Intro to Marine Engineering 2E Episode 11 docx

The flapper movement will be very minute andwhere measurement of a reasonable movement is necessary a system of Nozzle Orifice Air supply To measuring unit Flapper Pivot f Output Figure

conduct the current to the coil The moving coil assembly is surrounded

by a permanent magnet which produces a radial magnetic field Currentpassed through the coil will result in a force which moves the coil againstthe spring force to a position which, by a pointer on a scale, will readcurrent or voltage

The instrument is directional and must therefore be correctlyconnected in the circuit As a result of the directional nature ofalternating current it cannot be measured directly with this instrument,but the use of a rectifying circuit will overcome this problem

Figure 15.15 Mechanical tachometer

sliding collar, through a link mechanism, moves a pointer over a scale

As the driven shaft increases in speed the weights move out undercentrifugal force, causing an axial movement of the sliding collar This

in turn moves the pointer to give a reading of speed

Electrical

The drag cup generator device uses an aluminium cup which is rotated

in a laminated iron electromagnet stator (Figure 15.16) The stator has

Winding Stater Insulation

Bearing

A.C.supply

Figure 15.16 Drag cup generator-type tachometer

two separate windings at right angles to eaeh other An a.c supply isprovided to one winding and eddy currents are set up in the rotatingaluminium cup This results in an induced e.m.f in the other statorwinding which is proportional to the speed of rotation The outputvoltage is measured on a voltmeter calibrated to read in units of speed.Tachogenerators provide a voltage value which is proportional to thespeed and may be a.c or d.c instruments The d.c tachogenerator is asmall d.c, generator with a permanent field The output voltage isproportional to speed and may be measured on a voltmeter calibrated inunits of speed The a.c tachogenerator is a small brushless alternatorwith a rotating multi-pole permanent magnet The output voltage isagain measured by a voltmeter although the varying frequency willaffect the accuracy of this instrument

Various pick-up devices can be used in conjunction with a digitalcounter to give a direct reading of speed An inductive pick-uptachometer is shown in Figure 15.17(a) As the individual teeth pass thecoil they induce an e.m.f pulse which is appropriately modified andthen fed to a digital counter A capacitive pick-up tachometer is shown inFigure 15.17{b) As the rotating vane passes between the plates acapacitance change occurs in the form of a pulse This is modified andthen fed to the digital counter

Torsionmeters

The measurement of torsion is usually made by electrical means Thetwisting or torsion of a rotating shaft can be measured in a number ofdifferent ways to give a value of applied torque Shaft power can then becalculated by multiplying the torque by the rotational speed of the shaft

Pick up~»>fl U

C°" rL^ Signal

nmOigitai

f*s ^ modifier counter

(a) inductive

Rotating shaft

Capacitor j

Signal Digital modifier counter (b) Capacitive

Figure 15.17 Pick-up tachometers, (a) inductive; (b) capacitive

Strain gauge torsionmeter

With this device four strain gauges are mounted onto the shaft, as shown

in Figure 15.18 The twisting of the shaft as a result of an applied torqueresults in a change in resistance of the strain gauge system or bridge.Brushes and sliprings are used to take off the electrical connections andcomplete the circuit, as shown More recently use has been made of theresistance change converted to a frequency change A frequencyconverter attached to the shaft is used for this purpose: this frequency

signal is then transmitted without contact to a digital frequency receiver.When a torque is applied to the shaft, readings of strain and hencetorque can be made.

Differential transformer torsionmeter

Two castings are used to provide a magnetic circuit with a variable airgap The two are clamped to the shaft, as shown in Figure 15.19, andjoined to each other by thin steel strips The joining strips will transmittension but offer no resistance to rotational movement of the two

Transformer soft iron core

differential transformer

Figure 15.19 Differential transformer torsionmeter

castings with respect to each other, A differential transformer is fittedbetween the two castings, the two coils being wound on one casting andthe iron core being part of the other Another differential transformer isfitted in the indicating circuit, its air gap being adjusted by a micrometerscrew The primary coils of the two transformers are joined in series andenergised by an a.c supply The secondary coils are connected so thatthe induced e.m.f.s are opposed and when one transformer has an airgap different to the other a current will flow

When a torque is applied to the shaft the air gap of the shafttransformer will change, resulting in a current flow The indicator unittransformer air gap is then adjusted until no current flows The air gaps

in both transformers must now be exactly equal The applied torque isdirectly proportional to the width of the air gap or the micrometer screwmovement Shaft power is found by multiplying the micrometer screwreading by the shaft speed and a constant for the meter

Viscosity measurement

Viscosity control of fuels is essential if correct atomisation andcombustion is to take place Increasing the temperature of a fuel will

Pressure tapping led to differential pressure gauge

Oilflow

Constant speed gear pump

Gear pump suction

Thermometer

(b)

Figure 15.20 Viscosity sensor, (a) diagrammatic; (b) actual

reduce its viscosity, and vice-versa As a result of the varying properties

of marine fuels, often within one tank, actual viscosity must becontinuously measured and then corrected by temperature adjustment.The sensing device is shown in Figure 15.20 A small constant speedgear pump forces a fixed quantity of oil through a capillary (narrow

bore) tube The liquid flow in the capillary is such that the difference inpressure readings taken before the capillary and after it is related to theoil viscosity A differential pressure gauge is calibrated to read viscosityand the pressure values are used to operate the heater control tomaintain some set viscosity value.

Salinometer

Water purity, in terms of the absence of salts, is essential where it is to beused as boiler feed Pure water has a high resistance to the flow ofelectricity whereas salt water has a high electrical conductivity Ameasure of conductivity, in Siemens, is a measure of purity

The salinity measuring unit shown in Figure 15.21 uses two small cellseach containing a platinum and a gunmetal electrode The liquid samplepasses through the two cells and any current flow as a result ofconductance is measured Since conductivity rises with temperature a

compensating resistor is incorporated in the measuring circuit Theinsulating plunger varies the water flow in order to correct values to20®C for a convenient measuring unit, the microsiemens/cm3 or dionicunit A de-gassifier should be fitted upstream of this unit to removedissolved carbon dioxide which will cause errors in measurement.

Oxygen analyser

The measuring of oxygen content in an atmosphere is important,particularly when entering enclosed spaces Also inert gas systems useexhaust gases which must be monitored to ensure that their oxygencontent is below 5% One type of instrument used to measure oxygencontent utilises the fact that oxygen is attracted by a magnetic field, that

is, it is paramagnetic

A measuring cell uses a dumb-bell shaped wire which rotates in amagnetic field The presence of oxygen will affect the magnetic fieldand cause rotation of the dumb-bell The current required to align thedumb-bell is a measure of the oxygen concentration in the cell

The sampling system for an inert gas main is shown in Figure 15.22.The probe at the tap-off point has an integral filter to remove dust The

PRESSURE REGULATING

VALVE

VACUUM BREAK

gas then passes through a separator, a three-way valve and a flow valve.The gas sample, after further separation and filtering, passes to themeasuring cell and part of it is bypassed The flow valve is used to obtainthe correct flow through the measuring cell and a meter provides thereading of oxygen content The three-way valve permits the introduc-tion of a zeroing gas (nitrogen) and a span gas (air) The span gas gives a21% reading as a calibration check.

Oil-in-water monitor

Current regulations with respect to the discharge of oily water set limits

of concentration between 15 and 100 parts per million A monitor isrequired in order to measure these values and provide both continuousrecords and an alarm where the permitted level is exceeded

The principle used is that of ultra-violet fluorescence This is theemission of light by a molecule that has absorbed light During the shortinterval between absorption and emission, energy is lost and light of alonger wavelength is emitted Oil fluoresces more readily than water andthis provides the means for its detection

Diverting valves

| i Controller/

recorder

T

^

1n

u

Alar

Overboard discharge

Figure 15.23 Oil-in-water monitoring system

A sample is drawn off from the overboard discharge and passesthrough a sample cell (Figure 15.23) An ultra-violet light is directed atthe sample and the fluorescence is monitored by a photoelectric cell.The measured value is compared with the maximum desired value inthe controller/recorder Where an excessive level of contamination isdetected an alarm is sounded and diverting valves are operated Thedischarging liquid is then passed to a slop tank.

Control theory

To control a device or system is to be able to adjust or vary theparameters which affect it This can be achieved manually orautomatically, depending upon the arrangements made in the system.All forms of control can be considered to act in a loop The basicelements present in the loop are a detector, a comparator/controller and

a correcting unit, all of which surround the process and form the loop(Figure 15.24) This arrangement is an automatic closed loop if theelements are directly connected to one another and the control actiontakes place without human involvement A manual closed loop wouldexist if one element were replaced by a human operator

It can be seen therefore that in a closed loop control system the controlaction is dependent on the output A detecting or measuring elementwill obtain a signal related to this output which is fed to the transmitter.From the transmitter the signal is then passed to a comparator Thecomparator will contain some set or desired value of the controlled

Desired value

Figure 15.24 Automatic closed loop control

condition which is compared to the measured value signal Anydeviation or difference between the two values will result in an outputsignal to the controller The controller will then take action in a mannerrelated to the deviation and provide a signal to a correcting unit Thecorrecting unit will then increase or decrease its effect on the system toachieve the desired value of the system variable The comparator isusually built in to the controller unit.

The transmitter, controller and regulating unit are supplied with anoperating medium in order to function The operating medium may becompressed air, hydraulic oil or electricity For each medium varioustypes of transmitting devices, controllers and regulating units are used

Transmitters

Pneumatic

Many pneumatic devices use a nozzle and flapper system to give avariation in the compressed air signal A pneumatic transmitter is shown

in Figure 15.25 If the flapper moves away from the nozzle then the

transmitted or output pressure will fall to a low value If the flapper

moves towards the nozzle then the transmitted pressure will rise to almost

the supply pressure The transmitted pressure is approximatelyproportional to the movement of the flapper and thus the change in themeasured variable The flapper movement will be very minute andwhere measurement of a reasonable movement is necessary a system of

Nozzle Orifice

Air

supply

To measuring unit

Flapper

Pivot

f

Output

Figure 15.25 Position balance transmitter

levers and linkages must be introduced This in turn leads to errors inthe system and little more than on-off control

Improved accuracy is obtained when a feedback bellows is added toassist in flapper positioning (Figure 15.26) The measured value acts on

one end of the pivoted flapper against an adjustable spring whichenables the measuring range to be changed The opposite end of theflapper is acted upon by the feedback bellows and the nozzle Inoperation a change in the measured variable may cause the flapper toapproach the nozzle and thus build up the output signal pressure Thepressure in the feedback bellows also builds up, tending to push theflapper away from the nozzle, i.e a negative feedback An equilibrium

Measured variable

Feedback force

Feedback bellows

A

Pivot

Nozzle 'Flapper

Range

spring

Figure 15.26 Force balance transmitter with feedback

position will be set up giving an output signal corresponding to themeasured variable

Most pneumatic transmitters will have relays fitted which magnify oramplify the output signals to reduce time lags in the system and permitsignal transmission over considerable distances Relays can also be usedfor mathematical operations, such as adding, subtracting, multiplying ordividing of signals Such devices are known as 'summing' or 'computingrelays'

Electrical

Simple electrical circuits may be used where the measured variablecauses a change in resistance which is read as a voltage or current anddisplayed in its appropriate units

Another method is where the measured variable in changing creates apotential difference which, after amplification, drives a reversible motor

to provide a display and in moving also reduces the potential difference

to zero

Alternating current positioning motors can be used as transmitterswhen arranged as shown in Figure 15.27 Both rotors are supplied fromthe same supply source The stators are star wound and when the tworotor positions coincide there is no current flow since the e.m.f s of bothare equal and opposite When the measured variable causes a change in

Output

a.c supply

Figure 15.28 Force balance electronic transmitter

the transmitter rotor position, the two e.m.f.s will be out of balance Acurrent will flow and the receiver rotor will turn until it aligns with thetransmitter The receiver rotor movement will provide a display of themeasured variable.

An electrical device can also be used as a transmitter (Figure 15.28).The measured variable acts on one end of a pivoted beam causing achange in a magnetic circuit The change in the magnetic circuit results

in a change in output current from the oscillator amplifier, and theoscillator output current operates an electromagnet so that it produces anegative feedback force which opposes the measured variable change,

An equilibrium position results and provides an output signal

Hydraulic

The telemotor of a hydraulically actuated steering gear is one example

of a hydraulic transmitter A complete description of the unit and itsoperation is given in Chapter 12

Controller action

The transmitted output signal is received by the controller which mustthen undertake some corrective action There will however be varioustime lags or delays occurring during first the measuring and then thetransmission of a signal indicating a change A delay will also occur in theaction of the controller These delays produce what is known as thetransfer function of the unit or item, that is, the relationship between theoutput and input signals

The control system is designed to maintain some output value at aconstant desired value, and a knowledge of the various lags or delays inthe system is necessary in order to achieve the desired control Thecontroller must therefore rapidly compensate for these system variationsand ensure a steady output as near to the desired value as practicable

Two-step or on-off

In this, the simplest of controller actions, two extreme positions of thecontroller are possible, either on or off If the controller were, forexample, a valve it would be either open or closed A heating system isconsidered with the control valve regulating the supply of heating steam.The controller action and system response is shown in Figure 15.29 Asthe measured value rises above its desired value the valve will close.System lags will result in a continuing temperature rise which eventuallypeaks and then falls below the desired value The valve will then open

Closed ~

Time

Figure 15.29 Two-step or on-off control

again and the temperature will cease to fall and will rise again This form

of control is acceptable where a considerable deviation from the desiredvalue is allowed

Continuous action

Proportional action

This is a form of continuous control where any change in controlleroutput is proportional to the deviation between the controlled condition

and the desired value The proportional band is the amount by which the

input signal value must change to move the correcting unit between itsextreme positions The desired value is usually located at the centre of

the proportional band Offset is a sustained deviation as a result of a load

change in the process It is an inherent characteristic of proportionalcontrol action Consider, for example, a proportional controlleroperating a feedwater valve supplying a boiler drum If the steamdemand, i.e load, increases then the drum level will fall When the levelhas dropped the feedwater valve will open An equilibrium position will

be reached when the feedwater valve has opened enough to match thenew steam demand The drum level, however, will have fallen to a newvalue below the desired value, i.e offset See Figure 15.30

Integral action

This type of controller action is used in conjunction with proportionalcontrol in order to remove offset Integral or reset action occurs whenthe controller output varies at a rate proportional to the deviation

Figure 15.30 System response to proportional controller action

between the desired value and the measured value The integral action

of a controller can usually be varied to achieve the required response in aparticular system

Derivative action

Where a plant or system has long time delays between changes in themeasured value and their correction, derivative action may be applied.This will be in addition to proportional and integral action Derivative orrate action is where the output signal change is proportional to the rate

of change of deviation A considerable corrective action can thereforetake place for a small deviation which occurs suddenly Derivative actioncan also be adjusted within the controller

Mttltipte-term controllers

The various controller actions in response to a process change are shown

in Figure 15.31 The improvement in response associated with theaddition of integral and derivative action can clearly be seen Reference

is often made to the number of terms of a controller This means thevarious actions: proportional (P), integral (I), and derivative (D) Athree-term controller would therefore mean P-H-fD, and two-termusually P+I A controller may be arranged to provide either split range

or cascade control, depending upon the arrangements in the controlsystem These two types of control are described in the section dealingwith control systems