Two points analysis on the application of the circ

2022-08-07
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Two point analysis on the application of the belt weigher's cyclic chain code inspection device Abstract: This paper tries to analyze and elaborate on two issues related to the application of the belt weigher's cyclic chain code inspection device: the first issue is to analyze and elaborate the difference between the metrological characteristics of the cyclic chain code and the ordinary chain code, and to explain why the cyclic chain code has the inspection repeatability precision that the ordinary chain code cannot achieve; The second topic is to analyze the current cyclic chain code device with drive, and think that it is a harmful, useless and undesirable design

key words: belt weigher; Cycling chain weights; Simulation load test

the belt scale cyclic chain code inspection device is a new type of belt scale simulation load test device, which passed the national type approval in 1997. The device has the advantages of convenient installation and use, accurate inspection, high degree of advanced technology such as thermosetting resin injection molding process, low cost and use cost, and creates a new way for the inspection of belt weigher. This verification method has been incorporated into the national metrological verification regulation JJG "continuous accumulative automatic weighing instrument (belt scale)". In recent years, the application of belt weigher's cyclic chain code inspection device is becoming more and more extensive. In this paper, two issues concerning the application of the belt scale's cyclic chain code inspection device are analyzed and expounded

I. difference in metrological characteristics between cyclic chain codes and common chain codes

1 Obvious data contrast

a large number of tests have proved that although the cyclic chain code and the ordinary chain code are both simulated load test devices, the inspection repeatability of the cyclic chain code device is significantly better than the ordinary chain code (also known as plane chain code or rolling chain code). For example, in December2000, The verification method comparison group of the national quality and Metrology Technical Committee, headed by the Chinese Academy of metrology, conducted a comparison test on an electronic belt scale (made by East China electronic instrument factory, with a belt speed of 3.29m/s, a maximum flow of 2880t/h, a level 0.5, four idlers and double lever scale frame) with a circulating chain code device (made by Beijing Chunhai company) and a physical verification device and an ordinary chain code on the No. 3 belt conveyor of Jiangsu Changshu Power Plant, The repeatability of the indicated value of the belt scale detected by the circulating chain code device is 0.11%, while the repeatability of the indicated value of the belt scale detected by the common chain code is 1.01% [1]. At the same site, the thermal metering and testing center of the former Ministry of electric power used the cyclic chain code device of Chunhai company to track and inspect the belt scale for three months from February 28 to May 26, 2000. "In three months, the maximum variation when simulating the physical detection device (referring to the cyclic chain code device - the author's note) to detect the belt scale is -0.085%, not more than 0.1%." [2]

of course, the above repeatability error also includes the error of the belt scale itself. The data with much better repeatability can be detected by using the cyclic chain code, which shows that the uncertainty brought by the detection method itself has obviously exceeded the error of the belt scale itself when using the ordinary chain code for verification

2. Why common chain code has poor repeatability

interview to analyze the source of uncertainty in the detection value of common chain code and how circular chain code can effectively overcome these uncertain and stable factors

there are many reasons for the indication error of the belt scale, but the error brought by the chain code detection is basically only related to the weighing transmission error. Now take the belt scale with single weighing roll and single weighing frame as an example to see the transmission relationship between the belt material mass and the force received by the weighing sensor (see Figure 1)

Figure 1 belt scale gravity transfer

when the material on the belt passes through the belt scale, the weight of the material is pressed on the scale frame through the weighing roller, and the scale frame takes the trunnion as the fulcrum to transfer the load to the sensor. When the belt is set to move the animal material to a certain position, the force F caused by the material mass on the sensor can be expressed by the following formula:

where: Q (x) is the linear density of the material at the coordinate X (where x is set to be along the belt direction and x=0 at the weighing roller), kg; That is, the material quality within the DX range here is Q (x) DX

δ Is the inclination angle of the belt conveyor, (°)

c1 is the load distribution coefficient, indicating that the load distribution of the material with a mass of Q (x) DX at x acting on the weighing roll is CQ (x) DX

c2 is the leverage ratio coefficient, where c2=l1/l2

from a to B refers to an interval of materials at both ends of the belt scale along the belt direction, and the load of the symmetrical roll of materials in this interval has an impact

in case of belt scale with multiple weighing rollers and double weighing frames (except suspension belt scale), the transfer relationship of weighing gravity is relatively complex, but the principle is the same

during material operation, f changes with Q (x), so the measured output value of the weighing sensor corresponds to the weight of the material on the belt scale. When it is integrated with the belt speed output by the speed sensor, the cumulative quantity of materials can be obtained

why does the additional uncertainty factor obviously increase in the force transmission when the belt scale is tested by simulating the physical material with the ordinary chain code

1) the common chain code is easy to excite random vibration during detection, which interferes with the working output of the load cell

common chain code detection device is shown in Figure 2

Figure 2 Schematic diagram of common chain code

when the belt advances, the chain code rolls on the belt. Both ends of the chain code are fixed on the belt frame with ropes and have a certain pre tension. As shown in the figure, the pretension

f ≥ WLG sin δ (N)

where: W is the total mass of chain code, kg

chain codes form a mass spring system. When there is external interference, vibration will occur. The possible external interference factors are: the runout caused by the non concentricity of the chain code, the runout caused by the non concentricity of the idler, the runout in the belt movement and the vibration of the frame, etc. Due to the diversity of interference factors, the actual vibration is an irregular random vibration composed of different frequencies, phases and directions. The inertial force generated by vibration, especially along the Y direction, will seriously interfere with the working output of the sensor

2) the position effect of chain code load leads to the uncertainty of weighing gravity transfer

it can be seen from equation 1 that, except for uniformly distributed load, if objects with the same mass are at different positions (x coordinates) of the belt, the output of the sensor may be different. This characteristic is characterized by the coefficient C in the equation. When the material runs with the belt, each component of the material will pass through the weighing area of the belt scale in turn, that is, it will be affected by the whole process of C. Therefore, there is a relatively fixed relationship between the quantity transfer from the material mass to the output of the weighing sensor and then to the accumulated value of the belt scale instrument. However, the ordinary chain code is different: it does not run with the belt, and it is roller shaped. The concentrated load at equal intervals is applied to the belt along the X direction, rather than uniformly distributed load. When its position changes relative to the weighing roll of the belt scale, the influence of C will appear

Fig. 3 position effect of common chain code

it can be seen from Fig. 3 that when the position of chain code roll has a displacement of △ x, the load shared by each roll on the weighing roll will change, resulting in changes in sensor output. Set a safe distance. That is to say, the transmission of mass value is uncertain. We call it "location effect" here. If there are multiple weighing rollers or double weighing frames, the position effect can be compensated to some extent, but it is impossible to completely eliminate this effect. It can be seen from the data provided in the document [1] that when the same common chain code is used to test the same belt scale, the difference between the arithmetic mean values of the two groups of indicated values reaches 0.92% of the calculated value. It can be seen that the position effect is serious. The reasons for △ x (i.e. position uncertainty) of common chain code in inspection are:

(1) the vibration of chain code along the X direction causes the chain code roller to oscillate back and forth at their respective balance positions

(2) the serpentine movement of the chain code causes the position change of the chain code roller

(3) the uncertainty of chain code installation status or the change of chain code working position due to dynamic factors during operation

3. What is the difference between the cyclic chain code device (see Figure 4)

Figure 4 Schematic diagram of cyclic chain code

1) the cyclic chain code device will not form a mass spring vibration system like the ordinary chain code. The reason for the operation of the cyclic chain code is that the code block on the belt is driven forward by the belt under the action of friction between the code block and the belt; The circular chain code falls on the belt naturally, and the whole chain code is in the state of flexible chain cable. Generally, the chain code will not have elastic displacement and elastic restoring force that cause vibration. Therefore, when there are external interference factors, the chain code itself will not have mechanical vibration, and the inertial interference force that may be caused by the common chain code will not appear

2) the cyclic chain code is designed as a cyclic chain with uniform mass distribution. The chain code falling on the belt exerts a uniform load along the X direction on the belt. It can be seen from equation 1 that at this time, q (x) is a constant Q, so no matter which segment of chain code is above the belt scale in operation, the load shared by the weighing roller is always a constant value:, so the force F on the sensor is also a constant value:

this shows that the transmission relationship between the chain code mass and the sensor load is determined, and there is no change in the sensor output caused by the change of the concentrated load point of the common chain code

from the above two points, it can be seen that the cyclic chain code has the function of user-defined experiment more than the ordinary chain code; It is close to the working state of bulk materials, which obviously avoids and reduces the uncertainty brought by the simulation test device itself in the inspection. Because the cyclic chain code device has high repeatability, it has laid an essential foundation for improving its inspection accuracy

II. Problems existing in the verification of belt driven circulating chain code device

recently, it has been seen from relevant data that a belt driven circulating chain code device has emerged, that is, the circulating chain code operates under the drive of a driving device, and the driving device forms a closed loop with the speed measuring sensor of the belt conveyor, in order to keep the circulating chain code synchronized with the belt speed. I think this method has the following problems, and the analysis is as follows:

1 When the circulating chain code is equipped with a driving device, it will cause the impermanent change of the belt tension, increase the uncertainty of the test indication, and thus reduce the credibility of the test results

when the cyclic chain code is equipped with a driving device, the chain code is under two kinds of driving at the same time: the driving device of the cyclic chain code itself and the belt driving:

p1+p2= r

where: P1 is the driving force of the driving device of the cyclic chain code itself

p2 is the belt driving force, and its value is the tension increment at the belt position (as shown in "a" in Figure 4) at the front end of the chain code on the corresponding belt before and after the circular chain code is put down. Here, it is expressed as:

r is the total resistance when the circular chain code operates smoothly

under normal design, either the driving device of the circulating chain code itself or the driving device of the belt conveyor has the potential to drive the chain code independently for stable operation. The circulating chain code operates under the drive of the driving device. If the linear speed of the chain code is slightly greater than the linear speed of the belt, the friction between the chain code and the belt is toward the direction of belt advance, that is, the driving device of the circulating chain code not only drives the chain code, but also participates in the drive of the belt conveyor, that is, P2 (i.e. △ T) may have a negative value. On the contrary, if the linear speed of the chain code is slightly less than the linear speed of the belt under the driving device of the circulating chain code, the friction force of the chain code on the belt faces the direction that hinders the advance of the belt, that is, the belt conveyor not only drives the chain code, but also acts the force (torque) on the driving wheel of the driving device of the circulating chain code through the chain code; That is, P2 (i.e. △ T) may exceed the R value. Especially when there is speed

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