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With strain gauge sensors, a Wheatstone bridge may not bridge the gap This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide. 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.