WO2026017976A1

WO2026017976A1 - Laser encoder system

Info

Publication number: WO2026017976A1
Application number: PCT/GB2025/051536
Authority: WO
Inventors: Alexander Stephen VAN RUSSELT; David Oury King
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 2024-07-15
Filing date: 2025-07-11
Publication date: 2026-01-22
Anticipated expiration: 2027-01-15
Also published as: GB202410260D0

Abstract

A modular laser encoder system is disclosed which comprises one or more laser units (2) and one or more detector heads (4), with any one of the laser units (2) being connectable in use to any one of the detector heads (4) to provide laser light to the connected detector head (4) and with the connected detector head (4) using the laser light to generate a detection signal, wherein a signal level (S) associated with the detection signal varies depending on which combination of laser unit (2) and detector head (4) is used, and wherein each of the laser units (2) and detector heads (2) is associated with one or more coefficients (c1, c2) which aim to normalise a response of that laser unit (2) or detector head (4) such that a corrected signal (S) is derivable from the detection signal by a signal processing unit (50) using the coefficients (c1, c2) associated with the laser unit (2) and detector head (4) of the combination concerned, with the corrected signal (S) having a signal level that is substantially consistent regardless of what combination of laser unit (2) and detector head (4) is used.

Description

Laser Encoder System

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

Figure 8 of the accompanying drawings shows the front panel of the laser unit 2 of Figure 1 in more detail. The laser unit 2 of Figures 1 and 8 is adapted to support two detector heads 4 simultaneously, which might for example be arranged to measure along two orthogonal machine axes X and Y as described above with reference to Figure 4. Accordingly, many of the connectors for the laser unit 2 are duplicated between the two axes and are differentiated by AX1 and AX2 marked on the relevant labels visible in Figure 8. With the system setup as shown in Figure 1, only the first of the two available axes is in use, with the detector head 4 being connected via cable 3 to connector DI (of pair DI, D2) and the interface 6 (which can also be referred to as a controller) being connected via cable 7 to connector Cl (of pair Cl, C2). The laser light itself is delivered to the detector head 4 via optical fibre assembly 32 which is connected to connector Al (of pair Al, A2). Also shown in Figure 8 is a pair of status lights LI, L2, a pair of sensor connectors Ml, M2, a set of configuration switches W (normally covered but shown partially revealed in Figure 8), a pair of reference mark connectors Rl, R2, a shutter connector S, a reset connector R, an AUX I/O connector A, a laser status light L, and a 24V power supply input P. These are labelled as follows:

LI : STATUS AX1 (similarly for L2)

Cl : SIGNAL OUT AX1 (similarly for C2) Ml : SENSOR AX1

W: CONFIGURATION SWITCHES (COVERED) S: SHUTTER R: RESET

R1 : REF. MARK AX1 (similarly for R2) DI : DETECTOR AX1 (similarly for D2) Al : LASER FIBRE OPTIC AX1 (similarly for A2) A: AUX I/O

L: LASER STATUS

P: 24 V POWER SUPPLY INPUT

Further information regarding the purpose and function of each of the above can be found in the literature relating to the RLU10 or RLU20 laser unit products made and sold by Renishaw pic.

As mentioned above, the interference signal sent from the detector head 4 to the laser unit 2 is in the form of an analogue quadrature signal, which will now be described in more detail with reference to Figures 9 and 10 of the accompanying drawings. Figure 9 illustrates the reference and measurement beams forming a fringe pattern 29 on the light detector 19 in the detector head 4, while Figure 10 shows a simplified circuit diagram of the detection and processing circuitry 41 in the detector head 4. The fringe pattern 29 will move across the light detector 19 as the target 8 moves towards and away from the detector head 4. The light detector 19 is a multi-channel light detector comprising four photodiodes 19a, 19b, 19c and 19d (see Figure 10) providing four corresponding respective detection signals, which are effectively samples at 90° intervals across the fringe pattern 29 (where a single fringe period covers 360°). These provide a cosine and minus cosine signal pair, which are fed to a differential amplifier to give a clean cosine signal (cleaned of noise), and a sine and minus sine signal pair, which are likewise fed to a differential amplifier to give a clean sine signal. The clean cosine and sine signals are each fed through a DC offset adjustment stage and a gain adjustment stage to provide a quadrature output signal which comprises the adjusted cosine and sine signals, which are made available at the quadrature signal output 42. The cosine and sine signals of the quadrature signal can be referred to as constituent signals of the quadrature signal (or component signals or simply components).

If the sine and cosine signals of the quadrature output signal are plotted on the X and Y axes of a graph, they produce a circular “Lissajous” figure as shown in Figure 10. When the target optic 8 moves, the fringe pattern 29 on the light detector 19 will likewise move, and hence so will the sine and cosine signals from the detection and processing circuitry 41. The moving sine and cosine signals will therefore sweep out a circular path on the graph as the target optic 8 moves, and when the target optic 8 stops, so will the point that is being swept out on the circular path. The distance moved by the target optic 8 is measured by counting revolutions around the circular path (or Lissajous). By way of example, with the present laser encoder systems from Renishaw pic one revolution around the Lissajous corresponds to 316 nm for a single pass interferometer (this would be described as having a “316 nm Lissajous”) and 158 nm for a double pass interferometer (this would be described as having a “158 nm Lissajous”). If the laser unit 2 is set to output digital quadrature signals, then the analogue sine and cosine quadrature signals received from the detector head 4 are digitised and interpolated in the laser unit 2 by a signal conversion unit before being output to the interface 6. Otherwise, an analogue quadrature signal would be output from the laser unit 2 and the digitising and interpolating would be performed elsewhere. The laser unit 2 can be switched between analogue and digital quadrature output using the configuration switches W (the output from the detector head 4 is analogue, with the digitisation being in the laser unit 2).

Interpolation enables a higher resolution of measurement to be obtained, effectively enabling sub-revolutions around the Lissajous to be counted rather than just complete revolutions. For example, for the double pass interferometer mentioned above that has a 158 nm Lissajous (i.e. where the analogue output signal period is 158 nm), nominal output resolutions for the digital quadrature signal output from the laser unit 2 (when using a signal conversion unit within the laser unit 2 itself) include 10, 20, 39.5 and 79 nm. An even higher resolution of 0.39 nm is achievable using the REE interpolator from Renishaw pic (receiving an analogue quadrature signal from the laser unit 2), and an even higher resolution of 38.6 pm is achievable with the RPI20 and RPI30 parallel interfaces from Renishaw pic (which would be used as the interface 6 described above). The concepts of digitisation and interpolation are also discussed further below.

The present applicant has appreciated that, due to manufacturing tolerances the optical power of the laser light produced by the laser unit 2 (and delivered to the detector head 4 through the fibre optic cable 11 of the optical fibre assembly 32) will vary from one laser unit 2 to the next. The optical power will even vary between the two laser outputs Al, A2 of the same laser unit 2 in a dual axis system such as illustrated in Figures 1 and 8. Variation in optical power can result from tolerances in the optical power of the laser tube itself, and also of variation in the optical coupling efficiency, which is in turn determined by how accurately the launch end of the fibre is positioned relative to the coupling lens. It can also be affected by tolerances on the focal length of the coupling lens and the mode field diameter of the fibre. The optical power of the laser light delivered to the detector head via laser outputs Al, A2 has a direct impact on the signal strength received back from the detector head 4 via connectors DI, D2, with signal strength being directly proportional to the optical power of the laser light. In simple terms, the brighter the light falling on the light detector 19 within the detector head 4 (see Figure 2), the stronger the output signal from the detector head 4 (via cable 3) will be. Furthermore, there will in practice be variations in the sensitivity of the light detector 19 between different detector heads 4, and manufacturing variations will also be present elsewhere in the optical and electrical pathways.

Accordingly, each laser unit 2 is currently matched to one or two specific detector heads 4 prior to shipping to the customer as a complete fibre optic laser encoder system 1. The matching process is designed to give the best chance of achieving a nominal 100% signal strength for the signal sent from the laser unit 2 to the interface 6, which is equivalent to 1 Vpp measured on the appropriate signal strength output pin on the AUX I/O connector A using a multimeter M as shown schematically in Figure 11 of the accompanying drawings. This ideal 1 Vpp is also shown in relation to the output signal of Figure 10. This matching process is currently achieved by careful manual adjustment of a potentiometer associated with each of the gain and offset stages within the detector head 4 (see Figure 10) as part of the production process. As illustrated in the table shown in Figure 12 of the accompanying drawings, the system should be set up to achieve a signal strength in a range from 25% to 120%, though for optimum performance this should normally be around 100%. The system will still function with a signal strength in a range from 12.5% to 25% and above 120% but it may not achieve the best possible accuracy. The system is not intended to function with a signal strength below 12.5% and will typically assert an error output.

Because of the sensitivity of signal strength to laser output power (as well as other optical and electronic components), it is strongly recommended to the customer that the detector head(s) 4 that are supplied with the laser unit 2 are used only with that laser unit 2 to ensure optimal performance. For this reason, a label showing the serial numbers of the supplied detector heads 4 is currently affixed to the rear of the supplied laser unit 2. The consequences of changing the detector head 4 that is connected to a laser unit 2 may include one or more of the following:

(a) low signal strength (below the acceptable threshold), which will trigger a beam low error such that the relevant axis status light LI, L2 will appear continuously amber.

(b) high signal strength (exceeding 120%), which will trigger a beam saturation error such that the relevant axis status light LI, L2 will appear continuously amber, and which may also cause interpolation errors (see below).

(c) low signal strength (but still within the acceptable range), which will make system alignment more difficult, especially at longer distances.

(d) non-ideal offset and/or amplitude of the sine and cosine components of the quadrature signal, leading to a sub -divisional error or SDE.

Referring to part (d) above, unequal offset between the sine and cosine signals and unequal sine and cosine signal levels can both cause imperfect Lissajous plots, as represented respectively in Figures 13 and 14 of the accompanying drawings. In turn, this results in a sub -divisional error (also referred to as a non-linearity error or interpolation error), which is typically expressed in nanometres (nm). The interpolation error E is represented as the difference between the same corresponding point on the ideal Lissajous 35 and the actual Lissajous 36. Sub- divisional error is cyclic, occurring within each full signal period based on a difference between where the point is on the actual Lissajous 36 and where it should be on the ideal Lissajous 35, and does not therefore accumulate.

If a detector head 4 does have to be moved from one system to another, this would currently require the detector head 4 to be matched to the new laser unit 2, by adjusting an internal gain and offset of the detector head 4 to ensure that it is tuned correctly to the laser unit 2. Under normal circumstances there should be no requirement to adjust the gain and offset of the detector head 4. Incorrect adjustments to the detector head 4 can cause system performance errors as described above. Furthermore, the making of such adjustments is a highly skilled task which the normal user would not typically be capable of performing, thereby requiring the system to be returned to the manufacturer for tuning, which is inconvenient for the customer.

In view of the above, the present applicant has appreciated that it would be beneficial to provide a laser encoder system which offers the customer the flexibility to swap detector heads 4 between different laser units 2 more easily, and in particular to allow the customer to so this without having to return the system to the manufacturer for a specialised reconfiguration and matching process to be performed.

According to a first aspect of the present invention there is provided a laser encoder system comprising one or more laser units and one or more detector heads (or detector units), with any one of the laser units being connectable in use to any one of the detector heads to provide laser light to the connected detector head and with the connected detector head using the laser light to generate a detection signal. A signal level associated with the detection signal varies depending on which combination (or pairing) of laser unit and detector head is used. Each of the laser units and detector heads is associated (or provided) with one or more coefficients (or correction values) which aim to normalise a response of that laser unit or detector head (for example relative to or based on a reference laser unit or detector head) such that a corrected (or adjusted) signal is derivable from the detection signal by a signal processing unit using the coefficients associated with the laser unit and detector head of the combination (or pairing) concerned, with the corrected (or adjusted) signal having a signal level that is substantially (or at least more) consistent regardless of what combination (or pairing) of laser unit and detector head is used.

These features provide a laser encoder system with a modular or “plug and play” nature, automatically adapting or accounting for the inevitable variation in gain and/or offset response which exists in practice between different units, such that a consistent output signal is produced regardless of these variations and regardless of which combination or pairing of laser unit and detector head (or detector unit) is used. This makes the system very flexible and easy to use.

The signal processing unit may be or may form part of or may be implemented as a functional unit of the laser unit.

The coefficients may comprise one or more gain coefficients for correction of a gain associated with the detection signal.

The coefficients may comprise one or more offset coefficients for correction of an offset associated with the detection signal.

The detection signal may be an electrical signal.

The detection signal may be a quadrature signal comprising first and second constituent signals.

First and second different sets of coefficients may be provided for correction of the first and second constituent signals respectively.

The first and second constituent signals may be electrical signals.

The first and second constituent signals may be sinusoidal signals (as a function of time).

The first and second constituent signals may be at least nominally 90 degrees out of phase.

The first and second constituent signals may be cosine and sine signals respectively (as a function of time).

The detection signal may be an analogue signal.

The correction may be performed before converting the analogue signal to a digital signal.

The correction may be performed after converting the analogue signal to a digital signal. Conversion of the analogue signal to a digital signal may be performed by an interpolator.

At least one of (for example all of) the coefficients associated with the laser unit may be stored in the laser unit itself, for example in persistent memory.

The laser unit may be operable to send the required coefficients to the signal processing unit on request.

At least one of (for example all of) the coefficients associated with the detector head may be stored in the detector head itself, for example in persistent memory.

The detector head may be operable to send the required coefficients to the signal processing unit on request.

At least one of (for example all of) the coefficients associated with the laser unit or the detector head may be stored in a remote database and retrieved by the signal processing unit when required. The coefficients associated with the laser unit may be retrieved based on an identifier associated with the laser unit. The coefficients associated with the detector head may be retrieved based on an identifier associated with the detector head.

The signal processing unit may be or may form part of or may be implemented as a functional unit of a single system component, such as the laser unit. In this way, the adjustment is performed in one place. This is to be contrasted with a system in which the signal processing is performed partly in the laser unit and partly in the detector head.

The signal level associated with the detection signal output from the detector head may vary depending on which detector head is used with a specific laser unit. In other words, the signal level of the detection signal output from the detector head may be different for different detector heads connected to the same laser unit. In this respect, adjustments that account for variations in both the detector head and the laser unit may be made subsequently, in one place, at the signal processing unit. This is to be contrasted with a system in which the signal from different detector heads, paired to the same laser unit, is substantially unchanged or constant, for example if the different detector heads have already been preadjusted to provide a consistent output signal level for the same laser unit.

The corrected (or adjusted) signal may be derivable from the detection signal by the signal processing unit using the coefficients associated with the laser unit and detector head of the combination concerned as well as at least one user-adjustable coefficient.

The coefficients may be stored in memory (such as some form of persistent memory) and loaded dynamically by the signal processing unit when required.

The signal processing unit may be arranged to output the corrected signal to a signal conversion unit which is adapted to digitise and interpolate the corrected signal.

The signal conversion unit may be or may form part of or may be implemented as a functional unit of the laser unit or a separate system component such as an interface component.

The signal conversion unit may be arranged to output the digitised and interpolated signal to a positional data deriving unit.

The positional data deriving unit may be adapted to derive positional data from the digitised and interpolated signal. The positional data deriving unit may be or may form part of or may be implemented as a functional unit of the laser unit or a separate system component such as an interface component or controller.

The detection signal may relate to a distance (absolute or relative) between the detector head and a target (or target optic).

The detection signal may be an interference signal resulting from interference between a reference laser beam and a measurement laser beam. The measurement laser beam may be directed onto and at least partially reflected back from the target.

The laser unit may be connected in use to the detector head via an optical fibre assembly to provide the laser light to the detector head.

The laser unit may be connected in use to the detector head via an electrical cable to receive the detection signal from the detector head.

According to a second aspect of the present invention there is provided a laser unit which is adapted for use in a system according to the first aspect of the present invention.

According to a third aspect of the present invention there is provided a detector head (or detector unit) which is adapted for use in a system according to the first aspect of the present invention.

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

Figure 1, discussed hereinbefore, illustrates a known fibre optic laser encoder system comprising a laser unit, detector head and machine interface; Figure 2, also discussed hereinbefore, is a schematic view of some of the internal components of the detector head of Figure 1;

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

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;

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

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;

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

Figure 8, also discussed hereinbefore, shows the front panel of the laser unit of Figure 1 in more detail;

Figure 9, also discussed hereinbefore, illustrates a fringe pattern formed on a multi-channel light detector by the reference and measurement beams in the detector head;

Figure 10, also discussed hereinbefore, shows a simplified circuit diagram for the detection and processing that is carried out in the detector head, including gain and offset correction, and which also explains the concept of a quadrature signal and a Lissajous plot; Figure 11, also discussed hereinbefore, shows the laser unit of Figure 8 connected to a multimeter to measure a signal level of the detection signal from the detector head;

Figure 12, also discussed hereinbefore, is a table showing different ranges for the signal level and the associated condition which is indicated via a status light on the front panel of the laser unit;

Figure 13, also discussed hereinbefore, illustrates an interpolation error which results from a DC offset between the sine and cosine parts of the quadrature signal from the detector head;

Figure 14, also discussed hereinbefore, illustrates an interpolation error which results from unequal sine and cosine levels in the quadrature signal from the detector head;

Figure 15 shows a simplified model of gain for a laser encoder system and is useful for explaining the concept underlying an embodiment of the present invention;

Figure 16 is a schematic illustration of the way in which variations in gain associated with the laser unit and detector head are accounted for in the previously considered scheme;

Figure 17 is a schematic illustration of the way in which variations in gain associated with the laser unit and detector head are accounted for in an embodiment of the present invention;

Figure 18 is a table with example gain and coefficient values for two laser units which for use in a modular laser encoder system embodying the present invention; Figure 19 is a table with example gain and coefficient values for four detector heads which for use in a modular laser encoder system embodying the present invention;

Figure 20 is a schematic illustration of a modular laser encoder system embodying the present invention in which laser units and detector heads can be coupled interchangeably to one another in a flexible way;

Figure 21 is a schematic diagram showing an extension of the scheme of Figure 17 to handle a quadrature detection signal from the detector head;

Figure 22 is a schematic illustration of an alternative arrangement in which the signal correction is performed at a separate signal processing unit rather than at the laser unit;

Figure 23 is a schematic illustration of an alternative arrangement in which the required coefficients are fetched from a separate storage location;

Figure 24 shows a general representation of a modular laser encoder system embodying the present invention, noting that the signal processing unit and positional data deriving unit could be located anywhere in the system;

Figure 25 is a schematic illustration of the signal pathways carrying the detection signal from the detector head to a signal conversion unit in the laser unit or interface where the detection signal is digitised and interpolated;

Figure 26 provides a graphical representation of how the analogue quadrature signal is converted into a digital quadrature signal by the signal conversion unit; and

Figure 27 is a schematic illustration of an alternative implantation of a modular or “plug and play” system compared to that shown in Figure 24.

As described above, the present applicant has appreciated that there is variation in the amplitude and offset of the analogue quadrature signal produced by a laser encoder system 1 such as that shown in Figure 1. This variation, which is caused for example by variation in the parts sourced from suppliers and in the assembly processes, is undesirable because it can result in: (a) signal strength variation; and/or (b) sub -divisional error (SDE). Signal strength variation could mean that in some units the signal strength is not in the optimal range for the interpolator, which could increase measurement error. A high variation in signal strength between units may also give the customer a negative perception of the product’s quality. A non-zero offset or a mismatch in the sine and cosine amplitudes will increase the SDE, and this will reduce the accuracy of the measurement.

A simplified model of gain for the laser encoder system 1 is provided in Figure 15, which is a schematic diagram showing just the laser unit 2 and the detector head 4. In this model, for the sake of simplicity it is assumed that the laser unit 2 has a single laser and that the detector head 4 has a single light detector 19 producing a single corresponding (non-quadrature) signal. For the time being, offset is also ignored in this simplified model.

The laser in the laser unit 2 takes a current I, produces a laser beam with optical power P, which is incident on the light detector 19 in the detector head 4, which outputs a detection signal having strength S. In this simplified model, it is assumed that P is proportional to I (i.e. the higher the current I provided to the laser, the higher the optical power P from the laser), such that P = gl x I where gl is the gain of the laser in the laser unit 2. It is also assumed that S is proportional to P (i.e. the brighter the light that is incident on the light detector 19, the higher the signal strength S from the light detector 19), such that S = g2 * P where g2 is the gain of the light detector 19 in the detector head 4. It is assumed that the current I will be constant for all laser encoder systems 1, but that gl will vary between different laser units 2 and that g2 will vary between different detector heads 4 (for the reasons explained above). Substituting P = gl x I into the expression for S, it can be seen that S = gl x g2 x I. Therefore, S will vary depending on a combination of gl and g2.

Figure 16 illustrates the way in which these variations are accounted for in the previously considered scheme (as described above). As shown in Figure 16, and as described above, the detection signal from the detector head 4 is passed to the laser unit 2 and then onwards to the interface 6, such that the interface 6 can process the detection signal to derive positional data, and the interface 6 will be expecting a signal strength S that is close to the ideal value of 1 Vpp. This is achieved in the previously considered scheme by adjusting the gain g2 manually (e.g. using a potentiometer) as part of the production process until a signal strength S of 1 Vpp is seen at the output pin on the AUX I/O connector A using a multimeter M as described above with reference to Figure 11.

As a result, the detector head 4 must remain with that specific laser unit 2 throughout its life, and these can be considered as a single functional entity 37 as indicated by the dotted outline in Figure 16. Swapping to a different detector head 4 with the same laser unit 2 will result in a change in signal strength S and a higher position error seen at the interface 6.

Incidentally, referring to the above assumption that P = gl / I, for many modern laser technologies a higher current does indeed result in a higher laser power, but this not always the case (for example, with HeNe lasers). In that case, the laser power P would be determined for example by the quality of the mirrors and how well they are aligned, so that in the above model gl could represent the combined quality of the mirrors and their relative alignment. Similarly, the gain g2 associated with the detector head 4 might be dependent on more than just the sensitivity of the light detector 19. However, the simplified model is still applicable because each laser unit 2 can be considered to have a representative gain gl (due to various factors) and each detector head 4 can be considered to have a representative gain g2 (due to various factors).

With the above in mind, a scheme according to an embodiment of the present invention is illustrated schematically in Figure 17. From a comparison between Figure 16 (the previous scheme) and Figure 17 (the new scheme) it can be seen that the new scheme is quite different to what has been previously used. As will be discussed in more detail below, the detector head 4 of Figure 17 has not been specifically matched to the laser unit 2 (there is no manual adjustment provided for the gain g2, unlike in Figure 16). Accordingly, the signal strength S of the detection signal from detector head 4 in Figure 17 is uncertain, and may not be within the preferred range as discussed above.

Instead, with a scheme according to an embodiment of the present invention, each laser unit 2 is assigned its own individual gain coefficient cl and each detector head 4 is assigned its own individual gain coefficient c2. These coefficients can also be referred to as correction values. The gain coefficient cl in the laser unit 2 is intended to correct for gain variation in the laser unit 2 relative to a reference that is common across all laser units 2, and similarly the gain coefficient c2 in the detector head 4 is intended to correct for gain variation in the detector head 4 relative to a reference that is common to all detector heads 4. The coefficients cl and c2 are intended to normalise the gain response of the laser unit 2 and detector head 4 respectively, for example relative to a reference that is common to all of the laser units 2 and detector heads 4 that are intended for use in the laser encoder system 1, such that a corrected signal is derivable from the detection signal from the detector head 4 using the coefficients associated with the laser unit 2 and detector head 4 of the connected pair, with the corrected signal having a signal level that is substantially consistent (or at least more consistent than would otherwise be the case) regardless of what combination of laser unit 2 and detector head 4 is used in the pair. Correcting the gain of each part (i.e. laser unit 2 and detector head 4) relative to a separate reference allows these parts to be treated in a more modular or “plug and play” fashion than for the previous scheme and enables performance to be maintained even parts are swapped around between different systems. Accordingly, these parts are interchangeable in a flexible manner, rather than being restricted to use only in very specific pairings. When the parts are connected, the coefficients cl, c2 are communicated between them, combined into a single number, and applied using a single adjustment stage. Additional coefficients can be added to allow the user to adjust the gains using software while maintaining the original factory settings in memory should the user need to revert to it.

Such a plug-and-play scheme embodying the present invention will now be described in more detail with reference to the schematic drawing of Figure 17 and the tables of Figures 18 and 19. Firstly, the user connects the detector head 4 to the laser unit 2. These could be any laser unit 2 and any detector head 4 that have been configured to operate in accordance with an embodiment of the present invention. Accordingly, the laser unit 2 has been configured with a gain coefficient cl stored in memory 42, with the gain coefficient cl being specific to that laser unit 2 based on its actual gain gl . Similarly, the detector head 4 has been configured with a gain coefficient c2 stored in memory 44, with the gain coefficient c2 being specific to that detector head 4 based on its actual gain g2. However, the detector head 4 has not been pre-adjusted to match the laser unit 2.

After having been connected together, the laser unit 2 reads the gain coefficient c2 from the detector head 4, such that it now has both coefficients cl and c2 in memory 42. When a detection signal is received at the laser unit 2 from the detector head 4, a processor 46 in the laser unit 2 reads the coefficients cl and c2 from memory 42 and controls a suitable gain adjustment (not shown) to apply a gain correction to the detection signal such that the adjusted signal strength S is cl x c2 x S. The adjusted signal is then output to the interface 6 for processing to determine position data. As will be more apparent from the examples below, by applying the coefficients cl and c2 to the original detection signal, the strength S of the adjusted signal will be closer to the ideal of 1 Vpp that is expected at the interface 6.

In order to determine suitable coefficients cl for the various laser units 2 that are to be used in a modular system embodying the present invention, and also suitable coefficients c2 for each of the detector heads 4, a reference is first established. As an illustrative example it is determined that the gain values gl and g2 associated respectively with the laser unit 2 and detector head 4 in a reference laser encoder system 1 are both equal to 10, and that this gives a signal S with the ideal signal strength of 1 Vpp. This is the reference against which all further laser units 2 and detector heads 4 will be adjusted.

Now consider two laser units LI and L2 and four detector heads Hl to H4 which are being made and sold to a customer. The actual gain value gl for the two laser units LI and L2 are determined to be 5 and 20 respectively. From the actual gain value gl and the reference value gl, a coefficient cl is determined as (reference gl / actual gl), so that coefficient cl is 2 and 0.5 respectively for laser units LI and L2. It can be seen that the actual value for gl can notionally be corrected (to recover the ideal or reference value for gl of 10) by multiplying the coefficient cl by the actual value for gl. This information is collated in the table of Figure 18. A similar analysis can be made in respect of the four detector heads Hl to H4, and example values for the actual g2, the coefficient c2 and the corrected g2 are shown in the table of Figure 19.

Now that coefficients cl and c2 have been determined (and stored in memory 42 and 44 respectively), any laser unit of this group can be combined with any detector head of this group, using the relevant coefficients cl and c2 to adjust or correct the signal output from the relevant detector head. As an example, the customer connects laser unit LI to detector head Hl and finds that the signal output from the detector head Hl for this combination is 0.4 Vpp, which at the lower end of the acceptable range of values shown in the table of Figure 12 (though far from the ideal value of 1 Vpp). However, taking account the values of 2 and 1.25 for the coefficients cl and c2 respectively (see Figures 18 and 19), and using the scheme embodying the present invention as shown in Figure 17, the adjusted signal for this combination is determined to be cl * _c2 * S = 2 * 1.25 x 0.4 = 1 Vpp. Therefore, by using an embodiment of the present invention, the signal has been automatically adjusted so as to have a signal strength of 1 Vpp, without any manual gain adjustment being required.

Now the customer decides to use detector head H4 instead of Hl, so they disconnect Hl from LI and instead connect H4 (which has a coefficient c2 of 1.6 as shown in Figure 19). The output signal from the detector head H4 is found to be 0.3125 Vpp, which is apparently even further from the ideal of 1 Vpp than it was for the previous combination of LI and Hl, but after correction using the new coefficient pair cl, c2 the signal output from the laser unit LI is actually cl * c2 * S = 2 x 1.6 x 0.4 = 1 Vpp. Again, the signal has been automatically adjusted so as to have a signal strength of 1 Vpp, without anything needing to be done by the user.

As a final example, the customer now connects laser unit L2 with detector head H3 and finds that the output signal from the detector head H3 is 2 Vpp, which is apparently well beyond the top end of the range shown in Figure 12. However, referring to the coefficient values from Figures 18 and 19, the signal is automatically adjusted by operation of an embodiment of the present invention to give an output signal from the laser unit L2 of cl x c2 x S = 0.5 x 1 x 2 = 1 Vpp.

As illustrated schematically in Figure 20 (which corresponds to the example described above with reference to Figures 18 and 19), an embodiment of the present invention provides a very convenient and flexible laser encoder system 1 for the user, allowing them to couple any laser unit 2 with any detector head 4, rather than being forced to connected these in specific pairings, and also without having to undertake a difficult manual adjustment (which is likely to introduce errors as well as being very time consuming) or send anything back to the supplier (which takes the units concerned out of use for a prolonged period of time). For any combination of laser unit 2 and detector head 4 it is ensured that a consistent signal S is output from the relevant laser unit 2 to the interface 6, at least within a predetermined acceptable range.

In the embodiment described above with reference to Figure 17 the gain coefficients cl, c2 are stored in memory 42, 44. It will be appreciated that these gain coefficients cl, c2 could instead be stored in a central database and read by the laser unit 2 based for example on respective identifiers assigned to the laser unit 2 and detector head 4 concerned.

It was also described above that the adjusted signal strength S is determined to be cl x c2 x S. Effectively, the final gain for the combination of parts has been determined based on laser gain x head gain. It would also be possible to provide a user-adjustable gain coefficient c3, such that the adjusted signal strength S is determined to be cl ^x c2 x c3 x S. Effectively, the final gain for the combination of parts would then be determined based on laser gain x head gain x user gain.

The above concept can easily be extended to include offset correction, by configuring offset coefficients ol and o2 for the laser unit 2 and detector head 4 respectively. These would be held in memory 42 and 44, respectively, and the offset coefficient o2 would be communicated from the detector head 4 to the laser unit 2 along with the gain coefficient c2 exactly as described above. These offset coefficients ol and o2 would combine additively rather than multiplicatively, such that the final offset for the combination of parts would been determined based on laser offset + head offset (or laser offset + head offset + user offset if a user- adjustable offset coefficient is also made provided). Laser and head offsets are corrected for applied gain (so that changing the gain also changes the offset). In other words, the gain is applied after the offset. Therefore, the processor 46 of Figure 17 would be operating according to the following expression:

S = cl x _C2 x (S + ol + o2)

Or in the case where a user-adjustable offset coefficient o3 is also provided (along with a user-adjustable gain coefficient c3 as described above):

S = cl x c2 x c3 x (S + ol + o2 + o3)

As described above with reference to Figure 10, the detection signal is in practice typically a quadrature signal, comprising a pair of constituent signals: a cosine signal and a sine signal. The above simplified explanation can easily be extended to such a quadrature signal just by applying a separate adjustment to the signals in the quadrature pair, so that there would be a first set of coefficients for making corrections to the cosine signal and a second set of coefficients for making corrections to the sine signal. Such an arrangement is shown in Figure 21, which is a straightforward extension of Figure 17 so that a detailed explanation is not required, with like reference numerals referring to like parts and with an ‘s’ postfix denoting those parts 42s, 44s, 46s which relate to and act on the sine part of the quadrature signal S_quad and a ‘c’ postfix denoting those parts 42c, 44c, 46c which relate to and act on the cosine part of the quadrature signal S_quad. A simple further extension of this would be to duplicate the arrangement to account for two laser outputs Al, A2 in the same laser unit 2 in a dual axis system such as illustrated in Figures 1 and 8.

The gain adjustment applied to the detection signal can be implemented in the detector unit 2 via a digital potentiometer. This could be set by software but would tend to have relatively limited bandwidth and therefore this solution might not be appropriate in a laser interferometer system without adversely limiting the maximum measurement velocity.

Instead, an analogue multiplier could be used for the gain adjustment. An analogue multiplier is a component that takes two analogue signals and outputs the product of those two signals. The cost would typically be higher compared to a digital potentiometer but such a solution would typically be better suited for higher-frequency applications.

The correction could also be applied after the analogue quadrature signal is digitised, and this would typically require an additional piece of hardware. One possible downside with this option is that the customer may not be able to use their own interpolator.

The offset can be adjusted using a digital-to-analogue converter (DAC) to add/ subtract a DC voltage to/from the outputs.

Of those mentioned above, the preferred implementation is likely to be the use of an analogue multiplier option for gain adjustment. A field programmable gate array (FPGA) can be configured to read the stored coefficients, combine them, and then output appropriate signals to the analogue multipliers and offset DACs.

It should be noted that an embodiment of the present invention is not limited to the analogue domain but can be applied also in the digital domain. In this respect, the coefficients can be applied to the signals after digitisation but an advantage of doing it in the analogue domain is that the signal presented to the digitisation analogue-to-digital converter (ADC) is in the optimal range and makes the digitisation design more straightforward.

An embodiment of the present invention as set out above accounts for variation in gain and offset within the laser unit 2 and the detector head 4, to enable these parts to be swapped around easily in a modular or “plug and play” fashion. With the detector head 4 now being connectable to any laser unit 2, there are two separate fibres: one that is internal to the laser unit 2 and which delivers light from the fibre launch within the laser unit 2 to the connector panel (the equivalent of what is shown in Figure 8), and an external fibre 11 (within the optical fibre assembly 32) to transport light from the connector panel to the detector head 4. There will typically be an optical loss associated with the core of the fibre 11 being slightly misaligned to the core of the fibre inside the laser unit 2 when the optical fibre assembly 32 is connected to the chosen laser unit 2, thereby preventing efficient transfer of light from one fibre to the other (this is referred to as “insertion loss”). This optical loss will vary depending on which combination of laser unit 2, detector head 4 and optical fibre assembly 32 is used. As discussed above, for existing laser encoder systems without “plug and play” functionality the optical fibre assembly 32 cannot be disconnected from the laser unit 2 at all so this issue does not arise. However, this variation is not accounted for in an embodiment of the present invention as set out above, so this variation will result in a variation in signal strength S.

However, there are two main benefits of the “plug and play” approach embodying the present invention: (a) to minimise (or at least reduce) the variation in signal strength from unit to unit; and (b) to minimise (or at least reduce) differences in sine/cosine amplitude and offset, as this results in measurement errors (usually referred to as non-linearity error, or sub -divisional error). The optical fibre assembly 32 only contributes to (a) and not (b), i.e. swapping the optical fibre assembly 32 might change the signal strength S but it will not affect measurement accuracy. Hence, the system as a whole is still considered to benefit from being modular or “plug and play” in nature even in respect of having interchangeable optical fibre assemblies 32.

It will of course be appreciated that any absolute values given above for the detection signal voltage, such as those shown in the table of Figure 12 and the ideal signal level of 1 Vpp, are purely by way of example and are not intended to restrict the scope of the invention as set out in the appended claims. The same applies to the example coefficient values shown in Figures 18 and 19, and any other specific example values given elsewhere herein.

It is described above that the detection signal is sent from the detector head 4 to the laser unit 2 via the electrical cable 3, and that the signal adjustment is performed at the laser unit 2 based on the coefficients relevant to that pairing of laser unit 2 and detector head 4. As illustrated in the arrangement shown in Figure 22 (which is based more closely on the simplified arrangement of Figure 17 than that of Figure 21), it would be possible instead to send the detection signal from the detector head 4 to a signal processing unit 50 located somewhere other than the laser unit 2 to perform this signal adjustment. The coefficient cl is communicated from the laser unit 2 to the detector head 4, and both coefficients cl and c2 are communicated from the detector head 4 to the signal processing unit 50.

Alternatively, as already alluded to above, as shown in the alternative arrangement of Figure 23 the signal processing unit 50 could fetch these coefficients cl and c2 from a remote storage 55 based on respective identifiers idl and id2 assigned to the laser unit 2 and detector head 4. It would also be possible to perform the signal adjustment at the detector head 4 itself, in which case the detector head 4 would retrieve the relevant coefficients and use these in an entirely equivalent way to what is described above. It would also be possible for the signal adjustment to be performed at the interface 6. It will therefore be apparent that the signal adjustment can be performed anywhere and is not limited to being performed at the laser unit 2.

In view of the above, Figure 24 shows an alternative representation of a modular laser encoder system embodying the present invention, in which the signal processing unit 50 is shown as receiving the relevant coefficients cl, c2 by whatever means and from whatever source. The signal processing unit 50 could be implemented as a functional unit as part of the laser unit 2, or as part of the detector head 4, or as part of the interface 6, or it could be located separately to any of those parts. The adjusted signal S is passed from the signal processing unit 50 to a signal conversion unit 60 which is adapted to digitise and interpolate the analogue quadrature signal AQ to produce a digital quadrature signal DQ. The role of the signal conversion unit 60 will be discussed in more detail below. The digital quadrature signal DQ is then passed to a positional data deriving unit 70 which is adapted to derive positional data PD (relating to the position of the target optic 8 relative to the detector head 2) from the digital quadrature signal DQ.

Again, the signal conversion unit 60 and positional data deriving unit 70 could be implemented wherever is appropriate and convenient for the application concerned. For example, the signal conversion unit 60 could be implemented as part of the laser unit 2 (for example along with the signal processing unit 50) with the positional data deriving unit 70 being implemented as part of the interface 6, or both could be implemented in the interface 6. As discussed above, the laser unit 2 could be switchable between a digital output mode in which the signal conversion is performed by a signal conversion unit 60 implemented in the laser unit 2 and an analogue output mode in which the signal conversion unit 60 implemented in the laser unit 2 does not perform the signal conversion but instead an analogue signal is output from the laser unit 2. In the latter case, the signal conversion would be performed by a separate signal conversion unit 60 for example implemented in the interface 6 (for example the RPI20 or RPI30 parallel interface mentioned above), typically in a situation where a higher interpolation factor is required than that which is available at the laser unit 2.

Figures 25 and 26 will assist in a further understanding regarding the role of the signal conversion unit 60. Figure 25 schematically shows the core functional components of the signal conversion unit 60. In this respect, the adjusted analogue constituent signals received at the input of the signal conversion unit 60 from the signal processing unit 50 are converted into digital form by A/D converters 61 and the digitised versions of these signals are then passed through an interpolator algorithm (IA) 63 to generate a pair of digitised and interpolated output signals, which are illustrated schematically at the output of the signal conversion unit 60 as a digital quadrature signal DQ. For example, the interpolator algorithm 63 could base the interpolation on the most significant bit (MSB) of the digitised signal from the A/D converter 61, such that the interpolated outputs would follow the zero crossings of the sine and cosine signals. The digital quadrature signal DQ is passed to the positional data deriving unit 70 which is operable to derive positional data PD from the digital quadrature signal DQ.

To illustrate the digitisation and interpolation graphically, Figure 26 shows the analogue quadrature signal AQ (comprising a pair of cosine and sine signals), and below that shows two example conversions into digital quadrature signals DQ1 and DQ5. For the first conversion an interpolation factor of 1 is used by the interpolator algorithm 63, so that the digital quadrature signal DQ1 (which comprises a pair of digital signals Al, Bl corresponding respectively to the original cosine and sine signals) has the same frequency as the original analogue quadrature signal AQ. For the second conversion an interpolation factor of 5 is used by the interpolator algorithm 63, so that the digital quadrature signal DQ5 has a frequency that is five time that of the original analogue quadrature signal AQ.

The A and B signals (marked as Al, Bl and A5, B5 in Figure 26) are used to drive a digital counter which may form the basis of the positional data derived by the positional data deriving unit 70. Accordingly, the spatial resolution in the positional data PD derived from the DQ5 signal will be five times that derived from the DQ1 signal, so that smaller movements of the target optic 8 can ultimately be measured. The resolution of the DQ1 signal is represented as R1 in Figure 26, and similarly the resolution of the DQ5 signal is represented as R5. The signal conversion unit 60 is sometimes just referred to as an interpolator. Figure 27 is a schematic illustration of an alternative implantation of a modular or “plug and play” system compared to that shown in Figure 24. As before, the laser unit 2 receives current I and converts that to an optical power P at a ratio gl (which is fixed but can vary between different laser units 2). Optical power P is transferred to the detector head 4, where it is converted to a signal S at a ratio g2 (which is manually adjustable via a potentiometer). Signal S goes back through the laser unit 2 where an adjustment pl is applied (again manually adjustable via a potentiometer). Signal S (which is now normalised to 1 Vpp) is sent to the interface 6. Similar to the existing laser encoder system, each laser unit 2 and detector head 4 is pre-adjusted in the manufacturing facility before being sent out to the customer. But with the scheme shown in Figure 27 each side is preadjusted separately (via the potentiometers in the laser unit 2 and detector head 4 respectively) to normalise them relative to a reference, i.e. not as a pair but separately. Then, because each side is normalised separately, any detector head 4 can be used with any laser unit 2 and have a consistent signal level of 1 Vpp at the output from the laser unit 2.

With the system of Figure 24, the adjustment is applied in one place, at the signal processing unit 50, and based on coefficients that are loaded dynamically depending on which combination of laser unit 2 and detector head is connected. This is not the case with the system of Figure 27, where the adjustment is applied both at the detector head 4 (to normalise the response of the detector head 4) and also at the laser unit 2 (to normalise the response of the laser unit 2). Therefore, with the implementation of Figure 27, two gain/offset adjustment stages are required. Furthermore, the adjustments are built into the system at the time of the manufacture and, although in theory can be re-adjusted later, this is practically only possible by sending the units back to the manufacturer. With the dynamic system of Figure 24, where the coefficients are stored and loaded electronically, it becomes possible for the user very easily to apply custom gain and offset adjustments for example via additional user-adjustable coefficients (as mentioned above).

Furthermore, with the system of Figure 27, although the output signal from the detector head 4 is marked as having an uncertain signal “S = ?”, it should be noted that, when different detector heads 4 are paired with the same laser unit 2, the signal level S from the head 4 will actually stay roughly the same (albeit not necessarily at 1 Vpp), because each head 4 has been pre-adjusted or normalised. This would not be the case with system of Figure 24, where the signal from the detector head 4 would not be consistent when using different detector heads 4 with the same laser unit 2, because all the adjustment, for both the detector head 4 and the laser unit 2, are performed subsequently, in one place, at the signal processing unit 50.

Claims

1. A laser encoder system comprising one or more laser units and one or more detector heads, with any one of the laser units being connectable in use to any one of the detector heads to provide laser light to the connected detector head and with the connected detector head using the laser light to generate a detection signal, wherein a signal level associated with the detection signal varies depending on which combination of laser unit and detector head is used, and wherein each of the laser units and detector heads is associated with one or more coefficients which aim to normalise a response of that laser unit or detector head such that a corrected signal is derivable from the detection signal by a signal processing unit using the coefficients associated with the laser unit and detector head of the combination concerned, with the corrected signal having a signal level that is substantially consistent regardless of what combination of laser unit and detector head is used.

2. A system as claimed in claim 1, wherein the signal processing unit is or forms part of or is implemented as a functional unit of the laser unit.

3. A system as claimed in claim 1 or 2, wherein the coefficients comprise one or more gain coefficients for correction of a gain associated with the detection signal.

4. A system as claimed in claim 1, 2 or 3, wherein the coefficients comprise one or more offset coefficients for correction of an offset associated with the detection signal.

5. A system as claimed in any preceding claim, wherein the detection signal is a quadrature signal comprising first and second constituent signals.

6. A system as claimed in claim 5, wherein first and second sets of coefficients are provided for correction of the first and second constituent signals respectively.

7. A system as claimed in claim 5 or 6, wherein the first and second constituent signals are sinusoidal signals.

8. A system as claimed in claim 5, 6 or 7, wherein the first and second constituent signals are cosine and sine signals respectively.

9. A system as claimed in any preceding claim, wherein the detection signal is an analogue signal.

10. A system as claimed in claim 9, wherein the correction is performed before converting the analogue signal to a digital signal.

11. A system as claimed in any preceding claim, wherein at least one of the coefficients associated with the laser unit are stored in the laser unit.

12. A system as claimed in claim 11, wherein the laser unit is operable to send the required coefficients to the signal processing unit on request.

13. A system as claimed in any preceding claim, wherein at least one of the coefficients associated with the detector head are stored in the detector head.

14. A system as claimed in claim 13, wherein the detector head is operable to send the required coefficients to the signal processing unit on request.

15. A system as claimed in any preceding claim, wherein at least one of the coefficients associated with the laser unit or the detector head are stored in a remote database and retrieved by the signal processing unit based on an identifier associated with the laser unit or detector head, as the case may be.

16. A system as claimed in any preceding claim, wherein the signal processing unit is or forms part of or is implemented as a functional unit of a single system component.

17. A system as claimed in any preceding claim, wherein the signal level associated with the detection signal output from the detector head varies depending on which detector head is used with a specific laser unit.

18. A system as claimed in any preceding claim, wherein the corrected signal is derivable from the detection signal by the signal processing unit using the coefficients associated with the laser unit and detector head of the combination concerned as well as at least one user-adjustable coefficient.

19. A system as claimed in any preceding claim, wherein the coefficients are stored in memory and loaded dynamically by the signal processing unit when required.

20. A system as claimed in any preceding claim, wherein the signal processing unit is arranged to output the corrected signal to a signal conversion unit which is adapted to digitise and interpolate the corrected signal.

21. A system as claimed in claim 20, wherein the signal conversion unit is or forms part of or is implemented as a functional unit of the laser unit or a separate system component such as an interface component.

22. A system as claimed in claim 20 or 21, wherein the signal conversion unit is arranged to output the digitised and interpolated signal to a positional data deriving unit, with the positional data deriving unit being adapted to derive positional data from the digitised and interpolated signal.

23. A system as claimed in claim 22, wherein the positional data deriving unit is or forms part of or is implemented as a functional unit of the laser unit or a separate system component such as an interface component or controller.

24. A system as claimed in any preceding claim, wherein the detection signal relates to a distance between the detector head and a target.

25. A system as claimed in any preceding claim, wherein the detection signal is an interference signal resulting from interference between a reference laser beam and a measurement laser beam, with the measurement laser beam being directed onto and at least partially reflected back from a target.

26. A system as claimed in any preceding claim, wherein the laser unit is connected in use to the detector head via an optical fibre assembly to provide the laser light to the detector head.

27. A system as claimed in any preceding claim, wherein the laser unit is connected in use to the detector head via an electrical cable to receive the detection signal from the detector head.

28. A laser unit adapted for use in a system as claimed in any preceding claim.

29. A detector head adapted for use in a system as claimed in any one of claims 1 to 27.