Probe for measuring workpieces

A probe for measuring dimensions of a workpiece in coordinate positioning apparatus, comprises a base (20), a stylus holder (21) having an axis (21A), an intermediate member (22, 30) having a fixed region (22A) connected to said base and a free region (22B) connected to said holder, said intermediate member being resilient so as to be responsive to a displacing force (FW) acting on said holder in the sense tending to tilt or axially move said holder relative to said base, and sensing means (30) connected between said free region and said base for sensing the occurence of a said displacing force. The intermediate member is only sufficiently resilient to make possible said sensing. To cope also with the requirement of the probe over-running the sensing position there is provided a spring (25) connected in a state of partial deflection between said holder and said free end to urge the holder into engagement therewith with a spring force corresponding to said partial deflection, the holder being tiltably and axially displaceable from said engagement in opposition to said spring and being returnable to said engagement by said spring when said displacing force ceases, and wherein said spring is of lesser stiffness than said intermediate member.

This invention relates to a probe for measuring workpieces. 
British Pat. No. 1,573,447 shows a probe for measuring dimensions of a 
workpiece in coordinate positioning apparatus, comprising a base, a stylus 
holder having an axis, an intermediate member having a fixed region 
connected to said base and a free region connected to said holder, said 
intermediate member being resilient so as to be responsive to a displacing 
force acting on said holder in the sense tending to tilt or axially move 
said holder relative to said base, and sensing means connected between 
said free region and said base for sensing the occurrence of a said 
displacing force. 
In the known probe said intermediate member is sufficiently flexible to 
accommodate relatively large excursions, e.g. 10 mm, of the holder from 
its initial position. Although only a small excursion, e.g. 0.1 mm, is 
required for the purpose of said sensing, the relatively large excursion 
is provided to avoid collision damage when the stylus and the workpiece 
are brought into engagement. 
It has been found that the spring properties required for large excursions 
and/or the participation of the sensing means in the (large excursions) 
are not always compatible with properties required for sensitive sensing. 
It is an object of this invention to overcome or reduce this difficulty. 
The invention claimed provides spring means connected in a state of partial 
deflection between said holder and said free end to urge the holder into 
engagement therewith with a spring force corresponding to said partial 
deflection, the holder being tiltably and axially displaceable from said 
engagement in opposition to said spring means and being returnable to said 
engagement by said spring means when said displacing force ceases, and 
wherein said spring means is of lesser stiffness than said intermediate 
member. 
When a said displacing force occurs, then for so long as the displacing 
force is less than said pre-load, the force is accommodated entirely by 
said intermediate member. If the displacing force exceeds the pre-load, 
then, in view of the stiffness of the spring means being less than that of 
said intermediate member, the stylus can make a relatively large excursion 
accommodated substantially solely by said spring means. Accordingly, said 
intermediate member can be designed substantially solely with said sensing 
purpose in mind. In particular, relatively sensitive sensing means may be 
used because they are not subject to large excursions and because the 
spring means cushions the sensing means against the displacing force 
insofar as that force occurs in the form of impacts. Also, insofar as the 
sensing means have electrical cables connected to the base, these cables 
are protected insofar as they do not need to participate in the large 
excursions.

Referring to FIG. 1 the probe denoted 10 is intended to be supported by 
slides 11, 12 of a coordinate positioning aparatus 13 forming part of a 
measuring machine or other machine tool. The slides are movable relative 
to a workpiece 14 to bring the probe 10 into engagement therewith for the 
purpose of sensing the position of a surface of the workpiece and thereby 
determine the position of said surface relative to a datum. 
Referring to FIGS. 2 and 3, the probe 10 has a housing or base 20, a stylus 
holder 21 having an axis 21A a support member 31 for the stylus holder and 
three intermediate members 22 each of which is connected both to the base 
20 and through the intermediary of support member 31 to the stylus holder 
21. The support member 31 extends substantially in a plane A--A transverse 
to the axis 21A. The member 22 has a fixed axially upper region 22A 
secured to the base 20 and a free axially lower region 22B connected to 
the suport member. The stylus holder 21 has secured thereto a stylus 23 
having a free end 23A whereby to engage the workpiece 14. A spring 25 is 
connected between the holder 21 and the support member 31 in a state of 
pre-load, i.e. a state of partial deflection, thereby to urge the holder 
21 into engagement with the support member 31 with a force Fs 
corresponding to said pre-load. The holder 21 is supported on the support 
member 31 at a kinematic support 26 comprising a seat defined by three 
radial V-grooves 26A (FIG. 3) secured to the support member 31 and engaged 
by respective spherical elements 26C secured to the holder 21. The support 
26 ensures positive location of the holder 21 on the support member 31 in 
a rest position thereon. Sensors 30 (incorporated in the intermediate 
member 22) are situated between the support member 31 and the base 20 and 
are adapted to sense the occurrence of a force Fw acting on the free end 
of the stylus 23 due to engagement thereof with the workpiece 14. 
In the present example the sensors 30 are three piezoelectric crystals, 
secured, e.g. adhered, to the support member 31 and the base 20 in 
position therebetween. The crystals 30 are arranged at equispaced 
locations about the axis 21A of the stylus and effectively connect the 
support member 31 at its radially outer region to the base 20 thus 
indirectly connecting the stylus holder 21 to the base 20. In this example 
the crystals form part of the intermediate member and constitute resilient 
structure or strain-sensitive elements capable of responding to forces 
tending to tilt the holder 21, or to move the holder axially, relative to 
the base 20. The stiffness of the crystals 30 is greater than the 
stiffness of the spring 25. 
Referring now also to FIG. 4, the force Fw may rise from zero to a value B 
at which it balances the force Fs of the spring 25. This rise of the force 
Fw causes a deflection D1 in the sensors 30 and produces corresponding 
rise from zero in output signals 32 (FIG. 3) from the respective sensors. 
The condition of the probe in which the force Fw lies between zero and B 
is referred as the "sensing condition" and any position of the probe in 
that condition is referred to as the "sensing position". If the force Fw 
exceeds the value B, so as to cause a deflection D2 of the spring 25 
beyond the pre-load deflection, the probe is said to be in an "over-run 
condition". In the latter condition, and in view of the lesser stiffness 
of the spring 25 than the crystals 30, the holder 21 tilts relative to the 
support member 31, i.e. on the kinematic support 26, and the stylus can 
make an excursion which is significantly larger than its excursion during 
the sensing condition without a significant increase in the force Fw. In 
this way damage to the probe or the workpiece is avoided if, in operation, 
the slides 11, 12 cause the probe to over-run the sensing position. In the 
present example the value B of force Fw is of the order of 5 gram, the 
deflection D1 is less than 1 micrometer and the deflection D2 is of the 
order of 10 millimeter. 
As shown, the force Fw has a direction transverse to the axis 21A. If the 
force Fw acts axially on the stylus, the sensors 30 respond 
correspondingly and, in the over-run condition, the holder 22 separates 
axially from the support member 31. In any case, when the force Fw ceases 
on withdrawal of the probe from the workpiece, the spring returns the 
support member 31 to its rest position on the kinematic support 26 and the 
sensors return to a zero output condition. 
There are three said sensors 30 arranged in the plane A--A in equispaced 
relationship around the axis 21A (FIG. 3) and their outputs are connected 
to a circuit system 33 (known per se from British Pat. No. 1,573,447) for 
discriminating between forces acting on the stylus in the respective 
directions X, Y and Z of the coordinate system. Four or more sensors 30 
may be employed, all arranged in the plane A--A and around the axis 21A. 
The circuit system 33 may be embodied in a casing 34 (FIG. 2) arranged 
within the base 20, and each sensor 30 has cables 35 (FIG. 2) connecting 
the sensor to the circuit system. 
The arrangement illustrated provides good protection for the sensors 30 and 
the cables 34. Since the spring 25 is provided between the stylus and the 
sensors 30, the spring 25 cushions the sensors 30 against high impact 
forces which may act on the stylus if the probe is moved relative to the 
workpiece at high speeds. Further, the cables 35 undergo no significant 
flexing in the over-run condition of the probe. 
Referring to FIGS. 5 and 6, a probe 110 has a housing or base 120, a stylus 
holder 121 having an axis 121A, and an intermediate member 122 connected 
to the base 120 and stylus holder 121 in position therebetween. The member 
122 extends substantially in a plane A--A transverse to the axis 121A and 
comprises three limbs 127 extending tangentially to a circle about the 
axis 121A. The one ends of the limbs 127 are integral with a ring 128 
secured to the base 120 and defining a fixed radially outer region 122A of 
the member 122. The other ends of the limbs 127 which define a free 
radially inner region 122B of the member 122 are integral with a centre 
body 129 defining a support member for the holder 121 which is supported 
thereon through a kinematic support 126. The holder 121 has secured 
thereto a stylus 123 having a free end 123A whereby to engage the 
workpiece 14. A spring 125 is connected between the holder 121 and the 
centre body 129 in a state of pre-load, i.e. a state of partial 
deflection, thereby to urge the holder 121 into engagement with the 
support 126 with a force Fs corresponding to said pre-load. The support 
126 ensures positive location of the holder 121 on the centre body 129 in 
a rest position thereon. Sensors 130 connected between the free region 
122B and the base 120 are adapted to sense the occurance of a force Fw 
acting on the free end of the stylus 123 when the stylus engages the 
workpiece 14. The limbs 127 are capable of resilient bending about their 
fixed ends and out of the plane A--A. The stiffness of the limbs 127 is 
greater than that of the spring 125. 
The sensors 130 are strain gauges secured to reduced thickness portions of 
the limbs 127. Four strain gauges 130 are provided on each of the limbs 
127 and are connected in a bridge circuit 136 (known per se) to produce a 
single output 132 indicative of the bending strain in the limb. The 
outputs of the respective circuits 136 are connected to a circuit system 
133 similar to the system 33 (FIG. 3). In the sensing condition of the 
probe 110 the force Fw is sensed by the strain gauges 130 responding to 
deflection of the limbs 127 and in the over-run condition the holder 121 
tilts on, or axially separates from, the centre body 129. 
Referring to FIGS. 7 and 8, a probe 210 has a housing or base 220, a stylus 
holder 221 having an axis 221A, a support member 231 for the stylus holder 
and an intermediate member 222 connected to the base 220 and to the 
support member 231. The member 222 extends substantially in a plane A--A 
transverse to the axis 221A and comprises three limbs 227 extending 
radially in respect of the axis 221A. The one ends of the limbs 227 are 
integral with a ring 228 secured to the base 220 and defining a fixed 
radially outer region 222A of the member 222. The other ends of the limbs 
227 define a free radially inner region 222B of the member 222 and are 
integral with the support member 231 which supports the holder 221 through 
a kinematic support 226. The holder 221 has secured thereto a stylus 223 
having a free end 233A whereby to engage the workpiece 14. Springs 225 are 
connected between the holder 221 and the support member 231 in a state of 
pre-load, i.e. a state of partial deflection, thereby to urge the holder 
221 into engagement with the member 231 with a force Fs corresponding to 
said pre-load. The support 26 ensures positive location of the holder 221 
on the member 231 in a rest position thereon. Sensors 230 connected 
between the free region 222A and the base 220 are adapted to sense the 
occurance of a force Fw acting on the free end of the stylus 223 when the 
stylus engages the workpiece 14. The limbs are capable of resilient 
bending about their fixed ends and out of the plane A--A. The stiffness of 
the limbs 227 is greater than that of the spring 225. 
The sensors 230 are strain gauges secured to the limbs 227. The strain 
gauges 230 have outputs 232 processed through a circuit system 233 in the 
same way as the outputs of the sensors 130 (FIG. 5). The member 222 
differs from the member 122 (FIG. 5) in that the limbs 227 extend radially 
in respect of the axis 221A. This simplifies the construction of the 
member 222 but makes it virtually impracticable to unite the inner ends of 
the limbs 227 by a centre body such as 129 (FIG. 5) because, on 
application of the force Fw, the limbs 227 would then have to deflect in 
tension as well as in bending. To cope with this situation the support 
members 231 attached to the radially inner ends of the limbs 227 are left 
free and the very small amount of relative movement, required between the 
limbs 227 and the holder 221 when the limbs are deflected, is accommodated 
by the kinematic support 226. To this end the support 226 comprises at 
each member 231 two radial, mutually confronting V-grooves 226A, 226B 
provided respectively in the members 221, 231. A ball 226C engages both 
V-grooves 226A, 226B and the balls 226C at the three members 231 are 
connected by a cage 226D. Deflection of the limbs 227 is accommodated by 
the balls 226C rolling in the grooves 226, 226B. In the sensing condition 
of the probe 210, the force Fw is sensed by the strain gauges 230 
responding to deflection of the limbs 227 and in the over-run condition 
the holder 221 tilts on, or axially separates from the members 231. 
Referring to FIGS. 9 to 11, a probe 310 has a housing or base 320, a stylus 
holder 321 having an axis 321A, a support member 331 for the stylus holder 
and an intermediate member 322 connected to the base 320 and support 
member 331. The member 322 extends substantially in a plane A--A 
transverse to the axis 321A and comprises three limbs 327 extending 
annularly around the axis 321A. The one ends of the limbs 327 are integral 
with the base 320 and define a fixed region 322A of the member 322. The 
other ends of the limbs 327 are integral with the support member 331 and 
define a free region 322B of the member 322 connected to the holder 321. 
The limbs 327 are capable of resilient bending about the ends thereof at 
the fixed region 322A and out of the plane A--A. The holder 321 has 
secured thereto a stylus 323 having a free end 323A whereby to engage the 
workpiece 14. A spring 325 is connected between the holder 321 and the 
support member 331 in a state of pre-load, i.e. a state of partial 
deflection, thereby to urge the holder 321 into engagement with the member 
331 with a force Fs corresponding to said pre-load. The engagement between 
the holder 321 and members 331 occurs at a kinematic support 326 which 
ensures positive location of the holder 321 on the member 322 in a rest 
position thereon. Sensors 330 connected between the free region 322B and 
the base 320 are adapted to sense the occurrence of a force Fw acting on 
the free end of the stylus 323 due to engagement thereof with the 
workpiece 14. The stiffness of the limbs 227 is greater than the stiffness 
of the spring 327. 
The sensors 330 are inductive devices known per se and each comprising a 
core 330A movable along the axis of a coil 330B, the core being secured to 
the plate 331 and the coil being secured to the base 320. There are three 
sensors 320 equispaced about the axis 321A and each having an output 332. 
The coil axis of each sensor is parallel to the axis 321A. The outputs 332 
are connected to a circuit system 333 the same as the system 33 (FIG. 3). 
In the sensing condition of the probe 310 the force Fw is sensed by the 
sensors 330 responding to deflection of the limbs 327 and in the over-run 
condition the holder 321 tilts on, or axially separates from the member 
331. The inductive sensors 330 may be replaced by capacitative sensors, 
i.e. any reactive sensor may be provided for the purpose of the invention. 
On the other hand the sensors 330 may be replaced by optical devices known 
per se, involving e.g. reflective mirrors, to provide a signal of the 
movement of the member 331. 
The limbs 327 are integral with the base 320 and the member 331 
respectively as shown in FIG. 11. The sensors 330 are sited adjacent the 
free regions 322B. The spring 325 is arranged between the member 321 and a 
member 335 secured to the plate 331 by pillars 336.