Performance Simulation Analysis of Synchronous Hydraulic System for Variable-Diameter Threshing Drum

To meet the requirement that the traditional combine harvester can adjust the threshing gap in real time according to different working conditions, a hydraulic driven variable-diameter threshing drum and a synchronous hydraulic system were developed in the early stage. However, the performance of synchronous hydraulic system has not been studied in detail. Therefore, AMESim software was used to simulate and analyze the synchronous hydraulic system to verify its feasibility in this study. The results showed that the synchronization error of synchronous hydraulic system was less than 0.02 mm. When the hydraulic system was self-locking, the position fluctuation of the piston rod of hydraulic cylinder was less than 0.06 mm. When the hydraulic cylinder did not work, the logic valve effectively ensured that the hydraulic oil could directly return to the oil tank, resulting in the minimum energy loss of the system. The flow control valve could regulate the flow rate of the hydraulic system, causing the adjusting speed of the drum diameter can be adjusted. The hydraulic system pressure was determined to be 8 Mpa.


Introduction
The threshing gap was generally changed by adjusting the position of the concave grid and the diameter of threshing drum for grain combine harvester.Su et al. [1] used the discrete element method to establish a flexible rice stalk model and simulated the threshing process of the two adjustment methods.The results showed that at the same time, the adjustment type of drum diameter had better material separation and conveying ability than that of the concave grid, and the output mixtures were more evenly distributed in the receiving box.The application of hydraulic technology in the field of agricultural machinery reduces the energy consumption of agricultural machinery, improves the reliability and automation level of agricultural machinery, and makes the easy operation of agricultural machinery has been significantly improved [2][3][4][5].Therefore, Liu et al. [6] designed a hydraulic-driven variablediameter threshing drum and synchronous hydraulic system.However, the performance of synchronous hydraulic system has not been studied in detail.
In this paper, according to the principle of synchronous hydraulic system, the model of synchronous hydraulic system was established in AMESim software.To verify the synchronization and self-locking performance of the synchronous hydraulic system and determine the parameters of the selected Introduced the principle of synchronous hydraulic system: The static, the retraction and the extension status of the piston rod of hydraulic cylinder were realized using the electromagnetic reversing valve with three-position and four-way.The simultaneous motion of the piston rods of two hollow hydraulic cylinders was realized using two synchronous motor shunt and collector valves.Two hydraulic locks were used to prevent the flow of hydraulic oil to realize self-locking after adjusting the diameter of threshing drum.The logic valve was used to open and close the system flow rate by sensing pressure signals from load sensors, ensuring minimal loss of the system energy.When the hydraulic cylinder was stationary, the hydraulic oil returned to the oil tank.According to the working mechanism maximum pressure value, the pressure value of the working mechanism was changed using the direct acting overflow valve 2 to ensure that the main circuit, under low pressure, was unloaded.The motion speed of the piston rod was adjusted using the flow control valve.The pressure value of the system was set to prevent the pressure damage to the main pump using the direct action relief valve 1.

Simulation Model Parameter Setting
According to the hydraulic system pressure of the combined harvester is generally 10 ~ 18 MPa, the system pressure and flow parameters in the simulation model were set as 16 MPa and 20 L/min, respectively.Therefore, the opening pressure of the direct acting relief valve 1 was set to 16 MPa, the displacement of the hydraulic pump was set to 10 cc/rev, and the speed of the motor was set to 2000 rev/min.According to experience, the maximum flow rate of the logic valve P-B and P-A oil circuit was set to 100 L/min and 80 L/min, respectively.The oil circuits maximum flow rate of electromagnetic reversing valve was set to 100 L/min.The opening pressure of the direct acting relief valve 2 was set to 10 MPa.The maximum flow rate of the synchronous motor shunt and collector valve was set to 20 L/min.Therefore, the displacement of the hydraulic motor was set to 100 cc/rev, and the rotational speed of rotary inertia module was set to 100 rev/min.The opening pressure of the hydraulic control check valve was 3 bar.The piston of double rod hydraulic cylinder was set to 115 mm in diameter.The outer diameter of the piston rod at one end was set to 80 mm to indicate the rod-free cavity end of the hollow hydraulic cylinder.The outer diameter of the piston rod at the other end was set to 90 mm, indicating that the rod cavity end of the hollow hydraulic cylinder, which was connected with the mass block.The mass of the mass block was set to 10 kg.The stroke of the hydraulic cylinder was set to 60 mm, and the initial position was 30 mm.All other model parameters were default values.

Self-locking and Synchronization Performance Analysis
In the actual operation process of the combined harvester, the variable-diameter threshing drum will be subjected to the reaction force when the grain was compressed, and the loads of the hydraulic cylinders at both ends were different and will fluctuate.Therefore, the preset load variation is shown in figure 2    At 0-1 s, the piston rod position fluctuates greatly, but was less than 0.2 mm.This was caused by the sudden change of the pressure of the hydraulic system at the beginning of the simulation.At 1.5 s, the positions of the two ends of the piston rod moved to 32.54 mm and 32.56 mm, respectively.The synchronization error was 0.02mm, and the position fluctuation was less than 0.01 mm during selflocking.At 3 s, the position of the two ends of the piston rod moved to 25.12 mm and 25.10 mm, respectively.The synchronization error was 0.02mm, and the position fluctuation was less than 0.06mm during self-locking.At 4.5 s, the position of the two ends of the piston rod moved to 28.317 mm and 28.32 mm, respectively.The synchronization error was 0.003 mm, and the position fluctuation was less than 0.01 mm during self-locking.Therefore, when the diameter of the variable diameter threshing drum was adjusted or not. the synchronization and self-locking performance of the hydraulic system can meet the requirements.

Logic Valve Performance Analysis
When the control signal of the electromagnetic reversing valve changed as shown in figure 3, the flow of the logic valve ports A and B changed as shown in figure 5.When the control signal was 0 mA, the hydraulic cylinder did not work, the flow of oil port A was basically kept at 20 L/min, and the flow of oil port B was almost 0. When the control signal was 40 mA and -40 mA, the hydraulic cylinder worked, the flow rate of oil port A was almost 0, and the flow rate of oil port B was basically kept at 20 L/min.When the control signal was switched, it will cause the abrupt change of the flow.Therefore, when the hydraulic cylinder was not working, the hydraulic oil can be directly returned to the tank, and the logic valve effectively ensured that the system energy loss was minimal.

Performance Analysis of Flow Control Valve
Too fast speed of the piston rod is easy to cause damage to the actuator, and too slow the adjusting time of the variable-diameter threshing drum becomes longer.The flow rate of the hydraulic system determines the moving speed of the piston rod of the hydraulic cylinder, so the flow rate of the synchronous self-locking hydraulic system is adjusted by adjusting the flow area of the throttle of the speed-regulating valve, thereby adjusting the moving speed of the piston rod.The maximum opening diameter of the throttle port was 5 mm, and the control signal K value was set to 0.25 and 0.5, respectively.The corresponding opening diameters were 1.25 mm and 2.5 mm, respectively.When the control signal of the electromagnetic reversing valve was -40 mA, the electromagnetic reversing valve was in the right position, and the piston rod was extended.The simulation time was 0.6 s, and the other parameters were unchanged for comparative simulation analysis.Seeing figure 6a, at 0.6 s, when the opening diameter was 1.25mm and 2.5mm, the piston rod position moved about 4 mm and 0.5 mm, respectively.Therefore, the piston rod moving speed decreased with the increase of the opening diameter, which was mainly caused by the decrease of the flow rate at the rod-free cavity, as shown in figure 6b.Therefore, the flow control valve can adjust the flow of the synchronous hydraulic system to adjust the moving speed of the piston rod of the hydraulic cylinder, making the adjustment speed of the diameter of the variable-diameter threshing drum adjustable.

Determination of the Opening Pressure for the Direct Acting Relief Valve
The load of the hydraulic cylinder determines the pressure required by the hydraulic system.Too much pressure can not ensure the unloading of the main circuit under low pressure, and too little pressure can not meet the working requirements of the actuator.When the diameter of the variable diameter drum was the largest, the maximum load was -11762 N. Therefore, assuming that the piston rod was subjected to ±12000 N, the oil pressure of the rod-cavity and rod-free cavity of the hollow hydraulic cylinder was analyzed.The simulation time was 4 s.At 0 ~ 1 s, the control signal of electromagnetic reversing valve was 0, the piston rod did not move.At 1 ~ 2 s, the control signal was -40 mA, and the piston rod was extended.At 2 ~ 4 s, the control signal was 40 mA, and the piston rod was retracted.
When the load was -12000 N, the change of oil pressure is shown in Figure 7a.When the piston rod was stationary at 0 ~ 1 s, the piston rod had a tendency to extend under the action of load force.When the piston rod was extended in 1 ~ 2 s, it can be extended without the thrust generated by the rod-less cavity.Therefore, the pressure was only produced on the end of the rod-cavity when the piston rod was static or extended, while the oil pressure of the rod-free cavity was 0. When the piston rod was retracted in 2 ~ 4 s, the rod cavity needed to provide pressure to push the piston rod into retraction, and the hydraulic oil without the rod cavity will be subjected to a certain pressure, resulting in a rise in oil pressure.Therefore, when the load was -12000 N, the oil pressure of the rod-cavity was always greater than that of the rod-free cavity.
When the load was 12000 N, the change of oil pressure is shown in Figure 7b.When the piston rod was stationary at 0 ~ 1s, the piston rod had a tendency to retract under the action of load force, causing the piston rod produced pressure on the end of the rod-free cavity, while the oil pressure of the rod cavity was 0.
When the piston rod extended in 1 ~ 2 s, the rod-free cavity needed to provide pressure to push the piston rod out, and the hydraulic oil with the rod cavity will be subjected to a certain pressure, resulting in a rise in oil pressure.Therefore, when the load was -12000 N, the oil pressure of the rod-free cavity was always greater than that of the rod cavity.Seeing figure 7, the peak oil pressure with or without the rod cavity was less than 7 Mpa.Therefore, the opening pressure of the direct-acting relief valve 2 was set to 8 Mpa.

Conclusion
In this paper, according to the principle of synchronous hydraulic system, the hydraulic standard library of AMESim was used to build the system simulation model, and simulation analysis was conducted to verify the feasibility of synchronous hydraulic system.The main conclusions were as follows: 1.The synchronization error of the synchronous hydraulic system was less than 0.02 mm, and the position fluctuation of the piston rod during self-locking was less than 0.06 mm.The synchronization and self-locking performance of this hydraulic system can meet the requirements.
2. When the hydraulic cylinder was not working, the logic valve effectively ensured that the hydraulic oil can be directly returned to the tank, causing the system energy loss was minimal.The flow control valve can adjust the flow of the synchronous hydraulic system to adjust the moving speed of the piston rod of the hydraulic cylinder, making the adjustment speed of the diameter for the variable-diameter threshing drum adjustable.The opening pressure of the hydraulic system was set to 8 Mpa.
This hydraulic system can provide theoretical basis and reference value for the research of hydraulic drive system of synchronous motion structure in agricultural machinery field, and provide device basis for promoting the automation development of agricultural mechanization.

Figure 1 .
Figure 1.Simulation model of synchronous hydraulic system.
, and the fluctuation range was 8362 ~ 11652N.The input control signals of the electromagnetic reversing valve are shown in figure 3, which were switched between 0 mA, -40 mA and 40 mA, and the directional valve was respectively in the middle, right and left positions.It can be seen from figure 4 that when the simulation time was between 1 ~ 1.5 s and 4 ~ 4.5 s, the electromagnetic reversing valve was in the right position and the piston rods at both ends extended synchronously.When the simulation time was 2.5 ~ 3 s and 5.5 ~ 6 s, the electromagnetic reversing valve was in the left position, and the piston rods at both ends were retracted synchronously.When the simulation time was 0 ~ 1 s, 1.5 ~ 2.5 s, 3 ~ 4 s, 4.5 ~ 5.5 s, the electromagnetic reversing valve was in the middle position, and the hydraulic lock cut off the hydraulic oil circuit.

Figure 2 .
Figure 2. Schematic diagram of preset hydraulic cylinder load variation.

Figure 3 .
Figure 3. Schematic diagram of control signal change of electromagnetic proportional reversing valve.

Figure 4 .
Figure 4. Schematic diagram of piston rod position change.

Figure 5 .
Figure 5. Schematic diagram of flow changes of oil ports A and B of logic valve.

Figure 6 .
Figure 6.Performance analysis of flow control valve.