ABE425 Engineering Measurement Systems
Bridge methods ABE425 Engineering Measurement Systems Agenda Null methods (calibration and highly accurate measurements)
DC bridges (Resistance measurement) Wheatstone Capacitor and inductance (coil) models AC bridges (Inductance / capacitance measurement) Null-type Parallel-Resistance-Capacitance bridge for capacitance and dissipation factor measurement Maxwell bridge for inductance (coil) and quality factor measurement Wien bridge Approximate measurement of Inductance and Capacitance Deflection methods (control systems) Deflection-type Wheatstone bridge and non-linearity Null-type DC Wheatstone bridges are used for accurate resistance measurement
The bridge is balanced when the voltage Vd is adjusted to zero by tuning R1 while R2 and R3 are known and kept constant. The null-detector is usually some type of galvanometer The unknown resistance value can then be computed using the values of the other resistances Since there are no inductances (coils) or capacitances, a DC source is sufficient This type of bridge is used for strain gage measurements This is also a balance method
Roberval design Measurement procedure using a Galvanometer and a decade resistor box
Decade Box Unknown resistor Galvanometer Known, constant In balance: Galvanometers are VERY SENSITIVE instruments to detect zero current
D'Arsonval galvanometer Thompson mirror type galvanometer (1880)
Thompson mirror type galvanometer (1880). Note the antenna to compensate for electric/magnetic fields Null-type AC Wheatstone bridge for impedance measurement
The bridge is balanced when the voltage Vd is adjusted to zero by tuning Capacitor and Inductance (coil) models
Capacitor model with Capacitance and dissipation resistance Inductor (coil) model with Inductance and series resistance Inductances have a quality factor Null-type Parallel-Resistance-Capacitance bridge for capacitance and dissipation factor measurement Maxwell bridge to measure inductance, resistance and quality factor of low quality coils (Q10) Wien bridge for frequency measurement The coil characteristics inductance and series resistance can be measured by equalizing the voltage across a variable resistor and the coil itself Series resistance of the coil RL measured with a DVM Approximate method of measuring capacitance
Measure the AC Voltages for a known input frequency across resistor R and capacitor C Resistance measured with a DVM Deflection type DC Wheatstone bridge Output (deflection) for R2, R3 = 1,000 Ohm showing significant non-linearity
Bridge balance Output (deflection) for R2, R3 = 10,000 Ohm showing reduced non-linearity
Bridge balance Measuring the drag coefficient of a sphere using a compensation method
Electric current returns sphere to original position Air flow pushes sphere to the right Drag coefficient ~ Electric current Links Schneider, N Electrical instruments and testing. Spon and Chamberlain, New York. ABE425 Engineering Measurement Systems
The End ABE425 Engineering Measurement Systems