Module 1Module 1.1 - The Very BasicsIntroduction

This part of the course starts with an outline and overview of basic control concepts. Questions which process engineers routinely have to answer about process control include the following:

I have this process. What should I control?

Where on the process do I put my control loops?

As I proceed with the design of a process, what aspects of control should I consider at which stages?

Most books with the words `process control' in the title do little to answer these questions. Classical linear control theory, which forms the basis of most books on control, is much concerned with how to design controllers and is less helpful on how to design complete control systems. Other problems with this classical approach, for most process engineers wishing to design control systems for real chemical processes, are the restriction of most of its methods to idealised process models, and the extensive use of rather specialised mathematics.

Satisfactory answers to questions such as the above frequently require little conventional mathematics. What they do require, however, is a good understanding of what a process is intended to do and how it works.

In this book we will approach process control from the standpoint of a chemical or process engineer, and address these questions and others like them. We will consider the process and its control system in the language of process engineering. We will use mathematics, as such, only when necessary, and the language of classical control engineering only when it is unavoidable, or will add very significantly to the process engineer's understanding.

Why Control?

Chemical plants are intended to be operated under known and specified conditions. There are several reasons why this is so:

Safety:

Formal safety and environmental constraints must not be violated.

Operability:

Certain conditions are required by chemistry and physics for the desired reactions or other operations to take place. It must be possible for the plant to be arranged to achieve them.

Economic:

Plants are expensive and intended to make money. Final products must meet market requirements of purity, otherwise they will be unsaleable. Conversely the manufacture of an excessively pure product will involve unnecessary cost.

A chemical plant might be thought of as a collection of tanks in which materials are heated, cooled and reacted, and of pipes through which they flow. Such a system will not, in general, naturally maintain itself in a state such that precisely the temperature required by a reaction is achieved, a pressure in excess of the safe limits of all vessels be avoided, or a flowrate just sufficient to achieve the economically optimum product composition arise.

Control Objectives

Control systems in chemical plants have, as noted, three functions.

Safety.

Operability, i.e. to ensure that particular flows and holdup are maintained at chosen values within operating ranges.

To control product quality, process energy consumption etc.

To a large extent these are quite separate objectives. Indeed, in the case of safety systems separate equipment is generally used. The aims of control for operability are secondary to those of strategic control for quality etc., which directly affect process profitability.

Control for Safety

Concern for safety is paramount in designing a chemical plant and its control systems. Ideally a process design should be `intrinsically safe', that is, plant and equipment should be such so that any deviation, such as an increase in reactor pressure, will itself change operating conditions so that it is rapidly removed, for example by a fall in reaction rate. For many perturbations this type of responsive, passive safety system will not be possible and active systems will be required.

These active safety systems must be robust and of high integrity. Current processes achieve this through simplicity. The ultimate safety system is in most cases the mechanical relief valve which simply vents the plant to atmosphere, possibly through a flare or scrubber.

We will not discuss control for safety explicitly in this book. Generally speaking a complete and separate system is provided to handle emergency control action. The need for this, and its design requirements, is established in hazard and operability or hazop studies. These are typically carried out on the complete process with its `normal' control systems in place.

A number of safety issues will be addressed in the course of developing the design of the control systems for normal operation, but it must be emphasised that our treatment of this vital issue will be relatively restricted.

Control for Operability

The operator of a process quite simply has to

know what it is doing

be able to make it do what he or she wants, rather than to follow its natural inclinations.

The issue of making a plant behave in this way is called operability.

The majority of control loops in a plant control system are associated with operability. Specific flow rates have to be set, levels in vessels maintained and chosen operating temperatures for reactors and other equipment achieved.

Control for Profitability

There is no point in building a plant which is totally safe and can be made to take up any (safe) conditions of flow, temperature etc., if the conditions under which it is operated do not produce the correct amount of product to the correct specification, thus allowing its operators to make a profit.

The top level of process control, what we will refer to as the strategic control level is thus concerned with achieving the appropriate values principally of:

Production rate,

Product quality, and

Energy economy.

Techniques of ControlBasic Concepts of Feedback Control

The task of maintaining these required conditions falls to one or, more usually several, process control systems with which the plant will be equipped. The practical aspects of these will be discussed more fully in the following module. The underlying principle of most process control, however, is already understood by anyone who has grasped the operation of the domestic hot water thermostat:

The quantity whose value is to be maintained or regulated, e.g. the temperature of the water in a cistern, is measured.

Comparison of the measured and required values provides an error, e.g. `too hot' or `too cold'.

On the basis of the error, a control algorithm decides what to do.

Such an algorithm might be:

If the temperature is too high then turn the heater off. If it is too low then turn the heater on.

The adjustment chosen by the control algorithm is applied to some adjustable variable, such as the power input to the water heater.

This summarises the basic operation of a feedback control system such as one would expect to find carrying out nearly all control operations on chemical plants, and indeed in most other circumstances where control is required. The diagram below is a feedback control loop.

Notice that this extremely simple idea has a number of very convenient properties. The feedback control system seeks to bring the measured quantity to its required value or setpoint. The control system does not need to know why the measured value is not currently what is required, only that this is so. There are two possible causes of such a disparity:

The system has been disturbed. This is the common situation for a chemical plant subject to all sorts of external upsets. However, the control system does not need to know what the source of the disturbance was.

The setpoint has been changed. In the absence of external disturbance, a change in setpoint will introduce an error. The control system will act until the measured quantity reaches its new setpoint.

A control system of this sort should also handle simultaneous changes in setpoint and disturbances.

Advantages of Feedback Control

Not only does the feedback control system require no knowledge of the source or nature of disturbances, but it requires minimal detailed information about how the process itself works.

Feedback control action is entirely empirical, so long as an adjustment is being made in the correct `sense', e.g. more heat means increasing temperature and vice versa, then the control system should remove the effect of an external disturbance.

As we will see, it helps to know more than this, but the minimum information required to make a feedback control system work is whether the adjustment makes the measurement go up or down.

Disadvantages of Feedback Control

The main disadvantage of feedback control is that the disturbance enters into the process and upsets it. It is after the process output is different from the setpoint that the controller takes some corrective actions. Although most processes allow some fluctuation of controlled variable within a certain range, there are two process conditions which can make the overall effectiveness of feedback control quite unsatisfactory. One of these is the occurrence of disturbances of a large magnitude that is strong enough to seriously affect or even damage the process. The other is the occurrence of a large amount of lag (time delay) within the process. These are discussed further below.

Large Magnitude Disturbance

An example where the occurrence of disturbances of large magnitude that are strong enough to seriously affect the process, is temperature control of a catalyst reactor in which strong exothermic reaction takes place. The reaction heat is very high; therefore the reactant gas mixture is diluted by a inert gas to carry away most reaction heat, although the temperature of the reactor is maintained by feedback control of a coolant flowrate in coils inside. Assuming a large magnitude disturbance, the sudden large increase in the reactant concentration in the feed, enters the reactor, a sudden increase in the temperature is so large and so quick that the catalyst is burnt out before the control system senses the change and takes any actions. A diagram of this situation is shown below.

Large Time Delay

A simple example of a large time delay is the distillation column as outlined in the figure below. If we use feedback control to regulate the purity of the top product, when the feed composition changes (disturbance), the control system is not aware any takes no action until the effects of the disturbance travels and arrives at the sensor position at the top. When the controller takes the correction, the whole column may be far away from the designed conditions.

The question of importance of either occurrence is defined in economic terms. In either case, the principle concern is the existence of errors that have significant economic consequences in the overall process operation. In these cases, feedforward control can be used to deal with these disadvantages or inadequacies of feedback control.

Module 1.2: Control SystemsIntroduction

The key characteristic of control is to interfere, to influence or to modify the process. This control function or the interference to the process is introduced by an organization of parts (including operators in manual control) that, when connected together is called the Control System. Depending on whether a human body (the operator) is physically involved in the control system, they are divided into Manual Control and Automatic Control. Due to its efficiency, accuracy and reliability, automatic control is widely used in chemical processed.

The aim of this section is to introduce the concept of control systems, what their function is and what hardware and software is required by them.

Manual Control System

First start with a simple manual control system, to examine how control is introduced, how the control system is constructed and how it works.

A diagram of the system is shown below.

To begin with the shower is cold. To start the heating process the valve in the hot water line is opened. The operator can then determine the effectiveness of the control process by standing in the shower. If the water is too hot, the valve should be closed a little or even turned off. If the water is not hot enough then the valve is left open or opened wider.

Functions of a Control System

It can be seen that this control system, completed by the operator, possesses the following functions:

Measurement

This is essentially an estimate or appraisal of the process being controlled by the system. In this example, this is achieved by the right hand of the operator.

Comparison

This is an examination of the likeness of the measured values and the desired values. This is carried out in the brain of the operator.

Computation

This is a calculated judgment that indicates how much the measured value and the desired values differ and what action and how much should be taken. In this example, the operator will calculate the difference between the desired temperature and the actual one. Accordingly the direction and amount of the adjustment of the valve are worked out and the order for this adjustment is sent to the left hand from the brain of the operator. If the outlet water temperature is lower, then the brain of the operator will tell the left hand to open the steam valve wider. If there is any disturbance, or variation of flow rate in water to the shower inlet, some adjustment must be made to keep the outlet water temperature at a desired value.

Correction

This is ultimately the materialisation of the order for the adjustment. The left hand of the operator takes the necessary actions following the order from brain.

Therefore, for a control system to operate satisfactorily, it must have the abilities of measurement, comparison, computation and correction.

Of course, the manual operation has obvious disadvantages e.g. the accuracy and the continuous involvement of operators. Although accuracy of the measurement could be improved by using an indicator, automatic control must be used to replace the operator. In industry, it is automatic control that is widely used.

Automatic Control System

Based on the above process, we can easily set up an automatic control system as shown in the next figure.

Firstly, we can use a temperature measurement device to measure the water temperature, which replaces the right hand of the operator. This addition to the system would have improved accuracy.

Instead of manual valves, we use a special kind of valve, called a control valve, which is driven by compressed air or electricity. This will replace the left hand of the operator.

We put a device called a controller, in this case a temperature controller, to replace the brain of the operator. This has the functions of comparison and computation and can give orders to the control valve.

The signal and order connections between the measurement device, control valve and controller are transferred through cables and wires, which replace the nerve system in the operator.

Hardware of a Control System

Examining the automatic control system, it is found that it contains the following hardware.

Sensor - a piece of equipment to measure system variables. It serves as the signal source in automatic control. These will be discussed at length in a later module.

Controller - a piece of equipment to perform the functions of comparison and computation. The actions that a controller can take will be discussed at length in a later module.

Control Element - a piece of equipment to perform the control action or to exert direct influence on the process. This element receives signals from the controller and performs some type of operation on the process. Generally the control element is simply a control valve.

Software of a Control System

Associated with a control system are a number of different types of variables.

First we have the Controlled Variable. This is the basic process value being regulated by the system. It is the one variable that we are especially interested in - the outlet water temperature in the example above. In feedback control the controlled variable is usually the measured variable.

An important concept related to the controlled variable is the Set point. This is the predetermined desired value for the controlled variable. The objective of the control system is to regulate the controlled variable at its set point.

To achieve the control objective there must be one or more variables we can alter or adjust. These are called the Manipulated Variables. In the above example this was the input hot water flow rate.

Conclusively, in the control system we adjust the manipulated variable to maintain the controlled variable at its set point. This meets the requirement of keeping the stability of the process and suppressing the influence of disturbances.

Module 1.3 - Practical Control ExamplesIntroduction

Many different operations take place in a chemical plant. The classical approach of Unit Operations might thus be extended to process control, and we could consider in turn the control of heat exchangers, chemical reactors, distillation columns etc.

This turns out not to be a useful approach in most cases. The reason for this is that we are in the end concerned with the control of processes which consist of several operations, and these cannot be considered in isolation. This makes the engineer's task of designing a control system a difficult one, since it is hard to find just where to start!

The starting point we shall choose here is to consider how we regulate each of the basic quantities we may wish to keep constant in a process.

These quantities are

Flow

Inventory - level or pressure

Temperature

Composition

Pressure - two phases

The following sections discuss simple, but real, examples of how feedback control is applied to these basic quantities in a chemical plant. They are primarily examples of control for operability, and most of them will refer to single items of equipment or very simple combinations. A number of safety issues will be identified.

Strategic control for profitability will be dealt with in a later section in the context of control of complete plants and processes.

A number of fundamental concepts will be illustrated in the course of these examples. They are `graded' in the sense that the simplest examples come first; the reader is advised to follow the sequence we have presented. Even apparently trivial examples may be used to introduce important ideas.

Terminology

At this early stage there are three terms that should be used correctly. They are:

Control

Regulate

Adjust

The word `control' is sometimes loosely used to mean either regulation or adjustment. We have not actually seen the sentence 'In a control system the controller controls the controlled quantity by controlling a control valve position' However the term is regularly misused in this way. Control should refer only either to the actions of the controller element itself or to the function of the complete system. We are not just being pedantic, it is possible to misunderstand what is happening through misuse of terminology. We may also slip up ourselves, particularly in talking loosely about e.g. 'flow control' when we strictly mean 'flow regulation'.

Control Hardware

In all the examples which follow there will be various types of measurement sensors and transducers mentioned. These are simply considered here as black boxes and will be discussed in greater depth at a later stage.

Flow Control Systems

The most basic requirement in any chemical plant is to be able to make the flow through a pipe take a particular value. Consider first therefore the simplest item of plant equipment, namely a pipe, as shown below.

The basic pipe has had the following parts added to it, to make a control system.

A flow measuring device or Flowmeter. This consists of two parts

Firstly an Orifice Meter. This is shown in the diagram by two parallel lines.

This is connected to a sensor or Flow Transducer labelled FT in the figure.

An adjustable valve or Control Valve which alters the flowrate. This is shown by its conventional flowsheet symbol.

Finally these are connected by the Controller itself identified by the element FC.

This completes a control system to regulate the measured quantity, here the flow, by adjustment of the valve position. Compare this with the block diagram which we used earlier to introduce the feedback control system.

Positioning of Elements

One of the problems with designing control systems is that, as in any design problem, we are faced with alternatives. We have an alternative here in the positioning of the elements.

Should the measurement element be placed upstream of the valve as shown in the diagram?

Or should it be downstream?

Consideration of the properties of flowmeters and valves suggests that we were correct in our first choice. If the valve were upstream of the flowmeter then there are a number of ways in which it might affect the flowmeter calibration.

Control Algorithms

In the simple illustrative example of the water heater the rule for making the adjustment was:

If the temperature is too high then turn the heater off.

If it is too low then turn the heater on.

This is an example of an on-off control algorithm. The heater is either on (full) or off (completely). What will happen if we try to use such an algorithm here, where the objective is to maintain a particular flow?

If the flow is too high then shut the valve off.

If it is too low then open it.

Clearly, this is unlikely to serve, as rather than maintaining a specified flow the conditions will switch between zero and some maximum value. To achieve a specified steady flow we require something like:

If the flow is too high then shut the valve some more.

If it is too low then open it more.

This is a proportional control algorithm; the larger the error in the measured quatity, the larger will be the adjustment. This arrangement should result in the system settling at or near the required flow.

In practice, on-off control is seldom used. Most adjustment elements are valves, or occasionally other mechanical elements. These do not take kindly to being regularly or rapidly swung accross their full range of adjustment; they very quickly wear out or break down.

In most cases, therefore, proportional control or some variant is used. More detailed investigation of control algorithms requires quantitative information about the process. This aspect will be dealt with in a later section.

Inventory Control Systems

The next most basic requirement in a plant is a control system to regulate the amount of material or inventory in an item of equipment or over part of the process.

Inventory may be measured in a number of ways. Mass holdup may sometimes be determined directly, but usually volume is measured. In liquid systems volume is measured by level. In gas or vapour systems pressure is used as a measure of inventory.

Level Control Systems

Here we will consider simple feedback control of the level in a tank. This being the case it is necessary to measure the level directly and adjust the flow into or out of the tank to keep it constant.

Alternative Control Systems

Below is a diagram showing the two alternative control systems available for feedback control of the level. Both are equally valid and the decision as to which to use is based on

What is upstream or downstream of the tank?

Which streams are already being controlled?

The relative sizes of the flowrates, if there are several input or output flows.

Etc.

As can be seen the control system consists of

A Level Transducer denoted by LT in the diagram.

A Control Valve.

A Level Controller denoted by LC.

The level can be regulated by altering the flow via the adjustment of the valve position.

Control Algorithms

It is possible to control the level in a tank using

On/Off control.

Proportional control.

Extensions to proportional control.

The theory behind the algorithms will be found in a later section. There is also a level control experiment based on an actual experiment carried out by the undergraduates in the laboratory. Note that this can be found in the Case Study Section and in the Virtual Control Laboratory.

Pressure Control Systems

In gas or vapour systems we regulate inventory as pressure. A typical system is shown below. Both the inlet and outlet are gas or vapour. Therefore if the control valve is shut then the pressure in the tank will rise and vice versa.

In principle we might, like the level control system, have the valve either upstream or downstream of the tank. In practice in gas systems it is more likely to be downstream for the following reason.

Raising the pressure of a gas requires energy, and normally this energy is imparted by some mechanical device, such as a compressor. Both the compressor itself, and the energy to drive it, are expensive. To minimise the first cost we try to minimise the number of compressors in a process. Where possible we would use only one, locate it at the front of the process, and perform any subsequent manipulations to obtain the required pressure by downstream valves.

The energy used in compression is expensive, and throttling through a control valve throws this energy away. Therefore in proesses where compressor costs are very significant we may sometimes avoid such valves and manipulate the compressor speed in order to maintain the system at the required pressure. This control system is shown in the diagram below.

When we have vapour we usually also have liquids. Regulating pressure in two phase systems can be somewhat different. This is dealt with later.

Temperature Control Systems

In this section the control of temperature is to be discussed. Again only simple feedback loops are considered.

To change the temperature of something it is necessary to add or take away energy. This can be achieved in one of two ways.

Transfer energy indirectly, using a second stream, through coils, tubes, jackets etc. The second stream could be, for example, steam, cooling water, another process stream or even a source of power as in an electric element.

Mix in a second stream directly. This stream will have a different energy content from the original.

There are advantages and disadvantages for both methods. With the first there is the problem of transferring heat through the walls of the 'coil'. In the second the energy is absorbed directly but with the additional problem of increased flowrate/volume.

Diagrams of these alternative schemes can be found below.

Composition Control Systems

The control of composition is probably the most important objective in the chemical industry due to the requirement for specification on products. It is thus a strategic rather than an operational control problem and can only be considered sensibly in the context of whole process control. This will be discussed fully later but it is relevent to include a short example here.

Simple Composition Control Problem

To illustrate composition control consider the simplest process in which composition can be changed, namely blending. Here two streams of different compositions are mixed together e.g. a concentrate and a diluent as shown in the diagram below.

Either of the above schemes could be used although the first is preferred. The reasons why are discussed in a later section.

It is worth mentioning that the composition of a stream is rarely measured directly.

Typical composition analysers include

Gas chromatagraphs

Spectroscopic analysers

Features of this type of hardware which make them ineffective for control purposes are

Large time delay in their response

Low operational reliability

Relatively high cost

Thus an alternative method has to be sought to control the composition. This could be via the

Temperature of the mixture

Pressure of the mixture

Pressure Control in Two Phase Systems

An example of a process which contains both a vapour and a liquid is distillation. We generally wish to regulate the pressure in the column, which contains mainly vapour. This could be done by placing a valve in the vapour line leaving the column, exactly as we did with the simple tank.

There are several disadvantages to this system. One is that the control valve is on a vapour line. These are generally much bigger than liquid lines and hence require a much bigger valve i.e. of a much increased cost.

However, we remember that in a two phase system temperature and pressure are not independent. We can thus change the pressure of a vapour which is in equillibrium with a liquid by changing the temperature of the system. Raising the temperature raises the vapour pressure of the liquid which must equal the equillibrium pressure of the system.

Hence we can manipulate the temperature in the condenser by means of a small valve on the cooling water line, thus changing the pressure in both condenser and column.

There are a large number of different ways of manipulating pressure in two phase systems. These are discussed in a later section under Control of Distillation Process.

Module 1.4 - Sensors and TransducersBasic Requirements

In order to control the process performance, we need a control system, which consists of a sensor, a controller and a final control element. Obviously, the sensor is a very important part of the control system. It monitors the process and serves as a signal source for the control system. In our previous discussion, we always assumed, there was some suitable measuring device available, but not all measuring devices can be used in automatic control. The basic requirements for a sensor used in a control loop are the abilities:

to indicate the values of measured variables

to transmit the signals to the controller

The signals could be transmitted through either an electric circuit or a pneumatic pipeline; therefore, in order to transmit the signal, the sensors must have the ability to convert the measured variable values into either electric signals or pneumatic signals.

In this section of the course, we will very briefly discuss some kinds of sensors used in process control. However it is not intended to examine their working mechanism in any detail. Common variables in Chemical Engineering covered in the following discussion are pressure, temperature, flowrate, and liquid level. Analytical instruments for chemical composition measurement are usually specially designed for the specific purpose and hence are not included.

Pressure Transducers

Many kinds of pressure transducers are widely used in industry for pressure measurement. Although devices like the manometer and Bourdon tube etc are quite common, they are not suitable for control purposes due to the difficulties for signal transmission.

In most pressure transducers, there is a diaphragm to contact the fluids and protect the measuring setup isolated from the measured fluids, most of which may be corrosive. Due to the existence of this diaphragm, most pressure transducers can be used as pressure differential transducers, as long as the second (lower) fluid is introduced into the other side of the diaphragm. Actually, when measuring the pressure, it is measuring the pressure difference between the measured pressure and the pressure of atmosphere gauge pressure.

Strain meter (resistance):

In this sensor, under the diaphragm there is a pressure sensitive electric resistance. It is a spring type of resistance wire. Under pressure, some of the spring is pressed together causing a short circuit and reducing the resistance. The resistance will be inversely proportional to the strain on it, or the pressure. This changing resistance may be measured by being included into a electric circuit. The electric signal, current or voltage, is easily transmitted.

Piezo - electric sensor:

In this sensor, under the diaphragm is a quartz crystal. The working principle is based on the characteristics of quartz crystal. If the crystal is cut in a special way and placed between two plates, then the electro motivated force (e.m.f) set up between the plates will be a measure of the pressure applied to the crystal. This property of crystal is called piezo-electric effect. By measuring this e.m.f. setup, the applied pressure can be indicated and transmitted. This technique is mainly used for higher pressure measurements.

Traditional transducer (air pressure transmitted):

The principle of this system is that the pressure on a diaphragm is arranged to control the flow of air into, or out of, a chamber on the opposite side of the diaphragm, until a balance is obtained. The balancing pressure is an indication of the measured pressure. In this case, the measured signal is transmitted in a pneumatic circuit through the air pipeline.

Level MeasurementPressure operated sensor:

As there exists a unique quantitative relationship between the liquid level (head) and the static pressure at the bottom of the tank, the latter is widely used as an indication of the liquid level. Thus this is another case of pressure measurement and pressure transducers discussed above could be used for level measurement.

Float operated sensor:

In this kind of system, there is a float on the surface of liquid. The change in liquid level will cause movements of this float. By monitoring this movement a signal of the level is generated and transmitted.

Capacity bridge sensor:

This equipment consists of an electrode, an electronic unit and an indicator (or transmitter). The electrode is in the form of a long metal rod which reaches from the top to the bottom of the vessel. The electrode is bare when the liquid is electrically non conducting, but is sheathed in polymer like polyethlene etc when the liquid is conducting. The electronic unit is merely a power supply and a highly stabilized capacitance measuring bridge. One arm of the bridge is formed by the capacitance between the level sensing electrode and the earth (the vessel wall). A change in the capacitance owing to the rise and fall of the material around the electrode produces an out-of-balance current flow from the bridge which is measured and transmitted.

Optical sensor:

This kind of sensor is based on the difference in the reflecting and transparent properties of liquid and the gas above it. It takes the form of a light source and a receiver to the reflected light. When there is no liquid around, the receiver can detect the reflected light, and this light signal is converted to a electric signal which can be transmitted. When the sensor is surrounded by liquid, the receiver can't get the same amount of light reflected. The change in the light received is then converted into the change in the electric signal which is indicative to the amount of liquid. Thus to monitor level, you need a number of these kinds of sensors in series, spread over the whole height of the tank. This signal can also be transmitted.

Flow MeasurementDifferential Pressure Method:

This is still the most commonly used method. Whatever the construction of the meter, the principle involved is same. The net cross-section area of the stream is reduced, causing an increase in the velocity, and hence an increase in kinetic energy. This increase in kinetic energy is obtained at the expense of pressure energy, so that the pressure of fluid is reduced. By measuring this pressure reduction, or the pressure differential, the velocity of the fluid can be calculated. Examples of this kind are orifice plates and Venturi tube. The nature of this measurement is to measure the pressure differential and then to use pressure transducers.

Rotating vane meter:

Liquid passing through the meter is directed on to the rings of the vane, and rotates it at a rate which depends on the velocity of the liquid. This rotation can be arranged to drive some electric transducers to give out electric signal, like frequency.

Temperature MeasurementResistance thermometer and thermistor:

The electrical resistance of metals depends on temperature. By measuring the changing resistance, the temperature can be determined. The change in resistance can easily be converted to a electrical signal transmittable. Commonly used thermometers are made of Platinum or Nickel because they have a stable and preferable resistance-temperature coefficient.

A thermistor is made of semiconductor, a mixture of metal oxide. Unlike metals, the semiconductors have a negative resistance coefficient. This is the main difference between a thermometer and a thermistor.

Thermocouple:

If an electric circuit consists of all metallic conductors and all parts of the circuit are at the same temperature, there will be no electric force in the circuit, and hence there is no current. However, if the junctions between two metals are at different temperature, then there will be an e.m.f. and a current will flow. This e.m.f is called the thermoelectric e.m.f. , and the junction between the two metals is a thermocouple. The e.m.f will depend on the temperature difference between the two junctions. Therefore, when one junction (cold end) is kept at zero degrees, the e.m.f will indicate the temperature of the heated junction (measured temperature).

It has been shown the electric circuit for transmitting this signal does not alter the signal itself, which is indicated by the law of intermediate metals which states: In a thermoelectric circuit composed of two metals A and B, with junctions at temperature T1 and T2, the e.m.f is not altered if one or both the junctions are opened and one or more metals are interposed between A and B, provided that all the junctions by which the single junction at temperature T1 may be replaced are kept at T1, and all those by which the junction at temperature T2 may be replaced are kept at T2.

Silicon semiconductor:

Diodes have an important parameter called pass-required voltage. Below this voltage, there is no current through the diode. Above this voltage, the diode allows current to pass through.

The pass required voltage of silicon diodes depend uniquely on temperature, and thus this voltage signal can be used to indicate temperature. The main advantage of silicon semiconductor thermometers is that this pass-required voltage has a temperature coefficient which is essentially the same for all silicon devices of -2mV/ degC, and this linear change feature is a great advantage for control porposes.

Module 1.5 - Control ActionsThe Controller

The controller plays an essential role in the control system. Of the four basic functions of a control system, (measurement, comparison, computation, and correction) comparison and computation are solely achieved by the controller. The correction is materialized by the final control element, but this is done according to the controller's calculation.

The control mechanism in the controller may be considered as consisting of two sections

The Comparator

The Controller

The purpose of the first is to compare the measured and the desired values of the controlled variable and then compute the difference between them as the error. If there is no error, i.e. the controlled variable is at the setpoint, then no action is taken.

If an error is detected, the second section of the controller operates to alter the setting of the final control element in such a way as to minimize the error in the least possible time with the minimum disturbance to the system. To achieve this objective, different actions could be taken by the controller and hence different signals are sent to the final control element.

The remainder of this section is concerned with the different control actions that the feedback controller could take.

On-off or two-position action

This is the simplest and most commonly experienced type of control. A typical example is the thermostatically controlled domestic immersion heater. Depending on the temperature of the water in the tank, the power supply to the heater is either on or off. In The Virtual Control Laboratory and Case Study Section there is an experiment which shows on-off control of the temperature in a hot water tank.

Advantage

Advantage of this type of control action is that it is inexpensive and extremely simple.

Disadvantage

The disadvantage mainly lies in the oscillatory nature of the control, which makes it suitable only for those applications where it can be used alone and close control is not essential.

Note that it is impractical to use on-off control when trying to regulate a flowrate. It is necessary to have some sort of capacity available. This can be illustrated by means of an experiment which uses as a scenario someone taking a shower. Once again the experiment can be found in The Virtual Control Laboratory and Case Study Section

Thus the application of on/off control in industry is severely limited. In most industrial applications continuous control modes, such as those described below are widely adopted.

Proportional Control Action

Proportional action is the simplest and most commonly encountered of all continuous control modes. In this type of action, the controller produces an output signal which is proportional to the error. Hence, the greater the magnitude of the error, the larger is the corrective action applied.

Mathematical Description

Mathematically, proportional control could be expressed as:

Where

V is the adjustment or signal for the adjustment from the controller.

is the error.

= S - L

L is the measured value of the controlled variable.

S is the setpoint.

K is the proportional constant, named as the gain which shows the sensitivity of the control.

Vo is the signal output when no error exists.

The gain is often replaced with another parameter, called the proportional band, PB. 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. What is the relationship between K and PB.

According to this definition we can see that the whole range of the final control element adjustment should be Vmin to Vmax.

At point Vmin

At point Vmax

The error required to move from Vmin to Vmax will be

Therefore

Recall that the proportional band, PB, is defined as the error required to move the final control element over it's whole range expressed as a %. So for the controlled variable, L, with its total range Lmin to Lmax the definition for the proportional band is

or

Therefore we have the relationship between gain K and proportional band PB as

With proportional band, the relationship between the adjustment and the error can be expressed as

It can be seen both from the expression above and by running the experiments in the Virtual Laboratory that the larger the gain K, or equivalently the smaller the proportional band PB, the higher the sensitivity of the controller's actuating signal to deviations will be.

Dynamic Response

Now let's examine the dynamic response of the proportional control. Assume the process is at steady state and the level is at the setpoint. At time = 0, an increase in the inlet flowrate, regarded as a disturbance, enters into the process. If no control action is taken, i.e. the outlet flowrate is not altered, the level (controlled variable) will increase.

With proportional control, the level is brought back and maintained in a certain range near the setpoint. The history curve could typically be like that shown below. Different responses are obtained depending on the proportional band, B, of the controller.

As can be seen the smaller the proportional band the closer to the setpoint the controlled variable becomes but the more oscillatory the response.

Advantages

The advantages of this type of controller are

It is relativly simple and easy to design and tune

It provides good stability

It responds very rapidly

Dynamically it is relatively stable

Disadvantages

From the response curve to a step change in the input two features should be noted. These are two points which make proportional control unsatisfactory.

Offset

For a sustained change of load, the controlled variable is not returned to the original or desired value, but attains a new equilibrium value termed control point. The difference between the control point and the desired value (set point)is referred to as offset.

The reason for this offset with proportional action is that the control action is proportional to the error. Consider the above simple level control system. For the step increase in the flow of liquid into the tank, in order to maintain the level, the valve on the outlet must be opened wider. This will only occur if there is a continuous output from the controller. The output itself can only exist if there is an error signal supplied to the controller. In order to maintain this error, the level will rise above the desired level at the new control point, hence create an offset.

Overshoot

There is a significant time of oscillation, or in other words, overshoot. Although the period of this oscillation is moderate, this, in some cases, could be highly undesirable.

Integral and Derivative Control

Two other common control techniques are used to eliminate the problems found when using proportional only control. These are known as integral and derivative control. Sometimes they are used individually but more often they are combined with proportional control. A more mathematical definition can be found in a later section.

Integral Action

Integral action gives an output which is proportional to the time integral of the error. It is also called reset control. It is possible to use integral action itself, but this is not a common situation.

P and I Control

Integral action is generally applied with proportional control, yielding so-called proportional and integral control (P+I). This combination is favourable in that some of the advantages of both types of control action are available.

The main advantage of P+I is that it can eliminate the offset in proportional control.

The disadvantages of P+I are that it gives rise to a higher maximum deviation, a longer response time and a longer period of oscillation than with proportional action alone. This type of control action is therefore used where the above can be tolerated and offset is undesirable.

Derivative Action

This kind of action gives an output which is proportional to the derivative or the rate of change of the error. It is also known as rate control. This kind of action could not be used alone in practice. This is because its output is only related to the rate of change of the error. The error could be huge, but if it were unchanging, the controller would not give any output. Thus although it is theoretically possible, it is practically impossible.

P and D Control

Derivative control is usually found in combination with proportional control, to form so-called P+D. By adding the derivative action, lead is added in the controller to compensate for lag around the loop, and so P+D can eliminate excessive oscillations. A disadvantage is that it can not eliminate the offset although somehow it makes it smaller.

Proportional, Integral and Derivative Action

In applications, sometimes the above three action are combined together to set up the proportional plus integral plus derivative action, i.e, P+I+D.

This combined action is able to:

eliminate the offset due to the existence of integral action

reduce the maximum deviation and time of oscillation, which is a compromise between the advantage and disadvantage of P+I and P+D

Module 1.6 - Variations on Basic Feedback ControlFeed forward Control

In this configuration, a sensor or measuring device is used to directly measure the disturbance as it enters the process and the sensor transmits this information to the feed forward controller. The feed forward controller determines the needed change in the manipulated variable, so that, when the effect of the disturbance is combined with the effect of the change in the manipulated variable, there will be no change in the controlled variable at all. The controlled variable is always kept at its set point and hence disturbances have no effect on the process. This perfect compensation is a difficult goal to obtain. It is, however, the objective for which feed forward control is structural. A typical feed forward control loop is shown in the figure below.

Another name for feed forward control is open loop control. The reason is that the measured signal goes to the controller parallel to the process. This can be seen in the next figure. This is in contrast to feedback or closed loop control.

As mentioned previously the main advantage of feed forward control is that it works to prevent errors from occurring and disturbances have no effect on the process at all. However, there are some significant difficulties.

Complex Computation

The feed forward control computation involves determining exactly how much change in manipulated variable is required for a specific change in disturbance. To be able to make this computation accurately requires significant quantitative understanding of the process and its operation. There is also a tremendous escalation of the theoretical know-how required in the feed forward controller's computation activities.

Knowledge of Process

The structure of feed forward control assumes that

1. The disturbances are known in advance.

2. The disturbance will have sensors associated with them (measurable).

3. There will not be significant unmeasured disturbances.

This limitation on the disturbances constrains the application of feedforward control, simply as most disturbances in the industrial processes are unpredictable and immeasurable.

Limitations

In pure feedforward control, there is no monitoring on the controlled variable. If the controlled variable strays from its setpoint there is no corrective action to eliminate the error. This makes pure feedforward control somewhat impractical and a rarity in typical process application.

Specific Controller Required

The feedforward controller must be specifically and uniquely designed for the one particular control application involved, because of the necessity of accurate and quantitative calculations.

It can be seen that feedforward control requires a significant increase in technical skills and capabilities. As a result, feedforward control of specific variables is limited to the most economically significant cases. In practical industrial application, only few cases are handle with feedforward control. While the number of application is small, their importance is quite significant.

Cascade Control

The second alternative to simple feedback control is cascade control. In this setup there is

one manipulated variable

more than one measured variable

An inner and outer control loop is formed each with an individual feedback controller. The outer loop controller is also known as the master or primary controller.

The input to this controller is the measured value of the variable to be controlled.

The setpoint is supplied by the operator.

It passes its output signal to the inner control loop.

The inner loop controller is known as the slave or secondary controller.

It measures a second variable whose value affects the controlled variable.

The setpoint is supplied by the output from the outer loop.

Its output signal is used as the signal to the manipulated variable.

The above points can be shown clearly in a diagram.

The major benefit from using cascade control is that disturbances arising within the secondary loop are corrected by the secondary controller before they can affect the value of the primary controlled output. Cascade control is especially effective if the inner loop is much faster than the outer loop and if the main disturbances affect the inner loop first.

Below are described examples of cascade control in practise. It should be noted that in two of the three examples, the secondary loop is used to compensate for flowrate changes. In process systems this is generally the case.

Example 1 - Reactor Temperature Control

In this example the aim is to keep T2 at its setpoint. The primary control loop detects and eliminates changes in T1, the temperature of the reactants. The secondary control loop detects changes in the temperature of the cooling water. Hence it can adjust the flow accordingly before the effects are detected by the primary control loop. If there was no second controller the effect of the cooling water would take a long time to materalise and hence eliminated.

Example 2 - Distillation Bottoms Temperature Control

In this example the primary loop detects changes in the temperature brought about by changes in composition, pressure, etc. The secondary loop detects changes in the steam flowrate and hence eliminates anticipated effects on the temperature.

Example 3 - Heat Exchanger Temperature Control

This is similar to example 2. The aim is to keep T2 constant. Again the secondary loop is used to compensate for flowrate changes.

Split Range Control

The final alternative to simple feedback control to be discussed in this section is Split-Range Control. This is distinguished by the fact that it has

one measurement only (the controlled variable)

more than one manipulated variable

The control signal is split into several parts each associated with one of the manipulated variables. A single process is controlled by coordinating the actions of several manipulated variables, all of which have the same effect on the controlled output.

Below are described two situations where split-range control is used in chemical processes.

Example 1 - Control of Pressure in a Reactor

The aim of this loop is to control the pressure in the reactor. It may be possible to operate this system with only one of the valves but the second valve is added to provide additional safety and operational optimality.

In this case the action of the two valves should be coordinated. Thus for example if the operating pressure is between 0.5 and 1.5 bar then the control algorithm could be

If the pressure is below 0.5 bar then valve 1 is completely open and 2 is completely closed.

If the pressure is between 0.5 and 1 bar then valve 1 is completely open while 2 is opened continuously as the pressure rises. Note that both these actions lead to a reduction in pressure.

If there is a large increase in pressure and it rises to above 1 bar then valve 2 is completely open while 1 is closed continuously.

If the pressure reaches 1.5 bar then valve 1 is shut and 2 is open.

A graph of these valve positions with respect to pressure is shown below.

Example 2 - Control of Pressure in a Steam Header

The aim of this control loop is to maintain a constant pressure in the steam header subject to differing demands for steam further downstream. In this case the signal is split and the steam flow from every boiler is manipulated. An alternative manipulated variable could be the steam production rate at each boiler via the firing rate. A similar control scheme to the above could be developed for the pressure control of a common discharge or suction header for N parallel compressors.