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.
FUEL / OIL TEMPERATURE
In most forms of temperature measurement, the variation of some property of a substance with temperature is utilised. These variations may be summarised as the following:
- Expansion due to temperature increases
- Molecular changes. I.e. changes to the molecules with increased temperatures.
- Changes to its electrical resistance
- Emissivity - Radiation emitted by a fluid as a temperature rises.
The utilisation of these various methods provides us with a very convenient means of classifying temperature - measuring instruments. The majority of these instruments currently in use are, however, of the resistance and thermoelectric type and are applied to the measurement of the temperature of such liquids and gases as fuel, engine lubrication Oil, outside Air, Carburettor Air and turbine gas exhaust.
The monitoring of Oil and Fuel temperature on an aircraft is an essential operation mainly due to the environment and conditions that it is subjected to, not just in flight, but also on the ground as well. Other affects that this variable could have upon the aircraft would be to hinder performance, lessen flight ranges, and endanger passengers and crew due to high explosive and flammable risks.
For Fuel Temperature a resistance thermistor temperature transducer is used. This type of transducer is a temperature sensitive resistor indicates changes in the resistance. The resistance of the thermistor decreases as the temperature increases.
Below is a Schematic arrangement of a temperature sensor. The bulb acts as a safety barrier of protection around the coil assembly. The coil is wound at the bottom end of the former only. This ensures that the coil is well immersed into the hottest part of the liquid, minimising the errors due to radiation and conduction losses in the "bulb" which gives a more accurate reading. A Balancing coil is provided so a standard temperature / resistance characteristic can be obtained. This allows for any deviations in the characteristics of the coil.
These types of transducers, thermistor's are mainly constructed from semi - conductive disc's or beads, which are enclosed in vitreous enamel or in glass envelopes. Due to the efficiency of these the feedback response is extremely rapid.
For this application either of the options for a display could be used, Analogue, Digital or a Cathode Ray Tube. But I have chosen the digital display for reasons that are described in that section (see display units)
OIL PRESSURE
In many of the systems associated with the operation of aircraft and engines, liquids and gases are used the pressures of which must be measured and indicated.
Pressure is measured by instruments systems, which in the majority of applications are of the remote indicating type i.e. their sensor transmitter units, are connected, to a pressure source. Which pass data through an electrical transmission circuit. Which are connected to indicators. On jet aircraft a capsule system is used.
This system would use a resistive transducer, the output could be sent to a conditioning device such as a Wheat stone bridge, this would record maximum output which could then be sent directly to a reading device or could be further conditioned before displayed in the cock pit.
ADC
Analogue to digital converter, converts a continuous analogue signal produced by a digital (off and on) signal for computer processing.
This is an electronic circuit that converts an analogue signal to a digital output. Such a circuit is required to convert the signal device to a digital signal for input into a computer. For example many sensors designed to measure temperature and pressure, produce an analogue signal in the form of voltage and this must be passed through an ADC before computer input and processing. For signals of digital to analogue output, the process is therefore reversed.
WHEATSTONE BRIDGE – Measure an unknown resistance
The Wheatstone Bridge is an old system yet it is still applied to temperature indicators used in some types of aircraft. The most appropriate system to use in this case would be the Radiometer system. The Radiometer system employs two coils moving in a permanent magnetic field, the circuit is formed so that the current supplied to the two coils is in opposite directions, the movement of the coils is proportional to the current that they are supplied with. The direction of the movement of the measuring element is dependent upon the flow of the current.
BACKGROUND INFORMATION – Theory of the Wheatstone Bridge
A current flows in an electrical circuit driven by the potential difference at a battery. Resistance, current and voltage are connected by Ohm’s law.
U = I x R
The current and potential difference (= measured voltage) in each part of the circuit can be calculated with the help of Kirchoff’s rules.
- The sum of the potential drops around any circuit loop must equal the sum of the potential increases.
- At a junction point in a circuit where the current can divide, the sum of the currents into the junction must equal the sum of the currents out of the junction.
Look at the following set-up:
When the transducer feeds information through into the conditioning element it is being converted into a signal that can be represented by the display unit. If that conditioning element happened to be a Wheatstone Bridge then the output to the display would be of a higher degree of accuracy due to the resistance measurements.
DISPLAY UNITS
There are 3 main types off display units and they fall under the following headings:
- Analogue
- Digital
- Cathode Ray Tubes
ANALOGUE
This is by far the oldest method of the three, but never the less, still as effective in certain applications, often consists of a pointer arm or hand coinciding with a calibrated scale in the background. These are mainly used for displaying speed rates and therefore are used for Machometers and tachometers for example. They are also used for speedometers and rev counts on all car ranges.
Can be interfaced with digital displays to provide high standard outputs, but mainly are stand-alone units, which provide reasonably accurate accounts ‘at a glance’.
DIGITAL
As from the word, Digital, it can be explained that these are used as they give approximate readings to the cockpit and the degree of accuracy as at the pilot’s discretion. Many versions of this unit use seven segment displays to provide as their output. Therefore exact numbers are given to pilots and used in applications were accuracy is required as priority. Could be used to display either of the following variables:
- Oil Pressure
- Fuel Temperature
- Fuel Quantity
- Exhaust Gas Temperature. E.t.c.
Cathode Ray Tube
A CRT is a thermionic device, i.e. one which electrons are liberated as a result of heat energy. As shown below, the CRT consists of an evacuated of an indirectly heated cathode biased negatively with respect to the screen, a cylindrical grid surrounding the cathode, and two anodes. When the cathode is heated, electrons are liberated and in passing through the anodes they are made to form a beam (see diagram below) which is what is displayed in the cockpit.
CRT's are also capable of producing a wide rage of colours, which make it possible to differentiate between displays.
The most common colours found in the cockpit are and they translate to:
RED A Warning requiring immediate action
AMBER A Warning requiring immediate attention but not immediate action
GREEN An indication of normal operation
Other displays which are advancing with technology include:
VDU
Computer terminal consisting of a keyboard for input data and a screen for displaying output. The oldest and most popular type of VDU screen is the Cathode Ray tube.
PLASMA
Type of flat display, which uses an ionised gas between 2 panels containing grids of wires. When current flows through the wires the pixel is charged causing it to be illuminated.
LIQUID CRYSTAL
This type of display consists of a liquid crystal sandwiched between polarized filters similar to Polaroid sunglasses. When a segment of the 7-segment display is electrified, the liquid crystal twists the polarized light from the front filter, allowing the light to bounce of the rear reflector and illuminate the segment.
SYSTEM DAMPING
The definition of damping can be explained as the following:
- The deadening of the shock of sudden movement
- The rate at which an electrical oscillation dies away.
As you can observe from the above diagram you can see how the effect of damping, over a period of time, T, decreases the initial movement until it has been eliminated back to a steady stead.
The prime example of this would be a suspension system, on a motor vehicle, the suspension system and shock absorbers on the axis of the car take the effect of the impact and enabling the car to continue on a steady level course.
Excessive and critical damping are also variables that need taking into account, these are states which if uncontrolled can lead to instrumentation damage and breakdown. Staying on the suspension theory as an example if the suspension was altered in any form it would affect the performance of the system. I.e. if the springs and absorbers were tightened excessively then tension would be exerted and the steady state, which is required, wouldn’t be able to be maintained. The damping of instrumentation works on the same principles. If instrumentation was free to vibrate without any additional damping or protection (padding or packaging) then the performance would be affected and also the signals being transferred and converted. With this application of damping accuracy is maintained, but dampening can only be provided to a certain extent, hence, Critical and Excessive.
To ensure that vibration, sudden movements or changes to the environment do not affect the performance of instruments they are all fitted with damping systems. In there simplest form these could be damper pads, which house components acting as a form of protection or absorber. With the implementation of accelerometers, damping can be further advanced.
ACCELEROMETERS
Either mechanical or electro-mechanical, for measuring acceleration or deceleration – that is, the rate of increase or decrease in the velocity of a moving object.
The mechanical types have a spring-supported mass with a damper system, with indication of acceleration on a scale, which a light beam is reflected from a mirror on the mass. The electro-mechanical types use (1) a slide wire, (2) a strain gauge (3) variable inductance, or (4) a piezo-electric or similar device that produces electrically measurable effects of acceleration.
Accelerometers are used to measure the efficiency of the braking systems on road rail vehicles; those used in aircraft and spacecraft can determine accelerations in several directions simultaneously. There are also different types of accelerometers for different applications for example detecting vibrations in machinery and upon systems.
INERTIAL NAVIGATION
Navigation system that makes use of gyroscopes and accelerometers to monitor and measure movement of bodies. A computer calculates the position relative to its initial (starting) position using information supplied by sensors. Inertial navigation is used in aircraft, submarines, spacecraft and guiding of missiles.
BIBLIOGRAPHY
The references used in this assignment were the following:
- Hutchinson's Encyclopaedia
