# Optimization of Mode-Switching Quality of Hybrid Tractor Equipped with HMCVT
## 3 Optimization of Control Policy for Driver Source Switching
### Fuzzy Control Strategy of Clutch Oil Pressure  
To a large extent, the friction torque of the clutch depends on the control of the clutch engaging oil pressure. Accurate control of the clutch requires reasonable adjustment of the size and rate of change of the engaging oil pressure to ensure that the clutch can provide stable and reliable transmission effect during the working process. The adjustment of the initial engaging oil pressure and the oil pressure change rate is very important for the normal operation of the clutch. Proper setting of the initial engaging oil pressure can affect the smoothness and sensitivity of the clutch when starting, while controlling the oil pressure change rate can affect the clutch transition process and transmission efficiency [22]. This paper adopts the fuzzy control strategy of clutch oil pressure, as shown in Figure 8. The fuzzy control system is divided into two parts: initial engaging oil pressure fuzzy control and oil pressure change rate fuzzy control, which can effectively control the engaging process of the clutch. (1)  
Engaging the initial oil pressure fuzzy control stage. The gas pedal opening \(\alpha\) and the gas pedal opening change rate \(\dot{\alpha}\) are taken as the input of the initial oil pressure fuzzy control stage, and the initial oil pressure increment \(\Delta P\) is taken as the output. The initial clutch oil pressure increment \(\Delta P\) is added to the preset initial engagement oil pressure \(P_{1}\) to obtain the initial clutch engagement oil pressure \(P_{0}\). \[P_{0}=\Delta P+P_{1} \tag{11}\]  
The Mandani fuzzy controller was adopted to clarify the output fuzzy quantity by the gravity center method, and the rule surface of the initial oil pressure fuzzy control for clutch engagement was obtained, as shown in Figure 9.  
(2)  
Fuzzy control stage of oil pressure change rate  
The accelerator pedal opening change rate \(\dot{\alpha}\) and the absolute value \(|\Delta\omega|\) of the speed difference between the clutch driving plate and the driven plate are used as the input of the oil pressure change rate fuzzy control stage, and the clutch oil pressure change rate \(\Delta P\)  
Figure 8: Fuzzy control system. Figure 9: Clutch engagement initial oil pressure fuzzy control rule surface. is used as the output. The clutch oil pressure \(P\) is obtained by integrating the change rate of clutch oil pressure and adding the initial clutch oil pressure \(P_{0}\). \[P=P_{0}+\int_{t_{2}}^{t_{3}}\Delta\dot{P}dt \tag{12}\]  
where \(t_{2}\) is the start time s of clutch sliding and \(t_{3}\) is the end time s of clutch sliding. The Mandani type fuzzy controller was used to clarify the output fuzzy quantity using the center of gravity method, and the clutch oil pressure change rate fuzzy control rule surface was obtained, as shown in Figure 10.  
When the vehicle starts to switch modes, clutch C0 engages. The friction torque \(T_{c0}\) transmitted by clutch C0 is controlled by the fuzzy controller. The engine starts to accelerate under the drag of the friction torque \(T_{c0}\) of clutch C0 and the self-starting torque. To ensure the smooth acceleration of the engine, the fuzzy controller is used to adjust the change rate of the clutch oil pressure at this stage. When the engine speed rises to 700 r/min or so, the engine fires and starts. When the engine speed control is carried out and the engine speed continues to rise to close synchronization with the motor speed, the clutch is quickly engaged [23]. Before the engine ignition and start, the motor should not only maintain the torque required by the vehicle, but also offset the torque \(T_{c0}\) caused by the friction of the clutch. The target torque \(T_{m\_tar}\) of the motor is:  
\[T_{m\_tar}=T_{req}+T_{c0} \tag{13}\]  
Due to the small torque of the engine after ignition, the engine torque rises slowly, so the motor compensates for the engine torque.