Automatic Control Engineering and Safety Devices
Posted by: MarinerGalaxyIntroduction
Open loop control and closed loop control are the two main control strategies used in designing a control system. In open loop control system, there is no feedback and the controller action is unrelated to the final output. In closed loop control system, there is feedback and the controller action is related to the final output. There are two types of control systems used onboard. They are process control and kinetic control systems.Process control system is concerned with measurement and control of quantity or condition such as temperature, pressure, flow and level.
Open and Closed Control Loops
Control strategies
The main control strategies used in designing a control system are:
- Open loop control
- Closed loop control
An open loop control system has no feedback and controller action is not related to the final output. For instance, a purifier desludging timer when set at four hours operates only after four hours, even if the purifier is dirty.
A closed loop control system has a feedback from the system and controller action is related to the final output. For example in ALCAP purifier the water content in the output is measured and if water content increases above the set limit then the purifier desludges.
Closed loop control systems are further classified as:
- Feedback control system
- Feed forward control system
The following image explains about principle of control system.
Feedback control system is used in nowadays. It corrects the deviation between set point and the measured value in a system. The basic function of a closed loop feedback control system is shown schematically in the block diagram.
Types
Control systems used onboard can be classified as:
Process control system
It controls the physical quantity or condition of a process like pressure, flow or level.
Kinetic control system
It controls the displacement, velocity or acceleration of the controlled device.
Based on the condition controlled these two systems are further classified as shown.
Process control system is used in:
- Alarm systems
- Remote control of machinery for propulsion such as diesel engines and steam turbines.
- Auxiliary machinery such as boilers, bow thrusters, refrigeration and air conditioning.
- Fire detection, alarm and extinguishing systems.
- Safety systems such as emergency shut down in gas carriers. Bilge and ballast systems.
- Cargo systems
- Inert gas systems in oil tankers.
Kinetic control system is used in:
- Bridge control of steering and propulsion. Power management systems.
- Control of watertight doors in passenger vessels and ferries.
Simple theory of all Control Systems
The feedback controller consists of:
A measuring unit, which measures the state of process, like temperature and pressure transducers.
An input set point device to set the desired value.
Comparator for comparing the measured value with the set point and calculating the deviation. Control unit to correct this deviation so that the process returns to its desired value.
Output unit converting the output from the controller to physical action.
Feed forward control system attempts to correct the effect of disturbances by measuring them in advance and predicting a control action to counteract the disturbances.
Sensors and Transmitters
Introduction
RTD type sensors work on the principle that electrical resistance of electrical conductors increase with increase in temperature. Another type of sensor used for measuring temperature is thermocouple. The flow of liquid with a free surface in a channel is known as open channel flow. Pressure sensor is used to measure the pressure of gases or liquids. Pressure sensor is also known as pressure transducer and pressure transmitter. Viscometer is an instrument used for measuring viscosity. Similarly, torque can be measured using a strain gauge or by Prony brake method or cradled shaft bearing method.
Main and emergency switch boards, power panels and motor controllers are the most vital elements of an electrical distribution system in a ship. The electrical circuit and layout diagrams should be studied properly to find out the role and functions of each key component in a system.
Resistance Temperature Devices
RTD Sensor
Resistance Temperature Detector (RTD) type Sensors
These temperature sensors are based on the principle of increase in electrical resistance of electrical conductors with the increase in temperature. The conductor is usually wound over a rectangular strip of insulting material. The strip is then placed in between the two layers of the stator winding of an alternator. Winding and slot temperatures are monitored with these sensors. They are also used in transformers to measure the core and winding temperature. The two leads of the conductor is connected to a remote resistance measuring instrument whose dial is calibrated in terms of temperature, using the resistance temperature characteristic curve of the conductor.
Platinum, Nickel and Copper are some of the widely used conductors.
Platinum Conductors have high resistance properties and give stable value for a particular range of temperature. They also have high oxidation resistance property. The demerit is that they are much expensive.
Nickel Conductors have largely changing resistance properties. They need temperature compensation at the point of application, They also have high oxidation resistance property. Another merit is that they are not much expensive.
Copper conductors stable as like Platinum conductors but possess low resistance. They do not have high oxidation resistance property. They are cheap.
Most commonly used RTDs are Pt 46, Pt 100, Pt 53, Ni 110 & Cu 9.042.
Thermocouples
Thermocouple is a sensor which is used for measuring temperature. The wires of two different metals are joined together to form a thermocouple circuit. When the junctions are heated or cooled, a voltage will be produced related to temperature difference. These sensors are cheap.
Flow, Pressure and Level Measurement
Introduction
The flow of liquid with a free surface in a channel is known as open channel flow. Pressure sensor is used to measure the pressure of gases or liquids. Pressure sensor is also known as pressure transducer and pressure transmitter. Different types of pressure sensors are bellow gauge, bourdon gauge and diaphragm gauge. Hydrostatic pressure sensor or submersible pressure transducer consists of non- corrosive metal, housing a silicon diaphragm and piezoelectric crystal. This sensor is used in ground water level measurement.
Flow measurement
Open Channel Flow Measurement
Open channel flow is defined as the flow of liquid with a free surface in a channel. Open channel includes Rivers, Irrigation channels, Sewage treatment plants, Industrial waste applications. The flow rate in an open channel can be measured by the following methods.
- Hydraulic structures
- Area-Velocity and
- Slope-hydraulic radius method
Among these, the hydraulic structure method is used most widely in the industries for open channel flow measurement.
1. Hydraulic Structures Method
The Hydraulic structure consists of Primary and Secondary measuring devices. In this method, a calibrated restricting structure is inserted into the channel. This structure provides the proper shape and the velocity for the liquid flow. These restricting structures are called as the Primary measuring devices.
Primary Measuring Devices
These Primary measuring devices are classified into Weirs and Flumes. A Weir is a kind of restriction or dam built across an open channel. Weirs can be classified based on their shapes namely, Triangular (or V- notch weir) , Rectangular weir and Trapezoidal (or Cipoletti weir). A Flume is a specially designed open channel which provides the proper shape, restriction and a slope change in the channel. However flumes are more expensive than the weirs.
Secondary Measuring Devices
The Secondary measuring devices includes the flow meters such as Ultrasonic sensors, Bubblers and Submerged Pressure transducers. These secondary devices are normally used to measure the level of the liquid in the channel. The measured level of the liquid is converted into the flow rate based on the relationship between the flow rate and the liquid level. In modern technology, suitable software is used to perform this conversion.
2. Area-Velocity Method
In the Area-Velocity method, the flow rate can be measured by the known values of average velocity and area of the flow. This can be calculated from the continuity equation given below. X(flow rate)= A* V
The equation shows the product value of the average velocity(V) and area of the flow(A). The average flow velocity and level are measured by Doppler Ultrasonics and Pressure transducers. The Pressure transducers converts the level value into area of the flow.
Advantages
The advantage of the area velocity method is that it can be used to measure the flow rate in case of open channel, full pipe, submerged and reverse flow conditions. This method does not require any installation of weirs or flumes in the channel.
3. Slope-Hydraulic Radius Method
In Slope-hydraulic radius method, the flow rate can be measured based on the Manning formula. The Manning formula depends on the size, shape, roughness and the liquid depth of the channel.
Advantages
The main advantage of this method is that it does not require any weir or flume as in the case of the hydraulic structure. However, the accuracy won't be perfect as in the Hydraulic and Area velocity methods.
Solid Flow Measurement
Solid flow meter is a measuring instrument which is used to measure and to monitor the rate of flow of solid material flow in industries. These flow meters can be used for many solid bulk materials like coal, cement, seeds, sugar, grains, powdered materials, etc. In large industries, these flow meter data can be transmitted to main control panel and can be also used in feedback control system. In industry applications, to get reliable result, proper installation, calibration and regular maintenance are needed.
Flow transducer
Flow transducers measure the flow of materials in a process. This flow of materials can be in solid, gas, or liquid form. All flow control applications utilize the term Q or rate of flow, to define flow measurement in the system. In this section, we will discuss the rate of flow for each kind of material—solid, liquid and gas —and the transducers used to measure them.
Solid flow transducer - It is typically measured with a strain gauge based load cell transducer, which measures the weight of the product. Solid flow measurement frequently integrates the use of a conveyor or belt product transporter with a load cell. The units generally used for this type of flow measurement are lbs/min or kg/min. Figure illustrates an example of how a load cell measures the flow of solids. The equation that describes the flow (Q) is, Q = WV / L. Where, Q = the rate of flow, W = the mass/weight of the solid, V = the velocity/speed of the moving transporter, L = the length of the weight transducer (load cell)
Where, Q = the rate of flow, W = the mass/weight of the solid, V = the velocity/speed of the moving transporter, L = the length of the weight transducer (load cell)
Fluid flow transducer measures fluid flow, where it measures one of two conditions in the process line: pressure differential or fluid motion. The two most common devices for measuring the pressure differential in a process line are Venturi tubes and orifice plates. One of the most common fluid flow transducers for detecting fluid motion is the turbine flow meter. This transducer transforms the flow directly into electrical signals.
Both the Venturi tube and the orifice plate are based on the Bernoulli Effect, which relates the flow
velocity to the pressure differential between two points. These fluid flow meters use pressure transducers, which transform pressure into an electrical signal to determine the pressure differential. The strain gauge and the Bourdon C-tube are the two types of transducers most commonly used in pressure-based flow meters.
These transducers use the bridge circuit and LVDT techniques, respectively, to convert measured pressure values into electrical signals. If low pressures are to be measured, a Venturi tube or orifice plate may incorporate a low-pressure transducer, such as a bellows, diaphragm, or capsule to enhance the pressure reading resolution.
Pressure Measurement
Introduction
The Pressure sensor measures pressure typically of gases or liquids. Pressure sensor is used to control and monitor the thousands of everyday applications. Pressure sensors can be alternatively called pressure transducer and pressure transmitter. In this topic we are going to discuss about working principles, advantages and disadvantages of different types of pressure sensors, namely
Bellow Gauge Bourdon Gauge Diaphragm Gauge
Bellow Gauge
Bellow gauge is type of pressure sensor which is used for measuring low pressure or small pressure difference. Bellow gauge is used in places where the pressure sensor is not only used for measurement of pressure but also used as a recording device. The pressure range determined by this device is mainly by the effective areas of the bellows and the spring gradient.
Bourdon Tube Pressure Gauge
Bourdon tube pressure gauge is a mechanical device used to measure the pressure of gas or fluid. It uses a closed coil tube known as bourdon tube to measure the pressure. It was invented by Eugen Bourdon in the year 1849. The bourdon tube is a device which converts the pressure into displacement. The change in pressure can be mechanically amplified and it is denoted by a pointer. The bourdon tube is available in various shapes namely: c–shaped, spiral and helical. The bourdon tube can be classified in to:
Low-pressure bourdon tube - can measure pressure up to 2000psi High- pressure bourdon tube – can measure pressure above 2000psi.
Bourdon tube pressure gauge generally uses c-shaped bourdon tube. The shapes, sizes, and the material for the bourdon tube depend on pressure range and the type of gauge used.
Diaphragm Type Pressure Gauge What is Pressure?
The force acting per unit area on a surface is called pressure. Pressure can be expressed as,
P=F/A , N/m2 or Pascal where P denotes pressure,F denotes force and A denotes area.
SI unit of pressure is pascal(1 Pascal=1 N/m2).
The Pressure gauge is commonly used to measure the pressure of a gas or liquid. It is also Known as Vacuum gauge.
Diaphragm Pressure Gauge
In Diaphragm Pressure gauge, instead of using liquid levels to measure difference between known and unknown pressures, the elastic deformation of the diaphragm is used.
Merits
Compared to U- tube faster frequency response.
Accuracy up to ± 0.5 of full scale. Good linearity.
Demerits
More expensive than others.
Applications
It does filtering in water treatment process. Pharmaceutical applications.
To find Pressure value from membrane deflection click here
Level Measurement
Hydrostatic Type Liquid Level Measurement
Hydrostatic type liquid level measurement is a method of measuring liquid levels in an open tank or in an atmospheric pressurized closed tank. Hydrostatic pressure means that when an uniform density liquid is at rest, the height of the liquid column creates a static pressure.
The formula to calculate the hydrostatic pressure of a liquid column is: Hydrostatic pressure(N/m2) = Density of the liquid(kg/m3) X height of the liquid column(m) X gravity(m/s2).
Therefore, hydrostatic pressure is proportional to liquid density and the height of the liquid column. The hydrostatic liquid level measurement technique is used in many places like rivers, boreholes, manholes, tanks and waste water plants.
A Hydrostatic pressure sensor or submersible pressure transducer is used in this measurement. It consists of non-corrosive metal, housing a silicon diaphragm and piezoelectric crystal. Depending on the liquid density, the proper range of hydrostatic pressure sensors have to be chosen. While measuring the liquid level, the hydrostatic pressure sensor is placed at the bottom of the tank. Due to the height of the liquid column in the tank, pressure is created at the bottom of the liquid tank. This pressure is called hydrostatic head pressure. The amount of pressure created depends on the height of the liquid column. And this pressure is not affected by the volume of the liquid in the tank. So, if the liquid level increases, hydrostatic pressure also increases at the bottom of the tank. The silicon diaphragm of the hydrostatic pressure sensor senses the changing pressure. Then the piezoelectric crystal in the hydrostatic pressure sensor generates corresponding electrical signals. From the crystal, the electrical signal is sent to the measuring meter which is placed outside the liquid tank. The meter converts this electrical signal into a corresponding level measuring reading.
This sensor is also used in ground water level measurement. At this application, the connecting wire between the sensor and the meter may be affected while lightning hits a nearby area. To avoid this problem, a copper shielding is provided over the connecting wire and sensor parts. Then transient surge protectors are also connected in both ends to protect the sensor and meter.
Nucleonic Gauge
Gamma rays are used as non-contact type liquid level measurement. The device used for detection of liquid levels, using gamma rays, is called a Nucleonic gauge. The schematic diagram of a Nucleonic gauge is shown. A source of gamma rays is mounted from outside at the top of the tank. A radiation detector, like a Geiger Muller tube, is attached at the opposite side to the gamma ray source. The detector continuously measures the liquid level. It is working based on the principle that absorption of gamma rays varies with respect to the liquid level in the tank.
Gamma rays can penetrate the tank walls to reach the liquid inside the tank. The intensity of gamma rays depends on the liquid level. When the tank is empty, maximum radiation reaches the detector. When the tank contains the liquid, some of the gamma rays will be absorbed by the liquid. So lesser radiation will reach the detector. Thus output of detector is inversely proportional to the liquid level in the tank. Geiger Muller tube measures the radiation in the form of pulses. These pulses are counted by an electronic counter which is calibrated in terms of liquid level.
Displacer Type Level Indicator
Mechanical level indicator is used to measure the level of any liquid in both open and closed systems. There are many types namely,
- Float type
- Displacer type
- Diaphragm type Differential pressure gauge Air bubbler
Displacer level indicator works based on Archimede's principle. The displacer weight will be higher when it is measured in air than in water. Consider a cylindrical displacer having mass m and height h, is connected to a spring balance. When the displacer is dipped into liquid, it displaces some amount of liquid. The volume of liquid displaced is proportional to the height of the displacer up to which it has been immersed in the liquid. The displacer type level indicator suits for measuring clean liquid substances. Basically, a displacer mechanical level indicator is of two types namely,
Torque-tube displacer Spring-balance displacer
Float Level Indicator
A float level indicator is used for indicating the level of the fluid. The float can be metal spheres, disc or cylindrical shaped floats.
Optical Level Indicator
The optical level indicators provide very accurate results under critical conditions. This type of level indicators are widely used in industrial applications. There are four main types of optical level indicators used in industries. They are laser based level indicators, optical fiber based level indicators, refraction based level indicator and reflection based level indicator.
Ambient Temperature Compensation
Ambient temperature variations are generally faced in industrial based environments. Individually characterized transmitters are used to compensate such variations. As a result, the precision and constancy has been improved in the measurement. This decline in variability permits the process to be done closer to the set point. This in-turn results in greater output.
In a filled thermometer, the total internal pressure is considered as the measured variable. This pressure is due to the following two factors:
- The temperature around the bulb.
- The ambient temperature around the remaining system.
The main use of the installation is to determine the process temperature around the bulb. Hence, it is necessary to remove the influence of ambient-temperature variations on the reading. The error produced by ambient-temperature variations is not the same for all the types of fills. The error rises as the bulb or span becomes smaller or as the length of the capillary increases.
The thermal system can be classified into the following classes:
- Class I – liquid-filled systems/li>
- Class II – vapor-filled systems
- Class IIA – liquid in the bulb is in balanced state with its own vapor
- Class IIB – Only the bulb has the liquid and rest of the system is filled with vapor Class IIC – Can be operated as Class IIA or Class IIB systems
- Class IID – Two liquids are present in the bulb (one is volatile and the other is non-volatile)
- Class III – Gas-filled systems
- Class V – Mercury-filled systems
If the influences of ambient temperature are compensated in both the capillary and in the readout device, the design is said to be fully compensated. Fully compensated design is assigned by the letter “A” and is being included to the class designation number I, III or V. If the ambient temperature of both the case and the capillary are the same and the capillary’s length is comparatively small, it might be adequate to keep the capillary portion uncompensated and provide the compensation only for the case. When the class designation numbers I, III or V is case-compensated, the letter “B” is included to the designation.
Class II (also called as vapor-filled) systems do not require any such compensation. It is because the liquid or vapor interface in this system is within the bulb all the time. For class IIA filling, it is essential to retain all the portions of the capillary to have a temperature less than that at the bulb. Else, the liquid in the capillary might vaporize and this in-turn may produce large errors.
Figure 1: Full Compensation
The figure given above illustrates about the method of fully compensated I, III or V systems. We can see that, both the capillary and the spiral pressure sensor are replicated. Here, the compensating capillary is locked. The length of this compensating capillary is equal to the length of the active capillary without including the bulb. In order to avoid the ambient effects, both capillaries are made to run parallel to each other and both bourdon spirals are connected.
Figure 2: Class V - full compensation with internal wire
In Class V systems (mercury-filled), full compensation can be offered without the expenditure of replicating most of the thermal system. This cost-saving attitude is described in figure-2 given above. A Invar wire is introduced inside the single active capillary. Invar is actually a nickel alloy whose coefficient of thermal expansion is very less. Hence it is probable to choose the diameters of both the Invar wire and the capillary in such a way that a rise in ambient temperature will result in increasing the annular space between the Invar wire and the capillary. Hence, we can say that there is no variation in the internal pressure of the thermal system even if the ambient temperature around the capillary changes.
Case Compensation
When the ambient temperature of both the capillary and the case are equal and the volume of the capillary is very small when compared to the volume of the entire system, it is probable to keep the capillary portion uncompensated and fix the compensation only to the case.
Case compensation (Class IB, Class IIIB, or Class VB) can be attained by adding a bimetallic spring which produces a force approximately equal to the force caused by the ambient temperature variation (see figure 3). This is because whenever there is a change in the internal pressure with respect to the measured temperature, the volume of the bourbon spiral also varies.
Bimetallic spring offers the case compensation that is effectual only at the bulb temperature. The bimetallic spring is generally fixed at midrange. As a result, under- compensation occurs at the top, whereas, over-compensation occurs in the bottom half of the range.
Figure 4: Liquid-filled Capillary tube thermometer
It is possible to do uncompensated operation (figure 4) with Class I (liquid-filled) systems when the length of the capillary is very small and with all categories of Class II (vapor-filled) systems. Class II and Class V systems must be compensated at all times. In gas-filled Class III systems, the capillary error increases quickly with respect to the bulb temperature and can be minimized by raising the volume and/or span of the bulb.
Viscosity and Torque Measurement
Introduction
Viscometer is an instrument used for measuring viscosity. Various types of viscometer include rotational viscometer, rheometers, bubble viscometers, falling piston viscometers, etc. Similarly, torque can be measured using a strain gauge or by Prony brake method or cradled shaft bearing method.
Viscosity Measurement
A fluid's resistance to flow is known as viscosity of that fluid. That is low viscosity fluids flow easily and high viscosity fluids flow slowly. Viscosity of the fluid is directly proportional to the pressure applied to that fluid and inversely proportional to the temperature.
An instrument used for measuring viscosity is known as viscometer. This is also known as viscosimeter. This viscometers measure the viscosity of the fluid only under one flow condition. Viscosity of the fluid with varying flow conditions can be determined by rheometer.
Types of viscometer
- U-tube viscometers or Ostwald viscometer
- Falling piston viscometers
- Rotational viscometers
- Bubble viscometers
- Rheometers
Ostwald viscometer is most commonly used viscometer. In this viscometer, flow time of the fluid of whose viscosity is to be determined is compared with flow time of the known viscosity fluid. Commonly water is used as known viscosity fluid.
Torque Measurement
Measurement of Torque using Strain Gauge
The force which gives rotating movement to a body is known as torque. Methods of torque measurement:
- Cradled shaft bearing method Prony brake method
- Measurement using a strain gauge
Cradled shaft bearing method
In this method, torque measurement is done by cradling the reaction force in the shaft bearing. Then the torque is calculated by the following formula. T = F.L
Where,
T = Torque
F = Reaction force L = Arm length
Prony brake method
In this method, a rope is wound around the flywheel which is mounted on the shaft. One end of the rope is connected to a spring balance and the other end is connected to a mass. This spring balance is graduated on a scale. Because of the rotation of the fly wheel, some moment is applied on the rope. This moment is equal to the applied torque. By measuring this moment, torque can be calculated.
Measurement using strain gauge
The method most commonly used is the torque measurement method. This method does not disturb the measuring system and output will be in electrical quantity. Compared to the above two methods, accuracy of the measurement is high.
Magnetostrictive Transducer
The permeability of magnetostrictive material is changed when it is placed in the magnetic field. This effect is known as magneto strictive effect. A transducer which utilizes the magnetostrictive effect is known as magnetostrictive transducer. It is used to convert magnetic energy into mechanical energy.
Oil/Water Interface and Oil in Water Monitoring
Arrangement of Oil Monitoring System
This arrangement is shown in the figure below.
The oily water discharge from the separator passes via a sample chamber. A beam of light is made to fall directly on a photo electric cell. At the same time, a section of light beam is made to pass through the sample chamber and fall on another P.V. cell. The intensity of light passing through the sample chamber gets reduced depending upon the quantity of oil in the water in the sample chamber. Both the lights falling on the P.V cell are compared treating the light falling directly on the P.V cell as the reference one. The difference between the lights is registered and calibrated in a meter to show the quantity of oil content. The oil monitoring system is used to stop the discharge automatically when the oil content reaches a set value.
The following inputs are measured for the arrangement to function effectively.
The discharge quantity rate of the pump. The speed of the ship. The oil content of the water discharged.
These inputs are transferred and processed by a computer into various outputs which carry out the desired operations to stop the discharge. All the parameters like the time and instantaneous discharge etc are recorded.
Such a system has some problems. Choking of the filter happens frequently leading to reduction of oily – water flow. The glass in the detector gets smeared with oil making it less transparent causing misinterpretation of the measurement. The solenoid valve also gets stuck up due to the accumulation of oil affecting its closing and opening timings. Even the closing and opening operations may get affected some times.
Emergency Lighting
Based on the ship's classification and cargo carrying capacity, the Safety of Life at Sea has imposed the minimal requisites for emergency lighting throughout the ship. The emergency lighting receives power from the ship's emergency switchboard which is of 220 V A.C. Some of the other emergency lights receive power from the ship's 24 V D.C battery.
There are few shipping companies which fit special battery supported lighting fittings along main escape paths in the engine room, accommodation and even at the lifeboat positions on the deck. This type of emergency lights are fitted to provide light instantly even if the mains are under failure.
If there is a requirement for the boat station emergency lights, then the lights are switched on. These lights are battery supported lighting systems. The battery functions only for few hours. This type of power supply arrangement is termed as uninterruptible power supply.
Essential requirements on all ships is periodic checking and testing of all emergency lights.
Pneumatic Flapper/Nozzle System
Pneumatic Flapper/Nozzle System
Pneumatic 20 – 100 kPa, Analogue 4 to 20 mA Signals Pneumatic Pilot Relays
Control Air Supply
Pneumatic Control
Pneumatic control system uses low pressure air of about one to two bars as its working medium.
A flapper nozzle arrangement is used for generating and transmitting the pneumatic signal. This signal is proportional to the input signal and is used for controlling a process or condition.
Controllers are designed to provide the required control action for automatic control of various systems. The control action can be proportional, derivative, integral or a combination of these actions.
Pneumatic 20 – 100 kPa, Analogue 4 to 20 mA Signals
P/I converter or Pneumatic pressure-to-Current is a transducer. It converts pressure input to a current in miliampere range. P/I converter is a common instrument in automation system. Like a temperature senor or level sensor it transmits data to other source like PLC, instrument to control systems only here the input is pressure.
Bellow gauge
Bellow gauge is type of pressure sensor which is used for measuring low pressure or small pressure difference. Bellow gauge is used in places where the pressure sensor is not only used for measurement of pressure but also used as a recording device. The pressure range determined by this device is mainly by the effective areas of the bellows and the spring gradient.
Pneumatic Pilot Relays
When the imbalance detection system is most sensitive, self-balancing mechanisms are more accurate. One such self-balancing mechanism is the fictitious pneumatic laboratory scale. It can also be said that the relationship between the measured variable (mass) and output signal (air pressure to the gauge) will be more precise when the baffle/nozzle mechanism responds aggressively to slight out-of-balance conditions.
Reduction in the size of the orifice can make a plain baffle/nozzle mechanism to be extremely sensitive. However, the ability of the nozzle to provide increasing backup pressure to fill the bellows of significant volume is reduced correspondingly by a problem caused by decreasing size of the orifice. Thus it can be said that a greater sensitivity to baffle motion can be obtained from a smaller orifice. On the other hand the smaller orifice will also limit the rate of airflow available to fill the bellows thereby slowing the response of the system. Also the smaller orifices become more susceptible to plugging because of the impurities in the compressed air.
Alternatively, the baffle/nozzle mechanism can be made more sensitive by amplifying its output pressure with the help of some other pneumatic device. This technique is similar to increasing the sensitivity of voltage-generating electronic detector by passing its output voltage signal through an electronic amplifier. The self-balancing system is made even more precise by making small changes in detector output to bring about bigger changes in the amplifier output. Then we need a pneumatic amplifier that amplifies small changes in air pressure and converts them into larger changes in air pressure. We need to find a pneumatic device similar to the electronic transistor that lets one signal control another.
Analysis of pneumatic mechanism and its electrical analogue is given below.
The distance between the plug and the seat changes with the upward and downward movements of the control rod by an external force. There is a change in the amount of resistance experienced by the escaping air. As a result, the pressure gauge registers varying levels of pressure. Only a slight difference in function is noted between this system and a baffle/nozzle system. Both the mechanisms are based on the principle of one fixed (the orifice) and one variable restriction. In this mechanism, the pressure of the compressed air source is reduced to some lesser value.
By extending the control rod and adding a second plug/seat assembly, we can improve the sensitivity of the pneumatic mechanism. The mechanism thus obtained with dual plugs and seats is called as pneumatic pilot valve. The diagram given below illustrates a pilot valve along with its electrical analogue.
With the movement of the control rod, both variable restrictions change in complementary fashion. When one restriction opens up, the other one shuts down. This combination of changing in opposite direction of two restrictions leads to much more aggressive change in output pressure as registered by the gauge.
A pilot valve of the similar design changes the directions of the two plugs and seats. The only functional difference between this pilot valve and the other one mentioned above is an inverse relationship between control rod motion and pressure.
All that we have managed to accomplish at this point is to build a better baffle/nozzle mechanism. However, we do not have a pneumatic equivalent of an electronic transistor at this time. In order to achieve that we need to allow an air pressure signal to control the motion of the contrtol rod of a pilot valve. This can be accomplished by adding a diaphragm as shown in the figure give below.
The diaphragm is a thin disk of metal sheet. The incoming air pressure signal presses upon the diaphragm. Force on the diaphragm is given by a simple standard force-pressure-area equation with signal pressure (P) and diaphragm area (A). F = PA
The elasticity of the metal allows the diaphragm to function as a spring if the diaphragm is taut. This allows the conversion of force into displacement (motion) forming a definite relationship between control rod position and applied air pressure. Therefore the applied air pressure input will exert control over the output pressure. The pilot valve will be turned into a pneumatic relay with the addition of an actuating system to the pilot valve. This pneumatic relay is equivalent to the electronic transistor.
The two figures given below illustrate how the input air signal exerts control over the output air signal.
The pneumatic relay can be classified as a direct-acting relay because it has a direct relationship between input and output pressures. We can obtain a reverse-acting relay if we add an actuating diaphragm to the first pilot valve design as shown below:
As the gain of any electronic amplifier circuit, the gain of any pneumatic relay is defined as the ratio of output change to input change.
For example, consider the change of input pressure to be ∆2 PSI when an output pressure change is ∆12 PSI. Then the gain of the pneumatic relay is 6.
The performance of the pneumatic system can be improved by adding a force-balance system like our hypothetical laboratory scale to the pressure-amplifying relay.
Since the nozzle’s back pressure is amplified by the relay, the response of the force-balancing bellows will be even more aggressive to any change in baffle position than it is without the relay. Thus the scale is made more sensitive with better ability to sense even small changes in the applied mass than when compared without an amplifying relay.
Many pneumatic instruments with high sensitivity amplifying relay have been designed by the Foxboro Corporation.
A pair of valves was required for the motion of the diaphragm; one valve with a cone-shaped plug and the other valve with a metal ball for a plug. The ball-plug was used to supply air to the output port and the cone-shaped “stem valve” plug was used to release the excess air pressure to the vent port.
A different style of amplifying relay in some of the pneumatic instruments was adapted by the Fisher Corporation:
The gain obtained by the Fisher relay was much less when compared to the Foxboro relay. This is because in the Fisher relay, the output pressure acted against the input pressure by exerting force on a sizable diaphragm. Also in the Fisher relay, the movable vent seat design made it a non-bleeding type, i.e., it had the ability to close both the supply valve and the vent valve simultaneously. This feature allowed it to hold an output air pressure between saturation limits without venting a considerable amount of air outside through the vent valve. On the other hand, the Foxboro relay design was a “bleeding-type”, in which the ball and stem valves could never close at the same time. This allowed bleeding of some compressed air outside as long as the output air pressure remained somewhere between saturation limits.
Pneumatic Control
Pneumatic control system uses low pressure air of about one to two bars as its working medium.
A flapper nozzle arrangement is used for generating and transmitting the pneumatic signal. This signal is proportional to the input signal and is used for controlling a process or condition.
Controllers are designed to provide the required control action for automatic control of various systems. The control action can be proportional, derivative, integral or a combination of these actions.
Working and applications of Operational Amplifier
OP-amp consist of one are more differential amplifiers, level translators and output stages. Operational amplifier is a direct coupled high gain amplifier. OP-amp is very popular device handling in analog signals such as in computer mathematical functions.
Functions
Amplification
Buffering Integration/Differentiation Addition/Subtraction Golden rules of op-amp analysis Rule1: VA=VB
Due to the mismatch of similar transistors in differential stages,
op-amp can’t produce zero output while it has some input signal. By applying threshold voltage the differential output become perfect zero level.
Op-amp verifies with the input terminals and swings the output terminal because so that external feedback network gets the input differential to zero.
Rule2: IA=IB=0
Since the op-amp input impedance is infinite it draws no current. The inputs are connected to its essential open circuit.
Ideal characteristics of op-amp
Open loop gain infinite Input impedance infinite Output impedance low Bandwidth infinite
Zero offset, ie, V0=0 when V1=V2=0
