A New Type of Oil Depot Liquid Level Measurement System

1 System Working Principle
Real-time monitoring and recording of oil volume in oil depots is a regular task of the oil depot system. For safety reasons, electrical equipment is not allowed in the oil depot, and smoking and fire are strictly prohibited. Traditional oil depot level measurement relies on manual operation. Each oil tank has size marks inside and outside. When the oil level needs to be measured, someone is sent to climb to the top of the oil tank, open the top cover, and use visual method to determine the position of the oil surface and record the change in oil volume. This method is more troublesome and the measurement error is also large, but electricity is not allowed on the oil tank, which brings great difficulties to automated measurement. The author has cooperated with a research institute to develop a system that uses a photoelectric isolation system to measure the oil depot level, which solves this problem well: use a balanced transmission device on the top of the oil tank, use a light source to measure the change in oil level, and then transmit this optical signal that changes with the oil volume to the monitoring center through optical fiber. In this way, the monitoring center can monitor and control the oil volume in the oil depot in real time, which not only ensures the safety of the oil depot, but also achieves the purpose of automated management of the oil depot. At the same time, it also greatly improves the measurement accuracy of the inlet and outlet oil volume, which can significantly improve the economic benefits of the oil depot system. This method can be extended to real-time monitoring of oil density and temperature, and has a very broad market prospect. Figure 1 is a block diagram of the system.
In Figure 1, the buoy and the cone act as a balance. When the oil surface is stationary, the cone remains stationary, and the light emitted by the light source remains unchanged through the gear. When the oil surface floats up and down, the gear rotates. At this time, the light passing through the gear hole will change at a certain rate. This changing light signal is transmitted to the subsequent equipment through the optical fiber for processing. Whether the oil level rises or falls can be determined based on the different forward and reverse conductivities of the light passing through the gear hole.

In order to determine whether the oil tank is entering or exiting the oil tank, two optical signals need to be generated: the reference signal and the comparison signal. The two optical conversion and amplification circuits are exactly the same, but the delay of the optical signal is different. The optical signal is converted into an electrical signal through the optical converter, and after two stages of amplification and calibration, it is converted into a TTL level and sent to the data processing equipment. Figure 3 is the waveform observed when the oil is entering and exiting. As the gear rotates, the circuit will output a series of pulse signals. The faster the gear rotates, the higher the pulse frequency, and the faster the oil volume changes. In other words, when the gear rotates one grid, the circuit will output a pulse signal, which means the distance represented by the increase or decrease of one grid in the oil depot. The function of the data processing circuit to be introduced below is mainly to accurately count the number of pulses output by the amplifier circuit, and then convert the number of pulses into the corresponding height of the oil depot through the conversion circuit.

2 Hardware Design of Data Processing Circuit

The data processing circuit is the core of the whole system. All the circuits are integrated in a programmable logic device EPLD. The device uses the EPM7000 series produced by Altera. The main features of this chip are small size, low price, flexible pin selection, and 3,000 valid gates. It is particularly suitable for the design of small and medium-sized circuits. In the specific design process, the circuit to be designed is described and input in VHDL language or circuit diagram using Altera's development software MAX+PLUSII. The software automatically compiles, lays out and routes, generates programmable POF files and SNF simulation files, and can be loaded into EPLD through the programmer after the simulation results are correct.
The main functions of this circuit are: estimating the clock frequency of the input signal, selecting the appropriate clock division signal, synchronously shaping the two input signals, eliminating the burrs caused by the edge effect, and avoiding the error of pulse number statistics caused by the burrs. Identify the input signal, and judge whether the circuit works in mode 1, 2, or mode 1, 3 according to the different delays of the comparison signal to the reference signal. If the counter works in mode 1 or 2, it counts forward; if the counter works in mode 1 or 3, it counts backward; if the input is a constant level, the counter is in a hold state. Finally, the counter result is output to the LCD display module or computer. Figure 4 is a data processing block diagram.

The quality of the shaping recognition circuit design is directly related to the performance indicators of the entire system. Before designing the shaping recognition circuit, we analyzed the working conditions of the oil depot. Since the volume of the oil tank is generally large, the speed of oil inlet and outlet is relatively slow. We have used water instead of oil to do simulation experiments. The period of the optical signal becoming a pulse electrical signal is generally more than 500ms. In such a long-period signal, there will be some large glitches (the maximum measured glitches are 2ms wide). If this signal is used as the counter counting clock, it will cause a large error to the system. Considering that glitches mostly appear on the rising or falling edge stage, you can choose to latch the input signal with a 500μs clock. In this way, it can play a synchronization role and eliminate some small glitches. The remaining glitches are glitches greater than 500μs. Then use this signal as the reset signal of the 161 counter, and use the 500μs clock as the counting clock of the 161 counter. Connect as shown in Figure 5 to eliminate the interference pulses of 2ms on the leading and trailing edges. Send the two shaped signals to the recognition circuit.

The circuit generates forward and reverse control signals, among which the reference signal is sent to the forward and reverse counter circuit as a counting pulse. The identification circuit distinguishes according to the difference in the rising edge of the reference signal and the comparison signal. When the oil is in, the rising edge of the reference signal and the comparison signal is approximately aligned. When the oil is out, the rising edge of the reference signal and the comparison signal differs by half a clock beat. The number of counting pulses in the positive level is compared to determine the oil in and out. The specific circuit implementation is shown in Figure 5.

3 Conclusion
Practice has proved that this system has solved the problem of measuring the oil volume in the oil depot. The simulation experiment has good results. The measurement error is controlled within 2 pulse signals. It can accurately and timely reflect the changes in oil volume. It is easy to form a reliable monitoring system with a microcomputer, which provides a useful attempt for the automation of the oil depot management system.

1 Liu Baoqin, Zhang Fanglan, Tian Lisheng. Application of Altera Programmable Logic Devices. Beijing: Tsinghua University Press, 1995