WO2009007824A1

WO2009007824A1 - Two-axes-compensation and anticollision device mounted between an industrial robot and its tool

Info

Publication number: WO2009007824A1
Application number: PCT/IB2008/001777
Authority: WO
Inventors: Giovanni Borsa
Original assignee: Tekno Alfa S.R.L.
Priority date: 2007-07-11
Filing date: 2008-07-07
Publication date: 2009-01-15
Also published as: ATE502735T1; EP2190634B1; DE602008005765D1; EP2190634A1; ITTO20070512A1

Abstract

According to the invention, the two-axes-compensation and anticollision device comprises a structure (11, 12, 14, 14', 15, 15.1, 16) for connection between a horizontal plate (Bl) fixed to a manipulator arm (B) and a horizontal, plate (B2) for attachment of the operating tool (P) having a variable-geometry articulated parallelogram configuration, under the action of forces a component of which is directed along a cartesian axis in the horizontal plane (X, Y), and controlled translational sliding mobility, under the action of forces a component of which is directed according to the other cartesian axis in the horizontal plane (Y, X), so as to exceed, by means of variation of geometrical configuration and/or by means of controlled mobility, a physical barrier with respect to which said tool operatively interferes, if the order of the interference between said parts falls within a range of predetermined dimensional tolerances with respect to the cartesian axes (X, Y) in the horizontal plane. Spring means (15.2, 17, 17.1, 17.2, 18, 19; 16, 16.1, 16.2, 21, 22) for compensation of said variation of geometrical configuration and/or of said controlled mobility, after said physical barrier has been exceeded, automatically restore the initial geometry and arrangement of said structure.

Description

tWO-AXES-COMPENSATION AND ANTICOLLISION DEVICE MOUNTED BETWEEN AN INDUSTRIAL ROBOT AND ITS TOOL

5 The present invention relates to a two-axes-compensation and anticollision device associated to the operating tool of an industrial robot.

A range of application, which is provided purely by way of non- limiting example, of the present invention is that of industrial

10 robots operating at the service of machine tools. A typical case is represented by a robot of the above sort, in which a rigid and vertical manipulator arm is mounted mobile in a horizontal direction ( axis X) along a rectilinear guide constituted by a suspended bar. The manipulator arm carries at its bottom end an

15 operating tool, for example a collet, which is mobile in a vertical direction between a raised position, in which it supports - during a forward travel parallel to the axis X - a workpiece, and respective lowered positions of start and end of travel, in which it picks up the workpiece from the magazine and

20 places it in a working position. The return travel of the arm along the rectilinear guide is carried out, for example, in unloaded conditions.

During said operations of the manipulator arm, there may arise the drawback that positioning of the workpiece in a working

25 position is carried out by the collet in an improper way on account of interference with respect to a ^' physical barrier that rigidly delimits positioning of the workpiece itself for its further machining/manipulation. Said error of positioning can be of two orders of magnitude; namely:

30 a) it falls within a range of predetermined dimensional tolerances with respect to the axes X and Y ( in the horizontal plane) ; or else b) it falls outside the aforesaid predetermined tolerances. In known industrial robots, even though said error of positioning

35 falls in modest dimensional tolerances, it entails arrest of the system, with consequent interruption of the working rate. The present invention is aimed at providing a remedy to the aforesaid drawback.

Consequently, a purpose of the present invention is to provide a two-axes-compensation and anticollision device associated to the operating tool of an industrial robot that will enable automatic correction of the error of positioning of the workpiece in a working position by the operating tool, if it falls within a range of predetermined dimensional tolerances with respect to the axes X and Y ( in the horizontal plane) , or else, if the error falls outside this range, will automatically bring about arrest of the robot.

Another purpose of the present invention is to provide a two- axes-compensation and anticollision device associated to the operating tool of an industrial robot that can be selectively activated or de-activated, during operation of the robot, according to the working cycle to be performed.

A further purpose of the invention is to provide a two-axes- compensation and anticollision device associated to the operating tool of an industrial robot that will have a simple structure, will be safe and reliable in operation, and will present a relatively contained cost.

With a view to achieving the above purposes, the present invention provides a two-axes-compensation and anticollision device associated to the operating tool of an industrial robot, the essential characteristic of which forms the subject of Claim 1.

Further advantageous characteristics are described in the dependent claims. The aforementioned claims are understood as being integrally incorporated herein.

The present invention will emerge more clearly from the ensuing detailed description with reference to the attached drawings, which are provided purely by way of non-limiting example, and in which: - Figure 1 is a view in elevation of the two-axes-compensation and anticollision device associated to the operating tool of an industrial robot, according to an example of embodiment of the invention;

- Figure 2 is a view in cross section according to the line II-II of Figure 1; - Figure 3 is a view in the direction of the arrow III of Figure

1;

- Figures 4 and 5 are views in the direction, respectively, of the arrow IV and of the arrow V of Figure 3; and

- Figure 6 is a cross-sectional view according to the line VI-VI of Figure 3.

With reference to the drawing, designated by B is the bottom part of a vertical manipulator arm ( represented with dashed lines) , which carries a free-end fixed horizontal plate Bl. Said manipulator arm B is, for example, rigid and is mounted mobile in a horizontal direction ( here assumed as cartesian axis X, Figure 5) along a horizontal rectilinear guide constituted by a suspended bar ( not represented in the drawings) .

Mounted on said plate Bl is the two-axes-compensation and anticollision device 10, according to the invention, associated to the operating tool of the arm B.

Said device 10 comprises four fixed blocks 11 ( Figures 2, 6) , mounted in pairs on the plate Bl, on opposite sides of the arm B. Said blocks 11 extend parallel to the direction of the horizontal cartesian axis Y ( Figure 5) and are aligned in pairs to one another. They have a basically U-shaped cross section open upwards. Mounted by linear sliding within each pair of blocks 11 is a corresponding rectilinear guide 12, which is mobile parallel to the axis Y. Each guide 12 carries fixed at the top a respective box 13, provided within which is a cylindrical seat with axis parallel to the axis Y and formed by coaxial end bushings 13.1, housed coaxially in which in such a way that it can turn is a corresponding shaft 14. Each shaft 14 supports a respective vertical frame 15 shaped like a U turned upside down, provided with a pair of vertical uprights 15.1 with holes for housing the axial ends of said shaft 14 and of a horizontal top cross member 15.2, which overlies the box 13, without making contact therewith, and is parallel to the axis Y. Mounting-plate means, fixed to said uprights 15.1 by means of screws and appropriately perforated, co-operate with two opposite axial screws 14.1 so as to render said frame 15 fixed with respect to the respective shaft 14.

The vertical uprights 15.1 of the frame 15 extend with their bottom ends further down than the plate Bl and support in a rotating way, within respective holes aligned according to an axis parallel to the axis Y, a corresponding bottom shaft 14' with axis parallel to the axis Y. Mounting-plate means, fixed to said uprights 15.1 by means of screws and appropriately perforated, co-operate with two opposite axial screws 14' .1 so as to render said frame 15 fixed also with respect to the respective bottom shaft 14' . It will be noted that, at rest, the axes of the shafts 14, 14' associated to one and the same frame 15 lie substantially in one and the same vertical plane.

Provided above the cross member 15.2 of the front frame 15 with respect to the direction of advance of the arm B according to the axis X, is a horizontal shaft 17 with axis parallel to the axis Y, which extends in a specularly symmetrical way with respect to the vertical median plane that cuts the shafts 14, 14' and is fixed, by means of an intermediate insert 17.1, to the arm B. Coaxially fitted on parts 17.2 of said shaft 17, axially opposite to one another and of reduced diameter, are respective helical compression springs 18. Said axially opposite parts 17.2 of said shaft 17 with respective coaxial springs 18 are received in corresponding coaxial cylindrical cavities of casings 19, fixed to said cross member 15.2 in a specularly symmetrical way with respect to said median plane. It will be noted that the cylindrical cavity of the casings 19 is of a diameter such as to receive in axial sliding the shaft 17, also in its part of larger diameter. Washers having an external diameter corresponding to that of the shaft 17 are fixed by means of screws against the axial ends of the parts 17.2 of the shaft 17 and prevent the springs 18 from sliding out axially.

The insert 17.1 carries at the top, in a position corresponding to said median plane, a sensitive member 20.1, for example a pin with cylindrical head, for a proximity sensor 20, the detector 20.2 of which is fixed with respect to the cross member 15.2 of the frame 15 by means of a bracket 20.3. An intermediate horizontal plate 16 is supported beneath and fixed to the plate Bl, at a distance therefrom, and at the top fixed to the bottom shafts 14' , which are housed rotating in corresponding end bushings 14' .2, which are fixed with respect to a respective guard 14' .3 that supports said plate 16 from beneath.

An insert 16.1 is fixed along one side of said plate 16 and in a position corresponding to the vertical median plane parallel to the axes of the shafts 14, 14' . A fixed median vertical upright 16.2 of said insert traverses a corresponding wide hole BlO of the plate Bl and supports a fixed shaft 21 with axis parallel to the axis X and extending symmetrically with respect to said median plane. Fitted coaxially on axially opposite parts of said shaft 21 that have a reduced diameter ( not visible in the drawing) are respective helical compression springs. Said axially opposite parts of said shaft 21 with respective coaxial springs are received in corresponding coaxial cylindrical cavities of respective casings 22, fixed to said plate Bl in a specularly symmetrical way with respect to said median plane. It will be noted that the cylindrical cavity of each casing 19 has a diameter such as to receive in free axial sliding the shaft 21, also in its part of larger diameter, as well as washers having an external diameter corresponding to that of the shaft 21, which are fixed by means of screws against the axial ends of said opposite parts of the shaft 21 and prevent the corresponding springs from sliding out axially.

Said guards 14' .3 of the bottom shafts 14' support a fixed bottom horizontal plate B2, which is provided for anchorage of the manipulator tool, for example a collet, and which only in Figure 1 is illustrated schematically and designated by P. It will be noted that in the vertical uprights 15.1 of the front frame 15, anterior with respect to the direction of displacement according to the axis X, the mounting holes 15.3 of the respective top shafts 14 are slot-shaped, as likewise the holes in the mounting plates corresponding to the shaft 14 and to the fixing screw of the plate. The major axis of said slot-shaped holes is vertical.

The arrangement of the two slidable guides 12 in the blocks 11, of the four shafts 14, 14' with corresponding two frames 15, 15.1 and intermediate plate 16 and bottom plate B2 provides a connection between the plate Bl fixed to the arm B and the plate B2 for attachment of the collet P having a geometry that varies under the action of forces directed in the direction of displacement of the arm ( i. e. , according to the axis X) and having a controlled transverse mobility under the action of forces directed orthogonally to the direction itself ( i. e. , according to the axis Y) . Operation

On the one hand, said arrangement behaves as an articulated- parallelogram variable-geometry system ( viewed, for example, in the plane of Figure 4), when said bottom plate B2 is subject to a component of force directed according to the cartesian axis X, for example, on account of the interference between the collet P in the descending step and a physical barrier that rigidly delimits positioning of the workpiece carried by the collet for its further machining/manipulation. In this case, the vertical uprights 15.1 of the frames 15 oscillate with respect to the axis of the top shafts 14 ( as represented schematically in said Figure 4 by thin lines) , whilst the bottom horizontal plate B2, and along with this the collet, displaces with respect to the arm B, horizontally and in a direction parallel to the axis X so as to overstep said barrier, if the order of magnitude of the interference is, for example, smaller than or equal to ±10 mm. In this case, said displacement reflects on the intermediate horizontal plate 16 and determines a corresponding horizontal translation according to the axis X also of the shaft 21 in a direction opposite to that of said horizontal component of force, elastically loading, in a way proportional to the displacement, the compression spring supported by the part of said shaft 21, which is brought to re-enter partially within the respective casing 22. Once the barrier has been overstepped, on account of the elastic reaction of the aforesaid pre-loaded spring and of the reverse deformation of the aforesaid variable-geometry system, the arrangement of the four shafts 14, 14' with corresponding two frames 15, 15.1 and intermediate horizontal plate 16 and bottom horizontal plate B2 is automatically brought back into the normal configuration, in which the top shafts 14 and bottom shafts 14" are arranged in pairs according to respective basically vertical surfaces. In Figure 4, the double- headed arrow FX indicates the possibility described above of compensation according to the axis X of the aforesaid arrangement of parts 14, 14' , 15, 15.1, 16, B2 of the device 10 according to the invention. Illustrated schematically with thin lines in said Figure 4 is the possibility of deformation in parallelogram fashion of said arrangement.

On the other hand, the arrangement of the four shafts 14, 14' , corresponding frames 15 and intermediate horizontal plate 16 and bottom horizontal plate B2 behaves as a rigid body, which is mobile in the direction of the axis Y with respect to the fixed plate Bl and hence with respect to the arm B by means of sliding of the guides 12, which are fixed with respect to said rigid body, with respect to the corresponding blocks 11, when said bottom plate B2 is subjected to a component of force directed according to said axis Y, for example on account of the interference between the collet P in the descending stage and a physical barrier that rigidly delimits positioning of the workpiece carried by the collet for its further machining/manipulation. In this case, the frames 15, 15.1 with the corresponding shafts 14, 14' and intermediate horizontal plate 16 and bottom horizontal plate B2 translate parallel to the axis Y in the same direction as the horizontal component of the force applied on the plate B2. Together with the plate B2 there displaces accordingly, in a horizontal direction parallel to the axis Y, the collet P with respect to the arm B so as to overstep said barrier, if the order of magnitude of the interference is, for example, smaller than or equal to fLO mm. In this case, said displacement reflects on the cross member 15.2 of the frame 15 that supports the casings 19 and determines a corresponding 5 horizontal translation according to the axis Y of said casings with respect to the shaft 17 ( fixed to the arm B) in a direction concordant with that of said horizontal component of force, elastically loading, in a way proportional to the displacement, the compression spring 18, which is compressed by the

10 corresponding casing. Once the barrier has been exceeded, thanks to the elastic reaction of the aforesaid pre-loaded spring 18 and to the reverse translation of the aforesaid system with controlled transverse mobility, the arrangement of the four shafts 14, 14' with corresponding two frames 15, 15.1 and

15 intermediate horizontal plate 16 and bottom horizontal plate B2 is automatically brought back into the normal configuration, in which the top shafts 14 and bottom shafts 14' are arranged in a way symmetrically specular with respect to the vertical median plane of which the line of section VI-VI has been drawn in Figure

203. In said Figure 3 the double-headed arrow FY indicates the possibility of compensation according to the axis Y described above of the aforesaid arrangement of parts 14, 14' , 15, 15.1, 16, B2 of the device 10 according to the invention. Schematically represented with the thin dashed lines in Figure 3 are the

25 positions of extreme translation of said system in the two directions of the axis Y.

In the case where said physical barrier extends beyond the range of the aforesaid predetermined dimensional tolerances with respect to the axes X and Y ( UO mm in the horizontal plane) , the

30 corresponding variable-geometry deformation, or the corresponding transverse translation of said arrangement of the four shafts 14, 14' , corresponding frames 15 and intermediate horizontal plate 16 and bottom horizontal plate B2, determines a relative displacement of the sensitive member 20.1 associated to the

35 detector 20.2 of the proximity sensor 20 such as to induce θ issuing of a general-error signal, which causes instantaneous arrest of the arm B, or a signal that is discriminated to identify whether the error refers to the cartesian axis X or the cartesian axis Y. Likewise, if the collet P during the working travel in the direction X encounters an insurmountable obstacle, the latter causes, as illustrated in Figure 1, an oscillation upwards of the plate B2 about the axis of the bottom shaft 14' that is distal with respect to said obstacle. Said oscillation entails sliding upwards of the other bottom shaft 14' ( which is not prevented by the corresponding guard) and hence of the frame 15, 15.1 fixed with respect thereto, whilst the corresponding upper shaft 14 remains immobile within the respective slots 15.3. The displacement upwards of said frame 15, 15.1 determines the corresponding translation upwards of the respective cross member 15.2 and of the detector 20.2 of the sensor 20 along with this, which, by moving away from the respective sensitive member 20.1, determines issuing of a general-error signal, which causes instantaneous arrest of the arm B, or a signal that is discriminated to identify whether the error refers to the anticollision means.

The present invention moreover provides clamping means 30 ( Figure 6) designed to fix, selectively and in a dismantlable way, the arm B monolithically with respect to the bottom plate B2 for attachment of the collet so as to prevent the corresponding mobility of said plate with respect to any cartesian axis on account, for example, of inertial forces and/or accelerations acting on the arm itself. Said clamping means 30 comprise a cylindrical rod 31 with tapered tip at the bottom and with vertical axis set on the intersection of the two vertical median surfaces of the plate Bl. Said rod 31 is supported slidingly with respect to the plate Bl by means of a framework 32 erected on the plate itself and in which two bushings 33 are provided coaxial with respect to said rod and having the function of guide for said rod. A through hole 34 is provided in the plate Bl coaxial to said rod for free passage of the tapered tip thereof. The intermediate plate 16 is provided with a corresponding coaxial hole 35, fixed in which is a bushing 36, which has a corresponding conical seat, coaxial and facing the tip of the rod 31. Fixed to the top of the framework 32 is a pneumatic operating cylinder 37, the stem of which is operatively connected with respect to the end of the rod 31 opposite to its conical tip. Said cylinder 37 is connected in a sealed way with respect to a corresponding circuit for supply of pressurized air so as to be supplied therewith according to software-program instructions pre-loaded into a PLC processing calculation unit ( not illustrated) , which governs the working cycle of the arm B. When said cylinder 37 is pneumatically supplied, its own stem extends, and the rod 31 is brought to engage with its tapered tip the corresponding conical seat of the bushing 36, rendering fixed in a monolithic body said intermediate plate 16, and hence the bottom plate B2 for attachment of the collet P, with respect to the bottom plate Bl of the arm B.

Retraction of the stem into the operating cylinder 37 determines the corresponding retraction of the tip of the rod 31 from the bushing 36, as far as into the hole 34 of the plate Bl, a fact that again renders the connection between said plate Bl of the arm B and the collet plate B2 a connection with variable geometry and with controlled transverse mobility, if the plate B2 is affected by forces acting along the axes X and/or Y in the horizontal plane.

As emerges from the foregoing description, the present invention enables the purposes set forth in the premise to be achieved in a simple and effective way.

Claims

1. A two-axes-compensation device associated to the operating tool of an industrial robot, in which a manipulator arm ( B) is mobile in a horizontal direction ( so-called cartesian axis

X) and carries at its bottom end an operating tool ( P) , which is mobile between a raised position, in which it supports - during a travel parallel to said axis ( X) - a workpiece, and respective lowered positions of start and end of travel, in which it picks up and places the workpiece in a working position, said compensation device being characterized in that:

- it comprises a structure (11, 12, 14, 14', 15, 15.1, 16) for connection between a horizontal plate ( Bl) fixed to the arm ( B) and a horizontal plate ( B2) for attachment of the operating tool ( P) having a variable-geometry configuration, under the action of forces a component of which is directed along a cartesian axis in the horizontal plane (X, Y) , and controlled mobility, under the action of forces a component of which is directed according to the other cartesian axis in the horizontal plane (Y, X) , so as to exceed, by means of variation of geometrical configuration and/or by means of controlled mobility, a physical barrier, with respect to which said tool operatively interferes, if the order of the interference between said parts falls within a range of predetermined dimensional tolerances with respect to the cartesian axes ( X, Y) in the horizontal plane;

- it comprises means (15.2, 17, 17.1, 17.2, 18, 19; 16, 16.1, 16.2, 21, 22) for compensation of said variation of geometrical configuration and/or of said controlled mobility, which, after said physical barrier has been exceeded, restore automatically the initial geometry and arrangement of said structure; and

- it moreover comprises sensor means ( 20) , which determine issuing of a general-error signal, which causes instantaneous arrest of the arm ( B) , or a signal that is discriminated for identifying whether the error refers to a cartesian axis (X, Y) , and this in the case where the deformation of the variable- geometry configuration and/or the controlled translation of said structure, caused by said barrier, exceed the range of said predetermined dimensional tolerances with respect to the cartesian axes ( X, Y) in the horizontal plane.

2. The two-axes-compensation device associated to the operating tool of an industrial robot according to Claim 1, characterized in that said structure ( 11, 12, 14, 14' , 15, 15.1, 16) moreover comprises anticollision means ( 15.3) , which, if said operating tool ( P) during a travel of the arm ( B) encounters an insurmountable obstacle, cause a displacement of at least one part of said structure ( 15, 15.1, 15.2) that determines issuing by said sensor means ( 20) of a general-error signal, which brings about instantaneous arrest of the arm ( B) , or a signal that is discriminated for identifying whether the error refers to said anticollision means.

3. The two-axes-compensation device associated to the operating tool of an industrial robot according to Claim 1 and/or Claim 2, characterized in that it comprises clamping means ( 30) that fix, selectively and in a dismantlable way, said arm ( B) monolithically with respect to said plate ( B2) for attachment of the operating tool ( P) so as to prevent the corresponding mobility of said plate with respect to any cartesian axis, on account of inertial forces and/or accelerations and the like acting on the arm itself.

4. The two-axes-compensation device associated to the operating tool of an industrial robot according to one or more of the preceding claims, characterized in that said structure comprises:

- blocks ( 11) fixed on the horizontal plate ( Bl) with respect to the arm ( B) , said blocks extending parallel to a horizontal cartesian axis ( Y) and being arranged on opposite sides of said arm; - two rectilinear guides ( 12) , coupled by linear sliding in said blocks ( 11) and arranged on opposite sides of said arm; - two first shafts ( 14) with axis parallel to that of said cartesian axis ( Y) and supported fixed each by a respective one of said guides ( 12) ;

- two vertical frames ( 15) shaped like a ϋ turned upside down, each supported by a respective one of said first shafts ( 14) and each provided with a pair of vertical uprights ( 15.1) , with holes for rotary housing of the axial ends of the respective shaft

( 14) , and with a horizontal top cross member ( 15.2) ;

- two second shafts ( 14' ) with axis parallel to said cartesian axis ( Y) and each supported in a rotary way by a respective frame

( 15) , within holes of its uprights ( 15.1) underlying said plate ( Bl) that is fixed with respect to the arm ( B) ;

- an intermediate horizontal plate ( 16) , supported beneath and fixed to said plate ( Bl) with respect to the arm ( B) , at a distance therefrom, and at the top to said second shafts ( 14' ) by means of a respective guard ( 14' .3) of said second shafts; and

- said fixed bottom horizontal plate ( B2) , provided for anchorage of the manipulator tool ( P) , fixed at the bottom to said second shafts ( 14' ) .

5. The two-axes-compensation device associated to the operating tool of an industrial robot according to one or more of the preceding claims, characterized in that said means ( 15.2, 17, 17.1, 17.2, 18, 19; 16, 16.1, 16.2, 21, 22) for compensation of said • variation of geometrical configuration and/or of said controlled mobility comprise:

- a horizontal shaft ( 17) with axis parallel to one cartesian axis ( Y) in the horizontal plane, set on the cross member ( 15.2) of a frame ( 15) , extending in a specularly symmetrical way with respect to the vertical median plane that cuts the shafts ( 14, 14' ) and is fixed to the arm ( B) ; and

- elastic means (18) fitted on parts (17.2) of said shaft (17), which are axially opposite and are received in corresponding coaxial cylindrical cavities of casings ( 19) , which are fixed to said cross member ( 15.2) in a specularly symmetrical way with respect to said median plane, so that when said structure (14, 14', 15, 15.1, 16) is subjected to forces that translate it in direction parallel to said cartesian axis ( Y) with respect to the fixed plate ( Bl) and hence with respect to the arm ( B) , by means of sliding of the guides ( 12) with respect to the corresponding blocks ( 11) , said displacement is reflected on the cross member ( 15.2) of the frame ( 15) that supports the casings ( 19) and determines a corresponding horizontal translation along said axis ( Y) of said casings with respect to the shaft ( 17) , thus elastically loading the elastic means ( 18) that is compressed by the corresponding casing, whilst, as the action of said forces ceases, thanks to the elastic reaction of the pre-loaded elastic means ( 18) , said structure with controlled transverse mobility is automatically brought back into the normal configuration.

6. The two-axes-compensation device associated to the operating tool of an industrial robot according to the preceding claim, characterized in that said means ( 15.2, 17, 17.1, 17.2, 18, 19; 16, 16.1, 16.2, 21, 22) for compensation of said variation of geometrical configuration and/or of said controlled mobility comprise:

- an insert ( 16.1) , fixed along a side of said intermediate plate ( 16) and in a position corresponding to the vertical median plane parallel to the axes of the shafts ( 14, 14' ) ; - a fixed median vertical upright ( 16.2) of said insert, which traverses a corresponding hole ( BlO) of said plate ( Bl) fixed to the arm ( B) and supports a fixed shaft ( 21) with axis parallel to the other cartesian axis ( X) in the horizontal plane and extending symmetrically with respect to said median plane; - elastic means fitted on parts of said shaft ( 21) , which are axially opposite and are received in corresponding coaxial cylindrical cavities of respective casings ( 22) , which are fixed to said plate ( Bl) in a specularly symmetrical way with respect to said median plane, so that when said structure (14, 14', 15, 15.1, 16) is subjected to forces which are directed along said other cartesian axis ( X) and determine the variation of its geometrical configuration, said variation reflects on said intermediate horizontal plate ( 16) and causes a corresponding horizontal translation along said other cartesian axis ( X) of the shaft ( 21) , elastically loading an elastic means supported by a part of said shaft ( 21) and, as said forces cease, thanks to the elastic reaction of said preloaded elastic means and to the reverse deformation of the aforesaid variable-geometry system, said structure is automatically brought back into the normal configuration.

7. The compensation device according to Claim 5 and/or Claim 6, characterized in that said shaft ( 17) is fixed to the arm ( B) by means of an insert ( 17.1) that carries, in a position corresponding to said median plane, a sensitive member ( 20.1) for a sensor ( 20) , the detector ( 20.2) of which is fixed with respect to the cross member ( 15.2) of said frame ( 15) .

8. The compensation device according to one or more of the preceding claims, characterized in that said anticollision means comprise slot-shaped holes ( 15.3) for mounting said first shafts ( 14) in the vertical uprights ( 15.1) of the front frame ( 15) , in the direction of displacement along a cartesian axis ( X) in the horizontal plane, so that, if the operating tool ( P) during the working travel parallel to said cartesian axis ( X) encounters an insurmountable obstacle, the latter causes an oscillation upwards of the bottom plate ( B2) about the axis of the second shaft ( 14' ) that is distal with respect to said obstacle and, hence, a displacement upwards of the frame ( 15, 15.1) close to the obstacle, this determining the corresponding translation upwards of the respective cross member ( 15.2) and along with this of the detector ( 20.2) of the sensor ( 20) , which, by moving away from the respective sensitive member ( 20.1) , causes issuing a general- error signal, which brings about instantaneous arrest of the arm ( B) , or a signal that is discriminated for identifying whether the error regards a cartesian axis ( X, Y) or else the anticollision means.

9. The compensation device according to one or more of the preceding claims, characterized in that, in the case where said physical barrier extends beyond the range, of the aforesaid predetermined dimensional tolerances with respect to the cartesian axes ( X, Y) in the horizontal plane, the corresponding variable-geometry deformation, or the corresponding transverse translation of said structure, determines a relative displacement of the sensitive member ( 20.1) associated to the detector ( 20.2) of the proximity sensor ( 20) such as to induce issuing of a general-error signal, which causes instantaneous arrest of the arm ( B) , or a signal that is discriminated for identifying whether the error regards a cartesian axis ( X, Y) or else the anticollision means.

10. The compensation device according to one or more of the preceding claims, characterized in that said clamping means ( 30) comprise: - a cylindrical rod ( 31) with a tapered tip at the bottom, supported slidingly with respect to the plate ( Bl) fixed to the arm ( B) ;

- a through hole ( 34) , provided in said plate ( Bl) coaxial to said rod, for free passage of the tapered tip thereof; - a bushing ( 36) in said intermediate plate ( 16) , which has a corresponding conical seat, coaxial and facing the tip of said rod ( 31) ; and

- an operating cylinder ( 37) , the stem of which governs said rod in axial extension and retraction with respect to said through hole ( 34) , so that, when said cylinder ( 37) is supplied, the rod ( 31) is brought to engage with its tapered tip the corresponding conical seat of the bushing ( 36) , rendering fixed in a monolithic body said intermediate plate ( 16) , and hence the bottom plate ( B2) , with respect to the plate (Bl) of the arm ( B) .