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Robust speed measurements with standard wireless devices

IP.com Disclosure Number: IPCOM000237310D
Publication Date: 2014-Jun-12
Document File: 6 page(s) / 3M

Publishing Venue

The IP.com Prior Art Database

Abstract

The speed of moving objects can be measured using several methods such as radar guns, the Doppler Effect, or the signal strength of radio signals. However, existing approaches require either relatively bulky physical sizes or complex signal processing, which in turn increase the costs of the measuring tools. This disclosure proposes a new method for measuring the speed of metal objects moving on a fixed track, e.g., cars on a road, trains on rails, goods sliding on a conveyor belt. The new approach is based on the signal strength of received messages as perceived by standard wireless devices. The method relies on the constructive and destructive interference patterns created by the reflections from the moving target. The method requires only minimal signal processing and only two commodity wireless transceivers, which are independent of the moving target. As a consequence, the physical size and the cost of the proposed method are significantly lower when compared to existing approaches.

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Robust speed measurements with standard wireless devices

    Disclosed is a new approach for measuring the speed of moving metal objects, based on the received signal strength of messages sent and perceived by standard wireless devices. Key to the approach are the constructive and destructive interference of the signal, which are produced by reflections from the moving target. The proposed method does not require the moving objects to be equipped with a wireless transceiver. Furthermore, the method requires only one sender and one receiver, both independent of the moving target. Last, the proposed method uses only basic properties of the received signal strength of messages and is therefore more robust to noise and measurement imprecision.

    The proposed method, makes use of the multipath effect between a wireless sender and receiver, i.e. the signal is received at the receiver due to two signal paths (c.f., Figure 1). If the sender transmits a message the moving object reflects the signal. Depending on the distance of the object to the wireless sender and receiver, the strength of the signal at the receiver increases (constructive interference) or decreases (destructive interference) compared to the situation where no object is present. This variation in signal strength can be measured. In particular, it suffices to measure the time between peaks of the variation.

    The proposed technique can be implemented with commodity hardware, which makes it very cheap. Furthermore, since peak detection suffices rather than an accurate estimate of the received signal strength, the technique is much more robust compared to (localization) techniques that rely on precise signal strength measurements.

Figure 1: Schematic presentation of the measurement principle

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    The proposed method encompasses a sender, a receiver and a moving object, which speed is to be measured. The sender and receiver are assumed to be standard wireless devices, e.g., following the WLAN IEEE 802.11 or the WPAN IEEE 802.15.4 standards. The sender transmits continuously short messages, e.g., every 2 milliseconds. The receiver measures the signal strength of each received message and the time when the message was received. The receiver stores up to a few hundred measurements and applies signal processing, e.g., a moving average filter for the set of signal strengths of the received messages. Then, the receiver determines the sequences of maxima and minima in the set of signal strengths. The minima and maxima occur due to the radio signal being reflected by the moving target.

    The velocity of the moving object can be computed using the time between the minima and maxima of the received signal and the wavelength of the radio signal. For example, the wavelength is 0.125 meters for devices operating in the 2.4 GHz ISM band as specified by the IEEE 802.15.4 standard. More precisely, assuming that the distance of the moving object is large compared to the distance between the...