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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Lead wire effects

easiest solution:

2 wire connection

Quarter bridge (only one strain gauge active)

V0

R2 R3

R4

VE


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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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Lead wire effects

RCable

RCable

Equivalent circuit diagram:



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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 ==

==

=

=

==



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Lead wire effects


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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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%.



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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?


Conclusion: A two wire connection is troublesomeand should be avoided!

Conclusion: A two wire connection is troublesomeand should be avoided!


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Lead wire effects

V0

R2 R3

R4

VEThree wire connection




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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



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Lead wire effects

V0

R2 R3

R4

VE


Rwire

Rwire

R2, R3 and R4 = passive resistors

R1


( )

=

+=

+=

+=

kR

R

4

k

V

V

RRR

RRR

4321

E

0

2wire)total(2

gaugestrainwire)total(1


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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

===

=

=

== ==


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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Lead wire effects

Quarter bridge(3 wire connection)


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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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Lead wire effects

Shunt calibration?

Change in resistance: GaugeStrainShuntGaugeStrain

ShuntGaugeStrainR

RR

RRR

+

=


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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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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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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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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



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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.



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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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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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004-lead-wire.pdf

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