The principle and common problems of multihead weigher in industrial production

Author: Smartweigh–Multihead Weighter

With the improvement of the continuous and precise metrological verification control regulations for raw materials, especially solid raw materials, in the whole process of industrial production, and in the 1990s, a new type of metrological verification equipment that can take into account the regulations was created.——multihead weigher (English Loss-in-weight). The multihead weigher is based on the change of the net weight of the raw material on the scale body to carry out continuous and accurate measurement and verification of the raw materials. The emergence of the multihead weigher gradually replaced the original electronic belt scale, spiral scale, and even the accumulation scale, as a new upgraded measurement The verification method is more and more widely used in the fields of metallurgy, mining, chemical plants, and chemical fiber energy. 1multihead weigher (as shown in Figure 1) Figure 1 for Mettler·Toledo's multihead weigher consists of a weighing platform (a sensor is fixed on the weighing platform base), a feeding variable frequency motor (which can additionally drive the transport screw and horizontal mixing), a feeding bin, vertical mixing, and electrical conductivity. It is composed of soft connection and multihead weigher control instrument (IND560CF).

To complete the continuous measurement and verification operation, the field must also be equipped with a large hopper, a fully automatic gate valve (the function is to continuously replenish the multihead weigher feeding silo), and the receiving equipment (the function is to continuously accept the multihead weigher feeding), etc. 2 Operation block diagram 3 Principle Weighing platform, feeding bin and all the machinery and equipment functioning on the weighing platform are used as the whole scale body, and the sensor continuously transmits the net weight change on the scale body to the multihead weigher control instrument (the control instrument is The key solution part of the multihead weigher, all the manipulation and solution functions are carried out by it), the control instrument calculates the net weight elasticity coefficient of the scale body per unit time as the specific instantaneous total flow according to the data signal, and then compares it with the set The overall target total flow is relatively developed. After the PID calculation is carried out, the current flow data signal of 4-50mA is output to change the output frequency of the soft starter of the feeding motor, and then the speed ratio of the motor is changed to make the specific feeding amount as close to the overall set as possible. The target total flow, in order to achieve the purpose of accurate feeding. In order to complete the continuous feeding of the multihead weigher and the accuracy of the measurement verification, the top of the feeding silo must be equipped with a large hopper that can continuously feed the material and a fully automatic gate valve to control the feeding.

In the control instrument, set up an upper limit value of replenishment (Refill_Stop) and a lower limit value of replenishment (Refill_Star). When the control instrument weighs the net weight on the scale to reach the lower limit value of replenishment, an open replenishment limit value will be sent out. The data signal of the gate valve makes the gate valve open, the raw material of the large hopper will be put into the feeding bin according to the conductive soft connection, and the net weight on the scale body will increase. When the limit value is reached, a data signal to close the gate valve will be sent out to make the gate valve close. In this whole process, the feeding motor has been in operation, in other words, the feeding is continuous. For these raw materials with poor circulation, relatively light and relatively thin, within a short time after the gate valve is closed, a part of the net weight will not be added to the scale body. At this time, if the multihead weigher is developed according to the data signal transmitted by the sensor If the PID control is successful, because the net weight change felt by the sensor will be reduced in this time range, which will cause the data signal to lose the frame and prohibit the operation, so the feeding time (Timer2) is also set up in the control instrument, which is To close the gate valve just started timing.

At the beginning of the replenishment, it is expected that the feeding time will end. During this period, the feeding motor will keep the frequency before feeding and will not change. In other words, the multihead weigher is at a fixed frequency during the whole process. Operation—Static data manipulation. When the feeding time is over, the multihead weigher automatically restores the real-time control, that is, controls the feeding motor according to the data signal transmitted by the sensor. The whole process of the operation of the multihead weigher is repeated in this way.

In order to ensure the linearity of the multihead weigher, in addition to these key main parameters mentioned above, there are also the following main parameters in the control instrument: SetP (proportional coefficient P value); SetI (integration time I value); SetD (differential time) Caltime (current total flow sampling time); Calcount (current total flow sampling frequency); Target-F (flow monitoring target); Limit-E (flow monitoring tolerance range); Hig_Weight (high material level value) ); Low_Weight (low material level value); Load-Max (frequency specified value); Load-Min (frequency minimum value); SampleFlux1 (dynamic calibration total flow value 1); SampleFlux2 (dynamic calibration total flow value 2) ; SampleFlux3 (dynamic calibration total flow value 3); WorkMode (work mode selection); BatchSelect (batch number (quantitative analysis) role selection); FluxFactor (total flow adjustment main parameters); ProportionFactor (raw material ratio adjustment main parameters). 4 The common problem when designing the multihead weigher is to improve the linearity of the multihead weigher. The following aspects should be considered when designing the solution: 1) Select a suitable application frequency, and it is best to maintain the application frequency at 35Hz~40Hz. When it is low, the reliability of the system software is poor; 2) The selection of the sensor measurement range is appropriate, and it is used in 60%~70% of the measurement range, and the data signal conversion range is wide, which is beneficial to improve the linearity; 3) The mechanical system design scheme must ensure raw materials Good circulation, in addition to ensure that the feeding time is short, and the feeding should not be too frequent. Generally, 5min~10min feeding is required; 4) The transmission device of the supporting facilities should ensure stable operation and good linear shape. 5Common problems in the whole process of installation and application of multihead weigher: In order to ensure the precision of multihead weigher, the following key points must be paid attention to in the whole process of installation and application: 1) The weighing platform must be fixed firmly, and the sensor is elastic Deformation components, external vibration will affect them. The application work experience tells that the most taboo of multihead weigher in the whole process of application is the vibration hazard of the natural environment; 2) There should be no cyclone fluidity in the natural environment, because it is To improve the precision of weighing, the selected sensor is very smart, so all movements will have an impact on the sensor; 3) The upper and lower conductive soft connections should be light and soft to prevent the lower and lower equipment from affecting the multihead weigher causes impact.

The most ideal raw material used at this stage is smooth, soft and silky; 4) The smaller the connection distance between the large hopper and the feeding silo, the better, especially for these materials with relatively strong adhesion, when the large hopper and the feeding The longer the connection spacing in the middle of the bin, the larger the raw material adhered to the wall thickness. When the raw material on the wall thickness adheres to a certain level, once it falls, it will have a very large impact on the multihead weigher; 5) Try to avoid contact with external materials. The purpose is to better reduce the harm of external interaction force to the scale body; 6) The feeding rate should be fast, so it must be ensured that the whole feeding process is Smoothness of opening. For raw materials with poor circulation, in order to avoid their railway bridges, the best solution is to add mechanical stirring to the large hopper. The biggest taboo is cyclone arch breaking, but the stirring cannot be operated all the time. The most ideal is the mixing and The whole feeding process is consistent, that is, the same as the feeding gate valve; 7) The setting of the lower limit value of the feeding material and the upper limit value of the feeding material should be suitable. The apparent density in the silo is basically the same. This can be obtained by carefully observing the frequency transition of the soft starter. When the apparent density of the raw materials in the silo is basically the same, most of the frequency transition of the soft starter is not large.

The lower limit value of feeding and the upper limit value of feeding are suitable to improve the linearity of the whole feeding process. As mentioned earlier, the multihead weigher is in static data operation during the feeding process. If the feeding can be maintained The frequency basis of the front, rear, left and right soft starters will not change, and the measurement accuracy of the whole feeding process is also largely guaranteed. In addition, under the condition of ensuring that the apparent density is basically the same, try to avoid the frequency of feeding, that is, try to add more material each time. These two are contradictory and should be taken into consideration.

This is also the basis for ensuring the precision of the whole feeding process; 8) The time setting of the feeding time should be appropriate. The guideline for setting is to ensure that all raw materials have already fallen on the scale body, and the less the setting time, the better it is good. It has already been said that the multihead weigher is in static data manipulation during the feeding time, so the less time the better. This time can also be obtained by careful observation. In the adjustment stage, the time can be set longer first, and observe how long the total weight on the scale can not fluctuate (not easy to increase) after each feeding is completed. tends to stabilize (the total weight on the scale body decreases steadily).

Then this time is the right time to feed the ingredients. 6 Results The paper introduces the principle of multihead weigher in detail and some matters that should be paid attention to in the whole process of design scheme and application, especially these key points in the whole process of application, which is a precious experience sharing, and I look forward to sharing it with everyone. With assistance, the multihead weigher can be applied more strongly. Only by paying attention to this key point problem can the linearity of the multihead weigher be ensured, so that products that meet the standards can be produced.

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