How to form strain gage bridges Gage method Connection system 1 1-active-gage 2-wire Nr. of gage: 1 2 Application Monoaxial stress (uniform tension or compression) Circuit Output Gage factor Strain Bridge voltage Output voltage Gage resistor Fixed resistor No temp. compensation; x1 output; nonlinearity correction needed for large strain No temp. compensation; thermal effect on leadwires cancelled; x1 output; nonlinearity correction needed for large strain 1-active-gage 3-wire Nr. of gage: 1 Remarks Monoaxial stress (uniform tension or compression) Bending 3 1-active-gage (2 in series) 2-wire (cancelling bending strain) Nr. of gages: 2 Strain Strain Monoaxial stress (uniform tension or compression) No temperature compensation; bending strain cancelled; x1 output Fixed resistor Bending 4 1-active-gage (2 in series) 3-wire (cancelling bending strain) Nr. of gages: 2 Strain Strain Monoaxial stress (uniform tension or compression) Fixed resistor No temperature compensation; thermal effect on leadwires cancelled; bending strain cancelled; x1 output Active gage Active-dummy (2-gage method) 5 Monoaxial stress (uniform tension or compression) Nr. of gages: 2 6 Poison’s ratio 2-active-gage (at right angles) Nr. of gages: 2 7 Dummy gage Gage factor Strain Bridge voltage Output voltage Strain Fixed resistor Strain Monoaxial stress (uniform tension or compression) 2-active-gage (cancelling bending strain) Gage resistor Strain Strain Fixed resistor Strain Strain Fixed resistor Nr. of gages: 2 Temperature compensation; thermal effect on leadwires cancelled; x1 outpput Temperature compensation; thermal effect on leadwires cancelled; x (1+v) output Temp. compensation; thermal effect on leadwires cancelled; comp.(tension) strain cancelled; x2 output Bending stress 8 Opposite 2-active-gage 2-wire Nr. of gages: 2 Monoaxial stress (uniform tension or compression) Strain Strain Fixed resistor No temp. compensation; bending strain cancelled by gages cemented on front and back; nonlinearity compensation needed for large strain; x2 output Bridge boxes DB-120P, 350P Gage method Connection system 9 10 Opposite 2-activegage Opposite 2-active3-wire gage 2-wire Nr. of gages: 2 Nr. of gage: 2 Application Output Circuit Strain Strain Fixed resistor Monoaxial stress (uniform tension or compression) 4-active-gage (bending strain measuring) Bending strain Nr. of gages: 4 Bending strain Bending stress 11 4-active-gage (gages at right angles) Poison’s ration Strain Nr. of gages: 4 Monoaxial stress (uniform tension or compression) Active-dummy Active-dummy (4-gage method) (4-gage method) Monoaxial stress (uniform tension or compression) Nr. of gages: 4 Nr. of gages: 4 13 Strain Dummy gage Strain 2-active-dummy 2-Active-dummy (twisting (twisting strain strain measuring) measuring) Twisting strain Twisting strain Nr.of ofgagese: gages: 22 Nr. 14 No temp. compensation; thermal effect on leadwires cancelled; bending strain cancelled by gages cemented on front and back; x2 output; correction of nonlinearity needed Temp. compensation; thermal effect on leadwires cancelled; compression (tension) strain cancelled; x4 output Temperature compensation; thermal effect on leadwires cancelled; x2(1+v) output Temperature compensation; thermal effect on leadwires cancelled; bending strain cancelled by gages cemented on front and back; x2 output Temperature compensation; thermal effect on leadwires cancelled; x2 output Fixed resistor 4-active-gage (twisting strain measuring) Twisting strain Nr. of gages: 4 15 Bridge boxes DB-120P, 350P Strain Active gage 12 Remarks Twisting strain 4-active-gage (mean strain measuring) Rg2 Rg4 Rg3 Rg2 Rg1 Nr. of gages: 4 Rg3 ●Relation between strain and voltage The output of a strain gage bridge is expressed by a strain quantity (x10–6 strain) or by an output voltage (mV/V or µV/V) against a bridge voltage. The relation between the two is expressed by the formula below. e0 = E Ks · 4 Fixed resistor R =Rg1=Rg2=Rg3=Rg4 Temperature compensation; thermal effect on leadwires cancelled; bending strain cancelled; x4 output No Temp. compensation; mean stress measuring; x1 output; correction of nonlinearity needed for large strain Rg1 Rg2 Rg3 Rg4 Now, suppose that Bridge voltage (E) = 1V and Gage factor (Ks) = 2.00 The following then holds. 2e0 = ε0 This means that strain output is always 2 times larger than a bridge’s output voltage. Example: 3000 x 10–6 strain ➞ 1500µV/V = 1.5mV/V