Probe head mount for a deflectable probe or the like

Truncated pyramid mounting configurations and relationships are disclosed as inherently and reproducibly determining an accurate zero-position as between a probe head and a probe pin that is deflectably carried by the probe head. And the same principle of accurately determining zero position is also described in application to the automated releasable chucking of interchangeable probe pins and/or of an auxiliary probe head, as to the installed probe head of a coordinate-measuring machine. In one described case of truncated triangular pyramid mountings, the zero position is determined by concurrent engagement of six seating points which are distributed radially and axially with respect to the common central longitudinal axis of the involved geometric pyramids. The mounting configurations have high torsional rigidity and a relatively large region within which self-centering to the zero position is achieved. Furthermore, thise mounting configurations are characterized by an azimuth-angle distribution of relatively uniform force in opposition to a probe-pin or the like deflection, and associated lift-off, from the fully seated zero position.

Background of the Invention 
Mechanical probes of the so-called "switching" type, as are used in 
coordinate-measuring techniques, have a movable probe or probe pin which 
is spring-urged into a bearing system which reproduces with high precision 
the zero position of the probe or probe pin. At the instant of contact 
with a workpiece, the probe is lifted out of its bearing against the force 
of the spring and then produces a pulse-like switch signal which 
characterizes the contacting. In mounting the probe pin, it is customary 
to use the so-called "kinematic three-point mount" described in U.S. Pat. 
No. 4,153,998 in which three radially extending cylindrical arms can seat 
in V-bearings which are defined by balls or rollers. This arrangement 
provides excellent reproducibility, and it is furthermore torsionally 
rigid with respect to rotation about the longitudinal axis of the probe 
pin. 
Similar mounts are also used for attaching a selected one of a plurality of 
replaceable probe pins to a replacement holder on the probe head of a 
coordinate-measuring instrument. Such a replacement holder or chuck is 
described, for example, in U.S. Pat. No. 4,637,119. 
From the journal "Microtecnik" of 2/1986, pages 43-46, it is furthermore 
known to provide such a mount for the replaceable attachment of complete 
probe heads on the measurement arm of a multiple-coordinate measuring 
machine. 
The described three-point mounts have various disadvantages. On the one 
hand, the region within which the part to be held is self-centering when 
returning to its seated or at-rest position is relatively small, since 
this region is determined by the diameter of the pairs of balls of the 
mount. And a further disadvantage is encountered when mounting a probe pin 
in a switching probe head. Specifically, the contacting force, i.e., the 
force which the probe pin exerts on the workpiece which is to be measured, 
is direction-dependent; it changes by a factor of two depending on 
whether, when deflected, the probe pin tilts directly above one of the 
three points of at-rest support, or is tilted about the connecting line 
formed by two of these three points. This angular dependence of contact 
force is a source of measurement error in the determination of 
work-contact coordinates. Corresponding studies have been published in the 
journal "Technisches Messen TM" 1979, issue No. 2, pages 47-52 and 
161-169. 
Federal Republic of Germany Patent No. 3,229,992 discloses a probe head 
having a probe mount which is a combination of a truncated cone and a 
torus. Admittedly, work-contact force is independent of the direction of 
probe deflection, since the bearing is symmetrically engaged for the zero 
or at-rest condition. However, the probe pin is not secured against 
rotation about its longitudinal axis. Thus, with this probe mount, it is 
not possible to use probe pins having any probe balls which are 
eccentrically positioned, i.e., off the longitudinal axis of the probe 
pin. 
Federal Republic of Germany Patent No. 3,603,269 discloses a probe head 
with a probe-pin mount which has an anti-rotational feature, to prevent 
rotation of the probe pin about its longitudinal axis. For this purpose, 
the base end of the probe pin is elongate and relies upon longitudinally 
spaced reference locations within a cylindrical probe-head housing. At one 
of these locations, a first part of the probe-pin shaft is developed as a 
truncated pyramid which will seat against three balls in a fixed part of 
the housing. A second similar mount at a different one of these locations 
involves a second part of the probe-pin shaft. The two parts of the 
probe-pin shaft are ball-guided for relative longitudinal displaceability 
and a spring urges the probe-pin shaft parts in their respective 
directions to seat at their respective longitudinally spaced locations. 
Probe-pin deflection is accompanied by an unseating of one of the mounts. 
The latter probe head has various disadvantages. On the one hand, the 
construction is complex and manufacture is relatively difficult; also, the 
quality of the ball guide affects the accuracy of probe-pin mounting 
Furthermore, the torsional rigidity of the probe pin is not particularly 
great, since the surfaces of the truncated-pyramid part of the probe pin 
contact the balls of the mount centrally, i.e., at locations where forces 
to produce restoring torque are very small. 
Brief Statement of the Invention 
The object of the present invention is to provide a mount for a probe head 
on a coordinate measuring instrument, for a probe in a probe head, or for 
a replaceable element on a probe head, which mount is of the simplest 
possible construction, is torsionally rigid, and reproducibly centers 
itself automatically to its zero position, from within the largest 
possible deflection range. 
The invention achieves this object by providing a geometric array of at 
least six seating points of mounting contact to establish the zero or 
at-rest position, as between a deflectable-probe or the like component and 
a mounting component with respect to which the probe or the like component 
is deflectable or otherwise movable. One of these components has a 
longitudinal axis about which the at least six seating points are 
distributed in radially spaced, axially spaced and angularly spaced 
relation; and this distribution is over each of the geometric lateral 
sides of the frustum which results from truncating a regular pyramid, 
wherein the said longitudinal axis is the longitudinal axis of the 
pyramid. 
In addition to the advantages which have been stated within the above 
object, it must be emphasized that a probe which has been thus mounted to 
a probe head exhibits practically no dependence upon work-contact force 
directed away from the zero position. Since, in accordance with the 
invention, punctiform or areal resting surfaces cooperate with relatively 
large mating surfaces, the self-centering action of the mount is assured 
within a very large region. 
As compared to the probe head described in said German Patent No. 
3,603,269, a probe head of the invention is of substantially simpler 
construction, inasmuch as the zero position of the mounted component is 
determined by at least six simultaneously operative seating points of a 
single bearing support, and a second, axially spaced mating mount can be 
avoided. And since the seating points are spaced from each other and are 
offset from the center lines of the side surfaces of the truncated 
pyramid, the mount is of great torsional rigidity. 
When the pyramid is three-sided, i.e., a triangular pyramid, it is 
advisable to develop the mount such that the movable component develops 
its zero position in the mounting component, via six discrete points of 
seating contact. The at-rest position of the movable component is then 
kinematically unambiguously determined; furthermore, bearing friction is 
minimized by reason of punctiform seating-point contacts, so that 
reproducibility of the zero position of the movable component is 
excellent. Elongate regions of line contact, as established by knife edges 
or by cylindrical rollers, are also suitable for accurate zero-position 
determination, in that each line of contact is in reality a linear 
succession of point contacts. And the indicated advantages of point 
contacts or line contacts can also be achieved by provision of fluid or 
air-bearing action between matched coacting surface areas of like 
truncated positive and negative pyramid configuration, for friction-free 
determination of the at-rest or zero position. 
In order to reduce the friction and increase the precision of attaining the 
zero position, it is advisable for the seating-contact points of the mount 
to be formed by inserts of a material which is hard as compared with 
remaining body material of the respective components. 
If balls are selected to establish the discrete seating points, they can be 
retained with rotational mobility in a ball cage, thus further reducing 
friction. It is then advisable to produce the ball surfaces and 
ball-engaging mating surfaces with such accuracy that a displacement of 
the balls and thus of the seating points does not result in positional 
uncertainty of the movably mounted component. 
With respect to torsional rigidity of the movably mounted component, it is 
particularly advantageous for the truncated pyramid of the mount to have 
the shape of a three-sided pyramid, i.e., a triangular pyramid. Such a 
pyramid makes it possible so symmetrically to arrange the seating points 
so that each of the three surfaces of the pyramid has two of six seating 
points. In such an arrangement, the reaction force sustained by the mount 
by reason of a given tilting deflection of the probe-pin axis is very 
uniform, regardless of the direction of such deflection. 
Present use of the expression "truncated pyramid" or "pyramid frustum" is 
to be understood as being descriptive of geometric surface configurations 
which determine the plural seating points in space. Admittedly, the two 
components of the mount itself, i.e., the bearing seat in the "fixed" 
component and the body of the movable component which has a received zero 
position with respect to the "fixed" component can have the shapes of 
matching positive and negative pyramids, respectively. However, this is 
not absolutely necessary. Thus, it is always sufficient if only one of the 
two coacting parts, i.e., either the fixed or the movable component, has 
the shape of a truncated pyramid, depending (1) on which component is to 
provide each particular seating-surface area, and (2) if the coacting 
seating points on the other component are suitably interconnected and 
positioned with respect to each other. It is also to be understood that 
the seating-surface areas alone are the only fragments of the pyramid that 
are needed in the region of the seating points. The rest of the involved 
component can be replaced by some other shape, as long as suitable 
precaution is taken to avoid interference with the self-centering action 
of the movable component. 
Thus, for example, the corners of the truncated pyramid can be cut off or 
partially rounded, as a result of which the bearing base, i.e., the 
distance between spaced seating points, referred to a given outside 
diameter of the mounting, can be increased.

The probe head of FIGS. 1 and 2 has a cylindrical housing 1 which can be 
secured by means of a cap nut 2, directly or via an extension piece, to 
the measurement arm of a coordinate-measuring instrument. At its top end, 
housing 1 is provided with a connector 10 having contact pins for 
different electrical connections to the probe head. 
Housing 1 tapers down conically at its opposite end, where it is internally 
configured to define a bearing part 7 for mounting the base 5 of a probe 
pin 6 that is movable with respect to housing 1. The base 5 which is 
movably mounted in bearing part 7 is continuously urged into its zero 
position by a spring 3. A piezoelectric sensor 17 between probe parts 5 
and 6 provides recognition of a workpiece contact in the course of a 
measurement procedure, by producing an electrical signal at the instant 
when the probe pin 6 makes contact with the workpiece. The operation of 
such a sensor is described in detail in U.S. Pat. No. 4,177,568 and 
therefore need not now be repeated. 
In order to recognize the zero position of probe pin 6, a light-emitting 
diode 11 and a four-quadrant diode 13 are fixedly mounted to and within 
housing 1, and a mirror-backed lens 12 is fixedly mounted to the upper end 
of the probe base 5. Lens 12 focuses the luminous surface of 
light-emitting diode 11 on the quadrant diode 13, and the signal output of 
diode 13 serves to verify the initial work-contact signal of the 
piezoelectric sensor 17, the same being produced in the course of a 
work-contacting procedure. The electronic system in which these two 
signals of the probe are processed is designated 14 (FIG. 3a). Such a 
device is also known from said U.S. Pat. No. 4,177,568 and therefore need 
not be described here. 
As can be noted from the sectional view in FIG. 2 and the perspective view 
in FIG. 3a, base 5 of the probe pin 6 has the shape of a truncated 
triangular pyramid which, at six discrete seating points, makes contact 
with correspondingly shaped mating surfaces of the bearing mount 7 at the 
lower part of housing 1. The seating points derive from convex formations 
on the probe base 5, and these formations are designated 9(a-c) and 8(a-c) 
in FIGS. 2 and 3a. 
In this first embodiment, each of the three faces of the truncated pyramid 
establishes two associated seating points. These pairs of points 8a/9a, 
8b/9b and 8c/9c are not only radially offset from the central axis of 
probe 5, 6 but they also lie off the centerlines, designated 10(a,b and c) 
in FIGS. 2 and 3(a), of the faces of the truncated pyramid; specifically, 
three seating points 8(a-c) near the broader base end of the truncated 
pyramid are arranged to the left of the centerlines 10(a,b,c), and the 
three remaining seating points 9(a-c) are spaced apart axially in the 
direction towards the vertex of the pyramid and are disposed to the right 
of the corresponding centerlines 10(a,b,c). This radial and axial spacing 
of the seating points assures the torsional rigidity of probe mounting to 
the probe-head housing 1. 
The six seating points 8 and 9 can be either convex-spherical or 
cylindrical formations, and they may be fixedly located either on the 
probe base 5 or on the inner surfaces of the bearing part 7. However, for 
wear-resistance and low friction, it is recommended, that, whether 
spherical (FIG. 4b) or cylindrical (FIG. 4a), the point formations 8 and 9 
shall be features of the probe base and that seating engagement shall be 
to local-surface inserts 24(34) of hard metal, in the otherwise relatively 
soft material, e.g., aluminum 25(35), of the bearing part 7. The actual 
point for seating engagement at each of locations 8 and 9 is then 
established by the polished end of a hard-metal pin 26 (or by a hard ball 
36) retained in the otherwise relatively soft material of the probe base 5 
and in contact with a corresponding hardened seating surface 24(34). 
The precision of the six-point mounting depends, inter alia, on the bearing 
base, i.e., the radial distance of the seating points from the axis of 
symmetry of the truncated pyramid. For a given outside diameter of the 
probe head, this bearing base can be increased by beveling the corners of 
the truncated pyramid, as in the embodiment 105/106 shown in FIG. 3b. 
Another embodiment, which constitutes an alternative with respect to the 
arrangement of the six seating points, is shown in FIG. 3c, wherein three 
seating points 19(a,b and c) are associated with one face of the pyramid, 
two seating points 19d and 19e are associated with a second face, while 
the third face has only one point 19f of seating engagement with the 
corresponding surface of the bearing part 7 in the probe head. In this 
case again, several (namely four) of the seating points are arranged on 
opposite sides of the centerlines of surfaces of the truncated pyramid 
(namely, points 19(a,b) of the one face, and points 19(d,e) of the second 
face), thereby assuring torsional rigidity of the probe-mounting 
configuration. 
In the probe embodiment of FIG. 5, no discrete seating points are provided; 
rather, seating regions are developed along a straight-line seating 
engagement for each of the three faces of the base 45 of probe pin 46, and 
base 45 is developed as a truncated pyramid, having a cylindrical roller 
49(a,b,c) of hard metal embedded in each of the three faces of the 
pyramid. The rollers 49(a,b,c) are arranged obliquely with respect to the 
centerlines of the faces of the truncated pyramid, and they cross said 
centerlines to assure the torsional rigidity of the probe-mounting 
configuration. 
The embodiment of FIG. 10 shows that the probe base to be seated at 7 need 
not itself have the shape of a pyramid or of a truncated pyramid. In FIG. 
10, three arms project radially from the probe-pin shaft 96, at an angular 
spacing of 120.degree., and at each of two axially spaced locations 95a 
and 95b; each of the three longer arms of the upper set terminates in a 
seating ball 98(a,b,c), and each of the shorter arms of the lower set 
terminates in a seating ball 99(a,b,c). These balls are so arranged in 
space that they lie as paired balls (98a-99a, 98b-99b, 98c-99c) on the 
respective geometric inner faces of the frustum of a truncated triangular 
pyramid. Since the upper and lower three-ball sets 98(a,b,c) and 99(a,b,c) 
are at angular offest with respect to each other about the longitudinal 
axis of the probe pin, the principle of FIGS. 1/2 is again satisfied, for 
torsionally rigid mounting of probe pin 96, when the associated bearing 
part 7 is formed as a negative truncated triangular pyramid having the 
same apex angle. 
Experiments have shown that reproducibility of the fully seated (zero) 
position of the probe pin is particularly good if the apex half-angle that 
each truncated-pyramid face makes with the central axis of the probe pin 
is the same, and when such angle is selected in the range 35.degree. to 
55.degree.. 
In the probe head of FIG. 6, the seating relationship between (a) the 
truncated-pyramid base 55 of the probe pin 56 and (b) the bearing 
configuration within part 57 of the housing 51 is an area-to-area 
relationship between corresponding truncated-pyramid surfaces, as 
distinguished from the punctiform or straight-line seating engagements (of 
above-described embodiments) with local surfaces of a geometric pyramid. 
In order to avoid friction between the surfaces which seat upon each other 
and which would prevent a reproducible return of the probe pin 57 to its 
zero position, each of the inner wall surfaces (of truncated-pyramid 
configuration) of the mounting part 57 is developed as an air bearing. 
This does not result in any particularly great expense since 
coordinate-measuring instruments are usually mounted on air bearings, and 
thus a compressed-air supply for the inlet port 59 of probe head 51 is 
readily available. The base 55 of probe pin 56 therefore rides on an air 
cushion 58 which is about 2-micrometers thick; consequently, in the 
absence of a probe-deflecting force, the probe-pin base 55 can at all 
times return free of friction to the zero position determined by its 
bearing mount 57. 
In each of the embodiments described above, probe pins are shown in which a 
deflectable or yieldable probe is served with a mounting embodiment of the 
invention. FIG. 7 now shows application of the invention to a chucking 
device, suitable for use on measurement machines for the automatically 
controlled interchangeable chucking of different probe pins on a given 
probe head, or of a complete auxiliary probe head on a given probe head. 
In FIG. 7, the "fixed" receiving part of the chuck is designated 61. This 
receiving part 61 has the shape of a truncated triangular pyramid at its 
bottom end, the pyramid faces visible in the section of FIG. 7 being 
designated 67a and 67b. Within this receiving part 61, a combined 
permanent/electromagnet 62 is axially displaceable against the force of 
the spring 63, and in the direction toward the apex of the pyramid. Magnet 
62 attracts an armature plate 70 in the chuck-compatible part 65 to be 
interchangeably held and thus holds this part, which is internally 
developed as a negative truncated pyramid, against the pyramid surfaces 
67(a-c) of the mount. 
The operation of the electromagnetic chucking device need not be described 
here since it operates, in principle, in the same way as the chucking 
device described in the aforementioned U.S. Pat. No. 4,637,119. 
The interchangeable part 65, which is to be releasably chucked, is provided 
with six discrete seating points on its mating surfaces, and these seating 
points can, for example, have the three-dimensional arrangement shown in 
FIG. 3a or FIG. 3c, with respect to the faces of the truncated pyramid. At 
its lower end, the interchangeable part 65 is shown to include a mounting 
cube 66 to which several probe pins can be threaded in different 
alignments. With correspondingly large dimensioning, however, complete 
probe heads can also be carried by the interchangeable part 65, in which 
case not only mechanical probe heads but also optical probe heads such as, 
for example, so-called triangulation probes, can be used. 
In FIGS. 8 and 9, the interchangeable part of FIG. 7 is again shown in a 
slightly modified embodiment, wherein the carrier or receiving part is 
designated 71, and the combined permanent/electromagnet is designated 72. 
In contrast to FIG. 7, the seating points are developed as retained balls 
which are free to roll and which lie between (1) the three faces of the 
truncated pyramid on the receiving part 71 and (2) the corresponding three 
faces of the negative pyramid on the interchangeable part 75. As can be 
noted from the enlarged showing in FIG. 9, the balls 78 and 79 are 
retained between the surfaces 77b and 77a in a ball retainer 80, as of 
suitable plastic; and retainer 80 is held by pressing an integrally formed 
detent portion 81 of the retainer into an undercut groove or socket 82 in 
the receiving part 71. The ball retainer 80 will be understood to have the 
shape of a truncated pyramid and to position all six balls of the mount, 
being fastened to the receiving part 71 at each of several places, in the 
manner described for the detent engagement 81/82 in FIG. 9. 
It is to be understood that, for simplicity of description, all embodiments 
have been described in the context of punctiform or other configurations 
involving truncated triangular pyramids, which are to be understood as 
preferably having three like isosceles-triangular sides, rising from an 
equilateral-triangle base, and truncated at a smaller 
equilateral-triangular section which is parallel to the base. But what has 
been said as to usefulness of triangular-pyramid relationships for 
repeatably determining a zero position is also applicable to 
polygonal-pyramid relationships having greater numbers of sides, as for 
example, the four sides of a square pyramid, truncated as described. 
Furthermore, the seating points need not necessarily protrude from the 
retained part, they can also be arranged in the bearing mount, or they can 
be arrayed partially in the mount and partially on the retained part.