System for monitoring workpiece and tool carriages movement using a two-axis scale attached to a carriage cooperating with a reading head attached to the other carriage

• US 6,298,278 B1
• Filed: 07/07/1998
• Issued: 10/02/2001
• Est. Priority Date: 02/19/1996
• Status: Expired due to Fees

• Current Assignee: Cinetic Landis Ltd.
• Original Assignee: Unova Incorporated (Honeywell International Inc.)
• Inventors: Pierse, Michael George
• Primary Examiner(s): Lee, Thomas
• Assistant Examiner(s): Cao, Chun
• Application Number:
  US09/111,159
• Time in Patent Office: 1,183 Days
• Field of Search: 700/13, 700/108, 700/180, 700/186, 700/188, 700/159, 700/196, 700/174, 451/8, 451/9
• US Class Current: 700/174
• 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 19/404 characterised by control arrangements for compensation, e.g. for backlash, overshoot, tool offset, tool wear, temperature, machine construction errors, load, inertia G05B19/19, G05B19/41 take precedence
• CPC Class Codes: G05B 2219/34047 Dsp digital signal processor
• CPC Class Codes: G05B 2219/34242 For measurement only
• CPC Class Codes: G05B 2219/37098 X y scale plate instead of two ruler scale, two dimensional scale
• CPC Class Codes: G05B 2219/37306 Two sensors and two scales for same measurement of relative movement between x y
• CPC Class Codes: G05B 2219/37307 Detector in line, in plane of tool to avoid parallax
• CPC Class Codes: G05B 2219/37576 Post-process, measure worpiece after machining, use results for new or same
• CPC Class Codes: G05B 2219/37611 Relative movement between tool and workpiece carriage
• CPC Class Codes: G05B 2219/45141 Turret lathe
• CPC Class Codes: G05B 2219/49193 Orthogonality of axis, deviation from 90-degree correction
• CPC Class Codes: G05B 2219/49194 Structure error, in slide or screw
• CPC Class Codes: G05B 2219/49195 Slide, guideway, robot arm deviation
• CPC Class Codes: G05B 2219/49219 Compensation temperature, thermal displacement
• CPC Class Codes: G05B 2219/50391 Robot
• CPC Class Codes: Y10T 29/5126 Swiss type
• CPC Class Codes: Y10T 82/2506 And tool turret

Abstract

A machine tool has a carriage, has a workpiece carriage for carrying a workpiece from which material is removed by a tool carried by a tool carriage. A two-axis scale is attached to one of the carriages, and cooperates with a reading head attached to the other carriage. The output from the reading head is processed by signaling processing means to provide an indication of the position and/or movement of the carriages relative to each other and for generating positional data about one or both of the carriages to assist in controlling their movement in order to perform a machine operation. By directly measuring the relative positions of the two carriages, problems arising from, for example, distortions of the machine frame on which the carriages are mounted are at least mitigated. A preferred embodiment of the machine incorporates a second reading head attached to one carriage and incorporating the scale on another carriage so that the outputs from the two reading heads can be used to determine whether there has been any relative angular displacement of the paths followed by the carriages. There is also provided a method of determining the linear movement of one of the carriages relative to the other, and the machining process achieved using the machine tool.

Claims

1. A method of determining the linear movement of a first carriage relative to a second carriage in a machine tool, wherein each carriage carries a respective one of a workpiece and a tool for machining the workpiece, and wherein two-axis scale means is attached to one carriage so as to move therewith, and a reading head which cooperates with the scale means is attached to the other carriage to move therewith, the method comprising the steps of:

deriving positional data from the reading head and scale means for the first carriage, exerting a force on at least one of the two carriages to effect a movement thereof relative to the machine, and deriving a value for the linear displacement of the first carriage relative to the second by determining the difference between the positional data at the beginning and that at the end of the carriage movement, wherein a second reading head is attached to one of the two carriages at a position thereon separate from the position of attachment of the first reading head measured parallel to the axis of movement of the other carriage, the second reading head cooperating with said two-axis scale means attached to the other carriage, the method further comprising the steps of deriving second positional data relating to the first carriage from the second reading head, and computing an error value from any difference between the displacement values derived from the positional data from the two separate reading heads, and correcting the linear displacement value obtained from the first reading heads by using the said computed error value.

2. A method according to claim 1, in which both reading heads are attached to the same carriage, both heads cooperating with a single two-axis scale means attached to the other carriage.

3. A method according to claim 1, in which the first carriage is moved in incremental steps and the position value at the end of each step adjusted, so accurate positioning of a carriage can be achieved.

4. A method according to claim 1, in which the method includes the step of adjusting the position of the workpiece or tool carriage, or both, to compensate for any difference in the positional data obtained from the reading heads consequent on any unwanted movement of one carriage relative to the machine and/or the other.

5. A method according to claim 4, in which the difference is monitored and compensated for by adjusting the drive to the workpiece carriage, or tool carriage, or both, so as to reposition one or the other or both to the position it or they should have adopted had the movement been undistorted, so as to compensate for any distortion and maintain the point of engagement of the tool and the workpiece independent of distortion between the workpiece and tool carriage axes.

6. A method according to claim 1, in which viscous damping is applied to the carriage movements.

7. A machine comprising a movable workpiece carriage for carrying a workpiece to be machined and a movable tool carriage for carrying a tool for removing material from the workpiece, a two-axis scale means mounted on one of the carriages so as to move therewith, a reading head adapted to cooperate with the scale means attached to the other said carriage, and signal processing means for receiving signals from the reading head indicative of the position and/or movement of one carriage relative to the other, and generating positional data about one or both of the carriages to assist in controlling the movement of one or both carriages to effect a machining operation, and in which the machine tool includes a further reading head also adapted to cooperate with the scale means, and mounted on the same carriage as the reading head but at a different position from that at which the head is mounted, and in which the signal processing means is receptive of signals from both heads and provides a first output signal indicative of the linear movement of one carriage relative to the other, and a second output signal indicative of any difference between the positional signals derived from the two heads.

8. A machine tool according to claim 7, in which the reading head is attached to the underside of the tool carriage close to the machining point and the scale is mounted in a similar manner to the workpiece carriage.

9. A machine tool according to claim 7, in which the scale means comprises two two-axis scales, one for each respective head, arranged in substantially the same plane.

10. A machine tool according to claim 9, in which the two scales are attached to one carriage, and are arranged as close as possible to the point of engagement between the tool and the workpiece, and in which the two reading heads for cooperating with the two scales are carried by the other of the two carriages.

11. A machine tool according to claim 7, in which the machine tool includes feedback means for adjusting the position of a workpiece or tool carriage to take account of any error signal obtained from signal processing means receptive of positional data from the two reading heads.

12. A machine tool according to claim 7, in which the machine tool includes viscous damping means comprising a first plate fitted to the tool carriage and a second plate fitted to the workpiece carriage, the two plates being in close proximity to the scale system, and means for applying viscous damping to the two plates.

13. A machine tool according to claim 12, in which said means for applying viscous damping to each plate comprises a respective separate viscous damping medium, each of which is adjustable for fine tuning.

14. A machine tool according to claim 12, in which the application of damping to the two plates is achieved by means of a single viscous damping medium into which the plates protrude.

15. A machine tool according to claim 14, in which one of the plates overlies the other and a viscous fluid is introduced between the two plates to provide the said damping.

16. A machine tool according to claim 7, in which the carriages move along axes defined by respective guideways mounted on orthogonal vertical faces of a machine structure so as to reduce the distance between the two guideways.

17. A machine tool according to claim 9, in which one two-axis scale is mounted in close proximity to the workpiece carriage and the other two-axis scale is also mounted to the workpiece carriage but spaced therefrom by a distance similar to that between the first mentioned scale and the workpiece, measured along the workpiece axis.

18. A machine tool according to claim 17, in which the first scale preferably extends horizontally substantially in alignment with the tool and tool mounting whilst the second mentioned scale extends horizontally substantially in alignment with the workpiece and its support, so that parallax errors as between tool and first scale and between the workpiece and the second scale can be reduced.

19. A method of reducing or eliminating machine induced errors which affect the orthogonality of the axes of a first and a second carriage of a machine tool the carriages being moveable substantially orthogonally with respect to one another, each carriage carrying a respective one of a workpiece and tool for machining the workpiece, comprising the steps of attaching a two-axis scale means to one carriage so as to move therewith, attaching a reading head which cooperates with the scale means to the other carriage to move therewith, the reading head being disposed in substantially the same plane as the two-axis scale means, deriving positional data from the reading head and scale means for the first carriage, exerting a force on at least one of the two carriages to effect a movement thereof relative to the machine, and deriving a value for the linear displacement of the first carriage relative to the second by determining the difference between the positional data at the beginning, and at the end, of the carriage movement.

20. A machine tool comprising a moveable workpiece carriage for carrying a workpiece to be machined by rotation about its axis and a tool carriage for carrying tool for removing material from the workpiece, the two carriages being moveable in orthogonal directions, two-axis scale means mounted on one of the carriages so as to move therewith, reading head means adapted to cooperate with the scale means attached to the other carriage, and signal processing means for receiving signals from the reading head means indicative of the position and/or movement of one carriage relative to the other, and for generating positional data about one or both of the carriages to assist in controlling the movement of one or both carriages to effect a machining operation, the reading head means being disposed in substantially the same plane as the scale means but spaced apart a minimal distance on opposite sides respectively of the workpiece axis such that parallax errors as minimised.

21. A machine tool according to claim 20, in which the reading head means comprises a first reading head, and a second reading head also adapted to cooperate with the scale means and mounted on the same carriages as the first reading head but at a different position from that at which the first reading head is mounted, and in which said signal processing means is receptive of signals from both said reading heads and provides a first output signal indicative of the linear movement of one carriage relative to the other, and a second output signal indicative of any difference between the positional signals derived from said reading, heads.

22. A method of machining so as to remove material from a rotating workpiece using a tool which may be static or rotating, the method comprising the steps of moving the tool so as to engage the surface of the workpiece to remove material therefrom, determining the position of the tool carriage using a two-axis scale measuring system comprising a reading head mounted to one of the workpiece carriage or tool carriage and two-axis scale means mounted to the other, positioned close to the working region, and controlling the advance of the tool, and therefore the extent of the machining, using positional data from the reading head, and further including the steps of determining the tool carriage position by means of a second reading head also cooperating with said two-axis scale means, and mounted to the same carriage as the first mentioned reading head, and measuring any distortion and utilizing any measured divergence between scale readings and predicted scale readings following any given linear carriage movement of to enable a correction to be made in the linear carriage movement and thereby compensate for any distortion due to machining forces.

23. In a machining process in which a tool is progressively moved by means of discrete movement instructions relative to a rotating workpiece so as to remove material from a rotating, surface thereof, first tool position defining signals are obtained from a first reading head which is attached to and movable with the tool support and which cooperates with two-axes scale means attached to the workpiece support for movement parallel to the axis of rotation of the workpiece as the workpiece support is moved;

and second tool position defining signals are obtained from a second reading head also attached to and movable with the tool support and cooperating with the two-axis scale means; and

error signals indicative of any positional error of the tool are obtained by determining the values of the signals which should be obtained from the second reading head for a given linear movement of the tool from the first position to a second position using tool position defining signals obtained from the first reading head, and making a comparison therewith of the actual second tool position defining signals obtained from the second reading head.

24. A method according to claim 23, in which the position of one support or the other (or both) may be adjusted in response to any error signal derived from the said comparison, to achieve a desired position of the tool relative to the workpiece, or the next movement instruction so the tool support may be modified to take account of the actual position of the tool as a result of the previous movement instructions.