Design of detection system for the hottest CAV424

The detection system design of CAV424 inclination sensor

Introduction

the differential capacitive sensor has high sensitivity and small nonlinear error. At the same time, it can also reduce the working principle and advantages of electrostatic attraction to the mechanical performance testing machine. 1. The working principle of the mechanical performance testing machine. The components of the equipment include computer control system, force measuring mechanism, hydraulic system, workbench and sensor; Its working procedure also uses the hydraulic cylinder piston rod to approach the impact of the test data measurement, and can effectively improve the error caused by the environmental impact such as temperature. Therefore, in many measurement and control occasions, the capacitive sensors used are mostly differential capacitive sensors. However, the capacitance value of capacitive sensor is very small, and the change of small capacitance must be converted into the change of voltage, current or frequency proportional to it with the help of signal conditioning circuit, so that it can be displayed, recorded and transmitted. At present, most capacitive sensor signal conditioning circuits use discrete components or develop application specific integrated circuits (ASIC). Because the capacitance of the differential capacitive sensor is very small, the conditioning circuit of the sensor is often affected by parasitic capacitance and environmental changes, so it is difficult to achieve high-precision measurement; The CAV424 integrated circuit developed by AMG company in Germany can effectively reduce the errors caused by these effects, so it has greater application flexibility

the inclination sensor in the design is a new modified area capacitive inclination sensor. The inclination sensor technology is one of the few inclination sensor technologies that can have the advantages of simple structure, high reliability and general sensor integrated circuit. It is widely used in measuring instruments, construction machinery, antenna positioning, robot technology, automobile four-wheel positioning, etc

1 system working principle

the system hardware structure module block diagram is shown in Figure 1, which is mainly composed of differential capacitance, CAV424, operational amplifier, single chip microcomputer and display circuit. The system detects the inclination angle of the installation position of the inclination sensor by the differential capacitance, and converts the angle change into the electric capacity change. The differential capacitance increases while the other decreases, and then the change values of the two capacitors are sent into two CAV424 respectively, and the change values of the capacitors are converted into two different voltage values by two CAV424. After differential amplification, the two voltages are sent to the single chip microcomputer for processing. Finally, the display circuit shows the tilt angle of the detected object. It can be seen from the above principle that the tilt angle of the detected object has been processed by three-stage differential, and CAV424 is equipped with a temperature sensor. The output signal of this sensor is sent into the single chip microcomputer for temperature compensation. Therefore, the system has high accuracy and sensitivity

2 circuit design of each part of the system

2.1 design of differential capacitance/voltage conversion circuit

considering that the capacity of differential capacitance is very small, the conditioning circuit of sensor is often vulnerable to parasitic capacitance and environmental changes, so CAV424 developed by German AMG company is used as the signal conditioning circuit of differential capacitance. Because only one capacitor can be detected by a single CAV424, two CAV424 are used to complete the detection of differential capacitance

(1) introduction to CAV424

cav424 is a multi-purpose integrated circuit for processing various capacitive sensor signals. It has the functions of signal acquisition (relative capacitance change), processing and differential voltage output at the same time, and can measure the difference between a measured capacitance and a reference capacitance. Compared with the reference capacitance value (10 pf~1nf), the service temperature of general special engineering plastics is mostly in the range of about 180 ℃ to 100%. The capacitance value of 0pf-2nf can be detected, and the maximum output differential voltage can reach 1.4 V; At the same time, CAV424 also has a built-in temperature sensor, which can directly provide temperature signals to the microprocessor for temperature compensation, so as to simplify the whole sensor system. The principle is shown in Figure 2

(2) detection principle of CAV424

a reference oscillator whose frequency is determined by capacitance COSC drives two integrators with symmetrical structure and synchronizes them in time and phase. The amplitudes of these two integrators are determined by capacitors CXL and CX2 (as shown in Figure 2). Here, CXL is the reference capacitance and CX2 is the measured capacitance. Because the integrator has a high common mode rejection ratio and resolution, the signal obtained by comparing the difference between the two amplitudes reflects the relative changes of the two capacitors CXL and CX2. The differential signal is converted into a DC voltage signal through a two-stage low-pass filter and output through an adjustable differential classification. As long as a few components are simply adjusted, the filtering constant and magnification of the low-pass filter can be changed

the external oscillator capacitance COSC of the reference oscillator and its related internal parasitic capacitance COSC, par, int and the external parasitic capacitance cosc.par.ext are charged and then discharged. The capacitance of the oscillator is approximately taken as coc=1.6cxl. Reference oscillator current iosc=vm/ROSC. The output waveform of the measured oscillator, that is, the 12 pin output waveform of any CAV424, is shown in Figure 2 of reference [1]

the working mode of the capacitive integrator is similar to that of the reference oscillator. The difference is that the discharge time of the former is half that of the reference oscillator. Secondly, the discharge voltage of the former is clamped on an internally fixed voltage vclaamp. The 14 pins and 16 pins of two CAV424 (the output voltage of the electric volume divider) are measured. The output waveform can be found in reference [1]

the output voltage of the two integrators after internal signal conditioning should ideally be

vlpout=vdiff+vm

where the differential signal vdiff=3/8 (vcx1-vx2), VM is the reference voltage

(3) actual hardware circuit and circuit parameter design

the actual differential capacitance/voltage conversion circuit is shown in Figure 3

when the inclination sensor is placed in the horizontal position, the differential capacitance c10=c20=50pf, so the reference capacitance of CAV424 c11=c21=50pf, the oscillation capacitance c12=c22=1.6c11=80pf, the low-pass filter capacitance c13=c14=c23=c24=200c11=10nf, the capacitance load of the stable reference voltage VM c15=c25=100 NF, the current adjustment resistance r11=r12=r21

=r22=500k Ω. Reference oscillator current setting resistance r13=r23=250k Ω. In order to adjust vlpout, adjust the output stage resistance to a potentiometer of 100k Ω. In addition, in order to improve the stability of the circuit, 10nf capacitors C16 and C26 are connected between pin 4 of CAV424 and the ground

2. 2 operational amplifier circuit design

the operational amplifier circuit is used to synthesize and amplify the voltage signal output by two pieces of CAV424 and convert it into O ~ 5V DC voltage that is easy to be processed by single chip microcomputer. According to the general design principle, the instrument amplifier should be selected here; However, considering the high cost of the instrument amplifier and the high output voltage of the two CAV424 used in the front stage, the four op amp tl084 with high cost performance and high chemical stability is selected as the signal conditioning circuit. The experiment shows that the precision of the system meets the design requirement. Considering the simplicity of the later stage circuit, two-stage operational amplifier is used here. In the first stage, the vlpout of two pieces of CAV424 are used as the positive and negative input of the op amp respectively, so that when the inclination sensor changes by ± 90 °, the Vol output is ± 2.5V, and any VM end of two pieces of cv424 is used as the in-phase input end of the second stage op amp, so that the output voltage of V02 is 0 ~ 5V. Then, this signal is used as the analog input signal of the single chip microcomputer. The actual circuit is shown in Figure 4

here, select r1=r2=r3=r4=r5=rf2=10 K Ω, rf1=rp1=100 K Ω, then

uol=rfl/r1 (vlpout1-vlpout2) (1)

uo=vm UOL (2)

substitute formula (1) into formula (2), and uo=vm+rf1/R1 (vlpout1-vlpout2) can be obtained; At the same time, adjust RF2 and RP1 so that when the inclination sensor changes within ± 90 °, uo changes within 0 ~ 5V

2.3 the joint team of China and Russia is paying close attention to formulating the overall technical scheme of the aircraft. The software and hardware design of the chip machine and its display system

(1) hardware design

considering that the output of the operational amplifier is 0 ~ 5V analog voltage signal, and the output of the temperature sensor of CAV424 is also analog voltage signal, which can not be directly processed by the general single chip microcomputer, Therefore, pic16f872 produced by microchip company is selected as the microprocessor of the system. In addition to the features of RISC (reduced instruction set computer), Harvard dual bus and two-stage instruction pipeline structure of general pic series microcontrollers, it also comes with five 10 bit a/D conversion components, 2K × The 14 bit flash memory provides great convenience for the development of the system

in addition, considering that the inclination sensor should not only display the size of the inclination angle, but also display the positive and negative of the angle, and considering the convenience of programming and the display accuracy of the inclination sensor, HD7279 is selected as the 8-segment nixie tube display driving circuit in this design to display the size and positive and negative of the inclination angle

the design circuit of this part is shown in Figure 5

(2) software design

the software design of this system mainly includes a/D conversion, engineering quantity conversion and display. The main program flow is shown in Figure 6

conclusion

experiments show that the measurement accuracy and sensitivity of the inclination sensor meet the expected requirements. The design is a general module. By replacing the differential capacitance of the inclination sensor with other differential capacitance sensors, the vibration, acceleration, differential pressure, liquid level and other accurate measurements based on the principle of differential capacitance can be carried out. Therefore, the design scheme of the system has great application value. (end)