Regarding Digital Load Cells
At the end of the last century, digital load cells were all the rage. Almost all major foreign manufacturers launched digital load cells, especially column load cells, which could house integrated circuits within a housing and directly output digital signals, forming a complete digital load cell. However, most strain gauge load cells could not combine integrated circuits and strain gauges to form a complete digital load cell. Therefore, many manufacturers launched so-called "digital sensors" that integrated A/D conversion and control circuits into "channels" and connected to strain gauge load cells.
At the beginning of this century, digital load cells were widely used both domestically and internationally, and many articles were published introducing their advantages and application examples. I have also written many articles about digital load cells. I published three articles consecutively in the American journal "Weighing & Measurement," among which "Digital Load Cells Gain Acceptance" is a relatively comprehensive introduction to digital sensor technology and its applications.
The main advantages of strain gauge digital load cells are:
(1) Improved anti-interference capability of signal output, enhanced reliability of signal transmission, and overcoming the influence of temperature on long-distance transmission.
(2) Each sensor has its own independent communication address, facilitating the query of each sensitive element and enabling correction, diagnosis, and compensation of each individual signal. Early analog weighing instruments essentially used all sensors in parallel, thus providing only a comprehensive signal. In contrast, each sensor in a digital load cell can provide its own signal, greatly increasing the amount of information and facilitating the processing of weighing signals and the establishment of mathematical models.
(3) Each load cell can establish a unified and accurate nominal sensitivity value, greatly simplifying the installation, debugging, and replacement of load cells.
(4) Enables weightless calibration of the weighing instrument.
These features represent a qualitative leap in the development of strain gauge load cells. The strain gauge digital load cells mentioned above are not, in essence, "true" digital load cells. They are essentially "digitalized" strain gauge sensors with an A/D converter, CPU, and connectivity to digital channels integrated within the sensor itself.
A true digital load cell is a sensitive element that converts force or strain into a "frequency signal." Common types of such sensors include vibrating wire sensors, tuning fork sensors, and capacitive sensors.
Strain gauge digital load cells essentially utilize the technology of converting the analog signal output from a bridge circuit into a digital signal via an A/D converter. Since the advent of digital instruments, this analog-to-digital conversion technology has been widely used in various measuring instruments. Therefore, the so-called strain-type digital load cell is simply a combination of this technology and strain gauge technology to achieve analog-to-digital change. Combined with mature technologies such as computer addressing, it expands the application of the original analog strain sensor. Starting with Digital Load Cells □ Zhou Zulian, National Institute of Metrology, China
[Abstract] This article provides a brief review of some widely used weighing instruments, hoping to provide some reference for the future development and manufacture of new weighing instruments.
[Keywords] Digital load cell; nuclear belt scale; discontinuous accumulation scale; cyclic chain code
Document Code: B Article Number: 1003-1870 (2023) 09-0011-03
1 Capability and Scope.
One view holds that digital strain sensors are simply A/D converters installed inside the sensor, or analog signals converted to digital signals through a digital channel. This is not much different from the earlier implementation of load cell functions through digital instruments, and there is no essential difference. Although Rob Woodward's article calls digital load cell technology a "One Giant Leap for the Weighing Industry," its application only lasted for 10 years. I believe the main reasons for this are as follows: The market and user acceptance of digital strain sensors was primarily due to the widespread use and significant price reduction of integrated circuits. Abroad, sensors were expensive, but the substantial decrease in integrated circuit prices meant that the price increase of digital sensors compared to analog sensors was not significant. While domestically produced sensors were much cheaper than their foreign counterparts, integrated circuits were almost entirely imported, making domestically produced digital load cells significantly more expensive than analog sensors. Early on, almost all digital load cells used in China were imported, which was very expensive. Furthermore, the operating systems of these imported products were all in foreign languages. Therefore, except for some special cases, the early promotion and use of digital load cells in China faced many inconveniences. Besides the two factors mentioned above, because digital load cells integrate components such as A/D converters within the sensor or near it, the demands on these integrated components increase in harsh outdoor working environments, naturally raising the price. Due to the addition of numerous electronic components at the measurement point of an instrument, the reliability and stability of digital load cells are significantly lower than those of analog sensors, based on reliability principles. This is a major disadvantage, as current display controllers offer equivalent capabilities in addressing, control, diagnostics, and modeling. Furthermore, sensor manufacturers generally lack strong R&D capabilities for secondary instrument functions, making further development of intelligent digital load cells not only difficult but also increasing product costs.
I believe that although digital load cells appear to represent a significant improvement, their functionality can be almost entirely replaced by display and control instruments. Therefore, considering cost and reliability, the widespread application of digital load cells is limited.
2. About Nuclear Belt Scales
Nuclear belt scales, due to their simple structure and near-maintenance requirements, typically only require minor modifications to existing conveyor belts for operation.
Therefore, they were widely used in China for a period. However, like digital load cells, this widespread use was short-lived, and they are now rarely used.
Essentially, a nuclear belt scale is a solid flow meter. Its measurement result is based on the volume of material on the conveyor belt. Early nuclear belt scales were equipped with a material density measuring device, but its accuracy in measuring the dynamic density of materials was not very high. Furthermore, if the measured material is coal, the moisture content of the coal significantly affects the measurement results. In practical terms, adding a density measuring device only increases cost and does little to improve the actual measurement accuracy. Therefore, subsequent nuclear belt scales used in China no longer include a density measuring device.
The measurement principle of a nuclear belt scale is to measure the volume of material on the conveyor belt by measuring the amount of radiation absorbed by the material being measured and the intensity of the radiation source. Because different materials absorb radiation sources in vastly different ways, nuclear belt scales, like ordinary belt scales, can only be calibrated using physical samples.
Nuclear belt scales use radioactive materials, and due to exposure to radiation, especially long-term exposure, strict regulations are in place to protect the health of users, incurring additional costs. Differences in public understanding of radiation protection and its hazards often hinder the proper use of nuclear belt scales. Furthermore, occasional theft of radioactive sources in China, leading to the risk of radioactive material spreading and causing injury, also contributes to the difficulties in using nuclear belt scales.
Comparing the "weighing" of nuclear belt scales with that of conveyor belts is, in my opinion, untenable because the two measuring instruments measure different objects: one uses volume, the other weight. Therefore, the "conclusions" drawn from these comparative measurements are not rigorous and, strictly speaking, have little practical significance. However, in some special situations, the nuclear belt scale is still convenient and effective as a "weight/mass" monitoring device.
2.3 Regarding Discontinuous Accumulator Scales
For a period of time, discontinuous accumulation scales were widely used as grain scales in grain silo weighing and measurement devices. "Jinzhong" Weighing Instrument Company introduced technology from the Dutch company MOLEN. I also visited the headquarters of MOLEN in the Netherlands to conduct type approval tests and verification certifications for the product's use in China.
Discontinuous accumulation scales, as measuring devices for bulk materials, are undoubtedly a weighing instrument with high measurement accuracy. Since its basic weighing device is based on the mature bucket scale technology, manufacturing such a scale is not very difficult for most weighing instrument manufacturers. Its weighing is "quasi-static," allowing for relatively high accuracy.
MOLEN in the Netherlands made a small patented technical improvement to its measurement, thereby greatly improving the long-term stability of its measurement results.
Both discontinuous cumulative weighing scales and belt scales are automatic weighing instruments capable of continuously weighing bulk materials. The primary difference between them lies in the fact that modern belt scales can weigh materials with flow rates of tens of thousands of tons per hour, a capability far beyond the reach of discontinuous cumulative weighing scales. However, when weighing small flow rates (i.e., tens of tons per hour), discontinuous cumulative weighing scales offer advantages in accuracy, long-term stability, and price, and can be implemented as a portable system. Therefore, although discontinuous cumulative weighing scales are currently less commonly used, I believe they are still a promising weighing instrument worthy of promotion.
4. Circulating Chain Codes
Since the late 20th century, circulating chain codes have been widely used as simulation calibration devices for belt scales. Prior to this, rolling chain codes were primarily recognized as simulation calibration devices. What are the differences between these two simulation calibration methods? I believe that, in essence, there is no difference in the verification process. Both involve applying a known weight load to the idlers in the weighing section of the belt scale to simulate the weighing process of bulk materials transported on a belt.
Because the load of the circulating chain code moves with the belt, while the load of the rolling chain code does not move with the belt and remains relatively stationary with respect to the weighing idlers of the belt scale, these differences give the illusion that the circulating chain code is closer to the "true" weighing state of the belt scale. In reality, the basic weighing principle of a belt scale is to accumulate the weight signals on the weighing idlers over time—that is, the sum of the weight signals over a given period of time—regardless of the method of loading the weight signals. Therefore, whether it's a circulating chain code or a rolling chain code, the effect of the load applied to the weighing idlers is the same.
Neither the circulating chain code nor the rolling chain code can simulate the "additional" tension generated by the belt effect. For belt scales, physical verification remains the only correct method. In the early stages of the application of cyclic chain codes in China, manufacturers and even many professionals believed that they could replace physical verification. However, this resulted in significant expenditure of manpower, resources, and time without success. The fundamental error lay in a misunderstanding of the basic weighing principle of belt scales. Both types of chain codes have their advantages and disadvantages, which will not be discussed in detail here. Although the use of cyclic chain codes is now more widespread, I believe that in the JJG195-2002 Continuous Cumulative Automatic Weighing Instruments (Belt Scales) Verification Regulations (Appendix D), the requirement for belt scale users (mandatory) in Clause D regarding the verification of the indicated value during use, if combined with the "zero-load maximum deviation test" verification of the belt scale, could detect the influence of belt tension changes. In foreign ports, belt scales undergo zero-point and range stability checks before daily use to ensure correct operation—a very effective and convenient method. However, the aforementioned chain codes used in our country cannot detect the influence of belt tension changes during verification.
Essentially, the chain code is not a traceable verification standard for belt scales. Therefore, the "calibration value" of the chain code line weight (weight per unit length) must be traced and recalculated based on the results of actual belt scale tests, and this recalculated value needs to be corrected periodically. Furthermore, for cyclic chain codes, if there is relative movement between the chain code and the belt during use, additional errors will occur, which is often overlooked in practical applications.
