Constant force generator and apparatus including it

A constant force generator comprising:

• a case extending in an axial direction, the case being made of a tube-shaped ferromagnetic material;
• a tube-shaped stator extending in the axial direction, the tube-shaped stator being disposed in the case;
• and
• a rod-shaped shaft extending in the axial direction, the rod-shaped shaft being disposed in the tube-shaped stator via a gap, the rod-shaped shaft being movable in the axial direction relative to the tube-shaped stator,
• wherein the tube-shaped stator comprises:
• N main permanent magnets disposed adjacently at regular angular intervals of (360/N) degrees, each main permanent magnet being magnetized in a radial direction, where N represents an integer which is not less than two;
• and
• an even number of secondary permanent magnets disposed so as to sandwich each of the N main permanent magnets on both sides in a circumferential direction, each secondary permanent magnet being magnetized in the circumferential direction, the even number of secondary permanent magnets being configured to sandwich the both sides of each main permanent magnet so that magnetic poles of the secondary permanent magnets, facing the both sides of the sandwiched main permanent magnet, have a polarity the same as an inside magnetic pole of the sandwiched main permanent magnet,
• wherein the rod-shaped shaft at least includes a shaft portion made of a magnetic material.