GB2298488A - Articulating probe head - Google Patents

Description

1 ARTICULATING PROBE READ 2298488 The present invention relates to an

articulating probe head used, for example, on a coordinate positioning machine such as a coordinate measuring machine or machine tool.

Typically, such coordinate positioning machines comprise an arm, movable relative to a table with three linear degrees of freedom. In use, the arm carries an operating module such as a probe used for determining the dimensions of components. It is frequently necessary to orient such a probe relative to the arm about one or more rotational axes, in order that differently oriented surfaces of the component may be inspected. For this purpose, coordinate positioning machines are equipped with articulating probe heads, such as the Renishaw PH9. This probe head includes a base upon which first and second rotors are serially mounted for rotation about substantially perpendicular axes, thereby providing two rotational degrees of freedom.

The first and second rotors are rotatable between a plurality of indexed positions, each of which is highly repeatable. As a result, a probe mounted to such a probe head will occupy substantially the same location in space relative to the movable arm of the machine each time the first and second rotors of the probe head are engaged in particular indexed positions corresponding to a given angular orientation. A mechanism is provided for disengaging the first and second rotors from their repeatable locations on the base and first rotor respectively in order to enable rotation to a further indexed position, at which the mechanism is once again actuated to engage the first and second rotors into locations provided on the base and first rotor respectively.

A first known type of probe head includes a locking/unlocking mechanism which disengages the first rotor from the base axially along a first axis of rotation, 2 and disengages the second rotor from the first rotor axially along the second axis. Such engagement/disengagement is effected by means of a pivoting locking lever mounted on the first rotor; pivoting of the lever axially urging the second rotor into engagement against the action of a biasing spring, which effects disengagement when the pivoting force on the lever is removed. Upon engagement of the second rotor, the lever ceases pivoting, and the application of further force to the lever causes bodily movement of the first rotor (to which the second rotor is now engaged) into engagement with the base. The applied force, which initially causes pivoting of the lever, and when the lever is locked movement of the first rotor, is provided by means of a cam operably connected to the locking lever by means of a further lever and tie-rod.

The present invention relates to aspects of an alternative form of probe head.

Accordingly, a first aspect of the present invention provides an articulating probe head for use on a movable arm of a coordinate positioning machine, the probe head comprising:

base by which the probe head may be connected to the arm; first rotor rotatable substantially about a first axis relative to said base, the first rotor and base having at least one set of mutually engageable first engagement elements engageable in a plurality of angular orientations about said first axis, to provide a corresponding plurality of first angularly indexed positions at which said first rotor has a stable and repeatable location on the base; a second rotor, rotatable substantially about a second axis relative to said first rotor, the first and second rotors having at least one set of mutually engageable second engagement elements, engageable in a plurality of angular orientations about said second axis, to provide a 3 corresponding plurality of second angularly indexed positions at which said second.rotor has a repeatable location on the first rotor; the first and second sets of engagement elements being engageable and disengageable by axial movement of the first and second rotors along the first and second axes respectively; a lever mounted on said first rotor and having first and second limbs, the application of a force to said first limb causing: pivoting of said second limb to apply an axial force to said second rotor; and bodily movement of said lever to apply an axial force to said first rotor; thereby to effect engagement of said first and second sets of engagement elements; is a motor mounted on the base for applying a force to said first limb of said lever, wherein said motor is connected to said first limb of said lever by means of a cable.

Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which: Fig 1 is an external view of a probe head according to the present invention; 25 Fig 2 is a perspective view of a locking mechanism utilised in the probe head of Fig 1; Fig 3 is a plan view on Fig 2; Fig 4 is a further perspective view of the locking mechanism illustrated in Figs 2 and 3; and 30 Fig 5 is a further perspective of a detail of the probe head.

Referring now to Figs 1 to 4, a probe head includes a base 1, by means of which the probe head may be rigidly mounted upon the movable arm of the coordinate positioning machine. An operating module, which in this example is a touch probe 2 having an elongate stylus 4 and a spherical sensing tip 6 at the free end thereof is provided with two rotational 4 degrees of freedom relative to the base 1 (and hence the movable arm of the machine) by.means of first and second rotors Rl,R2. The first rotor Rl is mounted directly to the base 1, is rotatable about an axis Al, and may be engaged into one of a plurality of repeatable indexed positions on the base 1, each of which lies at a discrete angular orientation about the axis Al. The indexed positions are defined by sets of mutually engageable engagement elements; in the present example the engagement elements have the form of balls 5 on the base 1 and vgroove seat elements 7 on the first rotor Rl, and provide a kinematic location (see Fig 4). The second rotor R2 is mounted to the first rotor Rl, and may rotate relative thereto about an axis A2. The second rotor R2 may be engaged in a plurality of repeatable indexed positions on the rotor Rl, each of which positions has a different orientation about the axis A2. The repeatable positions are again provided by sets of mutually engageable engagement elements in the form of balls 9 on the second rotor R2, and v- groove seat elements 11 on the first rotor Rl, and provide kinematic locations (see Fig 4). The probe 2 may thus be positioned with two degrees of rotational freedom relative to the base 1 in any one of a number of discrete angular orientations, whose number is equal to the product of the number of indexed positions of rotor Rl relative to support I and of the rotor R2 relative to rotor Rl. On each occasion on which the probe 2 occupies a given indexed position, its position in space, and therefore the position in space of the spherical sensing tip 6, relative to the base I is substantially the same, due to the repeatability of the individual locations for the rotors R1 and R2 relative to the support 1 and rotor Rl respectively. This property is desirable in order to avoid the need to recalibrate the position of the sensing tip 6 relative to the base 1 each time the probe 2 is moved to a new indexed position for the purpose of inspecting a differently oriented surface. NB No degree of accuracy is either implied or required with respect to the use of the term repeatability, beyond the requirement that the specific configuration of engagement elements employed enables the probe head to function as required by the user.

The rotors Rl and R2 are axially disengageable along axes Al and A2 from respective indexed positions on the base 1 and rotor R2 in order to allow rotation to a new angular orientation corresponding substantially to a new indexed position, whereat the rotors Rl and R2 are axially engageable into the new indexed position with the base 1 and rotor R1.

Referring now particularly to Figs 2-4, the locking mechanism by means of which the rotors Rl and R2 are engaged and disengaged will now be described. The locking mechanism includes a locking motor M, provided on the base 1 (not illustrated in Figs 2-4 for reasons of clarity) having an eccentrically rotating shaft 10, to which a flexible linkage in the form of a cable 12 is attached.

The motor M rotates shaft 10 between two predetermined angular limits, corresponding to locking and unlocking of the probe head. Cable 12 passes around a first cable tensioning or biasing device in the form of a springloaded pulley 14, pivotally mounted on the base 1 by means of pivot in the form of a ball joint 16, and over a further vertical pulley 18. The cable 12 is connected at its end remote from the shaft 10 to an actuating device in the form of a right angle locking lever 20. The locking lever 20 is attached at its angle to a locating and pivoting mechanism in the form of a planar fixing spring 22, which is in turn fixedly and rigidly mounted to the rotor Rl at its lower end by means of two bolts 24, 26 (rotor Rl has been omitted from Figs 2-4 for reasons of clarity). Planar spring 22 acts as a support for locking lever 20 against twisting - and lateral movements of the lever 20, while allowing pivoting action of the lever on the rotor Rl. To accommodate the spring the two limbs 30,32 of locking lever 20 are mutually displaced along the pivoting axis of the lever 20. A 6 knife-edge member 34 is provided on limb 32 of locking arm 20 to ensure good pivoting action upon a bearing surface of rotor R1 (not shown). The lower end of limb 32 supports a horizontal push rod 36 which abuts rotor R2.

When the motor M is actuated to rotate shaft 10, and pull cable 12 in an upward direction, locking lever pivots and a force is applied to rotor R2 by push rod 36 substantially along the axis A2, in the direction of arrow A, by virtue of the consequential pivoting of locking arm 20. This force engages rotor R2 in a location with respect to rotor R1 against the action of a bias (not shown), which acts between rotor R2 and rotor R1 to bias the rotor R2 out of the indexed position. The lifting action of cable 12 will also cause the rotor R1 to be urged upwardly into engagement with the base 1 along axis A1 against the action of a further bias (not shown), by virtue of the connection of locking arm 20 to rotor R1 by the planar spring 22 and bolts 24 and 26.

The sequence of events during a locking operation depends, inter alia, upon the relative forces applied by the spring biases which act to effect disengagement of the base 1, rotor R1, and rotor R2 when motor M is rotated to relieve the locking force. For example, the relative biasing forces may be such that the bias acting between the base 1 and the rotor R1 is greater than that acting between the rotor R1 and the rotor R2, so that rotation of the motor M to lock the probe head initially causes pivoting of the lever 20 to effect engagement of the rotor R2, and when this engagement has occurred, subsequent upward bodily movement of the rotor R1 into engagement with the base 1. Alternatively, the biasing force between rotors R1 and R2 may be greater than that between rotor R1 and base 1, with engagement of the elements 7 and 5 on the rotor R1 and base 1 occurring first; equal forces and simultaneous engagement is also possible.

7 The locking force, i.e. the force withwhich rotor R2 is urged into engagement with rotor R1, and rotor RI urged into engagement with support 1, is determined by spring 40, whose biasing action on pulley 14 has the effect of tensioning the cable 12 when all elements of the probe head have been brought into engagement. Preferably, the spring 40 has a very low spring rate, and is pre-loaded to a relatively high degree to provide the required spring force. This ensures that small variations in eg the length of the cable due to eg thermal expansion, do not cause significant changes in the force with which the elements of the probe head are urged into engagement.

Referring to Fig. 5, in a modification, the motor M is supported on the base 1 in a cantilevered manner by means of a pair of ball races 50,52, which extend around the shaft of the cam, and are bonded to the base 1. Rotation of the motor is prevented by the interconnection of a bracket 60 fixed to the motor M, and plate 62 fixed to the base 1 by four springs 64. This method of mounting the motor M avoids the application of undesirable loads on the motor bearings. This mounting method constitutes a further independent aspect of the present invention which may be employed on probe heads which do not have all the features of claim 1.

The present invention need not be embodied by any of the specific features or functions described above, and each feature or function referred to may be substituted by other features performing the same or a similar function, and other functions achieving the same or a similar result. Further, the different features of the invention described above are not necessarily limited to their association with the embodiments in connection with which they were described. Many aspects of the invention are generally applicable to other embodiments of the invention described herein.

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Claims (5)

1. A first aspect of the present invention provides an articulating probe head for use on a movable arm of a coordinate positioning machine, the probe head comprising: a base by which the probe head may be connected to the arm; a first rotor rotatable substantially about a first axis relative to said base, the first rotor and base having at least one set of mutually engageable first engagement elements engageable in a plurality of angular orientations about said first axis, to provide a corresponding plurality of first angularly indexed positions at which said first rotor has a stable and repeatable location on the base; a second rotor, rotatable substantially about a second axis relative to said first rotor, the first and second rotors having at least one set of mutually engageable second engagement elements, engageable in a plurality of angular orientations about said second axis, to provide a corresponding plurality of second angularly indexed positions at which said second rotor has a repeatable location on the first rotor; the first and second sets of engagement elements being engageable and disengageable by axial movement of the first and second rotors along the first and second axes respectively; a lever mounted on said first rotor and having first and second limbs, the application of a force to said first limb causing: pivoting of said second limb to apply an axial force to said second rotor; and bodily movement of said lever to apply an axial force to said first rotor; thereby to effect engagement of said first and second sets of engagement elements; a motor mounted on the base for applying a force to said first limb of said lever, wherein said motor is connected to said first lever by means of a cable.

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2. A probe head according to claim 1 wberein said lever is mounted on said first rotor by.a planar spring, and said first and second limbs of said lever are mutually displaced along an axis about which said lever pivots.

3. A probe head according to claim 1 or claim 2 wherein biasing means are provided for applying a predetermined tension to said cable when said motor has been actuated to urge said first and second engagement elements into engagement.

4. A probe head according to claim 3 wherein said cable passes around a pulley, and said pulley is biased by said biasing means.

5. An articulating probe head substantially as described herein and illustrated in the accompanying drawings.