The control loop

The loop described above is an example of a feedback control loop. Other types of control strategy are possible but the feedback controller is the simplest and most widely used. The elements of the control loop will now be examined in turn. 

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The speed of the controller is adjusted by the proportional band and reset rate (proportional and integral gains). These parameters also influence the stability of the control loop. All control loops are limited to a gain of less than one at their critical frequency. Higher closed-loop gain will make the loop unstable. 

To prevent surges, a well-trained operator would put the controller in manual mode and freeze the valve in an open position. This stops the control loop oscillations and decreases the compressor discharge resistance, thus breaking the surge cycle. Unfortunately, the operator has no way of knowing how much to open the valve and, subsequently, how much to close it. 

All inputs to the control loop (changes in set-point or disturbances) are generically represented by V(s). The input V(s) is found by passing a mathematically bounded normalized input V (s) through a transfer function block lV(s), called the input weight, as shown in Figure 9.22. 

The user interface and the simplicity of usage are important issues. Likewise, stabilized and qualified control for some of the control loops must he as simple as possible. 

FIGURE 9.54 The controlling loop with sensors, actuators, and controller. 

Establish the rado of the naximurn anticipated flow rate for system, Qjj, to the design basis rate, Q,d or Q,m/Q.d-When Qv] is not known, nor can it be anticipated, use Qm/Qd of 1.1 for flow control and 1.25 for level pressure and temperature control valves to anticipate the flow rate transients as the control loop recovers from a disturbance [9]. 

The ARC is controlled by its own hardwired control module. The temperature is monitored by a set of seven thermocouples connected in series which measure the difference between the temperature of the sample and that of its surroundings. The temperature is maintained by heaters which receive their inputs from the control module. A pressure transducer is attached to the sample container, giving both an analog readout on a pressure gauge and a digital readout on the control module panel. It should be noted that pressure is monitored but it is not part of the control loop. 

Note that the system takes on the order of 15000 seconds to get to within 1 % of steady state with no controller (or the control loop in manual). 

The cost functional is the indicator of how well the control loop is functioning. The lAE criterion used essentially says "measure the cumulative difference between the actual value and the desired set point" this cumulative score is a measure of control system performance. With this code in the model, the commands to do the controller tuning are ... 

In this chapter only the first step in the specification of the control systems for a process will be considered the preparation of a preliminary scheme of instrumentation and control, developed from the process flow-sheet. This can be drawn up by the process designer based on his experience with similar plant and his critical assessment of the process requirements. Many of the control loops will be conventional and a detailed analysis of the system behaviour will not be needed, nor justified. Judgement, based on experience, must be used to decide which systems are critical and need detailed analysis and design. 

Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analysers are available they can be incorporated in the control loop, but more complex control equipment will be needed. 

The schemes used for reactor control depend on the process and the type of reactor. If a reliable on-line analyser is available, and the reactor dynamics are suitable, the product composition can be monitored continuously and the reactor conditions and feed flows controlled automatically to maintain the desired product composition and yield. More often, the operator is the final link in the control loop, adjusting the controller set points to maintain the product within specification, based on periodic laboratory analyses. 

For the rest of the control loop, Gc is obviously the controller transfer function. The measuring device (or transducer) function is Gm. While it is not shown in the block diagram, the steady state gain of Gm is Km. The key is that the summing point can only compare quantities with the same units. Hence we need to introduce Km on the reference signal, which should have the same units as C. The use of Km, in a way, performs unit conversion between what we dial in and what the controller actually uses in comparative tests. 2... 

With a given problem statement, draw the control loop and derive the closed-loop transfer functions. 

So far, we know that the secondary loop helps to reduce disturbance in the manipulated variable. If we design the control loop properly, we should also accomplish a faster response in the actuating element the regulating valve. To go one step further, cascade control can even help to make the entire system more stable. These points may not be intuitive. We ll use a simple example to illustrate these features. 

The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the "error signal."... 

The actuating signal passes through the two control elements the temperature controller and the temperature control valve. The temperature control valve responds by adjusting the manipulated variable (the cooling water flow rate). The lube oil temperature changes in response to the different water flow rate, and the control loop is complete. 

Occasionally an incident occurs that results in a common mode failure. This is a single event that affects a number of pieces of hardware simultaneously. For example, consider several flow control loops similar to Figure 11-4. A common mode failure is the loss of electrical power or a loss of instrument air. A utility failure of this type can cause all the control loops to fail at the same time. The utility is connected to these systems via OR gates. This increases the failure rate substantially. When working with control systems, one needs to deliberately design the systems to minimize common cause failures. 

The measuring and final control elements in the control loop are described by transfer functions which can be approximated by constants of unit gain, and the process has the transfer function ... 

Determine the open-loop response of the output of the measuring element in Problem 7.17 to a unit step change in input to the process. Hence determine the controller settings for the control loop by the Cohen-Coon and ITAE methods for P, PI and PID control actions. Compare the settings obtained with those in Problem 7.17. 

The uncertainty in the identified number of ineffective elements, indicated by the unknown class, grows with the position of the element in the control loop, e.g. the uncertainty about the number of ineffective interventions is larger than the uncertainty about the number of ineffective judgements. 

Including the double-loop of the control loop, it can be concluded that for at least 60% of the ineffective control elements, the origin of the ineffectiveness has to be sought in the steering element instead of in the ineffective control element itself. From this it is concluded that the steering element of the operational control level (which are the higher control levels in an organization) is the main cause for ineffective control loops. 

Let s start from the beginning of the control loop, at the sensor. Instruments for on-line measurement of many properties have been developed. The most... 

The interface with the process at the other end of the control loop is made by the final control element. In a vast majority of chemical engineering processes the final control element is an automatic control valve which throttles the flow of a manipulated variable. Most control valves consist of a plug on the end of a stem that opens or closes an orifice opening as the stem is raised or lowered. As sketched in Fig. 7.5, the stem is attached to a diaphragm that is driven by changing air pressure above the diaphragm. The force of the air pressure is opposed by a spring. 

The basic reason for using different control-valve trims is to keep the stability of the control loop fairly constant over a wide range of flows. Linear-trim valves are used, for example, when the pressure drop over the control valve is fairly constant and a linear relationship exists between the controlled variable and the flow rate of the manipulated variable. Consider the flow of steam from a constant-pressure supply header. The steam flows into the shell side of a heat exchanger. A process liquid stream flows through the tube side and is heated by the steam. There is a linear relationship between the process outlet temperature and steam flow (with constant process flow rate and inlet temperature) since every pound of steam provides a certain amount of heat. 

The part of the control loop that we will spend most of our time with in this book is the controller. The job of the controller is to compare the process signal from the transmitter with the setpoint signal and to send out an appropriate signal to the control valve. We will go into more detail about the performance of the controller in Sec. 7.2. In this section we will describe what kind of action standard commercial controllers take when they sec an error. 

Integral action degrades the dynamic response of a control loop. We will demonstrate this quantitatively in Chap. 10. It makes the control loop more oscillatory and moves it toward instability. But integral action is usually needed if it is desirable to have zero offset. This is another example of an engineering trade-off that must be made between dynamic performance and steadystate performance. 

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