How does a speedometer calculate the speed of a vehicle

The speedometers fitted in most of the Indian cars are of mechanical type, which however involves some electrical components.

This is called the eddy current speedometer. Any instrument to display an output needs some input, hence for the speedometer, the input signal in the form of mechanical rotation is taken from the transmission side of the power unit of vehicle. The transmission is that part of the power system of the vehicle which couples or connects to the wheels.

Here, a distinction is to be made with another instrument called “tachometer” which takes the input signal from the Engine side, as it is required to display the rotation per minute (rpm) of the crank shaft. The input from the transmission is transmitted via the drive cable to a magnetic rotor assembly, which is present behind the dashboard.

A drive cable is a flexible mandrel twisted with wires to make it sturdy and at the same time to bend sharp corners. The magnetic rotor assembly consists of an aluminum cover, often called the “cup”, and an internal magnetic rotor. The drive cable connects to this rotor. When the motion is transmitted, the drive cable rotates and hence makes the magnetic rotor to rotate.

But by Lenz’s law, the rotating magnetic field will tend to produce an opposing magnetic field in the stationary aluminium cup; this produces eddy currents on the surface of the cup.

So we have a current carrying conductor “cup” in a magnetic field, and this will get deflected angularly. The cylindrical aluminum cup is connected to the needle on the speedometer dial.

There is also a hair spring which tries to bring back the needle to original position (zero) when the car is stationary.

The needle deflects to the amount, where the magnetic torque equals the resisting torque of the spring.

Also, the amount of deflection of the needle depends upon the speed of rotation of the magnetic rotor, and hence when the vehicle is travelling faster, the rotor spins faster and hence a larger deflection of the coil will be observed.

The drive cable or the magnetic motor will not spin in the same speed of the transmission shaft or the wheels, due to the gears present while connecting the drive cable at the transmission side.

The gears will reduce the speed of rotation of the drive cable, but will maintain a constant speed ratio. Due to this the calibration of speedometer is essential and done by the manufacturer according to the car’s wheel and tyre specifications. Calibration is the process by which the deflection of the needle is made to correspond exactly to the speed of wheels.