In modern industrial production,many programmable control systems are used.Programmable controllers can operate normally under harsh operating conditions.However,the control system formed by them sometimes has defects due to planning,equipment,interference and other factors.Some problems are formed due to improper consideration in system planning.Based on the experience in practice,this paper discusses the problems that should be paid attention to when planning the programmable control system.
1.At least 75%of the old skills should be used in a system
“Mature skills”:firstly,programmable controller products or similar plans that have been tested by certain production practices may be able to withstand testing in future production practices;Second,the planning operators should have a resume or have the ability to grasp the skills they need to use.It is feasible to plan and equip the selected skills and planning plan of a programmable control system.Because of a production process control system,once it is made,it is difficult to find opportunities for repeated correction.Some shortages of the planned hardware system and programming software may always be hidden in the completed system.If there are conditions for damage effects,it is difficult to predict the results.
2.The hardware structure and network of the system should be concise and clear
The hardware structure should not be too complicated,and the network configuration should not be too many elements.It should be adjusted by using the networking provided by the programmable controller itself.When forming the template supporting the I/O chassis,it is recommended that the type be brief and strive to be together,and the template density should not be too large.The number of junction points used should not be too many.From the perspective of the manufacturing and wiring process of the current case,the input and output wiring density should not be too high.
3.The function of the control system and the function of the processing system should be severely different
Because the real-time requirements of the process control system composed of programmable controllers are very high,and network communication is allowed to temporarily lose communication contact,and you can recover from the beginning later,but the programmable controller will be out of control before the recovery.In addition to using the network communication of the programmable controller itself to transmit its information,it is better to use the hardware interlocking made of its I/O points,especially the processing of”emergency stop”between the two when using multiple programmable controller systems to form a large system;Although the two systems are exchanging”stop”or”emergency stop”instructions in their own communication scanning,the results are difficult to measure because one party has interrupted the operation during the emergency stop defect,and the other party has not received the interrupted information.The key”emergency stop”of the PLC control system should first cut off the power supply of the implementation organization,and then send its signal to the PLC,so as to obtain the time for safe maintenance of the equipment.
4.The program of programmable controller should be concise and readable
The writing of user software is”plain”.User software can be seen as an orderly”black box”series.Each”black box”can be divided into several typical sentences according to structural language differences.Each sentence and method may be very monotonous,but it must be clear.When planning and writing these statements,if there are too many logic relations that are not easy to reason,perhaps there are too many sentence elements and too many special conditions,it will be very difficult for people to read these statements.Therefore,the readability of user software of a programmable controller,that is,the software written can be read by most people,and it is very important to understand”what happened”when the programmable controller implements this statement.Each program should strive to have a single function and circulation,which is an important condition for software application and maintenance.
5.The programmable control system is preset in hardware and software,and has the key supervision conditions for operation detection
The programmable control system is equipped with colorful graphic operation station/screen supervision,but the screen supervision is not convenient from the perspective of price and the time to reflect the site situation.For critical defects,perhaps in the vicinity of critical mechanical equipment,some indicator lights can be equipped,which can be made of digital output to monitor the normal operation of the program,or to debug the program.A function sign is provided next to the indicator light,which can help the operator acknowledge the normal operation of the programmable sequence control system and timely reflect the defects.
6.Don’t use up its hardware and software resources when planning large and medium-sized programmable control systems
With regard to the planned new system,at least about 15%of the redundancy should be saved in the hardware.In the software development,the user software needs and consumption of computer resources should also be estimated.In particular,there should be room for the application of central relay and counter/timer.Because after debugging and operation,the software will always be corrected,compensated,or even compiled from scratch.The software that has been compiled cannot be modified and improved,and is not practical in engineering.
7.Reasonably equip the redundancy of PLC system
The programmable control system may make redundancy in many ways,and the dual-computer hot standby and cold standby redundancy of the CPU are common methods.In addition,dual-system redundancy,that is,the CPU and all the inputs,outputs and networking communications are completely redundant.Although its price and usefulness are difficult to be accepted in many engineering projects,it is necessary in toxic and harmful chemical production environment.In the planning system,it is necessary to make the redundancy method more economical and useful,and strive to reduce the defects on the equipment.Do not impact the operation or condition of relevant equipment in the process flow due to the defect of a certain equipment.
The above mentioned aspects are the issues that should be paid special attention to when planning the overall plan of the programmable control system.As long as the system is planned carefully,and the system is put into operation,the planners will be less disappointed.
