Abstract:
Measurement apparatus is described that comprises a measurement portion for acquiring object measurements and an output portion for outputting measurement data relating to the acquired object measurements. A deactivation portion is provided for inhibiting normal operation of the measurement apparatus such that output of the measurement data is prevented. The deactivation portion, in use, reads apparatus usage information from an apparatus usage module and inhibits normal operation of the measurement apparatus if said apparatus usage information fails to meet one or more predetermined criteria. The apparatus usage module may be provided as an integral part of the measurement apparatus or as a separate activation button. The measurement apparatus may comprise a measurement probe such as a touch trigger measurement probe.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/216,576, filed on Jul. 8, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/996,984, filed on Dec. 13, 2007, and of European Patent Application No. 07252959.7, filed on Jul. 26, 2007, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     i) Field of the Invention 
     The present invention relates to measurement apparatus and methods of operating such apparatus. In particular, the invention relates to measurement apparatus having a deactivation portion for inhibiting normal operation. 
     ii) Description of Related Art 
     Many different types of measurement apparatus are known. Such apparatus includes so-called dimensional measurement apparatus for measuring a physical dimension of an object. Examples of dimensional measurement apparatus include measurement probes, optical position encoders etc. Non-dimensional measurement apparatus is also known for measuring a property of an object other than a dimension; for example, temperature probes, Raman spectrometers, Fourier transform infrared spectrometers etc. 
     Measurement apparatus is usually manufactured to acquire measurements with a certain level of accuracy. It is also possible that periodic updates to the firmware of the measurement probe or software of the associated computer controller may be necessary to ensure that optimum levels of device performance are maintained. For certain measurement devices, periodic recalibration may also be required. Such updates can, however, be unintentionally overlooked resulting in the measurement device providing reduced measurement accuracy over time and/or with use. 
     SUMMARY 
     According to a first aspect of the invention, measurement apparatus comprises; a measurement portion for acquiring object measurements; an output portion for outputting measurement data relating to the acquired object measurements; and a deactivation portion for inhibiting normal operation of the measurement apparatus such that output of the measurement data is prevented; characterised in that the deactivation portion, in use, reads apparatus usage information from an apparatus usage module and inhibits normal operation of the measurement apparatus if said apparatus usage information fails to meet one or more predetermined criteria, wherein the apparatus usage information provides a measure of the amount of measurement apparatus usage. 
     Measurement apparatus is thus provided that, in normal operation, is able to acquire measurements of an object or objects and output measurement data relating to such measurements. The deactivation portion is arranged to read apparatus usage information that provides a measure of the amount of measurement apparatus usage from an apparatus usage module (e.g. a memory or a clock) and inhibit normal measurement operation (e.g. by stopping onward transmission of the measurement data) if certain predetermined criteria are not met. It should be noted that the deactivation portion may also be thought of as an activation portion that only allows normal operation of the measurement device when the one or more predetermined criteria are met. 
     The apparatus usage information may comprise any suitable measure that relates to the amount of measurement apparatus usage. The apparatus usage information may comprise. for example, a count value relating to a number of measurements previously taken, the amount of time the measurement apparatus has been operating, the amount of time that has elapsed since the measurement apparatus was initially activated or a measure of absolute time. The deactivation portion may be arranged to periodically compare the apparatus usage information (which may be updated with time and/or usage of the measurement apparatus as appropriate) against the one or more predetermined criteria and inhibit normal measurement apparatus operation if those criteria are not met. In this manner, measurement apparatus is provided that will deactivate itself after a certain predetermined time or event; for example, deactivation may occur after the acquisition of a certain number of measurements or after a certain amount of time. The apparatus usage module may, as described in more detail below, be an integral part of the measurement apparatus or may be contained in an associated unit (e.g. an activation button, smart card etc) that is attachable to a part of the measurement apparatus. 
     The present invention can thus be seen to be beneficial over known measurement devices (e.g. measurement probes and the like) that have no specified or predetermined lifetime. Although certain consumable components (e.g. batteries, styli etc) of known measurement apparatus may wear and need to be replaced, such apparatus is typically used until it becomes inoperable through some sort of mechanical failure. This has the disadvantage that such apparatus may continue to be used well after the measurement accuracy has degraded to unacceptable levels or when replacement products or updates are available that would provide improved measurement performance. In contrast, measurement apparatus of the present invention allow the operational lifetime (e.g. the time and/or amount of usage) of the measurement apparatus to be predefined by a manufacturer. Although, as described below, it is possible to “reset” the measurement apparatus after it has become deactivated by refreshing or replacing the apparatus usage module, control of such a refresh operation can be kept by the manufacturer. For example, a procedure for refreshing measurement apparatus of the present invention by updating/replacing the apparatus usage module may also include providing updates to the firmware or software of the measurement apparatus or may require an appropriate calibration process to be performed. In this manner, it is possible to ensure that a certain level of measurement accuracy is always maintained. 
     An additional benefit of the present invention arises from the ability to limit the operational lifetime of the measurement apparatus. At present, precision measurement devices are often expensive to manufacture and the high upfront cost in purchasing such devices can be a disincentive to potential users. For example, in the machine tool field the high upfront cost of adding a measurement probe to a machine tool may prevent the uptake of such technology by potential users who are unaware, or uncertain, of the cost savings that could be achieved by implementing automated machine tool based production processes using such measurement probe devices. The present invention may thus allow users to take advantage of the benefits of measurement technology at a much lower upfront cost. If the measurement apparatus is found to be useful, further usage of the equipment after the initially defined operational lifetime may be purchased. 
     As mentioned above, the apparatus usage module may comprise a memory (e.g. an electronic memory chip) that stores apparatus usage information that can be read by the deactivation portion. Advantageously, the apparatus usage information stored in such a memory is updated with apparatus usage; for example, the apparatus usage information may comprise information related to the amount of usage of the measurement apparatus (e.g. the number of acquired measurements) or the length of time that the apparatus has been operating. Although a separate means (e.g. a clock) could be used to update the apparatus usage information, the deactivation portion is advantageously arranged to update the apparatus usage information stored by the memory as the measurement apparatus is used. Alternatively or additionally, the apparatus usage module may comprise a clock that independently generates the apparatus usage information. The apparatus usage module may be provided as part of a separate device (e.g. as a separate activation button or fob of the type described in more detail below) or it may form an integral part of the measurement apparatus. 
     Advantageously, the apparatus usage information comprises a measurement count value relating to the number of (acquired) object measurements. Preferably, the apparatus usage module comprises a memory for storing such a measurement count value. If the measurement apparatus comprises a measurement probe, the measurement count value may, for example, comprise a “trigger count” value. Although the measurement count value could be a number that is incremented with each acquired measurement, the measurement count value stored in the memory is preferably decremented for each object measurement that is acquired by the measurement apparatus. In this manner, the memory stores a measurement count value that reduces with each use of the measurement apparatus. In such an example, the one or more predetermined criteria may comprise a single criterion which is met if the measurement count value is greater than zero. In other words, normal measurement apparatus operation occurs until the count reduces to zero, whereupon the deactivation portion causes normal operation of the measurement apparatus to cease. 
     It should be noted that the apparatus usage information may be provided in the form of units or blocks of time and/or measurement counts. For example, the apparatus usage information may comprise measurement count units, where each measurement count unit relates to N measurements (N being greater than one). Similarly, the apparatus usage information may comprise time units or blocks that each relate to a certain (predetermined) period of apparatus operation. For example, the apparatus usage information may be provided in the form of time units that each relate to a certain length of time (e.g. one minute or five minutes etc) of measurement apparatus usage. 
     Advantageously, the measurement apparatus comprises an integral apparatus usage module that preferably comprises an internal memory for storing said apparatus usage information. In other words, the memory that is read by the deactivation portion may be an integral (e.g. non-removable) part of the measurement apparatus. As outlined above, the apparatus usage information will typically be updated with measurement apparatus usage; for example it may comprise a measurement count value that is decremented with use. After the apparatus usage information fails to meet the predetermined criteria (e.g. when a measurement count value decrements to zero) the deactivation portion inhibits normal probe operation. This may be a permanent deactivation that requires the measurement apparatus or a component thereof to be replaced. It is, however, preferable that the apparatus usage information stored in such an internal memory can be refreshed. 
     Refreshing apparatus usage information stored within the internal memory of the measurement apparatus may involve also securely storing a plurality of unique release codes within the measurement apparatus. If a matching release code is entered (e.g. via a key pad) the apparatus usage information is updated (e.g. the measurement count value may be increased by a certain amount). In such an arrangement, each measurement probe (which could be identified by a serial or identification number) would store unique release codes that could be supplied by the manufacturer/supplier to the end-user on request. 
     Advantageously, the measurement apparatus comprises an interface for communicating with an associated unit, the associated unit storing information for updating the internal memory of the measurement apparatus. If the associated unit updates the internal memory of the measurement apparatus (e.g. by increasing a measurement count value), the information stored by the associated unit may be updated accordingly (e.g. by decreasing a measurement count value stored in the associated unit). Conveniently, the apparatus may be provided as a kit that also comprises an associated unit. As described in more detail below, the associated unit may be an activation button, smart card etc. 
     The measurement apparatus advantageously comprises an interface for communicating with an associated unit, wherein the associated unit comprises the apparatus usage module. The apparatus usage module of the associated unit preferably comprises a memory that stores apparatus usage information. In other words, the main memory for storing the apparatus usage information can be located in an associated unit that is separate from, but interfaced to, the measurement apparatus. Conveniently, a kit may be provided that comprises the measurement apparatus and the associated unit. It should be noted that the measurement apparatus may also comprise an internal memory buffer such that the main memory store of the associated unit only needs to be read and/or updated periodically. 
     Providing an associated unit having the main memory for storing the apparatus usage information has the advantage that it allows apparatus usage information (e.g. measurement count values) to be readily transferred between different measurement apparatus. In this manner, a certain number of measurement counts can be obtained (e.g. purchased) that can be expended on any one of a number of different pieces of measurement apparatus. Similarly, a plurality of associated units may be used in combination with the measurement apparatus; e.g. a new associated unit may be used with the measurement apparatus after a previous associated unit has been spent. 
     Advantageously, the measurement apparatus comprises an authentication module for verifying the authenticity of an associated unit. The associated unit may comprise an analogous or complementary authentication module. The measurement apparatus may be arranged to only interact with associated units that are found to be authentic; this prevents the measurement apparatus acting on apparatus usage information or update information from non-authentic devices. The provision and use of such an authentication module is described in more detail in Applicant&#39;s co-pending patent application that claims priority from European patent application 07252965.4. 
     The above mentioned communications link between the measurement apparatus and the associated unit may be implemented in a number of ways. Advantageously, the interface of the measurement apparatus comprises one or more electrical contacts for allowing electrical connection with the complimentary electrical contacts of an associated unit. In other words, a physical (conductive) link may be provided between the measurement apparatus and associated unit. Alternatively, the interface may comprise a wireless communications unit. A wireless link can then be conveniently established between the measurement apparatus and an associated unit. The associated unit may thus take the form of a smart card or activation button of the type described in detail below. Alternatively, the associated unit may be a wireless fob or the like. 
     Preferably, the deactivation portion is permanently disabled on receipt of a total release code thereby allowing ongoing (unlimited) normal operation of the measurement apparatus. In other words, the measurement apparatus may include means for stopping the deactivation portion operating. This may allow measurement apparatus of the present invention to be converted into measurement apparatus that operates as normal. 
     The measurement apparatus may be provided as a single integrated unit. For example, the apparatus may solely comprise a measurement device that incorporates each of the measurement portion, the output portion and the deactivation portion. The measurement device may also include a housing to contain the various components. In such an example, the measurement device is preferably arranged to transfer any measurement data to a remote interface via a wireless (e.g. RF or optical) link. The output portion thus advantageously comprises a wireless transmitter for transmitting measurement data to a separate, remote, interface. Advantageously, the deactivation portion inhibits output of the measurement data by deactivating the wireless transmitter. The apparatus usage module may be integral with the measurement device or interfaced thereto. The wireless link provided by the output portion may also provide a wireless link to an associated unit of the type described above. 
     Instead of providing an integrated measurement device, the various components of the measurement apparatus may be provided as a plurality of discrete distributed units that are interconnected in some manner. For example, the apparatus may thus comprise a measurement device (e.g. a measurement probe) linked to an interface (e.g. a probe interface). The measurement device may include the measurement portion and the interface may include the output portion. The deactivation portion may then inhibit normal operation of the apparatus by preventing output of measurement data from the output portion of the interface when the above mentioned predetermined criteria are not met. In other words, the deactivation portion may be provided as part of the interface and/or the measurement probe and may control whether the measurement data is output by the interface. In such an example, measurement data may be passed between the measurement probe and the interface via a wired or wireless link. The measurement apparatus may also include a computer controller (e.g. the computer that controls overall operation of a machine tool) that is linked to a measurement device (e.g. a measurement probe) optionally via an interface (e.g. a probe interface). In such an example, the deactivation portion may be implemented by software running on the controller that can prevent usage of the measurement data by other parts of the software if the predetermined criteria are not met. 
     The invention may be applied to any type of measurement apparatus; for example, the measurement portion may comprise a Raman spectrometer or similar for acquiring data from objects provided in the form of samples. The measurement apparatus may also comprise a temperature probe or the like. Advantageously, the measurement apparatus comprises a contact or non-contact (e.g. optical, video etc) measurement probe. If a contact measurement probe is provided, the measurement portion may comprise a deflection measurement mechanism and a deflectable stylus. The measurement probe may be a touch trigger probe that issues a trigger signal whenever stylus deflection exceeds a certain threshold. Alternatively, the measurement probe may be an analogue or scanning probe in which the amount of stylus deflection is measured (e.g. using strain gauges) and an output is provided containing information about the position of the stylus tip relative to the body of the measurement probe. In either case, the stylus may be releasably retained by a stylus holder that forms part of the deflection measurement mechanism thereby allowing stylus replacement. 
     According to a second aspect of the present invention, a method of operating measurement apparatus comprises the steps of: (i) using the measurement apparatus to acquire object measurements; (ii) outputting measurement data from the measurement apparatus that are related to the measurements acquired in step (i); characterised by performing the step of (iii) reading apparatus usage information and only performing step (ii) if said apparatus usage information meets one or more predetermined criteria. 
     Also described herein is a measurement device including a measurement portion for acquiring a series of object measurements, wherein the device comprises a device usage counter for counting the number of measurements acquired by the measurement portion. A deactivation device may also be provided for inhibiting any further operation of the measurement device after the count reaches a certain value. 
     A measurement device is also described herein that comprises; a measurement portion for acquiring object measurements; an output portion for outputting measurement data relating to the acquired object measurements; and a deactivation portion for inhibiting normal operation of the measurement device such that output of the measurement data is prevented; wherein the device disabler inhibits operation of the measurement device after the occurrence of a predetermined event (e.g. after a certain number of measurement) and in that said device disabler only permits continued operation of the measurement device on receipt of an appropriate reactivation instruction. Conveniently, the predetermined event comprises a certain amount of usage (e.g. a certain length of time of usage and/or the acquisition of a certain number of measurements) of the measurement device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example only, with reference to the accompanying drawings in which; 
         FIG. 1  shows a measurement probe and activation button according to the present invention; 
         FIG. 2  shows the components of the activation button in more detail; 
         FIG. 3  illustrates the principles behind a two-way authentication process; 
         FIG. 4  shows a measurement probe kit for use on a machine tool; 
         FIG. 5  shows an integrated battery and activation button holder, 
         FIG. 6  shows a measurement probe having a slot for receiving a smart card, 
         FIG. 7  shows a measurement probe having an integral memory for storing a trigger count value, 
         FIG. 8  shows a measurement probe and an associated activation fob, 
         FIG. 9  illustrates two-part measurement probe apparatus; 
         FIG. 10  shows a measurement probe storing a plurality of trigger count release codes; and 
         FIG. 11  illustrates application of the invention to non-dimensional measurement apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1 , a measurement probe  2  of the present invention is shown. The measurement probe  2  is a so-called touch trigger probe having a deflectable stylus  4  releaseably attached to a deflection measurement unit  6 . The deflection measurement unit  6  is of known type and comprises a stylus holder mounted to the measurement probe housing via a set of balls and rollers. Deflection of the stylus causes disengagement of the balls from the rollers thereby breaking an electrical circuit and producing a so-called trigger signal. The measurement probe  2  comprises a wireless (RF) communications unit  8  for transmitting trigger signal data to a remote probe interface (not shown) in a known manner. Although a wireless RF link is described herein, it should be noted that any type of wired or wireless link may be used. For example, the RF communications unit  8  could be substituted for an optical communications unit. 
     The measurement probe  2  also comprises a deactivation device  10 . The deactivation device  10  is arranged to prevent normal operation of the measurement probe if certain criteria are not met. Deactivation of the measurement probe may be implemented in a number of ways. For example, the deactivation device  10  could force the measurement unit  6  to power down or enter some kind of standby mode. Alternatively, the measurement probe could continue to produce trigger signals as normal but the transfer of trigger signal data to the remote interface via the wireless communications unit  8  could be blocked. In short, the deactivation device  10  is arranged to stop normal measurement probe operation thereby making the measurement probe inoperable. The measurement probe also includes an authentication module  13  that comprises an authentication device  12  and associated electronic memory  14 . An externally accessible electrical connection pad  16  is also provided that allows electrical connections between the authentication module  13  and an associated activation button  18  to be established. It should be noted that the measurement probe will typically include various additional components (e.g. filtering or data processing electronics, batteries etc) but these are not shown for clarity. 
     Referring now to  FIG. 2 , the activation button  18  is shown in more detail. The activation button  18  includes an authentication module  19  comprising an authentication device  20  and an electronic memory  22 . The memory  22  comprises a permanent memory portion  24  and a rewritable memory portion  26  for storing a trigger count value. 
     Referring to both  FIGS. 1 and 2 , operation of the measurement probe  2  with an activation button  18  attached will be described. 
     Firstly, a two-way authentication process is used to verify the authenticity of the measurement probe  2  and the activation button  18 . Details of a suitable authentication technique are described in more detail below with reference to  FIG. 3 , but the basic principle is that a secret key is stored in the electronic memories  14  and  24  of the measurement probe  2  and the activation button  18 . The authentication device  12  of the measurement probe  2  and the authentication device  20  of the activation button  18  communicate with one another to perform an authentication check which, without disclosing the secret key, confirms that the electronic memories of the measurement probe  2  and the activation button  18  hold the same secret key. 
     Once the measurement probe  2  has established that an authentic activation button  18  is attached to its external electrical connection pad  16 , the trigger count value stored in the rewritable memory portion  26  of the activation button is read by the measurement probe. If the trigger count value is non-zero, the deactivation device  10  permits normal measurement probe operation. Thereafter, the trigger count value stored in the rewritable memory portion  26  is decremented by one for each trigger signal that is generated by the measurement probe. It should be noted that the trigger count value stored in the rewritable memory portion  26  of the activation button  18  may be decremented after each trigger signal is issued or the measurement probe  2  may have some kind of temporary memory buffer (e.g. part of the memory  14 ) for storing trigger counts and means for periodically updating the main trigger count value stored in the rewritable memory portion  26  of the associated activation button. For example, the trigger count value stored in the rewritable memory portion  26  may be updated at regular time intervals or whenever a certain number (e.g. ten, fifty, one hundred etc) of trigger signals have been issued by the measurement probe. The use of a memory buffer within the measurement probe reduces the required number of updates to the value stored in the rewritable memory portion  26  of the activation button. However, any buffer is preferably not too large because the main count stored by activation button may not be decremented properly if the activation button is removed prior to an update event. 
     A measurement probe of the present invention thus operates normally in the presence of an activation button  18  containing a non-zero trigger count; i.e. the measurement probe issues a trigger signal whenever the stylus is deflected. However, removal of the activation button  18  or the reduction of the stored trigger count to zero causes the deactivation device  10  to stop normal probe operation thereby preventing measurements being made with the measurement probe. In this manner, the operational lifetime of the measurement probe can be set by a manufacturer. For example, a measurement probe may be sold with an activation button that stores a certain trigger count value (e.g. five or ten thousand trigger counts). After the trigger count is expended, a further activation button may be obtained from the manufacturer to reactivate the measurement probe. The new activation button may be provided with instructions for verifying the measurement probe is operating within the necessary tolerances and/or any appropriate firmware updates for the measurement probe may be provided with the replacement activation button. In this manner, the requirement to periodically refresh the measurement probe can also have the advantage of forcing a user to periodically update or check the operational performance of the measurement probe thereby ensuring the required measurement accuracy is maintained. 
     Although  FIG. 1  illustrates a measurement probe  2  having an authentication module  13 , an electrical contact pad  16  and a deactivation device  10 , it should be noted that such components may alternatively or additionally be provided as part of the remote probe interface. In such an example, the measurement probe may pass all measurement data to such a probe interface and the probe interface may then only pass on measurement data (e.g. to a machine controller) if an authentic activation button storing a non-zero trigger count is attached to its electrical contact pad. As a further alternative, the measurement probe may include the authentication module and an electrical contact pad for reading a trigger count from an activation button whilst the probe interface may comprise a deactivation device. The data transmitted by the probe to the interface may then contain information that indicates whether an authentic activation button storing a non-zero trigger count is attached to the electrical contact pad of the measurement probe. If the measurement probe provides an indication that there is no authentic activation button storing a non-zero trigger count attached thereto, the deactivation device of the probe interface may be arranged to prevent the output of any measurement data. 
     It should be noted that although the above examples work by storing and decrementing a trigger count value, other values could be stored and measured. For example, the measurement probe could include a clock that measures the length of time that the measurement probe is actively operating. In such an example, the activation button could then include a certain operational time value that is decremented by the operational time value accrued as the measurement probe operates. A combination of time and trigger count values could also be used. For example, the activation button could store separate counts related to the time of operation and the number of triggers. The deactivation device  10  could then allow normal measurement probe operation until the stored trigger count or the stored time of operation count is expended. It should also be noted that the trigger count could alternatively increment with use and the deactivation device could stop normal operation when a maximum count value is reached. Although the above described activation button includes a memory for storing some kind of count or time value this is by no means essential. The activation button could, for example, alternatively comprise a clock or similar that separately measures elapsed time. 
     Referring now to  FIG. 3 , the basic principle of the two way authentication technique employed by the apparatus described with reference to  FIGS. 1 and 2  is illustrated. 
     As outlined above, the measurement probe  2  and the activation button each include an authentication device. Each authentication device runs the SHA-1 algorithm developed by the National Institute of Standards and Technology (NIST) of the USA. The SHA-1 algorithm is a so-called one-way hash function that generates a fixed length Message Authentication Code (MAC) from input data. The SHA-1 algorithm has the properties of being irreversible; i.e. it is computationally infeasible to determine the input that corresponds to a generated MAC. The algorithm is also collision-resistant such that it is impractical to find more than one input message that produces a given MAC. Furthermore, the algorithm has a high avalanche effect meaning that any minor change in the input produces a significant change in the MAC that is generated. Although use of the SHA-1 algorithm is described in detail herein, it should be noted that many alternative algorithms could be used to implement similar types of authentication. 
     The two-way authentication process, which can also be termed challenge and response authentication, relies on the measurement probe and activation button both storing the same secret key in a secure (i.e. externally inaccessible) memory. When authentication is required, for example when an activation button is located in the electrical contact pad  16  of the measurement probe, the activation button sends message data (e.g. the activation button serial number plus the stored trigger count value) to the measurement probe. The message data contains no secret information and there is no threat to the security of the authentication process if the message is intercepted. The measurement probe responds by sending a random data string as a “challenge” to the activation button. 
     The measurement probe then applies its SHA-1 algorithm to an input that includes the secret key, the message data and the random data string and produces a MAC therefrom; this MAC can be termed MAC1. The activation button takes the same input data (i.e. the secret key, the message data and the random data string) and uses its SHA-1 algorithm to generate a MAC; this MAC can be termed MAC2. The measurement probe then compares MAC1 and MAC2. If MAC2 matches MAC1 it is certain (to a very high level of confidence) that the same secret key is stored by both the measurement probe and the activation button. The measurement probe then assumes that the activation button is genuine. It should be reemphasised that the authentication process does not compromise the secrecy of the secret key; i.e. the secret key itself is never passed between devices. 
     A similar two-way authentication check is also performed before data is written to the rewritable memory  26  of the activation button  18 . In such a process, the activation button  18  generates the random number and performs the MAC comparison. This authentication process prevents the security of the activation button  18  being compromised by ensuring that only an authentic device (such as measurement probe  2 ) can alter the stored trigger count value. In other words, the authentication check guards against unauthorised users tampering with the trigger count value that is stored by the activation button  18 . 
     A number of authentication devices suitable for incorporation into a measurement probe are available commercially and are described in more detail elsewhere. For example, suitable apparatus is the Maxim/Dallas i-button available from Maxim Integrated Products Inc, Sunnyvale, Calif., USA. 
     Referring to  FIG. 4 , measurement kit for use with a machine tool is illustrated. The measurement kit comprises a spindle mountable measurement probe  40 , a table top (tool setting) measurement probe  42  and a probe interface  44 . The spindle measurement probe  40  and the table top measurement probe  42  (which are hereinafter collectively termed the measurement probes) communicate with the probe interface  44  over a wireless radio frequency (RF) link. The measurement probes  40  and  42  are both touch trigger probes that issue a trigger signal whenever stylus deflection exceeds a certain threshold value. The trigger signal can be used to freeze machine position information; e.g. the location of the spindle can be determined in the x, y and z machine co-ordinates system as measured by machine position encoders. The spindle mountable measurement probe  40  has a spindle mountable shank  39  and a stylus having a ruby ball tip  41 ; this allows points to be measured on the surface of a workpiece. The table top measurement probe  42  has a tool setting cube  43  mounted to its stylus tip and is used to determine the position of cutting tools held by the machine tool spindle. For clarity, the associated machine tool on which such apparatus could be used is not shown in  FIG. 4 . 
     In order to overcome the various problems associated with hardwired measurement probe systems, the interface  44  communicates with the measurement probes  40  and  42  via a spread spectrum wireless RF link. To allow multiple systems to operate side by side, each measurement probe prefixes all of its data transmissions with a probe identification (ID) code. An initial “pairing” procedure is performed in which the interface  44  learns the ID code of the measurement probe that is intended for use with that particular interface. After pairing, the interface  44  will only process received data that contains the ID code of the paired measurement probe thereby ensuring that data transmissions are ignored that originate from any other measurement probes (i.e. probes having different ID codes) that may be in the vicinity. Once paired, the measurement probe and interface will frequency hop in a predefined manner to mitigate the effects of noise from other RF sources. More details about the spread spectrum, or frequency hopping, communications link are outlined in WO2004/57552. A variant of WO2004/57552 is also described in detail in PCT application WO2007/28964. The apparatus of WO2007/28964 allows multiple probes to be paired to a single interface by allowing the probe IDs of a measurement probe to be set by a user or by allowing the interface to recognise transmissions that contain any one of a plurality of different ID codes. Such an arrangement allows two or more probes to be used (non-concurrently) with a single interface. 
     To implement the frequency hopping RF link mentioned above, the spindle mounted measurement probe  40  and the table top measurement probe  42  each comprise wireless communications units  46   a - 46   b . The interface  44  includes a corresponding wireless communications unit  48  for communicating with the communications unit  46  of a measurement probe. In normal use, the wireless communications units  46  and  48  allow data transfer between any one of the measurement probes  40  and  42  and the paired interface  44  in the known manner outlined above. 
     The interface  44 , spindle measurement probe  40  and table top measurement probe  42  contain authentication modules  50   a - 50   c . Each authentication module  50  comprises an authentication device  52  for running the SHA-1 hash algorithm, a secure memory portion  54  for storing a secret key and a random data string generator  56 . The interface  44  and the measurement probes  40  and  42  also comprise deactivation devices  58   a - 58   c  for inhibiting normal operation. As outlined above, deactivation may be implemented in various way; for example, a deactivated measurement probe may not transmit trigger signals via the wireless communications unit whilst a deactivated interface may not output any data on its trigger signal output line  60 . 
     In use, a set-up routine is performed in which a measurement probe (e.g. spindle mountable probe  40 ) and the interface  44  are placed in “pairing” mode. In common with systems of the type described in WO2004/57552, the pairing procedure involves the measurement probe repeatedly transmitting its ID code. The interface searches for any ID codes transmitted by an unpaired probe and, when the relevant measurement probe ID code is received, it is stored by the interface. After pairing, the interface ignores any data it receives that does not contain the stored ID code. As outlined in WO2007/28964, the interface may also be paired with a further measurement probe (e.g. the table top measurement probe  42 ) by storing a second probe ID code or by loading the stored probe ID code into the further measurement probe. It can be seen that a potential weakness of such a pairing procedure is that it allows any components to be paired so long as the requirements of the communication protocol are met. The communications protocol can, however, be easily copied which would allow replica or incompatible measurements probes and/or interfaces to be used with genuine ones. This can seriously and unpredictably degrade the measurement performance of the kit. 
     As outlined above, the probes and interfaces of  FIG. 4  also include authentication modules  50  having a secure memory portion  54  in which a secret key is stored. After a measurement probe has been paired with the interface, an authentication step is performed in which the measurement probe verifies that the interface is authentic (i.e. that it stores the same secret key) and vice versa. The challenge and response authentication process is analogous to that described with reference to  FIG. 3 , with each authentication device  52  applying its SHA-1 algorithm to input data that includes the secret key stored in its associated secure memory portion  54 , a message (e.g. the probe ID code) and a random data string generated by one of the random data string generators  56 . Exchanges of the MACs, messages and random data strings are performed using the wireless communications units  46 . If the measurement probe or interface confirms, by comparing self-generated and received MACs, that it has been paired with a genuine counterpart (i.e. a counterpart storing the same secret key) normal operation of the apparatus is permitted. However, if a probe or interface fails to establish the authenticity of its counterpart, the deactivation device  58  prevents normal operation. 
     The authentication process described above may be performed only after pairing, each time a measurement probe is turned-on, at predetermined time intervals and/or during periods in which measurements are not being acquired. If required, the authentication process may also be performed before the pairing operation. In this manner, it is ensured that authentic measurement probes only ever operate normally with authentic interfaces and vice versa. Apparatus of this type can thus guarantee, to a high level of certainty, that only fully compatible measurement probes and interfaces can be used in combination. Providing an authentication process of this type thus prevents an interface being used with a certain type of measurement probe if that interface is unable to properly process the measurement probe data it receives because, for example, the format of the received data differs to that expected by the interface or requires the application of different processing techniques. The authentication process thus means that, for example, a manufacturer can provide different ranges of measurement probes and interfaces that use the same communications protocols. Compatible equipment can be assigned a common secret key, whilst it is ensured that incompatible equipment stores different secret keys. In this manner, the user is unable to use incompatible equipment in combination thereby reducing the chances of apparatus malfunction and/or the introduction of unacceptably large measurement errors. Such an arrangement also prevents third party, possibly inferior quality, apparatus being used with authentic devices which again ensures that measurement accuracy is not compromised. 
     Although by no means essential, the measurement probes  40  and  42  shown in  FIG. 4  may be measurement probes of the type described above with reference to  FIG. 1 . In particular, each measurement probes may comprise a deactivation device (which may be the same or different to the deactivation device  58 ) that only permits normal probe operation if an authentic activation button storing a non-zero trigger count value is attached to an electrical connection pad provided on the measurement probe. In such an arrangement, the kit will only operate normally if the interface and measurement probes are authentic and if the measurement probes each have an authentic activation button attached thereto that contains a non-zero trigger count. 
     The measurement probe described with reference to  FIG. 1  includes an external electrical connection pad  16  for receiving an activation button. In certain circumstances it is, however, preferable for the activation button to be sealed inside the measurement probe during use. This ensures that the activation button does not become accidentally detached from the measurement probe or damaged; this may occur, for example, during the process of loading a spindle probe into the machine tool spindle using an automated tool change device. A measurement probe may thus be provided that includes a separate, preferably sealable, compartment for receiving the activation button. Alternatively, the battery retaining compartment of the measurement probe may be adapted to also hold the activation button as will now be described in more detail. 
     Referring to  FIG. 5 , a battery holder  70  for a measurement probe is illustrated. The battery holder  70  includes a compartment  72  in which batteries  74  are located. In addition, a slot  76  is provided in which an activation button  18  can be placed. Electrical contacts  78  are also provided for establishing the necessary electrical connections between the batteries and activation button and the electronics of the measurement probe. A locking mechanism  80  may also be provided to securely retain the battery holder  70  in the probe body. This arrangement ensures good electrical contact is maintained even in a harsh operating environment and also prevents damage to the activation button. 
     The battery holder of  FIG. 5  also has the advantage that removal of the activation button also requires removal of the batteries. This ensures that the probe is powered down whenever the activation button is removed. In such apparatus, the authentication process need only be performed on power-up of the measurement probe because it is impractical to remove or replace the activation button after the measurement probe has been switched on. 
     It is important to note that the use of an activation button as described above provides a convenient way to implement the invention but is by no means the only solution. In other words, the use of an activation button of the type described above is advantageous but by no means essential. Many alternative types of device could be used to securely store a trigger count and implement some kind of authentication or encryption technique. For example, a smart card or other similar device may be used. 
     Referring to  FIG. 6 , a measurement probe  90  is illustrated that comprises a slot  92  for receiving a smart card  94 . The slot  92  may be sealable. The smart card  94  includes a memory to store a secret key, a processor for implementing the SHA-1 algorithm and a rewritable memory for storing a trigger count value. The measurement probe contains complimentary apparatus such that a challenge and response authentication process of the type described above can be carried out between the measurement probe and smart card. If required, the slot  92  for the smart card may be formed as part of the battery holder thereby physically protecting the card from damage. 
     The measurement probes described above are arranged to operate only when an activation button, smart card or similar device storing a trigger count data is attached to the probe. It is, however, also possible for the measurement probe itself to comprise a rewritable memory that stores the trigger count value. The activation button (or similar) is then only required when the trigger count stored in the probe needs to recharged or refreshed. 
     Referring to  FIG. 7 , a measurement probe  100  is shown that is a variant of the measurement probe of  FIG. 1 . In common with the measurement probe described with reference to  FIG. 1 , the measurement probe  100  comprises a deflectable stylus  4  attached to a deflection measurement unit  6 , a wireless communications unit  8  for communicating with a remote interface and a deactivation unit  10 . An electrical connection pad  16  provides a connection to an associated activation button  118 . 
     The measurement probe also comprises an authentication module  113  comprising an authentication device  112  and a memory  114 . The memory  114  stores a secret key in a permanent memory portion  114   a  and also includes a rewritable portion  114   b  for storing a trigger count value. In use, the deactivation unit  10  only permits normal measurement probe operation when the trigger count value stored in the rewritable memory portion  114   b  is non-zero. Each time a trigger signal is generated, the count stored in the rewritable memory portion  114   b  is decremented accordingly. Once the stored trigger count value reaches zero, normal measurement probe operation is inhibited by the deactivation unit  10 . 
     In order to reactivate the measurement probe, an activation button  118  storing a non-zero trigger count is placed in contact with the electrical contact pad  16 . The above described authentication process is then used to ensure that both the measurement probe and the activation button contain the same secret key. Once authenticity has been established, trigger counts are transferred or loaded from the activation button to the measurement probe. In other words, the trigger count stored in the rewritable memory of the activation button is decremented by a certain value and, at substantially the same time, the trigger count value held in the rewritable memory portion  114   b  is increased by that value. Following the loading of trigger counts, the activation button can be removed from the measurement probe. In this manner, trigger count credits are transferred in bulk from the activation button  18  to the measurement probe  100  thereby allowing continued operation of the measurement probe until the new trigger count is expended. 
     The measurement probe  100  may be configured to take all the trigger counts that are stored in the activation button  118 . Alternatively, the measurement probe  100  may be configured to take fewer trigger counts than are stored in the activation button. If necessary, the transfer of trigger counts may also be performed in the opposite direction. For example, trigger counts may be transferred from the measurement probe  100  back to an activation button  118 . Alternatively, the activation button  118  may be arranged such that the trigger count can only ever be decremented. It should also be noted that the activation button  118  may be identical to the activation button  18  and hence may also be used with the measurement probe  2  described with reference to  FIG. 1 . 
     The activation button described above is designed to be brought into physical contact with corresponding electrical contact pads of the measurement probe. As mentioned above, activation buttons are simply one way of implementing the invention and many different types of secure technologies (smart cards etc) could be connected to the measurement probe and used for the same purpose. Furthermore, if the measurement probe itself is capable of securely storing trigger count values, additional methods of refreshing the trigger counts stored in the measurement probe can be implemented. 
     Referring to  FIG. 8 , a further measurement probe  120  is shown. The measurement probe  120  includes a wireless communications unit  8  for passing trigger information to a remote probe interface  122  over a wireless RF link. The RF link may be as described previously in WO2004/57552 or may be arranged to implement an authentication process as described above with reference to  FIG. 4 . The measurement probe  120  also includes a further wireless communications unit  124  that is connected to an authentication module  113  that comprises an authentication device  112  and a secure memory  114 . The physical electrical contact pad  16  of the measurement probe  100  described with reference to  FIG. 7  is thus replaced in the measurement probe  120  by the wireless communications unit  124 . 
     A separate fob  126  is also provided that includes a wireless communications unit  128  for communicating with the wireless communications unit  124  of the measurement probe  120 . The communications unit  128  of the fob  126  is linked to an authentication module  131  comprising an authentication device  130  and an electronic memory  132  having a secure portion for storing the secret key and a rewritable portion for storing a trigger count value. The fob also includes a plurality of keys  134  that allow a user to control the transmission process. A liquid crystal display  136  is provided for displaying fob status information such as the number of trigger counts remaining and/or the number of counts to be loaded into the measurement probe. 
     In use, a user selects the number of trigger counts that are to be uploaded to a measurement probe using the keys  134 . The fob is then placed in the vicinity of the relevant measurement probe  120  and a key is pressed to initiate the trigger count upload. The challenge-response authentication process is performed over the wireless link to verify that the fob  126  and the measurement probe  120  are authentic. After a successful authentication step, the selected number of trigger counts are transferred from the memory  132  of the fob  126  to the memory  114  of the measurement probe. The use of a wireless link means that the measurement probe  120  does not have to include accessible electrical contacts; the count stored by the measurement probe  120  can thus be updated without having to touch or in any way access the measurement probe. 
     To ensure that the probe triggers are uploaded to the desired measurement probe, it is preferred that the RF communications link between the fob  126  and the measurement probe  120  is a relatively short range link (e.g. operable only over distances of less than 20 cm or so). Alternatively, an optical link may be used instead of the RF link. If an optical link is provided, the directionality of the transmitted light can be used to ensure that trigger counts are uploaded to the correct probe. Although separate communications units are shown for communicating with the probe interface and the fob, it should also be noted that a single wireless communications unit may be used to perform both functions. 
     Although a dedicated fob  126  is described, the measurement probe may be interfaced with a general purpose computer (e.g. a laptop or PDA) via a standard wireless communications link (e.g. Wi-Fi, Bluetooth etc) or a wired link (USB, Firewire etc). In such an embodiment, the computer may also be interfaced to an encryption module or card that runs the authentication check, securely stores the secret key and maintains a probe trigger count value. In other words, an activation button or chip type device may be provided that communicates with the measurement probe via an intermediate (general purpose) device. 
     Referring to  FIG. 9 , a two-part measurement probe  150  will now be described. The measurement probe comprises an upper part  152  and a lower part  154 . The lower part  154  comprises a stylus  156  attached to a deflection measurement unit  158 . The lower part  154  also includes an authentication module  159  comprising an authentication device  160  and an associated memory  162 . The memory  162  comprises a secure portion for storing a secret key and a rewritable portion for storing a trigger count value. The upper part  152  comprises a wireless communications unit  8  for communicating with an associated probe interface (not shown) and a deactivation device  10  for inhibiting normal operation. The upper part also includes an authentication module  170  comprising an authentication device  172  and a memory portion  174  for storing a secret key. 
     The upper and lower parts may be assembled to form a measurement probe. Once assembled, electrical links are provided between the upper and lower parts by appropriate sets of electrodes (not shown). After assembly, a challenge and response authentication process of the type described above is performed in order to verify that the upper and lower parts of the device are authentic. If authenticity is confirmed, the deactivation device  10  allows trigger events from measurement unit  158  to be output via the wireless communications unit  8  provided that there are still trigger counts stored in the memory  162  of the lower part. Each trigger event decrements the stored count and when the trigger count value equals to zero, the deactivation device  10  of the upper part  152  prevents further operation with that particular lower part  154  attached. The lower part is then discarded and replaced with a new lower part (i.e. a lower part having stored trigger counts). 
     The lower part  152  can thus be considered as the combination of an activation button to store a trigger count and the (moving) mechanical parts of the measurement probe. All the moving parts that will wear with use are thus contained in the (disposable) lower part of the measurement probe, whereas the bulk of the (expensive) electronics are contained in the re-usable upper part. The number of trigger counts initially stored in the memory of the lower part may correspond to, or be slightly less than, the expected operational lifetime of the stylus or deflection measurement unit  158 . In other words, the lower part may store a trigger count value that causes operation of the measurement probe to cease before the measurement probe fails or its measurement accuracy decreases to unacceptable levels. In this manner, the accuracy of measurements from the two-part measurement probe system can be assured. 
     The above embodiments use an authentication process which offers a high level of flexibility in that any authentic components can be used in combination. For example, trigger count credits stored by activation buttons can be transferred to any number of authentic measurement probe. This has the advantage of allowing activation buttons to be swapped between different measurement probes as required. Although such flexibility in using trigger counts is advantageous, it may be desirable to provide non-transferable trigger counts in certain circumstances. 
     Referring to  FIG. 10  an alternative measurement probe  200  is illustrated. The measurement probe  200  comprises a wireless (RF) communications unit  8  for transmitting data to a remote probe interface  202 . In addition, a deactivation device  204  is provided to stop normal measurement probe operation when the trigger counts stored in a rewritable memory portion  206  are expended. The measurement probe also includes a secure memory portion  208  that securely stores a number of (secret) pre-programmed codes for releasing further trigger counts. Entering a code that matches a stored code will thus increase the stored trigger count by a certain amount. These release codes are known only to the manufacturer and are sufficiently complex to ensure that it is not practically possible to find such codes by a trial and error process. The codes are also unique to the particular measurement probe; the measurement probe being identifiable by a unique probe identification or serial number. 
     The measurement probe  200  is thus supplied with a certain number (e.g. five or ten thousand) of trigger counts already stored in its rewritable memory. The stored trigger count reduces with probe use in the manner described above. When the trigger count reaches, or approaches, zero an appropriate release code can be acquired from the manufacturer. Entry of a release code that matches a stored code causes the release of further trigger counts thereby permitting continued operation of the apparatus. Each release code can only be used once to increase the trigger count. 
     The measurement probe  200  also comprises an interface  210  via which the release codes can be input. The interface may comprise one or more keys into which a code is typed. Alternatively, the interface may comprise a wireless link to a remote device (such as a fob) into which the appropriate code has been entered. Alternatively, the interface may receive data via a stylus deflection data entry process such as the trigger logic technique described previously in U.S. Pat. No. 7,145,468. Alternatively, the interface may establish a link (e.g. by telephone or over the interne) to a computer server of an authentic manufacturer, distributor or retailer etc. On receipt of appropriate payment, the necessary code may then be passed over the link to the measurement probe thereby reactivating the measurement probe. 
     It should also be noted that measurement probes may be provided in which the secure memory portion storing the trigger count can not be accessed after manufacture. In such a case, the measurement probe will only work for the preset number of triggers before becoming permanently inoperable. The probe may then be disposed of, or returned to the manufacturer for refurbishment. Although the above examples describe topping up a trigger count value, it is also possible for the measurement probe to be switched into a permanent (i.e. not trigger count or time limited) mode of operation. For example, an activation button or release code may be provided that permanently deactivates the deactivation device such that the measurement probe operates from that point forward as a standard measurement probe. 
     The above described embodiments all relate to measurement probe apparatus. It is, however, important to note that the same techniques could be applied to a wide range of other measurement apparatus. For example, the technique may be applied to any dimensional measuring apparatus such as position encoder systems, co-ordinate measuring machines, scanning apparatus etc. The techniques may also be used with non-dimensional measuring apparatus such as spectroscopy kits. 
     Referring to  FIG. 11 , a Raman spectroscopy system is illustrated in which a Raman spectrometer  250  is interfaced to a computer  252 . The spectrometer  250  comprises a measurement unit  254  that is arranged to acquire, under the control of the computer  252 , Raman spectra from samples  256  placed on a sample stage  258 . The spectrometer  250  also comprises a deactivation device  260  that can prevent measurement data being passed to the computer  252 . The deactivation device  260  is linked to an authentication module  261  comprising an authentication device  262  and a secure memory  264  in which a secret key is held. An electrical contact pad  266  for receiving an activation button  268  is also provided. The activation button  268  may be the same as that described with reference to  FIG. 2 , with the stored count value relating to measurement counts rather than trigger counts. 
     In use, the activation button  268  storing a number of measurement counts is placed on the electrical contact pad  266 . In the manner described above, the authentication module  261  of the spectrometer communicates with the corresponding authentication module of the activation button  268 . If the activation button  268  is found to be authentic and also holds a non-zero measurement count, the deactivation unit  260  allows normal spectrometer operation. If the activation button  268  is not authentic, or if it hold no measurement counts, the deactivation device  260  prevents normal spectrometer operation. In this manner, a spectrometer can be provided which can perform a certain number of measurements before a replacement activation button is required. The number of counts provided on an activation button may be linked to the number of measurements that can be taken before recalibration or servicing of the device is necessary, thereby ensuring operation does not occur when the spectrometer may be out of calibration. As described above, a variant of the apparatus may be provided in which measurement counts are uploaded to a secure memory store within the spectrometer. 
     It should be noted that herein the term “authentic” is used to describe devices that store the relevant secret key and does not necessarily relate to the origin of the manufactured device. In particular, the authentication process may allow only certain models of measurement probe to be paired with certain models of interface thereby preventing measurement probes and interfaces that are not designed to be operable with one another being used in combination. 
     It should also be remembered that the examples described above with reference to the associated drawings are only examples of the present invention. A skilled person would be aware of the many alternatives and variations of the above examples that would be possible. In particular, the various authentication modules, authentication devices, electronic memories etc described above are illustrated as separate functional blocks. These functions may be provided by discrete chips or circuits or may be implemented as parts of a computer program running on a general purpose computing module. The above examples should thus be seen as in no way limiting the physical manner in which the invention is implemented.