Abstract:
The invention provides a modular measurement device  100  having, attachable to the quill  12  of a CMM, any one of a plurality of stylus support modules  120   ab,    120   cd , or  120   ef . Attachable to a stylus support module is a range of stylus modules  110   a-f . The stylus support modules have different spring rates to ensure that a near optimum response can be obtained for each stylus module used. The stylus modules may be configured so that each cannot be mistakenly fitted to the wrong stylus support module. The modules are held together by magnetic attraction, and the strength of attraction is cascaded so that the stylus module is removed first if pulled off.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a modular measurement device for example a measurement probe, for use on positioning apparatus such as a coordinate measurement machine, a machine tool, robots, non cartesian machines or the like. 
     2. Description of the Related Prior Art 
     Measurement probes are known (e.g. U.S. Pat. No. 5,404,649) which have a number of stylus modules releasably attachable to a sensor module for producing a signal indicative of the movement of the stylus module relative to the sensor module. A range of stylus modules are provided which have different lengths or configurations, allowing the probe to be used for different measurement tasks. 
     However, a sensor module that has to have a large range of styli is generally designed as a compromise and probing forces for each stylus used will be different. The sensor module has to be strong enough to support the stylus used and to allow movement of the stylus. If a short stylus is used and a sensor is designed for a longer stylus then the measurement device will be less than optimally sensitive. A plurality of sensor modules each of which is designed for use with just one stylus would be ideal, but is very expensive for the customer. U.S. Pat. No. 6,430,833 shows a separable sensor and stylus support mechanism. However, the use of a range of styli is not addressed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a modular measurement device comprising: a retaining module attachable to a positioning apparatus; at least one stylus support module being exchangeably attachable to the retaining module; and at least one stylus module being exchangeably attachable to a stylus support module, 
     wherein the stylus support module has a suspension system for movably supporting the stylus module, wherein the retaining module and the stylus support module cooperate to measure displacement of the suspension system, and wherein the exchangeable attachment of the stylus module to the stylus support module is formed by complementary mountings on the stylus module and the stylus support module, allowing a repeatable rest position of the stylus module relative to the stylus support module following exchange. 
     Preferably the complementary mountings form a kinematic mounting. Preferably a plurality of stylus support modules and stylus modules are provided, the stylus modules are grouped and each group of stylus modules is configured such that it may be attached only to one of the plurality of stylus support modules. 
     Preferably each respective exchangeable attachment includes a magnetic attraction device. 
     Preferably the magnetic attraction force exerted by each respective attachment differs. 
     The invention provides also a modular measurement device comprising: a retaining module attachable to a positioning apparatus; a plurality of stylus support modules being exchangeably attachable to the retaining module; and a plurality of stylus modules being exchangeably attachable to a stylus support module, 
     wherein each stylus support module has a suspension system for movably supporting the stylus module, the retaining module and the stylus support module cooperate to measure displacement of the suspension system and the suspension system of at least one of the stylus support modules has a different configuration from that of at least one of the other of the stylus support modules. 
     Thereby embodiments of the invention provide for one sensor module which can have a range of styli attached thereto via a smaller range of styli support modules intermediate the sensor and the styli. 
     Preferably the suspension system includes spaced resilient elements and the different configuration includes a different spacing between the elements. 
     Preferably each stylus has a work contacting area and the degree of movement per unit force applied to the work contacting area of each stylus module is approximately equal when attached to a stylus support module. 
     Preferably the plurality of stylus modules are grouped and each group of stylus modules is configured such that it may be attached only to one of the plurality of stylus support modules. 
     Preferably the configuration of each group allows only one orientation of the stylus modules in the group relative to the stylus support module to which they are attachable. This means that a stylus module can be removed and replaced in the same place, giving better repeatability of measurement if stylus modules are removed and replaced. 
     Preferably the releasable attachments between the retaining module and the stylus support modules or between the stylus support modules and the stylus modules, forms a kinematic mounting. 
     Preferably the exchangeable attachments between the retaining module and the stylus support modules or between the stylus support modules and the stylus modules includes magnetic attraction. 
     Preferably there exists magnetic attraction between the retaining module and the stylus support modules, and between the stylus support modules and the stylus modules and the magnetic attraction is greater between the retaining module and the stylus support modules. 
     In use of modular measurement systems for example of the type mentioned above, it is often desirable to have machine driven exchange of various modules in the system. U.S. Pat. No. 5,404,649 shows such an arrangement. 
     If modules are stacked and held together by magnetic attraction it will be difficult to control the sequence of release of the stack e.g. when trying to remove the outer most module of the stack the inner most module might come away instead. 
     The invention extends also to a stacked modular measurement system having at least three modules in a stack held together by means of magnetic attraction between adjacent modules wherein the attraction between one pair of the adjacent modules is not equal to the attraction between another pair of adjacent modules. 
     Thus in embodiments of the invention the module at the free end will be removed before the module at the machine end if the stack is pulled at its free end. This allows a more simple routine for changing modules of the machine. In embodiments the invention allows simple movements to change modules when modules are stored on a rack on a measurement machine. 
     Preferably the stack has an end mountable to a machine and a free end and the module at the free end has less attraction to its adjacent module than the attraction of the module at the machine end and its adjacent module. 
     The invention extends also to a modular measurement system comprising a modular measurement device including: a retaining module attachable to a positioning apparatus; at least one stylus support module being exchangeably attachable to the retaining module; at least one stylus module being exchangeably attachable to a stylus support module, wherein the stylus support module has a suspension system for movably supporting the stylus module, the retaining module and the stylus support module cooperate to measure displacement of the suspension system and the exchangeable attachment of the stylus module to the stylus support module is formed by complementary mountings on the stylus module and the stylus support module allowing a repeatable rest reposition of the stylus module relative to the stylus support module following exchange, and the system further comprising a rack for storing any of the modules when not in use. 
     The invention extends also to a modular measurement system comprising a modular measurement device including: a retaining module attachable to a positioning apparatus; a plurality of stylus support modules being exchangeably attachable to the retaining module; a plurality of stylus modules having a work contact area and being exchangeably attachable to a stylus support module, wherein the stylus support module has a suspension system for movably supporting the stylus module, the retaining module and the stylus support module cooperate to measure displacement of the suspension system and the suspension system of each stylus support module has a different configuration, and the system further comprising a rack for storing any of the modules when not in use. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will now be described with reference to the drawings, wherein: 
     FIG. 1 shows a measurement machine incorporating a modular measurement device according to the invention; 
     FIG. 2 shows a storage rack for storing elements of a modular measurement device according to the invention; 
     FIG. 3 shows one set of elements from a plurality forming a measurement device according to the invention in a disassembled condition; 
     FIG. 4 shows the measurement device of FIG. 3 in an assembled condition; 
     FIG. 5 shows a complete set of modular elements performing a modular measurement device according to the invention; and 
     FIG. 6 shows details of the measurement device according to the invention as shown in FIGS.  3  and  4 . 
    
    
     FIG. 1 shows a coordinate measurement machine (CMM)  10 . The machine has a measurement device  100  which can be moved by the machine in the directions X, Y and Z. The measurement device  100  includes a stylus  110  which can be used to contact a workpiece (e.g. block  50 ). The measurement device produces a signal when contact between the workpiece and the stylus  110  is made. In this embodiment the signal is an analogue signal but a more simple on/off signal is possible. 
     The CMM needs varying shapes of styli in order that all shapes of workpiece can be measured. Short straight styli can be used for many situations but deep bores, overhangs and the like will require a different stylus. This CMM has a storage rack  20  in which those different styli  110  can be stored. The CMM has a controller (not shown) which as well as moving the stylus around the workpiece  50 , can move the measurement device to the rack  20  where stylus changing can take place. 
     Now, since the leverage exerted on the stylus support modules by short styli is different to the leverage exerted by long styli it is desirable to have stylus support modules  120  which can be changed also to provide approximately equal probing force for each stylus module used. The support modules  120  can thus be matched to the length of the stylus. This matching is even more important when the measurement device  100  is attached to an articulating head so that the stylus can extend for example horizontally. In such instances a stronger stylus support is required if a longer or heavier stylus is used. 
     FIG. 3 shows in detail the modular measurement device  100 . The so-called quill  12  of the CMM  10  is shown, to which the modular measurement device  100  is attachable. A tee connector  16  and location points  14  are used to secure and hold a sensor module  130 . The sensor module  130  has complementary location points  132  and a hooked cam  135  for holding the tee connector  16 . 
     Whilst the sensor module  130  is removable from the quill  12 , in normal use it is envisaged that it will be attached permanently to the quill  12 . The sensor module includes electronics  136 , a light emitter and light receiver pair  138 . Also included in the sensor module are magnets  131 , location points  134  and a removal groove  139 . 
     A stylus support module  120  is attachable releasably to the sensor module  130  by means of magnetic attraction. Location points  122  are provided which are complementary to the location points  134  on the sensor module  130 . The stylus support module includes magnets  121  which are adjacent magnets  131  when the stylus support module is attached to the sensor module. A magnetic attraction force between the magnets  131  and the magnets  121  holds the stylus support to the sensor module with an attraction of approximately 20N. 
     The stylus support module includes a yoke  123  which is mounted to an outer casing  125  via a suspension system comprising support springs  126 . The yoke is capable of constrained resilient movement in the aforementioned X, Y and Z directions. A mirror  127  is mounted to top of the yoke for reflecting light emitted by the light emitter of sensor  138 . The stylus support module has a groove  129  in casing  125  for its removal from the sensor module  130 . 
     Releasably attachable to the stylus support module is a stylus module  110 . The stylus module is held to the stylus support module by means of the magnetic attraction force between magnet  128  in the stylus support module and magnet  111  in the stylus module. The attractive force between the stylus module and the stylus support module is approximately 10N. Complementary location points  124  and  112  are provided also. The stylus module includes a stylus stem  113  and a stylus ball  114  which contacts the workpiece  50 . The stylus module has a groove  119  to aid its removal from the stylus support module  120 . 
     The arrangement shown in U.S. Pat. No. 6,430,833 is similar to that described above and the arrangement in U.S. Pat. No. 6,430,833 could be used as an alternative, its disclosure being incorporated herein by reference. 
     FIG. 4 shows an assembled modular measurement device  100 . Light from the emitter of the sensor  138  is shown being reflected from mirror  127  to the light receiver of the sensor  138 . In use of the modular measurement device mirror  127  is moved by the sprung yoke  123 , connected to the stylus module  110 . Thus movement of the stylus ball  114  causes movement of the mirror which is detected by the sensor  138  and turned into a signal by electronics  136 . 
     FIG. 5 shows a quill  12 , a sensor module  130 , a plurality of stylus support modules  120   a,b , 120   cd  and  120   ef , all connectable to the stylus module  130 . Each stylus support module has springs  126  of varying configuration. Springs  126 ′ fitted to the stylus support module  120   ab  are used to support the shortest stylus modules  110   a  and  110   b . Springs  126 Δ which are further apart than springs  126 ′ are fitted to stylus support module  120   cd  and are used to support stylus modules  110   c  and  110   d . Similarly springs  126 ′″ are further apart again than springs  126 ″ and are used with stylus support module  120   ef  for supporting longer stylus modules  110   e  and  110   f . The springs  126 ′, 126 ″ and  126 ′″ provide the appropriate (and approximately equal) level of force at the stylus ball  114 . 
     The modular measurement device  110  is made up of a stack consisting of a sensor module  130 , any one of the three stylus support modules  120  and an appropriate stylus module  110 . The configuration of the springs  126  can thus be approximately optimised for each group of stylus modules  110   a/b , 110   c/d  or  110   e/f.    
     In order that the wrong stylus is not attached to a stylus support module a unique configuration of a protrusion and recess can be incorporated into the top of the stylus module, and/or into the stylus support module so that only one group of stylus modules will fit the appropriate stylus support module. 
     Referring to all the figures during stylus module changing the groove  119  of each module is inserted horizontally into a slot  22  on the storage rack  20 . The quill  12  is moved up and the stylus is pulled off the stylus support module. The stylus support module does not stay with the stylus module because the attractive force between the stylus support module and the sensor module is greater than the attractive force between the stylus support module and the stylus module. 
     If it is required that the stylus support module is changed also then the quill moves the groove  139  of the stylus support module into a slot  22  on the rack and moves upwardly to pull the stylus support module from the sensor module. 
     Replacement of the stylus support module and stylus module is the reverse of their removal. 
     FIG. 6 shows a typical set of complementary location points  14 / 32 ,  134 / 122 , or  124 / 112 . The illustrated location is of a kinematic type which allows repeatable repositioning of the locators in the same rest position following separation and replacement. In this instance three equi-spaced balls  210  form one half of a set of location points. The other half of the location points is formed by three pairs of roller  215 , each ball being seated between each roller pair. Other arrangements of location points are possible provided six points of contact are established between the modules which are located together. The location points provide a means for exchangeably attaching one module to another. 
     Many variants are possible within the ambit of the invention. Modular contact measurement devices have been described and illustrated, however other modular measurement devices are envisaged, e.g. devices that do not contact a workpiece but merely measure the change in capacitance or some other variable as the measuring implement moves closer to the workpiece. 
     In the embodiment shown a simplified sensor  138  is shown. In practice this sensor will include a further light emitter and receiver pair for sensing movement in more than one direction. However other displacement sensors could be used. Two parallel springs  126  are shown. However, other arrangements of springs could be used. It is preferred that the spacing between the springs is varied, however the spring&#39;s stiffness could be altered to give similar results. 
     The sensor  138  has been shown in the sensor module  130 . However, the sensor may be mounted in the stylus support module  120 . Thus in the claims the sensor module is referred to as a retaining module. 
     Three stylus support modules are illustrated for use with three groups of stylus modules each group having two stylus modules. More or less stylus support modules or stylus modules are possible. 
     The invention has been described for use with a CMM however any machine capable of measurement could employ the modular measurement device described herein.