EE T55 MEASUREMENTS AND INSTRUMENTATION

Transcription

1 EE T55 MEASUREMENTS AND INSTRUMENTATION UNIT IV: DISPLAY AND RECORDING DEVICES LED & LCD Display Dot Matrix Display, 7 Segment Display Strip Chart Recorders Single point and multipoint Recorders X-Y Recorders-MagneticTape Recorders-Data Loggers Electromagnetic and Electrostatic interference. MAGNETIC DISC: A magnetic disk is a storage device with each disk in the stack divided into tracks and sectors PHYSICAL LAY OUT OF MAGNETIC DISK: - 1

2 Performance: Access time = seek time + rotational time + transfer time MECHANICAL CONFIGURATION OF MAGN TIC DISC: Magnetic tape is a storage device with the tape divided into tracks. Access to the data is sequential. - 2

3 SURFACE ORGANISATION OF MAGNETIC DISC: The width of the type is divided into nine tracks; each spot ( 點 ) of a track can store 1 bit of information. Nine vertical spots can store 8 bits of information related to a byte plus a bit for error detection To retrieve a specific block on the tape, you need to pass through all of the previous blocks Writing density: 8000 bits per inch - One bit occupies (25400/8000)*10-6 meters So, one bit occupies ~3.2*10-6 meters The diameter of your hair is ~70*10-6 meters O heads move to correct cylinder So, you can place 3 bytes on the diameter of your hair Seek time O depends entirely on where head is/where it has to go O advertised as (min, average, max) e.g. 20 ms for average O a clever OS exploits data locality Rotational latency O wait for disk to rotate data under the heads O depends on rotational speed of disk e.g. 8 ms Transfer rate O read or write the data O depends on rotational speed, sector size, bit density e.g. 10 MB/s

4 Some disks keep their own cache Average seek time<20 ms Disk rotates from 3600 to 7200 RPM, so Rotational latency max = 0.5/(3600/60) sec = 500/60 msec = 8.3 msec Rotational latency min = 0.5/(7200/60) sec = 500/120 msec = 4.2 msec Transfer time (suppose sector 512 bytes, block equals 8 sectors, transfer rate10mb/s), Transfer time = 8*512/10*106 = 0.41ms Time to read a block = seek time + rotational latency + transfer time = = ms Disk controller overhead not taken into account Hard Disk Physical size has dramatically decreased during past 30 years Capacity has dramatically increased during past 30 years Access time: - Slow access to the start of a block (miliseconds) Cylinders, Fast transfer rate (MB/sec) Magnetic disk internal organization: Tracks, Sectors Data stored in blocks (pages) of.5 to 8 KB TAPE RECORDERS: Observation about recorders: They put sound or sound information on tape They can reproduce the recorded sound A tape can hold several channels of sound The speed of the tape s motion matters 4

5 Tapes are vulnerable to heat and magnetism A tape s leader can t record sound SOUND IN AIR: Moving pressure fluctuations Created by compressing & rarifying the air Heard by detecting pressure fluctuations REPRESENTING SOUND WITH CURRENT: Microphone measures air pressure changes Produces current in a wire that is proportional to the air pressure shift, up or down This current isn t sound, itrepresentssound It contains enough info to recreate the sound Recording headuses sound current Produces magnetization on a tape that-is proportional to air pressure shift, up or down Magnetization isn t sound, representssound It contains enough info to recreate the sound Ferromagnets atomic magnets aligned Antiferromagnets atomic magnets anti-aligned Cancellation is perfect Ferrimagnets atomic magnets anti-aligned Cancellation is imperfect SOFT AND HARD MAGNETS: Ferromagnets usually hide their magnetism Material spontaneously forms magnetic domains

6 Domains randomly align so as to cancel Exposure to magnetic fields aligns the domains Domain walls shift so as to align with the field Soft magnetic materials domains shift back Hard magnetic materials domains stay put PERMANENT MAGNETS: Made from hard magnetic materials Difficult to magnetize with outside field Difficult to demagnetize when field is removed Fabricated in non-magnetic state Then magnetized by an intense, pulsed field Demagnetized by heat, shock, and strong fields SINGLE DOMAIN PARTICLES: Tiny particles contain only one domain Single domains magnetize by magnetic-flipping Long, thin particles are hard to flip They make excellent tiny permanent magnets They are the basis for magnetic tape Magnetic tape is covered with such particles RECORDING TAPE: Sound current sent through ring-shaped electromagnet Split in ring develops north and south poles Nearby tape region becomes magnetized PLAYING BACK TAPE: Tape moves past gap in ring-shaped electromagnet Fluctuating magnetism in ring induces current in playback coil

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8 RECORDING DETAILS: Louder sound deeper magnetization Higher pitch closer magnetic reversals Stereo two separate magnetic tracks/heads Noise reduction high pitch expansion Pitch control tape speed control Sound degradation magnetization damage PLOTTERS: Plotters are usedtoproducehighquality precision graphics usually on large sheets of paper. They are slow, but can draw continuous colors often in a variety of colors. They are especially useful for architectural drawings, building plans and CAD (Computer Aided Design) applications, where precision drawing is required. A mechanical arm holds a pen which can be moved across the page. The paper is sometimes laid on a flat bed (flat bed plotter) or on a rotating drum- (Drum plotter). A plotter is a vector graphics printing device that connects to a computer.

9 The two main ty pes of printer are impact and non-impact.impact Plotters print their output by moving a pen across the surface of a piece of paper. This means that plotters are restricted to line art, including text, but do so very slowly because of the mechanical movement of the pens. Plotters are used primarily in technical drawing where they have the advantage of working on very large paper sizes while maintaining high resolution. PRINTERS: OUTPUT DEVICES: Output devices are pieces of equipment that are used to get information or any other response out of a computer. Output can be divided into two types soft copy output and hard copy output. Softcopy output is temporary output and it refers to information displayed on a screen or in audio or voice form through speakers. This kind of output disappears when - the computer is switched off. PRINTERS: Hardcopy output is permanent output and refers to output printed onto paper. You can print out information that is in the computer onto paper. By printing you create what is known as a 'hard copy'. There are different kinds of printers which vary in their speed and print quality. strike an inked ribbon placed between the print head and the paper. 8

10 Some print heads have only 9 pins to make the dots to build up a character; some have 24 pins which produce a better resolution. The main types are: Dot Matrix Printer Daisywheel Printer DOTMATRIX PRINTER: - Characters are formed from a matrix of dots. EEE The speed is MVIT usually characters per second (cps). This is the cheapest and noisiest of the printer family. The standard of print obtained is poor. These printers are cheap to run and relatively fast. They are useful for low quality carbon copy printing.daisywheel 9

11 Molded metal characters like those in a typewriter are mounted on extensions attached to a rotating wheel and are printed onto the paper by means of a hammer and print ribbon. This results in a great deal of movement and noise during the printing of documents, so printing is slow (less than 90 cps). The standard of print is similar to that produced by an electric typewriter. As the characters on the wheel are fixed, the size and font can only be changed by using a different wheel. However, this is very rarely done. NON IMPACT PRINTERS: Non-impact printers are much quieter than impact printer as their printing heads do not strike the paper. Most non-impact printers produce dot-matrix patterns. Several different technologies have been used to provide a variety of printers. The main types of non-impact printer are: - Thermal Printer Laser Printer Ink Jet Printer Characters are formed by heated elements being placed in contact with special heat sensitive paper forming darkened dots when the elements reach a critical temperature. 10

12 Thermal printer paper tends to darken over time due to exposure to sunlight and heat. The standard of print produced is poor. Thermal printers are widely used in battery powered equipment such as portable calculators. LASER PRINTER: Laser printers use a process similar to machine. - EEE a photocopying Laser Printers use a laser beam and dry powdered ink toner to produce a fine dot matrix pattern being transferred to the page and then fused on to it by heat and pressure. This method of printing can generate about 4 pages of A4 paper per minute. Colour laser printers are expensive They work by printing 4 times...once with The standard of print is very good and laser printers can also produce very good quality printed graphic images too. Prices of laser printers are falling rapidly, so they are becoming one of the most popular printers. INKJET PRINTERS: 11

13 Inkjet printers produce an image by spraying ink onto the paper. Colour graphics may be produced by spraying cyan, magenta, yellow and black inks. Very common for most home users of computers as they are cheap to buy and can print in both colour and black and white. CATHODE RAY TUBE: A cathode ray tube contains a wire cathode that, when heated, emits electrons. A voltage source causes the electrons to travel to the anode. 12

14 The electrons can be steered using electric or EEE magnetic fields. Televisions and computer monitors (except for LCD and plasma models) have a large cathode ray tube as their display. Variations-in the field steer the electrons on their way to the screen An oscilloscope displays en electrical signal on a screen, using it to deflect the beam vertically while it sweeps horizontally The back of the tube has a negatively charged cathode, so called electron gun. The electron gun shoots electrons down the tube and onto a positively charged 13

15 screen. The screen is coated with a pattern of red, green and blue phosphor dots that will glow when struck by the electron stream. The three dots together make up a pixel. The most common type of display screen, the technology is the same as that of a television screen. LIGHT EMITTING DIODE: Basics of Light Emitting Diodes (Electrical) Basics of Light Emitting Diodes (Optical) High internal efficiency designs High extraction efficiency structures Visible Spectrum LED s White-Light LED s The promise of solid state lighting EEE - LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The quanta of light energy released is approximately proportional to the band gap of the semiconductor. Advantages of Light Emitting Diodes (LEDs) Longevity: The light emitting element in a diode is a small conductor chip rather than a filament 14

16 which greatly extends the diode s life in comparison to an incandescent bulb ( hours life time compared to ~1000 hours for incandescence light bulb)efficiency: Diodes emit almost no heat and run at very low amperes.greater Light Intensity: Since each diode emits its own light Cost: Not too bad Robustness: Solid state component, not as fragile as incandescence light bulb Luminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of energy. Electroluminescence excitation results from the application of an electric field In a p-n junction diode injection electroluminescence occurs resulting in light emission when the junction is forward biased Under forward bias majority carriers from both sides of the junction can cross the depletion region and entering the material at the other side. Upon entering, the majority carriers become minority carriers For example, electrons in n-type (majority carriers) enter the p-type to become minority carriers The minority carriers will be larger minority carrier injection Minority carriers will diffuse and recombine with the majority carrier. For example, the electrons as minority carriers in the p-region will recombine with the holes. Holes are the majority carrier in the p-region. The recombination causes light to be emitted Such process is termed radiative recombination. Ideal LED will have all injection electrons to take part in the recombination process In real device not all electron will recombine with holes to radiate light Sometimes recombination occurs but no light is being emitted (non-radiative) Efficiency of the device therefore can be described 15

17 Efficiency is the rate of photon emission over the rate of supply electrons (a) p n+ (b) p n+ E g E g h =E g ev o

18 Electrons in CB Holes in VB CONSTRUCTION OF LED: - Light output Al SiO 2 p Electrical contacts n Substrate Efficient light emitter is also an efficient absorbers of radiation therefore, a shallow p-n junction required. Active materials (n and p) will be grown on a lattice matched substrate. The p-n junction will be forward biased with contacts made by metallisation to the upper and lower surfaces. Ought to leave the upper part clear so photon can escape. The silica provides passivation/device isolation and carrier confinement Need a p-n junction (preferably the same semiconductor material only different dopants)

19 Recombination must occur Radiative transmission to give out the right coloured LED Right coloured LED hc/ = Ec-Ev = Eg so choose material with the right Eg Direct band gap semiconductors to allow efficient recombination All photons created must be able to leave the semiconductor - Little or no re absorption of photons LCD (liquid crystal display) are used in laptop computers, digital clocks, watches, microwave ovens, CD players etc Liquid crystals are neither a solid nor a liquid They require an external light source. Liquid crystal materials emit no light of their own U Светлина Twisted nematic (TN), is naturally twisted liquid crystal. It reacts to electric field in such a way as to control light passage. 17

20 If electric field is applied to liquid crystal molecules, they untwist to varying degrees, depending on the applied voltage. When they straighten out, consequently, no light can pass through that area. DOTMATRIX DISPLAY: Uses a simple grid to supply the charge to a particular pixel on the display Has slow response time and imprecise voltage control (normally the pixels around the controlled one also partially untwist, which makes images appear fuzzy and lacking in contrast) Thin Film Transistors (TFT) 18

21 LCD To can show colors a LCD must have three subpixels with red, green and blue EEE color filters to create each color. - Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. 19