GB2061554A

GB2061554A - Control System for Producing Crankshafts

Info

Publication number: GB2061554A
Application number: GB7936258A
Authority: GB
Original assignee: GFM Gesellschaft fuer Fertigungstechnik und Maschinenbau AG
Current assignee: GFM Gesellschaft fuer Fertigungstechnik und Maschinenbau AG
Priority date: 1979-10-18
Filing date: 1979-10-18
Publication date: 1981-05-13
Also published as: GB2061554B

Abstract

Work-holders support a crankshaft 6 centered on a fixed axis and are rotated by first drive 7. A second drive 15 displaces a tool transverse to said axis. A first controller 17 is connected to the first drive and causes it to impart a desired angular movement to said work-holders. A second controller 18 is connected to said second drive and controls said displacement. Drive control computer means 19 process workpiece and tool data and compute increments of said displacement, and ranges of positions of said tool carriage. Interpolating means apply reference inputs to both said controllers in dependence points of inflection and characteristic values defining a series of linear approximations of a generally nonlinear relationship of the movements of the two drives. <IMAGE>

Description

SPECIFICATION Control System for a Machine for Milling or Grinding Crankshafts This invention reiates to a control system for a machine for milling or grinding crankshafts, comprising drive means for rotating the crankshaft, and a tool carriage, which is movable transversely to the axis of the crankshaft by drive means which are controlled by a controller in dependence on the angular position of the crankshaft.

In a known system of that kind, the controller for the drive means for displacing the tool carriage is controlled by a memory, in which the displacements of the tool carriage associated with each increment of angular movement of the crankshaft are stored and the movement imparted to the tool by the tool carriage in dependence on said reference inputs and the rotation of the workpiece result in combination in a machining of a contoured surface. That known control system has the disadvantage that the data for the displacement of the tool carriage in dependence on the angular movement of the crankshaft, which data depend on data relating to the workpiece, must be computed and stored.A decisive other disadvantage resides in that any variations of the actual angular movement of the crankshaft will directly influence the drive means for the tool carriage because the angular position data are derived from the rotating crankshaft by a pulse generator. As a result, the inevitable vibrations and angular oscillations of the crankshaft being milled influence the movement of the tool carriage and may even be amplified as they are transmitted. Owing to this virtually rigid coupling between the drive means for rotating the crankshaft and the drive means for displacing the tool carriage, the tolerance with which the crankshafts can be machined cannot be decreased as much as would be desired.

It is an object of the invention so to improve a control system of the kind described first hereinbefore for a machine for milling or grinding crankshafts that the inevitable oscillations imparted by the tool to the crankshaft will not influence the control of the tool carriage and that the machine can be adapted to different workpieces and tools by an input of data which reflect such differences.

This object is accomplished according to the invention in that a separate controller is provided for the drive means for rotating the crankshaft and both controllers are controlled by common linear interpolating means which are connected to drive control computing means, which are fed with workpiece and tool data and from said data compute data representing points of inflection and characteristic values which define a series of linear approximations of a generally nonlinear relationship between the angular movement of the crankshaft and the displacement of the tool carriage.

As a result of these measures, a reference input is applied via the interpolating means to the controller for the drive means for rotating the crankshaft and to the controller for the drive means for displacing the tool carriage so that irregularities of one movement cannot influence the other movement because the actual movement of the member driven by one drive means is not relied upon in controlling the other drive means. The linear interpolating means are connected to drive control computing means and control the two controllers in accordance with the points of inflection determined by the drive control computing means. As a result, the predetermined relationship between the movement of the carriage and the increments of the desired angular movement of the crankshaft is maintained.That relationship depends on the geometrical configuration of that crankshaft portion which is being machined. A series of linear approximations is derived from the generally nonlinear relationship between the angular movement of the crankshaft and the displacement of the carriage. The drive control computing means provide data which represent the points of inflection which are required to ensure that the machining will be effected within a predetermined tolerance range, and the factors of proportionality which are changed at those points of inflection. The drive control computing means calculate the points of inflection and the factors of proportionality in accordance with a predetermined program from input data relating to the workpiece and tool.

A separate linear interpolator must be clockcontrolled at the rate required to ensure a desired machining rate. The expenditure involved in a separate linear interpolator can be avoided if, in accordance with a further preferred feature of the invention, the drive control computing means and the linear interpolating means combined in a central computer, which in accordance with a fixed program delivers control pulses and direction signals at a clock frequency determined by the program. Because that central computer delivers control pulses rather than only the results of arithmetic operations, expensive separate interpolators, which cannot be adjusted, are no longer required. When such central computer is used as a control unit, the control pulses must not be delivered at a frequency which depends on the computing rate but must be delivered at a predetermined rate.In accordance with its program, the computer selects the required frequency from a plurality of frequencies, which determine respective machining rates.

Embodiments of the invention are shown by way of example on the accompanying drawings, in which Figure 1 is a top plan view showing a milling machine embodying the invention, Figure 2 is a diagrammatic side elevation showing the tool carriage of that machine, Figure 3 is a block circuit diagram showing the control system according to the invention for a machine as shown in Figures 1 and 2.

Figure 4 is a block circuit diagram illustrating different means for controlling the two controllers for the drive means for the crankshaft and the drive means for the tool carriage, and Figure 5 is a block circuit diagram showing one of the two controllers.

The machine for milling crankshafts shown in Figure 1 comprises a frame 1 having end brackets 2 and 3, in which holders 4 and 5 for the crankshafts 6 to be machined are mounted. These holders are adapted to be driven by a motor 7, which drives via a transmission 8 a main drive shaft 9, from which the drive of the workpiece holders 4 and 5 is derived via respective worm gear trains 10. The frame 1 comprises rails 1 which are parallel to the axis of the crankshaft 6 and on which a guide carriage 12 is slidably mounted. A tool carriage 13 is carried by the guide carriage 12 and on guide rails 14 of the carriage 12 is movable by means of a motor 15 and a power screw 1 spa transversely to the axis of the crankshaft.The displacement of the carriage 1 3 and the rotation of the crankshaft 6 can be coordinated so that the revolving milling cutter 1 6 is moved, e.g., around a crankpin so as to machine a contoured surface thereon.

In accordance with Figure 3, the motors 7 and 1 5 are controlled by respective controllers 1 7 and 18, which are preceded by a central computer 19.

Workpiece and tool data are read into the central computer 19 by data input means 20. From said data, the central computer 19 in accordance with its fixed program computes points of inflection which define a series of linear approximations of the nonlinear relationship between the angular movement of the crankshaft 6 and the displacement of the tool by carriage 13, and the factors of proportionality which change at said points of inflection, and stores the data thus computed. In accordance with the data thus computed, the central computer 19 delivers control pulses as reference inputs to the controllers 17 and 18 at the ratio which is required to ensure that the tool is moved around the workpiece so as to form a contoured surface thereon.In accordance with its program, the central computer 19 selects the control pulse output rate in dependence on the setting of the machining rate selector 21. These control pulses are applied not only to the controllers 17 and 18 but also to respective frequency-voltage converters 22 and 23, which deliver respective reference input voltages to the motor controllers 24 and 25, respectively. As a result, the motors 7 and 1 5 are controlled in dependence on the required relationship between each increment of the angular movement of the crankshaft and the displacement of the tool carriage 13. Data representing said movements are delivered from respective pick-ups, 26 and 27, such as pulse generators, to the controllers 17 and 18, respectively, and are compared in the latter with the reference input delivered by the central computer 19.In case of a deviation, the error signal is converted in a digital-analog converter 28 or 29 into a correcting voltage, which is applied to the motor controller 24 or 25. The accuracy of the automatic control can be further increased by a comparison of the desired and actual speeds of each motor. For this purpose, suitable tachometer 30 and 31 are connected to the motors 7 and 15, respectively.

In accordance with Figure 4, the controllers 17 and 18 for the motor controllers 24 and 25 are controlled by a separate linear interpolator 32, which is supplied from a drive control computer 33 with data indicating the required points of inflection and the linear relationship between the increments of the angular movement of the crankshaft 6 and the displacement of the tool carriage 13. That linear relationship is changed.at said points of inflection. For each increment of angular movement, the drive control computer 33 computes the corresponding displacement and delivers data corresponding to the ratio of said movements to the interpolator 32, which delivers control pulses to the controllers 17 and 18 in accordance with said ratio.As a result, the drive control computer 33 has time for computing and storing new data until the next point of inflection has been reached. The interpolator 32 must be clock-controlled at a rate determined by the machining rate selector 21 so that the desired machining rate is maintained.

Figure 5 shows the basic arrangement of the two controllers 17 and 18. The control pulses delivered by the central computer 19 or the interpolator 32 are applied to a desired-value counter 34, which is connected to a desired value-actual value comparator 35.

The actual value is delivered to the controller 1 7 or 18 by a pick-up 26 or 27 and is applied therein to an actual-value counter 37, the output of which is also connected to the desired valueactual value comparator 35. The motor controller 24 or 25 can then be controlled in response to an error signal delivered by the desired value-actual value comparator 34. In thus way, the error can be restricted to a desired tolerance range. For this purpose the output signal of the desired valueactual value comparator can be applied to a comparator 38 in which the error signal representing the difference between the desired and actual values is compared with a tolerance value set by the tolerance selector 39. If the difference between the desired and actual values exceeds the preset tolerance in spite of a continuous automatic control of the two drive means, e.g., because the drive means respond too slowly or owing to mechanical obstructions, the comparator 38 will deliver a signal which causes the operation to be stopped and the trouble to be indicated. The operation cannot be resumed until the cause of the trouble has been eliminated.

Claims

1. A control system for a machine for milling or grinding crankshafts, comprising drive means for rotating the crankshaft, and a tool carriage, which is movable transversely to the axis of the crankshaft by drive means which are controlled by a controller in dependence on-the angular position of the crankshaft, characterized in that a separate controller is provided for the drive means for rotating the crankshaft and both controllers are controlled by common linear interpolating means which are connected to drive control computing means, which are fed with workpiece and tool data and from said date compute data representing points of inflection and characteristic values which define a series of linear approximations of a generally nonlinear relationship between the angular movement of the crankshaft and the displacement of the tool carriage.

2. A control system according to claim 1, the drive control computing means and the linear interpolating means combined in a central computer, which in accordance with a fixed program delivers control pulses and the direction signals at a clock frequency determined by the program.

3. A control system constructed and arranged substantially as herein described and as shown in the Figures of the accompanying drawings.