I will be showing the best possible display units and the best possible transducers to be used for the above tasks. Also the effects on the output with the introduction of a Wheatstone Bridge and information on system damping. A monitoring or measuring system in its simplest form is shown here below. The transducer, converting the input signal into a different output, feeds information into the conditioning element, i. e. Wheatstone Bridge, Computer processor, this then affects the output from the transducer and enables signals to be received by the display units,
I. e. analogue or digital, for the pilots observation. TRANSDUCER – A power transforming device for which the input and output are of different type, e. g. Electrical, Acoustic and Optical. Device that converts one form of energy into another, for example a Thermistor is a transducer that converts heat into an electrical voltage and an electric motor is a transducer that converts an electrical voltage into mechanical energy. EXHAUST GAS TEMPERATURE (E. G. T) To monitor the temperature of the exhaust Gas the application of Thermo-electric Transducers are implemented.
When 2 dissimilar metals or alloys are joined together at each end to form a thermocouple as shown below and the ends are at different temperatures, an electro – motive force (e. m. f) will be developed causing a current to flow around the circuit. The magnitude of the e. m. f. depends on the temperature difference between the 2 junctions and on the materials used. This thermo-electric effect is known as the Seebeck effect and is widely used in temperature-measurement control systems. l ? 1 i?? C ? 2 i?? C The main problems with thermocouples are corrosion, oxidation, or general contamination by the atmosphere of their location.
These problems can be overcome by the selection of a protective sheath that does not react with the atmosphere or fluid. Although they do give a direct voltage, this is generally small – in the order of mill-volts – and often requires amplification. Advantages of using thermocouples are that temperature at localised points can be determined, because of the small size of the thermocouple, and they are robust, with a wide operating range from -250i?? C to 2600i?? C. The most suitable instrumentation output for Exhaust Gas Temperature would be an analogue display unit.
The gas-turbine aircraft engine vibration-indicating system features include: engine curls rotation frequency measurement with two independent measuring channels, vibration level measurement with three engine-mounted piezoelectric sensors using digital broadband filters on each vibration sensor or digital monitoring narrow-band filters on curls main harmonics frequencies, as well as a comparison of measured and extreme limits of vibration, ‘excessive vibration’ and ‘vibration limit’ signals formed when limits are exceeded.
Engine vibration is, of course, something that is unwanted, but unfortunately it cannot be entirely eliminated even with turbine engines, which have no reciprocated parts. It can only be kept down to the lowest possible levels. In order, therefore, to monitor vibration and to indicate when the maximum amplitude on any engine exceeds a present level, systems have been developed which come within the engine control group of instrumentation.
A system contains essentially of a vibration pick – up unit mounted on the engine at right angles to the axis, an amplifier monitoring unit and a moving coil microammeter calibrated to show vibration amplitude in thousands of an inch. (mils) From the diagram on the opposite page, the pick – up unit is a linear – velocity detector that converts the mechanical energy of vibration into an electrical signal of proportional magnitudes. It does this by means of a spring-supported permanent magnet suspended in a coil attached to the interior of the case.
As the engine vibrates, the pick – up unit and the coil move with it; the magnet, however, tends to remain fixed in the space because of inertia. The motion of the coil causes the turns to cut the field of the magnet thus inducing a voltage in the coil and providing a signal to the amplifier unit. A warning light on the instrument panel is incorporated in the system to warn the pilot if an unacceptable level of vibration is reached, enabling the engine to be shut down and so reduce the risk of damage. FUEL QUANTITY
The measurement of the quantity of fuel in the tanks of an aircraft fuel system is an essential requirement, and in conjunction with measurements of the rate at which the fuel flows to the engine. For larger aircraft fuel levels are measured in terms of an electrical capacitance, which provides more accurate fuel gauging. A float type fuel quantity indicating system can be used. The components of these are shown schematically below. The float may be a cork specially treated to prevent fuel absorption.
This is attached to an arm pivoted to permit angular movement that is transmitted to an electrical element consisting of both a wiper arm and potentiometer, or a Desynn type of transmitter. In it basic form, using capacitive transducers, a capacitance – type fuel – gauge system consists of a variable capacitor located in the fuel tank, an amplifier and an indicator. The complete circuit forms an electrical bridge that is continuously being rebalanced as a result of difference’s between the capacitance’s of the tank capacitor and a reference capacitor.
The fuel control and monitoring system for the Airbus A340-500/600 is the highest performance and most comprehensive system of its type today. The system provides CG and transfer control – burn sequence, lateral balance and CG management, automatic manual refuel defuel jettison control, fuel quantity measurement and indication to MIL-G-26988 Class III accuracy, and fuel temperature measurement and indication. Two Capacitors are fully immersed in the fuel tank. Fuel does not act as an electrolyte this allows the two plates to store an electric charge.
Plates take the form of two tubes mounted concentrically with a narrow airspace between them. A potential difference is set up between the dielectric therefore the electric charge (capacitance) decreases which gives a signal voltage to change the pointer on the display housed in the cockpit. With this method accurate fuel measures are recorded but weight and cost is increased due to the equipment required. The best suitable display would be an analogue unit. ENGINE REVS
The measurement of the engine speed is of considerable importance, since together with such parameters as manifold pressure, torque pressure and exhaust gas, it permits an accurate control over the performance of the appropriate type of engine to be maintained. With reciprocated engines the speed measured is that of the crankshaft, while with turboprop and turbo jet engines the rotation speed of the compressor shaft is measured, such measurement serving as a useful indication of the thrust being produced.
The indicating instruments in the cockpit as referred to ass tachometers. The above method involves a magnet, which is continually rotated by a flexible shaft coupled to a drive outlet at the engine. An alloy cup – shaped element (known as a drag cup) fits around the magnet such that a small gap exists between the two. The drag cup is supported on a shaft to which is attached the pointer and a controlling spring. As the magnet rotates it induces eddy currents in the drag cup, which tend to rotate the cup at the same speed as the magnet.
This, however, is restrained by the controlling spring in such a manner that for any one speed, the eddy current drag and spring tension are in equilibrium and the pointer arm then indicates the corresponding speed on the tachometer dial. The best possible display unit being analogue, as tachometers and machmeters are prime examples. With the use of inductive transducers the speed in relation to revolutions per minute can be achieved. This is due to the engine shaft has apertures (teeth) in its design.
An Inductor would be positioned next to these and as the engine operates the shaft rotates and the teeth do also cutting into the magnetic field of the inductor. This then produces a signal which is proportional to the size of the shat a number of teeth. As the teeth cut the magnetic field of the coil an electromotive force (e. m. f. ) is induced with an alternating frequency, the frequency is proportional to the rotational speed of the rotor, therefore the frequency is a measure of the engine speed.