Method for automatically correcting the systematic errors in measurement and manufacturing machines and apparatus for implementing the method

• US 20040254673A1
• Filed: 03/17/2004
• Published: 12/16/2004
• Est. Priority Date: 08/08/2001
• Status: Active Grant

• Current Assignee: FPT Industrial SPA (Iveco Group NV)
• Original Assignee: FPT Industrial SPA (Iveco Group NV)
• Inventors: Tomelleri, Raffaele
• Granted Patent: US 6,985,792 B2
• Time in Patent Office: Days
• US Class Current: 700/176
• CPC Class Codes: G05B 19/401 characterised by control arrangements for measuring, e.g. calibration and initialisation, measuring workpiece for machining purposes G05B19/19 takes precedence
• CPC Class Codes: G05B 2219/49181 Calculation, estimation, creation of error model using measured error values

Abstract

A method for automatically correcting the systematic errors in measurement machines and in manufacturing machines, in particular in machine tools, characterised by comprising the following operative steps in succession: identifying the parametric mathematical model of the machine, initially assigning, to the parameters of said parametric mathematical model, numerical values considered to be close to the effective values, to obtain a provisional mathematical model of the machine, effecting a plurality of mutually independent distance measurements within the working field of the machine, determining the differences between the experimentally measured distance values and the corresponding distance values deduced from said provisional mathematical model, to define the corresponding provisional inaccuracy index of the machine, varying said parameters in the sense of improving the predefined provisional inaccuracy index, to hence obtain a new provisional mathematical model of the machine which is closer to its definitive mathematical model, proceeding iteratively with new determinations of the difference between these and the corresponding values deduced from the mathematical model until said parameter variations do not produce any further significant improvement in said inaccuracy index, to hence obtain the definitive mathematical model of the machine, obtaining from said mathematical model the position and possibly orientation errors of the final machine member, and utilizing the error data to execute the correction.

Claims

1-23. -23. Cancelled

24. A method for automatically correcting the systematic errors in measurement machines and in manufacturing machines, in particular in machine tools, comprising the following operative steps in succession:

identifying the parametric mathematical model of the machine,

initially assigning, to the parameters of said parametric mathematical model, numerical values considered to be close to the effective values, to obtain a provisional mathematical model of the machine,

effecting a plurality of mutually independent distance measurements in whatever direction within the working field of the machine,

determining the differences between the experimentally measured distance values and the corresponding distance values deduced from said provisional mathematical model, to define the corresponding provisional inaccuracy index of the machine,

varying said parameters in the sense of improving the predefined provisional inaccuracy index, to hence obtain a new provisional mathematical model of the machine which is closer to its definitive mathematical model,

if the new inaccuracy index so obtained is not satisfactory, proceeding iteratively with new determinations of the difference between these and the corresponding values deduced from the mathematical model until said parameter variations do not produce any further significant improvement in said inaccuracy index, to hence obtain the definitive mathematical model of the machine,

obtaining from said mathematical model the position and possibly orientation errors of the final machine member, and

utilizing the error data to execute the correction.

25. A method as claimed in claim 24, further comprising defining said provisional inaccuracy index of the machine as maximum value among all of said differences and by varying said parameters in the sense of annulling them.

26. A method as claimed in claim 24, further comprising effecting said distance measurements between separate positions assumed by the final machine member, using a laser interferometer.

27. A method as claimed in claim 26 further comprising effecting the distance measurements using a laser interferometer of automatically orientatable type.

28. A method as claimed in claim 27, further comprising effecting the distance measurements using a laser interferometer of automatically orientatable type towards a reflector fixed to the final machine member.

29. A method as claimed in claim 24, further comprising effecting said distance measurements between a point on the final machine member and a predetermined geometrical spot.

30. A method as claimed in claim 24, further comprising effecting distance measurements from a fixed point representative of the final machine member, in three mutually perpendicular directions.

31. An apparatus for implementing the method claimed in claim 24, further comprising:

measurement means, associated with the machine, for effecting distance measurements in whatever direction under different machine operating conditions,

a processing unit provided with means for assigning numerical values to said parameters of a parametric mathematical model, means for determining distances using said mathematical model, means for comparing the distance values determined in this manner with the experimentally measured distance values, and means for transmitting the error data from said comparison means to the numerical control system and to the machine.

32. An apparatus as claimed in claim 31 further comprising:

a reflector applied to the final member of the machine,

a laser interferometer, the beam of which is automatically orientatable towards the reflector,

processing means, which during the distance measurement step orientate the laser beam towards the reflector, during the mathematical model identification step assign the best values to the parameters, and during the correction step acquire the error data and transmit them to the numerical control system.

33. An apparatus as claimed in claim 32 further comprising a mirror facing the interferometer, to reflect the beam emitted thereby, towards the reflector and to also reflect towards said laser interferometer the beam reflected by said reflector, with said mirror there being associated means for its manipulation in the sense of ensuring that the reflected beam originating from said reflector strikes said laser interferometer independently of the position of said reflector.

34. An apparatus as claimed in claim 33 wherein the mirror is supported by a support with two mutually perpendicular rotating arms connected to respective actuators operated under the control of sensors associated with said laser interferometer.

35. An apparatus as claimed in claim 34, wherein each of said actuators consists of an electric motor provided with an angular transducer and controlled by said sensors, which are located on an annular element provided in said interferometer in a position surrounding the exit of said laser beam.

36. An apparatus as claimed in claim 31, further comprising a triaxial device for measuring distance from a fixed point.

37. An apparatus as claimed in claim 36, wherein said triaxial device comprises a sphere fixed in place of the final machine member, and three fixed sensors which measure the distance from said sphere in the three directions of said sensors.

38. An apparatus as claimed in claim 36, wherein said triaxial device comprises a fixed sphere and three sensors fixed in place of the final machine member, to measure the distance from said sphere in the three directions of said sensors.

39. An apparatus as claimed in claim 36, wherein the three sensors are rotary and of type suitable to each assume at least two separate positions.

40. An apparatus as claimed in claim 31, further comprising, for correcting the orientation of the final machine member, means consisting of the said devices for fixing the support head for said final member to the machine saddle.

41. An apparatus as claimed in claim 40, further comprising, in addition to said fixing devices, at least one conical peg for positioning said head.

42. An apparatus as claimed in claim 40, wherein each fixing device is mounted on the machine saddle and comprises a locking gripper cooperating with a stem fixed to the support head for the final machine member, said locking gripper being associated with a hydraulic piston able to exert on command an axial traction on said stem, acting against a deformable part present between said saddle and said head.

43. An apparatus as claimed in claim 42, wherein sensors are interposed between the saddle and the support head for the final machine member, for monitoring their mutual position.

44. An apparatus as claimed in claim 43, further comprising several individually controllable fixing devices exerting different tractions on the corresponding stems such as to induce rotations of said head with respect to said saddle about mutually perpendicular axes parallel to the surface of said saddle facing said head.

45. An apparatus as claimed in claim 41, wherein with the conical seat housing said conical peg there is associated a pusher acting on the lateral conical surface of said peg to ensure stability of the connection independently of its axial position in the respective seat.

46. An apparatus as claimed in claim 45, wherein elastic means are associated with said pusher to maintain it adhering to said conical peg.

47. An apparatus as claimed in claim 45, further comprising two spaced-apart conical pegs each provided, in its respective housing seat, with a pusher acting in the opposite direction to the other peg to induce, on the basis of the axial position of each peg in its respective seat, a rotation of said head with respect to said saddle about an axis perpendicular to that surface of said saddle facing the head.