Speedometer

Enlarge picture
Speedometer gauge on a car, showing the speed of the vehicle in kilometres per hour. Also shown is the tachometer, which displays the the rate of rotation of the engine's crankshaft.


A speedometer is a vehicle instrument that measures the instantaneous speed.

Traditional automotive speedometers are driven by a flexible, sleeved cable that is rotated by a set of small gears in the tail shaft of a transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

The most common form of speedometer relies on the interaction of a small permanent magnet affixed to the rotating cable with a small aluminum cup affixed to the shaft of the pointer. 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 'drags' the cup--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 spring. The torque on the cup increases with the speed of rotation of the magnet (which, recall, 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. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, 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 to a 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. The speedometer mechanism often also drives an odometer plus a small switch that sends pulses to the vehicle's engine computer.

Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.

The speedometer was invented by Josip Belušić of Croatia in 1888. Modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display, the latter of which is more common nowadays. Pulse counts may also be used to increment the odometer.

As of 1997, federal standards in the United States allowed a maximum 5% error on speedometer readings (per "Auto Tutor", American Automobile Association of California magazine, Oct. 17, 1997). Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.

Speedometers for other craft have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is a Pitot Tube.

Error

Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Modern speedometers are said to be accurate within 5% but as this is legislated accuracy, this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed. Although levels of some 3 km/h, or 3% are also used, where tough enforcement is used. This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue[1] is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced leaving this matter in limbo for many countries. Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the

percent error = 100x("standard diameter"/"new diameter" - 1).


Nearly all tires now have their size shown as "T/A_W" on the side of the tire, and the tire's

diameter in inches = TxA/1270 + W.


For example, a standard tire is "185/70R14" with diameter = 185x70/1270 + 14 = 24.196850 in. Another is "195/50R15" with 195x50/1270 + 15 = 22.677165 in. Replacing the first tire (and wheels) with the second (on 15" wheels), a speedometer reads 24.19../22.67..=1.0670139 times the correct speed or 6.7% too high.

GPS

GPS devices may indicate the true speed of travel on the user interface. Unlike instrumental speedometers which provide a continious reading, the GPS speed readouts have a one second update interval.

The reading is based on reception of data from the satellites in orbit, and is therefore independent of the car's transmission components. Discrepancies between the two readings may be caused by instrument error (on the vehicle), or by changing directly influencial factors, such as tire sizes.

See also

References

1. ^ Fight Unjust Victorian Speeding Fines. fightfines.info. Retrieved on September 18, 2007.

External links

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Speed is the rate of motion, or equivalently the rate of change in position, many times expressed as distance d traveled per unit of time t.

Speed is a scalar quantity with dimensions distance/time; the equivalent vector quantity to speed is known as
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cable is one or more wires or optical fibers bound together, typically in a common protective jacket or sheath. The individual wires or fibers inside the jacket may be covered or insulated. Combination cables may contain both electrical wires and optical fibers.
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A gear is a component within a transmission device that transmits rotational force to another gear or device. A gear is different from a pulley in that a gear is a round wheel which has linkages ("teeth" or "cogs") that mesh with other gear teeth, allowing force to be fully
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range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined 'higher' ratio.
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eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a moving (or changing) magnetic field intersects a conductor, or vice-versa.
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Calibration refers to the process of determining the relation between the output (or response) of a measuring instrument and the value of the input quantity or attribute, a measurement standard.
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differential is a device, usually consisting of gears, that allows each of the driving wheels to rotate at different speeds, while supplying equal torque to each of them.

Purpose

A vehicle's wheels rotate at different speeds, especially when turning corners.
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Tires or tyres (see American and British English spelling differences) are pneumatic enclosures used to protect and enhance the effect of wheels.

Tires are used on all types of vehicles, from cars to earthmovers to airplanes.
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An odometer (often known colloquially as a mileometer or milometer) is a device used for indicating distance traveled by an automobile or other vehicle. It may be electronic or mechanical.
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Anthem
Lijepa naša domovino
Our beautiful homeland


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Electronics is the study of the flow of charge through various materials and devices such as, semiconductors, resistors, inductors, capacitors, nano-structures, and vacuum tubes. All applications of electronics involve the transmission of power and possibly information.
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driveshaft, driving shaft, or Cardan shaft is a mechanical device for transferring power from the engine or motor to the point where useful work is applied. Most engines or motors deliver power as torque through rotary motion: this is extracted from the linear motion
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display device, also known as an information display is a device for visual or tactile presentation of images (including text) acquired, stored, or transmitted in various forms.
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An odometer (often known colloquially as a mileometer or milometer) is a device used for indicating distance traveled by an automobile or other vehicle. It may be electronic or mechanical.
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Motto
"In God We Trust"   (since 1956)
"E Pluribus Unum"   ("From Many, One"; Latin, traditional)
Anthem
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Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system.
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A Pitot (IPA: [pito]) tube is a pressure measuring instrument used to measure fluid flow velocity, and more specifically, used to determine the airspeed of an aircraft.
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automotive navigation system is a satellite navigation system designed for use in automobiles. It typically uses GPS to acquire position data to locate the user on a road in the unit's map database.
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Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its
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The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.

Markings

Light aircraft


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An odometer (often known colloquially as a mileometer or milometer) is a device used for indicating distance traveled by an automobile or other vehicle. It may be electronic or mechanical.
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A hubometer (hub, center of a wheel + -ometer, measure of), or hubodometer, is a device mounted on the axle of an automobile or other vehicle that measures distance traveled.
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Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system.
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tachometer is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the rate of revolutions per minute on a calibrated analog dial, but digital displays are increasingly common.
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A taximeter is a mechanical or electronic device installed in taxicabs, similar to an odometer, which calculates passenger fares based on a combination of distance travelled and waiting time. It is the shortened form of this word that gives the "taxi" its name.
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Vehicle instrument is an instrument that measures some parameters in the vehicle, often found on its control panel or dashboard.

Types

  • speedometer
  • Malfunction Indicator Lamp
  • Fuel gauge

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electronic instrument cluster, digital instrument panel or digital dash for short, is a set of instrumentation, including the speedometer, that is displayed with a digital readout rather than with the traditional analog gauges.
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September 18 is the 1st day of the year (2nd in leap years) in the Gregorian calendar. There are 0 days remaining.

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