Speed sensor

At a speed measurement is determined with the aid of technical devices which speed has an object in a particular direction or in the room. This article provides an overview of the principles of action; for acceleration measurement see also accelerometry.

Calculating method to determine the average speed in the considered position or time section. If this section is very small, the instantaneous velocity is measured approximately. Use measurement methods, the physical effects and do not react slow, measure the instantaneous velocity. Further, in the evaluation of the velocity distribution of the maximum and the minimum speed can be determined.

• 2.1 Electromagnetic induction
• 2.2 Doppler effect
• 2.3 dynamic pressure measurement
• 2.4 heat dissipation
• 2.5 runtime measurement of sound

• 4.1 On the Ground
• 4.2 On the Water
• 4.3 In the air

• 5.1 Astronomy
• 5.2 atomic physics

Calculating method

Time measurement of a distance

With photoelectric sensors, ultrasonic sensors, microwave sensors, or other switching sensors measuring the time it takes for the object for a particular path.

The speed is calculated. The tachymeter scale of the stopwatch, the velocity is calculated by a simple multiplication. Application in modern speedometer, sport, at the speed monitoring road traffic, the Log ( meter ).

Displacement measurement at fixed time intervals

When the position, distance or the distance covered () to two times ( and ) are known, the speed calculated. The position can be with GPS, runtime measurement of laser or radar pulses or measured optically with cameras. Other methods see distance measurement. Application, for example in the laser gun, optical measurement speed in rolling mills, and the particle image velocimetry in fluids.

Integration of the acceleration

From the measured acceleration a can be found by integrating the speed.

Error in the integration can be eliminated by control to other sensors. Application, inter alia, in mechanical engineering, with vibrating tables and in avionics (see Inertial Navigation System)

Physical effects

Electromagnetic induction

According to the law of induction, the voltage in the coil proportional to the rate of change of magnetic flux. This principle use tachometers, impeller, Tauchspulgeräte, Eddy

Doppler effect

When sound, microwave, or laser beams are reflected from an object, the echo has a higher frequency when the object is moving toward the viewer. This difference in frequency due to the Doppler effect is evaluated.

Some applications, in order of decreasing frequency, the laser Doppler anemometry for fluids, the laser surface velocimeter uneven surfaces, the Doppler radar sensor odometry and the avionics, the precipitation radar, and medical ultrasound Doppler and the meteorological sound - Doppler.

Dynamic pressure measurement

With a Prandtlsonde the difference between stagnation pressure and static pressure is measured. The flow must be invertebrate. Then:

Here, the density of the medium. This method is used for determining the velocity of airplanes, ships, and for the flow speed of gases and liquids.

Heat dissipation

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

Application in the hot-wire anemometer, Fast Receivers

Runtime measurement of sound

Spreads a sound wave parallel or at an angle from the velocity direction of a fluid whose velocity can be calculated from the running times.

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

Other methods

• Often the rectilinear motion is converted into a speed, which is then measured: see speed measurement.
• About the evaluation of the flow velocity also flows in pipes and the resulting forces can be determined. see flow measurement.
• There are visual methods of velocity measurement, where one evaluates image sequences of cameras and the optical flow determined. That is how, for example, optical mice and similar applications.
• For 2-D velocity measurement, the optical spatial filter method of the Fraunhofer -Gesellschaft is used (see web link). A camera observes metrics and uses statistical irregularities of the surface.
• The terrestrial navigation in turn works with angular measurements and bearings by landmarks as well as the calibration of airspeed indicators at known distances (miles running).

Applications

On the Ground

• The speed of a vehicle is displayed on the speedometer which measures the speed of the transmission output.
• In road traffic speeds are often measured by the unpopular to motorists speed traps. Here, for the measurement of the Doppler effect is used. Other measurement methods such as photocells, laser gun and more are described in the main article speed monitoring in road traffic.
• The speed of measurement of air flows in industrial plants is carried out by means of an anemometer.
• In the private sector, may be ( rpm) of a turntable measured and then adjusted by a stroboscope, the playback speed.
• In sports, speed measuring systems are widely used. From auto and motorcycle racing competitions, the applications range to winter sports such as skiing and ski jumping. The passage of laser light barriers at the beginning, permitted at intermediate points and at the end of the track both timekeeping as determination of speed.
• In industrial production speeds in almost all continuous productions (eg rolling mills, paper mill ) measured by measuring wheel or optical ( eg laser surface velocimeter ).

On the Water

• In the nautical the speedometer called the log or the Sign. The airspeed indicator determines the speed through water, from the ocean current follows the speed over the ground.
• Login operate mostly hydro-mechanically or hydrodynamically (propeller ); also generic magnet and electrical effects are available.
• An impeller is used for speed detection of vessels. In this case, the impeller drives a generator, the electrical power to the drive is converted.

In the air

• When an aircraft knowing the airspeed is essential. The back pressure is used in aircraft to measure speed, va with the pitot tube. For slow aircraft, the venturi ( suction effect ) is more favorable. The rising velocity measures the variometer on the air pressure change.
• The trip measurement to be performed with the Doppler radar, GPS or indirectly via the change in position (IN-, radio navigation ).
• The important for flight planning and drift wind speed at different heights is determined meteorological or by microwave radar. The latter is, inter alia, used in weather satellites of the new EUMETSAT Polar System (EPS ), which supplements the Meteosat system since 2006.

In science and research

Astronomy

In astronomy, the spatial movements of the stars of interest. You will astrometric ( slow change in the star positions ) and detected by spectroscopy:

However, the space motions of the stars included systematic portions, especially by the local difference in rotation around the Milky Way center ( approximately circular, about 200-250 km / s) and in so-called moving clusters ( jointly formed stars, about 20-100 km / s).

Atomic physics

Particle physics is, inter alia, the measurement of important atomic motions. They succeeded for the first time in 1920 the physicist Otto Stern as a direct measurement of the velocity of silver atoms.