The Structure Principle and Performance Characteristics of Electronic Weighing Instrument

Weighing instrument refers to the electronic weighing instrument in the display of the quality of the weighing and weighing status of the instrument, also known as the weighing display.The electronic weighing instrument is an analog indicating type originally, which is composed of error amplifier, reversible motor, balance bridge, excitation power supply, dial and pointer, etc. It works according to the principle of automatic balance electronic potential difference meter.It is slow weighing speed, single function, low accuracy, has been basically eliminated.The current weighing display is digital display.

Structure principle

There are many kinds of digital display weighing instruments. Figure 1 is one of them.The digital display device receives and processes the electrical signal output by the weighing sensor.Electrical signals have analog and digital quantities, the most common being analog voltages ranging from a few millivolts to tens of millivolts.The circuit principle of digital display is shown in Fig. 2.The excitation power supply provides the working power supply of the weighing sensor and the reference voltage of A/D (analog/digital) conversion unit at the same time, and its stability is generally above 0.1%.Amplification unit usually adopts the measurement amplifier structure to receive and amplify the signal of the weighing sensor.Magnification is usually hundreds of times.Filtering units filter out electrical noise from outside the machine and generated by the amplifier itself.A/D conversion unit to convert analog to digital quantity, conversion number usually take more than 14 bits of binary number.The data processing unit is composed of microprocessor as the core and peripheral supporting chip. It completes a series of operations such as data collection, calculation and storage under the control of the program, and sends the processing results to the corresponding interface.The display unit displays the weighing value and status in the form of numbers or words, charts and so on, and can be contacted with external equipment through interfaces.

The performance feature

The performance of digital weighing instrument includes measuring performance, function, environmental adaptability, safety and reliability.Compared with the general digital weighing instrument, the digital display instrument has five characteristics: (1) its own sensor excitation power supply, easy to use;(2) the use of ratio type A/D conversion and frequency doubling technology, the long-term stability of measurement performance is good;(3) The software can truly simulate the weighing characteristics such as vibration, air balance fluctuation, material drop and so on, showing fast, accurate and stable;(4) The setting unit of parameters such as zero setting, zero tracking, calibration, maximum scale and indexing position in the machine is convenient to change and has strong versatility;⑤ With output interface, can connect a variety of external equipment, convenient system control.

The International Organization for Legal Metrology (OIML) Recommendation No. 3 stipulates that non-automatic weighing instruments are divided into four levels of accuracy.According to the level of the weighing apparatus, China also divides the electronic weighing apparatus into four grades, which are respectively labeled with the corresponding level codes of the electronic weighing apparatus: Ⅰ, che, che and che.The maximum allowable error of the weighing instrument of each grade shall be 0.7 times of the allowable error of the weighing instrument of the corresponding grade.

The net friend explains the basic structure principle of the weighing instrument

Instrument architecture is very simple, a power supply for all devices, an AD part responsible for sensor analog signal amplifier, modulus conversion to digital signal, a central processing unit (CPU), which is often said that the single chip microcomputer (MCU), the weight of the part to handle AD read signal, a series of conversion processing, best decoding to read the signal can be shown,Display on the LCD screen, co-workers and keyboard circuit, to accept the user control, many instruments with parallel print port, micro drive, RS232 or RS485 interface and PC or other instrument of interconnected communication, and digital current loop interface, to connect the big screen, and industrial control with a lot of the instrument with 4 ~ 20 ma current loop interface,To connect with PLC.High-end CAN also be a field CAN bus or Ethernet port, and some meters have USB ports, though USB is less useful for weighing meters because it is transmitted too close, so few meters have USB ports.

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