Monitoring & Feedback Control

Monitoring & Feedback

Lesson ObjectivesWhen you finish this lesson you will understand:• Monitoring and control of power circuits• Review of sensor techniques• Sensing for weld quality

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line

workbook

KeywordsMonitoring, Feedback Control, Force Gage, Piezoelectric Gage, Voltage, Manganen Shunt, Meter Calibration, Rogowski Coil, Hall Effect Transducer, Power Factor Monitoring, Automatic Voltage Control, Current Regulation, Constant Current, Nugget Expansion, Optical Fringe Sensor, Dynamic Resistance, Dynamic Power Factor, Current Stepper, Acoustic Emission, Ultrasonic, Direct Temperature, Infrared

• Sensors for Monitoring• Control of Welder• Control of Weld Quality

Electrode

Electrode

Force Gage

Secondary Current Shunts

• Must have geometry - no skin effect HF problems• Return Conductors not Couple with Magnetic Field• Sometime high weight for robotics• Not completely secondary impedance non-intrusive

Meter Calibration

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

Can not use:IRMS = 0.707 Ipeak

dt/tMdIte

M

dtteI

Non-ferromagnetic Material In CoreNo Saturation

X-Ray

Bowling, DeLeon, Pietsch,Senior Project report, 1999

Rogowski Coils Are:

• Extremely Amplitude-Linear Devices• Not Frequency Sensitive (good Coils have 8 Octaves)• Position Sensitive (Coil Winding Imperfections)• Sensitive to Return Conductor Position

Hall Effect Devices Are:

• Very Position Sensitive• Temperature Sensitive• Proportional to Current not Rate of Change

Can Use on 3 phase• Sensitive to Location of Return Conductors

Power Factor: Current Through &Voltage Across Primary

Control of Welder• Automatic Voltage Control (AVC)• Current Regulation• Constant Current

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

AVC Heat Control (constant voltage)

Compensates for primaryvoltage changes

Act. % Avail Current = [Act. Volt/Set Volt] x Programmed % Current

AVC:• Holds Prim & Sec Voltage Constant on a 1/2 cycle basis• Changes total available current based on voltage changes• Does not compensate correctly for change in sec Resistance or Impedance• Usually has current pick up coil to alarm problems

Current Regulation Heat Control

Current Regulation:• Uses Current Pick-up Coil & Analog Feedback Circuit• Requires variable gain, reset, dead band, that effect current output• Normally requires 3 to 5 cycles to stabilize• Therefore, used mostly with seam welding not spot welding

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

Constant Current Heat ControlProgram Controller Directly in Secondary Amperes, then controller holds secondary current within +/- 1%

Controller Needs to Perform:• Digital Sampling of Primary and Secondary Current or Use Turns Ratio to Calculate Secondary• Calculate RMS Current on 1/2 cycle basis• Respond to process changes on 1/2 cycle basis (Use Predictive Feedback)• Self teach relationship between current and % available current• Primary benefit is its ability to compensate for changes in secondary impedance

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

Predictive Feedback

• Use Large Firing Angle (small current) for first 1/2 cycle• Read results of current• Calculate a ratio between % available current and actual• Predict where to fire the next 1/2 cycle using tables

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

Androvich “Resistance Welding Constant Current Heat Control”,AWS Sheet Metal ConfV, 1992

Lower Resistance

Higher Resistance

Longer Fire Angle

Shorter Fire Angle

Note: an increase in primary voltage is required to overcome the increase in secondary resistance

Benefit Extends to Seam Welding Also

Circumferential seam welders have short, small secondary with constant inductance

Longitudinal Seam welders have large, deep secondary , and inductance changes as more ferromagnetic material goes into throat.

Constant Current Adjusts Cuff, Seam Welding with Constant Current ControlsWelding Journal, Sept 1998

Control of Weld Quality

60 C

ycle

AC

Secondary Monitoring

Tsai, Experimental Study of Weld Nugget Expan, Paper B1, Sheet Metal Welding Conf V, AWS, 1992

Tsai, Experimental Study of Weld Nugget Expan, Paper B1, Sheet Metal Welding Conf V, AWS, 1992

4 Cycle

6 Cycle

8 Cycle

10 Cycle

12 Cycle

14 Cycle

16 Cycle

22 Cycle

Tsai, Experimental Study of Weld Nugget Expan, Paper B1, Sheet Metal Welding Conf V, AWS, 1992

9.8 ka

10.8 ka

12.2 ka

13.8 ka

14.0 ka

Real Time Equipment Arrangement

PowerSupply

Strain Gage Signal Conditioner

VoltageLeads

Tap

Displacement Transducer

IsolationBox

Integration

DataRecorder

Print

Oscilloscope

Dynamic Resistance = Voltage/Current

Current Voltage

0dt

dI

V

I

VR Two Points per Cycle (+ & - 1/2 cycles)

Liang, “Fundamental Study of Contact Behavior in RSW”OSU Dissertation, 2000

Dickinson, Welding in Auto Industry,AISI, 1981

Dickinson, Characterization of Spot Weld..”Welding Journal, 1980

continued

Dickinson, Welding in Auto Industry,AISI, 1981

Expulsion

Surface ExpulsionSurface Expulsion• Excessive CurrentExcessive Current• Low ForceLow Force• Dirty MaterialDirty Material• Poor Fit-upPoor Fit-up• Worn ElectrodesWorn Electrodes

Interfacial Expulsion• Excessive Current• Excessive Time

R=Resistance

X=R

eact

ance

Z=Impedance

Power Factor = Cos

Reactance is basically fixed; Changes only if size or shape of secondary changes

= 100% if pure Resistive= 0% if pure Reactive

Dynamic Power Factor Monitoring

So Dynamic Changes in Resistance Result in Dynamic PF Changes

Power Factor Can Be Measured at Controller, No Wires at Electrode Needed

Boilard, “Automatic Current Steppers for Improved Weld Quality”AWS Sheet Metal Welding Conf. V, 1992

= Negative Half Cycle+ = Positive Half Cycle

}

}Count # of Expulsions Per 25 Welds

If 5 Expulsions/25 WeldsCurrent Decrease 1%

If No ExpulsionCurrent Increase 1%

Automatic Current StepperBased on Dynamic Power Factor

Havens “Controllin Spot Welding Quality and Expulsion”SME Paper AD76-279 1976

Havens “Controllin Spot Welding Quality and Expulsion”SME Paper AD76-279 1976

Bhattacharya “Temp Measurement of Resistance Spot Welds”Welding & Metal Fabrication, Nov 1969

Snee “Infrared Monitoring of Resistance Spot Welding”Metal Construction April 1972

Beevers “Temp Measurement to Determine Post Weld Heat Treatment..”British Welding Journal, April 1963

Burbank “Ultrasonic In-Process Inspection of Resistance Spot Welds”Welding Journal May 1965

Burbank “Ultrasonic In-Process Inspection of Resistance Spot Welds”Welding Journal May 1965

Dynamic Temperature Measurements Using Ultrasonics

Waschkies, E. “Process-Integrated Resistance Spot-Welding Testing Using Ultrasound Techniques” Welding In the World, Vol 39, No 6, 1997

UltrasoundTransm itter

UltrasoundReceiver

Pulse

Generator

Signal

Processor

Real Tim e

Spot W eld

Evaluation

W eld Control

System