DATA PRESENTATION SYSTEMS

INTRODUCTION Measurement system consist of three elements: sensor, signal conditioner and display or data presentation element.  Data presentation elements can be classified as: Indicators and recorders.  Indicators: give an instant visual indication of the sensed variable.  Recorders: record the output signal over a period of time and give automatically a permanent record.  Applicable if the event is high speed or transient and can not be followed by an observer.  There are large amounts of data, or it is essential to have a record of the data. 

CONT… Both Indicators and Recorders can be subdivided in to two groups: Analogue and Digital.  Example: 

Analogue Indicator: a meter which has a pointer moving across a scale.  Digital meter: a display of a series of numbers.  Analogue Recorder: a chart recorder which has a pen moving across a moving sheet of paper.  Digital Recorder: output printed out on a sheet of paper as a sequence of numbers. 

MOVING COIL METER o Is an analogue indicator with a pointer moving across a scale. o The accuracy of such a meter depends on a number of factors, temperature, magnetic fields or ferrous materials, the way the meter is mounted, bearing friction, inaccuracies in scale marking during manufacturing, etc… o Errors are also involved in reading the meter, Eg. Parallax errors.

CONT… The moving coil movement uses a coil former of aluminum, around a center pole and 'immersed' in a strong magnetic field.  The coil is most commonly supported by jeweled bearings. 

The coil is maintained at the zero position by the tension of the hairsprings, and one of these (almost always the top) is made adjustable from outside the meter case.  This allows the user to zero the pointer.  Current to the coil is carried by the hairsprings. 

DIGITAL VOLTMETER Gives its readings in the form of a sequence of digits.  Such a form of display eliminates parallax and interpolation errors  Can give accuracies as high as ±0.005%.  It is essentially a sample and hold unit feeding an ADC with its output counted by a counter. 

ANALOGUE CHART RECORDERS There

are three basic types: Direct reading recorder, the galvanometric recorder and the potentiometric or closed loop recorder. The data can be recorded on paper by:  Fiber-tipped

ink pens  Impact of a pointer pressing a carbon ribbon against the paper  Use of thermally sensitive paper which changes color when a heated pointer moves across it.  By a tungsten wire stylus moving across the surface of specially coated paper  A thin layer of aluminum over colored dye and the electrical discharge removing the aluminum and exposing the dye

DIRECT READING RECORDER Has a pen or stylus directly moved by the displacement action of the measurement system.

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 Eg:

temperature measurement-movement of bimetallic strip  : for pressure gauge- movement of Bourdon tube

A circular chart is used and rotates at a constant rate, typically one revolution in 12 hours, 24 hours or 7 days.  the pen moves along curved radial lines and thus paper with curved lines has to be used for the plotting. This makes interpolation difficult. It has an accuracy of the order ±0.5% of FS deflection. 

CONT…

A circular chart recorder tracking temperature in a building

GALVANOMETRIC CHART RECORDER Works

on the same principle as the moving coil meter movement. The coil is suspended between two fixed points by a suspension wire. When a current passes through the coil a torque acts on it, causing it to rotate and twist the suspension. The coil rotates to an angle at which the torque is balanced by the opposing torque resulting from the twisting of the suspension. The rotation of the coil results in a pen being moved across a chart. If R is the length of the pointer and θ the angular deflection of the coil, then the displacement y of the pen is 𝑦 = sin θ , since θ is proportional to the current i through the coil, then y is proportional to sin 𝑖 . 0 If the angular deflection is restricted to less than ±10 then the relationship is reasonably linear, the nonlinearity error being less than 0.5%. A great problem is, however, the fact that the pen moves in an arc rather than a straight line and thus curvilinear paper has to be used for the plotting.

POTENTIOMETRIC RECORDER The position of the pen is monitored by means of a slider which moves along a linear potentiometer.  The position of the slider determines the potential applied to an operational amplifier.  The op amp subtracts the slider signal which is obtained from the input signal, from the sensor/signal conditioner.  The output from the amplifier is thus a signal that is related to the difference between the pen and sensor signal.  This signal is used to operate a servo motor which in turn controls the movement of the pen across the chart.  The pen thus ends up moving to a position where there is no difference between the pen and sensor signal.  The pen is thus made to track the sensor signal.  They have slower response time. They can thus be used for slowly changing signals. 

CATHODE-RAY OSCILLOSCOPE Provides accurate time and amplitude measurements of voltage signals over a wide range of frequencies. The heart of the CRO is a cathode-ray tube shown schematically in Fig. 

CONT… 

A cathode ray tube (CRT) contains four basic parts: 

electron gun

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focusing and accelerating systems,

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deflecting systems, and

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evacuated glass envelope with a phosphorescent screen that glows visibly when struck by the electron beam.

The cathode ray is a beam of electrons which are emitted by the heated cathode (negative electrode) and accelerated toward the fluorescent screen.

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The assembly of the cathode, intensity grid, focus grid, and accelerating anode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control its intensity and focus.

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CONT… Between the electron gun and the fluorescent screen are two pair of metal plates - one oriented to provide horizontal deflection of the beam and one pair oriented to give vertical deflection to the beam. These plates are thus referred to as the horizontal and vertical deflection plates.  The combination of these two deflections allows the beam to reach any portion of the fluorescent screen.  Wherever the electron beam hits the screen, the phosphor is excited and light is emitted from that point.  This conversion of electron energy into light allows us to write with points or lines of light on an otherwise darkened screen.  The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), echoes of aircraft detected by radar, etc. 

PRINTERS

Provide a record of data on paper. There are a number of versions of such printers:

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Dot matrix printer Ink/bubble jet printer Laser printer 

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Dot matrix printer:  The term dot matrix refers to the process of placing dots to form an image.  Its speed is usually 30 to 550 characters per second (cps).  This is the cheapest and the most noisy printer and has a low print quality.  Dot Matrix were 1st introduced by Centronics in 1970.

HOW DOT MATRIX PRINTER WORKS They consists either 9 or 24 pins in a vertical line.  Each pin is controlled by an electromagnet which when turned on propels the pin onto the inking ribbon.  This impacts a small blob of ink on to the paper behind the ribbon.  A character is formed by moving the print head in a horizontal lines back-and-forth across the paper and firing the appropriate pins. 

ADVANTAGE AND DIS-ADVANTAGE Advantages:  In-expensive.  Low per page cost.  Energy efficient.  Dis-advantages:  Noisy  Low resolution  Limited fonts flexibility  Poor quality graphics output. 

CONT… 

Ink jet printer: It is a non-impact printer producing a high quality print. It was introduced in the later half of 1980s and are very popular owing to their extra-ordinary performance.  Uses a conductive ink which is forced through a small nozzle to produce a jet of very small drops of ink of constant diameter at constant frequency. 

HOW INK JET PRINTER WORKS? Print head having four ink cartridges moves.  Software instructs where to apply dots of ink, which color and what quantity to use.  Electrical pulses are sent to the resistors behind each nozzle.  Vapor bubbles of ink are formed by resistors and the ink is forced to the paper through nozzles.  A matrix of dots forms characters and pictures. Color cartridge showing inkjet nozzles. 

CONT… 

Laser printer:  Use very advanced technology and produce a high quality output.  Laser printers produce the highest quality images, but are relatively expensive.  A laser printer works like a photocopier to produce images on a page.  A laser beam draws images on a light-sensitive drum. The drum picks up a fine powdered ink called toner, and then transfers the toner to the paper to create the images.

HOW A LASER PRINTER WORKS Paper is fed and the drum rotates.  A laser beam conveys information from the computer to a rotating mirror and thus an image is created on the drum.  The charges on the drum are ionized and the toner sticks to the drum.  Toner is transferred from drum to paper.  Heat is applied to fuse the toner on the paper. 

CONT…

LED DISPLAY 

Light emitting diodes are used in LED displays.

LED displays are available in many different sizes and shapes.

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Usually LED displays radiate red, orange, yellow or green light.

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They have a wide operating temperature range, are inexpensive, easily interfaced to digital logic, easily multiplexed, do not require high voltages and have fast response time.

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The viewing angle is good and display of arbitrary numbers of digits is easily assembled.

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SEVEN SEGMENT DISPLAY A

seven-segment display may have 7, 8, or 9 barshaped LEDs on the chip. Usually leads 8 and 9 are decimal points. The figure below is a typical component and pin layout for a seven segment display. .

Each segment (labeled A-G) contains an LED which may be individually controlled.

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CONT… The light emitting diodes in a seven-segment display are arranged in the figure below.

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CONT… There

are two types of displays available, common anode and common cathode. The wiring for a common anode is shown below.

CONT… The

wiring for the common cathode is shown below.

CONT…  To convert the binary numbers to signals that can drive the LED’s in the display you need a display driver.  Eg: MC14511 chip. The pin outs are shown below.  A, B, C, and, D are the binary inputs.  a, b, c, d, e, f, and g are the driver signals to the display elements.  LT is the Light Test control, turns all segments on, active low.  BI blanks all the segments when activated, active low.  LE is the latch enable control.

CONT… The

truth table shown below is used to confirm that the digital signal sent to the display lights up the correct segment.

LCD The heart of all liquid crystal displays (LCDs) is a liquid crystal itself.  A liquid crystal is a substance that flows like a liquid, but its molecules orient themselves in the manner of a crystal.(intermediary substance between a liquid and solid state of matter.)  The material is a compound with long rod-shaped molecules which is sandwiched between two sheets of polymer containing microscopic groves.  The upper and lower sheets are grooved in directions at 900 to each other.  The molecules of the LC material align with the grooves in the polymer and adopt a smooth 900 twist between them. 

CONT… Liquid crystal molecules can move freely while maintaining their orientation. It align itself to a polyimide film to the inside of a panel glass.  When plane polarized light is incident on the LC material its plane of polarization is rotated as it passes through the material. Thus if it is sandwiched between two sheets of polarizer with their transmission directions at right angles, the rotation allows the light to be transmitted and so the material appears light. 

CONT… However, if an electric field is applied across the material, the molecules become aligned with the field and the light passing through the top polarizer is not rotated and cannot pass through the lower polarizer but becomes absorbed. The material then appears dark.

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CONT… The arrangement is put b/n two sheets of glass on which are transparent electrodes in the shape of the sevensegment display and thus the application of voltages to the various display elements results in them appearing black against the lighter display where there is no electric field.  This is the form of display used in battery-operated devices such as watches and calculators. 

CONT… The

light passes through the polarizer. The voltage applied to the electrodes controls the liquid crystal orientation The liquid crystal orientation controls the rotation of the incoming polarized light. Color filters are used in color LCD, where each color sub-pixel is controlled individually

ALARM INDICATOR A wide variety of alarm systems are used with measurement and control systems. Commonly met ones are:

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Temperature alarms  Current alarms  Voltage alarms  Weight alarms 

Alarm indicators take an analogue input from some sensor, possibly via a signal conditioner, and turn it into an on-off signal for some indicator.

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Analogue Input Alarm level Set point

Comparator

Logic

Switching element

Indicator

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