What Makes Normally Open Solenoid Valves Stand Out in the Automotive Industry?

Solenoid valves are widely used in automotive electronic control systems and are a key actuating unit in the transmission control system; they complete rapid valve core movement by receiving a drive signal from the electronic control unit. The solenoid valve uses an energized solenoid to generate magnetism to change the opening and closing of the valve hole, thereby changing pressure or flow. Its physical structure is shown in Figure 1(1); in circuit principles, the solenoid valve is generally regarded as a circuit structure of a resistor and an inductor in series, and the corresponding simplified circuit model is shown in Figure 1(2).

Common solenoid valve control is divided into three types: ON/OFF control, PWM (pulse width modulation) open-loop control, and PWM closed-loop control. The first two are simple and have lower requirements; PWM closed-loop control can meet the demand for high-precision current control of the solenoid valve. The solenoid valve closed-loop control system is shown in Figure 2: the input gives a current, the piston-type normally open solenoid valve takes the difference between this and the sampled current, then performs PI (proportional-integral) adjustment on the difference and works out a new PWM duty cycle, to generate a stable current to drive the solenoid valve. This is the principle of solenoid valve current closed-loop control. The solenoid valve controls the size of the valve hole according to the magnetic field force generated by the current flowing through the solenoid, thereby establishing the corresponding pressure and adjusting the valve hole size to change the flow.

A simplified diagram of the hardware solution of the solenoid valve closed-loop control system is shown in Figure 3. This diagram mainly consists of a microcontroller chip (uController), a solenoid pre-driver module (Solenoid Pre-Driver IC), MOSFETs, and peripheral circuits. In this solution, the current detection range of the solenoid valve driver chip is 0~3A, the current setting resolution is 0.125mA/bit, and the current control accuracy is within 1%, with a second current feedback path. It integrates a sampling resistor and an operational amplifier internally and has a 20Mhz HSPI interface (compatible with the SPI interface). The solenoid pre-driver chip and the microcontroller obtain the corresponding commands via SPI communication, and at the same time feed back the output current, voltage, temperature, and faults to the microcontroller. This chip has a very comprehensive safety redundancy design and can realize a system design of ASIL D level in ISO 26262.

When the solenoid pre-driver chip receives the current preset value command, it begins to perform the current closed-loop adjustment work. Feedback is the prerequisite for promoting closed-loop control. The system feeds the sampled current back to the pre-driver chip; the pre-driver chip takes the difference between the preset value and the feedback value and, through control in the PI controller, outputs a PWM with the corresponding duty cycle. If the difference between the preset value and the feedback value is positive, the output PWM duty cycle increases and the output current increases; if the difference between the preset value and the feedback value is negative, the output PWM duty cycle decreases and the output current decreases. Adjusting repeatedly in this way completes the stable control output of the current.

To ensure good driving comfort, the electronically controlled normally open solenoid valve transmission control system has very high requirements for clutch control. Precise clutch control depends on precise control of the solenoid valve, to ensure that the clutch has good continuous controllability in the designed hydraulic system. Therefore, the current control accuracy in the solenoid valve drive design is very important. In hardware design, United Electronics will improve the current control accuracy from three levels.