Override Controls

Override Control In override control, either the highest or the lowest signal from two or more input signals is automatically selected. Common examples include  

There are situations where the control loop should be aware of more than just one controlled variable. This is particularly true in highly automated plants where the operator cannot be expected to make all the decisions that must be made under abnormal conditions. This includes the startup and shutdown of the process. 

Override control (or selective control as it is sometimes called) is a form of multivariable control in which a manipulated variable can be set at any point in time by one of a number of different controlled variables. 

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While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

Selective and Override Control When there are more controlled variables than manipulated variables, a common solution to this problem is to use a selector to choose the appropriate process variable from among a number of available measurements. Selec tors can be based on either multiple measurement points, multiple final control elements, or multiple controllers, as discussed below. Selectors are used to improve the control system performance as well as to protect equipment from unsafe operating conditions. 

For centrifugal and axial compressors, some form of override control is recommended for constant speed motor drivers to sense motor over load and override the process control until the cause of overload has... 

These should not be confused with elevators fitted with a fire officer s override control. A firefighting elevator is required for any building exceeding 18 m in height or 9 m in depth below the evacuation level, and is to be used to allow fire officers to maneuver personnel and equipment during firefighting operations. 

Slide valves will have an independent low differential pressure override controller to prevent the reaction temperature controllers from opening the slide valves to the point where low differential pressure could allow feed back to the regenerator. 

A host of gadgets and software are available to perform a variety of computations and logical operations with control signals. For example, adders, multipliers, dividers, low selectors, high selectors, high limiters, low limiters, and square-root extractors can all be implemented in both analog and computer systems. They are widely used in ratio control, in computed variable control, in feedforward control, and in override control. These will be discussed in the next chapter. 

However, over the years a number of slightly more complex structures have been developed that can, in some cases, significantly improve the performance of a control system. These structures include ratio control, cascade control, override control, etc. We will devote much of this chapter to these subjects. 

If an operator saw this problem developing, he would switch the temperature loop into manual and cut back on the steam flow. The control system in Fig. 8,4fl will perform this "override control automatically. The low selector (LS) sends to the steam valve the lower of the two signals. If the steam valve is air-to-open, the valve will be pinched back by cither high temperature (through the reverse-acting temperature controller) or by low base level (through the low-base-level override controller). 

In level control applications, this override controller can be a simple fixed-gain relay which acts like a proportional controller. The gain of the controller shown in Fig. 8.4n is five. It would be zeroed so that as the level transmitter dropped from 20 to 0 percent of full scale, the output of the relay would drop from 100 to 0 percent of scale. This means that under normal conditions when the level is above 20 percent, the output of the relay will be at 100 percent. This will be higher than the signal from the temperature controller, so the low selector... 

A sustained error signal can occur for a number of reasons, but the use of override control is one major cause. If the main controller has integral action, it wfll windup when the override controller has control of the valve. And if the override controller is a PI controller, it will windup when the normal controller is setting the valve. So this reset windup problem must be recognized and solved. 

Avoid saturation of a manipulated variable. A good example of saturation is the level control of a reflux drum in a distillation column that has a very high reflux ratio. Suppose the reflux ratio (R/D) is 20, as shown in Fig. 8.10. Scheme A uses distillate flow rate D to control reflux drum level. If the vapor boilup dropped ouly 5 percent, the distillate flow would go to zero. Any bigger drop in vapor boilup would cause the drum to run dry (unless a low-level override controller were used to pinch back on the reflux valve). Scheme B is preferable for this high reflux-ratio case. 

Specify the range and action of the override control elements required to achieve this control strategy. 

Design an override control system for the chilled-water loop considered in Prob. 7.11. The flow rate of chilled water is not supposed to drop below SOO gpni. Your override control circuit should open the chilled-water control valve if chilled-water flow gels below 500 gpm, overriding the temperature controller. 

There is a first-order dynamic lag of t minutes between a change in the signal to the steam valve and vapor boilup. The low base-level override controller pinches the reboiler steam valve over the lower 25 percent of the level transmitter span. 

There are several ways to achieve this variable structure control strategy. The elegant approach is to use model predictive control (MPC). The simple approach is to use override control. The latter technique is demonstrated in this section. 

Override controls are used to protect against fouling of the heat transfer surfaces, when the water outlet temperature exceeds 50°C (122°F) or to prevent... 

Greg Shinskey (1988), over the course of a long and productive career at Foxboro, has proposed a number of advanced control" structures that permit improvements in dynamic performance. These schemes are not only effective, but they are simple to implement in basic control instrumentation. Liberal use should be made of ratio control, cascade control, override control, and valve-position (optimizing) control. These strategies are covered in most basic process control textbooks. 

Now the liquid level loops must also be modified, since we no longer can specify production rate and reactor level control cannot use TV This is easily accomplished by using low level override controllers on each of the three levels. Low stripper level pinches product base product flowrate B. Low separator level pinches separator liquid flowrate L. Low reactor level pinches the condenser cooling water flowrate CWc. In an override situation the level control structure has been reversed from the basic structure and now levels are held in the direction of flow. 

Figure 8.7 shows how the process rides through the loss of F A, disturbance IMV(6). The override controller takes action almost immediately, and production rate is reduced after about 5 hours when liquid levels drop. 

Figure 8.8 shows the responses to IMVt l), a change in the composition of A and C in the FoC stream. As the amount of A in the system drops, the override controller cuts feed streams FoD and TV. When reactor... 

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