Stroboscope: Working Principle, RPM Measurement, Types and Uses

A stroboscope is an instrument that lights a moving object with short, regular flashes. When the flash rate equals the rate at which the object rotates or vibrates, the object is lit in the same position every time and appears to stand still, so its speed can be read straight off the flash-rate dial without touching it. Set the flash rate slightly below the true rate and the object seems to creep forward slowly; set it slightly above and it seems to turn backwards.

Stroboscope working principle

The eye cannot follow a shaft turning at 1500 rpm, but it holds an image that is refreshed often enough. Each flash lasts only a few microseconds, so the object barely moves while it is lit, and between flashes it is in darkness. What you see is a string of sharp snapshots.

• Flash rate = rotation rate: the object has made exactly one turn between flashes. Every snapshot shows it in the same place, so it looks frozen.
• Flash rate slightly lower: the object has turned a little more than one revolution between flashes. Each snapshot is slightly further on, so it appears to rotate forward slowly.
• Flash rate slightly higher: the object has not quite completed a turn, so it appears to rotate slowly backwards. This is the same effect as wagon wheels turning backwards in films.

The apparent (beat) speed is the difference between the two rates:

N_apparent = N – f

where N is the true speed in rev/min and f is the flash rate in flashes/min. A positive result means slow forward motion, a negative one means slow reverse. Flash rates are quoted in flashes per minute (FPM) so they compare directly with rpm; divide by 60 for hertz.

How short must the flash be?

Blur length = surface speed x flash duration. A 200 mm pulley at 3000 rpm has a rim speed of π x 0.2 m x (3000/60) rev/s = 31.4 m/s. With a 10 µs flash, the rim moves 31.4 x 10 x 10^-6 = 0.31 mm per flash, so the image stays sharp. A 1 ms flash would smear it over 31 mm.

How does a stroboscope measure RPM? The harmonic trap

A single stationary image does not prove the flash rate equals the speed. If the flash rate is half the speed, the shaft makes exactly two turns between flashes and still looks frozen. The same happens at one third, one quarter, and so on. At twice the speed, you see two images of the mark, 180° apart; at three times, three images.

The method used by instrument makers such as Monarch Instrument:

1. Mark the object in one place only, with reflective tape or paint, if it is symmetrical. A three-bladed fan with no mark also looks frozen at three times its speed, because every blade looks the same.
2. Start at the highest flash rate and turn it down.
3. The first single stationary image is the true speed. Above it, any frozen pattern shows two or more images of the mark.
4. Check by halving the reading (many strobes have a ÷2 key): you should again see a single image. Doubling it should give two images.

Monarch also notes that below about 300 flashes per minute the eye is no longer fooled into seeing a stopped image, so very slow shafts need another method.

Worked example

Problem. A fan with a tape mark on one blade appears stationary, with one image of the mark, at 1500 flashes/min and again at 750 flashes/min. At 3000 flashes/min two marks appear, 180° apart. What is the speed? What will be seen at 1480 and 1520 flashes/min?

• Single images at 1500 and 750, and a double image at 3000 = 2 x 1500. So 1500 is the highest rate giving one image, and N = 1500 rpm (25 rev/s). The image at 750 is the N/2 subharmonic.
• At f = 1480: N_apparent = 1500 – 1480 = +20 rev/min. The mark creeps forward, completing one apparent turn every 60/20 = 3 s.
• At f = 1520: N_apparent = 1500 – 1520 = -20 rev/min. The mark creeps backwards, also once every 3 s.

Turning the dial up from zero and stopping at the first frozen image would have given 750 rpm, half the true value.

Types of stroboscope

A neat mechanical example is the turntable strobe ring. Under 50 Hz mains lighting, the lamp flickers 100 times a second, or 6000 times a minute. A ring of 6000 / 33.33 = 180 dots therefore looks stationary when the platter turns at exactly 33 1/3 rpm.

In medicine, videostroboscopy is used because vocal folds vibrate at roughly 100 to 300 Hz in speech, far too fast to see directly. The strobe is fired slightly off the voice frequency, so the folds appear to move in slow motion. It needs a steady, periodic voice; for irregular voices, clinics use high-speed video instead.

Stroboscope uses, including motion analysis

• Non-contact speed measurement of shafts, fans, pulleys, spindles and couplings where a contact tachometer cannot reach.
• Motion analysis: freezing or slowing belts, gears, cams, springs, valves and linkages while running, to see flutter, slip, misalignment and wear.
• Vibration and balancing: synchronised to a vibration analyser’s signal, the strobe freezes a reference mark to show the phase angle for placing balance weights. See our notes on vibrations.
• Printing-web and packaging inspection: a strobe locked to line speed makes a moving printed web appear still, so register, colour and print defects can be checked without stopping the press.
• Engine ignition timing: the timing light used on petrol engines is a stroboscope triggered by the spark plug lead.

The stroboscopic effect as a safety hazard

Lighting can act as an unwanted stroboscope. Discharge lamps on older magnetic ballasts, and some cheap LED drivers, flicker at twice the supply frequency: 100 Hz, or 6000 times a minute, on India’s 50 Hz supply. A lathe chuck, saw blade or fan running at a speed whose ratio to 6000 is a whole number (or close to one) can look stationary or slowly turning while it is actually at full speed. Machines at 1500 and 3000 rpm, both common for 50 Hz induction motors, are the classic cases.

Workplace lighting guidance, such as the UK HSE’s HSG38, lists the usual fixes: high-frequency electronic ballasts or flicker-free LED drivers, spreading adjacent fittings across the three phases, and local task lights on rotating machines.

Limitations

• Harmonic readings (N/2, N/3) are easy to mistake for the true speed, as shown above.
• Needs a clear line of sight and dim surroundings; bright sunlight washes out the flashes.
• Not reliable below about 300 flashes/min, and the speed must be steady for the image to hold.
• Flashing light can trigger seizures in people with photosensitive epilepsy.

For lecture courses on measurement and instrumentation, see NPTEL – Online Engineering Courses.

FAQs

How does a stroboscope work?

It lights a moving object with short, regular flashes. When the flash rate equals the object’s rotation or vibration rate, the object is in the same position at every flash and appears to stand still. A slightly lower flash rate makes it appear to move slowly forward, a slightly higher one slowly backwards.

How do you measure RPM with a stroboscope?

Put a single mark on the shaft, start at the highest flash rate and turn it down. The first rate that gives one stationary image of the mark is the true speed. Check by halving it (single image again) or doubling it (two images). Lower frozen readings at N/2 or N/3 are harmonics, not the speed.

Why does an object appear frozen at half its speed?

At a flash rate of N/2, the object makes exactly two full turns between flashes, so every flash still catches it in the same position. The same is true at N/3, N/4 and so on, which is why the true speed is the highest flash rate that gives a single image.

What are the uses of a stroboscope?

Non-contact RPM measurement, motion analysis of running machinery, vibration phase and balancing, printing-web inspection, engine ignition timing and, in medicine, examination of the vocal folds.

What is the stroboscopic effect in lighting?

Lamps that flicker at 100 Hz on a 50 Hz supply can make rotating machinery at certain speeds, such as 1500 or 3000 rpm, look stopped or slowly turning. It is a safety hazard, fixed with flicker-free drivers, fittings spread over three phases or local task lighting.

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