Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Patent
1997-11-24
2000-03-14
Grant, William
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
700188, 700187, 700160, 700193, 700161, 700165, 700195, 82106, 82118, 82133, 2988808, G06F 1900, G06G 778
Patent
active
060384891
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method of controlling a machine tool so as to remove material from a rotating work-piece to form the latter into a desired shape which, is circularly asymmetric about the axis off rotation of the work-piece. The invention is particularly applicable to machines for the grinding of components such as, for example, crankpins and camshafts. The invention is also applicable to other machines, for example milling machines, brooch turning machines and turning machines (such as lathes).
BACKGROUND TO THE INVENTION
A work-piece rotating about an axis can be ground into parts which are circularly asymmetric with the axis of rotation by means of a grinding wheel which is moved laterally to that axis. In order to grind the parts rapidly, and to a high degree of geometric accuracy, a high performance servo system is used to control the movement of the grinding wheel towards and away from the work-piece as the latter rotates.
The servo system includes sensors for measuring the position of the wheel and the velocity with which it moves towards or away from the work-piece, and these are connected in respective position and velocity feedback loops which facilitate the accurate control of those parameters by a control computer. Typically, the position feedback loop has a bandwidth of 10 Hz and the bandwidth of the velocity feedback loop is 200 Hz.
The controlling computer can calculate very accurately the exact grinding wheel position required for any angular portion of the work-piece, (the work-piece angle) so as to generate the required offset diameter. The same computer can measure the instantaneous position of the grinding wheel (through the position feedback loop) at any work-piece angle by using a high speed, high accuracy measuring system such as a linear scale to a typical position resolution of 0.0001 mm.
The computer, knowing the required position and the actual position of the grinding wheel can calculate a position error and feed an error signal derived therefrom to the servo system controlling the movement of the grinding wheel, in order to correct its position and thereby minimise the position error for all work-piece angular positions.
In order to achieve the required geometric tolerances in the finished component, grinding wheel position errors of typically less than 0.001 mm are required.
However, it is difficult to maintain the required error envelope to achieve such accuracy, especially if the work-piece is to rotate at speeds of more than 20 rpm, as a result of the limited bandwidth of the position control loop. Furthermore, there is no correction of time varying contributions to the error (for example electronic or servo drift) which can become significant over a protracted period of time.
It is an object of the present invention to provide an improved control system and method for controlling a machine tool so as to achieve the desired error envelope with higher speeds of rotation of the work-piece.
SUMMARY OF THE INVENTION
According to the invention, there is provided a method of controlling a machine tool so that a component on the machine tool removes material from a rotating work-piece so as to form the latter into a desired shape which is circularly asymmetric, the method comprising the steps of: positions and positional velocities of the component relative to the workpiece, and which causes the component to move relative to the work-piece in directions lateral to the axis of rotation of the latter; respective position of the component relative to the work-piece when the work-piece is in each of a succession of angular positions; and storing data indicative of any differences between the measured and theoretical positions; and work-piece so as to compensate for said differences and thereby reduce the magnitude of any error which would have resulted from the measured positional differences.
The component and the work-piece are conveniently mounted on a frame of the machine. The component may be fixed relative to the frame, in which case said
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Griffiths Selwyn Jonathan
Pickles John David
Calcano Ivan
Grant William
Unova U.K. Limited
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