Control action direct acting

Two main control schemes exist feedback control and feed-forward control (Fig. 6). In feedback control (by far the most common), system performance is monitored, deviations from desired conditions are quantified, and controlled variables are modified to return the system to the desired state. In feed-forward control, process inputs are monitored. As they deviate from desired values, their effect on the system is predicted, and controlled variables are modified to minimize their effect. Feedback control is safer, since it guarantees performance by controlling it directly, but it is also slower corrective action is taken only after the perturbation has affected process performance. Feed-forward control is faster it acts on input deviations as soon as they are detected. However, it is riskier if the detected... 

Another, perhaps the most important, controller parameter is the control action, which is set as either direct or reverse. By convention, if the valve position is to increase as the measurement increases, then the controller is considered direct acting. The actual output signal from the controller will further depend on the specified failure mode of the valve. For example, a fail-closed valve will require an increase-to-open signal, whereas a fail-open valve will require an increase-to-close signal. In order to minimize confusion, rather than displaying actual output, most controllers display an implied valve position, which indicates the desired position of the valve. 

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In Figure 21.1a we can see the general form of a feedforward control system. It measures the disturbance directly and then it anticipates the effect that it will have on the process output. Subsequently, it changes the manipulated variable by such an amount as to eliminate completely the impact of the disturbance on the process output (controlled variable). Control action starts immediately after a change in the disturbance ) has been detected. In Figure 21.1b we have repeated the schematic of a typical feedback loop so that you can contrast the two control systems directly. It is clear that feedback acts after the fact in a compensatory manner, whereas feedforward acts beforehand in an anticipatory manner. 

The controller action, either direct or reverse acting, which defines the direction of its effect. For a direct-acting controller, when the PV rises above the setpoint (SP), the OP increases, and vice versa. In this case, the static process gain is negative, as illustrated for a level controller in Figure 21.21a. Here, the liquid level is the PV, the flow rate of the effluent stream, Q , is the OP, and the controller action is set to Direct. In contrast, for a reverse-acting controller, when the PV rises above the SP, the OP decreases, and vice versa. In these cases, the static process gain is positive, as illus-... 

Note that the risk index s control actions should be of different type both act directly on the product, R D and production requirements, or both, or on procedural and organizational methods. 

The closed-loop system will be stable if this root is negative. Because time constants are always positive (t > 0), the feedback control system will be stable if KcK Kp > -1. This means that as long as the controller has the correct control action (i.e., reverse- or direct-acting, as per Section 8.3), then the system will be stable. For example, if > 0 and Ky > 0, then the controller must be made reverseacting so that Kc > 0. By contrast, if Kp < 0, then a direct-acting controller Kc < 0) is required. 

Solenoid valves Power control signal fiom SIS output relay. 5 Stuck open after long periods 6 Pilot driven valves prone to blodrages, stick open. 7 Direct acting soloioids with spring action to block supply and to vent actuator are preferred. 

Let us first look at the action of the process element of the controller. Consider a fiimace that heats your home in the winter. When the energy that drives the furnace increases, the temperature in the surrounding rooms increases as well. This is known as an increase/increase (I/l) relationship, or a direct-acting element [5]. Direct action refers to a control-loop element that, for an increase in its input, also experiences an increase in its output. 

All controllers, whether implemented as stand alone or as part of a distributed control system (DCS) application, have a switch which will allow either direct or reverse action. In general, the action of the controller is the last to be specified, since there is typically little choice in the action of the other elements in the loop. Once the other elements actions are known, the controller action may be set such that the overall loop action is reverse acting, or I/D. 

For the components shown in Figure 3.15, assume the action shown in Figure 3.16, and also assume an air to open actuator (I/I) for the valve. Note that the valve, process, and sensor/transmitter are all direct acting. Therefore, in order to get the desired negative feedback action (I/D overall loop action) the controller must be set to reverse action. 

There are two reasons for the improvement in control. The first has already been discussed the elimination of inverse response. The second is equally important the two temperature controllers in the CS7-RR structure have the same action (direct). This means that when both control loops see a positive vapor boilup disturbance, which increases both tray temperatures, the two controllers will increase both fresh feeds. This helps to maintain the delicate stoichiometric balance between the reactants that is essential for neat operation of a reactive distillation column. Because a reactive distillation column acts like a pure integrator with respect to the reactants, this similar initial response is very important for CS7-RR, where feedstreams are used as manipulated variables. 

The results from Chapter 10 suggest that the two temperature controllers should have the same action (both direct acting). Therefore, a tray somewhere in the region where the gain between temperature and Fqb is negative should be selected. Tray 12 exhibits a negative gain, but it is small. 

Directional control valves may be operated by differences in pressure acting on opposite sides of the valve elements or may be positioned manually, mechanically, or electrically. Often two or more methods of operating the same valve will be used in different phases of its action. 

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