With strain gauge sensors, a Wheatstone bridge may not bridge the gap

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A G

Inc.

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

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).

3)

Pre-presentation Self Assessment Activity

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.

1) The following op-amp configuration is a non inverting output configuration.

Yes No

VOut

+

-

2) The following op-amp configuration is an inverting output configuration.

VOut

+

-

Yes No

Often this voltage difference value must be amplified.

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.

V-

V+

R1

R2

R3

R4

(a)

(b)

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.

R4

= 0V Load cell

Often this voltage difference value must be amplified.

Amplifying Wheatstone bridge output

V-

V+

R4

(a)

(b)

Circuit block represents this voltage amplification process.

V = 0

Circuits can be built to amplify the resistance change when the weight monitored by the load cell (R4) changes.

R1

R2

R3

This output voltage signal is proportional to input voltage difference between input (a) and input (b).

V-

V+

R4

(a)

(b)

V = 0

Amplifying Wheatstone bridge output

R1

R2

R3

Circuit block represents this voltage amplification process.

R4

Wheatstone Bridge circuit block

V-

V+

This high magnitude output voltage signal is proportional to input voltage difference value.

Amplifier circuit block

V+

V-

(a)

(b)

Circuit block represents the Wheatstone bridge voltage difference detection process.

Interface circuit overview

Circuit block represents this voltage amplification process.

V

This low voltage difference signal is the input to the amplifier circuit block. It is proportional to the change in load cell resistance value.

Load cell

This high magnitude output voltage signal is proportional to input voltage difference value.

Amplifier circuit block

V+

V-

(a)

(b)

Circuit block represents the Wheatstone bridge voltage difference detection process.

Interface circuit overview

Circuit block represents this voltage amplification process.

V

This low voltage difference signal is the input to the amplifier circuit block. It is proportional to the change in load cell resistance value.

Load cellAn resistance based sensor goes here.

R4

Wheatstone Bridge circuit block

V-

V+

Interface circuit overview

Amplifier circuit block

V+

V-

(a)

(b)

V

Circuit block represents this voltage amplification process.

Circuit icon for Operational Amplifier

This low voltage difference signal is the input to the amplifier circuit block. It is proportional to the change in load cell resistance value.

This high magnitude output voltage signal is proportional to input voltage difference value.

Operational Amplifier Operational Rule

Circuit icon for Operational Amplifier

Output voltage is equal to zero only when both input signals are equal.

This voltage value is the voltage difference between point (a) and point (b).

Operational Amplifier Operational Rule

(a)

(b)

V = 0

Circuit icon that represents this voltage amplification process.

A non zero voltage value here indicates weight on the load cell has left its expected (steady state) value.

Operational Amplifier interface to load cell sensor Wheatstone bridge

(a)

(b)

V = 0

Voltage signal here keeps changing if the two inputs to the operational amplifier are not equal.

V-

V+

R4

R1

R2

R3

Load cell

V-

V+

R4

R1

R2

R3

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.

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.

Operational Amplifier interface to load cell sensor Wheatstone bridge

V-

V+

R4

R1

R2

R3

Load cell

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.

Operational Amplifier interface to load cell sensor Wheatstone bridge

There are several possible feedback circuit arrangements.

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-

VOutR4

R1

R2

R3

Load cell

V+

+Vou

t

Rf

Rf R4R4

This operation amplifier configuration selection is known as a non-inverting amplifier.

Operational Amplifier interface to load cell sensor Wheatstone bridge

+

-

If the voltage difference across the amplifier inputs increases (becomes more positive) the output signal increases (becomes more positive).

Strain gage sensor interface example

(see chapter 14 of class notes) There are several possible feedback circuit arrangements. This operation amplifier configuration selection is known as an inverting amplifier.

If the voltage difference across the amplifier inputs increases (becomes more positive) the output signal moves to its maximum negative value.

VOut

Operational Amplifier interface to load cell sensor Wheatstone bridge

+

-

Post Presentation Self Assessment Activity1) The following op-amp configuration is a

non inverting output configuration.

Yes No

X

Input voltage to be amplified is applied across these points.

VOut

Operational Amplifier interface to load cell sensor Wheatstone bridge

+

-

Post Presentation Self Assessment Activity2) The following op-amp configuration is

an inverting output configuration.

Yes No

X

Input voltage to be amplified is applied across these points.

Post Presentation Self Assessment Activity

V-

VOutR4

R1

R2

R3

Load cell

V+

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.

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).

Post Presentation Self Assessment Activity2)

V-

VOutR4

R1

R2

R3

Load cell

V+

Rf

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.

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.

End of Presentation

A G

Inc.