GB1573330A

GB1573330A - Oscillating system having a linear displacement versus time characteristic

Info

Publication number: GB1573330A
Application number: GB3402177A
Authority: GB
Original assignee: Centre National dEtudes Spatiales CNES
Current assignee: Centre National dEtudes Spatiales CNES
Priority date: 1976-08-19
Filing date: 1977-08-12
Publication date: 1980-08-20
Also published as: DE2737359C2; DE2737359A1

Description

(54) AN OSCILLATING SYSTEM HAVING A LINEAR DISPLACEMENT VERSUS TIME CHARACTERISTIC (71) We, CENTRE NATIONAL D'E TUDES SPATIALES, a corporate body administered by the State, of 129, Rue de l'Universite, 75 Paris 7, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to oscillating systems, more particularly to oscillating scanning systems, for example oscillating mirrors for optical scanning.

In oscillating systems of this kind, it is often required to have a linear motion, that is to say a condition such that the oscillatory movement plotted against the time of oscillation resembles a sawtooth curve, a curve of this kind corresponding to an oscillation occuring at a constant speed between the two points at each end of the oscillatory travel or movement.

As cases where linear motion of this kind may be necessary, there may be mentioned the mirror type optical scanners used on board observation satellites, where a mirror is adapted to oscillate about one of its axes and the angular variation of mirror position about its oscillation axis must be linear against time for the ground trace of the image of a detector operatively associated with the mirror to be related to a straight line. A facility of this kind considerably facilitates the processing of information received by the system.

Basically, this invention proposes a magnetic device for linearizing the movement of an oscillatory system of the kind hereinbefore referred to.

Accordingly the present invention provides a device for producing an oscillating movement of linear displacement versus time characteristic, comprising a frame, a movable member which is free to oscillate relative to the frame, a magnetic structure and a magnetic system each mounted on a respective one of said frame and said movable member and forming a magnetic circuit whose reluctance remains constant substantially throughout the range of oscillating movement of the movable member and increases abruptly at each end of such movement.

The increase in reluctance at both ends of the movement or travel is responsible for the provision of a return or restoring force which tends to restore the oscillatory system to its central position. In other words, the magnetic forces which the device produces at both ends of the oscillatory movement have a contactless end stop effect with resilient restoration. The moving system experiences no force through most of its travel and experiences restoring forces at the two end points. The resulting pattern of movement is a constant-velocitydisplacement between two end points, i.e.

linearization of the oscillation.

The invention also relates to oscillatory systems comprising one or more such magnetic oscillation linearization devices which are associated with means for maintaining the oscillation. The oscillatory system for which the invention is more particularly of use is, as previously stated, the oscillating mirrors of optical scanners.

According to a further aspect the invention provides a device for producing an oscillating movement of linear displacement versus time characteristic, comprising a stator member and a rotor member mounted for pivotal movement relative to the stator, the rotor comprising a lightweight rim having a plurality of ferromagnetic strips secured thereto at equal pitches therearound, and said stator comprising two magnetic material armatures extending around the axis of the rotor member and each formed with a plurality of magnetic poles disposed opposite respective poles of the other arma ture and spaced around the armature at pitches equal to the rotor strip pitches, the rotor strips and the armature poles forming a magnetic circuit the reluctance of which remains substantially constant throughout the range of oscillating movement of the rotor member and increases abruptly at each end of such movement.

Preferably, the strips of the rotor are bevelled at their edges so as to concentrate the magnetic flux which passes through them when they are opposite a pole of the stator and thus increase the restoring force applied to them when they reach the end of the pole.

According to a further aspect the invention provides a device for producing an oscillating movement of linear displacement versus time characteristic, comprising a stator member and a rotor member mounted for pivotal movement relative to the stator, the stator member comprising two magnetic armatures extending around the axis of the rotor member, the armatures each having a plurality of poles in the form of strips extending towards the rotor at opposed points around the stator, and the rotor member comprising a plurality of magnetic members disposed around the periphery therof to form with the stator poles a magnetic circuit the reluctance of which remains substantially constant throughout the range of oscillating movement of the rotor and increases abruptly at each end of such movement, and a plurality of magnets interposed between said magnetic members, and wherein electrical windings are provided around at least some of the stator armature poles to produce an electromagnetic force acting in an axial direction on the magnetic members.

These features of the invention lead to a rotor which has a low inertia and therefore may have a relatively high operating frequency.

In order to promote a fuller understanding of the invention an embodiment will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of the device according to the invention associated with an oscillating system; Figure 2 is a diagrammatic side elevation of the magnetic device of Figure 1; Figure 3 is a section on the line III-III of Figure 2; Figures 4a, 4b and 4c are detail views of Figure 3 showing the magnetic lines of force for three relative positions of the magnetic system and of the structure; Figures 5a, 5b, and 5c show, plotted against displacement of the magnetic system relative variations of the reluctance of the magnetic circuit embodied by the magnetic system and the magnetic structure, of the force applied to such system and of the pattern of oscillation; Figure 6 is a developed diagrammatic view of the rotor and stator of an embodiment of the oscillatory device according to the invention, and Figure 7 is a partial perspective view of another embodiment.

Figure 1 is a very diagrammatic view of an oscillating system in the form of an arm B oscillating around a pivot axis A-A.

The oscillatory system diagrammatically represented by the arm B has associated with it a magnetic system 100, and a magnetic structure 200 is associated with a fixed frame of the complete system.

The kinetic functions of the integers 100 and 200 can of course be reversed, subject only to them being movable relatively to one another in a predetermined guided relative movement (accurate guiding is necessary to ensure that the air gaps of the magnetic circuit remain constant throughout the central portion of the oscillatory movement).

In the example shown in Figures 1-3, the structure 200 is embodied by two ferromagnetic armatures 201, 202 in the form of concentric arcuate portions centred on the axis A-A. Disposed between the armatures, and separated from each of them by a very small air gap, the system 100 has two assemblies 110, 120 each having two thin pole pieces of soft iron 111, 112 and 121, 122 respectively, having enlarged bases 113, 123 and disposed on either side of a central body comprising a high-coercivity magnet 114, 124 made, for instance, of ferrite or samarian cobalt.

The assemblies 110, 120 are substantially coplanar and their common plane contains the pivoting axis A-A so that the air gap between the pole pieces and the stationary armatures remains constant.

As will be readily apparent from Figures 1 and 2, both of the magnets 114, 124 are disposed so that their lines of force are perpendicular to the armatures 201, 202 but extending in opposite senses.

Consequently, the two assemblies 110, 120 and the two armatures 201, 202 together form a substantially rectangular magnetic circuit, as can be gathered from the arrows.

Since the small air gap between the pole pieces and the armatures remains constant when the system 100 moves relatively to the armatures 201, 202, the reluctance of the circuit does not vary when the system 100 oscillates within the scope of the armatures 201 and 202.

However, at the end of travel, corresponding to the chain-line positions of the system 100 in Figure 3, the circuit reluctance tends to increase abruptly when the system 100 moves out from the scope of the armatures. Figures 4a, 4b and 4c are diagrammatic view of the lines of force of the magnetic circuit for three positions of the system, namely the left-hand end position, the central position and the right-hand end position. Clearly, in the end positions the lines of force change and become asymmetrical about the moving element. Consequently, a restoring force Fr occurs at both ends of the oscillation and has a value proportional to the derivative of the reluctance of the circuit.

The phenomenon is shown graphically in Figures Sa, 5b and Sc, in relation to the oscillatory movement between two end points E which are symmetrical of a centre position 0. Figure 5a shows the variation of reluctance and Figure 5b shows the variation of restoring force.

Figure Sc shows relative variations of the position of the system in time. Clearly, the oscillation is substantially linear substantially throughout the whole of the travel.

The embodiment just described does not of course limit the invention, which may be embodied in any system having a structure and a moving system which produces a magnetic circuit whose reluctance remains constant substantially throughout the oscillation and increases abruptly at end positions.

More particularly, the armatures can be plane parallel armatures perpendicular to the pivot axis A-A of the arm which carries the moving system.

Also, of course, the oscillation can occur along a circular path or along a linear path, subject only to the moving system and the armature being properly guided in their movements relatively to one another.

Guidance of this kind can be provided by any known kind of low friction bearing; and advantageously, magnetic bearings can be used. The oscillatory system shown in-Figure 1 is purely schematic.

In actual practice, a number of devices according to the invention can be arranged around a circumference whose axis is the axis of oscillation; of course, the various devices are arranged around the circumference at regular intervals and so adapted that their end and effects are in phase with each other.

The oscillations of the system can be maintained by any known means; preferably, however, they are maintained by giving one energy pulse per cycle to the system at a point outside the linearized zone of the movement, as a rule for the 'stroke' or 'out' movement of a scanning system.

The invention can be used with particular advantage for the oscillatory mounting of a scanning system, for example for the mounting of an oscillating mirror, as previously stated.

Other improved embodiments, such as those shown in Figures 6 and 7, are possible.

Referring to Figure 6, a rotor 100 is disposed between armatures 120 and 140 of a stator, the armatures being disposed opposite one another. Rotor 100 forms a moving system carrying e.g. a mirror (not shown) which is required to perform a very linear oscillatory motion, the rotor reciprocating in the direction of arrows 160, 180 visible in Figure 6 which is a developed view. The rotor 100 is formed by a number of evenly spaced ferromagnetic strips 200. Preferably, the same extend in radial planes of the device. They are fitted to a light metal rim 220, e.g. of beryllium which is rigid enough to keep all the strips 200 properly centred between the stator armatures 120 and 140. The rotor assembly is mounted in a bearing (not shown). The strips 200 are preferably secured by screws, the rim 220 accordingly having locating and securing brackets 240 at various places.

Preferably, the ferromagnetic strips 200 are made of a low-saturation substance such as an iron cobalt alloy, e.g. AFK 502.

The resulting rotor has a low inertia and can oscillate linearly at a high frequency, more particularly appreciably higher than has been possible with magnet-carrying rotors of the past. The rotor strips 200 can be bevelled at their ends adjacent the stator armature poles so as to concentrate the flux produced between the stator and the strip ends, for this step increases the flux density at the strip ends, with a consequent increase in the restoring force (which is proportional to the flux density) tending to restore the strips to a position between the ends of the stator poles when the strips tend to move away therefrom.

The first stator armature 120 comprises a number of magnets 260 arranged at the same periodicity as the rotor strips.

Associated with each magnet 260 is a ferromagnetic pole piece 280 which has a plane surface 300 with clear end edges adjacent the path of the rotor strip ends.

A flux return plate 320 is disposed on the other side of the magnets 260 from the pole pieces 280. Preferably, the pole pieces 300 are of laminated construction, i.e. they consist of metal laminations to reduce hysteresis losses, the laminations being disposed in a disc 340 made of aluminium or of some other non-magnetic light metal, the surface of the laminations being machined with the disc after assembly.

The armature 140 can be of identical construction, i.e. it can also comprise magnets and pole pieces. It can also, as shown in Figure 6, be devised more simply and just have ferromagnetic poles 360 opposite the poles 280 of the armature 120.

The armature 140 can take the form of a metal strip spirally wound about the axis of the device (forming a laminated armature), the strip being formed with recesses between the projecting portions which form the poles 360. If required, the pole pieces can be made of a saturable magnetic material, such as an iron-nickel alloy or of F metal, to reduce the stiffness of unstable restoring force components acting in the direction of the arrows 170 and 190.

It is possible for there to be an additional air gap (i.e. additional to the air gaps between the strips 200 and the poles 280, 360), the additional air gap being in the middle of the rotor strips 200, in which event the same are embodied in two separate parts. The advantage of an additional air gap is of further reducing unstable stiffness in the directions indicated by the arrows 170, 190.

The rotor is centred very accurately between the stator armatures, with a low air gap, e.g. of the order of 0.3 mm.

If required, the rotor can be suspended magnetically by the provision of windings for the stator and magnets for the rotor, to produce a vertical force tending to lift the rotor; the magnets can be distributed regularly near the rotor strips (the magnets are so disposed as to produce a field in the tangential direction) and the windings extend around the stator pole pieces.

An arrangement of this kind is shown in Figure 7, where there can be seen a part of a construction of a further embodiment of the invention, pole pieces 380 being provided not on the rotor but on the stator, on the two armatures 120, 140. Windings 400 extend around the pole pieces 380. The rotor comprises magnets 420 which produce a tangetial field and which are arranged alternately in opposite senses. Disposed between the magnets 420 are ferromagnetic members 440 at the same spacing as the stator pole pieces. The path for the flux of the magnets is closed by the members 440 and the pole pieces 380, a small air gap remaining between the pole pieces 380 and the weights 440. The windings 400 are electrically energized so as to produce a lift of the rotor system, having regard to the direction of the flux produced by the magnets.

To produce this lift, oppositely disposed windings (on each armature) are energized by currents of the same angular direction to produce magnetic fields of the same sense, while the adjacent windings of a given armature are supplied with currents of opposite angular senses.

If required, just some of the stator strips can have magnetic bearing windings.

Also, the winding currents can be adjustable so as to have a variable force for one or more stator strips.

Another possibility in this embodiment is for proximity detectors to be placed at one or more places on the stator to detect rotor position relatively to a theroretical centred position between the stator armatures, the detectors being connected to variablecurrent supply systems to produce attraction forces tending to restore the rotor to its theoretical position.

The invention is not of course limited to the embodiments hereinbefore described but covers all devices which fall within the scope of the appended claims.

WHAT WE CLAIM IS: 1. A device for producing an oscillating movement of linear displacement versus time characteristic, comprising a frame, a movable member which is free to oscillate relative to the frame, a magnetic structure and a magnetic system each mounted on a respective one of said frame and said movable member and forming a magnetic circuit whose reluctance remains constant substantially throughout the range of oscillating movement of the movable member and increases abruptly at each end of such movement.

2. A device according to Claim 1, in which said magnetic structure and said magnetic system are guided accurately relative to one another over their relative movement.

3. A device according to Claim 1 or 2, in which said movable member is mounted for pivotal movement on said frame.

4. A device according to Claim 1, 2 or 3 in which said magnetic structure comprises at least one ferromagnetic armature, and the magnetic system comprises at least one ferromagnetic pole piece which is disposed perpendicularly to the armature and which can move with a constant air gap relative thereto and at least one magnet associated with the armature to form said magnetic circuit.

5. A device according to Claim 1, 2 or 3 in which the magnetic structure comprises two parallel armatures and the magnetic system comprises two coplanar pole piece assemblies disposed perpendicular to the armatures, each pole piece assembly having a central magnet producing a magnetic field in a direction substantially perpendicular to the armatures, the fields of the two pole piece assemblies being disposed in opposite directions so as to produce a rectangular magnetic circuit extending perpendicularly to the armatures and to the direction of oscillatory movement.

6. A device according to Claim 5, characterised in that the armatures are plane.

7. A device according to Claim 5, characterised in that the armatures are arcuate and concentric.

8. A device according to any preceding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The armature 140 can take the form of a metal strip spirally wound about the axis of the device (forming a laminated armature), the strip being formed with recesses between the projecting portions which form the poles 360. If required, the pole pieces can be made of a saturable magnetic material, such as an iron-nickel alloy or of F metal, to reduce the stiffness of unstable restoring force components acting in the direction of the arrows 170 and 190. It is possible for there to be an additional air gap (i.e. additional to the air gaps between the strips 200 and the poles 280, 360), the additional air gap being in the middle of the rotor strips 200, in which event the same are embodied in two separate parts. The advantage of an additional air gap is of further reducing unstable stiffness in the directions indicated by the arrows 170, 190. The rotor is centred very accurately between the stator armatures, with a low air gap, e.g. of the order of 0.3 mm. If required, the rotor can be suspended magnetically by the provision of windings for the stator and magnets for the rotor, to produce a vertical force tending to lift the rotor; the magnets can be distributed regularly near the rotor strips (the magnets are so disposed as to produce a field in the tangential direction) and the windings extend around the stator pole pieces. An arrangement of this kind is shown in Figure 7, where there can be seen a part of a construction of a further embodiment of the invention, pole pieces 380 being provided not on the rotor but on the stator, on the two armatures 120, 140. Windings 400 extend around the pole pieces 380. The rotor comprises magnets 420 which produce a tangetial field and which are arranged alternately in opposite senses. Disposed between the magnets 420 are ferromagnetic members 440 at the same spacing as the stator pole pieces. The path for the flux of the magnets is closed by the members 440 and the pole pieces 380, a small air gap remaining between the pole pieces 380 and the weights 440. The windings 400 are electrically energized so as to produce a lift of the rotor system, having regard to the direction of the flux produced by the magnets. To produce this lift, oppositely disposed windings (on each armature) are energized by currents of the same angular direction to produce magnetic fields of the same sense, while the adjacent windings of a given armature are supplied with currents of opposite angular senses. If required, just some of the stator strips can have magnetic bearing windings. Also, the winding currents can be adjustable so as to have a variable force for one or more stator strips. Another possibility in this embodiment is for proximity detectors to be placed at one or more places on the stator to detect rotor position relatively to a theroretical centred position between the stator armatures, the detectors being connected to variablecurrent supply systems to produce attraction forces tending to restore the rotor to its theoretical position. The invention is not of course limited to the embodiments hereinbefore described but covers all devices which fall within the scope of the appended claims. WHAT WE CLAIM IS:

1. A device for producing an oscillating movement of linear displacement versus time characteristic, comprising a frame, a movable member which is free to oscillate relative to the frame, a magnetic structure and a magnetic system each mounted on a respective one of said frame and said movable member and forming a magnetic circuit whose reluctance remains constant substantially throughout the range of oscillating movement of the movable member and increases abruptly at each end of such movement.

8. A device according to any preceding

Claim, in which said movable member comprises an optical scanning mirror.

9. A device according to any preceding Claim, comprising exciting means to maintain oscillation of the device, such means transmitting one energy pulse per cycle to said movable member.

10. A device according to Claim 9, in which said exciting means is arranged to apply said energy pulse to the movable member at a point in the cycle outside the range where the reluctance of said magnetic circuit is constant.

11. A device according to any one of Claims 1 to 10, comprising a plurality of said magnetic structures with respective magnetic systems disposed around a circumference extending around the axis of oscillation of the oscillating system.

12. A device for producing an oscillating movement of linear displacement versus time characteristic, comprising a stator member and a rotor member mounted for pivotal movement relative to the stator, the rotor comprising a lightweight rim having a plurality of ferromagnetic strips secured thereto at equal pitches therearound, and said stator comprising two magnetic material armatures extending around the axis of the rotor member and each formed with a plurality of magnetic poles disposed opposite respective poles of the other armature and spaced around the armature at pitches equal to the rotor strip pitches, the rotor strips and the armature poles forming a magnetic circuit the reluctance of which remains substantially constant throughout the range of oscillating movement of the rotor member and increases abruptly at each end of such movement.

13. A device according to Claim 12, in which the ferromagnetic rotor strips extend in radial planes.

14. A device according to Claim 12 or 13, in which the rim is made of beryllium.

15. A device according to Claim 12, 13 or 14 in which the rotor strips are made of a low-saturation ferromagnetic material.

16. A device according to any one of Claims 12 to 15, in which the rotor strips are bevelled at their ends opposite the poles of the stator armatures.

17. A device according to any one of Claims 12 to 16, in which the rotor strips are formed in two parts with an air gap therebetween.

18. A device according to any one of Claims 12 to 17, in which one of the stator armatures comprises a spirally wound metal strip formed with recesses between projecting parts which form the magnetic poles.

19. A device for producing an oscillating movement of linear displacement versus time characteristic, comprising a stator member and a rotor member mounted for pivotal movement relative to the stator, the stator member comprising two magnetic armatures extending around the axis of the rotor member, the armatures each having a plurality of poles in the form of strips extending towards the rotor at opposed points around the stator, and the rotor member comprising a plurality of magnetic members disposed around the periphery thereof to form with the stator poles a magnetic circuit the reluctance of which remains substantially constant throughout the range of oscillating movement of the rotor and increases abruptly at each end of such movement, and a plurality of magnets interposed between said magnetic members, and wherein electrical windings are provided around at least some of the stator armature poles to produce an electromagnetic force acting in an axial direction on the magnetic members.

20. A device according to Claim 19, in which the rotor magnets are disposed such that their field acts tangentially to the rotor and alternately in opposite senses around the rotor, the stator windings which are opposite one another are connected for current flow of the same angular direction to produce magnetic fields of the same direction, and the adjacent windings around the stator are connected for current flow of opposite angular direction.

21. A device for producing an oscillating movement of linear displacement versus time characteristic substantially as herein described with reference to the accompanying drawings.