A tachometer or speedometer is a device for measuring and displaying the amount of speed of a land vehicle. Usually, an analog display is made on a scale by the “tachometer needle” (pointer tachometer). In rare cases, a bar display – also analog – is used (roller tachometer) or digital display by means of a numerical display (digital tachometer) or a bar display by means of an LC display. If the values are recorded, this is referred to as a tachograph. If the speed signal comes from a computer to which several speed sensors may be connected, the visible part is technically called the indicator. The legislator refers to the whole as a “speed measuring device”, and the part containing the display as a “speedometer”. In most cases, the odometer (hodometer) is also connected to the tachometer, as they have a common drive. The RNS 510 is Volkswagen’s navigation unit, and is often located close to the speedometer.
Most speedometers are actually tachometers in terms of the measuring principle. The first, purely mechanical tachometers were based on centrifugal force. Diedrich Uhlhorn first used this for textile machines in 1817. From 1844, this measuring principle was used on locomotives – the Daniel tachometer for locomotives did not have a circular display, but still had a cardboard disc with a pin – a centrifugal pendulum caused the pin to be raised and a clockwork mechanism turned the disc.
The History of Tachometers
In the eddy current tachometer patented in 1888 by Croatian Josip Belušić under the name “Velocimeter”, a permanent magnet rotates and generates eddy currents in a metal disk or bell made of aluminum mounted in front of it. The additional field energy of the eddy currents would be avoided if the rotatably mounted disk were to rotate along with it. However, it is prevented from doing so by a return spring. The magnetic field increases linearly with the speed, the field energy quadratically, the force effect as its derivative again linearly, as does the restoring force of the spring with its angular deflection. Thus the deflection of the disc provided with a pointer is proportional to the speed. The pointer is backed by a calibrated scale. The magnetic material used has a high Curie temperature to keep the temperature response of the measurement error small.
In 1902, German inventor Otto Schulze developed the eddy current tachometer, a design for road vehicles. Schulze relied on a flexible shaft (tachometer shaft) to transmit the speed of the wheel or transmission to the tachometer, where it caused a magnet to rotate. This tachometer shaft was susceptible to wear and caused an erratic display due to jerking if the friction was too great. For long transmission distances, such as in rail vehicles, a rotary transducer was used instead of the tachometer shaft. The rotary transducer at the end of the electrical connection sat in the indicator and drove an eddy current tachometer there. It was simpler to rectify the AC voltage of a tachogenerator and display the DC voltage – which was, however, wavy at low speed – with a moving coil instrument.
Incremental encoders generate a frequency proportional to speed, which can be converted into a voltage with an analog circuit. The first production vehicle with such a system was the Porsche 911 Turbo. The encoder in the differential consisted of a magnetic pole wheel and a reed contact. Modern cars have encoders on each wheel (for ABS, ASR, ESP, Navi) whose frequency is measured by a microcontroller. The primary measurement result is the number of pulses within a certain measurement interval or, better, the period between signal edges.
The displays have also changed. Pointers are moved over a much larger angle completely linear and independent of temperature by a stepper motor or simulated on a display, often in connection with various other displays, see combination instrument and multifunction display. In other variants, it is important that certain important speed ranges are displayed in a spread-out manner so that the driver can read them more precisely. Today, speed is also often displayed as a numerical value, especially in head-up displays.