How to use Strain Gauges ?

Strain Guage Calculate Required Gain For Amplifier Circuit

Application

• Use a Strain Gauge to measure the torque of a rotating shaft

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• Wheatstone bridge circuit is required for this torque measurement
• Wheatstone bridge configuration
• Half-bridge  -2 strains sensors and two fixed resistors
• Full bridge - 4 strains sensors - max range and max accuracy

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• Wheatstone bridge supply voltage
• Negative and Positive - full range (5V to -5V or +3.3V to -3.3V)
• Positive and GND (5V to  GND or +3.3V to GND )

 

Let's take a commercial strain gauge from Vishay

Application - max measuring torque 95Nm of the cylindrical shaft

Perform a simulation Ansys to find max stain at 95Nm 

The software tool will need the following in order to perform simulation 

• Dimensions of shaft
• Material of shaft
• Environment temperature etc mechanical details

Max strain at 95Nm was found to be 340 micro stains

• Deformation 0/0408mm
• Following calculations can be done based on the above findings in the Ansys software tool

Let's calculate the required gain for above strain range

• The non-invritng amplifier can provide the gain of 2941, filtering will be required before gaining and after gaining
• Differential op-amp also can be used to gain 2941, has advantages compared to the non-inverting amplifier - can get a full range of measurement  
• A tuning circuit to set zero torque offset will be needed. otherwise, at zero torque the Wheatstone bridge will have some voltage difference which leads to less measuring range

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