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With strain gauge sensors, a
Wheatstone bridge may not
bridge the gap
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AInc.
Pre-presentation Self Assessment Activity
Your comfort level with your responses to
the following four design challenges should
suggest if the presentation that follows will
increase you knowledge base on the topic
embedded within these questions.
Pre-presentation Self Assessment Activity
1)
2)
The following op-amp
configuration is a non inverting
output configuration.
VOut
The following op-amp
configuration is an inverting
VOut
output configuration.
Yes
No
Yes
No
+
-
+
-
3)
Design a Wheatstone bridge load cell sensor interface
circuit that will permits the interface of an amplified
voltage output that indicates the sensor’s reaction to a
change in load (weight on the sensing element).
4)
Design a load cell sensor interface circuit that will
provided an amplified voltage output that is proportional
to the sensor’s response to a weight change.
Review Wheatstone bridge circuit
This resistance change must become an amplified voltage signal
if the resistance change is to be used in an instrumentation
system to control the amount of mass on the load cell.
(a)
Often this voltage
difference value
must be amplified.
R1
R3
V
+
V
V=0
R4 changes in resistance value
as the weight changes and
the voltage between points
(a) and (b) in the
Wheatstone bridge circuit
also changes proportionally.
(b)
Load cell
R
4
R2
Amplifying Wheatstone bridge output
Circuits can be built to amplify the resistance change when the
weight monitored by the load cell (R4) changes.
(a)
Often this voltage
difference value
must be amplified.
V
R3
R1
+
V
V=0
R4
R2
Circuit block
represents this voltage
amplification process.
(b)
Amplifying Wheatstone bridge output
This output voltage
signal is proportional to
input voltage difference
between input (a) and
input (b).
(a)
V
R3
R1
+
V
V=0
R4
(b)
Circuit block
represents this voltage
amplification process.
R2
Interface circuit overview
This high magnitude
This low voltage difference signal is
output voltage signal the input to the amplifier circuit
is proportional to
block. It is proportional to the
+change in load cell resistance value.
input voltage
V
difference value.
Circuit block represents
the Wheatstone bridge
voltage difference
detection process.
(a)
Amplifier
circuit block
V
V
R4
Wheatstone
Bridge circuit
block
(b)
Load cell
V
Circuit block represents this
voltage amplification process.
+
V
Interface circuit overview
This high magnitude
This low voltage difference signal is
output voltage signal the input to the amplifier circuit
is proportional to
block. It is proportional to the
+change in load cell resistance value.
input voltage
V
difference value.
Circuit block represents
the Wheatstone bridge
voltage difference
detection process.
(a)
Amplifier
circuit block
V
V
R4
(b)
V
Wheatstone
Bridge circuit
block
An resistance based
Load cell
sensor goes here.
Circuit block represents this
voltage amplification process.
+
V
Interface circuit overview
This high magnitude
This low voltage difference signal is
output voltage signal the input to the amplifier circuit
is proportional to
block. It is proportional to the
+change in load cell resistance value.
input voltage
V
difference value.
(a)
Amplifier
circuit block
V
(b)
V
Circuit icon for
Operational Amplifier
Circuit block represents this
voltage amplification process.
Operational Amplifier Operational Rule
Output voltage is equal
to zero only when both
input signals are equal.
Circuit icon for
Operational Amplifier
Operational Amplifier Operational Rule
(a)
This voltage value is
the voltage difference
between point (a) and
point (b).
V=0
Circuit icon that
represents this voltage
amplification process.
(b)
Operational Amplifier interface to load cell sensor Wheatstone bridge
A non zero voltage
value here indicates
weight on the load
cell has left its
expected (steady
state) value.
(a)
V
R3
R1
+
V
V=0
R4
Voltage signal here
keeps changing if the
two inputs to the
operational amplifier
are not equal.
R2
Load cell
(b)
Operational Amplifier interface to load cell sensor Wheatstone bridge
The Wheatstone bridge is “balanced” (output of operational amplifier is
zero volts) when the weight on the load cell is at its steady state value.
V
R3
R1
+
V
R4
R2
Load cell
If the weight of the load moves away from its steady state value, the output
voltage will rapidly move to one of its limit saturation voltage values.
Operational Amplifier interface to load cell sensor Wheatstone bridge
A feedback circuit from the output terminal of the amplifier to one of the
input terminals of the amplifier is used to stabilize the output signal at a
value that is between zero volts and a maximum (a saturation) value.
V
R3
R1
+
V
R4
R2
Load cell
Operational Amplifier interface to load cell sensor Wheatstone bridge
A feedback circuit from the output terminal of the amplifier to one of the
input terminals of the amplifier is used to stabilize the output signal at a
value that is between zero volts and a maximum (a saturation) value.
Rf
R
V
+
V
Out
3
R
1
+
V
R
4
-
R
Load cell
V out
a
Rf +
R4
R4
2
If the voltage difference
across the amplifier inputs
increases (becomes more
positive) the output signal
increases (becomes more
positive).
There are several possible feedback circuit arrangements.
This operation amplifier configuration selection is known
as a non-inverting amplifier.
Strain gage sensor interface example
If the voltage difference
across the amplifier inputs
increases (becomes more
positive) the output signal
moves to its maximum
negative value.
(see chapter 14 of class notes)
There are several possible feedback circuit arrangements.
This operation amplifier configuration selection is known
as an inverting amplifier.
Operational Amplifier interface to load cell sensor Wheatstone bridge
Post Presentation Self Assessment Activity
1)
The following op-amp configuration is a non
inverting output configuration.
+
V
Out
-
Input voltage to be
amplified is applied
across these points.
Yes
X
No
Operational Amplifier interface to load cell sensor Wheatstone bridge
Post Presentation Self Assessment Activity
2)
The following op-amp configuration is an
inverting output configuration.
+
V
Out
-
Input voltage to be
amplified is applied
across these points.
Yes
X
No
Post Presentation Self Assessment Activity
3)
Design a Wheatstone bridge load cell sensor interface
circuit that will permits the interface of an amplified
voltage output that indicates the sensor’s reaction to a
change in load (weight on the sensing element).
R
V
V
Out
3
R
1
+
V
R
4
R
2
Load cell
Note:
This comparator configuration for the operational amplifier will
drive the output signal to one of the two possible saturation
values when the weight on load cell leaves its steady state
value.
Post Presentation Self Assessment Activity
2)
Design a load cell sensor interface circuit that will
provided an amplified voltage output that is proportional
to the sensor’s response to a weight change.
Rf
R
V
V
Out
3
R
1
+
V
R
4
R
2
Load cell
Note:
This addition of a feedback resistor, Rf, will stabilize the output
signal from the operational amplifier and make Vout proportional
to the resistance change detected by strain gauge in the laod
cell.
End of Presentation
AInc.