A sensor system for measuring human motions is presented. The system consists of a number of miniature wireless inertial sensors that are attached to limbs of a person, and a PC with a wireless receiver that interprets and presents the measurement data. Each of the sensors measures 3D-acceleration, 3D-magnetization (earth magnetic field) and 3D-angular speed (gyroscopes). The data is transmitted via a proprietary protocol on an 802.15.4 radio. The angular accuracy of the calibrated system was found to be better than 3 degrees. The system is currently being applied in a pilot trial for motor rehabilitation of stroke patients.

A general formalism for hopping electron transport over insulators sustained by secondary electron emission is presented. Steady-state electron transport takes place when the charging of the insulator, which turns out to be self-stabilizing, is such that the average secondary electron yield becomes equal to unity. The steady-state potential distribution for the electron transport is determined for various insulating geometries with the aid of Monte Carlo calculations and compared with the low-hopping approximation. The Monte Carlo results show that the steady-state potential distribution can exhibit several interesting features such as spontaneous symmetry breaking of statistical origin and the occurrence of local repulsive parts in the geometry. In several cases the numerical results, including the above two features, are found to agree well with the results of experiments.

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