Laser encoder device

Abstract

A laser encoder device is disclosed which is operable to emit a laser beam, the device comprising a beam steering arrangement for steering the beam towards an intended target, the beam steering arrangement comprising a rotatable carrier on which a steering optic is mounted and through which steering optic the beam is arranged to pass before being emitted from the device, and a rotatable pinion gear which engages with a corresponding carrier gear provided on the carrier such that rotation of the pinion gear causes a corresponding rotation of the steering optic and thereby a corresponding steering of the beam, and further comprising a biasing member to bias the pinion gear into engagement with the carrier gear.

Description

[0001] Laser Encoder Device

[0002] The present invention relates to a laser encoder device. The present invention relates in particular, but not exclusively, to improvements in the setup and operation of such a laser encoder device.

[0003] Figure 1 of the accompanying drawings illustrates a fibre optic laser encoder system 1 which is made and sold by Renishaw pic. The laser encoder system 1 provides position feedback signals suitable for use in precision position feedback applications such as machine calibration and motion control.

[0004] The main components of the laser encoder system 1 are a laser unit 2, a detector head (or detector unit) 4 and a machine interface 6. The detector head 4 is the core of the optical measuring system and will be described in further detail below with reference to Figure 2 of the accompanying drawings. The laser unit 2 comprises a laser source and signal processing electronics, with an electrical cable 3 for receiving signals from and providing power to the detector head 4 and a fibre optic conduit 5 that delivers laser light directly to the detector head 4 through a fibre optic cable (not visible in Figure 1) within the fibre optic conduit 5. The machine interface 6 forms part of a controller 31 and communicates with the laser unit 2 via an electrical cable 7.

[0005] To complete the configuration, a target optic 8 is provided in the path of a laser beam 9 emitted from the detector head 4, such that the laser beam 9 is reflected off the target optic 8 and returned to the detector head 4. In this example, the target optic 8 is in the form of a retroreflector but with a variant of the detector head 4 the target optic 8 could instead be a plane mirror. The beam 9 is a measurement beam, with the return measurement beam interfering with a reference beam which in this example is internal to the detector head 4, with the distance to the target optic 8 (or rather changes in this distance relative to a chosen datum position) being determinable from the interference signal in a known way.

[0006] Figure 2 is a schematic illustration of the main components of the detector head 4 of Figure 1. The fibre optic cable 11 passes through into the body or housing 24 of the detector head 4 and into a collimator 17. The fibre optic cable 11 is terminated within and held in place via a ferrule 21, with laser light being emitted from the end of the fibre optic cable 11 in a diverging cone. The role of the collimator 17 is to collimate this diverging beam, using a lens 23, before it passes further through the detector head 4. As shown by the arrows, the collimated beam passes first to a beam splitter 14, with some of the light being reflected up to a reference retroreflector 18 (as a reference beam) and the remainder of the beam passing out through the laser aperture 16, via a beam steerer 30, and onwards (as the measurement beam 9) to the retroreflector target optic 8. Also shown in Figure 2 is a circuit board 12, which supports various processing, detection, and control electronics (such as a light detector 19), as well as an optical shutter 26 which can be used to shut off the measurement beam 9.

[0007] The return measurement beam from retroreflector target optic 8 re-enters the detector head 4 via the laser aperture 16, and through the beam splitter 14 where it joins (and interferes with) the measurement beam from the reference retroreflector 18 and is incident on a light detector 19. An analogue quadrature interference signal (or detection signal) from the light detector 19 then passes out from the detector head 4 via the electrical cable 3 where it is received at the laser unit 2 shown in Figure 1. Although there may be some processing performed on the interference signal at the laser unit 2, the main processing is typically performed at the interface 6, having received the interference signal from the laser unit 2 via electrical cable 7.

[0008] By digitising, interpolating and processing the interference signal the interface 6 can determine with high accuracy how far the retroreflector target optic 8 has moved by counting fringes, or rather pulses in the digitised / interpolated version of the signal. The positional data from the interface 6 can then be used by the controller 31 for the intended purpose, such as machine calibration or motion control. It should be noted that the laser unit 2 can be set to output a digital rather than analogue quadrature output signal, in which case the digitising and interpolating would be performed at the laser unit 2 rather than at the interface 6.

[0009] The exterior of the detector head 4 of Figure 1 is shown in more detail in Figure 3 of the accompanying drawings. The fibre optic cable 11 can just be seen within the fibre optic conduit 5. The fibre optic conduit 5 is itself coupled to the body of the detector head 4 via a strain relief 15, which is intended to prevent or at least limit forces on the fibre optic conduit 5 and the enclosed fibre optic cable 11 being transferred to any internal optical components to which the fibre optic cable 11 is connected. A laser aperture 16 is also apparent in Figure 3, through which both the outgoing and returning measurement beams 9 will pass.

[0010] The detector head 4 as shown in Figures 1 to 3 is just one type of detector head made and sold by Renishaw pic. Figure 4 of the accompanying drawings shows another type of detector head 4, which differs from that shown in Figure 3 mainly in that the measurement beam 9 is emitted at a ninety-degree angle to main axis of the detector head 4 (rather than zero-degree angle). The detector head 4 of Figure 4 would typically be used as a pair, with one detector head 4 of the pair measuring along an X machine axis and the other measuring along a Y machine axis, and would typically use a target optic 8 in the form of a plane mirror (though a retroreflector target could also be used). It can be seen that the laser unit 2 of Figure 1 has a spare set of connections available for accommodating a second detector head 4 in this way.

[0011] Figure 5 of the accompanying drawings shows yet another type of detector head 4, which differs more substantially from that of Figures 3 and 4. The detector head 4 of Figure 5 is a differential interferometer detector head 4, with a pair of measurement beams 9M emitted from the laser aperture 16 as well as a pair of reference beams 9R. By using an external reference beam 9M, the differential interferometer detector head 4 is able to measure the relative displacement between two plane mirror targets, one of which (the reference target) would typically be in a fixed position, for example on a fixed column of the machine, and the other of which (the measurement target) would be moving, for example on a moving stage of the machine on which a semiconductor wafer or other workpiece is supported. This helps to ensure accurate positioning between process critical components and to eliminate common mode errors.

[0012] For a detector head 4 of a type as shown in Figures 3 and 4 the electrical cable 3 is fixedly connected to the detector head 4, but for a detector head 4 of a type as shown in Figure 5 an electrical connector 13 is provided for releasably connecting the electrical cable 3 to the detector head 4. For all of these detector heads 4 the electrical cable 3 is releasably connectable at the other end to the laser unit 2. On the other hand, for all of these detector heads 4 the fibre optic conduit 5 (with enclosed fibre optic cable 11) is detachable from the detector head 4 (as will be explained in more detail below) but is fixedly coupled into the laser unit 2. In this respect, the fibre optic cable 11 is continuous from the fibre launch within the laser unit 2 all the way to the collimator 17 and therefore cannot easily be disconnected from the laser unit 2. The detector head 4 typically incorporates a feature that prevents the laser beam 9 being emitted if either the fibre optic cable 11 or electrical cable 3 is disconnected.

[0013] The collimator 17, the strain relief 15, fibre optic conduit 5 (with enclosed fibre optic cable 11) and fibre optic connector 34 (see Figure 1) can be considered to form a single optical fibre assembly 32, with the collimator 17 being considered as an integral part of the optical fibre assembly 32 because it terminates the fibre optic cable 11 in a manner required by the detector head 4 (and the laser encoder system 1 as a whole). The components of the optical fibre assembly 32 (and the internal components of the collimator 17) are assembled and aligned precisely in the manufacturing facility, and supplied to the customer as a unit, and as such are considered to be inseparable in normal use. The way in which the detector head 4 is adapted to receive the optical fibre assembly 32 will be more apparent from Figures 6 and 7 of the accompanying drawings.

[0014] When the laser encoder system 1 is being configured for operational use, the supplied optical fibre assembly 32 is connected to the detector head 4 simply by pushing the collimator 17 through a correspondingly sized opening formed in the housing 24 and into the body of the detector head 4, with the strain relief 15 remaining outside the body and in contact with the housing 24. This connection operation is illustrated in Figure 6 for a differential interferometer detector head 4 of a type shown in Figure 5, but the connection would be entirely equivalent for a detector head 4 of a type shown in Figures 3 and 4. The combination of the strain relief 15 and collimator 17 can be referred to as a fibre barrel 10, with the optical fibre assembly 32 being terminated by the fibre barrel 10.

[0015] Figure 7 shows a view of the differential interferometer detector head 4 of Figure 4 with the upper part (or lid) of the housing 24 removed. This illustrates how the collimator 17, having been pushed into the body of the detector head 4, is held in place tightly by a clamp 20, which is in turn tightened via a locking screw 22. It will also be apparent from Figures 6 and 7 that the collimator 17 is rigidly coupled to the strain relief 15, consistent with the above explanation that the collimator 17 and the strain relief 15 form part of a unitary optical fibre assembly 32. The strain relief 15 and collimator 17 are prevented from being pulled away from the detector head 4 by action of the clamp 20 on the collimator 17, but there could be an additional coupling of the strain relief 15 to the housing 24 (e.g. via a screw thread connection).

[0016] For a proper connection, the fibre barrel 10 (and in particular the collimator 17) must be inserted into the detector head 4 in the correct orientation. To achieve this, and as shown in Figure 6, a line L marked on the fibre barrel 10 is aligned visually with an alignment dot D marked on the detector head 4. The fibre barrel 10 is inserted fully and then rotated slightly until a ball bearing inside the clamp 20 can be felt to engage in a recess 38 in an outer surface of the collimator 17 (specifically in a lid of the collimator 17), with the ball bearing being biased by a spring 39 (see Figure 7) into the indent 38 to form a detent feature. There would also be an end stop in the lid of the detector head 4 which sets the position along the insertion axis, so that the indent 38 is in the correct axial position to receive the ball bearing that is biased into it by the spring 39.

[0017] As can be seen in Figure 8 of the accompanying drawings, a differential interferometer type of detector head 4 (as described above with reference to Figure 5) contains four integrated beam steering components so that both pitch and yaw adjustments can be made on the pair of measurement beams 9M and independently on the pair of reference beams 9R.

[0018] To perform alignment, the user simply adjusts the four controls 40 the back of the detector head 4 with a driver. Each control 40 has a suffix in the form “be” where “b” denotes the beam (or beam pair) that is being controlled (“r” for the reference beams 9R and “m” for the measurement beams 9M) and where “c” denotes the type of control performed (“p” for pitch and “y” for yaw). Accordingly, controls 40rp and 40ry are for adjusting pitch and yaw respectively of the pair of reference beams 9R, while controls 40mp and 40my are for adjusting pitch and yaw respectively of the pair of measurement beams 9M. If either or both parts of the suffix is not stated then the reference is generic in respect of the missing part; for example reference 40p would apply to both 40rp and 40mp while reference 40 would apply to all four variants. The same convention is used for other similar parts described herein that are replicated for each of the four controls 40.

[0019] There are up / down (pitch) or left / right (yaw) pictorial representations alongside each of the controls 40 as a reminder to the user of what each control does. Each pictorial representation has four dots, representing a pair of reference beams 9R above and a pair of measurement 9M beams below, and with an arrow showing the adjustment obtained (bearing in mind that the beams are emitted orthogonally from the detector head 4 in a direction into the page).

[0020] Figure 9 of the accompanying drawings shows two optical wedges 42p, 42y which are used as a basis for adjusting pitch and yaw respectively by turning the pair of controls 40p and 40y. The arrangement of Figure 9 is referred to as a Risley prism pair and has been previously proposed for beam steering applications (see for example “Risley Prism Beam Pointer” by Ostaszewski et al, Proc. SPIE 6304, Free-Space Laser Communications VI, 630406, 2006). It will be understood that the order of the optical wedges 42p, 42y could be reversed.

[0021] The optical wedges 42p and 42y are coupled mechanically to the corresponding controls 40p and 40y respectively (i.e. there is a pair of optical wedges 42rp and 42ry coupled to controls 40rp and 40ry respectively, and another pair of optical wedges 42mp and 42my coupled to controls 40mp and 40my respectively), such that rotation of each control 40 causes a corresponding rotation of the coupled wedge 42 around its optical axis (which is aligned with the beam path) by a predetermined amount. Figure 9 shows the wedges 42p, 42y in a “neutral” position in which the thick part of wedge 42p is aligned with the thin part of wedge 42y and vice versa. In this “neutral” position there is no overall deflection of the beam, since a deflection one way is exactly matched by a corresponding deflection the other way, as illustrated in the corresponding ray diagram of Figure 10 of the accompanying drawings.

[0022] Starting from the neutral position shown in Figures 9 and 10, rotating wedge 42y around its optical axis will adjust yaw, while rotating wedge 42p around its optical axis will adjust pitch. This is shown in Figures 11 and 12 of the accompanying drawings (which correspond respectively to Figures 9 and 10) for an extreme rotation of the wedge 42p by 180 degrees, so that the thickest part of the wedges 42p, 42y are aligned after the rotation, and this results in a change in pitch angle of 0. A similar adjustment would be made in respect of yaw by rotating wedge 42y.

[0023] Typically, the adjustments would in practice be less extreme than represented in these drawings, with the beam moving along arcs but approximately in pitch and yaw directions for these small adjustments. For example, the current differential interferometer detector head from Renishaw pic (when using optical wedges each of which has opposed faces at four degrees relative to one another) would typically give a change in beam direction of around ±1.7° for a wedge rotation of ±25° (amounting to a ratio of around 15: 1). A wedge rotation of ±25° requires the corresponding control 40 to be turned by ±162.5° (amounting to a ratio of 6.5: 1). The ratio between the control angle and the beam angle is around 96:1.

[0024] In view of the above, it will be apparent that the beam steering arrangement used in the differential interferometer detector head 4 shown in Figures 5 to 8 requires a total of four beam steering wedges 42: a first pair of beam steering wedges 42rp, 42ry for adjusting pitch and yaw of the reference beam (or beam pair) 9R and a second pair of beam steering wedges 42mp, 42my for adjusting pitch and yaw of the measurement beam (or beam pair) 9M. For the other types of detector head 4 as shown in Figures 3 and 4, there is a single beam steering component 30 (see Figure 2) for adjusting pitch only, with the pitch adjustment being made by rotating a ring around the laser aperture 16; yaw adjustment would be achieved simply by reorienting the body of the detector head 4 itself.

[0025] When setting up a detector head for operational use, pitch and yaw are adjusted separately, one at a time, using a beam steering arrangement as described above to achieve the desired beam alignment. The present applicant has appreciated that this can be a somewhat iterative and time-consuming process, with a certain degree of trial and error involved due to one of more of the following factors: (a) backlash or play in the drive mechanism; (b) residual stress in parts of the drive mechanism which can cause the beam alignment to drift or relax away from what it is set to initially (this can be referred to as hysteresis); (c) poor concentricity (or uncertain centration) of the beam steering wedges as they rotate during a beam steering adjustment leading to a lack of repeatability (where a slightly different beam angle results from the same control angle); and (d) interdependence between the adjustments made to each of the four beam steering wedges, so that making an adjustment to one can have an effect on an adjustment already made to another (this can be referred to as cross torque). The present applicant has also appreciated that there is the risk of internal damage being caused by excessive torque being applied to the adjustment controls, which may also slow down the setup procedure if the user is having to take extra care to ensure that this does not happen. Although these are relatively small effects that would not negatively impact most applications, they can be apparent in the most demanding of applications.

[0026] Accordingly, the present applicant has appreciated that it would be desirable to make some improvements to the mechanical design of the beam steering arrangement to make the setup and alignment procedure quicker, easier and more predictable for the user, without risk of internal damage, as well as to help provide a stable beam alignment that does not drift during operational use.

[0027] According to a first aspect of the present invention there is provided a device which is operable to emit a laser beam, the device comprising a beam steering arrangement for steering the beam towards an intended target, the beam steering arrangement comprising a rotatable carrier on which a steering optic is mounted and through which steering optic the beam is arranged to pass before being emitted from the device, and a rotatable pinion gear which engages with a corresponding carrier gear provided on the carrier such that rotation of the pinion gear causes a corresponding rotation of the steering optic and thereby a corresponding steering of the beam, and further comprising a biasing member which is adapted and arranged to provide a biasing force which biases the pinion gear into engagement with the carrier gear. The device may be a laser encoder device.

[0028] The laser encoder device may be a detector head or detector unit.

[0029] The laser beam may be a measurement or reference laser beam.

[0030] The laser beam may be used as the basis for an interferometric measurement of distance from the device to a target.

[0031] The carrier gear may function as a rack in a rack-and-pinion arrangement.

[0032] The biasing of the pinion gear into engagement with the carrier gear by the biasing member may provide a nesting force for a kinematic coupling which is adapted to constrain the carrier kinematically within the device in two translational degrees of freedom relative to a predetermined axis of the steering optic, with a rotational degree of freedom around the predetermined axis being unconstrained by the kinematic coupling. This thereby provides a stable rotation axis for the steering optic.

[0033] The other three degrees of freedom may be constrained non-kinematically.

[0034] The predetermined axis may be an optical axis of the steering optic.

[0035] The nesting force may be arranged to bias the carrier into contact with first and second contact features which provide two corresponding respective constraints for the carrier in the two translational degrees of freedom.

[0036] The carrier may comprise first and second concentric surfaces having different radii which are biased by the nesting force into contact with the first and second contact features respectively. The biasing of the pinion gear into engagement with the carrier gear by the biasing member may ensure that contact is made between both sides of each tooth (or at least a plurality of teeth) on the pinion gear and a corresponding pair of adjacent teeth on the carrier gear as the pinion gear is rotated. This thereby avoids or at least reduces backlash, eliminating or at least reducing slack in either direction of rotation for the pinion gear, even after wear of the teeth of one or both of the pinion gear and carrier gear.

[0037] The biasing of the pinion gear into engagement with the carrier gear by the biasing member may form part of an overtravel feature in which at least one tooth of the carrier gear that is contacted by the pinion gear during an overtravel condition (in which the carrier gear is prevented from further movement by a stop member) is adapted so as to have a profile which causes the pinion gear to slide over and lift away from the carrier gear, against the bias provided by the biasing member, rather than attempt to drive the carrier gear further. This thereby avoids causing damage to one or both of the pinion gear and carrier gear, or to any associated drivetrain components.

[0038] The pinion gear may be coupled rigidly to a control via a drive shaft.

[0039] The drive shaft may be provided with a ball joint arrangement to allow the drive shaft to pivot about the ball joint when the pinion gear lifts away from the carrier gear.

[0040] The at least one tooth of the carrier gear may be formed with a pressure angle that is sufficiently large to produce a radial force on the pinion gear that is sufficient to overcome the bias provided by the biasing member.

[0041] At least two teeth of the carrier gear may be so adapted.

[0042] A trailing (or following) tooth of the pinion gear may make contact with one of the at least two teeth before a leading tooth of the pinion gear loses contact with another of the at least two teeth. This thereby avoids a double click sound as the pinion gear is turned during an overtravel condition.

[0043] The teeth of the pinion gear and / or the carrier gear may have or may be based on an involute profile.

[0044] The carrier may be supported on a support member of the device.

[0045] A contact feature may be formed on the support member which is arranged to act as a contact for the kinematic coupling.

[0046] A relief pocket may be formed in the surface of the support member adjacent to the contact feature to prevent imperfections in the contact feature affecting the kinematic nature of the coupling.

[0047] The other three degrees of freedom may be constrained non-kinematically via a surface-to-surface contact between the carrier and the support member.

[0048] The beam steering arrangement may comprise a plurality of such rotatable carriers and associated features. These may be provided for use in steering the beam in different respective directions (e.g. pitch and yaw) and / or for use in steering different respective beams that are emitted from the device.

[0049] Each of the carriers may be provided with its own individual brake flexure member to hold the carrier in position. This thereby avoids or at least reduces cross torque between them, allowing an adjustment to be made to one without affecting an adjustment already made to another.

[0050] The beam steering arrangement may comprise a self-supporting and / or self- contained assembly having a frame (or chassis) which carries the pinion gears and associated drive shafts for all the carriers.

[0051] A stop member for one of the plurality of carriers may also act as a contact for the kinematic coupling of another of the plurality of carriers, or vice versa.

[0052] A feature may be formed on the support member which acts both as a stop member for one of the plurality of carriers and as a contact for the kinematic coupling of another of the plurality of carriers.

[0053] The steering optic may be an optic wedge.

[0054] According to a second aspect of the present invention there is provided a device which is operable to emit a laser beam, the device comprising a beam steering arrangement for steering the beam towards an intended target, the beam steering arrangement comprising a rotatable carrier on which a steering optic is mounted and through which steering optic the beam is arranged to pass before being emitted from the device, and a rotatable pinion gear which engages with a corresponding carrier gear provided on the carrier such that rotation of the pinion gear causes a corresponding rotation of the steering optic and thereby a corresponding steering of the beam, wherein the beam steering arrangement comprises a plurality of such rotatable carriers and associated features, and wherein each of the carriers is provided with its own individual brake flexure member to hold the carrier in position.

[0055] Reference will now be made, by way of example, to the accompanying drawings, in which:

[0056] Figure 1, discussed hereinbefore, illustrates a known fibre optic laser encoder system comprising a laser unit, detector head and machine interface;

[0057] Figure 2, also discussed hereinbefore, is a schematic view of some of the internal components of the detector head of Figure 1;

[0058] Figure 3, also discussed hereinbefore, shows in more detail a detector head of the type shown in Figures 1 and 2;

[0059] Figure 4, also discussed hereinbefore, shows a different type of detector head in which the laser beam is emitted from the body at a different angle;

[0060] Figure 5, also discussed hereinbefore, shows a differential interferometer type of detector head which emits both measurement and reference beams;

[0061] Figure 6, also discussed hereinbefore, shows how the fibre barrel of an optical fibre assembly is connected to a differential interferometer type of detector head;

[0062] Figure 7, also discussed hereinbefore, shows a view of the detector head of Figure 6 with the upper part of the housing removed;

[0063] Figure 8, also discussed hereinbefore, shows four controls that are used to adjust pitch and yaw of four corresponding beam steering components (or wedges) in a differential interferometer detector head of a type as shown in Figure 5;

[0064] Figure 9, also discussed hereinbefore, shows a pair of beam steering components that are used as a basis for adjusting pitch and yaw respectively of a beam (or pair of beams), with the components arranged in a neutral steering position so that there is no deflection of a beam passing through the beam steering arrangement;

[0065] Figure 10, also discussed hereinbefore, is a ray diagram corresponding to Figure 9 to show that there is no deflection of a beam passing through the beam steering arrangement;

[0066] Figure 11, also discussed hereinbefore, shows the pair of beam steering components of Figure 9 after the beam steering component associated with pitch adjustment has been rotated relative to the other beam steering component of the pair to cause a change in pitch of the beam;

[0067] Figure 12, also discussed hereinbefore, is a ray diagram corresponding to Figure 11 to show how a beam passing through the beam steering arrangement has been deflected to cause a change in pitch;

[0068] Figure 13 shows a perspective section view of a detector head embodying the present invention with a vertical section through the reference beam controls;

[0069] Figure 14 is a perspective section view of just the beam steering arrangement from the detector head, with the same vertical section as Figure 13;

[0070] Figure 15 shows a similar perspective section view to that of Figure 14, but with the beam steering arrangement separated into two self-supporting assemblies;

[0071] Figure 16 is a plan view looking down onto a support member of the beam steering arrangement, with a top layer of geared carriers supported thereon;

[0072] Figure 17 shows a perspective view of the support member, for clear visibility of various features formed on the support member, such as those which define stop positions and kinematic locations for the geared carriers;

[0073] Figure 18 shows a close-up plan view of the lower-most geared carrier from Figure 16, with the carrier in a middle position of its overall range of travel, and is used to explain how the carrier is provided with a kinematically-defined axis of rotation;

[0074] Figure 19 shows a simplified version of the gear and biasing arrangement of Figure 18; Figure 20 shows a close-up plan view of the upper-most geared carrier from

[0075] Figure 16, with the carrier at one end of its overall range of travel and in contact with a stop member and with the pinion gear in an overtravel condition;

[0076] Figure 21 is the first in a sequence of sixteen drawings for explaining operation of a beam steering arrangement embodying the present invention, with each drawing in the sequence showing the pinion gear and carrier in a different rotational position;

[0077] Figure 22 is the second in the sequence of drawings starting with Figure 21;

[0078] Figure 23 is the third in the sequence of drawings starting with Figure 21;

[0079] Figure 24 is the fourth in the sequence of drawings starting with Figure 21;

[0080] Figure 25 is the fifth in the sequence of drawings starting with Figure 21;

[0081] Figure 26 is the sixth in the sequence of drawings starting with Figure 21;

[0082] Figure 27 is the seventh in the sequence of drawings starting with Figure 21;

[0083] Figure 28 is the eighth in the sequence of drawings starting with Figure 21;

[0084] Figure 29 is the ninth in the sequence of drawings starting with Figure 21;

[0085] Figure 30 is the tenth in the sequence of drawings starting with Figure 21;

[0086] Figure 31 is the eleventh in the sequence of drawings starting with Figure 21;

[0087] Figure 32 is the twelfth in the sequence of drawings starting with Figure 21; Figure 33 is the thirteenth in the sequence of drawings starting with Figure 21;

[0088] Figure 34 is the fourteenth in the sequence of drawings starting with Figure 21;

[0089] Figure 35 is the fifteenth in the sequence of drawings starting with Figure 21;

[0090] Figure 36 is the sixteenth in the sequence of drawings starting with Figure 21;

[0091] Figure 37 is the first in a sequence of four drawings for explaining the standard concept of an involute gear profile and how it is characterised by a pressure angle, with each drawing in the sequence showing two meshed gears in different rotational positions;

[0092] Figure 38 is the second in the sequence of drawings starting with Figure 37;

[0093] Figure 39 is the third in the sequence of drawings starting with Figure 37;

[0094] Figure 40 is the fourth in the sequence of drawings starting with Figure 37;

[0095] Figure 41 is the first in a sequence of two drawings similar to the sequence of Figures 37 to 40 but shorter and for gears having a lower pressure angle;

[0096] Figure 42 is the second in the sequence of drawings starting with Figure 41;

[0097] Figure 43 shows an enlarged version of the carrier gear used in a beam steering arrangement embodying the present invention, and is for use in explaining how certain of the teeth are reprofiled away from the standard involute form;

[0098] Figure 44 shows a comparison between a geared carrier used in a known beam steering arrangement and a geared carrier used in a beam steering arrangement embodying the present invention;

[0099] Figure 45 shows a view looking towards the right side of Figure 14 with various parts removed to show the use of a kinematically supported ball joint provided on each drive shaft in a beam steering arrangement embodying the present invention; and

[0100] Figure 46 shows a view similar to that of Figure 14 but with various parts removed to show a brake flexure provided independently for each steering optic in a beam steering arrangement embodying the present invention.

[0101] Various views of a detector head 4 comprising a beam steering arrangement 50 embodying the present invention are shown in Figures 13 to 20. In these various views, like parts are numbered with like reference numerals. Figure 13 shows a perspective section view of the detector head 4 with a vertical section through the controls 40ry, 40rp of the beam steering arrangement 50, which are those for the reference beams 9R and also those that are situated closest to the beam end of the detector head 4. The section shows that each control 40 is rigidly coupled via a drive shaft 41 to a pinion gear (or just pinion) 43, such that the pinion gear 43 is rotated in a 1 :1 ratio via rotation of the control 40.

[0102] Figure 14 is a perspective section view of just the beam steering arrangement 50 from the detector head 4, with the same vertical section as Figure 13. Marked on Figure 14 is a carrier 45ry on which the optical wedge 42ry is supported. The carrier 45ry can also be referred to as a sub-mount. Carrier 45ry comprises a carrier gear 46ry on an edge surface therefore, which engages or meshes with the pinion gear 43ry to form a rack-and-pinion arrangement, with the rack side of this arrangement being provided by the carrier gear 46ry. Rotation of the pinion gear 43ry (via rotation of the control 40ry) causes rotation of the carrier 45ry and accordingly also of the supported optical wedge 42ry. This thereby enables yaw adjustment of the reference beams 9R. Similarly, a carrier 45rp comprises a carrier gear 46rp on an edge surface therefore, which engages or meshes with a pinion gear 43rp such that rotation of the pinion gear 43rp (via rotation of the control 40rp) causes rotation of the carrier 45rp and accordingly also the supported optical wedge 42rp. This thereby enables pitch adjustment of the reference beams 9R. There are corresponding features (partially hidden in the view shown in Figure 14 but more visible in other views) for yaw and pitch adjustment of the measurement beams 9M. The optical wedges 42 have a substantially circular profile in plan view, similar to those shown in Figures 9 and 11.

[0103] Figure 15 shows a similar perspective section view to that of Figure 14, but shows that the beam steering arrangement 50 can conveniently be separated into two self-contained and self-supporting assemblies. The upper assembly comprises a frame (or chassis) 54 which supports the controls 40, drive shafts 41 and pinion gears 43. By grouping these components into two self-contained assemblies this allows for a very convenient way of assembling the detector head 4, such that each of the two assemblies can be assembled separately, and then later slotted together when assembling the final product. As is more apparent from Figure 14, when in place within the detector head, the assembly is arranged such the upper cover for the detector head 4 does not touch the assembly and therefore does not move it (or any of the supported component) out of alignment if flexed or knocked. Figure 15 also shows the toothed profile of the pinion gears 43my and 43mp more clearly. The frame 54 may house or span optical components allowing the drive shafts 41 (and pinion gears 43) to be placed close to the optical path to reduce hysteresis and backlash. Steering may be carried out by manual adjustment of the controls 40 or by using a piezo or stepper motor arrangement.

[0104] Also shown in Figures 14 and 15 is a biasing member 44ry which is arranged to bias pinion gear 43ry into the carrier gear 46ry. The biasing member is compressed between the pinion gear 43ry and a lower housing part of the detector head 4 (see Figure 13), thereby biasing pinion gear 43ry towards the carrier gear 46ry. There is a corresponding biasing member 44 associated with each of the other three pinion gears 43. The interaction between these biasing members 44 and their corresponding respective pinion gears 43 is shown most clearly in the plan view of Figure 16, looking down onto the top layer of geared carriers 45ry and 45mp. The upper pair of geared carriers 45ry and 45mp are supported on a profiled support member 52, with the lower pair of geared carriers 45rp and 45my being supported on the underside of the support member 52; hence the support member 52 can be referred to as a mid plate because it sits in the middle between (and supports) two pairs of the carriers 45 (and is in the form of a plate). The lower pair of geared carriers 45rp and 45my is mostly not visible in Figure 16 but the corresponding carrier gears 46rp and 46my are just visible (and meshing with corresponding pinion gears 43rp and 43my respectively).

[0105] Figure 16 shows the geared carrier 45ry in a position in which it has been turned to the fullest extent in an anticlockwise direction by the pinion gear 43ry, with a leading surface of the geared carrier 45ry up against a stop member formed on the support member 52. Geared carrier 45mp is in a position in which it has been turned to the fullest extent in a clockwise direction by the pinion gear 43mp, with a leading surface of the geared carrier 45mp up against another stop member formed on the support member 52. These stop members, which will be discussed in more detail below, are also shown clearly in the perspective view of the plate 50 in Figure 17.

[0106] An advantageous feature of the beam steering arrangement 50 will now be discussed with reference to Figure 18, which shows a close-up plan view of the lower-most geared carrier 45ry from Figure 16, with the carrier 45ry having been driven to a position near the middle of its overall range of travel (between two stop positions) by the associated pinion gear 43ry. The carrier 45ry has a planar form (it can be formed from a plate material) and rests with a surface-to-surface contact against a planar surface of the support member 52 (as will be apparent for example from the section view of Figure 14). Accordingly, in the absence of any other constraints to motion, the carrier 45ry would be free to slide in three degrees of freedom along the surface of the support member 52: two translational degrees of freedom (lateral translation along the surface of the support member 52) and one rotational degree of freedom (rotation around an axis perpendicular to the surface of the support member 52).

[0107] In the beam steering arrangement for the existing version of the detector head 4, lateral movement of the carrier 45ry (and optical wedge 42ry) is achieved simply by surrounding the optical wedge 42ry (which has a circular profile in plan view) within a circular guide, so that it is free to rotate within the circular guide but not move laterally. The present applicant has appreciated that this can lead to the optical wedge 42ry being in a slightly different lateral position each time it returns to the same rotational position, which makes beam steering adjustment more difficult and time consuming.

[0108] Accordingly, in the beam steering arrangement 50 embodying the present invention, and as illustrated in Figure 18, the carrier 45ry is constrained in a kinematic manner to provide a more repeatable lateral position for the optical wedge 42ry which it supports. In this respect, of the three degrees of freedom available to the optical wedge 42ry it is required to constrain two of them (lateral translation in two orthogonal directions along the surface of the support member 52) leaving just one unconstrained degree of freedom (rotation around an axis perpendicular to the surface of the support member 52). To comply with kinematic design principles, this amounts to providing exactly two corresponding constraints, no more and no less. Accordingly, two raised features are formed on the support member 52 (see also Figure 17) which are profiled so as to provide exactly two points of contact C 1 and C2 with a circular outer edge of the carrier 45ry. For space saving reasons, the outer edge of the carrier 45ry is formed not as a single circular surface but rather as two part-circular surfaces SI and S2 having different respective radii, with the two part-circular surfaces SI and S2 being completed in dotted line in Figure 18 for clarity. The two points of contact Cl and C2 provide kinematic constraint (or exact constraint) for the carrier 45ry such that it rotates around an axis of rotation (marked in Figure 18 at the centre of the circular surfaces SI, S2) that is positionally very stable and repeatable.

[0109] In the context of locating a first body relative to a second body, kinematic design considerations are met by constraining the degrees of freedom of motion of the first body relative to the second body using the minimum number of constraints (also referred to as exact constraint). Where there is over constraint in a coupling between the two bodies, i.e. where the coupling provides more than the minimum number of constraints by providing at least one redundant constraint, it is not possible to determine with any certainty which combination of constraints will determine the actual position of the first body relative to the second body. Accordingly, the position of the first body relative to the second body is not repeatable, because it is not known at which of the several possible positions the first body will come to rest relative to the second body when they come together again.

[0110] Furthermore, where there is over constraint, the first body does not assume a stable position relative to the second body because it may move between two or more of the different possible positions when a force is applied. An example of this is a four-legged table which will often rock between two different positions when placed on a flat surface. This is because the minimum number of constraints in this context is three, to constrain relative motion in three corresponding degrees of freedom (two rotational degrees of freedom and one translational degree of freedom), whereas a four-legged table has four constraints (created by contact between each of the four legs and the flat surface), such that the coupling between the table and the flat surface is over constrained.

[0111] In general, a single constraint is required for each degree of freedom to be constrained, and a single point contact between two bodies creates a single constraint. Therefore, to constrain a first body relative to a second body in all six degrees of freedom, the coupling between them would need to define six points of contact that are mutually arranged to constrain the two bodies relative to one another in six corresponding degrees of freedom without any redundancy. It will be understood that these points of contact need not be (and in practice would not be) mathematical points in the pure sense. Instead, in practice, each of these would typically be a small area that approximates a point. However, even though each point of contact may not be pure in a mathematical sense, the coupling can still be referred to as a kinematic coupling because it can still be considered to follow kinematic design principles. The term kinematic, as used herein in the context of a coupling between two bodies, is to be interpreted accordingly as meaning kinematic or at least pseudo-kinematic, and similarly for like terms such as kinematically.

[0112] In view of the above, it will be understood that use of a kinematic coupling between a first body and a second body provides a kinematically-defined position for the first body relative to the second body that is one of more of: unique, discrete, repeatable, consistent, predictable and stable (at least in respect of the degrees of freedom that are constrained by the coupling, i.e. ignoring any unconstrained degrees of freedom).

[0113] Because of its reliance on the principle of exact constraint, a kinematic coupling between two bodies is also by its nature readily couplable and decouplable. This is because, in order to prevent the two bodies from coming away from the constraints that define their relative position, it would be necessary to provide an additional (and opposing) constraint, such as a clamp, and the additional constraint would itself result in over constraint and would therefore turn it from a kinematic coupling into a non-kinematic coupling.

[0114] That said, a kinematic coupling does typically require some form of nesting force (or biasing force) to hold the coupled bodies together, but without creating an additional constraint. In this respect, a theoretical constraint is not just a contact point alone, but also includes a corresponding nesting force that maintains the contact. The nesting force is a force vector that goes through the contact point normal to the surfaces of contact, but these can be vectorially combined into a single force. For example, a gravitational or magnetic force can be used as a nesting force.

[0115] By way of further background, these concepts are explored further in: (a) “Mechanical Design of Laboratory Apparatus” by H. J. J. Braddick, Chapman & Hall, London, 1960, pages 11-30; (b) “Exact constraint” by James G. Skakoon, Mechanical Engineering, September 2009; (c) “Kinematic Couplings: A Review of Design Principles and Applications” by Alexander Slocum, International Journal of Machine Tools and Manufacture 50.4 (2010): 310-327; and (d) “Principles and Techniques for Designing Precision Machines” by Layton C. Hale, Ph.D. thesis, Massachusetts Institute of Technology, February 1999.

[0116] In the kinematic coupling shown in Figure 18, the above-mentioned nesting force is provided by the biasing member 44ry which is arranged to bias the pinion gear 43ry into the carrier gear 46ry, and thereby biasing the surfaces SI and S2 of carrier 45ry into contact at Cl and C2 respectively. This provides kinematic constraint in two translational degrees of freedom, leaving a single unconstrained degree of freedom which is rotation around the perpendicular axis marked in the centre of the circles defined by the surfaces SI and S2. Of course, because there is surface-to-surface contact between the carrier 45ry and the support member 52, constraint in the other degrees of freedom is not kinematic in nature, but the stability and repeatability of the carrier 45ry (and supported wedge 42ry) in those other degrees of freedom is not critical.

[0117] The kinematically-defined rotation axis achieved with the above-described arrangement provides an advantage over the existing beam steering arrangement, in which the beam steering wedge is itself used as a centring feature. In this respect, the cylindrical beam steering wedge in the existing beam steering arrangement is located within a cylindrical bore having a slightly larger diameter than the beam steering wedge itself, so that the beam steering wedge is constrained by and rotates within the bore. Because there is some clearance between the beam steering wedge and the inside surface of the bore, and because the contact between them is non-kinematic in nature, there is inevitably some lateral movement and therefore the centration or concentricity of the beam steering wedge within the bore is somewhat uncertain. In addition, because there is some load transmitted to the beam steering optic by direct contact with the surface of the bore, this can place some stress on the glue joint between the beam steering optic and the carrier. With a kinematic support arrangement as used in an embodiment of the present invention (see Figure 18), the load is transmitted through the kinematic contact points onto the carrier 45, rather than onto the supported optical wedge 42 itself, so the above-mentioned problem is overcome. So, there is a dual advantage of a stable and repeatable rotation axis as well as a mechanically more robust construction.

[0118] A simplified version of the gear and biasing arrangement shown in Figure 18 is provided in Figure 19, showing just the carrier 45ry being biased into contact with the two contact features Cl, C2 by the pinion gear 43ry. The sprung pinion gear 43ry can be considered to serve at least four purposes. Firstly, it engages with the carrier gear 46ry to provide a rotational drive for the carrier 45ry (and supported wedge 42ry) by turning the control carrier 40ry, thereby providing the primary beam steering function. Secondly, it acts as a biasing member to provide a nesting force for the kinematically-defined rotation axis provided for the carrier 45ry as discussed above, thereby providing centring force for the steering optics. Thirdly, in addition to providing a nesting force for the kinematic constraint as discussed above, the biasing of the pinion gear 43ry into the carrier gear 46ry (by the biasing member 44ry) also ensures that both sides of each of the pinion teeth remains in contact with the involute gear form of the carrier gear 46ry, thereby eliminating backlash caused by clearances (e.g. due to wear and manufacturing tolerances) that would typically be present between two meshing gears. Fourthly, since the pinion gear 43ry enables the centre distance between the gears to float or flex instead of being fixed, this along with a modified design for the teeth of the carrier gear 46ry also provides an overtorque or overtravel system which protects drivetrain components, as will now be discussed in more detail with reference to Figures 20 to 36.

[0119] Figure 20 shows a close-up plan view of the upper-most geared carrier 45mp from Figure 16, with the carrier 45mp having been driven to a position at one end of its overall range of travel by the associated pinion gear 43mp. Two stop positions Pl and P2 are defined by two features formed in the support member 52 (see Figure 17), with the carrier 45mp being driven up against stop position Pl by anticlockwise rotation of pinion gear 43mp. The carrier gear 46mp has eleven teeth (with ten roots in between), and in the normal range of travel (for example as shown in Figure 18) each tooth of the pinion gear 43mp is pushed into the root between two teeth of the carrier gear 46mp as pinion gear 43mp revolves around. However, in the stop position shown in Figure 20 two of the teeth of the pinion gear 43mp are shown as sliding up the last two teeth of the carrier gear 46mp, with these final teeth having a modified form compared to the others, such that the pinion gear 43mp is being pushed outwards (against the biasing force from biasing member 44mp) as shown by the arrow. This forms the basis of the overtravel or overtorque protection in a beam steering arrangement 50 embodying the present invention, as will now be explained further with reference to the sequence shown in Figures 21 to 36.

[0120] Figure 21 shows the pinion gear 43 and carrier 45 (and associated gear 46) at a starting point for the sequence with the carrier 45 in a central position within its overall range of travel. To avoid unnecessary clutter in the sequence of drawings shown in Figures 21 to 36 reference numerals have been omitted. A dashed line is shown in Figure 21 between the centre of rotation for the carrier 45 (which is defined kinematically as discussed above) and the centre of rotation for the pinion gear 43. The carrier gear 46 is symmetrical about the dashed line, and everything is orientated such that the dashed line is vertical in the drawing. A circle is also shown in dashed line as a continuation of the outer surface of the carrier 45 in a similar manner to what is shown in Figure 18. In this position, a flat surface within the carrier 45 is also oriented at zero degrees to the vertical, parallel with the dashed line. The two stop positions Pl and P2 discussed above with reference to Figures 18 to 20 are also illustrated schematically in Figure 21, as is the biasing or nesting force applied to the pinion gear 43 along the dotted line between the respective centres of rotation for the pinion gear 43 and the carrier 45. A marker dot has been added to the representation of the pinion gear 43 to help in an interpretation of the sequence drawings as the pinion gear 43 rotates. Also shown in Figure 21 is a horizontal line passing through the centre of the pinion gear 43 in this position, which will act as a reference against which to compare the vertical height of the pinion gear 43 in other drawings in the sequence. It should be noted that the form or profile of the gear teeth on the pinion gear 43 and carrier gear 46 depicted in this sequence of drawings, and indeed the form of the various other surfaces of the carrier 45, are close to an actual working form or profile and are therefore not merely schematic in nature.

[0121] Figure 22 shows these parts after a small anticlockwise rotation of the pinion gear 43, which causes a corresponding clockwise rotation of the carrier 45 (and therefore also the supported wedge 42, thereby providing a beam steering function). Because of the gear ratio, with more teeth on the carrier gear 46 than the pinion gear 43 (albeit with only a subset of the full set of teeth being required on the carrier gear 46 because of the limited range of travel required), the angle rotated by the carrier 45 is less than that of the pinion gear 43, which helps to steer the beam more accurately (with a finer resolution) than would be the case for a 1 : 1 gear ratio.

[0122] Both the pinion gear 43 and the carrier gear 46 are based on the well-known involute gear profile in which the profiles of the gear teeth are involutes of a circle. A defining characteristic of the involute gear profile is something called the pressure angle, and the concept of a pressure angle will be discussed briefly with reference to the Figures 37 to 42, before returning to the sequence of Figures 21 to 36.

[0123] The pair of gears in the sequence shown in Figures 37 to 40 both have a completely standard involute profile. The upper gear has eight teeth (with adjacent tooth spaced apart by an angle A of 45°) while the lower gear has sixteen teeth (with adjacent tooth spaced apart by an angle A of 22.5°), giving a gear ratio of 1 :2. The sequence shows the upper gear being turned clockwise by an angle of 15° each time, with the lower gear turning anticlockwise as a result by half that due to the 1 :2 gear ratio.

[0124] As illustrated in this sequence, contact between a pair of gear teeth occurs at a single instantaneous point, and rotation of the gears causes the location of this contact point to move across the respective tooth surfaces. A force is transmitted between the gears along a line of action (or line of the force) LA. Where the line of action crosses the line LC between the two gear centres, as is the case in Figure 37, this defines the pitch point of the gears, where there is no sliding contact. The solid overlaid circle is a pitch line marking the radial position of the pitch points. The pressure angle is the acute angle between the line of action LA and a line LN that is normal to the line connecting the gear centres in this position. Each gear in a pair of gears must typically have the same pressure angle in order for the teeth to mesh properly, and with the example gear profile shown in this sequence the pressure angle of both gears is 27°. As the gears turn, it can be seen that although the contact point moves, the point of contact remains on the fixed line of action LA such that the force exerted on the driven gear is always at the same angle.

[0125] While any pressure angle can be manufactured, it is most common for gears to have a pressure angle of 20°. By way of comparison, a pair of gears with such a pressure angle is shown in the briefer sequence of Figures 41 to 42. The upper gear in Figure 42 is turned clockwise by 22.5° (i.e. half the angle A of 45° between adjacent teeth) compared to the position in Figure 41. It can be seen that the line of action LA is shallower than in the previous sequence, with the force on the driven gear with a pressure angle of 20° being more tangential (less radial) in comparison to a pressure angle of 27°.

[0126] With the above in mind, and referring to an enlarged version of the carrier gear 46 shown in Figure 43, an overtravel mechanism provided in a beam steering arrangement 50 embodying the present invention is based on a reprofiling of the end-most teeth Tl, T2, T10 and T11 on the carrier gear 46 away from the standard involute form, and in particular changing the pressure angle and / or truncating these teeth. This reprofiling is done in a way such that, when the carrier gear 46 has reached an end stop in either direction, further turning of the pinion gear 43 will cause the pinion gear 43 to slip and disengage from the carrier gear 46 rather than attempt to drive the carrier gear 46 further around (which would likely cause damage to the drivetrain components). Gear teeth T3 to T9 have a standard involute profile with a pressure angle of 20°. The outer edges (or outer facing flanks) of penultimate teeth T2 and T10 have been reprofiled to have a pressure angle that is higher than 20°, while the outer edges (or outer facing flanks) of first / last teeth Tl and Ti l have an even higher pressure angle (and these teeth Tl and Ti l are truncated compared to the main teeth T3 to T9).

[0127] In one embodiment, the modified profile for the outer edges (or outer facing flanks) of penultimate teeth T2, T10 provides an effective pressure angle of approximately 40°, which is approximately 20° higher than (or approximately double) the pressure angle of 20° used for the mid-range teeth T3 to T9. In this embodiment the pressure angle of 40° is not derived from one side of an involute tooth form having a pressure angle of 40°. Rather, it is derived from one side of an involute tooth form having a pressure angle of 20° (like that for teeth T3 to T9) and with that one side being rotated about its root by 20° to make an effective pressure angle of 40°. Manipulating the pressure angle in this way changes the ratio of the radial and tangential loads between the gears. Increasing the pressure angle will increase the radial load relative to the tangential load and will thereby make the gears tend to push apart from each other. Since the carrier gear 46 is not rotating during overtravel (only the pinion gear 43 is rotating), it can be considered that the penultimate teeth T2, T10 act more like a cam than a gear tooth, pushing the sprung pinion gear 43 away based on the cam profile. It would also be possible to form the penultimate teeth T2, T10 with one side having an involute profile with a 20° pressure angle and the other side having an involute profile with a 40° pressure angle. Of course, the values for these pressure angles are merely examples and are not intended to be limiting. Furthermore, the exact shape (and pressure angle) of the outer edges (or outer facing flanks) of the penultimate teeth T2, T10 is not particularly important to operation of the overtravel mechanism, so long as it allows the tooth of the pinion gear 43 to slip and rise over it (in the manner of a cam) rather than drive the carrier gear 46 further into the stop (in the manner of a gear). In particular, it will be apparent that the outer edges (or outer facing flanks) of the penultimate teeth T2, T10 need not be derived from an involute form at all.

[0128] As will be more apparent below, the modified profile for the outer edges (or outer facing flanks) of end-most teeth Tl, Ti l takes account of the movement of the pinion gear 43 over them as the pinion gear 43 is turned when in an overtravel condition. In particular, the profile of the outer facing flank (facing away from the stop) of the end-most tooth Tl (or Tl 1) follows the swept path of the approaching tooth of the pinion gear 43 resulting from the action of the lead tooth of the pinion gear 43 on the penultimate tooth T2 (or T10), with the end-most teeth Tl, Tl 1 therefore being truncated compared to other teeth as a result, before merging with a conventional 20° involute form (albeit truncated) on its inner flank (facing towards the stop) and a conventional root form. The profile of the outer facing flank of end-most teeth Tl, Ti l is therefore derived from or dependent on the profile of the outer facing flank of penultimate teeth T2, T10, and (as described above for the penultimate teeth T2, T10) the end-most teeth Tl, Tl 1 effectively act as a cam during overtravel to allow the approaching tooth of the pinion gear 43 to skip over them smoothly in the direction against the stop. Again, the exact shape (and pressure angle) of the outer edges (or outer facing flanks) of the end- most teeth Tl, Ti l is not particularly important to operation of the overtravel mechanism, so long as it allows the tooth of the pinion gear 43 to slip over it rather than drive the carrier gear 46 further into the stop, and preferably so that the motion of the pinion gear 43 transitions smoothly from the penultimate tooth T2, T10 to the end-most tooth Tl, Tl 1 as described above. Again, it will be apparent that the outer edges (or outer facing flanks) of the end-most teeth Tl, Tl 1 need not be derived from an involute form at all.

[0129] Returning to the sequence of Figures 21 to 36, in the range of travel depicted in Figures 21 and 22 (i.e. well away from the end stops), as the contact point moves one tooth is sliding and the other is rolling, and this switches over at the pitch point mentioned above. When the contact point is nearer the tooth tip it is sliding, and when it is nearer the tooth root it is rolling (the tangential velocity of a point on the surface of the tooth increases the further it is from the centre of rotation, so the relative speed at the contact point is different and changes over at the pitch point). The gears have a standard involute profile with a pressure angle of 20° during this range of travel. Figure 23 shows these parts after a further small anticlockwise rotation of the pinion gear 43, with the carrier gear 46 nearing (but not yet at) the end of its range of travel. In the position shown in Figure 23, a tooth of the pinion gear 43 is biased into the root between the second and third teeth T2, T3 of the carrier gear 46. The interaction between the gears at this point is still based on the standard involute form because the modified part of the gear profile mentioned above has not yet been reached.

[0130] Moving on to Figure 24, the carrier gear 46 has now rotated a further small amount clockwise and has just reached the right-most end stop. The engaged tooth of the pinion gear 43 is now in the root between the first and second teeth Tl, T2 of the carrier gear 46, the profiles of both of which have been modified away from the standard involute form. Accordingly, because of the modified profiles of these teeth, the pinion gear 43 has dropped down slightly as indicated by the downward arrow, though this particular movement is not particularly important to the overtravel function. At this point, the carrier gear 46 has rotated 27.7° clockwise from the starting position of Figure 21, with the flat part of the carrier gear 46 at 27.7° to the vertical, and with the vertical dashed line being roughly through the middle of the first tooth.

[0131] With a further small anticlockwise rotation of the pinion gear 43 as shown in Figure 25, there is no further movement of the carrier gear 46 because it is up against the end stop. Due to the higher pressure angle of the second tooth T2 compared to teeth T3 to T9, in this position there is an increased radial reaction on the pinion gear 43 from the rotational driving force applied to the pinion gear 43 (and a reduced tangential reaction) compared to the normal range of travel. This increased radial force on the pinion gear 43 is sufficient to overcome the biasing force applied on it by the biasing member 44, enabling the leading tooth of the pinion gear 43 to rise up and slide over the second tooth T2 of the carrier gear 46 rather than driving tangentially into it. The outward radial movement of the pinion gear 43 is apparent from a comparison between its centre position and the marked reference level and the movement is indicated by the arrow. The pinion gear 43 effectively disengages from carrier gear 46. In this position, because of the truncated form of the first tooth T1 of the carrier gear 46, the trailing (or following) tooth of the pinion gear 43 has not yet made contact with the first tooth T1 of the carrier gear 46.

[0132] With a further small anticlockwise rotation of the pinion gear 43 as shown in Figure 26, the leading tooth of the pinion gear 43 has slid further along and up the second tooth T2 almost to its tip. The first tooth T1 of the carrier gear 46 has been profiled such that the trailing tooth of the pinion gear 43 makes contact with the first tooth T1 of the carrier gear 46 just before the leading tooth of the pinion gear 43 loses contact with the second tooth T2 of the carrier gear 46. This allows a continuous, smooth motion and prevents the trailing tooth of the pinion gear 43 shooting forwards (after the leading tooth of the pinion gear 43 loses contact with the second tooth T2 of the carrier gear 46) and creating an audible “click” when it makes contact with the first tooth T1 of the carrier gear 46. The profile of the first tooth T1 of the carrier gear 46 also allows the leading tooth of the pinion gear 43 to remain in contact with the modified profile of the second tooth T2 all the way up to near its tip, thereby allowing the modified profile of the second tooth T2 to take full effect, before the trailing tooth of the pinion gear 43 makes contact with the first tooth T1. This is what is meant above when it was stated that the profile of the end-most tooth T1 follows the swept path of the approaching tooth of the pinion gear 43 resulting from the action of the lead tooth of the pinion gear 43 on the penultimate tooth T2, before re-merging with the conventional involute form. Therefore, in the position shown in Figure 26 both the leading and trailing teeth of the pinion gear 43 are in contact with the first and second teeth Tl, T2 respectively of the carrier gear 46.

[0133] With a further anticlockwise rotation of the pinion gear 43 as shown in Figure 27, what is now the leading tooth of the pinion gear 43 has slid along and up almost to the tip of the first tooth Tl, which has a conventional involute form in this part with a very shallow angle (due to its very high pressure angle). The pinion gear 43 is now almost at the most radial outmost end of its overtravel range. Because of the shallow angle of the first tooth Tl it does not contribute much to the radial displacement of the spring pinion gear 43 during the overtravel. And because the profile of the first tooth Tl has also been specifically arranged so that it is completely out of the way of the trailing tooth of the carrier gear 46 until the leading tooth of the carrier gear 46 has almost lost contact (see above), the first tooth Tl has not apparently contributed much to the overtravel behaviour so far in the sequence. But of course the first tooth Tl cannot be dispensed with entirely because the other side of it is needed when the direction of rotation of the pinion gear 43 reverses (see below).

[0134] With a further anticlockwise rotation of the pinion gear 43 as shown in Figure 28, the leading tooth of the pinion gear 43 loses contact with the first tooth T1 and the rotational force being applied to the pinion gear 43 causes it to shoot around (briefly without any reactive force from the carrier gear 46) until it makes contact with the second tooth T2 of the carrier gear 46 with an audible “click”. This “click” sound provides an audible feedback to the user which alerts them that the beam steering arrangement 50 has reached the end of its travel and is now in an overtravel condition, so that they know that further turning of the 40 will have no further beam steering effect. And because the first two teeth Tl, T2 have been profiled to ensure that the trailing tooth makes contact with the first tooth Tl before the leading tooth loses contact with the second tooth T2 (as mentioned above), this is just a single “click” rather than a double “click”, which is a more reassuring feedback. Truncating the first tooth Tl also helps to reduce the overall radial displacement of the pinion gear 43.

[0135] If the user does continue to turn the pinion gear 43 in an anticlockwise direction, then as shown in Figures 29 and 30 the process will repeat, with the pinion gear 43 rising up again over the first tooth Tl and then dropping down to make contact with the second tooth T2 with a “click”, and so on. This happens advantageously without any damage being done to the drivetrain components.

[0136] When the pinion gear 43 starts to turn in a reverse (clockwise) direction as shown in Figure 31, the leading tooth of the pinion gear 43 makes contact with the other (inner) side of the first tooth Tl of the carrier gear 46. Although the left (outer) surface of the first tooth Tl has a modified (higher) pressure angle to allow the pinion gear 43 to slide over it, the right (inner) surface of the first tooth Tl is of involute form and provides a pressure angle that is the same as (or at least closer to) the normal pressure angle of 20°. Furthermore, although the first tooth Tl is truncated compared to the others, it is still high enough that the leading tooth of the pinion gear 43 can make normal contact with it. Therefore, a small further clockwise rotation of the pinion gear 43 as shown in Figure 32 causes a small anticlockwise rotation of the carrier gear 46 away from the stop position, and with further clockwise rotations of the pinion gear 43 as shown in Figures 33 to 36 the carrier gear 46 is rotated anticlockwise all the way to the other end stop (with the carrier gear 46 having rotated 27.7° anticlockwise from the starting position of Figure 21). The overtravel mechanism operates exactly as described above if there is any further clockwise rotation of the pinion gear 43 from the position shown in Figure 36.

[0137] To highlight the difference between a geared carrier 45 used in a beam steering arrangement 50 embodying the present invention, having an useful overtravel mechanism enabled by modified tooth profiles for the carrier gear 46, Figure 44 shows a comparison between a geared carrier 55 (and carrier gear 56) used in a beam steering arrangement in the current version of the differential interferometer detector head 44 from Renishaw pic, and the geared carrier 45 (and carrier gear 46) used in a beam steering arrangement 50 embodying the present invention. Although not particularly relevant to the present invention, and not intending to be limiting, the gear ratio is the same, but the number of teeth and gear modulus is different. In this respect, the existing product has a ratio of 52 teeth on the carrier gear 56 to every eight teeth on the pinion gear (a ratio of 6.5:1). The teeth of the carrier gear 56 have an involute form but those on the pinion gear (not shown) do not. The above-described and illustrated embodiment of the present invention has a ratio of 65 teeth on the carrier gear 46 to every ten teeth on the pinion gear 43 (a ratio of 6.5: 1), with the teeth of the carrier gear 46 and pinion gear 43 both having an involute form (though with modifications to the involute form of certain teeth on the carrier gear 46 as described above).

[0138] Since the overtravel mechanism described above relies on an outward lateral movement of the pinion gear 43 away from the carrier gear 46, and since the pinion gear 43 is rigidly connected to the drive shaft 41 (see e.g. Figure 14), the drive shaft 41 is supported towards its upper end via an integrated ball joint arrangement to account for the lateral movement of the pinion gear 43 at the bottom of the drive shaft 41. In this respect, Figure 45 shows a view looking towards the right side of Figure 14 with parts of the frame 54 as well as the drive shaft 41mp (and associated elements) removed for visibility. A ball joint arrangement is formed by a spherical element 58rp provided towards an upper end of the drive shaft 41rp, concentric with a longitudinal axis of the drive shaft 41rp. The spherical element 58rp sits into a socket 59rp formed in the frame 54 and is held in place with a beam spring arrangement 60p (removed for Figure 45 but shown in Figures 14 and 15) to allow the drive shaft 41rp to rotate and articulate. With the other drive shaft 41mp not being shown in Figure 45, the features of its associated socket 59mp is more clearly visible, showing three pads formed by a cross-drilled hole and pockets. The socket 59rp similarly comprises three pads, which in conjunction with the beam 60p spring allows the drive shaft 41rp to be kinematically positioned and protected from thrust loads which may cause the mechanism to bind up. The drive shaft 41rp is then free to rotate around a well- defined axis and also to pitch about the centre of the ball joint when there is outward lateral movement of the pinion gear 43rp away from the carrier gear 46rp. Referring to the above discussion regarding kinematic constraint, the three pads provide three corresponding constraints to relative motion between the drive shaft 41rp and the frame 54 in a kinematic manner. Lateral (radial) and axial movement of the drive shaft 41rp are constrained kinematically (in three degrees of freedom) leaving pitch, yaw and rotation around the axis unconstrained (in three degrees of freedom), and therefore absorbing lateral movement of the pinion gear 43rp and also allowing rotation of the drive shaft 41rp via the control 40rp when adjusting the beam steering. It should be noted that, although yaw of the drive shaft 41rp is not constrained via the ball joint arrangement shown in Figure 45, it is constrained towards the lower end of the drive shaft 41rp via a key way on the shaft guide, thereby maintaining alignment of the pinion gear 43rp.

[0139] It is also worth noting that the surfaces of the carrier 45 which make contact with the end stops at Pl, P2 (see for example Figures 16, 18, 20, 24 to 30, and 36) are advantageously angled such that the normal reaction force helps to prevent the carrier 45 from unseating when being driven around in an overtravel state. Returning briefly to Figure 20, it should be noted that each of the pair of post features that sit in between the two carriers 45mp and 45ry on the support member 52 (see also Figures 16 and 17) have been arranged and adapted to act both as a stop position Pl, P2 for one carrier 45mp, 45ry and also as the kinematic contact Cl, C2 for the adjacent carrier 45ry, 45mp. In doing so, this advantageously provides space saving and therefore allows the overall size of the detector head 4 to be reduced.

[0140] It is also worth mentioning the presence of a relief pockets 46 on the surface of the support member 52 adjacent to each of the contact points Cl, C2, as can most clearly be seen in Figure 17. Without these, when the posts are formed on the support member 52 (for example by machining) due to manufacturing imperfections it might be the case that the base of the post does not meet the surface of the support member 52 at a clean right angle, and the extra material in this vicinity will tend to lift the carrier 45 away from the surface or prevent it from engaging against the vertical part of the post. By having these relief pockets this helps to ensure that the carrier 45 engages more precisely at the contact points Cl, C2 and this in turn ensures a more accurate centring and rotation of the optical wedges 42 via the kinematic coupling mentioned above. Arched relief pockets 57 are similarly provided to prevent any burring on the teeth of the carrier gear 46 due to manufacturing imperfections having an effect on alignment.

[0141] Another advantageous feature of the beam steering arrangement 50 embodying the present invention will now be described with reference to Figure 46, which shows a view similar to that of Figure 14 but with various parts removed for improved visibility of the key features, and also focussed on the geared carrier 45mp and associated optical wedge 42mp. A brake flexure 47mp holds the geared carrier 45mp in position, clamped on one side thereof and biased onto the top surface of the geared carrier 45mp by biasing members (springs) 48mp. The brake flexure 47mp may be formed of a laminated composite, comprising a flexible member which is stiff and resistant to torsional loads but which can hinge under a spring force, and a thicker pad through which a compressive load from the biasing members 48mp can be applied to the geared carrier 45mp keeping it flat against the mid plate 54 and providing a holding force regardless of orientation. The arrangement is mirrored around the support member 52 for a pair of steering optics 42mp, 42my.

[0142] The yoke shape of the brake flexures 47 allows the arrangement to be placed closer to another set of steering optics. The flexures 47 are advantageously independent for each of the four optic, which eliminates potential cross-torque effects during alignment (where adjusting one optic affects the position of the other). The arrangement is rigid in torsion and the carriers 45 are held in place under compression, which prevents torsional strain from being transmitted to the steering optics (i.e. following adjustment) which would manifest as hysteresis in the adjustment and creep following alignment.

Claims

CLAIMS1. A laser encoder device which is operable to emit a laser beam, the device comprising a beam steering arrangement for steering the beam towards an intended target, the beam steering arrangement comprising a rotatable carrier on which a steering optic is mounted and through which steering optic the beam is arranged to pass before being emitted from the device, and a rotatable pinion gear which engages with a corresponding carrier gear provided on the carrier such that rotation of the pinion gear causes a corresponding rotation of the steering optic and thereby a corresponding steering of the beam, and further comprising a biasing member to bias the pinion gear into engagement with the carrier gear.

2. A laser encoder device as claimed in claim 1, wherein the biasing of the pinion gear into engagement with the carrier gear by the biasing member provides a nesting force for a kinematic coupling which is adapted to constrain the carrier kinematically within the device in two translational degrees of freedom relative to a predetermined axis of the steering optic, with a rotational degree of freedom around the predetermined axis being unconstrained by the kinematic coupling.

3. A laser encoder device as claimed in claim 2, wherein the other three degrees of freedom are constrained non-kinematically.

4. A laser encoder device as claimed in claim 2 or 3, wherein the predetermined axis is an optical axis of the steering optic.

5. A laser encoder device as claimed in claim 2, 3 or 4, wherein the nesting force is arranged to bias the carrier into contact with two contact features which provide two corresponding respective constraints for the carrier in the two translational degrees of freedom.

6. A laser encoder device as claimed in claim 5, wherein the carrier comprises two concentric surfaces having different radii which are biased by the nesting force into contact with the two contact features respectively.

7. A laser encoder device as claimed in any preceding claim, wherein the biasing of the pinion gear into engagement with the carrier gear by the biasing member ensures that contact is made between both sides of each tooth on the pinion gear and a corresponding pair of adjacent teeth on the carrier gear as the pinion gear is rotated.

8. A laser encoder device as claimed in any preceding claim, wherein the biasing of the pinion gear into engagement with the carrier gear by the biasing member forms part of an overtravel feature in which at least one tooth of the carrier gear that is contacted by the pinion gear during an overtravel condition is adapted so as to have a profile which causes the pinion gear to slide over and lift away from the carrier gear, against the bias provided by the biasing member, rather than attempt to drive the carrier gear further.

9. A laser encoder device as claimed in claim 8, wherein the pinion gear is coupled rigidly to a control via a drive shaft, and wherein the drive shaft is provided with a ball joint arrangement to allow the drive shaft to pivot about the ball joint when the pinion gear lifts away from the carrier gear.

10. A laser encoder device as claimed in claim 8 or 9, wherein the at least one tooth of the carrier gear is formed with a pressure angle that is sufficiently large to produce a radial force on the pinion gear that is sufficient to overcome the bias provided by the biasing member.

11. A laser encoder device as claimed in claim 8, 9 or 10, wherein at least two teeth of the carrier gear are so adapted.

12. A laser encoder device as claimed in claim 11, wherein a trailing tooth of the pinion gear makes contact with one of the at least two teeth before a leading tooth of the pinion gear loses contact with another of the at least two teeth.

13. A laser encoder device as claimed in any preceding claim, wherein theteeth of the pinion gear and / or the carrier gear have or are based on an involute profile.

14. A laser encoder device as claimed in any preceding claim, wherein the carrier is supported on a support member of the device.

15. A laser encoder device as claimed in claim 14, when dependent on claim 2, wherein a contact feature is formed on the support member which acts as a contact for the kinematic coupling.

16. A laser encoder device as claimed in claim 15, wherein a relief pocket is formed in the surface of the support member adjacent to the contact feature to prevent imperfections in the contact feature affecting the kinematic nature of the coupling.

17. A laser encoder device as claimed in claim 15 or 16, when dependent on claim 3, wherein the other three degrees of freedom are constrained non- kinematically via a surface-to-surface contact between the carrier and the support member.

18. A laser encoder device as claimed in any preceding claim, wherein the beam steering arrangement comprises a plurality of such rotatable carriers and associated features.

19. A laser encoder device as claimed in claim 18, wherein each of the carriers is provided with its own individual brake flexure member to hold the carrier in position.

20. A laser encoder device as claimed in claim 18 or 19, wherein the beam steering arrangement comprises a self-supporting and / or self-contained assembly having a frame which carries the pinion gears and associated drive shafts for all the carriers.

21. A laser encoder device as claimed in claim 18, 19 or 20, when dependenton claims 2 and 8, wherein a stop member for one of the plurality of carriers also acts as a contact for the kinematic coupling of another of the plurality of carriers, or vice versa.

22. A laser encoder device as claimed in any one of claims 18 to 21, when dependent on claims 2, 8 and 14, wherein a feature is formed on the support member which acts both as a stop member for one of the plurality of carriers and as a contact for the kinematic coupling of another of the plurality of carriers.

23. A laser encoder device as claimed in any preceding claim, wherein the steering optic is an optic wedge.

24. A laser encoder device which is operable to emit a laser beam, the device comprising a beam steering arrangement for steering the beam towards an intended target, the beam steering arrangement comprising a rotatable carrier on which a steering optic is mounted and through which steering optic the beam is arranged to pass before being emitted from the device, and a rotatable pinion gear which engages with a corresponding carrier gear provided on the carrier such that rotation of the pinion gear causes a corresponding rotation of the steering optic and thereby a corresponding steering of the beam, wherein the beam steering arrangement comprises a plurality of such rotatable carriers and associated features, and wherein each of the carriers is provided with its own individual brake flexure member to hold the carrier in position.

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