Traditional engine starting program control systems for military and civilian aircraft generally employ a combination of electromechanical methods. Because electromechanical timing mechanisms control related relays and contactors to achieve engine starting program control, this not only increases the size and weight of the control system, leading to higher power consumption and lower reliability, but also results in a lack of system versatility due to fixed wiring hardware design. More importantly, mechanical wear gradually reduces the system's control accuracy. PLCs, on the other hand, combine the advantages of computer programming flexibility, comprehensive functions, and wide application with the advantages of relay systems, such as simple control, ease of use, and strong anti-interference capabilities. Furthermore, PLCs themselves offer advantages such as small size, light weight, and low power consumption. Therefore, replacing electromechanical timing mechanisms with PLCs to complete engine starting program control will significantly improve the performance of engine starting control systems.
2. Engine starting program control principle
The process of an engine transitioning from a stationary state to its lowest operating speed capable of generating power is called engine starting. To ensure the engine turbine (rotor) can smoothly and without impact rotate from a stationary state, the timing mechanism must adjust the starting torque of the starter motor in stages, gradually increasing the torque and controlling the timely injection and ignition of fuel into the engine combustion chamber. The starting procedure control principle of a certain type of aircraft engine is shown in Figure 1.
Figure 1. Engine starting procedure control principle
The timing mechanism's program control divides the starter motor's operation into the following stages:
The first stage, lasting 1 to 3.6 seconds after pressing the start button, involves the starter motor operating in a compound-excited state with the armature connected in series with the starting voltage-reducing resistor. The starter torque is limited to a very small range, allowing the starter motor to smoothly pass through...
The transmission device drives the engine turbine to rotate.
The second stage: within 3.6 to 9 seconds after pressing the start button, the starting voltage reduction resistor is shorted, the voltage across the starter increases, the starter torque increases rapidly, and the turbine speed rises rapidly.
The third stage: within 9 to 15 seconds after pressing the start button, the two batteries in the starting power vehicle switch from parallel connection to series connection, the voltage across the starter motor increases from 28V to 56V, the starter motor torque increases sharply, and thus the turbine speed increases sharply.
The fourth stage: within 15 to 22 seconds after pressing the start button, the starter motor shunt coil is connected in series with a voltage-reducing resistor, which reduces the starter motor's excitation flux, reduces the back EMF, increases the armature current, and increases the torque again, thereby further accelerating the turbine.
3PLC control system
3.1 System Hardware Design and I/O Address Allocation
Figure 2 Electrical control circuit diagram of engine starting procedure
In the engine starter program control system, the PLC adopts the Mitsubishi FX2 series FX2N-48MR-001 model. This series of PLCs has high reliability, strong anti-interference ability, and is suitable for use in military and civilian aircraft. It is also flexible in configuration and has a high cost performance [1]. As can be seen from Figure 1: In order to realize the four-stage control of the starter, from the press of the start button, the engagement time of contactors KM1 and KM2 is 9S~21S, KM3 is 3.6S~22S, KM4 is 1S~3.6S, KM5 is 1S~15S, and KM6 is 15S~22S. According to the control requirements of the system, the PLC control system needs to introduce two input relays corresponding to the stop button and the start button, six output relays corresponding to the four contactors and two relays, and four power-on delay time relays and two power-off delay time relays to control the above four contactors and two relays to work in different time periods. The electrical control circuit diagram of the engine start program and the I/O address encoding table of the PLC are shown in Figure 2 and Table 1, respectively.
Table 1 I/O Address Encoding Table
3.2 Software Design
Figure 3 Ladder diagram of the control system
The software design adopts the most widely used PLC ladder diagram graphical programming language. The ladder diagram is very similar to the circuit diagram of the relay control system. It is intuitive and easy to understand, and can be easily mastered by electrical personnel who are familiar with electrical control. It is especially suitable for switch logic control[2]. The ladder diagram of the control system is shown in Figure 3.
In Figure 3: X0 and X1 are input relays; Y1, Y2, Y3, Y4, Y5, and Y6 are output relays; T1, T2, T3, and T4 are power-on delay time relays; T5 and T6 are power-off delay time relays; and M0, M1, M2, M3, and M4 are intermediate relays.
4. Conclusion
By applying programmable logic controllers (PLCs) to the engine starting program control system, the performance of the control system can be greatly improved. This not only enhances the system's control accuracy and anti-interference capabilities, but also gives the system advantages such as small size, light weight, low power consumption, and strong versatility.
