GB2560999A

GB2560999A - Welding method and apparatus

Info

Publication number: GB2560999A
Application number: GB1705256.4A
Authority: GB
Inventors: Arthur Barber Richard; Aherne Mark; Kundrat Jorg
Original assignee: MacGregor Welding Systems Ltd
Current assignee: MacGregor Welding Systems Ltd
Priority date: 2017-03-31
Filing date: 2017-03-31
Publication date: 2018-10-03
Also published as: GB201705256D0

Abstract

A method of controlling a weld head comprises: calibrating a weld head position with a force applied (Fig 5; 50); determining a target position corresponding to a desired force (Fig 5; 52); positioning the weld head in said target position (Fig 5; 54); receiving a first force measurement; if the first force measurement is outside a predetermined range halting the weld process (Fig 5; 64); and if the force measurement is within a predetermined range performing the weld (Fig 5; 58). A second force measurement may be received after forming the weld (Fig 5; 59), and the first force measurement may be compared with the second force measurement to check that the change is consistent with a weld being completed (Fig 5; 60). Calibrating the weld head position may comprise repeating the steps of positioning the weld head in a predetermined position, receiving a force measurement and storing the predetermined position together with the corresponding force measurement. A welding apparatus comprises a servo controller 22, an actuator 18, a load sensor 25 and a force check and processing unit 31. The actuator 18 may be a linear, lead or ball screw actuator, and the actuator may be linearly adjustable.

Description

(54) Title of the Invention: Welding method and apparatus

Abstract Title: Method of calibrating a weld head and welding apparatus (57) A method of controlling a weld head comprises: calibrating a weld head position with a force applied (Fig 5; 50); determining a target position corresponding to a desired force (Fig 5; 52); positioning the weld head in said target position (Fig 5; 54); receiving a first force measurement; if the first force measurement is outside a predetermined range halting the weld process (Fig 5; 64); and if the force measurement is within a predetermined range performing the weld (Fig 5; 58). A second force measurement may be received after forming the weld (Fig 5; 59), and the first force measurement may be compared with the second force measurement to check that the change is consistent with a weld being completed (Fig 5; 60). Calibrating the weld head position may comprise repeating the steps of positioning the weld head in a predetermined position, receiving a force measurement and storing the predetermined position together with the corresponding force measurement. A welding apparatus comprises a servo controller 22, an actuator 18, a load sensor 25 and a force check and processing unit 31. The actuator 18 may be a linear, lead or ball screw actuator, and the actuator may be linearly adjustable.

Fig.4

1/5

tuo

2/5 r-

l/l U N

A

Γ\| _ώ

CM

3/5

CM

Fig. 3

4/5 r-

Fig. 4

5/5

-1 WELDING METHOD AND APPARATUS

BACKGROUND

a. Field of the Invention

The present invention relates to resistance welding apparatus and to a method of welding. The invention is of particular use in, but is not limited to micro-resistance welding.

Micro resistance welded joints are formed by passing high current and low voltage through a work piece to heat up the joint interfaces to form a weld. Pressure is applied to contain the hot metal and consolidate the joint.

Applications for the micro resistance welding process vary enormously by virtue of the component combinations involved and the manufacturing environments required. In some cases they may be applied to high value, low volume products such as internal human body products e.g. heart pacemakers (one micro joint every few minutes), whereas other applications include high volume manufacturing devices such as LED lamps (one micro joint every second).

These applications are continually demanding ever higher standards and current state-of-the-art micro welding equipment performance and capability has become a limiting factor.

Welding force applied between the electrodes in a resistance welding application is a critical parameter in determining the outcome of the welding process. It is therefore desirable to monitor and control the welding force in order to create a consistent and reliable process.

Weld heads generally comprise an electrode holder which is coupled to an actuator by a spring or rotational coupling. The actuator moves the holder so that

-2the electrode makes contact with the product and then, by further linear movement, the actuator compresses the spring to a preset distance, thereby setting a pre-load welding force by virtue of the fixed spring compression. In an opposed weld, a fixed return electrode provides a stationary mechanical point. As the weld progresses material deformation and displacement occurs, so the spring also provides a continuous follow up force.

Traditionally the weld head was positioned manually with no means of checking or changing the force applied. However, with the proliferation of electronic servo drive based actuators, the industry has begun to develop programmable weld heads by which the force applied to the electrode is determined by the servo system and driven through a user interface.

This invention relates to electronic/electrical servo drive and actuator applications, offering a new method of achieving a predetermined force.

In known servo drive and actuator applications, electronic/electrical servo drives actuate a spring coupled electrode holders in a servo weld head. Servo drive controllers generate an electrical drive signal to drive an electrical actuator and typically operate in a current control mode or in a position control mode. The actuator may translate the drive signal into rotational (if the actuator a lead ball/screw) or linear motion.

In a simple force programmable servo system, the current supplied to the actuator is approximately proportional to the force applied. In more complex systems, a load cell sensor in series with the actuator may be used in order to measure and hence adjust the force. The load cell sensor signal can provide feedback so that a target force may be achieved.

Further welding integrity checks may be performed by using a displacement sensor to measure the movement of the electrode before, during and after welding. This allows for the detection of the part to be welded and its subsequent

-3collapse after welding, albeit at the relatively high cost of the displacement sensor.

As stated previously, in current state of the art in spring based servo force systems the electronic servo inherently operates in current control mode or position control mode, while a feedback loop based on a load cell sensor measurement, seeks to achieve a target force. In precision micro welding, the servo controller is generally operated in position control mode. This allows the programming of intermediate positions, speeds and accelerations for the electrode during its travel profile. However, using load cell sensor feedback has the effect of compromising the speed at which the operation of the motion control can operate since continual force checking and feedback is required. On the other hand, one advantage of such a system is that the consumable electrode length is automatically compensated for.

As well as compromised control speeds, if the load cell sensor fails, the system cannot assure that the correct force is applied. For example, a 20% drift in the sensor would cause a 20% drift in the applied force

This invention introduces a new approach to setting a programmable force for a spring coupled servo actuator weld head system.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of controlling a weld head comprising the steps of: calibrating a weld head position with a force applied; determining a target position corresponding to a desired force; positioning the weld head in said target position receiving a first force measurement; in the event the first force measurement is outside a predetermined range halting the weld process; and in the event the force measurement is within a predetermined range performing the weld.

-4The method may advantageously further comprise the steps of: receiving a second force measurement after performing the weld; comparing the first force measurement and the second force measurement; and checking the change in force is consistent with a weld having been completed.

Even more advantageously the method may further comprise the steps of: in the event the first force measurement is within the predetermined range; comparing the first force measurement with any force measurements received previously; in the event of a systematic change in the force measurement modifying the target position to compensate for said change.

In a preferred embodiment the step of calibrating a weld head position with force applied comprises repeating the sub steps of: positioning the weld head in a predetermined position; receiving a force measurement; storing the predetermined position together with the corresponding force measurement.

According to a second aspect of the invention there is provided a welding apparatus comprising a servo controller; an actuator; a load sensor; and a force check and processing unit; in which the force check and processing unit is arranged in operation to perform the steps of calibrating the weld head position with force applied; determining a target position corresponding to a desired force; and sending said target position to the servo controller; and in which the servo controller is arranged to drive the actuator to position the weld head in said target position; and in which the force check and processing unit is arranged in operation to further perform the steps of receiving a first force measurement; in the event the first force measurement is outside a predetermined range sending a command signal to the servo controller to halt the weld process; and in the event the force measurement is within a predetermined range sending a command signal to the servo controller to execute the weld.

In one embodiment the actuator is a linear actuator. Alternatively the actuator may be a lead or ball screw actuator.

-5BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a conventional spring coupled weld head;

Figure 2 illustrates a conventional servo based system with current and positional feedback;

Figure 3 illustrates a conventional servo based system with a force feed back loop;

Figure 4 illustrates a servo based system according to the present invention; and

Figure 5 is a flow chart illustrating a method of operating the system of Figure 4 in accordance with the present invention.

DETAILED DESCRIPTION

Figure 1 illustrates a weld head 20 comprising an actuator 18 connected to an electrode holder 10 by a spring coupling 11. A first electrode 13 is mounted on the electrode holder. An opposed return electrode 14 remains stationery as pressure is applied via the spring 11. First weld part 15 and second weld part 16 are welded as weld zone 17 when a current is applied between first electrode 13 and return electrode 14.

In Figures 2 to 4 it is assumed the coupling is a spring coupling driven by a linear motion actuator or by a rotationally driven lead/ball screw actuator.

Figure 2 illustrates a servo weld head 21 which includes a servo controller 22. Servo controller 22 drives the actuator 18 with an electrical drive signal 23. The

-6actuator 18 translates the drive signal 23 into movement at the spring coupling 11, either directly or via a rotational movement translation.

The actuator 18 sends current and positional feedback signals 24 to the controller 12. The current may be used to determine an approximation of the force applied.

Figure 3 illustrates a welding apparatus 21' which includes a load cell sensor 25. The load cell sensor 25 measures the force provided and sends a force feedback signal 26 to the servo controller 22. A displacement sensor may also be used.

Figure 4 illustrates a welding apparatus 30 in accordance with the present invention. The load cell sensor provides force feedback signal 26 to a force check and processing unit.

Figure 5 is a flow chart illustrating a method of welding in accordance with the present invention.

The integral position/speed control system in the servo controller 22 is able to operate the weld head very efficiently if it can drive the actuator to achieve a target position for the electrode rather than using load cell feed back to achieve a target force for the weld.

At step 50 the weld head is calibrated for a known electrode length and weld part dimensions by determining the force achieved at various weld head positions.

At step 52 a nominal position at which the desired target force is achieved is determined.

Steps 54 to 58 are then performed until the desired number of the welds are complete at step 60 or until the measured force falls outside a predetermined range at step 56.

-7 Should any of the mechanical aspects change in this nominal setpoint condition, then a force outside the predetermined range will be measured, possibly indicating a fault or a need to modify the target position.

One advantage of using the load cell sensor 25 as a checking device only, means that should a fault develop in the load cell sensor, the nominal position target will result in a different force measurement and hence flag the potential error in the load cell sensor.

Once a nominal position is determined as being associated with a target force, the system is free to operate at high speed, with the force only being checked at the critical point in the cycle i.e. at step 56 before the weld is performed at step 58.

In an optional step 59 the force may be checked after weld collapse during the welding process, a second check should yield a small reduction in force as a result of the mechanical movement of the electrode, thereby revealing useful data to verify the weld occurred.

In an improved method the target position may be gradually modified if the force change is within the predetermined range but shows a systematic change within that range.

In this case at step 57 if the measured force is within range but shows a small systematic change at step 57 then at step 55, the target position may be modified accordingly.

For example, as the electrode is consumed, the overall force applied will gradually reduce, based on a fixed target position of the actuator. By checking this data from weld to weld, the nominal target position and hence force, may be adaptively modified in small increments so as to effectively adapt and automatically compensate for this gradual mechanical change.

-8The advantages of the method of the present invention include but are not limited to the following:

The servo controller is able to operate in a high speed positional control mode and 5 is therefore much faster and controllable when compared to using a force feedback loop.

The system is fault tolerant and as such, benefits from being able to detect changes and hence adapt if necessary or flag a fault.

Different forces can be applied sequentially at high speed, simply by learning and using the associated nominal positions.

Claims

1. A method of controlling a weld head comprising the steps of:

calibrating a weld head position with a force applied; determining a target position corresponding to a desired force; positioning the weld head in said target position;

receiving a first force measurement;

in the event the first force measurement is outside a predetermined range halting the weld process; and in the event the force measurement is within a predetermined range performing the weld.

2. A method according to claim 1 further comprising the steps of: receiving a second force measurement after performing the weld; comparing the first force measurement and the second force measurement;

and checking the change in force is consistent with a weld having been completed.

3. A method according to claim 1 or claim 2, further comprising the steps of: in the event the first force measurement is within the predetermined range; comparing the first force measurement with any force measurements received previously;

in the event of a systematic change in the force measurement modifying the target position to compensate for said change.

4. A method according to any one of the preceding claims in which the step of calibrating a weld head position with force applied comprises repeating the sub steps of:

positioning the weld head in a predetermined position; receiving a force measurement;

storing the predetermined position together with the corresponding force measurement.

-105. A welding apparatus comprising a servo controller;

an actuator;

a load sensor; and a force check and processing unit;

in which the force check and processing unit is arranged in operation to perform the steps of calibrating the weld head position with force applied; determining a target position corresponding to a desired force; and sending said target position to the servo controller;

and in which the servo controller is arranged to drive the actuator to position the weld head in said target position;

and in which the force check and processing unit is arranged in operation to further perform the steps of receiving a first force measurement;

in the event the first force measurement is outside a predetermined range sending a command signal to the servo controller to halt the weld process; and in the event the force measurement is within a predetermined range sending a command signal to the servo controller to execute the weld.

6. A welding apparatus according to claim 5, in which the actuator is a linear actuator.

7. A welding apparatus according to claim 5 in which the actuator is a lead or ball screw actuator.

8. A welding apparatus according to claim 5 in which the actuator linearly adjustable.

Intellectual

Property

Office

Application No: Claims searched: