Monitoring & Feedback Control. Monitoring & Feedback Lesson Objectives When you finish this lesson...
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Transcript of Monitoring & Feedback Control. Monitoring & Feedback Lesson Objectives When you finish this lesson...
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
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