1 Introduction

Dosing is a very important process in the production process of fine chemical plant, backward dosing system is not only low efficiency and dosing is not allowed, manual operation and human factors will lead to dosing link, so that the process formula is difficult to realize in the production, seriously affecting the stability of product quality and further improve the quality of the product, so the realization of high-precision automatic dosing of the production of industrial enterprises is of great significance. The original feeding system using analog circuit control slip motor speed control method for speed control, due to the slip motor speed control method in the low-speed characteristics of poor, low efficiency, the site of the external working environment is very poor, a lot of industrial dust, the dust is very easy to get into the slip motor inside the wear and tear, jamming and other phenomena, maintenance, maintenance trouble, resulting in a lot of faults, affecting the normal production of the rapid development of computer technology, the original feeding system is a very important part of industrial enterprise production, so the realization of high-precision automatic dosage. With the rapid development of computer technology, the original feeding system for technological transformation, stepless variable speed inverter control closed squirrel cage motor speed mode, with programmable controller to control the frequency converter, the batching machine will be produced according to a certain proportion of raw materials and auxiliary materials, such as conveyor belts through the conveyor belt in order to improve the feeding system of the batching accuracy and stability.

2 Frequency speed regulation principle

2.1 The working principle of frequency conversion speed control

According to the frequency conversion speed control when not changing the structure of the motor, if the power supply frequency fl continuously adjustable, the synchronous speed of the motor is also continuously adjustable. As the rotor speed n of asynchronous motor is always slightly lower than the synchronous speed, when n0 is continuously adjustable, n is also continuously adjustable. Therefore, change the parameter f1 that is, by changing the power supply frequency can also realize the motor speed regulation. When the frequency f is varied in the range of 0-50Hz, the motor speed adjustment range is very wide. In fact, just change the frequency of the motor can not get good frequency characteristics, the flux is too weak, not fully utilize the motor flux, is a waste, if you want to increase the flux, and will make the flux saturation, which leads to excessive excitation current, serious damage to the motor because of overheating of the windings. For this reason, in the frequency reduction at the same time also need to reduce the voltage, the need for frequency and voltage coordinated control, that is, variable frequency variable voltage speed control (VVVF), the abbreviation is variable frequency speed control.

2.2 The function of the frequency converter inverter is a fixed frequency usually 50HZ) AC power (three-phase or single-phase) into a continuously adjustable frequency (most of the 0 ~ 400H2) of the three-phase AC power supply. Figure 1 shows the structure of the inverter, generally by the rectifier inverter, intermediate circuit, protection circuit and control circuit and other components. The control circuit completes the control of the main circuit, the rectifier circuit converts the alternating current into direct current, the intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit inverts the direct current into alternating current.

3 automatic dosage system control program

3.1 System control program

Weighing and mixing machine by the programmable controller and belt scale composed of a two-level computer control network, through the field bus to connect the field instrumentation, control computers, PLC, frequency converter and other high degree of intelligence, fast processing speed equipment. PLC is mainly responsible for the system failure detection, display and alarm, and at the same time to the frequency converter output signals, the frequency converter to complete the role of adjusting the speed of the belt conveyor.

Belt scale weighing is the instantaneous flow, the upper computer gives the set flow, the two in real-time measurement deviation. The most widely used PID regulation in industrial control is used for actual flow control, according to the flow deviation, the control quantity is calculated by using proportional, integral and differential, and the speed of the belt scale is adjusted by the frequency converter to adjust the amount of feed to make it equal to the set value, and complete the automatic dosage process.

3.2 System control flow

When the system starts to work, start the batching production line, weighing and real-time measurement by the belt scale, the CPU calculates the real-time flow rate and cumulative flow rate. On the one hand, according to the proportion of each auxiliary material at the same time mixed and measured, and added according to the recipe process requirements. On the other hand, if there is a deviation between the set flow rate and the actual flow rate, the regulator compares the deviation between the set value and the actual flow rate according to the system control requirements and changes the output signal by PID regulation to control the speed adjustment of the frequency converter on the conveyor motor, so as to realize the flow control.

4 Frequency conversion speed control system composition

4.1 Functional characteristics of Mitsubishi FR-A540 series inverter

Mitsubishi inverter is a high-performance inverter using the flux vector control technology pulse width modulation (PWM) principle and intelligent power module. Its power range is 0.4~315kW.Mitsubishi FR-A540 series inverter has the following features.

(1) with flux vector control technology. The speed range is 1:120 under open-loop control without a speed sensor, and 1:1000 under closed-loop control with a speed sensor, and the torque is uniform at low-speed rotation.

(2) SPWM principle and intelligent power module (IPM), inverter output power is better, noise is lower, anti-interference performance is stronger.

(3)Deceleration and stopping function during power failure, built-in PID control function, inverter/frequency switching function and sequence control function.

(4) Fieldbus communication function in line with international standards.

(5) With over-current, overload, over-voltage, under-voltage, grounding over-current, output short-circuit, stall prevention and other protection.

(6) Easy installation and maintenance of fan terminal and control terminal.

Convenient.

(7) Attached to the cumulative power monitoring and cumulative running time monitoring function 4.2 Inverter function design

Mitsubishi FR-A540 series inverter comes with inverter with PID adjustment function, PID control according to the production process requirements of automatic batching system, part of the parameters to be tested and set are set as follows.

1）Pr.1=50Hz,Pr.2=5Hz，this system Pr.18=120Hz unchanged.

(2) Pr.19 = 9999, the same as the power supply voltage

3) Pr.7=2s, acceleration time (7.5K below the factory P set value 5s,0~3600s/0-360) Pr.8=2s, deceleration time (7.5K below the factory set value 5s, 0~3600s/0~360)

4) Pr.9 is determined by the motor rated value

5）Pr.14=0,Applicable to constant torque loads

6) Pr.79=3 external/PU combination operation mode

(7) Pr.183=8, realize RT switch=REX switch

(8) Pr.128, Pr.129, Pr.130, Pr.131, Pr.132, Pr.133, Pr.134 according to the specific requirements of the site PID regulation to set.

4.3 Electrical principle of inverter

The RS and T terminals of the inverter are directly connected to the 50HZ three-phase 380V AC power output U, V, W output frequency within a certain range of continuously adjustable three-phase alternating current, which is directly connected to the AC motor. The output of the frequency converter is also three-phase alternating current, but in the frequency converter internal through the alternating current has been transformed, and then through the inverter, the use of set parameters for the inverter, so that the output for a corresponding set frequency of alternating current. In the circuit, in order to ensure the normal operation of the safety, the equipment must be reliably grounded, so the inverter ground terminal should also be reliably grounded. In the control input circuit of the frequency converter is connected to the frequency setting circuit, the digital or analog output from the PLC to control the output frequency of the frequency converter, and the speed feedback signal is directly connected to the auxiliary input or current input.

5 Conclusion

Automatic batching system using Mitsubishi FR-A540 frequency converter plus PID closed-loop operation process, regulating the speed of the electronic belt scale drive motor, real-time control of the flow, not only to ensure the accuracy of the batching, to meet the process requirements, but also to improve the reliability and stability of the system operation, and reduce the maintenance costs and energy savings, energy saving and operation with significant results.