Design of Control Instrument for Weighing Vibrating Feeder

1 Introduction
In industrial applications, it is often necessary to quantitatively package or control some granular or powdery materials. For example, quantitative small packaging of powdered pesticides or edible seasonings, batching system in smelting system, etc. In the past, the constant volume metering control method is often used, that is, the container with specific volume is used to achieve the quantitative purpose by controlling the volume of materials, such as screw propulsion feeder, etc. With the continuous improvement of precision control and speed requirements in application, the constant volume metering control method can not meet the needs gradually. Weighing vibrating feeder is an ideal alternative.
2 working principle analysis
The main working process of the weighing vibrating feeder is as follows: after the system is started, input the weight of the materials to be packaged into the instrument through the weighing control instrument keyboard, and then press the start key to make the large and small vibrators start vibration and blanking. When the material falls into the scale bucket, when the difference between the weighed weight and the set weight value is no more than 50g, the large vibrator stops vibrating and the small vibrator continues to vibrate and discharge; When the weighed weight is equal to the set weight value, the small vibrator stops discharging and enters the unloading waiting state. After the photoelectric detector 1 detects that a packaging bag is placed at a predetermined position, start the motor and drive the weighing bucket to turn over; When the photoelectric detector 2 detects that the scale bucket is overturned to the predetermined position, turn off the motor to stop the overturning, wait for a period of time to unload the materials, and then the motor drives the scale bucket to start automatic inversion; When the photoelectric detector 3 detects that the scale bucket returns to the original position, stop the motor, start the large and small vibrators and start discharging. So repeatedly, complete the whole task.
Before putting into use, because the scale bucket itself has a certain weight, it should be calibrated. Through calibration, the weighing slope and the weight of the weighing bucket can be stored in the off-chip power down holding memory for calling its value to calculate the actual weight of the material during weighing. The formula is as follows: actual weight = weighing slope × (data from a / D conversion - bucket weight)
3 system hardware
The system is mainly composed of weighing bucket, vibrating feeder and control instrument. In the whole system, the control instrument is the core part. The hardware circuit of the control instrument is mainly composed of unit module circuits such as signal acquisition and processing, a / D conversion, off chip data storage, LED display, keyboard and so on.
3.1 selection of single chip microcomputer
In order to save the expansion of off chip memory and improve the running speed of MCU, we decided to use W77E58 produced by Taiwan Huabang company compatible with 89C51.
3.2 selection of operational amplifier
Because the voltage signal output from the sensor is very weak, the sensitivity of the sensor is set to 2mV / v. when the power supply voltage is 9V, the full negative output of the sensor is only 18MV. Therefore, this signal needs to be amplified by the operational amplifier before a / D conversion. Because the accuracy requirements of the subsequent A / D conversion are high, the accuracy requirements of the operational amplifier are relatively high. Based on the above considerations, the operational amplifier with high precision and low drift is adopted here, and the resistor used to set the operational magnification also adopts high-precision resistor. The operational amplifier used in this system is opa2277 of BB company, and the resistor adopts E2 series high-precision resistor of Bengbu BEDIS. The specific calculation formula is as follows:
The magnification of the system is R6 / R5 = 160
Resolution: in this system, the resolution of 24 bits is only 16 bits, expressed as a percentage
1÷65536=0.00015%
Because the full-scale output of the pressure sensor of the system is 18MV, the minimum voltage value of the A / D converter is
18 × 0.00015=0.00027mV
3.3 selection of a / D converter
Due to the real-time performance of the weighing instrument, the requirements for the speed and accuracy of the A / D converter are relatively high. In order to improve the accuracy of sampling, the resistance strain gauge pressure sensor is used to sense the change of pressure, which is amplified and transmitted to the A / D converter through the conversion bridge. In order to improve the conversion accuracy, we use a 24 bit serial A / D converter ADS1210.
4 system software design
From the working principle, the working process of the whole system can be roughly divided into four steps:
(1) Keyboard setting and control process. The weight to be weighed, display resolution, weighing accuracy, etc. can be set through the keyboard.
(2) The single chip microcomputer reads the sampling data from the A / D converter, and carries out the corresponding operation and processing process. This process is the core part of the weighing control instrument and the focus of software program editing.
(3) Displays the process. It has certain real-time performance.
(4) External control process. Through this process, we can timely know when to stop the large vibrator, when to stop the small vibrator, when to weigh, when to receive and unload materials, etc.
To some extent, these four processes can be completed at the same time, which requires continuous cyclic scanning in programming.
The main program starts running from the time when the system is powered on. Firstly, carry out various initialization (including the initialization of ADS1210, some initialization of MCU itself, reading the calibration value from AT24C02, etc.). After that, the program enters the formal main cycle, that is, scan and judge the above four processes respectively, so as to start again and again until all weighing work is completed.
5 Conclusion
At present, the equipment has been put into use. The actual operation shows that the equipment has stable and reliable operation, simple operation and high performance price ratio, which meets the design requirements. As an electronic weighing instrument that can automatically add ingredients, weigh and unload materials, the accuracy of static weighing meets the requirements of the national three-level scale and the national compulsory appraisal standard for the packaging accuracy of the packaging industry. It has high popularization and application value.

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