0Introduction

Metering system can be used in the production line instead of manual inspection of the filling level, can be fast, efficient and accurate rejection of the level height does not meet the production specifications of the filling bottles, effectively improve production efficiency and ensure the quality of production. Foreign countries mostly use ultrasonic, radar measurement module for detection of high accuracy but its price is expensive. In this paper, in order to achieve low-cost precision detection for the purpose of designing a capacitive metering module based on the liquid level metering system, and can be set through the human-computer interface level Min value range, real-time data recording, to meet the needs of industrial field detection.

1 capacitive measurement module principle

The detection part of the system is composed of two parallel electrodes parallel plate capacitor, its basic characteristic is the external non-electricity is converted to capacitance value. Level detection module of the transmitting pole plate by the capacitance digital conversion chip AD7745 to provide the excitation source signal receiving pole plate connected to the AD7745 capacitance metering pins. Excitation source signal and air, container wall and bottle level coupling in the receiving pole plate to produce a response to the bottle level height of the signal. AD7745 will be measured capacitance value into a digital quantity and after subsequent conversion will be mapped to the liquid level value displayed on the screen.

Filling solution through the photoelectric trigger device into the detection area, the occurrence of capacitive coupling, is the launch of the receiving pole plate through the air directly coupled to the capacitance; for the pole plate and the container wall of the air between the coupling capacitance, if the container can be in the standard situation in the detection of capacitance is in the middle of the actual level of liquid in the container and the ground between the coupling capacitance. The capacitance together constitute the equivalent capacitance between the receiving and transmitting pole plates. When the excitation source signal is stable, the equivalent capacitance is only related to the volume of the liquid in the container and its dielectric constant, the size of the equivalent capacitance can be reacted to the liquid level in the container value.

2 System Design

2.1 General framework

The system consists of photoelectric trigger module, capacitance measurement module, the main controller human-machine interface, reject device 5 parts. The microprocessor in the main controller uses STM32F103VET6 chip to communicate with AD7745 and upload the processed data to the screen display.

2.2 Capacitance measurement circuit

The capacitance is measured using a capacitance-to-digital converter, the AD7745, which combines all the circuits from capacitance to digital conversion on a single chip and can transmit data to the MCU for processing via the Ic bus. Using the single-ended mode of the AD7745, a square wave signal is added to the equivalent capacitor during a conversion cycle, the AD7745 built-in-modulator continuously samples the charge passing through the equivalent capacitor, which is processed by the digital filter X-into the modulator and output, and the data is read out by the IC serial interface.

Literature focuses on the study of the excitation source circuit, pointing out that the excitation source operating frequency should be between 3.5 ~ 10MHz when the filling level of the equivalent inductance capacitance resistance of the change is obvious. After experimental verification, the final choice of the excitation source frequency of 8MHz, the choice of AD8615 as a low-impedance signal source for the emitter plate to provide a stable 8MHz excitation, on the other hand, can ensure that the AD7745 in the beginning of the acquisition of capacitance value, the equivalent capacitance is in a fully charged state.

AD7745 detection circuit itself has a certain amount of stray capacitance suppression ability; in order to improve the accuracy of capacitance measurement, the connection between the transmitter and receiver pole plate wire matched to equal length and twisted-pair cable with shielding function: the software is programmed to set the CAPDAC, not only to a certain extent to expand the range of capacitance input, but also to improve the accuracy.

2.3 Master Controller

This system converts industrial power into 24V DC power supply for the main controller. Considering that the output ripple voltage of the switching regulator chip is high and prone to electromagnetic interference, a linear regulator chip is used. The 24V DC power through the linear regulator chip 78L08, 78L05 and low dropout voltage regulator AMS1117 will be rung to 5V and 3.3V for subsequent circuits. The human-machine interface adopts MT8071IE touch screen, which has high stability, and the operating environment humidity range of 10%~90%RH can meet the demand of high humidity at the test site. The touch screen comes with Modbus communication protocol, and the main controller through the RS485 interface connection communication; general-purpose host computer programming software EasyBuilder8000, easy to design the human-machine interface, and support for remote control.

2.4 Software Flow

AD7745 has 18 internal registers, the host has to send commands to these registers to set the state, and read the data from the registers, and set the corresponding registers to specific high and low power sub through software programming.

3 electromagnetic compatibility design

The working environment of the device is complex, the site has a frequency converter, motor, automatic filling machine and other equipment radiating electromagnetic waves will interfere with the signal waveform, the detection results have a certain impact. Grounding, shielding, filtering is the main electromagnetic interference suppression method common to dosage equipment and systems in the process of electromagnetic compatibility design.

In the design of PCB, the use of analog and digital ground and digital ground laying method, the digital ground and analog ground common ground resistance to a minimum; attenuate the digital signal harmonics on the analog signal interference and analog signals for high-frequency when the impact of digital circuits; system design, transmitter modules and receiver modules were connected to the chain of channels, and to ensure that the chain of channels bottom shell access to the earth; plus metal shielding and grounding, attenuating electromagnetic interference; the use of the electromagnetic interference of the batching equipment and systems for electromagnetic compatibility design process common main electromagnetic interference suppression methods. Weaken the electromagnetic interference; the use of 24V AC power through the power supply filter to the main controller power supply.

4 test data

4.1 Pole plate erection method

Determined by the detection principle of capacitive coupling, the closer the two pole plates, the stronger the signal received by the receiving pole plate. Considering the actual production needs in the test object for a brand of 600mL bottled beer, the pole plate distance is set to 5.5cm; pole plate height to the liquid level in the detection of the pole plate within the city is appropriate: to ensure that the modules are connected to the chain and common ground; beer bottles along with the chain with the road to travel, to test.

4.2 Test results

The level of the beer in a bottle after filling is set as the standard 0 level, and the height of the filling level in the rest of the test bottles has a certain height difference from the standard 0 level. This device can achieve a detection accuracy of at least 2mm, which can meet the requirements of industrial field metrology.

5 Conclusion

Build a model test, in the no-foam state, the system error: within 0.2mm, the error is small, the test performance is good. In addition, the system also reserves a variety of expansion interfaces to facilitate the measurement and processing of more parameters, thus further improving the control accuracy and enhancing the function of the system. At the same time, by changing the shape of the pole plate and the installation method and other measures, the device can have better detection results for glass bottles, ceramic bottles plastic bottles and other bottle types: beer, liquor, black tea and other media can be detected. However, when the bottle with a metal label or metal bottles, the liquid level is no longer the main reason for the size of the coupling capacitance, so this type of bottles can not be detected; the subsequent need to increase the foam compensation program to improve the device.