Electronic weighing instrument structure principle and performance

Weighing instrument is also called weighing indicator. The electronic weighing instrument is originally an analog indicating type, which is composed of an error amplifier, a reversible motor, a balance bridge, an excitation power supply, a dial and a pointer, and works according to the principle of automatic balance electronic potentiometer. It weighs slowly, has a single function, and is low in accuracy. It has basically been eliminated. The current weight indicator is a digital display.

Electronic weighing instrument composition and structure principle

There are many varieties of digital display scale instruments. Figure 1 shows one of them. The digital display accepts the electrical signal output by the load cell. Electrical signals have both analog and digital quantities, the most common being analog voltages of several to several tens of millivolts. Digital display circuit principle shown in Figure 2. Excitation power is supplied to the power supply of the load cell, and the reference voltage of the A/D conversion unit is supplied at the same time. The stability is generally above 0.1%. The amplifying unit usually uses a measuring amplifier structure to receive and amplify the signal of the load cell. The magnification is generally several hundred times. The filtering unit filters out electrical noise generated from the outside of the machine and the amplifier itself. The A/D conversion unit converts the analog quantity into a digital quantity, and the number of conversion bits is usually more than 14 bits in the binary number. The data processing unit is composed of a microprocessor and a peripheral support chip. It performs a series of operations such as data acquisition, calculation, and storage under the control of the program. The processing result is sent to the corresponding interface. The display unit displays the weighing value and the weighing status in the form of numbers or texts, charts, etc., and can communicate with external devices through the interface.

Performance characteristics

The performance of the digital weighing instrument includes five aspects: measurement performance, function, environmental adaptability, safety and reliability. Compared with the common digital weighing instrument weighing instrument, the digital display has 5 characteristics: 1 self-powered sensor excitation power, easy to use; 2 using ratio type A / D conversion and frequency multiplication technology, long-term stability in the measurement performance is good ; 3 software can really simulate the weighing characteristics such as vibration, air-weighing change, material drop, etc., display fast, accurate, stable; 4 machine has empty balance zero, zero tracking, calibration, maximum capacity, indexing and other parameters The setting unit is easy to change and has strong versatility. 5 With output interface, it can connect with various external devices to easily realize system control.

The accuracy level OIML Recommendation No. 3 stipulates that non-automatic weighing devices are divided into four levels according to the accuracy level. According to the scale used with weighing instruments, China will also divide the electronic weighing instrument into four grades, and respectively use the grade codes corresponding to the electronic scales: I, II, III, and IV. The maximum allowable error for the weighing instrument of each level is 0.7 times the allowable error of the corresponding level instrument.

Weighing instruments basic architecture principles <br> <br> instrument architecture is very simple, a power supply for the entire device, an AD moiety is responsible for amplifying the analog sensor signal, analog to digital conversion to digital signals, a central processing unit, i.e. The microcontroller (MCU) often used to process the weight signal read by the AD section performs a series of conversion processing. It is better to decode it as a display-reading signal and display it on the display screen. At the same time, there is generally a keyboard circuit. To accept some of the user's control, many instruments also have a parallel printer port, micro drive, RS232 or RS485 interface with the host computer or other instruments interconnected communication, digital current loop interface, to connect the big screen, and other industrial control Many meters have a 4-20mA current loop interface to match the PLC. There are also high-end CAN bus or Ethernet interface, some instruments also have a USB interface, but because the USB transmission distance is too close, less use in instrumentation, so there are few instruments have a USB interface.