one end to the reduction motor 123 and is turnably connected at the other end to the rod 121 by a rotating pivot point 126 while the end of the rod 121 that is not connected to the rod 122 is rotatably secured to the plate 11 b at a pivot point 125 the rods 121 and 122 are formed in the shape of a mountain with respect to the plates 11 a and 11 b the reduction motor 123 may be secured to either of the plates 11 a or 11 b and if the reduction motor 123 is secured to the plate 11 b the rod 121 is turnably secured to the plate 11 a the hinges 13 are not limited in arrangement or number as long as the set of plates can be pivotally secured and the hinges 13 need not necessarily be arranged at the opposite extreme ends between the connected sides of the plates 11 a and 11 b and the number of the hinges 13 may not be two further the reduction motor 123 is provided with measuring means 124 capable of measuring the angle formed by the plates 11 a and 11 b by measuring the angle of rotation of the rod 122 and the measuring means 124 uses a potentiometer or an encoder fig3 b is a side view of the set of plates viewed in the direction indicated by an arrow y in fig3 a and fig3 c is a side view of the set of plates viewed in the direction indicated by.