A Method For Measuring Fan Speed Without Speed Feedback

When there is no speed feedback, measuring the speed of a fan can indeed become complex because there is no direct method to obtain the actual speed value. Here are some feasible methods:
Photoelectric sensor or encoder: A small optical marker can be pasted onto the fan blade, and then a photoelectric sensor or encoder can be installed near the fan. In this way, when the fan blades pass through, the photoelectric sensor will detect changes in light, which can detect the number of times the fan blades pass through, and thus calculate the fan speed.
Hall sensor: Hall sensor can detect changes in magnetic field, thereby determining the rotation of fan blades. You can install a small magnet on the fan blade, and then place one or more change Hall sensors next to the fan. When the blade rotates, the magnet passes through the sensors and generates signals, which can be used to calculate the fan speed.
Sound frequency: When the fan rotates, it produces noise, and the frequency of the noise is related to the fan speed. You can use a sensor or microphone to capture the sound generated by the fan, convert the captured sound signal into frequency, and then detect audio changes in the main frequency. The accuracy of this method may be relatively low, as noise is influenced by the environment and fan design.
Vibration sensor: The rotation of the fan generates vibration, and the frequency of the vibration can be used to estimate the speed of the fan. Installing a vibration sensor near the fan can detect the vibration caused by the rotation of the blades, and then estimate the fan speed by analyzing the vibration data collected by the sensor.
The above are some methods for measuring fan speed, but the accuracy of these methods may be affected by environmental factors, sensor performance, and fan design, so sufficient testing and verification are required before practical application.