Stroboscopic Instrument

a monitoring and measuring device that makes use of stroboscopic effects and is employed in the observation of rapid periodic motions. Such instruments are used, for example, to measure the oscillation frequencies of mechanical and electronic systems, to measure resonance frequencies, to study the vibrations of various bodies, and to visually monitor rapidly moving parts of machines.

The principle of operation of stroboscopic instruments is as follows: the object performing periodic motion is illuminated and made visible in separate time intervals that are very small by comparison with the period of the object’s motion. If the frequency f_str of the light pulses is the same as the frequency f_obj of the period of the object’s motion, then the object appears stationary. When these two frequencies are somewhat different, the object appears to be executing a motion that is slower than the actual motion. The frequency F of the slowed motion is the difference between the two frequencies—that is, F = f_obj – f_slr.

Modern stroboscopic instruments can be divided into the following types: mechanical, electrooptical, electronic, and oscillo-graphic. Mechanical stroboscopic instruments are instruments with mechanical shutters (choppers) in the form of disks or hollow cylinders with slits through which the object is observed. By measuring the disk’s speed of rotation at which the object viewed through the shutter appears stationary, the frequency of the periodic motion of the object can be determined. Such instruments are called stroboscopic tachometers. The principal advantage of the stroboscopic tachometer is that it permits the angular speeds of rotation of objects to be measured without contact between the instrument and the object. Consequently, speeds can be measured for objects that are visible but not easily accessible. This advantage also permits measurement of the speeds of low-power objects without the speed being affected by the use of the instrument. The SEF-54 hand stroboscopic tachometer can be used in the measurement ranges 300–3,000 and 3,000–30,000 rpm with an error of ±1 percent.

In electrooptical stroboscopic instruments, the action of the shutters used to interrupt the light is based on such electrooptical effects as the Kerr effect and the Pockels effect. Such shutters provide a high pulse frequency, 10⁴–10⁵ hertz (Hz), and a high off-on ratio. Disadvantages are that the luminous efficacy is low and it is difficult to obtain sufficiently large luminous fluxes.

The most advanced stroboscopic instruments for industrial use are of the electronic type. They consist of an oscillator that controls the pulse frequency and of a gas-discharge tube that serves as the source of the light pulses. The frequency of the oscillator and, consequently, of the flashes can be smoothly adjusted by varying the parameters of the electric circuit, usually between 2 and 2,500 Hz. The measurements are accurate to within 1 to 2 percent. The commercially produced ST-32 instrument is designed for the observation of moving machine parts and the non-contact measurement of speeds of rotation between 250 and 3,200 rpm. The ST-150 general-purpose industrial instrument with battery feed is designed for conducting investigations in laboratories, in factories, and under field conditions. It has a pulse repetition frequency of 2 to 2,500 Hz and thus permits measurement of speeds of rotation of 110 to 150,000 rpm. The instrument can be operated by remote control and can provide a flash delay of 30 microseconds to 600 milliseconds.

Examples of low-frequency flash-lamp spectroscopic instruments are the SSh-1 instrument, with a flash frequency of 10 to 100 Hz, and the SET-1 and SET-2 theatrical electronic stroboscopes, with flash frequencies of 1 to 10 Hz and 1 to 40 Hz, respectively. These stroboscopes are used to demonstrate various experiments and to create lighting effects in theaters.

Special-purpose electronic instruments are also produced. An example is the PAS stroboscope, which is used for setting the ignition timing of motor-vehicle engines. A laryngostroboscope, which permits study of the vocal cords, is produced for medical purposes. In a number of instruments, the pulse frequency is automatically controlled through feedback from the object being studied in accordance with the object’s natural oscillation frequency. The use in electronic stroboscopic instruments of master oscillators with a stabilized frequency permits a high accuracy of measurement (~0.001 percent).

Oscillographic stroboscopic instruments, stroboresonance galvanometers, and some other devices have found application in the study of periodic electronic processes and in the measurement of the amplitude and duration of electric pulses.

The SI-60 stroboscopic oscillograph permits study of the duration of electric pulses in the nanosecond range with an error not exceeding 4 percent.

Stroboscopic instruments were developed in the 1970’s that permit periodic processes to be observed not only in the visible region of the spectrum but also in other regions—for example, X-ray pulse devices for the stroboscopic observation of the action of heart valves.