How a speedometer works

Here is the complete guide about Speedometer. Here we provide Speedometer working, types and How accurate is a speedometer etc.

Of all the instruments you’ll find on a modern car dashboard, just one may be a legal requirement – the speedometer and its built-in milometer (also called an odometer ).

In common with other developments in car technology, the trend is now towards using electronics in speedometers. But most cars – even ones being built today – have a mechanical speedometer, usually with a needle and calibrated dial to point out the speed. the design of this type of speedometer has hardly changed within the last 50 years.

What is Speedometer

An instrument for indicating the speed of a vehicle, typically by measuring the rate of rotation of a wheel or fan whose rate of rotation depends on the speed of the vehicle. Compare odometer. Speedometers for other vehicles have specific names and use other means of sensing speed.

How Speedometers Work

The dashboard instrument cluster in your car organizes a variety of sensors and gauges, including the oil pressure gauge, coolant temperature gauge, reserve gauge, tachometer and more. But the foremost prominent gauge — and perhaps the most important, a minimum of in terms of how many times you look at it while you’re driving — is that the speedometer. the work of the speedometer is to point the speed of your car in miles per hour, kilometers per hour or both. Even in late-model cars, it’s an analog device that uses a needle to point to a specific speed, which the driving force reads as variety printed on a dial.

As with any emerging technology, the first speedometers were expensive and available only as options. It wasn’t until 1910 that automobile manufacturers began to incorporate the speedometer as standard equipment. one among the primary speedometer suppliers was Otto Schulze Auto meter (OSA), a legacy company of Siemens VDO Automotive AG, one among the leading developers of recent instrument clusters. the primary OSA speedometer was inbuilt 1923 and its basic design didn’t change significantly for 60 years. during this article, we’re getting to check out the history of speedometers, how they work and what the longer term may hold for speedometer design.

Types of Speedometers

There are two types of speedometers:

  • Mechanical (eddy-current) speedometers
  • Electronic speedometers

Because the electronic­ speedometer is actually a relatively new invention — the first all-electronic speedometer didn’t appear until 1993 — this article will focus totally on the mechanical speedometer, or the eddy-current speedometer.

Otto Schulze, an inventor from Strasbourg, filed the first patent for the eddy-current speedometer in 1902. Schulze conceived of the revolutionary device as a solution to a growing problem. Cars weren’t only becoming more popular, they were also traveling faster. the average automobile’s top speed just after the turn of the 20th century was 30 miles per hour, slow by today’s standards but sizzling fast at a time when much of the world still moved at the leisurely pace of a horse-drawn carriage. As a result, serious accidents began to extend dramatically.

Schulze’s invention allowed drivers to ascertain exactly how briskly they were traveling and to form adjustments accordingly. At an equivalent time, many countries established speed limits and used cops to enforce them. Early solutions required automobiles to have speedometers with two dials — a little dial for the driving force and a way larger dial mounted so police could read it from a distance.

In the next section, we’ll check out this design to know the parts of an eddy-current speedometer.

Mechanical (eddy-current) speedometers

Let’s say a car is traveling along the highway at a constant speed. that means its transmission and driveshaft are rotating at a speed that corresponds to the vehicle speed. It also means the mandrel within the speedometer’s drive cable — because it’s connected to the transmission via a group of gears — is also rotating at the same speed. And, finally, the static magnet at the opposite end of the drive cable is rotating.

As the magnet spins, it sets up a rotating magnetic field, creating forces that act on the speed cup. These forces cause electrical current to flow within the cup in small rotating eddies, known as eddy currents. In some applications, eddy currents represent lost power and are therefore undesirable. But within the case of a speedometer, the eddy currents create a haul torque that does work on the speed cup. The cup and its attached needle turn within the same direction that the magnetic field is popping — but only as far because the hairspring will allow it. The needle on the speed cup involves a rest where the opposing force of the hairspring balances the force created by the revolving magnet.

What if the car increases or decreases its speed? If the car travels faster, the permanent magnet inside the speed cup will rotate faster, which creates a stronger magnetic field, larger eddy currents and a greater deflection of the speedometer needle. If the car slows down, the magnet inside the cup rotates more slowly, which reduces the strength of the magnetic field, leading to smaller eddy currents and fewer deflection of the needle. When a car is stopped, the hairspring holds the needle at zero.

The Electronic Speedometer

An electronic speedometer receives its data from a vehicle speed sensor (VSS), not a drive cable. The VSS is mounted to the transmission output shaft or to the crankshaft and consists of a toothed metal disk and a stationary detector that covers a magnetic coil. because the teeth move past the coil, they “interrupt” the magnetic field, creating a series of pulses that are sent to a computer. for every 40,000 pulses from the VSS, the trip and total odometers increase by one mile. Speed is also determined from the input pulse frequency. Circuit electronics within the car are designed to display the speed either on a digital screen or on a typical analog system with a needle and dial.

How accurate is a speedometer?

No speedometer can be perfectly accurate. for example, the measured speed is bound to change slightly from the true road speed if the tyres aren’t at their correct tyre pressure and also because the tyres affect . So for safety reasons the law requires speedometers to be accurate within a range of speeds: it must read no slower than the actual road speed of the car, and it’s allowed to give speeds of up to 10 per cent greater plus 2.5mph (4km per hour). The 2.5mph is included because a ten per cent error at very slow speeds would mean that the speedometer would need to be accurate to, say 0.5mph, which isn’t practicable.

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