IMC CANSAS - Bridge Measurements with Wire Strain Gauges (Wsgs)

182 Measurement Technique
imc CANSAS Users Manual - Doc. Version 1.9 - 05.12.2014© 2014 imc Meßsysteme GmbH
5.1.2 Bridge measurements with wire strain gauges (WSGs)
When connecting, observe the notes contained in the sections headed by "Block diagram" and "DC-
Bridge measurement (measurement target: Sensor)".
In the context of bridge amplifiers, strain analysis plays a major role. The configuration of a imc CANSAS
BRIDGE2 bridge module takes full account of this fact.
Strain in this sense refers to the ratio of a body's original length to the change in length due to a force
exerted upon it.
dL
L
By selecting "Strain gauge" as the measurement target on the virtual index card "Inputs", common bridge
circuits and configurations for wire strain gauges (WSG) are offered for selection. The scaling can be
adjusted in terms of typical parameters for strain measurements such as the gauge factor or Poisson's
ratio, the transversal expansion coefficient.
If a WSG adheres to a test object, the strain on the object is transmitted to the bridge circuit. The
changes in the lengths of the bridge arms causes their impedances to change. There is a correlation
between the changes in length and the changes in resistance:
dL
L
dR R
k
/
strain
strain
dL:
change in length
L:
original length
dR:
change in resistance
R:
resistance of strain gauge
k:
Gauge factor, describing the ratio of relative length change to change in
resistance
The changes in resistance caused by the strain are very small. For this reason, a bridge circuit is used to
translate these changes into voltage changes. Depending on the circuit, from one to four WSGs can be
employed as bridge resistors.
Assuming that all bridge resistors have the same value, we have
Ua : measurement voltage
Ue : excitation voltage
Ua
Ue k
*
*
4
For concrete measurement tasks, the arrangement of the WSGs on the test object is important, as well
as the circuitry of the bridge. On the card "Bridge circuit", you can select from among typical
arrangements. A graphic shows the position on the test object and the bridge circuitry. Notes on the
selected arrangement are displayed in the text box beneath.