With the advancement of technology, electronic weighing instruments made of weighing sensors have been widely applied to all walks of life and achieved rapid and accurate weighing of materials, especially with the advent of microprocessors, the automation of industrial production processes With the continuous improvement, the load cell has become a necessary device for process control, from the weight measurement of large tanks and hoppers that could not be weighed before, to the measurement and control of crane scales, car scales, etc., to a variety of mixed distributions. The weighing system for raw materials, the automatic detection in the production process, and the feed amount control of the powder and the like are all applied with the weighing sensor. At present, the weighing sensor is applied to almost all the weighing fields.
1. High-speed quantitative dispensing system
The system controls the weighing and comparison of the load cell by a microcomputer, and outputs a control signal, performs a fixed value weighing, controls the operation of the external feeding system, and carries out the tasks of automatic weighing and rapid packing.
The system uses MCS-51 microcontroller and V / F voltage frequency converter and other electronic devices, the hardware circuit block diagram shown in Figure 1, with 8031 ​​as the central processor, BCD pull code plate as a set value input device, materials installed in In the hopper, its weight distort the sensor's elastic body and output an electrical signal proportional to the weight. The sensor output signal is amplified by the amplifier and input to the V/F converter for A/D conversion. The converted frequency signal is directly sent to the 8031. In a microprocessor, its digital quantity is processed by a microcomputer. On the one hand, the microcomputer sends the instantaneous digital quantity of the heavy object to the display circuit to display the instantaneous weight, and on the other hand, it performs the weighing comparison, opens and closes the feeding port, discharges the material in the box and the like and a series of weighing setting control.
In the whole set value control system, the load cell is a key component that affects the measurement accuracy of the electronic scale. GYL-3 strain gauge load cell is selected. Four pieces of resistance strain gauges form a full-bridge bridge. Under the condition that the bridge pressure U is constant, the output signal of the sensor is proportional to the force acting on the sensor and the bridge pressure, and the pressure of the bridge pressure U is also changed. Directly affect the electronic measurement accuracy, so the bridge pressure is required to be very stable. After the millivolt-class sensor output is amplified, it becomes a 0-10V voltage signal output and is sent to the V/F converter for A/D conversion. The frequency signal output from the output terminal is added to the count of the timer 1 of the microcontroller 8031. , input terminal T1. Timer 0 is used to count timing within the microcomputer. Timer 0 timing is set by the required A/D conversion resolution.
The count value of timer 1 reflects the measured voltage, that is, the weight of the material. At the same time as the display, the computer also determines the value according to the set value and the measured value. The measured value is compared with the given value, and the difference value is used to provide the PID operation. When the weight is insufficient, the feeding is continued and the measured value is displayed. Once the weight is equal to or greater than the given value, the control interface outputs a control signal to control the external feeding device to stop feeding, display the final measurement value, and then issue an answer order indicating that the bag charge is finished and the next bag can be loaded and weighed. Figure 2 shows the automatic weighing and loading device. Each loaded box or bag moves along the conveyor belt until it is filled with the material under the electronic scale, the conveyor belt stops moving, the electromagnetic coil 2 is energized, the electronic weighing hopper is turned over, and the materials are all poured into the box or the bag. When the material is poured, the conveyor belt is finished. The motor is again energized to remove the filled bins or bags and to protect the conveyors from continuing until the next empty bag or empty box cuts off the light source of the photoelectric sensor. At the same time, the electronic weighing bins are reset and the solenoid 1 is energized. The electronic scale is automatically fed and the weight is controlled by a microcomputer. When the material in the electronic scale is equal to the given value, the electromagnetic coil 1 is powered off and the spring force closes the funnel door. The charging system starts the next charging cycle. When the material in the hopper and the box on the conveyor belt are sufficient, the process can continue continuously. If necessary, personnel can stop the conveyor belt at any time, enter a different setpoint value through the dial, and then start, you can change the weight of the box or bag.
Figure 2 Automatic weighing and loading device
This system uses different sensors to change the weighing range, which can be used in the automatic packaging of cement, sugar, flour processing and other industries.
2. Application of sensors in commercial electronic scales
At present, the use of commercial electronic pricing scales is very popular and will gradually replace traditional rod scales and mechanical case scales. Electronic weighing scales have a prominent feature in the weighing platform structure: a fairly large weighing platform, only a specially designed sensor is installed in the middle to bear all the weight of the material. Figure 3 Internal structure of the weighing scale
Quantity, as shown in Figure 3. The structure of the commonly used electronic pricing scale sensor is shown in Fig. 4, in which Fig. 4(a) is a double-connected elliptical hole elastic body, and the weighing pan is fastened with two screw holes on the upper plane of the cantilever beam; Fig. 4(b) For the plum four-hole elastic body, the three screws on the side of the cantilever beam side of the weighing pan are firm, and the strain gauge for compensation is pasted on the middle strut. These two types of sensors are used most in pricing scales. Fig. 4(c) is a three-beam bending elastomer that samples the bending stress and is sensitive to the weight reaction and should be used to make a small-scale weighing scale. Fig. 4(d) is a three-beam shear elastic body, which samples the shear stress of the middle sensitive beam and should be used to make a weighing scale weighing scale of several hundred kilograms.
Figure 4 Elastomer structure for pricing scales
These complex beam-type high-precision sensors are used to support a large weighing platform, and the objects to be weighted may be placed at any position on any weighing platform, inevitably resulting in a four-corner indication error, as shown in Fig. 4(a), (b ) Two types of sensors can be corrected for corner difference by honing. For Fig. 4(c) and (d), it has two partially weakened flexible auxiliary beams, which make the sensor have strong resistance to lateral force, lateral force and torsional moment, and can be used to honed the flexibility of the auxiliary beam. The location adjusts the sensor's sensitivity factor and quadrant error. Figure 5 is a block diagram of a commercial electronic pricing scale. The sensor adopts the plum four-hole structure shown in Fig. 4(b). The scale has functions of zero setting, automatic removal of unit price, automatic zero tracking, automatic debarking, number accumulation, sum accumulation, and print output. Segment green fluorescent digital display shows that it is very convenient to use.
Fig. 5 Circuit diagram of electronic pricing scale
Figure 6 is a schematic diagram of a portable household electronic hand-held scale made of CHBL3 type S-type double-hole elastic body load cell. It consists of four parts: a load cell, an amplifier circuit, A/D conversion and liquid crystal display. In the figure, E is a 9V laminated battery, R1-R4 are the four resistance strain gauges of the load cell, and R5, R6 and W1 form a zero adjustment circuit. When the load is zero, adjust RW1 so that the LCD shows zero. A1, A2 are the two unit circuits in the dual op amp integrated circuit LM358, which forms a symmetric non-inverting amplifier. The A/D converter uses ICL7106 dual-integrating A/D converter and the liquid crystal display adopts 3 1/2 LCD display. sheet. The electronic scale has high precision, is simple and practical, and is easy to carry.
The load cell is a high-precision sensor and must be used in accordance with the specified specifications. If not used in accordance with the specified specifications, not only can not play the role of weighing, but also easy to damage, in particular, is absolutely not allowed to use more than the load safety value.
Figure 6 Block diagram of a portable scale For the effect of temperature change on the bridge zero and output and sensitivity, even if the same batch of strain gauges are used, errors will be caused due to the difference in temperature characteristics between the strain gauges. Therefore, the accuracy of the requirements is higher. A high sensor must be temperature compensated by using an automatic compensator with appropriate temperature coefficients on the substrate to be affixed and externally compensated for it appropriately.
Non-linearity error is the most important point in sensor characteristics. There are many reasons for the nonlinear error. Generally speaking, it is mainly determined by the structural design. It can also be improved by linear compensation.
Hysteresis and creep are errors in strain gages and adhesives. Because the adhesive is a polymer material, its characteristics vary greatly with temperature, so the load cell must be used within the specified temperature range.
The use of sensors in the open air should also take into account the effects of direct sunlight and the effects of wind pressure.
The above information is the basic situation of the current domestic load cell. The current advanced load cells (high-frequency, high-precision, and high-range) have very different operating principles and electrical characteristics. For example, the load cells that are used underwater are in stark contrast.
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