Based on the principle of less investment, simple construction, reduced wiring of the air compressor control system before the hard modification of the previous control loop, reduction of common relays, and significant improvement of the stability of the air compressor control system, the comparison decided to select S7- 200 PLC system. This system is a micro system in the Siemens PLC series. It has weak network function, small capacity and low price. It is very suitable for use as a control black box with no display function in the control system such as air compressor.
Characteristics of PLC System PLC replaces a large number of intermediate relays, time relays, counters and other devices in the relay control system with software functions, which greatly reduces the design, installation and wiring of the control cabinet. The ladder program of PLC generally adopts the design method of sequential control. This programming method is very regular and easy to master. For complex control systems, the design time of the ladder diagram is less than the time of the system diagram of the relay. The PLC user program can be simulated and debugged in the laboratory. The input signal is simulated with a small switch. The status of the output signal can be observed by the LED on the PLC. After the system installation and wiring are completed, the problems found in the on-site tuning process can generally be solved by modifying the program. The system debugging is much easier than the relay system.
An air compressor PLC system consists of PLC system hardware, PLC application program, signal acquisition and control peripheral equipment, etc. Sometimes, although the most advanced PLC with the highest reliability is adopted, the whole system cannot work normally because the peripheral equipment cannot be matched. For example, in the process of this air compressor control system transformation, we found that if the signal cable is not shielded to ground, the signal transmitted to the PLC is seriously disturbed, and the transmitted signal has no value. Later, all the signal line shields were connected in parallel and grounded at one end of the control room, and the immediate effect was obtained, and the signal was stable without interference. In fact, the above problems are very easy to ignore in the construction of the instrument, because most of the secondary instruments now have strong anti-interference ability, and slight interference is not easy to detect. However, for industrial control equipment such as PLC, it must be strictly in accordance with the instrument. And the industrial computer wiring standards to go to construction. Therefore, the standard construction wiring, reliable peripheral equipment and careful and rigorous program logic are necessary conditions to improve the reliability of the air compressor control system. The reliability of the air compressor control system must be improved from the above three points.
The application hardware construction of the PLC control system causes PLC signal sampling errors mainly because of short-circuit or open circuit (due to mechanical pull, line aging, etc.).
When the transmission signal fails, the on-site signal cannot be transmitted to the PLC, causing sampling error; the mechanical contact is shaken, although the on-site electric shock is closed only once but the PLC is considered closed; although the PLC itself adds a filter circuit, the software increases the differential. The instruction, but because the PLC scan cycle is too short, it may still be in the error of counting, accumulating, shifting, etc.; when the field is faulty, the transmitter reacts to the site with a large deviation or does not work properly; external electromagnetic interference and AC interference When the signal line is unshielded or ungrounded, the external AC signal interference signal causes an error in the PLC sampling. Improve the reliability of the PLC input signal, using high-reliability switch contacts, high-quality transmission wires, reducing the number of terminals and a single meter is necessary. But the standard wiring and shielding grounding measures are more practical.
1) Strict wiring. The cable should be laid in layers, the power cable is in the upper layer, the switch cable is in the middle layer, the analog signal cable is in the lower layer, and the analog signal cable should be shielded. For the small signal line, the twisted pair and the split screen plus the total screen are used. The 420 mA signal line uses a total shielded computer cable.
2) Two-stage shield grounding is indispensable. In the construction, there must be a well-grounded system ground, that is, the cabinet and internal boards of the PLC system are connected to the same ground. When grounding, please pay attention to: A. Connect the internal parts to the cabinet with copper strips. The cabinets are also connected to the ground by copper strips. B. The grounding cables are installed by welding. C, PLC internal circuit boards should also be grounded directly.
The analog input signals must be grounded in a single-ended, common parallel.
For analog signals, in order to prevent different ground potentials at different points, it is easy to form a circulating current in the shielded loop circuit when grounding at both ends, thereby introducing additional interference, and generally performing single-ended grounding. The single-ended grounding has the single-ended grounding of the field and the single-ended grounding of the control room. In order to prevent the ground potential between the various field signal points from being unequal, additional interference is introduced. Therefore, one end of the control room is connected in parallel.
3) Improve the quality of the transmitted signal itself. For the analog input signal whose signal quality is not high, in order to filter out the AC component in the signal, the RC capacitor can be taken at the input of the PLC template to filter out the AC component in the signal. For the switching input signal with low signal quality itself, in order to prevent the external surge voltage from impacting the PLC template, it is possible to take measures at the input end and a Zener diode. The main causes of PLC control failure in the air compressor control system are: solenoid valve coil leakage, valve core stuck, and the solenoid valve can not be normally closed and disconnected after the PLC issues the command, so that the control fails; the control wire is short-circuited or Open the road, etc., to solve such problems can only require maintenance personnel to regularly check the solenoid valve and circuit.
Software programming To make the entire control system run more stable, software system reliability is also essential, so we have specially treated the control software. The input signal is filtered. To improve the reliability of the signal input from the field to the air compressor PLC control system, first select the high-reliability transmitter and various switches, standard wiring, and good grounding measures to prevent short-circuit of the transmission signal line for various reasons. , open circuit or poor contact. Secondly, the digital filtering program is added during program design to increase the credibility of the input signal. The programming method that can be used for digital filtering is to add a timer after the field input point. The timing time depends on the contact jitter and the response of the system. The speed is determined, typically in the tens of milliseconds, to ensure that the contacts are indeed stable and closed before other responses. The programming method used for analog filtering is to continuously sample the live analog signal three times. The sampling interval is determined by the A/D conversion speed and the rate of change of the analog signal. The data of the three samples are stored in three data words DB1.DWO, In DB1.DW2 and DB1.DW4, when the last sampling is completed, the average of the three numbers is obtained and the result of this sampling is stored in the data word DB1.DW4.
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