Patent Application
20080111430
The present invention relates to an iron core linear motor with a stator and a runner displaceably arranged along a running axis of the stator, in accordance with the introductory clause of the independent patent claim.
In modern automation technology, brushless linear motors are no longer unthinkable as actuators. They reach high speeds, and nevertheless reach a high precision into the nanometer range. For many applications of such linear motors, for example in linear axes or cross-tables, in addition as small a volume as possible of the linear motors is desired with, at the same time, as high an output as possible.
Linear motors are electric motors and consist of an immovable stator and a runner which is displaceable with respect to the stator along a running axis, said runner basically corresponding to the movable rotor in a rotating electric motor. The runner can be displaceably mounted with respect to the stator for example by a track/roller system. A construction which is specifically suitable for linear motors with compact dimensions, consisting of a monoblock and milled-in guide grooves, is disclosed in EP 1732197.
Linear motors can be produced in various types of construction, analogous to the rotating electric motors. In a variant which is especially suited for a high power density, several permanent magnets are arranged on the runner with an alternating pole alignment along the running axis, and several electromagnet coils are arranged on the stator, likewise along the running axis.
The coils are controlled so that a travelling magnetic field is produced, in which a force acts on the runner which moves the latter forward. The controlling of the coil takes place in modern, brushless linear motors electronically by means of field vector regulation and position measurement systems.
Alternatively in such a linear motor, the electromagnets can also be mounted on the runner, and the permanent magnets on the stator. This type of construction is more favourably priced in manufacture for track-shaped stators which are substantially longer than the runner. However, it requires a cable drag to the runner, whilst in the first variant the runner is freely movable.
In order to achieve higher propulsion outputs with, at the same time, a small motor volume, the electromagnets can be realized with iron core coils instead of with air core coils. In order to achieve outputs which are as high as possible, the air gap between iron poles and permanent magnets is, in addition, kept as small as possible. Furthermore, iron poles have the advantage that heat which occurs can be led off better, which likewise allows greater outputs.
However, the use of iron core coils has the considerable disadvantage, compared with air gaps, that a detent force occurs, the so-called “cogging”, a phenomenon which is also known from rotating electric motors. Strong static, therefore current-independent attractive forces (detent forces), which require a robust design of the mechanical guidance of the runner, occur between the iron set, in particular the iron poles of the stator and the permanent magnets of the runner. With the movement of the runner with respect to the stator, a modulated, alternately positive and negative detent force results, which is superimposed on the constant propulsive force of the linear motor. When the linear motor travels at a constant speed, this modulated detent force must be continuously controlled, because otherwise an undulation of the speed pattern results. A further disadvantage of the detent force is the adulterated relationship between current consumption of the motor and effective propulsive force; this is not linear and therefore the motor can not be used for precise force- or force/path measurement.
In addition to the actual detent force in the iron core linear motor is also an inlet and outlet effect, due to the final length of the runner and/or of the stator. When the runner travels into the region of one of the two stator ends, the magnetic system is unsymmetrical and the detent force increases in addition. For a compact type of construction, however, as far as possible the maximum path of travel of the system should be able to be used. Therefore, this inlet/outlet effect is advantageously reduced.
A known solution for the reduction of the detent force in rotating electric motors and linear motors is the inclination of the permanent magnets with respect to the iron poles or respectively the inclination of the iron poles with respect to the permanent magnets or alternatively the use of specially shaped permanent magnets.
This leads to the superimposing of attracting and repelling forces, and therefore to a reduction of the detent force. As an intensive degree of inclination (up to 20°) is necessary, the resulting propulsive force, however, automatically also falls thereby.
A similar solution is also shown by U.S. Pat. No. 5,214,323, in which a partial neutralizing of the detent force is achieved by the combination of two stators and two runners which are coupled with each other, in which the stators or the runners are offset with respect to each other by half a pole clearance. However, it is difficult to achieve a compact type of construction of the linear motor with this development.
US 2004/0256918 A1 shows a linear motor with a runner with electromagnets and with a track-shaped stator with permanent magnets, in which an additional coil, mounted at the end of the runner, is controlled so that it produces a counteracting force in opposition with respect to the detent force, and thus neutralizes the detent force. This solution is only suitable for runners which move on a long stator. The additional active element is, in addition, complex and therefore expensive.
US 2005/0236912 A1 and U.S. Pat. No. 4,912,746 use specially shaped blind pole blocks, i.e. iron poles without coils, at the two ends of the coil set. The reduction of the propulsive force is less, but the coil set becomes longer and heavier. The static attracting force between stator and runner additionally becomes greater, which makes a more robust design of the mechanical guidance necessary.
EP 1511164 A2 shows a linear motor with a runner equipped with electromagnets, in which a blind iron pole is arranged at the two ends of the runner. The said iron pole in the end position is intended to reduce the detent force, and has a curved surface, and also has bores over the entire width of the blind iron pole, in which magnets are arranged. Such a type of construction is complex and, accordingly, expensive.
US 2005/0134122 A1 shows a linear motor in which, in accordance with the nonius principle, the distance between the individual iron poles is slightly less than the distance between the permanent magnets. This leads to destructive interference of the detent forces, and hence to a reduction of the detent forces. However, the detent force problem is not solved thereby in the inlet/outlet region.
The use of T-shaped, ribbed iron poles is likewise known from the prior art in order to achieve a more homogeneous static attraction force between permanent magnets and iron poles. This type of construction has the disadvantage that the coils can not be simply be turned over the iron poles, but rather must be wound around them in a complicated manner. The overall height is likewise higher.
It is an object of the present invention to provide an iron core linear motor of the type initially mentioned, in which the above-mentioned problems are largely avoided, and which has in particular a low detent force even in the inlet/outlet region, allows a compact type of construction and can be produced at a favourable cost.
This and other problems are solved by a linear motor of the type initially mentioned in accordance with the independent Claim 1. Further advantageous embodiments of a linear motor according to the invention will be apparent from the dependent claims.
In an iron core linear motor according to the invention, the intrusive detent force and the in/outlet effect are reduced on the one hand by the height of the iron poles being selected differently in the two outer regions of the coil set, preferably by a gradual reduction of the height of the iron poles towards the ends of the stator, and on the other hand by a slight inclination (by a few degrees) of the permanent magnets, respectively iron poles. The coils themselves are preferably identical. The exact dimensioning of the iron poles is dependent, inter alia, on the dimensioning of the coils and other characteristics of the entire linear motor, and can be found and optimized by three-dimensional, finite element method (FEM) simulations. Here, an optimum is sought between as great a reduction of the detent force amplitude as possible and as small a reduction as possible of the motor power.
The different height of the iron poles in combination with the slight inclination of the magnets leads to the repelling and attracting static forces between the individual permanent magnets and the iron poles for the most part reciprocally overlying each other so that in the translatory direction the effectively resulting detent force only shows a low amplitude compared with a linear motor with identical pole heights.
An iron core linear motor according to the invention has substantially the same manufacturing costs as a conventional iron core linear motor without measures against the detent force, because no costly active devices are necessary and the manufacture of the stator does not become more complicated.
The mode of operation of a linear motor according to the invention is to be further explained below with reference to the attached drawings.
A possible form of construction of the iron set 52 of a stator of a linear motor according to the invention is illustrated diagrammatically in
The ideal height of the individual iron poles 522 for given overall dimensions of a linear motor can be determined by corresponding FEM simulations, in which the heights of the iron poles are varied until the amplitude of the detent force modulation is minimal. An iron set with a depth of 20 mm, a length in the running direction of 52 mm, a height of the iron core of 3.2 mm and a maximum height of 10.5 mm has six iron poles. All the iron poles have a length of 4 mm in the running direction and are separated by 4 mm-wide grooves. The height of the six coils which are turned over the iron poles is respectively 7.1 mm. The optimized heights of the individual iron poles were calculated by 3D FEM simulation for a stator which is dimensioned in such a way, in which the amplitude of the transversal component of the detent force is minimal in the running direction, with the two innermost iron poles with a fixed height over the iron core of 7.3 mm not having been varied. An optimum was found for a height of 6.70 for the two second-outermost iron poles and a height of 5.90 mm for the two outermost iron poles. The slight inclination of the permanent magnets with respect to the vertical to the running axis is between 5 and 8 degrees, preferably between 6 and 7 degrees, and particularly preferably 6.8 degrees.
Curve A shows a measurement with an iron set according to the example embodiment, whilst Curve B shows a measurement with an iron set with identical iron poles.
The amplitude of the modulated detent force of the linear motor according to the invention is approximately 1 N, whereas it is approximately 6 N without a pole height correction.
The amplitude of the detent force can be still further reduced by a more intensive inclination of the permanent magnets. Here, however, the propulsive force is also reduced. The design can be optimized to the desired characteristics by means of 3D FEM simulation.
The variant shown of a stator 2 for a linear motor according to the invention is longer than the stator of
The first-mentioned example embodiment for optimized dimensions of a linear motor according to the invention can be adapted to any given or desired geometry of a linear motor by a specialist in the art, without greater effort, by the application of known simulation methods, particularly FEM. In particular, it is also possible to also generalize the principle of the invention to other types of iron core linear motors.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01779/06 | Nov 2006 | CH | national |
