Dictionary:

speedometer

  (spĭ-dŏm'ĭ-tər, spē-)

1. An instrument for indicating speed.
2. 1. An instrument for indicating distance traveled as well as rate of speed.
   2. An odometer.

Background

A speedometer is a device used to measure the traveling speed of a vehicle, usually for the purpose of maintaining a sensible pace. Its development and eventual status as a standard feature in automobiles led to the enforcement of legal speed limits, a notion that had been in practice since the inception of horseless carriages but had gone largely ignored by the general public. Today, no automobile is equipped without a speedometer intact; it is fixed to a vehicle's cockpit and usually shares a housing with an odometer, which is a mechanism used to record total distance traveled. Two basic types of automobile speedometer, mechanical and electronic, are currently produced.

History

The concept of recording travel data is almost as old as the concept of vehicles. Early Romans marked the wheels of their chariots and counted the revolutions, estimating distance traveled and average daily speed. In the eleventh century, Chinese inventors came up with a mechanism involving a gear train and a moving arm that would strike a drum after a certain distance. Nautical speed data was recorded in the 1500s by an invention called the chip log, a line knotted at regular intervals and weighted to drag in the water. The number of knots let out in a set amount of time would determine the speed of the craft, hence the nautical term "knots" still applied today.

The first patent for a rotating-shaft speed indicator was issued in 1916 to inventor Nikola Tesla. At that time, however, speedometers had already been in production for several years. The development of the first speedometer for cars is often credited to A. P. Warner, founder of the Warner Electric Company. At the turn of the century, he invented a mechanism called a cut-meter, used to measure the speed of industrial cutting tools. Realizing that the cut-meter could be adapted to the automobile, he modified the device and set about on a large promotional campaign to bring his speedometer to the general public. Several speed indicator concepts were introduced by competing sources at the time, but Warner's design enjoyed considerable success. By the end of World War I, the Warner Instrument Company manufactured nine out of every 10 speedometers used in automobiles.

The Oldsmobile Curved Dash Runabout, released in 1901, was the first automobile line equipped with a mechanical speedometer. Cadillac and Overland soon followed, and speedometers began to regularly appear as a factory-installed option in new automobiles. Speedometers in this era were difficult to read in daylight and, with no lamp in the housing, virtually illegible at night. The drive cable in early models was attached to either the front wheels or the back of the transmission, but the integration of the drive cable into the transmission housing wouldn't happen for another 20 years. After that improvement was made, the basic technical design of a speedometer would remain untouched until the advent of the electronic speedometer in the early 1980s.

Raw Materials

Materials used in the production of speedometers vary with the type of gauge and intended application. Older mechanical models were entirely comprised of steel and other metal alloys, but in later years about 40% of the parts for a mechanical speedometer were molded from various plastic polymers. Newer electronic models are almost entirely made of plastics, and design engineers continually upgrade the polymers used. For example, the case of a speedometer's main assembly is usually made of nylon, but some manufacturers now employ the more water-resistant polybutylene terephthalate (PBT) polyester. The worm drive and magnet shaft are also nylon, as is the speedometer's gear train and spindles. The glass display lens of the recent past is now made of transparent polycarbonate, a strong, flexible plastic that is resistant to heat, moisture, and impact.

Design

In a mechanical speedometer, a rotating cable is attached to a set of gears in the automobile's transmission. This cable is directly attached to a permanent magnet in the speedometer assembly, which spins at a rate proportional to the speed of the vehicle. As the magnet rotates, it manipulates an aluminum ring, pulling it in the same direction as the revolving magnetic field; the ring's movement, however, is counteracted by a spiral spring. Attached to the aluminum ring is the pointer, which indicates the speed of the vehicle by marking the balance between these two forces. As the vehicle slows, the magnetic force on the aluminum ring lessens, and the spring pulls the speedometer's pointer back to zero.

Electronic speedometers are almost universally present in late-model cars. In this type of gauge, a pulse generator (or tach generator) installed in the transmission measures the vehicle's speed. It communicates this via electric or magnetic pulse signals, which are either translated into an electronic read-out or used to manipulate a traditional magnetic gauge assembly.

The Manufacturing
Process

Steel components

• To form molten steel, iron ore is melted with coke, a carbon-rich substance that results when coal is heated in a vacuum. Depending on the alloy, other metals such as aluminum, manganese, titanium, and zirconium may also be introduced. After the steel cools, it is formed into sheets between high-pressure rollers and distributed to the manufacturing plant. There, the individual parts may be cast into molds or pressed and shaped from bar stock by large rolling machines.

Plastic components

• The various plastics that arrive in an instrument manufacturing station were first created from organic chemical compounds derived from petroleum. These polymers are distributed in pellet form for use in the injection-molding process. To make the small parts for a speedometer assembly, these pellets are loaded into the hopper of a molding machine and melted. A hydraulic screw forces the plastic through a nozzle and into a pre-cast mold, where the plastic is allowed to cool and solidify. The parts are then gathered and transported to assembly stations.

Assembly

• The manner of assembly and degree of human interaction depends on the quality of speedometer. Some inexpensive speedometer systems are made to be "disposable," meaning that the instruments are not built for easy disassembly or repair. In this case, the hardware is fastened using a process called riveting, in which a headed pin is inserted and blunted on the other end, forming a permanent attachment. Higher-end speedometer systems consist of two major assemblies attached by screws; the advantage is that the inner hardware of the gauge is accessible for repair and recalibration.
• The inner shaft and speedometer assembly are then fused into place with rivets or screws. The permanent magnets used in mechanical speedometers are compressed and molded before arrival at the plant, and therefore only require mounting onto the worm drive. In the case of electronic speedometers, fiberglass-and-copper circuitry is also manufactured by vendors, and does require programming before it is screwed into the larger system. These larger components are transported to a separate assembly station, where they are mounted into the housing with stud-terminal or blade-terminal plastic connectors. Beyond its primary duty as a protective case, the housing also serves as a platform for attaching exterior features such as the dial face, needle, and display window. Again, these processes require automation due to large output, but human effort is needed at every step to inspect and ensure product consistency.

Calibration

• Calibration is the process of determining the true value of spaces in any graduated instrument. It is an especially vital process in the manufacture of speedometers because driver safety is reliant on an accurate readout. In a mechanical gauge, magnetic forces produce the torque that deflects the indicator needle. When calibrating this type of gauge, an electromagnet is used to adjust the strength of the permanent magnet mounted in the speedometer until the needle matches the input from the rotating cable. When calibrating an electronic gauge, adjustments are made when calibration factors are written into the memory of the meter. The system can then refigure the balance between input from the transmission and output of the needle. New automated systems for calibrating both mechanical and electronic speedometers are now available, saving an immense number of the man-hours usually required for this process.

Quality Control

Probably the most direct method of quality control is the calibration process. Auto parts manufacturers work under the measurement standards developed by International Organization for Standardization (ISO), which ensures that universal guidelines between gauge manufacturers are used. In-house quality assurance teams develop specifications for each new product before it moves to the assembly line, and the same teams later report whether those guidelines are adhered to on the factory floor. Gradual levels of assembly also involve inspection by factory personnel to make sure that the automation is working smoothly.

Byproducts/Waste

No byproducts result from the manufacture of gauges. Waste materials include scrap metals and plastics, some of which can be reused in later production runs. Because the raw materials involved are prepared outside of the factory, no significant amount of hazardous industrial waste results from manufacture. Emissions from factory automation are government-regulated and surveyed by environmental protection groups.

The Future

Design firms are currently experimenting with improvements in speedometer readout, an effort to eliminate the moment of distraction needed for a driver to look down and gauge his or her speed. Digital readouts projected onto the windshield appear to be the next developmental step. Some proto-types for these speedometers actually make the readout appear as though it is floating over the engine hood. Because this type of display looks as though it is several feet beyond the steering wheel, drivers will be able to continually monitor speed without having to take their eyes off the road. The mirrors and projection devices used in this system could also be adjusted to suit the driver's position, much in the same way that a rear-view mirror does. In addition, speedometer projection systems will eventually be integrated with navigation tools, allowing directional information to appear with gauge readouts.

Where to Learn More

Other

Devaraj, Ganesh, et al. "Automating Speedometer Calibration." Evaluation Engineering Web Page. December 2001. <http://www.evaluationengineering.com/archive/articles/1100auto.htm>.

"How a Tachometer/Speedometer Works Using a Magnetic Sensor." Manual. Stewart-Warner Co., April 2001.

"How an Electrical Gauge is Put Together." Manual. Stewart-Warner Co., April 2001.

"How Odometers Work." Marshall Brain's How Stuff Works. December 2001. <http://www.howstuffworks.com>.

"Speeding Through Time." Transport Topics Electronic Newspaper. November 1998. December 2001. <http://www.ttnews.com/members/printEdition/0000395.html>.

"Speedometer." Complete Computer Software Web Page. December 2001. <http://www.iao.com/howthing/Default.htm>.

"The Floating Speedometer." Siemens.com Web Page. December 2001. <http://www.siemens.com/page/1,3771,257095-1-999_5_4-0,00.html>.

[Article by: Kate Kretschmann]

A device for indicating the speed of a vehicle. There are three types of speedometers in general use: mechanical analog, quartz electric analog, and digital microprocessor.

The mechanical analog speedometer is driven by a cable housed in a casing and connected to a gear at the transmission. This gear is designed for the particular vehicle model, considering the vehicle's tire size and rear axle ratio. In most cases, the speedometer is designed to convert 1001 revolutions of the drive cable into registering 1 mi on the odometer, which records distance traveled by the vehicle. The speed-indicating portion of the speedometer operates on the magnetic principle. In the speedometer head, the drive cable attaches to a revolving permanent magnet that rotates at the same speed as the cable. Floating on bearings between the upper frame and the revolving permanent magnet is a nonmagnetic movable speed cup. The magnet revolves within the speed cup, producing a rotating magnetic field. The magnetic field is constant, and the amount of speed cup movement is at all times in proportion to the speed of the magnet rotation. A pointer, attached to the speed cup spindle, indicates the speed on the speedometer dial.

The quartz speedometer utilizes an accurate clock signal supplied by a quartz crystal, along with integrated electronic circuitry to process an electrical speed signal. This signal is generated by a permanent-magnet generator mounted in the transmission. This permanent-magnet generator, designed to be used with both quartz and digital speedometers, provides a sinusoidal speed signal that is proportional to vehicle speed at the rate of 4004 pulses per mile (2503 per kilometer).

In the digital microprocessor speedometer, the vehicle speed is monitored by the permanent speed sensor mounted in the transmission. The signal is transmitted to the microprocessor where the counter converts the speed signal to a digital signal and stores it in memory. The timing circuit has the capacity to handle the counter and memory storage in less than 0.25 s. Memory circuit signals are sent to the electronic display circuit, which selects the display numerals representing the vehicle's speed, according to the number of pulses received from the speed sensor. See also Automotive transmission; Electronic display.

A pattern of lights displayed on a linear set of LEDs (today) or nixie tubes (yesterday, on ancient mainframes). The pattern is shifted left every N times the operating system goes through its main loop. A swiftly moving pattern indicates that the system is mostly idle; the speedometer slows down as the system becomes overloaded. The speedometer on Sun Microsystems hardware bounces back and forth like the eyes on one of the Cylons from the wretched Battlestar Galactica TV series.

Historical note: One computer, the GE 600 (later Honeywell 6000) actually had an analog speedometer on the front panel, calibrated in instructions executed per second.

This article is missing citations or needs footnotes. Using inline citations helps guard against copyright violations and factual inaccuracies.

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.

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Speedometer

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

Main article: Automotive navigation system

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

• Airspeed indicator
• Odometer
• Hubometer
• Pit log
• Tachometer
• Taximeter
• Vehicle instrument
• Digital speedometer

References

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

External links

• Autoblog: Gauging changes
• Visual Tyre Size Calculator with speedometer error]

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - speedometer

Nederlands (Dutch)
snelheidsmeter

Français (French)
n. - compteur de vitesse, indicateur de vitesse

Deutsch (German)
n. - Tachometer

Ελληνική (Greek)
n. - ταχύμετρο, κοντέρ ταχύτητας

Italiano (Italian)
tachimetro

Português (Portuguese)
n. - velocímetro (m), aparelho que mede a velocidade de um veiculo

Русский (Russian)
спидометр, тахометр

Español (Spanish)
n. - velocímetro, indicador de velocidad

Svenska (Swedish)
n. - hastighetsmätare

中文（简体） (Chinese (Simplified))
速度计, 里程计

中文（繁體） (Chinese (Traditional))
n. - 速度計, 里程計

한국어 (Korean)
n. - 속도계, 주행 기록계

日本語 (Japanese)
n. - 速度計, 走行記録計

العربيه (Arabic)
‏(الاسم) عداد السرعه‏

עברית (Hebrew)
n. - ‮מד-מהירות, מד-אוץ‬

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