Speedometers are innovative devices that use electromagnetism to send signals from the physical wheel speed to the dash display. The how and why behind this is quite interesting from legal and technical standpoints and to learn the reasons, we need to go back a little in history.
Standard speedometers have been keeping a check on driving speeds since the early s. The very first standard mechanical speedometer, also called the eddy-current speedometer was patented and later developed by Otto Schulze, an inventor from Strasbourg. Speedometers in the 20 th century used to have two separate dials.
While the smaller dial was for the motorist to check and adjust speed accordingly, the bigger one was for the police for monitoring purposes. Today, we have moved to digital speedometers — an instrument cluster or a digital dash that shows an electronic readout of the speed and various other elements on the dashboard screen.
In a mechanical setup, a speedometer uses analogue devices that attach a drive cable directly from the transmission to the dial. We have never really thought how does a needle move on the meter? What makes the number change on odometer? We look at speedometer but never really notice it. But not anymore join me in this insight to learn all about speedometer. Right from its structure, to wiring to calibration everything is devised with perfection.
The moment you start your vehicle the speedometer starts working. The external view shows a circular display unit placed on a metallic cup. Display system is surrounded by chrome metal to provide it an extra finish. Eddy currents- are electric current generated in the conductors aluminum cup in speedometer due to changing magnetic field produced by magnet in the cup.
The circulating currents have inductance and thus induce magnetic fields. These fields can cause attraction, repulsion, drag and heating effects. The stronger the applied magnetic fields faster and greater are the effects. This is the part important to all of us because all the information processed by the speedometer mechanism is displayed here.
While the needle moves along the numbers to indicate speed, the odometer shows the distance. Bottom part is occupied by indicators. The last image is back part of the display unit. The Marking on the front is green due to the green colored back surface. Color selection depends on the manufacturer. External back view:. Bunch of wires of speedometer assembly can be seen. Important parts to be observed are rubber holders and a driver cable holder. Rubber holder holds the bulb used to illuminate indicators.
From the outside there is nothing catchy about the holder, just a place to set the bulb. But digging deep, it presents a well thought design. When we pulled out the bulbs we saw four terminals hooked to rubber casing. They are terminals help in griping the bulb tightly against mechanical shocks and provide electrical power. A bulb with ratings 12V and 1.
The wires coming out of the bulb are turned at the end in opposite directions which ensures proper grip with the terminals of the holder. These wires make contact with the copper terminals as shown below.
The bulb is placed in the holder and makes contact with terminal 1 and 3 always. Terminals 2 and 4 provide the grip that keeps the bulb in position. This is just the external design; heart of the speedometer is inside the metallic cup where the speedometer assembly is placed.
Knowing the parts will help in understanding the speedometer. After removing the screws, speedometer assembly and colored reflectors are visible. Four circular holes are cut on the metallic cup, smaller three for the bulbs and slightly larger one for driver cable holder.
This design allows the person to easily replace the defected bulb without disturbing other assembly. The colored reflector is responsible for converting the incandescent light into different colors on indicators.
From the above parts we know basic designing of speedometer now, we will understand how rotation of wheel affects the speedometer? How is wheel and speedometer connected to each other. The most important part that initiates the working of speedometer is a driver cable. Both are roughly similar in operation to the round dial type of speedometer - a cable driven by the gearbox output shaft turns a magnet which causes some sort of indicator to move against the force of a restraining hair spring.
In one type the indicator consists of a moving ribbon attached at each end to a spool. 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.
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