Speedometer

Speedometer

Definition of speedometer:

A speedometer or a speed meter 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.^[1] Speedometers for other vehicles have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.

Figure: speedometer

The History of the Speedometer:

 Regulating the speed of a vehicle by referring to a car's speedometer comes naturally to any driver.  In fact, without the speedometer it would be impossible to drive on the UK's roads without breaking the speed limit.  So where did this essential item of measurement originModern speedometers are mainly electronic with a rotation sensor mounted in the transmission delivering a series of electronic pulses whose frequency corresponds to the rotational speed of the drive shaft, and therefore the vehicle's speed.  The pulses are converted into speed by a computer and the displays the speed on an electronically controlled, analog-style needle or digit display.

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Working of speedometer:

The shaft that turns the car's wheels is connected to the speedometer by a long, flexible cable made of twisted wires. The cable is a bit like a mini driveshaft: if one end of the cable rotates, so does the other—even though the cable is long and bendy. At the top end, the cable feeds into the back of the speedometer. When it rotates, it turns a magnet inside the speedometer case at the same speed. The magnet rotates inside a hollow metal cup, known as the speed cup, which is also free to rotate, though restrained by a fine coil of wire known as a hairspring. However, the magnet and the speed cup are not connected together: they're separated by air. The speed cup is attached to the pointer that moves up and down the spedometer dial.

Fig : CONNECTING TO THE SPEEDOMETER BY A LONG FIELD CABLE

Ø  When the engine turns over ,the driveshaft turns to make the wheels spin round.

Ø  The speedometer cable ,powered by the driveshaft ,turns as well.

Ø  The cable spins a magnet around at the same speed inside the speed cup.

Ø  The magnet rotates continually in the same direction.

Ø  The spinning magnet creates eddy currents in the speed cup.

Ø  The eddy current make the speed cup rotate counter-clock wise as well in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together in any way- there’s- air in between them.

Ø  The hair spring tightens, restraining speed cup so it can turn only a little ways.

Advantages of speedometer:

·         The direction of rotation is directly indicated by the polarity of the output voltage.

·         The output  voltage is typically 10Mv/rpm and can be measured with conventional type of dc voltmeter.

Disadvantages of speedometer:

·         Brushes on small tachometer generators often produce maintenance problems, as their contact resistance may vary and produce appreciable error.

·         The input resistance of meter should be very high as compared with output resistance generator .This is required to limit the armature current to small value.

·         If the armature current is large, the field of permanent magnet is distorted giving rise to non linearity.   

           

       Raw material of speedomeeter

·         Materials used in the production of speedometers vary with the type of gauge and intended application. Older mechanical models were entirely comprised of steel and other metal alloys, but in later years about 40% of the parts for a mechanical speedometer were molded from various plastic polymers. Newer electronic models are almost entirely made of plastics, and design engineers continually upgrade the polymers used. For example, the case of a speedometer's main assembly is usually made of nylon, but some manufacturers now employ the more water-resistant polybutylene terephthalate (PBT) polyester. The worm drive and magnet shaft are also nylon, as is the speedometer's gear train and spindles. The glass display lens of the recent past is now made of transparent polycarbonate, a strong, flexible plastic that is resistant to heat, moisture, and impact.

          Design:

·        In a mechanical speedometer, a rotating cable is attached to a set of gears in the automobile's transmission. This cable is directly attached to a permanent magnet in the speedometer assembly, which spins at a rate proportional to the speed of the vehicle. As the magnet rotates, it manipulates an aluminum ring, pulling it in the same direction as the revolving magnetic field; the ring's movement, however, is counteracted by a spiral spring. Attached to the aluminum ring is the pointer, which indicates the speed of the vehicle by marking the balance between these two forces. As the vehicle slows, the magnetic force on the aluminum ring lessens, and the spring pulls the speedometer's pointer back to zero.

·         Electronic speedometers are almost universally present in late-model cars. In this type of gauge, a pulse generator (or tach generator) installed in the transmission measures the vehicle's speed. It communicates this via electric or magnetic pulse signals, which are either translated into an electronic read-out or used to manipulate a traditional magnetic gauge assembly.

           

Applications of speedometer:

·         Hubometer.

·         Train event recorder.

·         Buzzer.

·         Eddy current.

·         Cruise control.

·         Ford ATX Transmission.