Discussion on the technology of precise control of the rotational speed and temperature of the constant temperature culture oscillator
Tang Xinnan Shenshui Xingyan Guanghao, Shanghai Zhicheng Analytical Instrument Manufacturing Co., Ltd. Shanghai ~
The constant temperature culture oscillator (hereinafter referred to as the constant temperature shaker) is a sample pretreatment device that has important applications in medical, biological, molecular, pharmaceutical, food, environmental protection and other research applications. In view of the fact that a small amount of active substances such as bacteria and cells as samples need an excellent growth environment during growth, the control precision of a series of parameters such as temperature, rotation speed and even CO2 and illuminance of the constant temperature shaker is raised. Requirements.
How to achieve precise control of various indicators of the constant temperature shaker is a common concern of manufacturers in the industry.
I. Solutions for affecting the stability of operation and precise control of speed
1, sports institutions
The oscillation mode and rotation speed of the constant temperature shaker affect the uniformity of sample mixing and the amount of dissolved oxygen (the degree of fusion of oxygen, carbon dioxide and microorganisms), which directly affects the efficiency of biological reaction. At present, the common mode of the constant temperature shaker is eccentric rotation or linear reciprocating motion. The oscillation range is single fixed type, graded adjustable type and continuous stepless adjustable type. The eccentric shaker produced by ZHICHENG company mostly adopts continuous stepless adjustable type, and different speeds are matched with different eccentricities to obtain very satisfactory experimental results. The tray structure and movement mode of the constant temperature shaker have a great relationship with the stability, noise and equipment life of the constant temperature shaker. ZHICHENG has a number of such invention patents, such as multi-amplitude stepless adjustable technology, cantilever reciprocating drive technology, etc. These new technologies are used to ensure smooth operation of equipment, reduce noise, drive without dead angles, and deal with unbalanced loads. All aspects have played a positive role.
2, speed measurement
The measurement of the rotational speed can be done by using a speed measuring motor. In the digital instrument, the measurement of the rotational speed adopts a digital direct measurement technique, which can be realized by the M method (measuring frequency) and the T method (measuring period). The quartz crystal is used as the speed measurement reference to ensure the accuracy and long-term consistency of the speed measurement.
3, the choice of drive motor
The drive motor used in the constant temperature shaker can use various types of DC motors, DC brushless motors, single-phase AC motors and three-phase AC motors. Different motors need to be equipped with different types of governors.
The speed adjustment can be used with various types of DC motors, DC brushless motors, single-phase AC motors and three-phase AC motors. Different motors need to be equipped with different types of governors. Practice has proved that the torque control of the DC motor is the most stable and the speed regulation performance is the best. The DC motor is an inductive load, and the DC power supply can be used to perform chopper voltage regulation and speed control by PWM. It is also possible to use the SCR to directly phase-trigger the AC power supply to realize the voltage regulation and speed regulation. The speed controller needs to add the current feedback, voltage feedback and speed feedback required for the stabilizing torque. In addition, there is a need for a rising and falling speed control circuit and an overcurrent protection circuit. The DC motor starts smoothly, the starting torque is large, and the speed regulation performance is good. However, the brush wears and needs regular maintenance, so it is not recommended. The single-phase AC motor has a simple structure and convenient maintenance, and can also be used as a drive motor for a constant temperature shaker. However, since the constant temperature shaker needs to drive a constant torque load, it cannot be adjusted by the electric fan speed regulation method. The solution is to install a small speed measuring motor on the AC motor end cover and introduce the speed measurement into the circuit. Feedback can achieve stable speed regulation of the motor under constant torque load. The three-phase AC motor has a simple structure and can be equipped with a frequency converter to achieve stable speed regulation of the motor. The three-phase AC motor is suitable for large temperature shaker applications. The rated speed of the AC motor is generally higher than 1000-2000 (r/min), so it is necessary to reduce the speed through the speed reduction mechanism to meet the required speed requirements of the constant temperature shaker. The speed reduction device can be decelerated by a gear box, a worm gear or a pulley, and then the swing conversion device is driven to realize a linear or elliptical oscillation function.
At present, a new DC brushless motor direct drive technology has been used in ZHICHENG. The brushless DC motor is a driving technology that has developed rapidly in recent years. The internal permanent magnet acts as a fixed electrode to reduce the energy loss. The Hall element is used for position detection to realize the automatic commutation of the three sets of coils. With the motor governor to achieve motor start, commutation and continuous speed control operation. ZHICHENG adopts self-designed and self-supporting multi-pole low-speed and high-torque DC brushless motor, which can directly drive the direct connection between the motor and the pallet, omitting the complicated speed reduction mechanism. The constant temperature shaker realizes silent operation, long-term operation and long The effect of life use. This is a new and advanced method of oscillating speed regulation.
4, the speed control
The speed control of the constant temperature shaker motor can adopt the traditional PID control method or the fuzzy control method. Fuzzy control is an artificial intelligence control method. It does not need to control the precise mathematical model of the object. By summarizing the experience of skilled operators, applying the if-then statement combination, the artificial intelligence control methods are combined to form a control table, and then through En and ΔEn. The input quantity look-up table obtains the control quantity, and finally converts it into an output value to implement control. Stable control effects can be obtained through reasonable table design and data flow technology. The fuzzy control technology is especially suitable for objects that are difficult to control with mathematical model description, nonlinearity, large hysteresis, etc., to improve control accuracy and stability.
Considering the protection of experimental samples, the constant temperature shaker microcontroller should have an overspeed alarm and an overspeed protection. When the measured speed exceeds the alarm set value, the device emits an audible and visual alarm signal. When the measured speed exceeds the protection set value, the oscillating tray stops oscillating. The overspeed protection device should choose a separate overspeed control device. The control device should have independent speed detection function and control function. When the tray rotation speed exceeds the overspeed setting value, the power supply of the speed regulation system will be automatically cut off quickly to ensure the safety of samples and equipment. The speed setting value on the independent overspeed control device should be synchronized with the speed setting value on the main controller to prevent false alarm action.
The speed control of the constant temperature shaker also requires the introduction of a slow start function to prevent the sample on the tray from tipping or detaching due to the strong centrifugal force generated by the quick start of the tray. When the start is started slowly, the speed should accelerate slowly from zero, gradually accelerate to the speed set value and enter the constant speed state. In addition, the constant temperature shaker also needs to set the automatic door control mode. When the user opens the device door, the tray should stop running. After closing the door, the tray should resume operation in the slow start mode to ensure the safety of the sample on the tray.
Second, on the energy-saving temperature control technology
1, the choice of temperature measurement components
Temperature can be measured using precision thermal resistance (usually PT100), thermocouple, semiconductor temperature sensor or integrated digital sensor. The temperature is converted into a voltage signal by measuring the amplifying circuit, and then subjected to A/D conversion, and the temperature is measured by nonlinear correction.
2. Closed-loop feedback control of temperature
The open loop control is applied to a control occasion in which the control amount has a one-to-one correspondence with the controlled parameters. For example, lighting control in numerical control, program control and shaker applications. This control method is not suitable for temperature control. There is no direct correspondence between the heating power of the constant temperature shaker and the measured temperature value. Usually, the temperature is collected, compared with the set temperature, and the deviation value is obtained, and then the controller (commonly known as the regulator) calculates the control amount (output). Then, through the control mechanism (such as heater, voltage regulator) to control, by repeating this process, the deviation gradually becomes zero, and the whole repeating process is the control of closed-loop feedback.
3. Temperature PID control and parameter setting
The temperature controller can adopt the position control, that is, the measurement temperature is lower than the set temperature to turn on the heating, and the measurement temperature is higher than the set temperature to turn off the heating. The control output of the position control has only 0 or 1 states, and the actuator can be relatively simple. Since the control object has inertia and hysteresis, the effect of using the position control is that the temperature is always in the up and down state. The output of the proportional control is a continuous quantity (which can be defined as a 0-100% interval), from no heating at all to full power heating. The proportional controller can be understood as the larger the deviation, the larger the output control amount. There is a variable coefficient in the middle, that is, the proportional band P (0-100%, the proportional range of the input deviation when the output full amplitude changes, is the reciprocal of the magnification), and the smaller the P is, the larger the magnification is, the more sensitive the control is. . But when the deviation = 0, the control output = 0, but the actual output = 0 can not achieve temperature control, so the deviation can not be 0, in theory, this is an error control. In order to achieve error-free control, the integral I is usually added. In the integral control, as long as there is a deviation, the output accumulation increase or the accumulation decrease is determined according to the deviation polarity until the deviation is eliminated. Therefore, only the addition of points can finally eliminate the error. The integral parameter unit is seconds, and I=300 seconds means that after 300 seconds, the output accumulation is equivalent to the output of the proportional action. Obviously, the smaller I is, the faster the integral output, indicating the greater the ability to eliminate errors. The D (differential) action acts on the rate of change of the input and acts as a leading measure of temperature fluctuations. Theoretically, when the input step is abrupt, the output is infinite, but the maintenance time is zero, so that the control disturbance is too large, and the control effect on the inertial object is not obvious. The incomplete differential algorithm is used in the actual control. The differential gain is 5-10. The differential time refers to the differential decay time. Obviously, the larger D is, the stronger the differential action is. The D is set to 0 to indicate no differentiation. Practice shows that using PID closed-loop control algorithm, combining the three control functions of PID can get better control effect for most control systems.
Parameter selection for different control objects to achieve rapid system stability is called system parameter tuning. When the parameters are set incorrectly, the system may be unstable and produce large errors, which are manifested by the temperature being constantly fluctuating and even diverging. The ideal control object is a large time constant (large inertia, good thermal insulation) and small hysteresis (sensor measurement response sensitivity). In order to reduce the fluctuation caused when the set value changes, the PID control algorithm in which the measured value is differentiated may be used. In order to speed up the large deviation, the actual measured value changes rapidly to the vicinity of the set value. It can be considered that when the deviation is greater than a certain value, the position control (can be full power heating) is adopted. When the deviation is reduced to a certain value, the PID is entered to achieve precise control. Quasi-continuous control method, PID control output is a continuous quantity (0-100%), called continuous control, usually used for electronic regulators, electric or pneumatic continuous actuators. For the large inertia control object of temperature control, the zero-crossing trigger SCR switch can be used to work in the power-regulating mode, and the output average power is output according to the PID control law. In this way, the actuator is simple, low in price, and low in interference, although the control of the continuous control of the PID can be achieved by using a simple switch-type actuator control.
4. Design of refrigeration system and application of electronic stepping hot gas bypass valve
The method of electric heating to control the temperature rise is very simple, and the PID control algorithm can achieve accurate and stable control effects. When the temperature set point is lower than the ambient temperature (actually due to the system itself may generate heat and the system is stable fluctuation range, generally lower than the ambient temperature +5 degrees), the refrigeration system needs to be turned on. Commonly used refrigeration systems are semiconductor refrigeration systems, refrigeration systems using conventional compressors or inverter compressors. Among them, the advantages of the semiconductor refrigeration system are simple structure and convenient temperature control; the disadvantages are inconvenient heat dissipation and large cooling capacity, so it is rarely used in the constant temperature shaker; the compressor refrigeration system directly adopts the frequency conversion control technology, due to the frequency conversion control The precise control of the cooling capacity in the temperature mode is difficult to realize, so it is difficult to directly apply to the constant temperature shaker which requires high temperature control accuracy.
The compressor unit refrigeration system, which is currently widely used in the industry, is a waste of energy and a method of deviating from environmental protection requirements from the perspective of current energy conservation and environmental protection, which needs to be improved. Let's take a closer look at the reasons.
The conventional refrigeration system currently in common use has the disadvantage that the start-stop time of the compressor is limited. When the pipeline pressure at the high and low ends of the compressor has not reached the self-balancing state, and the refrigeration system detects the need to cool down and sends a signal to start the refrigeration system, the compressor will be in an unsuccessful thermal saturation state. This phenomenon is extremely rare. It may directly cause a "cylinder" to burn the compressor. Therefore, the compressor must adopt the method of delaying start and stop, but the application of this method increases the fluctuation of the control temperature, which does not meet the requirements of the constant temperature shaker for stable temperature requirements, so the refrigeration system cannot adopt simple start, Stop the compressor to control the temperature. The introduced thermal compensation method controls the temperature, that is, the refrigeration system is always turned on (or partially turned on), and then the heating compensation method is used to achieve precise temperature control under cold and heat balance. Although this method is good, the "cold heat balance" in the system is too much use of heaters to hedge the excess cold. This is undoubtedly a waste of energy. Under the general trend of promoting energy conservation and environmental protection, this The shortcomings of the program are obvious. So is there still an efficient and energy-efficient cooling solution? The answer is yes. The three-way solenoid valve is used to switch the throttle circuit to control the cooling capacity. This system uses a three-way solenoid valve in the throttle circuit to automatically switch between cooling or cooling bypass to achieve the adjustment of the cooling capacity. . When the set working temperature is at a higher value, the excess cooling amount will directly return to the compressor and the peripheral pipeline through the bypass line, which is likely to cause serious frosting of the relevant pipelines including the compressor. It is also possible that the abnormal operation of the refrigeration system causes the temperature to be out of control. In order to avoid frosting, it is also necessary to adopt a defrosting technique. At present, ZHICHENG has successfully developed a more advanced refrigeration control method based on introduction, digestion, absorption and re-innovation - step-by-step electronic bypass expansion valve throttling control technology.
Hot gas bypass is one of the best ways to defrosting or preventing frosting. The principle is that a part of the high-temperature steam generated by the compressor is directly sent to the evaporator through the hot gas bypass, so that the evaporator becomes a mixing chamber of high and low temperature gas, and the cavity is in a dynamic equilibrium state of cooling and defrosting. Since the evaporator temperature at this time is not very low, the occurrence of frosting is avoided. This technology not only solves the temperature control requirements, but also satisfies the long-term stable operation of the equipment under low temperature conditions. In addition, since this technology reduces the operation of the heater, energy consumption is relatively saved. The control of the hot gas bypass can be controlled by a solenoid valve to adjust the amount of bypass, and an electronic expansion valve can be used to precisely control the bypass. The stepper motor controller of the refrigeration system specially designed by ZHICHENG Company can realize the fine and precise control of the valve position. The power consumption of the controller is very small, especially after the refrigeration system enters the steady state of the control, the valve position remains basically unchanged, and the stepping motor has almost no energy consumption. In addition, the use of the electronic bypass expansion valve also avoids the solenoid valve noise that periodically and frequently moves, and realizes the silent operation of the refrigeration system. ZHICHENG adopts the imported electronic expansion valve, and has also developed the electronic expansion valve control driver which is specially equipped under the subdivision technology. The two are used together to obtain very precise and very stable control and very economical effect. . This technology has been widely used in various products of ZHICHENG's various constant temperature shakers.
Third, the factors affecting the accuracy of temperature control and solutions
A typical constant temperature shaker or cabinet exhibits a temperature resolution of 0.1 degrees, which has met the needs of a typical factory or laboratory. The resolution is determined by the circuit design. Generally, the actual internal circuit resolution should be higher than the display resolution, so that it is possible to keep the display value of the measured temperature unchanged. Accuracy is the repeatability of measuring temperature multiple times in a working environment. Accuracy is related to resolution, and is related to the accuracy of the measurement sensor, the thermal stability of the circuit and components, and the measurement method. The uniformity of the temperature inside the box depends on the structural form of the cooling heater in the box, the structure of the air duct, and the air volume of the circulating fan. For the temperature display value, ZHICHENG products are adjusted according to the average temperature of 9 points in the box. If the user has higher requirements for accuracy, the absolute value can be corrected under the display comparison of the standard measuring instrument that satisfies the accuracy (ATC function, which can correct 0 degree and full scale).
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