A double-sensor difference based algorithm to estimate segment rotation angles in 2 directions for lower limb segment orientation analysis in 3-dimensional (3D) space was presented. The results of the simulation suggest that the accuracy of the calculated angles correlate strongly with the range of the rotation angles. In order to achieve higher precision of the calculated angles in a range of -60° to 60°, a switching algorithm containing arcsine or arccosine according to the value of the angle was presented. Then, to qualitatively verify the method in practical application, a prototype of a wearable sensor system using only one kind of inertial sensor (accelerometer) was developed. The system was first tested on a board with one freedom of rotation in the sagittal plane. Then it was tested on the thigh of a volunteer to obtain the pitch and yaw angles for lower limb segment orientation. In the experiment, two accelerometers were used to measure the actual accelerations on the thigh. The volunteer swung his lower limb in the work space of an optical motion analysis system to obtain the orientation angles of the thigh as reference. It was assumed that there was very little trunk sway or skin artifacts and no significant internal/external rotation of the leg. The results show that, without integration and switching between different sensors, using only one kind of sensor (accelerometer), the prototype is suitable for ambulatory analysis of lower limb segment orientation in two directions.
2010 by The Japan Society of Mechanical Engineers