Programmable Logic Controllers

Learning Outcome
Upon successful completion of this module, the student will be able to describe the major components of programmable logic controllers (PLCs), their basic functions, and the types of tasks they perform.
Enabling Objectives
The student will be able to:
(a) Define the basic PLC applications.
(b) Define discrete-state process control.
(c) Define the sequence of events.
(d) Define flow charts in terms of the sequence of events.
(e) Define ladder logic as applied to control the sequence of events.
(f) State how to program the PLC using ladder logic.

INTRODUCTION
The majority of industrial process control installations involve more than simply regulating a dynamic variable, such as the pressure in a vessel or the level in a tank These simple control situations have applications in industry and are referred to as continuous control. There also are a great many processes in industry in which it is not a variable that has to be controlled, but a sequence of events. For example, a certain startup sequence must be followed before a boiler can run continuously. This startup sequence is made up of many discrete states, each discrete state specifying a condition of the process. An example would be the sequence for purging a boiler: dampers fully open, gas valve off, fan running, and so on.
Programmable logic controllers (PLCs) were originally designed to control processes that required a sequence of events to be followed. Today, PLCs have advanced to the stage of being incorporated in SCADA (Supervisory Control and Data Acquisition) systems, continuous control systems, and the like. However, for an introductory look into the operations of a PLC, this module will concentrate on discrete control, using PLCs to control a sequence of events.

COMPONENTS OF A PROGRAMMABLE LOGIC CONTROLLER
For the purposes of this discussion, the major components of a typical PLC will be broken down as follows:

1. Definition of inputs/outputs.
2. Sequence of events.
3. Flowcharts of the event sequence.
4. Ladder logic.
5. Programming a PLC.

Inputs/Outputs
PLCs are capable of inputting information from the process, and based upon theinformation at a particular point in time, generating a control signal. Fig. 1 shows a block diagram of the I/O (inputs/outputs) of a typical PLC.

Figure 1: Inputs/Outputs of Typlical PLC

The following terms are commonly used in the industry:
DIs - Discrete inputs are signals which have only two positions; for example, open/closed or on/off.
Als - Analog inputs are ever-changing signals, such as temperature. The temperature can be any value between its upper and lower range.
DOs - Discrete outputs are the control signals for devices which have only two positions; for example, to turn a pump on or off.
AOs - Analog outputs are ever-changing signals which control the process. An example is the control of a valve to perhaps 30% flow, or some other value between its upper and lower range.
It should be noted that the remainder of this module concentrates on the use of DIs and DOs. Other I/0s are also available on some PLCs, but the I/0s summarized in Fig. 1 make up the majority of PLC points.

Sequence of Events

Fig. 2 shows a schematic diagram of a simple boiler, illustrating the points to be monitored and controlled.

Figure 2: Boiler Schematic

LS - Level switch low(drum level low)
GH - Gas high pressure
GL - Gas low pressure
DS - Damper fully open (proof)
FS - Flame status
IC - Ignition control
The startup sequence for a boiler of this type is as follows:

1. Receive the start signal.
2. Check gas pressure high and low; if high or low, abort.
3. Check drum level; if low, abort.
4. Open dampers fully, if they fail to open, then alarm.
5. Start combustion fan.
6. Wait 30 seconds (the time could vary).
7. Position dampers to minimum fire position.
8. Start ignition
9. Open gas pilot valve.
10. Check for flame. If flame does not establish itself within 6 seconds, alarm and proceed with post purge.
11. Everything is normal; the pilot is established and the boiler can operate under another control system, such as modulating control.

Fig. 3 is a block diagram of a typical PLC, showing the same I/O points.

Figure 3: A Typical PLC

These I/O points are hardwired into the PLC and, under static conditions, do not change. The sequence of events listed has to be entered into the PLC, so that the PLC can control the boiler. Flow charts may be used in order to define the sequence of events into a logical flow.
Flowcharts
Flow charts are used to describe the sequence of events, to eliminate the possibility of ambiguities, and to more precisely document these sequences. Flow charts basically consist of three symbols, as shown in Fig. 4.

Figure 4: Three Symbols

 Although there are many sophisticated types of flowcharts and symbols, the symbols listed in Fig. 4 are adequate to describe the boiler example covered in this module. A suitable flow chart describing the boiler’s sequence of operations is shown in Fig. 5. This flow chart clearly defines the sequence of operations as started in the English language description. This flow chart now has to be described in terms that the PLC can understand, namely ladder logic.

Figure 5: Flow Chart of Boiler Startup

Ladder Logic
Ladder logic, or ladder diagrams, are used extensively when programming PLCs, but these are not the only method of implementing control sequences. Since it is so widely accepted, some of the simpler tasks performed by ladder logic will be described. The symbols that will be used to implement the boiler example are listed in Fig. 6.

Figure 6: Symbols

Rather than discuss the design methods used in developing ladder logic circuits, we will look at a suitable ladder logic diagram which could be used to control the boiler example. It is not intended that students understand the ladder logic completely, but rather that the student be exposed to the concepts of ladder logic. A suitable ladder logic diagram for the boiler control is shown in Fig. 7.

Figure 7: Ladder Logic Diagram

Programming and Using a PLC
Now that the sequence of events has been defined in terms of ladder logic, this has to be transferred into a PLC. Fig. 8 shows the connection of a PLC programmer to a PLC. A PLC programmer is a device which connects to the PLC so that someone can enter the ladder logic directly into the PLC.
Various forms of PLC programmers exist, but the point to note is that some physical device must be used to configure or program the PLC to carry out the proper sequence of events. The sequence can be altered any time a PLC programmer is connected. Normally, however, after the ladder logic has been entered, the PLC programmer is disconnected and the PLC is left to operate the process. Hence the PLC now becomes a dedicated controller. Under normal conditions, the PLC operates the process (in this case, the boiler) from the I/O points and the boiler start switch. PLCs are not limited to controlling a single process but can, if need be, control several processes at a time.

Figure 8: Connection of a PLC Programmer to a PLC

Basic Control Loop Components

Learning Outcome

Upon successful completion of this module, the student will be able to describe the basic types and functions of transmitters, recorders, controllers, and control valves.

Enabling Objectives

The student will be able to:

(a) Describe the operation and construction of a pneumatic transmitter.

(b) List controller types and terms, and describe the application of each.

(c) Describe the basic types of process recorders.

(d) Describe the construction of control valves.

INTRODUCTION

In Learning Module 027-11-02-06, Introduction to Process Measurement, some common measurement devices were discussed. This module will present a brief survey of the other major components of control loops, with some typical examples of each. It will deal mainly with pneumatic control loop devices, since these are among the simplest to understand. Although most control loops today are electronic or electronic/digital, their principles are similar to those of the earlier pneumatic devices. Furthermore, many control loops, such as local level controls, still employ pneumatic components.

TRANSMITTERS

Transmitters are used to send information about some process variable to a desired location, such as a central control room, when the sensing elements are located at various distant locations.

Pneumatic transmitters similar to those shown in Fig.1 can be used to transmit signals over long distances. The mechanism basic to all pneumatic transmitters is the flappernozzle-restrictor assembly shown in Fig. 1 (a). Process conditions affect the position on the link, and this motion is converted into a varying pneumatic output.

A regulated air supply equivalent to about 140 kPa (20 psi) is furnished continuously, and allowed to expand through a restriction which has a smaller bore than that of the nozzle. If the flapper is moved far away from the nozzle, the air will bleed out of the nozzle faster than it can pass through the restriction, so the pressure below the capsule will be zero. When the flapper is moved towards the nozzle, the increased nozzle pressure will cause the capsule to move upward slightly to reduce the exhaust opening, and to increase the opening for the air supply to pass to the output and the feedback bellows. An increase in output pressure will cause the feedback bellows to expand slightly to the left to reposition the top end of the flapper. Reverse action takes place if the flapper-nozzle clearance is increased. The output of the transmitter is directly proportional to the flapper-nozzle clearance and nozzle pressure.

The link of this basic transmitter is connected to a primary sensing element, such as a bourdon tube or bellows. As the process variable changes from minimum to maximum desired value, the output of the transmitter will increase in 20 to 100 kPa (3 – 15 psi).

Figure 1: Basic Pneumatic Transmitter

 

Fig. 1(b) illustrates a simplified commercial differential pressure transmitter that can be used for level and flow measurement. For flow measurement, the high-pressure side would be connected to the upstream side of the flow restriction in the line. An increase in flow will lead to an increase in differential pressure. This, in turn, will cause the capsule or diaphragm to move slightly to the left, and there will be slight clockwise rotation of the force bar that acts as a flapper.

This action causes a proportional decrease in the flapper-nozzle clearance, and an increase in the transmitter output line pressure that will be proportional to the increase in differential pressure across an orifice plate. The feedback bellows will expand slightly with an increase in output pressure to increase the flapper-nozzle clearance a small amount to ensure proportional action of the transmitter.

When this transmitter is applied to level measurement, the high-pressure side is usually applied to the tank level, so that an increase in level will cause a proportional increase in transmitter output.

Fig. 2 shows how a differential pressure transmitter looks when installed in a flow measurement system. Note the sensing lines from the high and low-pressure sides of the orifice plate connecting to the transmitter, and the supply and output air lines going to and from the transmitter.

Figure 2: Pneumatic Flow Transmitter

INDICATORS AND RECORDERS

There are many ways of presenting or indicating the values of variables that are measured. At times a simple mechanical indicator such as a Bourdon tube is quite adequate, but at other times it may be desirable to have a permanent record of performance that can be used for future reference.

Recorders

Recorders are used to maintain a permanent record of certain variables for future reference. The two most common types are the circular chart and the strip chart recorders.

1. Circular Chart Recorders

The circular chart recorder is an earlier design than the strip chart type. One hat is designed to measure static pressure is illustrated in Fig. 3.

Figure 3: Circular Chart Recorder

A spiral Bourdon tube is connected to a recording pen through a link-lever mechanism. When the pressure in the sensing element increases, the movement of the pen will be proportional to the change in pressure in the Bourdon tube.

The chart is in the form of a circular paper disc with a hole in the centre which fits over the chart drive motor. It also has concentric circles that form the scale on which the variable is read. The "time arcs" laid out at uniform distances divide the full or concentric circles into appropriate time intervals of the total period. The chart usually makes one revolution every 24 hours, although some charts rotate in up to 7 days. The clock may be powered by manually wound springs, by electricity, or by pressurized air or gas.

2. Strip Chart Recorders

Over the years, very little has been done to reduce the size of circular chart recorders, but in the design of strip chart recorders, similar to the one in Fig. 4, there has been a marked trend to size reduction.

Figure 4: Strip Chart Recorder

The standard strip chart recorder is characterized by the uniform linear motion of a strip of paper in a vertical or horizontal direction, as shown in Fig. 4. The time lines always run perpendicular to the direction of motion of the chart, while the measurement lines can be straight or curved.

Illustrated here is a three pen recorder that can record three different variables at once. Each pen on this type of recorder is generally operated by a different transmitter output. Instead of placing the primary sensing element directly in the recorder, as in the circular chart type, the output of a transmitter, which is proportional to the process variable, is admitted to the recorder. In most pneumatic recorders of this type, a bellows or capsule is used to position the recording pen.

CRT Trend Displays

Computerized control systems can display process conditions on the monitor or screen (often referred to by its technical name, a Cathode Ray Tube or CRT). The display looks similar to a strip chart record; several trends can be placed on the same screen and a printout can be taken if desired.

This system has both advantages and limitations when compared to the dedicated, paper type recorders discussed above. The operator has the flexibility to assign any transmitter output from the process to the trend display - provided, of course, that the transmitter is connected to the computer system. This means the operator can see the trend on almost any process condition; including the trend on things which are not normally recorded, such as valve positions, ambient weather conditions, and machinery speeds. Time periods can be assigned for the graph, stretching or compressing it, to get a closer look at Rapidly changing conditions.

Graph limits can be set higher or lower, so that small changes can be closely watched. The operator can also select the process conditions that appear on the screen, placing one above or below the other. This makes it possible to diagnose operating problems, since, by visual comparison, the first upset condition and then the subsequent effects can be determined.

The major limitation of this system is that the computer can only show one screen at a time. If it is necessary to access other displays, such as control loops, the operator may need to go back and forth between screens; this is a time consuming procedure when plant conditions are upset or changing quickly. For this reason, computer control panels are often supplied with a few dedicated recorders, which monitor critical operating conditions on a continual basis.

CONTROLLERS

As mentioned in Learning Module 027-11-02-04, Introduction to Instrumentation, the controller is a key device in a control loop. It compares the actual process conditions to a desired operating value, called the setpoint. If there is any difference between setpoint and actual conditions, it sends a corrective signal out to a control valve or other similar device.

Controllers can be organized into several categories, depending on how they react to maintain a setpoint. The first category includes on-off, two-position, and multi-position controllers. The second category, sometimes referred to as "modulating", includes proportional, integral, and derivative controllers. A detailed examination of these types is beyond the Fourth Class level; however, brief look at how each of these controllers responds is included here.

On-Off, Two-Position, and Multi-Position Controllers

These controllers compare actual conditions to setpoint values, and respond by either starting or stopping a final control element. Familiar examples are the common household thermostats on furnaces and refrigerators. The furnace remains off until a minimum temperature is reached. When the furnace turns on, fuel is admitted to the main burner at a constant rate. The main fuel valve is wide open during this period. This continues until the temperature reaches a higher cut-out point, when the main fuel valve closes completely.

Two-position and multi-position controllers react the same way, except that their output signals may be high-low, or high-medium-low, rather than on-off.

Proportional Controllers

A proportional controller also compares actual conditions to setpoint; however, unlike the on-off type, it sends a variable signal to the control valve, causing it to be open, closed, or somewhere in between. The control valve position is modulated. The controller output signal will vary depending on how much the actual conditions differ from setpoint. The controller output is thus proportional to the amount of error. The output signal from a pneumatic controller to a control valve will be between 20 - 100 kPa (3 - 15 psi). Both electronic analog and computer type controllers send out 20 -100 mA signals.

Proportional Plus Integral (Reset) Controllers

For reasons beyond the scope of this module, proportional controllers are not always capable of operating the process exactly on setpoint, although they are generally close. To operate exactly on setpoint, a modification called integral, or reset, action is required.

Proportional Plus Integral plus Derivative Controllers

Further modifications can be made to the proportional plus integral controller by adding derivative (also called "rate") action. This enables the controller to respond more quickly, almost like a temporary on-off controller, if the process begins to deviate widely from the setpoint. Derivative action is usually restricted to slow-responding processes such as heating systems.

Additional Controller Terms

The student should also become familiar with the following controller terms

2. Auto/Manual Control

Most (but not all) controllers have the option of selecting either automatic or manual control. Automatic, as discussed above, means the controller responds to differences between setpoint and actual conditions. Manual control means that the operator selects the controller output; the controller does not respond on its own to the process. Manual control is necessary, for example, when the control loop transmitter is being repaired or calibrated.

3. Feedback/ Feedforward

Most control loops respond to a specific process condition after the condition has changed. Since this information is fed back to the controller, it is called a feedback system. In some specialized control systems, the controller is alerted ahead of time that a change in the process is about to occur. The controller responds before the change takes place. This is referred to as feedforward control.

4. Cascade Control

In some control systems the output signal from one controller becomes the setpoint signal for another controller. Fig. 5 shows the relationship between single loop and cascade control.

Figure 5: Cascade Control

Introduction to Control System

Learning Outcome

Upon successful completion of this module, the student will be able to discuss the principles of control systems and compare differing controller terminology.

Enabling Objectives

The student will be able to:

(a) Describe the operation of an on-off controller.

(b) Differentiate between direct-acting and reverse-acting control.

(c) Define the term offset.

(d) State the purpose of integral mode in a controller.

(e) State the purpose of derivative mode in a controller.

(f) Describe the operation of a proportional controller.

INTRODUCTION

There are several variations in the design of control systems, each producing a different form of controller response to any particular error. These differing responses are referred to as control modes. In industry, most control actions fall within one of these four modes:

1. On-off action, sometimes denoted as two-position control, and usually used alone
2. Proportional action
3. Reset, or integral, action
4. Rate, or derivative, action

Certain combinations of these actions or modes are widely used, such as proportional plus integral, or proportional plus integral plus derivative.

CONTROL MODES

The mode of control chosen for a particular process loop will depend upon many factors, such as:

• Economics
• Preciseness of control required.
• Combined response delay of all the components in the loop
• Safety needs of operating personnel and process equipment.

On-Off Control

By definition, on-off control means that the controller output is at either its minimum or its maximum. The controller will never maintain the control valve in an intermediate or throttling position, If recognition of the existence and the direction of the control error is the basic requirement for a controller, it follows that the simplest controller of all is one which responds only to these two aspects of an error. The simplest of all controllers will respond by moving the control valve as soon as an error is sensed, and it will respond in the correct direction. However, this is as far as it will go. The extent of its response will not be related to the size of the error. In fact, whether the error is large or small makes no difference. The simplest controller always moves the control valve stem the same amount, and this means all the way to its wide open or shut position.

On line, an on-off controller will control the process variable, but continuous cycling will occur. For example, in a temperature control system the control valve will close when the temperature goes above the set point (positive error); when the temperature drops below set point, the valve controlling the steam flow to the heat exchanger will open wide. This results in temperature fluctuations and some overshooting of the process variable above the set point, followed by undershooting, which is illustrated in Fig. 1.

Figure 1: Process Variations

The temperature will oscillate about the set point with an amplitude and frequency that is dependent on the capacity and time response of the process. As the process lag approaches zero, the curve will tend to become a straight line; then, the frequency of the control valve open/close cycle will become high. The response curve will remain constant in amplitude and frequency, as long as the load on the system does not change.

Fig. 2 illustrates a pneumatic on-off controller. As the process variable, which in this case is the transmitter output, exceeds the set point pressure, the flapper is moved closer to the nozzle and the controller output increases. Since there is no controller feedback, as explained under transmitter principles, a slight change in the process variable will cause the controller output to go from minimum to maximum or vice versa.

Figure 2: On-Off Controller

Any controller can be converted from direct-acting, where an increase in error signal in a positive direction will result in increased output, to reverse-acting, where an increase in error signal causes decreased output, by interchanging the set point and process variable signals shown in Fig. 2.

During normal operation of the on-off controller, a neutral zone exists in which no control is initiated, as shown in Fig. 3. For example, if the set point is set at 50% of the temperature range, the process variable (temperature) may rise to 50.5% before the control valve is closed by the controller, and the valve may not be opened before the process variable drops to 49.5%. In the neutral zone of 1% width, valve position depends on the direction of change of the process variable.

Figure 3: Controller Neutral Position

Occasionally, the neutral zone is made adjustable, but the error must always change through some small value before control action occurs. Lost motion in the linkage, along with friction in the controller and control valve, win tend to increase the neutral or dead zone.

The following requirements are necessary for on-off control to produce satisfactory results:

• Precise control must be needed.

• The process must have sufficient capacity to allow the control valve to keep up with the measurement cycle.

• Energy inflow is small relative to the energy already existing in the process.

For proper on-off control, the speed of load changes must be fairly slow and the size of load change should be small. Under these conditions, an on-off controller can be applied with reduced installation costs, if variation of the process from the set point will not affect the final product.

On-off control is frequently found in air conditioning, refrigeration, and home heating systems. It is also widely used in safety shutdown systems to protect process equipment or operating staff.

On-off control is almost always the simplest and least expensive form of automatic control. It can be implemented with commercially-available mechanical, pneumatic, or electronic instrumentation.

Proportional Control

With on-off control, the controller output is either at minimum or maximum, and the controller cannot maintain the process variable at the desired condition. By using a proportional controller, the position of the final control element can be maintained anywhere between the two extremes; therefore, a smoother action can be expected.

Proportional-only control is a term usually applied to any type of control system where the absolute value of the position of the control valve is determined by the relationship between the measured variable and the set point. Change in output (corrective action) is proportional to the size of the error signal, or proportional to the deviation from the set point, so there is one and only one position of the final control element for every value of the process variable. Hence the name proportional controller.

Consider a very simple form of level control, as shown in Fig. 4, where a float operates a control valve at the input of the water supply to maintain the level in the tank. Assume the valve is closed when the tank is full, and fully open when the tank level falls to a minimum; also assume that valve opening causes a linear relation with the flow. For example, 25% valve opening causes 25% flow, 50% opening causes 50% flow.

Figure 4: Simple Proportional Control

If the output rate of liquid from the tank is 200 L/min, one can adjust the turnbuckle on the valve linkage until the set point is at 50% of maximum level. With this condition, the input and output flows would be equal. As the discharge rate is increased to 300 L/min the level in the tank drops, causing the float to drop. This, in turn,- increases the input valve opening so that the inflow is equal to the outflow. Now the level will stabilize below the original set point. Similarly, if the discharge rate is reduced to 100 L/min the level will stabilize above the set point. A change in the level (process variable) must take place before the final control element (the valve) can be repositioned. The difference between the set point and the actual value of the process variable is known as offset. Offset is an inherent characteristic of all proportional-only controllers, and may be defined as a sustained error which cannot he eliminated by means of the proportional mode of control, as shown in Fig. 5.

Figure 5: Offset as a Characteristic of Proportional Controllers

If the pivot F in Fig. 4 is moved to the left, so that the ratio of the lever arm AF/FB is decreased, a smaller change in flow will cause the control valve to go from minimum to maximum opening, and the offset will be reduced.

Fig. 6 shows a moment-balance pneumatic proportional controller. It is actually an on-off controller with a negative feedback bellows added.

Figure 6: Moment

For the controller shown in Fig. 6, initial discussions will assume that the pivot point is adjusted so that L1 and L2 are equal; also, the set point and process variable are both adjusted to a minimum value (assume a 20 to 100 kPa range is used), then adjust the force spring so the controller output is at the minimum value of 20 kPa.

When the process variable increases above the set point, the increase in output will bear a linear relationship with the deviation (process variable minus the set point pressure). As the process variable increases to the maximum value of 100 kPa, the controller output will also increase to be at maximum, as shown in Fig. 7; thus an 80 kPa deviation in the process variable will cause the controller output to increase by 80 kPa. With a proportional controller, this deviation is often referred to as the offset.

The output of the controller (V), or the valve position, is directly related to the process variable (PV). When the process variable goes through its full range of values, the controller output does likewise, and the final control element strokes through 100% of possible opening. The percent of the process variable range that causes 100% change in controller output is often called the proportional band.

Figure 7: Controller Output versus Process Variable

In the above example, the proportional band is 100%, because a 100% change of PV will cause a 100% change of V The ratio of change of output (DV) to change of input (DPV) is referred to as the gain (K) of a proportional controller.

Consider what happens if the pivot in Fig. 6 is now adjusted so that L2 /L1=2, and with a 20 kPa set point and PV pressure applied, the spring force is
adjusted so the output is 20 kPa.

Normally, this calibration is not required on an actual controller, but these design features are too complicated to show in a simple sketch. After this adjustment, if the PV input pressure increases above the set point, the PV signal has to increase only to 60 kPa, or 50%, before the output increases to maximum or 100%.

It can be seen that the width of the proportional band or the gain determines the output from the proportional controller, and the amount of valve movement for a given error (the difference between the value of the process variable and the set point). Also, as the gain is increased or the proportional band is made narrower or decreased, the offset of a proportional controller decreases, causing the process to remain closer to the set point.

Consider Fig. 7 with variations in process load. The gain of the controller can be increased only to a certain value beyond which the controller output will start to oscillate like an on-off controller. Any controller with a proportional band of 2% or less may be considered to operate exactly like an on-off controller. In Fig. 4, if the ratio AF/FB is made very small, a disturbance on the water surface can cause the valve to be positioned from the fully closed to the fully open position.

The fact that the proportional band is equal to the percentage change in the process variable (%PV) that causes a 100% change in the controller output (100% V), suggests that the following equation holds true:

Normally, better control of processes is achieved if the controller output is above minimum value when the error is zero, as any final control element such as a valve operates better at about mid-opening. To overcome this effect, a constant spring force, often called manual reset, is imposed by placing an opposing spring opposite to the negative feedback bellows.

When the process variable is at the set point, the clockwise moments will be equal to the counterclockwise moments, so the force provided by the negative feedback bellows must also be equal to the spring force.

The force of the spring can be adjusted to obtain the desired output when the process variable is at the set point, as indicated in Fig. 8.

Figure 8: Proportional Controller

When a proportional controller is used in a process, offset will always exist. As the gain is increased, the offset will decrease; but increasing the gain beyond a certain limit, depending on the process, will cause undesirable oscillation or instability in output and in the value of the process variable. In some processes, offset cannot be tolerated, as it will result in an inferior product. To overcome this problem, the constant spring force, which is actually manually reset, is replaced by automatic reset or integral bellows.

Proportional Plus Reset Control

Integral action, often called reset, is a controller mode that makes a corrective action whose rate of change is proportional to the size of error and the time it lasts. This is accomplished by adding a positive feedback bellows to a proportional controller, which will continue to change the output until the error is eliminated, or until the controller output is at either extreme of its range. This is shown in Fig. 9

Figure 9: Proportional Plus Reset Controller

Assume that a step change is introduced in the proportional plus integral controller shown in Fig. 9, so the process variable (PV) suddenly exceeds the set point. A step change is a vertical rise in PV. The controller output will immediately increase, due to proportional action, by an amount that depends on the gain and the size of error. This will create a pressure differential across the integral restriction valve. As the pressure differential decreases, the increase in force inside the integral bellows causes an increase in output, followed again by an increase in negative feedback, in order to maintain moment balance.

While this integration is occurring, the controller output is increased further than if proportional action were used alone, so that the final control element is moved further, causing the process variable to approach the set point. As the error approaches zero, or the PV approaches the set point, the pressure differential across the integral adjustment valve will also approach zero.

When the PV is at the set point, moment balance is achieved, so that the set point and PV pressures are equal, and likewise the pressure in the negative feedback bellows is equal to the pressure in the integral bellows. If the process variable drops below the set point, the action in the controller becomes reversed.

The capacity tank causes a delay in the integral action by providing a capacitance, and thus providing more stability in control. In proportional plus integral controllers, the offset due to proportional action is eliminated over a period of time.

The rate of change of the corrective output by the integral mode is expressed in terms of the output change due to proportional action alone; also, for any given deviation, the change in proportional controller output will depend on the gain. Integral or reset action is always expressed in terms of the time that it takes for the integral action to reproduce or repeat the output due to proportional action after a step change is introduced. The time that it takes integral action to reproduce the proportional action is known as reset time, expressed in minutes.

Figure 10: Reset Control Action

Integral action can also be expressed in terms of repeats per minute. This is the number of times per minute that the initial proportional action is repeated by integral action.

Reset or integral time can be varied by manipulating the integral adjustment valve. If the valve restriction is increased (assuming PV is above the set point, SP), the pressure in the integral bellows that provides positive feedback will increase more slowly, and the controller output will increase at a lesser rate. If the restriction valve is open wide, the pressures in both feedback bellows will increase almost simultaneously, so the positive feedback will cancel the effects of negative feedback immediately. This will result in a very short reset time, and the output of the controller oscillates like an on-off controller.

The output of a reset (integral) controller varies in accordance with the magnitude of the error signal, the difference between the set point and the controlled variable, and the duration of the error. The term integral originates from the mathematical expression that states that the output of the controller is equal to the time integral of the error signal.

The controller response is illustrated schematically in Fig. 10. Initially, the controlled variable is at the set point value and the system is in equilibrium. At time t, there is a disturbance to the process that changes the controlled variable in the form of a step. This creates a constant error signal in the time period from t1 to t2, which results in the controller output changing at a constant rate in an attempt to eliminate the error signal.

Fig. 10 does not show the controlled variable being brought back to the set point, but this would eventually happen. Basically, the output of this controller will change to any value within the output signal range, to alter the manipulated variable so that the controlled variable is maintained at the set point.

The reset action is adjustable, and it should be noted that the adjustment on a controller might be identified in two different ways:

1. Reset rate in repeats per minute, or the number of times that the error signal is duplicated as a change in the controller output signal within one minute. Electronic controllers have a broad range of adjustment of 0.1 to 100 repeats per minute.
2. Reset time is the time in minutes that is required for the error signal to be duplicated once in the controller output signal.

Fig. 11 illustrates how the gain and reset settings can influence the manner in which the controlled variable is returned to the set point. It is interesting to note that the reaction within the process is almost identical when either the gain or the reset is too high, and it is similar again when these settings are too low.

Figure 11: Proportional Plus Reset Controller Responce

Proportional Plus Reset Plus Rate Control

Rate (derivative) control action is normally used with proportional and reset control modes, but it is shown separately in Fig. 12 to help convey the unique action of this mode of control. Rate action creates a large initial controller output reaction which dissipates with time. Thus, it gives the corrective control action an initial boost, allowing the manipulated variable to change quickly and return the controlled variable to normal or near normal.

The rate adjustment of a controller, which is a measure of how quickly the initial action is removed from the controller output, might range from 0. 1 to 25 minutes.

Rate control is used to counteract the time lags in some processes, such as those involving the transfer of temperature changes. In many processes such as temperature control, there is considerable lag or delay from the time that a change in load or some other disturbance takes place, to the time that the change in the process variable is sensed by any controller. Derivative or rate action, which could not possibly control the process by itself, takes into account the speed at which the variable is deviating from the set point.

Figure 12: Rate Control Action

Fig. 13 illustrates the response of proportional action to a ramp change in the process variable and the controller response when rate action is added. At a certain time, To, the process variable starts to deviate from the set point. In another certain time, T2 (in minutes), proportional action alone will increase the controller output from A to B. If the same controller has rate action added to it at time To, when the process variable starts to deviate from the set point, the rate action will cause a vertical rise in output; then proportional action will cause the controller output to increase steadily, taking the controller only T1, minutes to increase the controller output from A to B.

Rate contribution occurs only when there is a change in rate of error. This change occurs only at time To, because after that the error is changing at a constant rate.

The speed of rate action is known as rate time, TD.

In Fig. 13

TD = T2 - T1

If the process variable starts to deviate from the set point at a faster rate, rate contribution to controller output will increase; also, T1 - T2 will become greater.

Figure 13: Rate Contribution to Controller Output

Introduction of Process Instrumentation

Learning Outcome #1

Upon successful completion of this module, the student will be able to describe the importance of process instrumentation as it relates to industrial production.

Enabling Objectives for Learning Outcome #1

The student will be able to:

(a)     State the three most important operation criteria for process plant instrumentation.

(b)     State the five purposes of instrumentation in a process plant.

Learning Outcome #2

Upon successful completion of this module, the student will be able to demonstrate how simple open-loop and closed-loop control systems are arranged, and identify their major components.

Enabling Objectives for Learning Outcome #2

The student will be able to:

(a)    List and - describe the function of the following control system components: controller, measurement, final control element (FCE), and process.

(b)    Sketch the arrangement of the components in a typical control system.

(c)    Use ISA symbols relating to control systems.

INTRODUCTION

A process plant is an arrangement of processing units, such as heat exchangers, distillation columns, reactors, and absorbers, which are integrated in a systematic and rational manner.  The plant's overall objective is to convert raw material into desired products, by the application of energy, in a safe and cost-effective manner.  A process plant that is designed correctly should address the following criteria:

Safety

The safe operation of the process plant is of primary concern, and the design of the plant should allow for the safe containment of process pressures.  Construction materials should be able to withstand the designated temperatures and chemical concentrations.  In addition, a control system should be specified that is capable of maintaining the process parameters at the design operating points, providing for an orderly equipment shut-down, and providing fail-safe capability.

Production

In order to maximize the return on investment to the plant operators, the design of the process plant should be such that the quality and quantity of the desired product meets or even exceeds the original design expectations.  In order to accomplish this, the process plant must utilize a control system that is capable of providing the necessary production and quality control.

Environmental Standards

The process plant must comply with federal, provincial, and municipal environmental regulations concerning air pollution and plant effluent if the plant is to remain operational, and avoid punitive action by government pollution control agencies.  For example, the plant design should incorporate a method of treating the plant effluent prior to discharge.  Instrumentation can play a large part in pollution control., by monitoring and controlling the discharge of plant effluent.

These requirements demonstrate the need for accurate process measurement, and precise and safe control of process parameters.  This is accomplished by proper design and specification of the elements in each control loop, and also requires skilled installation, configuration, and tuning of each piece of equipment.

PROCESS MEASUREMENT INSTRUMENTATION

The importance of process measurement in a process plant cannot be over emphasized.  Current process control and quality control technology relies on accurate and repeatable measurements, in order to safely produce a return on investment consistent with the design of the process plant.

In general, measurement instruments cannot measure the process parameter directly.  Rather, the device measures some other parameter, and infers that that is the desired parameter.  Consider the following two examples:

* Suppose it is required to read the temperature of a room using an alcohol-in-glass thermometer.  It is clear that, in reality, the expansion and contraction of the alcohol is what is being measured relative to the room temperature.  The height of the alcohol column is, in turn, compared to a calibrated scale and a temperature measurement is inferred.

* If the steam pressure in a boiler drum is to be measured by using a common pressure gauge, in reality the mechanical deflection of the measuring element inside the gauge is what is being measured relative to the steam pressure.  This deflection of the measuring element is converted to angular motion,  a pointer is linked to the measuring element and compared to a calibrated scale as an. inferred pressure measurement.

This phenomenon of inferred measurement causes the instrument to carry out a conversion from one energy form to another; in the case of the room thermometer, from heat energy to mechanical energy.  This conversion process increases the complexity of the instrument and, consequently, increases the potential sources of error.

Fig. 1 illustrates a typical filled-system temperature measurement system.  The instrument utilizes the volumetric expansion and contraction of the f'illed medium, which in turn, deflects a spiral expansion element.  The deflection of the spiral element is able to indicate temperature on a calibrated scale via a linkage.

Figure 1: Filled System Temperature Measurement

An instrument of this type is subject to numerous sources of error, such as:

1. Incorrect immersion of the bulb in the measured medium.

2. Changes in ambient temperature which would expand or contract the capillary.

3. Changes in atmospheric pressure which would affect the spiral expansion element.

4. Differences in elevation between the bulb and the spiral expansion element, resulting in a change in hydraulic head which would cause the spiral expansion element to deflect based on elevation rather than changes in temperature.

5. Incorrect calibration of the linkage mechanism.

There are specific techniques used to compensate for all of these sources of error, but any errors remaining can compound to affect the overall accuracy of the instruments

The most common parameters measured on a process plant are sometimes referred to as the bigfour: pressure, temperature, level, and flow.  These account for about 80% of all measurements on a typical process plant.  The remaining 20% include analytical measurements, and measurements of density, humidity, and speed.

Recording Instrumentation

Process parameters such as temperature, pressure, and flow require continuous measurements in real time.  If review of the measurements is desired, provision must be made to capture the parameters with respect to time.  The recorder shown in Fig. 2 is a device used to accomplish this task, and may take many forms depending on the application.  The usual method is to inscribe the measurement of the parameter on a chart with respect to time.  These charts can be circular or linear, and may be driven by a timing mechanism.  The process parameter is recorded by a pen which leaves a trace on the chart. thus producing a historical record.  The duration of the record is a function of chart speed (time base) and length of chart paper.

Figure 2: Two-Pen Recorder

In addition to recording parameters on a continuous basis, it may be desirable to record process parameters on a time interval basis.  This method is used to store large numbers of process parameters at preprogrammed intervals.  A device called a data logger or historical trend unit is used for this purpose.  The data is compiled continuously and can be stored for days, months, or even years.  From this stored data, long-term trends can be established, and the data can be used to investigate plant failures or other problems.

Indication Instrumentation

To provide visual indication of process parameters to the plant operators, instrument indicators are used extensively in process operations.  The indicators can be the traditional circular dial types, or linear analog and digital displays.  Indicators may be field mounted, or panel mounted in a control room.

The circular indicators are calibrated and ranged to give a reasonably accurate indication of a wide range of process parameters.  The indicator mechanisms . are selected to match the parameter signal and the desired range of measurement, and use a pointer on a calibrated scale to indicate.  In recent years, circular gages have been superseded by linear indicators displaying the process parameter in the form of a bar against a calibrated scale, as shown in Fig. 3.

Figure 3: Dual Indicator

Digital display units, which give a numerical indication of the process parameter, are becoming increasingly common; some indicators may give the operator a choice of analog or digital indication, or may display both simultaneously,

Alarm Instrumentation

The purpose of alarm instrumentation is to detect process parameters that deviate outside predetermined limits.  An alarm instrument loop usually consists of a switch or contact closure device as a field device, and an end device such as a horn, light, or bell.  The alarm loop is normally digital, having two conditions corresponding to an alarm state and a non-alarm state.  The field devices are typically diaphragm-operated pressure switches for pressure/level applications, filled system switches for temperature applications, and mechanical switches to detect flow.  End devices are designed to alert the operator of an undesirable condition by audio/visual means.  The end devices are often concentrated into a panel, known as an annunciator, designed to alert the operator if a field device is actuated, and to indicate its location.

Fig. 4 shows a typical process alarm loop designed to activate when the steam pressure is too high.  The detector is a pressure switch that is activated by closing the switch contacts at a predetermined level.  The end device is an incandescent bulb.  It is possible to combine alarms to give warnings of varying degree.  For example, the alarm loop could be designed to give a warning of low pressure, and specified as a PAL (pressure alarm low); an additional switch could be included in the same loop, set to operate at a lower pressure, and known as a PALL (pressure alarm low-low).

Figure 4: Typical Discrete Process Alarm

PROCESS CONTROL SYSTEMS

The three requirements listed in the introduction - safety, production, and environmental responsibility - suggest the need for continuous monitoring of the operation of a process plant.  In addition, some form of external intervention or control must be applied to ensure that these operations criteria are met.

The operations criteria can be achieved through the specific arrangement of equipment like pumps, process vessels, and valves, and through human intervention by plant operators.

The three main objectives of control systems are:

1. To suppress the influence of external disturbances on a process plant.

2. To ensure the stability and safety of a process plant.

3. To optimize the productivity of a process plant.

Cancelling the Effect of Disturbances

An attempt to suppress the influence of disturbances is the most common objective of a control system, although it is not necessarily the most important one.  This control objective attempts to return the process parameter back to the setpoint as effectively as possible.  The control system is called upon to make changes to the process in order to negate or cancel the effects of any external disturbances.  Fig. 5 represents a process that is designed to maintain a set outflow temperature.

Figure 5: Hot Water Process

In this process, the set outflow temperature (T out) is subject to influences from outside the process.  The disturbances could be varying feed flow (F in) and/or feed temperature to the process.  An increase in flow (F in) would normally require an increase in steam flow (F st,) to compensate for the increase in heat required to maintain the outlet temperature (T out ) at the desired setpoint.  Similarly, a decrease in feed temperature (T in ) would require a corresponding increase of steam flow (F st,) to balance the process.

Process Stability

Ensuring the stability of a process is an important aspect of control.  Without stability control, the behavior of the process can range from virtually inactive, with very little control action taking place, to a process that is totally unstable.  An unstable condition affects safety of plant and personnel, in addition to causing poor productivity.  Some processes are naturally stable or self-regulating; others are naturally unstable, and therefore non self-regulating.

Fig. 6 describes a self-regulating level application.  As the level in the vessel rises, the liquid head increases, raising the pressure difference across the outlet nozzle.  The increased pressure differential across the nozzle results in a greater flow from the nozzle.  At some point, the outflow of material from the process will balance the inflow of material into the vessel, and the level will stabilize and remain at that point until the inflow changes.

Figure 6: Self-Regulating Level Process

An example of a non self-regulating process is depicted in Fig. 7, showing a vessel which has a constant outflow due to the positive displacement pump.  The pump delivery volume is constant and not dependent oil the liquid head.  Since the discharge from the positive displacement pump is constant, a steady state vessel level will exist only when the inflow to the vessel exactly equals the outflow.

Figure 7: Non Self-Regulating Process

If the inflow is allowed to deviate from the steady-state condition by however small an amount, the vessel will either overflow or run dry.  It is clear that a level control system is required in order to maintain the stability of the process.

The Feedback Control Loop

A fundamental part of any industrial control system is the feedback control loop.  It consists of a controller, measurement element, final control element, and the process, as illustrated in Fig. 8. All of the components in the loop are interconnected so that information is passed continuously around the loop.  This is classic closed-loop control and, as long as the loop remains closed, automatic feedback control will exist.

Figure 8: Feedback Control Loop

Feedback is in the form of information gained by monitoring the controlled variable, and comparing the controlled variable signal to the setpoint.  If the feedback causes the difference between the setpoint and the controlled variable to increase, then the feedback is positive.  This situation will cause increasing loop instability, a dangerous and undesirable condition.  However, under negative feedback control, the setpoint (usually a fixed value) and the controlled variable are continually compared and the error between the two diminishes.

If the flow of information around the loop is interrupted in any way, (as, for example, when the controller is placed on manual control), then the loop is said to be open and automatic feedback control ceases.  This situation is illustrated in Fig. 9.

Figure 9: Open Control Loop

1.     Controller

The principle function of a controller is to generate an output signal that will manipulate the final control element in such a way that the equilibrium of the loop will be maintained or restored.  The controller is normally the only component in the loop that is capable of counteracting the effect of disturbances on the process.  The controller can be divided into two sections.  The first section is designed to function as an error detector, where the setpoint is compared to the controlled variable and the direction of the error is established.  The second section manipulates that error by solving a control algorithm, in order to produce an output that is of the correct magnitude.,

Fig. 10 identifies the components of a typical feedback controller.  In performing the control function, the automatic controller acts upon the error (e) which is determined by comparing the setpoint (sp) with the controlled variable (cv).  No control action will take place if no error exists (e = 0).  However, if an error does exist (e=<>0), then the control algorithm will act on that error to produce a change in output (m).  The output is applied to the final control element, which changes the manipulated variable in an effort to reduce the error to zero.

Figure 10: Feedback Controller

The control algorithm is a mathematical equation that is solved in real time, to produce a continuous output of the correct magnitude to restore the process to equilibrium.  The controller adjustments shown in Fig. 10 are manual tuning adjustments that affect coefficients in the control algorithm; they are adjusted to configure the algorithm to generate the most effective output necessary to maintain and restore loop stability.

The direction of the output is determined in the sign of the error.  If the error is plus (+e), then the output of the controller (m) will increase as the error increases.  The converse is also true: as the error decreases, so the controller output will decrease.  The sign of the error (e) can be reversed by switching the inputs of the setpoint and the controlled variable at the error detector.  This switching capability is a standard function of any controller, and is used to ensure that the controller output moves the final control element in the correct direction to maintain or restore loop stability.

2.      Measurement

The primary function of the measurement component is to produce a signal that represents the value of the controlled variable.  This is achieved by the use of a transducer or transmitter that is designed to convert the physical properties of the controlled variable to a standard electrical or pneumatic signal that is acceptable to the controller.  As an example, the controlled variable may be a flowing liquid which is converted to a differential pressure by a head-type flow meter.  In turn, the differential pressure is converted to a standard analog signal by a transmitter.

Table 1: shows typical standard analog signals currently used in the process control industry.

3.     Final Control Element

The final control element (FCE) is the loop component that changes the value of the manipulated variable, and regulates the supply of energy or materials to the process.  The final control element responds to the changes in output (m) of the controller that are necessary to maintain or restore loop equilibrium.  The final control element is typically a valve, but may also be a variable speed conveyor belt, a louvre actuator, a variable speed pump, or some other device.  The.selection and application of the final control element is critical to the performance of the entire loop.  Regrettably, the importance of this component is often overlooked, to the detriment of loop performance.

4.      Process

The process component of the control loop is by far the most complex and is usually the part of the loop that the instrument designer cannot change.  All processes have three major characteristics: capacity, resistance, and dead time.  All three characteristics occur in every process in varying degrees, and all affect the controllability of the process.

Fig. 11 illustrates a single capacity process with minimum resistance, and assumes a measurement dead time of short duration.  This would be an easy process to control, requiring only a single simple control loop.

Figure 11: Single Capacity Process

Conversely, a process with multiple capacities, high resistance, and long dead time, similar to the one illustrated in Fig. 12, could be extremely difficult to control with a simple single control loop.  It may be necessary to apply a multi-loop control configuration in order to maintain acceptable loop stability.

Figure 12: Multi-Capacity Process

In practice, a process will contain multiple capacitance and resistance characteristics.  Depending upon their size and relationship to each other, these characteristics affect the controllability of the loop to varying degrees.  Dead time, also referred to as delay, transportation lag, and distance velocity lag, is rarely found in a pure form, but occurs in combination with capacity and resistance.

In addition to the three characteristics found in all processes (capacity, resistance, and dead time), some processes exhibit non-linear gain characteristics and loop interactions which greatly increase the complexity of the control schemes necessary to maintain loop stability.

Fig. 13 illustrates the loop reaction curves of two processes: one with single capacity as shown by the solid line, and the multi-capacity process shown by a broken line.  By observing the process reaction curves of each process, it is possible to predict the degree of difficulty in controlling each process.  AS the reaction time of a loop increases, as in the case of the multiple capacity process, the degree of difficulty to control and maintain loop stability also increases.

Figure 13: Control Loop Response

Little can be done to improve the controllability of a process once the physical plant is in place.  It is, therefore, necessary to specify and correctly tune all loop components, especially the controller, in order to balance the effects of the process upsets.  Knowledge of process characteristics, and the ability to implement an effective control strategy to maintain loop stability, are essential to the study of industrial instrumentation.