The sensor is usually a toothed metal disc set between the coil and magnetic field sensor. As the engine operates, the magnetic field delivers a pulse to the sensor. The pulses given off correspond to the rotational speed of the drive shaft. This data is fed into the speedometer's internal computer and calculated out by converting rotations to miles or kilometers per hour. Modern speedometers do not depend on analog methods to read the drive shaft's spinning, so these devices provide more accurate readings.
Another benefit of the electric sensor, compared to the spinning cable, is longevity because traditional speedometers would wear out much faster. Electronic speedometers also provide tachometer and odometer readings with equal accuracy. For these reasons, many older vehicles exchange analog models for electronic speedometers. The magnet causes the ribbon to reel off one spool to the other against the force of the hair spring.
As the ribbon moves, a mark on it lines up with the calibrated scale to give a reading. The other type of indicator using a straight scale rather than a dial has a barrel marked with a line. The magnet causes the barrel to rotate until stopped by the hair spring.
Again, as the barrel moves, the mark on it lines up with the scale to indicate the speed. On both these types of speedometer the ribbon or barrel is usually a different colour on each side of the marker line to make the speed easier for the driver to read. In common with the trend towards electronic dashboard instruments, electronic speedometers are now becoming much more popular, even though mechanical ones are generally reliable.
The most common type has a magnet attached to the gearbox output shaft and an electronic unit positioned close by to act as a pickup. Every time the rotating magnet passes the pick-up unit, the unit sends a pulse of electric current down a wire to the speedometer. An electronic 'black box' inside the speedo uses these impulses to calculate the speed of the car. The Video Course teaches you everything about modern cars. Inside a mechanical speedometer. Engine Block 18 minutes. Every part explained There's ridiculous detail on every part.
But then nobody ever does. The ability to measure your rate of forward progress accurately is pretty key for both car and airplane safety. For example, you always want your pilot to have a idea about just how fast the airplane is moving when you are coming in for a landing. The speed with which your car is traveling may indeed be related to how hard you engine is working, and whether you are going uphill or downhill, but those factors have nothing at all to do with measuring the rate of speed.
Instead, the rate of rotation of a drive shaft is used to determine how fast your car is moving. The trick here is translating that rotation into its miles-per-hour equivalent.
On the drive shaft is a worm gear, which is threaded like a screw. That worm gear turns a notched gear which sits at one end of the speedometer cable. Inside that cable is a rotating wire which is driven directly by the rotation of the drive shaft.
Now at the other end of the wire inside the speedometer cable is a magnet. That magnet sits inside something called a drag cup, which is made of a non-magnetic metal, usually aluminum. The magnet spins as the wire rotates, creating a magnetic field that forces the drag cup to turn in the same direction. Attached to the drag cup is a spindle that connects to the pointer on your speedometer.
0コメント