# Measure Strain

Learn:

A typical quarter bridge strain measurement solution comprises

- the EMANT380 Bluetooth DAQ (or EMANT300 USB DAQ). Power supply for the EMANT380 not shown.
- Bridge Sensor Application Adaptor
- Bridge Completion Network
- Mounted Strain Gauge

When an object is stretched due to an external force and the length of the object increases from L to L+ΔL, the ratio ΔL/L is called strain.

ε = ΔL/L

As the ratio of deformation is often very small, it is often represented in a units of 10^{-6} or μstrain

A strain gauge (gage) can be used to measure the strain of this object. The most common type of strain gauge (gage) consists of a flexible backing which supports a metallic foil pattern etched onto the backing. As the object is deformed, the foil pattern is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge circuit, can be used to calculate the exact amount of deformation by means of the quantity known as the gauge (gage) factor.

The gauge(gage) factor of a strain gauge (gage) relates strain to change in electrical resistance. The gauge (gage) factor G_{F} is defined by the formula

where R_{G} is the resistance of the undeformed gauge, ΔR is the change in resistance caused by strain, and ε is strain.

In our example, we will use one strain gauge (gage) with G_{F}=2, R_{G} = 120 ohms and connected in a quarter bridge configuration. The bridge is excited at V_{EXC}=3.3V.

The voltage output of the wheatstone bridge V_{O} (seen at the differential input AIN3, AIN2) is given by

If the bridge is balanced V_{O} =0 since all the resistances are equal. When a strain is applied, R_{G} becomes R + ΔR and substituting R to all the other resistances, the equation becomes

If we assume 2 ΔR << 4R

Finally substituting V_{EXC}=3.3V, G_{F}=2, we obtain (in ustrains)

ε = -ΔV_{O} * 1000000 / 1.66

## Strain.py

import emant
import time
m = emant.Emant300()
m.Open("00:06:66:00:A1:D8")
print m.HwId()
m.ConfigAnalog(0.1,emant.Emant300.Bipolar,10)
# bridge output is connected to AIN3, AIN2
volt, binval = m.ReadAnalog(emant.Emant300.AIN3,emant.Emant300.AIN2)
zero = volt
for i in range(1, 10):
volt, binval = m.ReadAnalog(emant.Emant300.AIN3,emant.Emant300.AIN2)
ustrain = -1000000 * (volt-zero) / 1.66
print '%5.0f ustrain' % ustrain
time.sleep(1)
m.Close()

**Python with EMANT380**

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