(1) From the perspective of control methods
Electrical control systems typically use hardware wiring for control logic, employing series or cascade connections of relay mechanical contacts. This results in numerous and complex wiring connections, large size, and high power consumption. Once the system is configured, modifying or adding functionality is difficult. Furthermore, the limited number of relay contacts significantly restricts the flexibility and scalability of electrical control systems. In contrast, PLCs utilize computer technology. Their control logic is stored as a program in memory. Changing the control logic only requires modifying the program, making it easy to modify or add system functionality. PLCs have fewer wiring connections, smaller size, and lower power consumption. Moreover, the "soft relays" in PLCs are essentially the states of memory units, allowing for an virtually unlimited number of contacts. Therefore, PLC systems offer superior flexibility and scalability.
(2) From the perspective of working methods
In relay control circuits, when power is on, all relays in the circuit are in a constrained state; that is, relays that should be activated are activated simultaneously. Relays that should not be activated are prevented from activating due to certain conditions. This operating mode is called parallel operation. However, the PLC's user program executes cyclically in a specific order, so each soft relay is in a periodic cyclic scanning activation process. The activation order of relays constrained by the same condition is determined by the program scanning order. This operating mode is called serial operation.
(3) From the perspective of speed control
Relay control systems rely on the movement of mechanical contacts to achieve control, resulting in low efficiency. Mechanical contacts are also prone to chattering. In contrast, PLCs control semiconductor circuits through program instructions, offering high speed (program instruction execution time is in the microsecond range) and eliminating the contact chattering problem.
(4) From the perspective of timing and counting control
Electrical control systems typically use time relays with delayed actions for timing control. However, the delay time of time relays is easily affected by changes in ambient temperature and humidity, resulting in low timing accuracy. In contrast, PLCs use semiconductor integrated circuits as timers, with clock pulses generated by crystal oscillators, offering high accuracy and a wide timing range. Users can set timing values in the program as needed, making modifications convenient and unaffected by environmental factors. Furthermore, PLCs have counting functions, while electrical control systems generally lack this feature.
(5) From the perspective of reliability and maintainability
Because the electrical control system uses large mechanical contacts, it suffers from mechanical wear, arc burns, and has a short lifespan. The system also has numerous wiring connections, resulting in poor reliability and maintainability. In contrast, the PIZ system uses contactless semiconductor circuits for most of its switching actions. These circuits have a long lifespan and high reliability. The PLC also has self-diagnostic capabilities, allowing it to detect its own faults and display them to the operator in real time. Furthermore, it can dynamically monitor the execution of the control program, facilitating on-site debugging and maintenance.
Advantages of PLC control systems compared to relay control systems
PLC control systems are currently widely used in industrial automation control, and they have the following advantages compared to traditional relay control systems:
(1) Fast reaction speed, low noise, low energy consumption and small size.
(2) It is powerful, easy to program, and can be modified at any time.
(3) It has high control precision and can perform complex program control.
(4) It can automatically detect the control process.
(5) The system is stable, safe and reliable.
