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Measurement Techniques and Virtual Instruments for the Study of Electrostatic Processes

ABOUT:

The objective was to develop measurement techniques allowing a better control of electrostatic processes. A virtual instrument (VI) was designed to measure the electric charge of powders, and quantify the performances of a tribo-electrostatic painting process. A VI driven electrostatic voltmeter measured the discharge of insulating granular layers in contact with an electrode. The discharge curves of the components can predict the feasibility of separating them. A third VI analysed the variance of high voltage between the electrodes of an electrostatic separator; it evidenced a linear correlation between the high voltage standard deviation and the metal content of the granular mixture to be separated. Using Taguchi's method, we have modelled the separation process and studied its robustness. The techniques presented give the means of refining the analysis of the factors concerned, thus facilitating the use of statistical methods for process control.

KEYWORDS:

electrical measurments, electrical charge, electrical discharge, electrical charge distribution, high voltage, data aquisition (DAQ), electrostatic separation (technology), electrostatic, statistic process control

LAII-ESIP, UPRES EA 1219
Equipe « Electronique et Electrostatique »
IUT Angoulême
4, Avenue de Varsovie
16021 Angouleme


Thesis

"Because virtual instrumentation makes the measurement of process variables both accurate and versatile, it enables a deeper insight into electrostatic phenomena, paving the way for the improvement of existing technologies and the development of new ones."

Table of Contents


 

Acknowledgements

3

 

Contents

9

 

List of Figures

13

 

Abbreviations and Notations

19

 

Introduction

21

1

Electrostatic Separation Processes: Control Variables and Measurement Techniques

25

1.1

Electrostatic separation: a multifactor process

26

1.1.1

Roll-type corona-electrostatic separation

26

1.1.2

Free fall tribo-electrostatic separation

28

1.2

Measurement techniques for electrostatic processes

32

1.2.1

Charge measurements

33

1.2.2

High voltage measurements

34

1.2.3

Surface potential measurement

35

1.3

Virtual instruments

38

1.3.1

Data acquisition techniques

39

1.3.2

Graphical programming

39

1.3.3

Developing virtual instruments in LabVIEW

40

2

Charge Measurements of Finely Divided Matter

43

2.1

Charge of insulating particles

43

2.1.1

Experimental set-up for measuring the charge of insulating particles in roll-type corona separators

46

2.1.2

Virtual instrument for single particle charge measurement

47

2.1.3

Discharge of insulating particles on the surface of the grounded roll electrode of a corona separator

54

2.2

Charge of insulating powders

59

2.2.1

Experimental setup for measuring the tribocharge of insulating powder

59

2.2.2

Virtual instrument for powder charge measurements

62

2.2.3

Variables that express the charge of insulating powders

64

2.3

Statistical control of tribocharging processes

67

2.3.1

Capability of a tribocharging device

69

2.3.2

Charts for the statistical control of a tribocharging process

72

2.3.3

Monitoring tribocharging processes - perspectives

76

 

Conclusions to Chapter 2

77

3

Surface Potential Measurement of Insulating Granular Layers

79

3.1

Surface potential of insulating granular layers

80

3.1.1

Experimental set-up for surface potential measurements of charged insulating granular layers

80

3.1.2

Virtual instrument for surface potential decay measurement

83

3.1.3

Surface potential decay of charged insulating granular layers

84

3.2

Charge decay characteristics of insulating granular layers at the surface of grounded electrodes

86

3.2.1

Discharge of insulating particles in contact with a grounded electrode

86

3.2.2

Charge decay as function of the nature of the granular material

89

3.2.3

Charge decay as function of ambient conditions

92

3.3

Method for evaluating the feasibility of corona-electrostatic separation of granular insulators

94

3.3.1

Experimental procedure

95

3.3.2

Surface potential decay of HDPE and PVC granular layers

96

3.3.3

Corona-electrostatic separation of PE and rubber

101

 

Conclusions to Chapter 3

103

4

High Voltage Measurement in Corona Electrostatic Separators

105

4.1

High-voltage energizing of corona electrodes

106

4.1.1

Experimental set-up for measuring the high voltage in corona separators

107

4.1.2

Spark discharge monitoring

108

4.1.3

High-voltage supply response to spark discharges

109

4.2

High-voltage sensing of the metal content in granular mixtures

112

4.2.1

Virtual instrument for high voltage measurements

114

4.2.2

Statistical analysis of high-voltage measurement data

115

4.3

Measurement of low metal contents in granular mixtures using a high-voltage sensor

123

4.3.1

High-voltage signal processing

124

4.3.2

Monitoring metal content in granular mixtures processed by electrostatic separation

127

 

Conclusions to Chapter 4

133

5

Modelling Metal / Insulator Separation Process

135

5.1

A linear-interaction model for electrostatic separation processes

136

5.1.1

Problem formulation using Taguchi methodology

137

5.1.2

Experimental implementation of Taguchi methodology

144

5.1.3

Interpreting the linear-interaction model

147

5.2

Robust control of electrostatic separation processes

150

5.2.1

Taguchi's approach to robust design

151

5.2.2

Experimental setup and results

154

5.2.3

Model for a robust electrostatic separation process

157

5.3

Effect of ambient humidity on the outcome of electrostatic separation processes

161

5.3.1

Formulation of the robust design problem

162

5.3.2

The classic analysis approach

166

5.3.3

The interaction analysis approach

168

 

Conclusions to Chapter 5

175

 

Conclusion and Contributions

177

 

References

181

 

Appendix A: Charge measurements: LabVIEW instrument implementation

197

 

Appendix B: Discharge graph identification: results for insulating particles on a grounded plate

203