Analysis of the number of graduations for the verification of electronic truck scale

[Abstract] In recent years, according to the feedback from some weighing instrument manufacturers that use our loadcell, the verification divisions of various electronic weighing instruments have been continuously improved, and some have far exceeded the maximum verification divisions of the loadcells. Is this approach Can improve the precision and accuracy of the scale? the answer is negative. To clarify the confusion in understanding and solve the confusion in practice, this article combines OIML R60 and R76 to make an analysis of the problem of determining the division value of commonly used truck scales.

[Keywords] OIML International Recommendations Electronic Truck Scale Maximum Verification Index

Foreword

With the continuous development of electronic weighing instrument industry, electronic weighing instruments are used more and more widely in the market. Among them, various weighing scales and truck scales are used more and more, and the requirements for weighing instruments are getting higher and higher. The resolution of the scale is constantly improving, and the number of divisions is also increasing. For some weighing instrument manufacturers, they usually tell users how small the minimum division value of the scale we make and how large is the verification division number.

The scale has a large number of verification divisions, but the precision and accuracy of the scale are not necessarily very high. This article makes an analysis on the division value of the currently commonly used truck scale.

I. Overview 

In the past year, we have often received complaints from users, such as large zero drift of the scale, large full scale drift of the scale, and unstable display value. In addition, many phenomena are normal after replacing one or more loadcells, and the replaced loadcells are checked for various indicators after returning to the factory, and no abnormality is found. For this phenomenon, we have compared the relevant standards and carried out theoretical analysis. Calculation and analysis, from the perspective of OIML R76, analyze the cause of the problem, so that the weighing instrument manufacturer can use it as a reference. At present, the loadcells used in domestic platform scales or truck scales are generally C3 sensors. According to the requirements of OIML R60 international recommendations, the maximum number of verification divisions nmax of C3 loadcells is 3000, according to OIML R76 international recommendation 4.4.4 error distribution principle , C3 class loadcell is used to manufacture class III scales with verification division not more than 3000 divisions. my country's JJG555-1996 "General Verification Regulations for Non-Automatic Scales" equivalently adopts the OIML R76 international recommendation, in which the following provisions are made for the loadcell:

1. The maximum weighing capacity of the load cell should meet the following requirements:

Emax＞Q·Max·R/N

In the formula: Emax——the maximum weighing of the load cell;

N——Number of sensors;

R——reduction ratio; taking into account the most stringent requirements on the sensor, take R=1;

Q-correction factor. Generally Q>1.

2. The maximum number of divisions of the load cell The maximum number of divisions nLC of the loadcell should not be less than the scale's verification division number n.

nLC≥n

The minimum static load output recovery value DR of the load cell should meet the following conditions:

DR≤0.5eR/N

Take R=1, then DR≤0.5e/N

When the DR is unknown, the condition should be met: nLC≥Max/e

3. Minimum verification scale value of load cell

Vmin≤e·R/ N

Take R=1, then Vmin≤e·/ N

In actual use, many manufacturers have ignored nLC≥n, and this is the main problem. The scale's verification division number exceeds the loadcell's maximum verification division number, which fundamentally violates the metrology of scale sensors. The principle of error distribution in requirements and measurement. Theoretically speaking, the system is unstable. In fact, the various components of the system, such as loadcel and displays, are not supported by experimental data. Some typical cases in actual use are analyzed and explained below.

Two, application examples

According to the current use of weighing instruments in the market, when multiple loadcells  are used for weighing, most manufacturers set the scale of the scale at 3000 to 6000. The scale has fewer quality problems during use, and the scale of the scale is set The greater the value, the greater the frequency of problems. Examples of failures in a truck scale are as follows:

① A weighing instrument with a maximum weighing of 15t and a minimum scale of 1kg, using 4 10t loadcells. After installation, the zero point drift occurred, and the display number increased in one direction. Later, the problem was solved by replacing two loadcells.

The scale's verification division number n = 15000, the loadcell's maximum division number nLC = 3000

Vmin≤e·/ N =1kg / 4 = 0.5kg

Calculate according to Vmin as 0.5kg, corresponding to y=Emax/ Vmin=20000

DR≤0.5e/N＝0.5×1kg /4 = 0.125kg

Z = Emax/2DR = 10000/0.25 = 40000

② Weighing instrument with a maximum weighing of 60t and a minimum scale of 5kg, using 6 30t loadcells. After the scale was installed, the zero point of the meter was unstable, which showed irregular digital jumps. After removing one loadcell, it was still abnormal. Later, after replacing all the 6 loadcels, it was normal. The scale's verification division number n = 12000, the loadcell's maximum division number nLC = 3000

Vmin≤e·/ N ＝5kg / 6 = 2.041kg

Calculate according to Vmin as 2kg, corresponding to y=Emax/ Vmin=15000

DR≤0.5e/N＝0.5×5kg /6 = 0.4167kg

Z = Emax/2DR = 30000/0.8333 = 36000

③ Weighing instrument with a maximum weighing of 80t and a minimum scale of 10kg, using 8 40t loadcells. The meter displays irregular hops in the negative direction after power-on, and the displayed value keeps decreasing. It is normal after replacing 4 loadcells.

The scale's verification division number n = 8000, the loadcell's maximum division number nLC = 3000

Vmin≤e·/ N ＝10kg / 8 = 3.5355kg If Vmin is 3.5kg, it corresponds to y＝Emax/ Vmin=11428

DR≤0.5e/N=0.5×10kg /8 = 0.625kg Z = Emax/2DR = 30000/1.25 = 24000

④ A weighing instrument with a maximum weighing of 120t and a minimum scale of 20kg, using 10 40t loadcells. The scale body does not return to zero after 60t calibration, and it is normal after replacing two sensors.

The scale's verification division number n = 6000, the sensor's maximum division number nLC = 3000

Vmin≤e·/ N ＝20kg / 10 = 6.325kg

Calculate according to Vmin as 6kg, corresponding to y=Emax/ Vmin=6667

DR≤0.5e/N＝0.5×20kg /10 = 1kg

Z = Emax/2DR = 40000/2 = 20000

Three, cause analysis

For the composition of the above scale body, the minimum verification division value of all weighing platforms has exceeded 3000 divisions, and the quality of these weighing instruments has different quality problems, such as zero drift, full scale drift, unstable display, etc. phenomenon. What are the reasons for these problems?

First of all, the scale of the weighing instrument is too high, exceeding the maximum verification scale of the loadcell, or even 2 to 5 times the scale of the loadcell. According to the regulations in JJG-555-1996, nLC≥n, but these weighing platforms cannot meet the requirements. The requirements of this article.

Secondly, because the sensor y value and z value cannot meet the requirements of use, according to the requirements of OIML R60, the y value of the C3 loadcell should not be less than 7200, and the z value should not be less than 3000. Assuming that the ZTC index is 0.015%FS/10℃ Control, the corresponding y value can only reach 9300, and for the above problems, the y value is mostly greater than 10000. For this use case, if the effects of the same batch of loadcell on the zero temperature are more consistent, they will all change in the same direction. , There will be a phenomenon that the zero drift of each loadcell is superimposed. The occasional stability of the weighing platform can only indicate that the direction of the zero temperature drift of the loadcell selected for this scale body is inconsistent, and there is a phenomenon of mutual cancellation, so the zero point of the weighing platform is stable, but this is just a coincidence. Some weighing platforms appear to be stable after replacing one or more loadcells, but this is just an accidental coincidence. Some weighing platforms may be able to meet the coincidence phenomenon by replacing one loadcell.

Sometimes the stability of the displayed value can only be ensured after replacing a few loadcells. In the above example, the two phenomena that the y value is equal to 20000 and 15000 are caused by the failure of the typical y value to meet the requirements. Of course, for the loadcells used in truck scales, since the strain gauges used are symmetrical in the sensor manufacturing process, the zero point self-compensation ratio in the loadcell is relatively large, the y value is relatively large, and there are more zero points that meet the requirements. However, other properties may not be guaranteed.

For the requirements of the z value, if the loadcells manufacturer’s process control has a good creep consistency of the loadcell, the creep of each loadcell after the weighing will also be a superimposed process, and the instrument will be unstable and loaded. The phenomenon of not returning to zero afterwards. An accidental situation is that the creep direction of each loadcell  is different, and there is a phenomenon of mutual cancellation, and the creep and return to zero display normally. But this is an unstable state. Under different test conditions, it is difficult to obtain consistent results, or the influence of external factors will cause the instability and drift of the indicated value.

The above analysis of the loadcell y value and z value does not consider the influence of temperature changes on the instrument. For external temperature changes, it is clear that morning and evening, summer and winter will definitely affect the display value of the high-resolution instrument, even Even the outside air blowing will cause the display value to change. Among the domestically-produced weighing instruments currently on the market, there is no high-resolution instrument model that has passed OIML certification (the third-level 3000-division meters are rarely certified by OIML). In fact, there is no domestic high-resolution scale that supports high-resolution scales. Market for sensors and instrument products.

For a scale composed of multiple loadcells, considering the overload capacity, the total range of a general scale is only 1/5~1/2 of the total range of the loadcell. The maximum number of calibration divisions of the loadcell has actually been reduced proportionally. For a scale composed of a single loadcell, the rated load of the scale is only a part of the rated range of the loadcell  for example, 1/2 range. The sensor has been required to be greater than 3000 divisions, and various signal noises are relatively unchanged. With time drift, the sensitivity of each loadcell to external influences increases, and the loadcell is more affected by external interference signals and environmental factors.

Concluding remarks

Through the above analysis, for users who use C3 loadcell group scales, the number of verification divisions used should not exceed 3000. Excessive divisions will cause the scale to be unstable and unreliable. Some weighing instrument manufacturers unilaterally believe that as long as there are Once an accidental calibration is made with high accuracy, it is considered that the scale can be grouped according to the target of high accuracy, but it is ignored that this phenomenon is only an accidental coincidence. The unstable and unreliable scale formed in this way is inevitable. of. The true evaluation criteria should be based on reliability, considering the actual use of the scale, and in accordance with the relevant regulations of national standards and international recommendations. We strongly call on all weighing instrument manufacturers, loadcell and instrument manufacturers, all primary measurement supervision and law enforcement agencies, and users to jointly abide by the relevant national industry standards and the requirements of measurement regulations; honesty, credibility, production, measurement and appraisal according to the law, and provide the society with accurate and Reliable metering products. Make due contributions to the modernization of the motherland.