Abstract:
In the work-contacting probe head (or on the suspension of the deflectable work-contacting probe head) of a coordinate measuring machine, an electromagnet is provided to trigger a mechanical impact blow or vibration if the switch-contact circuit in the three-point mounting of the probe pin remains open for a predetermined short period of time after completion of a cycle of coordinate measurement of a given work-contacted measurement location. The delivery of the impact is sufficient to overcome any residual mechanical friction that may have interfered with return of the probe to its calibrated at-rest position, at completion of the measurement cycle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to probe-head construction for a coordinate-measuring instrument, wherein the probe head relies on self-contained electrical-switch means, in connection with assuring positioned reference for each measurement, and for other measuring purposes. Such probe heads are described, for example, in Federal Republic of Germany Pat. Nos. 2,347,633 and 2,712,181, and in Federal Republic of Germany OS No. 2,743,665; the corresponding U.S. patents for these German cases are U.S. Pat. Nos. 4,177,568, 4,153,998, and 4,136,458, respectively. In the known electrically switching probe heads, the mounting or seating location from which the probe pin lifts off, upon workpiece contact in the course of the probing process, is generally an isostatic three-point mounting formed of, e.g., paired balls; the paired balls of all three mounting points are connected as contact pairs in an electric ciccuit which is closed only when three electrically conductive mating members on the probe pin are fully seated against the respective ball pairs, to close all three switches and thus to electrically certify the at-rest position of the probe pin. The switching-contact feature is needed, for example, to supply machine-control means of the coordinatemeasuring instrument with trigger pulses for storing of length-measurement values at the time of work-contact. Furthermore, this switch-contact feature is used to report whether the probe pin has again returned to its seated reference position after completing each work-contacting procedure, it being understood that such seating determines the calibrated-zero position of the work-contact ball and therefore establishes that the probe head is ready for the next measurement. 
     It has now been found that, with an error frequency of about once every several thousand work-contact cycles, all three switch-contacts do not close upon termination of a work-contacting cycle. This error does not have any systematic cause but occurs due to the fact that the probe pin does not return entirely and precisely into its zero position, due to residual mechanical friction in its threepoint mounting. This frequency of error is, to be sure, very small, but in certain types of coordinate-measuring instrument operation, this error, i.e., a switch contact remaining open after a work-contact, has serious consequences. 
     Coordinate-measuring instruments are being used to an increasing extent in multiple-measurement operations and measure, for example during an unmanned night shift under CNC control (i.e., without attendants or operating personnel), a large number of workpieces which are clamped on a measurement table or fed continuously via a handling system. If such an error occurs in this type of operation, then the machine control stops the CNC operation for reasons of safety, and the coordinate-measuring instrument remains shut down until the beginning of the following shift, at which time the error can be noted and manually eliminated. It is clear that such interruptions in operation are extremely undesirable. 
     BRIEF STATEMENT OF THE INVENTION 
     The object of the present invention is reliably to assure closure of all switch contacts in the probe head of a coordinate measuring instrument, even in unattended, fully automatic operation of the instrument. 
     The invention achieves this object by providing, within the probe head or on the suspension for the probe head, means for delivering a mechanical-impact blow which can be triggered when the control system for a coordinate-measuring machine detects the occurrence of failure of switch-contact closure upon completion of a given measurement cycle, thus signifying failure of the probe to return to its zero-calibration or reference position. The mechanical-impact blow is sufficient to jar the probe-pin holder out of staticfriction retention of an erroneous at-rest position, allowing probe-head seating to complete the closure of all three switches. 
     The device for producing the impact blow can, for example, be developed as an electromagnet, an imbalance motor, electrically or pneumatically prestressed springs or a piezoelectric oscillator. In all cases, the closing of the switch contact takes place with the help of mechanical shock overcomes residual friction in the mounting of the probe pin. This energy blow is triggered, for example, by the machine-control means when all switch-closures have not been accomplished within a predeterminable period of time after the completion of the work-contacting cycle. 
     The the use of such a device, the reliability of coordinate measuring instruments in an automated series of operations can be substantially increased at relatively little expense. The expense necessary to obtain this result is particularly small for the case of probe heads which are already provided with an electromagnetic probe-pin changing device as described, for example, in European Pat. No. A2,128,464. In this case, the electromagnet which is relied upon to clamp a replacement probe pin the the probe-pin holder, and which is in any event already installed in the probe head, can be used to automatically produce the desired mechanical energy blow. 
     However, the device for producing the energy blow need not necessarily be installed directly in the probe head. It can, for example, also be arranged on the coordinate measuring instrument in the vicinity of the suspension or the mount for the probe head, since the mechanical blow is transmitted via mounting elements directly to the probe head. The work-contacting probe support is frequently developed as a disengageable mount in order, for example, to provide collision protection for the probe head. After a collision, the position of the probe head must be reproduced with high precision in order to avoid a recalibration of the probe-pin combination, and therefore the present invention offers the possibility of activating the mechanical-impact device after a contact-probe collision, as a result of which the contactprobe mount is &#34;shaken&#34; into its exact zero position by one or by several impacting blows. 
     In an advantageous use of the invention, the machinecontrol system of the coordinate measuring insturment contains a control module hwich, in the event of an erroneously open switch condition, interrupts the course of the measurement and moves the probe head back to the location of the most recently worked measurement cycle. 
     By this measure, the probe pin is again deflected at the most recent measurement location. Since there is only very small probability of the contact again remaining open during the course of this repeated measurement cycle, the error which has occurred may also be dependably eliminated by this procedure. The advantage of this solution resides in the fact that no change need to effected at the probe head itself or in its suspension and, furthermore, no electromagnetic probe-change device need to involved. Rather, the necessary measures are limited to an intervention in the electronic control system of the coordinate measuring instrument. 
     Although mention thus far has been made only in application to a coordinate measuring instrument, it is clear that the invention also has important application to a machine tool wherein a tool holder is automatically loaded with a probe head, in temporary replacement of a machining tool, for immediate measurement of the result of a given machining operation upon the workpiece; in this situation, the machine tool also operates as a measuring instrument. 
    
    
     DETAILED DESCRIPTION 
     The invention will be illustratively described in detail, in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a view in elevation of a probe head of electrical-switch tyhpe with means for producing a mechanical impact blow, the view being partly broken-away and in longitudinal section; 
     FIG. 1a is a simplified view in longitudinal section, to illustrate the invention in application to a probe-pin head having provision for interchangeable accommodation of different probe pins; 
     FIGS. 2 is a diagram schematically showing components for controlling the impact means of FIG. 1; and 
     FIG. 3 is a similar diagram for another embodiment. 
    
    
     The probe head of FIG. 1 comprises a cylindrical housing 1 having at its lower end an annular collar 1a which mounts three pairs of balls, at 120.° spacing between pairs. In the sectioned portion of FIG. 1, only one ball (6a) is visible. The three pairs of balls form the mounting or seat into which a probe-pin holder 2 having three radially protruding cylindrical members 5 is urged by a spring 4. The probe pin itself is designated 11 and is shown with a work-contacting probe-tip ball 3. Spring 4 rests via a pressure plate and a ball against an abutment 14 which is axially displaceable via an adjustment ring 15 which extends around the housing 1. Contact force of the probe pin 11 can be adjusted by means of ring 15. 
     As can be noted from FIG. 2, the three pairs of mounting balls 6 are electrically connected to one another and form contacts which are closed by the cylindrical members 5 of the probe-pin holder 2 in the at-rest position of probe pin 11. 
     Fixedly seated within the probe-pin holder 2, a soft-iron plate 7 (i.e., of low magnetic retentivity) faces a movable electromagnet 10; magnet 10 is urged by a second spring 8 against a stop ring 9 in such manner that a small air gap is produced between the magnet 10 and the soft-iron plate member 7. 
     The three switch contacts formed by the respective pairs of mounting balls 6 and the electromagnet 10 are separately connected to a control module 12 which may be a programmable &#34;firmware&#34; unit, interfacing the switch contacts and the electromagnet with a computer 13 associated with a coordinatemeasuring instrument (not shown). This control module 12 has the task, among other things, of transmitting to the computer 13 coordinates measured in the course of scanning displacements of the measuring machine and of reporting such malfunctioning errors as, for example, open contacts; and, in turn, control module 12 receives, from computer 13, control data for the measurement program to be executed. 
     The control module 12 will be understood to include a microprocessor so programmed that in the absence of closing all three switch contacts for a predetermined period of time, it briefly excites the electromagnet 10. In this connection, the electromagnet 10 strikes against the soft-iron member 7 and produces a mechanical-impact below by means of which a contact which has remained open as a result of static friction is jarred into closure of the circuit formed by the balls 6 and the cylindrical members 5. Accordingly, the indicated staticfriction malfunction is eliminated; no error is reported by the computer 13, and a CNC operation can be permitted to continue. 
     FIG. 1a shows application of the invention to a probe head having the probe-pin change feature of FIG. 1 of U.S. Pat. No. 4,637,119, to which reference is made for detailed description. It suffices here to identify the electromagnet of said patent and to state that, for convenience, significant reference numbers used in FIG. 1 of said patent are shown in 100-series notation in FIG. 1a herein; Thus, the electromagnet of FIG. 1 of said U.S. patent is identified 110 herein. In said patent, the electromagnet (110 herein) serves for probe-pin changing purposes, by so arranging and exciting the same as to selectively neutralize the field of a permanent magnet 111. The permanent magnet, via a steel armature plate 118, normally retains the probe pin in assembled relation to the probe-pin holder (116 to 121); but upon excitation of the electromagnet 110, the permanent-magnet field is neutralized, thus enabling the probe pin to be released. 
     In application of the invention to the changeable probepin holder of FIG. 1a, the electromagnet 110 is so controlled via its connection to control module 12 that to deliver the impact necessary to overcome static friction and thus close all pairs of switch contacts, the control module 12 is programmed to deliver to electromagnet 110 a polarized probe-pin releasing voltage pulse of only a few milliseconds duration, followed immediately by a pulse of opposite polarity, to aid the field of the permanent magnet and thus to attract the armature of the magnet. Due to its inertia, the probe pin responds to such a transient excitation of the electromagnet 10, by dropping only a fiew micrometers away from its mounting in the probe head, and the vibrational shock of the renewed clamping is sufficient to overcome the static-friction problem and to reset the switch-contact pairs for fully seated closure, thus eliminating the reported malfunction of incompleted switch closure. 
     Tests carried out with an interchangeable-probe device according to the embodiment of FIG. 1a were interrupted after 10 6  error-free work-contacting procedural cycles, since there was no occurrence of a switch contact remaining open after the described automatic instigation of the short jarring-impact cycle. 
     In the embodiment of FIG. 3, machine control of the coordinate-measuring instrument will be understood to include a control module 212 connected to the switch-contact pairs 205/206 in a probe head 201, wherein the microprocessor of control module 212 is so programmed that the measurement program transmitted by the associated computer 213 is interrupted only upon the absence of the switch contact. The probe head 201 is then retracted, via machine-displacement drive means 210, under control by module 212, to the most-recent prior position of contact with the workpiece 202. In this connection, another deflection of the probe pin 211 occurs, so that, upon retracing the machine displacement, all three switch contacts close with a very high degree of probability, and the error is eliminated.