Control action reverse acting

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. 

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-... 

A slightly more automated method would be to feed the water to each rotameter from a pressure-controlled header system (Fig. 11.46). The pressure would be measured with a diaphragm seal connected to a transmitter with a filled capillaiy. llie control valve would be a fail-open globe or V-ball valve with equal percentage characteristic. The controller would be reverse-acting with proportional and integral action. This arrangement allows... 

Every valve should be stroke-checked in the field from the controller or distributed control system to ensure that it is wired correctly, that its positioner is ranged correctly, that its action is smooth, and that valve action is in the correct direction (remembering that some valves are reverse-acting). 

Figure 3.15 shows the same disturbance but with the proportional-on-PV algorithm. Note that the vertical scale is much larger than that in Figure 3.14. As expected, there is no proportional kick and, since the action is now based on PV, the proportional part does not return to zero. It can be confusing that the proportional part reduces as the SP is increased but this is because the controller must be configured as reverse acting. The integral action compensates for this so that there is a net increase in controller output. The derivative action...

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. 

Stability. Thus, in the aggregate, the overall action of the control loop must be I/D or reverse acting. D/D or direct action generates, by definition, positive feedback. 

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. 

Covalent modification of proteins is a potent means of controlling the activity of enzymes and other proteins. Phosphorylation is the most common type of reversible covalent modification. Signals can be highly amplified by phosphorylation because a single kinase can act on many target molecules. The regulatory actions of protein kinases are reversed by protein phosphatases, which catalyze the hydrolysis of attached phosphoryl groups. 

The time course of action of PPADS at 5 pM, a concentration which inhibited the effects of endogenous ATP by about 70%, was examined in the field-stimulated rabbit vas deferens. In these experiments, PPADS acted as a slowly-equilibrating and a slowly-reversible P2X purinoceptor antagonist. It was found to reach apparent equilibrium in about 120 min, and the tissue took about 90 min to regain its control twitch height by continual washout of PPADS. Importantly, these findings indicate that PPADS is not an irreversible antagonist at P2X Purinoceptors in rabbit vas deferens. 

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