FIELD OF THE INVENTION
This invention relates to an improved voltage and current measurement system for
sparks.
This invention further relates to a voltage and current measurement system for sparks
with application to electro discharge machining.
PRIOR ART
Electro-Discharge Machining (EDM) is a process basically constituting material removal
from a work-piece through electric spark, the mechanics being applied in the process is
that of fusion and vaporization. Such a process is generally applicable for removal of
electrically conductive materials for example, superalloys, glass and semiconductors by a
controlled thermal erosion by generating a series of minute controlled sparks
(discharges) between a tool and work electrodes separated by a thin film of dielectric
fluid. In WEDM, a wire of 5-500 ym is used as cathode and the workpiece is connected
as anode. A unipolar D.C. pulse train of high frequency up to lOOKHz is applied across
the electrodes to cause the spark. When a discharge takes place between two points of
the anode and cathode, intense heat is generated near the sparking zone and the generated
heat melts the materials in the zone and evaporates the materials. The sparks travel all
over the surface of the workpiece thereby ensuring a uniform removal of the material.
A suitable gap known as the spark gap is maintained between the tool and work piece
surface. Most of the WEDM machines although have positional feed back means to
maintain the spark gap, but the process control is manual even for the most up-to -date
WEDM of the art. An electric circuitory is provided for the pulsating d.c. across the
workpiece.
In the wire-EDM machine of the art with voltage and current measurement system,
the power supply and controller unit charges the capacitors and controls the pulse
parameters (Ton, T0ff, Ip etc). The capacitors discharge over the electrodes.
A schematic diagram of a WEDM machine of the art (fig.l) including a detailed
diagram of the circuitory for voltage and current measurement (fig.2) indicate that the
current is measured by using a resistive shunt and the voltage history is collected
across the two electrodes. All the signals are drawn through series resistors to restrict
the flow of current through the instruments. All the wires and cables used for the
instrumention are shielded for noise reduction. The acquired voltage and current
history (fig. 3), indicates that signal to noise ratio is very low. The current wave form
is some times negative, which is unrealistic. Therefore, the circuitory requires
improvement to acquire a noise-free voltage and current signature.
OBJECTS OF THE INVENTION
An object of this invention is to propose a voltage and current measurement system
for sparks in which the voltage and current wave forms acquired showing the
signature of a good and stable condition.
Another object of this invention is to propose a voltage and current measurement
system for sparks which can be used for pulse counting .
Yet another object o f this invention is to propose a voltage and current measurement
system for sparks which can be used in low frequency applications for process
monitoring.
SUMMARY OF THE INVENTION
An improved voltage and current measurement system, incorporating the invention,
comprises a cathode; an anode; a power supply and controller unit; a capacitor bank
having a plurality of capacitor; a primary circuitory consisting of a plurality of
resistors; a secondary circuitory consisting of a current sensor, a resistor, and a
plurality of diode; and electrical cables connecting said circuitory . The voltage
tapping points across said cathode and said anodes are connected to the grounds, said
resistors constitute metal film resistors, said electrical cables constitute thick single
core wires, and said current sensor is a non-contact type 'Hall-effect" Sensor.
In accordance with this invention, the voltage tapping points have been changed
through modified ground connectors. The shielded cables, which act as active
elements (inductance) for higher frequencies, are replaced with thick single core
wires to avoid distortion of the signals during transmission. High quality metal film
resistors have been used for construction of the attenuating circuit. The voltage
waveform is thus improved but the current waveform was not satisfactory. This
problem was then solved by changing the sensor to a non-contact type 'Hall-effect'
current sensor.
Hall-effect is described as a phenomemon where a metal or semiconductor if carrying
a current I is placed in a transverse magnetic field of flux density B, an electric field
is developed along a direction perpendicular to both B and I.
The current bearing cable is passed through the hole of the sensor. The DC power
supply delivers +15 V to the secondary winding . Due to the flow of current during
sparking, a magnetic flux is generated, which modifies the flow of current in the
secondary circuitry. This is turn modifies the primary current flow, which is
measured from the voltage drop. The diodes are used to ensure unidirectional
current flow through the circuit.
The voltage and current waveforms acquired through the new circuit shows the
signature of a good and stable machining condition. The voltage waveform clearly
describes the history of charging and discharging of the capacitors. As the capacitors
are negatively charged, the whole voltage waveform is negative. The downward
trend in the voltage waveform describes charging and the upward trend describes
discharge (sparking). Corresponding to every discharge locations in the voltage
history, the current waveform shows a positive peak, and it is zero at all other
locations. The circuitry can even capture a cluster of sparks vividly while unstable
machining is taken place, an arcing condition which occurs at a higher frequency, and
also the wire rupture event.
Thus, on-line process monitoring is possible if the voltage and current signatures,
acquired with the new circuitry are used as the feed back signals in a properly designed
adaptive control strategy.
The invention is explained in more details hereinbelow usuig the exemplary embodiment
of the accompanying figures, in which,
Figure 1 shows a schematic diagram of a WEDM machine of the prior art
Figure 2 shows a system for measurement of voltage and current assigned to the
voltage and current measurement system of the prior art WEDM of fig. 1.
Figure 3 shows voltage and current history with excessive noise captured in the
circuitory of prior art.
Figure 4 4(a) and 4(b) shows the usual practice and the improved circuitory
respectively, assigned to the novel voltajje and current measurement
system of the invention where the voltage tapping points have been
changed and both the tapping points provided with modified ground
connectors.
4(c) shows details of the circuit around Hall-effect sensor clip.
Figure 5 shows a schematic diagram describing the principle of Hall-effect used in
the improved system according to the invention.
Figure 6 shows a noise free voltage and current waveform acquired with the
improved system.
Figure 7 shows a spark cluster during an unstable processing phase.
Figure 8 shows an arcing condition occurring at si high frequency during the
processing phase.
Figure 9 shows a wire rapture event which can be csiptured during the processing
by the improved system.
Figure 2 shows the tapping points for measuring the voltage across the electrodes through
the resistors r3 and r* The current is measured through ri and r? by using a resistive shunt
The acquired voltage and current history showed (Fig. 3) a high noise to signal ratio at
a high frequency. Fig 4(a) and 4(b) shows the improved circuitory assigned to the
voltage and current measurement system of the invention where the voltage tapping
points have been changed and as shown in fig 4(a) two grounding connections,
Grounding I, Grounding 2 have been provided. The electric wires used in the circuitry of
the art constitute shielded cables which at higher frequencies were found to be acting as
active elements and thus distorting the signals. In the embodiment of the invention, such
shielded cables have been replaced by thick single core wire-) to reduce the high noise to
signal ratio. The resistors r1, r2, r3, r4 were replaced with metal film resistors to obviate the
effect of capacitance present in the resistors of the art. Figure 4(b) shows the secondary
circuitory according to the invention which has been constructed using the principle of
hall-effect on the mobility of electron and this secondary circuitory constitutes an integral
part, of the improved voltage and current measurement, system of the invention. In this
circuitory, me resistive shunt of Fig. 2 for measurement of current signatory has been
replaced by a non-contact type sensor using the phenomenon call Hall effect Fig 4(c)
shows elaborate detail circuit around the Hall effect sensor chip. The primary current
Ijprimary flowing through the primary circuit of fig 4(a) and the secondary current
I secondary flowing through N turns of the secondary coil passes through a resistance.
The voltage drop across this resistance is proportional to Ijwimary / N. The Hall effect
modifies the current density initially in the secondary circuitory and subsequently in the
primary circuitory, and consequential voltage drop is measured across the metal film
resistor R„
The Hall Effect has been further explained in figure 5. According to figure 5. a
current I is flowing through a metal/semiconductor work piece in the direction M/N
under the influence of electric field (E^) applied through the primary circuitroy. Electron
comprising this current move along NM with a velocity of v„. One of such eieetoms
shown in the figure. The direction of the force exerted on it by the magnetic field B is
shown and the magnitute of force is Beve. Under the influence of this force an electrical
potential difference Vh (called Hall Voltage) is generated and hence an electric field Eh
Electron mobility, electron density can be measured by knowing El and B and measuring
Eh The Hall Co-efficient (Rh) can be determined after measuring the current density (J),
Rh=E»/JB.
The invention is not restricted to the embodiments: shown. It is clear to a
person skilled in the art that, such an invented system can be used in any spark-erosion
machine, in automobile industry during spark plug testing to capture voltage and
current history. This system can further be used for different non-conventional
manufacturing processes for example, electrochemical discharge grinding, electrical
drilling.

We claim:
1. An improved voltage and current measurement system across an electric spark at
high frequencies in a spark-erosion machine comprising a cathode; an anode; a
power supply and controller unit; a capacitor bank having a plurality of
capacitors; a primary circuitory consisting of a plurality of resistor, and a plurality
of diodes; and electrical cables connecting said circuitory characterized in that
said current sensor is a non-contact type "Hall-effect" sensor; voltage tapping
points across said cathode and said anodes are connected to two grounding points;
said resistors constitute metal film resistors; and said electrical cables constitute
thick single core wires.
2. The system as claimed in claim 1, wherein said cathode comprises a wire of
diameter between 5-500 urn separated by a thin film of dielectric fluid
3. The system as claimed in claim 1, wherein said anode comprises a work-piece
connected to said primary circuitory.
4. The system as claimed in claim 1, wherein said power supply and controller unit
comprises a unipolar D.C. pulse train of high frequency charging said capacitors
and controlling the pulse parameters.
-7-
5. The system as claimed in claim I, wherein said capacitors discharges
over said wire constituting work electrode.
6. The system as claimed in claim 1, wherein said diodes cause
unidirectional current flow through said secondary circuitory.
7. The system as claimed in claim 1, wherein said D.C. power supply
delivers ± 15 V to said secondary circuitory and a current bearing
wire is passed through a hole of said current sensor.
8. The system as claimed in any of the preceding claims, wherein a
magnetic flux is generated by flow of current through said hole of said
current sensor during the sparking, said magnetic flux modifying the
flow of current in said secondary circuitory.
9. The system as claimed in claim 1, wherein said modified current flow
in the secondary circuitory modifies the current flow in said primary
circuitory being measured from the voltage drop across said resistor of
said secondary circuitory.

The invention relates to an improved voltage and current measurement
system across the electrical spark at high frequencies in a spark- erosion
machine comprising a cathode; an anode; power supply and a controller unit;
a capacitor bank having a plurality of capacitors; a primary circuitory
consisting of a plurality of resistances, a secondary circuitory consisting of a
current sensor, a resistor, and a plurality of diodes; an electrical cables
connecting the said circuitory. A magnetic flux is generated by a flow of
current through a current sensor during the sparking, said magnetic flux
modifies the flow of current initially in the secondary circuitory and finally
to the primary circuitory. The voltage is measured across the electrodes and
the current is measured from the voltage drop across the resistor of said
secondary circuitory .