GB2305626A

GB2305626A - Electrical discharge machining

Info

Publication number: GB2305626A
Application number: GB9519048A
Authority: GB
Inventors: Carlton Dudley Manning; Peter Richard Holme
Original assignee: Loughborough University
Current assignee: Loughborough University
Priority date: 1995-09-18
Filing date: 1995-09-18
Publication date: 1997-04-16
Also published as: AU6993296A; GB9519048D0; WO1997010917A1

Abstract

In an electrical discharge machine the power supply is arranged with semiconductor switches S5 and S6 in parallel and in series respectively with the machining gap 18. The switches are operated in anti-phase to provide a pulsed electrical supply and the switch S5 is as close as possible to the gap to minimise inductance in the path including the gap.

Description

Electrical Discharge Machining This invention relates to the field of electrical discharge machining (EDM).

EDM is a process depicted schematically in Figure 1, wherein conductive material is removed via sparks generated between a tool anode 2 and a workpiece cathode 4, across a small gap filled with dielectric liquid 6.

In practical applications the EDM process is mainly used-to machine hard metals in order to produce complex shapes having very close tolerances. Typical applications ofEDM are drilling, slotting, die and mould making and general engineering applications. Multiple electrodes are often used to increase productivity especially in the aerospace industry where it is used for drilling cooling holes in aircraft engine blades.

A major application for the EDM process is in the manufacture of forging dies for the production of metals and plastic components used in the automotive and domestic goods markets.

Electrical pulses applied across the gap have previously been generated using a diode rectifier fed bipolar transitor (or mosfet) switched resistor scheme.

However, this scheme, using square wave switching, results in high power losses, especially at high switching frequencies. This results in a bulky, heavy and electrically inefficient design and forces the EDM power supply to be mounted a long way from the gap. This in turn causes high lead inductance and prevents the formation of current pulses with fast rise and fall times.

Another significant limitation ofpresent EDM is a tendency to suffer from arcing under poor machining conditions. These arcs (longer than the sparks produced during normal operating conditions) result in damage to the surface of the work piece.

The arcing can be caused by aggregation of debris in the gap due to insufficient flushing or by use of too small a gap width. Various methods, such as rf control, have been employed to prevent this arcing, but these schemes suffer from the disadvantage that machining must be interrupted in order to correct the problem.

The present invention addresses the aforementioned problems by utilising modern semiconductor devices and digital control techniques. The invention results in inter alia more efficient processing, smaller gaps and a reduction in problems associated with arcing.

According to one aspect ofthe invention there is provided a pulse width modulating (PWM) power converter for use with EDM.

The PWM power converter may be a non-isolated buck type power converter.

Altematively, the power converter may be a galvanically isolated foward converter or a ZVS phase shift bridge converter.

The power converter may be controlled by a digital control circuit, and may be substantially as hereinafter described with reference to the accompanying drawings.

According to a second aspect of the invention there is provided an EDM process control method comprising the steps of: providing EDM control means, a PWM power converter having PWM power converter digital control means, filter inductor means and load means; and modulating the operating characteristics and output of the PWM power converter via the digital control means.

The modulation may be accomplished by feed-foward control.

The modulation may be performed in order to control selected EDM operating characteristics, and these characteristics may comprise the gap current waveform. The modulation may be performed in response to measurements of other EDM operating characteristics.

The modulation may be performed to prevent arcing.

The process control method may include an adaptive control algorithm.

EDM power converters and process control methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram ofthe EDM process; Figure 2 is a Saber schematic ofthe EDM power supply; Figure 3 is a Saber schematic of the digital PWM controller; Figure 4 depicts digital averaging of current waveforms; Figure 5 is a simplified EDM load model; Figure 6 shows steady state inductor current and voltage waveforms; Figure 7 shows inductor current response to gap current demand; Figure 8 shows inductor current response to duty ratio; Figure 9 shows average inductor current response to line voltage; and Figure 10 shows average inductor current response to gap voltage.

An EDM power supply is desirably compact, high power, high frequency, and able to accomodate a range of machining conditions. The power circuit should provide rapid and accurate gap current control at a high level of electrical efficiency.

This is achieved with a preferred embodiment ofthe present invention, namely a buck derived high frequency PWM power converter, since the device permits the inductor current to be controlled directly thereby regulating the gap current.

A digital current mode control scheme is suitable for EDM applications due to its ability to regulate the gap current accurately. Also, the inherent programmability facilitates close cooperation with an EDM controller. Feed foward techniques can then be used to improve the power supply response to changes in the gap current and duty ratio demands.

Figure 2 shows a Saber model of an EDM power supply of the present invention, having an EDM process controller 10, a PWM power converter 12 having a PWM power converter digital controller 14, a filter inductor 16 and a EDM load 18. The EDM load 18 is understood to comprise the tool, workpiece and dielectric liquid. A nonisolated buck type PWM converter is shown in Figure 2, however it is noted that other configurations may be adopted. For example, if galvanic isolation is required, the converter may be a foward converter or a ZVS phase shift bridge converter.

The EDM process controller 10 provides the EDM gap current and duty ratio demands, based on infcrmation obtained by an EDM process monitor and the relevant machining programme. The task ofthe digital control circuit 14 is to take these demands and translate them as accurately as possible into the desired gap current waveform.

The basic mode of operation ofthe power circuit is as follows: Switch S1 (20) is turned on and off at the power supply switching frequency of 200 Hz, with a duty ratio Dp. Dp is set each cycle by the digital control circuit 14 so as to maintain the average current demanded in the gap flowing in inductor 16.

When switch S5 (22) is closed and S6(24) is open, the current in inductor 16 flows via S5 (22) and the gap is open circuit. Under this condition Dp is small, only allowing sufficient power to be drawn from the supply to top up component losses. Thus the current in inductor 16 is maintained at the demand level. When S5 (22) is opened and S6 (24) closed the current in inductor 16 rapidly charges the small capacitance C4 (26), since the gap is open circuit. When the gap voltage is sufficient to break down the dielectric oil between the anode and the cathode, current begins to flow via a plasma channel between said anode and cathode. The current in inductor 16 then rapidly transfers to the gap from C4 (26) and the gap voltage falls back from its breakdown level to its machining level.

Gap current is then maintained until switch S5 (22) is closed again and S6 (24) opened. At this point, the current in inductor 16 is transferred back via S5 (22) causing the plasma channel to narrow until the gap current falls to zero. In this way, the frequency and duty ratio of the EDM current pulses applied to the gap are controlled by switching S5 (22) and S6 (24) in antiphase.

Rise and fall times of the gap current pulses are influenced by the capacitance of C4 (26) and the speed with which the switches S5 (22) and S6 (24) are turned on and off. The voltage across C4 (26) varies between almost zero and ca. 170 V, with switching transients approaching 400 V. Vg must therefore be greater than 170 V to ensure an increase in the current through inductor 16 during all S1 (20) on-times.

The switch S6 (24), in series with the EDM load, is required to ensure the transfer of current from the gap to S5 (22) at the end of each EDM pulse. This is neccesary since, when machining with a small gap, the possibility of a dead short exists due to imprecise servo control of the tool.

Figure 3 shows a Saber model schematic ofthe digital control circuit 14.

This provides accurate control of the gap current by regulating the current flowing in inductor 16. Equation 1 and Figure 4 describe how the inductor current is averaged digitally.

I (n)= [(11(n)+12(n)).d1(n) + (i2(n)+i3(n)).d1(n) + (i3(n)+14(n)).d,(n) + (i4(n) +i,(n+l))-d4(n) ] (1) Note that this control method assumes that the frequency of EDM pulses is less than or equal to the switching frequency of the EDM power supply. Parameters dl(n) to d4(n) denote the proportions ofthe switching period for which each ofthe slopes of the inductor current exist.

Samples of inductor current taken in cycle n can be combined in cycle n+l to calculate Iav,(n), which is then available to set the duty ratio in cycle n+2. There is therefore a two cycle delay in the digital inductor current averaging process.

The averaged inductor current is subtracted from the gap current demand I~ref and passed through a digital filter H(). At its simplest, this is a proportional and integral filter to give the current loop high DC gain, whilst maintaining stability. It could, however, be of higher order if further optimisation of the current loop response is requireds The b orator (6 = z-1) is used, instead ofthe unit delay operator z' due to its lower coefficient sensitivity at higher sample rates.

The output from digital filter H() is multiplied by the feed foward gain block DEVjap/Vg to obtain the duty ratio of switch S1 (20). This feed foward term attempts to balance the volt-seconds applied to either side of inductor 16 in order for the inductor current to reject changes in the line voltage, the gap voltage and the EDM pulse duty ratio. Vg and V gap are sampled by the digital control circuit 14, with DE being supplied indirectly by the EDM controller 10. This allows zero delay feed foward Of DE on the basis that it is scheduled by the EDM controller 10.

This control circuit can be made to have a wide bandwidth, due to the essentially single pole nature of the system, providing excellent tracking of the gap current demand.

A simplified Saber schematic ofthe EDM load is shown in Figure 5. This models the behaviour ofthe actual, non-linear, EDM load 18.

When S5 (22) is opened and S6 (24) closed to initiate an EDM pulse, voltage controlled switch S7 (28) is open. The voltage on C4 (26) ramps up until it reaches the switch threshold level. At this point S7 (28) closes, causing current to start building up in L". When sufficient current is flowing, ZDw provides a fixed component of the gap width. Rpp provides a component of the gap voltage which increases with gap current. Lg,p prevents any instantaneous change in the gap current, modelling the inductance of the leads from the power supply to the gap, as well as any arising from the formation of the plasma channel in the gap.

When the gap current has built up to the level ofthe current in inductor 16, steady state conditions are attained until switch S5 (22) is closed and S6 (24) opened. At this point the current in inductor 16 transfers from S6 (24) to S5 (22). However, the current in Lw cannot fill to zero instantaneously, causing it to flow instead through a freewheel path via Dw . The negative source V tifroff increases the rate of fall of gap current at the end of each EDM pulse.

Circuit waveforms for the above described Saber model ofthe EDM power supply have been simulated, and are presented in Figures 6 - 10.

Figure 6 shows the inductor 16 voltage and current waveforms in the steady state. During the on-times of S (20), Vind is 200 V, falling to zero during the offtimes. When S5 (22) opens and S6 (24) closes, V gap rises rapidly to the gap breakdown voltage of 220 V, before falling back to the machining level of 180 V. The voltage waveforms Vind and Vjap combine to produce the characteristic inductor current waveform consisting of four different slopes.

Figure 7 shows the response ofthe current in inductor 16 and the digitally averaged inductor current to a 20% step change in the gap current demand. It can be seen that the current in inductor 16 attains the new demand level within 100 s under these conditions. Note the two cycle delay between the sensed inductor current and the digitally averaged inductor signal. All ofthe signals in Figure 7 are scaled, representing currents changing from 100 A to 120 A.

Figure 8 shows the response of the digitally averaged inductor current to a 20 % step change in the EDM duty ratio. This demonstrates the ability of the EDM power supply to almost totally isolate the response ofthe average current in inductor 16 from changes in the EDM duty ratio, via EDM duty ratio feed foward. Note that the average inductor current would, if plotted on the same graph, follow roughly the minimum points of the inductor current.

Figure 9 shows the response of the digitally averaged inductor current to a 20% step change in the line voltage. Again, 100 mA represents a current level of 100 A. It can be seen that the average current in inductor 16 deviates by less than 0.1% in either direction, illustrating the excellent line rejection provided by the line voltage feed foward.

Figure 10 shows the response ofthe digitally averaged inductor current to a 20% step change in the machining voltage. It can seen that the average inductor current in the inductor 16 deviates by less than 0.6% under these conditions, again demonstrating the benefits of gap voltage feed foward.

The present invention possesses a number of advantages over prior art EDM devices. The more efficient and compact power supply can be mounted closer to the gap, resulting in a reduced lead inductance and permitting the formation of current pulses with faster edges. As a result, electrical efficiency is increased. A secondary benefit is the narrow width ofthe gap current pulses, producing a finer surface finish on the work piece.

Numerous benefits accrue from the digital feedback control ofthe PWM power converter. Feed foward control of the filter inductor current permits accurate inductor volt-second balancing. Further, the instantaneous feed foward ofthe EDM duty ratio DE allows the optimal power converter duty ratio Dp for maintaining the filter inductor current to be selected. Importantly, the digital control permits rapid measurement and adjustment to the power converter settings. Thus, the detection of a deterioration in gap conditions may be achieved by AID conversion of the gap voltage and current. Steady state values during the spark, and transients across the EDM pulse edges provide valuable information. Knowledge of various machining conditions can then be applied to discriminate between good and poor conditions.In response, erosion rate can b varied rapidly by control of the current pulse amplitude, on-time and off-time.

It is noted that these parameters can be changed several orders of magnitude faster than the flushing pressure and the tool servo speed.

Using this combination of measurements and adjustments to the generator settings, it is possible to prevent gap conditions from deteriorating and to prevent arcs from occurring. This enables cutting to proceed at an optimum rate, given the capabilities of the servo, flushing, cooling and generator systems, without the need for interruptions to the machining due to servo retraction. As a result faster cutting speeds and lower tool wear is achieved.

It will be appreciated that it is not intended to limit the invention to the above examples only, many variations, such as might readily occur to one skilled in the art, being possible without departing from the scope thereof. For example, the topology and method ofpower supply control may be applied to any process requiring the accurate control of currents over a wide duty cycle range ; examples include laser machining, electromechanical machining, eletrochemical machining, welding applications and radar transmitters.

Claims

Claims

Apparatus for electric discharge machining, including: means for holding an electroconductive workpiece stationary; for earthing it, and for connecting it to one of two terminals across which a source of substantiallyconstant current is intended to be connected; servo means for feeding a conductive electrode to and fro the workpiece holder, the electrode being intended to be spaced from the workpiece when in place by a gap across which electric discharges are to take place to dislodge material from the workpiece; means for causing a dielectric liquid to fill the gap and to flow through it to carry away the decomposition products from the discharges; a switch positioned electrically in parallel with the gap and the two terminals, and close to the gap so that there is very little inductance in the path between the two terminals which includes the gap;; a switch in series with the electrode and the respective terminal, and a controller for the two switches by which the parallel switch is successively closed and opened at timed intervals to operate as an effective short-circuit across the terminals when there is to be no discharge across the gap, and to present a high impedance when energy from the source is to be discharged across the gap, and by which the series switch is opened and closed substantially in anti-phase to the first switch, the controller also providing a reference value for the gap current.
2 Apparatus as claimed in claim 1, in which the current is of either fixed or adjustable amplitude, in either case with an additional ripple component.
3 Apparatus as claimed in claim 1 or 2, in which each switch is a fast and controllable switch of appropriate rating.
4 Apparatus as claimed in claim 3, in which each switch is a semiconductor switch.
5 Apparatus as claimed in any preceding claim, in which the dielectric liquid is an oil.
6 Apparatus as claimed in any preceding claim1 in which the constant-current source includes an inductor in either or both leads to the terminals.
7 Apparatus as claimed in any preceding claim, in which the constant-current source topology includes a non-isolated buck type power converter, a galvanicallyisolated forward converter, or a zero voltage switched phase-shift bridge converter.
8 Apparatus as claimed in claim 6 or 7, in which the gap voltage and current waveforms are converted into digital form in order to monitor changes in the gap conditions.
9 Apparatus as claimed in any of claims 6 to 8, including means for adjusting the amplitude of the constant-current and/or the on-period and/or the off-period of the gap current to control the rate at which material is eroded from the workpiece.
10 A method of controlling the apparatus as claimed in any preceding claim, including the steps of: choosing an initial value for the gap current, on-period and off-period of each gap current pulse which corresponds to a desired rate of material removal and surface finish; converting the current through, and voltage across, the gap into digital form at a high-enough sampling rate to capture the transient information accurately; comparing the detected gap breakdown voltage with a set minimum value, and adjusting the amplitude and/or on-period and/or off-period of each gap current pulse to reduce the machining rate until the gap breakdown voltage rises above the set minimum value, thus eliminating the need to invoke corrective servo action.
11 The method as claimed in claim 10, in which, if the gap current is established before a significant gap voltage is reached, indicating an arc or a short-circuit, the parallel switch is immediately closed and the series switch immediately opened, thus terminating the gap current pulse and preventing damage to the workpiece.
12 The method as claimed in claim 10 or 11, including the step of using the gap breakdown voltage of each pulse as a feedback signal to control the position of the electrode to regulate the effective gap width, and extracting the gap breakdown voltage of each pulse from the digitised gap voltage waveform.
13 Apparatus for electric discharge machining of conductive workpieces, substantially as described herein with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.