Patent Publication Number: US-RE35510-E

Title: Probe head with indexing mechanism

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a probe head used on a coordinate positioning machine to orient a touch trigger probe (for example) relative to the movable arm (or quill) of the machine. The coordinate positioning machine may e.g. be either a coordinate measuring machine or a machine tool. 
     2. Description of Related Art 
     It is known (from &#34;Probing for Productivity on Coordinate Measuring Machines&#34; RENISHAW) to provide a two-axis motorised probe head for orienting a probe relative to the quill of a machine with two degrees of rotational freedom. Such a probe head comprises a support, which is connectable to the quill of a machine, and a pair of serially mounted rotors, each of which is engageable in, and may rotate between a plurality of indexed positions. A probe is connected to the &#34;free&#34; rotor, and is oriented relative to the quill by rotation of one or both of the rotors about their respective axes. Indexing of position between two relatively rotating parts is provided by a ring of balls on one of the parts, and three seating members on the other part, each of which seats in the convergent surfaces provided by an adjacent pair of balls. Thus, at each of the indexed positions each rotor is kinematically supported; this provides maximum positional repeatability of the probe at each indexed position. 
     It is often desirable, in order to perform different types of measuring operation, to exchange one probe which is carried by the probe head for another. EP 0142373 discloses a system which provides releasable kinematic engagement of a probe in order to facilitate probe change. 
     This obviates the need to calibrate the machine each time a given probe is removed and replaced on the machine, in order to determine the position of the probe relative to the quill. 
     However, in spite of the high positional repeatability of each of the individual kinematic locations formed between the probe and the quill for the purpose of supporting the probe, (i.e. the kinematic location of the probe on the probe head, and the kinematic location of each of the rotors in the probe head), the repeatability of the position of the probe relative to the quill in a given orientation of the probe head over a number of probe-change and/or probe head orienting cycles decreases with the number of kinematic locations formed between the probe and the quill which are coupled and uncoupled in the course of normal operation. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention a system is provided for interchanging a number of probes upon an indexing probe head of the type described above, in which each of the probes to be supported on the probe head is mounted to a carrying member which, in use, forms the rotor of the probe head. 
     Accordingly, the present invention provides a probe head for orienting a probe relative to the movable arm of a coordinate positioning machine, comprising: 
     a support by which the probe head is connectable to the arm of the machine; 
     a rotor for carrying a probe; 
     retaining means for releasably retaining the rotor on the support, to enable one rotor to be exchanged for another; 
     indexing means for providing a plurality of indexed angular positions of the rotor relative to the support, comprising a first set of engagement elements provided on one of the rotor and support, and a plurality of sets of second engagement elements provided on the other of the rotor and support, said first set of engagement elements being engageable with each set of said plurality of sets of second engagement elements to provide, at each indexed position, a stable, repeatable rest position of the rotor on the support; 
     coupling and decoupling means for engaging and disengaging said first set of engagement elements with a set of said second engagement elements; and 
     driving means for rotating said rotor relative to said support substantially about an axis when said first and second engagement elements are disengaged, thereby to rotate said rotor from one indexed position to another. 
     This aspect of the present invention reduces the number of parts which are coupled and uncoupled during normal operation by utilising the first and second engagement elements for both indexing and probe change functions. 
     In use, each probe required by the machine is connected to a separate rotor, all of which are retained on the machine. When a particular inspection cycle requires a change of probe, this is achieved by exchanging rotors. 
     Preferably, the rotor will be magnetically retained on the support. Preferably, an axially movable drive dog is engageable with the rotor to uncouple the first and second engagement elements and thus enable rotation of the rotor between indexed locations. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     An embodiment of the invention will now be described, by way of example, and with reference to the accompanying drawings in which: 
     FIG. 1 is a section through a probe head according to the present invention; 
     FIG. 2 is a section on II--II in FIG. 1; and 
     FIG. 3 is a further section through the probe head of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1-3, a probe head 10 comprises a support 12 by which the probe head 10 may be carried on the movable arm, .[.of.]. .Iadd.or .Iaddend.quill of a coordinate positioning machine (such as a coordinate measuring machine or machine tool). A rotor 14, whose purpose is to carry a probe 16, is releasably retained on the support 12 by retaining means in the form of a permanent magnet 18 provided on a drive dog shaft 20 which cooperates with a ferromagnetic element in the form of a further magnet 22 provided on the rotor 14. A first set of engagement elements in the form of three adjacently positioned, equispaced pairs of rollers 24 is provided on the support 12. A plurality of sets of second engagement elements in the form of a ring of balls 26 is provided on the rotor 14; when the rotor 14 is retained on the support 12 by the .Iadd.biasing .Iaddend.action of magnets 18,22, three of the balls 26 seat in the convergent surfaces defined by adjacently positioned pairs of rollers 24. The pairs of rollers 24 are included in a series of electrical circuit (not shown) which is completed when each of a set of three balls 26 seat in the convergent surfaces of adjacently positioned pairs of rollers, thus indicating that the rotor 14 is correctly retained on the support 12. 
     The rotor 14 may be oriented in any one of a plurality of indexed angular positions relative to the support 12 by disengaging each of a set of three balls 26 from the rollers 24, rotating the rotor 14 relative to the support 12, and engaging a further set of three balls 26 with the rollers 24. It is noted that the number of sets of three balls 26 thus determines the number of indexed angular positions which the rotor 14 may adopt relative to the support 12. 
     Disengagement of balls 26 and rollers 24, as shown in FIG. 3, is achieved by axial movement of the drive dog shaft 20 to engage each of three vee-groove detent elements 28 provided on the magnet 18 with three further balls 30 provided on the rotor 14, and subsequently axially displacing the rotor 14 until the balls 26 are moved clear of the convergent surfaces of adjacently positioned pairs of rollers 24. The magnetic attraction between the magnets 18 and 22 ensures that the rotor 14 does not fall off during this operation. Axial movement of the drive dog shaft 20 is actuated by a motor 32, which also rotates the drive dog shaft 20, thereby to orient the rotor 14 in a different indexed angular position. Once oriented, the motor 32 is controlled to retract the drive dog shaft 20 and bring a further set of three balls 26 into engagement with the rollers 24. The motor 32 continues to withdraw the drive dog shaft 20 until the magnets 18,22 lose contact with each other. However, the magnetic force between the magnets 18,22 is sufficient to ensure the rotor 14 is .Iadd.biased toward and .Iaddend.retained securely upon the support 12 in the new indexed position. The probe 16 may thus be positioned at a plurality of angular indexed locations, at each of which the rotor 14 is kinematically supported on the support 12. The rotor 14 is thus in a stable and repeatable position relative to the support at each of the aforementioned angular positions. 
     At each of the indexed angular positions, the centre point O of the rotor 14 will lie at a fractionally different position relative to the axis A of the drive dog shaft 20. This is a result of the tolerances associated with positioning the ring of balls 26 and the rollers 24. The rotor 14 may thus be described as having a &#34;wandering axis&#34; relative to the support 12; nonetheless for practical purposes the rotor 14 is rotatable about the axis A. 
     When it is desired to exchange the probe 26 for a different type of probe, the entire rotor 14 is removed from the support 12, and a new rotor is mounted thereon. The dual function of the balls 26 and rollers 24 as both indexing elements and .[.retaining.]. .Iadd.position retention .Iaddend.elements thus serves to eliminate the existing releasable connection between a probe head and a probe. In a modification to the present invention, the magnet 18 may be provided at a plurality of positions around the drive dog shaft 20 on the fixed part of the support 12; this eliminates the need to provide a magnet 18 at the end of the drive dog shaft 20.