Hardware failure
Sensor failure
Abnormal signal: The temperature sensor may have a large deviation in the measured value. For example, due to aging or damage of the sensor probe, the measured temperature is much higher or lower than the actual temperature. The pressure sensor may output an erroneous pressure signal, which may be due to corrosion of the sensor’s sensitive element or blockage of the pressure interface, making the pressure reading inaccurate.
Communication interruption: Some smart sensors are connected to the PLC through a communication interface (such as RS-485, etc.). When the communication line is short-circuited, broken, or electromagnetically interfered, the communication between the sensor and the PLC will be interrupted, and the PLC will not be able to obtain the sensor data.
Actuator failure
Motor failure: The stirring motor of the reactor may be overloaded, short-circuited, or burned. Overload may be caused by the stirring paddle being stuck by foreign objects, or the motor carrying too much load. Motor short circuit may be caused by damaged motor winding insulation, which will cause the motor to fail to start normally or stop suddenly during operation, and may cause the circuit breaker in the electrical control cabinet to trip.
Valve failure: It is a common problem that the valve cannot be opened or closed normally. For example, if the motor of an electric valve fails or the transmission mechanism is stuck, the valve will not be able to move according to the control instructions of the PLC. For pneumatic valves, it may be insufficient air source pressure, cylinder leakage or solenoid valve failure, which makes the valve opening unable to be adjusted.
PLC hardware failure
CPU failure: When the CPU (central processing unit) of the PLC fails, the entire control system will not be able to operate normally. CPU failure may manifest as program failure, system crash or continuous restart. This may be caused by overheating, static shock or quality problems of the chip itself.
I/O module failure: Input module failure may cause the PLC to fail to correctly receive sensor signals. For example, if the channel of the analog input module is damaged, the corresponding sensor signal cannot be transmitted to the PLC. Output module failure will affect the control of the actuator, such as a certain output point of the digital output module is damaged, and the control signal cannot be output to the motor or valve relay.
Software failure
Program logic error
This is a common type of PLC software failure. For example, in the program for controlling the temperature of the reactor, incorrect parameter settings of the PID (proportional-integral-derivative) controller may lead to unstable temperature control, temperature oscillation or failure to reach the set temperature. Errors in conditional judgment statements in the program can also lead to errors in the control process. For example, in the reactor level control program, incorrect judgment conditions may cause the feed valve and the discharge valve to not open and close correctly.
Program loss or damage
PLC programs are stored in the memory. When the memory fails (such as flash memory chip damage) or is affected by electromagnetic interference, power fluctuations and other factors, the program may be lost or damaged. Once the program is lost, the PLC will not be able to operate according to the predetermined control logic, and the control function of the reactor will be completely lost.
Communication failure
Internal communication failure
Inside the PLC control cabinet, there may be a communication failure between the PLC host and the I/O module. This may be caused by loose communication lines, damaged communication interfaces or incorrect communication protocol settings. For example, when the communication between the PLC host and the analog output module is interrupted, the control signal cannot be accurately sent to the actuator, affecting the normal operation of the reactor.
External communication failure
Communication between the PLC control system and the host computer (such as a monitoring computer) or other external devices (such as remote terminal devices) may have problems. For example, when communicating based on Ethernet, network cable damage, network switch failure, or IP address conflict will cause communication interruption. This will prevent operators from remotely monitoring and operating the reactor through the host computer, and also affect data transmission and backup.
Power failure
Power fluctuation
When the power supply grid has voltage fluctuations, it may affect the normal operation of the PLC control system. If the voltage is too high, it may damage electrical equipment such as PLCs, sensors, and actuators; if the voltage is too low, it may cause the equipment to fail to start normally or suddenly stop during operation. For example, when the stirring motor of the reactor encounters a low voltage during operation, the motor may stop rotating due to insufficient torque.
Power interruption
A sudden power interruption will cause the PLC control system to stop working immediately, resulting in an interruption of the reaction process in the reactor. If an uninterruptible power supply (UPS) is not configured, when the power is restored, the state in the reactor may become out of control because the startup process of the PLC takes a certain amount of time, and the data in the PLC may be lost because it is not saved in time.