EVALUATION OF THE "FLOCK OF BIRDS" ELECTROMAGNETIC TRACKING SYSTEM

Participants: M. Bhuta, L. Gallo, R.E. Hughes

Keywords: Flock of Birds, electromagnetic tracking device, kinematics

Introduction

The "Flock of Birds" electromagnetic tracking device utilizes a transmitter to produce an electromagnetic field and sensors ("birds") to measure location and orientation in the field. This technology was developed by the Ascension Technology Corporation, and it has been widely used in biomechanics research. Others have reported system accuracy and the effects of metal on accuracy (Milne et al., 1996). However, the effect of metal in the environment should theoretically be modified by the sampling rate. No published reports have investigated the dependence of accuracy on sampling rate. The objective of this research was to quantitatively evaluate the effect of sampling rate on measurement errors when metal is placed within the electromagnetic field.

Material and Methods

An apparatus was fabricated in order to mount and move the sensors at pre-determined positions and orientations. A plexiglass base platform was designed such that holes were placed in a grid forming 3 columns and 6 rows (18 points) at a spacing of 15cm. At the end of the platform, the transmitter was mounted. For translation, 4 towers (4, 15, 30 and 45 cm.) were constructed (only one sensor can be mounted upon each tower).

The sensor’s measured position was obtained for each tower at each of the 18 points of the platform. Six trials were conducted. For rotation, 2 towers (30-degree and 60-degree face) were constructed. The sensor’s measured orientation was obtained for each tower at 4 specific orientations at each of the 18 points. Six trials were conducted.

The measured error was calculated as the difference between the true value and the measured value for the position and orientation.

The procedure was repeated with aluminum, steel, and wood blocks in the electromagnetic field. Data collection rates of 40, 80, and 120 samples per second were used.

Progress

The test fixture has been fabricated. Data collection and analysis software has been developed. Data has been collected. Analysis is ongoing.

References

1. Milne, A.D., Chess, D.G., Johnson, J.A., and King, G.J.W (1996) Accuracy of an electromagnetic tracking device: A study of the optimal operating range and metal interference. J. Biomechanics 29: 791-793.