Characteristics of different control modes—offset

The response has a high maximum deviation and there is a significant time of oscillation (response time). The period of this oscillation is moderate. For a sustained change in load, the controlled variable is not returned to its original value (the desired value) but attains a new equilibrium value termed the control point. This difference between the desired value and the control point is called the offset or droop. The reason for offset with proportional action can be seen if it is remembered that the control action is proportional to the error. 

Consider again the temperature control system fitted to the heat exchanger in Fig. 7.1. Suppose that the temperature of the cold stream decreases. Then, clearly, the temperature at Y, i.e. 0, will also begin to fall. In response to this the controller will open the control valve further in proportion to the error—where the error is given by equation 7.1. In order to maintain this new steady state, i.e. with the increased rate of flow of the hot stream, a constant additional output must be applied to the control valve by the controller. This additional output can exist only if there is an 

A setting knob is provided on the more traditional stand-alone controllers for the adjustment of Kc as well as a pointer to set the desired value. The setting knob for Kc is sometimes graduated in terms of proportional band. This quantity is defined as the error required to move the final control element over its whole range and is expressed as a percentage of the total range of the measured variable (see Example 7.1). In the newer MBC installations, the control action is simulated in the form of software and values of Kc and of the desired value are entered via an appropriate keyboard with the relevant values displayed on a VDU (Visual Display Unit)—see Section 7.19. 

The level of liquid in a tank is controlled using a pneumatic proportional controller as shown in Fig. 7.6. The level sensor is able to measure over the range 1.85 to 2.2S m. It is found that, after adjustment, the controller output pressure changes by 4 kN/m2 for a 0.01 m variation in level with the desired value held constant. If a variation in output pressure of 80 kN/m2 moves the control valve from fully open to fully closed, determine the gain and the proportional band. 

As the level changes by 0.01 m whilst the desired value is held constant, the error must change by 0.01 m with a change in output pressure of 4 kN/m2. 

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