004-lead-wire.pdf
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Transcript of 004-lead-wire.pdf
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19.03.2008, Folie 2 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
First Step: Pay attention to environment
- Temperature.
- Humidity.- Mechanical.
- Electro-Magnetic.
All elements of the measurement chain must beshielded to withstand the environmental demands.
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19.03.2008, Folie 4 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
easiest solution:
2 wire connection
Quarter bridge (only one strain gauge active)
V0
R2 R3
R4
VE
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19.03.2008, Folie 5 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Attributes:
all changes in resistance will be measured
easy to realise and low cost solution
=
k
R
R
0
kR
R
0
=
Quarter bridge (2 wire connection)
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19.03.2008, Folie 6 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
RCable
RCable
Equivalent circuit diagram:
Quarter bridge (2 wire connection)
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19.03.2008, Folie 7 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Extra cable effect: shift of zero point
Resistance of the flexible wire: 0,432 /m,Adding 10m of this cable to the gauge, using a two wire connection
gives:
V/mV910730,174
03.2
U
U
4
k
V
V
m/m730,1701773.003.2
120
32.4
k
R
R
32.4Rm10m/432.0R
6
B
A
E
0 ==
==
=
=
==
Quarter bridge (2 wire connection)
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19.03.2008, Folie 8 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Quarter bridge (2 wire connection)
Extra cable effect: shift of zero point
Cutting of the cable when the resistance of thestrain gauges is inclusive the cable
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19.03.2008, Folie 10 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Gauge factor varies with cable length
Example: Resistance of the wire: 1gauge factor: 2,03
01.2121
12003.2
R
Rk
kRkRk
R
kR
Randk
R
R
1211120RRRR
newnewnewnew
new
new
newCablenew
=
=
===
=
=
=+=+=
The error is 1%.The error is 1%.
Quarter bridge (2 wire connection)
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19.03.2008, Folie 11 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Extra Cable: significant zero point shift (9mV/V)
Tare at the amplifier?
Temperature effect on the cable (16 times the valueof the strain gauge)
Control temperature around cables?
Influence of the cables on the gauge factor
Shunt calibration?
Quarter bridge (2 wire connection)
Conclusion: A two wire connection is troublesomeand should be avoided!
Conclusion: A two wire connection is troublesomeand should be avoided!
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19.03.2008, Folie 12 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
V0
R2 R3
R4
VEThree wire connection
Quarter bridge (3 wire connection)
Equivalent circuit diagram:
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19.03.2008, Folie 13 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Attributes:
One wire more than in two wire connection needed
Changes in resistances of the cables will becompensated by the Wheatstone bridge
Quarter bridge (3 wire connection)
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19.03.2008, Folie 14 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
V0
R2 R3
R4
VE
Quarter bridge (3 wire connection)
Rwire
Rwire
R2, R3 and R4 = passive resistors
R1
Equivalent circuit diagram:
( )
=
+=
+=
+=
kR
R
4
k
V
V
RRR
RRR
4321
E
0
2wire)total(2
gaugestrainwire)total(1
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19.03.2008, Folie 15 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Extra Cable effect: Zero point shift
Example:
Resistance of the cable: 0,216 /m for each wire, 10m length
( ) ( ) 0m/m8865m/m88654
03.2
U
U
4
k
V
V
m/m8865008865.003.2
120
16.2
k
R
R
16,2Rm10m/216,0R
16,2Rm10m/216,0R
B
A21
E
0
)total(2
)total(1
===
=
=
== ==
Quarter bridge (3 wire connection)
Resistance change due cable length have the same effect on R1 and
R2. These balance each other out.
Zero. No effect
using 3 wire
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19.03.2008, Folie 18 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Quarter bridge(3 wire connection)
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19.03.2008, Folie 19 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Extra cable, no zero point shift
No effect due to the temperature variation
on the connection cables
Gauge factor varies with cable length
Shunt calibration
Quarter bridge(3 wire connection)
?
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19.03.2008, Folie 20 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Shunt calibration?
Change in resistance: GaugeStrainShuntGaugeStrain
ShuntGaugeStrainR
RR
RRR
+
=
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19.03.2008, Folie 21 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
m/m5.668186600120
86600
07,2
11
RR
R
k
1
k4
1
R
R
4
1
V
V
RRR
RRR
ShuntGaugeStrain
Shunt
GaugeStrainE
0
GaugeStrain
ShuntGaugeStrain
ShuntGaugeStrain
=
+
=
+=
=
=
+
=
Consider: 120-strain gauge with 86.6k shunt resistor
Shunt calibration?
Shunt value
=
new
new
kR
RThe shunt value (strain) can be used to calculate a
correct gauge factor, k for the cables being used.
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19.03.2008, Folie 22 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
The problem of 2 and 3 wire connection:Potential drop along the wires, caused by cable resistances.
Voltage drop
Cable resistance are bridge balanced using a 3 wire connection,
but this fails to overcome gauge factor error.
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19.03.2008, Folie 23 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
The strain, obtained at the amplifier is lower than the physical strain!
Voltage drop
Solution: - Remove cable effects completely.
- Maintain a constant voltage at the strain gauge.
Essentially, a small part of the total
resistance does not vary with strain,
namely the cable resistance.
You need to apply a slightly different
gauge factor with different cableslengths to obtain the correct strain.
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19.03.2008, Folie 24 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
DMS
sense lead 2
RK
RK
RInt.
RH
RH
RS
RS
measuring lead 1
excitation voltage lead 2
excitation voltage lead 4
With HBM-Amplifier this will be done automatically: 4-wire connection
Quarter bridge (4 wire connection)
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19.03.2008, Folie 25 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Solution:
No zero point shift
No effect due to temperature variation on theconnection cables.
Gauge factor does not vary with cable length.
Cable length of approx. 500m are possible to
quarter bridge strain gauges.
Quarter bridge (4 wire connection)
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19.03.2008, Folie 26 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Lead wire effects
1 active strain gauge in 2 wire connection
1 active strain gauge in 4 wire connection
Example
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19.03.2008, Folie 28 Hottinger Baldwin Messtechnik GmbH Dirk Eberlein
Lead wire effects
Strain gauge 1
Strain gauge 2
VEStrain gauge 4
Strain gauge 3
RCable
RCable
V0
Full bridge circuit
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