VXI VT1422A - Manual (Page 94)

92 Programming the VT1422A & VT1529A/B for Remote Strain Measurement Chapter 3
Built-in Strain Conversion Equations
When using the VT1422A’s built-in strain conversion
(SENSe:FUNCtion:STRain:<bridge_type> <range>,(@<ch_list>)),
the following equations are used to convert voltage to strain.
Full Bridge Equation
(bridge_type=FBEN)
This equation is used by the VT1422A to convert bridge measurements to
Engineering Units of Strain for channels specified in the command
SENSe:FUNCtion:STRain:FBEN <range>,(@<ch_list>).
where: V
measured
= measured voltage value
V
excitation
= excitation voltage
V
unstrained
= unstrained voltage
gFactor = gage factor
Half Bridge Equation
(bridge_type=HBEN)
This equation is used by the VT1422A to convert bridge measurements to
Engineering Units of Strain for channels specified in the command
SENSe:FUNCtion:STRain:HBEN <range>,(@<ch_list>).
where: V
measured
= measured voltage value
V
excitation
= excitation voltage
V
unstrained
= unstrained voltage
gFactor = gage factor
Quarter Bridge Equation
(bridge_type=Q120, Q350
or USER)
This second-order equation is used by the VT1422A to convert bridge
measurements to Engineering Units of Strain for on-board strain SCP
channels only. Because VT1529A/Bs can expand the VT1422As strain
channel count to 512, use of this non-linear strain conversion equation
would require too much VT1422A memory. Instead, a quadratic
approximation of this equation is used. See below. For the following
equations, V
i
= bridge output while strained, V
u
= bridge output unstrained
and V
e
= excitation voltage at the bridge.
Quarter Bridge Equation for Strain SCPs only
Quarter Bridge Equation for VT1529A/B only
Strain V
measured
V
unstrained
–
( ) gFactor V
excitation
× ( ) ⁄
=
Strain 2 V
measured
V
unstrained
–
( ) gFactor V
excitation
Ч ( ) ⁄ Ч
=
Strain
4V
r
GF 1 2V
r
–
( )
------------------------------- = WhereV
r
V
i
V
u
–
V
e
----------------- =
Strain a
2
V
i
2
a
1
V
i
a
0
+ + =
Where a
2
8
GF V
e
2
×
---------------------, a
1
4 4V
u
V
e
–
( )
–
GF V
e
2
×
--------------------------------, a
0
4 2V
u
2 V
e
V
u
–
( )
GF V
e
2
Ч
------------------------------------ = = =