AU2005200642A1 - Voltage Reduction Device - Google Patents

Description

I

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Name of Applicant: Actual Inventor Address for service is: Safetac Welding Products Pty Ltd Brian John Snell WRAY ASSOCIATES Level 4, The Quadrant 1 William Street Perth, WA 6000 Attorney code: WR Invention Title: Voltage Reduction Device The following statement is a full description of this invention, including the best method of performing it known to me:- -2- "Voltage Reduction Device" Field of the Invention The present invention relates to voltage reduction devices applicable to electric arc welding apparatus.

Background Art Electrical welding equipment such as arc welders have conventionally operated by application of a voltage between the electrode and the work piece in the order of 80 to 120 volts. This voltage may be alternating current or direct current depending upon the application. This voltage has been found to be sufficiently high to impose a lethal risk to operators. As a result, devices have been developed to reduce the voltage to a safe level while the welder is idle. Such devices are commonly termed voltage reduction devices. The required performance of such devices is now designated in Australian Standard AS 3195.

Most voltage reduction devices provided previously have been adapted to control the output of the welder. This has often been achieved by the use of a contactor capable of handling the large output currents. In some designs, large current silicon controlled rectifiers are used. Such designs are expensive to implement, and are bulky and heavy. The also result in associated problems such as a requirement to dissipate considerable energy as heat.

A voltage reduction device capable of controlling the input was disclosed in Australian patent no. 675469 issued in the name of Sam Wha Engineering Co.

Ltd. While the device disclosed provided offered some important advantages, it had limitations which have prevented it from being adopted very broadly. Its circuitry is reasonably complicated, thus leading to it being a relatively expensive unit. It is limited to use with AC welding units. It does not monitor the output voltage directly and therefore would not detect a failure in insulation between primary and secondary windings.

-3- Virtually all of the units devised thus far incorporate a time delay within them in order to function properly. This time delay is a considerable cause of operator dissatisfaction.

An alternative way of controlling the welder has been disclosed in W003064097 filed in the name of Z Bruszewski. This device controls switching of the input to the primary winding without varying the voltage applied. It is therefore not technically a voltage reduction device in the sense that it does not cause the voltage to be reduced. Because it requires switching control be the operator, it is not suitable for the full range of welding units.

Hereinafter, the term VRD is used to refer to a voltage reduction device which applies a reduced voltage across the output terminals of an electric welding apparatus when the welder is in a no-load condition or more particularly when the resistance between the output terminals rises above 200 ohms.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge anywhere in the world as at the priority date of the application.

The present invention relates to an improved voltage reduction device which substantially alleviates the limitations of the prior art.

Disclosure of the Invention Accordingly, the invention resides in a VRD adapted to control the voltage applied to an energizing circuit of an electric welding apparatus, the VRD being associated with a current detection device adapted to detect the presence of a current in an output circuit connecting a pair of output electrodes of said welding apparatus above a predetermined value, the VRD comprising electronic switching means adapted to be switched by a signal from said output current detection device between a conducting mode when said current is greater than said -4predetermined value and a non-conducting mode when said current is less than said predetermined value, wherein when said electronic switching means is in said conducting mode, a first voltage is applied to said energizing circuit and when said switching means is in the non-conducting mode, a second voltage is applied to said energizing circuit where the first voltage is greater than the second voltage.

According to a preferred embodiment, the current detecting device detects current in the output circuit directly.

According to a preferred embodiment, the current detecting device detects current in the output circuit indirectly by detecting current in the energizing circuit.

According to a preferred feature of the invention, the second voltage is selected to provide a voltage at the output terminals at or below a predetermined level.

According to a preferred feature of the invention, the output current detection device is responsive to AC current, DC current and reverse-polarity DC current.

According to a preferred feature of the invention, the output current detection device comprises an omnipolar magneto-resistive sensor.

According to a preferred embodiment, the omnipolar magneto-resistive sensor is associated with a magnetically responsive toroid having a central aperture and the output circuit passes through the central aperture of the toroid so that the sensor is responsive to the magnetic field generated by the current in the output circuit.

According to a preferred embodiment, the omnipolar magneto-resistive sensor is mounted in a slot in the toroid.

According to a preferred embodiment, the electronic switching means is a solid state relay.

According to a preferred embodiment, the electronic switching means is a MosFET.

According to a preferred embodiment, said electric welding apparatus comprises a transformer and the energizing circuit comprises the primary winding of said transformer.

According to a preferred embodiment, the second voltage is provided by phase control.

According to a preferred embodiment, said electric welding apparatus comprises a motorised generator and the energizing circuit comprises the field winding of said generator.

According to a preferred embodiment, the second voltage is provided by control of the field current.

According to a preferred feature of the invention, the VRD further comprises output voltage detection means adapted to monitor the voltage across the output terminals of the welder.

According to a preferred feature of the invention, the output voltage detection means is adapted to be responsive to AC voltage, DC voltage or reverse polarity DC voltage.

According to a preferred feature of the invention, the VRD further comprises a indication circuit adapted to illuminate a light emitting device when second voltage is applied to the energizing circuit.

According to a further aspect, the invention resides in VRD adapted to control the voltage applied to an energizing circuit of an electric welding apparatus, the VRD being associated with a current detection device adapted to detect the presence of a current in an output circuit connecting a pair of output electrodes of said welding apparatus above a predetermined value, wherein said current detection -6device comprises an omnipolar magneto-resistive sensor such that when the current detected is above a predetermined level first voltage is applied to the energizing circuit and when the current detected is below a predetermined level, a second voltage is applied to said energizing circuit where the first voltage is greater than the second voltage.

According to a preferred feature of the invention, the omnipolar magneto-resistive sensor is associated with a magnetically responsive toroid having a central aperture and the output circuit passes through the central aperture of the toroid so that the sensor is responsive to the magnetic field generated by the current in the output circuit.

According to a preferred embodiment, the sensor is mounted in a slot in the toroid to thereby increase sensitivity to the magnetic field generated by the current in the output circuit.

According to a further aspect, the invention resides in current sensing device comprising an omnipolar magneto-resistive sensor and a magneticallyresponsive toroid wherein the sensor is positioned within a slot substantially parallel with the magnetic field.

The invention will be more fully understood in light of the following description of several specific embodiments.

Brief Description of the Drawings The description is made with reference to the accompanying drawings of which:- Figure 1 is a schematic diagram of a voltage reduction device according to the first embodiment; Figure 2 is a schematic diagram of a voltage reduction device according to a second embodiment; 7 Figure 3 is a schematic diagram of a voltage reduction device according to a third embodiment.

Figure 4a is a diagrammatic representation of an output current detection device of the prior art.

Figure 4b is a diagrammatic representation of an output current detection device using a an omni-polar magneto-resistive sensor as used in the first embodiment; Figure 4c is a diagrammatic representation of an output current detection device which is an adaptation of the sensor of Figure 4b.

Detailed Description of Preferred Embodiments Voltage reduction devices are required to operate by sensing of the resistance in the output circuitry of the welder. When the output resistance drops below 200 ohms, full output voltage is applied to the welder's outputs so that an arc can be struck and welding take place. When the arc is broken, the output voltage is required to be reduced to a safe level within 0.3 seconds. This delay has been permitted to enable compliance and so that welding is not interrupted by instantaneous variations occurring during the welding process, while being short enough to avoid any significant risk to the operator.

The figures disclose embodiments of a voltage reduction device (herein referred to as a "VRD") according to the invention. The embodiments are adapted to be connected within the circuitry of electric welding apparatus including consumable rod arc welding apparatus, metal-inert-gas (MIG) welding apparatus, tungsteninert-gas (TIG) welding apparatus, Plasma-arc welding apparatus and the like, hereinafter referred to as a "welder".

The VRDs of the embodiments are capable of use with welders providing alternating current (AC) direct current (DC) output or reverse polarity direct current where the input is an inductive type of transformer. They are also capable of use with DC welders powered by a rotating DC generator. They -8may be integral with the control circuitry of the welder or may produced as a separate unit adapted to be connected to an existing welder.

The first embodiment is a VRD as used in association with a single-phase AC or two phase welder employing a transformer adapted to be connected to mains power. The circuitry is the substantially the same whether the primary of the transformer is connected between a phase and neutral or between two phases.

As shown in Figure 1, a voltage reduction system 11 according to the first embodiment comprises the following elements: 1. control power supply PS; 2. primary controller 13; 3. output current detection device (separately packaged) 4. Indication circuit 17; over-voltage detection circuit 19.

A control power supply PS is provided to supply operating power to the detection 15 and indication circuitry 17. The power supply PS is of the switch mode type to enable the device to be used on all commonly provided mains power systems and to provide a suitable, regulated output voltage Vcc, typically of 12V DC. The input of the control power supply is connected to the mains power supply used to power the welding transformer T1. Use of the switch mode power supply PS also ensures a high level of electrical isolation between the welder's power circuitry and detection and indication circuitry (typically capable of 1500V isolation).

The primary controller 13 is adapted to control the voltage applied to the primary winding of welding transformer T1 which thereby energizes the secondary or output winding of the transformer T1. As is described below, full voltage is applied to the primary winding when welding is taking place while a reduced voltage is applied to the primary winding when the welder is idle. The method by which the reduced voltage of the welder output is generated is phase control.

-9- The primary controller comprises a pair of silicon controlled rectifiers SCR1 and SCR2 configured "back to back", that is one (SCR1) is arranged in a forward direction and the other (SCR2) in a reverse direction. A first input Tpl of transformer T1 is connected directly to the mains supply L1 while the other input Tp2 of the primary is connected to the two silicon controlled rectifiers SCR1 and SCR2. The gates of SCR1 and SCR2 are connected via respective diodes to a triac Tcl which in turn has its gate connected to a diac Dcl. The gate of the diac Dcl is connected to a resistor capacitor network and a bridge rectifier BD1 regulated by a pair of zener diodes ZD1 and ZD2 in substantially conventional configuration to provide a phase controlled supply to the primary winding of the transformer. The no load voltage is adjustable by trimmer resistance VR1. In an adaptation of the embodiment, input Ti2 may be controlled by a triac in series with input L2 rather than the two SCRs as used in the embodiment. The phase control circuitry applies a reduced voltage to the primary winding when the welder is idle.

When the resistance between the output terminals of the welder falls below the predetermined value (200 ohms), the output current detection device 15 triggers, in a manner described below, a solid state relay SSR1 into conduction to bridge out triac Tcl and thereby apply full input voltage to the inputs of the transformer T1.

The output current detection device of the embodiment is shown in Figure 4b and comprises an omnipolar magneto-resistive sensor U1 associated with a ferrite toroid 41. A suitable sensor is the Honeywell 2SS52M manufactured by Honeywell Inc. Such sensors have a magneto-resistive material integrated onto silicon and encapsulated into a suitable package. They are responsive to magnetic field in either direction but must be correctly aligned with that field.

The device provides a signal comparable to that of similar devices operating on the Hall effect principle. However, the omnipolar magneto-resistive device is more sensitive than the Hall effect devices and can operate at much greater sensor-to-magnet distances.

Nevertheless, as the magnetic field produced by the conductor is very weak, it is necessary to concentrate the field by use of a ferrite toroid. This technique is known is relation to bipolar or hall effect devices where the device is arranged in a radially-oriented slot, as shown in Figure 4a. As shown in that figure, the ferrite toroid 31 is provided with a radial slot 32 into which the bipolar device 33 is placed. However, this arrangement is not suitable for use with an omnipolar magneto-resistive sensor as the sensor is unresponsive when placed in this transverse orientation relative to the field. It is necessary to align the omnipolar magneto-resistive sensor substantially parallel with the field induced in the toroid.

As shown in Figure 4b, this is achieved in the embodiment by providing a toroid 41 which is incompletely formed so that there is provided a first face 42 oriented slightly obliquely from a radial orientation on one side and an opposed face 43 oriented considerably away from a radial orientation. The omnipolar magneto-resistive sensor U1 is inserted in the gap between the two faces 42 and 43 so that the end 44 of the sensor responsive to the magnetic field abuts the first face 42 and an adjacent side 45 lies substantially parallel to the second face 43.

The toroid 41 can be formed in this configuration by cutting a completed toroid appropriately.

The output conductor is arranged to pass through the toroid. The current detection device thus provided is very sensitive and also responsive to both AC and DC current. This is a significant advance over the prior art where such current sensing devices were only capable of sensing AC current.

In a further adaptation of the current detection device described, as shown in Figure 4c, the toroid 51 is pre-formed with a radial slot 52 to provide two opposed, parallel faces 53 and 54 forming magnetic poles. The omnipolar magneto-resistive sensor U1 is transversely aligned relative to the faces within the slot. This arrangement might be particularly applicable where a large toroid is required.

11 In operation, when a low resistance is placed across the output of the welding transformer, as in the case of commencement of welding, a small amount of current is caused to flow through the output circuit. This current is detected by the sensor Ul. The sensor U1 is thereby triggered to turn on to provide an operative signal. This signal is received by the input of SSR1 to cause the output of SSR1 to be switched into conduction, as described above.

When welding ceases and the resistance between the output terminals rises above the predetermined level, current in the output circuit stops or is reduced sufficiently and the output current detection device 15 switches off. In turn, SS2 is caused to turn off, which then reintroduces the phase controlled supply to the welder. The change from welding voltage to safe voltage is virtually immediate, the only delay being the result of response times of the electronic components, which are not perceptible, and the transformer. Unlike prior art VRDs, this delay is not perceptible by the operator.

In an adaptation of the embodiment, the current detection is monitored on the primary winding, the output current being directly related to the input current.

While such an arrangement is quite workable, it is not preferred. The output current is of course considerably greater than the input current, and therefore it is possible to obtain a more precise current determination by detecting the output current. As well, the Code specifies the performance in relation to the resistance between the output terminals. The output current is directly related to this parameter and therefore a more appropriate characteristic on which to control the device.

The indication circuit 17 comprises a green light emitting diode (LED) LED1 indicating a safe voltage condition and a red LED LED2 indicating an unsafe voltage. Both are supplied by control power Vcc from the control power supply

PS.

The LED1 is connected to a resistor R7 and to the load of a normally closed photoMOS relay U2 and then to the negative of the control power supply. The 12 input of U2 is connected to the output of the output current detection device.

When the output current detection device detects current in the output circuit of T1, the input of U2 is energised and U2 caused to cease conducting. LED1 is turned off.

The unsafe voltage indication LED2 is connected to the load of a normally-open photoMOS relay U3 and thereby connected to negative of the control power supply. When photoMOS relay U3 is urged into conduction, current is caused to flow through LED2 to emit a flashing warning light.

The over-voltage detection circuit 19 monitors the voltage between the output terminals of the welder to thereby operate photoMOS relay U3. As the output of the welder may be AC, DC or reverse polarity DC, input to photoMOS relay U1 passes initially through bridge rectifier BD2. This output from BD2 is fed through a resistor to the input of photoMOS relay U3 with silicon avalanche diode TZ2 determining the voltage above which conduction occurs. As a result of this circuit, the LED2 is turned on whenever high voltage is applied to the output terminals of the welder. This is not only the case where full welding voltage is applied during the welding process, but also should high voltage be applied due to a failure within the unit. This might occur for example in the event that an insulation failure occurs between the primary and secondary windings. Such failures, while rare, can result in full mains voltage being applied across the output terminals without rendering the welder inoperable. In such a situation an operator can be exposed to great danger. Previous prior art devices have given no indication of such a fault.

It should be that, while photoMOS relays are particularly suited to the present application, it would be possible to substitute alternative switching componentry, including mechanical relays. However, photoMOS relays provide isolation, simplified circuitry and reliability to make them the preferred means of providing the required switching function.

13 It should also be noted that the drawing of the embodiment incorporates a number of components which have not be mentioned in the above description.

These are identified by standard symbols and appropriate identifications.

Identifications commencing with C indicate that the component is a capacitor while identifications commencing with R indicate that the component is a resistor.

Those skilled in the art will recognize the function of these components and therefore they are not described.

The schematic diagram of a second embodiment of a VRD according to the invention is shown in Figure 2. The second embodiment is shown applied to a 3 phase welding unit of the type often used for welding tasks where high current is required. The second embodiment is substantially identical to the first embodiment so in the drawings like identification codes are used to depict like parts.

The second embodiment differs from the first embodiment only in that the welding transformer is of the 3 phase type and a solid state relay SSR2 is provided to switch mains power to the third phase. The connections to the first and second phases are connected to circuitry in an identical arrangement to that of the input connections Tpl and Tp2 of the primary winding. SSR2 is a zero crossover type to prevent high inrush currents should the transformer phase be energized whilst at a peak voltage. SSR2 is controlled by the output from the output current detection device 15. When the resistance between the welder output terminals is high, that is, under no load, SSR2 is switched off so that no voltage is applied to the third phase. While this is so the phase-controlled, reduced voltage only is applied to the first and second phase, thereby providing enough potential to provide the initiating current. When the arc is first struck, the output current detection device senses the small initiating current, placing SSR1 in conduction to apply full voltage to the first and second terminals of transformer primary winding in like manner to the first embodiment, and in addition, SSR2 is placed in conduction, thereby applying full potential to the third phase.

14 No VRD of the prior art has provided reduced voltage control to the primary windings of a 3 phase welder.

The schematic diagram of a third embodiment of a VRD according to the invention is shown in Figure 2. The third embodiment is similar to the first embodiment so in the drawings like codes are used to depict like parts. The third embodiment is shown applied to a motorised DC welder of the type often used on large construction sites. Such machines incorporate a secondary generator, usually providing 240V AC output, to provide power to the field of the welding generator. In such machines, voltage control of the welding output is achieved by varying the current in the field winding. This is usually achieved by adjusting a rheostat Vr. In the third embodiment, the VRD is therefore arranged to control this field current.

The VRD of the third embodiment incorporates the output current detection device; safe-Indication circuit and over-voltage detection circuit which are identical to the comparable features in the first and second embodiments.

However, in this device, phase control is inappropriate. Instead, simpler circuitry is used to limit the current when the welder is in a no-load (high resistance) condition. Power is supplied from the secondary generator G2 to power a power supply PS. This provides power for the control and detection systems. When in the no-load condition, field current is arranged to pass through resistor R6 to limit the current flowing in the field circuit. When welding commences, the output current detection device energizes mosfet F1 to bridge resistor R6 and thereby apply full current to the field, thus causing full output at the welder. Resistor and silicon avalanche diode TZ1 are provided to protect against voltages spikes.

In other respects, operation is the same as for the first embodiment. In particular, the VRD of the present disclosure does not impose any time delay of the voltage across the output reducing to the reduced voltage, beyond the response time of the welder.

15 The motorized welding machine may also be an AC welding machine, and it can be seen from the above embodiments that the circuitry can be adapted appropriately.

The VRD of the present invention can be seen to have a number of advantages over the prior art: By controlling the welder's energizing circuit, being the primary winding of an AC transformer or the field of a generator, effective control is achieved without the need for devices capable of controlling the high current. of welder's output; The VRDs according to the embodiments have considerably simpler circuitry than prior art devices. This means that there are a reduced number of points of failure; By use of the output current detection device of the embodiments, the VRD can detect AC, DC and reverse polarity DC without change in design; The VRD according to the invention is adaptable for controlling 3 phase welders; While the VRD controls the energizing circuit, it monitors the output and signals if the voltage is above a predetermined value; There is no perceptible time delay in the voltage returning to a safe level when the output resistance exceeds the predetermined value. As a result, there is no change in operating performance or procedures perceptible to the operator when compared with a comparable welder which does not have a VRD installed.

The control circuit is electrically isolated from mains power; The VRD can be used on wide range of mains voltages.

It is to be noted that, although there is no perceptible delay between the output voltage falling from full-load voltage to the reduced voltage when welding is ceased, no erratic welding performance is caused. This is partly as result of the fact that once welding has commenced, localised ionisation is caused in the gases around the molten weld bead. The resistance of this ionised gas is low 16 and prevents the resistance climbing above the predetermined level during an instantaneous break in the welding operation. The operator does not notice any difference from normal welding processes.

Modifications and variations as would be known to the skilled addressee are considered to be within the scope of this invention and it should be appreciated that the present invention need not be limited to the particular scope of the embodiment described above.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims (24)

1. A VRD adapted to control the voltage applied to an energizing circuit of an electric welding apparatus, the VRD being associated with a current detection device adapted to detect the presence of a current in an output circuit connecting a pair of output electrodes of said welding apparatus above a predetermined value, the VRD comprising electronic switching means adapted to be switched by a signal from said output current detection device between a conducting mode when said current is greater than said predetermined value and a non-conducting mode when said current is less than said predetermined value, wherein when said electronic switching means is in said conducting mode, a first voltage is applied to said energizing circuit and when said switching means is in the non- conducting mode, a second voltage is applied to said energizing circuit where the first voltage is greater than the second voltage.

2. A VRD as claimed in claim 1 wherein the current detecting device detects current in the output circuit directly.

3. A VRD as claimed in claim 1 wherein the current detecting device detects current in the output circuit indirectly by detecting current in the energizing circuit.

4. A VRD as claimed in any one of the previous claims wherein the second voltage is selected to provide a voltage at the output terminals at or below a predetermined level.

A VRD as claimed in any one of the previous claims wherein the output current detection device is responsive to AC current, DC current and reverse- polarity DC current.

6. A VRD as claimed in any one of the previous claims wherein the output current detection device comprises an omnipolar magneto-resistive sensor.

7. A VRD as claimed in claim 6 wherein the omnipolar magneto-resistive sensor is associated with a magnetically responsive toroid having a central 18 aperture and the output circuit passes through the central aperture of the toroid so that the sensor is responsive to the magnetic field generated by the current in the output circuit.

8. A VRD as claimed in claim 6 or claim 7 wherein the omnipolar magneto- resistive sensor is mounted in a slot in the toroid.

9. A VRD as claimed in any one of the previous claims wherein the electronic switching means is a solid state relay.

A VRD as claimed in any one of claims 1 to 8 wherein the electronic switching means is a MosFET.

11. A VRD as claimed in any one of the previous claims wherein said electric welding apparatus comprises a transformer and the energizing circuit comprises the primary winding of said transformer.

12. A VRD as claimed in any one of the previous claims wherein the second voltage is provided by phase control.

13. A VRD as claimed in any one of claims 1 to 10 wherein said electric welding apparatus comprises a motorised generator and the energizing circuit comprises the field winding of said generator.

14. A VRD as claimed in claim 13 wherein the second voltage is provided by control of the field current.

15. A VRD as claimed in any one of the previous claims wherein the VRD further comprises output voltage detection means adapted to monitor the voltage across the output terminals of the welder.

16. A VRD as claimed in claim 15 wherein the output voltage detection means is adapted to be responsive to AC voltage, DC voltage or reverse polarity DC voltage. 19

17. A VRD as claimed in any one of the previous claims wherein the VRD further comprises an indication circuit adapted to illuminate a light emitting device when second voltage is applied to the energizing circuit.

18. A VRD adapted to control the voltage applied to an energizing circuit of an electric welding apparatus, the VRD being associated with a current detection device adapted to detect the presence of a current in an output circuit connecting a pair of output electrodes of said welding apparatus above a predetermined value, wherein said current detection device comprises an omnipolar magneto- resistive sensor such that when the current detected is above a predetermined level first voltage is applied to the energizing circuit and when the current detected is below a predetermined level, a second voltage is applied to said energizing circuit where the first voltage is greater than the second voltage.

19. A VRD as claimed in claim 18 wherein the omnipolar magneto-resistive sensor is associated with a magnetically responsive toroid having a central aperture and the output circuit passes through the central aperture of the toroid so that the sensor is responsive to the magnetic field generated by the current in the output circuit.

A VRD as claimed in claim 19 wherein the sensor is mounted in a slot in the toroid to thereby increase sensitivity to the magnetic field generated by the current in the output circuit.

21. A current sensing device comprising an omnipolar magneto-resistive sensor and a magnetically- responsive toroid wherein the sensor is positioned within a slot substantially parallel with the magnetic field.

22. A VRD substantially as herein described.

23. A VRD substantially as herein described with reference to the accompanying drawings. 20

24. A current sensing device comprising an omnipolar magneto-resistive sensor and a magnetically- responsive toroid substantially as herein described. Dated this Fourteenth day of February 2005. Safetac Welding Products Pty Ltd Applicant Wray Associates Perth, Western Australia Patent Attorneys for the Applicant