Reset Control

The variables that need to be controlled in chemical processing are temperature, pressure, hquid level, flow rate, flow ratio, composition, and certain physical properties whose magnitudes may be influenced by some of the other variables, for instance, viscosity, vapor pressure, refractive index, etc. When the temperature and pressure are fixed, such properties are measures of composition which may be known exactly upon cahbration. Examples of control 

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The main limitation of the pilot-operated regulator is stability. When the gain in the pilot amplifier is raised too much, the loop can become unstable and oscillate or hunt. The two-path pilot regulator (see b) is also available. This regulator combines the effects of self-operated and the pilot-operated styles and mathematically produces the equivalent of proportional plus reset control of the process pressure. 

Figure 22 Response of Proportional Plus Reset Control. 33...

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 called an integral controller because it approximates the mathematical function of integration. The integral control method is also known as reset control. 

Proportional plus reset control is a combination of the proportional and integral control modes. 

EO 1.4 DESCRIBE the characteristics of the following types of automatic control systems d. Proportional plus reset control system... 

Let s once more refer to our heat exchanger example (see Figure 23). This time we will apply a proportional plus reset controller to the process system. 

The proportional action of the proportional plus reset controller, if acting alone, would respond to the disturbance and reposition the control valve to a position that would return the hot water out to a new control point, as illustrated by the response curves. You ll note that a residual error would still exist. 

Proportional plus reset controllers act to eliminate the offset error found in proportional control by continuing to change the output after the proportional action is completed and by returning the controlled variable to the setpoint. 

The proportional plus reset control mode is summarized below. 

Proportional plus reset control eliminates any offset error that would occur with proportional control only. 

Reset windup is an inherent disadvantage of proportional plus reset controllers that are subject to large error signals. 

For processes that can operate with continuous cycling, the relatively inexpensive two position controller is adequate. For processes that cannot tolerate continuous cycling, a proportional controller is often employed. For processes that can tolerate neither continuous cycling nor offset error, a proportional plus reset controller can be used. For processes that need improved stability and can tolerate an offset error, a proportional plus rate controller is employed. 

Most commonly it is known as proportional-plus-reset controller. Its actuating signal is related to the error by the equation... 

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