.If the position and the orientation of each body segment is known, then it is possible to calculate all the kinematic data. In the aerospace industry, gyroscopes and accelerometers are widely used to provide information on position and orientation. It is theoretically possible to use the same techniques for gait analysis. The signals from accelerometers and gyroscopes are acceleration and angular velocity, respectively. The raw signals from accelerometers and gyroscopes have been used to quantify human daily activities [6–10]. Joint angles are commonly used in gait analysis and can be derived by the integration of angular acceleration or angular velocity. However, data obtained from integration can be distorted by offsets or any drifts. To find the angular acceleration using accelerometers, a pair of accelerometers fixed on a rigid object is
88 K. Tong, M.H. Granat/Medical Engineering & Physics 21 (1999) 87–94
required. In order to eliminate any drift during integration, Morris (1973) [11] identified the beginning and the end of the walking cycles, and made the signal at the beginning and the end of the cycle equal. Willemsen et al. (1990) [12] developed a technique to find the joint angle without the need for integration, which used four accelerometers on each segment. The system used two metal bars with eight accelerometers for measuring a single joint angle. They also used a simplified version of their technique for the control of an FES system. Four accelerometers on a metal bar were used to calculate the joint acceleration, and different phases of gait could be detected for FES control without the need for the angular information [2]. Inclinometers have also been evaluated for use in controlling FES systems [3]. Inclinometers detect inertial forces. During the stance phase, when the angular acceleration is nearly zero, the inertial force is principally due to gravity, and the segment inclination can then be calculated. During the swing phase the angular acceleration affects the measurement and therefore inclination cannot be accurately calculated. Another promising alternative is to use gyroscopes directly to measure the angular velocity without the signal being affected by gravity or any linear acceleration. Gyroscopes can therefore theoretically be used to calculate the segment inclination and the relative joint angle. During walking the movement of the lower body segments occurs mainly in the sagittal plane, so only single uni-axial gyroscopes would be required on each segment. Heyn et al. (1996) [13] had showed that shank inclination could be measured with eight accelerometers and two gyroscopes fixed on two rigid metal plates. This experimental protocol did not include any turning which could be expected to effect the inclination. They also found that using these metal plates was cumbersome. The aim of this study was to investigate the possibility of using uni-axial gyroscopes to design a practical gait analysis system. The gyroscopes would be fixed directly to the skin making the system easy to apply and reducing subject encumbrance. The first objective was to evaluate the angular signals and derived signals from gyroscopes and compare these with data from the motion analysis system (Vicon). The second objective was to evaluate the problem of drifting during turning and to investigate solutions to this problem.
2. Methods
The dynamic performance of the uni-axial gyroscopes was evaluated from data collected while a subject walked in a straight line in a gait laboratory. The principle of operation of the gyroscope is the measurement of the Coriolis acceleration of a vibrating device. It con
sists of a triangular prism made of a special substance called ‘Elinvar’. If the prism is rotated about its sense axis the signal is proportional to the angular velocity. The gyroscope used was ENC-05EA (Murata, Japan), and the dimensions of this sensor were 20 3 7.2 3 10 mm.