adaptive welding controls for aerospace applications

Bob CohenWeldComputer Corporation

Copyright © 2014 WeldComputer CorporationAll Rights Reserved

Adaptive Controls for Aerospace Resistance Welding Applications

Adaptive control is about Recognizing when a process variability exists that would affect

the outcome of a production weld

Identifying the underlying condition responsible for the variability

Taking corrective action to compensate for the variability as the weld is taking place

The end result is to prevent the occurrence of a bad weld in the first place and to increase the consistency of all welds produced

When it’s impossible to correct the problem and make a good weld, notify the operation about the problem

Flattening electrodes

Surface contamination

Poor parts fit-up

Shunt condition and other geometry variations

Workpiece thickness variation

Electrode force variation

Wheel/brush contact resistance variation

Wheel velocity variation

Reduce reliance on destructive testing.

Prevent random problem welds from passing through production undetected.

Automatically take corrective pre-conditioning and compensating actions to prevent out of spec welds from being produced.

Increase consistency of all welds.

Substitute in process monitoring in place of manual surface resistance checks.

Substitute in-process monitoring of the weld machine in place of periodical machine inspection.

4.3.3.3 Contractor may substitute nondestructive evaluation for routine lot tests upon approval of the procurement activity provided he can demonstrate that the evaluation system will identify welds complying with size or strength requirements with a 99.5% reliability.

4.2.6 Control Adjustments: …settings may be varied by ±5% from the established certification values, or by ±10% when only one setting is adjusted.

Data collected with WeldComputer® Adaptive Control

Adaptive Weld Schedule Nominal Current and Thermal Expansion Response

ExpansionCurrent


Adaptive schedule detects greater than normal thermal expansion rate and reduces current to prevent expulsion from occurring.

Current Expansion


Adaptive control increases heat on cycle 6 by 1% in response to low expansion on cycle 5.

Current Expansion


Adaptive control increases heat on cycle 4 by 1% in response to low expansion on cycle 3.

Current Expansion


Adaptive control increases heat on cycle 4 by 1% in response to low expansion on cycle 3, and an additional 1% heat increase on cycle 6 in response to low expansion response on cycle 5.

Current Expansion

http://www.weldcomputer.com/adaptive-features/

Side view (left) and front view (right) of displacement sensor mounted on seam welder.

Displacement Monitoring:

No production slow-down

Speeds up production when used in conjunction with adaptive control

More reliable than destructive testing, because destructive testing doesn’t measure the size of any weld except the one that is destroyed.

Ability to meet the 99.5% reliability requirement demanded by the MIL-SPEC has been demonstrated.

X-RAY Testing

Severe impact on productivity.

Can detect porosity, cracks, lack of fusion. Difficult to determine weld penetration.

Ultrasonic Testing

Severe impact on productivity.

Can detect porosity, cracks, lack of fusion. Difficult to determine weld penetration.

Ability to meet the 99.5% reliability requirement demanded by the MIL-SPEC has not been demonstrated.

4.3.3.3 Contractor may substitute nondestructive evaluation for routine lot tests upon approval of the procurement activity provided he can demonstrate that the evaluation system will identify welds complying with size or strength requirements with a 99.5% reliability.

4.2.6 Control Adjustments: …settings may be varied by ±5% from the established certification values, or by ±10% when only one setting is adjusted.

5.2.3 Alternate Testing Requirements As an alternate to the testing requirements of 5.2.2(1) real time nondestructive system may be used when approved by the Engineering Authority. As a minimum the system shall address: part fitup, precleaning, electrode monitoring, and in-process monitoring of critical process parameters. This system of controls shall include but is not limited to, real time adaptive controls or in-process NDT methods. Destructive testing must still be used to establish and verify that the capability of this system will identify welds complying with strength or size requirements with 99.5% reliability.

5.1.5 Control Adjustments. The settings may be varied by ±5% from the established certification values, or by ±10% when only one setting is adjusted.

4.2.2.1 Preconditioning steps to compensate for fitupvariations that involve the controlled application of heat and/or force may be employed.

Current trace (left) & Expansion response trace (right) recorded with WeldView® Monitor.

Current and Expansion Response of Spot Weld Produced with Conventional Control

ExpansionCurrent

Material thermally expands while weld current is applied

Material thermally contracts after weld current stops


Spot Weld Produced with Conventional Control has Slight Fit-Up Problem

Current Expansion

Negative movement documents parts fitting together before material starts to thermally expand

Material has acceptable thermal response despite slight fit-up problem


Severe Fit-Up Problem with Conventional Control Results in Undersized Weld

Weld time is lost squeezing parts together

Not enough weld time remaining after parts fit together results in undersized weld

Current Expansion


Adaptive Weld Schedule Nominal Current and Thermal Expansion Response

ExpansionCurrent

Diagnostic/Pre-Conditioning Heat Pulse


Adaptive schedule produces pre-conditioning pulses to correct fit-up problem before proceeding to make weld.

Weld heat occurs after 3rd

pre-conditioning pulse succeeds in correcting fit-up problem.

Pre-conditioning pulse didn’t fully correct fit-up problem, so weld heat is inhibited, and a cool down delay applied before repeating process.

Pre-conditioning heat pulse 1



5.1.5.1 Control adjustments shall apply from start to finish of the weld nugget formation.

5.1.4.2 Use of in-process weld control monitoring capable of detecting when a micro-ohms shift outside of the specification range occurs may be substituted for the surface resistance checks as deemed appropriate by the Engineering Authority.


Substitution of In-Process Micro-Ohm Measurements

Diagnostic/Pre-Conditioning heat pulse measures resistance on every weld


Current Expansion

Adaptive schedule responds to expulsion occurrence by instantly terminating weld current to minimize part damage, then automatically performs a repair weld operation.

Instantly cut off heat upon detection of expulsion.

Keep electrodes clamped on part and wait for weld to cool.

Perform re-weld operation.

5.1.5.2 Any control adjustment made beyond the constraints set forth in 5.1.5 taken to minimize part damage during the occurrence of a welding fault shall be excluded as a condition that would require the establishment of a new certified welding procedure.

Control adjustments beyond the constraints set forth in 5.1.5 may be taken in instances when it can be demonstrated that not taking such actions would increase part damage and/or reduce weld consistency.

Current (7" Length Seam)

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Conductance (7" Length Seam)

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Conductance reveals 3.1 KMho drop after completing 4.45” of welding on seam.

Current is consistently maintained at 29 KA over length of seam.

4.3.4 Maintenance of Equipment. For machine characteristics wherein the behavior of the machine can be monitored, and criteria exists for those monitored parameters that would trigger maintenance when required, such monitoring techniques may be employed in place of periodical machine inspection.

Total RMS Current (for 22 welds)

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Conductance (4.2ms after start of weld)

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4.3.3 Jigs and Fixtures. Where shunting cannot be avoided due to part design, the effects of shunting shall be factored into the production weld schedule and necessary adjustments made to ensure acceptable welds are produced.

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Use right machine, control, electrodes, force & current to make weld.

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2nd weld is smaller than 1st because some current shunts through 1st weld.

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3rd weld is smaller than 2nd because current shunts through 1st & 2nd welds.

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Subsequent welds are all similarly reduced in size due to current shunting.

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Shunting makes welds hotter at start of seam.

produce smaller nuggets than they really want throughout the entire length of the seam, in order to avoid having the first few welds on the seam be too hot and possibly expulse material,

or

suffer from having the first few welds be too hot and expulse material, just so the rest of the welds in the seam are the size they want.

Scale heat down for 2nd spot.

Scale head down more for 1st spot.

All welds occurring at same wheel speed

Welding starts at slower speed

Welding stopsat slower speed

Wheel velocity is a major parameter of control, as significant as force and current.

For a given applied force and current.

Lower velocity causes hotter welds.

Higher velocity causes colder welds.

Upslope heat at start of seam.

Downslope heat at end of seam.

Hard to coordinate upslope heat and downslope heat with increasing and decreasing velocity.

Hard to synchronize heat profile with velocity profile.

Program the control to automatically adjust the heat up or down, in relation to instantaneous wheel velocity.

Heat is always coordinated and synchronized with wheel velocity.

Easy to manage because there are no operator settings or proximity switches to constantly adjust.

Dynamically adjusting weld heat to compensate for velocity fluctuations increases weld consistency and reduces leakers.

Constant Speed

Edge to edge adaptive control produces gas tight welds over the entire length of the seam.

Water heaters

55 Gallon drums

Pails

Aerosol Cans

Plot of wheel rolling up on front tank, across tank, & off back of tank.

Wheel starts rolling up on front of part.

Wheel starts rolling off back of part.

Data collected with WeldComputer® Adaptive Control.

Current starting here after wheel is already rolling up on part results in undersized weld on front edge

Wheel starts rolling up on part here

Displacement data (top) and current data (bottom) collected with WeldView® Monitor.

Wheel starts rolling up on part here

Current starting before wheel comes in contact with part overheats front edge of part.

Displacement data (top) and current data (bottom) collected with WeldView® Monitor.

Inability to control welds on front edge

Inability to control welds on back edge

Inability to prevent hot spots

Inability to prevent cold spots

Inability to produce consistent current

Heat envelop is precisely coordinated with front edge of part.

Current

Force

Wheel/Electrode Cooling

Machine Stability

Velocity fluctuations: can be

compensated for with adaptive control.

Force fluctuations: can be compensated

for with adaptive control.

Wheel starts rolling up on front of part.


Wheel overshoots rolling up on front of part.

Wheel bounces when it lands on top of part.

Wheels bounce against each other after rolling off back of part.

Displacement of wheel rolling up on front of part triggers current.

Velocity

Each 5ms duration weld current pulse (below) is adjusted every millisecond to control weld based on displacement, velocity & force.

Data collected and current synthesized with WeldComputer® Adaptive Control.

Wheel bounces when it lands on top of part.Wheel overshoots rolling

up on front of part.

Velocity

Heat automatically reduces itself and shuts off as wheel rolls off part.

Wheels bounce against each other after rolling off back of part.


Data collected and current synthesized with WeldComputer® Adaptive Control.

Apply right heat before hitting front edge of part.

Profile heat envelope rolling up on part.

Compensate for bounce of wheel landing on part.

Compensate for velocity fluctuations.

Profile heat envelope rolling off back of part.

Instantly cut off heat rolling off back edge of part.

Adjust heat for height variations on part.

Use control that delivers accurate repeatable heat.

Part proximity sensors used to synchronize heat with front of part entering machine have too much variability to accurately control welding on the front edge.

Inability to synchronize heat with back of part leaving machine.

Inability to compensate for machine force and velocity variations.

SCR based controls have limitations on current wave shape and regulation.

Traditional MFDC controls, provide ineffective regulation and heat repeatability, in applications requiring short duration high current pulses.

Current trace recorded with WeldView® Monitor.

Process is unregulated when control never reaches programmed current targets.

Seam performance is compromised when high currents persist during cool times.


Current only decays to 13.5kA at end of programmed cool time.


Process is unregulated when unstable current overshoots, has oscillations, and never reaches programmed current targets.


Current overshoot followed by current undershoot represents 73% current fluctuation during each weld.

Undershoot occurring 8 ms after overshoot. Current never decays to zero during

cool time between impulses.

Peak current fluctuates 15% from one impulse to the next.


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Big current fluctuations reduces stability of process

Long current decay during cool time means hotter wheels

Current still rising at end of programmed heat means process is uncontrolled


Big current fluctuations caused by MFDC reduces stability of process

Long current decay during cool time means hotter wheels

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Big mechanical disturbance from current fluctuations occurs twice per millisecond of applied heat.

Current decay time after each weld diminishes effectiveness of cool time & causes wheels to run hotter.

Magnetizes machine and part.

Causes heat imbalance from Peltier Effect.

Asymmetrical electrode wear.

Limited current adjustment rate.


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3 ms Weld Produced with 6 Pulses at a 2kHz Switching Frequency

3 ms Weld Produced with 12 Pulses at 4kHz Switching Frequency

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Archive File # Avg. Expansion Avg. Current Avg. Force

983 (sample - left) 5.77 6.77 2.64

982 (part - right) 3.45 6.12 2.88

Sample Actual Part