Permanent magnet speed detector

Claim 1

A vehicle having a chassis constituting a fixed support of the vehicle, a member rotatable with respect to said support, and means, mounted part on said chassis and part on said rotating member, to provide an electrical signal proportional to the speEd of rotation of the member with respect to said support, wherein the part of said means mounted on the chassis comprises an annular disc of non-magnetic material fixed to said chassis, a first annular plate of material of low magnetic reluctance secured to the face of said disc remote from said support, a second annular plate of insulating material secured to said first plate and carrying a printed circuit remote from the first plate, said printed circuit being formed by a continuous conductive zigzag track having a plurality of equally spaced radially extending parts alternately connected at the radially inner and outer ends by circumferentially extending parts, the pitch of the zig-zag track being uniform and there being provided adjacent output terminals at the opposite ends of the track, said disc and said plates being mounted coaxially with said rotating member, and wherein the part of said means fixed to said rotating member adjacent to the fixed support, and an annular magnet mounted on said supporting disc coaxially with the rotating member and counterposed to said track, said second plate and said magnet having substantially equal inner and outer radii and said magnet being formed of a number of radially extending sectors magnetized in a sense parallel to the axis of the rotating member, the individual sectors having a mean circumferential width equal to half the pitch of the zig-zag track with adjacent sectors being of opposite polarity.