Industrial and mining enterprises often need to carry out dynamic weighing of large quantities of materials to meet the requirements of the recipe in the production process. Common weighing methods include belt scales, bucket scales and so on. Belt scale adopts continuous feeding method, the weight does not change much, can realize the technology of adjusting the feeding amount while weighing and compensating afterwards, the requirement of time response is not high; while bucket scale is feeding between songs, the weight increases cumulatively, the material can be supplemented by the insufficiency of materials, but it is not easy to take out the material after the excess. Therefore, the bucket scale on the control accuracy and real-time requirements are higher.

1 system analysis

In practice, the requirements of multiple hopper scales composed of weighing group for material weighing and control of the material warehouse, each scale weighing range of 50 ~ 500kg material loaded in the material warehouse, the material warehouse and weighing hopper between the installation of vibrating screen, vibrating screen to control the amount of feed, when the weight of the material reaches the target value of the vibrating screen stops, the record of the weight of the material weighing of the static and according to the needs of the production of opening the bottom of the door unloading by the conveyor belt to send the material to the kiln tender firing, the material will be used for the kiln. The material is sent to the kiln for tender firing, and then repeat the above weighing and discharging process. It can be seen from the above process that the weight of the material is ultimately given by the more accurate static weighing value, and the difference between this value and the target yield 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 bucket scale cannot be adjusted at this time after weighing, so its dynamic weighing accuracy is important. Dynamic weighing in the feeding system has the following difficulties:

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

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 sensor, the instantaneous signal collected by the load cell 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 drop, the unit mass of 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 turned off, due to the inertia of the vibrating screen. Under the material will not stop immediately, there are still some materials continue to enter the weighing hopper, resulting in under the material overshoot, so the design process must take into account the time lag characteristics of the material process.

4) electromagnetic interference. The site has fans, elevators, belt conveyors, vibrating screens, winches and other equipment, we must take measures to reduce power grid fluctuations and electromagnetic interference to the weight signals and data acquisition impact.

In view of the above problems, we give a solution in the system realization part.

2 System design and realization

The feeding 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 printing peripherals.

The feeding system is a mechatronic computer control system, which controls six hopper scales. Each scale in addition to a weight signal there are other digital inputs and outputs such as monitoring the status and position of the warehouse door, control and monitoring of the frequency converter, control and monitoring of the transportation belt, automatic manual mode switching and other digital inputs and outputs. For the acquisition of weight signals. Symmetrical installation of three measurement modules to support the weighing hopper, the weight signal with an adder to merge into a weight analog signal, so as to avoid the material in the weighing hopper due to the accumulation of different positions caused by the difference in the weight signal, but also do not have to three load cells in the same level of precise adjustment, reducing the burden of maintenance.

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 the line number input line accidental high-voltage power damage, should be selected between the input and the system bus with isolation protection of the data acquisition card.

Batching machine also need to control the vibration of the lower material sieve to change the material flow, select the frequency converter to control the motor speed mode, can easily and quickly change the motor speed.

2.1 large flow of material weighing material flow requirements of the weight of the signal acquisition frequency and transmission rate is high, the Advantech production of high-speed PCI bus-based PCI-1713-type 32-channel analog acquisition 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 the line number transmission line unexpected high-voltage electric raises the harm in the transmission and PCI bus provides a 2500V (DC) between the DC light away from the protection. For this system will be set to PCI-1713 single-ended input internal trigger mode of operation gain code is set to 16, that is, the corresponding input signal voltage range of 0-10 V. The use of PCI-1713 card to achieve a 1ms/channel sampling period, the control cycle of 128ms or so, than the ordinary belt scales of 1s control cycle is greatly improved. Analog weight signal acquisition through the driver on the PCI-1713 operation of this system PCI-1713 analog input using multi-channel interrupt triggering mode, the use of FIFO and the use of user buffers. Sampling data is first stored sequentially in the first half of the FIFO and the second half of the cache, FIFO half-full to produce a medium call, and then this half-cache data to the user cache area for subsequent processing and to prevent the loss of data in the Windws multitasking operating system under the acquisition of a high number of data at the same time the new data to continue to fill the next half of the cache, high-speed real-time data acquisition can be realized. On this basis, the operating mode of the vibrating screen is further set. Let the control of the vibrating screen Li frequency converter can work in the software system under the control of high-speed and low-speed two modes, in the low-speed mode down low material flow 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 fluctuations in the power grid will also produce similar interference. To address this problem, in the data processing part of the data median value average filtering method, this method is the median value filtering method and arithmetic average filtering method of the synthesis. This method is a combination of median value filtering and arithmetic average filtering, i.e., firstly, the N data obtained in a sampling period are sorted according to their sizes, then the first m data and the last data are divided, and finally, the average value of the middle (N-Tn) data is calculated as the final sampling value. This method can remove the accidental pulse disturbance, and adjust the m value appropriately to 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 weight signal value. Running results found that this measure solves the problem of irregular impact and electromagnetic interference at the same time. The real-time material weight value displayed on the software interface in the process of discharging gradually and steadily grows. Even if the large-scale machinery start and stop caused by violent fluctuations in the power grid does not appear jump phenomenon.

2.3 Time lag processing method To solve the time lag problem. We have 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 under the material sieve fast under the material, the material to a larger flow into the weighing hopper, when the weight value of the material in the weighing hopper is greater than the target output minus the slow reserved impulse, then open the frequency converter for the low-speed state, vibration under the material sieve slow under the material. Material with a smaller flow into the weighing hopper: when the weight of the material in the weighing hopper is greater than the target output minus the stopping of the reserved impulse. Then the frequency converter is turned off, the vibrating discharging screen stops discharging, and the overshoot of the discharging screen will make up for the difference between the current material weight and the target output: this control flow 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 impulse can be set in the software interface according to the target weight and different material properties.

The actual debugging process we found that the 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, the vibration speed is too low is under the material time-consuming, too high is too fast under the material is difficult to control.

3 results and discussion

Automatic dosage system actual operation for half a year a total of more than 10,000 actual material data, No. 3 and No. 6 weighing due to weighing each time the target yield is small, and the material is often lumpy, time lag characteristics of the material under the precision control caused by a greater impact on the weighing error is greater. The rest of the weighers have reached the automatic weighing accuracy of 2.0 level. If the increase of overshooting retention device will greatly improve the system's controllability and weighing accuracy, but this will also make the weighing body mechanical part of the heavy for complex. In addition, if the target yield changes or the material properties change when the system is in use, it is necessary to manually increase or decrease the reserved impulse value appropriately. In the actual use of the process we found that inexperienced operators of the value of the setup sometimes improper, resulting in large weighing errors. Therefore, we consider adding the adaptive control of reserved impulse algorithm on the basis of manual intervention of reserved impulse to reduce the dependence of the appropriateness of the reserved impulse setting on the operator's experience.