The reliability of the entire PLC control system depends mainly on the peripheral equipment of the PLC, such as the stroke switch in the input device, the contacts in the output device, the relay and the solenoid valve. In addition, considering the preparation of the software program, if a program with monitoring can be compiled, it will also greatly improve the reliability of the system. Here are some suggestions on how to improve the reliability of the PLC control system. The main factors affecting the reliability of the programmable controller control system are briefly described, and the working environment of the programmable controller, the requirements of the power supply, the way of grounding and connecting lines, the design of the degraded operation, the input circuit and the output of the control system. Some feasible methods and measures are proposed in many aspects, such as circuit and programmable software programming.
Programmable controller (hereinafter referred to as PLC) is a product of the combination of microcomputer technology and relay conventional control concept developed on the basis of program controller and microcomputer control. In a broad sense, PLC is a computer system, than general The computer has a stronger input and output interface connected to the industrial process and has become the basic device of the automation control system. Senior Technician's papers published in the journal Science and Technology Innovation and Application magazine is a national journal published by the General Administration of Press and Publication of China. Heilongjiang Provincial Light Industry Federation is headed by Heilongjiang Light Industry Design Institute. It has a certain academic and application. The academic literature on value and the discourse articles reflecting new achievements, new processes, and new products in various disciplines and fields provide a platform for academic exchanges for scientific and technological workers.
PLC has been widely used in machinery, metallurgy, chemical, automotive, light industry and other industries, and has basically replaced the traditional logic control of relays and contacts. The use of PLC to control system equipment, the reliability of its work is much higher than the simple relay and contactor control. The reliability of the system also has great benefits. Here are some suggestions on how to improve the reliability of the PLC control system.
1. Consider improving the reliability of PLC from PLC peripheral equipment.
PLC is a control device designed for industrial production environments. When the working environment is harsh, such as strong electromagnetic interference, high humidity, power supply, input and output circuits, etc., the reliability of the control system will be affected.
1.1 Requirements for the working environment
The ambient temperature of general PLC operation should be in the range of 00C-550C, and avoid direct sunlight; keep away from the heat source during installation, ensure sufficient heat dissipation space and ventilation conditions; the relative humidity of air should be less than 85%, no knot Dew to ensure that the insulation of the PLC is good. The PLC should be avoided from being installed in a place with vibration; the conditions allowed for the vibration source should be installed in accordance with the requirements of the product manual, or the vibration-damping rubber pad or other anti-vibration measures should be taken. When there is dust and harmful gases in the air, the PLC should be closed and installed.
1.2 Power requirements
Different PLC products have different requirements on the power supply, including the voltage level, frequency, AC ripple coefficient and power supply mode of the input and output.
Where the electromagnetic interference is strong and the PLC reliability requirements are high, the power supply of the PLC should be separated from the power supply and the control circuit power supply. If necessary, the shielded isolation transformer can be used for power supply, and the series LC filter circuit can be used. In the design, the external DC power supply should be a regulated power supply, and the power supply should have a margin of 20%-30%. For the true-current power supply provided by the control itself, the maximum current that it can provide should be known to prevent damage caused by over-current.
1.3 PLC I / O circuit
(1) Since the PLC receives input signals such as switching quantity and analog quantity through the input circuit, the quality of the components of the input circuit and the connection mode directly affect the reliability of the control system, such as buttons, stroke switches, etc. Enter the contact amount of the contact contact is good, the wiring is firm, and so on. The mechanical limit switch on the equipment is a component that is more prone to failure. When designing, the high-reliability proximity switch should be used instead of the mechanical limit switch. In addition, the selection of the normally open and normally closed contacts of the button also affects the reliability of the system.
(2) When there is an inductive load at the input end, in order to prevent the recoil induced potential from damaging the module, connect capacitor C and resistor R (AC input signal) at both ends of the load, or connect the flow diode D (DC input signal). As shown in Figure 4, when the AC input mode is adopted, the CR selection should be appropriate to achieve better results. Through the test device test, when the load capacity is below 10VA, generally 0.1 micro-depletion +120 ohms; when the load capacity is above 10VA, it is generally suitable to select 0.47 micro-depletion +47 ohms. When the DC input mode is adopted, it is tested. The rated current of the diode should be selected as IA, and the rated voltage should be greater than 3 times of the power supply voltage.
(3) When there is an inductive load at the output, it is found through experiments that if the AC load is used, the CR surge absorber should be connected to both ends of the load; if the AC is 100V, 200V, the voltage and the power is about 400VA, The CR surge absorber is 0.47UF+47 ohms. The closer the CR is to the load, the better the anti-drying effect. If it is a DC load, the diode D is connected to both ends of the load, as shown in Figure 6. Close to the load, its reverse withstand voltage should be 4 times the load voltage.
2, from the PLC software program to improve the reliability of the control system
2.1 Health timeout detection
In order to improve the reliability of the PLC control system, a timer can be set up as part of the monitoring program to detect the running status of the system. If the program runs normally, the timer is cleared, if the program runs. If a fault occurs, such as an infinite loop, the timer cannot be cleared within the set time. At this time, the PLC issues an alarm signal. When designing the application, this method is used to monitor the operating status of each part of the system. If a PLC is used to control an object, a timer can be defined to monitor the running state of the object. The set time of the timer is the maximum time required for the object to work. When the object is started, the timer is also started. If the running program of the object ends at the specified time, a work completion signal is issued to clear the timer, indicating that the running program of the object is normal. Otherwise, it is not working properly, and an alarm signal or stop signal is issued.
2.2 logical error detection
When the system is in normal operation, the input and output signals of the programmable controller and internal signals (such as the state of the auxiliary relay) have a certain relationship with each other. If an abnormal logic signal occurs, it indicates that a fault has occurred. Therefore, you can write exception logic relationships for common faults. Once the exception logic relationship is NO, you should follow the fault. For example, in a mechanical movement process, the PLC input addresses corresponding to the two limit switches are X001 and X002 respectively. The two signals will not be in the NO state at the same time. If they are in the NO state at the same time, it means that there is at least one limit switch. If it is stuck, it should be stopped.
2.3. Degraded operation design
Degraded operations refer to designs that include manual operations at design time, such as the design of emergency stops, the design of the start and stop of critical equipment, and the design of restart functions. In this way, in the event of a failure, a degraded operation can be employed, that is, a manual opening and closing operation is performed on some or all of the equipment to avoid damage to the equipment or injury to personnel. In addition, it is also possible to consider from fully automatic to semi-automatic to manual operation in the design.
3, the conclusion
The reliability of the PLC control system is related to many factors. This objective factor also affects the stability of the control system. By adopting the design of the correct hardware circuit, selecting high-quality components, improving the working environment, and preparing monitoring procedures, etc. The operation reliability and stability of the PLC control system can be greatly improved.
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