Download Monitoring & Feedback Control

Monitoring & Feedback Control
Monitoring & Feedback
Lesson Objectives
When you finish this lesson you will
understand:
• Monitoring and control of power
circuits
• Review of sensor techniques
• Sensing for weld quality
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line
workbook
Keywords
Monitoring, 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
Force Gage
Electrode
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
Can not use:
IRMS = 0.707 Ipeak
Androvich “Resistance Welding Constant Current Heat Control”,
AWS Sheet Metal ConfV, 1992
Non-ferromagnetic Material In Core
No Saturation
et   MdIt  / dt
I
 et dt
M
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
AVC Heat Control (constant voltage)
Compensates for primary
voltage 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
Androvich “Resistance Welding Constant Current Heat Control”,
AWS Sheet Metal ConfV, 1992
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 Control
Program 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
Lower Resistance
Note: an increase in primary voltage is
required to overcome the increase in
secondary resistance
Longer Fire Angle
Higher Resistance
Shorter Fire Angle
Androvich “Resistance Welding Constant Current Heat Control”,
AWS Sheet Metal ConfV, 1992
Benefit Extends to Seam Welding Also
Circumferential
seam welders have
short, small
secondary with
constant
inductance
Cuff, Seam Welding with Constant Current Controls
Welding Journal, Sept 1998
Longitudinal Seam
welders have large, deep
secondary , and
inductance changes as
more ferromagnetic
material goes into throat.
Constant Current Adjusts
Control of Weld Quality
60 Cycle
AC
Secondary Monitoring
Tsai, Experimental Study of Weld Nugget Expan,
Paper B1, Sheet Metal Welding Conf V, AWS, 1992
4 Cycle
12 Cycle
6 Cycle
14 Cycle
8 Cycle
10 Cycle
16 Cycle
22 Cycle
Tsai, Experimental Study of Weld Nugget Expan,
Paper B1, Sheet Metal Welding Conf V, AWS, 1992
9.8 ka
12.2 ka
10.8 ka
13.8 ka
14.0 ka
Tsai, Experimental Study of Weld Nugget Expan,
Paper B1, Sheet Metal Welding Conf V, AWS, 1992
Real Time Equipment
Arrangement
Power
Supply
Strain Gage
Signal Conditioner
Oscilloscope
Voltage
Leads
Print
Displacement Transducer
Isolation
Box
Tap
Integration
Data
Recorder
Dynamic Resistance = Voltage/Current
dI
0
dt
V
Current
Voltage
R
V
I
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 Expulsion
• Excessive Current
• Low Force
• Dirty Material
• Poor Fit-up
• Worn Electrodes
Interfacial Expulsion
• Excessive Current
• Excessive Time
Z=Impedance

X=Reactance
Dynamic Power Factor Monitoring
R=Resistance
Power Factor = Cos 
= 100% if pure Resistive
= 0% if pure Reactive
Reactance is basically fixed;
Changes only if size or shape of secondary changes
So Dynamic Changes in Resistance Result in Dynamic PF Changes
Power Factor Can Be
Measured at Controller, No
Wires at Electrode Needed
}
= Negative Half Cycle
+ = Positive Half Cycle
Count # of Expulsions Per
25 Welds
Boilard, “Automatic Current Steppers for Improved Weld Quality”
AWS Sheet Metal Welding Conf. V, 1992
}
Automatic Current Stepper
Based on Dynamic Power Factor
If No Expulsion
Current Increase 1%
If 5 Expulsions/25 Welds
Current Decrease 1%
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
Ultrasound
Transmitter
Pulse
Generator
Weld Control
System
Real Time
Spot Weld
Evaluation
Signal
Processor
Ultrasound
Receiver
Waschkies, E. “Process-Integrated Resistance Spot-Welding Testing Using
Ultrasound Techniques” Welding In the World, Vol 39, No 6, 1997