Device for determining the torque exercised on a shaft

Device for determining a torque exercised on a shaft, wherein the shaft comprises a first shaft section and a second shaft section, wherein the two shaft sections can be rotated relative to another, with a multi-pole magnetic ring which surrounds the first shaft section and is connected thereto, and a stator holder which is mounted to the second shaft section, wherein two stator elements are mounted to a stator holder and each stator element comprises fingers projecting in an axial or radial direction which are distributed uniformly at least over part of the periphery with gaps between them, wherein the fingers of each stator element are interconnected via a magnetic flux ring, wherein the magnetic flux rings have a mutual separation and a magnetic field sensor is disposed between the magnetic flux rings, wherein at least one magnetic flux concentrator is associated with the magnetic field sensor, wherein the magnetic flux concentrator surrounds the magnetic flux rings.

This application claims Paris Convention priority of DE 103 16 124.4 filed Apr. 4, 2003 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a device for determining the torque exercised on a shaft, wherein the shaft comprises a first shaft section and a second shaft section, wherein the two shaft sections can be rotated relative to each other, a multi-pole magnetic ring which surrounds the first shaft section and is connected thereto, and a stator holder which is mounted to the second shaft section, wherein two stator elements are mounted to the stator holder and each stator element has fingers which protrude in an axial or radial direction and which are distributed uniformly at least over part of the periphery and have gaps inbetween, wherein the fingers of each stator element are interconnected via a magnetic flux ring, the magnetic flux rings having a mutual separation, and a magnetic field sensor is disposed between the magnetic flux rings, wherein the magnetic field sensor is associated with at least one magnetic flux concentrator.

U.S. Pat. No. 4,984,474 discloses a torque sensor which is formed substantially from one or more magnetic rings and two stator elements which have a low number of poles. The low pole number has the disadvantage that the signal measured by the sensor is modulated with a waviness when the steering shaft rotates, which can be compensated for only by suitable electronic addition of two signals which are offset by half a pulse width or by flux collecting rings of a completely annular shape. The torque sensor of this design is also relatively sensitive and susceptible to disturbances, since the magnetic flux concentrator is mounted radially outside of the stators. Such a design is also highly susceptible to concentricity tolerances. Finally, the stators comprise spacers formed by separate rings which render the assembly relatively complex.

FR 2,821,668 A1 discloses a device wherein the sensor consists of a discretely formed multi-pole magnetic ring and two nested soft-magnetic stators. These stators have finger-shaped structures on the radial inner side which scan the magnetic poles, and an annular gap on the radial outer side accommodating a stationary magnetic field sensor.

Pole division must be relatively coarse through discrete design of the magnet wheel (pole width 20°) which produces a likewise large linearity range which is not completely utilized since the range of the angle to be measured is only approximately ±3° to 5° due to the required rigidity of the torsion system. The magnetic flux cannot be optimally utilized since the air gap forming the magnetic return is uniformly formed across the entire periphery such that the magnetic flux is distributed over a large surface and is therefore only relatively small at the location of the magnetic field sensor.

Although highly remanent magnets are used, this device shows little sensitivity, and the measuring signal depends greatly on mechanical tolerances such as the width of the air gap where the flux density is measured.

DE 102 22 118 A1 discloses designs with annular flux conductors or magnetic flux collecting rings which are disposed on the outer side of the stators or magnet yokes. The design of the flux conductors disadvantageously leads to great expense and not all influences of radial and axial tolerances of the stators can be compensated for.

Mechanical tolerances in production and assembly of the components conducting the magnetic flux, in particular the stators, cannot be prevented. In all conventional constructions, these tolerances may have a direct effect on the size of the air gaps located in the magnetic circle and therefore a disturbing effect on the measuring signal thereby reducing the accuracy or producing erroneous measurements.

It is therefore the underlying purpose of the invention to further develop a device of the above-mentioned type to reduce the effect of the tolerances on the measuring result.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in that the magnetic flux concentrator surrounds the magnetic flux rings.

The stationary magnetic flux concentrators are designed such that both sides of the magnetic flux rings have flux-collecting surfaces. One single magnetic flux concentrator surrounds the neighboring magnetic flux ring on the inner and also on the outer side instead of on only one side. This inventive design has the great advantage in that tolerances in the flatness or run-out of the magnetic flux ring are averaged. When the inner air gap between magnetic flux ring and magnetic flux concentrator is reduced, the associated outer air gap increases. Since the magnetic flux is guided over both air gaps, both effects are compensated for.

In a preferred further development of the invention, the magnetic flux concentrator surrounds the magnetic flux ring in the shape of a C. The free ends of the magnetic flux concentrator thereby overlap the magnetic flux ring on both sides over a radial length of 20% to 80%, in particular 30% to 50%.

The magnetic flux sensor is preferably disposed between the magnetic flux concentrators. This is advantageous in that the magnetic field sensor is shielded from external influences and is also located close to the magnetic flux concentrators.

The invention is further optimized in that each magnetic flux ring is associated with two or more magnetic flux concentrators. The several magnetic flux concentrators are thereby uniformly arranged over the periphery of the magnetic flux ring which is substantially advantageous in that e.g. when two magnetic flux concentrators are used, a total of two magnetic field sensors can be used thereby producing a redundant signal which also permits signal averaging.

The magnetic flux concentrator can extend over an angular range of 10° to 180°, in particular an angular range of 25° to 90°, of the periphery of the magnetic flux ring.

In one embodiment, the magnetic flux concentrator is arranged in a stationary holder. Additional electronic components, an associated circuit board, plug contacts and/or soldering terminals may be provided in this holder. The holder is formed as separate component and is connected to the stator elements at a suitable location such that the stator elements are movable relative to the holder.

To minimize the play with the magnetic flux rings, the holder is supported on the stator holder via a sliding bearing. This direct contact minimizes the free gaps between magnetic flux rings and magnetic flux concentrators.

If the stator holder can be clamped or locked to a holder ring at the free end of the second shaft section, assembly, repair, and maintenance work are facilitated.

In a preferred embodiment, the magnetic flux concentrators are a stamped, bent component. Production of such stamped, bent components is simple and inexpensive. In an alternative embodiment, the magnetic flux concentrators are a sintered part or a MIM part (metal injection molding part).

Further advantages, features and details of the invention can be extracted from the following description which shows details of particularly preferred embodiments with reference to the drawing. The features shown in the drawing and described in the claims and description may be essential to the invention either individually or collectively in arbitrary combination.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1shows a steering shaft, referred to in total with10, of an automotive vehicle, of which two shaft sections12and14are shown. The two shaft sections12and14are connected to each other via a torsion rod spring16such that the free facing ends18and20are rotated relative to each other when a torque is applied to the steering shaft10. A magnetic ring holder22is mounted to the end20of the shaft section14which carries a multi-pole magnetic ring referred to in total with24.

This magnetic ring24is surrounded by a stator referred to in total with28which is mounted to a stator holder30. This stator holder30is mounted to the free end18of the shaft section12, wherein it is locked with a holder ring32via a locking device.

A first stator element26and a second stator element33are mounted to the stator holder30and surround the steering shaft10. The two stator elements26and33are axially facing and have fingers34and36(FIG. 2) which project radially inwardly. The fingers34and36are each carried by a respective magnetic flux ring,38and40, which surrounds the fingers34and36and guides the magnetic flux induced in each of the fingers34and36towards a magnetic field sensor42. This magnetic field sensor42is disposed between the two magnetic flux rings38and40and is carried by a sensor holder44.FIGS. 1 and 2clearly show that the magnetic flux ring38and also the magnetic flux ring40are each surrounded by a magnetic flux concentrator46and48. The two magnetic flux concentrators have a substantially C-shaped design and their free legs extend on both sides of the magnetic flux rings38and40. The two magnetic flux concentrators46and48are also borne by the sensor holder44and are held in their positions relative to the two magnetic flux rings38and40.

FIG. 1also shows a circuit board50for electronic components52, accommodated in the sensor holder44with plug contacts54being provided for cable connection. The sensor holder44is supported on the stator holder30via a sliding bearing56.

FIG. 2clearly shows magnetization of the magnetic ring24which consists e.g. of a plastic-bonded magnetic material which is injection-molded or compressed. The magnetic ring24is axially magnetized in a multipolar fashion, wherein the poles are disposed such that each pole pair is associated with a finger34or36of a stator element26or33, with the fingers34and36being located at the transition between one pole pair and another pole pair.

FIG. 3is a section III—III ofFIG. 2which clearly shows how the two magnetic flux concentrators46and48extend above the magnetic flux rings38and40which prevents changes of the air gaps58between the magnetic flux concentrators46and48and the magnetic flux rings38and40from affecting the measuring result. If one air gap58on the magnetic flux ring38or40is reduced by a wobble motion of the magnetic flux ring38or40, the length of the associated other air gap simultaneously increases, wherein the total magnetic flux collected by the magnetic flux concentrator46or48remains constant, irrespective of the associated motion of the second magnetic flux ring in the other magnetic flux concentrator.

FIG. 4shows a second embodiment with which the fingers34and36are bent in an axial direction, wherein the magnetic ring24of this embodiment is radially magnetized in a multipolar fashion. Each pole pair is again associated with one finger34or36and the fingers34and36are located in the transition region from one pole pair to the other pole pair. In this embodiment, the stator holder30can be injection molded to the holder ring32together with the stator elements26and33instead of being locked thereon.

FIGS. 5 and 6show the modular design of the inventive device, wherein each magnetic flux ring38or40is associated with two magnetic flux concentrators46and48which are opposite to each other relative to the longitudinal axis60of the steering shaft10. In this fashion, the system is given a certain redundancy and the signal can be averaged.

In the embodiment of the inventive device shown inFIG. 7, the magnetic flux rings38and40have fingers34and36which are bent radially inwardly and in the direction towards the magnetic ring24. This is advantageous in that the axial dimension of the magnetic ring24can be kept substantially smaller thereby still providing enough space between the magnetic flux concentrators46and48for receiving the magnetic field sensor. The reduced magnetic volume also reduces the overall weight of the device as well as the cost. The separation between the magnetic flux rings38and40which is still large, ensures that magnetic shunting remains small. The magnetic ring24may be of a sintered material, thereby increasing the field.

FIG. 8shows a further embodiment of the invention, wherein the magnetic field sensor42is mounted between shackles62which project downwardly from opposing sides of the magnetic flux concentrators46and48. The two shackles62extend approximately axially and are provided in one piece on the magnetic flux concentrators46and48. The free ends64of the shackles62are radially outwardly bent and each form one abutment surface with the magnetic field sensor42. This is substantially advantageous by providing a large region in which the magnetic field is homogeneous. Position tolerances of the magnetic field sensor42in a tangential and radial direction are negligible.

FIG. 9shows a perspective view of a magnetic flux concentrator48on which the free ends64of the shackles62are formed. The surface of the free ends64is approximately 3×3 mm2.

FIG. 4shows the stray flux between the fingers34and36of the two magnetic flux rings38and40in the direction towards the magnetic field sensor42and the shackles62. This stray field changes direction (in or out) along the periphery for each magnetic pole. Since the effective surface of the magnetic field sensor42is not exactly in the symmetry plane A but generally at least slightly axially offset from same, part of this stray field is detected by the magnetic field sensor42which modulates the signal with the number of magnetic poles.

FIG. 10shows a shielding device66in the form of a shielding plate68which is disposed between the magnetic field sensor42and the fingers34and36of the two magnetic flux rings38and40. The stray field acting on the magnetic field sensor42is thereby eliminated. The shielding plate68has a width which corresponds to approximately 50% of the separation between facing regions of the two magnetic flux concentrators46and48. The shielding plate68is also bent along its length which corresponds approximately to the angular segment over which the magnetic flux concentrators46and48extend.

The influence of the stray field is also reduced via the bent free ends64of the shackles62, since the magnetic field sensor42may thereby be placed radially further outwardly.