Abstract:
Motorized indexed measuring head for a machine for measuring three-dimensional coordinates. Comprises one, two or more re-orientable elements to orient a probe feeler according to a plurality of indexed orientations. The orientable elements are guided in their rotation and sliding movements by undercut bushings and undercut shafts. The arrangement of the undercuts allows an accurate guiding in the rotation movement without impairing the indexing accuracy.

Description:
REFERENCE DATA 
   This application claims priority from European patent applications N o  2004EP-106226 filed on Dec. 1, 2004, and N o  2004EP-106607 of Dec. 15, 2004, and the contents whereof are hereby incorporated by reference. 
   TECHNICAL FIELD 
   The present invention concerns a re-orientable measuring head for measuring three-dimensional coordinates of a mechanical part and notably, though not exclusively, a re-orientable measuring head designed to be used on a manual or automatic machine for measuring coordinates. 
   RELATED ART 
   Touch probes are measuring instruments used for example on production lines of mechanical parts, for checking dimensions or surfaces of machined parts. Touch probes are also used for capturing the three-dimensional shape of complex pieces, in order for example to reproduce or model them. 
   Probes generally comprise a measuring head, designed to be fastened onto the arm of a measuring machine and a mobile feeler, comprising a sphere at the end of an elongated rod and designed to be brought into contact with the piece to be measured. 
   In most applications, touch probes are fastened on the mobile arm of a machine whose position in space can be determined accurately by means of a manual or automatic measuring system, such as for example position encoders placed on the axes of the machine. The mobile arm is moved in space to bring the probe&#39;s measuring feeler into contact with the piece or surface to be measured. During contact, a deflective force is then applied on the feeler, moving it away from its initial resting position. A sensor reacts to the slightest displacement of the feeler, generating an electric signal that is sent either to the user, in the form of a light signal, or to the machine&#39;s control software which thus determines, on the basis of the data of the measuring system, the coordinates of the contact point within a given reference frame. For this purpose, the prior art uses electromechanical or optical sensors or movement sensors based on different principles, for example sensors comprising constraint gauges. 
   In the case of a three-dimensional touch probe, the connection between the feeler and the fixed part of the measuring head is usually realized according to the principle of the Boys connection, i.e. for example by three cylindrical pins resting on six spheres so as to define six contact points between the fixed organ and the feeler. Two- and one-dimensional probes are however also known. 
   When the probe is used for measuring pieces of complex shape, having cavities and protuberances, it is difficult or even impossible to bring the feeler into contact with the entire surface of the piece without the fixed part of the measuring head or the feeler&#39;s rod interfering with elements of the piece to be measured. To remedy this inconvenience, measuring heads are known that allow the contact feeler to be oriented in a plurality of directions in space. Generally, two independent rotation axes are required to cover all the possible orientations. An instrument of this type is described in European patent application EP0392660. 
   Use of this type of devices is however not limited to contact feelers and they can also be used with probes without contact, for example video cameras, for inspecting and checking machined parts for example. 
   The rotation axes are preferably indexed, in the sense that a sufficiently large but finite number of predetermined and accurately reproducible resting positions are provided. This arrangement avoids the measuring machine having to be re-calibrated after each change in orientation of the feeler. 
   The indexing of the feeler&#39;s rotation axes is achieved by indexing surfaces that engage mutually and define the desired resting positions, for example by a crown of spheres in which three pins engage. An example of this type of indexing mechanism is presented in European patent application EP1443299 in the name of the applicant. Optimum accuracy is achieved when the indexing surfaces define an isostatic connection with six independent contact points in each of the indexed positions. 
   For measuring complex pieces, it is desirable that the measuring head be motorized in order to orient the probe feeler automatically, upon command from the measuring machine&#39;s control program. For this purpose, the rotating and the locking of the feeler&#39;s axes are performed by electromagnetic actuators, for example engines or servomotors that move the indexing surfaces away and imprint a rotation to the axes. 
   One limitation of the known measuring heads, and particular of motorized measuring heads, is that the re-orientable elements must be guided in their rotation, for example by bearings or balls. These guiding organs, however, constitute additional mechanical constraints compared with the isostatic connection and impair the indexing accuracy. To avoid this inconvenience, the known measuring heads often adopt bearings of small diameter or having considerable tolerances. 
   The inertia forces linked to the mass of the probe feeler can also influence negatively the functioning of the measuring head, notably if massive feelers and considerable translation and rotation speeds are used. In these conditions, it is important to have efficient bearings or guiding organs for the re-orientable elements of the measuring head. 
   Without efficient guiding, the rotation speed of the re-orientable elements is necessarily limited. Furthermore, the trajectory of the probe feeler during rotation cannot be determined accurately. It is thus necessary to keep a considerable security distance between the probe feeler and the part to be measured, which increases the head&#39;s trajectories and reduces the measuring speed. 
   DESCRIPTION OF THE INVENTION 
   One aim of the present invention is to propose a measuring head free of the limitations of the known devices and, notably, a measuring head whose re-orientable elements are guided efficiently, without impairing the indexing accuracy, and wherein the trajectory of the feeler is completely controlled. 
   These aims are achieved by the device comprising the combination of characteristics that are the object of the main claim, and notably by a re-orientable measuring head for re-orienting a probe feeler relatively to a measuring apparatus comprising: a support element; a first re-orientable element capable of sliding in the direction of a first axis between a locked position and an unlocked position; a first guiding organ connected with said support element; a second guiding organ connected with said first re-orientable element; wherein said first and second guiding organs do not touch when said re-orientable element is in locked position; wherein said first guiding organ supports said second guiding organ so as to allow said re-orientable organ to rotate around said axis relatively to said support element when said re-orientable element is in said unlocked position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood by reading the description given by way of example and illustrated by the attached figures showing: 
       FIG. 1 , a view of a motorized measuring head according to the invention; 
       FIG. 2 , a cross section of the inventive measuring head in locked position; 
       FIG. 3 , a cross section of the inventive measuring head in unlocked position; 
       FIG. 4 , a detail of the locking/unlocking mechanism of the inventive measuring head; 
       FIG. 5 to 8 , different views of the actuator responsible for the locking and unlocking according to one aspect of the invention; 
       FIG. 9 , a detailed view of a crankshaft included in the actuator of  FIGS. 5–8 ; 
       FIGS. 10 and 11 , the position of the bearings of the measuring head&#39;s re-orientable elements in locked resp. unlocked position; 
       FIGS. 12 and 13 , details of  FIG. 10  resp. of  FIG. 11 ; 
       FIGS. 14 and 15 , another embodiment of the invention. 
   

   EMBODIMENT(S) OF THE INVENTION 
   With reference to  FIG. 1 , a re-orientable measuring head  10  according to the invention comprises a support  30  designed to be fastened onto the arm of a measuring machine, capable of moving, for example along three axes of coordinates X, Y and Z inside a measuring volume. It can be fastened for example by the rod  20  or by any other fastening means. 
   Hereinafter, for the sake of simplicity, the designation “vertical” will be used for referring to the orientation of the axis B in  FIG. 1 . This designation refers to the conventional orientation of the figures and also to the orientation in which the inventive device is normally used and usually coincides with the direction of the vertical axis Z of the measuring machine onto which the probe is mounted. However, the probe can be used with any orientation in space. 
   A first re-orientable element  40  is fastened to the support  30 , so as to be able to turn around the vertical axis B. The first re-orientable element  40  can preferably take up a plurality of indexed positions, corresponding to multiples of a small predetermined angle, for example 10 degrees. In known fashion, these indexed positions are determined for example by an isostatic connection defining six resting points between positioning elements whose position is determined with great accuracy. 
   The second re-orientable element  50  is free to turn around the horizontal axis A united with the first re-orientable element  40 . The rotation of the second re-orientable element  50  around the axis A can be continuous or indexed, motorized or manual, as for the first re-orientable element  40  here above. 
   A probe feeler  60  is fastened to the second re-orientable element  50  and bears, at its extremity, a sphere  70  designed to come into contact with the piece to be measured. A detection mechanism, not represented, thus responds to the slightest displacement of the sphere  70  relatively to the resting position with an electric signal that is sent either to the light display  35  or to the machine&#39;s control software, by a connector (not represented). 
   The locking and unlocking mechanism of the axes according to one aspect of the invention will now be described with reference to  FIGS. 2 and 3 . 
   The support  30  bears a series of balls  31  placed along a circumference with a usually constant angular distance, for example by 10°, so as to define a series of indexed position usually regularly spaced. The first re-orientable element  40  bears, corresponding to the balls  31 , three pins  41  at a distance of 120° and capable of engaging with the balls  31 . In locked position ( FIG. 2 ), the first re-orientable element  40  is brought, by pulling the rod  66 , against the fixed element  30 . Each of the pins  41  then touches two adjacent balls  31  so as to have an isostatic connection between the support element  30  and the re-orientable element  40 , according to the principle of the Boys connection. 
   In equivalent manner, in the frame of the present invention, it would be possible to exchange the position of the balls and of the pins, by placing the first on the re-orientable element and the latter on the support element. One could also replace the balls and pins by other positioning elements capable of defining six contact points between the support element  30  and the re-orientable element  40 . 
   One extremity of the vertical rod  66  is fastened in articulated manner to the support element  30  whereas the other extremity of the rod  66  is fastened in articulated manner to one arm of the lever  62 , capable of pivoting around the axis  65 , fixed relative to the first re-orientable element  40 . The rod  66  is preferably aligned with the rotation axis B. 
   In the locked state of  FIG. 2 , the rod  66  is tensioned and pulls the first re-orientable element  40  upwards so that the indexing pins  41  engage with the balls  31  of the support  30 . In this state, any rotation around the axis B is impossible and the re-orientable element  40  is locked in one of the indexed positions. 
   The force exerted by the rod  66  is applied centrally relatively to the contact points between the balls  31  and the pins  41 , and is oriented along the axis B. In this manner, one achieves an equal distribution of the contact forces between the balls  31  and the pins  341  for a maximum indexing accuracy. 
   The second re-orientable element  50  is also held against the first re-orientable element  40  by the tension of the horizontal rod  67  aligned with the axis A. The rod  67  is articulated on the one hand relatively to the re-orientable element  50  and on the other hand relatively to the lever  62 . 
   A second set of balls  43  and of pins  42 , placed between the first and second re-orientable elements, allows the rotation of the second re-orientable element  50  to be locked in an indexed position. 
   Optionally, the rods  66  and  67  comprise elastic elements (not represented), for example metallic springs, to ensure a constant indexing force between the pins  41 ,  42  and the balls  31 ,  43 . In equivalent manner, elastic elements could be included in the lever  62  or in the first and second re-orientable elements. 
   With reference to  FIGS. 5–9 , the position of the lever  62  is determined by the crankshaft  59 , represented in detail in  FIG. 9 , driven in rotation around the axis  75  by the electric motor  45  and the dented wheels  46 ,  51 . In equivalent manner, the crankshaft  59  could be driven directly by a motor placed on the same axis  75  of the crankshaft  59  or by any mechanical transmission, for example by a system of pulleys. 
   One arm of the lever  62  comprises a fork whose two branches  63  and  64  contact the two opposite sides of the crank pin  55  of the crankshaft  59 , so as to move the lever  62  from the locked position to the unlocked position when the crankshaft  59  turns by 180°. Optionally, a ball bearing is interposed between the crank pin  55  and the fork in order to reduce friction during locking and/or unlocking. In the embodiment illustrated in the figures, a bearing  54  is provided only to correspond to the branch  63  that transmits the locking force. To correspond to the other branch  64  of the fork, responsible for unlocking, the efforts required are less and a simple antifriction bearing can be used. 
   The rotation of the crankshaft  59  around the axis  75  is limited to a rotation angle slightly greater than 180° by the sector  53  and the pin  55  united with the first re-orientable element  40 . The stop positions of the pin  55  against the extremities of the sector  53  are disposed so as to overtake the points of equilibrium and thus to define stable resting positions corresponding respectively to the locked state and to the unlocked state. 
   The  FIGS. 3 and 4  represent the measuring head according to the invention in its unlocked state. In this case, the lever  62  is inclined and the rods  66  and  67  press on the support element  30  respectively on the second re-orientable element  50  so as to move the indexing elements  31 ,  41  respectively  42 ,  43  apart by a predetermined distance d 1 , respectively d 2 . 
   In a variant embodiment, the rods  66  and  67  could be driven by a pinion/rack unit. 
   The moving apart and the closing of the indexing surfaces take place thanks to the double action of the rods  66  and  67  which is independent of the direction of the weight force and of the inertia forces, and without springs or elastic elements having to be used. The inventive mechanism can thus also ensure a reliable and fast functioning whatever the orientation of the measuring head. 
   In unlocked position, the rotation around the two axes A and B is ensured by servomotors (not represented), controlled by the software of the measuring machine, or by other equivalent automatic actuators. 
   The embodiment described here comprises a single actuator for locking and unlocking the two axes A and B simultaneously. The invention however also includes variants in which each rotation axis is locked and unlocked by an independent actuator. 
   In one embodiment, the inventive measuring head comprises only a single rotation axis, for example a horizontal axis A. 
   With reference to  FIG. 10 , representing the guiding system in locked position, the first re-orientable element  40  is provided with a guiding bushing  82  into which the shaft  84 , united with the support element  30 , engages. The surface of the shaft  84  has protuberances  85   a ,  85   b  separated by undercuts on which the diameter of the shaft  84  is reduced relatively to the maximum diameter of the protuberances  85   a ,  85   b.    
   In the same manner, the inner surface of the bushing  82  has protuberances  83   a ,  83   b  separated by undercuts having a diameter greater than the inner diameter of the protuberances  83   a  and  83   b.    
   In the locked position of  FIG. 10 , the protuberances  83   a  and  83   b  face the undercuts of the shaft  84 . There is thus no contact between the guiding bushing  82  and the fixed shaft  84  that could impair the indexing accuracy. 
   In the unlocked position of  FIG. 11 , the first re-orientable element  40  is displaced axially along the axis B so as to move the balls  31  and the pins  41  apart. In this position, the protuberances  83   a  and  83   b  are juxtaposed over the protuberances  85   a  and  85   b  so as to support the rotation of the re-orientable element  40  around the axis B. Preferably, the bushing  82  and the shaft  84  each comprise two protuberances at a distance along the direction of the rotation axis to guide the rotation in optimal manner. 
   Thanks to the absence of contact between the guiding elements  82  and  84  in locked position, the latter&#39;s diameter can be considerable and the play between the bushing and the shaft can be essentially zero or negligible in unlocked position. In this manner, the probe feeler&#39;s position during rotation is fully determined. 
   The second horizontal rotation axis A is provided with a guiding mechanism visible in  FIGS. 12 and 13 , similar to that of the first vertical axis B. The guiding bushing  88  of the second axis bears two protuberances  89   a  and  89   b , whilst the shaft  86  comprises the two protuberances  87   a  and  87   b.    
   In locked position, visible in  FIG. 12 , the protuberances  89   a ,  89   b  of the bushing  88  and the protuberances  87   a  and  87   b  of the shaft  86  are shifted in relation to one another. In this manner, there is no contact between the bushing  88  and the shaft  86  that could impair the indexing accuracy. 
   In the unlocked position of  FIG. 13 , the protuberances  89   a ,  89   b  of the bushing  88  and the protuberances  87   a  and  87   b  of the shaft  86  are juxtaposed and form two bearings to guide the rotation of the second re-orientable element  50  around the axis A. 
   In a variant embodiment of the invention represented in  FIGS. 14 and 15 , the shaft  86  and the bushing  88  are provided with coaxial conical guiding surfaces  87   c  and  89   c . In the locked arrangement of  FIG. 14 , there is no contact between the guiding bushing  88  and the shaft  86 . In the unlocked position, represented in  FIG. 15 , the conical surfaces  87   c  and  89   c  stand in contact and guide the rotation of the second re-orientable element  50  around the axis A.