Speedometer - Analog and Digital

A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle. In an automobile, an electronic instrument cluster, digital instrument panel or digital dash for short, is a set of instrumentation, including the speedometer, that is displayed with a digital readout rather than with the traditional analog gauges. Electronic speedometer can be analog or digital.Values on analog devices are (normally) infinitely variable. A speedometer that shows a vehicles speed by means of a dial is an analog device. In a digital speedometer, values are represented by numbers and therefore do not have the variability of analog devices. A standard digital speedometer, for instance, will show a vehicle's speed as 58 kmph or 60 kmph, but it can't tell you when you're going 60.25 kmph.

Fig 1: Digital Speedometer

Digital speedometers manipulate and store information in binary form. A speedometer is a gauge that measures and displays the instantaneous speed of a vehicle. Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards. Speedometers for other vehicles have specific names and use other means of sensing speed.

Fig 2: Analog Speedometer

Working operation:

When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. A small permanent magnet affixed to the speedometer cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy current in the cup, which themselves produce another magnetic field. The effect is that the magnet exerts a torque on the cup, “dragging” it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.

Fig 3: Analog speedometer working

The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup are balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tyres.