Non-Contacting Shaft Position Sensor

The present invention relates to a sensor for non-contacting determination of angular offset of one shaft with respect to another, for example to determine relative angular rotary position of two shafts, or torsion in axially spaced positions of a shaft, which can be related to torque being transmitted by the shaft.

It has previously been proposed to determine the relative position of two shaft elements, which may both be stationary or rotating, with respect to each other by connecting concentric sleeves to respective shaft portions. Upon relative rotation of the respective shaft portions, for example due to torsion of the shaft upon being loaded by transmitting a substantial torque, or the like, the concentric sleeves will rotate with respect to each other. Electrical output signals can be obtained representative of the relative rotation of the sleeves.

The sleeves can be segmentally subdivided, in circumferential direction, into zones of high and low conductivity; they may be made, for example, of brass, bronze, or the like, and the zones of low electrical conductivity can be formed by cut-outs or air gaps in the circumferential portion, which can be left free or filled with a non-conducting material. Electrical current is induced in the sleeves by a winding surrounding the sleeves.

It is an object to improve a basic structure of this type to provide a measuring signal which has low drift and high accuracy, so that the system can be used to determine torque being transmitted by a shaft, by relative rotation of axially spaced shaft portions, or the like; and to optimize such a measuring system by a structure which is simple and easy to construct.

Preferably, the zones of conductivity or non-conductivity of the sleeves are so arranged that, measured in circumferential direction, the width b of the sleeves where they have high conductivity is about 5 to 10 times the width a of the zones of low conductivity.

In accordance with a feature of the invention, two sleeves are provided, one each located within the other when finally assembled to the respective shaft portions. Axially extending slots are then cut in the respective sleeves to thereby form the zones or regions of low conductivity, the slots having a width which is between 1/5 to 1/10 of the circumferential extent of the sleeve portions between the slots, so that the ratio of the dimension b, in circumferential direction, of the zones of high conductivity with respect to the width a of the zones of low conductivity is between 10:1 to 5:1.

FIG. 1 is an axial cross section through the sensor;

FIG. 2 is an axial cross section, partly cut away, through the inner one of the sleeves;

FIG. 3 is an axial cross section through the outer one of the sleeves; and

FIG. 4 is an axial cross section of the winding form or carrier.

The invention will be described in connection with a sensor to determine torque in a shaft or relative rotary position of two shaft elements, applied to a torsion bar 1.

The torsion bar 1 is located between two shaft stubs 2, 3 and is subjected to torque. Upon applying a torque Md to the shaft stub 3, for example on a square socket connector 4, and restraining the shaft stub 2, for example by clamping a square socket connector 5, relative rotation over a very small angle will occur between the shaft stubs 2 and 3. FIG. 1 assumes that a torque M.sub.d, towards the right, is applied to the socket connector 4 at the right-hand stub 3, the socket connector 5 at the left-hand stub 2 being clamped, so that a counter torque will be generated therein.

The measuring system to determine the torque M.sub.d is positioned within a tubular housing 6, retained and positioned by suitable ball bearings, which includes two concentric sleeves 8, 9, which are concentric with respect to a common longitudinal axis 7. The inner sleeve 9, as well as the outer sleeve 8, are made of electrically highly conductive material, preferably non-magnetic, for example having about 1 mm wall thickness. The outer sleeve 8 is secured by a spacer portion 8a to the stub shaft 3 to rotate therewith, while preventing relative rotation between stub shaft 3 and the spacer portion 8a and hence the sleeve 8. The inner sleeve 9 is similarly secured by a spacer 9a on the stub shaft 2. The outer sleeve 8 is shown in a longitudinal sectional view in greater detail in FIG. 3. It has a clearance diameter B, an end connecting portion 8a, and is formed along its circumference with eight uniformly distributed circumferential slits 11. The slits 11 are parallel to the axis 7, and are cut into the wall of the sleeve 8 by a milling cutter, by sawing, or the like. They extend axially close to the end zones 12, 13 of the sleeve 8. A circumferential surface portion 14 will remain of the sleeve between the respective slits 11. Thus, as far as the circumference of the sleeve is concerned, the electrical conductivity of the zones 11 will be substantially less--effectively zero--than the conductivity of the zones 14, which corresponds to that of the material of which the sleeves are made.

The inner sleeve 9 is shown in axial sectional view in FIG. 2. Sleeve 9 has an outer diameter (OD) E; the circumferential surface is cut by slits 16 to subdivide the surface in eight circumferentially continuous zones 15, which are congruent with the circumferential zones 14 of the outer sleeve 8 (FIG. 3). In contrast to the outer sleeve, the slits are formed in two groups 16, 17. A slit pair 16, 17, each, is so arranged that the respective slits 16, 17 are offset circumferentially by a distance v. The distance v corresponds approximately to the width of a slit. Slits 11, as well as the slits 16, 17, and the offset v, are about 1 mm wide. The axial length of the slits 16, 17 is about half the axial length of the associated slit 11 (FIG. 3), leaving a small continuous central strip 15a. The length of the slits 11 (FIG. 3) is shown at L; the length of the slits 16, 17 is roughly 1/2 L.

A cylindrical spool form or spool carrier 18 (FIG. 4), of insulating material, is positioned in the region of coverage of the inner and outer sleeves 9, 8. Two separate single-layer windings 20, 21, each of a width W, are wound on the form 18. The outer diameter of the form 18, forming the winding diameter of the windings 20, 21, is shown at d. The width W of the respective windings 20, 21 is a fraction of the axial length L of the surface region interrupted by the slits 11 (FIG. 3); preferably, the width W of any one of the coils 20, 21 is less than 1/3 L.

At least one of the two sleeves 8, 9, and preferably both, are made on non-ferrous material, such as brass or bronze, or copper alloy. The windings 20, 21 are so positioned that they are approximately centrally located with respect to the half-slits 16, 17 of sleeve 9 (FIG. 2), to cover the central region of the half-slits over a narrow width thereof. In operation, high frequency alternating current is connected to the windings, which can be separately connected by connecting leads 20a, 27a (FIG. 1).

Operation: By using two windings 20, 21, a differential measuring system can be used. Upon application of a torque Md to the stub shaft 3, and clamping the stub shaft 2, sleeve 8 will be rotated with respect to sleeve 9. One of the sleeves 16, 17 will be twisted out of partial-or at least edge-alignment with the sleeve 11, whereas the other one of the half-slits of sleeve 9 (FIG. 2) will be twisted into increasing congruence with the slit 11. Thus, in one of the windings 20, 21 the inductance will decrease whereas, in the other, the inductance will increase due to increased eddy current generation in the inner sleeve or, respectively, decreased occurrence of eddy currents (upon congruence of slit positions). Consequently, the differential change in inductance can be measured by connecting, for example, the coils 20, 21 in a bridge circuit or to another inductance measuring device. The two coils 20, 21 could, also, be individually connected, for example to different arms of a bridge.

High sensitivity of the measuring device is obtained by keeping the radial distance c (FIG. 1) between the inner diameter d of the windings 20, 21 and the outer diameter D of the outer sleeve 8 as small as possible. In accordance with a feature of the invention, this distance should be in the order of 1 to 1.5 mm; smaller distances are preferred, but difficult to obtain in actual constructions. The width b of the zones of high conductivity between the slits is about ten times the width of the slits, forming zones of low or, effectively, zero conductivity. The width b is measured in circumferential direction along the surfaces 14 of sleeve 8 (FIG. 3) and 15 of sleeve 9 (FIG. 2), so that the zones of low conductivity between the slits 11 (FIG. 3) and, respectively, 16 and 17 (FIG. 2), respectively, which have no conductivity, will be about ten times as great.

The effectiveness of differential measuring is improved by providing a shield ring 24, of conductive material, preferably of non-magnetic, or non-ferrous metal such as copper, copper-containing metal such as brass or the like, or aluminum, is positioned between the coils to form a shielding element therebetween and prevent inductive cross-coupling.

The spacing K between the outer diameter (OD) E of the inner sleeve 9 and the inner diameter (ID) B of the outer sleeve 8 should be as small as possible, and, preferably, a fraction of a millimeter; spacings between 0.05 to 0.25 mm are preferred. Such spacings can be obtained by making the sleeves of copper-containing metal, such as brass, bronze of the like, or of aluminum, in order to obtain circumferential surface areas 14 in sleeve 8 and 15 in sleeve 9 which have high conductivity, subdivided by the slits which have, effectively, zero conductivity. The shield ring 24 is preferably located in a plane of symmetry extending transversely to the axis 7 of the shafts stubs 2, 3.

The present invention is a further development of the invention of our application 06/213 589, filed Dec. 5, 1980, now U.S. Pat. No. 4,356,732 Nov. 2, 1982, the disclosure of which is hereby incorporated by reference.