Short communicationAccelerometer and rate gyroscope measurement of kinematics: an inexpensive alternative to optical motion analysis systems
Introduction
Optical motion analysis systems are often used in the study of human movement. However, these systems are expensive, only allow measurements in a restricted volume, and the markers are easily obscured from vision resulting in incomplete data. More recently, body-mounted sensors have also been used to obtain kinematic values (Bussmann et al., 1995; Bogert van den et al., 1996; Dai et al., 1996; Luinge et al., 1999; Tong and Granat, 1999; Veltink et al., 1996; Veltink, 1999). A fully portable system can be obtained if a portable data logger is used to gather the data from body-mounted sensors.
The system presented in this paper is general-purpose, combining accelerometers and rate gyroscopes, able to deliver the following kinematic parameters in the sagittal plane: shank angle, thigh angle, knee angle, shank angular velocity, thigh angular velocity, knee linear acceleration, shank angular acceleration and thigh angular acceleration. To verify the accuracy of the body-mounted sensor system, optical data were gathered simultaneously using a Vicon® System. Only the parameters of shank angle, shank angular velocity, knee linear acceleration and shank angular acceleration are presented in this paper as representative examples.
Section snippets
Model description
In the two-dimensional (sagittal plane) model, the shank and foot were considered as a single segment referred to as shank. Shank and thigh were represented by two rigid segments k and k+1 connected by a simple hinge knee joint (Fig. 1).
Calculation of linear acceleration
The accelerometer signal at any point i of a rigid body k can be expressed as a vector on the basis of three independent signal vectors at positions (Veltink and Boom, 1996)The coordinates
Measurement system verification
The number of cases (out of 20) in each comparison is indicated in Table 2, together with the RMS and CMC results. Missing cases are due to data lost to file archiving problems or to one or more of the markers being lost from vision. Fig. 3 shows examples of graphs of the four comparisons.
In three of the four comparisons, the RMS errors are <7% of the full range of the measurements and the found CMC are above 0.98. However, for the linear acceleration of the knee (comparison number 3), the
Accuracy of kinematic measurements
The body-mounted sensors give results that are very close to those of Vicon® presenting small RMS and large CMC values. Errors do increase at the highest speed for the accelerometer data, which for several subjects suffered from deformed peaks, probably due to the sensors being hit or vibrated during heel strike. This should be taken into account when designing future applications involving accelerometers. The rate gyroscopes were not affected in this way.
Contributions of this study and future work
In this study, a treadmill was used
Acknowledgements
We are grateful to the STW, Dutch Technology Foundation, for the funding for this project; to Andreas Heyn and Eric Camiro for their valuable assistance.
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