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"Universidade Federal do Rio Grande do Sul (UFRGS) \n",
"Programa de Pós-Graduação em Engenharia Civil (PPGEC) \n",
"\n",
"# PEC00144: Experimental Methods in Civil Engineering\n",
"\n",
"\n",
"### Part II: Instrumentation\n",
"[12. Strain gages and load cells](#section_12) \n",
"\n",
" [12.1. Strain gage principles](#section_121) \n",
" [12.2. Wheatstone bridge configuration](#section_122) \n",
" [12.3. Strain gage installation](#section_123) \n",
" [12.4. Load cell design](#section_124) \n",
" [12.5. Bridge signal conditioning](#section_125) \n",
"\n",
"---\n",
"_Prof. Marcelo M. Rocha, Dr.techn._ [(ORCID)](https://orcid.org/0000-0001-5640-1020) \n",
"_Porto Alegre, RS, Brazil_ \n"
]
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"## 12. Strain gages and load cells \n",
"\n",
"### 12.1. Strain gage principles \n",
"\n",
"\n",
"\n",
"
\n",
" | \n",
" | \n",
"
\n",
"\n",
"The gage equation is:\n",
"\n",
"$$ \\frac{\\Delta R}{R} = K\\varepsilon $$\n",
"\n",
"where $R$ is the gage resistance (usually 120 or 350Ω), $K$ is the gage factor\n",
"(approximatelly 2.1 for comercial foil gages) and $\\varepsilon$ is the local strain.\n"
]
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"0.735\n",
"1.344000000000051\n"
]
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"R = 350.\n",
"K = 2.1\n",
"e = 1/1000.\n",
"\n",
"DR = R*K*e\n",
"\n",
"print(DR)\n",
"\n",
"VA = 5*(350 + DR)/(2*R)\n",
"VB = 5*(350 - DR)/(2*R)\n",
"\n",
"G = 128\n",
"print(G*(VA - VB))\n"
]
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"### 12.2. Wheatstone bridge configuration \n",
"\n",
"A Wheatstone bridge can be seen as a double voltage divider.\n",
"The resistors can be replaced by strain gages as shown below:\n",
"\n",
"\n",
" | \n",
" | \n",
"
\n",
"\n",
" \n",
"\n",
"For the full bridge with four strain gages the voltage difference, $\\Delta V = V_2 - V_1$, \n",
"between the central nodes are given by:\n",
"\n",
"$$ \\Delta V = \\frac{1}{4}\\left( \n",
" \\frac{\\Delta R_A}{R_A} + \\frac{\\Delta R_B}{R_B} +\n",
" \\frac{\\Delta R_C}{R_C} + \\frac{\\Delta R_D}{R_D}\n",
" \\right) V_{\\rm in} $$\n",
"\n",
"To maximize this difference, variations $\\Delta R_A$ and $\\Delta R_C$ must \n",
"opose the variations $\\Delta R_B$ and $\\Delta R_D$. In the best case, with all\n",
"gages subjected to the same strain and with oposition ensured, we get:\n",
"\n",
"$$ \\Delta V = K \\varepsilon V_{\\rm in} $$\n"
]
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"### 12.3. Strain gage installation\n",
"\n",
"[A nice video showing how to glue a strain gage in place!](https://www.youtube.com/watch?v=7DXERPk_164)\n"
]
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"### 12.4. Load cell design\n",
"\n",
"\n",
"\n"
]
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