Present Situation And Development Trend of Electronic Weighing Technology

1. Present situation of electronic weighing technology at home and abroad

Electronic weighing device (or electronic scale) is equipped with electronic weighing device weighing device, mainly composed of weighing sensor, bearing device (commonly known as the scale frame), weighing instrument 3 parts.The working principle of the electronic weighing device is to convert the mass or force on the bearing device into electrical signals proportional to it through the weighing sensor, and it is displayed on the weighing instrument in the form of analog or digital quantity.

2. Development trend of electronic weighing technology

In developed countries, electronic weighing has reached a high level in terms of technical level, variety and scale, especially in terms of accuracy, long-term stability and reliability.The static commercial scale has achieved the 3 000 D stipulated by OIML, and the highest can achieve 6 000 D;In terms of stability, no out-of-tolerance is allowed within 1 year;In terms of reliability, the life of the load sensor under normal use conditions is generally more than 10 years, and the mean time between failures (MTBF) of the instrument is more than 2 000h, and some products are up to 5 000h.The accuracy of industrial electronic scale can be generally achieved about 0.1% ~ 0.3%.

2.1 Weighing sensor

Weighing sensor is the core component of electronic weighing, which can be divided into resistance strain type, piezomagnetic type, vibrating string type and so on.Resistance strain type weighing sensor occupies more than 90% of the market share of weighing sensor because of its simple manufacture, mature process and high accuracy (the highest accuracy can be non-linear, repeatability and lag index is better than 0.01%).The resistance strain type weighing sensor is used to measure the mass or force under static and dynamic conditions. It has been widely applied in the fields of industrial production process inspection, control and automatic weighing.With the development of industrial control system to digitization, the digital weighing sensor has been developed and applied in recent years. The digital weighing sensor can be compensated and adjusted by the microprocessor to the conventional bridge, and the performance of the digital weighing sensor can be improved greatly, such as non-linearity, hysteresis and creep.Because it outputs the digital quantity directly, it greatly improves the anti-jamming ability in the transmission, and makes the communication with the computer very convenient.Due to the cancellation of analog amplification, A/D conversion and other links in the instrument, the instrument is greatly simplified.The representative manufacturer of the piezomagnetic weighing sensor is ABB Company. The sensor has large output power and strong overload capacity, and can work for a long time under the harsh environmental conditions such as high temperature, humidity and dust.

The representative manufacturer of vibrating chord weighing sensor is Switzerland Hasler (Hasler) company. The electronic belt scale of SFT vibrating chord weighing sensor adopted by the company has been widely used in China.The characteristics of the weighing sensor are as follows: repeatability, linearity is 0.01% of the full scale, stability is 0.03%, and the resistance strain type weighing sensor index is similar;Small changes in measured force can produce large frequency changes, resolution up to one part per million, high sensitivity;Direct digital weighing, processed signal and output signal are digital signals, processing process without A/ D, D/A conversion, high accuracy;Strong anti-electrical interference and anti-vibration interference ability;The sensor is equipped with a temperature sensor, the temperature compensation sensitivity can reach 0.000 1%/ ℃;Non-displacement measuring principle, the measuring process almost no displacement occurs, does not affect the calibration state of the scale frame;With dynamic overload protection device;Up to 15 sensors can transmit signals on the same serial bus, and the maximum transmission distance is 500 m. Therefore, this kind of weighing sensor is a direction of future sensor development.

2.2.1 Single lever scale frame

There is only a group of weighing lever, which supports a group or several groups of weighing roller, the scale frame is usually composed of weighing lever, fulcrum, balance weight, etc., the structure is simple, but the weighing accuracy is not high, the accuracy is usually 1% ~ 2%.

2.2.2 Double lever scale frame

Consists of two groups of weighing lever, symmetrical, each weighing lever support a group or several groups of weighing roller, scale frame structure is complex, large scale body, but the two groups of weighing lever in the weighing process can offset some of the horizontal component of the (such as belt tension, the belt running deviation caused by the lateral force) to influence the accuracy of weighing, so scale performance is stable,Weighing accuracy is high, the accuracy is usually 0.25% ~ 0.5%.

2.2.3 Cantilever scale frame

It is usually used on the special short belt conveyor, the whole short belt conveyor is only by the fulcrum and the weighing sensor bearing, the scale frame is only used for short size quantitative feeder or special measuring belt scale, weighing accuracy is medium, the accuracy is usually about 1%.

2.2.4 Suspension scale frame

Suspension weighing platform is similar to static weighing platform, weighing platform supports a group or several groups of weighing roller, its characteristics are better than the above other kinds of structural forms of weighing frame.For example, in the scale frame with four groups of weighing idlers, the accuracy index of Type 17 double lever type is 0.25%, while that of Type 14 suspension type is 0.125%.The precision of suspension scale frame is usually 0.125% ~ 0.25%.Domestic Tongling Three Aisi Company's single roller scale SSS-50ICS, Xuzhou Sanyuan Company's single roller scale ICS-30A are using no fulcrum, no lever, double load sensor suspension scale structure.From the overall performance, the suspension structure of the scale frame will be a future scale frame development direction.

2.3 Weighing instrument

Weighing instrument from the early analog instrument, developed to the present microcomputer instrument.Instrumentation accuracy from 0.1%, 0.05% to 0.01%.Microcomputer type instrument has microcontroller, PLC, industrial PC, embedded PC and other forms.Microcomputer weighing instrument adopts low drift and high gain amplifier, high resolution A/D converter, flash memory, electrically erasable memory, non-volatile random access memory, RS-232/ RS-485 communication standard, field bus and other technologies, so that its performance and function have been greatly improved.At the same time, the new technology of ∑ -Δ modulation analog-to-digital converter and surface mounting of printed circuit board is also applied.The adoption of these new technologies further improves the performance and reliability of weighing instruments, and creates favorable conditions for miniaturization of instruments.In terms of performance, it can be achieved: the non-linearity is better than 0.01%, the sensitivity is better than 0.2V /D, the A/D conversion speed is generally 10 ~ 30 times/s, and can reach more than 100 times/s when used for dynamic weighing.Function includes: various parameters setting, such as indexing value, range, calibration parameters, calculation coefficient, tare weight setting;Zero automatic tracking;Automatic peeling;Automatic range calibration;Dynamic detection;Power-on self-check and fault diagnosis;Power outage data protection;Overload alarm;Nonlinear compensation;As well as gross weight, net weight, tare weight and accumulated value display.In order to facilitate communication with the computer, weighing meters are equipped with input/output interfaces, such as RS-232C, RS-485, 20 mA current loop, analog quantity (4 ~ 20 mA) and relay output.In order to meet the needs of various applications, the current trend of the development of weighing meters is to realize different functional requirements through the combination of hardware or software building blocks.Another development trend of weighing meters is miniaturization.Small or micro meters are mainly used in small production equipment and mobile weighing systems, such as quantitative packaging scale, forklift scale, truck scale, mobile platform scale, etc.

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