Tachometer, Odometer, Speedometer, Drehzanlmesser… there are too many instruments bunched up at the dashboard, displaying one or the other thing side by side. All of them look alike, but functions differently.
Let’s say, Tachometer. Tacho Meter is a Greek word forming from tacho means speed and metron meaning measure. Therefore, tachometer is an instrument that indicates the speed of an object or substance. This speed is usually measured in revolution per minute, at which an engine shaft is rotating and displayed on the tachometer instrument. The traditional tachometer is laid out as a dial, with a needle indicating the current reading and marking safe and dangerous levels. Recently, the most commonly used tachometers are digital tachometers, giving a direct numeric output.
How tachometer works?
A tachometer measures the speed at which a mechanical device is rotating. It measures the revolutions per minute i.e. RPMs of the engine shaft. It is important to have a tachometer that monitors correct engine RPMs, as running the engine at excessively high rates can drastically shorten engine life.
Tachometer works in Automobiles, Trucks, Tractors and Aircraft
Tachometers on automobiles, aircraft and other vehicles show the rate of rotation of the engine's crankshaft. The typical markings on the tacho meters indicate a safe range of rotation speeds. The tachometer assists the driver in selecting an appropriate throttle and gear settings for the driving conditions. On analog tachometers the maximum speed is typically indicated by an area of the gauge marked in red. The red zone in tachometers is superfluous on most modern cars, since their engines typically have a rev limiter which electronically limits engine speed to prevent damage. Diesel engines with traditional mechanical injector systems have an integral governor which prevents over-speeding the engine, so the tachometers in vehicles and machinery fitted with such engines often lack redline.
In vehicles such as trucks and tractors, the tachometer often has other markings, usually a green arc showing the speed range in which the engine produces maximum torque, which is of prime interest to operators of such vehicles. Tractors fitted with a power take off (PTO) system have tachometers showing the engine speed needed to rotate the PTO at the standardized speed. In many countries, tractors have a speedometer for use on a road. To save fitting a second dial, the vehicle's tachometer is often marked with a second scale in units of speed. Tractors with multiple 'road gears' often have tachometers with more than one speed scale.
Aircraft tachometers have a green arc showing the engine's designed cruising speed range.
Tachometer works in Trains and Light Rail Vehicles
Speed-sensing devices or tachometers are extensively used in rail vehicles. The commonly found types of devices of tachometer include Opto-Isolator Slotted disk sensors and Hall Effect sensors.
Hall Effect sensors type of tachometer typically use a rotating target attached to a wheel, gearbox or motor, which may contain magnet or be a toothed wheel. The teeth on the wheel vary the flux density of a magnet inside the sensor head. The probe is mounted with its head a precise distance from the target wheel and detects the teeth or magnets passing its face. But, the necessary air gap between the target wheel and the sensor allows ferrous dust from the vehicle's under-frame to build up on the probe or target, inhibiting the tachometer’s function.
Another type of tachometers like Opto-Isolator sensors are completely encased to prevent ingress from the outside environment. The only exposed parts are a sealed plug connector and a drive fork, which is attached to a slotted disk internally and measures the pulses per revolution requirements. These types of tachometer sensors typically provide 2 to 8 independent channels of output that can be sampled by other systems in the vehicle such as automatic train control systems and propulsion/braking controllers.