Evolvement of the intelligentized functions in digital load cells

Evolvement of the intelligentized functions in digital load cells

Abstract：Based on the hot topic of digital load cells recently in weighing industry,the writer brings forward the concep,requirement and evolvement of the intelligentized functions in digital load cells.

Key words:load cell, numeric conver, digital, intelligentize,digital load cell, analog load cell, wireless load cell

Overview:

In recent years, the application of digital load cell in my country's weighing instrument industry has developed rapidly. Some foreign-funded enterprises and domestic enterprise manufacturers with a long-term vision have all set their sights on this extremely vital product. Many companies call the amplification and A/D circuit originally placed in the weighing instrument in the weighing system, which is placed in the weighing sensor housing or in the junction box as an intelligent digital weighing sensor. In fact, the author believes that at best it can only be called a pre-conversion type load cell, of course it can also be called a "digital load cell". In the discussion of this article, it is called the “digital weighing load cell” in the initial stage, but it must not be called the “digital intelligent strain gauge load cell”. The description of the characteristics of digital load cells and the difference between "digital" and "digital" have been discussed (see References [1], [2]), and this article will not repeat them. This article only wants to describe the evolution of digital intelligent load cell functions, from the initial digital pre-conversion, to the second stage of intelligent compensation and correction, to the third stage of the evolution of the intelligent application of the weighing system, A more detailed discussion is given.

1:Intelligent closed loop control

Digital load cell in the initial stage Because the resistance-strain conversion principle of traditional analog load cell determines its inherent shortcomings such as small output analog signal, short transmission distance, poor anti-interference ability, and inconvenient installation and debugging. Therefore, as early as the 1980s, people paid attention to the shortcomings of analog load cells. On the basis of not changing the weighing mechanism of resistance strain load cells, the above shortcomings were turned into advantages. For this reason, some foreign load cell manufacturers have launched the first generation of “digital load cell” (see reference [1]), which means that the originalThe amplification and A/D circuits in the weighing instrument are placed in the load cell housing or in the junction box near it. Its basic configuration is: analog sensor + digital transmitter (amplification and A/D circuit) = digital sensor in the initial stage.Since the above-mentioned sensor outputs a digital signal, it overcomes the shortcomings of the analog load cell such as small signal, short transmission distance, and poor anti-interference ability.

However, the performance indicators of its various sensors are determined by its own manufacturing, compensation and adjustment processes. That is to say, if the manufacturing, compensation, and adjustment process of the sensor itself fails, it is impossible to rely on digital transmission to improve or compensate the mechanical and temperature indicators of the entire sensor (note that it is not the temperature indicator of the digital transmission circuit itself). At present, many domestic foreign-invested enterprise manufacturers and domestic enterprise manufacturers mainly promote products of this type. This type of load cell sensor has not broken the original function. Of course, it is not easy to achieve the first generation of "digital load cells". Because, first of all, it must be ensured that the designed and selected digital transmission circuits and devices cannot reduce the mechanical and temperature indicators of the entire sensor, that is to say, the temperature drift and time drift of the digital transmission circuit itself must not affect the manufacture of the sensor itself. Compensate and adjust the mechanics and temperature indicators determined by the process. In addition, some manufacturers haveHas improved. However, it still belongs to the first generation of "digital load cell".

This type of load cell is more representative and can be divided into the following three forms: The first is called the first generation of primary "digital load cell" represented by the early products of domestic manufacturers. Only the amplifier and A/D circuit originally placed in the weighing instrument are placed in the junction box in or near the load cell housing. The mechanics and temperature indicators of the original sensor are not broken at all. The general resolution of the digital transmission circuit of this type of sensor can achieve 60000 internal codes, the sampling rate can achieve 50 times/second, the temperature drift can achieve 200×10-6/10°C, and the time drift index is generally uncertain.The second type, represented by the C16i digital sensor from HBM, can be called the first-generation advanced "digital load cell". The biggest difference between this type of sensor and the first sensor is that the local function of the sensor is improved. The resolution of the digital transmission circuit can achieve 1 million internal codes, the sampling rate can achieve 100 times per second, and the temperature drift can achieve 100 ×10-6/10°C. It is said that the time drift can also be 100×10-6/year.

But the biggest disadvantage is that it cannot change the mechanical and temperature indicators determined by the traditional manufacturing, compensation, and adjustment process of the sensor itself, and only adds the linear compensation function. That is to say, if the mechanical and temperature indicators of the sensor itself are not good, the comprehensive performance indicators of the sensor cannot be improved either.The third type is a separate modular digital sensor, represented by the US SENSORTRONICS company, which was launched in 1992 to replace the first-generation primary integral digital sensor launched earlier. Modular digital sensor is to move the original A/D conversion circuit inside the sensor to the junction box. Usually the junction box with the A/D conversion module is called the digital junction box, and then the digital signal output by the digital junction box is transferred to Display controller. (See Reference [2])In short, the main feature of the digital weighing sensor at this stage is that it does not change the traditional manufacturing, compensation, and adjustment process of the sensor itself, and only moves the A/D conversion circuit in the weighing instrument to the sensor or junction box to realize the weighing. Transmission of heavy digital signals.

2. The second stage digital intelligent weighing load cell

With the development of computer software technology, people imagine whether the defects of the sensor itself can be solved by software technology? That is to say, almost all compensation processes of the sensor, such as zero point compensation, temperature compensation, linear compensation, hysteresis compensation, creep and recovery compensation, are completed by computer software. This can make the manufacturing process of the sensor itself extremely simple. First, there is no need to spend a lot of energy on the fine manufacturing process, and second, it can greatly improve the qualification rate of the sensor elastomer and patch. Of course, to complete the above-mentioned various software compensations, various mathematical models need to be established, and huge databases are needed to support them. It is impossible to complete without a lot of test data. Its basic configuration is: analog sensor + digital transmission (amplification and A/D circuit) + load cell software intelligent compensation = second stage digital intelligent load cell.

The digital transmission part of this type of sensor includes amplification, filtering, A/D converter, microprocessor, and temperature sensor. Through digital compensation circuit and digital compensation process, linearity, hysteresis, creep and other compensation can be performed; built-in temperature sensor, Real-time temperature compensation can be performed through compensation software; the address is adjustable for easy application and interchange; remote diagnosis and correction.

The more typical representative of this type of sensor is the digital sensor technology of TOLEDO Company of the United States. Its core is the intelligent compensation technology of sensor software. This load cell sensor is said to use fuzzy mathematics, artificial intelligence and other theories, and use reasonable data processing methods to achieve digital compensation of load cell sensor errors, avoiding the cumbersome and analog compensation methods in traditional load cells. This type of sensor already has the basic requirements of digital compensation intelligent technology. One adopts the neural network self-learning function to solve the measurement error effect caused by the change of the environment temperature on the characteristic difference between the sensor bridge arms. The specific method is: use the two output voltage signals of the electric bridge as calibration data, and use neural network data fusion to process the calibration data, which not only improves the environmental temperature adaptation range of the electric bridge measurement, but also improves its static characteristics.

At present, the non-linearity of domestic weighing load cell sensors is mainly solved by the manufacturing, compensation and adjustment process of the elastomer itself. An inverse nonlinear approximation of the input-output characteristics of the sensor calibration data using the nonlinear mapping capability of the BP software algorithm is used as the nonlinear correction software of the intelligent sensor system, so that the sensor can improve the measurement with the support of the software Accuracy. The network is trained with load cell sensor experimental data. According to the introduction, this method can improve the relative error of measurement (see Reference [5] for details). A smart sensor data preprocessing method, which is applied to non-linear correction temperature compensation, digital filtering and scale conversion of load cell sensors, can realize the front-end detection and processing of industrial field sensor test data, thereby improving the non-linearity of the sensors in the automatic detection operation system. Linear quality (see reference [7] for details). It can be seen from the various examples of the above-mentioned digital intelligent sensors that, as a whole, the digital intelligent sensors at this stage are mainly reflected in the intelligent compensation and correction of the load cell sensor itself.

3:The third stage digital intelligent weighing load cell
Strictly speaking, the intelligent function of the digital intelligent load cell is not only reflected in the intelligent compensation and correction of the sensor itself, but more importantly, it is necessary to realize the intelligent application. With the continuous expansion of the application field of digital load cells, how to make full use of the functions and characteristics of digital load cells is on the agenda. For this reason, the real “digital intelligent weighing sensor” suitable for different fields came into being. What is a "digital intelligent weighing sensor"? At present, the most consistent theory in the world holds that: Anyone with one or more sensitive functions can complete weighing signal detection and processing, logic judgment, closed-loop control, two-way communication, and cyclic automatic Weighing sensors with all or most functions such as inspection and self-diagnosis, automatic correction and compensation, and automatic calculation are called "digital intelligent load cells". From the structural point of view, it can be either a monolithic integrated structure or a separate modular structure. The basic configuration of the digital intelligent load cell is: analog sensor + digital transmission (amplification and A/D circuit) + sensor software compensation + intelligent automatic control software = the third stage of digital intelligent load cell.

The latest third-stage digital load cell has been applied to various applications such as intelligent closed-loop control, multi-purpose, intelligent multi-component measurement, high-speed dynamic digital signal processing, and network communication. Its typical products are as follows:

A:Intelligent closed loop control

Intelligent closed-loop control mainly refers to various applications such as intelligent closed-loop, high-speed and high-precision dynamic self-compensation weighing. The FIT series of dynamic high-speed digital intelligent weighing sensors from HBM in Germany, in addition to being used in the standard weighing process, also provides multi-purpose control functions for sorting and quantitative filling. The built-in signal processing module enables it to be used in fast, high-precision weighing occasions. 20-bit division, the measurement rate can reach 600 times per second. External factors can cause vibration. FIT load cells have a good digital filter to eliminate the weight caused by these vibrations. The built-in programmable filter allows customers to change the filter parameters according to different applications.

B:Intelligent multi-component control

Intelligent multi-function means that the load cell itself can detect other multi-component information in addition to the function of detecting weight information. For example: the electronic crane scale sensor can detect acceleration and complete the automatic correction of dynamic acceleration; the truck scale sensor can detect the lateral load in the horizontal direction and complete the automatic correction of the lateral force. The TS-MLC series of intelligent digital truck scales newly launched by Japan Yamato Check & Balance Co., Ltd., on a weighing platform, a one-way weighing process can not only complete the weighing of the whole vehicle, but also obtain the axle group, axle weight, All signals, such as wheel weight, do not require a combination of multiple vertical or horizontal weighing platforms and multiple weighing processes. Once an unbalanced load occurs during the weighing process, an alarm will be issued. The weighing process can be completed in 15 seconds. The printing of this parameter can be called one of the fastest intelligent digital truck scales in the world. The difficulty of this product lies in how to build mathematical models under different vehicle conditions on a weighing platform, automatically realize multi-component control and adaptive correction through calculation software, and quickly respond to a large amount of information and status code output.

C:High-speed dynamic signal processing technology

The high-speed dynamic nonlinear correction applied to the dynamic intelligent load cell can be realized through the modeling and software control of the system. An adaptive control theory that uses neural network affine nonlinear systems. The weights of neural network are derived from Lyapunov stability theory and adjusted online. Taking into account the existence of network approximation errors and external disturbances, using the characteristics of sliding modes that are not sensitive to parameters and disturbances, the nonlinear correction of dynamic systems can be achieved. In the industrial process control system, digital technology is developing towards intelligence, openness, networking and information. Using the data processing method of the industrial control software in the FCS (Fieldbus Control System), the hottest spot in the industrial process control system, first analyze the characteristics of the structure and data processing of the sensors in the system, and propose the data processing of FCS The method, according to the introduction, is adaptable to both stationary sensor data and non-stationary sensor data.

Vehicle dynamic weighing technology has become a key technology and development direction of my country's weighing instrument industry. Various types of intelligent resistance strain bending plate, vibrating wire, capacitive, quartz crystal digital sensors, especially optical fiber sensing technology, have begun to be applied to vehicle dynamic weighing detection systems. A distributed optical fiber micro-bending pressure sensor based on high-speed digital signal processing DSP (Digital Signal Processing) technology emerged (see reference [8] for details). DSP with cutting-edge data signal processing technology is a fast and powerful microprocessor. The unique feature is that it can process data in real time. It is this real-time capability that makes DSP the most suitable for high-speed and fast response applications. Optical fiber sensing technology analyzes the relationship between the attenuation and the distance in the optical fiber from the perspective of optical waveguide. DSP technology is used to analyze the characteristics of the processed signal, and the sampling integrator BOXCAR processing method is adopted to improve the signal-to-noise ratio before and after processing. According to the latest report, A frequency output fiber optic dynamic weighing sensor based on Mach-Zehnder interferometer. Compared with the dynamic weighing system composed of various sensors mentioned above, the system has high measurement accuracy and resistance to Strong interference ability, fast response speed, convenient networking, etc.

D:Application of Network Communication Technology

Networked communication technology is another area for the expansion of intelligent functions in digital intelligent load cells. Due to the continuous development of digital weighing sensors, a new revolution in weighing technology is about to be launched, and its depth and breadth will exceed any time in history. In the industrial process control system, the FCS based on the fieldbus is fully decentralized, fully digital, fully open and interoperable instead of on-site one-to-one 4-20mA analog signal lines, and uses twisted pair, optical cable or radio to transmit digital signals. Reduce a large number of wires, improve reliability and anti-interference ability. Computer communication technology is applied to two-way communication between sensor weighing data and various control information, multi-machine multi-unit communication, network communication, industrial field bus, wireless communication including remote SCADA technology, M2M technology (machine-to-machine wireless communication technology), wireless Data collection and even wireless network management system make the intelligent function of digital weighing sensor take another step.

4:End

From the above article on the evolution of digital intelligent load cell functions, from the initial stage of digital conversion, to the second stage of intelligent compensation of the sensor itself, to the third stage of the sensor to expand the application of the weighing system, especially In the discussion of the evolution of the "intelligence" function, it can be seen that the functions of the digital intelligent load cell, in addition to the digital or digital load cell sensor performance compensation and the digital long-distance transmission function, can not be ignored is the expansion of its It is applied to various intelligent closed-loop control multi-purpose, intelligent multi-component measurement, high-speed dynamic digital signal processing and even networked communication. In short, the digital intelligent load cell has changed from the single function and single detection of traditional sensors to multi-function and multi-variable detection, from open-loop data transmission to active closed-loop control and information processing, and from an isolated instrument to systematization and integration. , The development of network direction. The author believes that this should be the current and future development direction of my country's digital load cell manufacturing industry. I believe that in the near future, my country's load cell manufacturing industry will have more application occasions, with independent intellectual property rights, and truly intelligent digital weighing sensors spring up like mushrooms, thus creating weighing The new era of technology is what this article is looking forward to.