Speed & Measuring Equipments

7/27/2019 Speed & Measuring Equipments

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SPEED & ITS MEASURING EQUIPMENTSIn kinematics, the speed of an object is the magnitude of its velocity (the rate of change of

its position); it is thus a scalarquantity. The average speed of an object in an interval of time

is the distance travelled by the object divided by the duration of the interval; theinstantaneous speed is the limit of the average speed as the duration of the time interval

approaches zero.

Like velocity, speed has the dimensions of a length divided by a time; the SI unit of speed is

the metre per second, but the most usual unit of speed in everyday usage is the kilometre

per houror, in the USA and the UK, miles per hour. For air and marine travel the knot is

commonly used.

The Italian physicist Galileo Galilei is credited with being the first to measure speed by

considering the distance covered and the time it takes. Galileo defined speed as the

distance covered per unit of time. In equation form, this is

Where vis speed, dis distance, and tis time. A cyclist who covers 30 metres in a time of

2 seconds, for example, has a speed of 15 metres per second. Objects in motion often

have variations in speed (a car might travel along a street at 50 km/h, slow to 0 km/h,

and then reach 30 km/h).

In mathematical terms, the speed vis defined as the magnitude of the velocity v, that is,

the derivative of the position r with respect to time:

Ifs is the length of the path travelled until time t, the speed equals the time derivative

ofs:

In the special case where the velocity is constant (that is, constant speed in a

straight line), this can be simplified to v= s/t. The average speed over a finite

time interval is the total distance travelled divided by the time duration.

Instantaneous speedBy looking at a speedometer, one can read the speed of a car at any instant, or

its instantaneous speed.

Average speedDifferent from instantaneous speed, average speedis defined as the total distance covered

over the time interval.
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Tangential speedLinear speed is the distance travelled per unit of time, while tangential speed (or tangential

velocity) is the linear speed of something moving along a circular path.

Rotational speed(orangular speed) involves the number of revolutions per unit of time. All

parts of a rigid merry-go-round or turntable turn about the axis of rotation in the same

amount of time. Thus, all parts share the same rate of rotation, or the same number of

rotations or revolutions per unit of time.

Units

Units of speed include:

metres per second (symbol m s1 or m/s), the SI derived unit;

kilometres per hour(symbol km/h);

miles per hour(symbol mi/h or mph);

knots (nautical miles per hour, symbol kn or kt);

feet per second (symbol fps or ft/s);

Mach number(dimensionless), speed divided by the speed of sound;

in natural units (dimensionless), speed divided by the speed of light in vacuum

(symbol c= 299,792,458 m/s).

Measuring Equipments

1. Airspeed indicator

The airspeed indicatororairspeed gauge is an instrument used in an aircraft to display

the craft's airspeed, typically in knots, to the pilot. In its simplest form, an ASI measures the

difference in pressure between that which is generally around the craft and the increased

pressure caused by propulsion. The needle tracks pressure differential but the dial is marked

off as airspeed.

Diagram showing the face of a true airspeed indicator typical for a faster single engine

aircraft
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Operation

Airspeed indicator connections

Along with the altimeterand vertical speed indicator, the airspeed indicator is a member of

the pitot-static system of aviation instruments, so named because they operate by

measuring pressure in the pitot and static circuits.

Airspeed indicators work by measuring the difference between static pressure, captured

through one or more static ports; and stagnation pressure due to "ram air", captured through

a pitot tube. This difference in pressure due to ram air is called impact pressure.

Internal mechanism of an airspeed indicator

The static ports are located on the exterior of the aircraft, at a location chosen to detect the

prevailing atmospheric pressure as accurately as possible, that is, with minimum disturbance

from the presence of the aircraft. Some aircraft have static ports on both sides of

the fuselage orempennage, in order to more accurately measure static pressure

during slips and skids. Aerodynamic slips and skids cause either or both static ports andpitot tube(s) to present themselves to the relative wind in other than basic forward motion.

Thus, the alternative placements on some aircraft.

Icing is a problem for pitot tubes when the air temperature is below freezing and visible

moisture is present in the atmosphere, as when flying through cloud or precipitation.

Electrically heated pitot tubes are used to prevent ice forming over the tube.

The airspeed indicator and altimeterwill be rendered inoperative by blockage in the static

system. To avoid this problem, most aircraft intended for use in instrument meteorological

conditions are equipped with an alternate source of static pressure. In unpressurised aircraft,

the alternate static source is usually achieved by opening the static pressure system to theair in the cabin. This is less accurate, but is still workable. In pressurised aircraft, the
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alternate static source is a second set of static ports on the skin of the aircraft, but at a

different location to the primary source.

2. SpeedometerA speedometeror a speed meter is a gauge that measures and displays the

instantaneous speed of a land vehicle. Now universally fitted to motor vehicles, they started

to be available as options in the 1900s, and as standard equipment from about 1910

onwards.[1]Speedometers for other vehicles have specific names and use other means of

sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.

The speedometer was invented by the CroatianJosip Beluiin 1888, and was originally

called a velocimeter.

Eddy current

A speedometer gauge on a car, showing the speed of the vehicle in kilometres per hour.

Also shown is the tachometer, which displays the rate ofrotation of the engines crankshaft.

The eddy current speedometer has been used for over a century and is still in widespread

use. Until the 1980s and the appearance of electronic speedometers it was the only type

commonly used.

Originally patented by a German, Otto Schulze on October 7, 1902,[2]it uses a rotating

flexible cable usually driven by gearing linked to the output of the vehicle's transmission. The

early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front

wheel.

When the car or motorcycle is in motion, a speedometer gear assembly will turn a

speedometer cable which then turns the speedometer mechanism itself. A small permanent

magnet affixed to the speedometer cable interacts with a small aluminium cup (called

a speed cup) attached to the shaft of the pointer on the analogue speedometer instrument.

As the magnet rotates near the cup, the changing magnetic field produces eddy currents in

the cup, which themselves produce another magnetic field. The effect is that the magnet

exerts a torque on the cup, "dragging" it, and thus the speedometer pointer, in the direction

of its rotation with no mechanical connection between them.

The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup

increases with the speed of rotation of the magnet (which is driven by the car's

transmission). Thus an increase in the speed of the car will twist the cup and speedometer

pointer against the spring. The cup and pointer will turn until the torque of the eddy currents

on the cup is balanced by the opposing torque of the spring, and then stop. Given the torque
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on the cup is proportional to the car's speed, and the spring's deflection is proportional to the

torque, the angle of the pointer is also proportional to the speed, so that equally-spaced

markers on the dial can be used for gaps in speed. At a given speed the pointer will remain

motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds toa specific speed indication on the speedometer. This calibration must take into account

several factors, including ratios of the tailshaft gears that drive the flexible cable, the final

drive ratio in the differential, and the diameter of the driven tires.

3. Tachometer

A tachometer(revolution-counter, Tach, rev-counter, RPM gauge) is an instrument

measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device

usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital

displays are increasingly common. The word comes from Greek , tachos, "speed",

and metron, "measure".

History

The first mechanical tachometers were based on measuring the centrifugal force, similar to

the operation of a centrifugal governor. The inventor is assumed to be the German engineer

Dietrich Uhlhorn; he used it for measuring the speed of machines in 1817. Since 1840, it has

been used to measure the speed of locomotives.

4. Tachymeter

A 'tachymeter' scale is a scale sometimes inscribed around the rim of an analog watch. It

can be used to compute a speed based on travel time or measure distance based on speed.The spacings between the marks on the tachymeter dial are therefore proportional to 1/t

where t is the elapsed time, and hence the tachymeter scale is logarithmic and noticeably

non linear.

The function performed by a tachymeter is independent of the unit of distance (e.g. statute

miles, nautical miles, kilometres, metres etc.) as long as the same unit of length is used for

all calculations. It can also be used to measure an industrial production process in units per

hour. A tachymeter is simply a means of converting elapsed time in seconds per unitto units

per hour.

5. VariometerThe term variometeralso refers to a type of variable transformeror an instrument for

measuring the magnitude and direction of a Magnetic field.

A variometer(also known as a rate of climb and descent Indicator (RCDI), rate-of-climb

indicator, vertical speed indicator (VSI), orvertical velocity indicator (VVI)) is one of

the flight instruments in an aircraft used to inform the pilot of the near instantaneous (rather

than averaged) rate of descent or climb. It can be calibrated in knots, feet per minute

(101.333 ft/min = 1 KN) ormetres per second, depending on country and type of aircraft.

In powered flight the pilot makes frequent use of the VSI to ascertain that level flight is being

maintained, especially during turning maneuvers. In gliding, the instrument is used almostcontinuously during normal flight, often with an audible output, to inform the pilot of rising or
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sinking air. It is usual for gliders to be equipped with more than one type of variometer. The

simpler type does not need an external source of power and can therefore be relied upon to

function regardless of whether a battery or power source has been fitted. The electronic type

with audio needs a power source to be operative during the flight. The instrument is of little

interest during launching and landing, with the exception ofaerotow, where the pilot will

usually want to avoid releasing in sink.

Description

Schematic drawing of the internals of a classic aircraft variometer

Variometers measure the rate of change of altitude by detecting the change in air pressure

(static pressure) as altitude changes. A simple variometer can be constructed by adding a

large reservoir (a thermos bottle) to augment the storage capacity of a common aircraft rate-

of-climb instrument. In its simplest electronic form, the instrument consists of an air bottle

connected to the external atmosphere through a sensitive air flow meter. As the aircraft

changes altitude, the atmospheric pressure outside the aircraft changes and air flows into or

out of the air bottle to equalise the pressure inside the bottle and outside the aircraft. The

rate and direction of flowing air is measured by the cooling of one of two self-

heating thermistors and the difference between the thermistor resistances will cause a

voltage difference; this is amplified and displayed to the pilot. The faster the aircraft is

ascending (or descending), the faster the air flows. Air flowing out of the bottle indicates that

the altitude of the aircraft is increasing. Air flowing into the bottle indicates that the aircraft is

descending.

Newer variometer designs directly measure the static pressure of the atmosphere using a

pressure sensor and detect changes in altitude directly from the change in air pressure

instead of by measuring air flow. These designs tend to be smaller as they do not need the

air bottle. They are more reliable as there is no bottle to be affected by changes in

temperature and less chances for leaks to occur in the connecting tubes.

The designs described above, which measure the rate of change of altitude by automatically

detecting the change in static pressure as the aircraft changes altitude are referred to as

"uncompensated" variometers. The term "vertical speed indicator" or "VSI" is most often

used for the instrument when it is installed in a powered aircraft. The term "variometer" is

most often used when the instrument is installed in a glider or sailplane.

An "Inertia lead" VSI or ILVSI compensates for relative "g" forces experienced in a turn

(powered aircraft) and provides appropriate mechanical compensation to remove otherwiseerroneous indications of climb or descent.
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6. Radar gun

A radarspeed gun (also radar gun and speed gun) is a device used to measure the speed of

moving objects. It is used in law-enforcement to measure the speed of moving vehicles and

is often used in professional spectator sport, for such things as the measurement of the

speed of pitched baseballs, runners and tennis serves.

A radar speed gun is a Doppler radarunit that may be hand-held, vehicle-mounted or static.

It measures the speed of the objects at which it is pointed by detecting a change in

frequency of the returned radar signal caused by the Doppler Effect, whereby the frequency

of the returned signal is increased in proportion to the object's speed of approach if the

object is approaching, and lowered if the object is receding. Such devices are frequently

used forspeed limit enforcement, although more modern LIDAR speed gun instruments,

which use pulsed laser light instead of radar, began to replace radar guns during the first

decade of the twenty-first century, because of limitations associated with small radar

systems.

History

The radar speed gun was invented by John L. Barker Sr., who developed radar for the

military during World War II. After the war, he tested radar on the Merritt Parkway. In 1947,

the system was tested by the Connecticut State Police in Glastonbury, Connecticut, initially

for traffic surveys and issuing warnings to drivers for excessive speed. Starting in February

1949, the state police began to issue speeding tickets based on the speed recorded by the

radar device. In 1948, radar was also used in Garden City, New York.

Mode of Operations

Doppler EffectSpeed guns use Doppler radarto perform speed measurements.

Radar speed guns, like other types of radar, consist of a radio transmitterand receiver. They

send out a radio signal in a narrow beam, and then receive the same signal back after it

bounces off the target object. Due to a phenomenon called the Doppler Effect, if the object is

moving toward or away from the gun, the frequency of the reflected radio waves when they

come back is different from the transmitted waves. From that difference, the radar speed gun

can calculate the speed of the object from which the waves have been bounced. This speed

is given by the following equation:

Where cis the speed of light,fis the emitted frequency of the radio waves and f is the

difference in frequency between the radio waves that are emitted and those received

back by the gun. This equation holds precisely only when object speeds are low

compared to that of light, but in everyday situations, the velocity of an object is directly

proportional to this difference in frequency.

By rearranging terms we can see that f is proportional to the velocity:
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Stationary radarAfter the returning waves are received, a signal with a frequency equal to this

difference is created by mixing the received radio signal with a little of the

transmitted signal. Just as two differentmusical notes played together create a beat

note at the difference in frequency between them, so these two radio signals aremixed to create a "beat" signal (called a heterodyne) and an electrical circuit then

measures this frequency using a digital counterand displays the number on a digital

display as the object's speed.

Since this type of speed gun measures the difference in speed between a target and

the gun itself, the gun must be stationary in order to give a correct reading. If a

measurement is made from a moving car, it will give the difference in speed between

the two vehicles, not the speed of the target relative to the road, so a different

system has been designed to work from moving vehicles.

Moving radarIn so-called "moving radar", a gun receives reflected signals from both the target

vehicle and stationary background objects such as the road surface, nearby road

signs, guard rails and streetlight poles. Instead of comparing the frequency of the

signal reflected from the target with the transmitted signal, it compares the target

signal with this background signal. The frequency difference between these two

signals gives the true speed of the target vehicle.
http://en.wikipedia.org/wiki/Direct-conversion_receiverhttp://en.wikipedia.org/wiki/Musical_notehttp://en.wikipedia.org/wiki/Beat_(acoustics)http://en.wikipedia.org/wiki/Beat_(acoustics)http://en.wikipedia.org/wiki/Heterodynehttp://en.wikipedia.org/wiki/Digital_counterhttp://en.wikipedia.org/wiki/Digital_displayhttp://en.wikipedia.org/wiki/Digital_displayhttp://en.wikipedia.org/wiki/Digital_displayhttp://en.wikipedia.org/wiki/Digital_displayhttp://en.wikipedia.org/wiki/Digital_counterhttp://en.wikipedia.org/wiki/Heterodynehttp://en.wikipedia.org/wiki/Beat_(acoustics)http://en.wikipedia.org/wiki/Beat_(acoustics)http://en.wikipedia.org/wiki/Musical_notehttp://en.wikipedia.org/wiki/Direct-conversion_receiver

Documents

Speed & Measuring Equipments