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• Bridge circuits!
Vs +
− VO 1
4 – One arm active, use Z1 or Z3 ⋅ – Two arms active (same sign), use Z1 and Z3
– Two arms active (pushpull), use Z1 and Z4 1
2 ⋅ ⋅ε – Four arms active ME365 Bridge Circuits 1 Bridge Circuits
• Calibration 1
4 ⋅ Rc – Balance the bridge while the switch is
opened.
– Use a calibration resistor RC to simulate a
change in resistance ΔRCAL in arm 1 due
to an equivalent strain εCAL. Δ ME365 Bridge Circuits 2 Bridge Circuits ME365 Bridge Circuits 3 Bridge Circuits
Ex: To calibrate a bridge circuit, what should the
calibration resistance be to simulate the output of
a single active arm bridge with a strain of 0.02
in/in? The gage factor is 2.2 and R0 = 100 Ω. Rc Ex: Suppose that R0 = 120 Ω and the gage factor is
2.1. What equivalent strain will be indicated if
the calibration resistance RC = 100 kΩ. ME365 Bridge Circuits 4 Bridge Circuits ME365 Bridge Circuits 5 Variable Impedance Devices
• Potentiometer (Displacement Sensor)
Without output loading RL , the
potentiometer output voltage VTH is: VS 1x
+
− RP x VTH RL VOUT 1
To measure VTH , a voltmeter with input
impedance RL is used. The measured
output voltage VOUT is: ME365 Bridge Circuits ZOUT
VTH ZL VOUT 6 Variable Impedance Devices
• Potentiometer (Displacement Sensor)
1 1 1 1x OUT VS +
− RP x VTH ME365 Bridge Circuits RL VOUT 7 Variable Impedance Devices
• Variable Capacitance Displacement Sensor • For a typical device:
5 mm
100 mm 5 (relative permittivity of material between plates)
8.854 10 F/m (permittivity of space) Q: How would one interface with this displacement sensor?
ME365 Bridge Circuits 8 Variable Impedance Devices
• Variable Capacitance Displacement Sensor
+ Place C1 and C2 at the 1st and 4th
arms of a bridge circuit, respectively.
Let the resistance in arms 2 and 3 be
the same: Vs − VOUT Q: What kind of supply voltage (VS)
should we use?
 Constant DC voltage, i.e. 6 V (DC)
 Oscillating voltage source?
ME365 Bridge Circuits 9 Variable Impedance Devices
The output voltage VO is: ME365 Bridge Circuits 10 Variable Impedance Devices
• Variable Capacitance Displacement Sensor
– DC Supply Voltage constant • Capacitance is infinite at DC
• No capacitance change will be sensed – AC Supply Voltage sin ME365 Bridge Circuits 11 Variable Impedance Devices
• Variable Capacitance Displacement Sensor
AC excited bridge circuit with variable inductance sensor:
1
⋅
⋅⋅
2
Let ME365 Bridge Circuits sin 12 Ex: 1
2 ⋅ ⋅ ⋅ Let K = 2
VS = 5 sin(2π10t) [V] Output Voltage (V) x = 4 + 2 cos(2π1t) [μm] Input Distance (um) Variable Impedance Devices
4
2
0
2
4
6
0 0.5 1
Time (sec) 1.5 2 0.5 1
Time (sec) 1.5 2 40
20
0
20
40
60
0 ME365 Bridge Circuits 13 Variable Impedance Devices
• Variable Inductance Displacement Sensor
1 1 For a typical device:
d = 0.25 in dx 2d d +x L0 = 25 mH α = 1538.4 m1
Q: How would one use this displacement
sensor? ME365 Bridge Circuits 14 Variable Impedance Devices
• Variable Inductance Displacement Sensor
1 1 +
Vs Place L1 and L2 at the 1st and 4th
arms of a bridge circuit, respectively.
Let the resistance in arms 2 and 3 be
the same: − VOUT Q: What kind of supply voltage (VS)
should we use?
 Constant DC voltage, i.e. 6 V (DC)
 Oscillating voltage source?
ME365 Bridge Circuits 15 Variable Impedance Devices
The output voltage VO is: ME365 Bridge Circuits 16 Variable Impedance Devices
• Variable Inductance Displacement Sensor
– DC Supply Voltage constant • Inductance is zero at DC
• No inductance change will be sensed sin – AC Supply Voltage ME365 Bridge Circuits 17 Variable Impedance Devices
• Variable Capacitance Displacement Sensor
AC excited bridge circuit with variable inductance sensor:
1
21
Let ME365 Bridge Circuits ⋅ ⋅ ⋅ sin 18 Ex: 1
2 ⋅ ⋅ Let K = 2
VS = 5 sin(2π10t) [V] Output Voltage (V) x = 4 + 2 cos(2π1t) [μm] ⋅ Input Distance (um) Variable Impedance Devices ME365 Bridge Circuits 4
2
0
2
4
6
0 0.5 1
Time (sec) 1.5 2 0.5 1
Time (sec) 1.5 2 40
20
0
20
40
60
0 19 ...
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This note was uploaded on 12/26/2011 for the course ME 365 taught by Professor Merkle during the Fall '07 term at Purdue UniversityWest Lafayette.
 Fall '07
 MERKLE

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