Displays in Aircraft.

The pilot needs to have a lot of information about how his aeroplane is functioning.  If it breaks down, he can't simply pull over behind a cloud and open up the bonnet.  The aeroplane will return to the ground.  Taking off is optional; landing is not.  So he has a lot of instruments in front of him.  If he can tell that the aeroplane is not well, he can hopefully divert to the nearest aerodrome and land there.

Most aeroplanes still have analogue instruments as shown in the picture.

Analogue instruments are ones with needles and dials.  Some of the instruments have a light emitting diode (LED) display.

More modern aeroplanes have instrument panels based on liquid crystal display (LCD) units.  They are called glass cockpits and a picture of one is shown below:

Notice:

• The displays show some data as bars.
• Other data are shown in dials.  People prefer a dial rather than having to read out a series of figures.  You might not get a precise reading, but you have an overview that the reading is reasonable.  If it's in the red, the aeroplane is not well!
• This aeroplane still has analogue instruments in case the displays fail.  They allow the pilot to continue to fly his plane safely to the aerodrome.

This kind of idea is not new.  Cathode ray tube (CRT) displays were installed in aeroplanes many years ago.  However they are heavy and power-hungry.

Light Emitting Diodes

You will have seen the LED as an on/off indicator.  You will also have seen them in clock displays.  At the bottom of the picture above is an LED display (the one that says ALT 00000).  LEDs work like normal diodes.  There are two slices of semi-conductor material doped with different rare elements adjacent to each other:

• the n-material, which has an excess of electrons;
• the p-material, which has positive charge carriers called holes in the lattice.

The distribution of charge forms a layer called the p-n junction.

Diodes work by placing a forward-biased voltage which raises electrons into a conduction band.  This gives the charge carriers energy to cross the p-n junction, which also narrows in width.  In LEDs, as each electron loses energy and drops out of the conduction band, it emits a photon of light.

As each electron crosses the junction it acquires energy, given by the relationship:

energy (J) = charge (C) × voltage (V)

In Physics code:

DE = qV

For a photon we know that:

DE = hf

Since

f = c/l

it does not take a genius to see that:

DE = hc

       l

There are useful data for this kind of problem:

• Electronic charge e = 1.6 × 10^-19 C;
• 1 eV (electron volt) = 1.6 × 10^-19 J;
• Planck constant h = 6.63 × 10^-34 Js;
• Speed of light c = 3.00 × 10⁸ m s^-1.

Liquid Crystal Displays

These are relatively recent arrivals on the electronics scene.  Although they sound very sophisticated, liquid crystals are found in many places and can be described as molecules in the liquid state that have some kind of long-range ordered structure.  Examples include soaps.  The brain is 70 % liquid crystal.

Synthetic liquid crystals are used in displays.  Although they are electrically neutral, the distribution of positive and negative charge is uneven.  Such molecules are polar, and form an electric dipole.

LCD displays have a thin layer of liquid crystal between two glass plates with a very thin conducting layer.  These in turn are between crossed polaroids.

• When the display is off, the crystals are lined up, and polarised light passes through to be stopped by the crossed polaroid.
• When the display is on, the crystals form worm-like patterns and twist the light through 90 ^o so that it can go through the crossed polaroid.  This is because the dipoles turn when an electric field is applied.

Consider the electric field between two parallel plates:

The potential difference is DV (to show that it's definitely a difference).  You can say voltage if you want to!

We know that electric field strength is defined by force per unit mass.  So we can write:

E = F/q

Now if we place a small positive charge +q by the positive plate, it will be attracted by a force F to the negative plate and will gain energy:

DW = qDV

Alternatively we can say that a job of work is done by the force F:

DW = FDx

So we can write:

FDx = qDV

Since electric field strength is defined by force per unit mass, this leads us to the expression:

E = F = DV

       q     Dx

Strictly speaking, we should write:

E = -DV

       Dx

This is because the potential is going from positive to zero.  The potential gradient is negative.  But don't worry too much about it at this level.

In summary, we can say that electric field can be represented as:

• Force per unit charge (N C^-1);

• Potential difference per unit distance (V m^-1).

We can use base units to show that N C^-1 is the same as V m^-1.

Liquid crystal displays are falling in price all the time.  Colour LCDs are replacing the bulkier cathode ray tube (CRT) televisions and computer monitors.  They are an integral part of laptop computers.  Miniature LCDs are also found in data projectors.

Cathode Ray Tubes

They were initially mounted in military aircraft in the Second World War as displays for radar.  A ground radar system called H2S ("because it stinks") enabled more accurate high level bombing to be carried out.  Commercial aircraft used CRT displays for weather radar, since flight through thunderstorms can give an extremely bumpy ride.

Although CRT displays vary in shape, the general layout is like this:

The electron gun consists of:

• a heated filament (just like a light bulb);

• a cathode connected to 0 volts;

• a cylindrical metal anode connected to a voltage of about 5000 V.

Electrons are "boiled" off the cathode by a process of thermionic emission.  They are accelerated to the anode.  Most hit the anode, but a few pass through in a fine beam to strike a phosphor screen at the other end of the tube.  The electrons lose their kinetic energy to the molecules in the phosphor.  The atoms become excited and lose the gained energy by emitting photons.

This process would form only a dot (not very interesting).  Therefore the electron beam is swept forwards and backwards across the screen from top to bottom, and back to the top.  This is accomplished by varying the magnetic field from the two sets of deflection coils:

• The X-deflection coils move the beam from side to side across the screen;

• The Y-deflection coils move the beam up and down the screen.

To make a useful picture, the coils must work in a co-ordinated way.  The coils produce a uniform magnetic field of flux density B according to the equation:

The physics codes are:

• m₀ is the permeability of a vacuum (a constant).  m₀ = 4p × 10^-7 N A^-2.

• n is the number of turns in the coil.

• I is the current (A).

• r is the radius (m).

Deflecting Electrons

We have seen that a wire placed in a magnetic field has a force on it according to Fleming's Left Hand Rule:

If the wire is at an angle q and carries a current I in a magnetic field of flux density B, the force it experiences is given by:

F = BIl sin q

But in a CRT, the electrons are not in a wire.  They key to this is that the force acts on the charge carriers (electrons) in the wire, which in turn puts a force onto the wire.  So we can derive a relationship that links the force on the electrons to the magnetic field strength.

Consider a wire of length l with a number n of charges of charge q.  The charges travel at a velocity v.

The current I is the number of charge carriers passing point X every second.  So we can write:

I = nq

   t

So we can substitute this into our equation:

F = B× nq ×l sin q

t

We can rewrite this as:

F = Bnq × (l/t) sin q

Since the speed = length ÷ time we can rewrite this further:

F = Bnqv sin q

This covers any number of electrons.  So if we want the force on each electron, we make n = 1, giving us:

F = Bqv sin q

In a CRT tube, the electrons travel at 90 degrees to the magnetic field, so sin q = 1.

In a colour display there are three electron guns corresponding to red, blue, and green.  They are aimed at red, blue, and green phosphor dots.  When two or more of these are activated, the colours add up to make different colours.  The intensity is varied by the voltage applied to the anode.

Although getting beyond its sell-by date, the CRT still has a coupe of advantages over LCD.

• It is easier to change the brightness and contrast.

• The colours are more accurate.

• The screen can be view from a wider range of angles.

• Very rapid picture refreshment rates are the norm.  This is not the case with LCD.

People who work with photo-imaging software prefer to use a CRT monitor.