SELECTING CONTROLLER ACTION

Depending on the action of the valve, increases in measurement may require either increasing or decreasing outputs for control. All controllers can be switched between direct and reverse action.

· Direct action means that, when the controller sees an increasing signal from the transmitter, its output will increase.
· Reverse action means increasing measurement signals cause the controller output to decrease.

To determine which of these responses is correct, an analysis must be made of the loop. The first step is to determine the action of the valve.

In figure 1, for safety reasons the valve must shut if there is a failure in the plant air supply. Therefore, this valve must be air open, or fail close. Second, consider the effect of a change in measurement. For increasing temperature the steam flow to the heat exchanger should be reduced, therefore, the valve must close. To close this valve, the signal from the automatic controller to the valve must decrease. Therefore, this controller requires reverse, or increase/decrease, action. If direct action is selected increasing signals from the transmitter will result in a larger steam. flow, causing the temperature to increase further. The result would be a run-away temperature. The same thing will happen on any decrease in temperature, causing a falling temperature. Incorrect selection of the action of the controller always results in an unstable control loop as soon as the controller is put into automatic.

Assuming that the proper action is selected on the controller, how does the controller know when the proper output has been reached- In figure 3, for example, to keep the level constant, a controller must manipulate the flow in to equal the flow out, any difference will cause the level to change. In other words, the flow in, or supply must balance the flow out, or demand. The controller performs its job by maintaining this balance in steady state, and acting to restore this balance between supply and demand whenever it is upset.

UPSETS

Any one of three events could occur which would require a different flow to maintain the level in the tank. First, if the position of the output hand valve were opened slightly, then more flow would leave the tank, causing the level to fall. This is a change in demand, and to restore balance, the inlet flow valve has to be opened to supply a greater flow rate. A second type of unbalance condition is a change in the set point. Maintaining any other level besides midscale in the tank would cause a different flow out ; this change in demand would require a different input valve position. The third kind of upset is a change in the supply. If the pressure output of the pump were to increase, even though the inlet valve remained in the same position, the increased pressure would cause a greater flow, which would at first cause the level to begin to rise. Sensing the increased measurement, the level controller would have to close the valve on the inlet to hold the level at a constant value. In the same way, any controller applied to the heat exchanger shown in figure 1 must balance the supply of heat added by the steam with the heat taken away by the water. The temperature can only remain constant if the flow of heat in equals the flow of heat out.

PROCESS CHARACTERISTICS AND CONTROLLABILITY

The automatic controller uses changes in the position of the final actuator to control the measurement signal, moving the actuator to oppose any change it sees in the measurement signal. The controllability of any process is a function of how well the measurement signal responds to these changes in the controller output; for good control the measurement should begin to respond quickly, but then not change too rapidly. Because of the tremendous number of applications of automatic control, characterizing a process by what it does, or by industry, is an almost hopeless task. However, all processes can be described by the relationship between their inputs and outputs. Figure 4 illustrates the temperature response of the heat exchanger when the control valve is opened by manually increasing the controller output signal.

At first, there is no immediate response at the temperature indication, then the temperature begins to change; it rises steeply at first, and approaches a final, constant level. The process can be characterized by the two elements of its response. The first element is the dead time, or the time  before the measurement begins respond , in this example, the dead time arises because the heat in the steam must be conducted to the water before it can affect the temperature, and then to the transmitter before the change can be seen. Dead time is a function of the physical dimensions of a process and such things as belt speeds and mixing rates. Second, the capacity of a process is the material or energy which has to enter or leave the process to change the measurements. It is, for example, the gallons necessary to change level, the BTU's necessary to change temperature, or the standard cubic feet of gas necessary to change pressure. The measure of a capacity is its response to a step input. Specifically, the size of a capacity is measured by its time constant, which is defined as the time necessary to complete 63% of its total response. The time constant is a function of the size of the process and the rate of material or energy transfer. For this example, the larger the tank, and the smaller the flow rate of the steam, the longer the time constant. These numbers can be as short as a few seconds, or as long as several hours. Combined with dead time, they define how long it takes the measurement signal to respond to changes in the valve position. A process will begin to respond quickly, but then not change too rapidly, if its dead time is small and its capacity is large. In short, the larger the time constant of capacity compared to the dead time, the better the controllability of the process.

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Home

Function of automatic control .  - The feedback loop. - The measurement .  - The process .  

 The automatic controller .  -  Controlling the process .  -  Selecting controller action - Upsets . 

 Process characteristics and controllability . - Controller responses . - Proportional action . - Integral action (reset ).

 Derivative action . - Conclusion