Apparatus and method for detecting the location of an electrical discharge along the electrode wire of an EDM apparatus

In a travelling wire EDM apparatus, the location of an electric discharge along the portion of the electrode wire in the machining zone between the electrode wire and the workpiece, in the course of a cut being effected on the electrode workpiece by electrical discharges, is measured by measuring the currents flowing through each of the two lines connecting one output of the pulse generator associated with the EDM apparatus to the electrode wire on one side and the other of the machining zone. The measured currents are added and subtracted and the position, or location, of an electrical discharge is determined by a signal proportional to the quotient of the difference by the sum of the currents.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus and method for detecting the 
location of an electrical discharge in the machining zone of a travelling 
wire EDM apparatus. 
Travelling wire EDM apparatus are designed to cut an electrode workpiece, 
by machining electrical discharges, by means of an electrode wire. A pulse 
generator, providing the machining electrical discharges, is connected 
across the electrode workpiece. The connection to the electrode wire is 
generally effected by a pair of electrical conductors supplying the 
machining current, one conductor being connected to the electrode wire on 
one side of the machining zone and the other on the other side. 
Connecting the pulse generator to the electrode wire by way of a pair of 
electrical conductors is a necessary expedient for keeping under control 
the excessive thermal stress imposed upon the electrode wire due to 
heating caused by the electrical discharges themselves and by the 
resistive heating of the wire resulting from the flow of machining current 
through the wire. The heating of the wire, if remaining unchecked, may 
exceed acceptable limits and cause rupture of the wire, for example if the 
electrical discharges are concentrated at a specific location along the 
portion of the wire in the machining zone. Monitoring the concentration of 
the elecrical discharges by way of detecting the precise location of each 
electrical discharge permits to prevent electrode wire rupture. 
One known method for detecting the location of an electrical discharge 
which is disclosed in published Japanese patent application No. 53/64899 
consists in measuring the resistance of the electrode wire between the 
location of the electrical discharge and one of the contacts supplying 
machining current to the electrode wire. Such a method presents the 
inconvenience of measuring a voltage drop on a portion of the wire, which 
is a measurement that is very inaccurate due to the low impedance of the 
circuit in which the measurement is effected. 
Another method, also disclosed in the same Japanese patent application 
publication consists in measuring the voltage unbalance of a Wheatstone 
bridge, one of the branches of the bridge consisting of the length of wire 
between the two contacts supplying machining current to the wire, each 
contact being located on one side of the machining zone. Such a method is 
practical only if all the electrical discharges occur at the same current 
level. Otherwise, as is for example the case when the pulse generator is 
of the relaxation type, which is often the type of pulse generators used 
in travelling wire EDM apparatus, the intensity of the electrical 
discharges varies as a function of the triggering voltage of the 
discharges, and the amplitude of voltage unbalance of the bridge varies as 
a function of the intensity. Under such circumstances, the known method 
becomes useless. 
SUMMARY OF THE INVENTION 
The present invention has for principal object to overcome the 
disadvantages and limitations of the known method, by providing a 
circuitry comprising a pair of electrical conductors for supplying 
machining current to the EDM apparatus electrodes wire on each side of the 
machining zone. 
An arrangement according to the present invention provides means for 
subtracting and adding the intensities of the currents flowing 
respectively in each of the lines, in the course of each electrical 
discharge, and a circuit for obtaining a value which is proportional to 
the quotient of the subtraction by the addition. The invention provides 
the principal advantage of effecting very accurate measurements, even 
though the current level of the electrical discharges is constantly 
variable. 
The diverse objects and advantages of the present invention will become 
apparent to those skilled in the art when the following description is 
read in conjunction with the accompanying drawing schematically showing, 
for illustrative purpose, an example of apparatus according to the 
invention, and in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 illustrating schematically a travelling wire EDM 
apparatus, an electrode wire 1 is supplied from a reel or spool 2 and, 
after passing over a roller or pulley 3, travels through a machining zone 
5 wherein it effectuates, by electrical discharges, a cut on a workpiece 
6. After being passed over a second roller or pulley 8, the used electrode 
wire is wound on a reel or spool 9. The EDM apparatus further comprises a 
pulse generator 10 having a terminal connected to the electrode workpiece 
6 and the other connected, via two lines 11 and 12, to the electrode wire 
1 by two contacts 13 and 14, one contact being disposed on one side of the 
machining zone 5 and the other being disposed on the other side of the 
machining zone, for supplying an appropriate pulsed machining current for 
cutting the electrode workpiece 6 by electrical discharges. 
The location at which occurs an electrical discharge 15 is situated at a 
distance Z from the mid-distance, or D/2 between the contacts 13 and 14, D 
being the distance between the contacts or the length of the electrode 
wire 1 between the contacts. The distance Z is found by resolving the 
following equation: 
##EQU1## 
I.sub.1 being the current flowing through the line 11 and I.sub.2 being the 
current flowing through the line 12. 
The currents I.sub.1 and I.sub.2 are measured, and an appropriate computing 
circuit, generally designated at 16, provides at its output a value Z' 
which is equal to the quotient of the difference between the two current 
values by the sum of the two current values. The value Z' is proportional 
to the distance Z, or KZ'=Z and the constant, or proportion factor, K is 
equal to half the length D of the electrode wire 1 between the two 
contacts 13 and 14. 
As illustrated at FIG. 2, an example of computing circuit 16 comprises two 
current transformers 17 and 18 for determining the difference between the 
currents I.sub.1 and I.sub.2 and the sum of the currents. The primary 
windings 19 and 20 of the transformer 17 are wound in opposite direction 
such as to induce in the secondary winding 21 of the transformer 17 a 
current proportional to the difference between the magnetic fluxes induced 
by the currents I.sub.1 and I.sub.2, flowing respectively through the line 
11 and the line 12. The primary windings 22 and 23 of the second current 
transformer 18 are wound in the same direction such that the current in 
the secondary winding 24 of the transformer 18 is proportional to the sum 
of the magnetic fluxes induced by the currents flowing through the primary 
windings, or currents I.sub.1 and I.sub.2. 
The current flowing through the secondary winding 21 is supplied to a 
current-to-frequency converter 25, and the current flowing the secondary 
winding 24 is supplied to a current-to-frequency converter 26 for 
providing output signals respectively of the frequency Fi and the 
frequency F.DELTA.i. 
The output signal of frequency Fi corresponds to the sum of the currents 
I.sub.1 and I.sub.2 and is applied to one of the inputs of an up-down 
counter 27, while the other output signal of frequency F.DELTA.i, 
corresponding to the difference between the currents I.sub.1 and I.sub.2, 
is applied to the input of a frequency divider 28, consisting, for 
example, of a binary rate multiplier (BRM) circuit. The signal at the 
output of the frequency divider 28 is applied to another input of the 
up-down counter 27, and the digital signal at the output of the up-down 
counter 28 is applied to the control input of the frequency divider 28 to 
provide its instantaneous division factor. According to the illustrated 
arrangement, whose individual components are well known in the art, the 
signal at the output of the up-down counter 27 varies the division factor 
of the BRM frequency divider 28 until F.DELTA.i=Fi, such that the output 
signal of the up-down counter 27 remains constant. Under such conditions, 
Z'=F.DELTA.i. It is readily apparent that the signals of frequency Fi and 
F.DELTA.i are at a frequency much higher than the frequency of the 
electrical discharges in order to permit obtaining a stable value of Z', 
during the time intervals between consecutive electro-erosive electrical 
discharges. 
The signal Z' is in turn applied to a warning device, or is used for 
controlling the machining current. 
It will be appreciated that the operations effected by the example of 
circuit herein illustrated and described could be effected by different 
circuits. More particularly, signals proportional to I.sub.1 and I.sub.2 
can be added or subtracted by analog means, such signals being provided 
separately by a current transformer. The sum of the currents I.sub.1 and 
I.sub.2 can also be measured directly at the output of the pulse generator 
10, FIG. 1, and the difference between the currents measured by measuring 
the voltage differential between the two contacts 13 and 14 supplying 
machining current pulses to the electrode wire 1, for example by shunting 
the two contacts and measuring the current flowing through the shunt. 
Having thus described the invention by way of an example of structure well 
designed for accomplishing the objects of the invention, modifications 
whereof will be apparent to those skilled in the art, what is claimed as 
new is as follows: