High Precision Measurements

Transcription

“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
CAS2004
High Precision Measurements - Gunnar Fernqvist/CERN
1
High Precision Measurements
•
•
•
•
•
•
Precision
Precision power converters
Voltage transducers
Current transducers
Calibration infrastructure
Integration
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2
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
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3
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
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4
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
CAS2004
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
CAS2004
DCCT 2
6
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.
LF noise, 0.1-10 Hz
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
7
vs. actual performance …
Device performance
ppm of FS
Device
DCCT 120 A
Zero uncertainty (hyst etc.)
Settling after change
Repeatability
LF noise, 0.1-10 Hz
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.
LF noise, 0.1-10 Hz
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
CAS2004
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
8
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
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Specifications 2
Out
• Accuracy
– Offset
– Gain
– Linearity
• Temperature behaviour
–
–
–
–
–
CAS2004
In
Offset and gain change
Amplifiers
Resistors
Capacitors
Instability/Oscillations
10
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
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LEM Voltage Transducer
Accuracy range:
0.2 – 1 %
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13
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
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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
CAS2004
19
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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21
• Four terminals are
compulsory for low
value resistors
• Cooling can be by air,
oil, grease etc.
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• 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
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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
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
27
The Current Calibrator
CAS2004
28
DCCT testbeds
6 kA
CAS2004
20 kA
29
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
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30
1kΩ
10kΩ
1A
+
1Ω
1kΩ
A
Hi
10kΩ
Rg1
1mΩ
Hi sense
Output
signal
Lo sense
Lo
Analog common
Rg2
Power common
CAS2004
31
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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32
CAS2004
Time (h:mm:ss)
33
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
CAS2004
34
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
35
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
36
Future challenges
• Create a better burden resistor
• Create a better current-to-voltage
converter
• Create a truly digital DCCT
CAS2004
37

High Precision Measurements

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