Patent Publication Number: US-2013232805-A1

Title: Measuring machine provided with a system for compensating measuring errors due to thermal expansion of a scale of a linear transducer

Description:
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. 
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic perspective view of a measuring machine provided with an optical scale and a system for compensating the measuring errors caused by thermal expansion of the optical scale; 
         FIG. 2  is a diagram illustrating a first embodiment of the compensation system; 
         FIG. 3  is a schematic illustration of a second embodiment of the compensation system, in a first operating condition; 
         FIG. 4  illustrates the system of  FIG. 3 , in a second operating condition; 
         FIG. 5  is a schematic illustration of a third embodiment of the compensation system, in a first operating condition; and 
         FIG. 6  illustrates the system of  FIG. 5 , in a second operating condition. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , designated as a whole by  1  is a co-ordinate measuring machine. The machine  1  comprises a bench  2  provided with a horizontal plane top surface  3  or reference surface. The machine  1  further comprises a motor-driven carriage  5  that slides on the bench  2  along a first horizontal axis (axis Y) of a cartesian reference system X, Y, Z of the measuring volume. 
     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  FIG. 1 ), and a first reading head  15  fixed to the slide  9  and co-operating with the optical scale  14 . 
     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  FIG. 2 , which can be used for resetting of the reading head  15 . 
     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  FIG. 2 , the additional sensor is constituted by a second reading head  20  associated to the optical scale  14 . 
     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 ( FIG. 2 ). Each of the reading heads  15 ,  20  detects its own position on the optical scale  14 . The distance between the two heads  15 ,  20  can consequently be calculated as the difference between the respective readings L 1 , L 2 . 
     By comparing the known value of distance D with the difference between the readings L 1  and L 2 , it is possible to calculate a scale factor 
         SF=D /( L 1− L 2)   [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−T   ref ))/( L 1− L 2)   [2]
 
     where D ref is the value of D at the reference temperature T ref , for example 20° C. 
     In the embodiment illustrated in  FIG. 3 , the additional sensor is constituted by a distance sensor  24  configured for measuring the distance from a fixed reference  25 , which is fixed with respect to a point  26  of the optical scale  14  that is fixed with respect to the bench  2  of the machine  1 . 
     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 T ref  ( FIG. 2 ), where the distance from the fixed reference D ref  is measured by the distance sensor with the carriage  5  set with the reading head  15  at a reference sign  18 . 
     In conditions of operating temperature T ( FIG. 4 ), before carrying out a measuring cycle, the method according to the invention envisages the steps of:
         setting the carriage  5  with the reading unit  15  in the reference position at an operating temperature T;   measuring the distance D T  from the fixed reference by means of the distance sensor  24  at the operating temperature T;   calculating the difference S=D T −D ref , which, on the hypothesis that the distance between the reading head  15  and the sensor  24  is constant, is equal to the displacement E of the reference sign  18  with respect to the fixed point  26  of the optical scale  14  as a result of thermal expansion.       

     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  FIGS. 5 and 6  is a third embodiment of the present invention, where the additional sensor is constituted by an optical detector  30  configured so as to generate an enable signal  31  for reading the optical scale  14  by the reading head  15  when it detects the reference sign  18 . In other words, reading of the optical scale  14  by the reading head  15  is “frozen” at the instant in which the optical detector  30  passes over the reference sign  18 . 
     The operation is repeated at the reference temperature T ref  and the operating temperature T. Through the two readings L ref  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.