Control Systems Flashcards
What are the advantages and disadvantages of electrical and pneumatic control systems on ships ?
Electrical Control System
Advantages
No air compressor and associated machinery required.
High efficiency since there is no leakages.
Instantaneous response.
Very little affected by normal temperature variations.
Very accurate.
Electric cables are cheap and easy to lay.
Disadvantages
The system require uninterrupted power supply with no voltage fluctuations.
Standby batteries required in case of power failure.
There is risk of fire due to overheating. Expensive intrinsically safe / explosion proof equipment may be required.
Moisture may cause damage to the system.
Damage readily occurs in the event of a fire.
Pneumatic Control System
Advantages
Not affected by ship’s power supply variations for short term.
No heat generated and hence no risk of fire.
Actuators are cheaper and accurate than electric systems.
Simple control air piping system.
Disadvantages
Require air compressor and associated systems.
A clean, dry and oil free supply of good quality air is essential for satisfactory operation of the system.
Good quality air require driers, filters with drains, etc. which increases maintenance.
May have transmission lags in large systems.
What is meant by ‘fail safe’ and ‘fail set’ in a pneumatic control systems ?
On failure of control air supply, the pneumatic actuator may be arranged to move to a position that allows the plant to continue to operate safely, in other words, fully opened or fully closed. This is known as fail safe. For example, in the case of a jacket cooling water system, on failure of control air, the actuator opens fully to allow jacket water to the cooler without bypass. On the other hand a fuel oil control valve for boiler closes completely on such a failure. This ensures safety of the plant. In some other systems, control air supply failure locks the position of valve at that time of failure. This is called fail set. The advantage of this system is that the plant gets stable and have time for normal shut-down or can wait for re-establishment of control air supply for some time. Example for such a system is boiler water level control.
Explain two step control action, ratio controland proportional plus integral control and provide an application of use on ship?
Two step control action is the simplest form of control, in that it assumes one or two pre-set positions, a switch is ON or OFF or in the case of a valve it is OPEN or CLOSE. For example, a pressure switch controlling the starting air compressor is an acceptable form of control since there can be fairly large deviation between measured value and desired value. Another example is hydrophore pumps which cut in and cut off with the hydrophore pressure.
Ratio control is one in which one variable is kept at a fixed ratio to another variable. Ratio control is found in 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.
In a proportional plus integral controller, the proportional element acts if there is any deviation between the output (measured value) and the desired value, but the action results in a permanent difference between the measured and desired value (called offset). The integral element removes this difference or offset. For more details on the basics of proportional, integral and derivative control, please click on the links below. Application of the proportional-integral control is the water level control system for boiler on ships, which maintains desired water level always without offset.
What is the effect of a leaking proportional bellows or measurement bellows in pneumatic control systems ?
When proportional bellows start leaking, its ability to introduce the negative feedback is reduced. This results in increased gain of the controller and control valve starts moving to the extreme positions causing instability. When measurement bellows leak, it prevent the flapper from moving towards the nozzle, and the desired or set value is changed, again causing hunting of the system
Describe, with the aid of a sketch, a typical single element temperature control for a large lubricating oil system.
see notes for drawing
This system is another example of using a PID software to regulate the desired variable this being temperature in this case. This system contains a thermo transducer that sense the temperature and transmit a input signal to the PID control module which then compares temperature value to the desired value and then sends and output signal to the actuator to open or close the 3 way regulating valve. If valve is closed then its because the LO temperature is already at the required set point and so its just recirculated back to the main engine. 3 way regulating valve is opened or closed depending on the lube oil temperature so that flow rate and temperature can be adjusted through the use of a LO cooler (heat exchanger) to achieve required temperature. this LO cooler works like a heat exchanger so using sea water and the process of conduction heat is removed from the lube oil to prevent thermal stress damage to components. Once the main LO is cooled its supplied back to main engine for cooling purposes. The LO cools the main bearings, camshaft and camshaft drive.
Sketch and describe a method of remotely monitoring the contents of a fuel oil tank.
See EOOW ORAL/IAMI Sketch Pack
The sketch shown includes a radar level gauge suitable for non-contact level measurements and remote monitoring in fuel tanks and other types of tanks.
The radar level gauge sends a microwave signal with a continuously varying frequency towards the liquid surface. When the reflected signal returns it is mixed with the outgoing signal, the difference being proportional to the distance to the liquid surface. The tank fuel content level is then fed to the local and remote gauges.
With reference to a control systems,
a) list four examples of a measured variable (4)
b) Explain the meaning of the term “two-step control” giving one example of its use. (6)
c) Explain the meaning of the term “proportional control” giving one example of its use. (6)
a.) - High/Low Temperature for main engine jacket water
- High Low Pressure or High Low temperature for Main Engine Lub Oil
- Level and Temperature for Any Tank (i.e., Heavy Fuel Oil Tk)
- Viscosity, pressure, Temperature and or flow rate for main engine fuel system
b.) In two-step control, the output signal is either switched fully on or fully off. An example would be a fuel oil tank heating steam two-step controller. It consists of a PI controller that continuously senses fuel oil temperature. This is called measured value. The desired value or set point (temperature of fuel oil ) is fixed by the operator, so the engineer When the temperature of the fuel oil drops below the set point, the PI controller sends a signal to the steam valve to open and deliver steam to the steam heating coils provided inside the fuel oil tank As soon as this happens, the steam heating coil inside the fuel tank heats up the fuel to increase its temperature. When the temperature rises to the set point, the controller shuts the steam valve, and steam is then no longer being delivered to the steam heating coil.
c.) Proportional control is a control system in which the output is proportional to the difference between a setpoint value and the current value of a process variable. The main engine governor is a proportional controller. For small variations in load like small weather sea conditions, the engine response is good, but in bad weather conditions, it is controlled to shut off fuel to the engine. It does not have the capacity to respond to the large fluctuation of load.
what is the basic function of a PID controller?
Function of the controller is to maintain the parameter as per desired value (value set by the operator). The parameter could be jacket water temperature (for engine jacket water cooling system), lubricating oil temperature (for engine lube oil system), fuel oil pressure (for boiler fuel oil system), etc. PID controls are commonly used for these applications. Advantage of automatic indicating controller is that it controls as well as indicates the parameter reading. These controllers are normally located near the variable measuring point so that use of a transmitter (to transmit parameter signals to the controller) is not required.
explain what is meant by on/of or two step action/control
This is the simplest form of control. Here only two control positions are allowed; on or off. In the case of a valve, it opens or closes by the signal from the controller. Examples for such control system are; water level control for boiler cascade tank, air compressor cut in cut off control, temperature control for fuel oil storage tanks, etc. Let us consider the case of a fuel oil tank heating steam two step controller. It consists of a controller that continuously senses the system parameter (fuel oil temperature). This is called measured value. The desired value or set point (temperature of fuel oil to be maintained) is fixed by the operator. When the temperature of the fuel oil drops below set point, controller sent signal to the steam valve thereby passing steam through coils provided inside the fuel oil tank. Now temperature of fuel oil starts increasing. When the temperature rises to the set point, controller shuts the steam valve. Anyway disadvantage of this control is the large deviation from the desired value or set point. Hence the application of such control is limited.
explain with the aid of a sketch proportional action using a flapper-nozzle mechanism.
See EOOW ORAL/IAMI Sketch pack
Referring to the diagram above, as the measured value deviates from set point, flapper moves closer to the nozzle. This results in an increase in output air pressure. This increase in output air pressure changes the controlled condition (parameter to be controlled), say by closing or opening a valve. At the same time the increased output air pressure act on the feedback bellows also. This negative feedback pushes the flapper away from the nozzle, thereby reducing output air pressure. Now the system is in equilibrium. Remember that an offset is inevitable here. Moving the nozzle away from the feedback bellows increase sensitivity and hunting of the system. Also moving nozzle towards feedback bellows reduces hunting and increases offset. So position of the nozzle (orProportional Band) can be adjusted carefully to obtain a stable system with minimal offset and hunting.
explain what is proportional action
Proportional Action
This is a continuous control action in which the controller output is proportional to the deviation between measured value and desired value. Consider a water tank with an outlet valve, a water supply valve, control lever on a pivot, control strings, and float as shown in the figure. One end of the control lever is connected to the float while other end to the water supply valve. The system is designed in such a way that, when float goes down supply valve is opened more thereby increasing water supply to the tank. Similarly an elevation of the float results in closure of the supply valve. At this point quantity of water supplied and flown out of the tank is same or the system is in equilibrium. Suddenly demand of water increases or the discharge valve is opened more. This result in reduction in water level inside the tank and causes float to lower. As float lowers it raises the control string connected to the water supply valve, thereby increasing the supply of water. Once again supply and demand of water is same. But we can see a drop in water level from the desired value. This deviation from the measured value and desired value is known asoffset. It is an inherent property of the proportional control. Offset can be reduced but cannot be eliminated in such systems. The amount by which the input signal value must change to move the correcting unit between its extreme positions is known asproportional band. This concept can be made clear from the figures shown above. Move the position of the pivot towards right (closer towards the float). Now we can see that a small change in water level causes an amplified effect in opening or closing the supply valve. In other words the system becomes moresensitive. Also here the offset is reduced comparing to the scenario before. Similarly, moving the pivot towards the left cause minor changes for the supply valve even though float lowers or rises drastically. Here sensitivity of the system is less. It is clear that offset can be reduced to minimum when sensitivity is higher. But this results inhuntingof the system. Hunting means excessive fluctuation of the measured value around desired value.
what are instruments?
instruments are devices or sensors that are used for detecting and measuring the properties of elements
what is telemetering?
the remote signal transmission and conversion of a signal from one place or form to another
what is control theory?
control theory is basic concept of hydraulic, pneumatic and electronic control actions that are completed in response to predetermined variables. modern systems use mathematic algorithms completing the logic processes and deciding the control action required.
what are marine control systems?
marine control systems are the application of interdependent control loop functions to marine processes.
