A speedometer or speed meter is a gauge that measures and displays the vehicle's instantaneous velocity. Now universally installed for motor vehicles, they became available as an option in the 1900s, and as standard equipment began around 1910 onwards. Speedometer for other vehicles has a special name and uses other means of speed sensing. For boats, this is a pit log. For airplanes, this is an airspeed indicator.
Charles Babbage is credited with creating an early type of speedometer, which is usually installed for locomotives.
The electric speedometer invented by Croatia Josip Belu? I? in 1888 and was originally called a velocimeter.
Video Speedometer
Operation
Originally patented by Otto Schultze on October 7, 1902, he used a rotating flexible cable typically driven by gears connected to the vehicle transmission output. The early Volkswagen Beetle and many motorcycles, however, used cables driven from the front wheels.
When the vehicle is moving, the speedometer wheel assembly rotates the speedometer cable, which then changes the speedometer mechanism itself. The small permanent magnet attached to the speedometer cable interacts with a small aluminum cup (called speedcup ) attached to the pointer shaft on the analog speedometer instrument. When the magnet rotates near the cup, the changed magnetic field produces an eddy current in the cup, which in itself produces another magnetic field. The effect is that the magnet gives the torque on the cup, "dragging" it, and thus the speedometer pointer, toward the rotation without a mechanical connection between them.
The pointing shaft is held to zero by a fine torque spring. The torque on the cup increases with the magnetic rotation speed. Thus increasing the speed of the car will twist the cup and speedometer pointer against the spring. The cup and the pointer rotate until the eddy current torque on the cup is balanced by the torque versus the spring, and then stops. Since the torque of the cup is proportional to the speed of the car, and the spring deflection is proportional to the torque, the pointer angle is also proportional to the speed, so the same spacebar marker on the dial can be used for the gap in speed.. At a certain speed, the pointer will stay in motion and point to the corresponding number on the speedometer dial.
The spring is again calibrated in such a way that the speed of rotation of the given cable is in accordance with a certain speed indication on the speedometer. This calibration should take into account several factors, including tailshaft gear ratios that drive flexible cables, final drive ratios in differential, and the diameter of the driven tire.
One of the main disadvantages of an eddy current speedometer is that it can not show the speed of a vehicle when walking in reverse gear because the cup will turn in the opposite direction - in this scenario the needle will be pushed against the mechanical stop pin at the zero position.
Electronics
Many modern speedometers are electronic. In designs derived from previous eddy-current models, rotational sensors mounted in the transmission provide a series of electronic pulses whose frequency corresponds to the average rotation speed of the driveshaft, and therefore the vehicle speed, assuming the wheels have full traction. Sensors are usually one or more magnets mounted on the output shaft or (in transaxles) of differential gears, or toothed metal disks positioned between magnets and magnetic field sensors. When the inside of the question changes, the magnet or the tooth passes under the sensor, each time it generates pulses in the sensor as they affect the strength of the measured magnetic field. As an alternative, especially in vehicles with multiplex cables, some manufacturers use pulses derived from ABS wheel sensors that communicate with the instrument panel via CAN Bus. Most modern electronic speedometers have an additional capability above the eddy current type to show the speed of the vehicle as it moves in reverse gear.
Computers convert pulses to speed and display this speed in analog-style analogue needles or digital screens. Pulse information is also used for other purposes by the ECU or complete vehicle control system, e.g. trigger ABS or traction control, calculate the average travel speed, or to increase the odometer in place that is turned on directly by the speedometer cable.
Another early form of an electronic speedometer relies on the interaction between precision clock mechanisms and mechanical pulsed driven by wheels or car transmissions. The watch mechanism attempts to push the speed pointer to zero, while the vehicle-driven pulsator tries to push it in the infinite direction. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.
Bicycle Spedometer
A typical bicycle speed gauge measures the time between each wheel spin, and provides readings on a small digital display mounted on the handlebar. The sensor is mounted on the bicycle in a fixed location, pulsating when the magnet installed by the speaker passes. In this way, analogous to an electronic car speedometer uses pulses from the ABS sensor, but with a much more rugged time/distance resolution - typically a pulse/display update per revolution, or occasionally every 2-3 seconds of low speed with 26 inch wheels (2.07 m circle, no tires). However, this rarely becomes a critical issue, and the system provides periodic updates at higher road speeds where information becomes more important. The low pulse frequency also has little impact on measurement accuracy, since this digital device can be programmed based on wheel size, or additionally with wheels or tire circumference to make the distance measurement more accurate and precise than a typical motor vehicle gauge. This device, however, carries several small losses in requiring power from the battery which must be replaced so often in the receiver (AND sensor, for wireless models), and, in the cable model, the signal carried by the less powerful thin cable is used for brakes, gear, or cable speedometer.
Another, usually older bike's speedometer is a cable that is driven from one or the other wheels, as in the motorcycle speedometer described above. It does not require battery power, but it can be relatively large and heavy, and may be less accurate. The rotary force on the wheel can be provided either from the gear system in the hub (utilizing the presence of a hub hub, cylinder or dynamo gear) to an ordinary motorcycle, or with a friction wheel device pushing against the outer edge of the rim (the same position as the rim brake, but on the opposite side of the fork) or the side wall of the tire itself. The former is reasonably reliable and low maintenance but requires gauges and hub gearing that really match the rims and tire sizes, while the latter requires little or no calibration for fairly accurate readings (with standard tires, "distances" covered) in every rotation of the wheels with friction wheels mounted on the ream should be linearly lined with wheel sizes, almost as if rolled along the ground itself) but not suitable for off-road use, and should be fixed properly and cleanly from dirt roads to avoid slipping or jamming.
Maps Speedometer
Error
Most speedometers have a tolerance of about Ã, Â ± 10%, mainly due to tire diameter variations. Sources of error due to variation in tire diameter are wear, temperature, pressure, vehicle load, and nominal tire size. Vehicle manufacturers usually calibrate the speedometer to calibrate the same height as the average error, to ensure that their speedometer never shows a speed lower than the actual speed of the vehicle, to ensure they are not responsible for the driver breaking the speed limit.
An excessive speedometer error after manufacture can come from several causes but the most common is because the tire's diameter is not standard, in which case the error is
Almost all tires now have sizes shown as "T/A_W" on the tire side (See: Ban Code), and tires
For example, the standard tire is "185/70R14" in diameter = 2 * 185 * (70/100) (14 * 25.4) = 614.6 mm (185x70/1270 14 = 24.20 inches). Others are "195/50R15" with 2 * 195 * (50/100) (15 * 25.4) = 576.0 mm (195x50/1270 15 = 22.68 in). Replacing the first tire (and wheels) with the second wheel (at 15 "= 381 mm), the speedometer reads 100 * (1- (576/614.6)) = 100 * (1 - 22.68/24.20) = 6.28% higher than the actual speed At an actual speed of 100 km/h (60 mph), the speedometer will show 100 x 1.0628 = 106.28 km/h (60 * 1.0628 = 63.77 mph), approx.
In the case of wear, the new "185/70R14" tire with a diameter of 620 mm (24.4 inches) will have a ~ 8 mm tread depth, at this legal limit reduced to 1.6 mm, the difference being 12.8 mm or 0.5 inch ie 2% in 620 mm (24.4 inches).
International agreements
In many countries, errors set in speedometer readings are ultimately governed by the United Nations Economic Commission Regulation for Europe (UNECE) 39, which includes aspects of vehicle type approval with respect to the speedometer. The main purpose of the UNECE regulation is to facilitate the trade in motor vehicles by approving uniform type approval standards rather than requiring vehicle models to undergo different approval processes in each of the countries where they are sold.
EU Member States should also provide type approval for vehicles that meet similar EU standards. That includes speedometers similar to UNECE rules because they specify that:
- The speed shown should not be less than the actual speed, ie the speed should not be accidental because of incorrect speedometer readings.
- The speed indicated should not be more than 110 percent of the actual speed plus 4 km/h at the specified test speed. For example, at 80 km/h, the indicated speed should be no more than 92 km/h.
The standards define both accuracy limits and many details of how it should be measured during the approval process, for example that test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at ambient and road surface temperatures certain. There are slight differences between different standards, for example in the minimum accuracy of equipment that measures actual vehicle speed.
The UNECE Regulation eases the requirements for mass-produced vehicles after approval of the type. On Conformity of Production The upper limit audit at the indicated speed increases to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two- or three-wheeled vehicles with maximum speeds above 50 km/h (or cylinder capacity, if powered by a heat engine, more than 50 cm). The European Union Directive 2000/7/EC, associated with two-and three-wheeled vehicles, provides similar restrictions that are slightly relaxed in production.
Australia
No Australian Design Rule was in place for the speedometer in Australia before July 1988. They should be introduced when speed cameras are first used. This means there is no accurate speedometer for this older vehicle. All vehicles manufactured on or after 1 July 2007, and all vehicle models introduced on or after 1 July 2006, must conform to UNECE 39 Regulations.
Speedometers on vehicles manufactured prior to this date but after July 1, 1995 (or January 1, 1995 for front passenger vehicle passenger and off-road passenger vehicles) shall conform to previous Australian design rules. It specifies that they only need to display speeds to an accuracy of /- 10% at speeds above 40 km/h, and no accuracy is specified at all for speeds below 40 km/h. All vehicles manufactured in Australia or imported for supply to the Australian market must comply with Australian Design Rules.
State and territory governments may set policies for speed tolerance over posted speed limits that may be lower than 10% in previous versions of Australian Design Rules are permitted, such as in Victoria. This has caused some controversy as it would be impossible for the driver not to realize that he was speeding if his vehicle equipped with a speedometer below the reading speed.
United Kingdom
The Revised Road Vehicle Regulations (Construction and Use) of 1986 allow the use of speedometers that comply with the requirements of Council Regulation EC 75/443 (as amended by Regulation 97/39) or UNECE Regulation 39.
The 2001 Motor Vehicle Regulations (Permits) allow single vehicles to be approved. Like the UNECE rules and EC Directives, the speedometer should not indicate the speed shown at less than the actual speed. However it is slightly different from those in determining that for all actual speeds between 25 mph and 70 mph (or the maximum speed of a vehicle if lower than this), the speed shown should not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually traveling at 50 mph, the speedometer should not show more than 61.25 mph or less than 50 mph.
United States
Federal standards in the United States allow a maximum error of 5 mph at a speed of 50 mph on speedometer readings for commercial vehicles. Aftermarket modifications, such as different tire sizes and wheels or differential gearing differences, may cause speedometer inaccuracies.
Rules in the US
On 1 September 1979 NHTSA required a speedometer to have special emphasis on 55 mph and display no more than the maximum speed of 85 mph. On March 25, 1982, NHTSA revoked the rule because there were no "significant safety benefits" that could stem from maintaining standards.
GPS
The GPS device is the positioning speedometer, based on how far the receiver has moved since the last measurement. The speed calculation is not subject to the same error source as the vehicle's speedometer (wheel size, transmission/drive ratio). In contrast, GPS position accuracy, and therefore the accuracy of the calculated speed, depends on the quality of the satellite signal at that time. Speed ​​calculations will be more accurate at higher speeds, when the positional error ratios of position changes are lower. GPS software can also use moving average calculations to reduce errors. Some GPS devices do not take into account the vertical position of the car so it will report the speed by the road gradient.
As mentioned in the satnav article, GPS data has been used to reverse the oncoming ticket; The GPS log shows the defendant traveling below the speed limit when they are ticketed. That data coming from a GPS device is probably less important than the fact that it has been recorded; logs from the vehicle's speedometer may have been used instead, in case they existed.
See also
- GM Instrument Cluster Settlement
- Hubometer
- List of vehicle instruments
- The maximeter
References
External links
- Autoblog: Measures changes
Source of the article : Wikipedia
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