The system is with the control method of PLC combined with VFDs. Each drive point is with buttons for site speed regulation. The block diagram of the system is shown as Figure 1.
Figure 1. System Block Diagram
The VFD is Gozuk EDS1000 series. The whole system can provide the following functions:
- The drive points of each individual shall be able to start and stop separately and shall be adjustable for their speeds;
- The complete machine shall be sync speed increasing and decreasing;
- Speed regulation of one individual can result in a sync speed regulation at this individual and its subsequent individuals;
- The speed shall be automatically recovered as original operating speed after paper tensioning;
- The maximum design speed shall be adjustable according to production requirements;
- The system shall be with emergency stop function.
Control of Speed Chain
According to the paper machine and production process flow, papers on the paper machine prolong longitudinally at wet end due to traction action; papers continue to prolong longitudinally when the dry end starts; after the water content of papers reduces, such prolonging reduces; and when papers enter paper calendaring and reeling machines, they prolong once again due to traction. Therefore, in the whole production line of the paper machine, the speed of each individual is different so as to keep tension for paper web. Meantime, the speed of each individual of the paper machine shall be adjustable so as to avoid loose of paper web or breaking caused by tensioning, with speed regulating range of each individual: 10-15%.
Figure 2. Calculation and Control of Speed Chain
After transmitting the speed command of drive points to VFDs, visit location register to determine the node number of the sub-register. If the node number is not "0", conduct corresponding processing for this node until the whole chain is processed completely. After that, check the node number of brother registers and process another chain. Thus, it is only required to initialize the location register to form any branch speed chain.
As shown in Figure 1, the first individual point of the paper machine is regarded as the main node of the speed chain, namely its speed determines the working speed of the whole paper machine, so regulation of its setting speed is to regulate the speed of the complete paper machine. For example, in PLC, if speed regulation signal is detected, then change speed unit value; the speed at "1" point is just the operating speed setting value of the first VFD, which is sent to the first VFD for execution and to the second for calculation. The speed value of the first individual is multiplied with the ratio of the second individual (b1×a) is the setting value of the second VFD. If the speed of the second individual cannot meet operating requirements, this means the ratio of the second individual is not suitable. You can regulate the ratio of the second individual (b1) to meet required operating requirements. This regulation is equivalent to be with a high-accuracy gear box in PLC so that any stepless speed regulation is available.
During normal production, if the ratio is suitable and it is required for paper tensioning or releasing for some reasons, press appropriate buttons of this individual, and then PLC will add one positive or negative offset to appropriate speed chain to realize such paper tensioning or releasing functions. In the figure, the "2" point includes speed values for commands for speed regulating and paper tensioning and releasing etc., which are send to the second VFD for execution and to the next step for calculation at the same time. And so forth, the control system of speed chains is formed.
Reeling Tension Control
Tension control shall be used for the paper reeling part of the paper machine. If the paper machine has high production requirements for papers, the tension close-loop control can be added in the calendaring part. The tension control is with two methods available for selection: 1) the close-loop control with direct tension detection; 2) the close-loop control with indirect tension calculation and testing. The VFD for tension control shall be with EDS1000 series VFD.
1. Close-loop Control with Direct Tension Detection
The EDS1000 VFD module has two levels: one level for universal functional modules such as PID control, multi-step frequency, and automatic energy-saving operation etc.; another level for special functional modules such as location control, textile application, and constant-pressure water supply application etc. The EDS1000 VFD also has rich programmable modules with complete functions and flexible programming, including: 1 two multi-function comparators that can define faults by themselves; 2 two logical units that can carry out calculations such as "and", "or", and "xor"; 3 two timers that can realize various time-delay functions; 4 one counter that can preset values and can save data after power off; 5 four arithmetical units that can add, subtract, multiply, and divide and can calculate absolute values.
Besides, EDS1000 variable frequency drive also has built-in process PID with complete functions, which is essential in close-loop tension control. Details are shown below:
The input and feedback channels of PID have many categories for selection, and the feedback signals also can be set as many types of calculation results that are calculated from analog values. The PID can be preset and has two sets of parameters that can be switched over during operation. Users can freely carry out programming for resources of EDS1000 VFD, not only able to use its programmable functional modules to coordinate with special functional modules, but also able to use these two types of modules to realize special functions for various industries. The programmable functional modules of the EDS1000 VFD are like a group of jigsaw puzzles which can form numberless ideal patterns in users' hand. This makes it be able to provide solutions platform and integration solutions for various industry requirements and thus it is very valuable for reduction of system cost and increasing of system reliability. more...
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