1.Describe following Control Terminalogy
Controlled Condition :
Temperature, Flow, Pressure, Level
Detecting Element:
Thermometer bulb, Bellows, Orifice plate
Measuring Element :
Receives and evaluates the signal from the detecting element
Correcting Unit (regulating unit) :
A device capable of effecting change in the controlled condition
Controlling Unit :
Means of operating the correcting unit according to the difference between the desired and the measured value
Desired Value :
The value of the controlled condition that the operator desires to obtain
Set Value :
The command signal to a process system
Deviation :
Difference between the measured value of the controlled condition and the set value
Dead Time :
Time interval between a change in signal to the element and the initiation of a response to the change
Dead Zone (band) :
A range of values over which a signal may vary without initiating any response
Transmission of a signal from a late stage to an earlier stage in a control system
Feedback
Gain :
The ratio of output signal to input signal change in any part of a control system
Proportional Band :
Range of values of deviation corresponding to the full operating range of output signal from the controller due to proportional action only
Proportional Action :
Action of controller whose output signal is directly proportional to the deviation
Offset :
Sustained deviation of the controlled variable from the set value occurring when proportional control action is used alone
Overshoot :
Amount by which a changing process variable exceeds the desired value following a change in the system
Open Loop :
No feedback and the controller action has no relation to thefinal result
Closed Loop :
A combination of units by which the controlled condition is measured and compared with the desired value. If there is a deviation from the desired value, a signal is sent back to the final control element to correct or limit such deviation
Cascade Control :
Control in which one controller provides the command signal to one or more controllers.
Actuator :
Servo-motor or a complete self contained servomechanism producing a limited output motion
Reference Input Signal :
(desired or set point) information required before action can be taken
Gate :
Automatic switch allowing a pulse to pass to output only if specific combinations of pulse signals arrive simultaneously at its input
Address :
The identity of a particular store location or placing of data in a store
Positioner :
Part of the regulating unit, using power external to the loop, ensures precise adjustment of the regulating unit in accordance with controller output requirement
2.Explain with figure Closed Loop and Open Loop Control System
CLOSED LOOP
The input signal to the controller is dependent on the "ERROR" that is the difference between the system input and its output. A diesel generator fined with a governor will give small variations in speed with load changes.
OPEN LOOP :
In this the input signal to the controller is independent of the output from the controller, for example, a diesel generator without a governor, changes in load will give large speed variations.
3.State The advantages and disadvantages of Electrical and Pneumatic control system
Electrical Control System:
Advantages
a) No compressor and associated equipment required
b) Cables are cheap and easy to lay.
c) High efficiency (no leakage) .
d) Virtually instantaneous response .
e) Normal temperatures have little effect on operation.
f) Very accurate .
Disadvantages
a) Airborne moisture may cause damage .
b) Damage readily occurs in the event of a fire
c) Some risk of fire due to overheating .Intrinsically safe and/or explosion proof
equipment may be required
d) Equipment is affected by voltage fluctuations and/or supply failure
Therefore D.C supply from transformer / rectifier with fully charged batteries for
standby
Pneumatic Control System:
Advantages
a) Unaffected (in short term) by variations in ships power supply
b) Piping simple and easily installed
c) No heat generated therefore virtually no fire risk
d) Actuators are cheaper and have a faster response than electrical
Disadvantages
a) Transmission lags, especially in large systems
b) A supply of clean, dry , oil free air is required .Therefore filters, dryers, carefully
installed pipe work with drainage points are required .
4.Write Short Notes on
- Control Air Compressor
- Refrigeration type Air Driers
- Adsorption type air driers
COMPRESSOR
It is normal to have two oil free compressors, one of which is on standby and arranged to cut in at a preset pressure. The compressors will be fitted with unloading devices to allow light running when the desired pressure is reached, alternatively they may have preset cut in and cut out switches.
Typical pressures are:
Air supply from compressor 4 - 7 bar
Air control pressure 1.2 - 1.5 bar
Refrigeration Type Air dryers
This incorporates a small domestic refrigeration unit and contains controls that ensure that water is drained off does not freeze. The cooled air is reheated by the incoming air cooling the latter and increasing the volume of the former.
Adsorption Type
This type-of drier consists of twin chambers holding a drying chemical (silica gel) which has a fine honeycomb texture, this offers a very large surface area to which the water droplets in the air cling. The chemical is recharged by either heating or blowing through with dry air.
The unit maybe arranged to change over chambers automatically on a time basis.
Operation
With the valves in the positions shown, air flows through the valve A and through dryer '1'. Approximately 90% of the air flows through the outlet valve F. Valves 'C' and 'E' are open whilst valves 'B' and 'D' are closed allowing the remaining 10% of the dry air to flow through dryer '2 to reactivate the chemical Before the chemical in dryer '1' is saturated all the valves are switched and the main flow is now through dryer '2'. As previously stated this change over is done automatically on a time basis.
5.Draw a Control air supply system.
Explain why controlled air need to be oil, dirt free and dry.
CONTROL AIR SUPPLY SYSTEM CLEANLINESS :
The most important factor in obtaining a reliable, trouble :free control system is to ensure a supply of CLEAN DRY AIR.Free of dirt, water and oil The nozzle restrictor hole (can be as small as O.2mm diameter) can easily become blocked with dirt, whilst oil can react with rubber diaphragms and water will corrode the system.
Air lines should have a gradient of 1:50 and the instrument tapping are taken off the top of headers. Seamless copper, polythene and P.V.C tubes are used
The air used for a pneumatic control system should always be separate from the general service air system, since if either system fails then an alternative supply is available.
The air should be 100% oil free, having a dew point of between -20°C and -40oC under pressure and also having no solid panicles of a size that could cause damage to the instruments .
6.Explain with sketches
- Proportional Action
- Booster Relay
Draw Simple pneumatic proportional controller with booster relay.
PROPORTIONAL ACTION
Introducing a feedback (proportional) bellows into the flapper nozzle arrangement reduces flapper movement per unit error movement. This is termed negative feedback and increases the proportional bandwidth i.e decreasing the sensitivity of the unit.
The output of the unit is proportional to the difference between the desired value signal and the measured value signal (error)
The error signal now -moves the position of the flapper relative to the nozzle, varying the air leakage to atmosphere and hence the output signal.
PROPORTIONAL ACTION results in a steady hold after a change, but at a different value to the desired value.
BOOSTER RELAY
The output from the flapper / nozzle arrangement is very small and cannot be transmitted over long distances or operate large equipment, hence a booster or pilot relay valve is often fitted into the system to amplify the output signal.
The above booster relay shows a direct acting relay, i.e an increase in signal pressure from the controller results in an increased output signal from the relay.
An increased input signal moves the valve to the right thus reducing the leak- off at "D" and increasing the operating air flow at "B". This results in increased output signal at
SIMPLE PNEUMATIC PROPORTIONAL CONTROLLER WITH BOOSTER RELAY
7.Write Short notes on
- Multi function Relay
- Air to open and Air to close control valve
MULTI-FUNCTION RELAY
A multi-function relay combines the requirements for adding, averaging, subtracting and ratio ing in one relay.
The relay contains four bellows and a movable fulcrum.. which can be accurately positioned and clamped in the central position for all cases, except when it is used as a ratio relay.
The bellows required are connected to the input signals, any bellows not required being
vented to atmosphere.
Addition: Multiplication
P3= Zero P3=P4=Zero
A=B P1= A/B X P2
P1= P2 +P4
Substruction: Averaging
P2= Zero P1=P3, A=B
A=B 2P1=P2+P4
P1=P4-P3 P1=P2+P4
2
AIR TO OPEN CONTROL VALVES
Control air is used to open the control valve.Failure of control air will cause the valve to close by spring pressure.
AIR TO OPEN CONTROL VALVES
Control air is used to close the control valve.Failure of control air will cause the valve to remain full open by spring pressure.
It is normal to have a means of operating the valve in the event of complete loss of operating au
8.Explain with Sketches
- Proportional & Intregal Action ( Reset Action)
- Proportional & Intregal & derivative Action.
PROPORTIONAL & INTEGRAL ACTION (RESET ACTION)
To reduce "offset integral action is added by placing another bellows opposite the proportional bellows this gives positive feedback.
A variable restrict or is placed in the line before the integral bellows so that the integral action time can be changed.
When integral action is included in the controller it gives an output whose rate of change is proportional to the difference between the measured value and the desired value signals
PROPORTIONAL & INTEGRAL & DERIVATIVE ACTION Also called a three term controller.
Derivative action is obtained by placing a variable restrictor in the line into the proportional bellows. This gives a faster response from the controller, the output being proportional to the rate of change of the deviation
Derivative action has a stabilizing effect on the system and reduces overshoot.
9.Sketch a Single seated and double seated valve.
State advantages of above
State the seat materials
The valve plugs can be either Single or Double seated.
Single Seated when the valve is small or where the effects of fluid forces on the valve are small. They are cheap and tight seating can be readily achieved.
Double Seated (Double Ported) are arranged so that the fluid force across the plug are balanced. These usually require a smaller valve movement, or for a given movement, have larger flow capacities. If metal seats are used with double ported valves then complete shut off is not possible and to 2 % leakage is normal.
Seat materials can be metal (usually stainless steel or monel) which give high wear resistance, can be ground in, can cope with most fluids and can handle high temperatures. Metal seats are required for emergency shut off duties.
Alternatively soft seats can be used, where the material may be Nitrile Rubber (or similar). These can allow complete shut off with double ported valves and produce a softer action on closing.
10.Explain three types flow characteristics of Control valves
QUICK OPENING (Poppet Type)
This characteristic is common in conventional valves, the lift is small and the valve offers minimum resistance to flow. Use in control applications limited to ON / OFF situations, The flow-lift characteristic is geaually linear.
LINEAR FLOW TYPE
These are used when a large proportional of the systems pressure drop occurs across the valve its self
They give a flow - lift characteristic which is linear if plotted on a graph, 40% lift will give 40% flow
The plug can be modified to give a parabolic characteristic to get "fine" control at low flow rates .
EQUAL PERCENTAGE TYPE
It is also called the Logarithmic valve plug
The equal percentage plug is one of the common types in use in control systems as it gives good control over a wide range of pressure changes.
It is particularly useful when flow conditions are difficult to measure, as it has the flexibility for matching (or sizing) to systems.
It is used when most of the pressure drop take place in the system and where there are high system motion losses.
Equal increments of travel give equal percentage changes in existing flow., hence at low lift the flow rate will be low and changes in flow are also small, at large flows then changes in flow are also large.
11.Describe with sketches different type control valve glands.
CONTROL VALVE GLANDS
Control valve glands are fairly long to give good sealing with minimum friction. The type of gland and the packing will depend on the control valve application. For oil, packings are usually rubber proofed cotton,
For low pressure water, greasy packing of Indian Hemp is used.
With hazardous fluids or were the fluid is very expensive, a bellows seal is used, giving a completely tight seal.
Where P. T.F.E materials are used the packing is chevron shaped with the vee expanded against the gland (up to 230 degrees C no lubrication is required).
Where the temperature is very high a lubricant may be required as the temperature may quickly destroy any built in lubricant. The packing is normally asbestos, wire wound and impregnated with PTFE. The gland housing may also have cooling fins around it to help dissipate heat thus preventing expansion imposing increased resistance to valve movement.
12.State why valve Positioner is Used.
State where they are normally fitted
State their advantages.
VALVE POSITIONERS
In some systems (single ported valve with high pressure drop and high gland pressure) the direct signal from the controller may not always be positioned exactly each time it moves. for example at O.6b the valve may be at a different position when it is opening to that when it is closing with a O.6b signal. To overcome this problem a valve positioner is fitted into the system.
The valve positioner compares the valve position and the air signal from the controller to position the valve stem and valve quickly to a position proportional to the controller air signal, independent of counter acting forces.
Valve positioners are fitted where
1) there is a high pressure drop across the valve
2) the valve is remote from the controller
3) the medium being controlled is viscous
4) there are high gland pressures required
The advantages of fitting a valve positioner are:
1) faster response
2) helps overcome motion between the valve spindle and the gland packing
3) helps to overcome out of balance forces on the valve plug .
.14.Explain with a sketch operation of a valve positioner.
An increase in instrument air pressure expands the bellows causing the flapper to cover the nozzle. Through the relay action pressure to the control valve diaphragm increases. The valve stem moves downwards, this movement is fed back to the beam by the earn which moves the flapper away from the nozzle. The relay supply valve closes and the positioner is again in equilibrium but at a higher instrument pressure and a new valve stem position.
On a decreasing instrument pressure the bellows contracts and the flapper moves away from the nozzle. Relay operation causes a decrease in air pressure above the diaphragm by exhausting this air to atmosphere, allowing the valve stem to move upwards due to the force of the return spring. When equilibrium has been reached then the relay exhaust valve closes to prevent any further decrease in diaphragm air pressure.
15.Explain the terms
a. Fail safe
b. Fail set
Fail safe
On failure of the control air supply the valve maybe arranged to move to a position that allows the plant to continue to operate safely, that is fully open or fully closed this is known as fail safe.
Fail set
In some systems failure of the control air supply will cause the valve to be locked in the position at failure this is known as fail set this has the advantage that the system will remain stable and allow time for the plant to be shut down in a methodical method or the air supply to be re-established.
16.Explain the meaning of the following terms:
a.two step control action
b.proportional plus integral control
c.ratio control
d.Give a circumstance or condition where each of the above control modes might be suitable and give the reasons for its choice.
ANSWER
a i) Two step control action is the simplest form of control, in that it assumes one of two preset positions, i.e a switch is ON or OFF, in the case of a valve it is open or closed.
ii) Ratio control is one in which one variable is kept at a fixed ratio to another variable
iii) In a proportional plus integral control controller, the proportional element acts if there is any deviation between the output and the desired value, however the action results in a permanent difference between the desired value and the new output value (offset). The integral element of the controller acts to remove this offset
b) Typical areas of application of the above forms of control may be as follows:
i) A pressure switch controlling the Air start compressor, this is an acceptable form of control since there can be a fairly large deviation from the desired value
ii)Ratio control is found in a boiler combustion control system where the ratio between
the air / fuel must be controlled to ensure good combustion. The air flow is controlled by dampers to match the fuel flow.
iii)A boiler water level control system will employ P + I control, to maintain the water
level at the desired value at all times, removing offset and instability.
17.Suggest with reasons the effect of the following faults in pneumatic control systems:
a) A punctured diaphragm in the nozzle relay valve
b) Grit on the seat of the nozzle relay valve
c) Leaking proportional bellows in valve controller
d) Leaking measurement bellows in valve controller .
ANSWER
a)Because of the leaking diaphragm / bellows the valve will not close, hence the control valve will be unable to fully close
b)Grit on the seat of valve "E" will allow continuous leak of to atmosphere to take place therefore maximum operating pressure will not be obtained, hence the control valve will be unable to open fully
c)A leaking proportional bellows will reduce the ability of the bellows to introduce negative feedback. Hence the gain of the controller is increased. The control valve will move to extreme positions leading to instability
d)A leaking measurement bellows will binder the flapper from moving towards the nozzle. Hence the control valve will be moving from its equilibrium position, causing the system to hunt.
18.With sketch describe a Cascade control system used on Board
JACKET WATER TEMPERATURE CONTROL SYSTEM
In this system there are two measuring elements,
a)the primary measured value being the Jacket Water outlet temperature
b)the secondary measured value the coolant outlet temperature
The secondary measuring element detects changes in the cooler or heater, at the jacket water inlet to enable corrective action to be taken before significant temperature change in the engine occurs, to achieve this a second controller is needed.
The primary measuring element detects changes in JW outlet temperature, the signa] being sent to the master controller where it is compared with the set point. The output of the master controller passes to the slave to control the cooling or heating valve.
The secondary measurement, also passes to the slave controller, so that the value of the slaves output signal depends upon the error between the master controller output and JW inlet temperature, hence the heating / cooling control valves can be adjusted to compensate for changes in sea water or steam temperature, which leads to much quicker response.
The output from the slave controller fed to both the heating and cooling control valve, its value being split, so that only one valve will be open at a particular time.
To comply with the system fail safe requirements;
a) the cooling valve must be air to dose (ATC), so that in the event of a power loss the engine is cooled rather than heated.
b) the steam valve must be air to open (ATO)
The cooling valve (ATC) must therefore use the lower part of the slave output range and the steam valve the upper part of the signal.
To ensure that that the valves are never both open at once a small amount of dead band is introduce.
CASCADE: A number of controllers in series
SPLIT RANGE: Final controllers output signal range is split between two control
valves, one A TO, one A TC
TWO ELEMENT: There are two measured values
19.Sketch and describe automatic Package boiler control system
AUTOMATIC PACKAGE BOILER
On starting the circulating solenoid recirculates fuel until the correct temperature is reached. The solenoid then switches over to put the low flame spill line into operation. Once the flame is established the steam pressure switch operates the solenoid valve to change over between high and low flame as the steam pressure varies.
A shut down solenoid which shuts the fuel supply down is operated by:
a) High and low water level
b) High steam pressure
c) Flame failure
d) Low furnace air pressure
20.Sketch and describe Boiler water level controller .
Sketch and describe a control system which employs P +I control action.
BOlL.ER WATER LEVEL CONTROL
In simple single element boiler water level controllers, the boiler water level alone is measured, this can lead to the boiler tripping on high or low level due to the effects of boiler water shrink or swell.
On medium sized boilers it is common to employ two element controllers to control the boiler water level.
The primary measured value is the water level which may be measured by a D.P cell. the signal being compared to the set point, any deviation being fed into the slave controller as its set point.
The secondary measured value is the steam flow from the boiler, this being measure and the signal being sent to the slave controller where it is compared, with the set point (master controller output).
During the swell period the level signal calls for less water, even though the actual amount of water has actually decreased. Simultaneously the increase in steam flow calls for an increase in feed flow, this combination of signals will usually prevent the boiler from tripping.
21.Sketch a simple Hydraulic deck crane circuit
HYDRAULIC DECK CRANE CIRCUIT
The circuit is typical of a system using variable stroke pump and a fixed stroke hydraulic motor. The complete system would require three such circuits, one for hoist, luff and slew.
SAFETY FEA TIJRES
Braking _
The requirements are that it win hold the load securely and allow a smooth change over from the static position to winding and fail safe.
Normally the brake is spring loaded to the "ON" position, it being released by hydraulic pressure. The signal to operate the brake coming from a micro switch on the swash plate control lever, which opens the solenoid valve.
Override Controls
These are required to prevent damage or operating the equipment in unsafe areas, e.g slew angle, max and min luff angle, and slack rope protection
The operating pressure of the system will depend upon the position of the swash plate and the size of the load being lifted.
22.Sketch and Describe Constant pressure hydraulic system
CONSTANT PRESSURE HYDRAULIC SYSTEM
Hydraulic systems on deck normally use variable delivery pressure compensated pumps to supply the system with constant pressure oil.
The pump takes suction from a sump through a non-return valve and a filter, before discharging it into the main.
When the pumping capacity exceeds the load requirements, the systems pressure will increase to a predetermined value. Once this value is reached the pressure compensator acts to take the pump off stroke. Should the pressure compensator fail then the system is protected by a relief valve which returns the oil to the sump. Once the main line pressure has reduced to a preset value the compensator acts to put the pump back on stroke, increasing the system pressure.
To allow for slight fluctuations in pressure an accumulator is fitted into the system.
Hydraulic fluid is directed to the load via a 3-way valve.
23.Describe the components of simple hydraulic system
SIMPLE HYDRAULIC SYSTEM
All hydraulic power systems comprise of at least the following components
(1) Tank -Used to store the hydraulic fluid that is not currently in use
(2) Pump - Used to force the hydraulic fluid through the system, acts as the
pressure source
(3) Prime mover - The power source for the pump. b hydraulic systems the prime mover is usually an electric motor
(4) Valves - Installed to control liquid direction, pressure, and flow rates.
(5) Actuator - Devices that convert the energy of the liquid into mechanical
force or torque. Typically, single piston and cylinder arrangement that results in linear motion. A ship's steering system uses this design.
(6) Piping - Used to contain and direct hydraulic fluid from one point to
another.
Additional Components
Filters/strainers - Used to remove foreign particulate matter from hydraulic fluid
that could damage (by scratching close tolerance components) or clog the system.
Pressure regulator - A device that vents off or unloads hydraulic fluid from the high pressure side (pump outlet) when the pressure in the system exceeds set point (design pressure). The unloaded fluid usually . is returned to the low-pressure side of the system or the sump. By unloading hydraulic fluid, pressure is reduced. When hydraulic pressure returns to set point regulator stops unloading. By constantly loading or unloading, the pressure regulator maintains the pressure at set point.
Accumulator - A device that acts as a hydraulic shock absorber for the system.
It basically consists of a container, divided into two sections by a flexible divider or membrane. One section is open to the hydraulic system's liquid and the other section contains a gas (often nitrogen) under an appropriate pressure. It is used to store a certain volume of hydraulic fluid under pressure. The liquid side experiences the same pressures as the fluid in the hydraulic system. When the hydraulic system experiences a sudden pressure increase, the effect is reduced or dampened by fluid in the accumulator compressing the gas on the other side of the membrane. When the system experiences a sudden pressure decrease, the gas forces fluid out of the accumulator to increase pressure. The accumulator is designed to handle _brief system transients, not system casualties or long term degradations.
Relief valve - A device that protects Systems from excess pressure. A relief valve is much the same as a pressure regulator in that it unloads excess pressure. However, a relief valve has a higher set point than the pressure regulator and only operates under abnormal conditions such as a failure in the pressure regulator.
Check valve - A device that permits flow in only one direction.
Sequence valve - A device that controls the sequence of operation of one or more
other components.
Servo - A device that measures output or state such as the rotation of a motor or the distance of travel of a piston in a cylinder. The servo is output is compared to a standard or output signal. The results of the comparision are used to control a systems or actions.
24.Why it is necessary to have hydraulic system free of dirt used in Deck Machinery
What are the sources of contaminants.
HYDRAULIC DECK MACHINERY
The most important factor in obtaining a reliable, trouble free hydraulic system is to ensure a supply of clean hydraulic oil free of dirt, water The nozzle restrictor hole (can be as small as O.2mm diameter) can easily become blocked with dirt, causing malfunction of hydraulic control valves. water will corrode the system.
SOURCES OF CONTAMINATION
1 RESIDUES FROM THE MANUFACTURING PROCESS
a. Metallic particles swarf, grinding dust from machining process ,
b. Scale from heat processes such as forging welding rolling bending
c. Slag and sand from castings
d.Debris from sealants
e. Fibres from cleaning materials
f. Ingress of S.W. causing rust particles
g. Particles produced from fretting and vibration
h. Particles produced by the oxidation of oil, breakdown of additives g. Dirt from human sources hair and skin
CONTAMINATION OF THE HYDRAULIC SYSTEM BYWATER
The only source of contamination by water is from the leakage at the
water / oil interface.
Oil is capable of dissolving water in the order of hundreds of p.p.m.and
still remains clear and bright. This may reappear as condensate and cause rusting of ferrous components.
25.Describe Hydrostatic & pneumercator level gauge.
Hydrostatic level gauge
Hydrostatic pressure sensors can be used to measure liquid levels directly in tanks.
A pressure gauge located at the zero or empty level of a tank will register zero when the tank is empty and as the tank is filled with liquid a rise in pressure will occur due to the increased head of liquid acting on a diaphragm or bellows this pressure is then transmitted to a bourdon tube type gauge. The gauge is normally calibrated in tonnes, (if calibrated in meters then an allowance must be made for the relative density of the tank liquid)
The Pneumercator gauge
This instrument is in common use on fuel oil tanks and uses a mercury manometer and hemispherical bell. The hemispherical bell is fitted near the bottom of the tank and is connected by small bore piping to the mercury manometer. Compressed air pressurizes the system, the air supply is isolated and the system is switched to connect the hemispherical bell to the manometer via selector cocks, the head of liquid further increases the pressure of the compressed air in the system
which is measured on the mercury manometer.
The bubbler level gauge works by adjusting the air pressure entering a open ended tube immersed inside a tank, so that bubbles can just be seen leaving the bottom of the tube. A pressure gauge then measures the air pressure that is required to overcome the pressure of the liquid head above the bottom of the tube. The diaphragm bubbler gauge is a slight modification where a diaphragm isolates the air from the liquid, the air escaping to atmosphere. The varying head of oil acting on the diaphragm will vary the air escaping to atmosphere and hence the back pressure in the tube will alter. This type of gauge would be used lubricating oil tanks where the air may oxidize the oil
26.Explain Diesel engine Bridge Control System
DIESEL ENGINE BRIDGE CONTROL SYSTEM
The classification societies require that speed of the main engine must be able to be controlled from the bridge by means of a single lever, if:
a)the machinery spaces are periodically unmanned
b) only one man is on watch within the engine room.
The telegraph acts as a reference transmitter for the propeller speed and is graduated in r.p.m. Alterations of the speed of the main engine are carried out according to fixed programmes. These programmes are set up to meet the requirements of each engine and of the shi;> with respect to overloading and maneuverability.
It is desirable to include safety functions, e.g. low lub oil pressure etc. in order to obtain automatic speed reduction or to stop the machinery.
Starting and reversing of the engine are provide for by logic circuits, which control the injection of starting air, positioning of the camshaft and fuel injection.
The output from the logic circuits is converted to a suitable power level for control of the mechanical actuators on the main engine.
27. Describe following two types of torsionmeter
a.Angular Displacement
b. Magnetic stress sensitivity
Angular Displacement
Torsion meters are used for the measurement of power transferred through a propulsion shaft.
Principle
A torque of value T is applied to a shaft of fixed length L and radius r. An angle of twist θ is generated and is dependent of the modulus of torsional rigidity G and given by
T/r = Gθ/L
The modulus of rigidity, the raidus and the length of the shaft are all fixed thus the torque on the shaft is proportional to the angle of twist
Typical system
Two AC generators are mounted so that they are driven by the main shaft and area at set distance apart L. A sinusoidal waveform is produced. One of the generators is adjusted so that at minimum torque the generated waveforms are 180' out of phase. The outputs from the two generators are then added and the resultant voltage is used as the measurement of torque
As the torque is applied to the shaft so the twist causes the waveforms to shift in phase. When the two waveforms are now added an output ac current is produced which may be amplified and rectified to give an ouput voltage proportional to the torque appled to the shaft.
Another method of achieving this is to replace the generators wit sensors and toothed ring.
Power Calculation
Power is a product of the Torque and revs of the shaft, one of the generator outputs is used to measure the shaft rev/s and a calculation performed
Magnetic Stress Sensitivity
This type measures magnetic fields in the shaft surface, the distortion of these fields gives and indication of the torque. The principle behind this is that in some ferromagnetic materials reluctance ( magnetic resistance) is less along the plane of stress than across it.
In the torductor three rings are fitted around the shaft each ring having four electromagnetic poles. The centre ring acts as a transformer primary with the two outer secondary rings having their poles arranged 45' apart to the primary poles but in line with each other. The poles are held in a frame so that there is no contact between the poles and the shaft which have a gap of about 3 mm between them.
An alternating current is fed to the centre ring thus generating a magnetic field. This educes an emf in the outer two rings. The outer two ring coils are connected in series in such away that at zero stress the emf generated in each ring is opposite and equal in value giving an output of zero volts.
When torque is applied to the shaft the stress lines are distorted to to the axial. The distorion of this field affects the emf induced in the coils increasing on one side and reducing on the other. Thus a resultant emf of a few milivolts is available at the output. The size of the output is proportional to the stress applied.
28.Explain following terms
a. Two step control action
b. Proportonal action
c. Split range action
Two step control action
This can be defined as 'the action of a controller whose output changes from one state to another due to a variation in its input' One example of this control is that of a float operated filling v/v say for a cistern. In normal condition the output of the float is nil and no water passes through the valve, should the water level drop the float detects this and operates the valve to change to its second state which is open and water flows. When the level re-establishes then the float controls the valve to return to its primary state which is closed. In this way the float is controlling the water level by changing the valve between two different states. A more realistic system is shown below.
The system works as follows; the level drops until the lower float is uncovered, the controller detects this and opens the filling valve, the filling v/v remains open until the top float is covered and then the controller shuts the valve
The distance between the floats is termed the 'Overlap' i.e. the distance between the high and low controlling values ( on some systems this can be altered by altering the high or low set point of the controller, in the above system this would mean altering the position of the floats )
Proportional control action
This where the change in output signal from a controller is proportional to the change in input signal
The control can be summed up in the following;
Output = Constant x Deviation
Output - this is the output from the controller and goes to the control element ( say the filling v/v on the previous example i.e. the piece of equipment that actually alters the process.
Constant- This is the 'Gain' of the controller, as the output varies with the deviation, the amount it varies can be altered.
Say if the deviation changes by one unit the output changes by one unit, hence the gain is one. If the output varied by two for the same one change in deviation then the gain would be two. Similarly if the change in output was one half a unit for a one unit change in input then the gain would be half. Another way used to describe Gain is 'Proportional band', here a gain of one is described as a proportional band (Pb)of 100%. For a gain of two the Pb is 50%, and for a gain of a half the Pb is 200%, hence it can be seen that the magnitude of the Pb is opposite to the gain.
Deviation- This is the difference between the set point of the controller and the measured value. If the set point was one unit and the measured value was two units the deviation would be one unit.
Deviation = Set point - Measured value
The important think to remember is that the narrower the Proportional band the higher the gain and hence the higher the output varies for a change in deviation, this has the effect of making the controller control the process quicker by operating the controlling element more for smaller variations measured value. This has the negative
Split range control( negative and positive offset)
A system could be designed to control both the outlet valves and inlet valves (this is what is seen on the feed water system level control with the spill and filling being controlled from the one controller) ; here the controller would be set up so that when the level is at the set point its output is mid range ( say for a controller operating in the 3 to 15 psi range this would be 9 psi)
The control valves would be set up so that one, say the filling v/v would go from close at 9.5 psi to open at 15 psi, and the spill v/v would go from close at 8.5 psi to open at 3 psi. The 1 psi in the middle is called the 'Deadband' and is there to ensure both v/vs are not open at the same time.( The v/v acting to open with increasing input signal is called 'Direct Acting' and the v/v closing with increasing pressure is called 'Reverse Acting')
It can be seen that there can now be an offset in the positive i.e. water being used and hence the make up v/v has to be open and in the negative i.e. there is too much water entering the system and the spill v/v's have to be opened.
Offset is not a desired result of the control of a system, however for proportional only controllers this is a direct consequence. That is why for all controllers performing important functions; including the make up/spill system controller above other types of controlling action are added to remove the offset
