Jean-Pierre Merlet On the infinitesimal motion of a parallel manipulator in singular configurations. 320-325 1992 ICRA https://doi.org/10.1109/ROBOT.1992.220244 https://doi.ieeecomputersociety.org/10.1109/ROBOT.1992.220244 conf/icra/1992 db/conf/icra/icra1992.html#Merlet92