Human Body Posture Tracking Using Flexible,Wireless and Low-Power Sensor Nodes

Benoı̂t Huyghe

Supervisor(s): Jan Doutreloigne, Jan Vanfleteren

Abstract— Human posture tracking is per-formed using several sensor nodes attached to asubject’s body. A Kalman filter estimates the ori-entation of each of the nodes from the outputs ofthe sensors. This orientation is then mapped tothe appropriate bodypart allowing full body pos-ture reconstruction.

The design of flexible, wireless and low-powersensor nodes for orientation tracking purposesis presented. Hardware components are chosenbased on size and power consumption and soft-ware is written to implement a TDMA-like pro-tocol allowing 10 nodes to send 100 samples persecond to a base station receiver.

Keywords—Kalman Filter, Sensor Nodes, Wire-less, Low-Power, Orientation Tracking

I. INTRODUCTION

Several techniques are available for humanposture tracking. Among them, optical and in-ertial systems are most common. Optical sys-tems, which use one or more cameras, are re-stricted to a limited area and require a freeview to the subject at all times. Using iner-tial sensors, human posture tracking can be ac-complished by combining several sensor nodesplaced on the body. Obviously, these nodesmust be designed in an unobtrusive way in or-der to allow the subject to perform everydayactivities and wireless communication insuresfree view is not required and the range of thesystem can be controlled.

Each of the sensor nodes in an inertial track-ing system must contain a number of sensors

—————————————————–B. Huyghe is with the Centre of Microsystems

Technology, Department of Electronics and Informa-tion Systems, Ghent University (UGent), Gent, Bel-gium. E-mail: Benoit.Huyghe@UGent.be .

Figure 1. Top view of a fully assembled flexible sen-sor node.

that allow estimation of the full 3D orientation.Traditionally, 3 types of sensors are used: ac-celerometers, gyroscopes and magnetometers[1]. We have designed a Kalman filter that esti-mates full 3D orientation using only accelerom-eters and magnetometers [2]. This way, gyro-scopes can be omitted in the sensor node de-sign, resulting in smaller and less power con-suming sensor nodes.

II. SENSOR NODE DESIGN

A. Hardware

A top view of an assembled sensor nodeis displayed in Fig. 1. Each node contains alow-power TI MSP430F2132 microcontroller,a low-power Nordic nRF2401 RF transceiverand two fully integrated, 3D sensors: anST LIS302DL accelerometer and a YamahaYAS529 magnetometer. A board design wasmade in a single conductive layer allowing in-house production and assembly of the sensornode on a flexible poly-imide substrate.

Figure 2. Time graph of the current consumption inthe master node during 2 measuring cycles.

B. Software

The current consumption of the sensor nodeshas been minimized by careful implementationof the functionality in order to obtain a maxi-mal battery life. This involves maximal use oflow-power modes and interrupts of each com-ponent, which is especially valid for the RFtransceiver, as this chip generates the highestcurrent consumption when switched on. There-fore, a custom communication protocol basedon TDMA has been implemented.

The protocol assumes the presence of a mas-ter node which serves as a time reference forthe slave nodes. Every 10 ms, the master sendsa data package with the output of its sensorsto a receiver. The slaves will synchronize tothis transmission and, whenever they receive apackage, they will transmit their own sensordata in a different time slot. Slaves can dis-tinguish a package from the master from thatof another slave as both are transmitted in another RF channel. In order to limit the activeperiod of the RF transceiver, synchronizationwith the master is only performed every 256packets. All packets in between are sent in thecorrect time slot by relying on a watch crystal.

III. RESULTS

Fig. 2 shows a time graph of the current con-sumption in the master node during two cycles.Several regions can clearly be discerned and as-sociated to a certain component’s functionality.The average current consumption of the entire

Figure 3. Time graph of the current consumption intwo slave nodes during 2.5 measuring cycles.

node was measured to be lower than 3 mA.Fig. 3 depicts the current consumption of two

slaves which have been assigned the first andsecond time slot. Clearly, both send their dataat a different point in time, avoiding collision.The largest peak is due to the RF transceiveroperating as a receiver for the master data. Thispeak is only present every 256 packages andadds about 10 µA to the average current con-sumption of a slave in respect to the master.

IV. CONCLUSIONS

Flexible, wireless and low-power sensornodes for human body posture tracking havebeen designed. Careful implementation of thefunctionality has resulted in an average currentconsumption of less than 3 mA. A TDMA-likeprotocol allows 10 nodes to transmit data wire-lessly at a rate of 100 Hz to a single receiver.

ACKNOWLEDGMENTS

The work of B. Huyghe was supported by afellowship of the Fund of Scientific ResearchFlanders (FWO-V), Belgium.

REFERENCES

[1] X. Yun and E.R. Bachmann, Design, Implementa-tion, and Experimental Results of a Quaternion-Based Kalman Filter for Human Body MotionTracking, IEEE Trans. on Robotics, Vol. 22,No. 6, Dec. 2006, pp. 1216 - 1227.

[2] B. Huyghe, J. Doutreloigne and J. Vanfleteren3D Orientation Tracking Based on UnscentedKalman Filtering of Accelerometer and Magne-tometer Data, Proc. of IEEE Sensors Applica-tions Symposium 2009, pp. 148 – 152, Feb. 2009.