Setpoint

Some of the inherent advantages of the feedback control strategy are as follows regardless of the source or nature of the disturbance, the manipulated variable(s) adjusts to correct for the deviation from the setpoint when the deviation is detected the proper values of the manipulated variables are continually sought to balance the system by a trial-and-error approach no mathematical model of the process is required and the most often used feedback control algorithm (some form of proportional—integral—derivative control) is both robust and versatile. 

Because of the time constants and dynamics associated with the top level s control and manipulated variables, setpoints are usually ramped incrementally to their new values in a manner such that the process is not disturbed and the proximity to constraints can be periodically checked before the next increment is made. 

Fig. 12. Cascade control signal flow diagram, where SPP = primary control variable setpoint PVP = primary control variable measurement ...

SPS = secondary control variable setpoint and PVS = secondary control variable measurement. The + and — indicate to multiply the signal by +1 or —1... 

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

FC and TC = flow and temperature controllers, respectively SP = setpoint S/F = steam/feed ratio x = multiplication of signals and + = sum of signals, (a) Additive (b) multiphcative and (c) combined additive and multiphcative. 

Fig. 16. Methods of ratio control implementation, where -I- and x indicate the division and multiplication of signals, respectively (a) use of a divider and ratio controller and (b) use of a multiplier. Calculated ratio = actual value of control variable both setpoint and gain are equivalent to the desired ratio.

Ratio control and multiphcative feedforward control, in general, are subject to the same considerations. Ratio control can be of a steady-state or a dynamic form. It is often implemented using a setpoint as the load variable when the load variable has a controller associated with it and the controller is in auto mode. 

Multivariable control strategies utilize multiple input—multiple output (MIMO) controUers that group the interacting manipulated and controlled variables as an entity. Using a matrix representation, the relationship between the deviations in the n controlled variable setpoints and thek current values,, and the n controUer outputs, is... 

This equation relates the deviations from setpoints to the observed current values of the controlled variables. 

Fig. 18. Dead-time compensation (a) classical feedback and (b) Smith dead-time compensator. SP = setpoint C = controlled variable and (+) and (—)...

Fig. 19. Example of programmed adaptation. F/O SP indicates the fuel oil setpoint and F/G SP indicates the fuel gas setpoint.

FIG. 8-23 Both load regulation and setpoint response require high gains for the feedback controller. 

Intended Use The intended use of the model sets the sophistication required. Relational models are adequate for control within narrow bands of setpoints. Physical models are reqiiired for fault detection and design. Even when relational models are used, they are frequently developed bv repeated simulations using physical models. Further, artificial neural-network models used in analysis of plant performance including gross error detection are in their infancy. Readers are referred to the work of Himmelblau for these developments. [For example, see Terry and Himmelblau (1993) cited in the reference list.] Process simulators are in wide use and readily available to engineers. Consequently, the emphasis of this section is to develop a pre-liminaiy physical model representing the unit. 

Construction and equipment meet the design specifications. Obtaining field verification or performing document review for the new or modified process can validate design specifications for construction and equipment. If a change is not physical (such as a setpoint for an interlock shutdown), the method for the change and its anticipated effects should be reviewed. 

When a test run is performed using the actual materials for the toll, it is a prime opportunity for the toller and the client to document the capability of the equipment, instrumentation, and process steps. During such a test, frequency of sampling may be increased, additional analyses performed and yield capabilities checked to find the optimum setpoints and timing for the toll process. Health, safety and environmental staff may choose to provide close coverage of the test run to evaluate areas for improvement during the actual startup and long term operation. 

Bimetallie elements are widely used in instruments sueh as thermostats to sense or eon-trol temperatures. There are several bimetallie element types available, sueh as straight strips, eoils and dises, but all rely on the same working prineiple. In its most basie form, the bimetallie strip eomprises of two dissimilar metal strips bonded together, usually of the same surfaee area, but not neeessarily of the same thiekness thermostat. The eom-posite metal strip is elamped at one end to aet as a eantilever beam, and is horizontal at a partieular temperature. When the temperature is inereased, the strip defleets in the direetion of the metal with the least eoeffieient of linear expansion. Its working prineiple relies on the faet that the metals will expand at different rates as the strip is heated. The purpose of this defleetion is to typieally eause the strip to make eontaet with a switeh or eomplete an eleetrie eireuit at a partieular setpoint temperature above the ambient. 

For the purposes of meeting a eustomer speeifieation, a toleranee for the thermostat setpoint temperature ean be estimated at 3o-Ar) from whieh the approximate thermostat speeifieation beeomes ... 

This is a control algorithm that attempts to eliminate the offset (caused by proportional control) between the measurement and the setpoint of the controlled process variable. This control mode remembers how long the measurement has been off the setpoint. 

P.B. = proportional band setting overshoot = amount - -l- of setpoint, °F span = transmitter span, °F... 

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