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.