GB2464514A

GB2464514A - Capacitor discharge welding apparatus and method

Info

Publication number: GB2464514A
Application number: GB0819111A
Authority: GB
Inventors: Nigel Ian Dennis Lee
Original assignee: MacGregor Welding Systems Ltd
Current assignee: MacGregor Welding Systems Ltd
Priority date: 2008-10-16
Filing date: 2008-10-16
Publication date: 2010-04-21
Also published as: GB0819111D0

Abstract

Capacitor Discharge (CD) welding apparatus comprises a capacitor bank C1 which is initially charged from a charging circuit D1-D4, SCR1. A switching device connects and disconnects the capacitor bank C1 to a pulse transformer T1. The pulse transformer T1 reduces the voltage from that stored in the capacitor bank C1 to the required welding voltage. A secondary circuit electrically connects the output of the pulse transformer T1 to the material being welded. The welding apparatus includes control means to control the characteristics of the weld pulse. Preferably, the control means controls the switching device, which may be a semiconductor switching device. The switching device may comprise a Class D switched mode or pulse width modulation device, an IGBT, FET and/or a transistor. The control means preferably switches the semiconductor device at a high frequency. This enables the control means to control the characteristics of the weld pulse. In addition, the welding apparatus may include closed loop feedback means which monitors the output characteristics and/or the characteristics of the weld and may adjust the control means (for example, to adjust the input to the pulse transformer) in order to maintain pre-programmed welding characteristics.

Description

Capacitor Discharge Welding Apparatus

Field of the Invention

The present invention relates to capacitor discharge welding apparatus and to a method of welding. In particular, the present invention relates to current controlled capacitor discharge welding apparatus and to a method of welding using current controlled capacitor discharge welding apparatus.

Background of Invention:

Capacitor discharge (CD) welders have been used for many years and these welders operate by storing an electrical charge in a bank of capacitors and discharging the capacitors into the parts to be welded. The low resistive joint causes a very fast high pulse of current to flow between the two metallic parts which causes a very rapid rise in power due to the resistance and this power produces heat that welds the parts together.

An advantage of CD welders is the simplicity as shown in Figure 1. However because of its simplicity it also has problems and limitations. In particular, with CD welders it can be difficult to control the heat during the weld and as such the * technique has up to now only been suitable for simple welding tasks. *** S * S *S**

Further problems with CD welders and CD welding techniques include the S...

restriction that the charge in Watt seconds (Ws) of the capacitor has to be set before the weld, so any change in resistance of the weld since the last one can not be compensated for. In addition, the charge in Watt seconds (Ws) stored in the *:*. capacitor will reduce with the age and temperature of the capacitor as the capacitors get hot during charge / discharge reducing the welding current. The resistance of the output transformers windings and cables also increases with temperature thus reducing the welding current which causes problems in prior art CD Welders. In prior art total capacitor discharge techniques, the only way of controlling the charge in Watt seconds (Ws) stored in the capacitor has been to control the charge voltage.

Other types of resistance welding systems incorporate a closed loop feedback control technique to control either the voltage, current or power during the weld.

This feedback system will keep the controlled parameter constant and therefore keep the weld far more consistent than would be achievable without feedback.

A prior art transistorised CD welder (US 4 228 340) uses silicon controlled rectifiers (SCRs) for the rapid charge of the capacitor bank and a SCR to switch the charged capacitor into the output transformer. This welder technique includes a highly regulated welding energy power supply comprising a welding energy storage capacitor, circuits to charge the capacitor and discharge it and switching of the capacitor into the transformer.

Another prior art technique (US 5 149 933) relates to a method, and in particular software, to control the charge on the capacitor bank based on an equation that compensates for the variables including temperature rise of the capacitor and output transformer. This welding technique includes a method for controlling the energy delivered to a weld head by a capacitor bank using derived equations to * 20 control the charge on the capacitor bank and by using derived equations to compensate for the temperature rise of the capacitor and output transformer. S...

A further prior art technique (US 0173626) uses an insulated-gate bipolar transistor (IGBT) device in place of the normal SCR (thyristor) and the novel feature is that the pulse can be stopped before the capacitors are totally discharged thus controlling the pulse duration. This welding technique includes a CD welder using an IGBT to terminate the welding pulse and control the width. In addition, this technique uses an IGBT to switch on and off to slow down the effective rise time of the weld pulse. However, without a switching filter this will cause very high radio frequency interference (RFI) and/or electromagnetic interference (EMI) and heating. -3..

It is an aim of the present invention to overcome at least one problem associated with the prior art whether referred to herein or otherwise.

Summary of the Invention

According to a first aspect of the present invention there is provided capacitor discharge welding apparatus comprising; * a charging circuit; * a capacitor bank; * a pulse transformer; * a switching means for selectively connecting the capacitor bank to the pulse transformer; * a secondary circuit for supplying a weld pulse to a part to be welded; and * characterised in that the apparatus comprises control means to control the characteristics of the weld pulse.

Preferably the control means controls the switching means and more preferably controls the frequency of switching of the switching means.

Preferably the control means turns the switching means on and off at a controlled frequency. The frequency may be in the range of between 20 kHz and 50 kHz. *�*.

Preferably the control means turns the switching means on and off during an upslope section of the weld pulse and/or a peak time section of the weld pulse * and/or a downslope section of the weld pulse. Preferably the control means controls the gradient and/or duration of an upslope section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the * * weld pulse.

Preferably the switching means is connected to the pulse transformer through filter means. The filter means may comprise an LC filter. Preferably the filter reduces and/or minimises switching noise.

Preferably the apparatus comprises feedback means and more preferably comprises closed loop feedback means. Preferably the feedback means monitors at least one characteristic of the output pulse weld and may adjust the control means to control the input to the pulse transformer. The feedback means may control and adjust the frequency of the switching of the switch means.

The feedback means may monitor the output pulse weld throughout the duration of the pulse weld.

The feedback means may monitor and the control means may control throughout an upslope section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse.

Preferably the capacitor discharge welding apparatus is a controlled capacitor discharge welding apparatus.

Preferably the control means controls the characteristics of the weld pulse in real time.

Preferably the control means controls the shape of the weld pulse. Preferably the control means controls the shape of the weld pulse throughout the duration of the weld pulse. *e.. * S *.

The apparatus may be arranged to supply a weld pulse comprising an upsiope *S*. S. * 25 section, a peak time section and a downslope section. Preferably the apparatus controls the gradient and/or duration of the upslope section and/or the peak time section and/or the downslope section.

The control means may enable a user to preselect the duration of the upslope and/or of the peak time and/or of the downslope.

Preferably the control means controls the peak current of the weld pulse.

Preferably the control means maintains the current at a substantially constant level during the welding operation.

Preferably the apparatus comprises closed loop feedback means to feedback measurements from the secondary circuit to the control means.

Preferably the feedback means monitors the welding voltage and/or current and/or energy continuously throughout the welding pulse.

Preferably the switching device comprises a semiconductor switching device. The semiconductor switching device may comprise a plurality of semiconductors. The semiconductor switching device may comprise a pair of semiconductors and may comprise a plurality of pairs of semiconductors. Each pair of semiconductors may comprise a pair of phase shifted semiconductors.

The switching means may comprise a switching device and control means.

The switching means may comprise a Class D switched mode device.

The switching means may comprise a Class D pulse width modulation device.

Preferably the switching means comprises a high frequency switching device. * S S...

Preferably the switching means controls the current output from the capacitor bank. S. * . * S..

The switching device may comprise one or more of an IGBT, FET and/or a transistor.

Preferably the pulse transformer comprises an isolated pulse transformer.

Preferably the pulse transformer converts the high voltage output from the switching device to a lower voltage, higher constant current.

The apparatus may comprise a hysteretic pulse width modulation circuit.

The apparatus may comprise a filter and preferably comprises an LC filter.

According to a second aspect of the present invention there is provided a method of welding, the method comprising charging a capacitor bank though a charging circuit, discharging the capacitor bank to a pulse transformer through switching means which selectively connects the capacitor bank to the pulse transformer and supplying the output of the pulse transformer to a secondary circuit for supplying a weld pulse to a part to be welded and characterised by controlling the characteristics of the weld pulse by control means.

Preferably the method comprises controlling the characteristics of the weld pulse throughout the duration of the weld pulse.

The method may comprise operating switching means at a high frequency to control the discharge of the capacitor bank.

The method may comprise monitoring at least one characteristic of the weld pulse * *. and providing this information to the control means in order to control the weld **** pulse. **.. * * ****

: The method may comprise utilising a closed loop feedback system. * *25

The method may comprise turning switching means on and off during an upslope *..: section of the weld pu'se and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse. The method may comprise controlling the gradient and/or duration of an upslope section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to the drawings that follow, in which: Figure 1 is a circuit diagram of prior art CD welding apparatus.

Figure 2a is a capacitor discharge curve for prior art CD welding apparatus.

Figure 2b is a view of the output waveforms of conventional CD welding apparatus.

Figure 3 is a basic circuit for an embodiment of CD welding apparatus.

Figure 4 is a basic circuit for an embodiment of a Class D switched mode circuit.

Figure 5 is a view of waveforms from an embodiment of CD welding apparatus.

Figure 6 is a basic circuit of a 4 quadrant Class D switched mode circuit.

Figure 7 is a basic circuit of a Class D switched mode with an AC filter. * * * *..

Figure 8 is a basic circuit of a multi phase Class 0 switched mode circuit. ****

S.....

* 25 Figure 9 is a basic circuit showing a voltage polarity control of a Class D switched mode circuit. S. S *5

Figure 10 is a block diagram of an embodiment of a Class D switched mode control circuit.

Figure 11 is a schematic view of a pulse weld waveform.

Description of the Preferred Embodiments

The present invention relates particularly to micro welding including micro resistance and micro arc welding. One problem with CD welding apparatus is the uncontrolled nature of the output current this is a significant problem in precision micro welding applications.

Prior art CD welding apparatus may attempt to control the welding current by varying the voltage charge on the capacitor bank and controlling the pulse width by changing the taps on the output transformer. This is an of example open loop control. The present invention provides the ability to control the stored energy leaving the capacitors and to be able to control the welding pulse wave shape and current in the weld in real time during the weld pulse.

An advantage of the present invention is the ability to address and/or overcome the major drawbacks with basic CD welding techniques and therefore allow CD welding techniques to be used for the main area of micro resistance welding.

The present invention provides a current controlled CD welder using a high frequency Class D (Chopper) power switching circuit between the charged capacitor bank and the primary of the output transformer. This high frequency Class D power switching circuit acts as a variable impedance to control the peak current flowing into the output transformer and therefore the current into the weld.

The peak current will be controlled as long as the charge in the capacitor bank is greater than that required by the resistance of the load (welding materials).

A basic circuit for a CD welder is shown below in Figure 1. The circuit includes four diodes DI to D4 and a silicon controlled rectifier SCR1 to charge up a capacitor bank Cl. When the capacitors in the capacitor bank have charged to the required voltage, a second silicon controlled rectifier SCR2 is used to discharge the capacitor bank Cl to a pulse transformer Ti. The pulse transformer TI reduces the high stored voltage down to a low voltage suitable for welding.

The welding current then flows between the welding electrodes and the parts to be welded. Figures 2a and 2b show the capacitor discharge curve and the actual waveforms.

A capacitor discharge curve for a normal CD Welder is shown in Figure 2a. The total energy stored in the capacitor before the discharge pulse is calculated from the equation E (Watt seconds) = 0.5 CV 2 Accordingly, the output energy in the weld pulse is directly dependent on the value of the capacitance and the square of the charge voltage.

The waveforms of a basic CD welder are shown in Figure 2b. The top waveform is the current pulse from the pulse transformer and the lower waveform is the voltage pulse. The amplitude of the current pulse is adjusted by changing the charge voltage of the capacitor bank. The pulse duration is adjusted by changing taps on the primary of the pulse transformer.

The disadvantages with simple CD welder circuits include the charge in Ws (Watt seconds) of the capacitor must be set before the weld, so any change in resistance of the weld since the last weld or during the weld cannot be compensated for. The charge in Watt seconds (Ws) stored in the capacitor will reduce with the age and temperature of the capacitor as the capacitors get hot during charge I discharge reducing the welding current and producing a bad weld.

The resistance of the output transformers windings and cables increases with temperature thus reducing the welding current. If the resistance of the joint I..

* 25 changes during the weld due to pressure changes the weld current will also change producing a bad weld. The only way of controlling the Watt seconds (Ws) of the weld has been to control the charge voltage on the capacitor bank. Finally, the only way to control the pulse width is to select primary taps on the pulse transformer.

Figure 3 shows a basic circuit for controlled capacitor discharge welding apparatus in accordance with the present invention. The circuit includes four diodes Dl, D2, -10-D3, D4 and a silicon controlled rectifier SCRI to charge a capacitor bank Cl.

However, in alternative embodiments a power factor correction (PFC) step up circuit may be used to meet supp'y regulations. One difference in the present invention is the use of a wide range Class 0 pulse width modulation (PWM) switch circuit to control the discharge energy from the capacitor bank into the pulse transformer during the weld pulse. This control allows the wave shape to be controlled and also the current into the transformer to be controlled. This also enables the current into the part being welded to be controlled.

In use, the capacitor bank is initially charged from by a charging circuit. A semiconductor switching device connects and disconnects the capacitor bank to the pulse transformer. The pulse transformer reduces the voltage from that stored in the capacitor bank to the required welding voltage. A secondary circuit electrically connects the output of the pulse transformer to the material being welded. The welding apparatus includes control means to control the semiconductor device and preferably switches the semiconductor device at a high frequency. This enables the control means to control the characteristics of the weld pulse. In addition, the welding apparatus includes closed loop feedback means which monitors the output characteristics and/or the characteristics of the * *e weld and may adjust the control means (for example, to adjust the input to the pulse transformer) in order to maintain pre-programmed welding characteristics. ***

The advantages of current controlled CD welding apparatus in accordance with ***** * 5 the present invention include the control of the Watt seconds (Ws) energy into the pulse transformer in micro seconds (ps) during the weld pulse so any changes in ** resistance of the weld can be automatically compensated for. In addition, the present invention provides real time feedback from the welding voltage, current and energy during the weld and, therefore, any changes in the capacitor due to age will automatically compensated for. The automatic feedback control will also compensate for any resistance changes of the output transformers windings and cables due to temperature thus keeping the welding current constant during the -11 -weld. The constant current control will compensate for changes in resistance of the joint during the weld due to pressure and thus always produce good welds.

The Class D switched mode circuit will not only control the constant current but it will also start and stop the weld pulse and thus control the Watt seconds (Ws) into the weld. The basic Class D switched mode circuit switches at high speed (for example, 20 kHz -30 kHz) and this high frequency signal is then filtered by an LC filter before it feeds the pulse transformer. Multiple phase blocks (see Figure 8) can be used to increase this frequency many times and reduce the size of the filter components with smaller delays.

Figure 4 shows a basic Class D switched mode circuit. The circuit in Figure 4 is supplied by the charge on the main capacitor bank which typically may be in the range of 400 to 800 volts. The IGBT switch is controlled by a pulse width modulated (PWM) circuit and is switched on and off at 20 kHz to 50 kHz with the duty cycle controlled between 0% and 100%. The diode Dl is used to flywheel the current from the inductor Li during the off time of the IGBT. The high frequency chopped waveform is then fed into the filter comprising of the inductor Li and the capacitor Cl to remove the switching noise. The clean welding pulse is then *:*::* supplied to the primary of the pulse transformer. * S.. * 20 *S..

Figure 5 below shows typical wave forms of the basic Class D switched mode current controlled capacitor discharge unit. The top trace is of the main capacitor *.*..

* voltage showing the discharge during the weld pulse. The middle trace is the PWM switching voltage across Diode Dl and the lower trace is the weld current pulse at *:*. the output of the pulse transformer with a peak current of 2000 Amps. The weld pulse has been set for a rise time of ims and has a peak current duration of 2ms and a fall time of 2ms.

Figure ii shows a schematic view of a weld pulse waveform and indicates the different sections of the weld wave pulse. The weld pulse waveform includes an upslope section during which the current increases, as also shown in the bottom trace of Figure 5. The upslope section is controlled by the switching operation and can be used to provide a lower gradient than uncontrolled CD welding apparatus.

The weld wave pulse includes a peak time section. As shown in Figure 5 and Figure 11, the peak time section is substantially constant with respect to prior art peak time sections using uncontrolled CD welding. The weld pulse includes a downslope section which again can be controlled by the control means.

The control means can control the duration of a weld pulse which will affect the weld temperature achieved (in relation to the welding current), the rate of heating or cooling for the weld, and the time that the material is in a plastic or liquid state.

The time will therefore affect the weld strength as well as the weld collapse.

The duration may be programmed in three stages. The first of these is the upslope. The upslope is the time the output transitions from 0 to the programmed peak value. The use of a longer upsiope time can help prevent weld splash and instability, as well as control the rate of heating of the materials. A short rise time may be used for welding conductive materials or when welding on products that are heat sensitive, such as batteries.

*:*::* The second programmable timing variable is the peak time or hold time. The peak ***. . . . . . . *,*20 time is the time the programmed welding output is maintained. The value will have the most direct effect on the welding temperature in most applications compared to the upslope and the downslope values. The peak time essentially controls the *..S..

* time the welding temperature and flow of current is maintained. * S * ***

*:*. The third programmable time is the downslope time. The downslope time is the time the output transitions from the programmed peak value to 0. The downslope is used to control the rate of cooling of the welded materials. Of the three variables, the downslope is the least critical in most applications.

An improved four quadrant Class D switched mode circuit is show in Figure 6.

The circuit in Figure 4 provides an improvement over the basic circuit shown in Figure 4. The diode Dl is replaced by another IGBT switch that is turned ON when -13-the first switch is turned OFF. This enables the pair of IGBTs to provide much better control over the current in the inductor LI during the periods when the inductor current goes to zero and ensures that the output impedance of the switched mode circuit is always low and prevents voltage spikes and switching noise.

The output switching ripple from any Class D circuit should always be reduced as much as possible as this switching ripple will cause external radio frequency interference (RFI) and heating losses in the transformer and welding cables.

Another way to remove the switching noise is to use a second winding on the filter inductor to attenuate the AC ripple component from the output without removing the effectiveness of the LC filter to the low frequency pulse as shown in Figure 7.

Figure 7 shows a Class D switched mode circuit with AC filter.

The circuit uses a dual winding inductor to steer some of the switching AC ripple away from the output. This is achieved by the AC switching component only passing through the second winding and the two AC currents then cancel out.

However, the two currents may not cancel out completely due to leakage **::* inductance and magnetizing inductance distorting the switching waveform. * 1

*. *. .20 The output ripple and noise of all the above circuits is limited by the switching losses of the switch used (transistor, field effect transistor (FET) or IGBT). To * overcome this limitation it is possible to drive multiple Class D stages in parallel but phase shifted from one another so the output currents interleave with each "..3?5 other and multiply the output and input ripple frequency so smaller filters can be used, as shown in Figure 8. Figure 8 shows a multi phase Class D switched mode circuit.

As shown in Figure 9, if the output of the Class D switched mode circuit is supplied to a 4 SCR bridge then the output polarity of the welding pulse can be controlled by firing diagonal pairs of SCRs. To reduce the fall time of the welding pulse the transformer primary can be short circuited by firing either the top two SCRs or the -14 -bottom two SCRs at the end of the weld pulse.

A simplified block diagram of the Class D switched mode control circuit is shown in Figure 10. This block diagram demonstrates the control of the switching of the Class D switched mode circuit. It receives control voltages from the main capacitor (to vary the pulse width modulation (PWM) ratio with respect to the charge voltage) and the reference waveform (produced by a microprocessor) and it is the electrical template of the final weld current. The reference voltage drives the error amplifier that in turn drives an error voltage to the hysteretic PWM circuit. The other input to this circuit is a voltage proportional to the capacitor voltage and this is used to control the hysteresis of the switching points and control the switching frequency as the capacitor bank discharges.

The output of the hysteretic PWM circuit drives the isolated driver circuit and this turns on the main IGBT power switch. Each time the IGBT turns on its current is fed back to the Hysteretic PWM circuit and this current is compared with the error signal. When the switching current increases above an upper hysteresis level, the drive to the IGBT is switched off and stays off until the inductor current falls below *::. the minimum hysteresis level when it turns on again.

The weld current and weld voltage are monitored by the feedback amplifiers and these also condition and isolate the signals. These conditioned signals feedback to * the microprocessor for display and data logging and also into a multiplier to calculate the total energy in the weld. This accumulated value is then used to *:*.5 terminate the weld pulse when the energy exceeds the required amount.

As shown in Figure 10, the control circuit of the current controlled capacitor discharge welding apparatus monitors the welding voltage, current and energy continuously throughout the welding pulse with a closed loop feedback circuit.

This ensures that any changes in the power electronics or the welding process are compensated for and controlled thus overcoming all the major disadvantages with the basic capacitor discharge design.

Welding apparatus featuring closed loop control monitors the actual output to the weld and adjusts the output to maintain the programmed output level.

Accordingly, the CD welding apparatus allows more flexible programming of the weld, in particular with respect to the wave timing and shape. These features allow an increased control of the output power level, timing and shape. These advantages allow for weld profiles to be optimised further and material heating to be controlled and profiled more effectively.

The present invention provides a capacitive discharge welder comprising a Class 0 switched mode circuit used to control in real time the peak discharge current from the storage capacitors into the primary of the pulse transformer thus keeping the output current to the parts to be welded constant during the weld. The Class D switched mode control circuit controls the welding pulse shape and peak current constant by comparing them with a locally generated reference waveform and constantly correcting for any changes.

The preferred embodiment of the present invention provides capacitive discharge * ** *.. welding apparatus comprising a capacitor charging circuit, a capacitor bank for *.S.

storing the weld energy, a high frequency Class D switched mode (SM) circuit to control the current output of the capacitors and into the pulse transformer in real **I.

time, an isolated pulse transformer for converting the high voltage constant current output of the SM circuit to a lower voltage higher constant current, a pulse width modulation (PWM) control circuit to control the power switch and a ":.25 secondary circuit to measure the current I voltage and provide feedback to the power control circuit to keep the peak welding current constant throughout the weld pulse to the part to be resistance welded.

The Class D SM circuit between the capacitor bank and the transformer uses a fast semiconductor Insulated Gate Bipolar Transistor (IGBT), field effect transistor (FET) or transistor operating'as a high frequency (10kHz -500kHz) pulse width modulated square wave circuit to chop the stored capacitor voltage in to a square wave pulse train that is then removed by a LC filter before it is supplied to the part to be resistance welded.

The welding apparatus may comprise a 4 Quadrant Class D switched mode circuit between the capacitor bank and the transformer using a fast semiconductor IGBT, FET or transistor operating at a high frequency (10kHz -500kHz) pulse width modulated square wave to chop the stored capacitor voltage in to a square wave train that is then removed by a dual winding inductor as part of an LC filter before it is supplied to a pulse transformer and then to the part to be resistance welded.

The welding apparatus may further comprise a SM circuit between the capacitor bank and the transformer / welding load that is controlled from a hysteretic pulse width modulation circuit that in real time controls the switching of the power switch.

This may be dependant upon a master reference produced from the welder control circuit that is then compared with the stored capacitor voltage, the switched mode current and either the weld voltage, current and/or energy.

The CD welding apparatus may comprise Class D SM circuit between the " capacitor bank and the transformer using two fast semiconductors FET or IGBT to .,.2O actively control the positive and negative directions of the pulse waveform operating at a high frequency (10kHz -500kHz) square wave to chop the stored voltage in to a square wave that is then filtered by the SM filter into a low noise ***...

* welding pulse.

I * * * S..

*.25 The CD welding apparatus may comprise a Class 0 SM circuit between the capacitor bank and the transformer using multiple pairs of fast semiconductors FET, IGBT or transistors to actively control the positive and negative directions of the pulse waveform. Each pair of switches is phase shifted from each other by a set number of degrees depending on the number of phases to ensure that each phase is interleaved. For example, with three pairs of switches 120 degrees would be used between phases and for four pairs of switches 90 degrees would be used.

If each semiconductor switch is switched at 20kHz then the output ripple to the welding load will be this frequency multiplied by the number of switching phases, for example, for 4 phases it would be 80kHz, operating at a high frequency improves the waveform shape and lowers the switching ripple seen by the part to be welded.

The CO welding apparatus may include a low voltage capacitor bank (without a pulse transformer) and comprises a Class D SM circuit between the capacitor bank and the welding load using single or multiple pairs of fast semiconductors FET, IGBT or transistors to actively control the positive and negative directions of the pulse waveform. Each pair of switches is phase shifted from each other by a set number of degrees depending on the number of phases to ensure that each phase is interleaved. For example with three pairs of switches 120 degrees would be used between phases and for four pairs of switches 90 degrees would be used.

If each semiconductor switch is switched at 25kHz then the output ripple to the welding load will be this frequency multiplied by the number of switching phases, for example, for 4 phases it would be 100kHz, operating at a high frequency improves the waveform shape and lowers the switching ripple seen by part to be welded. * ** * * * * **

**.. . **** * * ****

S

5S55*5 * S *. * S * S. *

S

Claims

CLAIMS1. A capacitor discharge welding apparatus comprising; a charging circuit; a capacitor bank; a pulse transformer; a switching means for selectively connecting the capacitor bank to the pulse transformer; a secondary circuit for supplying a weld pulse to a part to be welded; and characterised in that the apparatus comprises control means to control the characteristics of the weld pulse.
2. A capacitor discharge welding apparatus according to Claim I in which the control means controls the switching means
3. A capacitor discharge welding apparatus according to Claim 2 in which the control means controls the frequency of switching of the switching means. * *** S. * * *s *v. S *-0
4. A capacitor discharge welding apparatus according to any preceding Claim in which the control means turns the switching means on and off at a controlled *S..frequency
5. A capacitor discharge welding apparatus according to any preceding Claim *..j25 in which the control means turns the switching means on and off during an upslope section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse.
6. A capacitor discharge welding apparatus according to any preceding Claim in which the control means controls the gradient and/or duration of an upsiope section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse.
7. A capacitor discharge welding apparatus according to any preceding Claim in which the switching means is connected to the pulse transformer through filter means.
8. A capacitor discharge welding apparatus according to any preceding Claim in which the filter means comprises an LC filter.
9. A capacitor discharge welding apparatus according to any preceding Claim in which the apparatus comprises feedback means.
10. A capacitor discharge welding apparatus according to any Claim 9 in which the feedback means comprises closed loop feedback means.
11. A capacitor discharge welding apparatus according to Claim 9 or Claim 10 in which the feedback means monitors at least one characteristic of the output pulse weld and adjusts the control means to control the input to the pulse transformer. * *. * * * * S. S...e* .20
12. A capacitor discharge welding apparatus according to any one of Claim 9 to Claim 11 in which the feedback means controls and adjusts the frequency of the *S* switching of the switch means. 501.e * .
13. A capacitor discharge welding apparatus according to any one of Claim 9 to .J25 Claim 12 in which the feedback means monitors the output pulse weld throughout the duration of the pulse weld.
14. A capacitor discharge welding apparatus according to any preceding claim in which the control means controls the characteristics of the weld pulse in real time.
15. A capacitor discharge welding apparatus according to any preceding claim in which the control means controls the shape of the weld pulse throughout the duration of the weld pulse.
16. A capacitor discharge welding apparatus according to any preceding claim in which the switching device comprises a semiconductor switching device.
17. A capacitor discharge welding apparatus according to Claim 16 in which the semiconductor switching device comprises a pair of semiconductors.
18. A capacitor discharge welding apparatus according to Claim 17 in which the pair of semiconductors comprise a pair of phase shifted semiconductors.
19. A capacitor discharge welding apparatus according to any preceding claim in which the switching means comprises a Class D switched mode device.
20. A capacitor discharge welding apparatus according to any preceding claim in which the switching means comprises a Class D pulse width modulation device. * .*...
21. A capacitor discharge welding apparatus according to any preceding claim * S..s. .20 in which the switching device comprises one or more of an IGBT, FET and/or a *1*S transistor. * S asSU.....*
22. A capacitor discharge welding apparatus according to any preceding claim in which the apparatus comprises a hysteretic pulse width modulation circuit. . 5 a **23. A method of welding comprising charging a capacitor bank though a charging circuit, discharging the capacitor bank to a pulse transformer through switching means which selectively connects the capacitor bank to the pulse transformer and supplying the output of the pulse transformer to a secondary circuit for supplying a weld pulse to a part to be welded and characterised by controlling the characteristics of the weld pulse by control means.-21 - 24. A method of welding according to Claim 23 in which the method comprises controlling the characteristics of the weld pulse throughout the duration of the weld pulse.25. A method of welding according to Claim 23 or Claim 24 in which the method comprises operating switching means at a high frequency to control the discharge of the capacitor bank.26. A method of welding according to any one of Claim 23 to Claim 25 in which the method comprises monitoring at least one characteristic of the weld pulse and providing this information to the control means in order to control the weld pulse.27. A method of welding according to any one of Claim 23 to Claim 26 in which the method comprises utilising a closed loop feedback system.28. A method of welding according to any one of Claim 23 to Claim 27 in which the method comprises turning switching means on and off during an upslope section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse.* 29. A method of welding according to any one of Claim 23 to Claim 28 in which the method comprises controlling the gradient and/or duration of an upsiope **�* section of the weld pulse and/or a peak time section of the weld pulse and/or a downslope section of the weld pulse. ** S.30. A capacitor discharge welding apparatus substantially as herein described) with reference to, and as shown in, any of Figures 3 to 11.31. A method of welding substantially as herein described, with reference to, and as shown in, any of Figures 3 to 11.