The liquid level weighing module supplies oil to the oil tank of the oil station through the oil pump. The lower limit of the liquid level is 0mm and the upper limit is 500mm. The oil flows out through the pump outlet valve and the flow rate is controlled by adjusting the opening degree of the regulating valve to maintain the liquid level at 200mm. The actual liquid level is measured by the weighing module, converted into a 4mA - 20mA DC signal, and input to the AI module through A/D conversion. The input range is converted to the actual liquid level value, and this value is compared with the set value to perform PLC control calculation to obtain the opening degree of the regulating valve. Then, the output range conversion and D/A conversion are carried out to obtain the 4mA - 20mA DC signal, which is used to control the opening degree of the regulating valve. Finally, the PLC control is achieved for the liquid level. The process variables and control outputs are both single-polarity analog quantities. In the PLC control program, it is required that the main program realizes input and output range conversion, calls the serial port data of the weighing instrument, converts the digital quantity after A/D conversion into a liquid level value within 0mm - 500mm, and converts the output operation value into a digital quantity. The result is output to the analog output memory. The initialization program is executed in the first scanning cycle of PLC power-on operation to initialize the parameters, set the water level value, PLC control parameters, assign initial values, and activate the PLC function. The loop interrupt program is called according to a fixed time interval to repeatedly call the PLC program block.

The weighing module collects the control system parameter data and transmits it to the upper computer. It receives corresponding instructions to control the actuators of the oil station. The collected signals include digital quantities, standard analog quantities, pulse signals, and digital signals, and the control output signals are controlled by switch, analog quantity, and PLC adjustment quantities. The main PLC technical indicators of the weighing module include input indicators, switch state detection, and output indicators. The input indicators include the digital and pulse quantity detection of the weighing sensor and transmitter without additional errors, and the analog quantity detection accuracy is better than ±0.1. The switch detection mainly detects the valve position, pump switch status, liquid level switch, gas alarm switch, etc. The PLC is used in the station control system of the oil station, and often collects the analog quantity signals at a 500ms/period timing, and stores the channel content in the corresponding data area. For the pulse quantity output by the on-site weighing module, the PLC treats it as a high-speed counting input and collects it through the unit, and performs PLC calculation on the frequency of the collected signal. To ensure a small precision conversion error of the system, the signal can be connected to the PLC through the communication port for overall control; at the same time, the acquisition sub-program is executed by the timing interrupt method to avoid pulse loss and signal interference problems. For analog quantity collection, the ladder diagram uses the average value method to filter and collect the analog quantity signal. The method of using PLC to monitor the station control system of the oil station mainly includes action response detection, and requires feedback return signals, adding I/O points for real-time monitoring.