Development of temperature detection and control s

2022-08-26
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Development of temperature detection and control system for electron beam smelting furnace

Abstract: This paper introduces the double closed-loop control loop scheme for temperature control of electron beam smelting furnace. Thyristor voltage regulation technology is the best choice for temperature control of electron beam smelting furnace with transformer as load. Several technical problems in the implementation of the scheme are analyzed, and the corresponding solutions are given. The operation results show that the performance and indicators of the system in furnace temperature detection and control are significantly improved, the adverse effects caused by human factors are reduced, and the quality of products is also greatly improved. It provides a theoretical basis for the development of electron beam smelting furnace

key words: electron beam smelting furnace; Double closed loop control; Thyristor voltage regulator; The temperature regulator

completes the reconstruction project of a 5kW electron beam smelting furnace. The author undertakes the electrical and instrument design tasks. The test run results show that the power is increased to 2okw. Due to the adoption of new devices and technologies, the performance and indicators of the system in furnace temperature detection and control are significantly improved, the adverse effects caused by human factors are reduced, and the quality of products is greatly improved

1 technical requirements of temperature control system

(1) the maximum working temperature of the furnace is 3500 ℃; Long term working temperature 3200 ℃

(2) furnace heating mode: high speed electron beam bombards the raw material rod

(3) heating power: 2O kW

(4) temperature control mode: PID self-tuning adjustment

(5) steady state control error: ± 2 ℃ (at 3200 ℃)

2 temperature detection

2.1 temperature measuring device

for the electron beam zone furnace, if the thermocouple is used to measure the temperature, the thermocouple is fixed on the surface of the raw rod. Because the electron beam directly acts on the surface of the raw rod, the electron beam will first exchange energy with the thermocouple, so it is difficult to measure the real temperature of the sample. Using infrared temperature measuring instrument to measure temperature is a feasible method. It is suitable for non-contact temperature measurement of furnace temperature above 1500 ℃, with fast response. It can be combined with the instrument to form a temperature measurement and control system to realize the automatic control of furnace temperature

2.2 temperature measurement compensation

the raw material rod and electron gun of the electron beam smelting equipment are located in the vacuum chamber. The infrared thermometer should measure the temperature through the observation window. Because the components volatilize and deposit on the observation window during the smelting process, it will bring great errors to the measurement results, and the observation window glass will also affect the accuracy of temperature measurement

in order to avoid the volatile deposition of components in the observation window during the smelting process affecting the accuracy of measurement, the following measures are taken: ① open the measurement window at the appropriate position of the furnace body, and reduce the component deposition effect by lengthening the length of the measurement cylinder to make the "infrared thermometer" lens away from the evaporation source, ② stick the infrared protective film on the inner wall of the measurement window glass, and replace the protective film every 500 hours to reduce the impact of deposited components, ③ The influence of component deposition on the infrared thermometer is obtained through experiments, and the temperature signal of the infrared thermometer is compensated accordingly. ④ the absorption coefficient of the observation window glass and the protective film is measured, and the temperature signal of the infrared thermometer is compensated. Through the above measures, the measurement error is reduced and the accuracy of measurement is improved

3 temperature control system

electron beam smelting furnace belongs to transformer load. For the temperature control of load, the domestic traditional methods are as follows: ① adopt multi tap transformer and switch; ② Connected to saturated reactor; ③ On one side of the transformer ③ the screw rod limit spring is pressed too tightly, and the induction voltage regulator and the coil shifting voltage regulator are used for smooth voltage regulation. Using the above method to control the furnace temperature, the furnace temperature stability is low, the energy consumption is high, the response is slow, and the effect is poor. It is not applicable to the electronic Cambodia smelting furnace with high requirements for furnace temperature control. The test shows that using thyristor voltage regulation technology can obtain good temperature control effect, and it is the best choice for transformer load temperature control

3.1 composition of temperature control system

because the filament of the electron gun in the electron beam zone furnace is tungsten, and the resistivity of the tungsten filament in the cold state is very low (only 0.056 Q · mm2/m at 20 ℃), it can only be powered by transformer after voltage reduction. The temperature of the furnace in the electron beam area is directly proportional to the cathode current, and the cathode current is directly proportional to the number of hot electrons in the filament. In short, the corresponding model can be selected according to the above three points. The filament current is sampled by the filament current sampling circuit and input into the PI regulator. After calculation, it is sent to the filament branch to control the thyristor gate trigger circuit, forming a temperature closed-loop control loop

the high-voltage branch adopts three groups of anti parallel thyristors in series with the primary of the transformer. The thyristor gate is controlled by the phase control trigger circuit, and the conduction angle of the thyristor is continuously changed, so as to smoothly adjust the primary voltage of the transformer. Then the furnace temperature is measured by the infrared thermometer, and its electrical signal is input to the temperature regulator after temperature compensation. After the temperature regulator is connected, it calculates, and the output signal controls the thyristor gate trigger circuit, Form another temperature closed-loop control loop. Through the above double closed-loop temperature control circuit, the accurate control of power and temperature is realized, and the temperature is adjusted by adjusting the thickness of the filament and the acceleration branch. Figure 1 is the block diagram of the control system

3.2 existing technical problems

(1) when the temperature is high, metal vapor will be produced in the furnace. The higher the temperature of the molten pool, the greater the radial drift speed of metal vapor; The higher the central temperature of molten pool liquid level and the greater the temperature gradient, the greater the evaporation. Metal evaporation capacity, speed and density seriously affect the stability of high-voltage current, voltage and cathode current, and then affect the stability of furnace power and temperature

(2) the impact of high-frequency shock wave generated by high-voltage circuit during smelting furnace discharge on the performance of temperature control system and the whole control system

(3) due to the nonlinearity of tungsten wire resistance coefficient, the size of load resistance is greatly affected by temperature. The cold resistance of tungsten is only one tenth of the hot resistance. If no measures are taken, the cold impulse current can reach more than ten times the rated current, affecting the reliability and service life of thyristor voltage regulator

(4) when the thyristor is triggered on, because the trigger phase is asymmetric or the parameters of the anti parallel thyristor are asymmetric, or the trigger pulse is unreliable, the output of the thyristor voltage regulator is asymmetric, producing a large DC component, saturating the transformer core, causing inrush current and burning out the fast fuse and thyristor

(5) inside the high-voltage oil tank, due to the special structure of the high-voltage power supply, there are inevitably parasitic capacitors, which introduce common mode interference signals, resulting in the decline of the ripple index of the beam

3.3 solution

(1) the metal evaporation capacity, speed and density are mainly related to the temperature and temperature gradient of the molten pool liquid level, so the temperature is directly sampled and the temperature is closed-loop controlled to stabilize the high-voltage current and voltage; Since the cathode current is proportional to the number of hot electrons in the filament, the filament current is sampled through the filament current sampling circuit and closed-loop controlled to stabilize the cathode current. Due to the double closed-loop rapid control and regulation, the influence of metal vapor in the furnace on the furnace power and temperature is well suppressed

(2) install capacitors and discharge gaps in electron guns and other positions

(3) select the temperature regulator with the output limiting function. In the cold state, the output amplitude of the thyristor voltage regulator can be limited by the temperature regulator to limit the cold impulse current within the rated current; At the same time, in the phase control trigger circuit, the overcurrent protection circuit is designed. When the thyristor current exceeds 1.5 times the rated current, the overcurrent cut-off link acts to effectively protect the thyristor

(4) try to choose the thyristor with the same or similar parameters as the anti parallel component. The phase control trigger circuit adopts wide pulse trigger and pulse train trigger technology, and its core circuit adopts high-precision and high reliability thick film integrated circuit to ensure the stability and reliability of the trigger pulse, so that the output waveform of the thyristor voltage regulator is basically symmetrical, and the DC component is close to zero

(5) consider EMC design. Due to the special structure of the high-voltage power supply, parasitic capacitance is inevitable. In the design of the high-voltage power supply, double-layer shielding measures and equipotential electromagnetic shielding measures are adopted, and multi-point grounding is adopted to reduce the system ripple and improve the stability of the system

3.4 selection of main components

infrared thermometer: 3ilml5 model of Raytek company of the United States, temperature measurement range: 600 ~ 3500 ℃, distance coefficient 300:1, response wavelength 1.0 m, single laser aiming measurement accuracy 05. Thyristor: through investigation, the pac35p three-phase thyristor regulator produced by Shimaden company in Japan can meet the requirements of electron beam zone furnace. The core circuit adopts the thick film circuit specially designed by the company, which not only has overcurrent protection design, but also has soft start function and high cost performance. The specific type selection of thyristor regulator is pac35p006137 No. The selection of temperature regulator should have PID self-tuning regulation function and output limiting function. The SR73 series temperature regulator of Shimaden company has this function, and the specific type selection is sr73-111-90-00000. There is no fault after operation, and the performance is stable and reliable

4 operation results

after chemical feeding and single crystal drawing, the trial results show that the steady-state control error of the system is ± 2 ℃ (at 3200 ℃), which meets the requirements of temperature control accuracy of electron beam smelting furnace, and the temperature control system operates normally. The purity, grain size, density and other indicators of the pulled molybdenum niobium single crystal are significantly improved, as shown in Table 1 and table 2. The data in Table 1 shows that the gas content in molybdenum metal decreases significantly after electron beam smelting, and table 2 shows that the density of molybdenum melted by electron beam basically reaches the theoretical density of molybdenum. When pulling molybdenum single crystal, due to the formation of a stable conical temperature gradient field in the molten pool,

produces dense, coarse columnar grains inclined to the center from bottom to top

5 Conclusion

the reconstructed electron beam furnace not only has good performance in temperature control, but also adopts PLC control in the supporting equipment. The temperature, controller and important data are connected with the main computer through the bus to realize the whole process control, computer control, display, storage and printing. The temperature control system is also equipped with manual control, and the system is equipped with three kinds of fault audible and visual alarms and interlocks: ① water pressure and water temperature alarms, disconnect the "heating power" and "temperature rise control", close the main valve and stop the diffusion pump; ② Vacuum alarm; ③ Heat it up so that it will not break during the deformation process. The temperature control system will give a fault alarm and disconnect the "temperature rise control". It has a high degree of automation, stable and reliable work, greatly simplifies the monitoring and operation of the furnace, and reduces the labor intensity

references

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