The invention relates to an apparatus for contact-less measuring the value of a difference angle between two parts rotating about a common axis.
It is possible to reduce the task of detecting rotational variables between two rotating parts, like, for example torque, angle of rotation, rotational speed or rotational acceleration, to a measurement of the difference angle between these two parts. If a torque is to be measured, e.g. between a power take-in shaft and a power take-off shaft, a counter torsion force with a defined spring characteristic has to be provided between the two parts for transforming the torque into a corresponding difference angle. This can be affected by a spring or torsion element acting between the two parts.
U.S. Pat. No. 4,784,002 discloses a torque sensor measuring a torsion between two shafts rotating about a common axis and connected to each other via a torsion element. A stator connected to one of the shafts comprises permanent magnets each forming one magnetic pole, the magnetic poles being arranged at equal intervals and with alternating axial magnetization in the direction of rotation about the common axis. A rotor connected to the other shaft has two separate rotor parts each forming a set of pole ends for magnetically scanning the permanent magnets of the stator. All pole ends are arranged at equal intervals in the direction of rotation about the common axis, each pole end of the one rotor part being arranged between two pole ends of the other rotor part. The total number of pole ends of the rotor is equal to the total number of magnetic poles of the stator. An axial first air gap is formed between the permanent magnets of the stator and the pole ends of the rotor. Further, a rotationally symmetric radial second air gap is formed between the two rotor parts, the magnetic flux across the second air gap being changed according to the difference angle between the rotor and the stator. This change is determined by a magnetically sensitive element which is arranged at a stationary position within the second air gap. There is no other magnetic flux from the rotor to the stator and back than the flux between the magnetic poles and the pole ends.
DE 198 16 598 A1 discloses a sensor for measuring difference angles between two parts rotating about a common axis. Here, both a rotor and a stator have a same number of pole ends. The pole ends of the rotor face the pole ends of the stator across a radial first air gap. A magnetic flux through the stator towards the rotor is provided by a radially magnetized permanent magnet. The magnetic flux through the rotor varies with the position of the pole ends of the rotor with regard to the pole ends of the stator, i.e. with the difference angle between the two parts. This variation is measured in a radially accessible and rotationally symmetric axial second air gap between the rotor and the stator by means of a magnetically sensitive element. However, the signal of the magnetically sensitive element does not indicate the direction of the difference angle here.
Thus, a simple but highly sensitive apparatus for contact-less measuring the value, inclusive of the direction, of a difference angle between two parts rotating about a common axis is still needed.
The invention provides an apparatus for contact-less measuring the value of a difference angle between two parts rotating about a common axis, the apparatus comprising an annular shaped stator attached to one of the two parts, the stator having a permanent magnetization on an inner circumference thereof forming of a number of at least two magnetic pole pairs, each pole pair consisting of two magnetic poles of opposite signs, and the signs of the magnetic poles of all pole pairs alternating in the direction of rotation about the common axis; a cylindrically shaped ferromagnetic rotor attached to the other of the two parts and positioned inside the stator, the rotor having a number of at least two pole ends for magnetically scanning the stator, the poles being successive in the direction of rotation about the common axis, and the number of the pole ends being the same as the number of the pole pairs of the stator; a first air gap between the pole pairs of the stator and the pole ends of the rotor, each pole end of the rotor facing one pole pair of the stator over the first air gap, all conditions of arrangement of all pole ends and the corresponding pole pairs being the same; an accessible and rotationally symmetric second air gap of continuous dimensions between the stator and the rotor, the magnetic flux across the second air gap being changed according to the difference angle between the rotor and the stator; and at least one magnetically sensitive element for determining the magnetic flux across the second air gap, the magnetically sensitive element being arranged at a fixed position in the second air gap and not being rotated with one of the two parts about the common axis.
In a preferred embodiment of the invention, the first air gap is a radial air gap, the permanent magnetization of the stator and the pole ends of the rotor having a radial orientation with regard to the common axis. Further, it is preferred that the second air gap also is a radial air gap. The second air gap may then be arranged at an axial position along the common axis of rotation differing from the position of the first air gap. The provision of only radial air gaps makes this embodiment of the invention particularly insensitive to axial play between the two parts in the direction of the common axis with regard to the signal of the magnetically sensitive element obtained for a certain difference angle.
In a further preferred embodiment of the invention, the second air gap is provided between a stator yoke and a rotor yoke.
In a further preferred embodiment of the invention, the stator consists of permanent magnets forming the pole pairs and a stator yoke, the stator yoke being continuous from the pole pairs up to the second air gap.
In a further preferred embodiment of the invention, the rotor consists of a rotor yoke, the rotor yoke forming the pole ends and being continuous up to the second air gap.
In a further preferred embodiment of the invention, the pole pairs of the stator are arranged at the same intervals in the direction of rotation about the common axis as the pole ends of the rotor, the magnetic poles of opposite signs of each pole pair being arranged directly side-by-side.
In a further preferred embodiment of the invention, the magnetic poles of the pole pairs of the stator have the same dimensions in the direction of rotation about the common axis as the pole ends of the rotor.
In a further preferred embodiment of the invention, the magnetically sensitive element transforms the magnetic flux across the second air gap into an electrical signal. To this end, the magnetically sensitive element comprises a Hall probe. In a further preferred embodiment of the invention for measuring torque between the two parts, a spring having a defined spring characteristic over the difference angle is arranged between the two parts, the spring transforming a certain torque between the two parts into a certain difference angle. The spring may be a mechanical spring. As an alternative, the spring may be a magnetic spring. In this alternative, the magnetic spring may make use of an intrinsic magnetic holding force between the pole ends of the rotor and the pole pairs of the stator.
In a further preferred embodiment of the invention, the stator and the rotor are encapsulated by a housing made of a material which is selected from the group consisting of paramagnetic and diamagnetic materials. This housing may not encapsulate the magnetically sensitive element. Instead, this element may be placed outside the housing.
In a further preferred embodiment of the invention, at least one further magnetically sensitive element is arranged at a fixed position adjacent the rotating pole pairs of the stator but not rotating with one of the two parts for determining the absolute rotational position of the stator by scanning the permanent magnetization of its pole pairs.
The main components of the present invention are the stator with the permanent radial magnetization, like for example a magnetized ferrite ring or a ferromagnetic ring to which permanent magnets are attached, and the ferromagnetic rotor, for example made of iron, forming the radially orientated pole ends for magnetically scanning the stator. Between the stator and the rotor there is the axially accessible, rotationally symmetric second air gap in which the magnetic flux changes according to the difference angle between the rotor and the stator. This change is detected by means of the magnetically sensitive element. At the stator there are pole pairs of magnetic poles of opposite sign, the signs of the magnetic poles of the pole pairs alternating in the direction of the difference angle between the rotor and the stator. Each pole end of the rotor is arranged underneath a magnetic pole pair of the stator in a same way. In a starting position, each pole end preferably is in a neutral position in the middle underneath the bordering between the two magnetic poles of opposite signs of one pole pair. A rotation of the rotor with regard to the stator results in the same change of the magnetic flux at all pole ends. This change also influences the flux at each point of the remaining magnetic circuit through the rotor and back to the stator. The rotor and the stator form the additional rotationally symmetric second radial air gap across which this flux is guided. Thus, the magnetic flux across the second air gap corresponds to a certain difference angle between the rotor and the stator. Because of the rotational symmetry of the gap this magnetic flux is, however, independent of the rotational position or motion of the whole arrangement about the common axis. In contrast to an electrical machine in which the total of the pole flux should have an average of zero, a total flux dependent on the difference angle between the stator and the rotor and running from the rotor yoke, across the second air gap and back to the stator yoke is enhanced and used here.
By a variation of the number of poles, the desired output signal for a given difference angle can be selected within a big range; also, by changing the form of the pole ends the curve of the output-signal of the magnetically sensitive element over the difference angle can be modified. Because of the compact and efficient magnetic circuit of the present invention a small number of pole pairs and pole ends is sufficient to achieve a high sensitivity with regard to the difference angle. As both the rotor and the stator may be freely rotated, there is a very big range of applications of the present invention. In case of large differences in the rotational speed of the rotor and the stator, however, eddy current losses are to be considered. If necessary, the rotor may be laminated.
The magnetically sensitive element arranged in the second air gap preferably enables a transformation of the magnetic flux into an electrical signal which is often necessary for an automated evaluation. The magnetically sensitive element, e.g. a Hall sensor or a field plate, is stationarily arranged in such a way that the active area of this element is within the second air gap. In case of a sufficient size of the second air gap the measurement signal shows nearly no dependency on small axial and radial tolerances and vibrations between the two parts. Because of the high sensitivity of the apparatus according to the invention, large signal amplitudes can be obtained even in case of small difference angles. These large amplitudes ensure a simple evaluation without the need of signal stabilization, signal offset correction, compensations, or the like.
If a mechanical or magnetic spring between the stator and the rotor transfers torques into rotational angles, the new arrangement enables the measurement of torques by means of measuring the difference angle between the two rotating parts. As a magnetic spring, the magnetic holding momentum (locking momentum) of the stator/rotor arrangement itself may be sufficient. In any case the spring characteristic of the forces between the stator and the rotor determine the ratio between the difference angle and the torque. An overload of a mechanical spring can be noted by too high amplitudes of the magnetic flux in the second air gap; over winding of a magnetic spring can be determined by the occurrence of an alternating flux in the second air gap. In the embodiments of the invention having no mechanical spring the functions of a reversible overload safety and of a torque determination are combined in an optimum way.
In a further advantageous embodiment of the invention the space between the rotor and the stator except for the air gaps is filled with an elastomeric material so that a closed, very compact vibration dampening unit is created which is suitable for a direct measurement of torque.
By means of para- or diamagnetically enclosing the arrangement of the stator and the rotor, the invention can also be used under difficult environmental conditions. For example, an enclosure can be provided by means of a housing in which lubricants are present. The housing can be designed in such a way that the second air gap is accessible for the magnetically sensitive element sensors through the wall of the housing. For example, if the housing is made of aluminum or plastics an opening in the housing is not necessary for the required access to the second air gap.
Further, the new arrangement provides the advantageous possibility of using the stator for an additional measurement of absolute rotational variables. The absolute rotational variables may be directly detected by a sensor sensing the rotating magnetic field of the rotating stator. Thus, for example, a precise determination of both the absolute rotational angle and the power-take-off torque, for example, are possible in a simple way.
The terms rotor and stator are only used for the reason of definition here. They are not intended to be limiting as such. Generally, the details of the rotor and the stator are free as in electrical machines.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.