I need detail working principle of digital and analog speedometer?

Question

I need detail working principle of digital and analog speedometer?

Answer 1

There are no motors in a tradition automobile analog speedometer. The cable turns a bar magnet. The spinning magnet causes eddys in an aluminum disk, the faster the spin, the more pull. This is exactly the way your house electric meter works, but instead of a spinning magent, the current flow creates an electro magnet that the aluminum disk responds to. take a look at your electric meter and you will see the edge of the aluminum disk turning.

Answer 2

There are several ways to implement either of these, but I'll cover two.

For an analog speedometer, there's some gearing from the final drive ratio that's transferred to the speedometer via a flexible cable. One way to transfer this to a speedometer is to have the end of that cable, perhaps via additional gearing, drive a magnet.... basically, functioning as a small generator. This, in turn, creates an electrical current that moves a needle against a counter-balancing spring. The faster the magnet spins, the more current is generated, and thus, the father the needle can move against the spring.

You could use this to make a digital speedometer, too, simply by changing the "meter" section to a digital current sensor. But a simpler way is to have that mechanical part drive a wheel with a slot past an LED and phototransistor.. similar to the workings of a mechanical computer mouse. The phototransistor will emit a pulse every time the slot in the wheel goes past, creating a series of pulses. Just as your mouse does, a computer in the car will translate the frequency of the pulses into a speed value.

Another way to do this is to drive a wheel with a magnet in one location past a small electronic sensor called a hall effect device. The hall effect device is a magnetic field sensor, and it can generate the same kind of series of pulses that the phototransistor did. I actually used this concept to create a speedometer kit for a digital R/C controller I helped to develop... you figure out where to mount the magnet and sensor, enter some calibration informat, and all of a sudden, reliable speed information from your R/C car.