Application of Frequency Control and Control Technology in Paper Machine Production Line
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1 Introduction
The paper machine is a combination machine consisting of a series of supporting equipment, divided into two parts: the wet end and the dry part. The wet part includes a sizing flow delivery system, a wire section, and a press section, and the dry part includes a portion such as drying, calendering, and coiling. The production process is generally: the slurry is transferred to the front end headbox of the paper machine production line through the sizing flow delivery system, and then the slurry flow passes through the wire section, the press, the front drying cylinder, the back press, the post-drying cylinder, and the calendering. The sub-machines such as the machine and the reeling machine become the base paper; the base paper can additionally enter the outer coating and rewinding machine to produce the finished paper.
Papermaking is a continuous production process, so continuous and orderly control of the production line becomes a bottleneck that limits the quality and yield of finished paper. As the most powerful control method after the mid-1990s, variable frequency speed control has entered the field of papermaking, which is originally DC speed regulation (suitable for large and medium paper machines) and slip motor (suitable for small and medium paper machines), and has achieved good results. Market effect.
AC variable frequency speed control technology is recognized as one of the most promising AC speed control methods due to its excellent speed regulation performance, remarkable power saving and wide applicability in the national economy. The DC speed control system occupies an important position in the development history of paper machines. However, due to the difficulty in maintenance of DC motors and poor environmental resistance, it has seriously restricted the performance price ratio of papermaking machines in the 1990s. The frequency conversion speed control technology can give full play to the inherent advantages of the AC motor (simple structure, sturdy and durable, economical and reliable, good dynamic response, etc.), coupled with the theory of variable frequency speed regulation has formed a relatively independent discipline, frequency conversion The era of speed control technology applied to the production line of paper machines has arrived. In this paper, the application of frequency control and its control technology in the paper machine production line is further elaborated.
2. Frequency conversion speed control of paper machine sizing system
The paper machine sizing system is responsible for conveying the pulp through the slurry pump, the slurry pump, the multi-stage sieve and other equipment to the headbox of the paper machine net section. The slurry pump and the slurry pump are the most important equipments because they are related to the forming. The quality of the paper, especially the weight, must be controlled by frequency conversion to meet the design requirements of the paper machine. Practice has proved that the installation of the frequency conversion device can adapt to different vehicle speeds and different types of usage changes, so that the operation of the slurry pump group is always in an efficient state. In addition, when the inverter control is used, the rotation speed of the motor and the slurry pump is lowered, the mechanical parts such as bearings are worn down, the pump end sealing system is not easily damaged, the failure rate of the pump is greatly reduced, and the maintenance workload is also reduced.
The sizing process must meet the following requirements:
(1) The amount of slurry delivered to the wire section of the paper machine must be stable, and the error does not exceed ± 5%;
(2) The ratio and concentration of the slurry should be stable and uniform;
(3) Purification and selection of the slurry. Therefore, the speed control of the slurry pump and the slurry pump is the most important to ensure the realization of items (1) and (2).
The following takes the slurry pump as an example to illustrate the flow control process: the frequency conversion control adopts the speed control mode of the double closed loop system (as shown in Figure 1), the outer loop is the speed closed loop (V-control), and the inner loop is the current or the revolution. Moment closed loop (I-control). The set value of the pumping speed is on the one hand the V setting obtained by the change of the slurry speed and the net speed ratio, and on the other hand the pressure controller (P-control) from the headbox. The former is the main tone and the latter is the fine adjustment. The pulp speed and the net speed ratio of the paper machine are basically constant, so the speed of the paper machine changes, the V setting also changes, and the rotation speed of the slurry pump also changes; in order to improve the accuracy of the speed regulator and Reflecting the actual process of the headbox, it is usually necessary to take a change of ±5% of the pressure PID control output value of the headbox as an additional speed setpoint for the slurry pump. The actual value of the speed (V actual) is taken from the actual speed sample of the drive motor and can be obtained by a rotating speed motor or a photoelectric rotary encoder.
The set value of the current (M setting) is taken from the output signal taken from the speed loop. The actual value of the current (M actual) is taken from the measured value of the current transformer at the output of the AC drive at each drive point.
Therefore, for the frequency conversion speed regulation of the slurry pump, it is necessary to carry out PID control, and it is necessary to correctly select various parameters of the speed feedback mode and the PID. The set value of the acceleration time of 0 to 100% is generally 60 seconds, and the deceleration time is approximately 30 seconds. The set input value of the inverter needs to have two to many inputs and can perform basic operations. The feedback signal needs to have a port for receiving an analog signal or a pulse signal. Knowing this is very important for selecting the model of the drive.
3. Variable frequency control of paper machine segment drive
China's paper machine sub-section transmission equipment, previously used SCR DC speed control method, due to the existence of slip ring and carbon brush caused by the reliability and accuracy is not high, resulting in the machine behind the machine, the speed is only about 200m / min, it is difficult Compared with foreign high-speed paper machines of 1000m/min. From this point of view, the frequency conversion of the paper machine sub-section transmission machinery has become the trend of the times.
The partial transmission involves many transmission points such as the net part of the paper machine, the press section, the front dryer, the rear press, the post-drying cylinder, the calender, etc., because the paper has the defects of thinness and fragility, so as to prevent the paper from breaking. , curl, wrinkles, indentations, high-speed speed control of each transmission point must be carried out, and the paper is stretched according to the extension rate defined by the paper forming direction. That is to say, from the sizing to the roll, the paper must maintain a certain speed cascade, so that there is tension. It is the variable frequency control based on the partial transmission that emphasizes the online stepless speed regulation and synchronous following performance of each transmission point. Therefore, the frequency converter used in the partial transmission must have the following characteristics:
(1) The speed adjustment range is wide, and the efficiency must be above 90% in the full speed range;
(2) The power factor is higher than 0.9;
(3) The total distortion of the input harmonic current is less than 3%;
(4) Adopting standard device IGBT with high reliability and mature technology;
(5) It can reduce the output harmonic component and effectively reduce the effect of dv/dt noise and torque ripple;
(6) Use the communication function to achieve high-speed serial transmission of data.
For example, the power of four sub-transmission transmission points of a 1092 type tipping paper machine of a paper mill is 11kW for pressing, 11kW for master cylinder, 7.5kW for group cylinder and 5.5kW for coil paper. According to the actual situation and requirements of the manufacturer, the inverter adopts the newly introduced MM420 series from Siemens, and the PLC adopts the Siemens S7-200 series. The network communication adopts RS485 interface, and the protocol method adopts Siemens USS protocol. The main engine speed regulation and the single transmission point fine adjustment adopt the button speed regulation, and can be adjusted in the front and the front of the equipment. Another example is a 2820 kraftliner paper machine in a factory. The inverter adopts the American Model 1336 high-performance inverter, the PLC adopts the AB SLC500 series, and the operation interface adopts the touch screen. The operating parameters storage, automatic generation of data tables, automatic printing of production reports, and analysis of fault alarm status are controlled by the host computer. The network communication adopts the international common PROFIBUSDP communication network. The motor uses an AC variable frequency motor and applies a flux vector closed loop control. In addition, the load distribution of each transmission point can be controlled by calculating the actual power usage of each transmission point, and local tensioning and slack can be performed by jogging without affecting the speed chain during the production process.
The above two cases are more commonly applied to the control principle of the partial frequency conversion control. The control principle of the partial transmission is to maintain the speed cascade and high-speed transmission. The former uses the upper computer (PLC or industrial computer) to calculate the speed cascade. The latter is achieved by the high-speed serial communication capability of the frequency converter itself.
In any case, the entire system is simple in wiring, high in automation, rich in information and compatible in interface, which facilitates the integration of FA and management.
The result of the speed cascade is to form a series of speed chains for the main drive point of the machine's partial drive. As soon as one of the point speeds is adjusted, each subsequent drive point is quickly adjusted. In actual paper machine control, it is often necessary to add a specific transmission control method according to a specific position, especially when the load ratio control is important.
The so-called load ratio control means that in the two intermeshing rolls, in order to maintain the tightness and rationality of the contact, the load distribution of the two rolls is required to achieve the best effect. In the control of the engaging roller of the paper machine press section, calender, etc., according to the process requirements, when the paper web breaks, the two rollers that are engaged are in the disengaged position, and the two rollers are in the closed position when the web passes. In the middle of the engagement roller, in order to maintain the close contact of the two rollers in the machine, the meshing requirement is met, and the load ratio control method is adopted.
The main drive of the meshing roller still adopts the speed cascade control mode, and the load ratio control method is adopted from the transmission. In the load ratio control, the set value of the speed closed loop is increased by 5% based on the main drive set value (V setting). In this case, the output value of the speed loop is the current loop setting. The fixed input value (M setting) will increase, then the output value of the main drive speed loop is multiplied by the load ratio percentage to define the set input value (M setting) from the drive current loop, and it will also be tight. Keeping up with the main drive, the two rollers are more closely meshed and fit.
4. Tension control of rewinding and unwinding systems such as reeling machine, rewinding machine and off-machine coating machine
There are two types of paper forming methods, flat paper and web. Due to the large number of high-speed printing presses, the demand for webs is increasing. Moreover, the web can increase the speed of the paper machine relative to the flat paper. However, the handling of the web by the paper machine is a relatively complicated process, especially in the process of center winding. As the winding diameter increases, the speed of the reel roller must be continuously reduced, and at the same time, the web must be secured. The tension is relatively stable. Therefore, for the rewinding and unwinding system, the tension control is the core technology and the difficulty of the frequency conversion.
For such a complex tension control system, the frequency conversion technology must overcome the following four technical problems:
(1) Simplify the mathematical model of a complex AC asynchronous motor;
(2) Considering the delay and overshoot of the tension feedback signal;
(3) describing the dynamic parameters of the winding tension control process as a time varying function;
(4) Ensure that the tension closed loop has high anti-fluctuation ability and good immediate adjustment.
Currently, the two most commonly used methods of control are:
(1) Speed control winding (SPW), using PID to pass the tension feedback of the load cell, or adjusting the position feedback of the roller to correct the speed reference;
(2) Current Control Winding (CPW), which uses PID to adjust the tension setting, can be applied to the center winding method. This type of control is generally open loop.
The variable frequency control system with tension control is generally composed of modules such as diameter, compensation, torque and speed calculation. Figure 3 shows the basic software structure of its coiling system:
5, the frequency conversion control of the auxiliary part of the paper machine
The auxiliary parts of the paper machine typically include a white water system, a vacuum system, a compressed air system, a chilled water system, a chemical dosing system, a broke recovery system, a ventilation system, and the like.
In the actual design of the paper machine, in order to enable the paper machine to operate continuously and evenly, its auxiliary part generally exceeds 15% to 35% of the maximum production capacity of the paper machine. Therefore, the use of variable frequency speed control and its control technology can effectively reduce the auxiliary energy consumption in actual operation, such as the fan and water pump that were originally controlled by adjusting the valve opening degree, and the belt conveyor that operates at a fixed speed. The following is a typical aspect of the wide range of applications of variable frequency control.
5.1 chilled water frequency control
In the auxiliary equipment of papermaking, coating and sizing often require the use of chilled water to treat the coating to maintain the stability of the chemical characteristics of the coating. The method currently used is the chilled water pump variable frequency speed control system. Its control principle is to use temperature or temperature difference control, because strictly speaking, the difference between the return water temperature and the outlet water temperature of the freezing main machine indicates the heat taken away by the chilled water from the paint tank, so the temperature difference can be used as a control basis. At the same time, since the outlet temperature of the freezing mainframe is generally stable, it is only necessary to control according to the return water temperature in the design.
The frequency control of the chilled water system should be designed as PID control, because most of the inverters have PID function, so the control loop is very simple, with the return water temperature as the feedback signal, and the target temperature can pass the inverter. The panel setting or the host computer outputs the analog signal setting.
5.2 Loss of paper belt machine frequency control
The broke belt conveyor is responsible for transporting the broken paper or processing paper of the cadre to the dry loss system, because the speed of the belt is usually designed according to the maximum speed of the paper machine. Usually, the speed of the paper machine runs at 50% to 80% of the maximum speed. Between the belt fixed speed operation will waste a considerable amount of energy, but also produce too much noise, at this time using frequency control, so that the speed of the belt conveyor follows the speed of the paper machine, and the starting frequency of the belt conveyor is 20 ~30Hz, so that the belt can quickly react to avoid paper jams and reduce energy consumption.
6, the conclusion
With the development and maturity of frequency conversion technology, the inverter has many advantages such as excellent speed regulation performance, simple operation, perfect function, easy to realize automatic adjustment, etc., which will replace the traditional DC speed control system on the paper machine and become the paper machine electrical. The leader of the transmission.