The situation when there is no specialized measuring equipment at hand, and it is urgent to find out the speed of rotation of the shaft of an electric motor, is familiar to many masters and enthusiasts. Modern mobile devices have computing power and sensors that allow you to turn a smartphone into a full-fledged measuring device with minimal error. Using a built-in microphone for analyzing sound waves or a high-definition camera for the optical method gives results comparable to the readings of industrial tachometers.
The accuracy of measurements depends on the correct preparation of the shaft surface and the choice of suitable software that can interpret the data obtained. Rotation frequency This is a critical parameter for bearing diagnostics, load assessment and frequency transducer tuning. In this article, we will discuss proven techniques that allow you to get reliable data about the operation of the mechanism using only the resources of your mobile gadget.
It's worth noting that safety when working with rotating machinery is always the first priority, regardless of the tool used. Even if you don't touch the engine physically, getting close to the exposed rotating parts requires concentration and understanding of the physical processes. Android and iOS We offer various signal processing algorithms that we will learn to use effectively.
Principles of operation of mobile tachometers and physical bases
The basis of any method of measuring rotational frequency is to convert a periodic event into an electrical signal, which is then digitized by the processor. In the case of a smartphone, we can use two basic physical phenomena: interruption of light flux or periodic sound vibrations. The optical method is based on the strobe effect, where the frequency of flashes of a camera or screen is synchronized with the frequency of rotation of the tag on the shaft.
The acoustic method, in turn, relies on the analysis of the sound spectrum emitted by the working engine. Sound wave.The sensing system, generated by fan blades or gear teeth, has a distinct basic frequency, a multiple of the rotation speed. The smartphone microphone picks up these fluctuations, and the app performs a rapid Fourier transform (FFT) to highlight the dominant frequency from the overall noise background.
⚠️ Attention: The acoustic method may be in error in conditions of heavy industrial noise or if the engine has design features that suppress the sound vibrations. Always double-check the data optically when possible.
For algorithms to work properly, you need to understand the difference between the speed of the shaft and the frequency of the pulses. If you have one label on the shaft, the pulse frequency is equal to the speed of the rotation. However, if you use the audible method and the fan of 5 blades, the frequency of the sound signal will be 5 times higher than the speed of the shaft. Conversion factor This is a critical parameter in this case.
Engine preparation and measurement shaft marking
The quality of the shaft surface preparation determines the accuracy of optical measurements. A smooth metal shaft often has a high reflectivity, which can lead to false alarms of the light sensor or blurring the image on the camera. To eliminate this effect, you need to create a contrast label that will clearly stand out against the general background when you rotate.
The ideal material for marking is matte black seal or marker if the shaft is light, and white paint or reflective sticker if the shaft is dark. Contrast of the label It has to be as high as possible so that the application can confidently record the moment the tag passes through the scan zone. The size of the tag also matters: too small can be overlooked by the algorithm, and too large can create a lubrication effect.
☑️ Preparation for measurement
When you apply the tag, make sure it doesn't disturb the shaft's balance, especially if the engine is running at high speeds. For low-power engines up to 1 kW, this isn't critical, but for high-speed spindles, the imbalance can cause vibrations that distort the results. Use thin strips of material, positioning them strictly along the rotational axis or across, depending on the geometry of the shaft.
Optical method: use of camera and strobe
The optical method is the most accurate of mobile solutions and does not require direct contact with equipment. There are two approaches: using the smartphone screen as a strobe and using high-speed video recording. In the first case, a flashing image of a certain frequency is displayed on the screen, which you adjust until the mark on the shaft becomes visually stationary.
When using video, you need to turn on the slow-motion mode (Slow Motion) with the maximum available frame rate, such as 240 or 960 FPS. By recording a few seconds of rotation, you can analyze the video frame by frame and count the number of full revolutions in a known time. Time stamp In the video editor allows you to achieve high accuracy of counting.
| Parameter | Description | Impact on accuracy |
|---|---|---|
| Frame rate (FPS) | Number of frames per second | The higher the FPS, the smaller the error |
| Label contrast | Difference in brightness of label and background | Critical for autofocus and tracking |
| Distance to the shaft | Distance from camera to object | Affects resolution and blurring |
| Lighting | Outer light or flash | Lack of light lubricates the frame |
Some applications use a camera to count the passages of the tag in real time, analyzing the change in the brightness of the pixels in a certain area of the frame, this method requires good lighting and stable position of the smartphone. Stabilization The device is mandatory at this point, as the trembling of the hands makes noise in the video stream.
Use a tripod or fix your smartphone on a stationary surface next to the engine to eliminate camera jitter and get a clear image for analysis.
Acoustic analysis: measurement through a microphone
The acoustic method is convenient because it allows you to take measurements from a distance and does not require labeling on the shaft, but it is more sensitive to external conditions. To implement this method, you will need a spectrum analyzer application (FFT analyzer), which displays the amplitude-frequency specifications of sound in real time. On the spectrogram you will see peaks corresponding to the speed and harmonics of rotation.
The challenge is to extract a useful signal from the overall noise. The engine makes sound not only because of the rotation of the shaft, but also because of the magnetic humming of the windings (network frequency of 50/100 Hz) and the bearings. Basic harmonica The rotation is usually the most powerful after the network frequency, but not always the lowest frequency.
To improve accuracy, it is recommended to bring the microphone closer to the fan casing or gearbox, where the rotational sound is most pronounced. If the engine is equipped with a cooling impeller, the sound of its blades will be dominant. NYou can calculate the speeds by formula: RPM = (Peak Frequency/N) * 60.
⚠️ Note: Do not direct the microphone of the smartphone directly into the exhaust holes of the hot engine or into the area of possible fragmentation of parts when the mechanism is destroyed.
Choosing software for Android and iOS
The mobile app market offers many solutions, but not all of them are suitable for technical measurement. Android The best results are shown by applications like the Physics Toolbox Sensor Suite or specialized Tachometers with FFT support. iOS You should pay attention to “Spectroid” or “Phyphox” which provide access to raw sensor data.
When choosing a program, pay attention to the presence of manual sensitivity settings and the possibility of calibration. Automatic algorithms often mistakenly mistake background noise for a useful signal. Manual setting The threshold of operation allows you to cut off quiet extraneous sounds and focus on the engine.
Secrets of sensitivity setting
If the app shows jumping values, try gluing unnecessary microphones (if there are several) with a piece of plasticine, leaving only one aimed at the sound source.
There are also applications that use a vibrometer by placing the phone directly on the engine body (for low-frequency vibrations only and with safety measures), a method less accurate for high revs, but one that allows the general condition of bearing units to be assessed by the nature of the vibration.
Interpretation of data and calculation of sliding of induction motors
Once you get the speed, it's important to interpret it correctly in the context of the type of motor. Asynchronous electric motors are characterized by the phenomenon of slipping, where the real speed of the rotor is always less than the synchronous speed of the magnetic field. Synchronous speed depends on the frequency of the network and the number of pairs of poles: for 50 Hz it is 3000, 1500, 1000 rpm, and so on.
If your smartphone showed a value, for example, 1440 rpm, then we are talking about a four-pole engine with a nominal speed of 1500 rpm. The difference of 60 rpm is this. slipThis is about 4%, and understanding this principle allows you to immediately determine how many pairs of poles an engine has, even if the nameplate is not readable.
For DC or collector motors, there is no such rigid grid frequency binding and voltage-controlled speed. In this case, the measured value simply reflects the current operating mode. Comparing the measured revolutions to data-sheet figures (if known) gives an indication of the shaft load: a heavy fall in revs under load may indicate power problems or mechanical jamming.
Knowledge of the nominal synchronous speed (3000, 1500, 1000 rpm) allows you to instantly estimate the percentage of slip and diagnose engine overload.
Frequently Asked Questions (FAQ)
Can I measure the speed without putting marks on the shaft?
Yes, this is possible using the acoustic method, analyzing the sound of fan blades or gear teeth, or using the method of video recording followed by frame-by-frame viewing of natural defects of the shaft surface.
What is the maximum error of measurement by smartphone?
When used correctly, the accuracy is less than 1%, but acoustic method in noisy rooms can give an error of up to 5-10% due to the difficulty of isolating a useful signal.
Will a regular voice recorder be suitable for recording sound?
The recorder itself will not show speeds, it will only record an audio file. You will need to additionally analyze the recording in the spectral editor on your computer or in a specialized application on your phone.
Does the screen refresh rate affect the accuracy of the strobe?
Yes, a standard 60Hz frequency can create limitations for measuring very high speeds, causing the effect of overlapping frequencies. For accurate measurements, it is better to use applications that work directly with the camera rather than with the screen.
Is it safe to keep your phone close to a working engine?
You need to keep a distance so that the rotating parts do not touch the device, as well as avoid areas with high temperature and a strong electromagnetic field, which can theoretically affect the operation of the electronics of the phone.