Study On The Methods For Measuring Mass Flow Rate Of Solid Granular Materials

The testing and measuring technology of flow rate, which is a very important way to develop agriculture and industry, save energy, improve product quality, increase economic benefits and advance management level, finds wide application in many fields of national economy. This technology is closely interrelated to people's daily life and plays a very important role in the national economy. Being compared to the testing and measuring technology of fluid flow rate, the technology for measuring solid granular materials is less-developed and less-advanced. Since the later of 1990's, the typical methods for measuring mass flow rate of solid granular materials were widely researched and applied. There are some problems to apply those methods, such as relatively rigorous conditions for application, relatively high product cost, and difficult evaluation for real-time measurement accuracy. With the development of agriculture and industry, the growing demands are exerted on the technology for measuring mass flow rate of solid granular materials. Especially in recent years, the new demands and challenges were put forward to this technology because of great deal of demands on yield monitor sensors being applied in precision agriculture. So launching the research in this field is of great realistic meaning and strategic significance.At first, the summary and analysis on the state-on-the-art of four mainly-used methods at present for measuring mass flow rate of solid granular materials were conducted in this paper. Those methods are loss-in-weight method, belt scale method, Coriolis method and impact-based method. Then, the measuring properties and exiting problems of the latter three methods were theoretically analyzed. Corresponding conclusions can be obtained as follows based on the theoretical analysis. The impact-based method demands all granular materials the uniform velocity when they impact the measuring board. In fact, the stable velocity is needed to grantee the high accuracy. When the measurand or measuring range changes, the relationship between the input and output signal of the sensor changes too. The appropriate proportional coefficient between the input and the output signal of the sensor cannot be determined before numerous calibration. The mass flow rate measured by belt scales is the average value along its effective measurement length, the measuring errors are related to the uniformity of mass distribution above the belt. High measuring accuracy can be obtained with Coriolis mass flow-meters, which is unrelated to the movement state of the granular material staying the flow-meters. But the Coriolis needs a high rotary speed during the measuring process, which results in high operation requirements to the device. According to the results from theoretical analysis on belt scales and Coriolis mass flowmeters, the amending formula were put forward respectively for these tow kinds of methods, which is helpful to improve the measuring accuracy of mass flow rate. In general, these three methods has a common characteristic:it is necessary to maintain consistent speed of granular materials, so an additional control on the equipment must be exerted and the complexity of the device increases.Aiming at some problems of commonly-used methods, two new types of methods were put forward to measure the mass flow rate of solid granular materials, one is the impact-based method with which the velocity of granular materials can be measured, another one is the symbolic method based on the measurement of gross mass of granular materials. The velocity and mass flow rate all can be calculated applying these two kinds of methods according to the real-timely measured data from the weighing sensors. An error evaluation method was also presented to assess the real-time measurement accuracy of impact-base method since lack of efficient methods to evaluate the real-time measurement accuracy at present. The designed experimental device can accomplish the replication experiments for both the impact-based method and the symbolic method at the same time. Simulation experiments and practical replication experiments for each type of methods were conducted to analyze the factors affecting the measuring accuracy as well as the theoretical analysis.The theoretical analysis of the impact-based method shows that the main factor affecting the measuring accuracy of the impact-based method is the width of measuring board, and the narrower the measuring board is, the higher accuracy can be obtained. Theoretically, the measuring accuracy of mass flow rate is always higher than that of velocity.Three different width of measuring boards,1.5cm,3cm and 6cm, were respectively employed in the simulation experiments adopting impact-based method. The results indicated that the average errors, the standard deviation and the maximum errors of velocity increased when more wider measuring boards were employed. The similar results were observed in mass flow rate. And the average errors, the standard deviation and the maximum errors of velocity were higher than those of mass flow rate under the condition with same width of measuring boards. This leads to the same conclusion as the theoretical analysis:the narrower width of the measuring boards means the higher measuring accuracy. But the measuring accuracy of the gross mass has not significant difference when different width of measuring boards are employed in experiments.The measuring boards with 3cm and 6cm were respectively employed in the practical experiments adopting impact-based method with 40kg soybean as the experimental material. During experiments, the stable granular material flow can be obtained when the discharging outlet was opened within its 1/3～2/3 range because no extra equipments were designed to promote the fluency of the granular material flow. If the discharging outlet was opened bigger than its 2/3 range, the jam occurred, however, if the discharging outlet was opened smaller than its 1/3 range, serious splashing material appeared which significantly affected the measuring accuracy. The results of the experiments with 1/3～2/3 range of discharging outlet being opened showed that the measuring accuracy of gross mass was 3.18% and 6.19% respectively corresponding to 3cm and 6cm wide measuring boards. The measured data of gross mass in the experiments with 6cm wide measuring boards had more fluctuation than those did with 3cm measuring boards. However, the averaged gross mass were respectively 39.79kg and 39.87kg with 3cm and 6cm wide measuring boards. This result indicates that random noises are easily drawn into in the experiments with wider measuring boards. During the error estimation under the 95% confidence level, the maximum relative error of velocity in the experiments with 3cm wide measuring boards was 9.69%, the average value was 5.03%, the corresponding values of the mass flow rate were respectively 4.00% and 1.44%. This indicates that the real-time measuring accuracy of mass flow rate is more higher than that of velocity. This error estimation also gives results: there is cause and effect relationship among the measuring accuracy of velocity, mass flow rate and gross mass. The smaller relative errors of velocity or mass flow rate results in higher accuracy of gross mass. Because the accuracy of gross mass obtained from the experiments with 3cm wide measuring boards is more higher than those from the experiments with 6cm wide measuring boards, it can be deduced that the measuring accuracy of velocity or mass flow rate is also higher during the experiments with 3cm wide measuring boards than those during the experiments with 6cm measuring boards. On other hand, according to the results of spectral decomposition, the signal output from the weighing sensors is a kind of low frequency one which can be efficiently extracted through low-pass filters.Similar to the simulation experiments using impact-based method, three kinds of measuring boards with different width, which are respectively 1.5cm,3cm and 6cm, were also employed in the simulation experiments adopting symbolic method. The simulation experiments showed that the gross mass had high accuracy, the average relative error of gross mass were respectively 0.19%,2.35% and 2.48% corresponding to 1.5cm,3cm and 6cm wide measuring boards. The calculated velocity and mass flow rate during the simulation has the same tendency as the true value of velocity and mass flow rate, but there were some bigger fluctuation around the true value. After being weighted, those calculated data embodied goodness of fit with the true data. More points were employed in weighing process, more smooth value of velocity and mass flow rate can be obtained, but this resulted in less instantaneousness. The general conclusion from the simulation was that the narrower measuring boards can obtain higher measuring accuracy.The 3 cm wide measuring boards were adopted during the practical experiments using symbolic method, the average relative error of gross mass was 4.83%, the maximum was 7.97%. Being compared with the corresponding data from the impact-based method, the measured data of velocity and mass flow rate with symbolic method exited big fluctuation though there were similar tendency as the data from impact-based method. In general, the symbolic method is available as a method to measure the gross mass, the measuring accuracy of velocity and mass flow rate are easily affected by the splashing of granular material during measurement, mutual movement among granular material and the difference of sensors in static or dynamic response properties and so on, and the symbolic method exerts more higher claim on hardware than the impact-based method does.According to the conclusions both from the theoretical analysis and practical experiments, the impact-based method embodies excellent performance in measuring the velocity and mass flow rate of solid granular materials and possesses practical application value. However, the symbolic method needs further study and improvement before application in practice. Finally, the shortcomings and the future direction of improvement are also pointed out.