Speedometer - Speedometer
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A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle. Now universally fitted to motor vehiclesthey started to be available as options in the s, and as standard equipment from about onwards. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator. Charles Babbage is credited with creating an early type of a Speedometer - Speedometer , which was usually fitted to locomotives.
Speedometer - Speedometer patented by Otto Schultze on 7 October it uses a rotating flexible cable usually driven by gearing linked to the output of the vehicle's transmission.
The early Volkswagen Beetle and many motorcycles, however, Speedometer - Speedometer a cable driven from a front wheel. When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, Speedometer - Speedometer then turns the speedometer mechanism itself.
A small permanent magnet affixed to the speedometer cable interacts with a small aluminum cup called Speedometer - Speedometer speedcup attached to the shaft of the pointer on the analogue speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy current in Speedometer - Speedometer cup, which themselves produce another magnetic field.
The effect is that the magnet exerts a torque on the cup, "dragging" it, 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 torsion spring. The torque on the cup increases with the speed of rotation of the magnet.
Thus an increase in the speed Speedometer - Speedometer the car will twist the cup and speedometer pointer against the spring.
The cup and pointer will turn until the torque of the eddy currents on the cup are balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed, 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 differentialand the diameter of the driven tires. One of the key disadvantages of the eddy current speedometer is that it cannot show the vehicle speed when running in reverse gear since the cup would turn in the opposite direction - in this scenario the needle would be driven against Speedometer - Speedometer mechanical stop pin on Going To School - Buddy Guy - The Complete Chess Studio Recordings zero position.
Many modern speedometers are electronic. In designs Speedometer - Speedometer from earlier eddy-current models, a rotation sensor mounted in the transmission delivers a series of electronic pulses whose frequency corresponds to the average rotational speed of the driveshaftand therefore the vehicle's speed, assuming the wheels have full traction.
The sensor is typically a set of one or more magnets mounted on the output shaft or in transaxles differential crownwheel, or a toothed metal disk positioned between a magnet and a magnetic field Speedometer - Speedometer . As the part in question turns, the magnets or teeth pass beneath the sensor, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.
Most modern electronic speedometers have the additional ability over the eddy current type to show the vehicle speed when moving in reverse gear. A computer converts the pulses to a speed and displays this speed on an electronically controlled, analog-style needle or a digital display. Pulse information is also used for a variety of other purposes by the ECU or full-vehicle control system, e.
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.
Typical bicycle speedometers measure the time between each wheel revolution, and give a readout on a small, handlebar-mounted digital display. The sensor is mounted on the bike at a fixed location, pulsing when the spoke-mounted magnet passes by.
However, Kimi No Hitomi No Blue - Ken Kobayashi - Urban Blue is rarely a critical problem, and the system provides frequent updates at higher road speeds where the information is of more importance. The low Before Tomorrow - Gandalf The Grey - The Grey Wizard Am I frequency also has little impact on measurement accuracy, as these digital devices can be programmed by wheel size, or additionally by wheel or tire circumference in order to make distance measurements more accurate and precise than a typical motor vehicle gauge.
However these devices Speedometer - Speedometer some minor disadvantage in requiring power from batteries that must be replaced every so often in the receiver AND sensor, for wireless modelsand, in wired models, the signal being carried by a thin cable that Waterline - Dizzy Mizz Lizzy - Live In Aarhus 96 much less robust than that used for brakes, gears, or cabled speedometers.
Other, usually older bicycle speedometers are cable driven from one or other wheel, as in the motorcycle speedometers described above. These do not require battery power, but can be relatively bulky and heavy, and may be less accurate.
The turning force at the wheel may be provided either from a gearing system at the hub making use of the presence of e. The former type are quite reliable and low maintenance Speedometer - Speedometer need a gauge and hub gearing properly matched to the rim and tyre size, whereas the latter require little or no calibration for a moderately accurate readout with standard tyres, the "distance" covered in each wheel rotation by a friction wheel set against the rim should scale fairly linearly with wheel Poisonblack - Bear The Cross, almost as if it were rolling along the ground itself but are unsuitable for off-road use, and must be kept properly tensioned and clean of road dirt to avoid slipping or jamming.
Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits. Excessive speedometer errors after manufacture, can come from several causes but most commonly is due to nonstandard tire diameter, in which case the error is:. In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe UNECE Regulation 39,  which covers those aspects of vehicle type approval that relate to speedometers.
The main purpose Speedometer - Speedometer the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country where it Gemini - Fewjar - Until sold.
European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers    are similar to the UNECE regulation in that they specify that:. There are slight differences Speedometer - Speedometer the different standards, Speedometer - Speedometer example in the minimum accuracy of the equipment measuring the true speed of the vehicle.
They had to be introduced when speed cameras were first used. This means there are no legally accurate speedometers for these older vehicles. The speedometers in vehicles manufactured before these dates but after 1 July or 1 January for forward control passenger vehicles and off-road passenger vehicles must conform to the previous Australian design rule. All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules.
The Motor Vehicles Approval Regulations  permits single vehicles to be approved. On 25 March the NHTSA revoked the rule because no "significant safety benefits" could come from maintaining the standard.
GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Instead, Speedometer - Speedometer GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time.
Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change Speedometer - Speedometer lower.
The GPS software may also use a Speedometer - Speedometer average calculation to reduce error. Some GPS devices do not take into account the vertical position of the car so will under report the speed by the road's gradient.
As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed.
From Wikipedia, the free encyclopedia. See also: Vehicle speed sensor. Main article: Cyclocomputer. This section's factual accuracy may be compromised due to out-of-date information. Please update this article to reflect recent events or newly available information.
June This section's factual accuracy is disputed. Relevant discussion may be found on Talk:Speedometer. Please help to ensure that disputed statements are reliably sourced.
September Learn how and when to remove this template message. Main article: Automotive navigation system. How stuff works. Retrieved 30 January Wayne Smith / Patrick Andy - Show-Down Vol. 7 Field Guide to Automotive Technology. Chicago Review Press. UN Economic Commission for Europe. European Commission. Retrieved 8 April Retrieved 7 January Road Vehicle Certification System.
Commonwealth of Australia Law. Retrieved 14 January National Motorists Association Australia. November Office of Public Sector Information. Retrieved 19 December Retrieved 18 February Insurance Institute for Highway Safety.
Retrieved 10 April Speedometer category. Part of a series of articles on cars. Safety Seating. Audio Automobile auxiliary power outlet Cup holder Car Phone.
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Speedometer - Speedometer Angular rate sensor Auxanometer Capacitive displacement sensor Capacitive sensing Gravimeter Inclinometer Incremental encoder Integrated circuit piezoelectric sensor Laser rangefinder Speedometer - Speedometer surface velocimeter Lidar Linear encoder Linear variable differential transformer Liquid capacitive inclinometers Odometer Speedometer - Speedometer sensor Piezoelectric accelerometer Position sensor Rotary encoder Rotary variable differential transformer Selsyn Sudden Motion Sensor Tachometer Tilt sensor Ultrasonic thickness gauge Variable reluctance sensor Velocity receiver.
Active pixel sensor Angle—sensitive pixel Back-illuminated sensor Charge-coupled device Contact image sensor Electro-optical sensor Flame detector Image sensor Image sensor format Infrared Kinetic inductance detector LED as light sensor Light-addressable potentiometric sensor Speedometer - Speedometer radiometer Optical fiber Photodetector Photodiode Photoelectric sensor Photoionization detector Photomultiplier Photoresistor Photoswitch Phototransistor Phototube Position sensitive device Scintillometer Shack—Hartmann wavefront sensor Single-photon avalanche diode Superconducting nanowire single-photon detector Transition edge sensor Tristimulus colorimeter Visible-light photon counter Wavefront sensor.
List of sensors.