Open-loop Control
The open-loop control of discrete quantities is the most basic control function of PLC. The instruction system of PLC has strong logical operation capabilities, which can easily realize various logical control methods such as timing, counting, and sequential (stepwise) control. Most PLCs are used to replace traditional relay contactor control systems. [9]
Closed-loop Control of Analog Quantities
For closed-loop control systems of analog quantities, in addition to having discrete input and output points, there also need to be analog input and output points to sample the input and adjust the output to achieve continuous regulation and control of parameters such as temperature, flow, pressure, displacement, and speed. Currently, PLCs not only have this function in large and medium-sized machines, but also some small machines also have this function. [9]
Digital Quantity Control
When the control system has rotary encoders and pulse servo devices (such as stepper motors), PLC can be used to realize the function of receiving and outputting high-speed pulses, achieving digital quantity control. More advanced PLCs have specially developed digital control modules, which can realize curve interpolation functions. Recently, new motion unit modules have also been launched, which can provide digital quantity control technology programming languages, making PLC digital quantity control more simple. [9]
Data Acquisition and Monitoring
Since PLCs are mainly used for on-site control, collecting on-site data is a necessary function. On this basis, connecting PLCs with upper computers or touch screens can not only observe the current values of these data but also conduct timely statistical analysis. Some PLCs have data recording units, which can use the storage card of a general personal computer to insert into this unit to save the collected data. Another feature of PLCs is that they have many self-checking signals. By taking advantage of this feature, PLC control systems can achieve self-diagnostic monitoring, reducing system failures and improving system reliability. [9]
PLC Applications in Intelligent Manufacturing
In the field of intelligent manufacturing, through real-time data analysis, intelligent PLC systems can automatically adjust operating parameters to adapt to changing production needs and environmental conditions. This not only speeds up the production process but also improves product quality and system stability. The application framework is as follows. [11]
Process Control Layer: This layer is the core of PLC and is responsible for direct control of the production process. It completes the functions of data acquisition, real-time storage, processing, and transmission of process variables. The human-machine interface should not be directly connected to the process control layer.
Operation Monitoring Layer: This layer is the main human-machine interface of PLC and is responsible for processing and storing data from the process control layer and achieving centralized operation management functions. The equipment of the operation monitoring layer should not directly have process interface units, and process variables should not be connected to PLC through the operation monitoring layer equipment.
Data Service Layer: This layer is the intermediate layer for data exchange between the internal network of PLC and the external network, used to provide data services to users indirectly involved in the production process. The data service layer should exchange data with the process control layer and operation monitoring layer through proxy servers or industrial-grade firewalls, and should not directly establish data communication. Data services related to production operations or system management (such as alarms, historical records, diagnostics, etc.) should not be implemented in the data service layer.