LINNETT, Laurie (Speedwell House, Dirleton, East Lothian EH39 5DZ, GB)
CLARKE, Stuart James (20 Blinkbonny Road, Currie, Edinburgh EH14 6AF, GB)
MORRISON, Steven (15 Tirran Drive, Dulloch ParkDunfermline, Fife KY11 8JG, GB)
MARSHALL, Thomas David (53 Pendreich Grove, Bonnyrigg, Mi EH19 2EH, GB)
PARKES, Richard (Haylands Paddock, Corbett RoadRyde, Iow PO33 3LF, GB)
LINNETT, Laurie (Speedwell House, Dirleton, East Lothian EH39 5DZ, GB)
CLARKE, Stuart James (20 Blinkbonny Road, Currie, Edinburgh EH14 6AF, GB)
MORRISON, Steven (15 Tirran Drive, Dulloch ParkDunfermline, Fife KY11 8JG, GB)
MARSHALL, Thomas David (53 Pendreich Grove, Bonnyrigg, Mi EH19 2EH, GB)
| CLAIMS: 1. A method of passing an electric current through an object, comprising: bringing an electrode and the object into contact and establishing a contact force therebetween; adjusting the contact force towards a target contact force; and passing an electric current between the electrode and the object. 2. The method according to claim 1 , comprising passing the electric current between the electrode and the object to resistively heat the object. 3. The method according to claim 1 or 2, comprising continuously adjusting the contact force towards the target contact force. 4. The method according to any preceding claim, comprising determining the contact force between the electrode and the object. 5. The method according to any preceding claim, wherein the target contact force comprises at least one of a discrete value and a range defined between maximum and minimum discrete values. 6. The method according to any preceding claim, comprising adjusting the contact force towards a first target contact force and then adjusting the contact force towards a second target contact force. 7. The method according to claim 6, wherein the second target contact force is less than the first target contact force. 8. The method according to any preceding claim, wherein the contact force is adjusted during the flow of electric current between the electrode and the object. 9. The method according to any preceding claim, comprising moving the electrode relative to the object. 10. The method according to any preceding claim, comprising adjusting the target contact force during the flow of electric current between the electrode and the object. 11. The method according to any preceding claim, comprising determining a target contact force in accordance with a sensed geometrical property of the electrode and/or the object. 12. The method according to any preceding claim, comprising determining a target contact force in accordance with a sensed temperature of the electrode and/or a temperature of the object. 13. The method according to any preceding claim, comprising determining a target contact force in accordance with a sensed electric current flowing between the electrode and the object. 14. The method may comprise determining the contact force between the electrode and the object by use of a load sensor associated with the electrode. 15. The method according to any preceding claim, comprising using an actuator to move the electrode. 16. The method according to claim 15, wherein the actuator is used to bring the electrode into contact with the object and to adjust the contact force between the electrode and the object. 17. The method according to claim 15 or 16, comprising measuring an actuator variable and then determining the contact force in accordance with the measured actuator variable. 18. The method according to claim 17, wherein the actuator variable comprises at least one of pressure, temperature, an electrical variable and a mechanical variable. 19. The method according to claim 17 or 18, comprising: determining the contact force as a function of the actuator variable; determining a value of the actuator variable; and using the determined contact force function and the determined actuator variable value to determine a contact force value. 20. The method according to claim 17, 18 or 19, comprising: determining the contact force as a function of a plurality of actuator variables; determining a value of each of the plurality of actuator variables; and using the determined contact force function and the determined plurality of actuator variable values to determine a contact force value. 21. The method according to any one of claims 15 to 20, wherein the actuator is operated by an actuator drive source and the method comprises the step of controlling the actuator drive source to control the contact force. 22. The method according to claim 21 , comprising: determining the contact force as a function of an actuator drive source variable; determining a value of the actuator drive source variable; and using the determined contact force function and the determined actuator drive source variable value to determine a contact force value. 23. An apparatus for passing an electric current through an object, comprising: an electrode adapted to contact an object; an electrical supply configured to pass an electric current between the electrode and the object; a sensor arrangement configured to determine a contact force between the electrode and the object; an actuator configured to move the electrode and/or the object; and a controller, wherein the controller is adapted to control the actuator so as to bring the electrode and the object into contact and to adjust the contact force towards a target contact force. 24. The apparatus according to claim 23, wherein the controller is configured to control the actuator so as to adjust a position of the electrode and/or of the object relative to one another during the supply of electric current between the electrode and the object. 25. The apparatus according to claim 23 or 24, comprising at least one sensor configured to sense a property of at least one of the electrode and the object, wherein the controller is configured to control the actuator to adjust the contact force in accordance with the sensed property. 26. The apparatus according to claim 25, wherein the sensed property comprises at least one of a geometric property, a thermal property, a mechanical property and an electrical property. |
AN OBJECT
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for passing an electric current through an object using an electrode and, in particular, but not exclusively, for passing an electric current through an object using an electrode for the purposes of resistively heating the object, for example, in a welding process. BACKGROUND OF THE INVENTION
When two objects are in physical contact and an electric current is driven between them, electrical energy is dissipated in the form of heat at an interface between the objects. The dissipation of heat may be represented by a resistance, conveniently known as the contact resistance, which depends on the nature of the interface between the objects.
The contact resistance between an electrode and an object may be important for applications that require an electrical current to be driven between the electrode and the object. In resistive heating applications, for example, in which an electric current is typically driven through an object between two electrodes, the value of the contact resistance may affect the efficiency or the effectiveness of the electrical heating method since electrical energy is dissipated and heat is generated across the contact resistance. The contact resistance may be particularly important for applications where it is necessary to precisely control the resistive heating process, such as in welding applications. The contact resistance may also affect the uniformity of the resistive heating process which may also be an important parameter for such applications. Resistive heating systems and methods may be particularly (though not exclusively) suitable for use in welding together components such as pipes which may be formed from an electrically conductive material such as steel for applications across different industries including the oil and gas industries.
In certain electrical measurement applications too, the value of the contact resistance between an electrode of an electrical probe and an electrical supply or measurement device may affect the accuracy of the measurements.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a method of passing an electric current through an object, comprising: bringing an electrode and the object into contact and establishing a contact force therebetween;
adjusting the contact force towards a target contact force; and
passing an electric current between the electrode and the object.
In use the method may permit control over the contact resistance by controlling the contact force.
The steps of the method need not be sequential. For example, the step of adjusting the contact force towards a target contact force may be performed before, during or after the step of passing an electric current between the electrode and the object.
The target contact force may be selected in accordance with, for example, a target contact resistance, a material of the electrode, a geometry of the electrode, a material of the object, a geometry of the object or any combination thereof. The target contact resistance may, for example, be a maximum contact resistance chosen so as to limit heat dissipated at an interface between the electrode and the object to an acceptable level.
The method may comprise establishing a contact force equal to or substantially equal to the target contact force.
The method may comprise continuously adjusting the contact force towards the target contact force. This arrangement may be advantageous when the contact force is caused to vary over time, for example, as a result of passing an electric current between the electrode and the object, as a result of a change in a property or properties of the electrode and/or the object over time and/or as a result of a change in geometry of the electrode and/or the object over time.
The method may comprise maintaining a contact force. For example, the method may comprise maintaining a contact force equal to or substantially equal to the target contact force.
The method may comprise maintaining a contact force for a fixed or specified period of time.
The method may comprise a step of determining the contact force between the electrode and the object.
The step of determining the contact force between the electrode and the object may be performed before, during or after the step of adjusting the contact force towards a target contact force.
The step of determining the contact force between the electrode and the object may be performed before, during or after the step of passing an electric current between the electrode and the object. The method may comprise selecting a target contact force so as to avoid arcing between the electrode and the object. It may be preferable to avoid arcing because arcing may lead to damage or deformation of the electrode and/or the object at an interface between the electrode and the object. Furthermore, arcing may lead to a localised increase in heating or formation of a 'hot-spot 1 at the interface between the electrode and the object. Formation of such a 'hot-spot' may, however, be undesirable for applications where uniformity of heating is important.
The method may comprise selecting a target contact force greater than or equal to a minimum contact force between the electrode and the object.
The method may comprise selecting a target contact force less than or equal to a maximum contact force between the electrode and the object.
The target contact force may comprise a discrete value. The target contact force may comprise a range, such as a range defined between maximum and minimum discrete values.
The method may comprise adjusting the contact force towards a first target contact force and then adjusting the contact force towards a second target contact force. For example, the step of adjusting the contact force towards the first target contact force may be performed prior to passing the electric current between the electrode and the object, and the step of adjusting the contact force towards the second target contact force may be performed during the flow of electric current.
Alternative sequences of steps are, however, possible. The first target contact force may be selected to avoid arcing when initiating an electric current. The second target contact force may be selected to avoid arcing once an electric current flow has been initiated.
The second target contact force may, for example, be less than the first target contact force. When the object is a steel pipe, for example, the first target contact force may have a value of 5 to 500, 100 to 300 or around 250kg force.
The method may comprise establishing the first target contact force then establishing the second target contact force. For example, the step of establishing the first target contact force may be performed prior to passing the electric current between the electrode and the object and the step of establishing the second target contact force may be performed during the flow of electric current.
The step of adjusting the contact force may comprise applying an external force to or adjusting, such as reducing or increasing, an external force applied to the electrode and/or to the object after the step of bringing the electrode and the object into contact. The method may comprise selecting a target contact force so as to at least partially suppress deformation of the electrode and/or the object at an interface therebetween. Suppressing deformation of the electrode and/or the object may be desirable in some applications where a dimension, shape and/or size of the electrode and/or the object during and/or after electric current flow is important. For example, in resistive heating applications such as resistive heating of an object in preparation for welding the object to a further object, the dimension, shape and/or size of the object during and/or after resistive heating may be critical.
The method may comprise selecting a target contact force less than or equal to a maximum contact force to at least partially suppress deformation of the electrode and/or the object at the interface.
The step of adjusting the contact force may comprise applying an external force to or adjusting, such as increasing or reducing, an external force applied to the electrode and/or to the object during the flow of electric current between the electrode and the object.
If resistive heating of the electrode and/or the object occurs during flow of the electric current, the electrode and/or the object may change size or shape on heating, for example, the electrode and/or the object may expand on heating. This may result in a change in the contact force. Adjusting an external force applied to the electrode and/or to the object during the flow of electric current may, therefore, be advantageous for controlling the contact force if resistive heating of the electrode and/or the object occurs during flow of the electric current.
The electrode may be formed from a resilient material.
The electrode may have a resilient construction.
The electrode may comprise a resilient element. For example, the electrode may comprise a resilient tip.
The electrode may comprise a tip mounted on a resilient member such as a spring or the like.
When the electrode and the object are in contact with a non-zero contact force therebetween, adjusting an external force applied to the electrode and/or to the object may affect the contact force. This may be the case regardless of whether the electrode and the object are both stiff or whether the electrode and/or the object are at least partially resilient. When the electrode and the object are both stiff, for example, urging the electrode and the object together may increase the contact force without relative movement of the electrode and the object, while urging the electrode and the object apart may result in loss of contact. When the electrode and/or the object are resilient, however, urging the electrode and the object together may still increase the contact force but may be accompanied by relative movement of the electrode and the object, whilst urging the electrode and the object apart may initially result in a decrease in the contact force before loss of contact occurs. Choosing the electrode to be resilient may, therefore, provide improved control over the contact force.
The method may comprise moving the electrode relative to the object. Additionally, or alternatively, the method may comprise moving the object relative to the electrode.
The method may comprise moving the electrode and/or the object relative to one another during the flow of electric current therebetween. For example, the method may comprise moving the electrode and/or the object away from one another during the flow of electric current therebetween.
The method may comprise moving the electrode and/or the object away from one another during the flow of electric current between the electrode and the object while maintaining contact between the electrode and the object. For example, the method may comprise moving the electrode and/or the object away from one another during the flow of electric current between the electrode and the object so as to maintain the contact force greater or equal to a minimum contact force, for example, a minimum contact force required to avoid arcing.
The mechanical properties of the electrode and the object at the interface, in turn, may depend on the temperature of the electrode and the object at the interface. For example, if resistive heating of the electrode and/or the object occurs during flow of the electric current, the material of the electrode and/or the material of the object may soften. When combined with any expansion of the electrode and/or the object occurring during flow of the electric current, softening of the material from which the electrode and/or object is formed may lead to additional changes in the contact force and/or deformation of the electrode and/or the object when a contact force is exerted between the electrode and the object.
The method may comprise adjusting the contact force during the flow of electric current between the electrode and the object so as to at least partially suppress deformation of the electrode and/or the object.
The method may comprise adjusting a target contact force during the flow of electric current between the electrode and the object so as to at least partially suppress deformation of the electrode and/or the object.
The method may comprise moving the electrode and/or the object relative to one another during the flow of electric current between the electrode and the object so as to at least partially suppress deformation of the electrode and/or the object. The method may comprise adjusting the contact force during the flow of electric current between the electrode and the object according to a dimension, size or shape of the electrode and/or a dimension, size or shape of the object.
The method may comprise adjusting a target contact force during the flow of electric current between the electrode and the object according to a dimension, size or shape of the electrode and/or a dimension, size or shape of the object.
The method may comprise moving the electrode and/or the object relative to one another according to a dimension, size or shape of the electrode and/or a dimension, size or shape of the object.
The method may comprise sensing the dimension, size or shape of the electrode.
The method may comprise sensing the dimension, size or shape of the object.
The method may comprise adjusting the contact force during the flow of electric current between the electrode and the object according to a temperature of the electrode and/or a temperature of the object. This may be advantageous because a dimension, size or shape of the electrode and/or the object may be dependent on the temperature of the electrode and/or a temperature of the object. For example, the electrode and/or the object may expand on heating.
The method may comprise determining a target contact force according to a temperature of the electrode and/or a temperature of the object.
The method may comprise moving the electrode and/or the object relative to one another according to a temperature of the electrode and/or a temperature of the object.
The method may comprise adjusting the contact force during the flow of electric current between the electrode and the object as a function of a temperature of the electrode and/or a temperature of the object so as to at least partially suppress deformation of the electrode and/or the object.
The method may comprise determining a target contact force as a function of a temperature of the electrode and/or a temperature of the object so as to at least partially suppress deformation of the electrode and/or the object.
The method may comprise moving the electrode and/or the object relative to one another as a function of a temperature of the electrode and/or a temperature of the object so as to at least partially suppress deformation of the electrode and/or the object.
The method may comprise sensing a temperature of the electrode.
The method may comprise sensing a temperature of the object. The method may comprise adjusting the contact force during the flow of electric current between the electrode and the object according to an electric current flowing between the electrode and the object, for example, according to an instantaneous or integrated electric current. This may be advantageous because a dimension, size or shape of the electrode and/or the object may be dependent on the temperature of the electrode and/or a temperature of the object which may be dependent, in turn, on the electric current flowing between the electrode and the object. For example, the electrode and/or the object may expand on heating.
The method may comprise adjusting a target contact force during the flow of electric current between the electrode and the object according to an electric current flowing between the electrode and the object, for example, according to an instantaneous or integrated electric current.
The method may comprise moving the electrode and/or the object relative to one another according to an electric current flowing between the electrode and the object, for example, according to an instantaneous or integrated electric current.
The method may comprise sensing an electric current flowing between the electrode and the object, for example, sensing an instantaneous or integrated electric current.
The method may comprise the following contact force control steps:
determining the contact force;
determining a difference between the contact force and the target contact force; and
adjusting the contact force to reduce the difference.
The method may comprise repeatedly performing the above control force control steps, for example, continually or continuously performing the above force control steps.
The method may comprise performing the force control steps before or during the step of passing an electric current between the electrode and the object.
The method may comprise using a controller to perform the force control steps.
The method may comprise using a control system to perform the force control steps.
The method may comprise using a closed-loop system to perform the force control steps.
Determining the contact force may comprise measuring the contact force, for example, using a load cell attached to the electrode. The load cell may, for example, comprise a strain gauge, a capacitive, piezoresistive or piezoelectric load cell or the like.
The method may comprise using an actuator to bring the electrode into contact with the object.
The method may comprise using an actuator to bring the object into contact with the electrode.
The method may comprise the step of controlling the actuator to control the contact force. The method may, for example, comprise the step of controlling the actuator to apply or vary an external force applied to the electrode or the object.
The actuator may comprise a pneumatic, hydraulic or mechanical actuator or any combination thereof.
The actuator may comprise a heater or a heat exchanger or the like.
The actuator may comprise an electrical actuator.
The method may comprise indirectly measuring the contact force. For example, the method may comprise the step of measuring an actuator variable and then determining the contact force in accordance with the measured actuator variable.
The actuator variable may comprise a pressure. For example, in embodiments where fluid actuation is utilised, the pressure of a fluid within an actuator may be measured. The pressure may be measured using a pressure sensor, such as a Bourdon gauge or the like. The pressure may be a pressure of a working fluid.
The actuator variable may comprise a temperature. For example, in embodiments where gas actuation is utilised, the temperature of a gas within an actuator may be measured. The temperature may be measured using a temperature sensor, such as a thermistor, thermocouple or the like. The temperature may be a temperature of a working gas.
The actuator variable may comprise an electrical variable, such as current, voltage, or electrical power provided to an actuator. For example, in embodiments where a solenoid or an electric motor is utilised, the electric current supplied to the actuator may be measured. The electric current may be measured using a current meter or the like.
The actuator variable may comprise a mechanical variable, such as displacement, applied load, strain and the like of or on the actuator. The displacement may be measured using a position sensor. The applied load may be measured using a force sensor such as a load cell or the like. The strain may be measured using a strain gauge or the like. For example, the actuator may comprise a compression spring and the actuator variable may comprise compression of the compression spring or a compression force associated with the compression spring.
The method may comprise:
determining the contact force as a function of the actuator variable;
determining a value of the actuator variable; and
using the determined contact force function and the determined actuator variable value to determine a contact force value.
The method may comprise storing the determined contact force as a function of the actuator variable.
The method may comprise measuring a value of the actuator variable.
The step of determining the contact force as a function of the actuator variable may, for example, comprise deriving the contact force as a function of the actuator variable from knowledge of the actuator construction.
Alternatively, the step of determining the contact force as a function of the actuator variable may comprise measuring the contact force as a function of the actuator variable. For example, a load cell may be used temporarily to measure the contact force exerted between the electrode and the object as a function of the actuator variable.
The method may comprise:
determining the contact force as a function of a plurality of actuator variables; determining a value of each of the plurality of actuator variables; and using the determined contact force function and the determined plurality of actuator variable values to determine a contact force value.
The method may comprise storing the determined contact force as a function of the plurality of actuator variables.
The method may comprise measuring a value of each of the plurality of actuator variables.
The step of determining the contact force as a function of the plurality of actuator variables may comprise deriving the contact force as a function of the plurality of actuator variables from knowledge of the actuator construction.
The step of determining the contact force as a function of the plurality of actuator variables may comprise measuring the contact force as a function of the plurality of actuator variables. For example, a load cell may be used temporarily to measure the contact force exerted between the electrode and the object as a function of the plurality of actuator variables. An actuator drive source may be provided to drive the actuator. The actuator drive source may comprise a compressor, a pump, an electrical power supply or the like.
The method may comprise the step of controlling the actuator drive source to control the contact force.
The actuator drive source may have an associated actuator drive source variable. For example, the actuator drive source variable may comprise a fluid pressure or temperature or an electric current, voltage or power or the like.
The method may comprise:
determining the contact force as a function of the actuator drive source variable;
determining a value of the actuator drive source variable; and
using the determined contact force function and the determined actuator drive source variable value to determine a contact force value.
The method may comprise storing the determined contact force as a function of the actuator drive source variable.
The method may comprise measuring a value of the actuator drive source variable.
The step of determining the contact force as a function of the actuator drive source variable may comprise measuring the contact force as a function of the actuator drive source variable. For example, a load cell may be used temporarily to measure the contact force exerted between the electrode and the object as a function of the actuator drive source variable.
The method may comprise using an electrical supply to pass the electric current between the electrode and the object.
The electrical supply may comprise an electrical power supply such as a constant electrical power supply.
The electrical supply may comprise a voltage source such as a constant voltage source.
The inventor has discovered that the contact resistance between the electrode and the object is a function of at least the contact force between the electrode and the object. Accordingly, when using an electrical power supply or a voltage source, the method of passing an electric current permits control over the electric current by controlling the contact force. Surface properties of the electrode and the object such as surface roughness and properties of any surface coating or surface oxide layer at an interface between the electrode and the object may also affect the flow of current. However, for a given electrode material property, electrode geometry, object material property, and object geometry, the flow of current between the electrode and the object is a function of the contact force between the electrode and the object, and by controlling the contact force between the electrode and the object, the flow of current may be controlled accordingly.
The electrical supply may comprise a current source such as a constant current source.
When the electrical supply comprises a current source, the current may be substantially constant independent of the contact force and the associated contact resistance. When using a current source, therefore, the method of passing an electric current permits the contact resistance to be varied without changing the electric current.
According to a second aspect of the present invention there is provided a method of resistively heating an object, comprising:
bringing an electrode and the object into contact;
adjusting the contact force towards a target contact force; and
passing an electric current between the electrode and the object.
The steps of the method may or may not be sequential.
The method of resistively heating an object may, for example, comprise passing an electric current through the object according to the method of the first aspect of the present invention. It should also be understood that the optional features disclosed above in relation to the first aspect may also apply either alone or in any combination in relation to the second aspect.
The method of resistively heating an object may be used to heat at least a portion of the object for welding to at least a further portion of the object.
The method of resistively heating an object may be used for heat treating the object, for example, for annealing and/or quenching the object.
According to a third aspect of the present invention there is provided a method of welding an object to a further object, comprising:
bringing an electrode and the object into contact;
adjusting the contact force towards a target contact force;
passing an electric current between the electrode and the object; and bringing the object and the further object into contact to weld the object to the further object.
The steps of the method may or may not be sequential.
The method of welding an object to a further object may, for example, comprise resistively heating the object and/or the further object according to the method of the second aspect of the present invention. It should also be understood that the optional features disclosed above in relation to the second aspect may also apply either alone or in any combination in relation to the third aspect.
The method may further comprise:
bringing a further electrode and the further object into contact;
adjusting the further contact force towards a further target contact force; and passing a further electric current between the further electrode and the further object.
According to a fourth aspect of the present invention there is provided an apparatus for passing an electric current through an object, comprising:
an electrode adapted to contact an object;
an electrical supply configured to pass an electric current between the electrode and the object;
a force sensor configured to determine a contact force between the electrode and the object;
an actuator configured to move the electrode and/or the object; and a controller, wherein the controller is adapted to control the actuator so as to bring the electrode and the object into contact and to adjust the contact force towards a target contact force.
The electrode may be resilient.
The electrode may, for example, be formed from a resilient material, or may have a resilient construction or comprise a resilient element.
For example, the electrode may comprise a resilient tip.
The electrode may comprise a tip mounted on a resilient member such as a spring or the like.
The controller may be programmed with a target contact force value which may be determined in accordance with, for example, a target contact resistance, a material property of the electrode, a geometry of the electrode, a material property of the object, a geometry of the object or any combination thereof.
The controller may be configured to control the actuator so as to adjust the contact force towards the target contact force before or during the supply of electric current between the electrode and the object.
The controller may be configured to control the actuator so as to adjust a position of the electrode and/or of the object relative to one another during the supply of electric current between the electrode and the object.
The apparatus may comprise a sensor adapted to sense a dimension, size, shape or location of the electrode and/or object. For example, the apparatus may comprise a sensor adapted to sense a dimension, size, shape or location of an external surface of the electrode and/or object.
The controller may be configured to control the actuator so as to control a position of the electrode and/or the object relative to one another in response to a sensed dimension size, shape or location of the electrode and/or object.
The apparatus may comprise a temperature sensor for sensing a temperature of the electrode and/or the object, for example, a surface temperature of the electrode and/or object.
The temperature sensor may be attached to a surface of the electrode and/or object. The temperature sensor may, for example, be a thermistor, thermocouple or the like.
The temperature sensor may be remote from the electrode and/or object. The temperature sensor may, for example, be a camera or an image sensor or the like.
The controller may be adapted to receive a sensed temperature from the temperature sensor.
The controller may be configured to control the actuator so as to control a position of the electrode and/or of the object relative to one another in response to the sensed temperature of the electrode and/or the object. For example, the controller may be configured to control the actuator so as to control a position of the electrode and/or of the object relative to one another in response to the sensed temperature of the electrode and/or the object and a dimension, size, shape or location of the electrode and/or object stored in the controller as a function of temperature.
The apparatus may comprise a current sensor adapted to sense an electric current flowing between the electrode and the object. For example, the current sensor may be adapted to sense an instantaneous or integrated value of the electric current.
The controller may be adapted to receive a sensed current from the current sensor.
The controller may be configured to control the actuator so as to control a position of the electrode and/or the object relative to one another in response to the sensed electric current flowing between the electrode and the object.
The force sensor may comprise a load cell for measuring the force exerted between the electrode and the object. The load cell may, for example, be mounted on or adjacent the electrode. The load cell may, for example, comprise a strain gauge, a capacitive, piezoresistive or piezoelectric load cell or the like. The actuator may be adapted to bring the electrode into contact with the object.
The actuator may be adapted to bring the object into contact with the electrode.
The actuator may comprise a pneumatic, hydraulic or mechanical actuator or any combination thereof. Such an actuator may comprise a piston and cylinder arrangement. The actuator may comprise a heater or a heat exchanger or the like. The actuator may comprise a piston and cylinder arrangement having a heater or a heat exchanger or the like.
The actuator may comprise an electrical actuator. The electrical actuator may be electro-mechanical. The actuator may comprise a motor, solenoid or the like.
The contact force may be determined indirectly. For example, an actuator variable value may be measured and the contact force may be determined from the actuator variable value. Accordingly, the force sensor may comprise an actuator variable sensor.
The actuator variable may comprise a pressure. For example, in embodiments where a fluid-activated actuator is utilised, the actuator variable may comprise a pressure of a working fluid within the actuator and the actuator variable sensor may comprise a pressure sensor, such as a Bourdon gauge or the like.
The actuator variable may comprise a temperature. For example, in embodiments where gas actuation is utilised, the actuator variable may comprise a temperature of a working gas within the actuator and the actuator variable sensor may comprise a temperature sensor, such as a thermistor, thermocouple or the like.
The actuator variable may comprise an electrical variable, such as current, voltage, or electrical power provided to an actuator. For example, in embodiments where a solenoid is utilised, the actuator variable may comprise an electric current supplied to the actuator and the actuator variable sensor may comprise a current meter or the like.
The actuator variable may comprise a mechanical variable, such as displacement, applied load, strain and the like of or on the actuator and the actuator variable sensor may comprise a position sensor, a load cell or a strain gauge or the like.
The electrical supply may comprise a power supply such as a constant power supply.
The electrical supply may comprise a voltage source such as a constant voltage source. The electrical supply may comprise a current source such as a constant voltage source.
According to a fifth aspect of the present invention there is provided an apparatus for resistively heating an object, the apparatus comprising:
an electrode adapted to contact an object;
an electrical supply configured to pass an electric current between the electrode and the object;
a force sensor configured to determine a contact force between the electrode and the object;
an actuator configured to move the electrode and/or the object; and a controller, wherein the controller is adapted to control the actuator so as to bring the electrode and the object into contact and to adjust the contact force towards a target contact force.
The apparatus for resistively heating an object may, for example, comprise an apparatus according to the fourth aspect of the present invention. It should also be understood that optional features disclosed in relation to any other aspect may also apply either alone or in any combination in relation to the fifth aspect.
According to an sixth aspect of the present invention there is provided an apparatus for welding an object to a further object, said apparatus comprising:
an electrode adapted to contact an object;
an electrical supply configured to pass an electric current between the electrode and the object;
a force sensor configured to determine a contact force between the electrode and the object;
an actuator configured to move the electrode and/or the object; and a controller,
wherein the controller is adapted to control the actuator so as to bring the electrode and the object into contact and to adjust the contact force towards a target contact force.
The apparatus for welding an object to a further object may, for example, comprise an apparatus according to the fifth aspect of the present invention. It should also be understood that optional features disclosed in relation to any other aspect may also apply either alone or in any combination in relation to the sixth aspect.
The apparatus may further comprise:
a further electrode adapted to contact the object; a further electrical supply configured to pass the electric current between the electrode and the object;
a further force sensor configured to determine a contact force between the electrode and the object; and
a further actuator configured to move the electrode and/or the object, wherein the controller is adapted to control the further actuator so as to bring the further electrode and the further object into contact and to adjust the contact force towards a further target contact force.
The apparatus may comprise an object actuator and/or a further object actuator. The object actuator and/or the further object actuator may be configured to bring the object and the further object into contact for the purposes of welding the object and the further object together. For example, the object actuator and/or the further object actuator may be configured to bring the object and the further object into contact during or after resistive heating of the object and/or the further object.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be further described by way of non-limiting example only with reference to the following figures of which:
Figure 1 is a schematic representation of an apparatus for driving an electric current between an electrode and an object constituting an embodiment of the invention, wherein the apparatus uses direct measurement of contact force;
Figure 2 is a schematic cross-section of selected parts of the apparatus of Figure 1 ;
Figure 3 shows the contact force F, temperature T 1 electric current I and electrode displacement s as functions of time t during use of the apparatus of Figure
1 ;
Figure 4 is a schematic cross-section of selected parts of an alternative apparatus for driving an electric current between an electrode and an object;
Figure 5 is a schematic representation of an alternative apparatus for driving an electric current between an electrode and an object constituting an alternative embodiment of the invention, wherein the apparatus uses indirect measurement of contact force; and
Figure 6 is a schematic cross-section of selected parts of the apparatus of Figure 5.
DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 shows an apparatus generally designated 1 for driving an electric current between a first electrode 2 and a second electrode 4 through an object 6. The apparatus 1 comprises an electrical power supply 7 which is adapted to drive the electric current between the first and second electrodes 2,4 through the object 6 for the purposes of resistively heating the object 6, for example, in preparation for welding to a further object (not shown). The object 6 may comprise a pipe as shown end-on in Figure 1. The electrodes may be arranged to drive current through the pipe in the vicinity of an end of the pipe in preparation for welding to an end of a further pipe.
The apparatus 1 comprises a first electrode actuator 8, an actuator drive source 9, a first force sensor 10 and a controller 11 , The apparatus 1 further comprises a second electrode actuator 12 and a second force sensor 14. The first electrode actuator 8 is arranged to move the first electrode 2 so as to bring the first electrode 2 into contact with the object 6 and to vary a contact force between the first electrode 2 and the object 6. The first force sensor 10 is mounted on or adjacent to the first electrode 2 and is adapted to sense the contact force between the first electrode 2 and the object 6. Similarly, the second electrode actuator 12 is arranged to move the second electrode 4 so as to bring the second electrode 4 into contact with the object 6 and to vary the contact force between the second electrode 4 and the object 6. The second force sensor 14 is mounted on or adjacent to the second electrode 4 and is adapted to sense the contact force between the second electrode 4 and the object 6.
As indicated by the dotted lines in Figure 1 , the controller 11 is adapted to communicate with the first and second electrode actuators 8,12 and the first and second force sensors 10, 14. More specifically, as indicated by the directions of the arrow heads on the dotted lines in Figure 1, the controller 11 is adapted to receive signals from the first and second force sensors 10,14 and to send signals to the first and second electrode actuators 8,12. In use, the controller 11 controls the first and second electrode actuators 8, 12 so as to control the contact force between the first and second electrodes 2,4 and the object 6 respectively as described in more detail herein below.
Figure 2 shows the first electrode 2 and the first electrode actuator 8 in more detail. In the embodiment shown, the second electrode 4 is identical in construction and operation to the first electrode 2, and the second electrode actuator 12 is identical in construction and operation to the first electrode actuator 8. However one skilled in the art will understand that the first and second electrodes 2,4 and/or the first and second electrode actuators 8,12 may have different constructions and/or may operate differently. For example, the first electrode actuator 8 may be constructed as shown in Figure 2 and may operate as described with reference to Figure 2, whilst the second electrode actuator 12 may be constructed as shown in Figure 4 and may operate as described herein below with reference to Figure 4.
As illustrated in Figure 2, the electrode 2 comprises a body portion 16 and a resilient tip portion 18. The electrode actuator 8 is pneumatically-operated and receives compressed air through a pipe 20 from the actuator drive source 9 in the form of a compressor. The electrode actuator 8 comprises a cylinder assembly generally designated 22 and a variable 3-way valve 24 which controls the flow of air to or from the cylinder assembly 22. The variable 3-way valve 24 comprises a first port 26, a second port 27 and a third port 28. The first port 26 is an inlet port and is connected to the compressor 9 by the pipe 20. The second port 27 is connected to the cylinder assembly 22 and the third port 28 is an outlet port which is open to the atmosphere.
The cylinder assembly 22 further comprises a cylinder 30, a compression spring 32, and a piston head 34. The piston head 34 is movable along a direction (henceforth known as the actuator direction) parallel to sidewalls 36 of the cylinder 30. The electrode 2 is connected to and moves with the piston head 34. The piston head 34 sealingly engages against the sidewalls 36 so as to form a chamber 38 between the piston head 34, the sidewalls 36 and a first end face 40 of the cylinder
30. An aperture 42 is formed in the first end face 40 to allow air to be transferred to or from the chamber 38 via the second port 27 of the variable 3-way valve 24. The cylinder 30 further comprises a second end face 44 having an aperture 46 through which the electrode 2 extends. The compression spring 32 is fitted around the electrode 2 and is compressed between the second end face 44 and the piston head
34. The cylinder 30 also comprises a stop flange 48 that extends into the chamber 38 and limits movement of the piston head 34 towards the first end face 40 under the action of the spring 32.
The force sensor 10 comprises a load cell 10 which is arranged to measure a contact force exerted between the electrode 2 and the object 6. In the embodiment shown in Figure 2, the load cell 10 comprises a strain gauge mounted on the electrode 2. In other embodiments, the load cell 10 may comprise a capacitive, piezoresistive or piezoelectric load cell.
In use, the electrode 2 has a range of movement along the actuator direction that is limited at a forward extreme by the position of the piston head 34 corresponding to a maximum compression of the spring 32 and at a rearward extreme by the stop flange 48. For a given air pressure supplied to the first port 26 of the 3-way valve 24, the maximum spring compression is dependent on the maximum air pressure achievable in the chamber 38 when the variable 3-way valve 24 is configured so that the first port 26 and the second port 27 are fully open and the third port 28 is fully closed such that the chamber 38 is connected directly to the compressor 9. When the object 6 is fixed with respect to the cylinder assembly 22 and is. separated from a distal end 50 of the electrode 2 by a distance less than the range of movement of the electrode 2, the electrode actuator 8 is operable so as to move the electrode 2 into or out of contact with the object 6. When the electrode 2 is in contact with the object 6, the electrode actuator 8 is also operable so as to vary the contact force and, therefore, the contact resistance between the electrode 2 and the object 6.
In use, the load cell 11 converts the measured electrode strain data to contact force data and transmits the contact force data to the controller 11. Alternatively, the load cell 10 transmits electrode strain measurement data to the controller 11 which converts the electrode strain data to contact force data. A predetermined target contact force is stored in the controller 11 and the controller 11 may subsequently control the electrode actuator 8 in response to the determined contact force data so as to achieve the predetermined target contact force and, therefore, a predetermined target contact resistance, between the electrode 2 and the object 6.
For example, to reduce the difference between the measured contact force and the target contact force when the contact force is greater than the target contact force, the controller 11 acts to reduce the air pressure in the chamber 38 by controlling the variable 3-way valve 24. The controller 11 may, for example, operate the 3-way variable valve 24 so as to augment the air flow through the third port 28. The controller 11 may, additionally, restrict air flow through the first port 26 simultaneously.
Conversely, if the contact force is less than the target contact force, the controller 11 acts to increase the air pressure in the chamber 38 by controlling the variable 3-way valve 24. The controller 11 may, for example, operate the 3-way variable valve 24 so as to restrict the air flow through the third port 28. The controller
11 may, additionally, augment air flow through the first port 26 simultaneously.
With reference to Figures 1 and 3, it has been discovered that, in the case where the object 6 comprises a steel pipe, a minimum contact force is required to avoid arcing during initiation of the current flow. Once current flow has been established, however, the contact force may be reduced without arcing taking place.
This requires that the controller 11 is programmed with a first target contact force F1 to avoid arcing during initiation of the current flow between each electrode 2,4 and the object 6. For the example of a steel pipe, a suitable value of the first target contact force to avoid arcing has been found to be approximately 250kg force. However, this may vary depending on, for example, a material and geometry of the electrode, a material and geometry of the object and the like.
During resistive heating of the object 6, however, the object 6 may expand and/or the material from which the object 6 is formed may soften. This may result in the electrodes 2,4 being pressed into an external surface 52 of the object 6 when the controller 11 adjusts the contact force towards a target contact force as described above resulting in formation of a "dimple" at respective interfaces between the electrodes 2,4 and an external surface 52 of the object 6. This has been found to be the case, for example, when the object 6 comprises a steel pipe and when an electrical current is passed through the steel pipe to resistively heat the steel pipe. To at least partially suppress the formation of such dimples during resistive heating, therefore, the controller 11 is configured to move the electrodes 2,4 in a direction away from the object 6 once the flow of electrical current has been initiated whilst still maintaining contact between the electrodes 2,4 and an external surface 52 of the object 6. It should be understood that during such an electrode position control scheme, the electrodes 2,4 remain in contact with the external surface 52 of the object 6 due to resilience of the electrode tips, for example, electrode tip 18. Furthermore, the controller 11 is configured to move the electrodes 2,4 in a direction away from the object 6 during resistive heating according to a temperature of the object 6. Accordingly, the apparatus 1 further comprises a first temperature sensor 54 for sensing a temperature of the object 6 in a vicinity of an interface with the first electrode 2 and a second temperature sensor 56 for sensing a temperature of the object 6 in a vicinity of an interface with the second electrode 4. The temperature sensors 54 and 56 are remote temperature sensors, for example, infrared image sensors such as infrared cameras. As indicated by the dotted lines in Figure 1 and the associated arrows, the controller 11 is adapted to receive signals from the first and second temperature sensors 54,56.
In addition, the controller 11 is programmed with respective positions of the first and second electrodes 2,4 as a function of the respective temperature values at the first and second electrodes 2,4 so as to substantially suppress dimple formation. Such information may, for example, be determined from a known, measured or predicted rate of expansion and a known, predicted or measured hardness of the object 6 as a function of temperature.
Figure 3 illustrates an exemplary sequence of events when passing an electric current through the object 6 using the apparatus of Figures 1 and 2, which electric current results in resistive heating of the object 6. Although the control of the first electrode 2 is described below with reference to Figure 3, it should be understood that the control of the second electrode 4 involves a sequence of events identical to those described for the first electrode 2 with reference to Figure 3.
As shown in the uppermost plot in Figure 3, at time t=0 the controller 11 begins controlling the electrode actuator 8 to adjust the contact force F between the electrode 2 towards a respective first target contact force F 1 . Once the contact force F reaches the first target contact force F 1 to within a predetermined degree of accuracy at a time I 1 , the controller 11 switches on the electric power supply 7 resulting in an initial flow of electric current I 1 between the electrode 2 and the object
6 as shown in the middle plot in Figure 3. At time \ u the controller 11 switches from contact force control to position control of the electrode 2 and begins controlling the electrode actuator 8 to adjust the displacement s of the electrode 2 according to the temperature value measured by the temperature sensor 54 and the position of the electrode 2 stored in the controller 11 as a function of the temperature value at the electrode 2. As the object 6 softens, the contact force F begins to fall until, at a time t 2 , the temperature of the object 6 sensed by the temperature sensor 54 reaches a steady-state temperature value T 2 , the contact force F reaches a second contact force value F 2 and the displacement s of the electrode 2 reaches a steady-state value S 1 . It should be understood that the predicted rate of expansion of the object 6 stored in the controller 11 as a function of temperature is selected to ensure that there is no loss of contact between the electrode 2 and the object 6 during electrical current flow. In addition, the rate at which the contact force F begins to fall after I 1 depends on how the predicted rate of expansion of the object 6 stored in the controller 11 compares with the actual rate of expansion of the object 6. For example, if the electrode 2 is withdrawn from the object 6 more rapidly than the actual rate of expansion of the object 6, the contact force F may fall more rapidly than shown in Figure 3. Conversely, if the electrode 2 is withdrawn from the object 6 more slowly than the actual rate of expansion of the object 6, the contact force F may fall more slowly than shown in Figure 3.
Depending on the application, a requirement to pass the electric current through the object 6 may end at a time t 3 and, accordingly, the controller 11 switches off the electric power supply 7 at time t 3 . In the case of a resistive heating application, for example, t 3 may correspond to an instant when a temperature T of the object at the electrode 2 has reached a target temperature value T 2 to within a predetermined degree of accuracy or when a temperature T of the object at the electrode 2 has been maintained to within a predetermined degree of accuracy of a target temperature value T 2 for a predetermined period of time. At time t 3l the controller 11 also controls the electrode actuator 8 to begin withdrawing the electrode
2 from the object 6 so as to achieve a predetermined separation between the electrode 2 and the object 6. This results in loss of contact between the electrode 2 and the object at a time U and a displacement value S 2 of the electrode 2 at a time t 5 .
An alternative embodiment of an apparatus for passing an electric current between an electrode 102 and an object 106 such as a pipe may be described with reference to Figure 4. Parts of the apparatus of Figure 4 perform corresponding functions to corresponding parts of the apparatus of Figure 2 and, as such, the parts in Figure 4 have reference numerals that are given by incrementing the reference numerals of the corresponding parts in Figure 2 by 100. The electrode actuator 108 shown in Figure 4 is a hydraulically-operated electrode actuator 108 which receives a fluid such as oil through a pipe 120 from an actuator drive source 109 in the form of a pump 109. The apparatus 160 further comprises a force sensor 110 in the form of a load cell 110 and a controller 111. The electrode actuator comprises a cylinder assembly 122 having a chamber 138 filled with oil and a variable 3-way valve 124, which valve 124 controls the flow of oil to or from the oil-filled chamber 138. The controller 111 acts to control the pressure of the oil in the chamber 138 according to the force sensed by the force sensor 110. In all other respects, the operation of the hydraulically-operated electrode actuator 108 of Figure 4 is so similar to the operation of the pneumatically-operated electrode actuator 8 of the apparatus of Figure 2, that the operation of the hydraulically-operated electrode actuator 108 of Figure 4 is not described in further detail.
Figure 5 shows an alternative embodiment of an apparatus generally designated 201 for driving an electric current between a first electrode 202 and a second electrode 204 through an object 206. The parts of the apparatus 201 of Figure 5 perform corresponding functions to corresponding parts of the apparatus 1 of Figure 1 and, as such, the parts in Figure 5 have reference numerals that are given by incrementing the reference numerals of the corresponding parts in Figure 1 by 200. The first and second electrodes 202,204 are connected to an electrical power supply 207 which is adapted to drive an electric current between the electrodes 202,204 through the object 206 for the purposes of resistively heating the object 206, for example, in preparation for welding to a further object (not shown).
The apparatus 201 further comprises a first electrode actuator 208, an actuator drive source 209, and a controller 211. The first electrode actuator 208 is arranged to move the first electrode 202 so as to bring the first electrode 202 into contact with the object 206 and to vary the contact force between the first electrode 202 and the object 206. The apparatus 201 further comprises a second electrode actuator 212. The second electrode actuator 212 is arranged to move the second electrode 204 so as to bring the second electrode 204 into contact with the object 206 and to vary the contact force between the second electrode 204 and the object 206. In contrast to the apparatus 1 of Figure 1 , however, the apparatus 201 does not directly measure the contact force. Instead, first and second actuator variables associated with the first and second electrode actuators 208 and 212 respectively are measured, and a first contact force between the first electrode 202 and the object 206 is determined from the first measured actuator variable and a second contact force between the second electrode 204 and the object 206 is determined from the second measured actuator variable.
As indicated by the dotted lines in Figure 5, the controller 211 is adapted to communicate with the first and second electrode actuators 208, 212. More specifically, as indicated by the directions of the arrow heads on the dotted lines in Figure 5, the controller 21 1 is adapted to send signals to and receive signals from the first and second electrode actuators 208, 212.
Figure 6 shows the first electrode actuator 208 in more detail. The parts of the electrode actuator 208 shown in Figure 6 perform corresponding functions to corresponding parts of the electrode actuator 8 of Figure 2 and, as such, the parts in Figure 6 have reference numerals that are given by incrementing the reference numerals of the corresponding parts in Figure 2 by 200. The electrode actuator 208 is pneumatically-operated and receives compressed air through a pipe 220 from an actuator drive source 209 in the form of a compressor 209.
The electrode actuator 208 comprises a cylinder assembly generally designated 222 and a variable 3-way valve 224 which controls the flow of air to or from the cylinder assembly 222. The construction and operation of the variable 3- way valve 224 are identical to the construction and operation of the variable 3-way valve 24 of Figure 2. The cylinder assembly 222 further comprises a cylinder 230, a compression spring 232, and a piston head 234. The piston head 234 is movable along a direction (henceforth known as the actuator direction) parallel to sidewalls
236 of the cylinder 230. The electrode 202 is connected to and moves with the piston head 234. The piston head 234 sealingly engages against the sidewalls 236 so as to form a chamber 238 between the piston head 234, the sidewalls 236 and a first end face 240 of the cylinder 230. An aperture 242 is formed in the first end face 240 to allow air to be transferred to or from the chamber 238 via a second port 227 of the variable 3-way valve 224. The cylinder 230 further comprises a second end face 244 having an aperture 246 through which the electrode 202 extends. The compression spring 232 is fitted around the electrode 202 and is compressed between the second end face 244 and the piston head 234. The cylinder 230 also comprises a stop flange 248 that extends into the chamber 238 and limits the movement of the piston head 234 towards the first end face 240 under the action of the spring 232. The cylinder assembly 222 differs from the construction of the cylinder assembly 22 in that a further aperture 262 is formed in the sidewalls 236, which further aperture 262 admits air from the chamber 238 to a pressure sensor 264.
The pressure sensor 264 may take any form. The pressure sensor 264 may, for example, be a Bourdon gauge type sensor that comprises an inflatable coiled tube (not shown) which moves in response to changes in the pressure in the chamber 238. Alternatively, the pressure sensor 264 may comprise a diaphragm (not shown) the deflection of which is measured using any mechanical, capacitive, optical technique or the like.
It will be understood by one skilled in the art that the contact force exerted by the electrode 202 on the object 206 is a function of a number of variables which may include the air pressure in the chamber 238, a surface area of the piston head 234 and a spring force exerted by the compression spring 232. Furthermore, the spring force is a function of a spring constant associated with the compression spring 232 and a compression of the compression spring 232. The cylinder assembly 222 further comprises a position sensor 266 for measuring a position of the piston head 234 from which the compression of the spring 232 may be determined thereby permitting the spring force to be determined for a known spring constant. If the surface area of the piston head 234 is also known, the contact force may be derived from the air pressure as measured by the pressure sensor 264 and the spring compression as derived from the piston head position measured by the position sensor 266. A contact force function may, therefore, be derived from knowledge of the construction of the electrode actuator 208 and stored in the controller 211. For example, the contact force may be stored in the controller 211 as a function of the air pressure and the spring compression as data in a look-up table comprising contact force values and corresponding air pressure and spring compression values. A value of contact force corresponding to an air pressure value intermediate two consecutive stored air pressure values may be determined by interpolation of the data in the lookup table. Similarly, a value of contact force corresponding to a spring compression value intermediate two consecutive stored spring compression values may be determined by interpolation of the data in the look-up table. Alternatively, the contact force may be stored in the controller 211 as a function of the air pressure and the spring compression as a functional form having associated constants.
In use, air pressure data is transmitted from the pressure sensor 264 to the controller 211. Similarly, piston head position data is transmitted from the position sensor 266 to the controller 211. The controller 211 subsequently calculates the contact force from the stored contact force function and the measured air pressure and piston head position data.
The contact force may, alternatively, be measured as an empirical function of the measured air pressure and the measured piston head position in a calibration step. During such a calibration step, the contact force exerted between the electrode
202 and the object 206 may be measured using a force sensor such as a load cell
(not shown) which is temporarily attached to or mounted adjacent to the electrode
202 during the calibration step. The force sensor may subsequently be removed once the calibration step is complete. Such an empirical contact force function may be stored as calibration data in the controller 211 and the controller 211 may subsequently calculate the contact force from the empirical contact force function and the measured air pressure and measured piston head position data.
In all other respects, the operation of the electrode actuator 208 is identical to the operation of the electrode actuator 8 and, accordingly, the operation of the electrode actuator 208 will not be described in further detail.
It should be understood that the embodiments described herein are merely exemplary and that various modifications may be made without departing from the scope of the present invention.
For example, in further alternative embodiments, the electrode actuator 8,12,208,212 may comprise an electric motor and threaded shaft or rack and pinion arrangement, or a solenoid or the like.
It should be understood that the number of electrodes may be greater than two and that each electrode may have a corresponding electrode actuator. In addition, each electrode may have a corresponding actuator drive source 9,209. Each actuator drive source 9,209 may be independently operable from the other actuator drive sources 9,209.
In addition to, or as an alternative to, controlling an electrode actuator 8,12,208,212 for the control of the contact force as described in the foregoing embodiments, the actuator drive source 9,209 may be controlled so as to provide control of the contact force. The actuator drive source 9,209 may, for example, be controlled by controlling an actuator drive source variable such as an actuator drive source fluid pressure (in the case of a compressor or a fluid pump) or an electrical variable such as current, voltage or electrical power (in the case of an electrical actuator drive source).
Any of the foregoing embodiments may also be used to control the electric current driven between an electrode and an object by controlling the contact force between the electrode and the object. Where more than two electrodes are used, for example, so as to define different current paths, the current flowing along one current path may be balanced relative to a current flowing along a different current path by controlling the contact forces between the electrode and the object.
In addition to controlling an electric current by controlling the contact resistance as described in the foregoing embodiments, the electrical power supply may also be controlled so as to provide additional control of the electric current.
The electrical power supply may comprise a constant electrical power supply.
In other embodiments, a voltage source such as a constant voltage source may be used instead of the electrical power supply. In yet further embodiments, a current source such as a constant current source may be used instead of the electrical power supply.
In some embodiments the target contact force may be determined in accordance with a target contact resistance or a target electric current flow between an electrode and an object.