Fabrication Process:

Various Open and Closed Mould Process:

OPEN MOLDED

Prototype: Working with your team the process begins with the creation of an exact prototype of your part based on your specifications.

Hand lay: The process of applying resin (usually polyester) to a reinforcement (usually fiberglass) and consolidating (removing the air) by hand with a brush and or roller within a one-sided open mold.

Spray up: Another option used to apply resin inside an open mold, the resin is sprayed, which speeds the process.

CLOSED MOLDED

LRTM: A composite molding process with two counter molds (male and female). Typically, the molds are joined, vacuum clamped, and resin is injected using vacuum assist into the mold cavity.

Closed molded parts feature a two-sided Class “A” exterior finish, and superior part-shape accuracy and quality. These parts can be fairly simple, flat shapes, or more complex shapes, such as head doors for a recreational boat.

Synthetic fibres:

Synthetic fibres are made from polymers, many of which are obtained from petroleum. Some common synthetic fibres are nylon, rayon, terylene, acrylon and cashmilon. They can be placed into two groups.

1. Fibres made from cellulose

2. Fibres made by joining monomers.

Synthetic materials are cheap, strong and attractive for clothing. They are easy to maintain as they are easy to wash, light in weight and resistant to wrinkles, moths and molds. When a new synthetic fibre is developed, it is given a new name by the trade commission. In order to receive such a name, the new product must have useful properties for the consumer.

Manufacture of synthetic fibres

Most synthetic fibres are made by forcing liquids through tiny holes in a metal plate and allowing them to harden. A wide range of liquids produces a great variety of fibres. The metal plates are called spinnerets. They are made of gold or platinum because these metals are not affected by most chemicals. The size of the spinneret is about the size of thimble and it has 10 to 150 small openings, depending on the thickness of the strand wanted. Different synthetic fibres are made from different raw materaials.

Rayon:

It is also called artificial silk. Rayon is made from cellulose. There are several varieties of rayon. Buyt rayon produced by the viscose process is the most important. The ingredients for making viscose rayon are

1. Cellulose (C6H­10O5),

2. Sodium hydroxide (NaOH),

3. Carbon disulphide (CS2), and

4. Sulphuric acid (H2SO4).

Manufacture of rayon:

The manufacture of rayon involves the following steps.

1. Cellulose in the form of wood pulp is treated with NaOH.

2. On adding CS2, it dissolves completely and a yellow syrup-like liquid called viscose is formed.

3. Viscose is forced through the fine holes of the spinneret into a solution of dilute H2SO4. Silk-like threads are formed. This product is viscose rayon.

Uses of rayon:

Rayon can be mixed with cotton or wool, which makes it more suitable for our needs. It is a good fabric or sarees. When mixed with cotton it makes a good dress material. Aprons and caps are preferably made of rayon. On mixing it with wool, it serves as a good fibre for making carpets. Bandages and lints for dressing wounds are made of rayon. Hosepipes and conveyor belts are also made from rayon.

Perspiration weakens rayon fibres and they lose strength when wet.

Acetate:

Acetate is another well-known fibre made from wood pulp. The reaction between wood pulp (cellulose) and acetic acid is the basis for this manufacture of this fibre. Acetate is made of fibres that do not wrinkle or shrink as much as rayon. It is an efficient smoke remover, thus it is used in cigarette filters.

Acetate fibres melt when burned. They are destroyed by pressing with very hot iron. Some dry-cleaning solvents dissolve the fibres.

Nylon:

Nylon is a polymer made of polyamide chains. The basic materials for making nylon are coal and petroleum. The polymer is squirted through spinneret holes to form nylon threads. The strands are then stretched four times their original length. The stretching forces the molecules to line up, giving nylon an increased strength and making it more elastic. Nylon is light weight, fine and durable. It is resistant to moths and molds. It absorbs very little water, therefore it dries quickly.

Uses of nylon:

Hammocks, fishing nets tyre cords, ropes, bristles of brushes and parachute fabrics are all made of nylon fibre. As nylon is elastic in nature, it is a good material for making stockings and socks. Nylon sarees are quite common in our country.

Nylon has a few weaknesses, as it absorbs very little moisture it is difficult to dye. It produces static electricity when rubbed. Being a non-cellulose fibre, it requires low to moderate ironing heat.

Polyester:

Acrylic and polyester are non-cellulose fibres. They are manufactured from petroleum products. Terylene and Dacron are examples of polyesters. These fibres are easy to wash; they dry quickly, and resist chemicals and wrinkles. They are difficult to dye. These fibres blend well with natural fibres in making cloth. Terylene is often mixed with cotton to make terycot, with wool, it gives terywool. Clothes made of these are more comfortable to wear than pure terylene.

Resin Types

The resins that are used in fibre reinforced composites can also be referred to as ‘polymers’. All polymers exhibit an important common property in that they are composed of long chain-like molecules consisting of many simple repeating units. Man-made polymers are generally called ‘synthetic resins’ or simply ‘resins’. Polymers can be classified under two types, ‘thermoplastic’ and ‘thermosetting’, according to the effect of heat on their properties.

Thermoplastics, like metals, soften with heating and eventually melt, hardening again with cooling. This process of crossing the softening or melting point on the temperature scale can be repeated as often as desired without any appreciable effect on the material properties in either state. Typical thermoplastics include nylon, polypropylene and ABS, and these can be reinforced, although usually only with short, chopped fibres such as glass.

Thermosetting materials, or ‘thermosets’, are formed from a chemical reaction in situ, where the resin and hardener or resin and catalyst are mixed and then undergo a non-reversible chemical reaction to form a hard, infusible product. In some thermosets, such as phenolic resins, volatile substances are produced as by-products (a ‘condensation’ reaction). Other thermosetting resins such as polyester and epoxy cure by mechanisms that do not produce any volatile by products and thus are much easier to process (‘addition’ reactions). Once cured, thermosets will not become liquid again if heated, although above a certain temperature their mechanical properties will change significantly. This temperature is known as the Glass Transition Temperature (Tg), and varies widely according to the particular resin system used, its degree of cure and whether it was mixed correctly. Above the Tg, the molecular structure of the thermoset changes from that of a rigid crystalline polymer to a more flexible, amorphous polymer. This change is reversible on cooling back below the Tg. Above the Tg properties such as resin modulus (stiffness) drop sharply, and as a result the compressive and shear strength of the composite does too. Other properties such as water resistance and colour stability also reduce markedly above the resin’s Tg.

Although there are many different types of resin in use in the composite industry, the majority of structural parts are made with three main types, namely polyester, vinylester and epoxy.

Properties of Resins:

Properties such as strength and durability are used to describe various types of resins’ physical and chemical characteristics. Resins are generally known for having superior strength and exceptional durability under various laboratory and environmental conditions. In addition, some types of resin can have variable adhesive and mechanical properties. Synthetic resin has properties similar to those of natural resin, but they are chemically different.

Engineering applications use chemical resin to produce a product that is resistant to both impact and fatigue. Other important resin properties for engineering and chemistry purposes include insolubility and fire resistance. Resin products are designed to encompass all of these properties, because the products undergo extreme conditions in terms of water abrasion, temperature changes or direct impact. Some common chemical resins include polyoxymethylene, also known as Acetal; polycarbonate; and tetrafluoroethylene, also known as Teflon TFE.

Products made with chemical resins can include centrifuge ware, safety shields and filter ware. These products are designed to withstand extreme temperatures and aqueous chemical environments. Acetal is especially resistant to organic solvents and may be reinforced with glass fibers. Polycarbonate is a type of clear thermo-plastic that is non-toxic and extremely rigid. Tetrafluoroethylene products have superior chemical resistance.

Application of resins:

The hard transparent resins, such as the copals, dammars, mastic and sandarac, are principally used for varnishes and adhesives, while the softer odoriferous oleo-resins (frankincense, elemi, turpentine, copaiba) and gum resins containing essential oils (ammoniacum, asafoetida, gamboge, myrrh, and scammony) are more largely used for therapeutic purposes and incense.

Resin in the form of rosin is applied to the bows of musical string instruments because of its ability to add friction to the hair to increase sound quality.

Ballet dancers, as well as boxers in the old days, may apply crushed resin to their shoes to increase grip on a slippery floor.

Resin has also been used as a medium for sculpture by artists such as Eva Hesse, and in other types of artwork.

In the early 1990s, most ten-pin bowling ball manufacturers started adding resin particles to the covers of bowling balls. Resin makes a bowling ball tackier than it would otherwise be, increasing its ability to hook into the pins at an angle and (with correct technique) making strikes easier to achieve.

Resin is also used in stereo-lithography.

Netting Analysis:

The analysis of filament-wound structures which assumes

(1) That the stresses induced in the structure are carried entirely by the filaments, and the strength of the resin is neglected.

(2) That the filaments possess no bending or shearing stiffness, and carry only the axial tensile loads.

SANDWICH CONSTRUCTION:

BASIC DESIGN PRINCIPLES:

Structural sandwich is a layered composite formed by bonding two thin facings to a thick core. It is a type of stressed-skin construction in which the facings resist nearly all of the applied edgewise (in-plane) loads and flatwise bending moments. The thin spaced facings provide nearly all of the bending rigidity to the construction. The core spaces the facings and transmits shear between them so that they are effective about a common neutral axis. The core also provides most of the shear rigidity of the sandwich construction. By proper choice of materials for facings and core, constructions with high ratios of stiffness to weight can be achieved.

A basic design concept is to space strong, thin facings far enough apart to achieve a high ratio of stiffness to weight; the lightweight core that does this also provides the required resistance to shear and is strong enough to stabilize the facings to their desired configuration through a bonding medium such as an adhesive layer, braze, or weld. The sandwich is analogous to an I-beam in which the flanges carry direct compression and tension loads, as do the sandwich facings, and the web carries shear loads, as does the sandwich core.

In order that sandwich cores be lightweight, they are usually made of low-density material, some type of cellular construction (honeycomb-like core formed of thin sheet material), or of corrugated sheet material. As a consequence of employing a lightweight core, design methods account for core shear deformation because of the low effective shear modulus of the core. The main difference in design procedures for sandwich structural elements as compared to design procedures for homogeneous material is the inclusion of the effects of core shear properties on deflection, buckling, and stress for the sandwich.

Because thin facings can be used to carry loads in a sandwich, prevention of local failure under edgewise direct or flatwise bending loads is necessary just as prevention of local crippling of stringers is necessary in the design of sheet-stringer construction. Modes of failure that may occur in sandwich under edge load are shown in figure 1-1.imageimage

Figure 1-1. Possible modes of failure of sandwich composite under edgewise loads: General buckling, shear crimping, dimpling of facings, and wrinkling of facings either away from or into the core.

Shear crimping failure (fig. 1-1B) appears to be a local mode of failure, but is actually a form of general overall buckling (fig. 1-1A) in which the wavelength of the buckles is very small because of low core shear modulus. The crimping of the sandwich occurs suddenly and usually causes the core to fail in shear at the crimp; it may also cause shear failure in the bond between the facing and core.

Crimping may also occur in cases where the overall buckle begins to appear and then the crimp occurs suddenly because of severe local shear stresses at the ends of the overall buckle. As soon as the crimp appears, the overall buckle may disappear. Therefore, although examination of the failed sandwich indicates crimping or shear instability, failure may have begun by overall buckling that finally caused crimping.

If the core is of cellular (honeycomb) or corrugated material, it is possible for the facings to buckle or dimple into the spaces between core walls or corrugations as shown in figure 1-1C. Dimpling may be severe enough so that permanent dimples remain after removal of load and the amplitude of the dimples may be large enough to cause the dimples to grow across the core cell walls and result in a wrinkling of the facings.

Wrinkling, as shown in figure 1-1D, may occur if a sandwich facing subjected to edgewise compression buckles as a plate on an elastic foundation. The facing may buckle inward or outward, depending on the flatwise compressive strength of the core relative to the flatwise tensile strength of the bond between the facing and core. If the bond between facing and core is strong, facings can wrinkle and cause tension failure in the core. Thus, the wrinkling load depends upon the elasticity and strength of the foundation system; namely, the core and the bond between facing and core. Since the facing is never perfectly flat, the wrinkling load will also depend upon the initial eccentricity of the facing or original waviness.

The local modes of failure may occur in sandwich panels under edgewise loads or normal loads. In addition to overall buckling and local modes of failure, sandwich is designed so that facings do not fail in tension, compression, shear, or combined stresses due to edgewise loads or normal loads, and cores and bonds do not fail in shear, flatwise tension, or flatwise compression due to normal loads.

The basic design principles can be summarized into four conditions as follows:

1. Sandwich facings shall be at least thick enough to withstand chosen design stresses under design1 loads.

2. The core shall be thick enough and have sufficient shear rigidity and strength so that overall sandwich buckling, excessive deflection, and shear failure will not occur under design1loads.

3. The core shall have high enough moduli of elasticity, and the sandwich shall have great enough flatwise tensile and compressive strength so that wrinkling of either facing will not occur under design— loads.

4. If the core is cellular (honeycomb) or of corrugated material and dimpling of the facings is not permissible, the cell size or corrugation spacing shall be small enough so that dimpling of either facing into the core spaces will not occur under design1 loads.

The choice of materials, methods of sandwich assembly, and material properties used for design shall be compatible with the expected environment in which the sandwich is to be utilized. For example, facing to core bonding shall have sufficient flatwise tensile and shear strength to develop the required sandwich panel strength in the expected environment. Included as environment are effects of temperature, water or moisture, corrosive atmosphere and fluids, fatigue, creep, and any condition that may affect material properties.

Certain additional characteristics, such as thermal conductivity, resistance to surface abrasion, dimensional stability, permeability, and electrical properties of the sandwich materials should be considered in arriving at a thoroughly efficient design for the intended purpose.

Detailed procedures giving formulas and graphs for use in structural design are given in subsequent sections of this Handbook. The formulas and graphs can be used to determine dimensions of facings and core as well as necessary core properties for sandwich components under various types of loads. Graphs and formulas are presented in terms of general parameters, and are not for specific materials. Design procedures involving buckling are based on theoretical buckling coefficients. These coefficients are in fair agreement with average test results, but allowance can be made in the final design to account for the scatter characteristic of buckling test results, perhaps by choosing a slightly thicker core, so that buckling of the sandwich component does not occur at design load.

Material used for Sandwich Construction:

2.1 FACING MATERIALS

2.1.1 Functions, Descriptions, Usual Forms

The facings of a sandwich part serve many purposes, depending upon the application, but in all cases they carry the major applied loads. The stiffness, stability, configuration, and, to a large extent, the strength of the part are determined by the characteristics of the facings as stabilized by the core. To perform these functions the facings must be adequately bonded to a core of acceptable quality. Facings sometimes have additional functions, such as providing a profile of proper aerodynamic smoothness, a rough non-skid surface, or a tough wear-resistant floor covering. To better fulfil these special functions, one facing of a sandwich is sometimes made thicker or of slightly different construction than the other.

Any thin, sheet material can serve as a sandwich facing. A few of the materials usually used are discussed briefly in the following:

2.1.1.1 Metals (ref. 2-35)

2.1.1.1.1 Aluminum Alloys. –The stronger alloys of aluminum, such as 7075-T6, 2024-T3, or 2014-T6, are commonly used as facings for structural as well as for non-structural sandwich applications.

2.1.1.1.2 Steel Alloys. –Stainless steel sheets are finding increasing use as a facing material in airframe sandwich construction. The chief advantage of stainless steel sheet is its high strength at elevated temperatures. Alloys such as 18-8, 17-7PH, and PH15-7Mo are currently finding use because high stresses can be realized. The 18-8 alloys can be rolled to various degrees of hardness to produce high strength but it should be understood that a sheet rolled full hard has a longitudinal compressive yield stress about one-half of the compressive yield stress in the transverse direction. This discrepancy can be closed by subsequent stress relief. Alloys of the 17-7PH and PH15-7Mo are precipitation hardenable and can be strengthened by heat treatment–usually to condition TH1050.

2.1.1.1.3 Titanium Alloys. –Alloys of titanium are currently of interest as facing materials because of their high strength-weight ratios and because they can be utilized for moderately high temperature applications.

2.1.1.1.4 Magnesium Alloys. –Magnesium alloy sheets have been used only experimentally as facing materials, but may find increasing application because of their low density.

2.1.1.1.5 Nickel Base Alloys. –Nickel base alloys such as René 41 can be utilized for heat-resistant sandwich at temperatures of 1200°-1500° F. René 41 is a precipitation-hardening alloy that needs protection from the atmosphere during heat treating. The alloy can be welded.

2.1.1.1.6 Cobalt Base Alloys. –Alloys of cobalt with chromium, nickel molybdenum, and tungsten are available for use in moderately stressed applications at temperatures of 1000°-1800° F. Alloys such as L605 can be brazed, or fusion or resistance welded.

2.1.1.1.7 Columbium Alloys. –Columbium alloys D-36, D-43, and Cb-752 are suitable for use at temperatures up to 2500° F if they are protected from oxidation by thin suicide coatings. These alloys can be brazed in an inert atmosphere or can be welded; however, degradation can be minimized by joining parts by diffusion bonding.

2.1.1.1.8 Molybdenum Alloys. –Alloy TZM of molybdenum can resist temperatures up to 2800° F. Need for protection and means of joining parts are the same as for the columbium alloys.

2.1.1.1.9 Beryllium. –The low weight and high elastic modulus of beryllium make it most attractive for use in sandwich composites. The metal is heat resistant in the range 1000°-1200° F. Parts can be joined by brazing or welding. Precautions must be taken to prevent individuals from inhaling toxic beryllium particles during fabrication of parts.

2.1.1.2 Reinforced Plastic Materials (ref. 2-34)

2.1.1.2.1 Glass-Fabric Reinforced. –Resin-impregnated glass-fabric facings possess acceptable properties for structural sandwiches when properly fabricated. Because of its excellent dielectric characteristics when fabricated with the proper resin, this type of facing is used almost universally for radomes of sandwich construction. A variety of weaves are available commercially, which makes it practicable, by orienting the fiber directions in the facing, to achieve a wide range of directional strength properties.

In many airframe applications, facings are exposed to moisture, either in the form of high humidity or free water. Even though the amounts of moisture absorbed by glass-reinforced plastic are quite small (on the order of 0.5 to 1.5 percent), the strength properties are decreased, with the amount of decrease depending upon the type of finish applied to the glass fabric. Current specification requirements permit only small losses of strength, after exposure to moisture, that are consistent with results of tests on fabrics made with more recent and effective finishes (such as Volan A, A-1100, Garan RS-49, T-31, NOL-24, and A-172). The most suitable finish for a given application is selected by the glass fabricator. For chemical resin types, such as phenolic, epoxy, and triallyl cyanurate-polyester resins, optimum properties are obtained by use of specific finishes with each resin formulation. The acceptable finishes for each approved resin are given in the qualified products lists that accompany the military specification for each chemical type of resin.

2.1.1.2.2 Glass Mats Reinforced. –Glass fibers are also commercially available in the form of mats, but owing to the relative non uniformity in thickness and resin content and because of the low strength when compared to glass fabric, mats have found little use in aircraft sandwich construction.

Method of Analysis:

Micro Mechanics:

Micromechanics are the analysis of composite or heterogeneous materials on the level of the individual constituents that constitute these materials.

Mechanics of materials approach in composites

Heterogeneous materials, such as composites, solid foams, poly-crystals, or bone, consist of clearly distinguishable constituents (or phases) that show different mechanical and physical material properties.

Given the (linear and/or nonlinear) material properties of the constituents, one important goal of micromechanics of materials consists of predicting the response of the heterogeneous material on the basis of the geometries and properties of the individual phases, a task known as homogenization. The benefit of homogenization is that the behavior of a heterogeneous material can be determined without resorting to testing it. Such tests may be expensive and involve a large number of permutations (e.g., in the case of composites: constituent material combinations; fiber and particle volume fractions; fiber and particle arrangements; and processing histories). Furthermore, continuum micromechanics can predict the full multi-axial properties and responses of inhomogeneous materials, which are often anisotropic. Such properties are often difficult to measure experimentally, but knowing what they are a requirement, e.g., for structural analysis involving composites. To rely on micromechanics, the particular micromechanics theory must be validated through comparison to experimental data.

The second main task of micromechanics of materials is localization, which aims at evaluating the local (stress and strain) fields in the phases for given macroscopic load states, phase properties, and phase geometries. Such knowledge is especially important in understanding and describing material damage and failure.

Because most heterogeneous materials show a statistical rather than a deterministic arrangement of the constituents, the methods of micromechanics are typically based on the concept of the representative volume element (RVE). An RVE is understood to be a sub-volume of an inhomogeneous medium that is of sufficient size for providing all geometrical information necessary for obtaining an appropriate homogenized behavior.

Most methods in micromechanics of materials are based on continuum mechanics rather than on atomistic approaches such as molecular dynamics. In addition to the mechanical responses of inhomogeneous materials, their thermal conduction behavior and related problems can be studied with analytical and numerical continuum methods. All these approaches may be subsumed under the name of “continuum micromechanics”.

Measurements and Extensometer:

Principles of Measurements:

Concepts for describing aspects of nature by numbers are called physical quantities. Examples may range from counting fruit to reading a thermometer gauge to determine temperature.

Acquiring such a number, a set of such numbers or related numbers directly from a natural system is called measurement.

In technical analysis, An intangible principle for finding minimum security price targets for traders. The measuring principle uses technical analysis to analyse chart patterns to detect stock levels that, if broken, could lead to a small down leg. More specifically, it allows traders to set a reasonable minimum price target on a stock by weighing the movements of the stock chart pattern against each other.

Mechanical Extensometer:

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It is an attachment to Universal Testing Machine and Tensile Testing Machines, used to find out the proof stress at the required elongation percentage.

It measures the elongation of a test piece on load for the set gauge length.

Least count of measurement 0.01 mm and maximum elongation measured upto 3 mm.

Adjustable gauge length from 30 mm to 120 mm

Round specimen from 1 mm to 20 mm dia and flat specimen from 1 mm to 20 mm thickness can be accommodated.

TECHNICAL SPECIFICATIONS – FOR MECHANICAL EXTENSOMETER 

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Our precision engineered mechanical extensometer measures elongation of test specimen under load. Our mechanical extensometer comprises of two long knife edges at top, two indicators and lower knife edges to actuate the dials. The bars are pressed against the specimen by means of clamps to ensure that the knife edges bite the specimen to avoid any slip. The under edges are adjustable to provide different gauge lengths for measurement. As deformation takes place the lower knife edges transfer the movement to the dial gauges and the deformation of specimen is shown on dials, so that the change in length of specimen by 1/100 mm. Equals to one division of the graduation of dial.

FLIGHT DECK AND COCKPITS:

Control and Display Technologies:

Multifunction CRT display system:

The MCDS on the orbiter crew compartment flight deck allows onboard monitoring of orbiter systems, computer software processing and manual control for flight crew data and software manipulation.

The system is composed of three types of hardware: display electronics units; display units that include the CRTs; and keyboard units, which together communicate with the GPCs over the display/keyboard data bus network.

The MCDS provides almost immediate response to flight crew inquiries through displays, graphs, trajectory plots and predictions about flight progress. The crew controls the vehicle system operation through the use of keyboards in conjunction with the display units. The flight crew can alter the system configuration, change data or instructions in GPC main memory, change memory configurations corresponding to different mission phases, respond to error messages and alarms, request special programs to perform specific tasks, run through operational sequences for each mission phase and request specific displays.

Three keyboards are located on the flight deck: two on the left and right sides of the flight deck center console (panel C2) and one on the flight deck at the side aft flight station (panel R12). Each consists of 32 momentary double-contact push button keys. Each key uses its double contacts to communicate on separate signal paths to two DEUs. Only one set of contacts on the aft station keys is actually wired because this keyboard can communicate with only the aft display electronics unit.

There are 10 number keys, six letter keys (used for hexadecimal inputs), two algebraic keys, a decimal key, and 13 special key functions. Using these keys, the flight crew can ask the GPC more than 1,000 questions about the mission and condition of the vehicle.

Each of the four DEUs responds to computer commands, transmits data, executes its own software to process keyboard inputs and sends signals to drive displays on the CRTs (or display units). The four DEUs store display data, generate the GPC/keyboard unit and GPC/display unit interface displays, update and refresh on-screen data, check keyboard entry errors and echo entries to the CRT (or DU).

There are three CRTs (or display units) on flight deck forward display and control panel F7 and one at the side aft flight deck station on panel R12. Each CRT is 5 by 7 inches.

The display unit uses a magnetic-deflected, electrostatic-focused CRT. When supplied with deflection signals and video input, the CRT displays alphanumeric characters, graphic symbols and vectors on a green-on-green phosphorous screen activated by a magnetically controlled beam. Each CRT has a brightness control for ambient light and flight crew adjustment.

The DEUs is connected to the display/keyboard data buses by multiplexer interface adapters that function like those of the GPCs. Inputs to the DEU are from a keyboard or a GPC. The CRT switches on panel C2 designate which keyboard controls the forward DEUs and CRT (or DUs). When the left CRT sel switch is positioned to 1, the left keyboard controls the left CRT 1; if positioned to 3, it controls the center CRT 3. When the right CRT sel switch is positioned to 2, the right keyboard controls the right CRT 2; if positioned to 3, it controls the center CRT 3. If the left CRT sel and right CRT sel switches are both positioned to 3, keystrokes from both keyboards are interleaved. Thus, flight crew inputs are made on the keyboards and data is output from the GPCs on the CRT displays.

The aft station panel R12 keyboard is connected directly to the aft panel R12 DEU and CRT (or DU); there is no select switch.

Each DEU/DU pair, usually referred to as a CRT, has an associated power switch. The CRT 1 power , on , stby , off switch on panel C2 positioned to stby or on allows control bus power to activate RPCs and sends MNA power to DEU/ DU 1. The stby position warms up the CRT filament. The on position provides high voltage to the CRT. The CRT 2 switch on panel C2 functions the same as the CRT 1 switch, except that CRT 2 is powered from MNB. The CRT 3 switch on panel C2 functions the same as the CRT 1 switch, except that CRT 3 is powered from MNC. The CRT 4 switch on panel R12 functions the same as the CRT 1 switch, except that CRT 4 is powered from MNC. The respective keyboards receive 5 volts of ac power to illuminate the keys. Each DEU/DU pair uses about 300 watts of power when on and about 230 watts in standby.

The CRT 1, 3, 2, major func, GNC, SM and PL switches on panel C2 tell the GPCs which of the different functional software groups is being processed by the keyboard units and what information is presented on the CRT. The CRT 4, major func, GNC, SM and PL switches on panel R12 function in the same manner.

Positioning the display electronics unit 1, 2, 3, 4 switches on panel O6 to load initiates a GPC request for data stored in mass memory through a GPC before operations begin. The information is sent from the mass memory to the GPC and then loaded from the GPC into the DEU memory.

It is possible to do in-flight maintenance and exchange DU 4 with DU 1 or 2. DU 3 cannot be changed out because of the control and display panel configuration. Also, either forward keyboard can be replaced by the aft keyboard. The DEUs is located behind panels in the middeck. DEUs 1 and 3 are on the left, and DEUs 2 and 4 are on the right. DEU 4 can replace any of the others; however, if DEU 2 is to be replaced, only the cables are changed because 2 and 4 are next to each other.

The DEUs and DUs are cooled by the cabin fan system. The keyboard units are cooled by heat dissipation.

Special Purpose Diodes

The unique characteristics of semiconductor material have allowed for the development of many specialized types of diodes. A short description of some of the more common diode types is given for general familiarization. Figure 10-205 illustrates the schematic symbols for some of the special purpose diodes.

Light-Emitting Diode (LED)

In a forward biased diode, electrons cross the junction and fall into holes. As the electrons fall into the valence band, they radiate energy. In a rectifier diode, this energy is dissipated as heat. However, in the light-emitting diode (LED), the energy is dissipated as light. By using elements, such as gallium, arsenic, and phosphorous, an LED can be designed to radiate colors, such as red, green, yellow, blue and inferred light. LEDs that are designed for the visible light portion of the spectrum are useful for instruments, indicators, and even cabin lighting. The advantages of the LED over the incandescent lamps are longer life, lower voltage, faster on and off operations, and less heat.

Liquid Crystal Displays (LCD)

The liquid crystal display (LCD) has an advantage over the LED in that it requires less power to operate. Where LEDs commonly operate in the milli-watt range, the LCD operates in the microwatt range. The liquid crystal is encapsulated between two glass plates. When voltage is not applied to the LCD, the display will be clear. However, when a voltage is applied, the result is a change in the orientation of the atoms of the crystals. The incident light will then be reflected in a different direction. A frosted appearance will result in the regions that have voltage applied and will permit distinguishing of numeric values.

Photodiode

Thermal energy produces minority carriers in a diode. The higher the temperature, the greater the current in a reverse current diode. Light energy can also produce minority carriers. By using a small window to expose the PN junction, a photodiode can be built. When light fall upon the junction of a reverse- biased photodiode, electrons-hole pairs are created inside the depletion layer. The stronger the light, the greater the number of light-produced carriers, which in turn causes a greater magnitude of reverse-current. Because of this characteristic, the photodiode can be used in light detecting circuits.

Varactors

The varactor is simply a variable-capacitance diode. The reverse voltage applied controls the variable capacitance of the diode. The transitional capacitance will decrease as the reverse voltage is increasingly applied. In many applications, the varactor has replaced the old mechanically tuned capacitors. Varactors can be placed in parallel with an inductor and provide a resonant tank circuit for a tuning circuit. By simply varying the reverse voltage across the varactor, the resonant frequency of the circuit can be adjusted.

Schottky Diodes

Schottky diodes are designed to have a metal, such as gold, silver, or platinum, on one side of the junction and doped silicon, usually an N-type, on the other side of the junction. This type of a diode is considered a unipoler device because free electrons are the majority carrier on both sides of the junction. The Schottky diode has no depletion zone or charge storage, which means that the switching time can be as high as 300 MHz. This characteristic exceeds that of the bipolar diode.

Plasma Panels:

A plasma display panel (PDP) is a type of flat panel display common to large TV displays 30 inches (76 cm) or larger. They are called “plasma” displays because the technology utilizes small cells containing electrically charged ionized gases, or what are in essence chambers more commonly known as fluorescent lamps.

Advantages:
  • Picture quality
    • Capable of producing deeper blacks allowing for superior contrast ratio
    • Wider viewing angles than those of LCD; images do not suffer from degradation at high angles like LCDs.
    • Less visible motion blur, thanks in large part to very high refresh rates and a faster response time, contributing to superior performance when displaying content with significant amounts of rapid motion.
Disadvantages:
  • Earlier generation displays were more susceptible to screen burn-in and image retention, recent models have a pixel orbiter that moves the entire picture slower than is noticeable to the human eye, which reduces the effect of burn-in but does not prevent it.
  • Due to the bistable nature of the colour and intensity generating method, some people will notice that plasma displays have a shimmering or flickering effect with a number of hues, intensities and dither patterns.
  • Earlier generation displays (circa 2006 and prior) had phosphors that lost luminosity over time, resulting in gradual decline of absolute image brightness (newer models may be less susceptible to this, having advertised lifespans exceeding 100 000 hours, far longer than older CRT technology).
  • Screen-door effects (black lines between rows of pixels) become noticeable on screen sizes larger than 127 cm (50 in); the effect is more visible at shorter viewing distances.
  • Other
    • Use more electrical power, on average, than an LCD TV.
    • Does not work as well at high altitudes above 2 km due to pressure differential between the gases inside the screen and the air pressure at altitude. It may cause a buzzing noise. Manufacturers rate their screens to indicate the altitude parameters.
    • For those who wish to listen to AM radio, or are amateur radio operators (hams) or shortwave listeners (SWL), the radio frequency interference (RFI) from these devices can be irritating or disabling.

Touch Screen:

Garmin has raised the bar with the announcement of the GTN 650 and GTN 750 series. These panel-mount units are certified and approved for installation in hundreds of makes and models of general aviation aircraft.

The GTN 650 and GTN 750 feature new capabilities for GPS/NAV/COM systems like touchscreen operation, graphical flight planning with victor airways and high-altitude jet routes, remote transponder, remote audio control (750 series only), Safe Taxi and electronic chart capabilities (750 series only).

“As the successors to the very popular GNS 430W and 530W, the GTN 650 and 750 have big shoes to fill. We’re confident that the GTN series will set a new standard on what avionics for general aviation aircraft should be, just as the GNS 430 and 530 did when they were announced in 1998,” said Gary Kelley, Garmin’s vice president of marketing.

“The GTN 650 and 750 are the first touchscreen avionics certified for general aviation aircraft. Although some may think the touchscreen operation is the most unique feature of these systems, we believe the interface and expansive new capabilities are even more innovative.”

The most notable physical difference between the GTN 650 and 750 is the screen size. The GTN 650 has the same exterior footprint as the GNS 430W, but has a 4.9-inch screen (diagonal) that has 53 percent more screen area than the GNS 430W.

The GTN 750’s large 6.9-inch screen (diagonal) has 98 percent more screen area than the GNS 530W, which makes it possible to view an entire chart via Garmin Flite Charts and Chart View, as well as display integrated audio and intercom functions (with the new optional GMA 35 remote mount audio processor).

In addition, both units display a greatly enhanced, higher resolution picture (GTN 650: 600×266 pixels; GTN 750: 600×708 pixel) that has over 5 times more pixels than the GNS 430W and 530W, respectively.

The touchscreen GTN 650 and 750 both feature a shallow menu structure, desktop-like menu interface with intuitive icons, audio and visual feedback, and animation so that pilots know exactly how the systems are responding to their input. The GTN has a touchscreen alphanumeric keyboard, and also utilizes a “back” icon for quick and easy operation.

Recognizing that hand stabilization will help make it even easier to enter data, both units have a finger anchoring bezel around the side of the display and fingerboard at the bottom of the screen.

For those who prefer traditional data entry via buttons and knobs, the GTN systems have a dual concentric knob for data entry, volume/squelch knob, “home” button and “direct to” button so that pilots can do all the basic fundamentals – like establish a route and change COM frequencies – without using the touchscreen. With the home key, pilots are seldom more than two taps away from all primary pages and functions.

Flight planning made easy, the GTN series offers graphical flight planning capability (patent pending) so that pilots can edit an active flight plan route on the map and easily enter a new waypoint or modify the sequence by tapping or dragging their finger on the screen. Victor airways and high-altitude jet routes can be overlaid on the moving map, and airway segments can be selected onscreen for instant entry into a flight plan.

The system has a unique “rubber band” feature that lets pilots select a flight plan leg on the screen and then alter it to accommodate a deviation or ATC amendment. In addition, pilots can pan across the map display by swiping their finger across the screen.

Enhanced situational awareness, thanks to built-in terrain, mapping and obstacle databases, the GTN provides a greatly enhanced, high resolution presentation of the surrounding area. A built-in terrain elevation database shows color-coded alerts when potential terrain conflicts are ahead.

Full Class B TAWS alerting is also available as an option. The SBAS/WAAS equipped GTN 650 and 750 let pilots fly GPS-guided LPV glide path approaches down to ILS-comparable minimums. In addition, precise course deviation and roll steering outputs can be coupled to select autopilots so that IFR flight procedures may be flown automatically.

Add weather, traffic and more because the GTN offers a wide array of compatibility with select Garmin avionics and sensors, Garmin has made it possible to have a consistent and intuitive interface to other systems – like audio and transponder – by creating simplified systems management functionality on the GTN flight deck.

Saving valuable panel space, Garmin’s new GMA 35 remote mount audio processor (optional) interfaces with the GTN 750 and makes it possible for the GTN to be used as a touchscreen control head for the aircraft’s audio and intercom functions.

The GMA 35 helps streamline cockpit communications with record/playback capability for copying clearances. It also includes an internal microphone that senses the amount of ambient noise and automatically adjusts the cockpit speaker and the headset volume based on the level of noise in the cockpit.

Garmin’s GTX 32/33/33D remote transponders (optional) also interface with the GTN 650 or 750 so that pilots can control transponder function from the GTN’s display. Optional versions of the GTX 33/33D mode S transponders are available which support ADS-B/Out.

Optional XM WX Satellite Weather™, lightning, and traffic system inputs are also supported and may be overlaid on the moving map. In addition, XM radio is available as an option (XM WX Satellite Weather™ and radio service is only available to U.S. and Canadian customers with a subscription and with an optional GDL 69 series data link receiver).

The standard GTN 650 and GTN 750 feature a 10-watt COM, and a field upgradeable 16-watt version is also available. In third quarter 2011, Garmin will make available a GTN 725, which is similar to the GTN 750, and is a GPS only unit. Also, a GTN 625 will be available that is a GPS only unit, and a GTN 635 that is a GPS unit with VHF Communications radio. All units are SBAS/WAAS enabled.

Direct voice input:

Direct voice input (DVI) (sometimes called voice input control (VIC)) is a style of human–machine interaction “HMI” in which the user makes voice commands to issue instructions to the machine. It has found some usage in the design of the cockpits of several modern military aircraft, particularly the Eurofighter Typhoon, the F-35 Lightning II, the Dassault Rafale and the JAS 39 Gripen, having been trialled on earlier fast jets such as the Harrier AV-8B and F-16 VISTA. A study has also been undertaken by the Royal Netherlands Air Force using voice control in a F-16 simulator.

The USAF initially wanted DVI for the Lockheed Martin F-22 Raptor, but it was finally judged too technically risky and was abandoned.

DVI systems may be “user-dependent” or “user-independent”. User-dependent systems require a personal voice template to be created by the pilot which must then be loaded onto the aircraft before flight. User-independent systems do not require any personal voice template and will work with the voice of any user.

In 2006 Zon and Roerdink, at the National Aerospace Laboratory in the Netherlands, examined the use of Direct Voice Input in the “GRACE” simulator, in an experiment in which twelve pilots participated. Although the hardware performed well, the researchers discovered that, before installation in a real aircraft their DVI system would need some improvement, since operation of the DVI took more time than the existing manual method. They recommended that:

  • The syntax must become simpler;
  • The recognition rate of the system must improve;
  • Response time of the system must decrease.

They suggested that all of these issues were of a technological nature and thus seemed feasible to solve. They concluded that in cockpits, especially during emergencies where pilots have to operate the entire aircraft on their own, a DVI system might be very relevant. During other situations it seemed to be interesting but not of crucial importance.

Multi-function display:

The primary flight instruments can all be displayed simultaneously on one reasonably easy-to-read video monitor much like the flat panel displays in laptop computers. These displays are called primary flight displays (PFDs). You must still cross-check around the panel and on the display, but more information is available in a smaller space in easier to read colors. These convenient displays receive data from sensors such as magnetometers or magnetic flux valves to determine heading referenced to magnetic north. The attitude (pitch and roll) of the aircraft is sensed by the attitude heading reference system (AHRS) and displayed as the attitude gyro would be in conventional instrumentation. The altitude, airspeed, and outside temperature values are sensed in the air data computer (ADC) and presented in the PFD on vertical scales or portions of circles.
The multi-function display (MFD) can often display the same information as the PFD and can be used as a backup PFD. Usually the MFD is used for traffic, route selection, and weather and terrain avoidance. However, some PFDs also accommodate these same displays, but in a smaller view due to the primary flight instrument areas already used in the display. You must learn and practice using that specific system.

It is important to be very careful in the selection (programming) of the various functions and features. In the event of failures, which have a large impact on flight safety and situational awareness, you must always be ready and able to complete the flight safely using only the standby instruments.

A multi-function display (MFD) (part of multi function structures) is a small screen (CRT or LCD) in an aircraft surrounded by multiple buttons that can be used to display information to the pilot in numerous configurable ways. Often an MFD will be used in concert with a primary flight display. MFDs are part of the digital era of modern planes or helicopter. The first MFD were introduced by air forces. The advantage of an MFD over analog display is that an MFD does not consume much space in the cockpit. For example the cockpit of RAH-66 “Comanche” does not have analog dials or gauges at all. All information is displayed on the MFD pages. The possible MFD pages could differ for every plane, complementing their abilities (in combat).

Many MFDs allow the pilot to display their navigation route, moving map, weather radar, NEXRAD, GPWS, TCAS and airport information all on the same screen.

MFDs were added to the Space Shuttle (as the glass cockpit) starting in 1998 replacing the analog instruments and CRTs. The information being displayed is similar, and the glass cockpit was first flown on the STS-101 mission.

In modern automotive technology, MFDs are used in cars to display navigation,

Head-up display:

A head-up display or heads-up display—also known as a HUD—is any transparent display that presents data without requiring users to look away from their usual viewpoints. The origin of the name stems from a pilot being able to view information with the head positioned “up” and looking forward, instead of angled down looking at lower instruments.

Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, computer gaming, and other applications.

Types:

Other than fixed mounted HUDs, there are also head-mounted displays (HMDs). Including helmet mounted displays (both abbreviated HMD), forms of HUD that features a display element that moves with the orientation of the user’s head.

Many modern fighters (such as the F/A-18, F-16 and Eurofighter) use both a HUD and HMD concurrently. The F-35 Lightning II was designed without a HUD, relying solely on the HMD, making it the first modern military fighter not to have a fixed HUD.

Generations:

HUDs are split into four generations reflecting the technology used to generate the images.

  • First Generation—Use a CRT to generate an image on a phosphor screen, having the disadvantage of the phosphor screen coating degrading over time. The majority of HUDs in operation today are of this type.
  • Second Generation—Use a solid state light source, for example LED, which is modulated by an LCD screen to display an image. These systems do not fade or require the high voltages of first generation systems. These systems are on commercial aircraft.
  • Third Generation—Use optical waveguides to produce images directly in the combiner rather than use a projection system.
  • Fourth Generation—Use a scanning laser to display images and even video imagery on a clear transparent medium.

Newer micro-display imaging technologies are being introduced, including liquid crystal display (LCD), liquid crystal on silicon (LCoS), digital micro-mirrors (DMD), and organic light-emitting diode (OLED).

Multifunctional Keypad (MFK)

The equipment in an aircraft are automatically controlled by a central computer, known as the Mission Computer (MC).
The MC interacts with the other avionics equipment through three MIL-STD-1553 communication buses. Multi Function Keyboard (MFK) is one such equipment. It is used to communicate with the pilot about the status of various equipment and the navigation.
The MFK has, at its heart, a MOTOROLA 68000 series processor with ROM, RAM and EEPROM memory to carry out its task. The MFK is the pilot interface for all communications with MC or other equipment through MC. It is connected to one of the 1553 buses, used by MC for data communication. It is also connected to the Multi Function Rotary switch (MFR) through a RS422 interface.
The MFK has a built-in display unit and a keyboard. The display unit is LCD based. The keyboard has provisions to enter alphabetic and numeric values. There are a number of control buttons to facilitate the pilot communication with MC.
The software has been developed as a real time, embedded application. The development platform includes the C Language, pSOS real-time kernel, and the 68000 series Assembly Language. Teamwork Case Tool was used during system analysis. It was also used for the preparation Software Requirement Specification.
The MFK software provides various functionalities to assist effective communication with MC. These messages are handled by MFK in the following three different modes of operations.

-Initialisation phase (Init)

-Operations Flight Program (OFP)

-Flight Line Maintenance (FLM)

The software is capable of receiving the page information from MC in binary format and validate them. Upon valid information, it converts them into pilot readable alphabetic and numeric values before displaying the page on the screen. The software can also process page requests from the pilot and send that message to MC to get the details for a new page.
The software allows the pilot to modify the flight parameters, validates the pilot input and sends them to MC for effecting the changes. The pilot can request the MC to send the status of other avionics equipment. Also, those equipment can be turned ON or OFF by the pilot depending on the need.
The system is capable of storing the data for waypoint library and airfield library. The data is sent from Mission Preparation and Retrieval Unit (MPRU). The system sends the results of the data loading to MPRU. The pilot can browse or change the waypoint library. The system also provides the ability to maintain alternate waypoints to enable the pilot to complete the mission.
The software is responsible for handling the time management functions of the MFR. It manages the real time display and the stop watch functions.
The software also has the following management functions:

-Identification

-CCU communication.

-Built-in tests

During identification, MFK sends the hardware and software version number to MC. The software sends all its MC messages to Coding and Control Unit (CCU) also. This enables the CCU to take charge of the system, in case the MC fails for some reasons. The software comes with a built-in test feature to ensure its proper functioning.
The development has followed the MIL-STD-2167 standards in all its phases, such as design, development and testing.

HOTAS:

HOTAS an abbreviation for Hands On Throttle-And-Stick, is the name given to the concept of placing buttons and switches on the throttle stick and flight control stick in an aircraft’s cockpit, allowing the pilot to access vital cockpit functions and fly the aircraft without having to remove his hands from the throttle and flight controls. Application of the concept was pioneered with the Ferranti AIRPASS radar and gunsight control system used by the English Electric Lightning[1] and is widely used on all modern fighter aircraft such as the F-16 Fighting Falcon.

HOTAS is a shorthand term which refers to the pattern of controls in the modern fighter aircraft cockpit. Having all switches on the stick and throttle allows the pilot to keep his “hands on throttle-and-stick”, thus allowing him to remain focused on more important duties than looking for controls in the cockpit. The goal is to improve the pilot’s situational awareness, his ability to manipulate switch and button controls in turbulence, under stress, or during high G-force maneuvers, to improve his reaction time, to minimize instances when he must remove his hands from one or the other of the aircraft’s controls to use another aircraft system, and total time spent doing so.

The concept has also been applied to the steering wheels of modern open-wheel racecars, like those used in Formula One and the Indy Racing League. HOTAS has been adapted for game controllers used for flight simulators (most such controllers are based on the F-16 Fighting Falcon‘s) and in cars equipped with radio controls on the steering wheel. In the modern military aircraft cockpit the HOTAS concept is sometimes enhanced by the use of Direct Voice Input to produce the so-called “V-TAS” concept, and augmented with helmet mounted display systems such as the “Schlem” used in the MiG-29 and Su-27, which allow the pilot to control various systems using his line of sight, and to guide missiles by simply looking at the target.

Introduction:

Historical Review:

Automatic control systems were first developed over two thousand years ago. The first feedback control device on record is thought to be the ancient Ktesibios‘s water clock in Alexandria, Egypt around the third century B.C. It kept time by regulating the water level in a vessel and, therefore, the water flow from that vessel. This certainly was a successful device as water clocks of similar design were still being made in Baghdad when the Mongols captured the city in 1258 A.D. A variety of automatic devices have been used over the centuries to accomplish useful tasks or simply to just entertain. The latter includes the automata, popular in Europe in the 17th and 18th centuries, featuring dancing figures that would repeat the same task over and over again; these automata are examples of open-loop control. Milestones among feedback, or “closed-loop” automatic control devices, include the temperature regulator of a furnace attributed to Drebbel, circa 1620, and the centrifugal flyball governor used for regulating the speed of steam engines by James Watt in 1788.

In his 1868 paper “On Governors”, J. C. Maxwell (who discovered the Maxwell electromagnetic field equations) was able to explain instabilities exhibited by the flyball governor using differential equations to describe the control system. This demonstrated the importance and usefulness of mathematical models and methods in understanding complex phenomena, and signaled the beginning of mathematical control and systems theory. Elements of control theory had appeared earlier but not as dramatically and convincingly as in Maxwell’s analysis.

Control theory made significant strides in the next 100 years. New mathematical techniques made it possible to control, more accurately, significantly more complex dynamical systems than the original flyball governor. These techniques include developments in optimal control in the 1950s and 1960s, followed by progress in stochastic, robust, adaptive and optimal control methods in the 1970s and 1980s. Applications of control methodology have helped make possible space travel and communication satellites, safer and more efficient aircraft, cleaner auto engines, cleaner and more efficient chemical processes, to mention but a few.

Before it emerged as a unique discipline, control engineering was practiced as a part of mechanical engineering and control theory was studied as a part of electrical engineering, since electrical circuits can often be easily described using control theory techniques. In the very first control relationships, a current output was represented with a voltage control input. However, not having proper technology to implement electrical control systems, designers left with the option of less efficient and slow responding mechanical systems. A very effective mechanical controller that is still widely used in some hydro plants is the governor. Later on, previous to modern power electronics, process control systems for industrial applications were devised by mechanical engineers using pneumatic and hydraulic control devices, many of which are still in use today.

Pneumatics Systems:

Pneumatics is a section of technology that deals with the study and application of pressurized gas to produce mechanical motion.

Pneumatic systems, which are used extensively in industry, and factories, are commonly plumbed with compressed air or compressed inert gases. This is because a centrally located and electrically powered compressor, that powers cylinders and other pneumatic devices through solenoid valves, can often provide motive power in a cheaper, safer, more flexible, and more reliable way than a large number of electric motors and actuators.

Pneumatics also has applications in dentistry, construction, mining, and other areas.

Advantages of pneumatics

Simplicity of design and control—Machines are easily designed using standard cylinders and other components, and operate via simple on-off control.

· Reliability—Pneumatic systems generally have long operating lives and require little maintenance. Because gas is compressible, Equipment is less subject to shock damage. Gas absorbs excessive force, whereas fluid in hydraulics directly transfers force. Compressed gas can be stored, so machines still run for a while if electrical power is lost.

· Safety—there is a very low chance of fire compared to hydraulic oil. Machines are usually overloading safe.

Hydraulic Systems:

A hydraulic drive system is a drive or transmission system that uses pressurized hydraulic fluid to drive hydraulic machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the kinetic energy of the flow.

A hydraulic drive system consists of three parts: The generator (e.g. a hydraulic pump), driven by an electric motor, a combustion engine or a windmill; valves, filters, piping etc. (to guide and control the system); the motor (e.g. a hydraulic motor or hydraulic cylinder) to drive the machinery.

Principle of a hydraulic drive:

Pascal’s law is the basis of hydraulic drive systems. As the pressure in the system is the same, the force that the fluid gives to the surroundings is therefore equal to pressure × area. In such a way, a small piston feels a small force and a large piston feels a large force.

The same principle applies for a hydraulic pump with a small swept volume that asks for a small torque, combined with a hydraulic motor with a large swept volume that gives a large torque. In such a way a transmission with a certain ratio can be built.

Most hydraulic drive systems make use of hydraulic cylinders. Here the same principle is used — a small torque can be transmitted into a large force.

By throttling the fluid between the generator part and the motor part, or by using hydraulic pumps and/or motors with adjustable swept volume, the ratio of the transmission can be changed easily. In case throttling is used, the efficiency of the transmission is limited. In case adjustable pumps and motors are used, the efficiency, however, is very large. In fact, up to around 1980, a hydraulic drive system had hardly any competition from other adjustable drive systems.

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Fig: Principle of hydraulic drive system

Nowadays, electric drive systems using electric servo-motors can be controlled in an excellent way and can easily compete with rotating hydraulic drive systems. Hydraulic cylinders are, in fact, without competition for linear forces. For these cylinders, hydraulic systems will remain of interest and if such a system is available, it is easy and logical to use this system for the rotating drives of the cooling systems, also.

Advantages of hydraulics:

· Liquid (as a gas is also a ‘fluid’) does not absorb any of the supplied energy.

· Capable of moving much higher loads and providing much higher forces due to the incompressibility.

· The hydraulic working fluid is basically incompressible, leading to a minimum of spring action. When hydraulic fluid flow is stopped, the slightest motion of the load releases the pressure on the load; there is no need to “bleed off” pressurized air to release the pressure on the load.

Thermal Systems:

In spacecraft design, the Thermal Control System (TCS) has the function to keep all the spacecraft parts within acceptable temperature ranges during all mission phases, withstanding the external environment, which can vary in a wide range as the spacecraft is exposed to deep space or to solar or planetary flux, and rejecting to space the internal heat dissipation of the spacecraft itself.

The thermal control is essential to guarantee the optimum performance and success of the mission, because if a component encounters a temperature which is too high or to low, it could be damaged or its performance could be severely affected. Thermal control is also necessary to keep specific components (such as optical sensors, atomic clocks, etc.) within a specified temperature stability requirement, to ensure that they perform as efficiently as possible.

The thermal control subsystem can be composed both of passive and of active items and works in two ways:

Protects the equipment from too hot temperatures, either by thermal insulation from external heat fluxes (such as the Sun or the planetary infrared and albedo flux), or by proper heat removal from internal sources (such as the heat dissipated by the internal electronic equipment).

Protects the equipment from too cold temperatures, by thermal insulation from external sinks, by enhanced heat absorption from external sources, or by heat release from internal sources.

Passive Thermal Control System (PTCS) items include:

· multi-layer insulation (MLI), which protects the spacecraft from excessive solar or planetary heating as well as from excessive cooling when exposed to deep space

· coatings that change the thermo-optical properties of external surfaces

· thermal fillers to improve the thermal coupling at selected interfaces (for instance on the thermal path between an electronic unit and its radiator)

· thermal washers to reduce the thermal coupling at selected interfaces

· thermal doublers to spread on the radiator surface the heat dissipated by equipment

· mirrors (secondary surface mirrors, SSM, or optical solar reflectors, OSR) to improve the heat rejection capability of the external radiators and at the same time to reduce the absorption of external solar fluxes

· radioisotope heater units (RHU), used by some planetary and exploratory missions to produce and store electrical power for TCS purposes

Active Thermal Control System (ATCS) items include:

· thermostatically controlled resistive electric heaters to keep the equipment temperature above its lower limit during the mission cold phases

· fluid loops to transfer the heat dissipated by equipment to the radiators. They can be:

· single-phase loops, controlled by a pump

· two-phase loops, composed of heat pipes (HP), loop heat pipes (LHP) or capillary pumped loops (CPL)

· louvers (which change the heat rejection capability to space as a function of temperature)

· thermoelectric coolers

Systems in Series:

When two or more systems are in series, they can be combined into a single representative system, with a transfer function that is the product of the individual systems.

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If we have two systems, f(t) and g(t), we can put them in series with one another so that the output of system f(t) is the input to system g(t). Now, we can analyze them depending on whether we are using our classical or modern methods.

If we define the output of the first system as h(t), we can define h(t) as:

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Now, we can define the system output y(t) in terms of h(t) as:

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We can expand h(t):

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But, since convolution is associative, we can re-write this as:

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Our system can be simplified therefore as such:

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Series Transfer Functions:

If two or more systems are in series with one another, the total transfer function of the series is the product of all the individual system transfer functions.

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In the time domain we know that:

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But, in the frequency domain we know that convolution becomes multiplication, so we can re-write this as:

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We can represent our system in the frequency domain as:

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Systems in Parallel:

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Blocks may not be placed in parallel without the use of an added. Blocks connected by an adder as shown above have a total transfer function of:

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Since the Laplace transform is linear, we can easily transfer this to the time domain by converting the multiplication to convolution:

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Analogous Mechanical and Electrical Components:

It is possible to make electrical and mechanical systems using analogs.  An analogous electrical and mechanical system will have differential equations of the same form.  There are two analogs that are used to go between electrical and mechanical systems.  The analogous quantities are given below.

Analogous Quantity:

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To see the analogies more clearly, examine the following table that shows the constitutive relationships for the various analogous quantities.  The entries for the mechanical analogs are formed by substituting the analogous quantities into the equations for the electrical elements.  For example the electrical version of Ohm’s law is e=iR.  The Mechanical I analog stipulates that e is replaced by v, i by f and R by 1/B, which yields v=f/B.

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