The present invention relates to a measuring machine provided with a system for compensating measuring errors due to thermal expansion of a scale of a linear transducer.
As is known, measuring machines comprise members that are mobile along co-ordinate axes in order to displace a measuring sensor in a measuring volume. Typically, the mobile members are constituted by a first carriage mobile with respect to a bench along a first axis, a second carriage carried by the first carriage and mobile with respect thereto along a second axis orthogonal to the first, and a spindle carried by the second carriage and mobile with respect thereto along a third axis orthogonal to the first two.
The displacement of the mobile members is generally detected via a transducer comprising a scale fixed, along the axis of motion, to the supporting and guide member, and a reading head fixed to the mobile member and co-operating with the scale for detecting the co-ordinates along the axis of motion. The transducer can be of an optical, capacitive, or inductive type, or of some other type. For example, in the case of an optical transducer, the scale is provided with a series of notches at constant intervals (for example 20 μm) that are detected by the reading head 14 and “counted” by the logic associated thereto.
Measuring machines are generally equipped with systems for compensating the measuring errors due to various causes (geometrical errors of the guides, static or dynamic deformation of the mobile members of the machine, thermal expansion of said members, etc.).
Normally, compensation of the thermal expansion of the scale of the transducer is also envisaged, which is based upon the measurement of the temperature and upon the knowledge of the coefficient of expansion of the scale. For this purpose, the latter is preferably made of a material with a low coefficient of thermal expansion, or in any case of a material having a certified thermal expansion, which renders it costly.
The aim of the present invention is to produce a measuring machine provided with an alternative system for compensating measuring errors due to thermal expansion of the scale of the linear transducer, as well as to a method for compensating said errors.
The aforesaid aim is achieved by a machine set forth in the claims.
For a better understanding of the present invention, three preferred embodiments are described in what follows by way of non-limiting examples and with reference to the attached drawings, wherein:
With reference to
The carriage 5 has a bridge structure and comprises two vertical uprights 6, 7 and a top horizontal cross member 8 that extends between the top ends of the uprights 6, 7.
The upright 6 comprises at the bottom a motor-driven slide 9 sliding on guides 10 parallel to the axis Y and provided, in a known way, in the proximity of a top longitudinal edge of the bench 2.
The cross member 8 carries a second carriage 11 designed to slide thereon along guides (not illustrated) in a direction parallel to a second axis (axis X) of the reference system.
The second carriage 11 carries a spindle 12 with vertical axis, mobile along its own axis parallel to a third axis (axis Z) of the reference system. The spindle 12 is designed to carry at the bottom a measuring sensor (not illustrated).
The machine 1 comprises, for each axis, a linear transducer for detecting the position of the corresponding mobile member on the axis itself. With particular reference to the carriage 5 and to the axis Y, the linear transducer comprises an optical scale 14 fixed to the bench 2 parallel to the axis Y (see enlarged detail in
The optical scale 14 has, in a known way, a lattice of notches 17 arranged at regular intervals apart, for example every 20 μm, and a plurality of reference signs 18 arranged at greater intervals apart, for example 50 mm or 100 mm, just one of which is visible in
According to the present invention, the machine 1 comprises a system 16 for determination and compensation of measuring errors due to thermal expansion of the optical scale 14. The system 16 basically comprises an additional sensor mounted on the slide 9 and a processing unit 19 connected to the first reading head 14 and the additional sensor.
In the embodiment illustrated in
The first and second reading heads 15, 20 are spaced apart from one another along the axis Y by a known distance D. The distance D is known in so far as it can be considered substantially constant as the temperature varies or else varies in a known way therewith. For example, the two reading heads 15, 20 can be mounted on a bar 21 made of material with an extremely low coefficient of thermal expansion, such as Zerodur® (registered trademark of Schott AG), or else of a material having a known coefficient of thermal expansion. The bar 21 is conveniently mounted on the slide 9 so as to enable differential thermal expansion between the slide 9 and the bar itself.
The method for compensation of measuring errors due to thermal expansion of the optical scale 14 in the example illustrated is described in what follows.
In the first place, the reading heads 15 and 20 are reset at one and the same reference sign 18 on the optical scale 14. For this purpose, the carriage 5 is displaced in succession into a first position in which the sign 18 is read by the first reading head 15, and into a second position in which the sign 18 is read by the second reading head 20.
Once both of the reading heads 15, 20 have been reset on the same sign 18, the carriage 5 is displaced into a third position distinct from the previous ones (
By comparing the known value of distance D with the difference between the readings L1 and L2, it is possible to calculate a scale factor
SF=D/(L1−L2) [1]
with which the co-ordinates detected by the machine in the measuring step can be corrected.
In the case where D is not constant with the temperature, Eq. [1] becomes
SF=D
ref(1+α(T−Tref))/(L1−L2) [2]
where Drefis the value of D at the reference temperature Tref, for example 20° C.
In the embodiment illustrated in
This embodiment, which is more economically advantageous than the previous one in so far as the distance sensor is less costly than a reading head, requires an operation of initial calibration in conditions of reference temperature Tref (
In conditions of operating temperature T (
The value S can be used for calculating a correction value and correcting the measuring data detected by said machine by means of said correction value.
For instance, if M is the distance of the reference sign 18 from the fixed point 26 at the reference temperature, the correction value can be defined by a scale factor FS calculated by applying the expression
SF=M/(M−S) [3]
Illustrated in
The operation is repeated at the reference temperature Tref and the operating temperature T. Through the two readings Lref and L of the reading head it is possible to determine the scale factor
FS=L
ref
/L [4]
The formulas [3] and [4] refer to the hypothesis whereby the distance between the reading head and the additional sensor is constant. The formulas can be easily modified in the case where said distance cannot be assumed constant but varies with the temperature in a known way.
In each of the embodiments described, the scale factor FS constitutes a correction value by means of which the measurements made by the measuring machine and acquired via the processing unit 19 can be corrected to compensate for the errors induced by thermal expansion of the optical scale.
Finally, it is clear that modifications and variations may be made to the machine 1 and to the compensation system 16 illustrated herein, without thereby departing from the sphere of protection defined in the ensuing claims.
In particular, the machine can be of a type different from the one described, the compensation system can be applied to any mobile member of the machine, the additional sensor can be of any type, and the signals of the reading head and of the additional sensor can be processed in any suitable way to generate one or more correction values for compensating the measuring errors due to thermal expansion of the optical scale.
Number | Date | Country | Kind |
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12425050.7 | Mar 2012 | EP | regional |