GB2344188A - Force decoupler system for mirror on flexible mount - Google Patents

Abstract

The apparatus 20 inhibits the transfer of lateral and torsional forces from a member 21, having a mirrored surface 22, to each piezoelectric actuator 27, used to vary the position of the member 21, or to compensate for movement of the member 21 caused by vibration impinging on the apparatus 20. The member 21 is flexibly mounted to a support pillar 23, having a central axis 29 extending therethrough, using a diaphragm 24. The member 21 has a predetermined centre of gravity 26 and is machined such that the centre of gravity 26 is arranged to be substantially co-axial with the central axis 29 of the support pillar 23 and in a plane passing through the diaphragm 24, the plane being normal to the central axis 29. Sensors 32, eg linear variable differential transformers, are shown.

Description

IMPROVEMENTS IN OR RELATNG TO A FORCE DECOUPLER SYSTEM The present invention relates to a decoupler system, in particular to a force decoupler system for inhibiting the transfer of lateral and torsional forces from a member to components used to vary the position of the member, or to compensate for movement of the member caused by vibration.

From Figure 1 it is known to provide two axis scanner 10 to vary the position of mirror 11.

The scanner 10 typically comprises a body 12 and head 13 formed from a single piece of material which has been machined to provide sections 14 which join the body 12 to the head 13. The mirror 11 is either adhered to the head 13, or formed integrally with the head 13.

The sections 14 allow the head 13 and mirror 11 carried thereon to be moved to a variety of positions. For this purpose the sections 14 are flexible and capable of a small degree of movement.

Figure 2, is a cross section through a line A-A of Figure 1, in which the same references as those used in Figure 1 are used to indicate similar integers. Typically, the body 12 houses four piezoelectric actuators 15 which are used to vary the position of the head 13. When a voltage is applied to one or more of the piezoelectric actuators 15 each will expand along its longitudinal axis 16. In this manner, by causing one or more of the piezoelectric actuators 15 to extend the head 13 will be moved with respect to the body 12 in correlation to the expansion of the piezoelectric actuator 15.

However, in this type of scanner 10, the mass of the head 13 and mirror 11 may cause lateral and/or torsional forces to be applied to each piezoelectric actuator 15. It is known that piezoelectric actuators 15 exhibit substantial compressive strength and typically are able to support loads of 100 newtons to 3000 newtons depending on the cross sectional area of the actuator 15. However these types of actuators are relatively weak when either a tensile, lateral or torsional force is applied and that such forces, if sufficient in magnitude, may damage a piezoelectric actuator 15. Any mechanical design incorporating these types of actuators 15 should ensure that lateral, torsional and tensile forces are kept to a minimum.

One way of protecting each actuator 15 from such forces is to apply a compressive pre-load to the actuator 15 and to ensure that the tensile stress on the actuator 15 does not exceed the initial pre-load.

Absolute position of each piezoelectric actuator 15 can be affected by a number of variables, for example, thermal expansion, hysteresis and variation in the external load applied to the actuator 15. These characteristics make it desirable that the actuator is used with a position sensor under closed loop control when absolute positional accuracy is required.

The resonant frequency of such a known scanner 10 is of the order 300Hz. However, in order to provide a stable closed loop feedback system it would be desirable to achieve a greater resonant frequency.

It is the object of this invention to obviate or mitigate the disadvantages associated with the prior art. According to the present invention a decoupler system comprises a member having a predetermined centre of gravity, a support having a central axis there through, a flexible mount operably arranged to couple the member to the support, and a driving means operably arranged to move the member, wherein the centre of gravity of the member is arranged to be substantially co-axial with the central axis of the support and to be substantially in a plane passing through the flexible mount and which is normal to the central axis.

In this manner, the flexible mount provides lateral and torsional support of the member with respect to the support thereby reducing the turning moment of the member about the centre of gravity. This reduces the sensitivity of the system to vibration and inhibits the transfer of lateral and torsional movements from the member to components of the driving means.

Preferably, the member may be a mirror. In this manner if the force decoupler is used in an optical system either the mirror may be moved by the driving means to compensate for external vibration on the optical system without transfer of lateral or torsional forces form the mirror to components of the driving means, or the mirror may be moved by the driving means in a manner to control light reflected by the mirror.

The flexible mount may be a diaphragm.

The driving means may comprise at least one actuator. Preferably each actuator may be formed from a piezoelectric material. In this manner a device which is unduly stressed by lateral and/or torsional forces can be protected from such forces At least one pair of actuators may be arranged to co-act in a push-pull arrangement.

Preferably, the driving means may comprise at least one sensor operably arranged to provide a measurement signal indicative of movement of the member with respect to a reference.

The driving means may comprise a controller which receives the measurement signal from each sensor and according to each measurement signal provides a driver signal to control each actuator. In this manner the force decoupler acts as an active system responding to movements of either the reference or member.

Alternatively, the driving means may comprise a controller arranged to produce a predetermined driver signal to control each actuator. In this manner the force decoupler acts as a passive system to move the mirror in a known manner.

Preferably, a flexure may be mounted between the member and each actuator. In this manner any lateral or torsional force not decoupled between the support and member through the flexible mount can be substantially inhibited between the member and the components of the driving means, in this case actuators.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a prior art apparatus for varying the position of a member; Figure 2 is a cross sectional drawing through A-A of Figure 1 ; Figure 3 is a schematic side elevation view of the present invention; Figures 4 to 6 illustrate different flexures to be employed with the present invention; Figure 7 illustrates reduction of the mass of the member, Figures 8 and 9 illustrate the effect of mounting actuators to the member.

The inventor has realised that the head 13 and mirror 11 associated with the prior art scanner 10 illustrated in Figures 1 and 2 provides a mass which may act upon and damage each piezoelectric actuator 15. Ideally only forces that are axial to each actuator 15 should be transferred to that actuator 15. Therefore, in the present invention, the inventor has reduced the mass acting on each actuator 15, thereby inhibiting transfer of lateral and torsional forces on each piezoelectric actuator 15.

In Figure 3, the apparatus 20 of the present invention, comprises a member 21 having a mirrored surface 22 thereon which is flexibly mounted to a support pillar 23 by a diaphragm 24. The support pillar 23 forms part of a base 25 and both can be formed of stainless steel.

The member 21 and mirror surface 22 can be separate integers or formed in one part. Either component can be formed of aluminium The member 21 and mirror surface 22 has a predetermined centre of gravity 26 and the member 21 is machined such as the plane of the diaphragm 24 is situated in the same horizontal plane as the centre of gravity 26. The diaphragm can be formed from a plastic material such as polypropylene, or metal material such as iron, stainless steel or phosphorbronze.

Drive means, such as piezoelectric actuators 27, are mounted on the base 25 and to the member 21 either directly or through an associated flexure 28 such that the actuators 27 are radially and equally spaced from the centre of gravity 26.

When a number of piezoelectric discs are stacked together they form a piezoelectric actuator 27. The piezoelectric actuators 27 changes length once a suitable signal voltage is applied across electrodes on opposite faces of the material, thereby transferring a force to the member 21 which will vary the position of the member 21 about the centre of gravity 26 in two axes. The expansion of the actuator 27 is dependant upon the applied voltage, for example, given a 15 micrometre/100 volt device, a voltage change of 0.1 volts would produce an incremental change in length of 15 nanometres. This feature provides excellent resolution in the control of small movement.

Rotation of the member 21 is achieved by driving diametrically opposed actuators 27 in opposite directions in a pus-pull arrangement. In this manner the member 21 can be driven in two axes simultaneously. For example, if each actuator 27 has a nominal expansion of 15 micrometres per 100 volts applied and is radially spaced from the centre of gravity 26 by 30 millimetres then the actuator 27 expansion required to produce rotation of the member 21 through plus or minus 50 microradians would be plus or minus 1.5 micrometres. The signal voltage required to produce this expansion would be plus or minus 10 volts with a nominal 50 volt offset.

In this manner, the diaphragm 24 provides lateral and torsional support of the member 21 with respect to the support pillar 23 and inhibits the transfer of lateral and torsional movements from the member 21 to the piezoelectric actuators 27.

The support pillar 23 has a longitudinal central axis 29 and the centre of gravity 26 of the member 21 is arranged to be substantially c-axial with this central axis 29. The piezoelectric actuators 27 can also be mounted to the member 21 such that each flexure 28 is arranged to be substantially in the plane of the centre of gravity 26. That is diaphragm 24 and each flexure 28 share a common plane through the centre of gravity 26. This arrangement minimises the turning moment of the member 21 that may be generated by linear accelerations but permits small rotations of the member 21 about two axes when driven by actuators 27.

Figures 4 to 6 illustrate flexures 28 which can be provided to inhibit any lateral or torsional forces which are transmitted by the diaphragm 24 to the piezoelectric actuators 27. Each flexure 28 has a central longitudinal axis 30 in which direction the flexure 28 is axially stiff such that translation of the piezoelectric actuator 27 transfers to the member 21 along axis 30. However, each flexure 28 has at least one lateral axis 31 about which the flexure 28 is flexible thereby any lateral or torsional forces transmitted by the member 21 is substantially inhibited from transfer to the piezoelectric actuators 27. The flexures 28 can be formed of stainless steel.

In this manner, the resonant frequency of the apparatus 20 can be in excess of lKz. This is significant in the design of close loop feedback technology to control movement of the mirror as it provides a much greater bandwidth so that a close loop system is less likely to become unstable.

Figure 3 also shows that sensors 32, in this case linear variable differential transformers, mounted between the member 21 and the base 25, adjacent each actuator 27, can be arranged to provide a measurement signal indicative of movement of the member 21 about the centre of gravity 24 with respect to a reference, in this case, the base 25. The sensors 32 are formed in two parts, a non-moving housing attached to the base 25 and a core attached to the member 21. The housing contains three coils, one primary and two secondary. When the primary coil is energised, voltages or measurement signals are induced in the secondary coils proportional to the relative position of the core. The advantage of using this type of sensor 32 is that it measures movement of the member directly. The measurement signal can be used in the feedback system in order to either move the member 21 to a desired position or to return the member 21 to a steady position in a vibration environment.

To maintain the balanced arrangement the coefficient of thermal expansion of each actuator 27, pillar 23 and each sensor 32 should be matched to ensure that the centre of gravity 26 of the member 21 tracks the horizontal plane of the diaphragm 24 over the operating temperature range of the apparatus 20.

With temperature change the member 21 and mirrored surface will move along the line of the axes of the actuators 27 by an amount proportional to the change in ambient temperature. This is caused by thermal axial expansion of sensors 32 which generates a shift in each sensor 32 null position. Since only 20% of the available travel of the actuator 27 is employed in this embodiment to move the member 21 plus or minus 15 microradiants, the remaining 80% of travel is available to follow any long term drift in the null of each sensor 32.

In a further embodiment, not illustrated, strain gauges are bonded directly onto each actuator and provide a measurement signal of expansion for the actuator. However, in some applications their use is limited by errors in the adhesive bond transferring strain from each actuator to the gauge and the elasticity and thermal expansion in the flexure located between the member and actuator.

In Figure 7, a member 40 can have mass removed by either machining away sections 41 of the member 40 or drilling holes 42 in the member 40. In this manner the resonant frequency of the apparatus can be increased.

Figures 8 and 9 indicate the avantage of mounting each actuator 50 to a position 56 which lies in the plane of the centre of gravity 52 as shown in Figure 9 rather than to a position 51 shown in Figure 8. Figure 8 shows a pair of actuators 50 each mounted to positions 57 not in the plane of the centre of gravity 52 and that greater force 54 are applied to the actuators 50 when the member 55 is moved about the centre of gravity 52. In Figure 9, when the pair of actuators 50 are mounted to a position 56 in the plane of centre of gravity 52, the lateral force 54 applied to the actuators 50 is reduced.

Although, this invention is described with reference to a member carrying a mirror, it will be understood that the member could carry any device which is required to be moved about a centre of gravity or that the member may be the device to be moved about the centre of gravity.

It will also be understood that the drive means of the invention is not limited to piezoelectric actuators. Actuators which should not be subjected to lateral or torsional forces are also included as possible drive means.

Claims (11)

1. CLAIMS 1. A decoupler system, comprising a member having a predetermined centre of gravity, a support having a central axis therethrough, a flexible mount operably arranged to couple the member to the support, and a driving means operably arranged to move the member, wherein the centre of gravity of the member is arranged to be substantial co-axial with the central axis of the support and to be substantially in a plane passing through the flexible mount and which is normal to the central axis.
2. 2. A decoupler system, as in Claim 1, wherein the member is a mirror.
3. 3. A decoupler system, as in Claims 1 or 2, wherein the flexible mount is a diaphragm.
4. 4. A decoupler system, as in any preceding claim, wherein the driving means comprises at least one actuator.
5. 5. A decoupler system, as in Claim 4, wherein each actuator is formed from a piezoelectric material.
6. 6. A decoupler system, as in Claims 4 or 5, wherein at least one pair of actuators are arranged to co-act in a push-pull arrangement.
7. 7. A decoupler system, as in Claims 4 to 6, wherein the driving means comprises at least one sensor operably arranged to provide a measurement signal indicative of movement of the member with respect to a reference.
8. 8. A decoupler system, as in Claim 7, wherein the driving means comprises a controller which receives the measurement signal from each sensor and according to each measurement signal provides a driver signal to control each actuator.
9. 9. A decoupler system, as in Claims 4 to 6, wherein the driving means comprises a controller arranged to produce a predetermined driver signal to control each actuator.
10. 10. A decoupler system, as in Claims 4 to 9, wherein a flexure is mounted between the member and each actuator.
11. 11. A decoupler system substantially as illustrated in and/or described with reference to the accompanying drawings.