A tachometer (also called a revolution-counter, "Tach", rev-counter, or RPM gauge) is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute Revolutions per minute is a unit of frequency of rotation: the number of full rotations completed in one minute around a fixed axis. It is used as a measure of rotational speed of a mechanical component (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The term comes from Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of Ταχος, tachos, "speed", and metron, "to measure".^[1]

History

The first mechanical tachometers were based on measuring the centrifugal force Centrifugal force represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation. In Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a, similar to the operation of a centrifugal governor. The inventor is assumed to be the German The German people are an ethnic group, in the sense of sharing a common German culture, descent, and speaking the German language as a mother tongue. Within Germany, Germans are defined by citizenship (Federal Germans, Bundesdeutsche), distinguished from people of German ancestry (Deutschstämmige). Historically, in the context of the German engineer Engineering is the discipline, art and profession of acquiring and applying technical, scientific, and mathematical knowledge to design and implement materials, structures, machines, devices, systems, and processes that safely realize a desired objective or invention Dietrich Uhlhorn; he used it for measuring the speed of machines in 1817. Since 1840, it has been used to measure the speed of locomotives A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th century to distinguish.

In automobiles, trucks, tractors and aircraft

Tachometers or rev counters on automobiles An automobile, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor. Most definitions of the term specify that automobiles are designed to run primarily on roads, to have seating for one to eight people, to typically have four wheels, and to be constructed principally for the, aircraft An aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines, and other vehicles show the rate of rotation of the engine Engines come in many types, a common type is a heat engine such as an internal combustion engine which typically burns a fuel with air and uses the hot gases for generating power. External combustion engines such as steam engines use heat to generate motion via a separate working fluid's crankshaft The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation. To convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to, 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 A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications. It generally uses a driver-operated clutch, typically operated by a pedal or lever, for regulating torque transfer from the internal combustion engine to the transmission, and a gear-shift, either operated by than to automatics. On analogue 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 Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of" 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 rev limiter which electronically limits engine speed to prevent damage. Diesel engines A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to spark ignition engines such as a petrol engine (known as a gasoline engine in North America) or gas engine (using a 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 sometimes lack a redline Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of.

In vehicles such as tractors A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction. Most commonly, the term is used to describe the distinctive farm vehicle: agricultural implements may be towed behind or mounted on the tractor, and the tractor may and trucks A truck or lorry (British English) is a motor vehicle designed to transport cargo. They vary greatly in size, power and configuration, with the smallest being mechanically similar to an automobile. Commercial trucks can be very large and powerful, and may be configured to mount specialized equipment, such as in the case of fire trucks and concrete, the tachometer often has other markings, usually a green arc showing the speed range in which the engine produces maximum torque Torque, also called moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist, which is of prime interest to operators of such vehicles. Tractors fitted with a power take off A power take-off is a splined driveshaft, usually on a tractor or truck, that can be used to provide power to an attachment or separate machine. It is designed to be easily connected and disconnected. The power take-off allows implements to draw energy from the tractor's engine (PTO) system have tachometers showing the engine speed needed to rotate the PTO at the standardised speed required by most PTO-driven implements. In many countries, tractors are required to have a speedometer 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 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. This scale is only accurate in a certain gear A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, magnitude, and, 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 tachometers with more than one speed scale. Aircraft tachometers have a green arc showing the engine's designed cruising speed range.

In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT) side of the ignition coil An ignition coil is an induction coil in an automobile's ignition system which transforms the battery's 12 volts (6 volts in some older vehicles) to the thousands of volts (20 to 30 thousand volts or more) needed to spark the spark plugs. Some coils have an internal resistor to reduce the voltage and some rely on a resistor wire or an external, while on others (and nearly all diesel engines A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to spark ignition engines such as a petrol engine (known as a gasoline engine in North America) or gas engine (using a, which have no ignition system) engine speed is determined by the frequency from the alternator An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used. In principle, any AC electrical generator can be called an alternator, but usually the word refers to small rotating machines driven by tachometer output. This is a special circuit inside the alternator to convert from rectified sine wave The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields. Its most basic form as a function of time is: to square wave A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels, and the electrical potential difference is directly proportional to engine speed. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata, water-raising machines, and water clocks such as the castle clock. The cam and camshaft later appeared in European mechanisms from at least the 14th century,) also exist - usually on simple diesel-engined machinery with basic or no electrical systems. On recent EMS An engine control unit , or engine control module (ECM), is an electronic control unit which controls various aspects of an internal combustion engine's operation. The simplest ECUs control only the quantity of fuel injected into each cylinder or the spark timing in an SI engine. More advanced ECUs found on most modern cars also control the found on modern vehicles, the signal for the tachometer is usually generated from an ECU An engine control unit , also known as power-train control module (PCM), or engine control module (ECM) is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running. It does this by reading values from multidimensional performance maps (so called which derives the information from either the crankshaft or camshaft An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata, water-raising machines, and water clocks such as the castle clock. The cam and camshaft later appeared in European mechanisms from at least the 14th century, speed sensor.

Hours meters

When used in stationary engines or vehicles where an odometer An odometer indicates distance travelled by a car or other vehicle. The device may be electronic, mechanical, or a combination of the two. The word derives from the Greek words hodós, meaning "path" or gateway and "métron", "measure" would not give an accurate reading of the vehicle's use (such as in aircraft An aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines, boats A boat is a watercraft of modest size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is something small enough to be carried aboard another or tractors A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction. Most commonly, the term is used to describe the distinctive farm vehicle: agricultural implements may be towed behind or mounted on the tractor, and the tractor may), tachometers frequently incorporate a display showing the total number of hours the engine has run. Service intervals are given and measured in hours. Generally, hours meters are accurate only at one specific engine speed — an hours meter calibrated for, say, 'Hours At 2000 RPM Revolutions per minute is a unit of frequency of rotation: the number of full rotations completed in one minute around a fixed axis. It is used as a measure of rotational speed of a mechanical component' will only advance one hour per hour if the engine is run at 2000 RPM. If the engine is run below this speed, hours will accumulate more slowly, and if the engine is run above the meter will gain hours more quickly. This discrepancy does not detract from the accuracy of service intervals, for an engine running at slow speeds may gain hours more slowly, but will also be put under less mechanical stress and will not require servicing work as frequently as an engine used generally at high speeds.

Traffic Engineering

Tachometers are used to estimate traffic speed and volume (flow). A vehicle is equipped with the sensor and conducts "tach runs" which record the traffic data. These data are a substitute or complement to loop detector data. To get statistically significant results usually requires a fairly high number of runs, and bias is introduced by the time of day, day of week, and the season. However, because of the expense, spacing (a lower density of loop detectors diminishes data accuracy), and relatively low reliability of loop detectors (often 30% or more are out of service at any give time), tach runs remain a common practice.

In trains and light rail vehicles

Speed sensing devices, termed variously "wheel impulse generators" (WIG), speed probes, or tachometers are used extensively in rail vehicles. Common types include opto-isolator In electronics, an opto-isolator is a device that uses a short optical transmission path to transfer an electronic signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated—since the electrical signal is converted to a light beam, transferred, then converted back to an electrical slotted disk sensors^[2] and Hall effect sensors A Hall effect sensor is a transducer that varies its output voltage in response to changes in magnetic field. Hall sensors are used for proximity switching, positioning, speed detection, and current sensing applications.

Hall effect sensors typically use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it may 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. One problem with this system is that the necessary air gap between the target wheel and the sensor allows ferrous dust from the vehicle's underframe to build up on the probe or target, inhibiting its function.

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 through a bearing A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle and seal. The slotted disk is typically sandwiched between two circuit boards A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks, or traces, etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring board. A PCB populated with electronic components is a containing a photo-diode In electronics, a diode is a two-terminal electronic component that conducts electric current in only one direction. The term usually refers to a semiconductor diode, the most common type today. This is a crystalline piece of semiconductor material connected to two electrical terminals. A vacuum tube diode is a vacuum tube with two electrodes: a, photo-transistor A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals, amplifier, and filtering circuits which produce a square wave pulse train output customized to the customers voltage and pulses per revolution requirements. These types of 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.

The opto devices, mounted around the circumference of the disk, provide signals that are phase-shifted relative to one another and thus allow the vehicle computer to determine the direction of rotation of the wheel. This is a legal requirement in Switzerland Switzerland , officially the Swiss Confederation (Confœderatio Helvetica in Latin, hence its ISO country codes CH and CHE), is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe[note 4] where it is bordered by Germany to the north, France to the west, Italy to to prevent rollback when starting from standstill. Strictly, such devices are not tachometers since they do not provide a direct reading of the rotational speed of the disk. The speed has to be derived externally by counting the number of pulses in a time period. It is difficult to prove conclusively that the vehicle is stationary, other than by waiting a certain time to ensure that no further pulses occur. This is one reason why there is often a time delay between the train stopping, as perceived by a passenger, and the doors being released. Slotted-disk devices are typical sensors used in odometer An odometer indicates distance travelled by a car or other vehicle. The device may be electronic, mechanical, or a combination of the two. The word derives from the Greek words hodós, meaning "path" or gateway and "métron", "measure" systems for rail vehicles, such as are required for train protection systems — notably the European Train Control System.

A weakness of systems that rely on wheel rotation for tachometry and odometry is that the train wheels and the rails are very smooth and the friction between them is low, leading to high error rates if the wheels slip or slide. To compensate for this, secondary odometry inputs employ Doppler radar A Doppler radar is a specialized radar that makes use of the doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been altered by the object's motion. This variation gives units beneath the train to measure speed independently.

As well as speed sensing, these probes are often used to calculate distance traveled by multiplying wheel rotations by wheel circumference.

They can also be used to automatically calibrate wheel diameter by comparing the number of rotations of each axle against a master wheel that has been measured manually. Since all wheels travel the same distance, the diameter of each wheel is proportional to its number of rotations compared to the master wheel. This calibration must be done while coasting at a fixed speed to eliminate the possibility of wheel slip/slide introducing errors into the calculation. Automatic calibration of this type is used to generate more accurate traction and braking signals, and to improve wheel slip detection.

In analogue audio recording

In analogue audio recording Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording. Acoustic analog recording is achieved by a small microphone diaphragm that, a tachometer is a device that measures the speed of audiotape Magnetic tape has been used for sound recording for more than 75 years. Tape revolutionized both the radio broadcast and music recording industries. It did this by giving artists and producers the power to record and re-record audio with minimal loss in quality as well as edit and rearrange recordings with ease. The alternative recording as it passes across the head. On most audio tape recorders the tachometer (or simply "tach") is a relatively large spindle near the ERP head stack, isolated from the feed and take-up spindles by tension idlers.

On many recorders the tachometer spindle is connected by an axle to a rotating magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials like iron and attracts or repels other magnets that induces a changing magnetic field Magnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field upon a Hall effect The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. Edwin Hall discovered this effect in 1879 transistor A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Other systems connect the spindle to a stroboscope, which alternates light and dark upon a photodiode.

The tape recorder's drive electronics use signals from the tachometer to ensure that the tape is played at the proper speed. The signal is compared to a reference signal (either a quartz crystal or alternating current from the mains). The comparison of the two frequencies drives the speed of the tape transport. When the tach signal and the reference signal match, the tape transport is said to be "at speed." (To this day on film sets, the director calls "Roll sound!" and the sound man replies "Sound speed!" This is a vestige of the days when recording devices required several seconds to reach a regulated speed.)

Having perfectly regulated tape speed is important because the human ear is very sensitive to changes in pitch, particularly sudden ones, and without a self-regulating system to control the speed of tape across the head the pitch could drift several percent. This effect is called a wow-and-flutter, and a modern, tachometer-regulated cassette deck has a wow-and-flutter of 0.07%.

Tachometers are acceptable for high-fidelity sound playback, but not for recording in synchronization with a movie camera. For such purposes, special recorders that record pilottone must be used.

Tachometer signals can be used to synchronize several tape machines together, but only if in addition to the tach signal, a directional signal is transmitted, to tell slave machines in which direction the master is moving.

See also

• Odometer
• Hubometer
• Speedometer
• Tachograph
• Taximeter
• Overspeed (engine)
• Rev limiter

References

1. ^ http://www.wisegeek.com/what-is-a-tachometer.htm
2. ^ HaslerRail Speed Sensors

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