Weighing instrument refers to the instrument that displays the quality and weighing status of the weighing object in the electronic weighing instrument, also called the weighing display. The electronic weighing instrument is originally an analog indicating type. It consists of an error amplifier, a reversible motor, a balanced bridge, an excitation power supply, a dial and a pointer. It works according to the principle of automatic balance electronic potentiometer. It has a slow weighing speed, a single function and low accuracy, and has been basically eliminated. The current weighing display is a digital display.
The composition of the electronic weighing instrument
Structural principle
There are many varieties of digital display scale instruments, one of which is shown in Figure 1. The digital receiver accepts the electrical signal output by the load cell. Electrical signals have analog and digital quantities, the most common being analog voltages of a few tens of millivolts. The circuit principle of the digital display is shown in Figure 2. The excitation power supply is supplied to the load cell operating power supply, and the A/D (A/D) conversion unit reference voltage is supplied, and the stability is generally 0.1% or more. The amplifying unit usually adopts a measuring amplifier structure to receive and amplify the signal of the load cell. Magnification is typically hundreds of times. The filtering unit filters out electrical noise generated from the outside of the machine and generated by the amplifier itself. The A/D conversion unit converts the analog quantity into a digital quantity, and the number of conversion bits is usually 14 bits or more. The data processing unit is composed of a microprocessor as the core and uses a peripheral support chip. It completes a series of operations such as collecting data, computing, and storing under the control of the program, and the processing result is sent to the corresponding interface. The display unit displays the weighing value and weighing status in the form of numbers or text, charts, etc., and can communicate with external devices through the interface.
Schematic diagram of the weighing instrument weighing instrument - circuit diagram
Performance characteristics
The performance of digital instrumentation includes five aspects: metrology performance, function, environmental adaptability, security and reliability. Compared with the universal digital weighing instrument, the digital display has five features: 1 self-contained sensor excitation power supply, easy to use; 2 ratiometric A/D conversion and frequency multiplication technology, long-term stability in metrology performance 3 software can realistically simulate the weighing characteristics such as vibration, air balance change, material drop, etc., display fast, accurate, stable; 4 machine with empty scale zero, zero tracking, calibration, maximum weighing, indexing and other parameters The setting unit is easy to change and has strong versatility; 5 with output interface, can connect a variety of external devices, and realize system control conveniently.
Accuracy Level The International Legal Metrology Organization (OIML) Recommendation No. 3 states that non-automatic scales are divided into four grades according to the accuracy level. According to the level of the instrument, China has divided the electronic weighing instrument into four grades, and respectively named the level codes corresponding to the electronic scale: I, II, III, IV. The maximum allowable error of the metering instrument of each class is 0.7 times the tolerance of the corresponding class instrument.
Netizens explain the basic architectural principles of scale instruments
The instrument architecture is actually very simple. A power supply supplies power to all devices. An AD part is responsible for amplifying the analog signal of the sensor, converting the analog to digital signal, and a central processing unit, also known as a single-chip microcomputer (MCU), to process the AD part. The read weight signal is subjected to a series of conversion processing, preferably decoded into a display-readable signal, displayed on the display screen, and generally has a keyboard circuit to accept some manipulations of the user, and many instruments also have Parallel print port, micro-drive, RS232 or RS485 interface, communication with host computer or other instruments, digital current loop interface to connect large screen, and many instruments for industrial control with 4~20mA current loop interface Match the PLC. The high-end also has a live CAN bus or Ethernet interface. Some instruments also have a USB interface. However, because the transmission distance is too close, the USB uses less on the scale instrument, so few instruments have a USB interface.
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Tag: Electronic Weighing Instrument Load Cell Keyboard Circuit Print Port Accuracy Level
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