Analysis of quantitative belt scale

[Abstract] Based on the principle of dynamic weighing, simply analyze the working characteristics of the batching belt scale, give the allowable response curve of the carrier and the expression of Laplace transform, and give the control diagram and system equation of the batching belt scale.

Keywords Batching belt scale Transient characteristics Loader response curve

Preface

Quantitative belt weigher is a weighing instrument often used in the process of industrial batching, mainly used for cement batching; China has been developing and producing since the early 1970s, and in 2016 established national standards for quantitative belt weighers for batching. The formulation of a standard for a product is a process of deepening the understanding of the product, which is of great benefit to the development of the product and the improvement of its quality. In this article, I want to talk about the cognition of quantitative belt scales.

The automatic quantitative belt scale can control the weight of the material conveyed by the belt in a predetermined unit time, and belongs to the dynamic scale of automatic weighing. However, many articles about automatic quantitative belt scales failed to describe the characteristics of its dynamic weighing. Most of them introduced the mechanical structure, materials and manufacturing characteristics of the quantitative belt scales of various manufacturers. Rarely introduce quantitative belt scales from the requirements of dynamic weighing. We are familiar with the heavy bag type quantitative packaging scale, which controls the predetermined weight through the amount of fine feeding so that the weight of the final material reaches the predetermined weight value. Quantitative belt scale is to use the process control method to adjust the amount of input and the speed of the belt during the whole weighing process, that is, adjust the flow rate of the material so that the final cumulative weight reaches the predetermined weight.

First, we analyze the physical process of conveying materials in a quantitative belt scale. The force problem of quantitative belt scale materials belongs to the category of variable mass mechanics. During the whole weighing process, the material to be weighed continuously flows in from the feed inlet, is transported by the belt and flows out from the outlet of the belt continuously.

According to the momentum theorem of variable mass mechanics, the expression of conveying materials on the belt can be written:

(1)where m 1 is the mass of the inflow belt, u 1 is the speed of the inflow mass relative to the belt, m 2 is the mass of the outflow belt, and u 2 is the speed of the outflow mass relative to the belt, which is equal to the conveying speed of the belt at this time. , Q = mv is the momentum of the conveyed material on the belt. The cumulative amount of materials sent remotely via the belt in time T is:

(2)Note that in the formula, F is the external force acting on the material, such as the tension of the belt, the shearing force of the shear gate, etc., not the weighing force of the load cell. The formula (1) is the expression of the momentum differential of the transported material, and the formula (2) is the expression of its integral. Usually, the speed u 2 of throwing out the material is also the conveying speed of the belt, that is, u 2 = v. The adjustment of the quantitative belt scale is usually by adjusting the quality of the input belt or adjusting the transmission speed of the belt so that the final accumulated total amount is equal to the preset amount. There are also belt scales that adjust the input quality and the belt transmission speed at the same time. However, this kind of quantitative belt weigher requires a pre-feeder, and the structure of the system is also more complicated.

(3)Here is a simple example to illustrate the control process and response expression of the feedback system. The regulating part can generally be a gate or a motor. For simplicity, use gate adjustment as an example.

(4)Figure 1 Simplified diagram of feedback control system

(5)Figure 2 is a block diagram of the feedback control system. x(P) and y(P) represent the Laplace transform of input and output variables x(t) and y(t). G(P) is the transfer function of the system, and H(P) is the transfer function (feedback function) of the regulation system. Feedback control process