Patent ID: 7191669

Claim:
A torque sensor comprising: a first shaft; a second shaft; an elastic member coupling said first shaft and said second shaft in alignment with each other, upon input of torque, said elastic member undergoing torsion; a hard magnetic member joined to said first shaft, said hard magnetic member having magnetic poles arrayed on a periphery thereof to produce a magnetic field therearound; an assembly of a first and a second soft magnetic member joined to said second shaft and placed around said hard magnetic member within the magnetic field produced by said hard magnetic member to form a magnetic circuit so that upon a change in position of the first and second magnetic members relative to said hard magnetic member arising from the torsion of said elastic member, density of magnetic flux developed within the magnetic circuit changes; and a magnetic sensor placed in non-contact with said first and second soft magnetic members to measure the density of magnetic flux within the magnetic circuit, wherein said first and second soft magnetic members are opposed to each other through a given gap in a direction of the alignment of said first and second shafts, said first and second soft magnetic members having as many claws as the poles of said hard magnetic member which are arrayed at regular intervals on peripheries of said first and second soft magnetic members, respectively, each of the claws of said first soft magnetic member being interposed between adjacent ones of the claws of said second soft magnetic member, each of the claws having a base portion and a head portion to form a substantially trapezoidal shape which defines dead zones where the density of magnetic flux developed within the magnetic circuit undergoes substantially no change, the dead zones lying in ranges across plus 90° and minus 90° in terms of a magnetic angle, respectively.