# Speed ​​measurement

With the speed measurement , technical equipment is used to find out how fast an object is turning. The speed can be measured using the following operating principles:

## Calculating procedures

### Time measurement of one revolution

With photocells or other sensors, the switching time is measured which requires the object for one revolution. The speed is calculated with ${\ displaystyle t_ {U}}$${\ displaystyle n}$

${\ displaystyle n = {\ frac {1} {t_ {U}}}}$

Application e.g. B. in bicycle computers , optical hand-tachometers

### Frequency measurement of vibration or noise

The speed can be determined using the sound spectrum of a rotating GyroTwister recorded with a microphone . The humming sound of the device had a frequency of 135 hertz when recording, which corresponds to a speed of 135 revolutions per second or 8100 revolutions per minute.

The speed of rotation of the rotating object is a main component in the frequency spectrum of the mechanical vibrations or acoustic noises generated on the machine. The speed can be determined by measurement (using a strain gauge , microphone , acceleration sensor , etc.).

### Angle measurement at fixed time intervals

If the angles φ 1 and φ 2 at two time points , and are well known, the rotational speed calculated by ${\ displaystyle t_ {1}}$${\ displaystyle t_ {2}}$

${\ displaystyle n = {\ frac {k \ cdot (\ varphi _ {2} - \ varphi _ {1})} {t_ {2} -t_ {1}}} = {\ frac {k \; \ Delta \ varphi} {\ Delta t}}}$

Here k is to be used in such a way that the numerator for a full angle results in the number one for one revolution, i.e. k  = 1 / 2π = 1/360 °.

The angle can be measured with an incremental encoder , potentiometer or gyroscope . See also angle measurement .

Application e.g. B. Measuring system for servomotors , yaw rate sensors for navigation

In industry, impulses on gears are often recorded without contact. The field plate sensors used for this supply pulses of up to 20 kHz.

### Stroboscope

The rotating object is flashed at fixed time intervals. If regular markings on the object rotate exactly by grid widths, the grid seems to stand still. The number of marks and the frequency of the stroboscope can be used to determine the speed. ${\ displaystyle x = 1,2,3 \ dots}$${\ displaystyle N}$

${\ displaystyle n = {\ frac {f \ cdot x} {N}}}$

Application e.g. B. the turntable .

## Physical effects

### Electromagnetic induction

According to the law of induction , the voltage in a coil is proportional to the speed at which the magnetic flux changes. A rotating permanent magnet generates an alternating voltage in the coil. The voltage level or the frequency can be used to measure the speed.

Application e.g. As the tachometer generator , inductance transmitters , Wiegand sensor

If a permanent magnet rotates in a metal bell, eddy currents occur, which generate a torque that is measured against a spring, see eddy current tachometer .

### Centrifugal force

Rotating weights are pushed outwards by centrifugal force . A spring or the force of gravity create the balance of forces. The resulting movement is mechanically transmitted to a pointer. Today this method is seldom used for measurements, but due to the reliable operation without auxiliary energy, this measuring principle is used for monitoring the maximum speed of elevators , cable cars, etc.

Application e.g. B. with mechanical hand tachometers, centrifugal clutches , historically: gyrometer , centrifugal governor