Tachometer
From Wikipedia, the free encyclopedia
A tachometer gauges the speed of rotation of a shaft or disk (from Greek: tachos = speed, metron = measure), as in a motor or other machine. The device usually displays the rate of revolutions per minute on a calibrated analog dial, but digital displays are increasingly common.
Contents |
[edit] History
The first, mechanical, tachometers were based on measuring the centrifugal force. The German engineer Diedrich Uhlhorn is assumed to be the inventor, he used it for measuring the speed of machines in 1817. Since 1840 it was used to measure the speed of locomotives.
[edit] Automotive
Automotive tachometers show the rate of rotation of the engine's crankshaft by measuring the spark rate of the ignition system, typically in revolutions per minute (RPM). This can assist the driver in selecting the most appropriate throttle and gear settings (more applicable to manual transmissions than automatics) that the driving conditions call for.
Tachometers fitted to cars, aircraft, and other vehicles typically have markings indicating a safe range of speeds at which the engine may be operated. Prolonged use at high speeds may cause excessive wear and other damage to engines. On an analog tachometer this maximum speed is typically indicated by an area of the gauge marked in red, giving rise to the expression of "redlining" an engine - i.e. running it at (dangerously) high speed. The red zone is superfluous on most modern cars, since engine speed is electronically limited to prevent damage (see rev limiter).
In older vehicles, the tachometer is driven by the pulses from the low tension (LT) side of the ignition coil, whilst on others (and all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output (a special circuit inside the alternator to convert from rectified sine wave to square wave) , which is directly proportional to engine speed. With modern engine management systems found in present day vehicles, the tachometer is driven directly from the engine management ECU.
[edit] Trains and Light Rail Vehicles
Speed sensing devices, termed variously "wheel impulse generators" (WIG), speed probes, or tachometers are used extensively in rail vehicles. Opto-isolator slotted disk sensors[1] are common, as are Hall Effect sensors.
Hall Effect sensors typically use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets or it may simply be a toothed wheel. In the latter case the teeth vary the flux density of a magnet bundled inside the sensor head itself. The sensor 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 function.
Opto-isolator sensors are completely encased to prevent ingress from the outside environment. The only exposed parts outside of the case are a sealed plug connector and drive fork, which is attached to a slotted disk internally through a bearing and seal. The slotted disk is typically sandwiched between 2 circuit boards containing a photo-diode, photo-transistor, 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, being 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 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 systems for rail vehicles; such as are required for train protection systems - notably the ETCS (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 units beneath the train to measure speed independently.
As well as speed sensing, these probes are often used to calculate distance travelled by multiplying wheel rotations by wheel diameter.
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 carried out 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/slide detection.
[edit] Medicine
In medicine, tachometers are used to measure the rate of blood flow at a particular point in the circulatory system. The specific name for these devices is haematachometer.
[edit] Analog audio recording
In analog audio recording, a tachometer is a device that measures the speed of audio tape 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 that induces a changing magnetic field upon a hall effect transistor. Other systems connect the tach 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 being played back at the proper speed. The signal from the tachometer is compared against 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!" A moment later the sound man calls back "Sound speed!" This practice 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. A modern, tachometer-regulated cassette deck has a wow-and-flutter (as the measurement is called) of 0.07%.
Tachometers are acceptable for high-fidelity sound playback, but are not acceptable 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 let the slave machines know not only how fast the master is going, but in which direction.
[edit] See also
