Posed by the bridge circuit measuring differential pressure sensor is a measurement method with high sensitivity. Diffusion resistance of silicon is mainly active transmitter full Wheatstone bridge circuit pressure measurement. It can achieve high accuracy and stability of the premise, so that the transmitter is practical, such as zero / span adjustments and move quickly and easily. However, the literature [1,2] on the diffusion of silicon bridge sensor description is too simple and limited to qualitative analysis. This quantitative analysis will be diffused silicon sensor circuit using the zero / span adjustment, output feedback stabilization and other technical measures.
A sensor bridge works
Sensor measuring circuit shown in Figure 1. No pressure, the electric current equal to the bridge arms. Differential signals to four silicon varistor, the varistor resistance changes with pressure, causing the bridge imbalance. Bridge output voltage is fed into the amplifier A1. A1 output voltage of the transistor Q8 through changes in the size of the output current I0. I0 flows through the resistor bridge feedback network, the bridge back into balance. Thus, changes in bridge output voltage and proportional to the corresponding pressure changes. Pressure changes which will be directly converted into electrical signals.
2, quantitative analysis and calculations
Adjustment of the measuring circuit and the range 2.1
Figure 2, including zero and span adjustment circuit, the feedback resistor network and measuring the differential pressure sensing bridge circuit. I0, said current source output current. Four arm resistance bridge is R1S, R3S, R4S and R6S. When the measured increase of positive pressure, R1S and R6S resistance decreases, R3S and R4S resistance increases, the bridge output signal VBr negative increase. This VBr measured pressure corresponds to the value of the voltage error signal. After measuring circuit the role of negative feedback resistor network, VBr signal will remain very close to zero.
Feedback voltage is the basic formula
Vfb = V1-V2 = IO1 [aR2 + R1]
Range corresponding to the maximum pressure sensors, varistors, and gives the greatest change in the bridge and the largest imbalance. At this point, in order to maintain VBr close to zero, the maximum feedback voltage required. When equation (1) a = 1, IO2 = 0 , Vfb maximum value, the sensor get the maximum range. A range of coarse bits and screws in the head at the bottom of R2 when slide to meet the above requirements. When a = 0, IO2 maximum measurement range narrowest, the situation is at the same time three coarse screw B, C, D position, and R2 and at the top of the first slide. R2 sliding through coarse screw head and the combination of other locations, access to the middle of the range. R2 sufficient adjustment range, in order to adjust the coarse adjustment screw from the range corresponding to the range of different coarse to provide overlap.
Bridge output feedback voltage Vfb at the weight of the
2.2 Zero adjustment and migration circuit
Figure 3 is a basic bridge circuit with ± 100% migration of the circuit. The middle of the bridge on both sides of the insertion of a 1000O resistor string. This series resistance has six points in order to provide the required voltage output zero adjustment range. Namely:
VBr = V press.signal-Vfb + Vsp (2)
Since Vfb approximate zero, so there
Where, a potentiometer R11 from the zero position on the sliding contact decision. From the equation (4) can be seen, zero potentiometers and screw at different positions corresponding to different Vsp, this is the output of the reasons for zero can be adjusted. Zero when the potential is in the top, a = 1, Vsp most. Coarse adjustment screws to provide zero, zero zero potentiometer provides fine-tuning.