Calibration apparatus

Abstract

A calibration apparatus for calibration of a thickness measurement instrument comprises a holder attached to a base. The holder holds a probe of the thickness measurement instrument and the base receives a substrate. The holder is movable relative to the base to cause movement of the probe relative to the substrate for calibration of the thickness measurement instrument. The calibration apparatus may comprise coupling to provide a first degree of freedom of movement of the holder with respect to the base. The holder may be releasably attached to the calibration apparatus. A set of holders may be provided, each holder in the set being for use with a corresponding different weight probe. Each holder may have a different weight such that each holder and probe pair has the same weight. The weight of the holder and probe may define a contact force between the probe and substrate.

Description

[0001] CALIBRATION APPARATUS

[0002] Technical Field of the Invention

[0003] The present invention relates to calibration apparatus. In particular to calibration apparatus for thickness measurement instruments. Most particularly to automated calibration apparatus for coating thickness measurement instruments. Background to the Invention

[0004] Coating thickness measurement instruments have a variety of applications in determining the thickness of a coating on a substrate, for example, the thickness of a protective coating of paint on the hull of a boat. These types of coating thickness measurement instrument typically have an inductive probe that is moved onto the surface of the coating to be measured and a gauge attached to the probe. The gauge monitors the electrical properties of the probe to determine the distance between the probe and the substrate, and therefore the thickness of the coating. In existing coating thickness measurement instruments, the probe and gauge may be integrated into the same device, or may be provided separately to allow different combinations of probes and gauge depending on the measurement required.

[0005] Both the gauge and probe require various electronics components to function, such as signal generators, amplifiers, analogue-to-digital and digital-to-analogue converters, and processors. Each of these components may have one or more different settings that must be calibrated to ensure the final measured coating thickness is correct. Examples of existing coating thickness measurement instruments and their components are disclosed in GB2367135A and GB2601158A.

[0006] To calibrate the coating thickness measurement instrument, the electrical properties of the probe and response of the gauge is monitored when the probe is provided at various different calibration distances from the substrate. The calibration process is manual and involves an operator selecting an appropriate calibration foil of known thickness, and then measuring the calibration foil with the probe and recording the measurement. This involves manual placement of the probe onto the calibration foil and substrate. Once several measurements have been taken, including placing the probe directly on the substrate without a calibration foil, the operator changes thesettings of the probe and / or gauge to calibrate the coating thickness measurement instrument.

[0007] However, small changes in the angle of the probe, placement of calibration foil, and contact forces between the probe and substrate can easily occur as an operator changes calibration foils or between different operators. These can cause calibration inaccuracies that compromise the effectiveness of the instrument. So, there remains a need to improve the consistency and speed of calibration.

[0008] It is therefore an object of embodiments of the present invention to at least partially overcome the above issues.

[0009] Summary of the Invention

[0010] In a broad sense, the invention relates to calibration apparatus. The calibration apparatus may be for calibration of a thickness measurement instrument. The calibration apparatus may comprise a holder. The calibration apparatus may comprise a base. The holder may be attached to the base. The holder may be for holding a probe of the thickness measurement instrument. The base may be for receiving a substrate. The calibration apparatus may be configured to move the holder relative to the base to cause movement of the probe relative to the substrate for calibration of the thickness measurement instrument

[0011] According to a first aspect of the invention there is provided a calibration apparatus for calibration of a thickness measurement instrument, the calibration apparatus comprising a holder attached to a base, the holder being for holding a probe of the thickness measurement instrument and the base being for receiving a substrate, wherein the calibration apparatus is configured to move the holder relative to the base to cause movement of the probe relative to the substrate for calibration of the thickness measurement instrument.

[0012] Advantageously, the apparatus enables automation of what is conventionally a manual task. This improves the repeatability of measurements as the holder and base may be designed to ensure that the probe angle, position and forces applied between the probe and substrate remain constant between successive calibration measurements. This improves the consistency of the calibration procedure and ensures that thicknessmeasurement instruments are all calibrated to have the same response to a given substrate or measurement condition.

[0013] The calibration apparatus may comprise a coupling comprising first and second coupling parts attached together. The first and second coupling parts may have a first degree of freedom of movement with respect to each other. The first coupling part may be connected to the probe. The second coupling part may be connected to the substrate. The first and second coupling parts may therefore be movable with respect to each other. The first and second coupling parts may be free to move with respect to each other as defined by the first degree of freedom of movement. The first degree of freedom of movement may be present during measurement of the substrate by the probe. By providing some degree of freedom of movement of the coupling parts during measurement, the errors in the accuracy or control capabilities of the apparatus are mitigated because the coupling parts can move to account for these errors.

[0014] The first degree of freedom of movement may be a translation in a first direction. The first direction may be a direction between the probe and substrate. The first direction may be perpendicular to the substrate. Advantageously, the first and second coupling parts may move relative to one another when the probe contacts the substrate, or an object on the substrate. This ensures that errors in movement or forces applied by the calibration apparatus between the holder and base do not get transferred into excessive forces acting between the probe and substrate. This could cause damage to the probe, substrate and / or an object on the substrate, as well as leading to inconsistent calibration results. It also allows the placement force, or contact force, of the probe onto the substrate to be controlled by other means, such as the weight of the probe as described below.

[0015] The probe may comprise a probe axis defining a measurement direction of the probe. The probe may be configured to measure a thickness of an object on the substrate. The probe may be configured to be placed onto the substrate with the object therebetween. The calibration apparatus may be configured to move the probe in a direction parallel to the probe axis onto the substrate. The first direction may be a direction parallel to the probe axis. The first direction may be a vertical direction. The calibration apparatus may be configured to move the probe towards the substrate untilthe first and second coupling parts move relative to one another. This allows the coupling parts to move and mitigate any errors in movement or forces applied along the probe axis. This is important as the force applied between the probe and substrate can be decoupled from the force applied by the calibration apparatus, this allows more repeatable force application that is independent of the control of the calibration apparatus which may be more prone to errors.

[0016] The first and second coupling parts may have only one translational degree of freedom with respect to each other. The first and second coupling parts may be fixed with respect to each other in directions other than the first direction. Fixed directions may include translations in directions other than the first direction and / or rotations. In particular, any rotation not about an axis defined by the first direction may be restricted. Rotation about the first direction may be permitted. This can be allowed as rotation of the probe along the probe axis typically does not affect measurement results. The probe and substrate may therefore be held securely with respect to each other in all but one direction. This helps ensure the calibration apparatus can accurately control other aspects of the movement of the probe and substrate with respect to each other.

[0017] The calibration apparatus may be configured to hold the probe above the substrate. The holder may be configured to move the probe down onto the substrate. This advantageously mimics the typical use of the probe in practice and allows gravity to bias the position of the coupling as defined below without additional biasing elements or actuators.

[0018] The first and second coupling parts may comprise a range of motion with respect to one another in the first direction. The range of motion may define the extent of the freedom of movement of the first and second coupling parts with respect to each other. The calibration apparatus may be configured to move the probe and the substrate together until the first and second coupling parts are towards a midpoint of the range of motion with respect to one another. An end of the range of motion may prevent continued movement of the first and second coupling parts with respect to each other. The calibration apparatus may be configured to move the probe and substrate together until the first and second parts are away from an end of the range of motion. Thus, the calibration apparatus may continue moving until the first and second couplings are in aposition where they are not limited by an end of the range of motion. This ensures the coupling can work more effectively.

[0019] An end of the range of motion may be a lower end or an upper end. When the first and second coupling are at the lower end of the range of motion, the holder may hold the probe closer to the substrate than when the first and second couplings are at the upper end. The calibration apparatus may be arranged such that when the probe and substrate are held apart, the first and second couplings are at the lower end of the range of motion with respect to one another. This ensures that when the calibration apparatus moves the probe and substrate together, a contact force therebetween will cause the first and second couplings to move out of the lower end of the range of motion towards the upper end.

[0020] The first and second couplings may be biased towards the lower end of the range of motion. The first and second couplings may be biased towards the lower end due to gravity. This helps ensure that when a contact force establishes between the probe and substrate, the coupling is free to move and prevent the transfer of forces directly from the calibration apparatus to the probe and substrate.

[0021] One of the first and second coupling parts may comprise a rail. The other of the first and second coupling parts may comprise a carriage that is coupled to the rail and movable along the rail. The first coupling part may be the carriage. The second coupling part may be the rail. The rail may define the freedom of movement of the first coupling with respect to the second coupling. The rail may define the range of motion of the first and second coupling parts. Therefore, the coupling is conveniently designed and simple to construct while providing an intuitive range of motion that is easily controlled and adapted for different uses.

[0022] The coupling may comprise a damper to damp movement of the first coupling part with respect to the second coupling part. Advantageously, this softens the relative movement of the probe and substrate and reduces the tendency of the probe to bounce off the substrate when they are brought together but remain free to move due to the coupling. The damper may be any suitable damper for the specific coupling arrangement used, for example a frictional damper, viscous damper, or the like.The second coupling part may be attached to the holder. Consequently, the coupling may be provided between the holder and the probe. The holder may hold the probe via the coupling. The holder may comprise the coupling. This enables more intuitive and simple operation of the apparatus with the weight of the first coupling part and probe controlling the contact force between the probe and substrate. Of course, in other embodiments, the coupling may be provided between other parts of the calibration apparatus, such as between the base and holder, or base and substrate, or as an integral part of the holder and / or base.

[0023] The holder may comprise a probe mount configured to attach to the probe. The first coupling part may comprise the probe mount. The probe mount may comprise an aperture to receive the probe. The aperture may be configured to prevent movement of the probe with respect to the probe mount. The aperture may be lockable to prevent movement of the probe. The probe mount may comprise a door. The door may be hinged with respect to the probe mount. The door may be lockable to the probe mount. The door may comprise a latch to lock the door closed against the probe mount. When the door is locked to the probe mount, the aperture may be formed between the door and probe mount. Therefore, the probe mount can conveniently allow the probe to be attached to the calibration apparatus. Of course, in other embodiments different types of mount could be used.

[0024] The calibration apparatus may comprise an arm connecting the base to the holder. The arm may be configured to translate the holder across the base. The arm may be configured to move the holder in directions perpendicular to the first direction. The arm may be pivotally connected to the base. This can allow the holder to move with respect to the base and select different positions on the base. The calibration apparatus may comprise an arm motor configured to move the arm with respect to the base. The arm motor may comprise a planetary gearbox. This can help provide improved performance where it is otherwise limited by the motor itself and is especially useful when the apparatus is for use with heavier or larger probes. The base may comprise a tower. The arm may be attached to the top of the tower. Thus, the arm may be raised above the base. This can provide sufficient clearance for the operation of the holder and base during calibration.The calibration apparatus may be configured to releasably hold the probe. This may enable quick release and / or interchange of probes to facilitation calibration of multiple probes in a short amount of time.

[0025] The holder may be releasably attached to the calibration apparatus. The holder may be releasably attached to the arm. This may allow the probe to be easily removed and attached from the calibration apparatus. The calibration apparatus may comprise a kinematic coupler formed of first and second plates couplable together. The first plate may be provided on the holder. The second plate may be provided on the arm. The kinematic coupler may comprise one or more kinematic mounts, such as a ball and plate, ball and groove, and / or ball and cone on the first and second plates respectively. The first and second plates may be couplable together magnetically, or by any other suitable means, such as a physical clamp. The kinematic coupler may comprise one or more adjustment screws to change the relative angle, position, and / or orientation of the first plate with respect to the second plate when coupled together. Advantageously, the use of a kinematic coupler ensures that the precise alignment of the probe onto the calibration apparatus is maintained when it is removed or re-attached multiple times.

[0026] The holder may be releasably attached to the calibration apparatus when the holder comprises the coupling. The coupling and probe mount may therefore be releasably attached to the holder. This can allow for easier removal and replacement of probes as the coupling can be adapted to be more ergonomically designed and may need to be large to accommodate the rail or other moving parts to enable the range of motion between the first and second couplings.

[0027] The calibration apparatus may comprise a set of holders. The set of holders may comprise two, three, four, or more holders. Each holder may be configured to releasably attach to the arm. The calibration apparatus may be for calibration of a set of different thickness measurement instruments. Each holder may have a different weight. Each holder may have a different weight from the other holders. The holder may comprise a ballast. The ballast allows easy control of the weight of the holder. This allows the holders to have a specific weight for a specific different probe as described below to provide constant contact forces between different probes and holders.Each holder may have a calibrated weight with respect to a specific one of the set of different thickness measurement instruments. The calibrated weight may be selected such that the combined weight of each holder and the probe of the corresponding thickness measurement instrument is substantially the same. “Substantially the same” in this context may mean the combined weight of a holders and its probe is within 10%, or 5% or 2%, or 1% of the combined weight of another holder and its probe. This helps to ensure that the calibration apparatus may effectively work with multiple different thickness measurement instruments and that, when combined with the coupling, the contact force between the probe of the thickness measurement instrument and substrate is always the same, as it is controlled by the combined weight of the holder and probe / thickness measurement instrument.

[0028] The calibration apparatus may comprise a trolley and a track. The trolley may be movable along the track to move the holder towards the base. The holder may be attached to the trolley. The arm may comprise the track. The trolley may slide along the track. This can allow smooth and accurately controlled motion of the holder relative to the base.

[0029] The track may be arranged parallel to the first direction. Advantageously, this allows the coupling to work effectively to ensure any errors in movement of the trolley along the track do not negatively affect calibration of the measurement instrument.

[0030] The calibration apparatus may comprise a compressed air input connected to the trolley. The trolley may be pneumatically powered along the track. Advantageously, this provides smooth power control with widely available components and without the need to have permanently energised electrical motors or actuators. Of course, in other embodiments other powered trolleys could be used such as hydraulically powered trolleys, electrically actuated trolleys, or screw driven trolleys.

[0031] The calibration apparatus may comprise a sample selector. The sample selector may be configured to hold one or more calibration foils. The sample selector may be configured to move the one or more calibration foils between the probe and the substrate. Advantageously, the calibration apparatus can therefore move a calibration foil into position for calibration. The sample selector may comprise a gap free ofcalibration foils. Advantageously, this may allow the probe to be lowered directly onto the substrate to take a zero reading.

[0032] The calibration apparatus may be configured to move the holder with respect to the base so that the probe can measure the thickness of the substrate. The calibration apparatus may be configured to move the holder with respect to the base until a contact force is present between the probe and substrate. The calibration apparatus may be configured to move the holder with respect to the base to trap a calibration foil between the probe and substrate. This allows the measurement instrument to be calibrated to provide a correct measurement of the thickness of a calibration foil.

[0033] The sample selector may comprise turntable configured to hold the one or more calibration foils. The sample selector may be configured to rotate the turntable to move the one or more calibration foils between the probe and the substrate. The sample selector may comprise a motor to rotate the turntable. A plurality of calibration foils may be arranged around the turntable. The calibration foils may each be equidistant from a rotation axis of the turntable. The base may be provided on an inside of the turntable. The turntable may be arranged to rotate around the base.

[0034] Advantageously, the turntable facilitates a more compact design where multiple calibration foils can be easily moved between the probe and substrate for calibration of the measurement instrument.

[0035] The calibration apparatus may be configured to change an offset of the respective positions of the calibration foil and the probe between calibration measurements.

[0036] The sample selector may comprise an offset mechanism. The offset mechanism may be configured to apply an offset to the respective positions of the calibration foil and probe. The offset may be in a direction perpendicular to the probe axis. The calibration foil may be substantially planar. The offset may be in a direction parallel to a plane of the calibration foil. The offset may select a position on the foil that the probe measures during calibration. The offset mechanism may be arranged to change the offset between each calibration measurement, and / or at the start of a calibration of a new measurement instrument. The offset may be randomly calculated from the possiblepositions of the probe on the calibration foil. The offset may cycle through the possible positions of the probe on the calibration foil. The offset mechanism may be arranged to move the probe, such as via rotation of the arm. The offset mechanism may be arranged to move the foil, such as via the sample selector. The calibration apparatus may be configured to change the offset such that the probe does not repeatedly contact the same location on the foil. This increases the longevity of the calibration foil and ensures it can be used for the calibration of many measurement instruments as the probe measures different points on the foil and does not tend to wear down a single location on the foil. This is typically not a problem encountered in existing manual calibration methods as manual placement of the probe onto a foil results in variations in the position of the probe with each measurement.

[0037] The calibration apparatus may comprise one or more support bearings. The one or more support bearings may be configured to support the sample selector. The one or more support bearings may be provided beneath the sample selector. The one or more support bearings may be adjustable to change the height of the sample selector with respect to the substrate when received by the base. Preferably, the support bearings may be configured to raise and lower the sample selector with respect to the substrate when received by the base. Each support bearing may be adjustable via a screw that is rotatable to translate the support bearing, preferably in the first direction. The sample selector may be configured to receive the one or more support bearings. The sample selector may comprise a channel to receive the one or more support bearings. This can ensure that the sample selector moves smoothly to ensure the calibration foil is not damaged or incorrectly positioned during calibrations.

[0038] The base may comprise a well to receive the substrate. The base may comprise an upper base surface. The well may be provided by a recess in the upper base surface. The base may comprise an access opening provided at an edge of the well. The access opening may be sized to allow a user to remove the substrate from the well. The base may comprise two access openings provided on opposites sides of the well. Therefore, the substrate may be slotted into the base. Advantageously, the substrate can be easily received and removed from the base.The base may be substantially planar in shape. The upper base surface may be substantially flat. The base may be circular. The base may be disc shaped. These shapes help the calibration apparatus to remain compact while working efficiently. Of course, many different shapes of the base could be used depending on the configuration of the apparatus.

[0039] The well may comprise a well depth measured from the upper base surface to the bottom of the well. The well depth may be greater than a thickness of the substrate. The substrate may comprise an upper substrate surface intended to face the probe during calibration. The substrate may comprise a lower substrate surface opposite the upper substrate surface. The thickness of the substrate may be measured from the upper substrate surface to the lower substrate surface. When the substrate is received in the well, the upper substrate surface may be recessed with respect to the upper base surface. The edges of the well may comprise a chamfer extending down from the upper base surface. The chamfer may extend down the well to the upper substrate surface when the substrate is received in the well. The calibration apparatus may comprise the substrate. The substrate may be a ferrous substrate. The substrate may be a non-ferrous substrate. Thus, the substrate may be conveniently received in the well. Advantageously, the substrate may not protrude from the well. This helps to protect the substrate. The chamfer may guide the calibration foil out of the well during movement of the sample selector with respect to the base. The upper base surface may be substantially flat around the chamfer. These features help ensure the calibration foil is not damaged as it moves over the base and into, or out of, the well.

[0040] The base may be configured to hold two or more substrates. The calibration apparatus may be configured to move the holder over the base to select a substrate for use with the probe. The arm may be configured to move the holder over the base to select a substrate for use with the probe. The arm may be rotatable around the same rotation axis as the turntable. The two or more substrates may comprise a ferrous substrate and a non-ferrous substrate. Advantageously, this allows the calibration instrument to calibrate measurement instruments that work with different substrates.

[0041] The sample selector may comprise a foil holder to hold a calibration foil. The foil holder may be configured to releasably hold a calibration foil. This can allow foilsto be easily changed as necessary. The foil holder may be pivotally mounted to the sample selector. The foil holder may comprise a clip, or any other suitable mechanism for holding a calibration foil. The calibration foil may be held in a position below the upper base surface. The calibration foil may bear on the upper base surface. This may hold the calibration foil at an angle to the upper base surface. Advantageously, this reduces the area of the foil in contact with the base, which reduces frictional forces therebetween and the likelihood of damage to the foils.

[0042] The calibration foil may be held in a position aligned with the upper substrate surface. This ensures that when the foil is placed on the substrate, it may be parallel to the substrate. This improves the accuracy of the calibration procedure.

[0043] The calibration apparatus may comprise two or more calibration foils. The two or more calibration foils may each have a different thickness. Each of the two or more calibration foils may be held by a respective foil holder of the sample selector. The calibration apparatus may comprise three or more, four or more, five or more, or ten or more calibration foils. Advantageously, by providing additional calibration foils then more accurate calibration of the measurement instrument can be realised.

[0044] The calibration apparatus may comprise a compressed air input. The calibration apparatus may comprise an air output nozzle. The air output nozzle may be arranged to direct air onto the substrate. The air output nozzle may be arranged to direct air onto the one or more calibration foils, preferably when positioned over the substrate. Advantageously, the compressed air then flows onto the substrate or foil and blows away any dust or other dirt that might compromise the calibration process.

[0045] The thickness measurement instrument may be for measuring the thickness of a coating on a substrate. The thickness measurement instrument may be a coating thickness measurement instrument. As described above, the thickness measurement instrument may comprise a probe. The probe may be for measuring the thickness of a coating on a substrate. The probe may be an inductive probe. The probe may comprise one or more drive coils. The probe may comprise one or more pick-up coils. The thickness measurement instrument may comprise a gauge. The gauge may be connected to the probe. The gauge may be separate from the probe or integrated with the probe. The gauge may be configured to receive electrical signals from the probeand output a thickness measurement. The gauge may comprise a user interface. The user interface may comprise one or more screens, buttons, sliders, toggles or the like. The thickness measurement instrument may comprise one or more electrical components, such as signal generators, amplifiers, analogue to digital convertors, and digital to analogue convertors. Each of these electrical components may be integrated into the probe and / or gauge. Each of these electrical components may be provided by software running on a processor. Calibration of the thickness measurement instrument may comprise adjusting the settings of any one or more of the electrical components and / or mathematical constants and / or weighting factors applied by the measurement instrument.

[0046] The calibration apparatus may comprise a communications system. The communications system may comprise a wired data connection and / or wireless data connection. The communications system may be configured to allow the calibration apparatus to communicate with external devices such as external computers and / or the measurement instrument.

[0047] The calibration apparatus may comprise a user interface. The user interface may comprise one or more user interface items that are configured to facilitate control of the apparatus by a user. The user interface items may comprise buttons, touch screens, sliders, toggles or the like. The user interface items may comprise a stop button. The stop button may be configured to safely cease control of the apparatus, for example by cutting power to motors. The stop button may also notify the processor that operation of the apparatus has been stopped. The user interface items may comprise a power button. The power button may be configured to power the apparatus on or off. Thus, the apparatus may be intuitively used and safety feature implemented to reduce the chance of damage to the apparatus or measurement instrument and / or harm to the user.

[0048] The calibration apparatus may be sized to fit onto a work bench and / or table. The calibration apparatus may comprise a cover that houses the electronics and other internal moving parts. The base may be provided on or over the cover. The arm and holder may be provided above the base. This can provide a compact and robust design that is portable and easily stored or used on a regular work bench or table.According to a second aspect of the present invention there is provided a method of calibrating a thickness measurement instrument using the calibration apparatus of the first aspect comprising: moving the holder with respect to the base to change the position of the probe with respect to the substrate and measuring the electrical properties of the probe; then using the measurements to calibrate the thickness measurement instrument.

[0049] The method may comprise moving the probe away from the substrate and measuring the electrical properties of the probe far from the substrate. The method may comprise moving the probe towards the substrate and measuring the electrical properties of the probe in close proximity to the substrate or on the substrate.

[0050] The method may comprise translating the probe with respect to the substrate. The translation may be in a direction parallel to the substrate.

[0051] The method may comprise identifying the measurement instrument. The method may comprise establishing a communications link between the measurement instrument and apparatus. For example, a wired or wireless communications link. This can allow the apparatus to automatically determine the appropriate calibration procedure for the measurement instrument. The method may comprise determining a calibration procedure, preferably based on the identity of the measurement instrument. The method may comprise receiving an indication of the thickness measurement range of the measurement instrument. The method may comprise selecting one or more calibration foils that have a thickness within the thickness measurement range for the calibration procedure. The method may comprise measuring each of the selected one or more calibration foils with the probe.

[0052] The method may comprise moving the holder towards the base. The method may comprise moving the holder towards the base past the point at which a contact force is present between the probe and substrate. The method may comprise allowing the first and second couplings to move with respect to each other as the holder moves towards the base. Advantageously, by moving past the point a contact force is present the contact force can be controlled by the weight of the probe, this makes the readings more reliable and less reliant on control of movements of the calibration apparatus.The method may comprise determining an offset for the sample selector. The method may comprise applying an offset to the position of the sample selector. The same offset may be applied for all measurements in the calibration of a measurement instrument. A different offset may be applied for different measurement instruments. A different offset may be applied for each measurement in the calibration of a measurement instrument. This helps ensure that the calibration foils are not worn down overly quickly.

[0053] The method may comprise moving the sample selector to select a calibration foil for calibration. This may be used to calibrate the measurement instrument against a specific thickness of foil. The method may comprise moving the sample selector to select no calibration foil for calibration. This may be used where a zero measurement is required.

[0054] The method may comprise assessing the quality of a measurement by the measurement instrument. The method may comprise determining if the measurement quality is below a threshold. The method may comprise discarding the measurement if the measurement quality is below a threshold. The method may comprise repeating the measurement if the measurement quality is below a threshold. The method may comprise measuring a measurement uncertainty. An increase in the measurement uncertainty may decrease the measurement quality. The method may comprise measuring the temporal stability of the measurement. A decrease in the temporal stability may decrease the measurement quality. The temporal stability may be measured by monitoring a temperature of the probe over a measurement period. The measurement quality may be below the threshold if the measured temperature varies by more than a temperature threshold over a measurement period. The temperature threshold may be 0.01°C, or 0.05°C, or 0.1°C. The measurement period may be 1 minute, 2 minutes or 4 minutes. Preferably, the temperature threshold may be 0.1°C and the measurement period 4 minutes. The method may comprise storing the measurement if the measurement quality is at least the threshold. Advantageously, measurement quality can be used to ensure only high quality measurements are used to calibrate the measurement instrument. Preferably, the measurement may be discarded if the measurement uncertainty is over a threshold proportion of an expected thickness.The threshold proportion may be 5%, 3%, 2%, 1% or 0.75% of the expected thickness. These preferred values provide improved confidence in the accuracy of the calibration procedure and help to make it more robust.

[0055] The method may comprise taking measurements of at least two calibration foils. The at least two calibration foils may have different thicknesses. The method may comprise comparing the measurements to known thicknesses of the measured calibration foils. The method may comprise calibrating the measurement instrument in response to differences between the measurements and the known thicknesses of the measured calibration foils. The step of calibrating may comprise adjusting the settings of the measurement instrument.

[0056] The calibration apparatus may be an automated calibration apparatus. The calibration apparatus of the first aspect may comprise an electronic processor configured to control the calibration apparatus and implement at least part of the method of the second aspect. The calibration apparatus may comprise a power source to provide electrical power to the apparatus, for example a connection to an electric power grid, generator, or battery.

[0057] The method may comprise weighing the probe. The method may comprise adjusting the weight of the holder such that the combined weight of the probe and holder matches a target weight. The method may comprise adjusting the weight of the probe mount such that the combined weight of the probe and probe mount matches a target weight. The method may comprise calculating a target weight based on a target contact force between the substrate and probe during calibration. Advantageously, the weight may be controlled to ensure a target contact force is applied during calibration.

[0058] The electronic processor may be provided within the calibration apparatus. The electronic processor may be provided by a separate computer in communication with the calibration apparatus.

[0059] According to a third aspect of the present invention there is provided a system comprising the calibration apparatus of the first aspect.The system may comprise the thickness measurement instrument. The thickness measurement instrument may comprise any one or more features of the thickness measurement instrument as described above.

[0060] The system may comprise a computer provided separately from the calibration apparatus and in communication with the calibration apparatus. The computer may be in communication with the thickness measurement instrument. The computer may comprise an electronic processor. The electronic processor may be configured to control the calibration apparatus and implement at least part of the method of the second aspect. The electronic processor may be configured to control the thickness measurement instrument. This can allow more efficient and automated calibration procedures.

[0061] The system may comprise a plurality of calibration foils. Each of the calibration foils may be interchangeably held by a foil holder of the calibration apparatus. This can allow multiple different foils to be used as needed, or for foils to be replaced if they are damaged or otherwise no longer suitable for use in a calibration procedure.

[0062] The system may comprise a set of holders. Each holder may be configured to releasably attach to the calibration apparatus. Each holder may be attachable to the calibration apparatus via a kinematic coupler. The system may be configured for calibration of a set of different thickness measurement instruments. The system may comprise the set of different thickness measurement instruments. The set of different thickness measurement instruments may comprise two, three, four or more thickness measurement instruments. Each of the set of different thickness measurements instruments may have a different weight to the other thickness measurement instruments. Each holder may be configured to hold a probe of a specific one of the set of thickness measurement instruments. Each holder may have a calibrated weight such that the weight of each holder when holding the probe of its specific thickness measurement instrument is substantially the same. That is, substantially the same as the respective weight of each of the other holders when holding the probe of their specific thickness measurement instrument. “Substantially the same” in this context may mean the weight of a holder when holding its probe is within 10%, or 5% or 2%, or 1 % of the weight of another holder when holding its probe. This helps to ensure thatthe calibration apparatus may effectively work with multiple different thickness measurement instruments and that, when combined with the coupling, the contact force between the probe of the thickness measurement instrument and substrate is always the same, as it is controlled by the combined weight of the holder and probe / thickness measurement instrument.

[0063] According to a fourth aspect, there is provided a set of holders for a calibration apparatus. The holders may be for a calibration apparatus of the first aspect. Each holder may have a different weight from the other holders. Each holder may be configured to hold a probe of a specific one of a set of different thickness measurement instruments. Each of the set of different thickness measurements instruments may have a different weight to the other thickness measurement instruments. The probes of each thickness measurement instrument may have a different weight. Each holder may have a calibrated weight such that the weight of each holder when holding the probe of its specific thickness measurement instrument is the same as the weight of each of the other holders when holding the probe of their specific thickness measurement instrument. This provides a simple way to effectively calibrate a set of different thickness measurement instruments as they can be prepared and attached to their respective holder, then easily calibrated via attachment to a calibration apparatus via their respective holder. The fourth aspect may therefore extend to a set of holders for a calibration apparatus and a corresponding set of thickness measurement instruments.

[0064] Of course, the calibration apparatus of the first aspect, the method of the second aspect, the system of the third aspect and the set of holders of the fourth aspect may share any one or more features of one another interchangeably, whether optional or otherwise.

[0065] Detailed Description of the Invention

[0066] In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0067] Figure 1 is a front view of a thickness measurement instrument;Figure 2 is a perspective view of a calibration apparatus for the thickness measurement instrument of Figure 1 ;

[0068] Figure 3 is a perspective front view of a holder of the calibration apparatus of Figure 2 with its door open to receive a probe of the calibration apparatus of Figure 1 ;

[0069] Figure 4 is a perspective front view of the holder of Figure 3 with the door closed; Figure 5 is a perspective rear view of the holder of Figure 3 with the door closed; Figure 6 is a perspective view of the arm of the calibration apparatus of Figure 2 with the holder removed;

[0070] Figure 7 is a side view of the calibration apparatus of Figure 2 holding a probe above the substrate;

[0071] Figure 8 is a side view of the calibration apparatus of Figure 2 with the probe partially lowered towards the substrate;

[0072] Figure 9 is a side view of the calibration apparatus of Figure 2 with the probe lowered onto the substrate;

[0073] Figure 10 is a side view of the calibration apparatus of Figure 2 holding a probe above the substrate with a calibration foil on the substrate; Figure 11 is a side view of the calibration apparatus of Figure 2 with the probe partially lowered towards the substrate and calibration foil; Figure 12 is a side view of the calibration apparatus of Figure 2 with the probe lowered onto the calibration foil;

[0074] Figure 13 is an exploded view of the base, sample selector and motor of the calibration apparatus of Figure 2;

[0075] Figure 14 is detail A from Figure 13;

[0076] Figure 15 is a cross-sectional view of the sample selector, base and a calibration foil of the calibration apparatus of Figure 2;

[0077] Figure 16 is a cross-sectional view of the sample selector, base, substrate and a calibration foil of the calibration apparatus of Figure 2;Figure 17 is detail B from Figure 16;

[0078] Figure 18 is a cross-sectional view of the sample selector, base and support bearing of the calibration apparatus of Figure 2;

[0079] Figure 19 is a schematic diagram of the pneumatic system of the calibration apparatus of Figure 2;

[0080] Figure 20 is a flow diagram of a method of calibration of the measurement instrument of Figure 1 using the calibration apparatus of Figure 2; Figure 21 is a schematic diagram of a set of holders for the calibration apparatus of Figure 2 and a set of different thickness measurement instruments. Referring to Figure 1, a thickness measurement instrument 1 is shown. The thickness measurement instrument 1 comprises a probe 2 connectable to a gauge 3. In this embodiment, the probe 2 and gauge 3 are separate devices that connect together, but in other embodiments, the probe 2 may be integrated into the same device as the gauge 3. The probe 2 is an inductive probe comprising one or more drive coils and one or more pick up coils (not shown). The probe 2 is configured to be placed onto or above a substrate 4 to determine the distance of the probe 2 from the substrate 4. This allows measurement of the thickness of a coating provided on the substrate 4. During calibration, a calibration foil 27 of known thickness is used instead of a coating as described further below. The probe 2 thereby makes thickness measurements in a direction parallel to the probe axis P shown in Figure 1.

[0081] In this embodiment, the probe 2 is connectable to the gauge 3 via a cable 6 and connector plug 7 that is received into a connector socket 8 on the gauge 3. The gauge 3 also comprises a user interface in the form of a screen 9a to allow visualisation of results and buttons 9b to control the gauge 3.

[0082] An example of an existing thickness measurement instrument and its operation is described in GB2601158A. As explained in GB2601158A a typical coating measurement instrument comprises various electrical components and processors that produce, receive and analyse the electrical signals in the drive and pick up coils to determine the thickness of the coating. These components all require calibration, or factory set-up, to ensure that when used in practice the measured thickness provided isaccurate. Existing calibration procedures are manual and rely on the skill and experience of the operator to ensure accurate calibration. For example, the measurement instrument reading may change depending on forces applied, incorrect calibration foil values used, angular placement variances, stability and finally positional accuracy.

[0083] Referring to Figures 2 to 19, a calibration apparatus 10 for the coating thickness measurement instrument 1 is shown. The calibration apparatus 10 comprises a holder 11 for holding the probe 2 and a base 12 for receiving the substrate 4. As described in more detail below, the calibration apparatus 10 is configured to move the holder 11 relative to the base 12 and cause movement of the probe 2 relative to the substrate 4 for calibration of the thickness measurement instrument 1. This provides more automated, controlled and repeatable calibration procedures that are less susceptible to errors then previous methods relying on manual handling of the probe.

[0084] In this embodiment, the base 12 is provided by a flat circular disc and the calibration apparatus 10 comprises an arm 13 that is rotationally mounted to the centre of the base 12 such that it can rotate in a plane parallel to the base 12. The arm 13 extends radially outward over the base 12 where it supports the holder 11. The arm 13 thereby connects the base 12 to the holder 13. To ensure there is sufficient clearance between the base 12 and holder 11, the base 11 comprises a tower 14 that extends upward from the base 12 around its centre. The tower 14 is an open-ended cylinder and supports an electric motor 15 (or arm motor) that connects to the arm 13 to drive the rotation of the arm 13 around the base 12, this allows power and control cabling to be provided centrally in the tower 14 and through the centre of the base 12. In some embodiments, the electric motor may be connected to the arm via a planetary gearbox, this can enable improved movement control of the arm where this is otherwise limited by the specifications of the motor.

[0085] In this embodiment, the holder 11 is removably attached to the arm 13 by a kinematic coupler. The kinematic coupler comprises a first plate 15a provided on the holder 11 and a second plate 15b provided on the arm 13. The first and second plates 15a, 15b comprise complimentary kinematic mounts 15c that magnetically couple the plates 15a, 15b together. In this embodiment, three kinematic mounts 15c are providedand each comprises a ball and groove geometry to control the position of the first and second plates 15a, 15b with respect to each other when coupled together. The first and second plates 15a, 15b each further comprise two complimentary plate magnets 15d that further enhance the magnet coupling strength between the plates 15a, 15b. An example of a suitable kinematic coupler is available from Thorlabs, Inc, USA under part number KB25 / M. The mounts and base thereby provide a reliable releasable mounting that ensures the holder 11 may be removed and replaced with very high accuracy. The probe is therefore held in a releasable manner to allow quick interchange of different probes as described further below.

[0086] In this embodiment, the holder 11 comprises a probe mount 16 configured to attach to the probe 2. The probe mount 16 comprises a door 17 that is hinged with respect to the probe mount 16. The door 17 is provided with a latch 18 that is configured to lock the door 17 closed against the probe mount 16. In this embodiment, the latch 18 is locked to the probe mount 16 via a set of spring plungers 18a when the door 17 is closed. When closed in this way, an aperture 19 is formed between the door and probe mount to retain the probe 2. The probe mount 16 can thereby attach efficiently to the probe 2.

[0087] In this embodiment, the holder 11 is held above the base 12 and moved down to move the probe 2 down into the substrate 4 as described in more detail below. To mitigate any errors in this movement which may cause damage to the substrate / probe, or cause errors in the calibration process, the probe mount 16 is connected to the holder 11 via a coupling formed of a carriage 20a moveable along a rail 20b. The carriage 20a provides a first coupling part and is connected to the probe 2 via the probe mount 16. The rail 20b provides a second coupling part and is connected to the substrate 4 via the base 12, arm 13 and holder 11. The rail 20b extends in a substantially vertical first direction which is parallel to the probe axis P. This provides a single degree of freedom of movement of the carriage 20a along the rail 20b in the first direction. This freedom of movement is also restricted by a range of motion of the carriage 20a with respect to the rail 20b, which is defined by the length of the rail 20b. Movement of the carriage 20a along the rail 20b is damped by a damper (not shown), which in this embodiment is a frictional damper. The coupling as described above, enables the weight of the probemount 16 and probe 2 to decide the contact force between the probe 2 and substrate 4 which improves calibration repeatability and reduces the likelihood of errors, as described in more detail below.

[0088] In this embodiment, the holder 11 is moved with respect to the base 12 via a track 22 and trolley 21. The arm 13 comprises the track 22 and the trolley 21, with the second plate 15b of the kinematic coupler attached to the trolley 21 to attach the holder 11 to the trolley 21. The trolley 21 is configured to move vertically and parallel to the first direction along the track 22 to enable the probe 2 to move towards the base 12.

[0089] In this embodiment, the trolley 21 is pneumatically powered to slide along the track 22 via the pneumatic system PS shown in Figure 19. The pneumatic system 23 comprises a compressed air input Pl for connection to a source of compressed air (not shown). The system also comprises three solenoid valves: input valve P2; up valve P3; and down valve P4, five pressure regulators: two air output regulators P5; a trolley regulator P6; an up regulator P7 and a down regulator P8, two air outputs P9, and connections to the track 22 and trolley 21. The input valve P2 is provided adjacent to the compressed air input Pl to control the pressurisation of the entire system PS. The two air outputs P9 are fed from the input valve P2 via their respective pressure regulators P5. The operation of the air outputs P9 are described further below.

[0090] The trolley regulator P6 controls the pressure of air delivered from the input valve P2 towards the track 22 and trolley 21. From the trolley regulator P6, there are two parallel connections to the track 22 and trolley 21: an up connection which can be pressurised to drive the trolley 21 up the track 22; and a down connection which can be pressurised to drive the trolley 21 down the track 22. The up connection comprises the up regulator P7 and up valve P3, and the down connection comprises the down regulator P8 and down valve P4. This allows the pressure delivered to drive the trolley 21 up or down to be varied. Each valve P3, P4 is configured to either connect the trolley 21 to the compressed air input Pl, or to vent the system to atmosphere. So, when the trolley needs to move upward, the up valve P3 is connected to the air input Pl and the down valve P4 vents to atmosphere. This allows air provided from the air input Pl to push the trolley 21 up the track. Similarly, to move the trolley 21 down the track 22, the down valve P4 connects to the air input Pl and the up valve vents to atmosphere.In this embodiment, the base 12 is configured to receive two substrates 4, one being ferrous and the other being non-ferrous. This allows the calibration apparatus 1 to calibrate a wider range of measurement instruments such as those that are for only ferrous substrates, those for only non-ferrous substrates, and those for both ferrous and non-ferrous substrates. The base 12 comprises two wells 23, each configured to hold a substrate 4. Both wells 23 are identical and are spaced apart around the outer circumference of the base 12, so only one is described in detail below.

[0091] The well 23 is substantially the same shape and size as the substrate 4. The main difference is that the well 23 has a depth, which is measured from an upper base surface 12a to the bottom of the well 23, which is greater than a thickness of the substrate 4. The upper base surface 12a being substantially flat. The thickness of the substrate 4 is measured from an upper substrate surface 4a to a lower substrate surface 4b, in a direction parallel to the probe axis P and first direction. This helps protect the substrate from potential damage with other objects and also helps control position of calibration foils used during calibration as described in more detail below.

[0092] The well 23 comprises a chamfer 24 around its edges that extends down from the upper base surface 12a to the upper substrate surface 4b. The well 23 also comprises two access openings 25 provided on opposite sides of the well 23 and outside the substrate 4. The openings 25 are sized to allow a user to remove the substrate from the well, such as by inserting their fingers into the openings and grasping the substrate.

[0093] In this embodiment, the calibration apparatus 10 comprises a sample selector in the form of a turntable 28 configured to hold one or more calibration foils 27, for example, the sample selector may hold 1, 2, 4, 10, or more calibration foils 27. Each calibration foil 27 is typically a different thickness and the turntable 28 is arranged to move the calibration foils 27 onto the substrate 4 so that the probe 2 can be calibrated using each foil 27.

[0094] In this embodiment, the turntable 28 is provided underneath the base 12 and extending circumferentially around the base 12. Around the edge of the base 12, the turntable 28 comprises one or more clips 29 that are each a foil holder configured to hold a respective calibration foil 27. The clips 29 are positioned such that when adjacent to the substrate 4 held in the well 23, the calibration foil 27 extends out overthe substrate 4 substantially parallel to and in contact with the upper substrate surface 4a. This ensures that the foil 27 does not move significantly when the probe 2 moves down onto the foil 27 during calibration and as described below. The turntable 28 is rotatable about the same axis as the arm 13 to allow the different foils 27 to be moved onto and off the substrate 4 during calibration.

[0095] Each clip 29 is pivotally mounted to the turntable 28 and is arranged to hold each foil slightly below the upper base surface 12a. Due to the pivotal mounting, each calibration foil 27 rotates and pitches upward with respect to the substrate 4 and base 23 when the clip 29 and foil 27 are not aligned with the substrate 4, as shown in Figure 15. This helps reduce build up of dust or dirt on each foil and reduces frictional drag between the base 12 and foils 27. The difference between the pitched and parallel positions of the foil 27 are shown in Figures 15 and 16 respectively. The chamfer 24 mentioned above provides a smooth transition between this pitched upward position and a position parallel to the substrate as each foil moves on and off the substrate.

[0096] In this embodiment, an electric stepper motor 30 is provided beneath the turntable 28 to control rotation of the turntable 28 with respect to the base 12. The motor 30 provides an offset mechanism for the apparatus 10 that is configured to alter the position of the foils 27 to ensure the same place on each foil is not measured repeatedly. This ensures the foils 27 do not wear out as quickly.

[0097] In this embodiment, the calibration apparatus 10 comprises one or more support bearings 31 provided beneath the turntable 28 and arranged to support the turntable 28 around its edges. The support bearings 31 are mounted on an adjustment screw 32 that allows the position of the bearing 31 to be adjusted to finely control the position of the turntable 28. A suitable bearing for use in this application is a plastic ball transfer unit that can be obtained from Omnitrack Etd, England under part number EP15, alternatively, other bearings may also be used such as bearing wheels or similar. Each bearing 31 is received in a circular channel 33 that runs around the underside of the turntable 28 to help ensure that the turntable 28 only rotates and does not translate with respect to the base 12.

[0098] In this embodiment, the calibration apparatus 10 also comprises a cover 34 that provides a platform for supporting the rest of the apparatus 10 on a work surface, suchas a bench or table. The cover 34 houses a computer to control the operation of the apparatus and provides a communications link in the form of a data connection socket (not shown) to allow the computer to communicate with other external devices and computers. This can allow for more complex calibration procedures and automatic export of calibration data. In this embodiment, the computer is arranged to communicate directly with the measurement instrument 1 via the data connection socket, this can allow fully automated calibration of the instrument 1. Of course, in other embodiments, wireless communications technology may also be utilised.

[0099] In this embodiment, the cover 34 also comprises various user interface items 35 such as buttons, led status indicator lights etc. to allow control of the apparatus. In one example, one user interface item 35 is a stop button that can be pressed to safely cease operation of the apparatus 10. When pressed, the stop button cuts power to the motors of the apparatus and also communicates with the computer to ensure it is aware the stop button has been pressed. Another user interface item 35 may be a power button. The cover 34 also conceals the pneumatic system PS and motor 30. Extending from the cover 34 adjacent to each well 23 and substrate 4 are air output nozzles 36. These are connected to the outputs P9 of the pneumatic system PS and are arranged to direct a flow of air down onto the substrate and / or calibration foil if moved into a position over the substrate. This helps to keep the substrate and foils free of dust and dirt that might otherwise compromise calibration measurements.

[0100] Figure 20 shows the method of calibrating a measurement instrument 1 using the calibration apparatus 10. First, at step SI, the calibration apparatus 1 is connected to a source of electric power, unless it comprises an internal battery power source, and connected to a source of compressed air. The apparatus 1 may then be connected to an external computer (not shown) to assist in the calibration process and powered on. This may involve control of the solenoid valves P2, P3, P4 to direct a flow of compressed air out of the output nozzles 36 and move the trolley 21 to an upward position.

[0101] At step S2, the measurement instrument 1 is identified, for example, the measurement instrument 1 may be connected to the apparatus 1 by a data cable, or wireless data link. This allows automatic identification of the serial number of the instrument 1 as well as automatic updating of the settings of the measurementinstrument 1 as required to finalise calibration. In other embodiments, the serial number may of course be manually entered into the apparatus 1 or external computer. This allows the apparatus 1 to automatically identify the appropriate calibration procedure and steps for this instrument 1. For example, the apparatus may determine the appropriate thickness of calibration foils 27 to use and the substrates 4 to use, such as whether to use a ferrous or non-ferrous substrate 4 in calibration.

[0102] In addition, where the measurement instrument 1 is formed as a separate probe 2 and gauge 3 like that shown in Figure 1 , the apparatus 1 can use the serial number to look up a calibration state of the probe 2 and gauge 3. For example, it may retrieve this information from a calibration database provided on a local, or remote, storage device. The apparatus 1 can thereby determine if the settings of the probe 2 and / or gauge 3 need to be calibrated and adjust the calibration steps accordingly.

[0103] At step S3, the apparatus 1 rotates the turntable 28 and arm 13 to their zero positions via motors 15, 30. In this embodiment, the offset mechanism provided in stepper motor 30 randomly selects an offset and re-zeros the turntable 28 with this offset. This will cause all rotations of the turntable 28 and foils 27 to include this offset. In this embodiment, a new offset is selected at the start of each calibration procedure, but in other embodiments, a new offset could also be selected before each measurement. As mentioned above, this helps to ensure the same spot on each foil is not repeatedly measured and therefore reduces wear on each foil.

[0104] At step S4, the details of the first measurement in the calibration procedure is used to identify the position of the arm 13 and turntable 28 required for the measurement. This will typically be a position where the probe 2 is above the substrate 4 and the turntable 28 is rotated to a position where there is a gap with no foils 27 between the probe 2 and substrate 4. This provides a “zero” measurement. The arm 13 and turntable 28 are then rotated to the required positions for the measurement.

[0105] At step S5, the solenoid valves P2 that control the trolley 21 are used to drive the trolley 21 downwards. This moves the holder 11 down towards the base 12 causing the probe 2 to move towards the substrate 4, and foil 27 if present. As shown in Figures 7-9 and 10-12, as the probe 2 moves downward, it is at a lower end of the rail 20b, which is the lower end of the range of motion of the carriage 20a with respect to the rail20b. The trolley 21 and track 22 are arranged such that the trolley 21 continues moving towards the base 12 after the probe 2 has made contact with the substrate 4, or foil 27 if present. Due to the damper provided between the carriage 20a and rail 20b, as the probe 2 makes contact with the substrate 4 or foil 27, the contact is smoothly controlled with minimal bouncing of the probe 2.

[0106] As shown in Figures 9 and 12, when the trolley 21 is completely lowered, the carriage 20a has moved slightly up the rail 20b, towards a midpoint of the range of motion and the carriage 20a is no longer at the lower end of the range of motion of the carriage 20a with respect to the rail 20b. In this position, the contact force between the probe 2 and substrate 4 is determined by the weight of the probe mount 16, door 17 and probe 2. This weight can be carefully controlled during manufacture of the mount 16 and door 17 to ensure that it is suitable for calibration. The main advantages of this are that control of the trolley 21 does not change the contact force applied during calibration. This greatly reduces errors in calibration as the contact force no longer relies on active control elements that might be susceptible to errors, wear or other defects. Due to the tolerance provided by the freedom of motion of the carriage 20a along the rail 20b, then these errors are much less likely to negatively affect the calibration procedure or cause damage to the probe 2, substrate 4 or foils 27.

[0107] Additionally, the rail 20b restricts the movement of the carriage 20a in other directions and in all rotational directions. This ensures that the probe 2 can move to and from the substrate but is otherwise fixed. This ensures that other factors that can affect measurement quality, such as the angle of the probe 2, are carefully controlled.

[0108] At step S6, the apparatus 1 instructs the gauge 3 to output the thickness as measured by the probe 2, and then the trolley 21 is raised back up and the probe 2 moved back up off the substrate 4. The measurement result is then assessed by the apparatus 1 to determine the quality of the measurement result. For example, the uncertainty in the measurement might be calculated. If the uncertainty is too high, the measurement quality may be below a threshold, and then the measurement is rejected. For example, in one embodiment, if the uncertainty is over a proportion of 1% of the expected measurement value, then the measurement is discarded and repeated. Similarly, if the measurement reading is not stable over time, the measurement qualitymay be below the threshold and the measurement may be rejected. In another example, the temperature of the probe is monitored and if the temperature varies by more than 0.1°C over a measurement period of 4 minutes, the measurement is discarded and repeated once the temperature is stable. If the measurement is rejected, the method moves back to step S5 directly to repeat the measurement. The measurement may be repeated multiple times until an acceptable measurement is obtained. In this embodiment, a measurement is repeated up to 3 times before the apparatus pauses the calibration procedure and notifies an operator of a suspected error.

[0109] Otherwise, once the measurement is accepted, it is stored by the apparatus 1 along with the details of the measurement, for example the substrate type and foil thickness, if any. The method then moves to step S7.

[0110] At step S7, the apparatus 1 loads the required arm 13 position and turntable 28 position for the next measurement in the calibration procedure. The arm 13 and turntable 28 are then rotated to the required positions for the measurement. Where a calibration foil 27 is used between the probe 2 and substate 4, the foil 27 is pitched upward when not aligned with the substrate 4. Then, as the turntable 28 moves, the foil 27 rotates to a position parallel to the substrate as it moves down the chamfer 24 of the side of each well 23. Similarly, if a foil 27 rotates off the substrate 4, then it smoothly rotates upward to clear the base 12 via the chamfer 24. Once the arm 13 and turntable 28 are rotated into position, the method returns to step S5 to conduct the required measurements.

[0111] Some of these measurements may also include “free air” measurements in which the probe 2 is held in free air and far from the foil 27 and substrate 4. This allows the apparatus 1 to determine the response of the measurement instrument 1 in this state.

[0112] Additionally, in some embodiments, individual measurements may be repeated and then combined to reduce the overall error in the measurement. For example, a mean average of two or more measurements may be calculated and stored for a given measurement condition.

[0113] Once all measurements have been completed, the method moves to step S8.At step S8, the stored measurement results are compared to the details of each measurement. Differences between the measurement results and the expected known thickness for each measurement, are identified and the settings of the measurement instrument 1 adjusted to minimise any differences. For example, the settings may be adjusted to minimise the root-mean-square deviation (RMSD) of the measurement results to the expected known thicknesses for each measurement.

[0114] At step S9, the calibration procedure is complete and the measurement instrument 1 and probe 2 may be removed from the calibration apparatus 10 and the next instrument loaded for calibration. This is assisted through the kinematic coupler that attaches the holder 11 to the arm 13. As mentioned above, the weight of the probe mount 16 and door 17 are important in determining the contact force between the probe 2 and substrate 4. As such, each probe mount 16 may be specific to a certain type of probe 2. To increase speed of the calibration procedure, each probe 2 can therefore be attached to a respective probe mount 16 and holder 11 prior to calibration, the kinematic coupler can then be used to quickly attach and detach different probes as they are calibrated. This ensures that the calibration process is sped up and less likely to cause bottlenecks in the manufacturing process for the measurement instrument.

[0115] In view of this, prior to calibration, the apparatus 1 and holder 11 may be set up by first weighing the probe 2 and then calculating a required weight of the holder 11 and / or probe mount 16 that will deliver the appropriate contact force between the probe 2 and substrate 4. The weight of the holder 11 and / or probe mount 16 may then be adjusted to match the required weight.

[0116] During steps S4 to S7, various different calibration measurements may be taken. For example, measurement of the probe response in free air, zero measurements, or measurements of a given foil as highlighted above. In addition, more specific measurements may also be taken, such as testing of analogue to digital and digital to analogue converters that are part of the probe. This may typically involve checking the probes response to free air and a zero measurement. Another set of measurements may be used to determine an appropriate gain and offset setting for the probe. This can be used to optimise the signal-to-noise ratio of measurements made by the probe.In the above examples, the processor contained within the apparatus 1 or an external computer / processor may be used to automatically control all or parts of the method. Where the computer is external, this may be provided as part of a system for calibration of a thickness measurement instrument comprising the computer and calibration apparatus.

[0117] Referring to Figure 21, a set 100 of holders Ila, 11b and a corresponding set 200 of different thickness measurement instruments la, lb are provided for use with the calibration apparatus 10 of Figure 2. Each thickness instrument la, lb being a different weight to one another due to their specific characteristics. Each of the holders Ila, 11b has a weight calibrated such that the weight of one of the holders Ila when holding the probe of the corresponding thickness measurement instrument la is the same as the weight of the other holder 11b when holding the probe of its corresponding thickness measurement instrument lb. As such, any difference in the weight of the probes is cancelled out by differences in the calibrated weight of the holders. This causes the contact force between the probe and substrate to be the same across different probe / holder combinations. This enables efficient and simple calibration of the different thickness measurement instruments la, lb by simply mounting and unmounting the holders Ila, 1 lb to the calibration apparatus.

[0118] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

CLAIMS1. A calibration apparatus for calibration of a thickness measurement instrument, the calibration apparatus comprising a holder attached to a base, the holder being for holding a probe of the thickness measurement instrument and the base being for receiving a substrate, wherein the calibration apparatus is configured to move the holder relative to the base to cause movement of the probe relative to the substrate for calibration of the thickness measurement instrument.

2. The calibration apparatus of claim 1 wherein the calibration apparatus comprises a coupling comprising first and second coupling parts attached together with a first degree of freedom of movement with respect to each other, wherein the first coupling part is connected to the probe and the second coupling part is connected to the substrate.

3. The calibration apparatus of claim 2 wherein the holder comprises the coupling.

4. The calibration apparatus of claim 2 or claim 3 wherein the first degree of freedom of movement is a translation in a direction between the probe and substrate.

5. The calibration apparatus of any of claims 2 to 4 wherein one of the first and second coupling parts comprises a rail and the other comprises a carriage that is coupled to the rail and movable along the rail.

6. The calibration apparatus of any of claims 2 to 5 wherein the first and second coupling parts comprise a range of motion with respect to one another in the first direction, and the calibration apparatus configured to move the probe and the substrate together until the first and second coupling parts are towards a midpoint of the range of motion with respect to one another.

7. The calibration apparatus of claim 6 wherein the calibration apparatus is arranged such that when the probe and substrate are held apart, the first and second couplings are at the lower end of the range of motion with respect to one another.

8. The calibration apparatus of any of claims 2 to 7 wherein the coupling comprises a damper to damp movement of the first coupling with respect to the second coupling.

9. The calibration apparatus of any preceding claim wherein the calibration apparatus comprises a trolley and a track wherein the trolley is movable along the track to move the holder towards the base.

10. The calibration apparatus of any preceding claim where the calibration apparatus is configured to releasably hold the probe with respect to the substrate.

11. The calibration apparatus of claim 10 further comprising a kinematic coupler formed of first and second plates magnetically coupled together, wherein the first plate is attached to the probe, and the second plate is attached to the substrate to releasably hold the probe with respect to the substrate.

12. The calibration apparatus of claim 11 comprising an arm connecting the base to the holder, wherein the holder comprises the first plate and the arm comprises the second plate such that the holder is releasably attachable to the arm.

13. The calibration apparatus of claim 12 comprising a set of holders, each of the holders being configured to releasably attach to the arm, wherein the calibration apparatus is for calibration of a set of different thickness measurement instruments and each holder has a different weight.

14. The calibration apparatus of any preceding claim further comprising a sample selector configured to hold one or more calibration foils, wherein the sample selector is configured to move the one or more calibration foils between the probe and the substrate, and the calibration apparatus is configured to move the holder with respect to the base so that the probe can measure the thickness of the substrate.

15. The calibration apparatus of claim 14 wherein the sample selector comprises a turntable configured to hold the one or more calibration foils and the sample selector is configured to rotate the turntable to move the one or more calibration foils between the probe and the substrate.

16. The calibration apparatus of claim 14 or claim 15 wherein the sample selector comprises one or more support bearings.

17. The calibration apparatus of any of claims 14 to 16 comprising two or more calibration foils wherein the two or more calibration foils each have a different thickness.

18. The calibration apparatus of any preceding claim further comprising a sample selector configured to hold one or more calibration foils, wherein the calibration apparatus is configured to move the holder with respect to the base to trap a calibration foil of the one or more calibration foils between the probe and substrate and wherein the calibration apparatus is configured to change an offset of the respective positions of the calibration foil and the probe between calibration measurements, the offset being in a direction parallel to a plane of the calibration foil.

19. The calibration apparatus of any preceding claim comprising a compressed air input and an air output nozzle, wherein the air output nozzle is arranged to direct air onto the substrate.

20. The calibration apparatus of any preceding claim wherein the base is configured to hold two or more substrates and the calibration apparatus is configured to move the holder over the base to select a substrate for use with the probe.

21. The calibration apparatus of claim 20 wherein two or more substrates comprise a ferrous substrate and a non-ferrous substrate.

22. A method of calibrating a thickness measurement instrument using the calibration apparatus of any preceding claim comprising: moving the probe away from the substrate and measuring the electrical properties of the probe far from the substrate; moving the probe towards the substrate and measuring the electrical properties of the probe; then using the measurements to calibrate the thickness measurement instrument.

23. The method of claim 22 comprising translating the probe with respect to the substrate in a direction parallel to the substrate between successive measurements.

24. A system comprising the calibration apparatus of any of claims 1 to 21 and a thickness measurement instrument.

25. A set of holders for the calibration apparatus of any of claims 1 to 21 wherein each holder is configured to releasably attach to the calibration apparatus and to hold a probe of a specific one of a set of different thickness measurement instruments, the probes of each thickness measurement instrument having different weights, wherein each holder has a different weight from the other holders such that the weight of each holder when holding the probe of its specific thickness measurement instrument is the same as the weight of each of the other holders when holding the probe of their specific thickness measurement instrument.

26. The set of holders of claim 25 and a set of different thickness measurement instruments, wherein each holder is configured to hold a probe of a specific one of the set of different thickness measurement instruments.

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