Digital Tachometer

The word Digital Tachometer is derived from two Greek words: tachos means “speed” and metron means “to measure”. It works on the principle of a Digital Tachometer generator, which means when a motor is operated as a generator, it produces the voltage according to the velocity of the shaft. It is also known as revolution-counter, and its operating principle can be electromagnetic, electronic or optical-based. Power, accuracy, RPM range, measurements and display are the specifications of a Digital Tachometer. DigitalTachometer can be analog or digital indicating meters; however, this article focuses only on the digital Tachometers.

Digital Tachometer Types

The Digital Tachometer are classified into four types based on the data acquisition and measurement techniques.

·        Contact type

·        Non-Contact type

·        Time measurement

·        Frequency measurement

The Digital Tachometer counters on cars, aircraft, and other vehicles show the rate of rotation of the engine's crankshaft, and typically have markings indicating a safe range of rotation speeds. This can assist the driver in selecting appropriate throttle and gear settings for the driving conditions. Prolonged use at high speeds may cause inadequate lubrication, overheating (exceeding capability of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring retracted valves) thus causing excessive wear or permanent damage or failure of engines. This is more applicable to manual transmissions than to automatics. On analogue Digital Tachometers, speeds above maximum safe operating speed are typically indicated by an area of the gauge marked in red, giving rise to the expression of "redlining" an engine — revving the engine up to the maximum safe limit. The red zone is superfluous on most modern cars, since their engines typically have a revolution 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 Digital Tachometers in vehicles and machinery fitted with such engines sometimes lack a redline.

Applications in Industries

In vehicles such as tractors and trucks, the Digital 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 Digital Tachometers showing the engine speed needed to rotate the PTO at the standardized speed required by most PTO-driven implements. The Digital Tachometer, in many countries, tractors are required to have a speedometer for use on a road. To save fitting a second dial, the vehicle's Digital Tachometer is often marked with a second scale in units of speed. This scale is only accurate in a certain gear, but since many tractors only have one gear that is practical for use on-road, this is sufficient. Tractors with multiple 'road gears' often have Digital Tachometers with more than one speed scale. Aircraft Digital Tachometers have a green arc showing the engine's designed cruising speed range.

The Digital Tachometer In older vehicles, the Digital Tachometer is driven by the RMS voltage waves from the low tension (LT) side of the ignition coil, while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator Digital Tachometer output. This is from a special connection called an "AC tap" which is a connection to one of the stator's coil output, before the rectifier. Digital Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) exist - usually on simple diesel-engine machinery with basic or no electrical systems. On recent EMS found on modern vehicles, the signal for the Digital Tachometer is usually generated from an ECU which derives the information from either the crankshaft or camshaft speed sensor.