Speed measurement

When measuring speed , technical equipment is used to determine the speed of an object in a certain direction or in space. This article provides an overview of the operating principles; for acceleration measurement see also accelerometry .

Calculating methods determine the average speed in the considered distance or time section. If this section is very small, the current speed is measured approximately. Measurement methods that use physical effects and do not react sluggishly measure the instantaneous speed. Furthermore, when evaluating the speed distribution, the maximum and minimum speed can be determined.

Calculating procedures

Time measurement of a distance

With light barriers , ultrasonic barriers , microwave barriers , or other switching sensors , the time that the object needs for a certain path is measured . ${\ displaystyle t}$ ${\ displaystyle x}$

The speed is calculated with . With the tachymeter scale of the stopwatch, the speed is more easily calculated by multiplying. Use in modern speedometers , in sports, in speed monitoring in road traffic, in the log (measuring device) . ${\ displaystyle v}$ ${\ displaystyle v = {x \ over t}}$

Distance measurement at fixed time intervals

If the position, distance or distance covered ( ) are known at two points in time ( and ), the speed is also calculated . The position can be measured with GPS , runtime measurement of laser or radar pulses or optically with cameras. For further procedures, see Distance Measurement . Application e.g. B. with the laser gun , optical speed measurement in rolling mills, or the Particle Image Velocimetry in fluids. ${\ displaystyle x = x_ {2} -x_ {1}}$ ${\ displaystyle t_ {1}}$ ${\ displaystyle t_ {2}}$ ${\ displaystyle v = {\ frac {x_ {2} -x_ {1}} {t_ {2} -t_ {1}}} = {\ frac {\ Delta x} {\ Delta t}} = {\ dot {x}} (t)}$

Integration of acceleration

The speed can be determined from the measured acceleration a by integration .

{\ displaystyle v _ {\ mathrm {end}} = v _ {\ mathrm {beginning}} + \ int _ {0} ^ {t} a \, \ mathrm {d} t}

Errors in the integration can be eliminated by checking with other sensors. Application u. a. in mechanical engineering , vibrating tables and in avionics (see inertial navigation system )

Physical effects

Electromagnetic induction

According to the law of induction , the voltage in a coil is proportional to the speed at which the magnetic flux changes. This principle is used by tacho generators , impellers , moving coil devices and eddy current sensors .

Doppler effect

When sound, microwaves, or laser beams are reflected from an object, the echo has a higher frequency as the object moves towards the viewer. This frequency difference due to the Doppler effect is evaluated.

Some applications, sorted by decreasing frequency, are laser Doppler anemometry for fluids, the laser surface velocimeter for moving surfaces, the Doppler radar sensor for odometry and avionics , the precipitation radar , as well as the medical ultrasonic Doppler and the meteorological sound. Doppler .

Dynamic pressure measurement

The difference between dynamic pressure and static pressure is measured with a Prandtl probe . The current must be invertebrate. Then: ${\ displaystyle p_ {dyn}}$

{\ displaystyle v = {\ sqrt {\ frac {2 \, p _ {\ mathrm {dyn}}} {\ rho}}}}

Where is the density of the medium. This method is used to determine the speed of aircraft, ships and for the flow speed of gases and liquids. ${\ displaystyle \ rho}$

Heat emission

A wire heated by an electric current is cooled by the air flowing around it. The change in resistance or length of the wire is measured.

Use in hot wire anemometers , high-speed receivers

Time of flight measurement of sound

If a sound wave propagates parallel or at an angle to the direction of velocity of a fluid, its velocity can be calculated from the transit times.

Used as an ultrasonic flow meter or anemometer or with much lower frequencies in tomography of the oceans .

Further procedures

Often the straight movement is converted into a speed, which is then measured: See speed measurement .
By evaluating the flow velocity, flow rates in pipes and the resulting forces can also be determined. see flow measurement .
There are optical methods of speed measurement, where one evaluates image sequences from cameras and determines the optical flow . This is how z. B. Optical mice and similar applications.
The terrestrial navigation in turn works with angle measurements and bearings for landmarks as well as the calibration of airspeed indicators on known routes ( running miles ).
For projectiles, the muzzle velocity is determined using the Cranz-Schardin method .

Applications

In this country

The speed of a motor vehicle is displayed on the speedometer , which measures the output speed of the transmission .

In road traffic, the Doppler effect is mostly used for speed monitoring. Other measuring methods include light barriers and laser pistols .

Air flow velocities are measured with anemometers .

The playback speed (rotational speed) of a record player can be measured by a stroboscope and then adjusted.

Speed measurement systems are widespread in sport. The possible uses range from car and motorcycle sport competitions to winter sports such as skiing and ski jumping . Passing laser light barriers at the beginning, at intermediate points and at the end of the route allows both timing and determination of speed.

In industrial production, speeds are measured in almost all continuously operating productions (e.g. rolling mills , paper mills ) using a measuring wheel or optically (e.g. laser surface velocimeter ).

To water

In nautical science , the speedometer is called the log or the log . The airspeed indicator determines the speed through water , the speed over the ground follows from the ocean current .
Logging mostly works hydromechanically or hydrodynamically (propeller); magnetic and electrical effects can also be used.
An impeller is used to determine the speed of watercraft. The impeller drives a generator, the electrical voltage of which is converted to the journey.

In the air

Knowing the airspeed is essential for an aircraft . The dynamic pressure is used in airplanes to measure the speed, v. a. with the pitot tube . The Venturi tube (suction effect) is better for slow aircraft . The variometer measures the rate of climb via the change in air pressure.

The speed measurement can also be carried out with the Doppler radar , with GPS or indirectly via the position change (IN, radio navigation).

The wind speed at different altitudes, which is important for flight planning and drift , is determined meteorologically or with microwave radar . The latter is u. a. used in weather satellites of the new EUMETSAT Polar System ( EPS ), which has been supplementing the Meteosat system since 2006 .

In science and research

astronomy

In astronomy , the spatial movements of the stars are of interest. They are recorded astrometrically (slowly changing the star locations ) and by spectroscopy :

two-dimensional proper motion on the celestial sphere (mostly less than 1 "per year).
scalar radial velocity in the direction of the line of sight (up to about 100 km / s). Together these 3 components give the star's spatial speed relative to our solar system.
The movement of our planetary system to the “ sun apex ” in the constellation Hercules is superimposed on this . This movement (about 30 km / s) must be taken into account when evaluating speed measurements in the Milky Way in order to obtain the actual galactic movement from the measured relative speed to the sun . The apex analysis allows further investigations.

The spatial movements of the stars contain systematic components, mainly due to the locally different rotation around the center of the Milky Way (approximately circular, around 200–250 km / s) and in the case of clusters of motion (stars formed together, around 20–100 km / s).

Atomic physics

For particle physics u. a. the measurement of atomic movements is important. It was first achieved in 1920 by the physicist Otto Stern as a direct measurement of the speed of silver atoms .

Web links

Wiktionary: speed measurement - explanations of meanings, word origins, synonyms, translations

Velocity measurement of silver atoms ( LEIFI )

Speed ​​measurement