Breakthrough of the hottest continuous measurement

The breakthrough of continuous measurement technology

the author of this paper, combined with years of work practice and extensive and in-depth research on dynamic weighing measurement at home and abroad, analyzes how to improve the measurement accuracy of continuous mixing equipment and obtains methods

I. current situation of measurement methods of continuous mixing equipment

1.1 the traditional continuous measurement methods

there are many methods in measuring or controlling ingredients of bulk materials such as building materials, grain and oil, mines, etc. Typical ones are: belt scale, punching plate flowmeter, nuclear scale and disc feeding scale. These measurement forms have their own characteristics, but they have great limitations, which are greatly affected by the mechanical changes of the equipment, the accuracy is not high, the installation and adjustment are cumbersome, and the maintenance is large

process introduction and process of belt weigher:

the belt weigher integrates the load signal received on the unit area (weighing section) and the change speed (belt speed) signal to obtain the flow value, which can be used as a controllable object

note: by controlling the speed of the drag belt, change the amount of material pulled out. After the material is shaped at the outlet of the feeding chute, its thickness is stable and uniform. The load on the belt is constant regardless of the speed of the belt conveyor. Compared with other feeding methods, this method has better measurement and control accuracy

note: the feeding and weighing functions are realized on two belts respectively

1.2 continuous measurement method is used on continuous mixing equipment

continuous mixing equipment includes: stabilized soil plant mixing equipment, cement continuous mixing equipment, asphalt continuous mixing equipment. In terms of measurement accuracy, at present, these devices cannot be compared with intermittent type. Therefore, continuous mixing method is not favored by the majority of users, which is also one of the reasons. Scientific analysis shows that the mixing process determined by these two measurement methods has its applicable occasions, and the application of continuous mixing cannot be affected due to temporary technical restrictions

at present, the continuous mixing equipment in China adopts the volumetric method or belt scale/screw scale considered by ISRI to measure. 7. The experimental machine adopts a high-speed test platform, and the continuous mixing process has been introduced from Europe in the 1980s. So far, there has been no breakthrough. In fact, these two measurement methods can achieve high accuracy in Europe. For example, Schenck's belt batching scale has a dynamic batching accuracy of 2%. But not in China, because it is restricted by China's basic industries such as machinery manufacturing and materials. At present, the measurement accuracy of belt weighers used in the highway industry in China is generally only about 5%, which is almost the same as the volume measurement, and the long-term stability is poor

II. Even waste foam granulators plus Enterprises above the manufacturing industry paid 1398.77 billion yuan for the main business of the continuous weighing revolution - the differential weight loss (weight loss) scale

weight loss scale (English loss in weight) was applied to the continuous weighing of industrial processes in the 1990s. Weight loss scale gradually replaces belt scale, screw scale, and even cumulative scale. As a new measurement method, it is gradually applied to more and more material processing

2.1 basic principle:

take the weighing bucket and feeding mechanism as the whole weighing body, continuously sample the weighing signal of the weighing body through the instrument or upper computer, calculate the change ratio of the weight in unit time as the instantaneous flow, and then process it through various software and hardware filtering techniques to obtain the "actual flow" that can be used as the control object

the acquisition of this flow is very important, which is the basis of whether the weight loss scale can measure accurately. What is shown in the figure is a classic method:

then FC performs the control operation of approaching the target flow through the PID feedback algorithm, and outputs the adjustment signal to control the feeder controller such as the frequency converter

2. 2 Application of differential weight loss scale (weight loss scale) in practice:

in principle, it is not affected by the mechanical changes of the scale body and the feeding mechanism. It only calculates the weight difference (differential weight). Compared with the traditional dynamic measurement means, its advantages are self-evident

when the control object is the flow (t/h, kg/min), and the material is transportable, and the measurement accuracy is high, the weight loss method can be used as a best scheme

2.2.2 technological process of weight loss scale:

2.2.3 matters that must be paid attention to in the design of weight loss scale and factors affecting accuracy:

weight loss scale has the characteristics of both static scale and dynamic scale. Therefore, when designing the system, it is required that:

1. The correct range of conveying rate is generally the best when the actual working range is 60% - 70% of the rated conveying capacity. If AC speed regulation is adopted, the strain frequency of Hz is the best. This ensures a wide adjustment range. Also, when the conveying rate is too low, the system stability is poor

2. The range of the sensor is appropriate. According to the formula

, that is, the sensor also uses 60% - 70% of its range, and the signal has a wide range of changes, which is very beneficial to improve the accuracy

3. The mechanical structure design should ensure good material fluidity and short replenishment time. Replenishment should not be too frequent. It is generally required to replenish once a minute

the supporting transmission system should ensure stable operation and good linearity

2.2.4 application prospect:

with the rapid development of electronic control technology, the weight loss scale adopts new technology, and the measurement accuracy is from 0.3% to 0.5%. The core of this new technology is the application of digital weighing sensors

2 the current equipment is being debugged. 2.4.1 application of digital weighing sensor:

in order to meet the needs of dynamic measurement, it is very important to use the sensor as the input of the system in the weighing system. Especially in the situation that requires intelligence, the direct or indirect digital security of the sensor is essential. At this time, the measurement uncertainty and measurement speed are often a pair of contradictions, and it is difficult to have both, but a compromise choice needs to be made according to the actual situation. In the field of weighing, traditional analog sensors are widely produced and applied in China, and the output of analog signals is small. Taking the load cell with the largest production and adopting the resistance strain principle as an example, the general maximum output is MV