Method and apparatus for controlling an electric discharge machining process

1. A method for controlling an electric discharge machining process, wherein a tool electrode is moved relatively to a workpiece with a working gap distance, wherein the process comprises a current and at least one previous erosion cycle, the current and the at least one previous erosion cycle each being divided into predetermined time intervals each comprising at least one discharge pulse, and wherein the current erosion cycle and the at least one previous erosion cycle are separated by a process pause cycle that is operable to restore a working gap condition, the working gap condition of the current erosion cycle and the previous erosion cycle being substantially similar, the method comprising the steps of:

measuring a value of an indicative process parameter within the time interval of the previous erosion cycle, the indicative process parameter being indicative of the working gap distance;

determining a deviation value based on the measured value and a desired value of the indicative process parameter of the time interval of the previous erosion cycle; and

in the current erosion cycle, controlling the relative movement of the tool electrode in a current erosion direction within the time interval of the current erosion cycle based on the deviation value determined for the time interval of the previous erosion cycle and at least one instantaneous process parameter that is indicative of an instantaneous process condition.

2. The method according to claim 1 , wherein each current and previous erosion cycles are divided into predetermined time intervals 1, . . . , k, . . . , N, and wherein the time interval k of the current erosion cycle and the time interval k of the previous erosion cycles have the same length.

3. The method according to claim 2 , wherein the relative movement of the tool electrode is controlled for the k th time interval of the current erosion cycle based on the deviation value determined for a . . . , (k−2) th , (k−1) th , k th , (k+1) th , (k+2) th , . . . time interval of the previous erosion cycle.

4. The method according to claim 1 , further comprising the step of generating a command value for controlling the relative movement of the tool electrode within the time interval of the current erosion cycle, based on the determined deviation value for the time interval of the previous erosion cycle and the instantaneous process parameter.

5. The method according to claim 1 , further comprising the step of computing a tool electrode position trajectory on the basis of the determined deviation value.

6. The method according to claim 5 , wherein the computing of a tool electrode position trajectory comprises the fitting with a mathematical function.

7. The method according to claim 5 , wherein the shape of a tool electrode position trajectory is preset and at least one process parameter is changed within an erosion cycle based on the determined deviation value.

8. The method according to claim 1 , wherein the process pause cycle comprises at least one of the following actions to restore the working gap condition:

a jump motion of the tool electrode,

a prolonged process pause without erosion pulses,

an active flushing pulse,

a magnetic field pulse, and

another tool electrode movement.

9. The method according to claim 1 , wherein the indicative process parameter of the previous cycle is at least one of:

an ignition delay time (td),

a mean pulse voltage,

a discharge voltage.

10. The method according to claim 1 , wherein an axis return position of the tool electrode is adapted on the basis of at least one characteristic of at least one previous electrode position trajectory.

11. The method according to claim 1 , wherein at least one actuation parameter of the process pause cycle is computed on the basis of the determined deviation value.

12. The method according to claim 1 , further comprising: within the time interval of the current erosion cycle, controlling at least one further actuation parameter based on the deviation value determined for the time interval of the previous erosion cycle.

13. The method according to claim 12 , wherein the at least one further actuation parameter is at least one of: an ignition voltage, a discharge current, a discharge duration, a pause time between two successive pulses, a pulse frequency, and the frequency and/or stroke of a short jump motion within an erosion cycle.

14. The method according to claim 1 , wherein the erosion cycle is represented by a complete micro electric discharge machining drilling operation.

15. An apparatus for controlling an electric discharge machining process, wherein the process comprises a current and at least one previous erosion cycle, the current and the previous erosion cycle each being divided into predetermined time intervals each comprising at least one discharge pulse, and wherein the current erosion cycle and the at least one previous erosion cycle are separated by a process pause cycle that is operable to restore a working gap condition, the working gap condition of the current erosion cycle and the previous erosion cycle being substantially similar, the apparatus comprising: a process controller that is operable to control the electric discharge machining process, and an axis drive configured to drive a tool electrode, wherein the process controller is configured to control the axis drive such that a tool electrode is moved relatively to a workpiece with a working gap distance and wherein the process controller is further configured to:

measure a value of an indicative process parameter within the time interval of the previous erosion cycle, the indicative process parameter being indicative of the working cap distance;

determine a deviation value based on the measured value and a desired value of the indicative process parameter of the time interval of the previous erosion cycle; and

in the current erosion cycle, control the relative movement of the tool electrode in a current erosion direction within the time interval of the current erosion cycle based on the deviation value determined for the time interval of the previous erosion cycle and at least one instantaneous process parameter that is indicative of an instantaneous process condition.

16. The apparatus according to claim 15 , wherein the apparatus is an electric discharge machine.