Design of Control Indicator for Weighing Type Vibrating Feeder

1. The introduction

In industrial applications, it is often necessary to carry out quantitative packaging or batching control for some granular or powdery materials.For example, quantitative small packages of powdery pesticides or edible seasonings, batching systems in smelting systems, etc.In the past application, constant volume measurement control method is often used, that is, the use of a specific volume of the container, through the control of the volume of the material to achieve quantitative purposes, such as the spiral push feeder.With the continuous improvement of precision control and speed requirements in application, the constant volume metering control method has been gradually unable to meet the needs.Weighing vibration feeder is an ideal replacement scheme.

2. Analysis of working principle

The main working process of the weighing vibration feeder is as follows: after the system is started, the weight of the material to be packaged is input into the instrument through the weighing control instrument keyboard, and then press the start button to make the large and small vibrators start to vibrate and blanking.When the material falls into the scale bucket, when the difference between the weighing weight and the set weight value is no more than 50 g, the big vibrator stops vibrating, and the small vibrator continues vibrating feeding;When the weighing weight is equal to the set weight value, the small vibrator stops feeding and enters the unloading waiting state.When the photodetector 1 detects that the packaging bag is placed at the predetermined position, the motor is started and the weighing hopper is turned over.When the photodetector 2 detects that the scale bucket turns over to a predetermined position, it closes the motor to stop turning, and waits for a period of time for the material to be unloaded, and then the motor drives the scale bucket to start automatic reversal;When the photodetector 3 detects that the scale hopper returns to its original position, it stops the motor, starts the large and small vibrators at the same time, and starts feeding.So again and again, complete the task.

Before putting into use, because the scale bucket itself has a certain weight, it should be calibrated.By calibration, the weighing slope and weighing bucket weight can be stored in the off-chip power retention memory, so that the actual weight of the material can be calculated by calling its value during weighing.The formula is as follows: actual weight = weighing slope ×(data obtained from A/ D conversion - weight of scale bucket)

3. System hardware

The system is mainly composed of weighing hopper, 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 signal acquisition and processing, A/ D conversion, off-chip data storage, LED display, keyboard and other unit modules.

3. 1 MCU selection

In order to save the expansion of the chip memory and improve the running speed of the SCM, we decided to use the 89C51 compatible W77E58 produced by Taiwan Huabang Company.

3. 2 Op-Amp Selection

Because the voltage signal output from the sensor is very weak, the sensor sensitivity set is 2 mV/ V, when the supply voltage is 9 V, the full negative output of the sensor is only 18 mV, so this signal needs to be amplified by operational amplifier before A/ D conversion, because the accuracy of the latter A/ D conversion is higher.So here the operational amplifier precision requirements are relatively high.Taking into account the above considerations, an op-amp with high precision and low drift is adopted here, and a resistor with high precision is also used to set the operational magnification.The operational amplifier used in this system is OPA2277 of BB Company, and the resistance is E2 series high precision resistor of Bengbu Beidis. The specific calculation formula is as follows:

The magnification of this system is: R6/ R5= 160

Resolution: In this system, the resolution of 24 bits is only 16 bits, which is expressed as a percentage

1 present 65536 = 0. 00015%

Because the full scale output of the pressure sensor in this system is 18mV, the minimum voltage value of the A/ D converter is

18× 0.00015 = 0.00027 mV

3. Selection of 3A/ D converter

Due to the real-time operation of the weighing apparatus, the requirements for the speed and precision of 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, and the amplifier is transmitted to the A/ D converter through the conversion bridge.In order to improve the accuracy of conversion we use 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, the resolution of the display and the precision of weighing can be set through the keyboard.

(2) MCU reads the sampling data from 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 key point of the software program editing.

(3) Display process.With a certain degree of real-time.

(4) External control process.Through this process, we can know in time when to stop the large vibrator, when to stop the small vibrator, when to finish weighing, when to receive and discharge material, etc.

These four processes to some extent can be said to be completed at the same time, which requires in the program design to carry on the continuous cycle scan.

Weighing control instrument, weighing vibration feeder

From the moment the system is powered on, the main program starts running.Firstly, various initializations are carried out (including the initialization of ADS1210, some initializations of SCM itself, reading calibration values from AT 24C02, etc.).Then the program enters the formal main cycle, that is, the above four processes are scanned, judged, and so on, until all the weighing work is completed.

5.Conclusion

At present, the equipment has been put into use, the actual operation shows that the equipment is stable and reliable, simple operation, high performance-price ratio, meet the design requirements.As an electronic weighing device which can automatically add ingredients, weigh and unload material, the precision of static weighing meets the requirements of the national three-level scale and conforms to the national compulsory appraisal standard of packaging precision in the packaging industry.It has high popularization and application value.

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