Dynamic weighing of large quantities of materials is often required in industrial and mining enterprises to meet the requirements of the recipe in the production process. Common weighing methods include belt scales. Hopper weigher and so on. Belt scales use continuous feeding mode, the pressure weight does not change much, can be realized while weighing and adjusting the amount of feed as well as compensation for the technology, the time response requirements are not high; while the hopper weighing is between the song feeding, pressure weight cumulative increase in the material can be supplemented, but the material is not easy to take out the excessive amount of material. Therefore, the hopper weighing control accuracy and real-time requirements are higher.

1 system analysis

In practice, the requirements of more than one hopper scales composed of weighing group for material weighing and control, each weighing range of 50 ~ 500kg.

Material loaded in the material warehouse, material warehouse and weighing bucket installed between the vibrating screen, the vibrating screen to control the amount of feed, when the weight of the material reaches the target value of the vibrating screen stops, the weight of the material to record the static weighing, and according to the needs of the production to open the bottom of the warehouse door unloading the material by the conveyor belt to send the material to the kiln tender burning, and then repeat the above process of weighing and discharging. From the above process, it can be seen that the material weight is finally given by the static weighing value with high accuracy, and the difference between this value and the target output is used to measure the accuracy of the weighing. However, the system needs to weigh dynamically during the feeding process so that the vibrating screen can be stopped in time, and the hopper cannot be adjusted in this time after weighing, so its dynamic weighing accuracy is more important. In this system, there are the following difficulties in dynamic weighing:

1) The material flow is large. As the density, particle size, fluidity of the material is not the same, so the feed flow of each scale is not the same, ranging from 8 to 23kg / s, the fastest a few seconds to reach the target yield, the measurement and control of real-time requirements.

2) Irregular impact of materials. Material falling from the vibrating screen into the weighing hopper has a certain falling speed, the material impact oscillation to form a series of false weight transfer to the weighing module, the instantaneous signal collected by the weighing module can not truly reflect the weight of the material. Due to the size of the material is not the same, the impact of its fall is random, it is difficult to estimate, and with the reduction of the material fall, the unit mass of the material impulse will also be reduced. These characteristics make the system can not accurately establish the correspondence between the impulse and weight.

3) Lag time of discharging material. When the vibrating screen is closed, due to the inertia of the vibrating screen, the discharging will not be stopped immediately, and there are still some materials continue to enter the weighing hopper, resulting in discharging overshoot, because the design process must take into account the discharging time lag.

This design process must consider the time lag characteristics of the material process.

4) electromagnetic interference. There are fans, elevators, belt conveyors, vibrating screens, winches and other equipment on the site, and measures must be taken to minimize the impact of power grid fluctuations and electromagnetic interference on the weighing signal and data acquisition.

For the above problems, the solution is given in the system realization part.

2 System design and realization

The system is roughly divided into weighing module part, signal conditioning part, signal acquisition, processing part and display recording part. Among them, the signal acquisition, processing part and display record are completed by PCI data acquisition card, industrial control computer and display and print peripherals. The system composition is roughly shown in Figure 2.This system is a mechatronic computer control system, which controls six hopper scales. In addition to a weighing signal, each scale has other digital inputs and outputs, such as monitoring the status and position of the warehouse door, controlling and monitoring the frequency converter, controlling and monitoring the transportation belt, and switching between automatic and manual modes. For the acquisition of the weighing signal, consider symmetrical installation of three weighing modules to support the weighing hopper, the weighing signal will be combined with an adder into a one-way weighing analog signal, which can avoid the difference of the weighing signal due to the different stacking positions of the materials in the weighing hopper, and does not require precise adjustment of whether the three weighing modules are at the same level or not, which reduces the burden of maintenance in the future.

Industrial computer with data acquisition card to form a system, this way compared to embedded systems and PLC also has the advantages of friendly interface, short development cycle, easy maintenance and updating, low price, etc., and the performance of the industrial computer is stable, electromagnetic compatibility and dust resistance to meet the needs of industrial applications. In order to protect the industrial control machine and peripherals from accidental high-voltage power damage on the line number transmission line, should be selected between the input and the system bus with isolation protection of the data acquisition card.

The system also needs to control the vibration of the material sieve to change the flow of material, select the frequency converter to control the motor speed, can easily and quickly change the motor speed.

2.1 Large flow rate material weighing

Material flow requirements of the pressure weight signal collection frequency and high transmission rate for the use of Advantech's production of high-speed PCI bus-based PCI-1713-type 32-channel analog acquisition and control card. The card realizes 12-bit high-speed A/D conversion sampling rate of up to 100KS / s, in order to protect the PC as well as peripherals from line number transmission line of accidental high-voltage power damage. Between the transmission and PCI bus provides a 2500V (DC) DC optical isolation protection. For this system will be set to PCI-1713 single-ended input internal trigger operating mode gain code is set to 16, that is, the corresponding input signal voltage range of 0 ~ 10V using the PCI-1713 card to achieve the ms/chamnel sampling period, the control cycle of about 12ms, than the ordinary belt scales 1s control cycle is greatly improved. Analog weighing signal acquisition through the driver on the PCI-1713 operation. The PCI-1713 analog input in this system is triggered by multi-channel interrupt, using FIFO and user buffer. Sampling data is first stored in the first half of the FIFO and the second half of the cache in turn, the FIFO half-full to produce a medium "and then this half-cache data to the user cache area for subsequent processing and to prevent the Windows multi-tasking operating system for a high number of data acquisition lost data at the same time the new data to continue to fill the next half of the cache, so as to realize the high-speed This enables high-speed real-time data acquisition.

On this basis, the working mode of the vibrating screen is further set. Let the frequency converter controlling the vibrating screen can work in two modes of high speed and low speed under the control of the software system, and lower the material flow in the low-speed mode to improve the control accuracy.

2.2 data processing methods due to the irregular impact of the material, weighing bucket oscillation, resulting in real-time material weight values displayed on the software interface suddenly large and small abnormal jumps, and other high-power equipment start and stop caused by power grid fluctuations will also produce similar interference. In response to this problem. In the data processing part of the data median value average filtering method. This method is a synthesis of the median value filtering method and the arithmetic mean filtering method. That is, the N data obtained in a sampling period are first sorted by size, then the first m and the last data are eliminated, and finally the average value of the middle (N-m) data is calculated as the final sampling value. This method can remove the accidental pulse interference, and the appropriate adjustment of the m value can adjust the size of the output value bias. In this system, 64 weight signals are collected in one data acquisition cycle, the first 16 and the last 16 data are divided, and the average value of the middle 32 data is taken as the value of the weight signal. The operation results found that this measure solves the problems of irregular impact and electromagnetic interference at the same time, and the real-time material weight value displayed on the software interface in the process of discharging grows gradually and steadily. Even if the large-scale machinery start and stop caused by violent fluctuations in the power grid did not appear jump phenomenon.

2.3 Time lag processing method to solve the time lag problem. Adopted a fuzzy control based on the method of setting the reserved impulse, the basic algorithm is: the beginning of the material to open the frequency converter for high speed, vibration sieve under the fast material, the material with a larger flow into the weighing head, when the weighing hopper in the weight of the material value is greater than the target yield minus the slow reserved impulse, then open the frequency converter for the low-speed state. Vibrating undercoating sieve slow down, the material with a smaller flow into the weighing hopper: when the weight value of the material in the weighing hopper is greater than the target output minus the stopping of the reserved impulse, then turn off the frequency converter, vibration undercoating sieve stops undercoating, undercoating sieve overshooting will be made up for the difference between the current weight of the material and the target output: this control process is shown in Figure 3. In this way. Fast and slow two-speed discharging and the method of setting the reserved impulse solves the problem of discharging overshoot caused by the time lag characteristic. The size of the reserved punch can be set on the software interface according to the target weight and material properties.

The actual debugging process found. Vibrating screen high-speed open 50H2, low-speed open 20H2, then the high-speed target is set to the set value minus 10 times the reservation is more appropriate. Vibrating speed is too low is under the material time-consuming, too high is under the material speed is too fast and difficult to control.

3 results and discussion

Statistics of the actual operation of the system for half a year a total of more than 10,000 pieces of actual feeding data, in accordance with the cumulative hopper scale automatic weighing error calculation method to get 1-6 weighing error and calculated the cumulative error.

No. 3 and No. 6 weighing due to weighing each time the target output is small, and the material is often lumped, the time lag characteristics of the material caused by the greater impact on the accuracy of control, weighing error is larger. The rest of the weighers have reached the automatic weighing accuracy of 2.0 level.

If the increase in the amount of overshoot retention device will greatly improve the system's controllability and weighing accuracy, but this will also make the weighing body mechanical part more complex. In addition, when the system is used, if the target yield changes or the material properties change, it is necessary to manually increase or decrease the reserved impulse value appropriately. In the actual use of the process found that inexperienced operators to the value of the setup is sometimes improper, resulting in large weighing errors. Therefore, it is considered to increase the adaptive control of the reserved impulse algorithm on the basis of manual intervention of the reserved impulse, in order to reduce the dependence of the appropriateness of the reserved impulse setting on the operator's experience.