Real time eddy current responsive Hall effect device tachometer

• US 4,441,077 A
• Filed: 11/23/1981
• Issued: 04/03/1984
• Est. Priority Date: 11/23/1981
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines SA (International Business Machines Corporation)
• Inventors: Siegl, Ludwig R., Dodgen, David F.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Assistant Examiner(s): Solis, Jose M.
• Application Number:
  US06/323,778
• Time in Patent Office: 862 Days
• Field of Search: 324/164, 324/166, 324/208, 324/239, 73/519, 73/520, 73/497
• US Class Current: 324/164
• CPC Class Codes: G01P 3/49 using eddy currents

Abstract

A real time tachometer is realized in an electromechanical device having a moving member, for example a disk, of nonmagnetic electrically conductive material driven by a suitable moving device, for example a rotating shaft, the velocity of which is to be determined. An assembly of at least one permanent magnet and at least one Hall effect device is arranged for inducing eddy currents in the moving member or rotating disk and measuring them for indicating the relative velocity. The placement and electric interconnection of the Hall effect devices measure the magnetic field provided by the magnet and in an alternate embodiment incorporate compensation for runout and adverse deflection with a rotating disk.

Claims

1. A tachometer for providing an analog electrical voltage signal whose amplitude indicates in real time the velocity of a flat, moving, nonmagnetic, electrically conductive member and whose polarity indicates the direction of movement of said member comprising in combination:

(1) a pair of permanent magnets and a C-shaped yoke of material for providing a magnetic circuit including an air gap defined by first and second opposed parallel pole faces corresponding to opposite poles of said pair of permanent magnets;

(2) means for moving said member through said air gap in a plane parallel to said faces so as to induce eddy currents in the surface of said flat member which flow generally in closed loop paths that include parallel segments disposed substantially normal to the direction of movement of said member, said eddy currents causing an eddy current flux field which at the mid-point of said air gap includes lines of induced eddy current flux which are substantially parallel to the lines of permanent magnetic flux in said air gap, said induced eddy current lines of flux disposed on opposite sides of said mid-point of said air gap being in opposite directions whereby the net value of flux at any point in said air gap varies as a function of (a) the velocity of said member;

(b) the location of said point relative to said center point; and

(c) the distance of said member from each of said parallel faces corresponding to said pole faces;

(3) a first pair of Hall effect sensing devices positioned between said moving member and one said pole face and disposed in spaced apart relationship relative to each other on opposite sides of said mid-point;

(4) a second pair of Hall effect sensing devices positioned between said moving member and said other pole face and disposed in correspondingly spaced apart relationship on opposite sides of said mid-point; and

(5) circuit means for (a) first, differentially combining the outputs of said Hall effect sensing device of each pair to provide potential output signals which are indicative of the velocity and direction of movement of said member, and (b) secondly, differentially combining said potential signals from said first and second pair of Hall effect devices to eliminate errors in said indication of velocity resulting from any wobble type movement of said member in said air gap in a direction parallel to said flux lines.

2. The combination recited in claim 1 in which said member is a circular disk mounted on a shaft for rotation about an axis normal to said disk surface and spaced from said magnetic circuit whereby said potential signals reflect the rotational velocity of said shaft and its direction.