An Ultrasonic Six Degrees-of-Freedom Pose Estimation Sensor

Motion capture and human body pose estimation systems have become a more common appliance nowadays because of the movie and video game industry. These measurement systems have been proven to be useful for other applications besides entertainment. One of these applications is motion analysis, which can be used for improving the form of athletes or for providing an objective validation tool for rehabilitation treatments. These analyses are done using high-accuracy measurement systems which result in high costs. Although there are some consumer products (e.g. the Microsoft Kinect) that offer movement tracking at a low cost, the accuracy does not suffice for clinical movement analysis applications. This paper therefore focuses on reducing the cost of a human body pose estimation system while retaining the required accuracy. The proposed solution comprises of an embedded ultrasonic transmitter and receiver subsystem. The receiver subsystem consists of multiple mobile nodes that are equipped with a small microphone array (at least three microphones). Each mobile receiver node captures the encoded simultaneously broadcast ultrasonic transmissions from a distributed transmitter array (which consists of at least three elements). Using signal processing, a distance can be calculated between each transmitter and microphone resulting in at least nine distances for each mobile node. Using these distances in combination with the position of the transmitters and the microphone array configuration, the XYZ-position of the mobile node and its rotation about these axes (six degrees-of-freedom) can be estimated. The combination of low-cost embedded and ultrasonics hardware that forms the transducer and receiver subsystem (consisting of multiple mobile receiver nodes) together with powerful signal processing techniques yields a high-accuracy pose estimation system, which can be used as an affordable tool in various fields and applications (e.g., gait analysis for rehabilitation purposes).