Vortex EM Wave-Based Rotation Speed Monitoring on Commodity WiFi

In industrial, various machinery involves spinning components, and monitoring their rotation speed is important for operation state prediction. There are many approaches for rotation speed estimation, but traditional contact methods require specialized equipment and non-contact methods such as optical methods are affected by illumination and occlusion, acoustic methods are susceptible to environmental noise. RF signals like WiFi can provide non-intrusive approach for rotation sensing. However, conventional WiFi sensing struggles to capture self-spinning objects when there is no radial motion towards or away from the WiFi transceivers. This paper proposes a rotation speed estimation method based on vortex electromagnetic (EM) waves. By using a q-shaped patch antenna on commodity WiFi, vortex EM waves with spiral phase front can be transmitted in WiFi communication channel. When these vortex EM waves interact with the surface of a self-spinning object, the echos exhibit a frequency shift compared to the original wave. This frequency shift is proportional to the object’s rotation speed, and is known as the rotational Doppler effect. This paper first derives the mathematical relationship between WiFi channel state information (CSI) and rotational Doppler frequency shift (RDS), and we design a series of signal processing methods to extract RDS from WiFi CSI, achieving the rotation speed estimation of self-spinning targets. Experimental results show that this method is applicable to a rotation speed range of 0-6000 rpm, with an error within 0.05.