plc未来发展趋势外文文献综述基于plc的自动售货机控制系统设计

plc未来发展趋势外文文献综述基于plc的自动售货机控制系统设计内容摘要：

experience that has collected over 5year practical work with PLCs. Strong and weak sides, positive and negative practices are discussed. Suggestions weather to use programmable controllers or not have been formed to help decision makingThe most important decision in planning PLCbased system is selecting processor type. Mistakes mean extra costs for modifications or even need for pletely new devices. The most mon error is overestimating programming possibilities of smallsized processors. There are several methods for selecting PLC. Unfortunately most of them focus on electrical side of PLCs and imum count of signals allowed. They don’t involve analysis to determine possible special needs for user program or municationsBased on several existing methods, practical experience and future trends a new method for selecting PLC was developed. Ahti Mikkor has gained his experience by taking part in more than 15 bigscale automation projects. These projects include development of power consumption monitoring system in AS Kunda Nordic Cement factory, renewing testing rig for flowmeters, building Ahtme powerplant turbine safety systems, water treatment plants in towns Rakvere and P?lva, waste water treatment plant in town J?geva and development of monitoring system for central heating work in Tartu. 2. PROS AND CONS IN USING PROGRAMMABLE LOGIC CONTROLLERS Positive arguments The main advantage that programmable controllers provide is flexibility Jack, 2020. Behaviour of the system can be easily changed via program without any other alterations. Special devices for example make any changes in control algorithm very hard to implement. Flexibility makes PLCs well suitable for frequently changed applications, for example in roboticsIn PLCs the relations between inputs and outputs are determined by user program. By using advanced programming technologies it is much easier to implement plex control algorithms than in any hardwired solutions. It makes PLCs very petitive for plex tasks, for example in controlling chemical processes Special modules allow vast amount of different signals to be connected to the PLC system. Use of PLCs should be considered in applications that require some “special” input or output signals. Typical example would be positioning using reference data from highspeed input Typically PC visualization software packages are made for PLCs. Some special devices have also PC software packages. Wide range of munication options between PLCs makes it possible to gather all information from field devices into one central control pointCommunication lines between PLCs allow using information collected from other parts of the system in local process control. Modern munication technologies enable remote diagnostics and configuration Jack, 2020. These two significantly reduce overall maintenance costs of the Negative arguments Programmable controllers are not equipped with enough memory to store big amounts of data. Although future trends show growth in PLC memory sizes, special devices recorders are still better suited for standalone datalogging applications. For worked solutions there is possibility to use visualization software packages together with PLCs to archive collected data in any database format necessary. If logged data amounts are small or there are also control functions included, it’s reasonable to still use PLC s. About visualization software packages it’s good to know that in standard versions most of them do not support offline recording so that after。