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Resistive Strain Gage Bridge Pressure Sensor Applications

Resistive Strain Gage Bridge Pressure Sensor Applications

(Summary description)Resistive strain gauge bridge pressure sensors, the use of more common, circuit is relatively simple, basically by the op amp circuit, filter circuit, following the circuit composition, can meet the ordinary sampling use; can be directly connected to the MCU with AD function for sampling, or connected to the AD conversion chip, and then access to the MCU, etc.; of course, the actual use of the process of voltage, gain, range, etc., according to the relevant sensor manual, chip manual, etc. can be adjusted.

Resistive Strain Gage Bridge Pressure Sensor Applications

(Summary description)Resistive strain gauge bridge pressure sensors, the use of more common, circuit is relatively simple, basically by the op amp circuit, filter circuit, following the circuit composition, can meet the ordinary sampling use; can be directly connected to the MCU with AD function for sampling, or connected to the AD conversion chip, and then access to the MCU, etc.; of course, the actual use of the process of voltage, gain, range, etc., according to the relevant sensor manual, chip manual, etc. can be adjusted.

Information

1. Principle of operation of resistive strain gauge bridge pressure sensors

电阻应变片

Resistive strain gauge pressure sensors are sensors that combine a measurement circuit consisting of resistive strain gauges and an elastic sensitive element. When the elastic sensitive element is subjected to external pressure, a strain is generated and the resistive strain gauges affixed to the surface also generate a strain and the resistance value changes. The deformation of the elastomer is thus translated into a change in the resistance value of the resistance strain gauges.
 
Generally speaking, the four strain gauges are connected in a bridge circuit, with a certain voltage applied to the two inputs, and the common mode voltage at the two outputs increases or decreases with the change in resistance on the bridge circuit. The correspondence of this variation has an approximately linear relationship. By finding the correspondence between the change in pressure and the change in output common-mode voltage, the pressure value can be obtained by measuring the common-mode voltage.
 
The bridge is usually measured using a four-armed, full-bridge method, which has a high sensitivity to the strain gauge displacement transducer and a high accuracy, and can play a role in automatic temperature compensation.
 
 
 
2. Resistive strain gauge bridge pressure transducer circuit design
 
The circuit is generally composed of an amplifier circuit, a filter circuit, a voltage follower and U0 as an analogue output.
 
(1) Amplification circuit: using differential amplification circuit;
 
Differential amplification circuit, more commonly used circuit, can use discrete op-amps to build differential op-amp circuit; however, the differential op-amp circuit built by discrete op-amps, more peripheral devices, and the device itself has differences, the impact of external interference signals, etc., may cause some noise and other errors; for example, R1, R2 resistance value is ideally the same resistance value, but in reality, due to the production process and other reasons However, in reality, due to the production process and other reasons, it is impossible to have the same resistance value of the resistor, and there will be more or less deviation, so the result of the op-amp will not be ideal.
 
Therefore, in order to reduce the number of peripheral devices and unnecessary errors, it is recommended to use instrumentation amplifiers, which have integrated differential circuitry inside, in order to reduce the number of peripheral devices, the circuit structure is simple and the interference is also reduced.
 
Commonly used instrumentation op amps are AD620, INA128, etc.; the following diagram shows the internal structure of INA128:
 
RG is used to condition the gain, according to the sensor and the actual circuit to condition the gain;
 
When using the instrumentation op-amp pay attention to options, supply voltage, gain adjustment range, input and output impedance, common mode rejection ratio CMR, bias value, and noise, etc.;
 
 
 
2) Filter circuit.
 
The function of the filter circuit is to allow the signal within the specified range to pass, while the signal outside the specified range cannot pass.
 
Filter circuits are divided into: low-pass filtering, high-pass filtering, band-pass filtering and band-stop filtering;
 
Low-pass filtering: allowing signals of low frequencies to pass through, attenuating those of high frequencies;
 
High-pass filtering: allowing signals of higher frequencies to pass through, attenuating those of lower frequencies;
 
band-pass filtering: allowing signals within a certain frequency band to pass through and attenuating those outside the band;
 
Band-stop filtering: allowing signals outside a certain band range to pass through, attenuating those within the band range;
 
Filter circuits are subdivided into: passive filtering, active filtering;
 
Edgeless filtering: a filter circuit consisting of only passive devices (resistors, capacitors, inductors); it has major disadvantages, such as: small circuit gain, poor driving load capability, etc.
 
Active filtering: a filter circuit made up of amplifiers, resistors and capacitors, mainly used for data transmission, interference suppression, etc.; of course it has disadvantages: limited by the op-amp band, this filtering is mainly used in the low frequency range.
 
 
 
The input RC forms a low-pass filter, with the characteristic frequency determined by the RC;
 
This circuit is characterised by its simplicity, but the blockband attenuation is too slow and the selectivity is poor.
 
In order to improve the resistance band attenuation characteristics, accelerate the attenuation and improve the filtering effect, we can add another section of RC low-pass filtering link, constituting a second-order low-pass filtering, the effect is much better than the first order;
 
 
 
3. Summary
 
Resistive strain gauge bridge pressure sensor, the use of more common, circuit is relatively simple, basically by the op amp circuit, filter circuit, following the circuit composition, can meet the ordinary sampling use; can be directly connected to the MCU with AD function sampling, or connected to the AD conversion chip, and then access to the MCU; of course, the actual use of the process of voltage, gain, range, etc., according to the relevant sensor The actual use of voltage, gain, range, etc. can be adjusted according to the relevant sensor manual, chip manual, etc.

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