Power output module for electrical discharge machining power supply circuit

Included in the power supply is an output module connected to the gap which module uses a power FET. This module has its current output and the current to the gap controlled as a linear function of the control voltage applied to the gate of the FET. A suitable input means for the control voltage, includes manually controlled potentiometers, microprocessors, keyboards and the like.

FIELD OF THE INVENTION 
The present invention relates to an improved power supply circuit for 
electrical discharge machining. It makes possible linear control over a 
broad range of the machining current being furnished to the gap. 
BACKGROUND OF THE INVENTION 
Electrical discharge machining circuitry has advanced from the early stages 
in which relaxation oscillators were used to provide machining power 
pulses. Independently timed and controlled pulse generators are now almost 
universally used and in those generators electronic switches are employed 
in the form of solid state switches or banks of parallel connected 
switches, particularly transistors. These switches are used to provide the 
machining power pulses to the gap. 
In the electrical discharge machining process, sometimes hereinafter 
referred to as "EDM", it is necessary that as the workpiece material is 
removed, a predetermined gap be maintained between the tool electrode and 
the workpiece through a servo feed system which provides a continuous 
advance into and toward the workpiece as the material removal progresses. 
During the electrical discharge machining process, a fluid coolant is 
circulated through the machining gap to flush the removed workpiece 
particles from the gap. The coolant is usually furnished under pressure by 
a pump through one or more openings provided in the electrode and/or 
workpiece. 
One defining characteristic of electrical discharge machining is that the 
coolant is a dielectric fluid such as kerosene, transformer oil, distilled 
water, or the like. The dielectric fluid is broken down in minute, 
localized areas by the action of the machining power pulses, passed 
between the closely opposed surfaces of the tool electrode and the 
workpiece. For control of the servo feed system, there is generally 
utilized an electrical signal from the machining gap in order to control 
the rate and the direction of servo feed. In many cases, this gap signal 
is compared to an adjustable reference voltage so that the machine 
operator can select the rate of servo feed desired. 
In precision EDM, it is necessary to precisely control current output from 
the power module connected to the gap. This control has been generally 
maintained in the prior art by switching one or more power limiting 
resistors into or out of circuit with the machining gap. This necessarily 
results in a non-linear type of current control. Additional problems arise 
from the relays involved in switching the resistors in series with the 
gap. The present invention, for the first time, makes possible a linear 
gap current control in a novel and simplified manner. 
It will be understood in the specification that when we refer to 
"electronic switch", we mean any electronic control device having several 
electrodes comprising at least two principal or power conducting 
electrodes acting to control current flow in the power circuit, the 
conductivity between the principal electrodes generally being controlled 
by a control electrode in the switch whereby the conductivity of the power 
circuit is controlled statically or electrically without movement of 
mechanical elements within the switch. Included within the definition are 
transistors in which turn-on is accomplished by a control voltage applied 
to the transistor control electrode and in which turn-off is accomplished 
automatically in response to the removal of that control voltage. Also 
included in the definition are devices of the gate type in which turn-on 
is accomplished by a control voltage applied to the control electrode, 
which control voltage may be then removed and in which turn-off is 
accomplished by application of a subsequent control voltage to the control 
electrode. An additional class of electronic switches, called "electronic 
trigger devices", falls within this definition and includes thyratrons, 
semi-conductor controlled rectifiers, and the like. By electronic trigger 
device, we mean any electronic switch of the type which is triggered on at 
its control electrode by a pulse and is turned off by a reverse voltage 
applied for a sufficient time across its principal electrode. 
The present invention further incorporates a particular type of electronic 
switch known in the art as a power field effect transistor. One such type 
of transistor, specifically a VMOS power field effect transistor sometimes 
hereinafter referred to as a power FET is included in the circuits used 
for out invention. Power FET's appropriate for inclusion in EDM power 
modules are currently manufactured and sold by Siliconix Incorporated, 401 
Broad Hollow Rd., Mellville, N.Y. 11746. 
SUMMARY OF THE INVENTION 
The present invention provides an improved output module capable of 
providing its output current to the EDM gap. The output current is 
controlled in a linear manner in accordance with an externally applied 
control voltage. The control voltage is connected to the gate of a power 
FET that is incorporated as the electronic switching element in the output 
module. The control voltage may be derived from a peripheral input means 
such as a keyboard or a microprocessor which contains a binary 
representation of the desired magnitude of current. The voltage is then 
passed through a digital to analog converter and then used to control the 
height of the voltage pulses being supplied to the power FET. 
It is also possible to provide the control voltage from a potentiometer 
that may be manually adjusted by the machine operator.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows the basic parts of an electrical discharge machining power 
supply circuit. Included in the system is a multivibrator 20 and a drive 
transistor 22. Triggering pulses are passed from the multivibrator 20 to 
the base of the drive transistor 22. A power module 24 is used to provide 
machining power pulses to an EDM gap including an electrode 26 and a 
workpiece 28. Included in the power module 24 is an electronic switching 
means embodied as a power FET 30. The power FET 30 has its gate electrode 
connected to the output of the drive transistor 22. It is thus switched on 
and off to provide discrete machining power pulses to the gap. The main DC 
power supply 32 is shown connected in circuit with the machining gap and 
the source and drain terminals of the power FET 30. A further current 
limiting resistor 34 is connected in circuit with the gap to provide a 
maximum safe limit for gap current. 
Also shown in FIG. 1 is the variable control voltage input means which 
provides a selectively variable DC voltage at point P. In the FIG. 1 
embodiment, voltage input is received from an input means used to furnish 
a binary representation of the output current desired. This may be 
provided by computer input, microprocessor input, keyboard input, or the 
like. The four bits are then available as inputs to a digital to analog 
converter 36. The output from the D/A converter 36 is passed through a 
voltage amplifier 38 and through a series resistor 40 to provide the 
control voltage at point P. Thus the voltage level at point P represents 
the control voltage presented to the gate electrode of the power FET 30 
and accordingly controls the current output from the power module to the 
gap. 
In the operation of the circuit the pulse output from the multivibrator 20 
turns transistor 22 on and off. The power module is operated through the 
power FET 30. After each turn-off of the power FET 30, the gate of the 
power FET 30 is returned to the voltage level preset at point P. As shown 
by the FIG. 2 diagram, the higher the input or control voltage at point P, 
the lower the effective resistance and accordingly, the higher the current 
output from the power module 24. This relationship is a relatively linear 
one as shown by the graph. 
FIG. 3 shows a different form of our invention in which the control voltage 
at point P is derived from several different EDM operating parameters. The 
circuit of FIG. 3 includes inputs at terminals S and T representing arc 
voltage and arc current, respectively. The signal at terminals T may be 
derived from an ammeter shunt resistor 52 in the EDM gap circuit. With 
respect to FIG. 3, it will be seen that the voltage output from the next 
following amplifier 50 represents the magnitude of the current flow in the 
shunt resistor 52. The arc voltage signal and the arc current signal are 
then passed as inputs to an analog multiplier 54 so that the voltage 
output resulting from it is representative of the power being expended in 
the gap. This voltage then passes through a voltage amplifier 38 and then 
through the series resistor 40 to point P which is representative of the 
contorl voltage applied to the gate electrode of the power FET in the 
following power module 24. Thus the higher the voltage preset at point P 
on the gate of the FET the lower the resistance of the power module and 
the more current is available from it. The series resistor 34 is included 
in the circuit to limit the current through the module to a maximum safe 
level. 
Once again the drive transistor 22 is triggered on and off to turn the 
power module on and off. When the power module is turned on, it will 
return to the control voltage set at point P. 
FIG. 4 illustrates a straightforward voltage control system in which a 
potentiometer 56 is used to provide the selectively variable voltage to 
control the current output of the power module 24. Again, current control 
is achieved by controlling the voltage on the gate of the power FET 30 in 
the power module 24 as best shown in FIG. 1. 
It will be noted that the control voltage applied can be applied from 
several different input means including the potentiometer 56 as shown in 
FIG. 4, the power circuit as shown in FIG. 3, or through a digital input 
means including I/O port 35. It is possible to preset or change a binary 
representation of the output current desired by writing it in from a 
microprocessor or providing an input from a keyboard or the like. With the 
four-bit capability shown or I/O port 35, there would be sixteen possible 
current limit positions, but this could be readily expandable with 
additional bits. It will thus be seen that we have provided by our 
invention a greatly improved EDM power supply circuit, particularly with 
respect to the power module and incorporation in it of a power FET in the 
configuration shown. The selectively variable voltage applied to the gate 
electrode of the power FET makes it possible to control the current output 
from the power module in a straightforward and linear manner.