Introduction to solenoid valve current closed loop control system

- Oct 06, 2019-

With the development of automotive electrical automation control, the number and types of solenoid valves are increasing, and the requirements for solenoid valve control accuracy are getting higher and higher. The accuracy of the solenoid valve control will affect the control accuracy of the clutch, and the accuracy of the clutch control is directly related to the operating performance and user experience of the car. Therefore, improving the accuracy of solenoid valve control is one of the important goals of hardware design.


Introduction to solenoid valve control


Solenoid valves have a wide range of applications in automotive electronic control systems and are key actuators in transmission control systems that achieve fast spool motion by receiving drive signals from electronic control units. The solenoid valve relies on the energizing solenoid to generate a magnetic force to change the opening and closing state of the valve hole, thereby changing the pressure or flow. The physical structure is shown in Fig. 1 (1). In the circuit design, the solenoid valve is generally regarded as a circuit in which the resistor and the inductor are connected in series. The structure, the corresponding simplified circuit model is shown in Figure 1 (2).


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(1) Physical structure of solenoid valve


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(2) Solenoid valve simplified circuit model


Figure 1 Physical structure and equivalent circuit model of solenoid valve


Common solenoid valve control is divided into ON/OFF control, PWM (pulse width modulation) open loop control and PWM closed loop control. The first two types of control are simple and require less, and the PWM closed-loop control can meet the requirements of high current precision control of the solenoid valve. The closed-loop control system of the solenoid valve is shown in Figure 2. The input current is input, and the current is compared with the sampled current. Then the difference is PI (proportional integral) to modulate the new PWM duty cycle to form a constant current. To drive the solenoid valve work, this is the principle of closed-loop control of the solenoid valve current. The solenoid valve controls the size of the valve hole according to the electromagnetic force generated by the current flowing through the solenoid, thereby establishing a corresponding pressure, and then adjusting the size of the valve hole to change the flow rate.



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Figure 2 Solenoid valve closed-loop control system


Hardware solution


A schematic diagram of the hardware solution for the closed-loop control system of the solenoid valve is shown in Figure 3. This diagram consists mainly of a microcontroller chip (uController), a Solenoid Pre-Driver IC, a MOSFET, and peripheral circuits. In this scheme, the solenoid valve driving chip current measurement range is 0~3A, the current setting resolution is 0.125mA/bit, the current control precision is within 1%, and the second current feedback path is provided. Internal integrated sampling resistor and operational amplifier with 20Mhz HSPI interface (compatible with SPI interface), solenoid pre-driver chip and microcontroller through SPI communication to obtain the corresponding command, while feeding back the output current, voltage, temperature and Faults, etc., the chip has a comprehensive safety redundancy design, can achieve the ASIL D system design in the ISO26262 class.



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Figure 3 Solenoid valve closed-loop control system hardware solution


When the solenoid pre-driver chip receives the current set value command, it starts to perform the current closed-loop adjustment work. The feedback is the basis of the closed-loop control. The system feeds the sampled current back to the pre-driver chip, and the pre-driver chip sets the set value and the feedback value. To do the difference, by adjusting the PI controller, output the PWM with the corresponding duty cycle. If the difference between the set value and the feedback value is positive, the output PWM duty cycle will become larger, and the output current will increase; if the difference between the set value and the feedback value is negative, the output PWM duty cycle will decrease, and the output will decrease. The current will decrease. This adjustment is repeated to achieve a stable control output of the current.


In order to ensure better driving comfort of the vehicle, the transmission control system has higher control requirements for the clutch. Precise control of the clutch relies on precise control of the solenoid valve to ensure good continuous controllability of the clutch in the designed hydraulic system. Therefore, the control accuracy of the current in the solenoid valve drive design is very important. In hardware design, the joint electronics will improve the current control accuracy at three levels.


Use high precision solenoid valve pre-driver chip


The device that samples the current in the feedback loop is the sampling resistor (shunt), and then the operational amplifier further amplifies the sampled voltage, and the accuracy requirements for both devices are high. In order to pursue higher sampling accuracy, the latest products from major chip suppliers have integrated sampling resistors and operational amplifiers inside the chip, so that some self-calibration work can be done at the chip level to calibrate the sampling resistor and the operational amplifier itself. 


Well designed circuit


After ensuring the detection accuracy of the closed-loop feedback, it is also necessary to ensure the stable output of the forward channel current to achieve stable driving of the solenoid valve. The actuators of the forward channel hardware circuit are MOSFETs and freewheeling diodes, which are optimized to reduce the effects on current accuracy control.


Accuracy improvement at the TCU (Transmission Controller) system level


The first two steps basically achieve the optimization of the accuracy of the entire loop, but due to the dispersion of the chip, and the chip is affected by temperature when working. In order to further improve the current control accuracy of the TCU, it is first necessary to reduce the inherent deviation of the chip itself, and at the same time analyze the accuracy variation of the chip due to temperature. For this reason, the joint electronic will also perform a TCU-level current accuracy calibration on the solenoid valve drive system to further Improve the control accuracy of the closed-loop control system of the solenoid valve.


to sum up


In the hardware design of the solenoid valve current closed-loop control system, closed-loop control is only the basic principle of achieving high-precision control. In order to achieve higher-precision control, the accuracy of the combined control of the current control from three layers is improved: through the self-calibration of the chip. At the chip level, the accuracy of the feedback path is improved. The precision of the drive path is improved from the circuit level through the optimized design of the circuit. The accuracy of the system is improved from the TCU level by the current accuracy compensation of the TCU. Improving the accuracy of solenoid valve control is an important goal of hardware design, and the future solenoid valve drive system will be more "excellent."