Strain Gauge Amplifier

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Connection and Wiring Guidelines

Load Cell

A load cell based on strain gauge sensors is basically a full wheatstone bridge which is specified by its voltage sensibility in [mV/V] of the applied excitation voltage and the load sensibility in [Applied Load/Rated Load].

The SGA-2 Amplifier can be connected either in 4- or 6 wire mode:

Sga 6wire full bridge.JPG Sga 4wire full bridge.JPG

The 6-wire mode compensate any voltage drops on the excitation voltage line between the amplifier and the sensor/load cell.

Formulas

G = 1  ;Amplifier Gain
S[mV/V] = 10 ; Sensitivity 
LR[Kg]  = 1  ; Rated Load
L[Kg]   = 0.5 ; Applied Load
Vex[V]  = 10  ; Excitation Voltage

Vout[mV] = G * Vex * S * L/LR

; Calculate the applied load from the output voltage

Load = Vout * LR / (G * Vex * S)

Single Strain Gauges

Strain Gauge sensors are available in different sizes, shapes and orientations. Depending on the application they can be mounted in combination or different orientations for having more sensibility or compensate temperature effects.

Common in all application cases is the fact that at the end a full Wheatstone bridge (Wikipedia) must be build up. The strain gauge itself can be either one, two or even all of the four resistors in the bridge. The completion of the bridge can be either be externally or internally of the strain gauge amplifier.

Quarter Bridge

If only one sensor is present, we speak of a Quarter Bridge as only a quarter of the bridge is active.

The SGA2 has a build in fixed half bridge (2 kOhm). The third resistor R4 of the bridge must be mounted manually and must have the same value as the nominal strain gauge resistance.

Sga 4wire quarter bridge.JPG

Formulas:

Quarter bridge formula.JPG

The blue part of the formula can be neglected as a first approximation.

Temperature Compensation

Strain Gauge sensors are very sensible on temperature changes. For compensating the change in resistance due the temperature change, a second strain gauge sensor can be used as completion resistor R4. This “dummy” sensor must be mounted as close as possible to the active sensor Rg but must not follow the deformation of the device under test. Even if two sensors are used, it is still a quarter bridge as only one sensor is active.


Half Bridge

In a half bridge configuration two strain gauge are active. By mounting one sensors in tension (Rg + deltaR) and one in compression (Rg - deltaR) the sensitivity of the bridge get doubled.

Formula

Half bridge formula.JPG

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