The electronic table scale with aluminum alloy load cell stability

The stability of the aluminum alloy weighing sensor is based on a relatively accurate sensitivity temperature

Under the premise of compensation, the elastic modulus E of the elastic element is eliminated by the sensitivity temperature compensation, the sensitivity coefficient K of the strain gauge and the geometrical dimensions of the elastic element are affected by the temperature change. As the sensitivity of the load cell temperature error is a systematic error, the sensitivity of the temperature compensation is not carried out one by one as zero temperature compensation, but in bulk. The sensitivity of the temperature compensation resistor and the parallel linearity adjustment resistance are not changed under the premise that the elastic element material, the resistance strain gauge, the strain adhesive and so on are unchanged. In fact, the sensitivity of the batch temperature compensation is not possible to make the sensitivity of the load cell temperature drift to zero, and the perfect compensation between the ideal value there is always a certain deviation. When this deviation is positive, the overcompensation is generated when it is negative. The result of the over-compensation causes the load cell to decrease as the ambient temperature increases, resulting in a high temperature difference. This requires measuring the temperature drift direction of the electronic table scale measuring circuit, controlling the load cell temperature compensation resistor to produce a temperature drift that is opposite to the direction of the temperature drift of the measuring circuit, that is, overcoming the defects of the bulk sensitivity temperature compensation , But also improve the stability of the electronic table scale.

The stability of the load cell, that is, in a certain period of time and under the same conditions, the degree of change in its characteristics, mainly zero and sensitivity of the degree of change, it can be said that the stability of zero and sensitivity.

     If the temperature can not be kept the same at each test, the calculation is corrected according to the measured sensitivity temperature coefficient. The stability of the load cell is extremely important quality indicators, but in the domestic and foreign manufacturers of product samples are rarely given. The reason is that there is no uniform standard at home and abroad, the industry consensus is that the sensor sensitivity changes should not exceed twice the repeatability error.

The effective way to improve the stability of the aluminum alloy load cell is to choose the aluminum alloy material and the heat treatment aging state reasonably; to optimize the protective sealing material and sealing process; to implement the effective stability treatment process.

    1. Reasonable choice of aluminum alloy material

As the electronic scale scale is smaller, so that the load cell that has a higher sensitivity and a larger

High strength, high strength, good plasticity, corrosion resistance, low temperature performance, and excellent mechanical processing performance of low elastic modulus aluminum alloy material. Although the aluminum alloy material has so many advantages, but if the type of aluminum and heat treatment aging status improper choice, will make the load cell creep error increases, the stability of worse.

More commonly used at home and abroad is the high strength, good heat resistance, are aluminum - copper - magnesium hard aluminum alloy, the United States, Japan grade 2024, China grade 2A12 (formerly brand LY12), there are two heat treatment aging , That is, heat treatment natural aging 2A12-T4 and heat treatment artificial aging 2A12-T6. As the 2A12 aluminum alloy after quenching to produce a greater internal stress, the surface of the tensile stress, the core of the compressive stress, the natural aging can not be effectively eliminated, processed into elastic components, due to stress relaxation and deformation. The natural aging state of the phase and the organization is not stable, the process will continue to precipitate the phase of the desorption process, and cause changes in size, resulting in zero and sensitivity of the load cell instability, resulting in a larger measurement error. It is necessary to select the heat treatment of artificial aging state 2A12-T6 aluminum alloy.

    As the domestic market for the heat treatment of natural aging state of the 2A12-T4 aluminum alloy, before the application

It is necessary to perform the re-aging treatment in which the heat treatment state is changed, and the natural aging state T4 is the artificial aging state T6.

Change the aging state of the re-aging process: 190 ℃ × 24h or 110 ℃ continuous heating, 150 ℃, 190 ℃ 6,12,24 hours (time continuous calculation). Re-aging treatment, not only changed the state, stabilize the organization, but also to eliminate the heat treatment and roughing the residual stress generated by the purpose.

    2. Optimized protective sealing material and sealing process

Aluminum alloy load cell protection and sealing the basis of brushing resistance strain gauge protective adhesive, brushing time and brushing before cleaning is essential. Generally after the patch, pressurized curing, pressure relief after curing, quality inspection, paste the terminal, wire group bridge and measure the insulation resistance of the process after the quality inspection pass. That is, after the whole cleaning of the elastic components as soon as possible into the high temperature test chamber, heating 40 ℃ and insulation for a certain time, remove the elastic components from the box under dry environmental conditions brushing protective adhesive [the purpose is to prevent the resistance strain gauge In the production process damp, and for the final protection and sealing to lay the foundation. For the resistance strain gage to protect the coating of more material, it is representative: the United States VISHAY company produced M-CoatC type protective adhesive, it is a solvent diluted RTV silicone rubber, after curing to form a tough and flexible Of the transparent film, and has good mechanical, electrical properties and chemical resistance of the film thickness of the film thickness of 0.4 ~ 0.5mm; Germany HBM produced SN-4 neoprene and BARRIER-C-type non-corrosive Silicone Rubber.

After the aluminum alloy load cell circuit compensation and adjustment process is completed, the surface multi-layer protective sealing process is implemented. The protective seal mechanism is the use of a variety of protective and sealing material characteristics and the complementarity between them to form a good protective performance of the surface seal structure. It consists of the inner protective adhesive, the middle layer of the sealing film and the outer layer of silicone rubber. Close to the resistance strain gauge inner protective adhesive mainly from the role of adhesive sealing film, the outer silicone rubber from a certain degree of sealing and mechanical protection, the real seal from the sealing film. In this kind of sealing film has a PTFE sealing film, in fact, is a double-sided special treatment of PTFE film, it has anti-damp heat, good hydrolysis resistance, anti-atmospheric aging and chemical erosion ability. The data given, the thickness of 0.1mm polytetrafluoroethylene film exposed to the outdoor for more than six years, its appearance and mechanical properties were no significant changes.

3. Implement an effective stabilization process

The stability treatment of the aluminum alloy load cell is carried out in two parts. The first stability treatment is carried out after the machining of the aluminum alloy elastic element. The second stability treatment is carried out after the completion of the whole manufacturing process.

According to the United States in the 20th century, 80 years Bassa Dina sensor technology seminar information, the first stability treatment using heat treatment, mainly constant temperature aging method and hot and cold cycle. Constant temperature aging can eliminate the residual stress generated by machining, but also to eliminate the heat treatment of the introduction of residual stress. LY12 hard aluminum alloy at 200 ℃ high temperature constant temperature aging, the residual stress release and aging time relationship shows that insulation for 24 hours, the residual stress can be reduced by about 50%. The hot and cold circulation process is -196 ℃ × 4 hours / 190 ℃ × 4 hours, the cycle 3 times, the residual stress can be reduced by about 90%, and the structure is stable, the micro plastic deformation resistance is high, the dimensional stability is good. The thermal stress generated by the hot and cold temperature gradient interacts with the residual stress and redistributes it to obtain the effect of the residual stress drop.

The second stability treatment using mechanical method, mainly pulsating fatigue, overload static pressure and vibration aging three methods. Pulsating fatigue method is the aluminum alloy load cell installed in a special multi-station low-frequency fatigue testing machine, the lower limit of the application (1/5 ~ 1/3) rated load, the maximum rated load or 120% rated load, 3 to 5 times the frequency of 5000 to 10000 times the cycle, which can effectively release the elastic element, resistance strain gauge, strain binder adhesive layer of residual stress. Overload static pressure method is in a special standard weight loading device or simple mechanical screw loading device, the aluminum alloy load cell applied 125% of the rated load, hold 4 to 8 hours, or apply 110% rated load, keep 24 hours , In order to achieve the purpose of releasing residual stress. As a result of overloading static pressure equipment used by the simple, low cost, good effect, for the aluminum alloy load cell manufacturing enterprises widely used. The vibration aging method is to install the aluminum alloy load cell in the rated sine thrust to meet the vibration requirements of the vibration table, according to the rated range to estimate the frequency to determine the applied vibration load, operating frequency and vibration time. For small-scale parallel beam load cell, at the frequency of 30Hz, vibration acceleration 10g, the vibration for 15 minutes to get better results. Resonant aging is better than vibration release effect of residual stress. Vibration aging and resonance aging process is characterized by: low energy consumption, short cycle, the effect is good, does not damage the surface of the elastic components, and simple operation. The above three kinds of stability treatment methods, to improve the aluminum alloy load cell zero and sensitivity stability of the effect is extremely obvious.