In the dashboard of modern cars today, it is no longer simply displaying speed, but also showing oil pressure, coolant temperature, fuel gauge, and many other parameters. However, the most basic, oldest, and perhaps the most important is the speedometer. As the name suggests, the speedometer indicates the speed of the vehicle, with units in mph or km/h.

The invention of the speedometer can be attributed to the German engineer Otto Schulze, who was granted a patent for the spiral flow speedometer on automobiles at the Imperial Patent Office in Berlin over 100 years ago. This significant invention foreshadowed the formation and development of other measuring and information devices in automobiles.

Otto Schulze patented the mechanical speedometer or analog speedometer in 1902. During those years, not only did cars become popular, but their speed also improved. The maximum speed at that time was 50 km/h, which is normal compared to today’s standards, but impressive compared to the time when horse-drawn carriages were still common. As a result, serious accidents began to increase significantly.

Therefore, Otto Schulze’s invention helped drivers accurately gauge the actual speed of their vehicles, allowing them to adjust accordingly. Over time, countries began to establish speed limits. Consequently, on a car’s instrument panel, the speedometer usually consists of two dials, a smaller one for the driver and a larger one that can be seen from a certain distance by law enforcement to ensure traffic safety.

Like many other technologies, when first introduced, the first speedometer was very expensive. It was only until 1910 that automakers officially equipped their models with speedometers as an essential component.

One of the first speedometer suppliers was Otto Schulze Autometer (OSA), a company established by Siemens VDO Automotive AG, a leading technology development corporation. OSA’s first speedometer was manufactured in 1923, and its basic design remained unchanged for the following 60 years.

To measure a vehicle’s speed, it is important to measure the revolving speed of the wheel or the gearbox. In most cars today, the vehicle’s speed is calculated based on the gearbox’s revolving speed through an active cable.

The active cable consists of multiple tightly wound coil springs in the form of a helix around a central axis. Due to its design, the active cable can be easily bent without breaking, creating a very small angle. This is crucial because the cable must loop to connect from the gearbox to the speedometer’s processing units. As the vehicle moves, each gear change also alters the internal central axis. This axis connects the gearbox’s revolving speed to the active cable, which then transmits it to the speedometer.

Another important component of the speedometer is a permanent magnet. This magnet is placed inside a metal box shaped like a cup, known as a speedcup. The speedcup is attached to an index needle, held by a hair spring. The driver can see the speed index needle on the speedometer’s face with a scale from 0 to the maximum speed, which varies depending on the model.

Assuming a vehicle is traveling at a constant speed, the gearbox and active axis rotate at a speed corresponding to the vehicle’s movement. Because the active cable’s core or inner axis connects to the gearbox through the gear selectors, the inner axis will also rotate at a speed equal to the gearbox’s speed. Consequently, the revolving magnet on the other end of the active cable will also rotate.

When the magnet rotates, it generates a swirling magnetic field, creating a force that acts on the speedcup. These forces generate eddy currents in the speedcup, causing a small rotational movement, hence the name “Eddy currents.” In some other applications, eddy currents are known as wasted energy. However, in the case of speedometers, eddy currents generate a slipping moment. The speedcup is attached to an index needle that can align accurately with the magnetic field. The index needle will stop when the counterforce from the hair spring balances the force generated by the rotating magnet.

In the case of accelerating or decelerating, if the vehicle accelerates, the revolving permanent magnet inside the speedcup will spin faster, creating a stronger magnetic field, resulting in larger eddy currents and consequently, the index needle rotates at a greater angle. The same applies when the vehicle slows down. When the vehicle comes to a complete stop, the hair spring will keep the needle at 0.

Thao Anh (TTTĐ)