Dynamic Strain Input Amp (Ar-Gxst) - Teac GX-1 Instruction Manual

Amplifiers

Dynamic Strain Input Amp (AR-GXST)

The AR-GXST provides a excitation voltage to a sensor
that uses a strain gauge, and extracts a signal. Input is bal-
anced input, and the amp has an balance adjustment func-
tion that corrects unbalances in the strain gauge, and
enables zero balancing up to ±700% of the rated value.
The adjustment range for a ±200 µST range becomes
approximately ±1400 µST.
When a sensor and the GX-1 are separated by a long dis-
tance, the lowered excitation voltages can affect measure-
ments. The AR-GXST has a remote sense function for
controlling this voltage.
When Using a Strain Gauge Directly
The strain gauge usually uses a general bridge box, and is
used with an internal Wheatstone bridge (see the diagram
below).
R4
R3
R1 is the strain gauge and is usually 120 Ω to 1000 Ω.
(Merging of direct currents might lead to higher resis-
tance values.) Taking 120 Ω as an example, and using a
resistance of 120 Ω for R2, R3, and R4, the voltage
change ∆e by a strain is:
E
∆e = ε
K
4
Where E is the excitation voltage, K is the gauge factor,
and ε is the strain.
For the excitation voltage E, in the AR-GXST you can
add 2 V or 5 V.
The gauge factor K means the proportion of the rate of
change (∆R/R) of the resistance value against the rate of
change (strain ε) in the length direction. In the AR-
GXST, this is assumed to be 2.0. A gauge factor of 2 is
thus a change of 2 times the resistance value against the
change (strain) in the length direction.
The strain ε is the comparative ratio (∆1/1) of the
changed part ∆1 of the length against the original length
1.
4-22
+BV
R1
–Out
R2
–BV
+Out
When the excitation voltage is 2 V and the gauge factor is
2, then the voltage change ∆e by a strain becomes
∆e = ε
So because the strain quantity and output voltage are
numerically the same (making the situation easy to under-
stand), a excitation voltage of 2 V is often used. For
example, when ε = 1000 µST (µST: micro strain. A strain
of 10 –6 in the length of the original. The value of 1000
µST means a change in the length direction of 1000 x
10 –6 , which is 1/1000.), then there is a change of 1/1000 V
(that is, 1 mV).
However, K = 2 is an ideal value, and the actually used
gauge factor of a gauge is usually a little larger than 2.
You can use Change Unit in GX Navi to correct the dif-
ference in this gauge factor. For example, when the gauge
factor is 2.13, specify 2.13 uST = 2.00 uST.
When Using a Strain Gauge Type
Transducer
The situation is as described above when using a direct
strain gauge for measurements, but in the case of a trans-
ducer that used a strain gauge, the output values are out-
put not as strain quantities but rather as voltage signals.
(Usually mV/V is indicated.)
For example, when the excitation voltage is 2 V in a load
cell with a rated output of 2 mV/V (±1%) and a rated
capacity of 20 kN, a signal of 4 mV for the 20 kN can be
obtained. When the excitation voltage is 2 V as described
above, 1 mV corresponds to 1000 µST so 20 kN corre-
sponds to 4000 µST.
However, there is variation (in this case, ±1%) in the
rated output of individual transducers. You can use
Change Unit in GX Navi to correct this variation. For
example, when the rated output is 1.98 mV/V, set the
input range to 5000 µST, and use Change Unit to specify
3960 µST = 20 kN.
For Reference
A strain gauge is usually called a sensor, but a strain
gauge that is packaged with a bridge configuration is not
usually called a sensor; rather it is called a transducer.
Usually 4 (not just 1) gauges are used internally. (All 4
resistances of a bridge are used for the strain gauge.)