time lags are caused by three properties of the process. They include: Capacitance Resistance Transportation time Capacitance is the ability of a process to store energy. This energy-storing…
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What is the Controller?
A controller is a device that generates an output signal based on the input signal it receives. The input signal is actually an error signal, which is the difference between the measured variable, and the desired value, or setpoint. The input error signal represents the amount of deviation between where the process system is actually…