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Most modern electronic speedometers have the additional ability over the eddy current type to show the vehicle's speed when moving in reverse gear.Ī computer converts the pulses to a speed and displays this speed on an electronically controlled, analog-style needle or a digital display. Alternatively, particularly in vehicles with multiplex wiring, some manufacturers use the pulses coming from the ABS wheel sensors which communicate to the instrument panel via the CAN Bus. 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. 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 sensor.
#Gps speedometer app accuracy full
In designs derived 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 driveshaft, and therefore the vehicle's speed, assuming the wheels have full traction. 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 its mechanical stop pin on the zero position. 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 return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. At a given speed, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial. 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. 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. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The torque on the cup increases with the speed of rotation of the magnet. The pointer shaft is held toward zero by a fine torsion spring.
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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. As the magnet rotates near the cup, the changing magnetic field produces eddy current in the cup, which themselves produce another magnetic field. A small permanent magnet affixed to the speedometer cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analog speedometer instrument. When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel. The speedometer uses a rotating flexible cable usually driven by gearing linked to the output of the vehicle's transmission. German inventor Otto Schultze patented his version (which, like Belušić's, ran on eddy currents) on 7 October 1902. His invention had a pointer and a magnet, using electricity to work. He presented his invention at the 1889 Exposition Universelle in Paris. The speedometer was originally patented by Josip Belušić (Giuseppe Bellussich) in 1888.