Arrangement for measuring dimensions of a workpiece

The arrangement of this invention for measuring the dimensions of a workpiece (14) includes a circuit in which a fluid circulates. Such circuit comprises a first branch (15) provided with an inlet nozzle (16) and at least one measuring nozzle (17) directed towards the workpiece the dimensions of which are to be measured, and a second branch (19) provided with an inlet nozzle (20) and opening into an output reference nozzle (21). A pressure transducer (22) furnishes an electrical signal representing the pressure difference in the two branches. The arrangement is original inasmuch as the fluid employed is a liquid and that the inlet nozzles are arranged to deliver a liquid flow at their output producing a minimum of noise on the electrical signal issuing from the pressure transducer. Utilization is for in-process measurement of cylindrical bores or outer diameters of cylindrical workpieces or indeed of planar surfaces.

This invention concerns an arrangement for measuring dimensions of a 
workpiece and comprises a circuit in which a liquid circulates, such 
circuit including a first branch provided with an inlet nozzle and at 
least one measuring nozzle directed towards the workpiece the dimensions 
of which are to be measured, and a second branch provided with an inlet 
nozzle and opening into an output reference nozzle, a pressure transducer 
furnishing an electrical signal representing the difference of the 
pressures prevailing in the two branches of the circuit, and means for 
connecting the inlet nozzles to a source of regulated pressure. 
BACKGROUND OF THE INVENTION 
Such a measuring arrangement is known and is described for instance in the 
standard DIN 2271 where air under pressure is employed in place of the 
liquid used in the present arrangement. A similar arrangement is described 
in the patent document EP-B-0 109 936 (=U.S. Pat. No. 4,538,449). 
As is seen in FIG. 1, the arrangement of the cited document consists in 
feeding via a pressure regulator 1 a pneumatic circuit comprising a first 
branch 15 provided with an inlet nozzle 16 and a measuring plunger 4 
adapted for instance to be introduced into a bore 5 the diameter of which 
is to be measured, as well as a second branch 19 provided with an inlet 
nozzle 20 opening into a reference nozzle 21, the flow through which may 
be regulated. A pressure transducer 22 connected between branches 15 and 
19 furnishes an electrical signal representing the difference of the 
pressures prevailing in such branches, which signal is amplified by an 
amplifying arrangement 10 prior to being led to an indicating arrangement 
11 for instance. It is thus clear that if the plunger 4 is introduced into 
a larger bore, for instance, the pressure prevailing in branch 15 will be 
reduced and the variation of the pressure differential detected by the 
transducer 22 will be representative of the increase in diameter of bore 
5. The essential interest of the differential assembly is to absorb 
automatically noise variations of pressure due, for instance, to the 
functioning of the pressure regulator 1 or to temperature effects. 
The object of the invention described in patent document EP 0 109 936 is 
that of proposing an improvement in the known arrangements which permits 
considerable reduction in the measurement noise as well as the time 
constant of the measuring arrangement and sensitivity to vibration, while 
enabling an adjustment which is simple and independent of the sensitivity 
of the arrangement and of the null point of the output signal. 
To this end, the cited invention provides the use of a differential 
pressure transducer using semi-conductor elements and arranging the 
assembly of the transducer and the pneumatic circuit within the measuring 
plunger. While leaving one at liberty to define the pneumatic circuit in 
an optimum manner so as to limit as far as possible the production of 
measurement noise, such solution enables the obtaining of a measuring 
arrangement entirely insensitive to vibrations and the time constant of 
which, on the order of a tenth of a second, is sufficiently small to 
enable obtaining extremely rapid measurements, for instance between two 
penetrations of an oscillating machine tool into a workpiece or measuring 
discontinuous surfaces of a workpiece in motion. 
In spite of the improvements which it brings, the system of the cited 
invention gives rise to several difficulties, all coupled to the fact that 
the fluid employed is air under pressure: 
The cutting liquid employed for cooling and lubricating the workpiece to be 
machined is often found to be a mixture based on petrol. If air under 
pressure is employed for the measurement of the dimensions of the 
workpiece at the same time as this is being machined, the cooling petrol 
may be pulverized by the air and thus bring about explosions should the 
cutting tool produce sparks. Generally as well, the cutting liquid thus 
pulverized may be harmful for the health or at least provoke general 
discomfort and dirtying of objects located in the surroundings of the 
machine. 
It has been indicated that the time constant of the measuring arrangement 
is on the order of a tenth of a second (100 ms). This reaction time is 
connected to the fact that the fluid employed is air, basically 
compressible. In certain cases, the attained time of 100 ms slows the 
machining process, for instance in the machining of the truing of interior 
bores where the coming and going of the grinder could be much more 
frequent within a predetermined time lapse if the dwell time of the 
measuring arrangement within the piece could be shortened. Because of the 
elasticity of the air employed for the measurement, the pressure requires 
a certain time to be established and to be stabilized at the places of 
measurement which thus prevents rapid machining since it is necessary to 
proceed with a measurement which lasts a relatively long time following 
each passage of the tool. 
It will be further noted that in the cited document the time constant of 
100 ms has been capable of attainment thanks to the fact that the 
measurement arrangement and the measuring plunger form only a single part, 
thus diminishing the length of the paths travelled by the air. This 
advantage leads however to the providing of a heavier and larger apparatus 
which is poorly suited to the measurement of small workpieces. 
The cited difficulties have already been anticipated by the applicant of 
this invention who has proposed, in an arrangement called Hydrocompar, the 
replacement of the air in the known systems by a liquid to overcome said 
difficulties. 
By employing a liquid in the place of air, it has been possible to reduce 
considerably the time constant of the system and times on the order of 10 
ms have been observed, which improves by 10 times the system response 
relative to systems formed according to the document cited hereinabove. 
This is due principally to the almost total incompressibility of the 
liquid. The liquid employed, as well as no longer being followed by 
pulverization effects, no longer requires construction of a clumsy and 
sometimes inconvenient compact system. 
The Hydrocompar arrangement cited hereinabove uses input nozzles identical 
to those employed with arrangements employing air, and this has the 
disadvantage of creating undesirable turbulence in the liquid. Such 
turbulences bring about substantial background noise onto the pressure 
transducer. 
SUMMARY OF THE INVENTION 
To avoid the cited difficulties, the measuring arrangement according to 
this invention is characterized by the fact that the inlet nozzles are 
equipped with means for furnishing at their output a liquid flow producing 
minimum noise on the electrical signal issuing from the pressure 
transducer. 
The invention will be understood now with reading of the following 
description illustrated by the drawings giving practical examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a very general manner, FIGS. 2 and 3 show two different embodiments of 
the invention in which the measuring arrangement includes a circuit in 
which a liquid circulates. Such circuit comprises a first branch 15 
provided with an inlet nozzle 16 and at least one measuring nozzle 17 
directed towards the workpiece 14, the interior dimensions 13 of which are 
to be measured. The circuit further comprises a second branch 19 provided 
with an inlet nozzle 20 and opening into an output reference nozzle 21. 
The arrangement further comprises a pressure transducer 22 furnishing an 
electrical signal on its terminals 23, such signals being representative 
of the difference in pressures prevailing in branches 15 and 19 of the 
circuit. FIGS. 2 and 3 further show that a nipple 24 enables coupling the 
inlet nozzles 16 and 20 to a regulated pressure source (not shown). 
More specifically, FIGS. 2 and 3 show arrangements furnished with a 
measuring plunger 2, respectively 12 on the surface of which open out the 
measuring nozzle 17. 
In the case of the arrangement of FIG. 2, the system includes a measuring 
unit 3 including the inlet nozzles 16 and 20, the output reference nozzle 
21 and the pressure transducer 22. A flexible conduit 9 connects plunger 2 
to the unit 3. In this case one may employ plungers of any dimensions 
whatsoever, indeed of very small dimensions with a standard measuring 
unit. It has already been mentioned that the measuring fluid being liquid, 
the reaction time of the system is extremely short, on the order of 10 ms, 
and this within reasonable limits of length of the conduit 9. 
In the case of the arrangement of FIG. 3, the measuring nozzles 17, inlet 
nozzles 16 and 20, the output reference nozzle 21 and the pressure 
transducer 22 are entirely housed within the interior of plunger 12. 
In the special cases of FIGS. 2 and 3, the measuring plungers 2 and 12 
include a cylindrical end 18 for measuring the diameter of a bore 13. Two 
measuring nozzles 17 open out symmetrically at two opposite places of the 
cylindrical end 18. 
The differential pressure transducer 22 is a semiconductor element. It is 
formed essentially by a semiconducting plate in which has been formed a 
membrane by chemical machining procedures as well as a bridge of piezo 
resistors formed on the membrane and amplifying elements. Such pressure 
transducers and their manufacture are described for instance in the French 
patent application 2 266 314. One may obtain them from the Semsym company 
under reference SPX 200. 
It appears of interest to dimension the reference nozzle 21 in a manner 
such that the differential pressure between the two branches be null at 
the instant when the dimensions of the workpiece 14 attain their intended 
value. Effectively, the stability and precision of measurement of the 
dimensions are maximum when such differential pressure is close to zero 
and the preceding arrangement enables assuring that the maximum precision 
is obtained, for instance during machining when one approaches the desired 
dimensions. 
In FIG. 7, there has been shown in a schematic manner a control circuit for 
the differential pressure transducer employed in the arrangement of the 
invention. Such transducer is schematically shown in the framework in 
dotted outline 40 by a bridge of piezo resistors 41 which are energized by 
a source 47, preferably a current source. An operational amplifier 42, one 
of the inputs of which is controlled through a variable potentiometer 43, 
enables assuring the zero setting of the output signal while the 
amplification gain is defined by the potentiometer 44 which acts on 
amplifier 45 at the output 46 of which may be collected the signal 
representing the measured dimension. 
It is to be mentioned that whatever be the manner of obtaining the 
measuring arrangement according to FIG. 2 or FIG. 3, the dimensional and 
dynamic characteristics of such arrangement enable applying it to numerous 
types of measurements within the domain of machining of workpieces. In 
particular, it is especially well adapted to be mounted within the 
workpiece spindle head of an interior truing machine and kinematically 
coupled to the tool spindle head piece so as to bring about a measurement 
of the bore diameter of a trued piece during each oscillation of the tool, 
and to thus permit regulating continuously the advance of the grinder 
throughout the truing operation and to stop it when the nominal diameter 
of the bore has been attained. 
FIG. 4 shows a measuring sensor 70 employed on the arrangement according to 
the invention, such sensor being adapted to measure the diameter of 
cylindrical workpieces. Here the arrangement includes a head 55 having an 
opening provided with two rectilinear ridges 56 and 57 arranged in V. The 
ridges bear against the workpiece 52 and are here materialized by 
cylindrical bar generators or jaws 58 and 59 formed of hard material as 
for instance tungsten carbide. On head 55 is mounted a sensor 70 arranged 
along the bisector of the angle formed by ridges 56 and 57, such sensor 
furnishing, in a well known manner, a signal representing the diameter of 
the part to be machined. The sensor 70, taking the place of the measuring 
nozzle, is here a sensor of the indirect leakage type. A liquid 65 under 
pressure is injected into the flexible tube 71 which is shrunk onto the 
sensor 70. The sensor includes a chamber 62 in which moves a piston 63. 
Such piston is provided with channels 64 to allow passage of the liquid. 
The piston terminates in a cone 66 and a feeler 67. When the sensor is 
free, the piston blocks orifice 68 by urging cone 63 against a flange 69 
present on such orifice 68. When the piston 67 is pushed back towards the 
right, which happens when the diameter of the part 52 diminishes under the 
effect of the operation of the grinder, liquid may flow according to arrow 
60 and 61. At this instant the liquid flow varies, which has as 
consequence to bring about variation of the pressure which then 
constitutes a measure of the penetration of the feeler into the sensor. 
It is clear that the head which has just been described may be coupled by 
means of a flexible tube 71 to the measurement unit 3 which has been 
described with reference to FIG. 2, the differential measuring system 
remaining the same. 
There will now be described two forms of inlet nozzles which call for 
substantial commentary. 
Nozzle 80 of FIG. 5 is of a form currently employed in order to be 
assembled onto pneumatic arrangements. Nozzle 80 is screwed into a 
threading 73 provided either in the measurement unit 3 (FIG. 2) or in the 
plunger 12 (FIG. 3). Such a nozzle is employed as inlet nozzle 16 and 20 
and generally includes an inlet cone 82 and a constriction 81. If a liquid 
is injected in channel 83 in the sense of arrow A, constriction 81 will 
have as effect to increase the velocity of the liquid at the output 84 of 
the nozzle in direction B and, consequently, bring about unwanted 
turbulence, this provoking substantial background noise on the pressure 
transducer. The turbulences are produced by a jet at B with high speed 
relative to the region C where such speed is less substantial. 
The nozzle of FIG. 6 has been conceived to avoid such turbulences and to 
assure at its output an homogeneous liquid flow. Such nozzle includes two 
parts. The first part 80 resembles that of FIG. 5 with its inlet cone 82 
and its constriction 81. Such first part is screwed into a threading 73 
provided in a second part 74 itself driven into the inlet channel 83. Such 
second part 74 includes a chamber 76 and at least one outlet duct 75 
arranged substantially perpendicular to the axis of the constriction 81. 
The arrangement is that the cross-section of duct 75 be greater than the 
cross-section of the constriction 81. Such difference of cross-section 
combined with the perpendicularity mentioned have as result the 
dissipation of the kinetic energy of the liquid jet, or if preferred, the 
breaking down of the velocity without production of the additional fall in 
pressure. One then finds in the output channels 85 and 84 a liquid flowing 
without much turbulence in the direction of arrow E and attaining the 
pressure transducer 22 without provoking noise. 
The invention is not limited to the nozzle described in FIG. 6. Any other 
form could be imagined which would enable furnishing at the nozzle output 
a liquid flow producing minimum noise on the electrical signal issuing 
from the pressure transducer. 
There have been seen the advantages brought about by the joint employment 
of a liquid and the special nozzle in the measuring arrangements for 
dimensions of workpieces, the main advantages residing in the fact that 
such reduces the time necessary for measurement in the arrangements of 
in-process measuring and, from there, enables a higher production cadence. 
It will be noted that one may employ with profit a measuring liquid which 
is the same as the liquid used for machining the workpieces, naturally 
under the condition that an appropriate filtering of the quality is 
carried out.