Control loops defined

To supplement the SVD analysis, rigorous d mamic simulations under closed loop operation were carried out. For the closed-loop analysis, several issues must be defined first, such as the control loops for each system, the type of process controller to be used, and the values of the controller parameters. 

The advantage of defining a transfer function in terms of Laplace transforms of input and output is that the differential equations developed to describe the unsteady-state behaviour of the system are reduced to simple algebraic relationships (e.g. cf. equations 7.17 and 7.19). Such relationships are much easier to deal with, and normal algebraic laws can be used to relate the various transfer functions of each component in the control loop (see Section 7.9). Furthermore, the output (or response) of the system to a variety of inputs may be obtained without classical integration. 

As can be observed in Figure 10.1, information from the process (value of the temperature measured by the sensor) arrives at the control loop, which defines it as a closed loop. This is the case for most controllers installed in bioreactors. If the decisions of a controller do not take into account any of the monitored information of the process, it is called an open loop. An example of an open loop is a fed-batch process, where it... 

Dead time can result from measurement lag, analysis, and computation time, communication lag or the transport time required for a fluid to flow through a pipe. Figure 2.27 illustrates the response of a control loop to a step change, showing that the response started after a dead time (td) has passed and reaches a new steady state as a function of its time constant (t), defined in Figure 2.23. When material or energy is physically moved in a process plant, there is a dead time associated with that movement. This dead time equals the residence time of the fluid in the pipe. Note that the dead time is inversely proportional to the flow rate. For liquid flow in a pipe, the plug flow assumption is most accurate when the axial velocity profile is flat, a condition that occurs when Newtonian fluids are transported in turbulent flow. 

The complete control system for positioning on an actual control axis includes several levels of control loops, which are acting in a synchronized manner, as shown in Figure 3.155. The intermediate control loops are called embedded loops, and together they guarantee that the original command for a specified movement is correctly completed. Any command for a movement, which is executed under closed-loop control, will result in a motion profile. This, usually trapezoidal profile, defines the way how the component is intended to be moved. 

The application of a redox sensor in a control loop has been reported by Memmert and Wandrey [274] who controlled xylanase production of Bacillus amyloliquefaciens by defined oxygen limitation redox electrodes refer essentially to dissolved oxygen concentration below 10 mmol 1 102. This property was also promoted to determine the quality of anaerobic processes [403]. 

The equipment used in the unit operations is complex and microprocessor controlled to allow the execution of process recipes. However, advanced control schemes are rarely invoked. The microprocessor adjusts set points according to some sequence of steps defined by the equipment manufacturer or the process operator. Flows, pressures, and temperatures are regulated independently by off-the-shelf proportional-integral-derivative controllers, even though the control loops interact strongly. For example, fluorine concentration, substrate temperature, reactor pressure, and plasma power all influence silicon etch rates and uniformity, but they are typically controlled independently. 

The central element in any control loop is the process to be controlled. Therefore, the control objectives must be defined (e.g., maintain a desired outlet temperature and/or composition, maintain the level in a tank at a certain height, etc.). Once the control objective is specified, variables are measured in order to monitor the operational performance of the process (sensing element). Next, the input variables. that are to be manipulated are determined. Finally, after the control objectives, the possible measurements, and the available manipulated variables have been identified, the control configuration is defined. The control configuration is the information structure used to connect the available measurements to the available manipulated variables. The two general types of control configurations are feedback control and feedforward control. Details on feedback control are discussed below in this problem. Feedforward control is discussed in the next problem, CTR.2. 

An existing column has a reboiler, no condenser, and two feeds, one of which is at the top of the column. The top feed is of fixed temperature, pressure, and composition, but its rate is variable. The other feed is of fixed rate, composition, and pressure, and flows through a heat exchanger upstream of the column so that its temperature may be controlled before entering the column. How many independent variables are required to define this process If the key components are ethane and propane, describe conceptual control loops to control the separation and performance of this process. 

Figure 7. An n-p-n bipolar transistor. B, base E, emitter and C, collector. The direction of flow for the electrons and holes in the on state is described with grey and white arrows, respectively. The primary current, defined as flow from + to -, is shown in the large black block arrow, and the current through the control loop is represented by the smaller black arrow.

The definition of the relative gains and their use in selecting the control loops are not limited to systems with two inputs and two outputs. The extension to general processes is straightforward. Thus the relative gain between an output y, and a manipulation mj is defined by... 

Examples 15.1 and 15.2 dramatized the effect that a feedback control loop may have on the stability characteristics of a process. In this section we organize and systematize our analysis, introducing and defining some appropriate terms. 

The basic function of the algorithm is to implement a feedback control loop, as defined by the controller responsibilities, along with appropriate checks to detect internal or external failures or errors. 

The recommended pairings depend on the operating conditions of the coolant subsystem, with significant interactions normally occurring between the control loops. To avoid this, the manipulated variables are defined as < > = F,., + F,2 and jl = FJ(F,i + F -j), transforming Eqs. (21.30) and (21.31) to... 

Many PID controller loops remain on their factory settings long after plant startup. When the controller action is in the right direction and the PV range is defined wisely, these settings often give adequate performance. 

---