CONTROL SYSTEM

AUTOMATIC CONTROLLER :

It is a device which measured the value of variable quantity or condition and operates to correct or lie it deviation of this measured value from a selected reference.

AUTOMATIC CONTROL SYSTEM :

It is any operable arrangement of one or more automatic controllers in closed loops with one or more processes.

SELF OPERATED CONTROLLER :

It is one in which all the energy needed to operate the final control element is derived from the controlled medium through the primary element.

RELAY OPERATED CONTROLLER :

It is one in which the energy transmitted through the primary element is either supplemented or amplified for operating the final control element by employing energy from another sources.

PROCESS :

A process comprises the collective function performed in and by the equipment in which a variable is to be controlled.

SELF REGULATION :

It is an inherent characteristic of the process which aids in limiting the deviation of the controlled variable.

CONTROLLED VARIABLE :

The controlled variable is that quantity and condition which is measured and controlled.

CONTROLLED MEDIUM :

It is that process energy or material in which a variable is controlled. The controlled variable is a condition or characteristic of the controlled medium. For e.g. where temperature of water in a tank is automatically controlled, the controlled variable is temperature and controlled medium is water.

MANIPULATED VARIABLE :

It is that quantity or condition which is varied by the automatic controller so as to affect the value of the controlled variable.

CONTROL AGENT :

It is that process energy or material of which the manipulated variation is a condition or characteristic. The manipulated variable is a condition or characteristic of the control agent. For e.g. when a final control element changes the fuel gas flow to burner the manipulated variable is flow the control agent is fuel gas.

ACTUATING SIGNAL :

The actuating signal is the difference at anytime between the reference input and a signal related to the controlled variable. This basically known as error signal.

DEVIATION :It is the difference between the actual value of the controlled variable and the value of the controlled variable corresponding with set point.

OFFSET :

It is the steady state difference between the control point and the value of the controlled variable corresponding with set point

CORRECTIVE ACTION :

It is the variation of the manipulated variable produced by the controlling means. The controlling means operates the final control element ( control value ) which in turn varies the manipulated variable.

REFERENCE INPUT :

It is the reference signal in an automatic controller.

SET POINT :

It is the position to which the control point setting mechanism is set.

CONTROL POINT :

It is the value of the controlled variable which under any fixed set of conditions the automatic controller operates to maintain.

PRIMARY FEEDBACK :

It is the signal which is related with the reference input to obtain the actuating signal. Simply stated primary feedback is the actual measurement of the controlled variable which when compared with the desired measurement of the controlled variable produces the actuating signal.

POSITIONING ACTION :

It is that in which there is a predetermined relation between the value of the controlled variable and the position of the final control element.

PROPORTIONAL ACTION :

It is that in which there is a continuous linear relationship between the value of the actual measurement of the controlled variable and the value position.

FLOATING ACTION :

It is that in which there is a predetermined relation between the deviation and speed of final control element.

DERIVATIVE ACTION :

It is that in which there is a predetermined relation between a time derivative of the controlled variable and position of final control element.

REST ACTION :

It is the value movement at a speed proportional to the magnitude of deviation.

RATE ACTION :

It is that in which there is a continuos linear relation between the rate of change of controlled variable and position of final control element. Rate action produces value motion proportional to the rate of change of actual measurement.

PROPORTIONAL BAND :

It is the range of values of the controlled variable which correspond to the full operating range of the final control element.

RESET RATE :

It is the number of times/minute that the effect of proportional position action upon the final control element is repeated by proportional speed floating action.

There are two ways of expressing reset action :

1. Reset time and  2. Reset Rate

1. Reset Rate : It is commonly expressed as a number of "repeats" per minute. It is determined by dividing.

a) Travel of final control element ( Value stroke ) in one minute as a result of the effect of proportional speed floating action.

b) The travel as a result of the effect of proportional position action with the same deviation in both cases.

2. Reset Time : It is the time interval by which the rate is commonly expressed in minutes. It is determined by subtracting.

a) The time required for a selected motion of the final control element resulting from combined effect of the proportional position plus rate action.

b) The time required for the same motion  as a result of the effect of proportional position action alone with the same rate of change of controlled variable in both cases or expressed in another way. It is the time lead in terms of air pressure on the control value produced by rate action compared with proportional position action for the same rate of change of actual measurement in both cases.

Que. :  Explain the application of proportional integral and derivative.

Ans. : 

PROPORTIONAL CONTROL ONLY :

Proportional control only attempts to return a measurement to the set point after a load upset has occurred. How ever it is impossible for a proportional controller to return the measurement exactly to the set point.

USE : It is normally used for level controls. It reduces the effect of a load change but it can not eliminate it.

PROPNT RESET CONTROL :

Reset action is introduced to eliminate offset. It will integrate any difference between measurement and set point and cause the controller's output to change until the difference between the measurement and set point is zero. Reset will act as long as the error exists.

USE: Proportional + Reset controllers are by far the common types used in industrial process control and where predominate dead times occur.

PROPNT RESET + DERIVATIVE :

Derivative or rate action helps the controller overcome system inertia and result in faster, more precise control. Derivative action occurs whenever the measurement signal changes. Under study conditions the rate action does not act. Derivative allows the controller to inject more corrective action.

USED : On temperature controls.

Que. :  What is difference gap control ?

Ans. :  Differential gap control is similar to on off control except that a band or gap exists around the control point.

USE : In industry differential gap control is often found in non critical level control applications where it is desirable only to prevent a tank from flooding or drying. When a measured variable exceeds the upper gap the control valve will open fully or be closed fully. Similarly when it exceeds the lower gap it will open or close fully.

Que. :  Where is on off control used ?

Ans. :  On off control is used when

1. Precise control is not needed.

2. Processes that have sufficient capacity to allow the final operator to keep up with the measurement cycle.

3. It is mainly used in refrigeration and are conditioning systems.

Que. : 

Ans. :  When reset action is applied in controllers Where the measurement is away from the set point for long periods the rest may drive the output to its maximum resulting in rest wind up. When the process starts again the output will no come off its maximum until the measurement crosses the so point causing large overshoots. This problem can be avoid by including antireset wind up circuit which eliminates the problem of output saturation.

Que. :  Why is reset called integral and Rate derivative ?

Ans. :  RESET is called integral because of the mathematical relationship to the output.

RATE is called derivative because

Oi = rd (e) / dt +O0

Oi = Output at any instant

e   = error signal

O0 = Output at zero error.

t     = time.

PROPORTIONAL ACTION :

            Oi = 100 / PB  x  e  + O0

Que. :  Explain tuning of controllers.

Ans. :  Tuning basically involves adjustment of proportional. Integral  and derivative parameters to achieve good control. The gain, time constants, and dead times around the loop will dictate the settings of various parameters of the controller.

Tuning methods are broadly classified into two :

1. Closed Loop Method : e.g. Ultimate Gain Method.

2. Open Loop Method : e.g. process Reaction curve.

ULTIMATE GAIN METHOD :

The term ultimate gain was attached to this method because its use require the determination of the ultimate gain (sensitivity) and ultimate period. The ultimate sensitivity Ku is the maximum allowable value of gain (for a controller with only Proportional mode) for which the system is stable. The ultimate period is the period of the response with the gain set at its ultimate value.

PROCESSORS REACTION CURVE :

To deter mine the process reaction curve, the following steps are recommended. :

1.   Let the system come to steady state at the normal load level.

2.   Place the controller on manual.

3.   Manually set the output of the controller at the value at which it was operating in the automatic mode.

4.   Allow the system to reach the steady state.

5.   With controller on manual, impose a step changes in the output of controller, which is an signal to value.

6.   Record the response of controlled variable.

7.   Return the controller output to its previous value and return the controller to auto operation.

Que. :  Explain the working of an electronic P.I.D. controller.

Ans. : 

Input from the measurement transmitter is compared with the set point voltage to produce a deviation signal. The deviation signal is combined with a characterized feed back signal to provide the input for the function generator amplifier. This amplifiers output is delivered to the feed back network, and to the final output which is a 10-50m.a. do signal for actuation of final operators.

PROPN ACTION : It is a obtained by adjusting the magnitude of feed back signal. An increase in negative feed back means less effective gain and thus a broader proportional band.

REST ACTION : It is obtained by charging the reset capacitor at a rate  determined by the value of reset resister. The reset resister is variable, and constitutes reset adjustment.

DERIVATIVE ACTION : The connection of a derivative capacitor across the feedback circuit delays feedback until the capacitor is charged to a value approaching amplifier output. This delay is controlled by value of derivative resister. This resister is variable and constitutes derivative adjustment.

Que. :  What is an analogue integrator and an analogue differentiators ?

Ans. :

ANALOGUE INTEGRATOR :

ANALOGUE DIFFERENTIATORS :

Que. :  What is anti reset wind up ?

Ans. :  If the limit acts in the feed back section of the control amplifiers integral circuit, the controller output will immediately begin to drive in the opposite direction as soon as the process signal crosses the set point. This approach is referred to as antireset wind up.

Que. :  What are De saturators ?

Ans. :  When, in some processes, e.g. batch process, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called "integral saturation of the control unit". When this condition.

Que. :  Explain the working of Rotameter.

Ans. :  Variable area meters are special form of head meters. Where in the area of flow restrictor is varied. So as to hold the differential pressure constant. The rotameters consists of a vertical tapered tube through which the metered fluid flows in upward direction. A "float" either spherical or cone shaped, actually more dense than the fluid being measured, creates an annular passage between its maximum circumference and the weight of the tapered tube. As the flow varies the "float" rises or falls to vary the area of the passage so that the differential across it just balances the gravitational force on the "float" i.e. the differential pressure is maintained constant. The position of the "float" is the measured of the rate of flow.

Que. :  Explain the working of a magnetic meter.

Ans. :  An electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a "wire"; the principle is equally applicable to a moving, electrically conductive liquid. The primary device of commercial magnetic meters consists of a straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube wall and located at opposite ends of a tube diameter. A uniform a.c. magnetic field is provided at right angles to electrode diameter and to the axis of the tube. The a.c. voltage developed at the electrodes is proportional to the volume flow rate of fluid, and to a magnetic field strength. This device is limited to electrically conducting liquids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids, since there are no location for solids to collect except the walls of the tube itself.

 Que. :  Explain the working of a turbine meter.

Ans. :  Turbine meters consist of a straight flow tube within which a turbine or fan is free to rotate, about its axis which is fixed along the center line of the tube. Straightening rances upstream of the turbine minimizes possible rotational components of fluid flow. In most units a magnetic pick-up system senses the rotation of the rotor through the tube wall. The turbine meter is a flow rate device, since the rotor speed is directly proportional to flow rate. The output is usually in the form of electrical pulses from the magnetic pick-up with a frequency proportional to flow rate. Turbine meter are primarily applied to measurement of clean and noncorossive hydrocarbons.

Que. :  Explain the working of a Pitot tube.

Ans. :  The pitot tube measures the velocity at point in the conduct. If quantity rate measurement is desired, it must be calculated from the ratio of average velocity to the velocity at the point of measurement.

PRINCIPLE : If a tube is placed with its open and facing into a stream of fluid, then the fluid impinging on the open end will be brought to rest, and the kinetic energy converted to pressure energy. This the pressure built up in the tube will be greater than that in the free stream by the impact pressure or pressure produced by loss of kinetic energy. The increase in pressure will depend upon the square of the velocity of the stream. The difference is measured between the pressure in the tube and static pressure of the stream. The static pressure is measured by a tapping in the wall of the main or by a tapping incorporated in the pitot static tube itself. The difference between the pressure in the tube and static pressure will be a measure of the impact pressure and therefore of the velocity of the stream oil.

Que. :  Where is the integral orifice used ?

Ans. :  Integral orifice is used to measure small flow rates. It is mounted directly on the secondary device. The integral orifice diameter varies between 0.020 inch and 0.250 inch diameter. The integral orifice finds considerable use in laboratory and pitot plants.

Calculation of flow rate :

            Qn / Fc = Ks  x  Cwi  x  Fa  x  Fm  x  ( Gp / Ge ) hw

Que. :  Explain the working of a target meter.

Ans. :

The target meter combines in a single unit both a primary element and a force balance flow rate transmitter. A circular disc (or target) supported concentrically in the pipe carrying the flowing fluid results in an annular orifice configuration. Pressure difference developed by the fluid flow through this annular orifice produces a force on target proportional to the square of the flow rate. This force is carried out of the pipe through a rod passing through a diaphragm seal, and is measured by a pneumatic or electronic force balance system identical with the mechanism of the force balance D.P. cell. The advantages of the target meter lies primarily in its single unit construction the primary device and responsive mechanism in a single structure. This eliminates the diff. pressure fluid connections in most heads meters. This is particularly used for sticky and dirty material which may plug up differential connections and for liquids which require elevated temperatures to avoid solidification, this elimination of liquid connection is useful.

{     Wm     } 2

F = {------------------------------ }

{ Cst   Fa   Fm   Fc cf }

Que. :  Where is a quadrant orifice used ?

Ans. :  If the fluid is viscous and the operating Reynolds number is low quadrant orifice is preferred

Que. :  What are types of taps used for orifices ?

Ans. : 

1. FLANGE TAPS :

This are most commonly used on pipe sizes of 2 inches or larger. They are located in the orifice flange 2 inch from upstream and 1 inch downstream from the faces 0 orifice plate.

2. CORNER TAPS :

On pipe sizes less than 2 inches corner taps located directly at the face of the orifice plate.

3. VENA CONTRACTA AND REDIUS TAE :

Vena contracta taps located at 1 pipe diameter upstream and at point of minimum pressure downstream. There are mostly widely used for measurement of steam.

Radius taps are located 1 pipe diameter upstream and 1/2 pipe diameter downstream for the inlet face of the orifice are a close approximation to vena contracta taps upto 0.72 d / D.

4. FACE FLOW TAPS :

Face flow taps are located at 2 1/2  pipe diameter upstream and B pipe diameter downstream. Full flow taps at 2 1/2 and B pipe diameter have the same advantage as vena contracta or radius taps.

QUE. : What is Reynolds number ?

Ans. :  Dynamic similarity implies a correspondence of fluid forces in two systems. In general situation there are many classes of forces that influence the behavior of fluids. Some of these are inertial viscous, gravitational, compressibility, pressure and elastic forces. Certain dimensionless ratio are developed based on fluid properties. Velocities and dimension, which are essentially force ratio.

The more important of these are Reynolds number

SVD

u

For most applications in practical flow measurement the Reynolds number is taken to be sufficient criterion of dynamic similarly. The magnitude of Reynolds number not only indicates whether the flow is laminar or turbulent but also furnishes the probable shape of velocity profile. Due to the strong role it plays as an indicator of varying flow characteristics, many of the deviation from the theoretical equations are called Reynaldo number effects.

Que. :  How would you choose differential range ?

Ans. :  The most common diff. range for liquid measurement is 0-100" H20. This range is high enough to minimize the errors caused by unequal heads in the seal chambers, differences in temps. of load lines etc. The 100" range permits an increase in capacity upto 400" and a decrease down upto 20" by merely changing range tubes or range adjustment.

Que. :  What are positive Displacement meters ?

Ans. : 

PRINCIPLE :

The principle of measurement is that as the liquid flows through the meter it moves a measuring element which seals off the measuring chamber into a series of measuring compartments each holding a definite volume. As the measuring element moves, these compartments are successively filled and emptied. Thus for each complete of the measuring element a fixed quantity of liquid is permitted to pass from the inlet to the outlet of the meter. The seal between measuring element and the measuring chamber is provided by a film of measured liquid. The number of cycle of the measuring element is indicated by means of a pointer moving over the dial, a digital totalizer or some other form of register, driven from the measuring element through an adjustable gearing.

The most common forms of positive displacement meters are :

1.   Reciprocating Piston type.

2.   Rotating or Oscillating Piston type.

3.   Nutating Disc type.

4.   Fluted Spiral Rotor type.

5.   Sliding vane type.

6.   Rotating vane type.

7.   Oval Gear type.