process control suggestion

8/12/2019 Process Control Suggestion

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1. What is good control? What is the function of quarter decay ratio?The most commonly defined criteria for a process control system to be described as a

good control system as described below:

(i) The decay ratio should be 0.25, where decay ratio is defined as the ratio of thesecond peak overshoot and the first peak overshoot.

(ii) The integral of the square error (ISE) should be minimum. The ISE is defined as: .

(iii) The integral of the absolute value of error (IAE) should be minimum. The IAE isdefined as:

.(iv) The integral of time-weighted absolute error (ITAE) should be minimum. The ITAE

is defined as:

.

Function of quarter decay ratio:

The decay ratio is defined as the ratio of the second peak overshoot to the first peak

overshoot of a system having transient response. The Ziegler-Nichols, Cohen-Coon and

many other controller tuning is based on tuning the controller to achieve the quarter

decay response. The characteristic of the quarter decay response is that each oscillation

has an amplitude that is one-fourth of the previous oscillation. It is illustrated in the

following diagram.

2. What are the basic functions of pneumatic controller and which are the basicparameters of pneumatic controller? Describe the pneumatic PI controller with

proper diagram.

Principles and working parameters of Pneumatic Controllers:

In a pneumatic system, information is carried by the pressure of gas in a pipe. If we havea pipe of any length and raise the pressure of gas in one end, this increase in pressure

will propagate down the pipe until the pressure throughout is raised to the new value.

The pressure signal travels down the pipe at a speed in the range of the speed of sound


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in the gas (say, air), which is about 330 m/s (1083 ft/s). Thus, if a transducer varies gas

pressure at one end of a 330-m pipe (about 360 yd) in response to some controlled

variable, then that same pressure occurs at the other end of the pipe after a delay of

approximately 1 sec. For many processescontrol installations, this delay time is of no

consequence

In general, pneumatic signals are carried with dry air as the gas and signal information

adjusted to lie within the range of 3-I5 psi. In SI unit systems, the range of 20-100 kPa is

used. There are three types of signal conversion of primary interest. This is usually

derived from a regulated air supply of 20-30 psi. As usual, we use the English system

unit of pressure because its use is so widespread in the process-control industry.

Eventual conversion to the SI unit of N /m2 or Pa will require some alteration in scale (of

measurement) to a range of 20 to 100 kPa.

Pneumatic proportional-integral controller:

Pneumatic proportional-integral controller control mode is also implemented using

pneumatics by the system shown in Figure.

In this case, an extra bellows with a variablerestriction is added to the proportional

system. Suppose the input pressure shows a

sudden increase. This drives the flapper

toward the nozzle, increasing output pressure

until the proportional bellows balances the

input as in the previous case. The integral

bellows is still at the original output

pressure, because the restriction prevents

pressure changes from being transmitted immediately. As the increased pressure on the

output bleeds through the restriction, the integral bellows slowly moves the flapper closer

to the nozzle, thereby causing a steady increase in output pressure (as dictated by theintegral mode). The variable restriction allows for variation of the leakage rate, and hence

the integration time.

3. Explain with block diagram the feed-forward control and ratio control. What aretheir differences?

Feedforward Control:

Conventional feed-back loops can

never achieve perfect control. It is

difficult for the conventional loops

to keep the process output

continuously at the desired setpoint

value in the presence of load or

setpoint changes. This is because of

feedback controller reacts only after

it has detected a diversion in the

value of the output from the desired

setpoioint. Unlike feedback systems

a feed-forward control configuration

measures the disturbance directly and takes control action to eliminate its impact on theprocess output. Feed-forward controllers have the theoretical potential for perfect control.

In feed-forward control strategy, information concerning one or more conditions that

might disturb the control variable is converted into corrective action to minimize

Controller Disturbance

Process Controlled

OutputManipulated

Variable


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deviation of control variable. The signals which have the potential to upset the process

are transmitted to the controller. The controller makes appropriate computation on the

signals and calculates new values for the manipulated signals and sends those to the

final control element, therefore, the control variable remains unaffected in spite of load

changes. The generalized block diagram of feed-forward control system is shown in the

figure alongside.

Ratio Control:

Ratio control is used to ensure that two or more

flows are kept a constant ratio even if the flows are

changing. Ratio control is special type of feed-

forward control where two disturbances are

measured and held at constant ratio with each

other. Sometimes the control of one of the streams

becomes difficult, in which case its flow is

measured; and then, the flow rate of the other

stream is controlled. The stream whose flow rate is

uncontrollable is called wild stream.

The configuration of a basic ratio-control is described below. In this configuration we

measure both the flow rates, and take their ratio. The ratio is compared with the desired

ratio, and the deviation between the two is used to generate the actuating signal for the

ratio controller.

Difference between feed forward and ratio control:

In feed-forward control strategy, information concerning one or more conditions that

might disturb the control variable is converted into corrective action to minimize

deviation of control variable. Whereas, ratio control is special type of feed-forward controlwhere two disturbances are measured and held at constant ratio with each other.

4. Describe the controller tuning method of Harriot.When it is undesirable to allow sustained oscillations, the Harriotts method of controller

tuning is used. The process is characterized by finding the gain at which the system has

a damping ratio of , and the frequency of oscillation at this point. Similar to the Ziegler-

Nichols method, the controller parameters are calculated from the gain and oscillation

frequency.

is the proportional gain for damping ratio of .is the period of oscillations in minutes.

Advantages and Disadvantages: In general, there are two major disadvantages to thedamped oscillation methods. First, it is essentially a trial-and-error method, since several

values of gain must be tested before the ultimate gain or the gain to give a 1/4 decay

ratio are to be determined. To make one test, especially at values near the desired gain, it

is often necessary to wait for the completion of several oscillations before it can be

Type ofController

P 1.1PI 1.1 /2.6

PID 1.1 /3.6 /9


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determined whether the trial value of gain is the desired one. Second, while one loop is

being tested in this manner, its output may affect several other loops, thus possibly

upsetting an entire unit. While all tuning methods require that some changes be made in

the control loop, other techniques require only one and not several tests, unlike the

closed-loop methods. Also, if the tuning parameters are too aggressive, the expected

response can be obtained by increasing the proportional band (or decreasing the

proportional gain). The integral and derivative settings probably need to be modified. The

proportional gain has to be reduced to 3.5 to have a quarter of amplitude decay.

5. What is the valve characteristics?All control valves are classified by relationship between the valve stem position and the

flow rate through the valve. Control valves exhibit and Inherent characteristic and

install or effective characteristic.

Inherent characteristic: This control valve characteristic is assigned with the assumption

that the set point indicates the extent of the valve opening and that the pressure

difference is determined by the valve alone.

(a) Quick Opening: In this type, the relationship between flow and valve opening isapproximately linear up to 60-70% of valve opening. After this limit, the flow

does not change rapidly with the change in the valve opening.

(b)Linear Opening: The flow is directly proportional ro the valve opening for aconstant pressure drop. The relationship thus can be expressed as a straight

line. It is given as:

Where,

Stem position (m) Maximum stem position (m) Flow rate (m3/sec) Maximum flow rate (m3/sec)

(c) Equal percentage: Equal increments of valve movements produces an equalpercentage changes in inflow. The gain of equal percentage valve is directly

proportional to flow through the valve. Gain in low when valve is nearly closed

and gain is high when valve is nearly open.

Installed / Effective Characteristic: The control valve when installed in a process pipeline

downstream and upstream equipment will exhibit a different flow rate stem position

relation and is called installed or effective characteristic.

6. Describe the basic block diagram of PLC. What is ladder logic? Explain with aproper example.

A Programmable Logic Controller (PLC) can be defined as a digital electronic device that

use a programmable memory to storte instructions and implement specific functions

such as logic, sequencing, coiunting, and arithmetic to control machines and processes.

The basic parts of a PLC are described

below:

(i) Processor Module: It is thebrain of the PLC system. The

intelligence of programmable

controllers is derived from the


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microprocessors which have tremendous computing and control ability. The

functions of the programming module includes scanning, program

execution, peripheral and external device communication and self-

diagnostics.

(ii) Input Module: There are many types of input modules to choose from.Choice of the input module depends on the application of the PLC. Most

commonly used inputs are limit switches, proximity switches, and push

buttons. Nature of the inputs can be classified as Analog/Digital, Low/High

frequency, and Maintained/Momentary.

(iii) Output Module: Output modules can be used for devices such as solenoids,relays, contractors, pilot lamps, and LED outputs.

(iv) Addressing Scheme: Each of the input and ouput devices used in the PLCare identified with a unique address for exchange of data, which needs to be

uniquely addressed during the programming of a PLC device.

(v) Programming Unit: It is an external, electronic, handheld device which canbe connected to the processor of the PLC when programming changes are

needed. Once the program is debugged, the programming unit is

disconnected; and PLC can operate the process according to the ladder

diagram or the statement list.

Ladder Logic:

Ladder Logic Programming is a graphical representation of

the program designed to look like a relay logic. It uses

symbols in horizontal rows called rungs, to represent

inputs and outputs. A program in this designing scheme

resembles a ladder, and is therefore, known as the ladder

diagram.

For example, the following diagram the ladder logicrealization where two limit switches connected in series are used to control a

solenoid.

8. (a) What do you mean by controller tuning? What are the different methods of

controller tuning?

The mechanism of selecting different parameters of controllers in a control loop so

that the process variable may be maintained at the desired set-point value without

any fluctuations, is called tuning of controller.


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The most commonly used tuning methods are summarized below:

(b) What are the basic tuning criteria of a controller?

The most commonly defined criteria for a process control system to be described as

a good control system as described below:

(i) The decay ratio should be 0.25, where decay ratio is defined as the ratio of thesecond peak overshoot and the first peak overshoot.

(ii) The integral of the square error (ISE) should be minimum. The ISE is definedas:

(iii) The integral of the absolute value of error (IAE) should be minimum. The IAE

is defined as:

(iv) The integral of time-weighted absolute error (ITAE) should be minimum. TheITAE is defined as:

Where e is defined as the generalized error, such that (d)Explain the tuning criteria of Ziegler-Nicholls method.

The Ziegler-Nichols technique of controller tuning is also called Ultimate Cycling

Method is based on adjusting a closed loop until steady oscillations occur.

Controller settings are then based on the conditions that generate the cycling. This

method is based on frequency response analysis.

Unlike the process reaction curve method which uses data from the open-loop

response of a system, the Ziegler-Nichols tuning technique is a closed-loop

procedure. It goes through the following steps:

(i) Bring the system to the desired operational level


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(ii) Reduce any integral and derivative actions to their minimum effect(iii) Using proportional control only and with the feedback loop closed, introduce

a set point change and vary proportional gain until the system oscillates

continuously. The frequency of continuous oscillation is the cross over

frequency .Let be the amplitude ratio of the systems response at thecross over frequency.

(iv) Compute the following two quantities :

(v) Using the values of & , Ziegler & Nichols recommended the followingsettings for feedback controllers.

Proportional Proportional-Integral Proportional-Integral-Derivative

The settings above reveal the rationale of the Ziegler-Nichols methodology.

(i) For proportional control alone, use a gain margin equal to 2(ii) For PI control use a lower proportional gain because the pressure of the

integral control mode introduces additional phase lag in all frequencies with

destabilizing effects on the system. Therefore lower maintainsapproximately the same gain margin.

(iii) The presence of the derivative control mode introduces phase lead with

strong stabilizing effects in the closed-loop response. Consequently the

proportional gain for a PID controller can be increased withoutthreatening the stability of the system.9. (a) What is process characteristics? Write the definition of manipulated variable,

load variable and control variable with proper examples.

Process Characteristics

A process is defined as a progressive operation that consists of a series of

controlled actions or movements systematically directed towards a desired result.

The features of a process are usually measured by process variables. The control of

process variables is achieved by controllers (hardware elements or softwareprograms) and final control elements like control valves.

The processes are situated in the production environment and are affected by time-

space aspects. These aspects determine the character of the process. The five main

process characteristics are: speed (slow/fast); spacing (lumped/distributed);

continuity (continuous/discrete); periodicity (cyclic/acyclic) and determinacy

(deterministic/stochastic) Time-space aspects also influence the complexity of a

particular process.

The Manipulated variable, Load variable and Controlled variable are defined as:

(i) Manipulated variable:is the one that can be changed in order to maintain the

controlled variable at the set point value. In other words, the variable chosen to

control the system's state is termed the manipulated variable. It is also called


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sometimes as controlling variable. Examples of manipulated variables are coolant

flow, fuel flow, feed water flow etc.

(ii) Load variables: are those variables that cause disturbances in the process.

They are also called disturbances. The load variable may change either

continuously or sporadically with some function of time. Sometimes it is fixed and

not a function of time. Examples are feed rate, feed composition, steam header

pressure, coolant temperature etc.

(iii) Controlled variable: is the one that must be maintained precisely at the set

point. Typically, the variable chosen to represent the state of the system is termed

the controlled variable. Examples of controlled variables are temperature,

pressure, flow rate, level, vacuum pressure, concentration, density etc.

(b) What is the basic process control loop? Explain each block with proper

example.

Process A process is defined as a progressive operation that consists of a series ofcontrolled actions or movements systematically directed towards a desired result.

Measurement To effect control of a variable in a process, we must have

information on the variable itself. Such information is found by measuring the

variable. In general, a measurement refers to the conversion of the variable into

some corresponding analog of the variable, such as a pneumatic pressure, an

electrical voltage, or current. A sensor is a device that performs the initial

measurement and energy conversion of a variable into analogous electrical or

pneumatic information. Further transformation or signal conditioning may be

required to complete the measurement function.

ErrorThe difference between the process output and the desired setpoint value is

the error that is detected by the error detector.

ControllerThe next step in the processcontrol sequence is to examine the errorand determine what action, if any, should be taken. The controller requires aninput of both a measured indication of the controlled variable and a representationof the reference value of the variable, expressed in the same terms as the measuredvalue. The reference value of the variable, you will recall, is referred to as thesetpoint. Evaluation consists of determining action required to bring thecontrolled variable to the setpoint value.

Final Control Element The final element in the processcontrol operation is thedevice that exerts a direct influence on the process; that is, it provides thoserequired changes in thecontrolled variable to bring itto the setpoint. This elementaccepts an input from thecontroller, which is thentransformed into someproportional operationperformed on the process.

Figure alongside shows ageneral block diagramconstructed from theelements defined previously. The controlled variable in the process is


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denoted by c in this diagram, and the measured representation of thecontrolled variable is labeled b. The controlled variable setpoint is labeled r,for reference.The error detector is a subtracting-summing point that outputs an errorsignal to the controller for comparison and action.

10. (a) Describe the construction and working principle of pneumatic control valve.

The pneumatic valve is the most commonly used final control element. It is a

system that exhibits inherent second order dynamics.

Consider a typical pneumatic valve shown in Fig. The position of the stem (or

equivalently of the plug at the end of the stem) will determine the size of the

opening for flow and consequently the quantity of the flow (flow rate).The position

of the stem is determined by the balance of all forces acting on it. These forces are:

Force exerted by the compressed air at the top of the diaphragm; pressure is the signal that opens or closes the valve & is the area of the diaphragm.

Force exerted by the spring attached to the stem & the diaphragm K is theHookes constant for the spring & x is the displacement, it acts upward.

Frictional force exerted upward & resulting from the close contact of thestem with valve packing; is the friction coefficient between stem & packing.

Apply Newtons law and take

()


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( )

Let and and take

The last equation indicates that the stem-position follows inherent second-orderdynamics. The transfer function is

Usually, and as a result, the dynamics of a pneumatic valve can beapproximated by that of first-order system.

(b) What are and? Explain their relationship.One of the most useful factors to determine the size of a control valve in the flow

coefficient or factor (or factor). Practically all control valve manufacturessupply factors for their valves. These factors form the basis for all calculations.

The flow coefficient indicates the amount of flow the control valve can handle under

a given pressure drop across the control valve.

Factor: The flow coefficient is defined as the flow rate of water in gallons perminute at 60

F through a valve at maximum opening with a pressure drop of 1 psi

measured in the inlet & outlet pipes directly adjacent to the valve body.

Factor: Whenever the flow coefficient is mentioned in metric units, it is denotedby the symbol which is defined as the flow rate of water in /hour at about30C flowing through the fully opened control valve at a pressure drop of 1kg/across the control.

The following relationship between & can generally be used. The flow coefficient is determined by the manufacturer for various types & sizes of

valves by actual experiments with water. The flow coefficient for 100% valve

opening is termed as (or ) of the particular valve size & the variation of (or) at different valve openings is given in the form of a graph, which is termed asvalve characteristic.

(c) What are the valve selection criteria?

The selection of control valves for a particular application depends on many

variables; such as the corrosive nature of the fluid, temperature of operation,

pressures involved, high or low flow velocities, volume of flow, and the amount of

suspended solids.


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Careful attention must be paid to the system requirements and manufacturers

specifications, only then can a careful valve selection be made. Some of the factors

affecting the choice of valves are as follows:

(i) Type of valve for two-way or three-way fail-safe considerations, and so on.(ii) Valve size from flow requirements; care must be taken to avoid both

oversizing and under sizing.

(iii) Materials used in the valve construction, considering pressure, size, andcorrosion.

(iv) Materials used in valves range from PVC to brass to steel.(v) Tightness of shutoff: Valves are classified by quality of shutoff by leakage at

maximum pressure. Valves are classified into six classes depending on

leakage from 0.5 percent of rated capacity to 0.15 mL/min. for a 1-in dia.

valve.

(vi) Acceptable pressure drop across the valve.(vii) Valve body for linear or rotary motion, i.e., globe, diaphragm versus ball,

butterfly, and so forth.

(viii) Percentage travel versus flow characteristics plus loop-and-processcharacteristics.(ix) Maximum permissible noise level.(x) Viscosity of fluid.

11. (a) Mention the advantages and disadvantages of cascade control.

The principal advantages of cascade control are the following:

(i) Disturbances occurring in the secondary loop are corrected by thesecondary controller before they can affect the primary, or main, variable.

(ii) The secondary controller can significantly reduce phase lag in thesecondary loop, thereby improving the speed of response of the primary

loop.

(iii) Gain variations due to nonlinearity in the process or actuator in thesecondary loop are corrected within that loop.

(iv) The secondary loop enables exact manipulation of the flow of mass orenergy by the primary controller.

The disadvantages are:

(i)

Cascade control cannot be employed indiscriminately; only when a suitableintermediate variable can be measured does this method of control fit in

properly

(ii) Cascade action fails to yield the desired results if the inner loop is closedaround the largest time constant of the part of the process. In fact, cascade

control is effective only when the secondary time constant is smaller than

the primary time constant.

(b) Why is the feed-forward control better than the feed-back control?

The advantages of a feedforward control over a feedback control are as follows:

(i) Feedforward control scheme compensates for disturbances before they affectthe process, whereas the feedback control waits until the disturbance has

affected the process before taking action.


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(ii) Feedforward control system can improve the reliability of the feedbacksystem by reducing the deviation from setpoint.

(iii) The feedback system is susceptible to disturbances when the process is slowand when significant dead-time is present. On the other hand, the

feedforward control scheme offers noticeable advantages for slow processes

with significant dead-time.

(c) What is multivariable control? Explain with a diagram.

The control system in which there is only one output of the interest is called single

variable system. But in many practical applications more than one variables are

involved. A control system with multiple inputs and multiple outputs in called a

multivariable system. The block diagram representation of a multivariable control

system is shown in the figure. The part of the system which is required to be

controlled is called plant. The controller provides proper controlling action

depending on the reference inputs. There are reference inputs .

There are

output variables

. The values of these variables

represent the performance of the plant. The control signals produced by the

controller are applied to the plant. With the help of feedback elements the closed

loop control of the plant is also possible. Due to the feedback, the controller takes

into account the actual output values to decide the control signals.

In case of multivariable systems,

sometimes it is observed that a

single input considerably affects

more than one outputs. The system

is said to be having strong

Interactions or coupling. This

coupling is nothing but the

disturbances for the separate

systems. The interactions inherently

present between inputs and outputs

can be cancelled by designing a decoupling controller. Thus the resulting

multivariable system is considered to have proper number of single input single

output systems and the controller is designed for each system. The other way is to

design a controller which will take care of all the inherent interactions present in

the multivariable system. In multivariable linear control system, each input is

independently considered. Only one input and one output is considered and the

total effect on any output because of all the inputs acting simultaneously is

determined by addition of the outputs due to each input acting alone. Thus law of

superposition is used to analyze multivariable linear control systems.

12. Write short notes on the following topics:

(a) Process Capacitance

Definition:The Capacitance of a process is a measure of its ability to hold energy

with respect to a unit quantity of some reference variable. It is related to capacitybut is not the same thing two processes with the same capacity might have very

different capacitances.


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Principles of large capacitance: Two principles emerge relating capacitances to

control in the face of load changes:

Large Capacitance tends to keep the controlled variable constant despiteload changes.

Large Capacitance tends to make it difficult to change the variable to a newvalue.

Effects of large capacitance: The overall effect of large capacitance on control is

generally favorable, but it does introduce a time lag between control action and

result.

When a liquid is heated in a vessel, it takes some time for the liquid to reach a

higher temperature after the heat supply is increased. How much time it takes

depends primarily on the thermal capacitance of the liquid relative to the heat

supply.

Capacitance does influence the corrective action required of an automatic

controller, and so it is a major factor in the analysis of any process and control

loop.

Heater Example: Both the heaters shown in figure are used to raise the

temperature of the liquid coming in. In heater A, heat is applied to a jacketed vessel

containing a considerable amount of liquid. The relatively large mass of the liquid

exercises a stabilizing

influence, and resists

changes in temperature

which might be caused by

variation in the rate of

flow, minor variations in

heat input, or suddenchanges in ambient

temperature.

Heater B illustrates a high

velocity heat exchanger.

The rate of flow through

this heater may be identical with that of heater A, but a comparatively small

volume is flowing in the heater at any one time.

Unlike heater A, the mass of liquid is small, so there is less stabilizing influence.

The total volume of liquid in the heater is small in comparison to the rate of

throughput, the heat transfer area, and the heat supply.

Slight variations in the rate of feed or the rate of heat supply will be reflected

almost immediately in the temperature of the liquid leaving the heater.

On the other hand, if a change in temperature of the liquid output was desired,

which heater would give the most rapid change? Heater B would give the most

rapid change if the setpoint were changed.

(b) Valve Positioner


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The main purpose of having a valve positioner is to guarantee that the valve does

move to the position where the controller wants it to be. By adding a positioner one

can correct for many variations including changes in packing friction due to dirt,

corrosion, or lack of lubrication; variations in the dynamic forces of the process

sloppy linkages or non linearities in the valve actuator. The effective dead band of a

valve/actuator combination can be as much as 5% with the addition of a positioner

it can be reduced to less than 0.5%. The function of the positioner is to protect the

controlled variable from being upset by any of the variations. In addition the

positioner can be used for split-ranging the control signal between more than one

valve, for increasing the actuator speed

for modifying the valve characteristics

by cams or electronic function

generators. But these reason do not

necessitate the use of positioners as

they can be achieved by other means

without using positioner also.

The valve positioner is a high-gain

plain proportional controller which

measures the valve stem position

compares that measurement to its set

point & if there is a difference corrects

the error. The open-loop gain of

positioners ranges from 10 to 200

(proportional band of 10% to 0.5%) & their periods of oscillation range from 0.3 10

10 seconds (frequency response of 3 to 0.1 Hz).In other words the positioner is a

very sensitively tuned proportional only controller.

(c) Electronic PI Controller

A simple combination of the proportional and integral circuits provides theproportional-integral mode of controller action. The resulting circuit is shown inFigure 10.16. For this case the relation between input and output is most easilyfound by applying op amp circuit analysis. We get (including the inverter)

()

The definition of PI controller includes the proportional gain in the integralterm, so we can write as follows:

() ()

The adjustments of this controller are:

(i) The proportional band which is adjusted through (ii) The integration gain which is adjusted through .


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(d) I to P Convertor

The current-to-pressure converter, or simply I/P converter, is an important elementin process control. Often, when we

want to use the low-level electric

current signal to do work, it is easier

to let the work be done by a

pneumatic signal. The I/P converter

gives us a linear way of translating

the 4~20-mA current into a 3~l5 psig

signal. There are many designs for

these converters, but the basic

principle almost always involves the

use of a nozzle/flapper system. Figure7.6 illustrates a simple way to

construct such a converter. Notice

that the current through a coil

produces a force that will tend to pull the flapper down and close off the gap. A

high current produces a high pressure so that the device is direct acting.

Adjustment of the springs and perhaps the position relative to the pivot to which

they are attached allows the unit to be calibrated so that 4 mA corresponds to 3

psig and 20 mA corresponds to 15 psig.

(e) Self-regulation process

A significant characteristic of some processes is the tendency to adopt a specificvalue of the controlled variable for nominal load with no control operations. Thecontrol operations may be significantly affected by such self-regulation.

As an example, consider the control ofliquid temperature in a tank, as shownin figure. The controlled variable is theliquid temperature. This temperaturedepends on many parameters in theprocess, for example, the input flow rate

via pipe, the output flow rate viapipe, the ambient temperature, thesteam temperature, inlettemperature, and the steam flowrate. In this case, the steam flow rate


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is the controlling parameter chosen to provide control over the variable (liquidtemperature). If one of the other parameters changes, a change in temperatureresults. To bring the temperature back to the setpoint value, we change only thesteam flow rate, that is, heat input to the process.

(1) Suppose we fix the steam valve at 50% and open the control loop so that nochanges in valve position are possible. 2) The liquid heats up until the energycarried away by the liquid equals that input energy from the steam flow. (3) If the

load changes, a new temperature is adopted (because the system temperature isnot controlled). (4) The process is self-regulating, however, because the temperaturewill not "run away," but stabilizes at some value under given conditions.

An example of a process without self-regulation is a tank from which liquid ispumped at a fixed rate. Assume that the influx just matches the outlet rate. Thenthe liquid in the tank is fixed at some nominal level. If the influx increases slightly,

however, the level rises until the tank overflows. No self-regulation of the level is

provided.

process control suggestion

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