At present, we have encountered some quality problems of electronic truck scales. Although the direct phenomenon is the problem of the load cell, the reason is the improper selection of the weighing sensor. The popularization of electronic weighing instruments in my country has been more than 20 years. The selection of weighing sensors seems to be some very common common sense, and these common senses affect the overall performance of the weighing instruments. For this reason, based on my personal experience in the use of weighing sensors in the past years, I have written many related articles such as "Comparison of weighing sensor accuracy", "Understanding of the protection requirements of weighing sensors", "How to use weighing sensors correctly", "Precautions for the use of digital weighing sensors", and "My opinion on digital electronic weighing instruments". This time, combined with the feedback from the trainees in recent years, I will organize and collect some knowledge that is prone to problems and often occurs, as well as some new personal experiences, for your reference.
1. Influence of thermal expansion and contraction
For the previous case where the length of the load cell of the truck scale was 12~14m, the thermal expansion and contraction of the load cell was not a prominent problem, but when the length of the load cell reached more than 18m, it became a problem that cannot be ignored. Although it has an impact on the design of the load-bearing device, it has a greater impact on how to choose the weighing sensor. We know that due to the structural stress characteristics of the weighing sensor, the linear expansion coefficient of the metal is determined by: the temperature range is 20-100℃ (if the actual use range is 60℃), carbon steel is 10.6-12.2/10-6℃-1 (11.4/10-6℃-1 is selected). For a 18m long truck scale, the change should be:
18000mm×60×11.4/10-6=12.312mm.
This will cause two problems: one is the spacing between the load-bearing device and the two end channels; the other is the anti-eccentric load performance of the weighing sensor.
1. Spacing
This spacing is not the load-bearing device spacing gap. The load-bearing device spacing gap is adjustable, and the spacing mentioned here is not adjustable and is fixed at the time of manufacturing. Therefore, it is necessary to consider the possible temperature range at the site of use and the possible change in the length of the load-bearing device caused by thermal expansion and contraction when manufacturing the foundation. The total size of the distance between the two ends must be greater than the deformation caused by thermal expansion and contraction.
2. Anti-eccentric load performance of weighing sensor
The anti-eccentric load performance of resistance strain weighing sensor is currently arranged in the following order:
Column → Cylindrical → Butterfly → Parallel beam → Cantilever beam → Bridge → Spoke (Of course, for columnar double diaphragm weighing sensor, it is better than ordinary single diaphragm structure; double spherical structure is better than single spherical structure).
It must be noted here that the above is only distinguished from the appearance. The arrangement order from the structural principle is:
Columnar normal stress → Beam normal stress → Single shear stress → Double shear stress.
From the above anti-eccentric load performance arrangement order, it can be clearly seen that when using columnar structure weighing sensor, attention should be paid to the problem of thermal expansion and contraction. For example, what is the specified eccentric load angle of the selected product? Can it meet the requirements of weighing performance?
Some manufacturers introduce in their product samples that the performance range of the column weighing sensors they produce is within 3° and can guarantee the measurement performance. This is a manifestation of a responsible enterprise. However, I don’t know how they tested it, because according to my superficial knowledge, it is very dangerous to test the weighing sensor on a workbench with a certain slope.
2. Maximum weighing impact
1. Sensitivity impact problem
(1) Among the commonly used weighing sensors, the sensitivity is usually 2mV/V. The sensitivity of the double shear beam (bridge) structure, single shear beam (cantilever) structure, and parallel beam structure is usually 2mV/V, and the sensitivity of the column structure is usually 1mV/V. There is also a small number of cantilever beam structures and S-type structures with a sensitivity of 3mV/V.
Dynamic electronic rail scales have a large impact load during weighing, so they use column-type weighing sensors with a sensitivity of 1mV/V and a maximum weighing capacity of 20 tons. For general static weighing scales, such as truck scales and platform scales, the impact load is relatively small, so weighing sensors with a sensitivity of 2mV/V are used. Some scales that use a lever system, or some hopper scales that are not easily affected by eccentric loads, and weighing liquid materials, can use weighing sensors with a sensitivity of 3mV/V.
(2) Some friends may say that in R76-1 International Recommendation for Non-Automatic Weighing Instruments (2006 Edition), when checking the compatibility of non-automatic weighing instrument modules, only a correction factor Q is given for weighing instruments of different structures, and the "sensitivity" of the weighing sensor is not considered at all. Are you being mysterious here?
In fact, if two weighing sensors with the same maximum weighing capacity have a sensitivity of 1mV/V and a sensitivity of 2mV/V, their usage is completely different. Because relatively speaking, the actual load capacity of the two is about half the difference. In other words, if the sensitivity of the two is the same, the load capacity of a weighing sensor with a sensitivity of 1mV/V is twice that of a weighing sensor with a sensitivity of 2mV/V.