Speedometer
A tachometer ( from Ancient Greek ταχύς tachýs "fast" and μέτρον métron " measure, measure ") is a device used to measure and display the speed of a land vehicle. Usually there is a analog display on a scale by the " speedometer " (pointer speedometer) In rare cases, a will -. Likewise analog -. Bargraph ( roll tachometer) or the digital representation via digital display ( digital speedo ) or a bar graph used by a LC-display Will recorded the values you originates speaks of a tachograph or tachograph., the speed signal from a computer, the possibly multiple speed sensors are connected, ie the visible part fachsprachlich Gazette. lawmakers called the whole thing as " speed meter ", the part that display the contains, as a " speedometer ". usually is also the odometer (odometer ) connected to the speedometer, as they have a common drive.
History and Principles of Operation
Most speedometers are the measuring principle actually tachometer. The first, purely mechanical tachometer based on centrifugal force. Was invented by the German mechanical engineer Diedrich Uhlhorn, 1817 first used this for textile machinery. From 1844 this measurement principle was used on locomotives.
In 1888 by the Croatian Josip Belušić under the name " velocimeter " patented eddy current speedometer, a permanent magnet rotating ( 1) and produced in a front -mounted metal disc or bell made of aluminum ( 2) eddy currents. The additional energy field of the eddy currents would be avoided if the rotatably mounted disk would rotate. Thereto but it is prevented by a return spring (3). The magnetic field increases linearly with the speed, the field energy square, the force effect than the discharge linear again, as well as the restoring force of the spring, with its angular. So that the deflection of the disk is provided with a pointer is proportional to the speed. The pointer is fitted with a calibrated scale deposit (4). The magnetic material used has a high Curie temperature in order to keep the temperature coefficient of the measurement error small.
1902 developed the German inventor Otto Schulze the eddy-current speedometer, a construction for road vehicles. It Schulze sat on a flexible shaft ( speedo ) at which the speed of the wheel or the gear is transmitted to the speedometer and there dealt a magnet in a rotating motion. This speedometer cable was prone to wear and caused due to excessive friction caused by jerking a restless display. For long transmission paths, such as rail cars, a resolver was used instead of the speedometer cable. The synchro receiver at the end of the electrical connection was sitting in the Gazette and drove there on an eddy current speedometer. Was Easier, rectify the AC voltage of a tachogenerator and - indicate DC voltage with a moving coil instrument - but at low speed wave.
Incremental generate a speed-proportional frequency, which can be implemented with an analog circuit into a voltage. First production car with such a system was the Porsche 911 Turbo. The encoder in the differential gear consisted of a magnetic rotor and a reed contact. Modern cars have encoders on each wheel ( for ABS, ASR, ESP, Navi ) whose frequency is detected by a microcontroller. Primary measurement result is the number of pulses within a given measuring interval, or rather the period of time between signal edges.
Even the ads have changed. Pointers are completely linear and independent of temperature moves over a much wider angle of a stepper motor or simulated on a display, often in conjunction with various other indications, see the instrument cluster and multi- function display. In other variants, one just considers it important that certain important speed ranges are shown splayed so that the driver can read it accurately. Often, the speed is now displayed as a numerical value, especially in head-up displays.
Measurement errors and tolerances
All the methods are not based on the direct measurement of the distance traveled in a certain time distance, but at a speed measurement and are therefore dependent on the circumference of the wheels. If larger or smaller wheels mounted a speedometer adjustment is necessary. Smaller error contributions that are not proportional to the speed, altitude, caused by centrifugal forces and slippage due to increasing air resistance.
Wear leads to cars to measurement errors of a few percent, in rail vehicles it is to be corrected to about 10% and must. Under ETCS the permissible limit is for the measurement error of less than 30 km / h constant 2 km / h and takes about linearly, down to 12 km / h 500 km / h For motor vehicles in Europe and many other countries, that the displayed speed is not below the actually traveled, but up to max. May not exceed 10 % v 4 km / h differ (so-called speedometer ).
Speedometer showing speed graphically often linear or with equidistant ticks, but for which there is no legal requirement. In some vehicles, such as the range 50-60 km / h is widened or the range of higher speeds shown with decreasing tick intervals. In order to achieve a proportional display to kinetic energy and the required braking distance and risk, the speed can be represented in a quasi - logarithmic scale ( see figure), as it was also similar in the models GS and CX from Citroën in the seventies. However, this leads to a correspondingly difficult -to-read speed in the lower speed range.
Speedometer
Speed Displays the roadside serve as feedback to operators of motor vehicles and exhort to comply with speed limits.
The Insurers Accident Research ( UDV ) resulted in the published 2010 study "Evaluation of dynamic speed feedback" dynamic speed displays a comparison of these with the so-called dialogue display through. Although the dialog screen does not display the speed driven and praises only by the display of " thank you" and "Slow" and blames himself stronger and lasting declines in average speeds showed.