Design of an automatic transmission controller based on a FreeSCALE microcontroller

0 Introduction

With the increase in car ownership and repair volume in my country, the automatic transmission remanufacturing industry has gradually developed. Since 2000, my country has begun to import and produce automatic transmission vehicles in large quantities. Currently, the number of vehicles with automatic transmissions exceeds 4 million. Considering that there are more new cars using automatic transmissions than used cars in my country, even if we calculate remanufacturing at 1.5% of the total car ownership, the demand would still be over 60,000 units. The market demand for automatic transmissions is strong, and the remanufacturing industry has great potential. This controller was developed specifically for automatic transmission remanufacturing.

The solenoid valve is one of the most important components of an automatic transmission, playing a crucial role in gear shifting. The accuracy of shift control is closely related to the solenoid valve's operation. It communicates with the VCU (Vehicle Control Unit) to obtain shift parameters, which are vital for determining the shift timing.

1. System Structure and Working Principle

The system has six independent output solenoid valve control channels and consists of an input module, an output module, and a communication module. The system structure is shown in Figure 1.

Figure 1 System Functional Block Diagram

This system is designed for the electronic control unit (TCU) of a transmission. The TCU communicates with the ECU via a CAN bus and obtains the current status of the transmission, such as position, speed, and temperature, through sensors. Gear shifting is achieved by a microcontroller outputting PWM to control solenoid valves, thereby altering the oil circuits of relevant hydraulic control valves and enabling the actuators to operate. The serial port displays the relevant information to the host computer.

2 System Hardware Design

The hardware of this system consists of a control module and a drive module. The control module generates PWM signals, receives information from the CAN bus, receives signals from sensors, and samples the current. The drive module amplifies the control signals to control the solenoid valve. The signal processing circuit converts the signals collected by the sensors into signals that the microcontroller can receive.

2.1 Main Chip Selection

The S12X series has a bus frequency up to 50MHz, full CAN functionality, and improved interrupt handling capabilities compared to the HCS12 series. The XEP100 has 117 interrupt sources and can access up to 8MB of all memory space (on-chip and off-chip). The XB, XD, and XS families in the S12X series have a maximum bus frequency of 40MHz, the XE family has 50MHz, while most S12 cores have a maximum bus frequency of 25MHz.
The XEP100 features 8 PWM outputs, 5 independent CAN controller modules, 2 16-channel AD conversion modules with a maximum precision of 12 bits, and an ECT-enhanced clock enhancement module.

2.2 Sensor Access Hardware Circuit Design

Hall effect sensors are used for the speed sensors in the gearbox. The speed sensor connection circuit designed for this purpose is shown in the figure. The function of this circuit is to condition the sensor output signal and to make the circuit design adaptable to a variety of sensor types.

Figure 3. Speed ​​sensor hardware circuit diagram

Figure 5 Speed ​​sensor signal

2.3 CAN Bus Access Hardware Circuit Design

Since the CAN controller is only a protocol controller and cannot provide physical layer drivers, the CAN node must be physically connected to the CAN bus through a transceiver. Each CAN module has two pins: transmit and receive. Here, we choose the PHILIPS TJA1050 transceiver. In environments with low electromagnetic interference, the isolation circuit can be omitted, thus connecting the CAN controller to the CAN transceiver. The circuit diagram designed in this circuit is shown in the figure:

Figure 6 CAN module access circuit

2.3 Design of Solenoid Valve Connection Hardware Circuit

Figure 7. Schematic diagram of the solenoid valve connection circuit.

The TCU (Transmission Control Unit) receives CAN information from the ECU and acquires signals from sensors. It then outputs PWM pulses via control commands to drive solenoid valves. These solenoid valves control the oil pressure at the ports of the hydraulic control valves, thereby altering the on/off state of the relevant oil circuits to control the final actuators. By using sensors to measure position, speed, and other information, as well as voltage acquired through A/D conversion, the PWM duty cycle is adjusted to ultimately achieve the set control value.

The TCU (Transmission Control Unit) receives CAN information from the ECU and acquires signals from sensors. It then outputs PWM pulses via control commands to drive solenoid valves. These solenoid valves control the oil pressure at the ports of the hydraulic control valves, thereby altering the on/off state of the relevant oil circuits to control the final actuators. By using sensors to measure position, speed, and other information, as well as voltage acquired through A/D conversion, the PWM duty cycle is adjusted to ultimately achieve the set control value.

3. Software Design and Experimental Results

Transmission electronic control unit main program diagram

In the software flowchart, normal TIC operation includes current detection during solenoid valve operation, which is obtained through A/D sampling resistors. During gear shifting, the VCU communicates with the transmission control unit in real time via the CAN bus using the SAE J1939 protocol. It drives the solenoid valve based on the shifting rules stored in flash memory, thus performing the corresponding shifting operation. While the XE series microcontroller is a dual-core microcontroller, it operates in a master-slave configuration. The slave CPU processes interrupts and returns the results to the master CPU; however, the master CPU can also process interrupts independently. To ensure real-time performance, interrupts are used to send and receive CAN frames during program design.

Table 1. Experimental data at a speed of 650 r/min under no-load conditions.

4. Conclusion

This system uses the MC9S12XEP100 MCU. A controller was developed for a high-power hydraulic automatic transmission for a certain company, with production planned for the following year. This controller generates eight PWM signals with adjustable duty cycles, rise times, and fall times. The duty cycle of the PWM signals can be adjusted from 0% to 99% in 1% steps. The system communicates with the ECU using the SAE J1939 protocol, which meets the reliability and real-time requirements for engineering vehicles. Currently, research on high-power hydraulic transmissions (up to 350kW) is still lacking in China. It is hoped that this paper will provide some reference for the development of the electronic control components of high-power automatic transmissions. The completed PCB board is shown in the figure.

References

[1] Zhao Qiang, Xie Feng, Yu Tianming. Current status and technological trends of automatic transmissions in automobiles [J]. Machinery: Review. 2012, 12 (37)
[2] Jorg Schauffele and Thomas Zurawka, authors. Translated by Zhang Ju et al. Automotive Software Engineering: Principles, Processes, Methods, and Tools [M]. Beijing: Electronic Industry Press, 2008.
[3] Shi Wenku (ed.). Modern Automotive New Technologies (First Edition) [M]. Beijing: National Defense Industry Press, August 2008.
[4] Ronald K. Jurgen, ed., translated by Lu Zhixiong et al., Automotive Electronics Handbook (Second Edition) [M]. Beijing: Electronic Industry Press, March 2010.

Dai et al. (University of Electronic Science and Technology of China, Postcode: 611731, Tel: 15828067451, Contact Address: Building 17, Master's Building, Qingshuihe Campus, University of Electronic Science and Technology of China, Chengdu, Sichuan)