Transcript
“When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science.” William Thomson, Lord Kelvin, 1824-1907
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High Precision Measurements - Gunnar Fernqvist/CERN
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High Precision Measurements • • • • • •
Precision Precision power converters Voltage transducers Current transducers Calibration infrastructure Integration
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Precision • Precision is a qualitative term • Accuracy and Uncertainty are quantitative terms • Device imperfections, measurement errors and measurement uncertainty • ISO GUM defines terms and methods to express uncertainty in a standardised way CAS2004
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Precision Power Converters • User specifications – – – – – –
Voltage output or current output ? Pulsed or DC ? Type of load Performance Reliability etc
• System (=converter) design specifications – Configuration – Power topology
• Component specifications CAS2004
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Analogue converter control To magnet load D/A converter
Regulation electronics
Power part
DCCT 1 Control
DCCT electronics
interface
Output amplifier DCCT electronics
A/D converter Output amplifier
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High Precision Measurements - Gunnar Fernqvist/CERN
DCCT 2
5
LHC converter control To magnet load
CONTROL ELECTRONICS Digital regulation loop
D/A converter
Power part
Fieldbus DCCT 1
Control interface
Σ−∆ filter
Σ-∆ modulator
DCCT electronics
cal wdg
DCCT electronics
cal wdg
Output amplifier
Σ−∆ filter
Σ−∆ modulator Output amplifier
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High Precision Measurements - Gunnar Fernqvist/CERN
DCCT 2
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Accuracy budget Device spec ppm of value
Stability 1/2 hr
20 3 50 10 0 0 0 0 10 40 3
0 0 0 0 10 10 100 5 0 0 0
0 0 0 10 10 0 0 0 0 0 0
0 3 0 10 0 10 0 25 10 0 15
20 3 50 10 0 0 100 50 0 40 30
20
73
303
0 10 0.4 0 0 0 0 0 0
0 10 0 0.5 0 10 0.2 0 3
45 10 0 0 100 20 0 50 6
10.4
23.7
231
5
10
100
35.4 50
106.7 100
634 1000
0
5
10
Device DCCT 120 A Zero uncertainty (hyst etc.) Repeatability Uncomp non-linearity LF noise, 0.1-10 Hz Stability 1/2 hr, 1-100 mHz Gain drift 24 hr Gain drift 1 year Gain Temp Coeff Offset drift 24 hr Offset drift 1 year Offset Temp Coeff DCCT total A/D converter, 16 bit succ. approx. Uncomp non-linearity LF noise, 0.1-10 Hz Stability 1/2 hr, 1-100 mHz Gain drift 24 hr Gain drift 1 year Gain Temp Coeff Offset drift 24 hr Offset drift 1 year Offset Temp Coeff A/D total Miscellaneous Total LHC committment Conditions Temp change (K) No special temp ctrl
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LHC machine impact Reproducibility Accuracy 1-day 1 year
ppm of FS
45 10 0 0 0 0 0.2 50 0.6
0 0 0.4 0.5 100 2 0 0 0
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vs. actual performance … Device performance ppm of FS
Device DCCT 120 A Zero uncertainty (hyst etc.) Settling after change Repeatability Uncomp non-linearity LF noise, 0.1-10 Hz Stability 1/2 hr, 1-100 mHz Gain drift 24 hr Gain drift 1 year Gain Temp Coeff Offset drift 24 hr Offset drift 1 year Offset Temp Coeff
ppm of value
Spec
Real
50
3
3 50 0 10
Spec
Real
0
30
3 50 3 15 10 100 5
10 40 3
10 100 10
10 40 2
DCCT total A/D converter, 16 bit succ. approx. Uncomp non-linearity LF noise, 0.1-10 Hz Stability 1/2 hr, 1-100 mHz Gain drift 24 hr Gain drift 1 year Gain Temp Coeff Offset drift 24 hr Offset drift 1 year Offset Temp Coeff A/D total Miscellaneous Total LHC committment Conditions Temp change (K) No special temp ctrl
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60 60
240 60 30 100 3
10 50 0.6
10 50 1
30 100 3
1/2 hr Stability Spec Real
LHC machine impact Reproducibility 1 day Accuracy 1 year Spec Real Spec Real
0
0
0
0
50
3
0 0 0 10 0 0 0 0 0 0
0 0 3 15 0 0 0 0 0 0
3 0 0 10 10 0 25 10 0 15
3 0 3 15 10 0 50 10 0 10
3 50 0 10 0 100 50 0 40 30
3 50 3 15 0 100 100 0 40 20
10
18
73
101
333
334
0 60 0 0 0 0 0 0 0
0 60 0 0 0 0 0 0 0
0 60 0 30 0 15 10 0 3
0 60 0 30 0 15 10 0 5
60 60 0 0 100 30 0 50 6
240 60 0 0 100 30 0 50 10
60
60
118
120
306
490
5
5
10
10
100
100
75 50
83 50
201 100
231 100
739 1000
924 1000
0
0
5
5
10
10
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Specifications 1 • Stability – Noise – – – – – – – – –
Ground noise - Common mode rejection Power supply noise - rejection Interference, conducted or radiated (Charroy) 50 Hz pickup Modulation residues Amplifier noise Reference noise Humidity influence – Leakage paths Contact resistance and emf’s
• Resolution CAS2004
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Specifications 2 Out
• Accuracy – Offset – Gain – Linearity
• Temperature behaviour – – – – – CAS2004
In
Offset and gain change Amplifiers Resistors Capacitors Instability/Oscillations High Precision Measurements - Gunnar Fernqvist/CERN
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Specifications 3 • Settling behaviour – Bandwidth related – Thermally related
• Repeatability and reproducibility • Long term drift – Material ageing or stress modification – Resistors, amplifiers – Humidity
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Voltage transducers • Problems you may face: – Isolation – High voltage – High frequency performance
• Solutions: – – – –
Isolation amplifiers High voltage dividers Precision resistors easily available Compensation for stray capacitance
• Relatively easy to verify performance CAS2004
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LEM Voltage Transducer
Accuracy range: 0.2 – 1 %
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Current Transducers, Principles • Current measuring resistors – – – –
Current range: 0 - 20 kA Accuracy range: 10-2 - 10-6 No isolation DC up to MHz with low inductance design
– – – –
Accuracy: 10-2 to 10-3 for 1-50 kA Needs magnetising energy Limited bandwidth, no DC Good isolation, kV easy
• AC passive current transformers
• Optical fibres
– Accuracy: 10-2 to 10-3 – Excellent isolation
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Magnetic Flux Principle • Measure field around conductor – Hall probe – open loop system • Flux compensation around conductor, sense zero flux – Hall effect sensor • 10-3 accuracy
– magnetic modulation • • • •
Second harmonic detector Peak current sensing Separate DC and AC loops 10-6 accuracy achievable in current ratio
– Burden resistor/output amplifier CAS2004
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LEM Current Transducer 1
Accuracy range: 1–2%
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LEM Current Transducer 2
Accuracy range: 0.2 – 1 % Linearity error: < 0.1 %
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DCCT Principle I comp Ip
Current output Oscillator Power amplifier
Burden resistor Zero-flux detector Optional output amplifier
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DCCTs on the Market
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Zero-flux transducer performance • Current ratio accuracy – 0.1 – 10 ppm
• Current/voltage conversion accuracy – 1 – 1000 ppm
• Accuracy vs. frequency – Loop gain important – Difficult to measure
• Noise and sources of noise • Hysteresis CAS2004
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Current measuring resistors 1 • Resistance is defined as R=U/I • It is a material property, not a constant • It changes with temperature, humidity, pressure, mechanical stress • Cu, Al, Ag, Au etc. ~ 4000 ppm/K • Good materials are NiCr, Manganin, Zeranin, Evanohm – 1-100 ppm/K • Packaging is crucial to performance
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Current measuring resistors 2 • Four terminals are compulsory for low value resistors • Cooling can be by air, oil, grease etc.
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Current measuring resistors 3 • The output voltage is a trade-off between noise/thermal emf’s and power dissipation • Temperature coefficient measured at low power • Power coefficient measured at one temperature • Hysteresis CAS2004
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Calibration infrastructure 1 Standards • Standards – – – –
Voltage, 10 V zener based Resistance, 1 Ω - 10 kΩ Current, 10 mA Accuracy 10-6
• Reference DCCTs
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Calibration infrastructure 2 • Current calibrator – Principle: inverted DCCT, multiplies current up to max 10 A – Calibrates DCCTs with special winding – Calibrates burden/output amp directly – Fully computer controlled
• DCCT testbeds – Calibrates DCCTs by providing the full primary current with a known value CAS2004
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The current calibrator principle 10.00000 mA 10 mA current source
Ext calib.
1
1
2
4
1024
2048
0-10 mA 16 bit DAC
1
Toroidal core
Zero-flux detector Power amplifier CAS2004
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0-10 A output
Range switching 26
The transfer scheme from the Standards lab Primary bank
O F M E T
On-site standard
Current standard
Current standard
Current standard
10mA
10mA
10mA
Automated voltage divider
mV
mV
10V
10mA
10mA
Volt standard
Current standard
CERN standards lab CAS2004
0-10A
Current calibrator
Standard resistor
B E R N
10mA
Portable standard
Current standard
LHC control point
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The Current Calibrator
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DCCT testbeds 6 kA
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Integration and other problems • • • • • • • • •
Grounding – Distance DCCT to electronics Common mode voltages Power supply noise – rejection “Negligible” resistance 4 wire configuration - not always a solution Avoid resistive loading – use buffer amps Insufficient amplifier gain Instrumentation amplifiers Amplifier stability – Decoupling – Power amplifiers – Cascade amplifiers
• Load problem – dR/dt => dI/dt @ V= const • External field sensitivity CAS2004
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1kΩ
10kΩ
1A +
1Ω 1kΩ
A
Hi
10kΩ Rg1
1mΩ
Hi sense
Output signal Lo sense Lo
Analog common
Rg2 Power common CAS2004
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EMC problems in high precision • Symptoms
– Non-linearity – Unusual and unstable offset
• Tests – – – –
Use oscilloscope frequently – your best friend RF exposure Burst generator Diagnose coupling mechanism
• Remedies
– Grounding and Shielding – Filters – Consultants
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CAS2004 High Precision Measurements - Gunnar Fernqvist/CERN Time (h:mm:ss)
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1:22:49
1:20:56
1:19:03
1:17:10
1:15:17
1:13:24
1:11:31
1:09:38
1:07:45
1:05:52
1:04:00
1:02:07
1:00:14
0:58:21
0:56:28
0:54:35
0:52:42
0:50:49
0:48:56
0:47:03
0:45:10
0:43:17
0:41:24
0:39:31
0:37:39
0:35:46
0:33:53
0:32:00
0:30:07
0:28:14
0:26:21
0:24:28
0:22:35
0:20:42
0:18:49
0:16:56
0:15:03
0:13:10
0:11:18
0:09:25
0:07:32
0:05:39
0:03:46
0:01:53
0:00:00
Output (ppm)
Offset drift after power-up
10.0
5.0
0.0
-5.0
-10.0
-15.0
-20.0
-25.0
Stability test of a DCCT
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Conclusions • Discourage exaggerated accuracy requests – direct and hidden costs • Build conservative, with good margins • Watch out for specmanship and quality control in industrial products • Test in the lab, not in the machine • Switch mode converters increase EMC problems at least an order of magnitude • Presumption is the mother of all screwups CAS2004
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References • ISO, Guide to the expression of uncertainty in measurements (GUM), 1995 • Ott, Noise reduction techniques in electronic systems, 2nd ed. 1988 • Horowitz, Hill, The art of electronics, 2nd ed., 1989 • Bendat, Piersol, Random data analysis and measurement procedures, 3rd ed. 2000 • Ramirez, The FFT-fundamentals and concepts, 1985 • Fernqvist et al, A novel current calibration system up to 20 kA, IEEE Trans. Instrum. Meas., vol. 52, Apr. 2003 • Moore, Miljanic, The current comparator, 1988 • Appelo et al., The zero flux DC current transformer – A high-precision bipolar wide-band measuring device, IEEE Trans. Nucl. Sci., Vol NS-24, No 3, June 1977 CAS2004
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Future challenges • Create a better burden resistor • Create a better current-to-voltage converter • Create a truly digital DCCT
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