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Author: Smartweigh–Multihead Weighter
The digital display multihead weigher designed in this paper is a weighing control display device based on the resistor strain force sensor and the single chip design as the key to control. The detection range is 0-10kg, and the measurement accuracy is±2g, the liquid crystal display screen displays the information of the precise measurement data, in addition, the information of the precise measurement data can be sent to the electronic computer to display the information according to the serial communication. The system software has the characteristics of high precision, stable characteristics and simple operation. The frame diagram of the multihead weigher design scheme is shown in Figure 1 below: 1. The hardware configuration circuit principle 1.1. The weight sensor resistor The strain force type weight sensor is composed of many key parts such as resistance strain gauges, polyurethane elastomers and inspection power circuits.
The polyurethane elastomer causes elastic deformation under external force, so that the resistance strain gauge attached to its surface also causes deformation. After the resistance strain gauge is deformed, its resistance value will change (expand or decrease), and then through relatively accurate measurement The power circuit converts this resistor into an electronic signal (working voltage or current), and then completes the whole process of converting the external force into an electronic signal. The test power circuit is shown in Figure 2, and the resistance of the resistance strain gauge is converted into the working voltage output. Because the Wheatstone bridge has many advantages, such as the ability to suppress the harm of temperature change, to suppress the influence of side force, and to easily deal with the compensation problem of the weight sensor, the Wheatstone bridge has been widely used in weight sensors. use.
The weight sensor generally has four lines of I/O, and the output resistance is generally 350Ω, 480Ω, 700Ω, 1000Ω. The input terminal will generally carry out some compensation for temperature and sensitivity. The input terminal resistor will be 20-100Ω higher than the output terminal. Therefore, the I/O terminals can be distinguished by measuring the resistance value with a digital multimeter. 1.2. The output data signal of the operational amplifier strain-type weight sensor is not strong (in the order of mV or even μV), and often accompanied by a lot of noise. For such a data signal, the first step in the power supply circuit solution is generally to select an instrumentation amplifier to first enlarge the small data signal.
Instrumentation amplifier power supply circuits have stronger common mode rejection capabilities than simple differential signal op amps. The most critical purpose of increasing is not the gain value, but only to improve the frequency stability of the power supply circuit. In this design, the instrumentation amplifier adopts the structure of OP07 three operational amplifiers.
As shown in Figure 3. When R1=R2, R3=R4, Rf=R5, the gain value of the power supply circuit is: G=(1+2R1/RG1) (Rf/R3). It can be seen from the formula calculation that the adjustment of the gain value of the power supply circuit can be completed by changing the resistance value of RG1.
1. 3. A/D conversion power supply circuit A/D converter selects electronic scale-specific integrated icHX711, which is a 24-bit A/D converter integrated ic specially designed for high-precision electronic scales. Compared with other integrated ICs of the same type, the integrated IC integrates the peripheral circuits necessary for other integrated ICs of the same type, including an adjustable regulated power supply, an on-chip digital clock oscillator, and the like. Enter the switch to select the safety channel A or the safety channel B at will, and the internal low-noise programmable controller amplifier is connected between the two.
The programmable controller gain value of safety channel A is 128 or 64, and the corresponding full credit limit differential signal input data signal amplitude value is respectively±20mV or±40mV. Safety channel B is a fixed gain value of 32, and the matching full-scale differential signal input working voltage is±80mV. Safety channel B is used to check the main parameters of the system software including the rechargeable battery.
This design scheme leads the output of the instrumentation amplifier to the input terminal of the safety channel A to simulate the analog differential signal, multihead weigher1.4, single chip design and communication interface single chip design selects AT89C51 integrated ic, and the communication interface with function keys, liquid crystal display screen and electronic computer shown in 5. The HX711 serial communication line leads to the single-chip design P1.0 and P1.1 ports. After the solution is designed by the single-chip microcomputer, the weighing data information is sent to the LCD screen.
In addition, several times of accurate measurement data information is sent to the electronic computer for display information according to serial communication.
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