Actuator

Abstract

Elimination of unwanted motion of limited rotation actuators of the moving iron, permanent magnet type, is described. In this type, biasing flux from a permanent magnet and control flux from a coil are applied to a permeable armature along different paths through split pole pieces. Damping of unwanted residual oscillation is achieved by an independent short-circuited coil surrounding a permeable member in the control flux path. Other erratic movement is found to be due to bearing tolerances and is avoided by intentional misalignment or biasing of the armature shaft relative to its bearings, this also improving bearing life. Telescoped construction of a torsion bar within a tube which mounts armature laminations between bearings is shown. Compensation for improper relation of movement vs. current is achieved by a distinctly different shaping of the curvature of portions of the surfaces relative to other cylindrical portions of the surfaces defining the gaps between armature and pole pieces, obtained for large size armatures by offcenter machining. In various combinations these features achieve improved sets of characteristics relating to overall size and the amount of delivered torque, deflection, damping, linearity and heating obtained. Electro-optical scanners and choppers having resonant frequencies in the 1,000 Hz. range are described.

Description

This application relates to limited rotation electroactuator devices and electro-optical devices.

In devices of the type here concerned a high permeability armature is mounted between two poles, at least one of which is split into two spaced pole pieces. A permanent magnet provides biasing flux through the armature, between pairs of pole pieces while a control coil provides control flux through the armature along different paths, between different pairings of the pole pieces.

Objects of the invention are to provide improved operating characteristics and life and simplified procedures for manufacture of such devices. Among the more specific objects are to remove the causes of unwanted motion of the armature, to compensate such actuators in a smooth manner over an extended range of armature deflection (e.g. to achieve a desired relation, e.g. linearity, between current and position or movement), to damp residual armature oscillation, and to reduce other sources of undesired motion. Other objects are to enable extended ranges of frequency and deflections to be achieved, including frequencies in the range of 500 - 1500 Hz. and deflections in the range up to 40.degree. included angle. Other objects are to drive mirrors and other loads of relatively large moments of inertia and to provide improved electro-optical devices such as scanners and choppers.

The invention features the various aspects mentioned in the abstract, to which reference is made.

These and other objects, features and advantages will be understood from the following description in connection with the drawings wherein:

FIG. 1 is a schematic view of an optical system employing devices according to the invention;

FIG. 2 is a longitudinal cross section of an actuator of FIG. 1, and FIG. 3 is a transverse cross section thereof taken on line 3-- 3 of FIG. 2;

FIG. 4 is a partially diagrammatic broken away perspective view of a portion of the embodiment of FIGS. 1-3;

FIG. 5 is a longitudinal cross section of another preferred actuator embodiment suitable for a larger, more powerful actuator, and FIG. 6 is a transverse cross section of the armature thereof taken on line 6-- 6 of FIG. 5;

FIG. 7 is a diagrammatic view in perspective illustrating the formation of an alternative armature for the embodiment of FIGS. 5 and 6, and FIG. 7 a is an end view of such an armature;

FIG. 8 is a plot of frequency response showing effects of different types of damping, and FIG. 9 is a plot of current vs. deflection for a preferred embodiment.

As shown in FIG. 1 a light beam 15 originating from light source 1 and columnated by columnator 2 passes to electro-optical chopper 3 comprising a shutter 5 rotatable by limited rotation actuator 4 with electrical input leads 6 where it is amplitude modulated. Thence beam 15 passes to electro-optical scanner 10, comprising a mirror 9 rotatable by limited rotation actuator 8 with a vertical axis, where its horizontal direction is changed in accordance with the position of mirror 9. Thence beam 15 passes to electro-optical scanner 11, with a horizontal axis but otherwise identical with scanner 10, where the vertical direction of the beam is changed. Thence beam 15 passes to screen 13.

Referring to FIGS. 2 and 3, actuator 8 (which is identical to the other actuators of FIG. 1) has a stator 17 comprising two poles 52 and 82, and two permanent magnets 110, all supported within case 23. Pole 52 has a bridge 74 connecting two pole pieces 70, 72, each of which provides a gap surface 71 and 73 respectively, lying closely outside reference circle 32. Control coil 76, connected to leads 7, is wound of insulated wire around bridge 74 extending between pole pieces 70 and 72.

Electrically independent, shorted (diagrammatically at 78 a, see FIG. 4) damping coil 78, having a total cross-sectional area of conductor approximately one-third as large as that of coil 76 is also wound around bridge 74. Coil 78 is formed of bare wire, to accomplish shorting. (As will be explained more fully below this shorted coil is found to produce a great amount of damping for its size, believed to be caused by action similar to that of a transformer rather than merely the effect of the back EMF generated by the moving element).

Similarly, pole 82 is supported by case 23 and divided into two pole pieces 84, 86 each of which provides a gap surface 88 and 90 respectively, lying closely outside reference circle 32. Control coil 94 is wound around bridge 92 extending between pole pieces 84 and 86. Electrically independent, shorted damping coil 95, having a total cross-sectional area of conductor approximately one-third as large as that of coil 94 is also wound around bridge 92.

Poles 52, 82 are made of lamina (.014 in. thick in preferred embodiment) of high permeability (.mu.=50,000) material such as 50% Ni-50% Fe alloy.

Permanent magnets 110 have one pole 114 abutting against pole 82 and their other pole 116 against pole 52. Spaces 101 and 103 around coils 76, 78, 94, 95, spaces 97 between the magnets 110 and the poles 52, 82 as well as the space 25 between the assembly and case 23 are filled with epoxy potting compound.

Armature 12 is made of a single piece of high permeability material such as soft iron and has projecting shafts 150, 152 (FIG. 2) fitted into radial bearings 14, 16 respectively. Torsion bar 20 is affixed coaxially to shaft 152 and affixed eccentrically at one end 21 to anchor 22 at an angle .psi.(e.g. 5.degree.) to armature axis 18 so as to be in a flexed state. This angle is effective to produce forces on the armature, pressing the armature tight against the left side of radial bearings 16 and the right side of bearing 14, thus taking up any clearance. Certain unwanted motion of the armature and e.g. its attached mirror, especially at high frequency, is discovered to be attributable to uncertain positioning of the rotor in its bearings. The intentional biasing or "binding" of the armature in its bearing, just described, is found to eliminate this problem. It extends bearing life as well, because of elimination of a Brinelling or chatter effect at high frequencies.

In an alternative actuator as shown in FIGS. 5 and 6, wherein counterparts of the above described armature are designated by a primed number, armature 12' is made of lamina (.014 in. thick in preferred embodiment) of high permeability material (such as 50% Ni-50% Fe alloy) affixed around hollow shaft 53, which is rotatably supported on bearings 14' and 16' around axis 18' . Torsion bar 20' passes through cylindrical passageway 24 in shaft 53 and is attached at one end 19 to armature 12' with the axis of torsion bar end 19 coincident with the axis of shaft 53. The other end 21' of torsion bar 20' is affixed to anchor 22' with its axis eccentric to the axis 18' of shaft 53 so that torsion bar 20' is held in a state of elastic flexure. Bearings 14' , 16' and anchor 22' are supported by case 23' .

Referring back to FIGS. 1- 4, armature 12 has two gap surfaces 30 and 31 which rotate closely inside reference circle 32 which is concentric with axis 18. Pole pieces 70, 72, 84 and 86 are thus each separated from opposed surface of armature 12 by narrow gaps 102, 104, 106, 108 respectively. The spaces 40, 42 between poles of different polarity (70 and 84; 72 and 86) are much larger than the gaps 102, 104, 106, and 108.

Portions 34 and 36 of gap surface 30 are sloped gradually inward from circle 32, the slope being such that, with armature 12 in its central position as illustrated in FIG. 3, surface portions 34, 36 come closer to axis 18 as one moves with greater angle from dividing space 40. Similarly portions 38 and 39 of gap surface 31 are sloped gradually inward from circle 32, the slope being such that with armature 12 in its central position surface portions 38, 39 come closer to axis 18 with greater angle from dividing space 42.

For the embodiment of FIGS. 5 and 6 sloped portions 34', 36', 38', and 39' are similarly provided, (FIG. 7) in this case extending along only a limited portion of the length of armature 12' . Sloped portions 34' and 38' preferably conform to a single eccentric surface of revolution having an axis XA situated about a plane of symmetry 148 of armature 12' and intersecting at an acute angle the rotation axis XX' of armature 12' . Similarly, sloped portions 36' and 39' conform to an eccentric surface of revolution centered on axis XB also situated on a plane of symmetry. As an alternative, the armature of FIG. 7 employing an exterior torsion spring as shown in FIG. 2 can be substituted for the hollow armature of FIGS. 5 and 6. The FIG. 7 armature is of machineable material and the sloped surface portions 34' , 36' , 38' , and 39' are preferably made by turning as shown in FIG. 7. Armature 12' is first turned around axis XA while a cutting tool simultaneously produces sloped surface portions 34' and 38'. Armature 12' is then turned around axis XB as shown in FIG. 7 while cutting tool 122 simultaneously forms sloped surface portions 36', 39'. A similar gradually varying effect can be obtained for small-sized armature such as illustrated in FIGS. 1- 3 by electrochemical metal removal, to remove the sharp edges. Varying the time of exposure to the chemicals along the length of the armature can produce a surface generally corresponding in form to that shown in FIG. 7.

The compensation achieved by my invention can be understood by considering the interaction of the magnetic flux from two different sources. The first of these is an invariant bias-flux .phi..sub.B (FIG. 4) originating from magnet poles 116, entering pole 52 and dividing to follow two paths to armature 12, the first path through pole piece 70, and across gap 102, the second through pole piece 72 and gap 104.

The second flux is the control flux .phi..sub.C arising from an imposed control current flowing in control coil 76. The control flux circulates through pole piece 70, across gap 102, through armature 12, thence across gap 104 and through pole piece 72 back to bridge 74. (The magnets 110 because of their low permeability as compared to pole 52 present no effective path for the control flux). The bias-flux thus flows in parallel across gaps 102 and 104 while the control flux flows in series through gaps 102 and 104. If the flux across gap 102 is designated by .phi..sub.1 and that across 104 as .phi..sub. 2 the following equations can be written:

.phi..sub. 1 +.phi..sub. 2 =.phi..sub.B (1)

.phi..sub. 1 R.sub. 1 -.phi..sub. 2 R.sub. 2 =F (2)

where R.sub. 1 and R.sub. 2 are the reluctances of gaps 102 and 104 respectively and F is the magnetomotive force produced by the control current. Equations 1 and 2 can be readily solved to give explicit expressions for the flux in each of the gaps.

The magnetic energy in the gaps is proportional to

U= 1/2(.phi..sub. 1 .sup. 2 R.sub. 1 +.phi..sub. 2 .sup. 2 R.sub. 2 ) (5)

The torque on armature 12 produced by the magnetic fields in gaps 102, 104 can be obtained by differentiating the quantity U with respect to the angle .alpha. describing the angular position of the rotor, the differentiation being carried out while the flux in each gap is considered constant. The torque is therefore proportional to ##SPC1##

The reluctances R.sub. 1 and R.sub. 2 associated with the airgaps 102 and 104 respectively and appearing in equation 7 are in general dependent upon the size and shape of the airgaps, which in turn depend upon the angular position of the armature 12.

The construction of the device is symmetrical with the lower half adjacent to poles 114 duplicating the construction and action of the upper half adjacent to poles 116 and producing an additional equal torque acting on armature 12.

In addition, to the magnetic torque given by equation 7, a restoring torque is applied to the armature by torsion bar 20. The restoring torque is essentially proportional to the angle the armature is rotated from its central position. For every value of control current the armature assumes some position such that the magnetic and restoring torques are in equilibrium. The current circulating through the control coil thus determines the equilibrium angular position of the armature. In general, as is indicated in part by equation 7, the relationship between the control current and the equilibrium angular position of the armature is complex and nonlinear. In particular if the gaps between the pole members and the armature were of uniform thickness the relationship is nonlinear.

In accordance with this invention, the slopes of curved portions 34 and 38 are determined to modify the shape of gaps 102, 104 and the relationship of the reluctances R.sub. 1 and R.sub. 2 to the angular position of armature 12 and thereby to compensate for the nonlinear effects indicated in equation 7 and provide a desired relationship (such as smooth and linear) between the current in control coil 76 and the equilibrium angular position of armature 12 over an extended range of values for the control coil current.

Referring to FIG. 7 a, for a typical example, an armature for an actuator may have an end form as shown, a width d of 1/2 inch, and a thickness t of 1/4 inch. Its central curved surfaces are formed by turning about center X', radius r=1/4 inch. For compensation the armature is mounted for machining on axis A, on the plane of symmetry perpendicular to the widthwise axis of the armature. Axis A is shifted for instance 1/16 inch from X', and two surfaces 34' and 38' are turned, with radius r.sub. 1 .sup.= 9/32 inch. The armature is then shifted to axis B, symmetrically located with A about axis X', and surfaces 36' and 39' are formed. While the armature thus formed in some instances may have uniform cross section along its length (i.e. axes B and A parallel to XX'), the advantage of easier adjustment is obtained by holding one end of the armature on the original axis, and displacing the other end, with fine adjustments during model-making based upon operational measurements.

With regard to the damping aspect of the invention, when a driving current applied to leads 7 a, 7 b changes quickly from one value to another, the driving flux changes, and the armature moves suddenly to a new position and tends to oscillate about that position for a certain settling time due to both electrical and mechanical effects. The oscillation of the armature position is accompanied by flux oscillations in bridges 74 and 92, as well as in other stationary permeable portions in the control flux path. I have realized that armature oscillations can be damped significantly by a relatively small short-circuited coil surrounding a stationary permeable portion of the control flux path. The effect can be likened to transformer action. Thus, referring to FIG. 3, damping coils 78 and 95 are linked to the flux in the bridges 74 and 92 and draw energy from the oscillations and dissipate it, thereby reducing the resonance factor Q of the coupled electromechanical resonance to a low value.

The damping coil can be of quite small size, i.e. less than one-half the cross-sectional area of the control coil, and for a given torque and frequency the actuator is accordingly reduced in size. Thus third order effects may be avoided, which along with the other features mentioned herein lead to predictable linearity. Also the heat dissipation problem is reduced, leading to operation below critical temperature levels, in turn leading to greater reliability and life.

The invention leads to highly effective electroactuators having characteristics heretofore unknown.

For example a galvanometer was produced having the following dimensions, referring to FIG. 2

W=0.875 in., square in cross section

L=1.06 in.

Armature dimensions : length 1/2 inch width 3/16 inch thickness 1/16 inch EXAMPLE 1 Each of the control coils 76, 94 was formed of 175 turns of No. 32 (0.0080 in. diameter) copper wire, insulated (cross section of conductor in coil 0.0088 in..sup. 2 ). On top of each of these was wound a damping coil (78, 95) of 30 turns of No. 28 (0.0126 in. diameter) copper wire, uninsulated and thus shorted (cross section of conductor in coil 0.0037 in. .sup. 2) the cross-sectional area ratio damping to control coils is about 1 to 2.4.

The rotor had a transverse cross section (see FIG. 7 a) with width d 1/2 inch and thickness t 1/4 inch. The rotor moment of inertia was 0.0125 gm-cm.sup. 2 .

This actuator weighed 3 ounces, had for coils 76, 94 in series a resistance of 7.5 ohms and consumed 2 watts. At a rated pp (peak to peak) rotation of 15.degree. it demonstrated a resonant frequency of 1,000 Hz. with maximum rotation of 20.degree., pp. The actuator drove a mirror with dimension, 7 mm..times. 11 mm..times. 1 mm. and a moment of inertia of 0.008 gm-cm.sup. 2 .

Similar constructions with certain parameters varied in accordance with desired characteristics were as follows:

EXAMPLE 2

Resonance 1,500 Hz.; rotation, rated pp 8.degree.; rotation, max. pp 10.degree.; rotor moment of inertia, 0.007 gm-cm.sup. 2 .

EXAMPLE 3

Resonance 700 Hz.; rotation, rated pp 30.degree.; rotation max. pp 40.degree.; rotor inertia, 0.015, gm-cm.sup. 2 .

The actuator of example 1 was modified and tested at different frequencies to compare the effectiveness of different modes of damping. The results are shown in FIG. 8, where amplitude is plotted vs. frequency. Curve A is with no damping. Curve B is with shorted damping coils 78, 95. Curve C is with damping coils 78, 95 inoperative and control coils 76, 94 shunted with an amplifier having zero output impedance. The resonance factor Q (computed as the ratio of amplitude at resonance to that at 10 Hz.) was found to be as follows:

Case A (no damping): Q=8; Case B (shorted damping coils): Q= 1.37; Case C (shunted amplifier): Q= 2.0. Taking into account the difference in the amount of copper present in coils 78 and 95 vs. that in the control coils 76, 94, the effectiveness of the damping coils was three and one-half times that of shunting.

Because of the large amount of damping available in small systems according to the invention the actuator can be equipped with a mirror having a moment of inertia greater than one-third that of the armature to achieve large aperture and optical flatness. Such large mirrors with very high resonant frequency (about 1,000 Hz.) and damping permits use with computers and other high speed components to create visual displays. Two such actuators combined together in FIG. 1 provide an optical scanning head A for high speed scanning of X-Y optical fields. It is well suited to such applications as:

Recording of Computer CRT displays

Microfilm memory-reading and writing

Flying spot scanners

Character recognition

Projection of oscillograph traces

Photographic X-Y recording curve follower,

Plotter transient reproducer.

When the actuator is equipped with a shutter it provides an optical chopper with the unusual characteristic that the frequency of operation can be modulated.

Larger scale actuators combining some or all of the features herein described lead to improved operation of high frequency (about 300- 500 Hz.) recording pens on strip chart records, servomechanisms and numerous other uses. The linearity of movement vs. current of the actuator that can be achieved when employing the compensation here described is illustrated in FIG. 9. The curve is straight, or will curve only gradually, with no abrupt discontinuities.

Referring back to FIG. 1 for a preferred system employing the actuator of the invention, actuator 4 is connected to an electrical modulating signal with very high frequency components through leads 6. It rotates shutter 5 in accordance with the applied signal. The shutter in one position permits the passage of light and in another blocks the light so that the light beam is modulated in accordance with the electrical signal. Actuator 8 is connected to a ramp-shaped electrical signal through leads 7 and in response to this signal first rotates mirror 9 at a uniform rate in one direction thereby deflecting beam 15 at a uniform rate horizontally across screen 13. Then it rotates the mirror quickly back to its initial position thereby directing the beam back to the starting point. As a result of the smooth linear relationship between the driving current and the angular position of armature 12 and the damping, the motion of the beam on the screen accurately corresponds to the input signal. The second scanner 11 is driven similarly by a ramp signal to advance the light beam, down the screen in correspondence to the desired display. The signal may be digital, i.e. pulsed increases of voltage rather than varying smoothly.

Numerous modifications and uses will be obvious in light of the foregoing.

Claims

What is claimed is:

1. A limited rotation actuator of the type having a high permeability armature mounted to turn relative to a stator, a permanent magnet providing bias flux through a first path that includes the armature and permeable stationary parts, and a stationary control coil providing control flux through a second path that includes the armature and permeable stationary parts including the improvement for eliminating unwanted armature motion comprising the combination of a damping coil independent of said control coil and short-circuited, said damping coil surrounding a permeable part of the control flux path, and opposed surfaces of the armature and stator each being smoothly curved but at least along part of the length having different, nonconcentric curvature, providing a gap that decreases the bias flux reluctance with increased displacement of the armature from its center position.

2. The actuator of claim 1 in which said armature is mounted in bearings including a mechanical mounting constructed to press the armature radially to a predetermined side in its bearings.

3. The actuator of claim 2 wherein motion of said armature is resisted by a torsion spring, said torsion spring mounted also to press the armature to said predetermined side in its bearings.

4. A limited rotation actuator of the type having a high permeability rotary armature mounted to turn relative to a stator, a permanent magnet providing bias flux through a first path that includes the armature and permeable stationary parts, and a stationary control coil providing control flux through a second path that includes the armature and permeable stationary parts, including the improvement for eliminating unwanted armature motion comprising a damping coil independent of said control coil and short-circuited, said damping coil surrounding a permeable part of the control flux path.

5. The limited rotation actuator of claim 4 wherein the stator comprises a pole having two separated pole pieces, each pole piece having a surface opposed across a gap to said armature, said control coil disposed so that a control current passing therethrough produces flux in a control flux path through one of said pole pieces to said armature, thence to said second pole piece and thence back to said first pole piece, and said permanent magnet providing a bias flux in paths from each of said pole pieces across the respective gaps to said armature, said magnet presenting to said control flux a reluctance high compared to the reluctance of said control flux path, and wherein said short-circuited damping coil surrounds a permeable stationary portion of said control flux path thereby providing damping for coupled electrical and mechanical oscillations of said armature.

6. The actuator of claim 5 wherein the cross-sectional area of said damping coil is less than about one-half the cross-sectional area of said control coil.

7. The actuator of claim 5 in which said control coil and said damping coil encircle a bridge between said two pieces of said pole.

8. The device of claim 5 wherein said actuator is formed with two structurally symmetric poles coacting on said armature and wherein on each pole the cross-sectional area of electrical conductor of the combined said control and damping coils is less than about 0.02 sq. in. and wherein the coupled electrical and mechanical resonance factor Q is reduced to a value less than about 2.

9. An electro-optical device comprising a member disposable in a light beam to impart characteristics to said beam depending upon the position of said member, a limited rotation actuator for moving said member to a plurality of positions in response to electrical signals, the actuator having a damping arrangement to prevent residual oscillation after said actuator moves from a first position rapidly to a second position, said actuator having a high permeability rotary armature with a surface opposed to a stationary pole, the pole having two pole pieces separated by a dividing space, each pole piece having a surface opposed across a gap to said armature, a control coil disposed so that a control current passing therethrough produces a flux in a control flux path through one of said pole pieces to said armature, thence to said second pole and thence back to said first pole piece, and a permanent magnet providing a bias flux in paths from each of said pole pieces across the respective gaps to said armature, said magnet presenting to said control flux a reluctance high compared to the reluctance of said control flux path, and the damping arrangement including a short-circuited damping coil electrically independent of said control coil, said damping coil surrounding a permeable stationary portion of said control flux path, thereby providing damping for coupled electrical and mechanical oscillations of said device.

10. The device of claim 9 wherein the cross-sectional area of said damping coil is on the order of one-third the cross-sectional area of said control coil.

11. A limited rotation actuator of the type having a high permeability rotary armature mounted to turn relative to a stator, a permanent magnet-providing bias flux through a first path that includes said armature and permeable stationary parts, and a stationary control coil providing control flux through a second path that includes the armature and permeable stationary parts, including the improvement wherein the opposed surfaces of the armature and stator have portions that are concentrically cylindrical, but at least along part of the length one of the opposed surfaces has a distinctly different nonconcentric curvature, to provide a gap that decreases the bias flux reluctance with increased displacement of the armature from its center position.

12. A limited rotation actuator wherein a high permeability rotary armature is supported to be rotatable about an axis within limits from a predetermined central position and has at least two gap surfaces opposed to stationary pole pieces of a stator,

each pole piece having a gap surface opposed across a gap to the corresponding armature gap surface and being at least approximately concentric with the axis of rotation of said armature,

a driving coil disposed so that a control current passing therethrough produces a control flux in a circuit from a bridge, through one of said pole pieces, across its gap surface to the corresponding opposed gap surface of said armature, through a portion of said armature, thence across a gap surface of said armature to the corresponding opposed gap surface of a second of said pole pieces and thence back to said bridge, with the reluctance of said path represented preponderantly by said gaps between said pole pieces and said armature,

a permanent magnet providing a bias flux in paths from each of said pole pieces across the respective gaps to said armature, said magnet presenting to said driving flux a reluctance high compared to the reluctance of said control flux path,

and wherein a portion of one of said gap surfaces slopes gradually relative to a circle centered on the center of armature rotation and projected through said gap, the slope arranged so that, with the armature in central position, the distance between said sloped surface portion and said axis gradually decreases with increasing angle from said dividing space, whereby upon rotation of said armature away from its central position gradual decrease in the overall reluctance to the bias flux occurs with said gradual increase in reluctance in the path of the control flux, thereby providing said compensation in a smooth manner over an extended range of rotation of said armature.

13. The actuator of claim 12 wherein said sloped surface portion occurs along only a limited portion of the length of said armature.

14. The actuator of claim 12 wherein said sloped surface portion is shaped to conform to at least one surface of revolution, the axis of which is offset from said axis of rotation of said armature.

15. The actuator of claim 14 wherein said armature has a plane of symmetry, said sloped portion lies on one side thereof and is opposed to a first pole piece, and said surface of revolution is centered on an axis lying in said plane, wherein there is a second sloped portion lying on the opposite side of said plane opposed to a second pole piece.

16. The actuator of claim 15 in which said axis of said surface of revolution intersects said axis of rotation of said armature at an acute angle.

17. The actuator of claim 12 having a damping element that is electrically independent of the circuit of said control coil and comprises a short-circuited damping coil surrounding a permeable control flux-carrying part of said actuator, and wherein said armature is mounted in radial bearings, with a torque element disposed to apply to said armature a torque of predetermined direction acting in a direction perpendicular to the rotation axis of said armature, thereby constraining the axis of rotation against movement due to the play of said bearings; said compensating means, damping means and torque arrangement cooperating to permit said actuator to operate linearly over an extended range including frequencies in excess of 500 Hz. and deflections in excess of 5 degrees.

18. The actuator of claim 17 including, secured to said armature, a driven member having a moment of inertia at least one-third that of said armature.

19. In combination with a limited rotation actuator of the type having a high permeability armature supported to be rotatable in bearings within limits from a predetermined central position and having a gap surface directed toward a stationary pole,

a control coil disposed so that a control current passing therethrough produces a control flux in a path through said armature, and

a permanent magnet providing a bias flux in a path through said armature;

a mechanical means acting perpendicular to the axis of rotation to apply forces to said armature in a direction to constrain said axis of rotation against movement due to the play of said bearings.

20. The actuator of claim 19 wherein said means comprises a torsion spring which applies a torque acting about an axis perpendicular to the rotation axis of said armature, said spring also providing a restorative torque to oppose deflection of said armature.

21. The actuator of claim 20 wherein said torsion spring comprises an elongated torsion bar secured at one end to fixed structure and at the other end to said armature, the axis of said torsion bar secured at a slight angle to the axis of said armature.

22. The actuator of claim 21 wherein said armature is hollow and said torsion bar is telescoped with said armature, extending from a point of connection therewith through the armature to a point of connection with fixed structure.

23. The actuator of claim 22 wherein said hollow armature is formed by a multiplicity of lamina of high permeability, difficult-to-machine material, said lamina having openings formed therein to provide said hollow space.

24. A limited rotation actuator of the type having a high permeability armature supported to be rotatable in bearings within limits from a predetermined central position and having a gap surface directed toward a stationary pole,

a control coil disposed so that a control current passing therethrough produces a control flux in a path through said armature, and

a permanent magnet providing a bias flux in a path through said armature;

an elongated torsion bar providing a restorative torque to oppose deflection of said armature,

said armature being hollow, formed by a multiplicity of lamina of high permeability, said lamina having openings formed therein to provide a hollow space, and said torsion bar being telescoped with said armature, within the hollow space formed by said lamina, extending from a point of connection with said armature through the armature to a point of connection with fixed structure.

25. The limited rotation actuator of claim 24 wherein said torsion bar extends beyond the bearings mounting said armature, said laminations mounted on a hollow tube supported at each end by said bearings, said torsion bar protruding beyond one end of said tube to a point where it is joined with said fixed structure and said bar joined at its other end to said tube.

26. The actuator of claim 25 in the form of a scanner wherein a mirror is mounted upon said tube, thereby being aligned with said armature.